Transient birefringence and dichroism in ZnO studied with fs-time-resolved spectroscopic ellipsometry. - In: Physical review research, ISSN 2643-1564, Bd. 3 (2021), 1, S. 013246-1-013246-12
The full transient dielectric-function (DF) tensor of ZnO after UV-laser excitation in the spectral range 1.4-3.6 eV is obtained by measuring an m-plane-oriented ZnO thin film with femtosecond (fs)-time-resolved spectroscopic ellipsometry. From the merits of the method, we can distinguish between changes in the real and the imaginary part of the DF as well as changes in birefringence and dichroism, respectively. We find pump-induced switching from positive to negative birefringence in almost the entire measured spectral range for about 1 ps. Simultaneously, weak dichroism in the spectral range below 3.0 eV hints at contributions of inter-valence-band transitions. Line-shape analysis of the DF above the band gap based on discrete exciton, exciton-continuum, and exciton-phonon-complex contributions shows a maximal dynamic increase in the transient exciton energy by 80 meV. The absorption coefficient below the band gap reveals an exponential line shape attributed to Urbach-rule absorption mediated by exciton-longitudinal-optic-phonon interaction. The transient DF is supported by first-principles calculations for 1020cm^-3 excited electron-hole pairs in ideal bulk ZnO.
Husimi functions for coupled optical resonators. - In: Journal of the Optical Society of America, ISSN 1520-8532, Bd. 38 (2021), 4, S. 573-578
Phase-space analysis has been widely used in the past for the study of optical resonant systems. While it is usually employed to analyze the far-field behavior of resonant systems, we focus here on its applicability to coupling problems. By looking at the phase-space description of both the resonant mode and the exciting source, it is possible to understand the coupling mechanisms as well as to gain insights and approximate the coupling behavior with reduced computational effort. In this work, we develop the framework for this idea and apply it to a system of an asymmetric dielectric resonator coupled to a waveguide.
InP and AlInP(001)(2 × 4) surface oxidation from density functional theory. - In: ACS omega, ISSN 2470-1343, Bd. 6 (2021), 9, S. 6297-6304
The atomic structure and electronic properties of the InP and Al0.5In0.5P(001) surfaces at the initial stages of oxidation are investigated via density functional theory. Thereby, we focus on the mixed-dimer (2 × 4) surfaces stable for cation-rich preparation conditions. For InP, the top In-P dimer is the most favored adsorption site, while it is the second-layer Al-Al dimer for AlInP. The energetically favored adsorption sites yield group III-O bond-related states in the energy region of the bulk band gap, which may act as recombination centers. Consistently, the In p state density around the conduction edge is found to be reduced upon oxidation.
Atomic force extrema induced by the bending of a CO-functionalized probe. - In: Nano letters, ISSN 1530-6992, Bd. 21 (2021), 5, S. 2318-2323
The control and observation of reactants forming a chemical bond at the single-molecule level is a long-standing challenge in quantum physics and chemistry. Using a single CO molecule adsorbed at the apex of an atomic force microscope tip together with a Cu(111) surface, bending of the molecular probe is induced by torques due to van der Waals attraction and Pauli repulsion. As a result, the vertical force between CO and Cu(111) exhibits a characteristic dip-hump evolution with the molecule-surface separation, which depends sensitively on the initial tilt angle the CO axis encloses with the surface normal. The experimental force data are reproduced by model calculations that consider the CO deflection in a harmonic potential and the molecular orientation in the Pauli repulsion term of the Lennard-Jones potential. The presented findings shed new light on vertical-force extrema that can occur in scanning probe experiments with functionalized tips.
