Junqi Xu

On-Nanowire Axial Heterojunction Design for High-Performance Photodetectors

We report the growth of high-quality CdS/CdSxSe1-x axial heterostructure nanowires (NWHs) via a temperature-controlled chemical vapor deposition method. Microstructural characterizations revealed that these NWHs have a single-crystalline structure with abrupt heterojunctions. Local photoluminescence and mapping near the heterojunctions show only two separated narrow band-edge emission bands from the two different adjacent semiconductors, further demonstrating the high-quality of these heterostructures. Moreover, the photodetector based on the single NWH shows a performance (higher responsivity (1.18 × 10(2) A/W), faster response speed (rise ∼68 μs, decay ∼137 μs), higher Ion/Ioff ratio (10(5)), higher EQE (3.1 × 10(4) %), and broader detection range (350-650 nm)) at room temperature superior to that of photodetectors based on single band gap nanostructures. This work suggests a much simpler route to achieve superior NWHs for applications in optoelectronic devices.

ELASTIC AND THERMODYNAMIC PROPERTIES OF NiAl AND Ni3Al FROM FIRST-PRINCIPLES CALCULATIONS

The elastic and thermodynamic characteristics of NiAl and Ni3Al crystals have been investigated by using a method of density functional theory within the generalized gradient approximation. The three independent elastic constants are C11 = 229.8 GPa, C12 = 124.7 GPa, C44 = 115.7 GPa for NiAl and C11 = 239.6 GPa, C12 = 151.7 GPa, C44 = 123.4 GPa for Ni3Al. The bulk moduli, shear moduli, Youngs moduli, Poissons ratios and ratios of B/G of NiAl and Ni3Al are also calculated. In addition, the dependences of the bulk moduli on temperatures and pressures as well as the linear thermal expansion coefficients (αL) versus temperatures are evaluated and discussed. Both NiAl and Ni3Al crystals are elastically anisotropic. Especially for NiAl crystal, which should be much brittler than Ni3Al, lies at the critical point of brittle behavior. In addition, the present results are well in line with experimental and other theoretical results.

Vortex phase transition and anisotropy behavior of optimized (Li1−x Fe x OH)FeSe single crystals

(Li1−x Fe x OH)FeSe single crystals have been successfully synthesized by a hydrothermal ion-exchange technique. We have carried out systematic research on the influence of the concentration of selenourea (c m) on the superconducting properties of (Li1−x Fe x OH)FeSe single crystals. The optimized specimens possess an onset superconducting transition temperature T c of up to 42 K obtained at c m = 0.8 mol l−1, and the corresponding residual resistivity ratio (RRR = ρ (300 K)/ρ (T c)) is estimated to be 11. The vortex liquid-to-glass phase transition has been discussed based on the collective pinning model. The large distance between the H g and H c2 lines, as well as the much narrower region of the pinned vortex-liquid phase, indicate a weak vortex-pinning ability in (Li1−x Fe x OH)FeSe single crystals. The big out-plane anisotropy behavior versus temperature has been explored according to the anisotropic Ginzburg–Landau theory, and the anisotropy factor Γ displays linear behavior towards temperature similar to that of SmFeAsO1−x F x superconductors.

Large-area, high-quality monolayer graphene from polystyrene at atmospheric pressure

Graphene films have been attracting great interest owing to their unique physical properties. In this paper, we develop an efficient method to prepare large-area monolayer graphene (97.5% coverage) by atmospheric pressure chemical vapor deposition on Cu foils using polystyrene in a short time (3 min). Raman spectroscopy, transmission electron microscopy and scanning electron microscopy are employed to confirm the thickness and uniformity of the graphene films. Graphene films on glass substrates show high optical transmittance and electrical conductivity. Magnetic transport studies demonstrate that the as-grown monolayer graphene exhibits a high carrier mobility of 3395 cm2 V-1 s-1 at 25 K. On the basis of the analysis, it is concluded that our method is a simple, safe and versatile approach for the synthesis of monolayer graphene.

Directed growth of graphene nanomesh in purified argon via chemical vapor deposition

Graphene nanomeshes (GNMs), new graphene nanostructures with tunable bandgaps, are potential building blocks for future electronic or photonic devices, and energy storage and conversion materials. In previous works, GNMs have been successfully prepared on Cu foils by the H2 etching effect. In this paper, we investigated the effect of Ar on the preparation of GNMs, and how the mean density and shape of them vary with growth time. In addition, scanning electron microscopy (SEM) and high resolution transmission electron microscopy (TEM) revealed the typical hexagonal structure of GNM. Atomic force microscopy (AFM) and x-ray photoelectron spectroscopy (XPS) indicated that large copper oxide nanoparticles produced by oxidization in purified Ar can play an essential catalytic role in preparing GNMs. Then, we exhibited the key reaction details for each growth process and proposed a growth mechanism of GNMs in purified Ar.