Juehan Yang

Electric-Field Tunable Band Offsets in Black Phosphorus and MoS2 van der Waals p-n Heterostructure

The structural and electronic properties of black phosphorus/MoS2 (BP/MoS2) van der Waals (vdW) heterostructure are investigated by first-principles calculations. It is demonstrated that the BP/MoS2 bilayer is a type-II p-n vdW heterostructure, and thus the lowest energy electron-hole pairs are spatially separated. The band gap of BP/MoS2 can be significantly modulated by external electric field, and a transition from semiconductor to metal is observed. It gets further support from the band edges of BP and MoS2 in BP/MoS2 bilayer, which show linear variations with E⊥. BP/MoS2 bilayer also exhibits modulation of its band offsets and band alignment by E⊥, resulting in different spatial distribution of the lowest energy electron-hole pairs. Our theoretical results may inspire much interest in experimental research of BP/MoS2 vdW heterostructures and would open a new avenue for application of the heterostructures in future nano- and optoelectronics.

Gas-dependent photoresponse of SnS nanoparticles-based photodetectors

SnS nanoparticles were synthesized with a facile hydrothermal method and characterized by X-ray diffraction (XRD), Raman, transmission electron microscope (TEM) and scanning electron microscope (SEM). The red light photoresponse of the SnS-based devices in different gas environments were also systematically investigated, and revealed that the adsorbed gas molecules play important roles in the photosensitive properties. Compared with that in vacuum, the photosensitivity was enhanced in O2 (or air) and reduced in NH3. The dynamic response time was much longer in a gas environment. These influences were ascribed to the charge transfer between the adsorbed gas molecules and SnS.

Synthesis and Transport Properties of Large-Scale Alloy Co0.16Mo0.84S2 Bilayer Nanosheets

Synthesis of large-scale highly crystalline two-dimensional alloys is significant for revealing properties. Here, we have investigated the vapor growth process of high-quality bilayer CoxMo1-xS2 (x = 0.16) hexagonal nanosheets systematically. As the initial loading of the sulfur increases, the morphology of the CoxMo1-xS2 (0 < x ≤ 1) nanosheets becomes hexagons from David stars step by step at 680 °C. We find that Co atoms mainly distribute at the edge of nanosheets. When the temperature increases from 680 to 750 °C, high-quality cubic pyrite-type crystal structure CoS2 grows on the surface of CoxMo1-xS2 nanosheet gradually and forms hexagonal film induced by the nanosheet. Electrical transport measurements reveal that the CoxMo1-xS2 nanosheets and CoS2 films exhibit n-type semiconducting transport behavior and half-metallic behavior, respectively. Theoretical calculations of their band structures agree well with the experimental results.

Abnormal photocurrent response and enhanced photocatalytic activity induced by charge transfer between WS(2) nanosheets and WO(3) nanoparticles.

Sun trap: Pure WS2 nanosheets are prepared that exhibit excellent photosensitive properties. After functionalization with WO3 nanoparticles, abnormal photocurrent responses, enhanced photocatalytic activity, and induced photoluminescence is observed.

Low temperature electrical transport and photoresponsive properties of H-doped MoO3 nanosheets

α-MoO3 nanosheets were synthesized by a water bath method using ammonium heptamolybdenum tetrahydrate and concentrated nitric acid as precursors. Hydrogen was doped by a chemical reduction in aqueous acidic media, with hydrazine hydrate used as the reducing agent. Temperature dependent resistance showed that the low temperature Peierls transition of H-doped MoO3 nanosheets breaks below 50 K, and its resistance is satisfied at temperatures lower than 37 K (37–10 K). This phenomenon was induced by thermal disturbance and the dominance of defects in low temperature transport, which was confirmed by photoresponse measurements taken before and after the break of the new phase.

Improving the field-effect performance of Bi2S3 single nanowires by an asymmetric device fabrication.

High-quality Bi2 S3 nanowires are synthesized by chemical vapor deposition and their intrinsic photoresponsive and field-effect characteristics are explored in detail. Among the studied Au-Au, Ag-Ag, and Au-Ag electrode pairs, the device with stepwise band alignment of asymmetric Au-Ag electrodes has the highest mobility. Furthermore, it is shown that light can cause a sevenfold decrease of the on/off ratio. This can be explained by the photoexcited charge carriers that are more beneficial to the increase of Ioff than Ion . The photoresponsive properties of the asymmetric Au-Ag electrode devices were also explored, and the results show a photoconductive gain of seven with a rise time of 2.9 s and a decay time of 1.6 s.

Effect of Electrical Contact on the Performance of Bi2S3 Single Nanowire Photodetectors

Bi2S3 single-crystalline nanowires are synthesized through a hydrothermal method and then fabricated into single nanowire photodetectors. Due to the different contact barrier between the gold electrode and Bi2S3 nanowires, two kinds of devices with different electrical contacts are obtained and their photoresponsive properties are investigated. The non-ohmic contact devices show larger photocurrent gains and shorter response times than those of ohmic contact devices. Furthermore, the influence of a focused laser on the barrier height between gold and Bi2S3 is explored in both kinds of devices and shows that laser illumination on the Au-Bi2S3 interface can greatly affect the barrier height in non-ohmic contact devices, while keeping it intact in ohmic contact devices. A model based on the surface photovoltage effect is used to explain this phenomenon.

Low temperature electrical and photo-responsive properties of MoSe2

MoSe2 was fabricated by a facile hydrothermal method, and a simple device based on it was prepared to investigate the low temperature electrical and photo-responsive (PR) properties. PR current of MoSe2 under 650 nm red illumination is 2.55 × 10−5 A and remains approximately at low temperatures, which demonstrates its fine PR property. As the temperature became lower, electrical conductivity of MoSe2 first decreased from 300 to 43 K and then increased at temperatures from 43 to 13 K. Mechanisms of such electrical and PR phenomenon were proposed. Our findings revealed potential method to adjust band gap of transition metal dichalcogenides and demonstrated their potential applications under special environment.

Synthesis of Bi2S3–Bi2O3 composites and their enhanced photosensitive properties

Bi2S3–Bi2O3 composites were synthesized by a facile hydrothermal method with surfactant as template. The structure and morphology of the as-synthesized products were characterized in detail. The photosensitive behavior of the Bi2S3–Bi2O3 composites was investigated carefully. Under the illumination of a 650 nm laser in air or vacuum, the device displayed enhanced photosensitive performance compared to the pure Bi2S3, pure Bi2O3 and mechanical mixture of Bi2S3 and Bi2O3 based devices. The photoswitch ratio (Iphoto/Idark) was as high as 30 in a vacuum with fast photoresponse speed, which indicated their potential applications in manufacturing photodetectors and optoelectronic devices. The possible mechanism of enhanced photosensitivity was also proposed.

Anisotropic photoresponse of layered 2D SnS-based near infrared photodetectors

We fabricated near-infrared (NIR) photodetectors with two-dimensional (2D) single crystal orthorhombic SnS nanosheets. The as-fabricated devices exhibited an excellent photoresponsivity of 365 A W−1 under 808 nm light illumination with an excellent external quantum efficiency of 5.70 × 104, which can be further increased to 635 A W−1 and 9.92 × 104 by Au nanoparticle decoration. An anisotropic photoresponse was found for the SnS-based NIR photodetectors and the conductivity and photoconductivity along the zigzag direction are much greater than those along the armchair direction. Treated with oxygen plasma, the effects of defects on the anisotropic photoresponse were further investigated. These results provide a deeper understanding of the electrical and photoelectrical properties of 2D SnS nanosheet based devices.