Biomass-derived highly dispersed Co/Co9S8 nanoparticles encapsulated in S, N-co-doped hierarchically porous carbon as an efficient catalyst for hybrid Na-CO2 batteries. - In: Materials today, ISSN 2468-6069, Bd. 19 (2021), 100594, S. 1-12
Im Titel sind "9", "8" und "2" tiefgestellt
Na-CO2 batteries are prospective in energy storage and CO2 recycling applications; development of a high-efficiency, low-cost electrocatalyst to promote CO2 reduction and carbonate decomposition is extremely vital for practical Na-CO2 batteries. Herein, a highly efficient cathode catalyst for rechargeable hybrid Na-CO2 batteries is successfully synthesized by encapsulating highly dispersed Co/Co9S8 nanoparticles into carbon skeletons, consisting of biomass-derived S, N-co-doped hierarchically porous carbon (Co/Co9S8SNHC). The conductive and hierarchically porous framework structure of the Co/Co9S8@SNHC can not only accelerate electron transport, electrolyte infiltration, and CO2 diffusion but also can inhibit overgrowth and agglomeration of Co/Co9S8 nanoparticles and expose numerous high density of active sites, as well as offer sufficient space to store discharge products. Benefiting from the synergistic effect among S and N dopants, carbon defects, and Co/Co9S8 nanoparticles in robust porous carbon structure, the hybrid Na-CO2 batteries displayed a low charge overpotential (only ˜0.32 V) at 0.2 mA/cm2 and repeatedly charged and discharged over 200 cycles at 0.1 mA/cm2. Besides, an ultrahigh areal capacity of ˜18.9 mAh/cm2 was obtained at 0.5 mA/cm2, the highest value to date for Na-CO2 batteries. Meanwhile, the hybrid Na-CO2 battery charging from Na2CO3@C catalytic cathode demonstrated the high catalytic activity of biomass-derived S,N-co-doped hierarchically porous carbon (Co/Co9S8@SNHC) for CO2 reduction and carbonate decomposition. Given this finding, this work might open up a potential avenue for the reasonable design of low-cost and highly efficient catalysts for advanced metal-CO2 batteries systems.
Efficient and inexpensive MPCVD method to synthesize Co3O4/MoS2 heterogeneous composite materials with high stability for supercapacitors. - In: Journal of materials research and technology, ISSN 2214-0697, Bd. 10 (2021), S. 953-959
Im Titel sind "2", "3" und "4" tiefgestellt
Large-sized metal oxide particles have the potential to constitute cheap, high-performance, and high-stability supercapacitor electrode materials. Herein, the marketable large-sized Co3O4 particles (˜6 [my]m) as the starting raw material, inexpensive Co3O4/MoS2 core-shell heterogeneous composites have been one-step fabricated via an improvised MPCVD system modified by a domestic microwave oven. After that, the surface morphology, composition structure, and valence state of elements were analyzed to the confirmed successful synthesis of MoS2 on the surface of Co3O4. Besides, the performance was tested by cyclic voltammetry and galvanostatic charge-discharge method. The results show that the synergistic effect of Co3O4 core and MoS2 shell can effectively improve the material's electrochemical performance. The specific capacitance of Co3O4/MoS2 composite can reach 337 F g-1 with a current density of 0.5 A g-1, which is six times more than the raw Co3O4 powder. Furthermore, it could maintain 93.6% of the initial specific capacitance after 2000 charges and discharges. Finally, the mechanism of material performance improvement is proposed.
Template-assisted fabrication of Ag-nanoparticlesZnO-nanorods array as recyclable 3D surface enhanced Raman scattering substrate for rapid detection of trace pesticides. - In: Nanotechnology, ISSN 1361-6528, Bd. 32 (2021), 14, 145302, insges. 9 S.
