SACPM2015

4 May 2015, 08:00 → 8 May 2015, 18:00 Africa/Johannesburg

Mabula Game Lodge

Jacqueline Nel (University of Pretoria)

6^th South African Conference on Photonic Materials

Paper Abstract_template.docx

summary SACPM2015_Programme_Book_Final_web2.pdf

Monday, 4 May

14:00 → 18:00 Registration

18:00 → 19:00 Welcome

Welcome reception

19:00 → 21:00 Dinner

Tuesday, 5 May

07:00 → 08:30 Breakfast

08:30 → 09:10 Plenary

08:40 Lattice location of dopants in group-III nitrides and ZnO

The properties (electric, optic, and magnetic) of impurities and dopants in semiconductors are strongly dependent on the lattice sites which they occupy. Although the main occupied site, for a given impurity-host combination, can often be predicted based on chemical similarities between impurity and host elements, such expectations fail in many cases. Furthermore, minority sites (in case of multiple-site occupancy) are even more difficult to predict, detect and identify. In this talk, we give an overview of recent lattice location studies for impurities and dopants in ZnO and GaN, as representative wide-gap semiconductors. These experiments are based on the emission channeling (EC) technique, using radioactive isotopes produced at the ISOLDE facility at CERN. EC makes use of the charged particles emitted by a radioactive isotope upon decay. The screened Coulomb potential of atomic rows and planes determines the anisotropic scattering of the particles emitted isotropically during the radioactive decay. Along low-index directions of single crystals and epitaxial films, this anisotropic scattering results in well defined channeling or blocking effects. Because these effects strongly depend on the initial position of the emitted particles, they result in emission patterns which are characteristic of the lattice site(s) occupied by the probe atoms. In this talk, we present some particular cases that illustrate the strengths of emission channeling when studying systems exhibiting multiple-site occupancy.

Speaker: Prof. André Vantomme (KU Leuven, Leuven, Belgium)

09:10 → 10:50 Presentations: Tuesday Session 1

09:10 Effective doping of ZnO quantum dots with Rare Earth ions for opto-electronic applications

Speaker: Mr GUY LEBA KABONGO (University of South Africa)

09:30 The Role of Titanium Valence on the Luminescence of ZrO₂ Materials

1. Introduction The optical properties of zirconia materials have received much attention though no outstanding applications have been established so far. The advantage of combining the efficient luminescence of the trivalent rare earths (R3+) and the stable and versatile zirconia host has finally been ruined by the incompatibility of these two systems: the charge compensation defects created by the aliovalent substitution of the tetravalent zirconium with trivalent (or divalent) rare earths have jeopardized the luminescence efficiency [1]. So far, no efficient tetravalent luminescent rare earth has been found and, undoubtedly, will not be in the future either. The room temperature UV excited luminescence of the non-doped ZrO2 or zirconates or even hafnates have been noted in several reports. In addition to Ti impurity luminescence, defects as oxygen vacancies and colour centers have been offered as origins to luminescence. The Ti impurities were mentioned as the luminescence center and a mechanism to the persistent luminescence was proposed recently [2]. Regarding the details of the luminescence mechanism, synchrotron radiation techniques such as X-ray absorption and vacuum UV (VUV) spectroscopy can unveil more details on the elements’ valences as well as energy transfer processes. 2. Results The monoclinic ZrO2 nanomaterials were prepared using the sol-gel route followed with annealing at 1000 oC for 5 h. The typical VUV excited luminescence was observed as a broad band centred at 500 nm (Fig. 1, left). Starting with opposite hypotheses to explain this emission, the materials were doped either with Ti3+ or Lu3+ (both 0.5 mole-%) supposed to enhance the Ti3+ or defect emission, respectively. The emission spectra at room temperature show clearly that the 500 nm luminescence is due to a Ti3+ impurity, albeit at a trace element level [2]. The analogy between Ti3+/IV and Tb3+/IV suggests the dopant can exist in both oxidation states in ZrO2 depending on the post-preparation treatment of the material. The XANES measurements (Fig. 2) on the Ti K edge proved inconclusive due to complicated pre-edge structure, however. Low temperature emission spectra with VUV excitation agree with the emission of both TiIV and Ti3+ species in the Lu3+ doped and non-doped material. Lu3+ doping enhances slightly the Ti3+ emission. The TiIV charge transfer emission centered at 390 nm agrees with the UV-vis absorption edge of TiO2. The broad bands at 750 nm may be attributed to TiIV Ti3+ polarons [3] supporting the presence of both ions. The Ti doped materials reveal only the 520 nm transition due to the eg → t2g transition of Ti3+ splitting when symmetry lowers from Oh. The UV-VUV excitation spectra (Fig. 1, right) show energy transfer from the ZrO2 host to Ti3+/IV with Eg at 5.25 eV which decreases with increasing temperature as usual.

Speaker: Mr Jose Carvalho (University of Sao Paulo)

09:50 Polarization Controlled Light Emitter based on Elongated Pyramidal Quantum Dots

1.Introduction Polarized light is the basis for a manifold of optoelectronic technologies ranging from telecommunication and LCD-displays to quantum cryptography. However, it is challenging to efficiently attain a strong polarization of spontaneously emitted light, in particular for the generation of single photons. In solutions of today, mainly filtering of unpolarized light is employed. In this process, a higher light transmission rate larger than 50 percent from the source can never be achieved. Such a loss is particularly devastating for quantum information technologies, in which each single quantum of light, i.e. each photon, literally counts. 2. Results Our approach is based on the unique properties of the III-nitride materials, known to be efficient light emitters in the blue and UV part of the spectrum. We have employed μm-sized GaN pyramids with six facets, formed in etched circular holes in a patterned substrate by Selective Area Growth (SAG). The holes are made in a SiN film on top of the substrate by means of standard UV lithography and RIE etching. The tips of the pyramids are made slightly truncated, as has been demonstrated to be advantageous for quantum dot (QD) formation. The pyramids are subsequently overgrown by a thin optically active InGaN quantum well and finally capped with a thin GaN layer. Due to the accumulated strain caused by the lattice mismatch between the GaN and InGaN well, InGaN QDs will evolve on the microscopic c-plane area in the apex of the pyramid. Well-defined single emission lines with a sub-meV line width have been monitored by means of μ-photoluminescence (μPL) from these deterministic QDs. So far, emission wavelengths have been demonstrated in the blue range, around 400 nm, but can be pre-defined by altering the growth conditions, such as the growth temperature, the well width and/or the In composition, for the InGaN layers. In a subsequent step, the circular holes in the patterned substrate have been replaced by elongated holes in a specified direction. As a result, elongated pyramids are formed with ridges characterized by typical dimensions of 1.0 µm length and 100 nm width on top of these asymmetric pyramids. The elongation directions 0o, 60o, and 120o are preferable due to the six-fold symmetry of the wurtzite crystal structure. The primary emission lines monitored are originating from electron-hole pairs, i.e. single neutral excitons, but also biexcitons and charged excitons have been monitored. Interestingly, the excitonic emission from the extended InGaN QDs on top of the elongated pyramids exhibit a strong degree of linear polarization, with a typical polarization ratio of about 85% achieved for the case of an elongation of the pyramid base by 1μm. For investigations of the exciton lifetime, μ-PL with a high spatial resolution combined with a streak-camera detector for recording the time spectral evolution under pulsed excitation with ps pulse duration was employed. The exciton lifetimes have been demonstrated to vary between 100 ps to 1 ns, with shortest lifetimes measured for the negatively charged QDs. In order to demonstrate the single photon characteristics of the dots, temporal photon correlation spectroscopy has been performed. These correlation measurements have been done in a setup of a Hanbury-Brown and Twiss interferometer equipped with sensitive single photon detectors. The excitonic single photon emission and biexcitonic photon bunching from the pyramidal dots are reported, confirming the sound single photon properties of these dots. 3. References 1.C.W.Hsu, A. Lundskog, K. F. Karlsson, U. Forsberg, E. Janzén and P.O.Holtz, Nano Letters Volume 11, 2415 (April 28, 2011) 2.A. Lundskog, C.W. Hsu, D. Nilsson, U.Forsberg, K. F. Karlsson, P.O. Holtz and E. Janzén, Nature: Light, Science & Applications (2014) 3, Article:139; doi:10.1038/lsa.2014.20 3.A. Lundskog, J. Palisaitis, C. W. Hsu, M. Eriksson, K. F. Karlsson, L. Hultman, P.O.Å. Persson, U.Forsberg, P. O. Holtz, E. Janzen, Nanotechnology 23, 305708 (2012) 4.A.Lundskog, C.W. Hsu, D. Nilsson, U. Forsberg, P.O. Holtz and E. Janzén, Journal of Crystal growth 363, 287 (2013) 5.A. Lundskog, U. Forsberg, P.O. Holtz and Janzén, Crystal growth and design 12, 5491 (2012) 6.S. Amloy, K. Fredrik Karlsson, P. O Holtz arXiv: 1311.5731 7.T. Jemsson, H. Machhadani, P.O. Holtz and K. F. Karlsson, Nanotechnology 26, 065702 (2015)

Speaker: Prof. Per Olof Holtz (Linköping University, Sweden)

10:10 Electrical characterization of Au/Ni/AlGaN Schottky barrier diodes

In this study, Au/Ni Schottky barrier contacts have been fabricated on AlGaN grown by hydride vapor-phase epitaxy (HVPE). After ohmic and Schottky contacts fabrication, the contacts were annealed at 500°C. The electrical characteristics of the diodes were investigated by using current-voltage measurements. The results show that the diodes characteristics improved after annealing, with reverse leakage current dropping to less than 10-8 A. Furthermore, the analysis of the temperature dependent electrical characteristics shows that the reverse current of the diodes increases with increasing temperature. The barrier height and ideality factors increased and decreased with increasing temperature respectively.

Speaker: Dr Mmantsae Diale (University of Pretoria)

10:50 → 11:10 Tea

11:10 → 11:40 Plenary

11:10 Plasmonic systems and their Interaction with Moleucles

We look on interaction between molecules and light at the nanoscale, by fabrication of metallic nanostructures (plasmonic systems). The interaction (coupling) is interesting both in the effect the molecules have on the plasmons and in the effect the plasmons have on the molecular transitions. Thus, molecules which are immeressed is the plasmonic field, may undergo photochemical processes which otherwise suppressed. We further discussed the coupling between localized plasmonic modes of metallic nano structures and their nonlinear properties, such as second harmonic generation (SHG) . We show that when strongly coupled the equilateral triangular nanocavities lose their individual three-fold symmetry to adopt a lower symmetry of the coupled system. The coupled system then responds like a single dipolar entity, and the SHG signal is either enhanced or suppressed depending on the incoming beam polarization. References [1] From Individual to coupled metallic nanocavities. Salomon, A.;Prior, Y.; Kolkowski, R.; Zyss, J. ; Journal of optics 2014 [2] Role of mode degeneracy in molecule-surface plasmon strong coupling. Salomon A. Wang, S. Hutchison, J.A., Genet, C. Ebbesen, T.W. J. Phys. Chem. C, 2013, 117 (43), pp 22377–22382 [3] Collective Plasmonic-Molecular Modes in the Strong Coupling Regime. Salomon, A et al. .; Phys. Rev. Lett 109, 073002, 2012 [4] Molecule – light complex: dynamics of hybrid molecule – surface plasmon states. Salomon, A. Genet, C. and Ebbesen, TW. Angewandte Chemie, 48, Pages: 8749-8751, 2009.

Speaker: Dr Adi Salomon (Chemistry department, BINA nanocenter, Bar Ilan University)

11:40 → 12:30 Presentations: Tuesday Session 2

11:40 Annealed Ce-doped ZnO nanoparticles synthesized by chemical bath deposition method

The X-ray diffraction results showed that All the ZnO samples possess a typical wurtzite structure for non-annealed and annealed. It was observed that the XRD diffraction intensities decrease with annealing temperature as shown in Fig. 1. This behaviour may be due the estimated crystallite size that decreased with annealing temperature. In Fig. 2 The XRD spectra of the annealed ZnO:Ce3+ nanostructures correspond to the various planes of a single hexagonal ZnO phase for the lower and higher Ce concentration samples. At higher Ce molar concentration there is secondary phase. The estimated average crystallite sizes decay exponential with an increase in the amount of Ce. Scanning electron microscopy observations showed the presence of flower-like for non-annealed samples and spherical nanoparticles for annealed samples. Doping the annealed samples with Ce it was observed that the nanoparticles increase in size with an increase in the amount of Ce. At higher molar concentration of Ce there was a mixture of spherical and hexagonal particles. The UV-Vis spectra showed that the absorption edges red shift slightly with an increase in the molar concentration of Ce. The photoluminescence results showed that the maximum intensity is obtained for udoped ZnO nanoparticle and doping with Ce there was decrease in luminescence intensity.

