Worasak Sukkabot

Tight-binding Calculation of Exciton States in InAs Nanocrystals

We theoretically study the electron-hole interactions in spherical InAs nanocrystals by incorporating coulomb energies into the sp3d5s* tight-binding model. The comparisons of excitonic gaps and single-particle gaps for InAs nanocrystals as a function of the radius with tight-binding method, pseudopotential method and experimental data are realized. Finally the calculated results are in agreement with other calculations and experimental data.

Zinc Oxide Nanostructures Synthesized by Thermal Oxidation of Zinc Powder on Si Substrate

ZnO nanowhiskers were formed by a simple oxidation of metallic zinc powder 99.9% at different temperatures from 400-900 °C for 2 hours on the silicon substrate. The result can be obtained after the thermal oxidation process, the ZnO nanowhiskers with different morphologies at different temperatures in which these morphologies and composition of ZnO nanostructures were characterized by scanning electron microscope, (SEM) and X-ray diffraction, XRD. It was found that the products were nanowhiskers, the structure are triangular shapes with average width of 10-50 nm at the root, 10-30 nm at the tip and length in the range of 1-3 μm was observed by SEM. Temperature in the range of 600-700 °C was suitable for the preparation of ZnO nanowhiskers. At higher temperature of 800 °C, the concentrations of ZnO structures become lower and stronger. At the oxidized temperature of 900 °C, ZnO nanowhiskers were not found and surface morphology become to porous. These results indicate that the oxidation rate is faster than the diffusion rate of Zn vapor on the surface of ZnO nuclei.

Atomistic tight-binding theory of electron-hole exchange interaction in morphological evolution of CdSe/ZnS core/shell nanodisk to CdSe/ZnS core/shell nanorod

Based on the atomistic tight-binding theory (TB) and a configuration interaction (CI) description, the electron-hole exchange interaction in the morphological transformation of CdSe/ZnS core/shell nanodisk to CdSe/ZnS core/shell nanorod is described with the aim of understanding the impact of the structural shapes on the change of the electron-hole exchange interaction. Normally, the ground hole states confined in typical CdSe/ZnS core/shell nanocrystals are of heavy hole-like character. However, the atomistic tight-binding theory shows that a transition of the ground hole states from heavy hole-like to light hole-like contribution with the increasing aspect ratios of the CdSe/ZnS core/shell nanostructures is recognized. According to the change in the ground-state hole characters, the electron-hole exchange interaction is also significantly altered. To do so, optical band gaps, ground-state electron character, ground-state hole character, oscillation strengths, ground-state coulomb energies, ground-state exchange energies, and dark-bright (DB) excitonic splitting (stoke shift) are numerically demonstrated. These atomistic computations obviously show the sensitivity with the aspect ratios. Finally, the alteration in the hole character has a prominent effect on dark-bright (DB) excitonic splitting.

Zinc Oxide Nano Walls Synthesized by Chemical Vapor Deposition

ZnO nanowalls were synthesized by chemical vapor deposition at temperature of 650 °C for 1 hour on the silicon substrate. The morphologies of samples were characterized by scanning electron microscopy (SEM). The result from X-ray diffraction (XRD) confirmed that the ZnO nanowalls were vertical c-axis orientation. A room temperature Photoluminescence peak at 378 nm is ultraviolet emission (UV) and the broad peak at wavelengths around 450-650 nm is corresponding to the green emission of ZnO nanostructure. This synthesis may be applicable for gas sensor or solar cells.

