Phase Of Transition Metal Dichalcogenides Research Articles

Regulated and Gradual Polymorph Transition By Thermal and Plasma Treatment on Transition Metal Dichalcogenides

Transition metal dichalcogenides (TMDs) have two different phase structure which exhibit different properties. Generally, it can be classified into 1T (metallic) and 2H (semiconductor) phase structures according to the relative atomic direction of the TMDs. Currently, the utilization of two-dimensional (2D) materials is limited due to the lack of an appropriate phase control method of TMDs. There are several phase control methods reported so far (post-treatment chemical process, lattice deformation of physical crystals due to ion collision, phase control of materials through strain, etc.), but the disadvantages of requiring additional processes and low yield are limited for practical usage of those method. In this research, we conducted a study on gradual phase control of TMDs according to substrate temperature (150, 200, 250, 300, and 350°C) while other plasma parameters were kept constant (power, pressure, processing time, and gas ratio). By controlling the phase, the versatility of TMDs is dramatically improved. 1T-TMDs basal plane plays a crucial role as catalytically active site to enhance electrochemical properties more than 2H-TMDs. 1T-TMDs used as a hydrogen generation catalyst has been proposed as an alternative to noble metals (e.g., Pt, Ir, Ru). This is because 1T-TMDs have a metallic property and thus have dense catalytically active sites as edge and basal plane, which can be used as an electrochemical catalyst. However, the problem of (1T-TMDs) manufactured in the conventional way is that the unstable properties at room temperature of them. During the low-temperature plasma process, ion bombardment onto the substrate induces a grain boundary of several nano sizes to expand the active site per area of the surface crystal structure resulting in high catalytic activity. In addition, the possibility of large-scale production was presented on large-scale wafer synthesis, and the original phase of 1T-TMDs was preserved after the 1,000 HER cycle to prove robustness and durability. On the other hand, in the case of (2H-TMDs), instead of showing low catalytic activity, the existence of band gap makes them favorable for semiconductor device applications such as photodiode, transistor, and neuromorphic computing. In this study, by using the sputtered MoS2 and WS2 to control the ratio of different polymorph, memristive switching (RS) is investigated. Similarly, by ion bombardment onto substrates in a low-temperature plasma process, a few nano-sized particle boundaries were induced, creating a defect on the material surface. An experiment was conducted in which filaments are mimicking artificial neural networks and then ruptured under electrical bias. However, the reproducibility of filament formation for 2H-TMDs was relatively low, and the formed filament was relatively unstable. Therefore, it was intended to form a kind of 'road' to solve this problem. Here, the road was using a 1T-TMDs structure with high catalytic activity. The mixed phase of TMDs consist of a different phase (1T and 2H) structure, which depends on their process temperature. Diversity of process temperature results in various phase ratio into the material structure. Gradual phase change from 1T to 2H at various temperature (150, 200, 250, 300, and 350 °C) confirmed by X-ray photoelectron spectroscopy. By utilizing the mixed phase of TMDs, the metal filaments were formed along the 1T structure within 2H structure, which showing uniform RS behavior. Higher reproducibility was obtained for mixed phase structure than the 2H-TMDs and secured stability. In conclusion, by presenting a research direction on how to control the TMDs’ phases, this study can increase the utilization of the 2D material while adjusting different phases together, rather than single usage of the 2D material phases such as 1T- and 2H-TMDs.Acknowledgement: This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education 2022R1A6A3A13063381 and 2022R1A3B1078163). And this work was also supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No.2022R1A4A1031182).

Phase-Controlled Growth of 1T'-MoS2 Nanoribbons on 1H-MoS2 Nanosheets.

