Christos S. Garoufalis

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.

High accuracy calculations of the optical gap and absorption spectrum of oxygen contaminated Si nanocrystals

We report accurate high level calculations of the optical gap and absorption spectrum of small Si nanocrystals, with hydrogen and oxygen at the surface. Our calculations have been performed in the framework of time dependent density functional theory (TDDFT) using the hybrid nonlocal exchange and correlation functional of Becke and Lee, Yang and Parr (B3LYP). The accuracy of these calculations has been verified by the high level multi-reference second order perturbation theory. The effect of oxygen contamination is studied by considering several different bonding configurations of the surface oxygen atoms. We show that for nanocrystals of sizes smaller than 20 angstroms, the widening of the gap due to quantum confinement facilitates the stabilization of Si[double bond, length as m-dash]O double bonds. For this type of bonding, the oxygen related states determine the value of the optical gap and make it significantly lower compared to the corresponding gap of oxygen-free nanocrystals. For diameters larger than 20 angstroms, the double bonds delocalize inside the valence band. We find that for small amounts of oxygen, the size of the optical gap depends strongly on their relative distribution and bonding type, while it is practically insensitive to the exact number of oxygen atoms.

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.

The optical gap of small Ge nanocrystals

Using the Density Functional Theory (DFT) with the hybrid nonlocal exchange correlation functional of Becke and Lee, Yang and Parr (B3LYP), we have calculated the optical gap of small Ge nanocrystals passivated by hydrogen and with diameters between 2 and 20 A. Our results show that the optical gap exhibits a size dependence (due to quantum confinement) with many similarities as in the case of Si quantum dots. The diameter of the smallest Ge nanocrystal emitting in the visible region of the spectrum, is approximately 19 A. This critical dimension is smaller than the one found for the case of Si nanocrystals [Phys. Rev Lett. 87 276402 (2001)].

Structural properties and magic structures in hydrogenated finite and infinite silicon nanowires

Unusual effects such as bending and “canting,” related with the stability, have been identified by ab initio real-space calculations for hydrogenated silicon nanowires. We have examined in detail the electronic and structural properties of finite and infinite nanowires as a function of length (and width) and have developed stability and bending rules, demonstrating that “magic” wires do not bend. Reconstructed 2×1 nanowires are practically as stable as the magic ones. Our calculations are in good agreement with the experimental data of Ma et al. [Science 299, 1874 (2003).].

Towards the local structure of the Co(II), Ni(II), Cr(VI) and W(VI) ionic species formed upon impregnation on titania

The mode of interfacial deposition and the local structure of the Co(II), Ni(II), Cr(VI) and W(VI) ionic species formed upon impregnation on titania surface has been studied using various techniques provided by the interface science.

Variation and adjustment of the optical gap of small Si nanocrystals by partial substitution of Si with Ge

In order to adjust the optimum region of diameters of Si based nanocrystals, which can emit in the visible region of the spectrum, we have partially substituted in Si nanocrystals layers of Si atoms by similar layers of Ge atoms and calculated the optical and HOMO-LUMO gaps as a function of Ge concentration and of the size of the nanocrystals, up to about 20 A in diameter. For the calculation of the optical gap of SixGey:Hz nanocrystals as a function of x , y, and z, we have used the framework of time dependent density functional theory (TDDFT) with the hybrid nonlocal exchange-correlation functional of Becke, Lee and Yang (B3LYP). Our results show that by proper adjustment of x, y and z we can optimize either the range of diameters for a desired gap, or the value of the optical gap for a given diameter.

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.