Zaiping Zeng

Linear and nonlinear optical properties of ZnO/ZnS and ZnS/ZnO core shell quantum dots: Effects of shell thickness, impurity, and dielectric environment

In the present work, we investigated theoretically the linear, nonlinear, and total absorption coefficients and refractive index changes associated with intersubband transitions in ZnO/ZnS core shell quantum dot (CSQD) and ZnS/ZnO inverted CSQD (ICSQD), emphasizing on the influence of the shell thickness, impurity, and dielectric environment. The effect of the polarization charges due to the possible existence of the dielectric mismatch between the system and its surrounding matrix is considered. The electronic structures are numerically calculated by employing the potential morphing method in the framework of effective mass approximation. We find that in both impurity-free CSQD and ICSQD, increasing the shell thickness red shifts significantly the threshold energy and enhances drastically the nonlinear absorption coefficients and all the refractive index changes, independently on the dielectric environments. Similar behaviour has also been observed in most of the cases studied when the impurity is displa...

Combination effects of tilted electric and magnetic fields on donor binding energy in a GaAs/AlGaAs cylindrical quantum dot

We have performed a systematic study on the ground-state binding energy of an on-center donor impurity confined in a GaAs/Al0.3Ga0.7As cylindrical quantum dot (QD), subjected to simultaneously applied electric and magnetic fields. The two fields are tilted with respect to the QD growth direction and they are either parallel or perpendicular to each other. All the calculations are based on the potential morphing method which is employed within the framework of the effective-mass approximation. Our results show that when the tilted electric and magnetic fields are parallel, the magnetic shift of the donor binding energy is a monotonic function of the magnetic field strength. On the other hand, when the two fields are perpendicular to each other, the magnetic shift of the donor binding energy varies nonmonotonically with respect to the magnetic field strength, exhibiting a minimum value at a critical magnetic field strength. The position of this minimum value and its dependence on the QD size, its aspect ratio and the orientation of the tilted magnetic field is systematically investigated. Moreover, we discuss in detail the competition effects which appear in the presence of the two fields, showing that the critical line which corresponds to zero shift of the donor binding energy can be manipulated by suitably adjusting the QD size, the aspect ratio and the relative orientation of the two fields.

Optical susceptibilities in singly charged ZnO colloidal quantum dots embedded in different dielectric matrices

Within the two-level system approximation, analytical expressions for the linear, third-order nonlinear and intensity-dependent susceptibilities in quantum dots (QDs) embedded in a dielectric matrix are developed by using density matrix equations, considering the local field effect due to the presence of dielectric mismatch. Based on the derived expressions, we perform a comparative study of the optical susceptibilities in singly charged zinc oxide QDs embedded in various dielectric matrices. Three commonly adopted matrices are considered. The electronic structure of the system is numerically calculated. In general, our results indicate that the optical susceptibilities are highly affected by the capped matrices. For example, QDs embedded in the matrix with the largest dielectric constant but the smallest energy band gap exhibit the largest linear and nonlinear optical susceptibilities, while that dispersed in a matrix with the largest energy band gap show the highest threshold energy. It is also found that the third-order nonlinear susceptibility exhibits a stronger dependence on the nature of the capped matrix as compared to its linear counterpart. Finally, we find that the total susceptibility in charged QD immersed in a matrix with a higher dielectric constant is more sensitive to the applied radiation intensity.

Tuning the binding energy of surface impurities in cylindrical GaAs/AlGaAs quantum dots by a tilted magnetic field

Using the potential morphing method in the framework of the effective-mass approximation, we have studied theoretically the effect of a tilted magnetic field on the binding energy of surface impurities in GaAs/Al0.3Ga0.7As cylindrical quantum dots. It is found that contrary to what was expected based on the existing literature for growth-direction magnetic fields, the presence of a tilted field does not always contribute positively to the binding energy of surface impurities. The shape (aspect ratio) and size of the cylindrical QD as well as the dopant positions at the QD surface play an important role. Furthermore, we find that decrease of the QD size can reduce the sensitivity of the variation of the donor binding energy with respect to the field strength (orientation), but it cannot change its general behaviour.Using the potential morphing method in the framework of the effective-mass approximation, we have studied theoretically the effect of a tilted magnetic field on the binding energy of surface impurities in GaAs/Al0.3Ga0.7As cylindrical quantum dots. It is found that contrary to what was expected based on the existing literature for growth-direction magnetic fields, the presence of a tilted field does not always contribute positively to the binding energy of surface impurities. The shape (aspect ratio) and size of the cylindrical QD as well as the dopant positions at the QD surface play an important role. Furthermore, we find that decrease of the QD size can reduce the sensitivity of the variation of the donor binding energy with respect to the field strength (orientation), but it cannot change its general behaviour.

