H2S Plasma Research Articles

(Invited) Low Temperature Atomic Layer Deposition Processes for Large-Area Synthesis of 2D Transition Metal Dichalcogenides

2D materials have been the focus of intense research in the last decade due to their unique physical properties. This presentation will highlight our recent progress on the large-area synthesis of two-dimensional transition metal chalcogenides for nanoelectronics using advanced atomic layer deposition cycle schemes. First, how we can use advanced cycle schemes to deposit wafer-scale polycrystalline MoS2 thin films at very low temperatures down to 100 °C. We have identified the critical role of hydrogen during the plasma step in controlling the composition and properties of molybdenum sulfide films. By increasing the H2/H2S ratio or adding an extra hydrogen plasma step to our ALD process, we are able to deposit pure polycrystalline MoS2 films at temperatures as low as 100 °C. To the best of our knowledge, this represents the lowest temperature for crystalline MoS2 films prepared by any chemical gas-phase method.[1]The most dominant methods for preparing MoS2 via ALD is to alternately expose a substrate to a metalorganic precursor and a hydrogen sulfide (H2S) or a plasma containing H2S. H2S is a corrosive, toxic, and flammable gas that is heavier than air, which makes it hazardous and expensive to store, install, and transport. Alternative sulfur precursors in the solid or liquid phase would be beneficial in terms of cost and safety and would require the installation of no additional hardware for most ALD reactors. In the second half of this contribution, the widely researched ALD process using bis(tert-butylimido)bis(dimethylamino)molybdenum(IV) ((tBuN)2(NMe2)2Mo) and H2S plasma is compared to a novel ALD process using (tBuN)2(NMe2)2Mo, hydrogen plasma, and di-tert-butyl disulfide (TBDS), which is an inexpensive, liquid-phase sulfur precursor.[1] M. Mattinen et al., Chem. Mater. (2022), 34, 5104

Parametric investigation and RSM optimization of DBD plasma methods (direct & indirect) for H2S conversion in the air

Hydrogen sulfide (H2S) is known as a harmful pollutant for the environment and human health, and its emission control is a high priority. Non-thermal plasma is an effective technology in this field. In this study, for the first time, the performance of direct and indirect H2S plasma conversion methods was compared, optimized, and modeled with the CCD method. H2S was diluted in zero air, and the study investigated the effect of discharge power, relative humidity, total flow rate, initial H2S concentration, and their interactions. ANOVA results showed that the models for H2S conversion efficiency and energy yield were significant and efficient. The direct method achieved a maximum conversion efficiency of 56 % and energy yield of 3.43 g/kWh, while the indirect method produced 68 % conversion efficiency and 1.59 g/kWh energy yield. According to the process optimization results, the direct conversion method is more optimal than the indirect conversion method due to the presence of active species and high-energy electrons in the plasma treatment, and it is a better choice if there are suitable working conditions.

Post O2 Plasma-Induced Fabrication of 1T-WS2/a-WO3 Heterostructure for Superior Hydrogen Evolution Reaction Electrocatalysts Via Hydrogen Spillover