Speaker: Dr Lehlohonolo Koao (UFS (Qwa Qwa))

12:00 CHARACTERIZATION OF CRYSTALLITE MORPHOLOGY FOR DOPED STRONTIUM FLUORIDE NANOPHOSPHORS BY TEM AND XRD

M.E. Lee1, J.H. O’Connell1, M.Y.A. Yagoub2, H.C. Swart2 and E. Coetsee2 1Centre for HRTEM, Nelson Mandela Metropolitan University, Port Elizabeth, 2Department of Physics, University of the Free State, Bloemfontein, South Africa Corresponding author e-mail address: michael.lee@nmmu.ac.za 1. Introduction Strontium fluoride (SrF2) is one of the most widely used optical materials due to its optical properties (wide bandgap and low phonon energy) as well as physical properties (low refraction index, high radiation resistance, mechanical strength and low hygroscopicity)[1,2]. Europium doped (optimally ~2%) strontium fluoride (Eu:SrF2) nanophosphors have been shown to possess improved scintillation properties [3]. Previous work on the characterization of the material by X-ray diffraction (XRD) has produced results on the average dimensions for the nanoparticles [4]. However, analysis of exact morphology and orientation for the nanocrystallites will require alternative techniques such as high resolution transmission electron microscopy (HRTEM). In this paper, complimentary results obtained by HRTEM will be compared to results obtained from XRD. Doped and undoped SrF2 samples were prepared by a hydrothermal process described in the literature [5]. The reaction product consists of spherical SrF2 particles having dimensions in the range 0.8-2.0 μm as measured by scanning electron microscopy secondary electron (SEM-SE) imaging. The FIB lamellae produced from the nanophosphor particles were imaged in a TEM. XRD analysis showed the structure of the material to agree with the expected space group Fm3m. The micron-sized particles were assumed to consist of nanocrystallites which are unimodal and single phased. Application of the Scherrer's equation to calculate the crystallite size produced dimensions in the range of 6-8 nm. The accuracy of the Scherrer equation will however depend on a number of factors such as grain size distribution, crystal defects and lattice strain. 2. Results The high angle annular dark field (HAADF) STEM imaging of the FIB lamella, clearly demonstrate the nanocrystalline composition of the particles shown in figure 1. The selected area diffraction patterns (SAD) clearly shows the presence of numerous, relatively randomly oriented crystallites. The hollow cone technique is used to produce dark field (DF) micrographs obtained by precessing the indicated diffraction ring through the objective aperture by applying hollow cone precession of the incident beam as shown in figure 2. Exposure time was an integer multiple of the precession period. The hollow cone DF TEM images have demonstrated evidence for a bimodal distribution of crystallite sizes. Firstly, a 5-10 nm size distribution which is in reasonable agreement with the XRD Scherrer calculation was measured. A second distribution of crystallite sizes in the range of 50-80 nm was also observed, as shown in figure 2. Data from the hollow cone TEM technique has therefore shown the crystallite size distribution to be bimodal which was not possible to determine by the XRD Scherrer equation. 3. References [1] K.V. Ivanovskikh, V.A. Pustovarov, M. Kirm, B.V. Shulgin. J. Appl. Spectrosc. 72 (2005) 564. [2] B.M. Van der Ende, L. Aarts and A. Meijerink. Adv. Mater. 21(2009) 3073. [3] S.Kurosawa, Y. Yokota, T. Yanagida, A. Yoshikawa Phys. Status Solidi C 9 (2012) 2275. [4] M.Y.A. Yagoub, H.C. Swart, L.L. Noto, J.H. O’Connel, M.E. Lee, E. Coetsee. J. Lumin. 156 (2014) 150. [5] J. J. Peng, S. Hou, X. Liu, J. Feng, X. Yu, Y. Xing, Z. Su. Mater. Res. Bull. 47 (2012) 328.

Speaker: Prof. Mike Lee (NMMU)

12:30 → 13:30 Lunch

13:30 → 14:00 Plenary: Session 3

13:30 Characteristics of amorphous transparent and conductive oxides grown by combinatorial pulsed laser deposition

Amorphous and transparent semiconductor oxides are key components of new thin film transistors (TFTs), solar cells electrodes and displays. By controlling their stoichiometry, they can be used as TFT channel (semiconductive behavior) or as transparent electrode (conductive behavior). Recently, room temperature deposited indium zinc oxide (IZO) and indium gallium zinc oxide (IGZO) thin films were shown to exhibit very good transparency in the visible range, low resistivity, and high mobility. Since the optical and electrical properties of these films depend on the In/(In+Zn) and Ga/(In+Ga+Zn) values, the accurate measurement of these ratios is important for future developments and applications. In this presentation we focus on the relationship between composition and properties of IZO and IGZO thin films synthesized using the Combinatorial Pulsed Laser Deposition technique. An accurate monitoring of the thin films elemental composition was performed by Laser-Induced Breakdown Spectroscopy (LIBS) based on plasma modeling in view of further in-situ and real-time technological developments and process control in case of ASOs fabrication. The cation fractions measured by LIBS were compared to values obtained by complementary measurements using Rutherford backscattering spectrometry, energy dispersive X-ray analysis and X-ray fluorescence. The optical properties (thickness profile and refractive index determination) of the thin films were inferred from spectroscopic ellipsometry data acquired in the visible range and optical reflectance measured from 30 cm-1 (4 meV) to 30 000 cm-1 (4 eV). Complementary investigations to obtain the thickness and density of the deposited films as well as their surface and interface roughness have been performed by fitting the measured X-ray reflectivity and X-ray diffuse scattering curves with simulated ones using dedicated models. The room temperature electrical properties were investigated using typical four-point probe geometry and Hall measurements and compared with the values estimated from the optical reflectance data. X-ray photoelectron spectroscopy was used to measure the energy discontinuities in the valence and conduction bands of various dielectric/IGZO and dielectric/IZO heterostructures. All these measurements helped design better transparent and conductive oxides containing lower amounts of In, an element that is rather scarce and therefore expensive.

Speaker: Dr Valentin Craciun (National Institute for Laser, Plasma and Radiation Physics)

14:00 → 15:00 Presentations: Tuesday Session 3

14:00 Chloride-based SiC growth on a-axis 4H-SiC substrates

SiC has during the last few years become increasingly important as a power-device material for high voltages. The thick low-doped voltage-supporting epitaxial layer is normally grown by CVD on 4˚ off-cut 4H-SiC substrates at a growth rate of 5-10 µm/h using silane and propane (or ethylene) as precursors. The concentrations of epitaxial defects and dislocations depend to a large extent on the underlying substrate but can also be influenced by the actual epitaxial growth process. Here we will present a study on the properties of the epitaxial layers grown by a Cl-based technique on an a-axis (90˚ off-cut from the c direction) 4H-SiC substrate

Speaker: Prof. Erik Janzen (Semiconductor Materials (IFM), Linkoping University)

14:20 Response of Ni/4H-SiC Schottky barrier diodes to alpha-particle irradiation at different fluences

We have investigated the effect of 251-Am alpha-particle irradiation of energy 5.4 MeV on 1.9 × 10^(16) cm^(–3) N-doped 4H-SiC SBDs at different fluences. Prior to bombardment, we observed four defects with energies 0.10, 0.12, 0.16 and 0.65 eV below the conduction band. Alpha-particle irradiation introduced one additional defect with a very broad peak after receiving a 2.8 × 10^11 cm^(–2) fluence, and the peak became conspicuous after several irradiations. The signature in term of energy and apparent capture cross section was estimated to be 0.37 eV and 5.5 × 10^(–16) cm^2, respectively. The introduction rate of this defect was determined to be 6500 cm^(–1).

Speaker: Mr Ezekiel Omotoso (University of Pretoria)

14:40 Transport characteristics of n-ZnO/p-Si heterojunction as determined from temperature dependent current-voltage measurements

Speaker: Mr Stive Roussel Tankio Djiokap (Nelson Mandela Metropolitan University)

15:00 → 15:15 Tea

15:15 → 17:00 Poster: Poster Session 1

15:15 Ab initio studies of Split<110> and Tetrahedral Di-interstitials of Germanium (Ge) using Hybrid functional HSE06

As a result of its narrow band gap, high carrier mobility and low voltage operation the application of Germanium (Ge) as a promising material for complementary metal-oxide semiconductors (CMOS) technology is attracting attention recently [1]. Studies on Several defects including self-interstitials, interstitials, vacancies and vacancy complexes [2-3] have been carried out. For Silicon, various interstitials and vacancies including mono-, di- and tri- has been studied[2], Ge mono-interstitials using local density approximation(LDA), generalized gradient approximation (GGA) was reported[3], but none have been studied on mono- and di-interstitials of Ge using HSE[4]. In this work we present ab-initio calculations results of Ge Tetrahedral (TT) split<110> (SP10) and split<110>/Tetrahedral (SPT) di-interstitials configurations in the frame work of density functional theory(DFT) using hybrid functional (HSE06)[4] exchange correlation functional. The formation and transition level energies of defected configurations charge states of -2, -1, 0, +1 and +2 were obtained. Depending on the Fermi level, the formation energies shows that the SP10 is less favourable than T while the combination of SPT forms the most stable defect. We find (+1/+2) transition charges states level of TT to be 0.76 eV above valence band maximum (VBM) and (+1/+2) for (SPT) configurations to be 0.72 eV above VBM. These di-interstitials exhibited the properties of shallow and deep donor at (+1/+2) above the Fermi level depending on the configurations. We compare this result with calculation of di-interstitials in silicon and available experimental data.

Speaker: Mr Emmanuel Igumbor (University of Pretoria)

15:15 Advances in phosphors based on purely organic materials for solid state lighting applications

Speaker: Dr Kashma Sharma (Department of Physics, University of the Free State, P.O. Box 339, Bloemfontein 9300, Republic of South Africa)

15:15 Analysis of temperature-dependent current-voltage characteristics and extraction of series resistance in Pd/ZnO Schottky diode

We report on the analysis of current voltage (I-V) measurements performed on Pd/ZnO Schottky barrier diodes in the 30-350 K temperature range. Assuming thermionic emission (TE) theory, the forward bias I-V characteristics were analysed to extract Pd/ZnO Schottky diode parameters. Comparing Cheung’s method in the extraction of the series resistance with Ohm’s law, it was observed that at lower temperatures (T<180 K) the series resistance decreased with increasing temperature, the absolute minimum was reached near 180 K and increases linearly with temperature at high temperatures (T>200 K). The barrier height and the ideality factor decreased and increased, respectively, with decrease in temperature, attributed to the existence of barrier height inhomogeneity. Such inhomogeneity was explained based on TE with the assumption of Gaussian distribution of barrier heights with a mean barrier height of 0.986 eV and a standard deviation of 0.015 eV. A mean barrier height of 0.994 eV and Richardson constant value of 37.48 A cm-2 K-2 were determined from the modified Richardson plot that considers the Gaussian distribution of barrier heights.

Speaker: Mr Meehleketo Advice Mayimele (university of Pretoria)

15:15 Characterization of TiO₂ nanostructures prepared by microwave method for gas sensing

1. Introduction The need for gas sensors to monitor and detect toxic, explosive and combustible gases in mines and in the environment has stimulated researchers to develop portable gas sensing devices that can detect at low concentration and at room temperature. Monitoring such gases can help to prevent occurrences of fatal accidents such as fire and explosions [1]. Amongst the metal-oxides (MOXs) semiconductors, TiO2 appears to be the most attractive material because of its diverse applications in various fields such as spintronics, solar cells, photocatalysis and gas sensors [2]. In addition, it has a good thermal stability and can operate in harsh environment. The structure and morphology of MOXs can be controlled by varying the synthesis procedure. In this study we report the synthesis of TiO2 nanostructures using a simple hydrothermal method in NaOH solution at 220 °C for 15 minutes. The influence of hydrochloric acid (HCl) aqueous solution as a washing agent on the surface morphology, structure and gas sensing properties of the TiO2 nanoparticles was investigated. Additionally, the effect of calcination temperature on the properties of TiO2 nanoparticles was also studied in detail. 2. Results Fig. 1 shows the XRD patterns of the as-prepared TiO2 nanoparticles washed with distilled water and different concentrations of HCl. Based on the results in Fig 1, the structure of the starting material, P25 Degussa shows a mixture of both anatase and rutile phases. The material washed with H2O and HCl aqueous solution after synthesis reveals only the anatase phase. Previous studies reported that pure anatase TiO2 phase has comparatively good sensing properties. The SEM images of P25 Degussa compared with that washed with H2O and various concentrations of HCl aqueous solution are shown in Fig. 2. The particle sizes of the as-prepared samples increased as the concentration of the HCl as washing agent was increased as depicted in Fig. 2(c-d). The insets of figure 2 show the particle size distribution. The results revealed that when the grain size increases, grain boundaries decrease providing a large surface area thus increasing the sensing capabilities.

Speaker: Ms zamaswazi portia Tshabalala (DST/CSIR National Centre for Nano-Structured Materials, Council for Scientific and Industrial Research, Pretoria, 0001, South Africa)

15:15 Effect of doping concentration on the conductivity and optical properties of p-type ZnO thin films

Zinc oxide (ZnO) is a multifunctional material with unique physical and chemical properties as well as good photo stability.In present work we study the nitrogen doping effect on conductivity and optical properties.

Speakers: Mr Trilok Pathak (Gurukula Kangri University, Haridwar, India) , Dr Vinod Kumar (Department of Physics, University of the Free state, Bloemfontein, ZA-9300)

15:15 Effects of the copper content on the structural and electrical properties of Cu2ZnSnSe4 bulks used in thin-film solar cells

We have investigated the concept of defect in CuxZnSnSe4 (x= 1.62.0) and Cuy(Zn0.9Sn1.1)Se4 (y= 1.6-2.0) bulks prepared by liquid-phase sintering at 600 oC for 2 h with soluble sintering aids of Sb2S3 and Te. All CuxZnSnSe4 pellets exhibited p-type conductivity regardless of Cu contents but Cuy(Zn0.9Sn1.1)Se4 pellets show p-type at y=1.6 and n-type at y= 1.8-2.0. SEM surface images showed the sintered CZTSe bulk exhibited a smooth, densely packed and homogeneous surface at the nearly stoichiometric composition. Increasing the copper excess also yields a rougher CZTSe morphology. The non-stoichiometric composition of CZTSe under various Cu contents caused the intrinsic defects, and the structural and electrical properties of the bulks can be explained based on the point defect properties. The deficiency of Cu content in CZTSe bulks easily leads to smaller unit cells.

Speaker: Dr Moges Tsega Yihunie (University of the Free State)

15:15 Influence of varying Cr3+ mol% in MgAl2O4:0.1% Eu3+, x% Cr3+ nanophosphor synthesized by sol-gel process

The XRD data revealed that all annealed samples consist of the pure cubic MgAl2O4 structure. The estimated crystallites size were in the range of 12.1 – 11.0 nm in diameter. SEM results showed that the dopant type and varying the Cr3+ concentration in the co-activated samples influences the surface morphology of the phosphor. The PL results showed that the host, 0.1% Cr3+ and Eu3+ activated nanophosphor emits at different wavelengths. Emission peak at 390 nm is attributed to the band-gap defects in the host material. Emission at 405 is attributed to arise from both the contribution from the host and Cr3+ (4T1 → 4A2 transition) emissions. The green emission peak at 565 nm is attributed to arise from either the host or Cr3+ (4T2 → 4A2 transition). An emission peak at 574 nm is attributed to the well-known orange emission from 5D0 → 7F1 transition in Eu3+ ion, while the emission peak at 619 nm is assigned to the Eu3+ electric dipole from 5D0 → 7F2 transition. An emission at 694 nm is attributed to the Cr3+ from 2E → 4A2 transition. Co-activating the host with Cr3+ ion quenches the host emission at 390 nm. The CIE color coordinates (see Fig. 2) shows that the emission color can be turned by varying the co-activator concentration.

Speaker: Mr Setumo Victor Motloung (University of the Free State (Qwaqwa Campus))

15:15 Low-Temperature Alpha-Particle Irradiation of Pd/4H-SiC Schottky barrier diodes

The effect of low-temperature alpha-particle irradiation on Pd/4H-SiC Schottky barrier diodes has been investigated. The motivation is to study the radiation damage of the sample after bombarded with 1.6 MeV a-particles [(He)^(2+)] at 20 K and the annealing of the radiation-induced defects taking place with increasing in temperature. The of fluence alpha-particles amounted to 3 × (10)^(13) (cm)^(–2). Thermal admittance and photo-capacitance spectroscopy were employed to characterize the diodes. The shallow donors D1 and D2 were detected in the as-grown as well as in the a-bombarded samples. The defects TαAnn was stable to both irradiation and annealing at room temperature.

