Towards metal chalcogenide nanowire-based colour-sensitive photodetectors

Financial support provided by Scientific Research Project for Students and Young Researchers Nr. SJZ/2016/6 realized at the Institute of Solid State Physics, University of Latvia is greatly acknowledged. Authors are grateful to Reinis Ignatans for XRD measurements.In recent years, nanowires have been shown to exhibit high photosensitivities, and, therefore are of interest in a variety of optoelectronic applications, for example, colour-sensitive photodetectors. In this study, we fabricated two-terminal PbS, In2S3, CdS and ZnSe single-nanowire photoresistor devices and tested applicability of these materials under the same conditions for colour-sensitive (405 nm, 532 nm and 660 nm) light detection. Nanowires were grown via atmospheric pressure chemical vapour transport method, their structure and morphology were characterized by scanning and transmission electron microscopy (SEM and TEM), X-ray diffraction (XRD), and optical properties were investigated with photoluminescence (PL) measurements. Single-nanowire photoresistors were fabricated via in situ nanomanipulations inside SEM, using focused ion beam (FIB) cutting and electron-beam-assisted platinum welding; their current-voltage characteristics and photoresponse values were measured. Applicability of the tested nanowire materials for colour-sensitive light detection is discussed.ISSP UL Nr. SJZ/2016/6; 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

Synthesis and characterization of ZnO/ZnS/MoS2 core-shell nanowires

The present research was supported by the Latvian National Research Program IMIS2 . Authors are grateful for Dr. Robert Kalendarev and Martins Zubkins for assistance in magnetron sputtering, Dr. Krisjanis Smits for TEM measurements, Dr. Roberts Zabels for AFM measurements and Reinis Ignatans for XRD measurements.Hybrid nanostructures composed of layered materials have recently attracted a lot of attention due to their promising electronic and catalytic properties. In this study, we describe a novel synthesis strategy of ZnO/ZnS/MoS2 core-shell nanowire growth using a three-step route. First, ZnO nanowire array was grown on a silicon wafer. Second, the sample was immersed in ammonium molybdate solution and dried. At the third step, the sample was annealed in a sulfur atmosphere at 700 °C. Two solid state chemical reactions occur simultaneously during the annealing and result in a formation of ZnS and MoS2 phases. Produced ZnO/ZnS/MoS2 core-shell nanowires were characterized by scanning and transmission electron microscopy, whereas their chemical composition was confirmed by selected area electron diffraction and micro-Raman spectroscopy.Latvian National Research Program IMIS2; 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

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

A comparative study of heterostructured CuO/CuWO4 nanowires and thin films

Authors are grateful to Reinis Ignatans for XRD measurements.A comparative study of heterostructured CuO/CuWO4 core/shell nanowires and double-layer thin films was performed through X-ray diffraction, confocal micro-Raman spectroscopy and electron (SEM and TEM) microscopies. The heterostructures were produced using a two-step process, starting from a deposition of amorphous WO3 layer on top of CuO nanowires and thin films by reactive DC magnetron sputtering and followed by annealing at 650 °C in air. The second step induced a solid-state reaction between CuO and WO3 oxides through a thermal diffusion process, revealed by SEM-EDX analysis. Morphology evolution of core/shell nanowires and double-layer thin films upon heating was studied by electron (SEM and TEM) microscopies. A formation of CuWO4 phase was confirmed by X-ray diffraction and confocal micro-Raman spectroscopy.Latvian National Research Program IMIS2; 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

Understanding of conversion process of magnetron deposited thin films of amorphous ReOx to crystalline ReO3 upon thermal annealing