2D transition metal dichalcogenide (TMDC) group has been widely spotlighted as hydrogen evolution reaction (HER) electrocatalysts to replace the conventional Pt electrocatalyst due to their distinctive features of robustness, low cost, and accessibility to structural modification. However, the electrochemical activity of TMDC materials is mostly originated from its exposed edge sites, whereas its basal plane is relatively inert. There have been attempts to increase the relative fraction of active edge sites in the basal plane or employ the phase transition from semiconductive 2H phase to metallic 1T phase. The practical uses have been restricted compared to nobel metal Pt.Plasma engineering is a facile method to functionalize their basal plane and generate vacancies into the lattice as active sites to improve electrocatalytic activity. In our previous works, MoS2 and WS2 has been synthesized to have abundant defects with maximized active edge sites to impart superior HER performance by employing Ar+H2S plasma treatment on tungsten (W) thin metal film with a thickness of 1 nm. However, to surpass the intrinsic electrochemical properites, we introduce 1T-WS2/a-WO3 (WSO) heterostructure via post O2 plasma treatment by exploiting a plasma enhanced-chemical vapor deposition (PE-CVD). There has been a limitation in obtaining the best electrocatalytic performances from the plasma assisted 1T-WS2 films because of its much lower hydrogen affinity of 1T-WS2. On the contrary, tungsten trioxide (WO3) shows strong hydrophilic property in that W atomic site of WO3 can easily absorb H2O. In this regard, 1T-WS2 with hydrophilic amorphous WO3 (a-WO3) can have the outstanding HER activity by inducing hydrogen spillover effect that is a hydrogen transport phenomenon from a-WO3 to 1T-WS2. a-WO3, which has relatively stronger hydrogen affinity, plays an important role in transferring the absorbed hydrogen to 1T-WS2 as active sites. As a result, the HER performance of WSO thin film can be optimized by means of controlling the amount of a-WO3 depending on the post O2 plasma treatment time. It was also confirmed from density functional theory (DFT) calculation that the hydrogen movement on the WSO surface related to hydrogen absorption energy difference between 1T-WS2 and WO3.

Plasma-enhanced atomic layer deposition of crystalline Ga2S3 thin films

Gallium (III) sulfide is a frontrunner for many energy storage and optoelectronic applications, which demand a deposition technique that offers a high level of control over thickness, composition, and conformality. Atomic layer deposition (ALD) is a potential technique in this regard. However, the state-of-the-art ALD processes for depositing Ga2S3 often lead to films that are amorphous and nonstoichiometric, and contain significant contaminations. Herein, we present a new plasma-enhanced atomic layer deposition (PE-ALD) process using the hexakis(dimethylamido)digallium precursor and H2S plasma coreactant to deposit high-quality Ga2S3 sulfide thin films and compare it to the thermal ALD process using the same reactants. While both cases exhibit typical ALD characteristics, substantial disparity is observed in the material properties. The PE-ALD process deposits crystalline Ga2S3 sulfide thin films at a temperature as low as 125 °C with a growth per cycle of 1.71 Å/cycle. Additionally, the PE-ALD process results in smooth and stoichiometric Ga2S3 films without any detectable carbon and oxygen contamination. Grazing incidence wide-angle x-ray scattering analysis indicates that the as-deposited Ga2S3 film crystallizes in a cubic structure with a preferred orientation along the [111] direction. The Ga2S3 film exhibits a transmittance of 70% and a bandgap of 3.2 eV with a direct transition.

Scrutinizing pre- and post-device fabrication properties of atomic layer deposition WS2 thin films

In this work, we investigate the physical and electrical properties of WS2 thin films grown by a plasma-enhanced atomic layer deposition process, both before and after device fabrication. The WS2 films were deposited on thermally oxidized silicon substrates using the W(NMe2)2(NtBu)2 precursor and a H2S plasma at 450 °C. The WS2 films were approximately 8 nm thick, measured from high-resolution cross-sectional transmission electron imaging, and generally exhibited the desired horizontal basal-plane orientation of the WS2 layers to the SiO2 surface. Hall analysis revealed a p-type behavior with a carrier concentration of 1.31 × 1017 cm−3. Temperature-dependent electrical analysis of circular transfer length method test structures, with Ni/Au contacts, yielded the activation energy (Ea) of both the specific contact resistivity and the WS2 resistivity as 100 and 91 meV, respectively. The similarity of these two values indicates that the characteristics of both are dominated by the temperature dependence of the WS2 hole concentration. Change in the material, such as in sheet resistance, due to device fabrication is attributed to the chemicals and thermal treatments associated with resist spinning and baking, ambient and UV exposure, metal deposition, and metal lift off for contact pad formation.