Two-dimensional (2D) heterostructures are emerging as alternatives to conventional semiconductors, such as silicon, germanium, and gallium nitride, for next-generation electronics and optoelectronics owing to their ultrathin thickness, absence of dangling bonds, and strong light-matter interaction. However, the direct growth of 2D heterostructures, especially for those with metastable phases still remains challenging. To obtain 2D transition metal dichalcogenides (TMDs) with designed phases, it is highly desired to develop phase-controlled synthetic strategies. Here, a facile chemical vapor deposition (CVD) method is reported to prepare vertical 1H/1T' MoS2 heterophase structures. By simply changing the growth atmosphere, semi-metallic 1T'-MoS2 can be in-situ grown on the top of semiconducting 1H-MoS2 , forming vertical semiconductor/semi-metal 1H/1T' heterophase structures with a sharp interface. The integrated device based on the 1H/1T' MoS2 heterophase structure displays a typical rectifying behavior with a current rectifying ratio of ∼103 . Moreover, the 1H/1T' MoS2 -based photodetector achieves a responsivity of 1.07 A/W at 532nm with an ultra-low dark current of less than 10-11 A. The aforementioned results indicate that 1H/1T' MoS2 heterophase structures could be a promising candidate for the future rectifiers and photodetectors. Importantly, our approach might pave the way toward tailoring the phases of TMDs, which can help us understand and utilize phase engineering strategies to promote the performance of electronic devices. This article is protected by copyright. All rights reserved.

Phase Engineering of Two‐Dimensional Transition Metal Dichalcogenides

Since the successful isolation of single‐layer graphene with an atomic thickness, various van der Waals (vdW) materials have been intensively studied owing to their unique properties. Among the families of vdW materials, transition metal dichalcogenides (TMDs) have served as representatives because of their diverse band structures and intriguing quantum states, unlike those observed in their bulk counterparts. Particularly, unconventional polymorphic phases of TMDs increase the degrees of freedom in device fabrication and property modulation. As variations in structural phases significantly change the electrical, physical, and chemical properties of materials, phase engineering is essential for the new paradigm of TMD‐based devices. In this review, diverse strategies that can induce and control structural phases in TMDs are explored. After introducing the polymorphic phase changes and the resulting electronic band structures, the various empirical approaches used for manipulating phases in vdW materials, including phase‐selective synthesis and post‐synthesis treatments, are summarized. The group‐VI TMDs are considered as reference, and the analysis is extended to other TMDs across various groups in the periodic table. In addition to providing a comprehensive survey of the recent progress in TMD applications, the challenges for TMD applications and potential opportunities in emerging fields are discussed.

Synchronous Surface-Interface and Crystal-Phase Engineered Multifaceted Hybrid Nanostructure of Fe-(1T)-VSe2 Nanosheet and Fe-CoSe2 Nanorods Doped with P for Rapid HER and OER, Kinetics.

Two different nanostructures of two dissimilar highly-potent active electrocatalysts, P-dopped metallic-(1T)-Fe-VSe2 (P,Fe-1T-VSe2 ) nanosheet and P-dopped Fe-CoSe2 (P,Fe-CoSe2 ) nanorods are hybridized and integrated into a single heterostructure (P,Fe-(VCo)Se2 ) on Ni-foam for high-performance water splitting (WS). The catalytic efficiency of VSe2 nanosheets is first enhanced by enriching metallic (1T)-phase, then forming bimetallic Fe-V selenide, and finally by P-doping. Similarly, the catalytic efficiency of CoSe2 nanorods is boosted by first fabricating Fe-Co bimetallic selenide and then P-doping. To develop super-efficient electrocatalysts for WS, two individual electrocatalysts P,Fe-1T-VSe2 nanosheet and P,Fe-CoSe2 are hybridized and integrated to form a heterostructure (P,Fe-(VCo)Se2 ). Metallic (1T)-phase of transition metal dichalcogenides has much higher conductivity than the 2H-phase, while bimetallization and P-doping activate basal planes, develop various active components, and form heterostructures that develop a synergistic interfacial effect, all of which, significantly boost the catalytic efficacy of the P,Fe-(VCo)Se2 . P,Fe-(VCo)Se2 shows excellent performance requiring very low overpotential (ηHER = 50 mV@10 mAcm-2 and ηOER = 230 mV@20 mAcm-2 ). P,Fe-(VCo)Se2 (+, -) device requires a cell potential of 1.48V to reach 10mA cm-2 for overall WS.