Speaker: Mr Ezekiel Omotoso (University of Pretoria)

15:15 Magneto-optical investigation of the cyclic redox R2O2S ↔ R2O2SO4 (R: Eu, Tb) reactions

1. Introduction Impurities and dopants’ inappropriate valences may deteriorate the performance of luminescent materials, cause waste of the high purity (rare earth) material and then incur significant financial losses [1]. The methods used to detect elements’ valence (XPS, Mössbauer and XANES) are not sensitive enough for low concentrations and EPR is not suitable for powders. Obtaining quantitative data leaves a lot to hope for, too. To make the things worse, the two most common rare earth dopants in phosphors, Eu3+ and Tb3+, may exist in different oxidation states, Eu2+ and TbIV [1], as well. For the Eu3+ or Tb3+ doped R2O2S:Eu3+ and R 2O2SO4, the Eu2+ or TbIV may be formed since the manufacture of these phosphors involve reducing and/or oxidizing conditions. The qualitative observation of Eu2+ can usually be made based on its intense luminescence due to the parity-allowed electric dipole 4f6↔4f55d1 transitions. In contrast, the Eu3+ line emission is weaker despite the high quantum yield. TbIV does not luminesce, but this species may absorb the emission of Tb3+ and, in addition, may facilitate non-radiative processes reducing the efficiency of Tb3+ doped phosphors even further [1]. 2. Results In this work, the comparison between the experimental and calculated temperature-dependent paramagnetic susceptibilities was used to obtain quantitatively the concentrations of the valence impurity in Eu2O2S (Eu2+) and Tb2O2SO4 (TbIV), both containing nominally only R3+. The wave functions for the calculations were obtained from the analysis (Fig. 1) of the Eu3+ luminescence spectra [2,3]. Minute (ppm level) Eu2+ impurities could be defined due to the huge difference in the paramagnetic susceptibility of Eu2+ and Eu3+. However, temperatures below 50 K are then needed (Fig. 2). In contrast, the TbIV impurity in a Tb3+ host can be determined already at higher temperatures with similar susceptibility measurements. The latter method is based on comparing the slopes of the Tb3+/TbIV paramagnetic susceptibility vs. temperature curves. The results for the Tb3+/TbIV couple are less sensitive than for the Eu2+/Eu3+ one, however. Finally, the host independent temperature evolution of the paramagnetic susceptibility was calculated for Gd3+ (or Eu2+ or TbIV) to yield an analytical expression to be used universally. 3. References [1] W. M. Yen, S. Shionoya and H. Yamamoto (Eds.). Phosphor Handbook, 2nd ed., (CRC Press, 2007). [2] J. Sovers and T. Yoshioka. J. Chem. Phys. 51 (1969) 5330. [3] P. Porcher, D. R. Svoronos, M. Leskelä and J. Hölsä. J. Solid State Chem. 46 (1983) 101.

Speaker: Prof. Jorma Hölsä (University of Turku)

15:15 Preparation of ZnO nanorods and their gas sensing properties

In this work, low dimensional ZnO nanorods with different sizes were successfully prepared using a microwave-assisted hydrothermal method. The influence of the microwave reaction time on the structural, and luminescence properties of the ZnO nanorods was studied using X-ray diffraction (XRD), Scanning electron microscope (SEM), and photoluminescence (PL). XRD analysis showed that all samples have the typical hexagonal wurtzite structure without any other new phases. The size of the ZnO nanorods was found to vary with variation of reaction time, respectively. The possible sensing mechanism and the relationship between the sensing response and the content of defects were proposed.

Speaker: Ms Katekani Shingange (CSIR and UFS)

15:15 Self-Assembled Nanotubular Mesoporous Layered Double Hydroxides with Tunable Photoluminescence

Self-assembled luminescent LDH nanotubes (Ø 20nm) combine the potential of RE3+ containing LDH with a high surface area and easily accessible mesopores (±175 m2/g; 0.75 cm3/g) suitable for hosting large sensitizing dyes and other interesting photonic species such as luminescent nanodots.

Speaker: Dr Danilo Mustafa (University of São Paulo)

15:15 Spectroscopic properties of Pr3+ ions embedded in lithium borate glasses

Speaker: Dr Durgaprasad Ramteke (Department of Physics, University of the Free State)

15:15 Structural and luminescence properties of SrAl2O4:Eu2+, Dy3+/Nd3+ phosphor thin films grown by pulsed laser deposition

1. Introduction Long afterglow or persistent phosphors have the ability of absorbing energy from UV or sunlight and then release it slowly in the dark [1,2]. Inorganic phosphors doped with rare earth elements show broad band emission from blue to red which makes them suitable for a variety of industrial applications, such as luminescent pigments, fluorescent lamps, color display, plasma display panels (PDP), radiation dosimetry and X-ray imaging [3]. The type and duration of emission from a phosphor is affected by a number of parameters such as the type and amount of activators or dopants, the structure of the host lattice, the method of preparation or growth conditions and other post-treatments. These parameters play a significant role in inducing a crystal field effect within the host matrix which in turn influences the emission wavelength, its intensity and lifetime. The main task would therefore be to optimize these factors to obtain a phosphor that gives the best performance for the desired application. In this study thin films of SrAl2O4:Eu2+, Dy3+ /Nd3+ were prepared using Pulsed Laser Deposition (PLD). The effect of varying argon gas pressure and substrate temperatures on the structure and photoluminescent (PL) properties of the SrAl2O4:Eu2+, Dy3+ /Nd3+ thin films were investigated. 2. Results X-ray diffraction (XRD) patterns of the SrAl2O4:Eu2+, Dy3+ /Nd3+ phosphor thin films deposited on Si(100) in vacuum, 10, 20 and 30 mTorr argon partial pressure as compared with that of the as-prepared powder are shown in figure 1. The peaks fitted well with the powder and the JCPDS card number 74-0794 for the monoclinic SrAl2O4 of space group P1211 (4). It can be seen that with increasing the argon pressure the peaks in the (220) direction shifted to the lower 2-theta angles from 29.1o in vacuum to 27.9o in 30 mTorr argon partial pressure. Ar has a higher mass density and therefore tends to reflect lighter atoms in the plume more and these results in a film with big particles [4] which results in lattice expansion inducing crystal field effect in host lattice causing the XRD peak shift to lower angles. Figure 2 shows PL emission spectra recorded with He-Cd laser at excitation of 325 nm from SrAl2O4:Eu2+, Dy3+/Nd3+ thin films deposited under different argon atmospheres whereby emission peaks are observed to shift towards the higher wavelength side. Larger particles will cause lattice expansion and increase the crystal-field interaction of Eu2+ resulting in red shift.

Speaker: Mr ALI HALAKE WAKO (University of the Freestate)

15:15 Structure and optical properties of undoped and Mn-doped ZnO(1-x)Sx nano powders prepared by precipitation method

1. Introduction With a wide band gap of 3.4 eV and a large exciton binding energy of 60 meV at room temperature, ZnO is attractive for blue and ultra-violet optoelectronic devices, and transparent conducting oxide films for photovoltaic applications. The large excitonic binding energies of ZnO and ZnS could enable efficient excitonic emission at temperatures well above room temperature and therefore lower threshold intensities for optoelectronic devices based on these semiconductors can be expected. Alloying ZnO by incorporating equivalent anions has not been extensively studied. Anion doping in ZnO, i.e., replacing oxygen by sulphur e.g. ZnO(1-x)Sx (ZnOS), has been reported recently [1, 2]. Due to the large electronegativity differences between O and S it would be expected that the bowing parameters of ZnOS are large. The change of anions in ZnO by isoelectronic impurities is important from the viewpoint of band gap engineering. In this work, high-quality undoped and Mn-doped ZnOS nano powders were prepared by the precipitation method. 2. Results Figure 1 shows the typical XRD pattern of the obtained product. All the strong peaks in this pattern can be readily indexed to hexagonal wurtzite ZnOS structures. ZnOS alloys with a wurtzite structure were achieved for small content of sulphur and no evident phase separation was observed in the investigated composition range as determined by X-ray diffraction. Scanning electron microscopy observations showed the presence of nano-crystallites that decrease in size with Mn-doping. The optical absorption measurements show strong excitonic peak emission without any defect emission in the visible spectrum. The absorption edges of the nano powders shift towards low-energy side with increasing the Mn-dopant content. The presence of the Mn dopant diminishes the excitonic emission. The bandgap energies of the ZnOS nano particles were calculated and found to change from 4.0 to 4.2 eV, showing a nonlinear variation with a bowing behavior that was previously reported. The photoluminescence emission spectra of ZnOS nanoparticles gives four bands centering at about 548 nm, 614 nm, 649 nm and 670 nm, wavelengths. Similar to observations, It has been reported that the dopants of S, Mn can shift the luminescence position of ZnOS nanocrystals.

Speaker: Mr Lehlohonolo Koao (UFS (Qwa Qwa Campus))

15:15 Synthesis and characterisation of copper sulphide quantum dots for photovoltaic cell applications

Speaker: Ms Leandre' Brandt (University of the Western Cape)

15:15 The effect of high temperatures on the electrical characteristics of Au/n-GaAs Schottky diodes

We investigated the rectification properties of Gold Schottky contacts fabricated on epitaxial n-type GaAs.The study was carried out using Laplace deep level transient spectroscopy together with the current-voltage characteristics and the capacitance voltage characteristics in the 80-480K range.

Speaker: Mr Shandirai Tunhuma (University of Pretoria)

15:15 The effect of the Optical System on the Electrical Performance of III-V Concentrator Triple Junction Solar Cells

The progression of the development of High Concentrated Photovoltaic (H-CPV) technology promises lower costs and higher cell and module efficiencies. However, development is limited by various materials and device aspects. One of these, the power production limiting effects of the optical system on a Concentrator Triple Junction (CTJ) cell in an H-CPV module, is the focus of this paper. With carefully designed experiments, which included spectral measurements, topographic intensity profiles in the cell plane and the analysis of I-V curves of the CTJ cell/H-CPV module, one can fully characterise loss mechanisms associated with the optical system and their effect on the electrical performance of the CTJ cell.

Speaker: Prof. Ernest van Dyk (NMMU)

15:15 The effect of urea ratio on structural and luminescence properties of YVO4:Dy phosphor

The effect of urea ratio on structural and luminescence properties of YVO4:Dy phosphor 1. Introduction Although the luminescence characteristics of vanadates phosphors have been reported, yttrium vanadates (YVO4) are good host materials for luminescence efficiency. Like Eu, Tm and other rare earth ions, Dy can also act as a useful activator. Many researchers have reported that YVO4 can be modified by Eu to be used as red phosphor in colour television and cathode because of its high luminescence [1]. Besides europium, Dy ions is a good activator for YVO4: Dy3+. Synthesis of YVO4 has previously been prepared by various methods. Here YVO4:Dy was prepared by combustion method at initiation temperature of 600 oC. Combustion method is one of an ideal technique, because exothermic reaction was initiated at the ignition temperature and it generates heat which was manifested in a maximum temperature of 100-1650 K. 2. Results Figure 1 shows the XRD patterns of the YVO4:Dy powders synthesized by combustion method at initiation temperature of 600 o C with different mole ratios of urea. The phosphor powder showed that the peaks were due to YVO4 tetragonal phase (JCPDS 17-0341). No other crystalline phase was detected on XRD spectra. Scanning electron microscopy results showed when increasing ratio of urea the agglomeration of particles decreases and the nanorod-like shape structure starts to form. Figure 2 shows the emission spectra obtained from excitation of 282 nm. The emission spectra consist of two main peaks, yellow band at 573 nm corresponding to 4F9/2→6H13/2 and the blue band (482 nm) corresponds to the 4F9/2→6H15/2 transition. There is a very week band at 663 nm which correspond to 4F9/2→6H11/2 transition. The intensity of the yellow emission is stronger than that of blue emission, this is because when the Dy3+ ions is located at low symmetry local sites with no inversion centers, the 4F9/2→6H13/2 transition is prominent in its emission spectrum. 3. References [1] J. Wang, Y. Xu, M. Hojamberdiev, Y. Cui, H. Liu and G. Zhu, J. of Alloys and Compounds. 479 (2009), 772-776. [2] H. Zhang, X. Fu, S. Niu and Q. Xin, J. of Alloys and Compounds. 457 (2008) 61-65.

Speaker: Mrs Kewele Emily Foka (University of the Fee Stae)

15:15 The influence of oxygen partial pressure on material properties of Eu3+- doped Y2O2S thin films deposited by Pulsed Laser Deposition method.

1. Introduction Eu3+- doping has been of interest to improve the luminescent characteristics of thin-film phosphors. Europium-doped Y2O2S exhibits strong UV and cathode ray-excited luminescence, so it is widely used as red phosphors for low-pressure fluorescent lamps, cathode-ray tubes and plasma display panels [1]. Also, the hexagonal Y2O2S is a good host material for rare earth ions. In recent years, the Y2O2S: Eu has received much attention for its tremendous potential applications in optical display and lighting materials and basic science research on special luminescent spectra. Nanoscale and thin film Y2O2S: Eu has remarkably different luminescent properties from those of bulk samples: such as emission line broadens, lifetime changes and its spectra shift [2]. In this study Y2O2S: Eu thin films have been deposited with the pulsed laser deposition technique in an O2 environment. The oxygen pressure was changed from 0 to 140 mtorr. 2. Results The X-ray diffraction patterns (Fig.1) show mixed phases of cubic and hexagonal crystal structures. As the oxygen partial pressure increased, the crystallinity of the films improved. Further increase of the O2 pressure to 140 mtorr reduced the crystallinity of the film. Similarly, both scanning electron microscopy and atomic force microscopy confirmed that an increase in O2 pressure affected the morphology of the films. The average band gap of the films calculated from diffuse reflectance spectra using the Kubelka-Munk function was about 4.75 eV. The photoluminescence measurements (Fig.2) indicate red emission of Eu3+ doped Y2O2S thin films with the most intense peak appearing at 619 nm, which is assigned to the 5D0-7F2 transition of Eu3+. This most intense peak is totally quenched at higher O2 pressures. This phosphor may be good promising material for applications in the flat panel displays.

