2016, Vetter, Michael, Schwuchow, Anka, Andrä, Gudrun

The room temperature photoluminescence (PL) spectrum due band-to-band recombination in an only 8 μm thick liquid-phase crystallized silicon on glass solar cell absorber is measured over 3 orders of magnitude with a thin 400 μm thick optical fiber directly coupled to the spectrometer. High PL signal is achieved by the possibility to capture the PL spectrum very near to the silicon surface. The spectra measured within microcrystals of the absorber present the same features as spectra of crystalline silicon wafers without showing defect luminescence indicating the high electronic material quality of the liquid-phase multi-crystalline layer after hydrogen plasma treatment.

2021, Wunderlich, Ralf, Staacke, Robert, Knolle, Wolfgang, Abel, Bernd, Meijer, Jan

Here, we investigate the magnetic field dependent photoluminescence (PL) of a fiber-coupled diamond single crystal with a high density of nitrogen vacancy (NV) centers. Angle-dependent magnetic field sweep measurements between 0 and 111 mT were performed using an oscillating illumination combined with lock-in techniques. Besides the expected superposed PL of differently oriented NV centers, a zoo of features in the PL are found. These features can be associated with level anti-crossings and cross relaxations. In particular, PL measurements allowed us to detect auto-cross relaxation between coupled NV centers. Moreover, the PL measurements at low magnetic fields show dips suggesting an interaction of NV centers with additional spin defects. The results presented here are not only a study for NV-based fiber-coupled sensors made of diamond, but also show a way to investigate with manageable effort and purely an optical multispin interaction with at least one NV center as a constituent.

2012, Möller, Michael, Hernández-Mínguez, Alberto, Breuer, Steffen, Pfüller, Carsten, Brandt, Oliver, de Lima Jr, Mauricio M., Cantarero, Andrés, Geelhaar, Lutz, Riechert, Henning, Santos, Paulo V.

The oscillating piezoelectric field of a surface acoustic wave (SAW) is employed to transport photoexcited electrons and holes in GaAs nanowires deposited on a SAW delay line on a LiNbO3 crystal. The carriers generated in the nanowire by a focused light spot are acoustically transferred to a second location where they recombine. We show that the recombination of the transported carriers occurs in a zinc blende section on top of the predominant wurtzite nanowire. This allows contactless control of the linear polarized emission by SAWs which is governed by the crystal structure. Additional polarization-resolved photoluminescence measurements were performed to investigate spin conservation during transport.

2011, Pfüller, Carsten

This thesis presents a detailed investigation of the optical properties of semiconductor nanowires (NWs) in general and single GaN NWs and GaN NW ensembles in particular by photoluminescence (PL) spectroscopy. NWs are often considered as potential building blocks for future nanometer-scaled devices. This vision is based on several attractive features that are generally ascribed to NWs. In the first part of the thesis, some of these features are examined using semiconductor NWs of different materials. On the basis of the temperature-dependent PL of Au- and self-assisted GaAs/(Al,Ga)As core-shell NWs, the influence of foreign catalyst particles on the optical properties of NWs is investigated. The effect of the substrate choice is studied by comparing the PL of ZnO NWs grown on Si, Sapphire, and ZnO substrates. The major part of this thesis discusses the optical properties of GaN NWs. The investigation of the PL of single GaN NWs and GaN NW ensembles reveals the significance of their large surface-to-volume ratio and that each NW exhibits its own individual recombination behavior. An unexpected broadening of the donor-bound exciton transition is explained by the abundant presence of surface donors in NWs. The existence and statistical relevance of these surface donors is confirmed by PL experiments of single GaN NWs which are either dispersed or free-standing. Furthermore, the influence of electric fields on the optical properties of GaN NWs is investigated and the coupling of light with GaN NWs is studied by reflectance and Raman measurements. The central results of this thesis motivate the introduction of a model that explains the typically observed nonexponential recombination dynamics in NW ensembles. It is based on a distribution of recombination rates. Preliminary simulations using this model describe the nonexponential decay of GaN NW ensembles satisfactorily and allow for an estimation of their internal quantum efficiency.