Activation of nitrogen species mixed with Ar and H2S plasma for directly N-doped TMD films synthesis

Among the transition metal dichalcogenides (TMD), tungsten disulfide (WS2) and molybdenum disulfide (MoS2) are promising sulfides for replacing noble metals in the hydrogen evolution reaction (HER) owing to their abundance and good catalytic activity. However, the catalytic activity is derived from the edge sites of WS2 and MoS2, while their basal planes are inert. We propose a novel process for N-doped TMD synthesis for advanced HER using N2 + Ar + H2S plasma. The high ionization energy of Ar gas enabled nitrogen species activation results in efficient N-doping of TMD (named In situ-MoS2 and In situ-WS2). In situ-MoS2 and WS2 were characterized by various techniques (Raman spectroscopy, XPS, HR-TEM, TOF–SIMS, and OES), confirming nanocrystalline and N-doping. The N-doped TMD were used as electrocatalysts for the HER, with overpotentials of 294 mV (In situ-MoS2) and 298 mV (In situ-WS2) at a current density of 10 mA cm−2, which are lower than those of pristine MoS2 and WS2, respectively. Density functional theory (DFT) calculations were conducted for the hydrogen Gibbs energy (∆GH) to investigate the effect of N doping on the HER activity. Mixed gas plasma proposes a facile and novel fabrication process for direct N doping on TMD as a suitable HER electrocatalyst.

Patchwork-Structured Heterointerface of 1T-WS2/a-WO3 with Sustained Hydrogen Spillover as a Highly Efficient Hydrogen Evolution Reaction Electrocatalyst.

Using tungsten disulfide (WS2) as a hydrogen evolution reaction (HER) electrocatalyst brought on several ways to surpass its intrinsic catalytic activity. This study introduces a nanodomain tungsten oxide (WO3) interface to 1T-WS2, opening a new route for facilitating the transfer of a proton to active sites, thereby enhancing the HER performance. After H2S plasma sulfurization on the W layer to realize nanocrystalline 1T-WS2, subsequent O2 plasma treatment led to the formation of amorphous WO3 (a-WO3), resulting in a patchwork-structured heterointerface of 1T-WS2/a-WO3 (WSO). Addition of a hydrophilic interface (WO3) facilitates the hydrogen spillover effect, which represents the transfer of absorbed protons from a-WO3 to 1T-WS2. Moreover, the faster response of the cathodic current peak (proton insertion) in cyclic voltammetry is confirmed by the higher degree of oxidation. The rationale behind the faster proton insertion is that the introduced a-WO3 works as a proton channel. As a result, WSO-1.2 (the ratio of 1T-WS2 to a-WO3) exhibits a remarkable HER activity in that 1T-WS2 consumes more protons provided by the channel, showing an overpotential of 212 mV at 10 mA/cm2. Density functional theory calculations also show that the WO3 phase gives higher binding energies for initial proton adsorption, while the 1T-WS2 phase shows reduced HER overpotential. This improved catalytic performance demonstrates a novel strategy for water splitting to actively elicit the related reaction via efficient proton transport.

The first progress of plasma-based transition metal dichalcogenide synthesis: a stable 1T phase and promising applications.

