A. Hospodková

Wavelength Effect on Photocatalytic Reduction of CO2 by Ag/TiO2 Catalyst

Photocatalytic reduction of CO2 by water was performed in the presence of a Ag/TiO2 catalyst under illumination by lamps with different wavelengths (254, 365, and 400 nm). The yields of the main products (methane and methanol) were higher with the 254 nm lamp than with the 365 lamp while no products were observed with the 400 nm lamp. This was because the electron-hole generation rate increased with increasing energy of irradiation (decreasing wavelength) and there were higher densities of electron states at higher energies in TiO2. The increased efficiency of electron-hole generation with a shorter wavelength irradiation increased the efficiency of the catalyst. The energy of the electrons excited by visible light (400 nm) was too low for CO2 photocatalytic reduction.

Transmission study of picosecond photocarrier dynamics in free-standing porous silicon

Abstract Pump and probe technique has been used to study time-resolved differential absorbance of free-standing porous silicon at room temperature. The second (532 nm) harmonics of a mode-locked Nd 3+ :YAG picosecond laser served as pump pulses and the fundamental frequency (1.06 μm) were used as probe pulses. Photocarrier dynamics shows a fast decay in time domain of hundred picoseconds. A model of bimolecular radiative recombination is suggested, the values of bimolecular radiative recombination coefficient B and excited carrier cross section σ have been found to be B = 4 × 10 −10 cm 3 s −1 and σ = 9 × 10 −18 cm 2 , very close to the values reported for direct gap semiconductors.

Optical characterisation of MOVPE grown vertically correlated InAs/GaAs quantum dots

Structures with vertically correlated self-organised InAs quantum dots (QDs) in a GaAs matrix were grown by the low-pressure metal-organic vapour phase epitaxy (MOVPE) and characterised by different microscopic techniques. Photoluminescence in combination with photomodulated reflectance spectroscopy were applied for characterisation of QDs structures. We show that combination of both methods allows detecting optical transitions originating both from QDs and wetting (separation) layers, which can be than compared with those obtained from numerical simulations. On the basis of obtained results, we demonstrate that photoreflectance spectroscopy is an excellent tool for characterisation of QDs structures wetting layers and for identification of spacer thicknesses in vertically stacked QDs structures.

Type I–type II band alignment of a GaAsSb/InAs/GaAs quantum dot heterostructure influenced by dot size and strain-reducing layer composition

The aim of this work is to red shift quantum dot (QD) photoluminescence (PL) towards telecommunication wavelengths by engineering the metalorganic vapour phase epitaxy (MOVPE) prepared structure of InAs/GaAs QDs covered by a GaAsSb strain-reducing layer. Our results proved that type I or type II band alignment can be controlled by both GaAsSb composition and QD size. Maintaining type I heterostructure is important for high luminescence efficiency and emission wavelength stability of the QD structure. The simulation of electron structure in InAs QDs covered with a GaAsSb strain-reducing layer as well as experimental results suggest the importance of increasing QD size for obtaining a longer wavelength PL from the type I heterostructure. The PL maximum wavelength 1371 nm was achieved for the MOVPE prepared type I QD structure with 14% of Sb in GaAsSb. This type of structure exhibits seven times higher PL intensity, twice narrower PL peak and 85 meV redshift in comparison with similarly prepared QDs covered by GaAs.

InGaN/GaN multiple quantum well for fast scintillation application: radioluminescence and photoluminescence study.

We prepare InGaN/GaN multiple quantum well (MQW) structure by metal-organic vapour phase epitaxy and characterize it by fine XRD measurements. We demonstrate its suitability for scintillator application including a unique measurement of wavelength-resolved scintillation response under nanosecond pulse soft x-ray source in extended dynamical and time scales. The photoluminescence and radioluminescence were measured: we have shown that the ratio of the intensity of quantum well (QW) exciton luminescence to the intensity of the yellow luminescence (YL) band IQW/IYL depends strongly on the type and intensity of excitation. Slower scintillation decay measured at YL band maximum confirmed the presence of several radiative recombination centres responsible for wide YL band, which also partially overlap with the QW peak. Further improvements of the structure are suggested, but even the presently reported decay characteristics of the excitonic emission in MQW are better compared to the currently widely used single crystal YAP:Ce or YAG:Ce scintillators. Thus, such a type of a semiconductor scintillator is highly promising for fast detection of soft x-ray and related beam diagnostics.

Optical non-linearity and hysteresis in porous silicon

Abstract We present results of experimental investigations of picosecond (decay of ∼ 200 ps) optical non-linearities and optical hysteresis in porous silicon. We consider the application of porous silicon (PS) for all-optical switching. We discuss the physical origin of the non-linearities observed and we propose a theoretical model describing well the optical absorption non-linearity and hysteresis in PS.

Picosecond dynamics of photoexcited carriers in free-standing porous silicon

We study the ultrafast dynamics of photoexcited carriers in luminescent free-standing porous silicon at room temperature using the experimental techniques of picosecond absorption and luminescence spectroscopy. Both the luminescence intensity and transient absorption signals show a fast decay on the scale of hundreds of picoseconds, followed by a slower nanosecond decay. We identify the faster component of the decay as being due to a bimolecular recombination process in the core of Si nanocrystallites with a quasi-direct gap energy structure (bimolecular recombination coefficent of 10 -10 cm 3 s -1 ), while the slower component is likely to be due to recombination via surface states

Elongation of InAs∕GaAs quantum dots from magnetophotoluminescence measurements

The authors have used magnetophotoluminescence for the determination of the lateral anisotropy of buried quantum dots. While the calculated shifts of the energies of higher radiative transitions in magnetic field are found to be sensitive to the lateral elongation, the shift of the lowest transition is determined mainly by the exciton effective mass. This behavior can be used for a fairly reliable determination both the effective mass and the elongation from spectra containing at least two resolved bands.

Combined vertically correlated InAs and GaAsSb quantum dots separated by triangular GaAsSb barrier

The aim of this work is to offer new possibilities for quantum dot (QD) band structure engineering, which can be used for the design of QD structures for optoelectronic and single photon applications. Two types of QDs, InAs and GaAsSb, are combined in self assembled vertically correlated QD structures. The first QD layer is formed by InAs QDs and the second by vertically correlated GaAsSb QDs. Combined QD layers are separated by a triangular GaAsSb barrier. The structure can be prepared as type-I, with both electrons and holes confined in InAs QDs, exhibiting a strong photoluminescence, or type-II, with electrons confined in InAs QDs and holes in GaAsSb QDs. The presence of the thin triangular GaAsSb barrier enables the realization of different quantum level alignment between correlated InAs and GaAsSb QDs, which can be adjusted by structure parameters as type-I or type-II like for ground and excited states separately. The position of holes in this type of structure is influenced by the presence of the tr...

Temperature behaviour of luminescence of free-standing porous silicon

Abstract The temperature dependence of the luminescence of free-standing samples of porous silicon was studied. Luminescence spectra are composed of three bands at 680 ± 20nm, 740 ± 20nm and 820 ± 20nm, their exact position and intensity differ in spectra taken from the top and bottom sides of the sample. The intensity of all components exhibits different temperature dependence which is responsible for the observed anomalous temperature behaviour of the luminescence spectra of PS. The intensity and the position of the bands are modified by an additional treatment and we suggest that it is caused by the change of the surface states, which substantially influence the luminescence properties of porous silicon.