Electron and impurity states in GaN/AlGaN coupled quantum dots: Effects of electric field and hydrostatic pressure

Based on the effective mass approximation, the ground-state donor binding energy of impurity located at the surface of the zinc-blende (ZB) GaN/AlGaN symmetric coupled quantum dots (SCQDs) is investigated variationally, considering the combined effects of the electric field to the right (along the growth direction) and the hydrostatic pressure. Numerical results show that the impurity localized inside the left dot, the donor binding energy is insensitive to the middle barrier width of the ZB GaN/AlGaN SCQDs if the middle barrier width is large. While the hydrostatic pressure increases the donor binding energy for any electric field and impurity position. Moreover, the hydrostatic pressure has a remarkable influence on the donor binding energy of impurity localized inside left dot. In addition, the competition effects between the electric field and hydrostatic pressure (between the electric field and quantum confinement) on the donor impurity states in the ZB GaN/AlGaN SCQDs have also been investigated in ...

Effects of applied electric field and hydrostatic pressure on donor impurity states in cylindrical GaN/AlN quantum dot

Based on the effective-mass approximation and variational procedure, the donor impurity states in the cylindrical zinc-blende GaN/AlN quantum dot (QD) are investigated, considering the influence of the applied electric field and hydrostatic pressure along the growth direction. Numerical results show that the donor binding energy is highly dependent on the impurity positions, quantum size, applied electric field, and hydrostatic pressure. It is found that the hydrostatic pressure has a remarkable influence on the donor binding energy of the hydrogenic impurity in the QD with a small size and a strong applied electric field. However, the applied electric field affects obviously the donor binding energy in the QD with a large QD size and a small hydrostatic pressure. In addition, the competition effects between the applied electric field and quantum confinement on the donor impurity states in the QD have also been investigated in detail, which is useful to understand impurity states in semiconducting nanostr...

Barrier width dependence of the donor binding energy of hydrogenic impurity in wurtzite InGaN/GaN quantum dot

Within the framework of the effective-mass approximation, the barrier width dependence of the donor binding energy of hydrogenic impurity in a cylindrical wurtzite (WZ) InGaN/GaN strained quantum dot (QD) is calculated by means of a variational procedure, considering the strong built-in electric field effect due to the spontaneous and piezoelectric polarizations. Numerical results show that the built-in electric field and the donor binding energy of the impurity located at any growth direction position are obviously dependent on the barrier width in WZ In0.1Ga0.9N/GaN strained QD with a small barrier width ( 8 nm). Moreover, the donor binding energy of the impurity located at the right boundary of the QD is independent of the barrier width with any dot height and indium compositi...

Shallow-donor impurity in zinc-blende InGaN/GaN asymmetric coupled quantum dots: Effect of electric field

We have performed a theoretical calculation of the shallow-donor impurity states in cylindrical zinc-blende (ZB) InGaN/GaN asymmetric coupled quantum dots (QDs), considering the effect of the electric field applied to the left (opposite to the growth direction). Numerical results show that the donor binding energy in ZB InGaN/GaN asymmetric coupled QDs is highly dependent on the impurity positions, asymmetric coupled QD structure parameters, and the electric field. In the presence of the electric field, if the left dot height is increased from zero, the donor binding energy of the impurity localized inside the middle barrier layer has a maximum value; if the right dot is increased from zero, the donor binding energy of impurity localized inside the right dot has a maximum value. It is also found that for the impurity located at the center of the right dot, the donor binding energy is insensible to the electric field when the electric field is large; however, the critical electric field is smaller if the r...

Excitonic optical properties of wurtzite ZnS quantum dots under pressure.

By means of atomistic empirical pseudopotentials combined with a configuration interaction approach, we have studied the optical properties of wurtzite ZnS quantum dots in the presence of strong quantum confinement effects as a function of pressure. We find the pressure coefficients of quantum dots to be highly size-dependent and reduced by as much as 23% in comparison to the bulk value of 63 meV/GPa obtained from density functional theory calculations. The many-body excitonic effects on the quantum dot pressure coefficients are found to be marginal. The absolute gap deformation potential of quantum dots originates mainly from the energy change of the lowest unoccupied molecular orbital state. Finally, we find that the exciton spin-splitting increases nearly linearly as a function of applied pressure.

Morphology control of exciton fine structure in polar and nonpolar zinc sulfide nanorods

Electron-hole exchange interaction in semiconductor quantum dots (QDs) splits the band-edge exciton manifold into optically active (“bright”) and passive (“dark”) states, leading to a complicated exciton fine structure. In the present work, we resolve by atomistic million-atom many-body pseudopotential calculations the exciton fine structure in colloidal polar and nonpolar zinc sulfide (ZnS) nanorods (NRs). We explore that polar NRs with high symmetry exhibit vanishing fine structure splitting (FSS), and are therefore ideal sources of entangled photon pairs. In contrast, nonpolar NRs grown along