Two‐dimensional (2D) PtSe2 shows the most prominent layer‐dependent electrical properties among various 2D materials and high catalytic activity for hydrogen evolution reaction (HER), and therefore, it is an ideal material for exploring the structure–activity correlations in 2D systems. Here, starting with the synthesis of single‐crystalline 2D PtSe2 with a controlled number of layers and probing the HER catalytic activity of individual flakes in micro electrochemical cells, we investigated the layer‐dependent HER catalytic activity of 2D PtSe2 from both theoretical and experimental perspectives. We clearly demonstrated how the number of layers affects the number of active sites, the electronic structures, and electrical properties of 2D PtSe2 flakes and thus alters their catalytic performance for HER. Our results also highlight the importance of efficient electron transfer in achieving optimum activity for ultrathin electrocatalysts. Our studies greatly enrich our understanding of the structure–activity correlations for 2D catalysts and provide new insight for the design and synthesis of ultrathin catalysts with high activity.

Among two-dimensional (2D) transition metal dichalcogenides (TMDs), platinum diselenide (PtSe2) stands at a unique place in the sense that it undergoes a phase transition from type-II Dirac semimetal to indirect-gap semiconductor as thickness decreases. Defects in 2D TMDs are ubiquitous and play crucial roles in understanding and tuning electronic, optical, and magnetic properties. Here we investigate intrinsic point defects in ultrathin 1T-PtSe2 layers grown on mica through the chemical vapor transport (CVT) method, using scanning tunneling microscopy and spectroscopy (STM/STS) and first-principles calculations. We observed five types of distinct defects from STM topography images and obtained the local density of states of the defects. By combining the STM results with the first-principles calculations, we identified the types and characteristics of these defects, which are Pt vacancies at the topmost and next monolayers, Se vacancies in the topmost monolayer, and Se antisites at Pt sites within the topmost monolayer. Our study shows that the Se antisite defects are the most abundant with the lowest formation energy in a Se-rich growth condition, in contrast to cases of 2D molybdenum disulfide (MoS2) family. Our findings would provide critical insight into tuning of carrier mobility, charge carrier relaxation, and electron-hole recombination rates by defect engineering or varying growth condition in fewlayer 1T-PtSe2 and other related 2D materials.

Two-dimensional (2D) metallic transition metal dichalcogenides (TMDCs), such as 1T-TiSe, are ideal systems for exploring the fundamentals in condensed matter physics. However, controlled synthesis of these ultrathin materials has not been achieved. Here, we explored the synthesis of charge density wave (CDW)-bearing 2D TiSe with chemical vapor transport (CVT) by extending this bulk crystal growth approach to the surface growth of TiSe by introducing suitable growth substrates and dramatically slowing down the growth rate. Sub-10 nm TiSe flakes were successfully obtained, showing comparable quality to the mechanically exfoliated thin flakes. A CDW state with 2 × 2 superstructure was clearly observed on these ultrathin flakes by scanning tunneling microscopy (STM), and the phase transition temperature of these flakes was investigated by transport measurements, confirming the existence of CDW states. Our work opens up a new approach to synthesizing 2D CDW and superconductive TMDCs for exploring new fundamentals and applications in novel electronics.

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Van der Waals (vdW) heterostructures synthesized through the chemical vapor deposition (CVD) method allow creation and tuning of intriguing electronic and optical properties of twodimensional (2D) materials. Especially, local structures in the heterostructures, such as interfaces, edges and point defects, are critical for their wide range of potential application. However, up to now atomic scale measurements of local structures in as-grown 2D heterostructures on insulating substrates are still rare. Here we report our scanning tunneling microscopy (STM) and spectroscopy (STS) study of as-grown MoS2 monolayer and WS2/MoS2 heterobilayer on SiO2. The heterobilayer appears smoother than the MoS2 monolayer, with root mean square (RMS) roughness of 0.230 nm in the former and 0.329 nm in the latter. For the first time to our knowledge, we directly observed a novel type of continuous interfaces between the MoS2 monolayer and the top layer of the heterobilayer with atomic resolution. Our STS results and density functional theory (DFT) calculations revealed the band gaps of the heterobilayer and the 2 MoS2 monolayer. The finding of the continuous interfaces and the systematic characterizations could have significant impacts on optimizing and designing new 2D heterostructures.