Decatungstate Photocatalyzed Acylation of 2D Transition Metal Dichalcogenides

2D Transition metal dichalcogenides (TMDs) and their unique chemistry have flourished in the recent years owing to their pertinence in a broad series of front-row applications including electronic, opto-epectronic and energy storage /harvest devices for the semiconductors,1, 2 while also catalysis and photo /electrothermal applications for the metallic phases.3 Since their ability to protagonist in such applications is dependent on their chemical structure and processing, a great deal of research effort has been focused on TMDs’ functionalization. The nucleophilic nature of the metallic phase is responsible for the uneven flourish, regarding covalent functionalization, in favor of the metallic phase vs the semiconducting. In parallel, the main inspiration for such chemical transformations on 2D TMDs has been organic chemistry. However, until now there is no report based on a catalytic protocol for the covalent functionalization of TMDs. Following our previous report on the catalytic transformation of single walled carbon nanotubes,4 we herein present the decatungstate-photocatalyzed acylation of 2H-MoS2 and 2H-MoSe2, employing aliphatic and aromatic aldehydes (Figure 1). The successful protocol is accompanied by detailed optimization and spectroscopic characterization. The availability of the various aldehydes as means to realize chemical modifications of choice on the surface of 2H phase of TMDs and the mild reaction conditions protocol, shape an ideal methodology to advance the chemistry of 2D TMDs bringing them one step closer to industry applications.

Giant Reduction of Stabilized Pressure for the Superconducting Re0.8V0.2Se2 Phase.

Access to new superconducting phases in transition-metal dichalcogenides (TMDs) via pressure treatment has been the primary target in this field. As equally essential as the fabrication of new superconducting materials at high pressure, maneuvering new superconducting phases at moderate pressures is also one of the core goals in the synthesis community. Here, we successfully reduced the synthesized pressure of the superconducting phase in ReSe2 by combining V-doping and high-pressure techniques, with a reduction in pressure of 50% in contrast to ReSe2. Our electrical transport measurements displayed that metallization appeared at 10 GPa and subsequently superconductivity appeared at about 52.4 GPa with Tc ∼ 1.9 K. There was a giant reduction in the stable pressure of the superconducting phase derived from the d-electrons and interlayer interaction changes, as evidenced by the Hall effect and X-ray diffraction measurements. These findings serve as ideal starting points and guidance for designing superconducting TMDs at moderate pressures.

Reversible Semimetal-Semiconductor Transition of Unconventional-Phase WS2 Nanosheets.

Phase transition with band gap modulation of materials has gained intensive research attention due to its various applications, including memories, neuromorphic computing, and transistors. As a powerful strategy to tune the crystal phase of transition-metal dichalcogenides (TMDs), the phase transition of TMDs provides opportunities to prepare new phases of TMDs for exploring their phase-dependent property, function, and application. However, the previously reported phase transition of TMDs is mainly irreversible. Here, we report a reversible phase transition in the semimetallic 1T'-WS2 driven by proton intercalation and deintercalation, resulting in a newly discovered semiconducting WS2 with a novel unconventional phase, denoted as the 1T'd phase. Impressively, an on/off ratio of >106 has been achieved during the phase transition of WS2 from the semimetallic 1T' phase to the semiconducting 1T'd phase. Our work not only provides a unique insight into the phase transition of TMDs via proton intercalation but also opens up possibilities to tune their physicochemical properties for various applications.

Metastable Polymorphic Phases in Monolayer TaTe2.

Polymorphic phases and collective phenomena-such as charge density waves (CDWs)-in transition metal dichalcogenides (TMDs) dictate the physical and electronic properties of the material. Most TMDs naturally occur in a single given phase, but the fine-tuning of growth conditions via methods such as molecular beam epitaxy (MBE) allows to unlock otherwise inaccessible polymorphic structures. Exploring and understanding the morphological and electronic properties of new phases of TMDs is an essential step to enable their exploitation in technological applications. Here, scanning tunneling microscopy (STM) is used to map MBE-grown monolayer (ML) TaTe2 . This work reports the first observation of the 1H polymorphic phase, coexisting with the 1T, and demonstrates that their relative coverage can be controlled by adjusting synthesis parameters. Several superperiodic structures, compatible with CDWs, are observed to coexist on the 1T phase. Finally, this work provides theoretical insight on the delicate balance between Te…Te and Ta-Ta interactions that dictates the stability of the different phases. The findings demonstrate that TaTe2 is an ideal platform to investigate competing interactions, and indicate that accurate tuning of growth conditions is key to accessing metastable states in TMDs.