Speaker: Mr ABDUB ALI (UNIVERSITY OF THE FREE STATE)

15:15 The Synthesis and Characterization of Magnetic/ Luminescent Fe¬3O4-InP/ZnSe Core-Shell Nanocomposite

1. Introduction Magnetic luminescent nanoparticles have shown great promise in various biomedical applications namely: contrast agents for magnetic resonance imaging, multifunctional drug carrier system, magnetic separation of cells, cell tracking, immunoassay, and magnetic bioseparation. However most of these nanoparticles are cadmium- based. Indium- phosphide is known to be less toxic than cadmium- based products. This experiment describes the synthesis of a core- shell nanocomposite material, which is composed of an iron oxide superparamagnetic core and an InP/ZnSe quantum dot shell. The magnetic nanoparticles (MNP’s) and quantum dots (QD’s) were synthesized separately before conjugation could occur. The MNP’s were functionalized with a thiol-group allowing the QD shell to bind to the surface of the MNP by the formation of a thiol-metal bond. 2. Results The synthesized nanocomposite was characterized with high- resolution transmission electron microscopy (HR-TEM), energy-dispersive X-ray spectroscopy (EDS), selective electron area diffraction (SAED), scanning electron microscopy (SEM), UV- visible spectroscopy, XRD and photoluminescence. These techniques yielded particle size, morphology, dispersion, and chemical composition including luminescence and florescence properties of the as prepared nanoparticles. The TEM micrograph showed crystalline nanoparticles which are monodispersed. These particles would be more useful in the in vivo applications after their solubility is tuned to a desired property. This experiment will continue further by investigating how various capping agents’ affects the particles solubility. All the peaks of XRD patterns were analyzed and indexed using ICDD data base, comparing with magnetite standards. The lattice constant a was found to be 8.330 Å, which was compared with the lattice parameter for the magnetite of 8.39 Å. Further results will be reported including their applications. 3. References [1] Pedro Tartaj1, Mar´ıa del Puerto Morales1, Sabino Veintemillas-Verdaguer, Teresita Gonz´alez-Carre ˜no and Carlos J Serna. 2003. [2] Shen Wu, Aizhi Sun, Fuqiang Zhai, Jin Wang, Wenhuan Xu, Qian Zhang, Alex A. Volinsky [3] Shouheng Sun, Hao Zeng, David B. Robinson, Simone Raoux, Philip M. Rice, Shan X. Wang, and Guanxiong Li

Speaker: Ms Zuraan Paulsen (Unversity of the Western Cape)

15:15 TOF SIMS Analysis, Structure and Photoluminescence Properties of Pulsed Laser Deposited CaS:Eu2+ thin films

1. Introduction Red-emitting alkali earth sulfide phosphor such as divalent europium (Eu2+) doped calcium sulfide (CaS: Eu2+) is a good material for blue pumped three-band phosphor-converted white LEDs since it has strong absorption in the blue region [1]. CaS: Eu2+ thin films were deposited on Si (100) substrates using the pulsed laser deposition technique to investigate the effect of Argon (Ar), Oxygen (O2), and vacuum deposition atmospheres on the structural, morphological and photoluminescence (PL) properties of the thin films. The phosphor target was ablated using a 266 nm Nd: YAG laser. X-ray diffraction, Atomic force microscopy, scanning electron microscopy, energy dispersive X-ray, fluorescence spectrophotometry, and time-of-flight secondary ion mass spectrometry (TOF-SIMS) were used to characterize the thin films. 2. Results The PL results for CaS: Eu2+ thin films deposited using different atmospheres are shown in Fig.1. Films prepared in Ar atmosphere showed better PL intensity than the films deposited in an O2, while the least intensity was observed from the films prepared in vacuum. The emission observed at around 650 nm for all the films is attributed to the transitions from the excited state to the ground state of the Eu2+ [2] ions. The emission at 618 nm, which is more prominent in the film prepared in O2, is ascribed to transitions in Eu3+ [3], suggesting that Eu2+ was unintentionally oxidized to Eu3+. TOF-SIMS images indicated that Eu2+ ions were evenly distributed in the CaS host and that the thicknesses of the prepared thin films depend on the atmosphere in which the films were grown. The overlay of Fig. 2 shows Eu in the +3 state, observed as EuO+ (m/z = 168. 8992) in the films prepared in an oxygen atmosphere. It is speculated that less oxidation of Eu2+ occurred during deposition in argon and vacuum atmospheres, since insignificant PL emission due to Eu3+ was observed. 3. References [1] Jia, X. Wang. Opt. Mater. 30 (2007) 375. [2] H. K. Yang, K. S. Shim, B. K. Moon, B. C. Choi, J. H. Jeong, S. S. Yi and J. H. Kim. Thin Solid Films 516 (2008) 5577. [3] P. Dorenbos. J. Lumin. 104 (2003) 239.

Speaker: Mr Raphael Nyenge (University of the Free State and Kenyatta University)

15:15 Wavelength-modulated photocurrent spectroscopy of GaSb/GaAs quantum ring solar cells

1. Introduction In recent years, many studies have been reported on the optical response characterization of type-II GaSb/GaAs quantum dot solar cells [1-2]. It has been shown that incorporating quantum structures into the junction can considerably extend the near band-edge response of the solar cell [2]. Unlike with conventional response measurements, where the intensity of the light source is typically modulated, differential spectroscopy is performed by modulating the wavelength of the pseudo-monochromatic excitation source [3-4]. However, due to the spectral dependence of most excitation sources, the optical intensity is inherently also modulated. As an example, in order to obtain the correct differential quantum efficiency (QE) spectrum of the solar cell, the photon flux of the excitation source would need to be measured separately and subtracted ...

Speaker: Mr Nambinintsoa Romeoh HASINJATOVO MANDANIRINA (Nelson Mandela Metropolitan University)

19:00 → 21:00 Dinner

Wednesday, 6 May

07:00 → 08:30 Breakfast

08:30 → 10:30 Plenary: Session 4

08:40 The Quantum Design of Photosynthesis

Photosynthesis has found an ultrafast and highly efficient way of converting the energy of the sun into electrochemical energy. The solar energy is collected by Light-Harvesting complexes (LHC) and then transferred to the Reaction Center (RC) where the excitation energy is converted into a charge separated state with almost 100% efficiency. That separation of charges creates an electrochemical gradient across the photosynthetic membrane which ultimately powers the photosynthetic organism. The understanding of the molecular mechanisms of light harvesting and charge separation will provide a template for the design of efficient artificial solar energy conversion systems. Upon excitation of the photosynthetic system the energy is delocalized over several cofactors creating collective excited states (excitons) that provide efficient and ultrafast paths for energy transfer using the principles of quantum mechanics. In the reaction center the excitons become mixed with charge transfer (CT) character (exciton-CT states), which provide ultrafast channels for charge transfer. However, both the LHC and the RC have to cope with a counter effect: disorder. The slow protein motions (static disorder) produce slightly different conformations which, in turn, modulate the energy of the exciton-CT states. In this scenario, in some of the LHC/RC complexes within the sample ensemble the energy could be trapped in some unproductive states leading to unacceptable energy losses. Here I will show that LHCs and RCs have found a unique solution for overcoming this barrier: they use the principles of quantum mechanics to probe many possible pathways at the same time and to select the most efficient one that fits their realization of the disorder. They use electronic coherence for ultrafast energy and electron transfer and have selected specific vibrations to sustain those coherences. In this way photosynthetic energy transfer and charge separation have achieved their amazing efficiency. At the same time these same interactions are used to photoprotect the system against unwanted byproducts of light harvesting and charge separation at high light intensities.

Speaker: Prof. Rienk van Grondelle (VU University Amsterdam)

09:10 Making every photon count: Optical nanoscopy and single molecule spectroscopy applied to natural light-harvesting materials

Artificial photosynthesis is envisioned by many to be an important component of mankind’s long-term energy solution. Bio-inspired photosystems appear most promising, but the first constructs over the past few years have clearly pointed to the infancy of this field [1]. To make progress, a very detailed understanding of natural photosynthesis is required in order to wisely extract the most important design principles. Here, the primary steps of photosynthesis – light harvesting and charge separation – are the most crucial to ensure that the energy of an absorbed photon is stored with a sufficiently high probability, which is commonly 90-100% under conditions of low solar radiation! The latter of the two – charge separation – is understood sufficiently well for the purpose of designing artificial devices. However, the former – light harvesting – has proven to be a very complex process and further experimental and theoretical advances are being awaited. One such promising technique is known as single molecule spectroscopy (SMS), where the averaging effect of an ensemble is overcome by investigating dynamical processes of a single light-harvesting unit. SMS, in fact, laid the foundation for optical nanoscopy, also known as superresolution microscopy, which has developed tremendously since the first experimental results appeared less than 10 years ago [2]. The new set of techniques enables optical imaging of structures down to the nanometer scale, a two orders of magnitude improvement over the diffraction limit. Application of the technique was found particularly useful in biology, because subcellular structures can now be unraveled using light. Optical nanoscopy is widely regarded as one of the most important developments in biophysical chemistry and chemical physics during the past two decades. It may therefore not be surprising that the founders were awarded the 2014 Nobel Prize in Chemistry. The first part of the presentation will highlight the principles of optical nanoscopy, point out the limitations in applying the techniques to autofluorescent systems, such as photosynthetic light-harvesting complexes, and encourage application to other areas of materials science research where optical imaging on the mesoscopic scale will be instructive. The second part of the presentation will highlight three design principles of photosynthetic light harvesting that were revealed using SMS. 2. Results In nature, proteins strongly bind a high density of chromophores to form extended, complex networks of light-harvesting antennae, operating at a remarkable speed and efficiency. Even more intriguing is the level of adaptability to environmental influences at the macromolecular scale to maintain a constant energy throughput at the photochemical reaction centre where charge separation occurs. One important part of this regulation involves very efficient absorption of solar energy and thermal dissipation of any excess excitation energy, leading to a finely tuned feedback self-protection mechanism. This should be done despite (or rather: with the help of) structural fluctuations of the protein on a wide range of timescales. By mimicking the natural conditions that would give rise to the self-protection state under levels of intense sunlight, we have shown that the main light-harvesting complex of plants – LHCII – uses intrinsically available thermal energy dissipation channels by finely controlling its structural disorder on timescales of ms to tens of seconds [3]. Furthermore, using a mutant of LHCII where the terminal emitter chromophore cluster is disrupted, it was demonstrated that the terminal emitter cluster in wild-type LHCII is responsible for energy transfer robustness due to the combination of the particular energy gaps of the lowest exciton states and the strong excitonic coupling of the chromophores in this cluster [4]. Finally, based on the spectral dynamics of light-harvesting complexes from plant photosystem I and II, we can conclude that the particular protein microenvironment of a chlorophyll dimer is responsible for considerable tuning of the extent of shade absorption of plants [5]. 3. References [1] R. E. Blankenship et al. Science (2011) 805; J. Barber and P. D. Tran. J. R. Soc. Interface (2013) 20120984. [2] E. Betzig, G. H. Patterson, R. Sougrat, O. W. Lindwasser, S. Olenych, J. S. Bonifacino, M. W. Davidson, J. Lippincott-Schwartz and H. F. Hess. Science (2006) 1642; S. T. Hess, T. P. Giriajan and M. D. Mason. Biophys. J. (2006) 4258; M. J. Rust, M. Bates and X. Zhuang. Nature Methods (2006) 793. [3] T. P. J Krüger, C. Ilioaia, M. P. Johnson, A. V. Ruban, E. Papagiannakis, P. Horton and R. van Grondelle. Biophys. J. (2012) 2669. [4] C. Ramanan, J. M. Gruber, P. Maly, M. Negretti, V. I. Novoderezhkin, T. P. J. Krüger, T. Mancal, R. Croce and R. van Grondelle. Biophys. J. (2015) 1047. [5] T. P. J Krüger, E. Wientjes, R. Croce and R. van Grondelle. Proc. Natl. Acad. Sci. USA (2011) 13516.

Speaker: Dr Tjaart Krüger (University of Pretoria)

09:40 Proteins as opto-electronic materials?

We have shown that electron transport (ETp), i.e., conduction, through protein monolay-ers in a solid state–like configuration is remarkably efficient, compared to most molecules, including conjugated ones.1 Some proteins also have a natural electron transfer (ET) function and ET and ETp are related, but while nature regulates ET via redox chemistry, where control over the process is achieved even at the expense of free energy and low rates (and ubiquity), in ETp no redox process is needed. This allows study of optically active, no-redox proteins, such as the rhodopsins. We studied ETp in the dark and under illumination, esp. in bacteriorohodopsin, but also in halorhodopsin and the light-oxygen-voltage (LOV) sensing domain proteins. The experimental data on ETp via proteins show poor fits with current ET models (pathway or average packing density), such as lack of distance dependence. We will discuss progress towards understanding solid-state ETp charge transport ,which will help to advance bio-opto-electronics. 1 N. Amdursky et al., Adv. Mater. 42,7142-7161 (2014) Electronic Transport via Proteins 10.1002/adma.201402304 (progress report).

