Chao Fan

Synthesis of MoSe2 flower-like nanostructures and their photo-responsive properties

Novel MoSe2 flower-like nanostructures were produced by a facile hydrothermal method. Devices containing such nanostructures were also fabricated and reveal obvious photo-responsive characteristics. SEM and HRTEM images show that the as-prepared products have a flower-like structure, and the diameter of a single particle is about 500 nm. The average atomic ratio between Se and Mo is 2.68 according to the results of the EDS measurements. The photo-responsive characteristics responding to red illumination of a device with MoSe2 flower-like nanostructures were investigated for the first time and a mechanism for the photoresponse was proposed. Our findings reveal that such nanostructures have excellent electric conductivity and new photo-responsive properties.

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.

Wavelength dependent UV-Vis photodetectors from SnS2 flakes

2D layered materials have attracted increasing interest, owing to their unique properties and large potential for versatile applications. As one of the 2D layered semiconductors, tin disulfide (SnS2) is rarely reported compared with other 2D materials like molybdenum disulfide (MoS2). Herein, high quality SnS2 flakes were grown by a facile and low-cost path, and photodetectors based on thin SnS2 flakes were fabricated and characterized. These flakes are of high quality according to the results of XRD, Raman and TEM measurements, and present hexagonal and half-hexagonal forms with an average diameter of 100 μm. The devices based on these SnS2 flakes showed wavelength dependent photo-responsive characteristics as the illuminating wavelength varied in the UV-Vis range (from 100 to 800 nm). They also showed excellent photo-responsive characteristics under monochromic illumination using three different wavelengths (533, 405 and 255 nm) with high photo-responsivity and high external quantum efficiency (EQE). The experimental results agree well with the first-principles calculated band structure and optical absorption coefficient curve.

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.

Composition-tunable 2D SnSe2(1−x)S2x alloys towards efficient bandgap engineering and high performance (opto)electronics

Efficient bandgap engineering is a significant strategy for the utilization of widely concerned two-dimensional (2D) layered materials in versatile devices such as nanoelectronics, optoelectronics, and photonics. Alloying transition-metal dichalcogenides (TMDs) with different components has been proved as a very effective way to get 2D nanostructured semiconductors with artificially designed tunable bandgaps. Here we report a systematically study of chemical vapor transport (CVT) grown SnSe2(1−x)S2x alloys with continuously bandgaps ranging from 1.37 eV (SnSe2) to 2.27 eV (SnS2). The carrier mobility of 2D SnSe2(1−x)S2x nanosheets can be tuned from 2.34 cm2 V−1 s−1 (SnS2) to 71.30 cm2 V−1 s−1 (SnSe2) by controlling the S composition in the alloy. Furthermore, the carrier mobility of SnSeS increase from 10.34 to 12.16 cm2 V−1 s−1 under illumination, showing excellent optoelectronic properties. Our study suggests that SnSe2(1−x)S2x nanosheets is a highly qualified 2D materials for next-generation nanoelectronics and optoelectronics application.

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.

Oxygen-induced abnormal photoelectric behavior of a MoO3/graphene heterocomposite

A MoO3 nanoflakes/graphene heterocomposite synthesized by a water bath method showed unusual photoelectric behavior. Its resistance increased under visible light irradiation in air. The behavior arises from the physical contact between the two materials, which leads to hole doping in graphene. Thus, the increase of electrons in the MoO3 nanoflakes induced by desorption of oxygen molecules from its surface under visible light was probably the main reason for the change in resistance. This was confirmed by the resistance of the heterocomposite in a vacuum being 102 times larger than that in air, and by the photoelectric measurements under Ar. The MoO3 nanoflakes/graphene heterocomposite exhibits great potential for use in oxygen gas sensing.

Two-dimensional plumbum-doped tin diselenide monolayer transistor with high on/off ratio

Doping can effectively regulate the electrical and optical properties of two-dimensional semiconductors. Here, we present high-quality Pb-doped SnSe2 monolayer exfoliated using a micromechanical cleavage method. X-ray photoelectron spectroscopy measurement demonstrates that Pb content of the doped sample is ∼3.6% and doping induces the downward shift of the Fermi level with respect to the pure SnSe2. Transmission electron microscopy characterization exhibits that Pb0.036Sn0.964Se2 nanosheets have a high-quality hexagonal symmetry structure and Pb element is uniformly distributed in the nanosheets. The current of the SnSe2 field effect transistors (FETs) was found to be very difficult to turn off due to the high electron density. The FETs based on the Pb0.036Sn0.964Se2 monolayer show n-type behavior with a high on/off ratio of 106 which is higher than any values of SnSe2 FETs reported at the moment. The estimated carrier concentration of Pb0.036Sn0.964Se2 is approximately six times lower than that of SnSe2. The results suggest that the method of reducing carrier concentration by doping to achieve high on/off ratio is effective, and Pb-doped SnSe2 monolayer has significant potential in future nanoelectronic and optoelectronic applications.