The low symmetry 1T′-MoS2 enabling the lithium directional diffusion through ferroelastic domain switching

The 1T phase of transition metal dichalcogenides (TMDs) is predicted to be an excellent anode material of lithium batteries (LIBs) by many previous theoretical investigations, especially in Mo- and W-TMDs, mainly because of its metallic character. However, the Peierls instability will induce a spontaneous transition from 1T to 1T′ phase, which was ignored by previous studies but should play an important role during the cycling process of LIBs. In this paper, the density functional theory calculations were adopted to systematically studied Li diffusion behavior on the different variants of low symmetry 1T′-MoS2. It was revealed that the diffusion barrier parallel to the Mo-Mo dimerization zigzag chain is almost two times to the perpendicular direction. Besides, the diffusion direction will be turned to ∼120° after Li crosses the domain boundary in a 1T′ multi-domain nanosheet. Similar results could also be observed in Na and K diffusion. Our finding not only revealed the Li (Na or K) diffusion behavior of 1T′-MoS2 but also propose to control the atom diffusion direction through strain engineering, which may inspire the further development of electronic devices.

Electron-Extraction Engineering Induced 1T''-1T' Phase Transition of Re0.75 V0.25 Se2 for Ultrafast Sodium Ion Storage.

Inducing new phases of transition metal dichalcogenides by controlling the d-electron-count has attracted much interest due to their novel structures and physicochemical properties. 1T'' ReSe2 is a promising candidate for sodium storage, but the low electronic conductivity and limited active sites hinder its electrochemical capacity. Herein, new-phase 1T' Re0.75 V0.25 Se2 crystals (P2/m) with zig-zag chains are successfully synthesized. The 1T''-1T' phase transition results from the electronic reorganization of 5d orbitals via electron extraction after V-atom doping. The electrical conductivity of 1T' Re0.75 V0.25 Se2 is 2.7×105 times higher than that of 1T'' ReSe2 . Moreover, density functional theory (DFT) calculations reveal that 1T' Re0.75 V0.25 Se2 has a larger interlayer spacing, lower bonding energy, and migration energy barrier for Na+ ions than 1T'' ReSe2 . As a result, 1T' Re0.75 V0.25 Se2 electrode shows an excellent rate capability of 203mAhg-1 at 50C with no capacity fading over 5000 cycles for sodium storage, which is superior to most reported sodium-ion anode materials. This 1T' Re0.75 V0.25 Se2 provides a new platform for various applications such as electronics, catalysis, and energy storage.

Direct Observation of the Topological Surface State in the Topological Superconductor 2M-WS2.

The quantum spin Hall (QSH) effect has attracted extensive research interest because of the potential applications in spintronics and quantum computing, which is attributable to two conducting edge channels with opposite spin polarization and the quantized electronic conductance of 2e2/h. Recently, 2M-WS2, a new stable phase of transition metal dichalcogenides with a 2M structure showing a layer configuration identical to that of the monolayer 1T' TMDs, was suggested to be a QSH insulator as well as a superconductor with a critical transition temperature of around 8 K. Here, high-resolution angle-resolved photoemission spectroscopy (ARPES) and spin-resolved ARPES are applied to investigate the electronic and spin structure of the topological surface states (TSS) in the superconducting 2M-WS2. The TSS exhibit characteristic spin-momentum-locking behavior, suggesting the existence of long-sought nontrivial topological states therein. We expect that 2M-WS2 with coexisting superconductivity and TSS might host the promising Majorana bound states.

