Simulation Of Growth Of Nanostructures In Solution And Investigation Of Spin Decoherence In Several Confined Semiconductor Systems

This dissertation includes two parts. We investigate the growth of the noble metal nano-structures in solution in the first part and the spin relaxation/coherence in several confined semiconductor systems in the second part.In the work of the first part, we apply the Monte-Carlo method to simulate the growth of dendritic nano-structures and noble metal nano-particles in solutions. The wet chem-ical synthesis method has been applied extensively due to its easy operation and flexible control. So far, many types of nano-structures have been obtained by this method in the experiments. However, the works that how different factors, including the solution concentration, growth velocity, surfactant and so on, affect the final morphology of the nano-structures, are not sufficient. The physical and chemical properties of these two types of nano-structures are both sensitive to their morphologies. Therefore, it is neces-sary to study the effects of various mechanisms to the morphology of the nano-structures. For the growth of the dendritic nano-structures, so far, although there are a lot of ex-perimental works focusing on the effects of various factors on the growth of the dendritic nano-structures, these works are only limited in simple comparison between the experi-mental conditions and the final morphologies and lack of the quantitative study of the growth mechanisms. Especially, it is lacking of the quantitative analysis of the effect of the surfactant. For the growth of the noble metal nanocubes, there is a considerable por-tion of cuboid-shaped nano-particles accompanying with the cube-shaped nano-particles in almost every case when the size of particles is small in experiments. It needs the de-tailed simulation of the growth process to find if this only comes from the thermodynamic fluctuation or other asymmetrical growth mechanisms. Furthermore, for the experiments of the formation of 1D nano-structures, several particular kinetic mechanisms have been proposed to account for the asymmetrical growth. It will helpful to do the quantitative simulation to explain the above two problems.In the simulation of the growth process, although different conditions and growth mechanisms are included, it is noticed that no matter what kind of nano-structure is synthesized and no matter which mechanism is considered, we can always introduce the growth velocity along different directions to describe the effect of different mechanisms phenomenologically. Therefore, base on the problems mentioned above, by introducing the growth possibilities determined by various mechanisms, we will focus on studying the growth of the dendritic nano-structures and the cube-shaped nano-particles in the first part of this dissertation. In the former work, we pay close attention to the effect of the surfactant. In the latter work, the influences of the thermodynamic and kinetic factors are investigated in detail. The main content of chapter one to five is addressed below.In the first chapter, We first introduce several typical experimental methods which can produce the dendritic nano-structures. Then the Monte-Carlo method which are used to simulate the growth of the dendritic nano-structures intensively is introduced.Next, in the second chapter, we first introduce the growth of the metal nano-particles in solutions, including the seed nucleation process, the selective adorbates in the growth of the single-crystalline and multiply twinned particles due to the surfactants. At last, we in-troduce the applications for shaped metal nanocrystal, including the catalysis, plasmonics, surface-enhanced raman spectroscopy and assembly.Then in the third chapter, our work focuses on the influences of different conditions on the growth and the morphology of the silver nano-structures. These conditions include the bias voltage and the surfactant. First, we investigate the effect of the bias voltage. The SEM images of as-deposited silver dendritic electrodeposited are showed and the diffusion-limited aggregation method is applied to simulate the growth of the dendritic nano-structures. Our study shows that the bias voltage can control the morphology of the dendritic structures. When the voltage increases, the dendritic nano-structures become denser and denser. Then we study the influence of the surfactant. Different surfactant can stick on certain crystalographic planes and change the order of the free energy for those crystalographic planes, which induces the change of the growth velocity of such crysta-lographic planes. The non-equilibrium of the growth will change the whole morphology. Our experimental results show that, by adding PVP, citrate or PVP/citrite, the diameter of each branch decreases and the dendritic structures are more regular ans symmetrical. Specially, when the bias voltage is small and the surfactant is added, It is interesting that the branches are assembly of hexagon Ag plates. To simulate the effect of the surfactant, we apply the biased on-lattice DLA model. Triangle particles are used on the 2-D square grids. The sticking possibilities of the particles sides are introduced to describe the effect of the surfactant. The simulation results can explain the experiments qualitatively.In the forth chapter, a Monte-Carlo simulation method is applied to investigate the asymmetrical growth of the cube-shaped nano-particle. Three different phenomenolog-ical growth models are set up to study the role of thermodynamic and kinetic factors in the asymmetrical growth. At first, following the Kossel-Stranski model, we present a model with only the thermodynamic factor. We find that even under a pure thermodymic-controlled growth, there are a considerable portion of nano-particles which would deviate from the thermodynamic equilibrium shape when the size of these nanoparticles is small (e.g.,5 nm). The reason is that, the number of atoms in a small nanoparticle is very limited, while the thermodynamic equilibrium shape can only be reached when the num-ber of atoms is large enough. Once the deviating occurs, some kinetic mechanism, such as the electric-field-directed growth proposed by Perez-Juste et al., may further increase the deviating. To study this, we set up a new model by adding the shape-dependent kinetic factor whose strength increases with the increasing of the aspect ratios