Two-dimensional Quantum Dot Superlattice Research Articles

Effects of size polydispersity on electron mobility in a two-dimensional quantum-dot superlattice

We found that the transitions between delocalized electronic states in quantum-dot superlattices with smaller size dispersion can account for higher electron mobility. In particular, we solved for the quantum states of a two-dimensional quantum-dot (QD) superlattice using a one-electron approximation. Electron transport properties were studied by considering hopping transitions among coupled delocalized electronic states. Molecular dynamics simulations were employed to introduce disorders in superlattice configurations as a function of QD size and size dispersion for calculation of electron mobility. The interparticle spacing, size, and temperature dependence of the electron mobility can be well explained within the framework of our analysis.

Novel Si Nanodisk Fabricated by Biotemplate and Defect-Free Neutral Beam Etching for Solar Cell Application

We have already developed a uniform silicon-based two-dimensional quantum dot superlattice (2D QDSL) (i.e., 2D array of Si nanodisks) on the x–y plane with high-density nanodisks (NDs) and controllable size and spacing between NDs. To integrate more nanodisks in the third dimension (z-axis), a new stacked nanodisk (S-ND) has been developed by using thin film deposition, bio-template and advanced alternate etching. The S-ND structure comprises of two NDs connected vertically by coupling a tunnel junction (TJ). The scanning transmission electron microscopy image clearly revealed the stacked structure. The quantum confinement of S-ND was confirmed by observing the Coulomb staircase at room temperature. Tunneling current could be controlled by changing TJ thickness. By simply changing the ND thickness, band gap energy (Eg) could be controlled. These characteristics of the quantum effect and Eg play important roles in QDSL solar cells.

Light absorption by a two-dimensional quantum dot superlattice

The monochromatic light absorption in an ideal two-dimensional quantum dot superlattice (QDSL) is considered theoretically. Calculations of the absorption coefficient are done in both the absence and presence of a homogeneous DC electric field with rational and irrational orientations. The explicit dependencies of the absorption coefficient on the frequency of the light, the QDSL parameters and the strength of the electric field are found. Some numerical results for GaAs — Ga 0.7 Al 0.3 As QDSL are obtained.

Comparison of Optical Properties of In0.4Ga0.6As/GaAs(311)B Two-Dimensional Quantum Dot Superlattices and Quantum Wells

We present a detailed comparison of the optical properties of a quantum well (QW) and a two-dimensional (2D) quantum dot superlattice (QDSL) made of In0.4Ga0.6As on GaAs(311)B substrates under the same growth conditions. The lowest electron to hole transition energy in strained In0.4Ga0.6As/GaAs(311)B QWs is calculated. While good agreement between the calculated transition energy and the measured photoluminescence (PL) peak energy is found in the QW, the PL peak energy of the 2D QDSL significantly shifts to a lower energy with respect to the calculated transition energy for a QW with the same nominal thickness. Based on the linewidth broadening induced by increasing excitation density and temperature, the exciton–exciton and exciton–phonon interaction strengths in the 2D QDSL are found to be much larger than those in the QW. As the exciton–phonon scattering is sufficiently strong, a transition of the exciton motion from coherent to incoherent occurs.

Capture, relaxation, and recombination in two-dimensional quantum-dot superlattices

We present a systematic study of the exciton capture, relaxation, and recombination processes in two-dimensional quantum-dot superlattices (2D QDSL's) based on time-resolved photoluminescence measurements. Due to the formation of minibands in 2D QDSL's, the capture of excitons from the miniband into some large islands is found to be a quantum capture process. The capture time increases significantly with increasing excitation density. In addition, the excitons relax rapidly within the miniband. However, the relaxation becomes slower at high excitation densities. Furthermore, the fast recombination in the miniband indicates the drastic elongation of the exciton coherence length and the significant delocalization of the wave functions. Finally, the observation of radiative recombination over a wide energy region implies the relaxation of momentum conservation or the small exciton effective mass in 2D QDSL's. All phenomena suggest that the exciton dynamics in 2D QDSL's is governed by the exciton coherence length in the miniband.

Realization and characterization of two-dimensional quantum dot superlattices forming “artificial crystals”

The realization of two-dimensional quantum dot superlattices (2D QDSLs) is briefly reported. The formation of minibands is manifested in the large red shift of the photoluminescence (PL) peak as well as the dramatic narrowing of the PL linewidth. This is confirmed more directly by various experiments intentionally designed to localize excitons.

The Procedure to Realize Two-Dimensional Quantum Dot Superlattices: From Incoherently Coupled to Coherently Coupled Quantum Dot Arrays

We will describe, from both the theoretical and experimental points of view, a procedure leading to the realization of two-dimensional quantum dot superlattices (2D QDSLs). Two kinds of laterally coupled quantum dot arrays in which the exciton motions are coherent and incoherent are studied by photoluminescence (PL) measurements. The formation of minibands with coherent exciton motion is demonstrated in the large red shift of the PL peak energy as well as the dramatic narrowing of the PL linewidth. Moreover, we indicate, using excitation-dependent PL spectra as an example, that the exciton coherence can be reduced by various scattering mechanisms.

Rehybridization of electronic structure in compressed two-dimensional quantum dot superlattices

Two-dimensional superlattices of organically passivated 2.6-nm silver quantum dots were prepared as Langmuir monolayers and transferred to highly oriented pyrolytic graphite substrates. The structural and electronic properties of the films were probed with variable temperature scanning tunneling microscopy. Particles passivated with decanethiol (interparticle separation distance of ∼1.1±0.2 nm) exhibited Coulomb blockade and staircase. For particles passivated with hexanethiol or pentanethiol (interparticle separation distance of ∼0.5±0.2 nm), the single-electron charging was quenched, and the redistribution of the density of states revealed that strong quantum mechanical exchange, i.e., wave-function hybridization, existed among the particles in these films.