Xingli Wang

Recently, two-dimensional layers of transition metal dichalcogenides, such as MoS2, WS2, MoSe2, and WSe2, have attracted much attention for their potential applications in electronic and optoelectronic devices. The selenide analogues of MoS2 and WS2 have smaller band gaps and higher electron mobilities, making them more appropriate for practical devices. However, reports on scalable growth of high quality transition metal diselenide layers and studies of their properties have been limited. Here, we demonstrate the chemical vapor deposition (CVD) growth of uniform MoSe2 monolayers under ambient pressure, resulting in large single crystalline islands. The photoluminescence intensity and peak position indicates a direct band gap of 1.5 eV for the MoSe2 monolayers. A back-gated field effect transistor based on MoSe2 monolayer shows n-type channel behavior with average mobility of 50 cm(2) V(-1) s(-1), a value much higher than the 4-20 cm(2) V(-1) s(-1) reported for vapor phase grown MoS2.

Due to the novel optical and optoelectronic properties, 2D materials have received increasing interests for optoelectronics applications. Discovering new properties and functionalities of 2D materials is challenging yet promising. Here broadband polarization sensitive photodetectors based on few layer ReS 2 are demonstrated. The transistor based on few layer ReS 2 shows an n‐type behavior with the mobility of about 40 cm 2 V −1 s −1 and on/off ratio of 10 5 . The polarization dependence of photoresponse is ascribed to the unique anisotropic in‐plane crystal structure, consistent with the optical absorption anisotropy. The linear dichroic photodetection with a high photoresponsivity reported here demonstrates a route to exploit the intrinsic anisotropy of 2D materials and the possibility to open up new ways for the applications of 2D materials for light polarization detection.

Accurate prediction of band gap for new emerging materials is highly desirable for the exploration of potential applications. The band gaps of bulk and monolayer TMDs (MoS 2 , MoSe 2 , WS 2 , and WSe 2 ) are calculated with the recently proposed by us GVJ‐2e method, which is implemented within DFT framework without adjustable parameters and is based on the total energies only. The calculated band gaps are in very good agreement with experimental ones for both bulk and monolayer TMDs. For monolayer MoS 2 , MoSe 2 , WS 2 , and WSe 2 , direct band gaps are predicted to be 1.88 eV, 1.57 eV, 2.03 eV, 1.67 eV correspondingly, and for bulk TMDs, indirect band gaps of 1.23 eV (MoS 2 ), 1.09 eV (MoSe 2 ), 1.32 eV (WS 2 ), 1.21 eV (WSe 2 ) are predicted. The GVJ‐2e method demonstrates good accuracy with mean absolute error (MAE) of about 0.03 eV for TMDs PL gaps (and 0.06 eV for QP gaps). GVJ‐2e method allows to equally accurately obtain band gaps for 3D and 2D materials. The errors of GVJ‐2e method are significantly smaller than errors of other widely used methods such as GW (MAE 0.23 eV), hybrid functional HSE (MAE 0.17 eV), TB‐mBJ functional (MAE 0.14 eV).

Due to the intriguing optical and electronic properties, 2D materials have attracted a lot of interest for the electronic and optoelectronic applications. Identifying new promising 2D materials will be rewarding toward the development of next generation 2D electronics. Here, palladium diselenide (PdSe 2 ), a noble‐transition metal dichalcogenide (TMDC), is introduced as a promising high mobility 2D material into the fast growing 2D community. Field‐effect transistors (FETs) based on ultrathin PdSe 2 show intrinsic ambipolar characteristic. The polarity of the FET can be tuned. After vacuum annealing, the authors find PdSe 2 to exhibit electron‐dominated transport with high mobility ( µ e (max) = 216 cm 2 V −1 s −1 ) and on/off ratio up to 10 3 . Hole‐dominated‐transport PdSe 2 can be obtained by molecular doping using F 4 ‐TCNQ. This pioneer work on PdSe 2 will spark interests in the less explored regime of noble‐TMDCs.