Huibing Mao

Photoluminescence properties of the Eu3+-doped ZnS nanocrystals and the crystal-field analysis

The Eu3+-doped ZnS nanocrystals prepared by the chemical method have a cubic structure. All the transitions 5D0 ? 7FJ (J = 0, 1, 2, 3, 4, 5, 6) of Eu3+ were observed in ZnS nanocrystals. In particular, the forbidden transitions of 5D0 ? 7F0, 5D0 ? 7F3 and 5D0 ? 7F5 were all observed and their observed values of energies were found to match well with those predicted from a complete diagonalization of the crystal-field Hamiltonian for Eu3+. The photoluminescence of the transition 5D0 ? 7F1 confirms the crystal-field analysis that in the first order approximation the crystal field has no effect on the multiplet 7F1. The crystal-field analysis shows that the multiplet 7F2 has a doubly degenerate sublevel and a triply degenerate sublevel, which also agrees with the photoluminescence results very well. Although not all sublevels of crystal levels of multiplet 7F4 were observed in the photoluminescence of the transition 5D0 ? 7F4, the crystal-field parameter was estimated from the width of the emission band of this transition. In accordance with the superposition model, the crystal-field parameters and were fitted to be ?59.8?meV and 41.6?meV, respectively.

Manipulation of band structures in wurtzite and zinc-blende GaAs/InAs-core-shell nanowires

Using first principles calculation based on density-functional theory, we adopted internal and external two mechanisms to manipulate the band structures of wurtzite and zinc-blende GaAs/InAs-core-shell nanowires (NWs) along the [0001] and [111] directions, respectively. Variational geometry size and chemical component are the internal approaches to tune the band structures. The band gaps are nonlinear composition dependence for the core-shell NWs with fixed diameter and linear composition dependence for the NWs with fixed core. Using external uniaxial strain is another alternative approach. We found that the relative band gap decreases evidently with tensile strain, while it gradually increases with increasing compressive strain. The higher the ratio of GaAs composition in the core-shell NWs, the larger the variations of the relative energy. More interestingly, in wurtzite core-shell NWs, we found a critical reflection point, which results from the two competition states between bonding and anti-bonding. ...

Whispering gallery modes with different polarizations in semiconductor microrod with rectangle shape

The whispering gallery modes (WGMs) of the ZnO photoluminescence emission were investigated in this paper. The transmission electron microscopy and X-ray diffraction results confirm that the rectangle microrod has a single crystalline wurtzite structure. Except for the band-edge emission, the visible luminescence band of the ZnO microrod presents the WGMs with two different polarizations. In a large waveband, the WGMs modes with two different polarizations have same mode spacing. Because of the phase difference in reflectance for two polarizations, the WGMs for s and p polarizations are distributed interlacedly. Due to the property difference, the WGMs for two polarizations have different appearances.

Electrically tunable electron g factors in coupled InAs/GaAs pyramid quantum dots

The electron g factors of coupled InAs/GaAs quantum dots under external magnetic and electric fields are investigated by using the eight-band k•p model. The resonant coupling between the two dots remains under electric fields below 8.2 mV/nm, and is broken above the critical field due to the quantum Stark effect. By applying electric fields, a sign reverse of g factors is observed, and an electric field tunable zero g factor is found in the quantum dot molecules. Spin-orbit interactions nicely explain the transition mechanism of g factors under external electric fields.

Photoluminescence eigenmodes in a single ZnO nanobelt covering the ultraviolet and visible band

The photoluminescence properties of a ZnO nanobelt are investigated. Both the band-edge emission and the green-yellow emission bands have a series of eigenmodes. The theoretical results demonstrate that in the band-edge emission region the photoluminescence modes are determined by the polariton modes. In the green-yellow band there is no coupling between the photons and excitons and the photoluminescence modes are determined by the transverse Fabry-Perot modes. The photoluminescence spectra at different spots confirm that the Fabry-Perot modes are determined by the transverse size. Furthermore, the fitting results show in the waveband in the ultraviolet and visible band the quality-factor Q of the cavity is decreased from 280 to 70 with the increase of the wavelength.

