ABSTRACT
Self-assembly of gold nanoparticles (AuNPs) is an important growth mode for fabricating functional materials. In this work we report a dendrite structure formed by slowing down the aggregation dynamics of AuNPs self-assembly. The obtained results show that the aggregation dynamics is dominated by the Reaction Limited Aggregation Model (RLA) more than the Diffusion Limited Aggregation Model (DLA). In which the repulsion due to electrostatic forces is dominant by the Van Der Walls attraction forces, and low sticking probability of nanoparticles. The aggregation dynamics of AuNPs can be slowed down if the water evaporation of the drop casted colloidal AuNPs on a quartz substrate is slowed. Slowing down the evaporation allows electrostatic repulsion forces to decrease gradually. At certain point, the attraction forces become higher than the electrostatic repulsion and hence cluster aggregation take place slowly. The slow aggregation dynamics allows the nanoparticles to sample all possible orientation in the sticking site, searching for the lowest energy configuration. The size distribution of the nanoparticles in liquid is confirmed using dynamic light scattering based on Stokes-Einstein equation for diffusion coefficient in water. X-ray and photoluminescence (PL) spectra of the sample after aggregation showed a shift which is related to the aggregation compared with non-aggregated colloidal nanoparticles in the solution. The study shows that dendrite self similar structure can be formed by slowing down the aggregation dynamics of nanoparticles as a result of minimizing the Helmholtz free surface energy of the system.
ABSTRACT
The Photoluminescence spectra (PL), their temperature and power dependence were investigated for the ground state in InAs quantum dots (QDs) embedded in InGaAs asymmetric quantum well (Asym. QW). In-atom segregation is well known phenomena in such structures, which result in altering the inter-atomic distances; as a consequence the thermo-dynamical parameters change as well, namely Debye temperature. The bigger value of Debye temperature for the studied sample with respect to the corresponding bulk value is attributed to In/Ga inter-diffusion during growth. The inter-diffusion process causes non-radiative defects in the sample. As a consequence, rapid decrease in the QDs integrated emission intensity as the temperature increases was occurred.
ABSTRACT
The dependence of the integrated photoluminescence on the excitation power intensity in Al(0.3)Ga(0.7)As/GaAs multi quantum well is studied. Four peaks are found in the photoluminescence spectra, which are corresponding to the four quantum wells in the sample. The temperature dependence of the exponent alpha of the power law shows peculiar behavior for the quantum well of width 11.2 nm (peak C). The value of the exponent alpha exceeds the quadratic value predicted by the steady state model near room temperature. All other peaks shows linear dependence in the low temperature range which switches to super linear in the high temperature range with values of alpha less than 2. Carriers thermal capture and re-trapping is discussed. The presented results are a sign of thermal dissociation of exciton in quantum well near room temperature. The peculiar behavior is attributed to the excess flow of the charge carriers to this QW by thermal escape from other QWs, and also due to excess free carriers because of exciton dissociation.
ABSTRACT
The carrier thermodynamics of InAs self assembled quantum dot (QD) are investigated. The investigated parameters include the dependence of quantum dot photoluminescence on temperature and the photoluminescence (PL) dependence on the excitation power density. Results are discussed on the basis of steady state model. The model predicts that the photoluminescence integrated intensity has linear dependence on the excitation power density in low temperature range, and super linear in the high temperature range. Our data matches the prediction of the steady state model. In our sample the super linearity starts to take place at T = 150 K and the super linear behavior of the photoluminescence on excitation power density proves that the carrier dynamics in our quantum dot sample are dominated by uncorrelated electron hole pair in the high temperature region.
