Jian-Bai Xia

Photoresponsive and Gas Sensing Field-Effect Transistors based on Multilayer WS2 Nanoflakes

The photoelectrical properties of multilayer WS2 nanoflakes including field-effect, photosensitive and gas sensing are comprehensively and systematically studied. The transistors perform an n-type behavior with electron mobility of 12 cm2/Vs and exhibit high photosensitive characteristics with response time (τ) of <20 ms, photo-responsivity (Rλ) of 5.7 A/W and external quantum efficiency (EQE) of 1118%. In addition, charge transfer can appear between the multilayer WS2 nanoflakes and the physical-adsorbed gas molecules, greatly influencing the photoelectrical properties of our devices. The ethanol and NH3 molecules can serve as electron donors to enhance the Rλ and EQE significantly. Under the NH3 atmosphere, the maximum Rλ and EQE can even reach 884 A/W and 1.7 × 105%, respectively. This work demonstrates that multilayer WS2 nanoflakes possess important potential for applications in field-effect transistors, highly sensitive photodetectors, and gas sensors, and it will open new way to develop two-dimensional (2D) WS2-based optoelectronics.

Electronic structural Moiré pattern effects on MoS2/MoSe2 2D heterostructures

The structural and electronic properties of MoS2/MoSe2 bilayers are calculated using first-principles methods. It is found that the interlayer van der Waals interaction is not strong enough to form a lattice-matched coherent heterostructure. Instead, a nanometer-scale Moiré pattern structure will be formed. By analyzing the electronic structures of different stacking configurations, we predict that the valence-band maximum (VBM) state will come from the Γ point due to interlayer electronic coupling. This is confirmed by a direct calculation of a Moiré pattern supercell containing 6630 atoms using the linear scaling three-dimensional fragment method. The VBM state is found to be strongly localized, while the conduction band minimum (CBM) state is only weakly localized, and it comes from the MoS2 layer at the K point. We predict such wave function localization can be a general feature for many two-dimensional (2D) van der Waals heterostructures and can have major impacts on the carrier mobility and other electronic and optical properties.

Binding energy of a hydrogenic donor impurity in a rectangular parallelepiped-shaped quantum dot: Quantum confinement and Stark effects

We calculate the binding energy of a hydrogenic donor impurity in a rectangular parallelepiped-shaped quantum dot (QD) in the framework of effective-mass envelope-function theory using the plane wave basis. The variation of the binding energy with edge length, position of the impurity, and external electric field is studied in detail. A finite potential model is adopted in our calculations. Compared with the infinite potential model [C. I. Mendoza et al., Phys. Rev. B 71, 075330 (2005)], the following results are found: (1) if the impurity is located in the interior of the QD, our results give a smaller binding energy than the infinite potential model; (2) the binding energies are more sensitively dependent on the applied electric field in the finite potential model; (3) the infinite potential model cannot give correct results for a small QD edge length for any location of the impurity in the QD; (4) some degeneracy is lifted when the dot is no longer cubic.

InAs/GaAs single-electron quantum dot qubit

The time evolution of the quantum mechanical state of an electron is calculated in the framework of the effective-mass envelope function theory for an InAs/GaAs quantum dot. The results indicate that the superposition state electron density oscillates in the quantum dot, with a period on the order of femtoseconds. The interaction energy E-ij between two electrons located in different quantum dots is calculated for one electron in the ith pure quantum state and another in the jth pure quantum state. We find that E-11]E-12]E-22, and E-ij decreases as the distance between the two quantum dots increases. We present a parameter-phase diagram which defines the parameter region for the use of an InAs/GaAs quantum dot as a two-level quantum system in quantum computation. A static electric field is found to efficiently prolong the decoherence time. Our results should be useful for designing the solid-state implementation of quantum computing

Electronic structure and binding energy of a hydrogenic impurity in a hierarchically self-assembled GaAs/AlxGa1-xAs quantum dot

We calculate the electronic structures and binding energy of a hydrogenic impurity in a hierarchically self-assembled GaAs/AlxGa1-xAs quantum dot (QD) in the framework of effective-mass envelope-function theory. The variation of the electronic structures and binding energy with the QD structure parameters and the position of the impurity are studied in detail. We find that (1) acceptor impurity energy levels depend more sensitively on the size of the QD than those of a donor impurity; (2) all impurity energy levels strongly depend on the GaAs quantum well (QW) width; (3) a donor impurity in the QD has only one binding energy level except when the GaAs QW is large; (4) an acceptor impurity in the QD has two binding energy levels, which correspond to heavy- and light-hole quantum states; (5) the binding energy has a maximum value when the impurity is located below the symmetry axis along the growth direction; and (6) the binding energy has a minimum value when the impurity is located at the top corner of the QD. (c) 2006 American Institute of Physics.

