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Tellurium (Te), as an elementary material, has attracted intense attention due to its potentially novel properties. However, it is still a great challenge to realize high-quality 2D Te due to its helical chain structure. Here, ultrathin Te flakes (5 nm) are synthesized via hydrogen-assisted chemical vapor deposition method. The density functional theory calculations and experiments confirm the growth mechanism, which can be ascribed to the formation of volatile intermediates increasing vapor pressure of the source and promoting the reaction. Impressively, the Te flake-based transistor shows high on/off ratio ≈10 4 , ultralow off-state current ≈8 × 10 −13 A, as well as a negligible hysteresis due to reducing thermally activated defects at 80 K. Moreover, Te-flake-based phototransistor demonstrates giant gate-dependent photoresponse: when gate voltage varies from −70 to 70 V, I on /I off is increased by ≈40-fold. The hydrogen-assisted strategy may provide a new approach for synthesizing other high quality 2D elementary materials.

Layered tin disulfide (SnS2) is a vital semiconductor with versatile functionality due to its high carrier mobility and excellent photoresponsivity. However, the intrinsic defects Vs (sulfur vacancies), which cause Fermi level pinning (significant metal contact resistance), hinder its electrical and optoelectrical performance. Herein, oxygen plasma treatment is employed to enhance the optoelectronic performance of SnS2 flakes, which results in artificial sub‐bandgap in SnS2. Consequently, the broadband photosensing (300–750 nm) is remarkably improved. Specifically, under 350 nm illumination, the O2‐plasma‐treated SnS2 photodetector exhibits an enhanced photoresponsivity from 385 to 860 A W−1, the external quantum efficiency and the detectivity improve by one order of magnitude as well as increase the photoswitching response improvement by two orders of magnitude for both rising (τr) and decay (τd) time. This artificial sub‐bandgap can both improve the photoresponse and broaden the response spectra, which paves a new path for the applications of optoelectronics.

Layered InI has a great potential in optoelectronic applications due to its direct wide tunable bandgap. However, there is no single report about its 2D synthesis. Here, the growth of high‐quality and ultrathin InI flake (as thin as 8 nm) is reported via space‐confined physical vapor deposition. Impressively, an InI flake‐based photodetector exhibits an ultralow off‐state current of ≈4.2 × 10−12 A, high on/off photocurrent ratio of 104, excellent detectivity of 4.2 × 1012 Jones, a high‐speed response time of 10 ms, as well as excellent effective quantum efficiency of 1600%, suggesting its promising applications in optoelectronics.

Two-dimensional (2D) ternary compounds (2DTCs) have attracted intensive attention due to the new degree of freedom of modulating physical and chemical properties. However, the controllable synthesis of 2DTCs still remains a great challenge impeding further research and applications. Here, for the first time, ultrathin (≈7.4 nm) ε-CaTe 2 O 5 flakes with high anisotropy are obtained by a chemical vapor deposition method using soda-lime glass as the capture substrate. The molten glass adsorbs Te vapor in the gas flow to its surface, which reacts with CaO in the molten substrate leading to the precipitation of ε-CaTe 2 O 5 . Interestingly, ε-CaTe 2 O 5 flakes display highly anisotropic band structures and optical properties. Furthermore, low-temperature electrical measurements show that the metal-semiconductor/ insulator transition of ε-CaTe 2 O 5 is exhibited at about 130 K, and optical phonon assisted hopping of small polarons becomes dominant within the temperature range of 130-300 K. Employing soda-lime glass as the capture substrate may provide a new approach for the synthesis of 2DTCs.freedom endows 2DTCs novel physical and chemical properties such as the tunable band gap and variable crystal structure, which may lead to great applications in electronics and optoelectronics. As an important member of 2DTCs, CaTe 2 O 5 has excellent ionic conductivity, [14,15] ferroelastic [16] and optical properties. [17] Owing to the special electronic configuration of tellurium (VI), its electron lone pair will trigger several coordination and unusual structures. Furthermore, the in-plane low symmetric structure may lead to some interesting phenomena such as anisotropic optical and electrical properties. However, there are few reports on 2D CaTe 2 O 5 structures to the best of our knowledge, thus their optical and electrical properties remain unexplored. The synthesis of 2D CaTe 2 O 5 is a huge challenge due to the coexisting hybrid phases in the products such as α, β, γ, ε, δ-phases. [18,19] Thus it is highly desirable to explore an effective way to realize the controllable synthesis of 2D CaTe 2 O 5 structures, which may promote its applications in electronics and optoelectronics.In this work, for the first time, ultrathin (≈7.4 nm) ε-CaTe 2 O 5 flakes are realized by chemical vapor deposition (CVD) method using soda-lime glass as the trapping substrate. The band structure of ε-CaTe 2 O 5 shows different strain-dependence along three directions (a, b, and c axes). The angle-resolved polarized Raman spectra indicate strong in-plane anisotropy in the ε-CaTe 2 O 5 flakes, and the Raman peaks at 124 cm −1 decrease linearly with increasing temperature. Impressively, low-temperature electrical measurements show metal-semiconductor/ insulator transition of ε-CaTe 2 O 5 at about 130 K. These characterizations prove the high anisotropy of ε-CaTe 2 O 5 indicating promising applications in novel electronics and optoelectronics. Results and DiscussionUltrathin, single-crystalline ε-CaTe 2 O 5 flakes were synthesized by CVD method, empl...