3,068 research outputs found
Excitation properties of the divacancy in 4H-SiC
We investigate the quenching of the photoluminescence (PL) from the divacancy
defect in 4H-SiC consisting of a nearest-neighbour silicon and carbon
vacancies. The quenching occurs only when the PL is excited below certain
photon energies (thresholds), which differ for the four different inequivalent
divacancy configurations in 4H-SiC. Refined theoretical ab initio calculation
for the charge-transfer levels of the divacancy show very good agreement
between the position of the (0/-) level with respect to the conduction band for
each divacancy configurations and the corresponding experimentally observed
threshold, allowing us to associate the PL decay with conversion of the
divacancy from neutral to negative charge state due to capture of electrons
photoionized from other defects (traps) by the excitation. Electron
paramagnetic resonance measurements are conducted in dark and under excitation
similar to that used in the PL experiments and shed light on the possible
origin of traps in the different samples. A simple model built on this concept
agrees well with the experimentally-observed decay curves.Comment: 28 pages, 6 figure
Synthesis of Improved Catalytic Materials for High-Temperature Water-gas Shift Reaction
In this investigation, we report the preparation and characterization of Co-, Cu- and Mn-substituted iron oxide catalytic materials supported on activated carbon. Co-precipitation method and low temperature treatment were used for their synthesis. The influence of chemical composition, stoichiometry, particle size and dispersity on their catalytic activity was studied. Samples were characterized in all stages of their co-precipitation, heating and spend samples after catalytic tests. The obtained results from room and low temperature Mössbauer spectroscopy were combined with analysis of powder X-ray diffraction patterns (XRD). They revealed the preparation of nano-sized iron oxide materials supported on activated carbon. Relaxation phenomena were registered also for the supported phases. The catalytic performance in the water-gas shift reaction was studied. The activity order was as follows: Cu0.5Fe2.5O4 > Co0.5Fe2.5O4 > Mn0.5Fe2.5O4. Catalytic tests demonstrated very promising results and potential application of studied samples due to their cost-effective composition
Changes in the structure and geography of tourist flows during the COVID-19 pandemic
Tourism suffered more than many other sectors amid Covid-19 travel restrictions. This article looks at the impact of the pandemic on the territorial redistribution of tourist flows in the Baltic region. The study draws on regional annual and monthly data on international and domestic tourist arrivals between 2019 and 2021, available on the websites of the statistical offices of the Baltic Region countries. The regions earlier dependent on tourist arrivals from Russia experienced the largest drop in inbound tourism during the Covid-19 pandemic. Yet, domestic tourism burgeoned in most of the region’s countries. In 2021, the most dramatic increase in domestic arrivals occurred, year on year, in the border regions of Russia’s Northwest Federal District; the growth was also notable in Finland and the Baltic States. Domestic tourist flow compensated, partially or even fully, for the decrease in inbound tourist flow in most regions. The structure of inbound tourism changed markedly during the pandemic, with the share of tourists from the Baltic States growing. The strongest rise was in some regions of southern Finland and more modest in Latvia and Lithuania
Selection rules in the excitation of the divacancy and the nitrogen-vacancy pair in 4H- and 6H-SiC
In this study, we address the selection rules with respect to the
polarization of the optical excitation of two colour centres in 4H-SiC and
6H-SiC with potential for applications in quantum technology, the divacancy and
the nitrogen-vacancy pair. We show that the photoluminescence (PL) of the axial
configurations of higher symmetry (C3v) than the basal ones (C1h) can be
cancelled using any excitation (resonant or non-resonant) with polarization
parallel to the crystal axis (EL||c). The polarization selection rules are
determined using group-theoretical analysis and simple physical arguments
showing that phonon-assisted absorption with EL||c is prohibited despite being
formally allowed by group theory. A comparison with the selection rules for the
silicon vacancy, another defect with C3v symmetry, is also carried out. Using
the selection rules, we demonstrate selective excitation of only one basal
divacancy configuration in 4H-SiC, the P3 line and discuss the higher contrast
and increased Debye-Waller factor in the selectively excited spectrum
Clustering and Morphology Evolution of Gold on Nanostructured Surfaces of Silicon Carbide: Implications for Catalysis and Sensing