of the nano-particles. The simulation results show that strong shape-dependent kinetic factor can lead to the formation of one-dimensional nano-structures, but, because this type ki-netic factor would enlarge the differences of aspect ratios during the growth process, the monodispersity of the aspect ratios of the final products is poor. We also set up a model with shape-independent kinetic factor which favors the growth along one certain direction with an unchanged strength. The competition between the thermodynamic factor and the shape-independent kinetic factor results in the formation of nanorods with uniform aspect ratios. The simulation results would benefit our understanding about the reason and manner of the asymmetrical growth of the nano-particles during growth process.At last, we give the conclusion for the first part. First, we simulate the growth of the dendritic nano-structures. By introducing the isosceles right-angled triangle particles in the two-dimensional square grids and the sticking possibilities of different sides of the particle, a modified biased DLA model is set up and applied to study the effect of the bias voltage and the surfactant to the morphology of the fractal trees. It is found that the dendritic nanostructures become denser and the sizes of the single branches decreases with the increasing bias voltage. What is interesting is the fractal trees joint together and become plates due to the surfactant. Then, we study the asymmetrical growth of the noble metal nanocubes in the solution. We set up three phenomenological models to explain the effects of different mechanisms. This first model can explain why there is a considerable portion of cuboid nano-particles accompanying with the cube-shaped nano-particles obtained in the experiments when the size of the nano-particles is small, no matter what the materials and experimental conditions are. The second and the third model explain the experiments in which nonuniform and uniform shaped products are obtained, respectively.Then, in the second part, we study the spin relaxation/coherence in several confined semiconductor systems. Spintronics is a multidisciplinary field whose central theme is the active manipulation of spin degree of freedom in solid-state system. So far, the progress of the spintronics can be divided into three stages. At the first stage, the spintronics emerges and mainly focuses on the investigation of the magnetic metal. Then at the second stage, people try to design spintronics devices corressponding to traditional electronic devices, such as spin transistor, spin valve and so on. At the third stage, the main goal is the generation, manipulation and probe of the spin for a single and several electrons. At each stage, there are a lot of theoretical and experimental works, some of which even have been applied in the industry. In the field of spintronics, the electron spin relaxation and dephasing are very important. So far, many people including us focus on this topic and do a lot of works. Therefore, in the second part of the dissertation, we first review the progress of the spintronics. Then we introduce our own works which are focusing on the electron spin relaxation and dephasing. The main content of chapter one to six is addressed below.In chapter one, We first give a simple review of development of the spintronics. Then we introduce several problems in the three stages of the development of the spintronics: magnetoresistance effect, spintronics devices, spin generation, and quantum dot system.Then in the second chapter, we review the spin decoherence mechanisms and the calculation methods of spin relaxation/dephasing time.In the third chapter, an ensemble Monte Carlo method is used to study the spin injec-tion through a ferromagnet-semiconductor junction where a Schottky barrier is formed. It is shown that the Schottky-barrier-induced electric field, which is confined in the depletion region and is parallel to the injection direction, is very large. This electric field can induce an effective magnetic field due to the Rashba effect and cause strong spin relaxation.Next, in the forth chapter, we propose a scheme to manipulate the spin coherence in vertically coupled GaAs double quantum dots. Up to ten orders of magnitude variation of the spin relaxation and two orders of magnitude variation of the spin dephasing can be achieved by a small gate voltage applied vertically on the double dot. Specially, large variation of spin relaxation still exists at O K. In the calculation, the equation-of-motion approach is applied to obtain the electron decoherence time and all the relevant spin decoherence mechanisms are included. The condition to obtain the large variations of spin coherence is also addressed.In the fifth chapter, the spin relaxation time in zinc blende GaN quantum dot is investigated for different magnetic field, well width and quantum dot diameter. The spin relaxation caused by the two most important spin relaxation mechanisms in zinc blende semiconductor quantum dots, i.e. the electron-phonon scattering in conjunction with the Dresselhaus spin-orbit coupling and the second-order process of the hyperfine interaction combined with the electron-phonon scattering, are systematically studied. The relative importance of the two mechanisms are compared in detail under different conditions. It is found that due to the small spin orbit coupling in GaN, the spin relaxation caused by the second-order process of the hyperfine interaction combined with the electron-phonon scattering plays much more important role than it does in the quantum dot with narrower band gap and larger spin-orbit coupling, such as GaAs and InAs.Finally, in the sixth chapter, we conclude for the second part. Due to the large Schottky-barrier-induced electric field, the spin relaxation for the injection electrons is strong. Then, we study the electron spin relaxation/dephasing in the vertical double quantum dots. We can obtain several orders of magnitude variation of the spin relax-ation/dephasing. At last, we investigate the electron spin relaxation in cubic GaN single quantum dot. We find that in GaN, in which spin-orbit coupling is small, the spin relax-ation induced by the second-order process of the hyperfine interaction combined with the electron-phonon scattering is much more important than it does in the quantum dot with narrower band gap and larger spin-orbit coupling, such as GaAs and InAs.