A. Pawlis

CuO and Co 3 O 4 nanoparticles: synthesis, characterizations, and Raman spectroscopy

Copper oxide and cobalt oxide (CuO, Co3O4) nanocrystals (NCs) have been successfully prepared in a short time using microwave irradiation without any postannealing treatment. Both kinds of nanocrystals (NCs) have been prepared using copper nitrate and cobalt nitrate as the starting materials and distilled water as the solvent. The resulted powders of nanocrystals (NCs) were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and atomic force microscopy (AFM) measurements. The obtained results confirm the presence of the both of oxides nanopowders produced during chemical precipitation using microwave irradiation. A strong emission under UV excitation is obtained from the prepared CuO and Co3O4 nanoparticles. The results show that the nanoparticles have high dispersion and narrow size distribution. The line scans of atomic force microscopy (AFM) images of the nanocrystals (NCs) sprayed on GaAs substrates confirmthe results of both X-ray diffraction and transmission electron microscopy. Furthermore, vibrational studies have been carried out using Raman spectroscopic technique. Specific Raman peaks have been observed in the CuO and Co3O4 nanostructures, and the full width at half maximum (FWHM) of the peaks indicates a small particle size of the nanocrystals.

Mechanism of the Living Lactide Polymerization Mediated by Robust Zinc Guanidine Complexes

Zinc bis(chelate) guanidine complexes promote living lactide polymerization at elevated temperatures. By means of kinetic and spectroscopic analyses the mechanism has been elucidated for these special initiators that make use of neutral N-donor ligands. The neutral guanidine function initiates the polymerization by a nucleophilic ring-opening attack on the lactide molecule. DFT calculations on the first ring-opening step show that the guanidine is able to act as a nucleophile. Three transition states were located for ligand rearrangement, nucleophilic attack, and ring-opening. The second ring-opening step was modeled as a representation for the chain growth because here, the lactate alcoholate opens the second lactide molecule via two transition states (nucleophilic attack and ring-opening). Additionally, the resulting reaction profile proceeds overall exothermically, which is the driving force for the reaction. The experimental and calculated data are in good agreement and the presented mechanism explains why the polymerization proceeds without co-initiators.

Lasing properties of non-polar GaN quantum dots in cubic aluminum nitride microdisk cavities

We demonstrate laser emission from optically pumped non-polar cubic GaN quantum dots embedded in cubic aluminum nitride microdisks. Power dependent micro-photoluminescence studies at low temperature (∼10 K) revealed S-shaped curves of the integral mode intensity. We observed whispering gallery modes with quality factors up to 5000 at the high energy side (4 eV, i.e., ∼310 nm wavelength) in photoluminescence spectra of microdisks with a diameter of 2.5 μm. Furthermore, we have determined the spontaneous emission coupling factors to β = 0.12 and β = 0.42 for resonator modes of different radial orders.

Lasing of donor-bound excitons in ZnSe microdisks

Excitons bound to flourine atoms in ZnSe have the potential for several quantum optical applications. Examples include optically accessible quantum memories for quantum information processing and lasing without inversion. These applications require the bound-exciton transitions to be coupled to cavities with high cooperativity factors, which results in the experimental observation of low-threshold lasing. We report such lasing from fluorine-doped ZnSe quantum wells in 3 and 6 micron microdisk cavities. Photoluminescence and selective photoluminescence spectroscopy confirm that the lasing is due to bound-exciton transitions.

Photon antibunching and magnetospectroscopy of a single fluorine donor in ZnSe

We report on the optical investigation of single electron spins bound to fluorine donor impurities in ZnSe. Measurements of photon antibunching confirm the presence of single, isolated optical emitters, and magneto-optical studies are consistent with the presence of an exciton bound to the spin-impurity complex. The isolation of this single-donor-bound-exciton complex and its potential homogeneity offer promising prospects for a scalable semiconductor qubit with an optical interface.