Financial support was provided by ERAF Project Nr. 1.1.1.1/18/A/073. Parts of this research were carried out at PETRA-III P64 beamline at DESY, a member of the Helmholtz Association (HGF). The synchrotron experiments have been supported by the project CALIPSOplus under the Grant Agreement 730872 from the EU Framework Programme for Research and Innovation HORIZON2020.Thin films of rhenium trioxide (ReO3) were produced by reactive DC magnetron sputtering from metallic rhenium target followed by annealing in the air in the range of temperatures from 200C to 350C. Nanocrystalline single-phase ReO3 films were obtained upon annealing at about 250C. The thin films appear bright red in reflected light and blue-green in transmitted light, thus showing an optical transparency window in the spectral range of 475-525 nm. The film exhibits high conductivity, evidenced by macro- and nano-scale conductivity measurements. The long-range and local atomic structures of the films were studied in detail by structural methods as X-ray diffraction and X-ray absorption spectroscopy. The oxidation state (6+) of rhenium was confirmed by X-ray photoemission and X-ray absorption spectroscopies. The nanocrystalline morphology of the annealed films was evidenced by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and atomic force microscopy (AFM). The obtained results allowed us to propose the mechanism of rhenium oxide conversion from the initially amorphous ReOx phase to cubic ReO3.ERAF 1.1.1.1/18/A/073; CALIPSOplus under the Grant Agreement 730872 from the EU Framework Programme for Research and Innovation HORIZON2020; 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

Comparison of the Resistivities of Nanostructured Films Made from Silver, Copper-Silver and Copper Nanoparticle and Nanowire Suspensions

Spray deposition and inkjet printing of various nanostructures are emerging complementary methods for creating conductive coatings on different substrates. In comparison to established deposition techniques like vacuum metal coating and lithography-based metallization processes, spray deposition and inkjet printing benefit from significantly simplified equipment. However, there are number of challenges related to peculiar properties and behaviour of nanostructures that require additional studies. In present work, we investigate electroconductive properties and sintering behaviour of thin films produced from nanostructures of different metals (Ag, Cu and Cu-Ag) and different shapes (nanowires and spherical nanoparticles), and compare them to the reference Ag and Cu magnetron deposited films. Synthesized nanostructures were studied with transmission electron microscopy. Morphology and crystallinity of produced metal films were studied with scanning electron microscopy and X-ray diffraction. The electrical parameters were measured by the van der Pauw method. All nanowires-based films provided high conductivity and required only modest thermal treatment (200 C). To achieve sufficient sintering and conductivity of nanoparticles-based films, higher temperatures are required (300 C for Ag nanoparticles and 350 C for Cu and Cu-Ag nanoparticles). Additionally, stability of nanowires was studied by annealing the samples in vacuum conditions inside a scanning electron microscope at 500 C

Unraveling the Structure and Properties of Layered and Mixed ReO3–WO3 Thin Films Deposited by Reactive DC Magnetron Sputtering

Tungsten trioxide (WO3) is a well-known electrochromic material with a wide band gap, while rhenium trioxide (ReO3) is a “covalent metal” with an electrical conductivity comparable to that of pure metals. Since both WO3 and ReO3 oxides have perovskite-type structures, the formation of their solid solutions (ReO3–WO3 or RexW1–xO3) can be expected, which may be of significant academic and industrial interest. In this study, layered WO3/ReO3, ReO3/WO3, and mixed ReO3–WO3 thin films were produced by reactive DC magnetron sputtering and subsequent annealing in air at 450 °C. The structure and properties of the films were characterized by X-ray diffraction, optical spectroscopy, Hall conductivity measurements, conductive atomic force microscopy, scanning and transmission electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray photoemission spectroscopy. First-principles density functional theory calculations were performed for selected compositions of RexW1–xO3 solid solutions to model their crystallographic structure and electronic properties. The calculations predict metallic conductivity and tetragonal distortion of solid solutions in agreement with the experimental results. In contrast to previously reported methods, our approach allows us to produce the WO3–ReO3 alloy with a high Re content (>50%) at moderate temperatures and without the use of high pressures. --//-- Article published under the CC BY license.The financial support was provided by ERAF Project Nr. 1.1.1.1/18/A/073. 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

Synthesis and properties of nanowire and transition metal dichalcogenide core-shell heterostructures

