Three-dimensional Quantum Dot Superlattice Research Articles

Control of Electronic Structures and Phonon Dynamics in Quantum Dot Superlattices by Manipulation of Interior Nanospace.

Quantum dot (QD) superlattices, periodically ordered array structures of QDs, are expected to provide novel photo-optical functions due to their resonant couplings between adjacent QDs. Here, we computationally demonstrated that electronic structures and phonon dynamics of a QD superlattice can be effectively and selectively controlled by manipulating its interior nanospace, where quantum resonance between neighboring QDs appears, rather than by changing component QD size, shape, compositions, etc. A simple H-passivated Si QD was examined to constitute one-, two-, and three-dimensional QD superlattices, and thermally fluctuating band energies and phonon modes were simulated by finite-temperature ab initio molecular dynamics (MD) simulations. The QD superlattice exhibited a decrease in the band gap energy enhanced by thermal modulations and also exhibited selective extraction of charge carriers out of the component QD, indicating its advantage as a promising platform for implementation in solar cells. Our dynamical phonon analyses based on the ab initio MD simulations revealed that THz-frequency phonon modes were created by an inter-QD crystalline lattice formed in the QD superlattice, which can contribute to low energy thermoelectric conversion and will be useful for direct observation of the dimension-dependent superlattice. Further, we found that crystalline and ligand-originated phonon modes inside each component QD can be independently controlled by asymmetry of the superlattice and by restriction of the interior nanospace, respectively. Taking into account the thermal effects at the finite temperature, we proposed guiding principles for designing efficient and space-saving QD superlattices to develop functional photovoltaic and thermoelectric devices.

Formation of three-dimensional quantum-dot superlattices in amorphous systems: Experiments and Monte Carlo simulations

Quantum dots ordered in regular lattices, called quantum dot superlattices, offer numerous possibilities for the creation of novel materials. The formation of such structures during multilayer deposition has been studied and explained satisfactorily only in crystalline materials. Here we are reporting the spontaneous formation of quantum dot superlattices in amorphous systems. The observed superlattices comprise Ge quantum dots embedded in amorphous SiO2 matrix. The internal structure and shape of Ge quantum dots can be controlled by post-deposition thermal annealing. The superlattices show collective behavior properties that appear to be the consequence of a regular ordering of quantum dots. The observed self-organized growth is explained and successfully simulated by a theoretical model based on the interplay of diffusion-mediated nucleation and surface morphology effects. The presented results can be applied more generally and show the ability of formation of regularly ordered, densely packed and uniformly-sized quantum-dot arrays in amorphous matrices.

Damping of bloch oscillations in one-, two-, and three-dimensional quantum-dot superlattices

In the preceding paper by the same authors, the density-matrix formalism was used to derive a quantum kinetic equation describing the damping of Bloch oscillations (BOs) in perfect one-, two-, and three-dimensional quantum-dot superlattices (QDSLs) and the conditions were determined under which the only process of the charge-carrier scattering by phonons in 2D and 3D QDSLs that contributes to the BO damping is the acoustic-phonon scattering within the transverse minibands of the Stark ladder of the carrier states. In this paper, the possibilities of suppressing this remaining scattering channel are analyzed. It is shown that the BO damping time in 2D and 3D QDSLs at room temperature may exceed the oscillation period by a factor of several hundreds and the conditions necessary for such strong suppression of the scattering are revealed. This makes a considerable difference between the QDSLs and the quantum-well superlattices, where, in reality, the BOs damping over a single oscillation period at room temperature.

Vertical alignment of laterally ordered InAs and InGaAs quantum dot arrays on patterned (0 0 1) GaAs substrates

We demonstrate vertical alignment of laterally ordered self-assembled quantum dot (QD) arrays stacked on artificially pre-patterned substrates with two-dimensional hole arrays. The initial InGaAs layer is directly grown on the periodically modulated surface in order to exactly control nucleation sites of QDs to be stacked. After growing three InGaAs dot layers with GaAs spacers as a buffer, laterally ordered InAs dots are grown as an optically active layer. The cross-sectional images of transmission electron microscopy reveal vertical alignment of the stacked QDs. Photoluminescence signal at room temperature is detected from the three-dimensional QD superlattice.

Self-organized growth of three-dimensional quantum-dot superlattices

The self-organization of three-dimensional quantum-dot superlattices that spontaneously form during heteroepitaxial growth is investigated by using a three-dimensional kinetic model. The model predictions show that the multilayer arrays of quantum dots can be fully or partially aligned vertically, or completely misaligned with respect to the buried islands, depending on the thickness of the spacer layer and the growth interruption time. In particular, there is a growth window in which a nearly uniform and regular array of dots is arranged in a squared lattice with an alternating stacking sequence; namely, the A–B–A–B stacking sequence.

Electron localization and bloch oscillations in quantum-dot superlattices under a constant electric field

It is demonstrated that the electron spectrum in ideal two-dimensional and three-dimensional quantum-dot superlattices (SLs) under a constant electric field can be either discrete or continuous depending on the field orientation with respect to the SL crystallographic axes. In the latter case, the width of the resulting transverse miniband depends exponentially on the crystallographic index corresponding to the direction of the field. The electron localization area undergoes dramatic variations with the field orientation in the vicinity of the directions corresponding to the continuous energy spectrum. The Bloch oscillations in this kind of SL are considered. It is established that the scattering of oscillating electrons can be strongly suppressed by an appropriate choice of the field strength and direction.

Self-Organization of CdSe Nanocrystallites into Three-Dimensional Quantum Dot Superlattices

The self-organization of CdSe nanocrystallites into three-dimensional semiconductor quantum dot superlattices (colloidal crystals) is demonstrated. The size and spacing of the dots within the superlattice are controlled with near atomic precision. This control is a result of synthetic advances that provide CdSe nanocrystallites that are monodisperse within the limit of atomic roughness. The methodology is not limited to semiconductor quantum dots but provides general procedures for the preparation and characterization of ordered structures of nanocrystallites from a variety of materials.