Fast-Response Single-Nanowire Photodetector Based on ZnO/WS2 Core/Shell Heterostructures

This work was supported by the Latvian National Research Program IMIS2 and ISSP project for Students and Young Researchers Nr. SJZ/2016/6. S.P. is grateful to the ERA.Net RUS Plus WATERSPLIT project no. 237 for the financial support. S.V. is grateful for partial support by the Estonian Science Foundation grant PUT1689.The surface plays an exceptionally important role in nanoscale materials, exerting a strong influence on their properties. Consequently, even a very thin coating can greatly improve the optoelectronic properties of nanostructures by modifying the light absorption and spatial distribution of charge carriers. To use these advantages, 1D/1D heterostructures of ZnO/WS2 core/shell nanowires with a-few-layers-thick WS2 shell were fabricated. These heterostructures were thoroughly characterized by scanning and transmission electron microscopy, X-ray diffraction, and Raman spectroscopy. Then, a single-nanowire photoresistive device was assembled by mechanically positioning ZnO/WS2 core/shell nanowires onto gold electrodes inside a scanning electron microscope. The results show that a few layers of WS2 significantly enhance the photosensitivity in the short wavelength range and drastically (almost 2 orders of magnitude) improve the photoresponse time of pure ZnO nanowires. The fast response time of ZnO/WS2 core/shell nanowire was explained by electrons and holes sinking from ZnO nanowire into WS2 shell, which serves as a charge carrier channel in the ZnO/WS2 heterostructure. First-principles calculations suggest that the interface layer i-WS2, bridging ZnO nanowire surface and WS2 shell, might play a role of energy barrier, preventing the backward diffusion of charge carriers into ZnO nanowire.IMIS; Institute of Solid State Physics, Chinese Academy of Sciences; Eesti Teadusfondi PUT1689; Rural Utilities Service 237; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART

First principles evaluation on photocatalytic suitability of 2H structured and [0001] oriented WS2 nanosheets and nanotubes

This study was supported by the EC ERA.Net RUS Plus Project No. 237 WATERSPLIT. R.E. acknowledges the financial support provided by the Russian Foundation for Basic Research (grant N 17-03-00130a) and High Performance Computer Center of St. Petersburg University for the assistance. The authors are indebted to D. Bocharov, O. Lisovski and E. Spohr for stimulating discussions.Pristine WS2 multilayer nanosheets (NSs), which thickness h NS varies from 1 to 12 monolayers (MLs), as well as single- and multi-walled nanotubes (SW and MW NTs) of different chirality, which diameter d NT exceeds 1.9 nm, display photocatalytic suitability to split H2O molecules. Obviously, such a phenomenon can occur since the band gap of these nanostructures corresponds to the energy range of visible light between the red and violet edges of spectrum (1.55 eV < Δϵgap < 2.65 eV). For all the studied WS2 NSs and NTs, the levels of the top of the valence band and the bottom of the conduction band must be properly aligned relatively to H2O oxidation and reduction potentials separated by 1.23 eV: ϵ VB < ϵO2/H2O < ϵH+/H2 < ϵ CB. The values of Δϵgap decrease with growth of h NS and increase with enlargement of dNT. 1 ML nanosheet can be considered as a limit of infinite SW NT thickness growth (d NT→∞), which band gap increases up to ∼2.65 eV. First principles calculations have been performed using the hybrid DFT-HF method (HSE06 Hamiltonian) adapted for 2H WS2 bulk. The highest solar energy conversion efficiency (15-18%) expected at Δϵgap = 2.0-2.2 eV (yellow-green range) has been found for 2 ML thick (stoichiometric) WS2 (0001) NS as well as WS2 NTs with diameters 2.7-3.2 nm (irrespectively on morphology and chirality indices n of nanotubes). Moreover, unlike discrete variation of hNS magnitudes, tuning of d NT values provides much higher energy resolution.Russian Foundation for Basic Research N 17-03-00130a; European Commission EC 237 WATERSPLIT; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART

Growth and characterization of PbI2-decorated ZnO nanowires for photodetection applications