Direct and in situ growth of 1T′ MoS2 and 1T MoSe2 on electrochemically synthesized MXene as an electrocatalyst for hydrogen generation

Crystal phase modulations in two-dimensional (2D) materials have garnered extensive interest because of their intriguing features and potential applications. Nonetheless, synthesizing some specific phases of metastable transition metal dichalcogenides (TMDs) remains a challenge due to the restricted selectivity under chemical and physical treatments, especially for direct growth of specific phase of TMDs on 2D substrates to form heterostructures. Taking the merits of MXene with tunable atomic vacancies and surface chemistry via surface functionalization, the phase of epitaxially grown TMDs on 2D MXene can be significantly affected. In this work, 1 T′-MoS2 and 1 T-MoSe2 were directly grown on electrochemically synthesized MXene under controlled oxidization conditions and selected MXene substrate. Owing to the synergistic effect of TMDs and MXene, the TMDs/MXene heterostructure catalysts exhibit good electrochemical activity under acidic conditions, with a low overpotential of 167 mV at a current density of 10 mA cm−2. Our work not only demonstrates the distinct chemical characteristic of various MXenes feasible for surface engineering which promotes the direct and in situ growth of phase controlled TMDs on MXene substrates but also displays the high electrochemical activity in TMDs/MXene heterostructure catalysts.

Non-centrosymmetric features in nanostructured MoS2 and WS2 exfoliated in liquid phase

Exfoliation in liquid phase of Transition Metal Dichalcogenides promotes the industrialization of low dimensional structures in photonic and material science applications, due to its high nonlinear optical and piezoelectric responses on monolayers and few odd layers. Here, we report piezoelectricity and prospects of non-linear optics through Switching Spectroscopy Piezoresponse Force Microscopy (SS-PFM) and Fluorescence spectroscopy on MoS2 - and WS2 - flakes exfoliated in liquid phase. These features are intrinsically related to non-centrosymmetric structures and depend on the number of layers. SS-PFM signal in a small region reveals that the strength of AFM tip deflection was considerable just for an odd number of layers and negligible for even layers. Thin films were evaluated with ellipsometric measurements by constructing models mixing mono and few layers, resulting in a detailed description of the optical and surface properties of the nanostructures studied.

Symmetry-allowed nonlinear orbital response across the topological phase transition in centrosymmetric materials

Nonlinear optical responses are commonly used as a probe for studying the electronic properties of materials. For topological materials, studies thus far focused on photogalvanic electric currents, which are forbidden in centrosymmetric materials because they require broken inversion symmetry. In this Letter, we propose a class of symmetry-allowed responses for inversion-symmetric topological insulators with two doubly degenerate bands. We consider a specific example of such a response, the orbital current, and show that the sign of the response reflects the ${\mathbb{Z}}_{2}$ topological index, i.e., the orbital current changes sign at the transition between trivial and topological insulator phases. This is illustrated in two models of topological insulators: the Bernevig-Hughes-Zhang model and the $1{T}^{\ensuremath{'}}$ phase of transition metal dichalcogenides.

Floquet band engineering and topological phase transitions in 1T’ transition metal dichalcogenides

Using ab initio tight-binding approaches, we investigate Floquet band engineering of the 1T’ phase of transition metal dichalcogenides (MX2, M = W, Mo and X = Te, Se, S) monolayers under the irradiation with circularly polarized light. Our first principles calculations demonstrate that light can induce important transitions in the topological phases of this emerging materials family. For example, upon irradiation, Te-based MX2 undergoes a phase transition from quantum spin Hall (QSH) semimetal to time-reversal symmetry broken QSH insulator with a nontrivial band gap of up to 92.5 meV. On the other hand, Se- and S-based MX2 undergoes the topological phase transition from the QSH effect to the quantum anomalous Hall effect and into trivial phases with increasing light intensity. From a general perspective, this theoretical work brings further insight into non-equilibrium topological systems.

Intrinsic Valley Polarization in Computationally Discovered Two‐Dimensional Ferrovalley Materials: LaI2 and PrI2 Monolayers

AbstractFerrovalley materials with intrinsic valley polarization present a novel space for materials exploration suitable for valleytronic applications. Here, two Ferrovalley materials, LaI2 and PrI2 monolayers, exhibiting intrinsic valley polarizations of 28 and 35 meV, respectively, mediated by strong magnetic exchange interaction and spin‐orbit coupling are computationally discovered. The unconstrained crystal structure prediction algorithm is used to predict the structures of bulk LaI2 and PrI2, which are layered and analogous to the 2H phase of transition metal dichalcogenides. The monolayers have small exfoliation energy (smaller than MoS2) and exhibit dynamical, mechanical, and thermodynamical stability, advocating their experimental realization. Valley polarization under the effect of bi‐axial strain is preserved and can be used to enhance valley splitting. Finally, the anomalous hall properties of both the monolayers under the effect of in‐plane electric field are discussed. Both the materials exhibit significant anomalous Hall conductivity with proper tuning of the Fermi level.