2023, Yang, Wei, Rosenkranz, Marco, Velkos, Georgios, Ziegs, Frank, Dubrovin, Vasilii, Schiemenz, Sandra, Spree, Lukas, de Souza Barbosa, Matheus Felipe, Guillemard, Charles, Valvidares, Manuel, Büchner, Bernd, Liu, Fupin, Avdoshenko, Stanislav M., Popov, Alexey A.

Nd-based nitride clusterfullerenes NdM2N@C80 with rare-earth metals of different sizes (M = Sc, Y, Lu) were synthesized to elucidate the influence of the cluster composition, shape and internal strain on the structural and magnetic properties. Single crystal X-ray diffraction revealed a very short Nd-N bond length in NdSc2N@C80. For Lu and Y analogs, the further shortening of the Nd-N bond and pyramidalization of the NdM2N cluster are predicted by DFT calculations as a result of the increased cluster size and a strain caused by the limited size of the fullerene cage. The short distance between Nd and nitride ions leads to a very large ligand-field splitting of Nd3+ of 1100-1200 cm−1, while the variation of the NdM2N cluster composition and concomitant internal strain results in the noticeable modulation of the splitting, which could be directly assessed from the well-resolved fine structure in the Nd-based photoluminescence spectra of NdM2N@C80 clusterfullerenes. Photoluminescence measurements also revealed an unprecedentedly strong nephelauxetic effect, pointing to a high degree of covalency. The latter appears detrimental to the magnetic axiality despite the strong ligand field. As a result, the ground magnetic state has considerable transversal components of the pseudospin g-tensor, and the slow magnetic relaxation of NdSc2N@C80 could be observed by AC magnetometry only in the presence of a magnetic field. A combination of the well-resolved magneto-optical states and slow relaxation of magnetization suggests that Nd clusterfullerenes can be useful building blocks for magneto-photonic quantum technologies.

2021, Maleki, Afshin, Seifi, Mehran, Marzban, Nader

The existence of a humic substance in water causes the growth of microorganisms and reduces the quality of water; therefore, the removal of these materials is crucial. Here, the ZnO nanoparticles doped using transition metals, copper (Cu) and manganese (Mn), were used as an effective catalyst for photocatalytic removal of humic substances in an aqueous environment under ultraviolet, visible light, and light-emitting diode irradiations. Also, we study the effect of the sonocatalytic method. A solvothermal procedure is used for doping, and the Cu- and Mn-doped ZnO nanocatalyst were characterized by means of FTIR, XRD, AFM, SEM, and EDAX analyses. We investigate the effect of operational variables, including doping ratio, initial pH, catalyst dose, initial HS content, and illuminance on the removal efficiency of the processes. The findings of the analyses used for the characterization of the nanoparticles illustrate the appropriate synthesis of the Cu- and Mn-doped ZnO nanocatalysts. We observe the highest removal efficiency rate under acidic conditions and the process efficiency decreased with increasing solution pH, when we tested it in the range of 3–7. Photocatalytic decomposition of HS increases with a rise in catalyst dose, but an increase in initial HS content results in decreasing the removal efficiency. We observe the highest photocatalytic degradation of humic acid while using the visible light, and the highest removal efficiency is obtained using Cu.ZnO. The Cu.ZnO also shows better performance under ultraviolet irradiation compared to other agents.

2014, Osminkina, L.A., Sivakov, V.A., Mysov, G.A., Georgobiani, V.A., Natashina, U.А., Talkenberg, F., Solovyev, V.V., Kudryavtsev, A.A., Timoshenko, V.Y.