Speaker: Prof. David Cahen (Weizmann Inst. of Science)

10:30 → 10:50 Tea

10:50 → 11:20 Plenary: Session 5

10:50 Preparation and measurements of MOVPE multiple InAs/GaAs/GaAsSb quantum dot structures for the telecom wavelength region emitted from 1.3 to 1.8 µm

Although the self-assembled InAs/GaAs quantum dots (QDs) are intensively studied during more than the last twenty years, the mass production of QD lasers started only in 2010 and the production of telecommunication QD lasers was announced even last year [1]. The main problem is the complexity of QD preparation process which includes not only the epitaxial layer growth but also Stranski-Krastanow formation, subsequent dissolution of QDs during capping, diversity of QD size and shape or the surfactant behavior of In atoms. The situation is even more complicated when QDs are covered by InGaAs or GaAsSb strain reducing layer (SRL) to shift the QD luminescence toward telecom wavelength. The GaAsSb SRL seems to be more suitable for MOVPE grown QD structures [2]. However, in these structures the surfacting behavior of In and Sb atoms is enhanced which complicates growth of multiple QD (MQD) structures for semiconductor laser application. In this talk I will present our results obtained on QD and MQD structures with InAs/GaAs QDs covered by GaAsSb SRL. The growth of these structures was studied in situ by reflectance anisotropy spectroscopy (RAS), which offer direct observation of processes during the structure growth such as quantum dot formation, and dissolution or surfacting of In and Sb atoms. The surfacting of In atoms in structures where QDs are covered only by GaAs or by GaAsSb SRL will be compared. Enhanced In surfacting was observed for structures with GaAsSb SRL. Possible ways how to suppress surfacting of both types of atoms and how to prevent their transport on epitaxial surface to subsequent QD layer in MQD structures will be suggested. Different interruption and growth rates of the separation layer growth under varying temperature or composition gradient of GaAsSb SRL will be discussed with respect to the suppression of undesired surfacting of In and Sb atoms. The conclusions derived from RAS measurements will be supported by TEM, AFM and photoluminescence (PL) results. Properties of InAs/GaAs QDs prepared by the MOVPE technology covered by GaAsSb SRL with extremely long emission wavelength at 1.8 μm will be presented. The prolongation of the emission wavelength was achieved by the introduction of GaAsSb SRL with Sb content of about 30% in the solid phase. The high Sb concentration in the SRL causes the preservation of QD size, which is about 15 nm wide at the base and 5 nm high. Increased QD size prolongs the PL wavelength. Furthermore, high content of antimony leads to a creation of type II heterostructure for which a red shift of the PL wavelength and decrease of the PL intensity is typical. Low PL intensity may complicate light emitting applications; however, fast separation of carriers in the type II structure is an advantage for detector or solar cell application, especially with the long wavelength. With respect to the perspective application of this structure, the photocurrent (PC) measurement was chosen as the complementary characterization method. A depression of PC for quantum well wavelength region (900-1200 nm) was observed for positive bias, while the PC from QDs (over 1200 nm) is not sensitive to the electric field orientation at all. Explanation of this unexpected phenomenon will be suggested. [1] Semiconductor lasers: Fundamentals and Applications, edited by A Baranov, ‎E Tournie, Elsevier 2013, ISBN 0857096400. [2] A. Hospodková et al, J. Cryst. Growth 370 (2013) 303. doi: 10.1016/j.jcrysgro.2012.08.007

Speaker: Prof. Eduard Hulicius (Institute of Physics, AS CR, v.v.i.)

11:20 → 12:40 Presentations: Session 5

11:20 VO2 nanorod as a new candidate for near infrared light harvesting

Vanadium dioxide (VO2) thermocromic properties have been widely applied in many devices in an attempt to solving the world energy crises. One of such devices is thermochromic/switchable/smart window, an energy saving window which is transparent to infrared light with high refractive index above 70 degree C (the transition temperature of VO2) and refracts infrared light with low refractive index below this temperature. In this current attempt, we have investigated the potentials of VO2 nanorod structure for near infrared light harvesting by coating the material on p-type Si wafers using a simple doctor blading method. The n-VO2|p-Si solar cell was annealed at 70 degree C and tested with a SF 150 small beam simulator by SCIENCETECH Inc. from Ontario Canada. Photon power conversion efficiency (PCE) was calculated form the values of the open-circuit voltage (Voc), fill factor (FF), and short-circuit current density (Jsc), and a decreasing trend of PCE was observed with the photon power.

Speaker: Mr Amos Akande (Dept. of Physics, University of Limpopo, DST/CSIR National Centre for Nano-Structured Materials, P O Box 395, Pretoria 0001, South Africa)

11:40 Evolution of stress in thin hard films by surface Brillouin scattering

1. Introduction Transition metal based thin films continue to attract tremendous research interest due to their excellent properties. As such they are widely used as protective coatings in optics and cutting tools due to their chemical properties, low wear and tear under extreme environments [1]. However delamination and intrinsic stress remain a great challenge especially in multilayer thin films. The mechanisms of thin film growth and intrinsic stress can be understood by investigating stress evolution through modification of elastic constants after insitu Ar+ incorporation during film growth. In this work, we present an alternative approach to investigate stress evolution of either crystalline or amorphous thin hard films using the components of the elastic constant tensor. Thin films of transitional metal carbide on etched (100) Si substrates have been grown by RF magnetron sputtering at 0 and - 60V bias to observe stress evolution by surface Brillouin scattering. A RF power of 175W and Ar2 working gas pressure of 1.0 x10-3mbar were used for film synthesis. X-ray Reflectometry has been used extract the film growth rate from measurements of film thickness, interfacial roughness and density. The density values were used to extract and simulate velocity dispersion curves obtained from surface Brillouin scattering spectra. A low surface roughness has been determined by X-ray Reflectometry for all films to  1.5 nm. Surface Brillouin studies on the - 60V biased and pristine samples have shown the propagation of Rayleigh surface acoustic wave and higher frequency peaks. The presence of the high frequency shift indicates a high film quality. The velocity dispersion curves show an increase in surface acoustic phonon velocity corresponding to an increase in elastic constants upon biasing. It is observed that Ar+ incorporation changes the C33 elastic constant by 38% with a resulting columnar thin film growth from the C11/C33 <1 values. 2. Results Fig 1 shows the typical surface Brillouin scattering spectra of transitional metal carbide. The true surface acoustic wave and the higher frequency peaks (Sezawa waves). The evidence of stress evolution is presented in Fig 2 which depicts an increase in the surface acoustic phonon velocity of the irradiated samples. 3. References [1] A. Palmero, E.D. van Hattum, W.M. Arnoldbik, F.H.P.M. Habraken, Surface & Coatings Technology 188-189, 392 (2004).

Speaker: Dr Daniel Wamwangi (wits university)

12:00 The defect passivation effect of hydrogen on the optical properties of solution-grown ZnO nanorods

Speaker: Dr Zelalem N. URGESSA (NMMU)

12:40 → 13:30 Lunch

13:30 → 15:00 Poster: Poster Session 2

13:30 A study of the interface kinetics affecting cylindrical phase separation in PS-b-PMMA copolymer thin films

In this study a controlled examination was undertaken in order to confirm and compare the parameters affecting PS-b-PMMA copolymer thin film nano-masks. A PS-b-PMMA copolymer with a molecular weight of 67 100 g.mol-1 (70:30) was used to produce thin film nano-masks on treated and untreated Si substrates. By simultaneously annealing samples of differing thin film thicknesses on the two types of treated silicon substrate in the same ambient (vacuum of 6 ×〖10〗^(-2) mbar) a direct comparison is made between the samples. It is shown that the average domain spacing’s for perpendicular-to substrate morphology is approximately 40 nm. Additionally, the effect of thin film thickness on the formation of a perpendicular morphology is clearly demonstrated, with a thickness repeat period of approximately 32 nm. Importantly, interface effects are seen between the thin film and both the substrate and vacuum interfaces, and a minimum vacuum quality is shown to be a necessary requirement for producing uniform perpendicular morphologies across the thin film.

Speaker: Dr Zelalem N. URGESSA (NMMU)

13:30 An Ab-initio Study into possible metastability of the Antimony-vacancy (Sb - V) complex in Germanium.

We are investigating the possibility of the Antimony-Vacancy complex (E-center) using the Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional within density functional theory (DFT). The results are then compared the predictions we obtained to experimental observations. We have in the past observed interesting results for the similar case of metastability in the Boron – Vacancy complex in silicon and wish to extend this research further to the Antimony-Vacancy complex in Germanium. We investigate the dependencies of the formation energy of the complex to the position of the Germanium vacancy to respect to the substitutional Antimony within the supercell. We examine the ability of the HSE06 functional to accurately predict the thermodynamic charge transition levels and whether or not charge-state controlled metastability exists in that of the Sb-V complex. The nearest neighbor and next nearest neighbor configurations of the Sb-V complex were also examined to give a greater understanding of the nature of this defect.

Speaker: Mr Geoffrey Webb (University of Pretoria)

13:30 Analysis of deep level emission bands in solution grown ZnO nanorods

Speaker: Mr Crispin Munyelele Mbulanga (Nelson Mandela Metropolitan University)

13:30 AZO as a transparent conductive oxide for inversion-layer silicon solar cells

1. Introduction Zinc Oxide (ZnO) has been attracting much attention in research activities for applications in light emitting diodes, spintronic devices, solar cells, etc. [1]. Doping the ZnO changes its properties especially in increasing its electrical conductivity [2]. This work focuses on the development of Aluminium doped Zinc Oxide (AZO) which has property-integrity to allow a research-in-progress hybrid inversion layer silicon solar cell to give maximum efficiency. The effects of thermal post-annealing in controlled air (N and O mixtures) on the electrical, structural and optical properties of undoped and aluminium-doped (i.e., 0%, 2% and 5%) AZO thin films deposited on glass and silicon substrates by the sol-gel method are investigated. 2. Results AZO was prepared using the sol-gel method with 0%, 2% and 5% Al. Thin films were then deposited on glass as well as crystalline silicon (c-Si) immediately after preparation, after 1 day, after 2 days, and after 3 days, pre-heating at 200˚C between layers. After a number of days, they were post-annealed at 400˚C, 450˚C, 500˚C, 550˚C, 600˚C and 650˚C. Various characterization techniques including four-point probe, PL, UV-Vis, Raman spectroscopy, AFM, SEM and XRD measurements were conducted on the prepared samples. The energy bandgaps for 0%, 2% and 5% of Al in ZnO were determined as 3.48 eV, 3.37 eV and 2.95 eV respectively for samples deposited on microscope glass slides after 1 day (‘Day 1’, see Fig. 1) from the time of the sol-gel preparation. The determined Eg results for AZO depositions on Day 0, Day 2 and Day 3 after the preparation of the sol-gel are consistent with the trend of reducing bandgap as the concentration of Al increases from 0% through 2% to 5% in the AlxZn1-xO polycrystalline material configuration. XRD measurements indicate that (002) is the most preferred orientation of the material. Peaks for (010), (011) and (110) orientations were also observed in the XRD pattern. Fig. 2 shows the de-convolution of peaks for AZO on glass Raman spectra, re-sulting in significant attribution of measured peaks. The results are being analyzed to understand the structural characterization of the AZO. The measurements from the other techniques will also be discussed.

Speaker: Mr Fred Joe Nambala (University of Pretoria)

13:30 Chemical and electrical characteristics of annealed Ni/Ir/Au and Ni/Au contacts on AlGaN

Aluminum gallium nitride (AlxGaxN) is a ternary wide direct bandgap semiconductor. The Al to Ga ratio can be varied to achieve various bandgap ranging from 3.4 eV for pure GaN to 6.2 eV for pure AlN. This property makes AlGaN based devices suitable for light emitting and light detecting devices such as LED’s and photodiodes. In fabricating a device such as a Schottky diode the metals deposited on the semiconductor play an important role in the operation of the device. This is because the metal-semiconductor contact of a Schottky diode influences the transport mechanisms of the device. It therefore becomes significant to understand the interaction of metals with the semiconductor substrate. Annealing has been used as a method of studying the evolution of the optical and electrical properties of semiconductor based devices [1,2]. In particular a study was performed on AlGaN Ni (20 Å)/ Au (50 Å) and Ni (20 Å)/ Ir (30 Å)/ Au (50 Å) Schottky photodiodes [3]. The samples were subjected to isochronous annealing for 5 min. under an Ar. ambient. The evolution of the chemical properties of these contacts with temperature is studied by using two surface characterization methods namely Time of Flight Secondary Ion Mass Spectroscopy (TOF-SIMS) and X-ray photo electron spectroscopy (XPS).

Speaker: Mr Phuti Ngoepe (University of Pretoria)

13:30 Effect of solvent medium on the material properties of ZnO nanoparticles synthesized by sol-gel method.

The abstract gives an overview of the content of my paper entitled, “Effect of solvent medium on material properties of ZnO nanoparticles synthesized by sol-gel method.” The issues on applications, optical and luminescence properties and the structure of ZnO were mentioned in introduction. In addition, the abstract reports that, in the research, high-quality zinc oxide (ZnO) nanoparticles were synthesized by sol-gel method using zinc acetate and sodium hydroxide precursors. The effect of varying volume ratios of water to ethanol solvent on zinc oxide (ZnO) nanoparticles prepared at constant temperature of 35 oC was studied. The material properties of the nanoparticles were examined using SEM, EDS,XRD,Uv-Vis Spectroscopy and PL systems. The abstract provides summary of the results and discussions. Essentially, reports on scanning electron microscopy, optical absorption, transmittance and the band gap were covered briefly. XRD and PL spectra were included in the abstract also. Finally, two items of references were captured.

Speaker: Mr Jatani Ungula (University of the Free State)

13:30 Electrical Characterisation of Electron Beam Exposure Induced Defects in Silicon

Silicon (Si) is one of the most important semiconductor materials and it has been studied extensively. This is mainly due to its low cost, thermal stability, and good durability. It is because of these properties that Si is a suitable candidate for exploring the electron beam exposure (EBE) technique. The main aim of developing the EBE technique was to see if electron beam deposition (EBD) induced defects could be introduced in a controlled manner.Deep level transient spectroscopy (DLTS) and high resolution Laplace-DLTS were used to characterise the defects introduced in epitaxially grown p-type Si during electron beam exposure. In this process, Si samples were first exposed to the conditions of EBD without metal deposition (EBE). After EBE, Aluminium and Nickel Schottky contacts were fabricated using the resistive deposition method. For the Aluminium contacts, the defect level H(0.33) was identified as the interstitial carbon (C_i ) related defect. It was a result of induced damage and could only be explained by the presence of donor-like traps. The capture cross-section was calculated to be 1.6×〖10〗^(-19) 〖cm〗^(-2) from the Arrhenius plot shown in figure 1.The defect level observed using the Nickel contacts had an activation energy of H(0.55) with a capture cross-section of 6.6×〖10〗^(-14) 〖cm〗^2. This defect has an activation energy similar to the I-defect. Pintilie et al observed a similar energy level, 0.545eV, with a capture cross-section of 1.7×〖10〗^(-15) 〖cm〗^2 and 9.0×〖10〗^(-14) 〖cm〗^2 after exposing their samples to high irradiation fluences. The defect level was detected using thermally stimulated current (TSC).

Speaker: Ms Helga Danga (University of Pretoria)

13:30 Electrical characteristics of a nearly ideal Ni/4H-SiC interface studied by I–V–T and Admittance techniques

Speaker: Mr Matshisa Legodi (University of Pretoria)

13:30 Interaction mechanism for energy transfer from Ce to Tb ions in silica

Energy transfer phenomena can play an important role in the development of luminescence materials. For example, in co-doped silica samples the Tb ions producing green luminescence useful in a variety of lighting and display applications can be excited indirectly by energy transfer from the Ce ions which have a longer, more acccessible excitation wavelength. As a result of energy tranfer away from the Ce ions, their luminescnce intensity and lifetime decreases as the Tb concentration increases. In this work we have compared experimental measurements of these effects with models developed by Inokuti and Hirayama [J. Chem. Phys. 47 (1967) 3211] for various interaction mechanisms and conclude that the energy transfer is as a result of dipole-dipole interactions.