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted attention as polymorphs depending on their phases (1T and 2H) when applying typical synthesis methods. The 2H phase is generally synthesised through chemical vapour deposition (CVD) on a wafer-scale at high temperatures, and many synthesis methods have been reported owing to their thermodynamic stability and semiconductor properties. By contrast, although the 1T phase is meta-stable with an octahedral coordination, thereby limiting the use of synthesis methods, the recent structural advantage in terms of the hydrogen evolution reaction (HER) has been emphasised. Despite this demand, no large-area thin-film synthesis method for 1T-TMDs has been developed. Among several strategies of synthesizing metallic-phase (1T) TMDs, chemical exfoliation (alkali metal intercalation) is a major strategy and others have been used for electron-beam irradiation, laser irradiation, defects, plasma hot electron transfer, and mechanical strain. Therefore, we suggest an innovative synthesis method using plasma-enhanced CVD (PECVD) for both the 1T and 2H phases of TMDs (MoS2 and WS2). Because ions and radicals are accelerated to the substrate within the sheath region, a high-temperature source is not needed for vapour ionisation, and thus the process temperature can be significantly lowered (150 °C). Moreover, a 4-inch wafer-scale of a thin film is an advantage and can be synthesised on arbitrary substrates (SiO2/Si wafer, glassy carbon electrode, Teflon, and polyimide). Furthermore, the PECVD method was applied to TMD-graphene heterostructure films with a graphene-transferred substrate, and for the first time, sequential metal seed layer depositions of W (1 nm) and Mo (1 nm) were sulfurized to MoS2-WS2 vertical heterostructures with Ar + H2S plasma. We considered the prospects and challenges of the new PECVD method in the development of practical applications in next-generation integrated electronics, HER catalysts, and flexible biosensors.

Plasma Hydrogen Sulfide Is Positively Associated With Post-operative Survival in Patients Undergoing Surgical Revascularization.

Objective: Hydrogen sulfide (H2S) is a gaseous signaling molecule and redox factor important for cardiovascular function. Deficiencies in its production or bioavailability are implicated in atherosclerotic disease. However, it is unknown if circulating H2S levels differ between vasculopaths and healthy individuals, and if so, whether H2S measurements can be used to predict surgical outcomes. Here, we examined: (1) Plasma H2S levels in patients undergoing vascular surgery and compared these to healthy controls, and (2) the association between H2S levels and mortality in a cohort of patients undergoing surgical revascularization.Methods: One hundred and fifteen patients undergoing carotid endarterectomy, open lower extremity revascularization or lower leg amputation were enrolled at a single institution. Peripheral blood was also collected from a matched control cohort of 20 patients without peripheral or coronary artery disease. Plasma H2S production capacity and sulfide concentration were measured using the lead acetate and monobromobimane methods, respectively.Results: Plasma H2S production capacity and plasma sulfide concentrations were reduced in patients with PAD (p < 0.001, p = 0.013, respectively). Patients that underwent surgical revascularization were divided into high vs. low H2S production capacity groups by median split. Patients in the low H2S production group had increased probability of mortality (p = 0.003). This association was robust to correction for potentially confounding variables using Cox proportional hazard models.Conclusion: Circulating H2S levels were lower in patients with atherosclerotic disease. Patients undergoing surgical revascularization with lower H2S production capacity, but not sulfide concentrations, had increased probability of mortality within 36 months post-surgery. This work provides insight on the role H2S plays as a diagnostic and potential therapeutic for cardiovascular disease.

In Situ Growth and Activation of Ag/Ag2S Nanowire Clusters by H2S Plasma Treatment for Promoted Electrocatalytic CO2 Reduction

AbstractNonmetallic activation of metal electrodes is effective and universal to improve the catalytic activity for the CO2 reduction reaction. Here, selective electrocatalytic reduction of CO2 is performed on a plasma‐activated Ag/Ag2S catalyst for the first time. By a facile H2S plasma treatment on Ag foil and subsequent electrochemical prereduction, Ag/Ag2S nanowire clusters are obtained in situ with different sizes. High Faradaic efficiency for CO generation on Ag/Ag2S is shifted to a significantly lower overpotential compared with untreated Ag foil with high current density. Density functional theory studies reveal that the promoted catalytic activity can be ascribed to the enhanced CO2 activation and reduced reaction energy barrier of the limiting step.