Atomistic tight-binding computations of the structural and optical properties of CdTe/CdX (X=S and Se)/ZnS core/shell/shell nanocrystals

Abstract A study of CdTe/CdX (X=S and Se)/ZnS core/shell/shell nanocrystals is carried out using atomistic tight-binding theory and the configuration interaction method to provide information for applications in bioimaging, biolabeling, display devices and near-infrared electronic instruments. The calculations yield the dependences of the internal and external passivated shells on the natural behaviours of CdTe/CdX (X=S and Se)/ZnS core/shell/shell nanocrystals. The reduction of the optical band gaps is observed with increasing numbers of monolayers in the external ZnS shell due to quantum confinement. Interestingly, the optical band gaps of CdTe/CdS/ZnS core/shell/shell nanocrystals are greater than those of CdTe/CdSe/ZnS core/shell/shell nanocrystals. In the presence of an external ZnS-coated shell, electron–hole wave function overlaps, oscillation strengths, ground-state exchange energies and Stokes shift are improved, whereas ground-state coulomb energies and fine-structure splitting are reduced. The oscillation strengths, Stokes shift and fine-structure splitting are reduced with the increase in external ZnS shell thickness. The oscillation strengths, Stokes shift and fine-structure splitting of CdTe/CdS/ZnS core/shell/shell nanocrystals are larger than those of CdTe/CdSe/ZnS core/shell/shell nanocrystals. Reduction of the atomistic electron–hole interactions is observed with increasing external ZnS shell size. The strong electron–hole interactions are more probed in CdTe/CdS/ZnS core/shell/shell nanocrystals than in CdTe/CdSe/ZnS core/shell/shell nanocrystals.

Atomistic Tight-Binding Theory Applied to Structural and Optical Properties of Silicon Nanodisks

The use of ultrathin crystalline silicon (c-Si) wafers in solar cells necessitates a highly effective light absorber to compensate for poor light absorption. One route to overcoming this problem is to use a periodic array of Si nanodisks on ultrathin c-Si. In the present manuscript, we numerically investigate the effects of the geometrical parameters of the Si nanodisks, including disk diameter (D) and length (L), on the structural and optical properties, using atomistic tight-binding theory. These computations confirm that the electronic structure and optical properties are sensitive to the structural parameters. As the disk diameter and length increase, the single-electron energies decrease, and the single-hole energies increase. These calculations also reveal that, because of the quantum confinement effect, the optical band gaps gradually decrease independently of the increasing disk diameter and length. The optical spectra can be tuned across the visible region by varying the disk diameter and length, which is a useful feature for optimizing light absorption in solar cell applications. As the disk diameter and length increased, the optical intensities also increased; however, the atomistic electron–hole interactions and ground electron–hole wave function overlap progressively decreased. The ground electron–hole wave function overlap, Stokes shift, and fine structure splitting decreased as the disk diameter and length were increased. Thus, Si nanodisks with a large diameter and length might be a suitable candidate source of entangled photons. The Si nanodisks in this study also show promise for applications to solar cells based on ultrathin c-Si wafers.

Effects of square electric field pulses with random fluctuation on state dynamics of InAs/GaAs double quantum dots

ABSTRACT The study investigated the influence of square electric-field pulses with random fluctuation on the state dynamics of InAs/GaAs double quantum dots (DQDs) charge qubits. The DQDs were proposed as a charge qubit under operation by a finite number of sequent square pulses of electric field, whose amplitude varied from −0.3 to 0.3 [mV/A°] with random fluctuation at various frequencies. Moreover, for a fixed period of a single pulse, the operation time can also have fluctuation. The state dynamics were analyzed by modelling the DQDs as a two-level quantum system with the effective Hamiltonian and solving the time-dependent Schrödinger equation with the fourth-order Runge-Kutta method using modelling parameters obtained by interpolating results from the tight-binding calculation. Without fluctuation, the state dynamics depicted on the Bloch sphere showed that the states can have different phases, while the probability profiles remain roughly the same with varying electric field strength. When the square pulses were added with uniform random fluctuations in amplitude of the electric field, it was found that the mean state dynamics remained close to that without fluctuation, and the standard deviation of the occupancy probability profiles was proportional to that of the random fluctuation. For fluctuations in operation time, such proportionality does not exist, and the effects on the standard deviation were more pronounced. By comparing the occupancy probability profiles with that without fluctuation, the accuracy of the averaged probability profiles is not monotonic, depending on the fluctuation strength, but there exist some moments in operation time when the profiles are close to the mean dynamics, for both types of fluctuations.