Low temperature CVD growth of WSe2 enabled by moisture-assisted defects in the precursor powder
Lucas M. SassiAravind KrishnamoorthyJordan A. HachtelSandhya SusarlaAmey ApteSamuel Castro‐PardoAlec AjnsztajnRóbert VajtaiPriya VashishtaChandra Sekhar TiwaryAnand B. PuthirathPulickel M. Ajayan
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Abstract Two-dimensional transition metal dichalcogenides (TMDs) have been proposed for a wide variety of applications, such as neuromorphic computing, flexible field effect transistors, photonics, and solar cells, among others. However, for most of these applications to be feasible, it is necessary to integrate these materials with the current existing silicon technology. Although chemical vapor deposition is a promising method for the growth of high-quality and large-area TMD crystals, the high temperatures necessary for the growth make this technique incompatible with the processes used in the semiconductor industry. Herein, we demonstrate the possibility of low-temperature growth of TMDs, using tungsten selenide (WSe 2 ) as a model, by simply using moisture-assisted defective tungsten oxide (WO 3 ) precursor powders during the growth of these materials. Density functional theory calculations reveal the mechanism by which moisture promotes the defect formation on the precursor crystal structure and how it dictates the reduction of the temperature of the growth. The results were compared with the standard growth at high temperatures and with a precursor mixture with alkali salts to show the high quality of the WSe 2 grown at temperatures as low as 550 °C. To conclude, the work improves the understanding of nucleation and growth mechanisms of WSe 2 at low temperatures and provides a useful strategy for the growth of TMDs at temperatures required for the back-end-of-line compatibility with current silicon technology.Crystal (programming language)
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Nucleation of barite (BaSO4) has broad implications in geological, environmental, and materials sciences. While impurity metals are common, our understanding of how they impact nucleation remains dim. Here, we used classical optical microscopy compared to fast X-ray nanotomography (XnT) to investigate heterogeneous nucleation of barite on silica in situ with Sr2+ as an impurity ion. The observed barite nucleation rates were consistent with classical nucleation theory (CNT), where barite crystals displayed a nonuniform size distribution, exhibiting distinct morphologies and incubation periods in Sr-free solutions. While undetectable with optical microscopy, nanotomography revealed that addition of Sr2+ enhances nucleation rates driven by the pre-factor in CNT, likely because both adsorbed Ba2+ and Sr2+ act as precursor sites on which nucleation occurs. Sr2+ simultaneously inhibits growth, however, leading to a homogeneous distribution of smaller crystals. This finding will enable an improved predictive understanding of nucleation in natural and synthetic environments.
Classical nucleation theory
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An experimental investigation to explore the interaction between bubbles forming at adjacent nucleation sites is presented. The results obtained are consistent with the results of Calka’s and Knowles’ experimental investigations and confirm that nucleation site activation/deactivation, whereby a bubble growing at a nucleation site is able to promote/hinder the formation of a bubble at an adjacent nucleation site by depositing/displacing a vapor nucleus in the nucleation cavity, is instrumental in determining how a bubble forming at a nucleation site influences the nucleation of the subsequent bubble at an adjacent nucleation site.
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A micro-structuring of the tungsten plasma-facing surface can strongly reduce near surface thermal stresses induced by ELM heat fluxes. This approach has been confirmed by numerical simulations with the help of ANSYS software. For experimental tests, two 10 × 10 mm2 samples of micro-structured tungsten were manufactured. These consisted of 2000 and 5000 vertically packed tungsten fibres with dimensions of Ø240 µm × 2.4 mm and Ø150 µm × 2.4 mm, respectively. The 1.2 mm bottom parts of the fibres are embedded in a copper matrix. The top parts of the fibres have gaps about of 10 µm so they are not touching each others. The top of all tungsten fibres was electro-polished. A Nd:YAG laser with a pulse duration 1 ms and a pulse repetition frequency of 25 Hz was used to simulate up to 105 ELM-like heat pulses. No damage on either of the micro-structured tungsten samples were observed. Neon plasma erosion rate and fuel retention of the micro-structured tungsten samples were almost identical to bulk tungsten samples.
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The induction times for electrodeposition of individual Ag nanoparticles on Pt nanodisk electrodes in acetonitrile were used to determine the critical nucleus size and activation energy barrier associated with the formation of Ag nuclei. Induction times for the nucleation and growth of a single Ag nanoparticle were determined following the application of a potential step to reduce Ag+ at overpotentials, η, ranging from −130 to −70 mV. Sufficiently small Pt electrodes (5.1 × 10–10–2.6 × 10–11 cm2) were used to ensure that the detection of a single Ag nucleation event occurred during the experimental observation time (150 ms–1000 s). Multiple measurements of Ag nucleation induction times were recorded to determine nucleation rates as a function of η using cumulative probability theory. Both classical nucleation theory (CNT) and the atomistic theory of electrochemical nucleation were employed to analyze experimental nucleation rates, without a requisite knowledge of the nucleus geometry or surface free energy. Using the CNT, the number of atoms comprising the critical size nucleus, Nc, was estimated to be 1–9 atoms for η ranging from −130 to −70 mV, in good agreement with 1–5 atoms obtained using atomistic theory. The experimental nucleation rates were also used to determine the activation energy barriers for nucleation from the CNT, which varied from 3.31 ± 0.05 to 13 ± 1 kT over the same range of η.
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Feasibility and cost data are supplied for the production of tungsten-rhenium alloys from tungsten targets in the N-Reactor. The two types of target elements assumed were: (a) tungsten containing 90 a/o tungsten-186, 9 a/o tungsten-184 and 1 a/o tungsten-183 and 182, and (b) tungsten of natural isotopic composition (28.4 a/o tungsten-186, 30.6 a/o tungsten-184, 14.4 a/o tungsten 183, and 26.4 tungsten-182). It is assumed that the average thermal neutron capture cross section for the tungsten-186 is 32 barns.
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