High Mobility 2D Palladium Diselenide Field-Effect Transistors with Tunable Ambipolar Characteristics.

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 (PdSe2 ), 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 PdSe2 show intrinsic ambipolar characteristic. The polarity of the FET can be tuned. After vacuum annealing, the authors find PdSe2 to exhibit electron-dominated transport with high mobility (µe (max) = 216 cm2 V-1 s-1 ) and on/off ratio up to 103 . Hole-dominated-transport PdSe2 can be obtained by molecular doping using F4 -TCNQ. This pioneer work on PdSe2 will spark interests in the less explored regime of noble-TMDCs.

Chemical vapor deposition growth of crystalline monolayer MoSe2.

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.

Identifying the mechanisms of p-to-n conversion in unipolar graphene field-effect transistors.

The mechanisms of p-to-n conversion and vice versa in unipolar graphene field-effect transistors (GFETs) were systematically studied using Raman spectroscopy. Unipolar p-type GFETs are achieved by decorating the graphene surface with a thin layer of titanium (Ti) film, resulting in a Raman D peak. The D peak is observed to recover by annealing the GFET in nitrogen ambient followed by silicon nitride (Si3N4) deposition, suggesting that the Ti adatoms are being partially removed. Furthermore, unipolar n-type GFETs are obtained after the passivation on p-type GFETs. The threshold voltage of the n-type GFET is dependent on the thickness of the Si3N4 layer, which increases as the thickness decreases. A comparison between the Si3N4 and SiO2 passivation layers shows that SiO2 passivation does not convert the GFET into n-type graphene, which identifies the significance of ammonia (NH3) for the formation of the n-type GFETs. This study provides an insight into the mechanism of controlling the conduction behavior of unipolar GFETs.