Financial support provided by Scientific Research Project for Students and Young Researchers Nr. SJZ/2017/1 realized at the Institute of Solid State Physics, University of Latvia is greatly acknowledged. The authors are grateful to Liga Bikse for XRD measurements.In this study, we demonstrated for the first time the growth of ZnO nanowires (NWs) decorated with highly crystalline few-layer PbI2 and fabricated two-terminal single-nanowire photodetector devices to investigate the photoelectric properties of the hybrid nanostructures. We developed a novel two-step growth process for uniform crystalline PbI2 nanosheets via reactive magnetron deposition of a lead oxide film followed by subsequent iodination to PbI2 on a ZnO NW substrate, and we compared as-grown hybrid nanostructures with ones prepared via thermal evaporation method. ZnO–PbI2 NWs were characterized by scanning and transmission electron microscopy, X-ray diffraction analysis and photoluminescence measurements. By fabricating two-terminal single-nanowire photodetectors of the as-grown ZnO–PbI2 nanostructures, we showed that they exhibit reduced dark current and decreased photoresponse time in comparison to pure ZnO NWs and have responsivity up to 0.6 A/W. Ab initio calculations of the electronic structure of both PbI2 nanosheets and ZnO NWs have been performed, and show potential for photoelectrocatalytic hydrogen production. The obtained results show the benefits of combining layered van der Waals materials with semiconducting NWs to create novel nanostructures with enhanced properties for applications in optoelectronics or X-ray detectors.ISSP UL SJZ/2017/1; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART

2d slab models of nanotubes based on tetragonal tio2 structures: Validation over a diameter range

This research was funded by the M-ERA.NET project ?Multiscale computer modelling, synthesis and rational design of photo(electro)catalysts for efficient visible-light-driven seawater splitting? (CatWatSplit). Institute of Solid State Physics, University of Latvia as the Center of Excel-lence has received funding from the European Union?s Horizon 2020 Framework Program H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under Grant Agreement No. 739508, project CAMART2.One-dimensional nanomaterials receive much attention thanks to their advantageous properties compared to simple, bulk materials. A particular application of 1D nanomaterials is pho-tocatalytic hydrogen generation from water. Such materials are studied not only experimentally, but also computationally. The bottleneck in computations is insufficient computational power to access realistic systems, especially with water or another adsorbed species, using computationally expensive methods, such as ab initio MD. Still, such calculations are necessary for an in-depth understanding of many processes, while the available approximations and simplifications are either not precise or system-dependent. Two-dimensional models as an approximation for TiO2 nanotubes with (101) and (001) structures were proposed by our group for the first time in Comput. Condens. Matter journal in 2018. They were developed at the inexpensive DFT theory level. The principle was to adopt lattice constants from an NT with a specific diameter and keep them fixed in the 2D model optimization, with geometry modifications for one of the models. Our previous work was limited to studying one configuration of a nanotube per 2D model. In this article one of the models was chosen and tested for four different configurations of TiO2 nanotubes: (101) (n,0), (101) (0,n), (001) (n,0), and (001) (0,n). All of them are 6-layered and have rectangular unit cells of tetragonal anatase form. Results of the current study show that the proposed 2D model is indeed universally applicable for different nanotube configurations so that it can be useful in facilitating computationally costly calculations of large systems with adsorbates. © 2021 by the authors. Licensee MDPI, Basel, Switzerland. Published under the CC BY 4.0 license.M-ERA.NET; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union's Horizon 2020 Framework Program H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under Grant Agreement No. 739508, project CAMART2

AB Initio Calculations of CU N @Graphene (0001) Nanostructures for Electrocatalytic Applications

Funding from European Union’s Horizon 2020 Research and Innovation Programme project under grant agreement No. 768789 is greatly acknowledged.Substitution of fossil-based chemical processes by the combination of electrochemical reactions driven by sources of renewable energy and parallel use of H 2 O and CO 2 to produce carbon and hydrogen, respectively, can serve as direct synthesis of bulk chemicals and fuels. We plan to design and develop a prototype of electrochemical reactor combining cathodic CO 2 -reduction to ethylene and anodic H 2 O oxidation to hydrogen peroxide. We perform ab initio calculations on the atomistic 2D graphene-based models with attached Cu atoms foreseen for dissociation of CO 2 and H 2 O containing complexes, electronic properties of which are described taking into account elemental electrocatalytical reaction steps. The applicability of the model nanostructures for computer simulation on electrical conductivity of charged Cu n /graphene (0001) surface is also reported.Horizon 2020 Framework Programme 768789; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART

Review of First Principles Simulations of STO/BTO, STO/PTO, and SZO/PZO (001) Heterostructures

We acknowledge the financial support from our funder the Latvian Council of Science. The funding number is Grant No. LZP-2020/1-0345. The Institute of Solid-State Physics, University of Latvia (Latvia), as a center of excellence, has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD01-2016-2017-Teaming Phase 2 under Grant Agreement No. 739508, project CAMART2.In this study, we review our first-principles simulations for STO/BTO, STO/PTO, and SZO/PZO (001) heterostructures. Specifically, we report ab initio B3PW calculations for STO/BTO, STO/PTO, and SZO/PZO (001) interfaces, considering non-stoichiometric heterostructures in the process. Our ab initio B3PW calculations demonstrate that charge redistribution in the (001) interface region only subtly affects electronic structures. However, changes in stoichiometry result in significant shifts in band edges. The computed band gaps for the STO/BTO, STO/PTO, and SZO/PZO (001) interfaces are primarily determined according to whether the topmost layer of the augmented (001) film has an AO or BO2 termination. We predict an increase in the covalency of B-O bonds near the STO/BTO, STO/PTO, and SZO/PZO (001) heterostructures as compared to the BTO, PTO, and PZO bulk materials. --//-- This is an open access article R.I. Eglitis*, D. Bocharov, S. Piskunov, R. Jia; Review of first principles simulations of STO/BTO, STO/PTO, and SZO/PZO (001) heterostructures; Crystals, 2023, 13, 799 (pp. 1-25); DOI: 10.3390/cryst13050799; https://www.mdpi.com/2073-4352/13/5/799 published under the CC BY 4.0 licence.Latvian Council of Science Grant No. LZP-2020/1-0345. The Institute of Solid-State Physics, University of Latvia (Latvia), as a center of excellence, has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD01-2016-2017-Teaming Phase 2 under Grant Agreement No. 739508, project CAMART2

Positron annihilation lifetime spectroscopy insight on free volume conversion of nanostructured MgAl2 O4 ceramics

H.K. and A.I.P. are grateful for the support from the COST Action CA17126. H.K. was also supported by the Ministry of Education and Science of Ukraine (project for young researchers No. 0119U100435). In addition, I.K. and H.K. were also supported by the National Research Foundation of Ukraine via project 2020.02/0217, while the research of A.I.P. was funded by the Latvian research council via the Latvian National Research Program under the topic ?High-Energy Physics and Accelerator Technologies?, Agreement No: VPP-IZM-CERN-2020/1-0002. In addition, the research of A.I.P. has been supported by the Latvian-Ukrainian Grant LV-UA/2021/5. The Institute of Solid State Physics, University of Latvia (Latvia) as the Centre of Excellence has received funding from the European Union?s Horizon 2020 Framework Programme H2020-WIDESPREAD01-2016-2017- Teaming Phase2 under grant agreement No. 739508, project CAMART2.Herein we demonstrate the specifics of using the positron annihilation lifetime spectroscopy (PALS) method for the study of free volume changes in functional ceramic materials. Choosing technological modification of nanostructured MgAl2 O4 spinel as an example, we show that for ceramics with well-developed porosity positron annihilation is revealed through two channels: positron trapping channel and ortho-positronium decay. Positron trapping in free-volume defects is described by the second component of spectra and ortho-positronium decay process by single or multiple components, depending on how well porosity is developed and on the experimental configuration. When using proposed positron annihilation lifetime spectroscopy approaches, three components are the most suitable fit in the case of MgAl2 O4 ceramics. In the analysis of the second component, it is shown that technological modification (increasing sintering temperature) leads to volume shrinking and decreases the number of defect-related voids. This process is also accompanied by the decrease of the size of nanopores (described by the third component), while the overall number of nanopores is not affected. The approach to the analysis of positron annihilation lifetime spectra presented here can be applied to a wide range of functional nanomaterials with pronounced porosity. © 2021 by the authors. Licensee MDPI, Basel, Switzerland. Published under the CC BY 4.0 license.European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD01-2016-2017-Teaming Phase2 739508; Latvian National Research Program VPP-IZM-CERN-2020/1-0002; Latvian Science Council; National Research Foundation of Ukraine 2020.02/0217; European Cooperation in Science and Technology CA17126; National Research Foundation of Korea; Latvijas Universitate; Institute of Solid State Physics, Chinese Academy of Sciences; Ministry of Education and Science of Ukraine 0119U100435; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART2