Elektroniskā versija nesatur pielikumusDisertācija ir veltīta dažādu pārejas metālu halkogenīdu saturošu nanovadu (NW) kodola-apvalka heterostruktūru izstrādei un to raksturošanai. Tika demonstrētas jaunas kristālisku kodola-apvalka NW sintēzes metodes, tika raksturota nanomateriālu struktūra, sastāvs, morfoloģija un fotoelektriskās īpašības. Darbā tika sintezēti ZnO-WS2, ZnO-MoS2, GaN-ReS2, ZnS-ReS2 un ZnO-ReS2 kodola-apvalka, kā arī PbI2-dekorēti ZnO NW. Pasivējot ZnO NW virsmu ar plānu PbI2 vai WS2 pārklājumu, šādi heterostruktūru fotodetektori uzrādīja samazinātu tumsas strāvu un fotoreakcijas laiku salīdzinājumā ar tīriem ZnO NW. Izstrādātās sintēzes metodes neaprobežojas tikai ar pētītajām heterostruktūrām un var tikt piemērotas arī citiem materiāliem. Atslēgvārdi: nanovadi, pārejas metālu halkogenīdi, 2D materiāli, cinka oksīds, fotodetektori, nanotehnoloģijasThis Dissertation is dedicated to the development and characterization of various transition metal dichalcogenide based core-shell nanowire (NW) heterostructures. Novel synthesis methods of highly-crystalline core-shell NWs have been demonstrated, the nanomaterials structure, composition, morphology and photoelectrical properties have been characterized. In this work, ZnO-WS2, ZnO-MoS2, GaN-ReS2, ZnS-ReS2 and ZnO-ReS2 core-shell, as well as PbI2-decorated ZnO NWs were synthesised. Surface of ZnO NWs was passivated using thin PbI2 or WS2 coatings, and photodetectors of such heterostructures exhibit reduced dark current and photoresponse time in comparison to pure ZnO NWs. The developed synthesis methods are not limited to the demonstrated heterostructures and can be applied for other materials. Keywords: nanowire, transition metal dichalcogenide, 2D materials, zinc oxide, photodetector, nanofabricatio

HOPG micropatterning for graphene stamp printing on Si wafer

Darbā apskatītas un pētītas dažādas augstas sakārtotības pirolītiskā grafīta (HOPG) gravēšanas metodes vēlamā virsmas raksta izveidošanai, kā arī izveidotas un attīstītas metodes grafēna lokšņu uznešanai uz dielektriskas pamatnes ar dažādiem spiedogiem. Gravēto HOPG struktūru kvalitātes raksturošanai, uz pamatnes uznesto grafēna lokšņu vizualizācijai un to biezuma noteikšanai tika izmantotas dažādas LU CFI pieejamās zinātniskās iekārtas. Darbs sastāv no literatūras pārskata daļas un no eksperimentālās daļas, kur aprakstītas izmantotās gravēšanas metodes, spiedogu izgatavošana, grafēna mehāniskās kontaktpārnešanas metodes un ietverti sasniegtie rezultāti. Pilnveidojot šādu ātru, vienkāršu un lētu metodi, un spējot kontrolēti uznest vēlamās formas grafēna loksnes nepieciešamajā pozīcijā uz pamatnes, pavērtos vairāk iespēju turpmākam zinātniskam darbam grafēna īpašību pētniecībā un uz grafēna bāzētu elektronisku ierīču prototipu izveidei laboratorijās.The thesis discusses and studies various surface patterning methods of highly oriented pyrolytic graphite (HOPG), as well as different graphene stamp printing methods for graphene transfer on dielectric surface were investigated. Quality of HOPG patterns were characterized with optical profilometer and Raman spectroscopy, graphene sheets on the substrate were visualized and measured with optical microscope, SEM and AFM. The thesis consists of the review of the literature and the experimental part, which includes both experimental methods of patterning HOPG, preparing stamps, graphene transfer, and results obtained. Developing such simple, quick and cheap method for graphene stamp printing on substrate while controlling the desired shape and position, would open up more opportunities for further scientific work in researching graphene properties and creation of graphene-based electronic devices