Manipulation of spin polarization by charge polarization in GaMnN ferromagnetic resonant tunneling diode

We have investigated the dependence of spin polarization on charge polarization at room temperature in GaMnN-based ferromagnetic resonant tunneling structures with different Mn doping positions. Our results show that compared to the nonpolarization case, the degree of spin polarization can be enhanced by two to four times and resonant current intensity increases about fourfold at a moderate polarized-charge concentration of 1012/cm2. In addition, the dipole right case in magnetic resonant diodes has stronger impact on spin tunneling than the opposite one. These results demonstrate that tailoring charge polarization is a feasible way to tune electron spin polarization in piezoelectric-type ferromagnetic heterostructures.

Enhancement of Curie temperature under low electric fields in Mn selectively δ-doped GaAs∕AlGaAs wide quantum wells

A prominent improvement of Curie temperature under low electric fields through Mn δ-doped GaAs∕p-AlGaAs wide quantum wells is presented theoretically. The electric-field-controlled Curie temperature for different δ-doping positions and well widths has been investigated by means of the numerical self-consistent calculation. For quantum wells with 40nm well width, an applied electric field of 0.3meV∕nm enhances TC up to five times than ones without the applied field. Our results indicate that wide quantum wells (>20nm) have more advantage than narrow quantum wells in the electric-field-controlled low dimensional ferromagnetic systems.

Luminescence Emission Mediated by Surface Plasmon in the Semiconductor Quantum Dots

The luminescence of the hybrid CdSe/ZnS QDs–Ag nanocrystals has a weak blue shift in comparison with the luminescence of the pure quantum dots (QDs). The luminescence of the CdSe/ZnS QDs only has a single exponential decay process, and the decay rates are almost same for different emission wavelengths. The luminescence of the hybrid CdSe/ZnS QDs–Ag nanocrystals structure has two decay processes: The first is a fast decay process, and then there is a slow decay process with nearly the same decay rate of the pure QDs. The fast decay process is due to the surface plasmon coupling and the coupling rate depends on the energy difference between the confined exciton in the QDs and the resonance energy of the Ag plasmon.

Enhanced efficiency of p-type doping by band-offset effect in wurtzite and zinc-blende GaAs/InAs-core-shell nanowires

Using first principles calculation based on density-functional theory, we investigated p-type electronic structures and the doping mechanism in wurtzite (WZ) and zinc-blende (ZB) GaAs/InAs-core-shell nanowires (NWs) along the [0001] and [111] directions, respectively. Comparing the doping in WZ and ZB core-shell NWs, we found it is easier and more stable to realize dopant in WZ NWs. Due to the type I band-offset, p-type doping in the GaAs-core of GaAscore/InAsshell for both WZ and ZB NWs makes that the valence band-edge electrons in the InAs-shell can spontaneously transfer to the impurity states, forming one-dimensional hole gas. In particular, this process accompanies with a reverse transition in WZ core-shell nanowire due to the existence of antibonding and bonding states.

Different acceptor behaviors in two-dimensional hole gas system formed in Mn-doped GaAs∕p-AlGaAs ferromagnetic heterostructures

The authors investigated the ferromagnetic property of dual acceptors (Be and Mn) doped GaAs∕AlGaAs heterostructures by means of numerical self-consistent field calculation. The dependence of the magnetic behavior on both acceptor doping concentrations and modes was studied in the two-dimensional hole gas system. The results show different dependences of Curie temperature on the concentration of each acceptor, which is attributed to different doping conditions and mechanisms. In addition, the ferromagnetic transition temperature can be increased by about 70K as a result of introducing Be δ-doping layer in AlGaAs barrier, as compared with that for Be bulk doping. The theoretical results indicate that double acceptor δ-doped ferromagnetic heterostructures have potential advantage in the realm of spintronic application.