GYY4137 attenuates functional impairment of corpus cavernosum and reduces fibrosis in rats with STZ-induced diabetes by inhibiting the TGF-β1/Smad/CTGF pathway

Erectile dysfunction (ED) is a common diabetic complication. Recent evidence has illuminated the role of hydrogen sulfide (H2S) as a dynamic mediator of the erection process. H2S is a potent endogenous relaxant gas. It has been shown to relax human and animal penile tissue in vitro and induce erection in animals in vivo. The reported penile expression of H2S-synthesizing enzymes also supports the potential role of the endogenous L-cysteine/H2S pathway in penile homeostasis. Several pathological changes take place in the diabetic penile tissue, including inflammation, oxidative stress, neuropathy and fibrosis of the corpus cavernosum (CC), the major erectile structure of the penis. The present study is experimental and has been performed in the diabetic rat model. The study will investigate the role of H2S as a potential protective mediator against diabetes-induced structural and functional alterations in the CC by examining if it: (1) reduces corporal contraction and/or enhances corporal relaxation following pharmacological stimulation, (2) attenuates fibromuscular changes in diabetic CC, and (3) whether there is a link with H2S plasma/urine level and CC tissue generation, as well as studying the expression of some proteins in the transforming growth factor (TGF)-β1-associated pathway. The major findings of the study reveal that- compared to the nondiabetic controls - the diabetic animals CC showed: (1) augmented contraction and attenuated relaxation in response to phenylephrine and carbachol, respectively, (2) marked fibromuscular degeneration with a significantly lower smooth muscle/collagen ratio and upregulation of TGF-β-1/Smad/CTGF fibrosis signaling pathway, (3) reduced H2S plasma and urinary levels and cavernosal tissue generation. Chronic GYY4137 treatment prevented most of these pathological changes in diabetic CC, thus may be considered a potential new strategy for the prevention and/or treatment of diabetes-induced ED.

Growth and Electrical Characteristics of PE-ALD Germanium Sulfide for 3D Cross-Point Memory

The 3D cross-point memory, known to contain chalcogenide material-based ovonic threshold switching (OTS) selector, is bringing new changes to the memory hierarchy of DRAM and NAND flash memory. The cross-point memory of current planar 3D structures, meanwhile, is already expected to limit scaling in terms of number of critical masks and normalized cost, so it is necessary to prepare a vertical 3D structures [1]. Atomic layer deposition (ALD) process, which is essential to realize vertical 3D structures, has been mostly studied for phase change materials (PCM) based on GeSbTe but OTS selector research is still in its early stage. So, we have studied plasma enhanced-ALD (PE-ALD) of germanium sulfide (GeS) films containing sulfur chalcogenide element with higher optical bandgap than Te and Se for low off current OTS selector for larger array implementation [2] [3]. The PE-ALD of GeS amorphous chalcogenide films was synthesized using commercially available GeCl4 precursor with H2S plasma reactant. The Ge1S2 film was identified quantitative composition and impurities through AES depth profile analysis, and excellent thermal stability up to 600 ℃ was studied by In-situ high temperature X-ray diffraction (HT-XRD). And PE-ALD Ge1S2 has confirmed the superior step coverage in the vertical 3D structure through FIB-TEM compared to chemical vapor deposited GeS film. In addition, the devices of the 50 nm bottom electrode contact size using PE-ALD Ge1S2 films demonstrated OTS characteristic with of remarkable high threshold field 3.0 MV/cm, and low normalized off-current of 25 nA at half threshold voltage. The achievement of PE-ALD research on simple binary Ge1S2 amorphous chalcogenide will contribute to development the future 3D cross-point memory scaling.AcknowledgmentsThis paper was result of the research project supported by SK Hynix Inc.Reference[1] T. Kim and S. Lee, “Evolution of Phase-Change Memory for the Storage-Class Memory and Beyond,” IEEE Trans. Electron Devices, vol. 1, pp. 1–13, 2020.[2] C. C. Wu, C. H. Ho, J. Y. Wu, S. L. Lin, and Y. S. Huang, “Characterization of Ge(Se1-xSx)2 series layered crystals grown by vertical Bridgman method,” J. Cryst. Growth, vol. 281, no. 2–4, pp. 377–383, Aug. 2005.[3] J. W. Park et al., “Optical properties of pseudobinary GeTe, Ge2 Sb2 Te5, GeSb2 Te4, GeSb4 Te7, and Sb2 Te3 from ellipsometry and density functional theory,” Phys. Rev. B - Condens. Matter Mater. Phys., vol. 80, no. 11, Sep. 2009.