Speaker: Dr Ted Kroon (University of the Free State)

13:30 Interplay between phase transitions and thermoluminescence in BaAl2O4

1. Introduction The Eu2+ and R3+ doped alkaline earth aluminates (MAl2O4; M: Ca, Sr, Ba) are among the best persistent luminescence materials [1]. The most efficient is, however, SrAl2O4:Eu2+,Dy3 – partly due to its emission in green to which the human eye is sensitive. Also the blue emitting CaAl2O4:Eu2+,Nd3+ is more efficient than the barium one. All three materials should have similar properties since they have the stuffed tridymite (SiO2) type structure. However, SiO2 is known for polymorphism and MAl2O4 derived from this do not behave dissimilarly. Although CaAl2O4 seems to have only one form and SrAl2O4 two, for BaAl2O4 one knows two hexagonal [2] and one orthorhombic forms. These polymorphs are found at high temperatures quite far from the usual operating range (<200 °C) of persistent luminescent materials. Nevertheless, the role of possible low temperature phase transitions should not be excluded as a reason for the less good persistent luminescence performance of BaAl2O4:Eu2+(,R3+) [3,4]. Further studies were carried out by using the high-temperature X-Ray Powder Diffraction (HT-XPD), Differential Scanning Calorimetry (DSC) and thermoluminescence (TL) methods. 2. Results The HT-XPD patterns of BaAl2O4 (Fig. 1) at room temperature (RT) correspond to the hexagonal P63 form. Additional weak reflections may indicate symmetry decrease to e.g. orthorhombic but this needs further proof. The RT structure is not stable for more than some 50 °C before significant changes occur in the intensity of all reflections. Characteristic to these changes is that they occur over a wide temperature range and smooth changes seem to be the rule rather than the exception. The intensity variation suggests displacement of atoms away from the reflection planes and changes may thus occur just in the space group. The DSC measurements were inconclusive since only a few broad signals and strong background variation were found instead of sharp signals. These features are in agreement with the slow and gradual nature of the changes in the XPD patterns. Both the HT-XPD and DSC results correlate well with the bands observed in the TL glow curves. When compared to the very simple (1 or 2 TL bands) glow curves of CaAl2O4:Eu2+ or SrAl2O4:Eu2+ (without R3+ co-doping), the five broad TL bands for BaAl2O4:Eu2+ between 100 and 400 °C (Fig. 2) indicate a more complex trap structure with energies from 0.8 to 1.4 eV. Thus a reason for the inferiority of the BaAl2O4 host vs. CaAl2O4 and SrAl2O4 is clear: the close to continuous distribution of traps does not allow long term storage of input energy but the bleaching of the traps occurs too rapidly. The persistent luminescence is then of short duration.

Speaker: Prof. Jorma Hölsä (University of Turku)

13:30 Luminescence properties of CaO:Bi3+ phosphor

In recent years, there has been growing importance focused on research in light emitting diodes (LEDs) because of their long operation lifetime, energy-saving feature and high material stability [1, 2]. During the past few years, white LEDs fabricated using a near ultraviolet (n-UV) LED (380–420 nm) coupled with red, green, and blue phosphors have attracted much attention [2]. Accordingly, it is necessary to develop new blue phosphors that could be effectively excited in the near ultraviolet range especially for wavelengths of 400 nm [1]. The spectroscopic properties of the Bi3+ ion in different hosts have attracted much attention due to its emission wavelength that varies from the ultraviolet to the red region depending on the host matrix [3]. Therefore, with the appropriate matrix, the emission of Bi3+ ions can be used as a candidate for n-UV for LED applications. CaO:Bi3+ phosphor powder was successfully synthesized by the sol-gel combustion method. The structure, morphology and luminescent properties of the phosphor were characterized by X-ray diffraction (XRD), Scanning electron microscope (SEM), photoluminescence (PL) and cathodoluminescence (CL) techniques.

Speaker: Dr Abdelrhman Mohmmed (University of the Free State)

13:30 Microwave assisted synthesis of ZnO nanoparticles for lighting and dye removal application

Speaker: Dr Vijay Kumar (Department of Physics, University of the Free State, P.O. Box 339, Bloemfontein 9300, Republic of South Africa)

13:30 n-WO3|p-Si tandem layer solar cells as new candidates on the Shockley-Queisser chart of photovoltaic efficiency

Speaker: Mr Funda Mpanza (Heriot-Watt University)

13:30 Pd doped ZnO nanostructures: Structural, luminescence and gas sensing properties

Undoped and Pd (0.5 and 0.75 mol%) doped ZnO nanostructures were synthesized by a sol-gel method and annealed at 350 oC. An increase in the particle size with Pd doping was observed. Room-temperature PL measurements showed that both oxygen and zinc related defects played a role in the defect emission process. PL and sensing analyses revealed that there is a correlation between the sensing properties and the relative concentration of the oxygen vacancies, zinc vacancies and zinc interstitials present on the surface of the ZnO- nanostructures.

Speaker: Dr Gugu Mhlongo (CSIR)

13:30 Rare earth doped lanthanum strontium borate (La2Sr3(BO3)4: xTb3+) polycrystalline green phosphors

Recently, great efforts have been made to develop efficient phosphor systems. Among these, a number of works have been carried out on investigations of the luminescent properties for borates, aluminates and gannets [1, 2]. Since the first white light emitting diodes(W-LED’s) became commercially available, it have attracted great attention for their obvious advantages such as long lifetime, high luminescence efficiency, low power consumption and environmental friendliness, consequently they are expected to replace incandescent and fluorescent lamps for general lighting application in the future. Good glass host is very important for efficient luminescence of rare-earth ions. Borate glass is a suitable optical material with high transparency, low melting point, high thermal stability and good rare-earth ions solubility. Among them, La2Sr3(BO3)4, is one of the best candidates for the desired host materials of phosphors, and La2Sr3(BO3)4:xTb3+, as a green phosphor could be possible used extensively in future in plasma display panels. Tb3+ -doped phosphors are considered more important due to their efficiency in the display of sharp and intense green emission at 544 nm due to an electronic transition of 5D4-7F5. In this paper, we report combustion synthesis of some important, borate based, PL phosphors using commonly available fuels and oxidizers.

Speaker: Ms Roz Madihlaba (University of the Western Cape)

13:30 Surface morphology and structural properties of iron oxide thin film photoanode prepared by dip coating: effect of electrochemical oxidation

Metal oxides have attracted a considerable attention as transparent electrodes, active layers and charge collectors in energy harvesting activities due to their diverse properties [1]. Due to its a nearly ideal band gap (~ 2.1 eV) for solar energy conversions, low cost, chemical stability, natural abundance, n-type conductivity, “rust”- iron oxide (α-Fe2O3) - is regarded as a promising semiconductor in photovoltaic (PV) solar cells and photoelectrochemical (PEC) cells [2]. However, despite the good characteristics α-Fe2O3 absorbs weakly and conducts poorly due to poor carrier transport and rapid recombination of photo-generated electrons and holes. Surface morphology and structural properties strongly influences photoactivity efficiency of nanostructured electrodes [3]. α-Fe2O3 thin films were prepared by dip-coating method on fluorinated tin oxide (FTO) conductive glass substrates from Fe (NO3)3•9H2O (28.0 g) and oleic acid (17.0 g) precursor. Four layers of hematite films were obtained after repeated dip coating and annealing at 500 ᵒ C for 2 hours. The photoanodes were electrochemically oxidized (anodized) in 1M KOH at a constant anodization potential of 500 mV for 1 min in dark and light conditions. The structural properties of α-Fe2O3 nanoparticles were investigated. 2. Results Fig 1 shows top view images of high resolution FE-SEM of pristine and anodized α-fe2O3 thin film photoanodes. The nanoparticles were spherical in shape. The micrographs depicts a denser surface after electrochemical oxidation of photoanodes in light. Fig 2 XRD diffractograms of the films. The results show decrease in linewidth of the peaks in anodized samples which indicates increase in crystallite sizes upon the treatment.

Speaker: Ms Maabong Kelebogile (University of Pretoria)

13:30 Synthesis and Characterization of a Novel Rare-Earth Oxyorthosilicates (R2SiO5) (R = La, Gd, Y) Doped Dy3+ Nanophosphors

1. Introduction Nowadays, phosphors have found applications in solid state lighting, phototherapy, information display technologies, and solar cells, among other things. Rare earth oxyorthosilicates of the form R2SiO5 (R = La, Gd, Y) doped with rare earth elements, have been of interest for the past decades due to their wide band gap, fast decay times, high quantum efficiency, high density and minimal self-absorption. Using urea-assisted solution combustions method, we prepared both single and mixed rare-earth oxyorthosilicates doped with Dy3+ powder nanophosphors. 2. Results The structures of our phosphors analysed using X-ray diffraction (XRD) confirmed that the phosphors crystalized in the pure monoclinic phases of La2SiO5, Gd2SiO5 and Y2SiO5 or in the mixture of any of the three compounds. We also analysed the morphologies, elemental composition, and the chemical and electronic states of our powders using field emission scanning electron spectroscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) respectively. The photoluminescence (PL) (Fig. 1) measured when the samples were excited using a 325 nm He-Cd laser showed broad blue emission assigned to self-trapped excitons in SiO2 [1] (which were not observed in the PL spectra measured in phosphorescence mode when the samples were excited using monochromatic xenon lamp) and 4F9/2→6H15/2 and 4F9/2→6H13/2 transitions of Dy3+ [2]. The colour purity of the samples calculated using the CIE coordinate calculator confirmed that the phosphors can emit tunable colours and white light. Furthermore, there was even distribution of the atomic and molecular ionic species on the surfaces of the samples as shown in the time-of-flight secondary ion mass spectrometer images in Fig 2. 3. References [1] C. Itoh, K. Tanimura, N. Itoh. J. Phys. C. Solid State Phys. 21(1988) 4693. [2] K.G. Sharma, N.R. Singh. New J. Chem. 37 (2013) 2784.

Speaker: Mr Simon. N Ogugua (UNIVERSITY OF THE FREE STATE)

13:30 Synthesis and characterization of structural and luminescence properties of TiO₂ nanoparticles for water treatment application

Introduction Solar water treatment is a low-technology solution that works to capture the heat and energy from the sun to make water cleaner and healthier for human use and consumption. The oxide nanoparticles synthesized by several methods appears more and more useful because these nanoparticles have good electrical, optical and magnetic properties that are different from their bulk counterparts [1]. Titania nanoparticles have received much interest for applications such as optical devices, sensors, and photocatalysis [2]. There are several factors in determining important properties in the performance of TiO2 such as particles, crystallinity and the morphology. TiO2 nanoparticles were synthesized by a simple sol-gel method. The structure and morphology of the synthesised nanocrystalline TiO2 were characterized by XRD, SEM, UV and PL. The effect of some parameters such as hydrolysis rate, concentration amount of the precursor constituents, and annealing temperature were investigated. Results A representative scanning electron microscopy image of the TiO2 nanoparticles synthesized presented in Fig. 1, indicates that the sample is composed of roughly spherical surface aspects. The anatase to rutile structural phase change occurs when annealed at a temperature greater than 500 °C when evaluated from X-ray diffraction pattern intensities of (101) and (110) peaks, respectively. Annealing samples at high temperature improves crystallinity as confirmed by both SEM and XRD measurements. It was observed that the band gap of TiO2 varied from 2.49 and 3.26 eV while the crystallite sizes calculated using the Scherrer equation obtained from each set of synthesis conditions changed from 8 to 10 nm with variation of hydrolysis rate. The emmissions spectra (λexc= 325 nm) of TiO2 nanoparticles for different hydrolysis rate display peaks at 336, 381 and 486 nm and the broad emission peak at about 381 nm is attributed to band gap transition.

Speaker: Prof. Francis Dejene (University of the Free Stet)

13:30 Systematic Study of Up-Conversion Luminescence in NaYF4:Yb3+,R3+

In up-conversion luminescence, the absorption of two or more low energy photons is followed by the emission of a high energy photon. NaYF4:Yb3+,R3+ (R: Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er or Tm) materials were prepared to study up-conversion luminescence. The materials were studied with TG-DSC, FTIR and XPD methods. Up-conversion luminescence was studied with NIR laser excitation at 976 nm. The DSC curves showed an exothermic signal at 400-500 oC due to the cubic-to-hexagonal phase transition of NaRF4. The TG curves showed only small mass losses (ca. 2-4 %) during heating to 500 °C. The FTIR spectra did not reveal other impurities apart from water absorbed in the KBr discs. The XPD patterns confirmed the hexagonal structure of the annealed materials. The up-conversion luminescence was observed of Pr3+, Nd3+, Eu3+, Tb3+, Ho3+, Er3+ and Tm3+. The up-conversion luminescence was not obtained of Sm3+ and Dy3+.

Speaker: Mr Tero Laihinen (University of Turku, Department of Chemistry, Turku, Finland)

13:30 The influence of reaction times on structural, optical and luminescence properties of CdTe nanoparticles prepared by wet-chemical process.

The title of the abstract is “The influence of reaction times on structural, optical and luminescence properties of CdTe nanoparticles prepared by wet-chemical process” In introduction, the abstract reports the study of CdTe nanoparticles made by a simple green synthesis using L-cystein as a capping agent, potassium tellurite and sodium selenosulphate as stable tellurium and selenium sources. The synthesis parameters have considerable influence on the particle size and photoluminescence quantum yield of the CdTe nanoparticles. The results part of the abstract, discusses briefly the properties of CdTe as characterised using PL, TEM, UV-vis and XRD. Spectra on a representative TEM micrographs of CdTe nanoaprticles and PL emission of CdTe QDs prepared at various reaction emitting at different wavelength when excited by a single excitation wavelength were attached. Finally, three references were appended at the foot of the abstract.

