Electrical and optoelectronic anisotropy and surface electron accumulation in ReS2 nanostructures.

Two interesting electronic transport properties including in-plane anisotropy and nonhomogeneous carrier distribution were observed in ReS2 nanoflakes. The electrical conductivity defined by the current parallel to the b-axis (‖b) is 32 times higher than that perpendicular to the b-axis (⊥b). Similar anisotropy was also observed in optoelectronic properties in which the ratio of responsivity ‖b to ⊥b reaches 20. In addition, conductivity and thermal activation energy with substantial thickness dependence were observed, which indicates a surface-dominant 2D transport in ReS2 nanoflakes. The presence of surface electron accumulation (SEA) in ReS2 has been confirmed by angle-resolved photoemission spectroscopy and scanning tunneling spectroscopy. The electron concentration (∼1019 cm-3) at the surface is over three orders of magnitude higher than that of the bulks. Sulfur vacancies which are sensitive to air molecules are suggested to be the major factor resulting in SEA and high conductivity in ReS2 nanostructures.

Photoconduction Properties in Tungsten Disulfide Nanostructures.

We reported the photoconduction properties of tungsten disulfide (WS2) nanoflakes obtained by the mechanical exfoliation method. The photocurrent measurements were carried out using a 532 nm laser source with different illumination powers. The results reveal a linear dependence of photocurrent on the excitation power, and the photoresponsivity shows an independent behavior at higher light intensities (400-4000 Wm-2). The WS2 photodetector exhibits superior performance with responsivity in the range of 36-73 AW-1 and a normalized gain in the range of 3.5-7.3 10-6 cm2V-1 at a lower bias voltage of 1 V. The admirable photoresponse at different light intensities suggests that WS2 nanostructures are of potential as a building block for novel optoelectronic device applications.

Composition-dependent photoconductivities in indium aluminium nitride nanorods grown by magnetron sputter epitaxy.

Photoconduction (PC) properties were investigated for ternary indium aluminium nitride (In x Al1-x N) nanorods (NRs) with different indium compositions (x) from 0.35 to 0.68, as grown by direct-current reactive magnetron sputter epitaxy. Cross-sectional scanning transmission electron microscopy (STEM) reveals single-crystal quality of the vertically aligned In x Al1-x N NRs. Single-rod photodetector devices with good ohmic contacts were fabricated using the focused-ion-beam technique (FIB), where the In-rich In0.68Al0.32N NR exhibits an optimal photocurrent responsivity of 1400 A W-1 and photoconductive gain of 3300. A transition from a positive photoresponse to a negative photoresponse was observed, while increasing the In composition x from 0.35 to 0.57. The negative PC was further enhanced by increasing x to 0.68. A model based on the coexistence and competition of deep electron trap states and recombination centers was proposed to explain the interesting composition-dependent PC in these ternary III-nitride 1D nanostructures.