Low Temperature Growth of Wafer-Scale 2D MoS2 Thin Films By Pulsed Plasma-Enhanced Chemical Vapor Deposition

Layered two-dimensional molybdenum sulfide (MoS2) has attracted great interest for a promising candidate material for opto-electronic, sensing and catalysis applications due to its outstanding mechanical, electrical and optical properties. In order to apply MoS2 to the industrial field, significant efforts have been placed in obtaining a wafer-scale uniform MoS2. There have been many progressions in various methods such as thermal-CVD [1] and ALD [2], but these methods have crucial limitation that they demand either high growth temperature or post-thermal treatment. In the present study, to overcome the limitation of traditional method, pulsed plasma-enhanced chemical vapor deposition was adopted for the growth of MoS2 thin films. By using H2S plasma reactant, the growth temperature could be significantly lower than in the previous way. The pulsing of Mo precursor injection properly regulates the Mo precursor partial-pressure ratio that is crucial for the synthesis of MoS2. Through these strategies, mono to few layers of MoS2 thin films was deposited uniformly on SiO2/Si substrates. X-ray photoelectron spectroscopy and transmission electron microscopy demonstrated the composition and crystallinity of MoS2 thin films depends on the plasma conditions. Furthermore, through optimizing the plasma conditions, the potential of MoS2 for electric device component was investigated.Fig1. (a) Raman spectra of MoS2 thin films (b) XPS spectra of Mo 3d(left), S 2p(right) obtained from MoS2 thin films deposited by pulsed PECVD.

Area-Selective Atomic Layer Deposition of Two-Dimensional WS2 Nanolayers

With downscaling of device dimensions, two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) such as WS2 are being considered as promising materials for future applications in nanoelectronics. However, at these nanoscale regimes, incorporating TMD layers in the device architecture with precise control of critical features is challenging using current top-down processing techniques. In this contribution, we pioneer the combination of two key avenues in atomic-scale processing: area-selective atomic layer deposition (AS-ALD) and growth of 2D materials, and demonstrate bottom-up processing of 2D WS2 nanolayers. Area-selective deposition of WS2 nanolayers is enabled using an ABC-type plasma-enhanced ALD process involving acetylacetone (Hacac) as inhibitor (A), bis(tert-butylimido)-bis(dimethylamido)-tungsten as precursor (B), and H2S plasma as the co-reactant (C) at a low deposition temperature of 250 °C. The developed AS-ALD process results in the immediate growth of WS2 on SiO2 while effectively blocking growth on Al2O3 as confirmed by in situ spectroscopic ellipsometry and ex situ X-ray photoelectron spectroscopy measurements. As a proof of concept, the AS-ALD process is demonstrated on patterned Al2O3/SiO2 surfaces. The AS-ALD WS2 films exhibited sharp Raman (E2g1 and A1g) peaks on SiO2, a fingerprint of crystalline WS2 layers, upon annealing at temperatures within the thermal budget of semiconductor back-end-of-line processing (≤450 °C). Our AS-ALD process also allows selective growth on various TMDs and transition metal oxides while blocking growth on HfO2 and TiO2. It is expected that this work will lay the foundation for area-selective ALD of other 2D materials.

Low-Temperature Phase-Controlled Synthesis of Titanium Di- and Tri-sulfide by Atomic Layer Deposition.