Speaker: Ms Sharon Kiprotich (University of the Free State)

13:30 Zn2SiO4:Mn2+ co-doped with Tm3+ and other Re ions (Re = Rare-earth): Synthesis, Structure and Optical Properties

Zinc silicate (Zn2SiO4) is a good host lattice for luminescence centers such as rare-earth ions and transition metals (TM) to prepare light emitting materials (phosphors) that can emit blue, green and red light upon excitation with high energy electrons or photons. In this work, combustion method was used to prepare undoped and manganese (Mn2+) and thulium (Tm3+)–co-activated zinc silicate (Zn2SiO4:Mn2+,Tm3+) nanoparticulate powder phosphors. In addition, a selection of Re ions (rare-earth ions) were used for co-doping to tune the emission colour. The structure, morphology and luminescence properties were investigated by X-ray diffractometer (XRD) and Field-Emission Scanning Electron Microscopy (FE-SEM) respectively , while the optical and luminescent properties were examined by Fourier Transform Infrared Spectroscopy (FTIR), ultraviolet visible (UV-vis) spectroscopy, Varian Cary-eclipse fluorescence spectrophotometer and 325 He-Cd laser equipped photoluminescence system. The XRD patterns matching with the willemite structure of Zn2SiO4 were observed. However, there was additional secondary peak assigned to (101) diffraction of the hexagonal wurtzite structure of ZnO, suggesting that our material was an admixture of ZnO and Zn2SiO4. ZnO was either formed from the reaction of Zn2+ and O2- during the combustion reaction in air or resulted from the incomplete decomposition of the precursors. A network of spherical (but faceted) agglomerated nanoparticles were observed from undoped, Mn2+-doped and Mn2+/Tm3+-codoped Zn2SiO4 powders. The PL spectra recorded from Zn2SiO4:Mn2+ nanophosphors with dopant concentration of Mn2+ ions ranging from 0.045 – 0.09 mol% show strong green-orange emission band at ~ 562 nm and a shoulder at ~523 nm and as the Mn2+ concentration increases the emission peak slightly shifted to the higher wavelength. This is a typical emission of Mn2+ in α-Zn2SiO4 and may be assigned to the electronic transition 4T1(4G) →6A1(6S). Tuning of the emission colour by Tm3+ co-doping and other selected Re ions is demonstrated and will be discussed in detail. These nanophosphors have potential applications in nanoelectronics and optoelectronics.

Speaker: Dr Pontsho Mbule (University of South Africa)

15:00 → 15:20 Tea

15:20 → 16:00 Poster: Poster Session 2 Continued

16:00 → 19:00 Game Drive

19:00 → 21:00 Dinner

Thursday, 7 May

05:30 → 07:00 GAME DRIVE

07:00 → 09:00 Breakfast

09:00 → 09:40 Plenary: Session 6

09:10 Defects in Zinc Oxide Grown By Pulsed Laser Deposition

ZnO is a wide band gap semiconductor having excellent properties for fabricating optoelectronic devices operating at the wavelength of ultra-violet (UV). The realization of fabricating UV optoelectronic devices with ZnO based technology is hindered by the asymmetric p-type doping difficulty, which is related to the poor understanding of the defects, defect compensation, and defect control in ZnO materials. Using pulsed laser deposition (PLD), undoped ZnO films with (002) orientation are grown on c-plane sapphire with the systematic variation of the fabrication parameters including the substrate temperature and oxygen pressure during the growth, as well as the post-growth annealing temperature [1]. Defects in the films were characterized by the positron annihilation spectroscopy (PAS), Raman spectroscopy, secondary ion mass (SIMS) and photoluminescence (PL). The electron concentration (~1018 cm-3) is similar to the hydrogen concentration measured by SIMS. SIMS study also reveals thermal induced Zn out-diffusion into the sapphire substrate and leaves out the VZn related defects at the ZnO film. Oxygen deficient defect related Raman lines 560 cm-1 and 584 cm-1 are identified and with their origins being discussed. PAS reveals two kinds of VZn related defects having different microstructures in the PLD grown films, which are different from those identified in the ZnO single crystals. Green luminescence (GL) with more than one origin is found in the films with annealing temperature lower than 900oC. At the annealing temperature of 900oC, the defect emission spectra (measured at 10 K) of all the films irrespective of the initial growth condition exhibit a GL peaked at 2.47 eV and originated from a single defect, and simultaneously the ~3.23 eV donor-to-acceptor-pair (DAP) emission is introduced. PAS study shows that the GL at 2.47 eV and the DAP are correlated to VZn defect having the ionization levels at EV+0.15 eV and EV+0.97 eV [2]. The result is compatible with the LDA+U calculation [3]. References [1] Zilan Wang et al, J. Appl. Phys. 116, 033508 (2014). [2] ZIlan Wang et al, submitted to Appl. Phys. Lett. [3] A. Janotti and C. G. Van de Walle, Phys. Rev. B 76, 165202 (2007).

Speaker: Prof. Francis Ling (The University of Hong Kong)

09:40 → 10:40 Presentations: Session 6

09:40 Inductively coupled plasma induced defects in n-Si using low energy Ar ions

Speaker: Dr Sergio Coelho (UP)

10:00 Effect of Substrate temperature on the defect related emission of ZnO thin films prepared by pulsed laser deposition

1. Introduction Zinc Oxide (ZnO) has attracted much attention in research activities with potential applications such as light emitting diode, spintronic device, transparent conductive electrodes, laser and solar cells due to its wide band gap (~3.37 eV) and large exciton binding energy (~60 meV) [1,2]. There are several deposition techniques used to grow ZnO thin films, including chemical vapour deposition (CVD) [3], magnetron sputtering [4], spray pyrolysis [5], the sol-gel method [6] and pulsed laser deposition (PLD) [7]. In the case of PLD prepared films, the degree of orientation is influenced by the deposition conditions such as temperature, background gas composition and pressure, and kinetic energy of the plume particles [7]. The trivalent rare earth (RE3+) doped ZnO belong to one kind of novel optical materials and have drawn an increasing amount of attention [8]. Terbium doped ZnO (ZnO:Tb3+) thin films were prepared by PLD at different substrate temperatures. In the present work, the effects of substrate temperature on the structure, optical and luminescence properties of ZnO:Tb3+ thin film were investigated in detail. A correlation was found between the defects (confirmed by X-ray photoelectron spectroscopy) and the Photoluminescence (PL) results. 2. Results Figure 1 shows the XRD patterns of the ZnO thin films deposited on Si substrates at different substrate temperatures ranging from room temperature (RT) to 400ºC. According to the XRD patterns, all ZnO films were oriented along the (002) plane. This is in line with the characteristics of the hexagonal ZnO wurtzite where the c-axis is perpendicular to the substrate plane [9]. The PL spectra of the ZnO films grown at the different substrate temperatures are shown in figure 2. It is worth noting that the films mainly exhibit emission in the UV region. The strong near-band edge emission at room temperature is due to free exciton recombination. Generally in ZnO, the visible light emission is ascribed to the structural defects [10] such as zinc vacancy (VZn), oxygen vacancy (Vo), interstitial zinc (Zni), interstitial oxygen (Oi) and antisite oxygen defects (OZn) [10]. The PL spectra of the ZnO:Tb3+ thin films are characterized by three different types of transitions, the one is due to exciton recombination emission, the second is due to defect level emission and the third is due to the Tb3+ f-f transitions. For the emission due to the Tb3+ ions, a green emission peak at 543 nm and a few minor peaks at 489 and 622 nm were detected. These peaks represent the 5D4-7F5, 5D4-7F6, and 5D4-7F3 transitions of Tb3+, respectively [10]. 3. References [1] U. Ozgur, Y.I. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S. Dogan, V. Avrutin, S.J. Cho, et. al, J. Appl. Phys. 98 (2005) 041301. [2] Vinod Kumar, N. Singh, V. Kumar, A. Kapoor, L.P. Purohit, O.M. Ntwaeaborwa and H.C. Swart, J. Appl. Phys. 114 (2013) 134506. [3] J. Hu and R.G. Gordon, J. Appl. Phys. 71 (1992) 880. [4] Y. Igasaki and H. Saito, J. Appl. Phys. 70 (1991) 3613. [5] A.F. Aktaruzzaman, G.L. Sharma and L.K. Malhotra, Thin Solid Films 198 (1991) 67. [6] Vinod Kumar, R. G. Singh, L. P. Purohit and Fouran Singh, J. Alloys & Comp. 544 (2012) 120. [7] A. Suzuki, T. Matsushita, N. Wada, Y. Sakamoto and M. Okuda, Jpn. J. Appl. Phys. 35 (1996) L56. [8] X. Chen and Wenqin Luo, J. Nanoscie. Nanotechn. 10 (2010) 1482. [9] S. Choopum, R.D. Vispute, W. Zoch, A. Balsamo, R.P. Sharma, T. Venkatesan, et. al, Appl. Phys. Lett. 75 (1991) 3947. [10] Vinod Kumar, S. Som, V. Kumar, V. Kumar, E. Coetsee, O.M. Ntwaeaborwa and H.C. Swart, Chemical Eng. J., 255 (2014) 54.

Speaker: Dr Vinod Kumar (Department of Physics, University of the Free state, Bloemfontein, ZA-9300)

10:40 → 10:50 Tea

10:50 → 11:20 Plenary: Session 7

10:50 Structural and Optical Properties of Group-III Nitride Nanorods

Introduction One-dimensional group-III Nitride nanorods (NR), have drawn a large interest during the past decade thanks to great prospects for improved optoelectronics by e.g., increased quantum efficiency, higher sensitivity, lower heat generation, etc. as compared to bulk and quantum well-based devices. Magnetron Sputter Epitaxy (MSE) allows for epitaxial growth of Al1-xInxN NRs as well as GaN NRs.[1,2] Moreover, low temperature group-III N epilayer growth is possible by MSE[3] and it is easily scalable to large areas, which make it an industrially potent, yet unexploited, technique. Results Al1-xInxN NRs grown on ZrTiN seed layers feature In-rich cores and Al-rich shells, as observed by high resolution electron microscopy (HREM) and quantitative valence electron energy loss spectroscopy using scanning transmission electron microscopy. Such nanorods exhibit near band-edge optical emission at ~5 eV, as observed by cathode luminescence. An internal composition gradient in Al1-xInxN nanorods leads to a curved-lattice epitaxial growth (CLEG) [4] which is utilized for high precision tailoring of nanorod morphologies, such as spirals and zig-zag shapes which opens the possibility to obtain new unique optical properties. For example, Al1-xInxN spirals with a pitch of ~200 nm can be designed to produce either fully right-handed or left-handed circularly polarized reflected light at specific wavelengths in the UV-regime (see Fig 1). High quality GaN NRs were grown by MSE at 1000°C on Si(111), 4-H SiC(0001) and SiOx substrates, using a liquid Ga-target. NRs can be grown at a thickness ~35 nm and lengths up to several µm without extended defects as seen in HREM. Low-temperature micro photoluminescence (µPL) reveal intense and sharp band-edge emission, characteristic of donor-bound exitons, with a FWHM = 1.7 meV at 3.48 eV. (see Fig. 2) In conclusion, we show that reactive Magnetron Sputter epitaxy (MSE) can be used to produce high quality group-III Nitride NRs, with optical properties comparable to e.g., MBE grown structures, as well as new tailored NRs by CLEG, featuring unique polarizing properties.

Speaker: Prof. Jens Birch (Linköping University)

11:20 → 12:40 Presentations: Session 7

11:20 ANOTHER METHOD TO DETERMINE THE REFRACTIVE INDEX OF AlXGa1-XN

1. Introduction AlGaN alloys continue to be of great interest due to the application of these alloys in high-power, high-temperature and high-frequency devices such as field-effect transistors, UV-light emitting LED’s and laser diodes1-3. A range of different electrical and optical properties can be obtained by varying the alloy composition of AlxGa1-xN by changing the amount of Al in the alloy. The characterization of these alloys to determine the various physical properties as a function of Al content is therefore necessary. Optical characterization is preferred as this technique has the advantage of being non-contact and non-destructive. In this work, the use of infrared reflection spectroscopy to evaluate AlxGa1-xN epilayers grown with varying Al content by metalorganic vapour phase deposition (MOCVD) on sapphire substrates was investigated. The layer thickness was readily determined using interference fringes in the reflectance spectra of the samples4. However, this requires knowledge of the refractive index n as function of the wavelength. Determination of the refractive index of AlxGa1-xN is challenging, since the refractive index is a function of wavelength and the aluminium content of an AlxGa1-xN sample, as well as the temperature of the sample5. A number of techniques have been reported for the determination of the refractive index for AlxGa1-xN, including from the refractive indexes of AlN and GaN, ellipsometry measurements or Sellmeier type equations6. The present investigation aims to provide an alternative method to determine the refractive index of AlxGa1-xN at room temperature. The method is based on the manipulation of earlier published experimental results6 (Fig.1) and relations from the slope and y-intercept of straight line graphs. The new formulation was then employed to obtain the thickness of the measured AlxGa1-xN epilayers, using the observed interference fringes (Fig. 2). 2. Results Fig. 1 Examples of straight-line fits of previous results Fig. 2 Interference spectra of two AlxGa1-xN epilayers 3. References 1. N. Maeda, T. Saitoh, K. Tsubaki, T. Nishida and N. Kobayashi, Jpn. J. Appl. Physics 38 (1999) L987-L989. 2. T. Takano, Y. Narita, A. Horiuchi and H. Kawanishi, Appl. Phys. Lett. 84 (2004) 3567-3569. 3. J. Han, M.H. Crawford, R.J. Shul, J.J. Figiel, M. Banas, L. Zhang, Y.K. Song, H. Zhou and A.V. Nurmikko, Appl. Phys. Lett. 73 (1998) 1688-1690. 4. F. Reizman, J. Appl. Phys. 36 (1965) 3804 5. U. Tisch, B. Meyler, O. Katz, E. Finkman and J. Salzman, J. Appl. Phys. 89 (2001) 2676-2685. 6. N.A. Sanford, L.H. Robins, A.V. Davydov, A. Shapiro, D.V. Tsvetkov, A.V. Dmitriev, S. Keller, U.K. Mishra and S.P. DenBaars, J. Appl. Phys. 94 (2003) 2980-2991.

Speaker: Prof. JAA Engelbrecht (NMMU)

11:40 Optoelectronic properties and structural dependence of carbon nanomaterials-based hybrid organic photovoltaic devices

We report on the incorporation of carbon nanomaterials (CNMs) such as single-walled carbon nanotubes (SWCNTs) into organic PV (OPV) cells for efficiency optimization. Although CNMs have been used before in OPVs, the focus is put here to elucidate the effect of the structural properties of the CNMs on OPV performance, which is poorly understood. More specifically, we address the issue of improving the performance of a new hybrid OPV device by combining the physical and chemical characteristics of light-sensitive conjugated polymers (CP), with the high electrical conductivity of SWCNTs by blending the both in a composite photoactive layer. The focus is put on exploring in depth the electronic and optoelectronic properties of the composite material in an OPV scheme and exploring its corresponding photo-conversion capability. The root-mean-square roughness, photoluminescence and optical absorption were found to increase with increasing SWCNTs content and a non linear correlation between the nanotubes loads and the open circuit voltage VOC was clearly pointed-out.