Evaluation of cytotoxicity, photoluminescence, bio-imaging, and sonosensitizing properties of silicon nanoparticles (SiNPs) prepared by ultrasound grinding of porous silicon nanowires (SiNWs) have been investigated. SiNWs were formed by metal (silver)-assisted wet chemical etching of heavily boron-doped (100)-oriented single crystalline silicon wafers. The prepared SiNWs and aqueous suspensions of SiNPs exhibit efficient room temperature photoluminescence (PL) in the spectral region of 600 to 1,000 nm that is explained by the radiative recombination of excitons confined in small silicon nanocrystals, from which SiNWs and SiNPs consist of. On the one hand, in vitro studies have demonstrated low cytotoxicity of SiNPs and possibilities of their bio-imaging applications. On the other hand, it has been found that SiNPs can act as efficient sensitizers of ultrasound-induced suppression of the viability of Hep-2 cancer cells.

2020, Morrow, Ryan, Sturza, Mihai I., Ray, Rajyavardhan, Himcinschi, Cameliu, Kern, Jonas, Schlender, Philipp, Richter, Manuel, Wurmehl, Sabine, Büchner, Bernd

Single crystal specimens of previously unknown garnet Eu2PbSb2Zn3O12 were grown in a reactive PbO:PbF2 flux medium. The crystals were characterized by a combination of X-ray crystallography, magnetization measurements, and the optical techniques of Raman, photoluminescence, and UV/Vis spectroscopy. The material exhibits Van Vleck paramagnetism associated with the J = 0 state of Eu3+, which was possible to accurately fit to a theoretical model. Band structure calculations were performed and compared to the experimental band gap of 1.98 eV. The crystals demonstrate photoluminescence associated with the 4f 6 configuration of the Eu3+ ions sitting at the distorted 8-coordinate garnet A site. The title compound represents a unique quinary contribution to a relatively unexplored area of rare earth bearing garnet crystal chemistry. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.

2016, Gonchar, Kirill A., Zubairova, Alsu A., Schleusener, Alexander, Osminkina, Liubov A., Sivakov, Vladimir

Silicon nanowires (SiNWs) were fabricated by metal-assisted chemical etching (MACE) where hydrofluoric acid (HF), which is typically used in this method, was changed into ammonium fluoride (NH4F). The structure and optical properties of the obtained SiNWs were investigated in details. The length of the SiNW arrays is about 2 μm for 5 min of etching, and the mean diameter of the SiNWs is between 50 and 200 nm. The formed SiNWs demonstrate a strong decrease of the total reflectance near 5-15 % in the spectral region λ < 1 μm in comparison to crystalline silicon (c-Si) substrate. The interband photoluminescence (PL) and Raman scattering intensities increase strongly for SiNWs in comparison with the corresponding values of the c-Si substrate. These effects can be interpreted as an increase of the excitation intensity of SiNWs due to the strong light scattering and the partial light localization in an inhomogeneous optical medium. Along with the interband PL was also detected the PL of SiNWs in the spectral region of 500-1100 nm with a maximum at 750 nm, which can be explained by the radiative recombination of excitons in small Si nanocrystals at nanowire sidewalls in terms of a quantum confinement model. So SiNWs, which are fabricated by environment-friendly chemistry, have a great potential for use in photovoltaic and photonics applications.

2007, Hafenbrak, R., Ulrich, S.M., Michler, P., Wang, L., Rastelli, A., Schmidt, O.G.

The radiative biexciton-exciton decay in a semiconductor quantum dot (QD) has the potential of being a source of triggered polarization-entangled photon pairs. However, in most cases the anisotropy-induced exciton fine structure splitting destroys this entanglement. Here, we present measurements on improved QD structures, providing both significantly reduced inhomogeneous emission linewidths and near-zero fine structure splittings. A high-resolution detection technique is introduced which allows us to accurately determine the fine structure in the photoluminescence emission and therefore select appropriate QDs for quantum state tomography. We were able to verify the conditions of entangled or classically correlated photon pairs in full consistence with observed fine structure properties. Furthermore, we demonstrate reliable polarization-entanglement for elevated temperatures up to 30 K. The fidelity of the maximally entangled state decreases only a little from 72% at 4 K to 68% at 30 K. This is especially encouraging for future implementations in practical devices. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.