Aijiang Lu

In Situ Atom Scale Visualization of Domain Wall Dynamics in VO2 Insulator-Metal Phase Transition

A domain wall, as a device, can bring about a revolution in developing manipulation of semiconductor heterostructures devices at the atom scale. However, it is a challenge for these new devices to control domain wall motion through insulator-metal transition of correlated-electron materials. To fully understand and harness this motion, it requires visualization of domain wall dynamics in real space. Here, domain wall dynamics in VO2 insulator-metal phase transition was observed directly by in situ TEM at atom scale. Experimental results depict atom scale evolution of domain morphologies and domain wall exact positions in (202) and (040) planes referring to rutile structure at 50°C. In addition, microscopic mechanism of domain wall dynamics and accurate lattice basis vector relationship of two domains were investigated with the assistance of X-ray diffraction, ab initio calculations and image simulations. This work offers a route to atom scale tunable heterostructure device application.

Fabrication of single-crystal/phase Cu2ZnSnS4 nanorods via a two-step spin coating route

In this study, we develop a two-step spin coating route without a template and use it to fabricate Cu2ZnSnS4 nanorods. Cu2ZnSnS4 nanorods with a diameter of about 30 nm were nearly perpendicular to the molybdenum-coated soda lime glass substrate. The results of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM), and Raman spectroscopy confirm that the as-synthesized products are single-crystal/phase Cu2ZnSnS4 nanorods. The effects of various factors on the formation of Cu2ZnSnS4 nanorods are also investigated. Hall effect measurement reveals that the Cu2ZnSnS4 nanorod film has p-type conductivity.

Photoluminescence of excitons and defects in ZnSe-based longitudinal twinning nanowires

Photoluminescence spectrum is a powerful tool to investigate the excitonic and defect-related information in condensed phase materials. Through temperature-dependent photoluminescence spectra, heavy- and light-hole free excitons, and the exciton bound to a neutral donor and the accompanied LO-phonon replicas of the zinc blende ZnSe are observed and verified in ZnSe-based longitudinal twinning nanowires. In the deep-level emission region, two peaks at 1.979 and 2.315 eV are attributed to the donor–acceptor–pair recombination and the acceptor impurity of Au, respectively.

Enhanced surface modification engineering (H, F, Cl, Br, and NO2) of CdS nanowires with and without surface dangling bonds

Semiconductor nanowires (NWs) can be applied in gas sensing and cell detection, but the sensing mechanism is not clearly understood. In this study, surface modification effect on the electronic properties of CdS NWs for different diameters with several species (H, F, Cl, Br, and NO2) is investigated by first principles calculations. The surface dangling bonds and halogen elements are chosen to represent the environment of the surface. Halogen passivation drastically changes the band gaps due to the strong electronegativity and the energy level of halogen atoms. Density of states analysis indicates that valence band maximum (VBM) of halogen-passivated NWs is formed by the p states of halogen atoms, while VBM of H-passivated NWs is originated from Cd 4d and S 3p orbitals. To illustrate that surface modification can be applied in gas sensing, NO2-absorbed NWs with different coverage are calculated. Low coverage of NO2 introduces a deep p-type dopant-like level, while high coverage introduces a shallow n-type...

Structural and stable properties of ZnSe/Si core-shell nanowire heterostructures: The first principles calculation

Relations between composition and structural and stable properties of cubic zinc selenide-silicon core-shell nanowires (NWs) are studied by first principles calculation. The diameter is between 1.1 and 2.7 nm, and the direction of the NWs considered is [110]. The lattice constants of the nanowires deviate from the Vegard’s law positively with compressed ZnSe core. Stability of the NWs is discussed by taking binding energy into account. Pure Si NWs show an increasing trend of binding energy as the diameter increases while ZnSe NWs do not. Further analysis shows that zinc blende ZnSe NWs might be unstable under small diameters and a phase transition to wurtzite structure would occur. Our findings might give some guidance for the application of ZnSe/Si core-shell NWs in photoelectronics.

Orbital electronic occupation effect on the metal-insulator transition in TixV1-xO2

A series of Ti x V1-x O2 (0%  ⩽  x  ⩽  4.48%) thin films on c-plane sapphire substrates have been fabricated by co-sputtering oxidation solutions, and the metal-insulator transition temperature (T MIT) of Ti x V1-x O2 films rises monotonically at the rate of 1.64 K/at.% Ti. The x-ray diffraction measurement results show that, after Ti4+ ion doping, the rutile structure expands along the c r axis while shrinking along the a r and b r axis simultaneously. It makes the V-O bond length shorter, which is believed to upshift the π * orbitals. The rising of π * orbitals in Ti-doped VO2 has been illustrated by ultraviolet-infrared spectroscopy and first-principles calculation. With the Ti4+ ion doping concentration increasing, the energy levels of π * orbitals are elevated and the electronic occupation of π * orbitals decreases, which weakens the shielding for the strong electron-electron correlations in the d|| orbital and result in the T MIT rising. The research reveals that the T MIT of VO2 can be effected by the electronic occupancy of π * orbitals in a rutile state, which is helpful for developing VO2-based thermal devices.

Growth of ZnSe-based longitudinal twinning nanowires by phase transformation

Zinc blende ZnSe longitudinal twinning nanowires (Type I) and a sandwich structure with the wurtzite ZnSe inserting into the zinc blende ZnSe longitudinal twinning nanowires (Type II) are fabricated via a simple thermal evaporation method. The growth of them might be caused by the crystal plane slip during the phase transformation process from wurtzite ZnSe to zinc blende ZnSe nanowire. The wurtzite ZnSe might have two origins: 1) The phase transformed wurtzite from zinc blende. At first, during the temperature rising stage in the experiment, before the temperature approached to the transformation temperature (Ttr), ZnSe in zinc blende phase might begin to nucleate and grow. Once the temperature is higher than Ttr, the zinc blende products would transform to wurtzite phase. 2) The new-born nuclei grown wurtzite phase at high temperature for it is reported that the wurtzite phase is more stable at higher temperature. During the cooling period, the source material is exhausted and no more nucleation would occur. Some of the wurtzite products would transform to zinc blende phase when the temperature is lower than Ttr. During the process, it is reasonable that the ZB phase begins to form from the outer sides of an individual nanowire. Once the process completes, the longitudinal twinning ZB nanowire would be obtained; otherwise, the sandwich-structured nanowire forms.

Magnetic and electronic structural properties of the GdGa7N8 cluster

Geometric optimizations and calculations of GdGa7N8 cluster were performed by a DMoL program using spin-polarized density functional theory (DFT). The binding energy, HOMO-LUMO gap, Mulliken charge and bonding characteristics were computed and analyzed. It is found that the Gadolinium substituting the Gallium would make the bonds between itself and neighboring atoms longer than that of the undoped cluster. The magnetic moment of GdGa7N8 was found to be 7 μB. And most of the magnetic moment was focused on the Gd atom owing to its half-filled 4f-shell.