Speaker: Dr Brahim Aissa (QEERI-QF)

12:00 Characterisation of Defects in Photovoltaic modules using Electroluminescence and Large-Area Light Beam Induced Current (LA-LBIC) techniques

1. Introduction The performance and longevity of photovoltaic (PV) modules can be severely limited by poor material, manufacturing defects and physical cracks. Cell mismatch can occur when a solar cell in a series-connected string produces a lower current than the other cells in that string[1]. The current output of the entire string is limited by the weakest cell in the string so damage to a single cell in a module can affect the entire module’s current output. These material defects are not always identifiable in visual inspection so additional characterisation techniques are necessary. In this study Electroluminescence (EL) imaging and Large-Area Light Beam Induced Current (LA-LBIC) measurements were used. EL is an effective, fast and non-destructive characterisation technique for PV modules and is widely used in PV module manufacturing to assess the quality of the finished module[2, 3]. The EL emitted under forward bias is related to the recombination, optical and resistive properties of the cell. LA-LBIC is also a non-destructive spatial characterisation tool capable of measuring the photo-response of PV modules[4]. The PV module is spot-illuminated using a light source mounted above the module on a motorised x-y scanning stage. At each point the current output of the module is measured at a set voltage level. This provides a photo-response map of the PV module related to the wavelength of the light source used. 2. Results Figure 1 shows an EL image of a mono-crystalline silicon module with striation rings visible as a dark circle in the centre of the cell. In this region of the cell the presence of oxygen precipitates results in the recombination of electron hole pairs. This defect occurs during the wafer growth stage and affects the efficiency of the cell and will lower the module efficiency. However, it is not regarded as a module failure. The corresponding LA-LBIC photo-response map is shown in figure 2. The striation ring is visible in the centre of the cell as an area of decreased photo-response. LA-LBIC and EL measurements have shown good agreement in measurement results and the two techniques have proved complementarity in PV module characterisation.

Speaker: Prof. Ernest van Dyk (NMMU)

12:40 → 13:30 Lunch

13:30 → 15:00 Presentations: Session 8

13:30 Non-rare earth doped persistent luminescence materials

1. Introduction Both the commercial markets as well as research and development of persistent luminescence phosphors have been dominated by materials doped with Eu2+. The domination of Eu2+ over other rare earths (R; e.g. Ce3+, Pr3+, Tb3+) is due to e.g. the strong and easily tunable emission (Fig. 1) as well as the favourable energy level scheme of Eu2+ vs. the host band structure [1]. For many reasons this situation may soon change: i) europium is needed for many other phosphor applications, ii) the recovery and recycling of europium from persistent luminescence materials is nearly impossible and iii) e.g. the biotechnical applications require strongly red emitting biomarkers. The economic pressure due to both aggressive and wildly fluctuating pricing policy of rare earths is probably the most important driving force to the change. 2. Results The pursuit for cheaper persistent luminescence materials has already resulted in the adoption of such inexpensive host materials as the alkaline earth aluminates, MAl2O4 (M: Ca, Sr), Sr2MgSi2O7 or Sr4Al14O25 [1]. The replacement of Eu2+ (and R3+ co-) dopants with non-rare earths is ensuing at a much slower pace despite many outstanding possibilities. For example, the first persistent luminescence material, the famous Bologna Stone from the beginning of the 17th century, uses doping with Cu+ in BaS [2]. The 3d elements such as Ti3+, Cr3+ and Mn2+ have been shown to give strong persistent luminescence, too [1,3]. Many main group metal ions such as Pb2+ and Bi3+ can also be used as dopants. To improve either the absorption of incident radiation, the efficiency of emission or its tunability, combinations with conventional dopants (e.g. Eu2+- Mn2+ or Cr3+-Pr3+) or with non-rare earth ones (e.g. Cr3+-Bi3+ or Mn2+-Bi3+) may be used [3]. As for the mechanism of persistent luminescence, it is necessary to consider first the redox behaviour of the dopants. It is noted at once that each dopant has an oxidized counterpart: Eu2+/3+, Ce3+/IV, Pr3+/IV, Tb3+/IV, Ti3+/IV, Cr3+/IV, Cu+/2+, Pb2+/IV and Bi3+/V. However, this does not necessarily mean that this counterpart is easily observed experimentally. The mechanism should thus be similar to the very efficient Eu2+ based persistent luminescence and the electrons act as charge carriers [1]. Eventually, the properties of a Eu2+-Mn2+ dopant combination is described (Fig. 2) [4]. Efficient persistent luminescence and also tuning of emission from red to blue via e.g. magenta can be achieved. System’s efficiency depends now not only on the individual dopants but also on the persistent energy transfer between them.

Speaker: Prof. Jorma Hölsä (University of Turku)

13:50 Effect of Ag doping on the luminescence of ZnO and ZnO:Tb

ZnO has attracted considerable attention as a photonic material and ZnO:Tb is a promising material for phosphor converted white light emitting diodes. It has been reported that metallic Ag nanoparticles will precipitate if an excessive amount of Ag is doped into ZnO and localized surface plasmon resonance effects associated with metallic nanoparticles are of great interest because they have the potential to significantly enhance the luminescence properties of phosphors. In this study we investigated the effect of doping ZnO and ZnO:Tb with Ag on their luminescence properties. The addition of Ag caused more than a twofold increase the near band edge recombination peak of ZnO near 390 nm. However, the addition of Ag to ZnO:Tb reduced the characteristic green luminescence from the Tb ions. Possible mechanisms for this and the influence of intrinsic defects in the ZnO are discussed.

Speaker: Mr Abd Ellateef Abbass (Sudanese)

14:10 Luminescence investigation of dual mode emitting Ho3+ doped tellurite glass

1. Introduction Lanthanides activated luminescent materials having increasing interest among researchers owing to its broad area of applications in photonic, sensing, security and biomedical fields [1]. Rare earth doped glasses are one of the best luminescent solids because of their multifunctional uses from daily live to scientific applications. Tellurite glasses are a better choice for preparing luminescent glasses rather than other oxides due to its low phonon frequency characteristics [2]. Triply ionized holmium is a good candidate for an activator due to its strong emission in the green region and dual mode emitting properties [3]. Several glass synthesis techniques are in used but the melt-quenching technique is the simplest and cheapest method for the preparing of glasses [2].The present work is focused upon luminescence emission from the Ho3+ doped TeO2-ZnO glass prepared by a melting and quenching method. 2. Results The amorphous nature of the prepared glass was checked by X-ray diffraction patterns of the prepared glass while the thermal behavior was discussed on the basis of thermogravimetric analyses. The luminescence property of the glass was determined via absorption and photoluminescence emission spectra. Fig. 1 shows the photoluminescence emission spectra of the prepared glass upon 980 and 488 nm excitations. The results suggest a tunable two photon upconversion emission and intense green emitting downconversion emission from the sample which is also explained via a suitable energy level structure diagram. Thus dual mode emission (up and down-conversion) observed from the prepared glass is useful in lighting and enhancing the efficiency of solar panels. Fig.1: Schematic diagram of dual mode emission from the Ho3+ doped TeO2-ZnO glass. 3. References [1]A. Pandey, S. Som, V. Kumar, V. Kumar, K. Kumar, V. K. Rai, and H. C. Swart. Sens. Actuators B 202 (2014) 1305. [2] D. K. Mohanty, V. K. Rai, Y. Dwivedi and S. B. Rai. Appl. Phys. B 104 (2011) 233. [3] A. Pandey, V. K. Rai. Dalton Trans. 42 (2013) 11005. Acknowledgements This research is supported by the South African Research Chairs Initiative of the Department of Science and Technology (84415) and National Research Foundation of South Africa. The financial support from the University of the Free State is also acknowledged.

Speaker: Dr Anurag Pandey (University of the Free State)

14:30 Synthesis and Characterization of luminescence magnetic nanocomposite for biomedicine application

1. Introduction In the recent past, magnetic materials and semiconductor inorganic materials have been developed and applied (as independent) in fields including biomedicine, especially biotechnological processes imaging, tracking, and separating biological molecules or cells. Processes of biomedical diagnosis and therapy require nanometer scale particles featuring characteristics such as magnetization and fluorescence [1-4]. In the last two decades research has been focused on fabrication of QDs constituting elements of groups II-IV owing to their potential application in lasers, light emitting diodes and biological studies. However, biological applications of the as-prepared QDs were limited by presence of highly toxic Cadmium core. Strategies have been employed to alleviate toxicity of Cd based core coating it with less toxic materials such as a ZnS shell. However, exposure to UV light or oxidation results in the release of cadmium via surface oxidation. In the current study new type of Indium based quantum dot was synthesized and conjugated to the magnetic nanoparticles. The QD were characterized by PL, HRTEM, XRD, SQUID and FTIR. 2. Results and Discussion The photoluminescence characteristics of the coupled and uncoupled indium based quantum dots were investigated to determine whether the fluorescing property could be retained in the bifunctional system. Generally, the PL intensity of the QDs was observed to remain almost the same with slight blue shift, most probably due to quenching effects of magnetic nanoparticles (MNPs). Equally the quenching effect of the QDs on the magnetic nanomaterials was also investigated. The toxicity of the as prepared nanoparticles was investigated and results will be reported during the conference.

Speaker: Mr Kiplagat Ayabei (University of the western cape)

15:00 → 15:20 Tea

15:20 → 16:30 Presentations: Session 9

15:20 Structure and Spectroscopic Properties of M(Qn)3 complexes

Metal quinolinates are known as key materials in the design of organic light emission diodes (OLEDs). The aluminium complex, tris-(8-hydroxyquinoline) aluminium(III), is the most well-known molecule of this family, although its gallium analogue has also been considered for use in OLEDs [1,2]. Factors that govern the efficacy these metal complexes for potential use in OLEDs include the type of isomer (mer versus fac), inter-molecular interactions (like π-π stacking) and solvento species “trapped” in the crystal lattice. Furthermore it was shown that the emission spectra of these complexes can be red or blue shifted by using derivatives of 8-hydroxyquinoline on the 5 and 7 positions of the ligand backbone with electron withdrawing or donating properties (as in 5,7dichloro-8-hydroxyquinoline (5,7-Cl2Qn) or 5,7 dimethyl-8-hydroxyquinoline (5,7-Me2Qn)) [3-5]. In this work, various complexes of M(Qn)3 (M = Al(III), Ga(III), In(III) and Eu(III); Qn = 8-hydroxyquinoline / 5,7dichloro-8-hydroxyquinoline / 5,7 dimethyl-8-hydroxyquinoline) have been synthesized and characterized with 1H NMR, single crystal X-ray spectroscopy and other means and their photoluminescence properties evaluated [6, 7]. A systematic evaluation of the structural versus luminescence properties of nine M(Qn)3 complexes plus interesting results of two Europium(III) metal complexes are reported here. 3. References [1] C.W. Tang and S.A. Vanslyke, Appl. Phys. Lett. 51 (1987) 913-915. [2] P.E. Burrows, L.S. Sapochak, L.S. McCarthy, S.R. Forrest and M.E. Thompson, Appl. Phys. Lett., 64 (1994) 2718. [3] A. Irfan, R. Cui, J. Zhang and L. Hao, Chem. Phys. 364 (2009) 39-45. [4] Y. Qin, I. Kiburu, S. Shah and F. Jakle, Org. Lett. 8 (2006) 5227-5230. [5] Y.W. Shi, M.M. Shi, J.C. Haung, H.Z. Chen, M. Wang, X.D. Liu, Y.G. Ma, H. Xu and B. Yang, Chem. Commun. (2006) 1941-1943. [6] M. Duvenhage, H.C. Swart, O.M. Ntwaeaborwa and H.G. Visser, Optical Materials, 25 (2013) 2366-2371. [7] M. Duvenhage, H.G. Visser, O.M. Ntwaeaborwa and H.C. Swart, Physica B, 439 (2014) 46 - 49.

Speaker: Prof. Hendrik Vissser (University of the Free State)

15:40 XPS investigation of the photon degradation of Znq2 green organic phosphor.

Although tris-(8-hydroxyquinoline) aluminium (Alq3) is used as a green emissive layer in organic light emitting diodes (OLED) it tends to degrade with time leading to a decrease in device performance and efficiency. It has been reported that by substituting Al with Zn to form bis-(8-hydroxyquinoline) zinc (Znq2), the Znq2 shows advantages over the Alq3 in electron transport and higher quantum yields in device performance which would result in lower operating voltages. There is a two fold increase in the photoluminescence intensity of Znq2 compared to that of Alq3. Znq2 powder was irradiated with an UV source for 400 hours and the intensity vs time was monitored. From the N1s and O1s high resolution XPS peaks, it can be seen that the pyridyl ring stayed intact during degradation, but that the phenoxide ring ruptured and formed new bonds like C=O and C-OH.

Speaker: Dr Mart-Mari Duvenhage (University of the Free State)

16:00 AB-INITIO CALCULATION OF THE ELECTRONIC STATES INDUCED BY Nb AND Cr DOPING OF RUTILE AND ANATASE TiO2 .

In this study, plane wave self consistent field calculations have been performed on the electronic structure of rutile and anatase phases of titanium dioxide (TiO2). To this end ab-initio calculations have been performed for a 48-atom rutile supercell and 96-atom anatase supercell using the plane wave code – Quantum Espresso. Density functional theory (DFT) with hybrid functionals have been used to obtain a more accurate description of the electronic properties of titanium dioxide in addition to the standard exchange-correlation (XC) functionals namely; the local density approximation (LDA) and the generalized gradient approximation (GGA). In this study, the screened hybrid functional of Heyd-Scuseria-Erzehoff (HSE06) has been used. HSE06 is known to overcome most of the shortcomings of both the LDA and GGA XC functionals. Both the titanium dioxide structures were fully optimized by minimizing the local total energy and atomic forces, after which band structure and density of states were calculated. Band gap energies 1.89 eV (rutile), 2.28 eV (anatase) and 2.25 eV (rutile), 2.65 eV (anatase) were obtained using GGA-PBE and HSE06 functionals respectively. Experimental value for band gap is 3.0 eV for rutile and 3.2 eV for anatase. In order to obtain the bulk equilibrium properties, the energy volume curves for the two structures were fitted to the Murnaghan equation of state. Substitutional chromium and Niobium at the titanium sites was found to introduced new electronic states within the band gap. These new states are discussed with respect to tuning doped titanium dioxide for the application in photocatalysis.

Speaker: Ms WINFRED MULWA (University of the Free State-Qwaqwa campus, Department of physics, Private Bag x13, Phuthaditjhaba, 9866, SOUTH AFRICA)

19:00 → 21:00 Dinner

Friday, 8 May

07:00 → 09:00 Breakfast

09:00 → 10:00 Departure