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.

Unidirectional Alignment of AgCN Microwires on Distorted Transition Metal Dichalcogenide Crystals.

Van der Waals epitaxy on the surface of two-dimensional (2D) layered crystals has gained significant research interest for the assembly of well-ordered nanostructures and fabrication of vertical heterostructures based on 2D crystals. Although van der Waals epitaxial assembly on the hexagonal phase of transition metal dichalcogenides (TMDCs) has been relatively well characterized, a comparable study on the distorted octahedral phase (1T' or Td) of TMDCs is largely lacking. Here, we investigate the assembly behavior of one-dimensional (1D) AgCN microwires on various distorted TMDC crystals, namely 1T'-MoTe2, Td-WTe2, and 1T'-ReS2. The unidirectional alignment of AgCN chains is observed on these crystals, reflecting the symmetry of underlying distorted TMDCs. Polarized Raman spectroscopy and transmission electron microscopy directly confirm that AgCN chains display the remarkable alignment behavior along the distorted chain directions of underlying TMDCs. The observed unidirectional assembly behavior can be attributed to the favorable adsorption configurations of 1D chains along the substrate distortion, which is supported by our theoretical calculations and observation of similar assembly behavior from different cyanide chains. The aligned AgCN microwires can be harnessed as facile markers to identify polymorphs and crystal orientations of TMDCs.

1T Phase Transition Metal Dichalcogenides for Hydrogen Evolution Reaction

Metallic (1T) phases of transition metal dichalcogenides (TMDs) are promising alternatives for Pt as efficient and practically applicable hydrogen evolution reaction (HER) catalysts. Group 6 1T TMDs are the most widely studied due to their impressively higher HER activity than that of their 2H counterparts. However, the mediocre electrochemical and thermal stability of these TMDs has limited their widespread application. Over the last decade, while immense attempts have been made to enhance the stability of group 6 1T TMDs, 1T TMDs based on other transition metals have gained increasing attention. To address the great potential of the 1T TMD family for industry-scale HER and inspire future breakthroughs in realizing their scalable utilization, a critical overview of 1T TMDs for application in HER is presented in this work. With an emphasis on the recent progress, the main contents include the elucidation of the “structure–performance” relationship in 1T TMD-based HER, the approaches for the synthesis and morphology control of 1T TMDs, and the types of 1T TMD-based materials that have been explored for efficient and long-term water splitting. Before the main discussions, the reaction mechanism of HER and the evaluation indexes for HER catalysts are introduced. Moreover, future perspectives on overcoming the primary challenges that hinder the practical application of 1T TMDs for HER are provided.

Growth of large scale PtTe, PtTe2 and PtSe2 films on a wide range of substrates

1T phase of transition metal dichalcogenides (TMDCs) formed by group 10 transition metals (e.g. Pt, Pd) have attracted increasing interests due to their novel properties and potential device applications. Synthesis of large scale thin films with controlled phase is critical especially considering that these materials have relatively strong interlayer interaction and are difficult to exfoliate. Here we report the growth of centimeter-scale PtTe, 1T-PtTe2 and 1T-PtSe2 films via direct deposition of Pt metals followed by tellurization or selenization. We find that by controlling the Te flux, a hitherto-unexplored PtTe phase can also be obtained, which can be further tuned into PtTe2 by high temperature annealing under Te flux. These films with different thickness can be grown on a wide range of substrates, including NaCl which can be further dissolved to obtain free-standing PtTe2 or PtSe2 films. Moreover, a systematic thickness dependent resistivity and Hall conductivity measurements show that distinguished from the semiconducting PtSe2 with hole carriers, PtTe2 and PtTe films are metallic. Our work opens new opportunities for investigating the physical properties and potential applications of group 10 TMDC films and the new monochalcogenide PtTe film.