A fundamental understanding of the behavior of gold (Au) nanostructures deposited on functional surfaces is imperative to discover and leverage interface-related phenomena that can boost the efficiency of existing electronic devices in sensorics, catalysis, and spintronics. In the present work, Au layers with nominal thickness of 2 nm were sputter-deposited on graphenized SiC substrates represented by buffer layer (BuL)/4H-SiC and monolayer epitaxial graphene (MLG)/4H-SiC. Morphometric analysis by means of scanning electron microscopy shows that Au on BuL self-assembles in nearly round-shaped plasmonically active islands, while on MLG, a fractal growth of considerably larger and ramified islands is observed. To correlate the experimentally established differences in surface morphology on the two types of graphenized substrates with energetics and kinetics of Au nanostructure growth, the deposit-substrate interaction strength was studied using density functional theory (DFT) calculations, molecular dynamics simulations, and optical measurements. The theoretical considerations involve participation of Au clusters with different sizes and energetics at the initial stages of the metal nanostructure formation. The results indicate that gold exhibits a considerably stronger interaction with BuL than with MLG, which can be considered as a key aspect for explaining the experimentally observed morphological differences. From the statistical analysis of Raman spectra, indications of Au intercalation of MLG are discussed. The current research shows that, due to its unique surface chemistry, buffer layer has peculiar affinity to gold when compared to other atomically flat surfaces, which is beneficial for boosting high-performance catalytic and sensing technologies based on low-dimensional materials
Determination of reducing carbohydrates in natural honey samples by optical micrometry method
The present article deals with the determination of the total amount of glucose and fructose in natural honey samples by the optical micrometry (OM) method. (Poly)vinyl alcohol spherical granules impregnated with a 0.05 mol/l borax solution were chosen as a sensitive element. It was shown that the formation of chelate esters of boron with polymer and carbohydrates is a pH-dependent process, and that the pH range 8.5–10.0 is the most appropriate for quantifying the total amount of reducing carbohydrates because glucose and fructose are undiscriminated. The impregnated polymer is not sensitive to the sucrose presence in the solution due to the absence of cis-diol fragments in it. Subsequently, the OM method was tested in the analysis of natural honey samples. The relative standart deviation in the case of OM method is less than 6%, and the results are similar to those obtained by the iodometric titration method. This makes the OM method suitable for laboratory-scale applications
Identification and tunable optical coherent control of transition-metal spins in silicon carbide
Color centers in wide-bandgap semiconductors are attractive systems for
quantum technologies since they can combine long-coherent electronic spin and
bright optical properties. Several suitable centers have been identified, most
famously the nitrogen-vacancy defect in diamond. However, integration in
communication technology is hindered by the fact that their optical transitions
lie outside telecom wavelength bands. Several transition-metal impurities in
silicon carbide do emit at and near telecom wavelengths, but knowledge about
their spin and optical properties is incomplete. We present all-optical
identification and coherent control of molybdenum-impurity spins in silicon
carbide with transitions at near-infrared wavelengths. Our results identify
spin for both the electronic ground and excited state, with highly
anisotropic spin properties that we apply for implementing optical control of
ground-state spin coherence. Our results show optical lifetimes of 60 ns
and inhomogeneous spin dephasing times of 0.3 s, establishing
relevance for quantum spin-photon interfacing.Comment: Updated version with minor correction, full Supplementary Information
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Quantum properties of dichroic silicon vacancies in silicon carbide
The controlled generation and manipulation of atom-like defects in solids has
a wide range of applications in quantum technology. Although various defect
centres have displayed promise as either quantum sensors, single photon
emitters or light-matter interfaces, the search for an ideal defect with
multi-functional ability remains open. In this spirit, we investigate here the
optical and spin properties of the V1 defect centre, one of the silicon vacancy
defects in the 4H polytype of silicon carbide (SiC). The V1 centre in 4H-SiC
features two well-distinguishable sharp optical transitions and a unique S=3/2
electronic spin, which holds promise to implement a robust spin-photon
interface. Here, we investigate the V1 defect at low temperatures using optical
excitation and magnetic resonance techniques. The measurements, which are
performed on ensemble, as well as on single centres, prove that this centre
combines coherent optical emission, with up to 40% of the radiation emitted
into the zero-phonon line (ZPL), a strong optical spin signal and long spin
coherence time. These results single out the V1 defect in SiC as a promising
system for spin-based quantum technologies
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