Low-threshold ZnSe microdisk laser based on fluorine impurity bound-exciton transitions

Impurity states in semiconductors, in which two long-lived ground states can be optically coupled to a single excited state, provide a powerful mechanism for applications including lasing without inversion, electromagnetically induced transparency, and optically addressable quantum memory for quantum information processing. We report low-threshold lasing from fluorine-doped ZnMgSe/ZnSe quantum wells in microdisk cavities. The lasing mechanism was studied by power-dependent photoluminescence spectroscopy. Lasing thresholds lower than 50Wcm^-^2 were observed and the fraction of spontaneous emission contributed to the lasing modes was about @b=0.03-0.1.

Excitonic emission of colloidal nano-crystals embedded in Molecular Beam Epitaxy grown ZnSe

We combine ZnSe layers grown by molecular beam epitaxy with colloidal CdSe core, CdSe/ZnSe and CdSe/ZnS core/shell nano-crystals (NCs) to achieve monolithic NC-semiconductor heterostructures. The NCs are prepared in solution and deposited by spray-coating on ZnSe buffer layers and subsequently overgrown by ZnSe. We find a blue shift of the photoluminescence of core/shell dots when they are overgrown by ZnSe. Rapid thermal annealing is used to improve the interface region between the NCs and the ZnSe matrix. The effect of different annealing temperatures on the optical properties of CdSe core, CdSe/ZnSe and CdSe/ZnS core/shell NCs overgrown with a cap layer of ZnSe is investigated. After annealing at 673 K the photoluminescence of these samples is red-shifted as compared to unprocessed samples. All photoluminescence results are explained by a model calculation with the following assumption about the 3D confining potential of the NCs: (i) the shell of core/shell NCs dissolves during ZnSe overgrowth, (ii) after overgrowth NCs are separated from the ZnSe matrix by an interface barrier, (iii) the height of this barrier is significantly reduced by annealing. For all three types of NCs we find an excellent quantitative agreement between the experimental and calculated NC transition energies. The absence of the barrier after annealing is further demonstrated by low temperature photoluminescence data of annealed samples which show enhanced diffusion of electron-hole pairs from ZnSe into the NCs.

Structural and Optical Investigations of ZnSe Based Semiconductor Microcavities

This paper describes structural and optical investigations of molecular beam epitaxy grown strained ZnSe/(Zn,Cd)Se quantum wells. Strain state, cadmium content, and quantum well layer thickness have a strong influence on the emission energy, which allows a luminescence tuning between 2.25 and 2.50 eV. Furthermore a ZnSe based microcavity structure completed with ZnS/YF 3 distributed Bragg mirrors was processed. These reflectors are an alternative to several epitaxial grown II-VI materials and other reported approaches with respect to oxide layers. A blueshift in the photoluminescence emission energy measured by angle resolved microcavity detuning indicates an influence of the cavity mode on the quantum well mode.

Exciton spectroscopy on single CdSe/ZnSe quantum dot photodiodes

We have investigated the properties of neutral and charged excitons in single CdSe/ZnSe QD photodiodes by @m-photoluminescence spectroscopy. By applying a bias voltage, we have been able to control the number of electrons in a single QD by shifting the energy levels of the QD with respect to the Fermi level in the back contact. Also the quantum-confined Stark effect was observed as a function of the applied electric field.

Micro-Raman imaging and micro-photoluminescence measurements of strain in ZnMgSe/ZnSe microdiscs

Semiconductor microdiscs are promising for applications in photonics and quantum-information processing, such as efficient solid-state-based single-photon emitters. Strain in the multilayer structure of those devices has an important influence on their optical properties. We present measurements of the strain distribution in ZnMgSe/ZnSe microdiscs by means of micro-photoluminescence and micro-Raman imaging. Photoluminescence measurements of microdiscs reveal substantially broadened emission lines with a shift to lower energy at the undercut part of microdiscs, indicating local relaxation in this area. The distribution of the strain in the microdiscs is obtained from an imaging micro-Raman analysis, revealing that the freestanding part of the microdiscs is free of defects.