Atomic and electronic structure of both perfect and nanostructured Ni(111) surfaces: First-principles calculations

article i nfo In this study, we perform first principles simulations on both atomically smooth and nanostructured Ni(111) slabs. The latter contains periodically distributed nickel nanoclusters atop a thin metal film gradually growing from adatoms and serving as a promising catalyst. Applying the generalized gradient approximation within the formalism of the density functional theory we compare the atomic and electronic structures of Ni bulk, as well as both perfect and nanostructured (111) surfaces obtained using two different ab initio approaches: (i) the linear combination of atomic orbitals and (ii) the projector augmented plane waves. The most essential inter-atomic forces between the Ni adatoms upon the substrate have been found to be formed via: (i) attractive pair-wise interactions, (ii) repulsive triple-wise interactions within a triangle and (iii) attractive triple-wise interactions within a line between the nearest adatoms. The attractive interactions surmount the repulsive forces, hence resulting in the formation of stable clusters from Ni adatoms. The magnetic moment and the effective charge (within both Mulliken and Bader approaches) of the outer atoms in Ni nanoparticles increase as compared to those for the smooth Ni(111) surface. The calculated electronic charge redistribution in the Ni nanoclusters features them as possible adsorption centers with increasing catalytic activity, e.g., for further synthesis of carbon nanotubes

N-Graphene Sheet Stacks/Cu Electrocatalyst for CO2 Reduction to Ethylene

The authors would like to express their gratitude for funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 768789 (CO2EXIDE project). Calculations and research were performed in Center of Excellence at Institute of Solid State Physics, the University of Latvia, which is supported by European Union Horizon2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under Grant Agreement No. 739508, project CAMART2. P.L. and J.K. thank ISSP UL research assistant Ingars Lukosevics for the experimental work and the results obtained.Renewable energy resources (wind, solar) are unpredictable, so it is wise to store the electricity they generate in an energy carrier X. Various PtX (power to useful energy-intensive raw material such as hydrogen, synthetic natural gas, fuel) applications have been proposed. At the heart of our work is widely used idea to convert residual CO2 from biogas plant into higher hydrocarbons using electricity from renewables (e.g., sun, wind, hydro). The specific goal is to produce ethylene-highly demanded hydrocarbon in plastics industry. The process itself is realised on electrocatalytic carbon/copper cathode which must be selective to reaction: 2CO2 + 12e− + 12H+→C2H4 + 4H2O. We propose a bottom-up approach to build catalyst from the smallest particles-graphene sheet stacks (GSS) coated with metallic copper nanocrystals. Composite GSS-Cu structure functions as a CO2 and proton absorber, facilitating hydrogenation and carbon–carbon coupling reactions on Cu-nanocluster/GSS for the formation of C2H4. In our design electrocatalytic electrode is made from nitrogen-doped graphene sheet stacks coated with copper nanostructures. The N-GSSitself can be drop-casted or electrophoretically incorporated onto the carbon paper and gas diffusion electrode. Electrochemical deposition method was recognized as successful and most promising to grow Cu nanocrystals on N-GSS incorporated in conducting carbon substrate. Gaseous products from CO2 electro-catalytic reformation on the cathode were investigated by mass-spectrometer but the electrode surface was analysed by SEM/EDS and XRD methods. © 2022 by the authors. --//-- Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).Horizon 2020, grant agreement No 768789 (CO2EXIDE project); Horizon2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under Grant Agreement No. 739508, project CAMART2