Phase-controlled synthesis of two-dimensional (2D) transition-metal chalcogenides (TMCs) at low temperatures with a precise thickness control has to date been rarely reported. Here, we report on a process for the phase-controlled synthesis of TiS2 (metallic) and TiS3 (semiconducting) nanolayers by atomic layer deposition (ALD) with precise thickness control. The phase control has been obtained by carefully tuning the deposition temperature and coreactant composition during ALD. In all cases, characteristic self-limiting ALD growth behavior with a growth per cycle (GPC) of ∼0.16 nm per cycle was observed. TiS2 was prepared at 100 °C using H2S gas as coreactant and was also observed using H2S plasma as a coreactant at growth temperatures between 150 and 200 °C. TiS3 was synthesized only at 100 °C using H2S plasma as the coreactant. The S2 species in the H2S plasma, as observed by optical emission spectroscopy, has been speculated to lead to the formation of the TiS3 phase at low temperatures. The control between the synthesis of TiS2 and TiS3 was elucidated by Raman spectroscopy, X-ray photoelectron spectroscopy, high-resolution electron microscopy, and Rutherford backscattering study. Electrical transport measurements showed the low resistive nature of ALD grown 2D-TiS2 (1T-phase). Postdeposition annealing of the TiS3 layers at 400 °C in a sulfur-rich atmosphere improved the crystallinity of the film and yielded photoluminescence at ∼0.9 eV, indicating the semiconducting (direct band gap) nature of TiS3. The current study opens up a new ALD-based synthesis route for controlled, scalable growth of transition-metal di- and tri-chalcogenides at low temperatures.

Thickness-dependent electrochemical response of plasma enhanced atomic layer deposited WS2 anodes in Na-ion battery

In spite of the promising future of the Sodium (Na)-ion batteries (NIBs), it still suffers with low specific capacity and stability mostly owing to the slow kinetics associated with Na ion. In this regard, transition metal sulfides (TMSs) are considered to be one of the best anode materials that can efficiently store Na ions not only owing to their graphite-like layered structure but also through conversion reactions facilitated by their multi-oxidation states. However, the poor cyclic stability of these TMSs attributed to the low electronic conductivity of the TMSs hinders the practical use. Therefore, understanding on an optimized mass loading (or physical dimensions) and configuration of such active electrode material are essential to improve the kinetics associated with Na-ion and electron pathway. To study this, plasma-enhanced atomic layer deposition (PEALD) is employed to grow WS2 using tungsten hexacarbonyl [W(CO)6] and H2S plasma as a precursor and reactant, respectively. The thin films of WS2 deposited directly on stainless steel coin with varying ALD cycles (200–600) are then tested as anode in NIBs without any binder or conducting carbon. Two-stage growth mode is observed with increasing number of ALD cycles which leads to WS2 nano-flakes formation on top of a two-dimensional film of the same. Reversible conversion and intercalation reactions from the cyclic voltammetry measurements are evident for the electrochemical stability of these pristine-WS2 films. While the highest areal capacity of ∼58.8 μAh/cm2 at a current density of 50 μA/cm2, after 50 charge-discharge cycles, is achieved with 400 ALD cycles, the highest capacity retention (∼72.5%) is observed for the film deposited with minimum (200) ALD cycles under same conditions. However, the capacity as well as its retention degrades drastically when the number of ALD cycles is further increased beyond 400. In this study, we address a critical issue associated with WS2 as an anode material for NIBs which should be similarly true for other TMSs as well.

Edge-Site Nanoengineering of WS2 by Low-Temperature Plasma-Enhanced Atomic Layer Deposition for Electrocatalytic Hydrogen Evolution.