Complex quantum ring structures formed by droplet epitaxy

Well-defined complex quantum ring structures formed by droplet epitaxy are demonstrated. By varying the temperature of the crystallizing Ga droplets and changing the As flux, GaAs/AlGaAs quantum single rings and concentric quantum double rings are fabricated, and double-ring complexes are observed. The growth mechanism of these quantum ring complexes is addressed. (c) 2006 American Institute of Physics.

Magnetic coupling properties of rare-earth metals (Gd, Nd) doped ZnO: First-principles calculations

The electronic structure and magnetic coupling properties of rare-earth metals (Gd, Nd) doped ZnO have been investigated using first-principles methods. We show that the magnetic coupling between Gd or Nd ions in the nearest neighbor sites is ferromagnetic. The stability of the ferromagnetic coupling between Gd ions can be enhanced by appropriate electron doping into ZnO Gd system and the room-temperature ferromagnetism can be achieved. However, for ZnO Nd system, the ferromagnetism between Nd ions can be enhanced by appropriate holes doping into the sample. The room-temperature ferromagnetism can also be achieved in the n-conducting ZnO Nd sample. Our calculated results are in good agreement with the conclusions of the recent experiments. The effect of native defects (V-Zn, V-O) on the ferromagnetism is also discussed

Electronic states of InAs/GaAs quantum ring

In the framework of effective mass envelope function theory, the electronic states of the InAs/GaAs quantum ring are studied. Our model can be used to calculate the electronic states of quantum wells, quantum wires, and quantum dots. In calculations, the effects due to the different effective masses of electrons in rings and out rings are included. The energy levels of the electron are calculated in the different shapes of rings. The results indicate that the inner radius of rings sensitively changes the electronic states. The energy levels of the electron are not sensitively dependent on the outer radius for large rings. If decreasing the inner and outer radii simultaneously, one may increase the energy spacing between energy levels and keep the ground state energy level unchanged. If changing one of two radii (inner or outer radius), the ground state energy level and the energy spacing will change simultaneously. These results are useful for designing and fabricating the double colors detector by intraband and interband translations. The single electron states are useful for studying the electron correlations and the effects of magnetic fields in quantum rings. Our calculated results are consistent with the recent experimental data of nanoscopic semiconductor rings

First-principles study of the electronic structures and magnetic properties of 3d transition metal-doped anatase TiO2

We study the electronic structures and magnetic properties of the anatase TiO2 doped with 3d transition metals (V, Cr, Mn, Fe, Co, Ni), using first-principles total energy calculations based on density functional theory (DFT). Using a molecular-orbital bonding model, the electronic structures of the doped anatase TiO2 are well understood. A band coupling model based on d-d level repulsions between the dopant ions is proposed to understand the chemical trend of the magnetic ordering. Ferromagnetism is found to be stabilized in the V-, Cr-, and Co-doped samples if there are no other carrier native defects or dopants. The ferromagnetism in the Cr- and Co-doped samples may be weakened by the donor defects. In the Mn-, and Fe-doped samples, the ferromagnetism can be enhanced by the acceptor and donor defects, respectively.

Effects of piezoelectricity and spontaneous polarization on electronic and optical properties of wurtzite III–V nitride quantum wells

A theoretical model accounting for the macropolarization effects in wurtzite III-V nitrides quantum wells (QWs) is presented. Energy dispersions and exciton binding energies are calculated within the framework of effective-mass theory and variational approach, respectively. Exciton-associated transitions (EATs) are studied in detail. An energy redshift as high as 450 meV is obtained in Al0.25GaN0.75/GaN QWs. Also, the abrupt reduction of optical momentum matrix elements is derived as a consequence of quantum-confined Stark effects. EAT energies are compared with recent photoluminescence (PL) experiments and numerical coherence is achieved. We propose that it is the EAT energy, instead of the conduction-valence-interband transition energy that is comparable with the PL energy. To restore the reduced transition rate, we apply an external electric field. Theoretical calculations show that with the presence of the external electric field the optical matrix elements for EAT increase 20 times