We present a template-assisted fabrication method for a large-scale ordered arrays of ZnO nanorods (ZnO-NRs) modified with Ag nanoparticles (Ag-NPs), which possess high-density three-dimensional (3D) hot spots uniformly dispersed all over the substrate, being beneficial to ultrahigh sensitivity of surface enhanced Raman scattering (SERS) detection. These achieved Ag-NPsZnO-NRs arrays show high sensitivity, good spectral uniformity and reproducibility as substrates for SERS detection. Using the arrays, both dye molecules (rhodamine 6G, R6G) and organic pollutants like toxic pesticides (thiram and methyl parathion) are detected, with the detection limits of thiram and methyl parathion being 0.79 x 10-9 M and 1.51 x 10-8 M, respectively. In addition, the Ag-NPs@ZnO-NRs arrays have a self-cleaning function because the analyte molecules can be photocatalytic degraded using ultraviolet irradiation, showing that the 3D recyclable arrays have promising opportunities to be applied in rapid SERS-based detection of toxic organic pesticides.
Band alignment at GaxIn1-xP/AlyIn1-yP alloy interfaces from hybrid density functional theory calculations. - In: Physica status solidi, ISSN 1521-3951, Volume 258 (2021), issue 2, 2000463, Seite 1-4
Im Titel sind "x", "x-1", "y" und "y-1" tiefgestellt
The composition dependence of the natural band alignment at the GaxIn1-xP/AlyIn1-yP alloy interface is investigated via hybrid functional based density functional theory. The direct-indirect crossover for the GaxIn1-xP and AlyIn1-yP alloys is calculated to occur for x = 0.9 and y = 0.43. The calculated GaxIn1-xP/AlyIn1-yP interface band alignment shows a crossover from type-I to type-II with increasing Ga content x. The valence band offset is essentially positive irrespective of the alloy compositions, and amounts up to 0.56 eV. The conduction band offset varies between 0.85 and 1.16 eV.
Field-cycling NMR and DNP - a friendship with benefits. - In: Journal of magnetic resonance, ISSN 1096-0856, Bd. 322 (2021), 106851, insges. 16 S.
Field-cycling relaxometry, or rather its electronic version with a resistive magnet which requires signal detection at a field strength of 1 Tesla or below, remains an inherently insensitive technique due to the construction compromise that goes along with the need for a fast-switching, low-inductance magnet. For the same reasons, signal lifetime is short and frequency resolution is typically not given, at least for the predominantly used hydrogen nuclei. Dynamic Nuclear Polarization (DNP) bears the potential to circumvent these disadvantages: not only has it been demonstrated to enhance magnetization by up to three orders of magnitude beyond its thermal value, but it also provides the possibility to address particular parts of a molecule, thus generating selectivity even in the absence of spectral resolution. At the same time, DNP requires the introduction of stable radicals giving rise to additional relaxation contributions. This article presents a straightforward way to recover the native relaxation rates of the undisturbed system, and shows examples in different research fields where field-cycling relaxometry is traditionally used for refining models of molecular dynamics and interactions.
Layer-by-layer polyelectrolyte assembly for the protection of GaP surfaces from photocorrosion. - In: ACS applied nano materials, ISSN 2574-0970, Bd. 4 (2021), 1, S. 425-431
Polyelectrolyte layer-by-layer assemblies are known as protective coatings for corrosion inhibition. Here, we demonstrate that polyelectrolyte multilayers of poly(ethyleneimine) (PEI) and poly(styrene sulfonate) (PSS)-(PEI/PSS)x-adsorbed at the GaP(100) photocathode surface remarkably mitigate the photocorrosion of GaP without decreasing its photoconversion efficiency. The activity of the polybase-polyacid complex is based on buffering pH changes at the solid-liquid interface. We carried out ab initio molecular dynamics-based simulations of the GaP(100) surface in contact with liquid water and demonstrated that an increase in the proton concentration enhances GaP dissolution. We used the scanning vibrating electrode technique (SVET) to characterize the distribution of photocorrosion activity areas over bare and polyelectrolyte-coated GaP surfaces and we showed that a polyelectrolyte coating impedes the dissolution kinetics. Data obtained using the SVET were compared to photoetched pores on the semiconductor surface. Voltammetric and chronoamperometric measurements were also performed to evaluate photoconversion efficiencies before and after the application of the protective coatings.