Edge-enriched transition metal dichalcogenides, such as WS2, are promising electrocatalysts for sustainable production of H2 through the electrochemical hydrogen evolution reaction (HER). The reliable and controlled growth of such edge-enriched electrocatalysts at low temperatures has, however, remained elusive. In this work, we demonstrate how plasma-enhanced atomic layer deposition (PEALD) can be used as a new approach to nanoengineer and enhance the HER performance of WS2 by maximizing the density of reactive edge sites at a low temperature of 300 °C. By altering the plasma gas composition from H2S to H2 + H2S during PEALD, we could precisely control the morphology and composition and, consequently, the edge-site density as well as chemistry in our WS2 films. The precise control over edge-site density was verified by evaluating the number of exposed edge sites using electrochemical copper underpotential depositions. Subsequently, we demonstrate the HER performance of the edge-enriched WS2 electrocatalyst, and a clear correlation among plasma conditions, edge-site density, and the HER performance is obtained. Additionally, using density functional theory calculations we provide insights and explain how the addition of H2 to the H2S plasma impacts the PEALD growth behavior and, consequently, the material properties, when compared to only H2S plasma.

Using H2S plasma to modify activated carbon for elemental mercury removal

Non-thermal plasma was applied to modify activated carbons (ACs) in the presence of H2S for improving the elemental mercury (Hg0) removal performance. The physical and chemical properties of raw and modified ACs were characterized by scanning electron microscopy with energy disperse X-ray spectroscopy, Brunauer-Emmett-Teller and X-ray photoelectron spectroscopy. The Hg0 removal performance of modified AC was tested by a bench-scale fixed bed. The results indicated that H2S plasma treatment enhanced S content of raw AC. Moreover, S content and Hg0 removal efficiency of modified AC increased with the increase of H2S concentration. The reason was that higher concentration H2S resulted in more S active sites on modified AC and S active site played a significant role during the Hg0 removal process. Besides, non-thermal plasma treatment in the presence of H2S had a weak impact on BET surface area. By further analysis, it could be inferred that the improvement of Hg0 removal performance was ascribed to the chemisorption rather than physisorption. Combined the results of temperature programmed desorption and XPS analysis, the mechanism of Hg0 removal was that elemental S on the surface of modified AC reacted with Hg0 to form HgS.

Enhanced mercury removal by transplanting sulfur-containing functional groups to biochar through plasma

Six biomasses (rice, tobacco, corn, wheat, millet and black bean straws) were used to prepare biochar. The mercury removal performance of the obtained biochars after H2S plasma modification was tested, and characterisation was by FTIR (including in-situ FTIR), XPS and TPD. The mercury removal efficiency of biochar was enhanced after H2S plasma modification. The initial efficiency of wheat straw biochar was raised from 26.4% to 95.5% after modification. The number of sulfur-containing and carboxyl functional groups on the surface of the biochar increased through plasma modification. C–S and carboxyl are considered as the main functional groups forming HgS and HgO in mercury removal. The biochar after modification exhibited better mercury removal capacity at low temperatures. Active S is released from the C–S bonds at high temperature and further oxidises Hg0 in the gas phase. The adsorption process of modified samples is controlled by physical and chemical adsorption at 30 °C, while the main part of the reaction at 150 °C takes place in the gas phase.

Low-temperature wafer-scale growth of MoS2-graphene heterostructures

In this study, we successfully demonstrate the fabrication of a MoS2-graphene heterostructure (MGH) on a 4 inch wafer at 300 °C by depositing a thin Mo film seed layer on graphene followed by sulfurization using H2S plasma. By utilizing Raman spectroscopy and high-resolution transmission electron microscopy, we have confirmed that 5–6 MoS2 layers with a large density of sulfur vacancies are grown uniformly on the entire substrate. The chemical composition of MoS2 on graphene was evaluated by X-ray photoelectron spectroscopy, which confirmed the atomic ratio of Mo to S to be 1:1.78, which is much lower than the stoichiometric value of 2 from standard MoS2. To exploit the properties of the nanocrystalline and defective MGH film obtained in our process, we have utilized it as a catalyst for hydrodesulfurization and as an electrocatalyst for the hydrogen evolution reaction. Compared to MoS2 grown on an amorphous SiO2 substrate, the MGH has smaller onset potential and Tafel slope, indicating its enhanced catalytic performance. Our practical growth approach can be applied to other two-dimensional crystals, which are potentially used in a wide range of applications such as electronic devices and catalysis.