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Solution nanostructure growth simulation system and a number of limited semiconductor spin relaxation studies

Author: WangYuanYuan
Tutor: WangGuanZhong
School: University of Science and Technology of China
Course: Condensed Matter Physics
Keywords: Silver dendritic nanostructures Electrochemical synthesis Surfactants Diffusion limited deposition modeling Monte - Carlo method Precious Nanocubes Thermodynamic factors Kinetic factors Asymmetric growth Spin relaxation / dephasing Spin - orbit coupling GaN quantum dots Vertical double quantum dots Spin injection Schottky junction Ferromagnetic - semiconductor interface
CLC: O469
Type: PhD thesis
Year: 2010
Downloads: 147
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Abstract

This thesis is divided into two parts. The first part is to simulate a number of in solution nanostructure growth; second part of the semiconductor structure in certain limited spin relaxation. In the first part of the work, we used Monte - Carlo simulation solution of dendritic growth of metallic nanostructures and metal nano cube solution asymmetric growth. Wet chemical solution method is a simple process, regulation and flexible nanostructure synthesis. To date, there have been a large number of different morphologies nanostructure successfully prepared using this method. However, wet chemical synthesis process solution concentration, growth rate, and how factors surfactant nanostructure size and morphology of the final impact, the findings in this area is still not clear enough. Dendritic nanostructures and nanoparticles of precious metals on the morphology of physical and chemical properties are more sensitive to study how they are regulated by changing the morphology of the experimental parameters is important. For dendritic nanostructures, there are a lot of experimental work of various factors on the solution of nanostructure formation of the tree, but often limited to the experimental conditions and the final morphology between simple phenomenological analogy, the lack of growth mechanism Quantitative aspects described, especially for the morphology of surfactant on the modulation, there is a lack of quantitative findings. For the growth of noble metal nano-cubes in the experiments are often part of the nanoparticles is rectangular. This is just a different crystal growth rate fluctuation? Still there cause asymmetric growth factors? If there is no growth process simulation is often difficult to make judgments. In addition, in some studies the growth of nanoparticles obtained asymmetric one-dimensional structure of the experimental work, different researchers have proposed different growth mechanisms. And different mechanisms of crystal growth is not completely understood. For the above problem by simulation for quantitative study helps to clarify the current problems. During growth simulation study, although involving different environments and different crystal growth mechanism, because the simulation without loss of generality, regardless of nanostructures and influencing the growth of this structure is what factors, we can use different directions ( or a different crystal faces) growth likely to phenomenological description. Thus, in the first part of this paper, we introduce the probability of different factors on the growth effects of the use of Monte - Carlo simulation solution dendritic nanostructures and the growth of noble metal nanoparticles. To the previous work, special attention to our work surface active agent. For the second task, we are focused on the thermodynamic and kinetic factors asymmetric growth. The chapters in this section reads as follows: In the first chapter we introduced the dendritic nanostructures several typical preparation methods, as well as several Monte - Carlo simulation of dendritic nanostructures growth-related research. In the second chapter we first introduced the solution of nanoparticles growth process, including nanoparticle nucleation process, monocrystalline and polycrystalline growth of nanostructures and nanoparticles during the growth of surface molecules in the crystal surface selectivity adsorption, as well as their final morphology of nanostructures. Finally, highlighting the noble metal nanoparticle applications, including their catalytic properties, plasma optics, surface-enhanced Raman spectroscopy as well as self-assembly of nanoparticles. In the third chapter, our work focuses on the effects of different factors on dendritic nanostructures growth process and the final morphology, these factors include electrochemical preparation process and surface bias voltage of selective adsorption of molecules . We obtained by scanning electron microscopy prepared under different bias dendritic morphology of silver nanostructures, and then through Monte - Carlo simulation qualitatively explain the microscopic mechanism of the formation of such morphologies and the bias effect. Our results show that the bias voltage can control the overall morphology of dendritic nanostructures. When the bias voltage is increased, the dendritic nanostructures will become more dense. Subsequently, we focus on consideration of the surface agent on non-equilibrium growth. Since the various agents can be adsorbed on the surface of some specific crystal surface, changing the crystal surface free energy, thereby changing the corresponding growth rate of the crystal face. The growth rate of local imbalances inevitably lead to last long into the overall morphology of silver nanostructures different. Our experiments show that the addition of PVP, citric acid, or a PVP / citric acid after the dendritic branches nanostructure becomes more regular overall reduced diameter. In particular, the addition of surface preparation and the applied bias is relatively small, it can be observed that dendritic nanostructures blades constituted by a hexagonal silver pieces. In order to simulate the surface agent added silver nanostructures grown, we use the bias diffusion limited aggregation (DLA) model used in the two-dimensional square lattice triangle particle to be simulated. We introduce particles stick to the group on different planes to simulate the probability of the effect of surface preparation, simulation results can be explained qualitatively bias on dendritic nanostructures dense, branched structure into a sheet structure and other major experimental results. In the fourth chapter, we use a Monte - Carlo simulations to study the asymmetric growth of cubic nanoparticles. We propose three different phenomenological model to study the growth kinetics and thermodynamic factors on the asymmetric growth. First, learn Kossel-Stranski model, we established a thermodynamic factors that contains only a model. Simulation results show that even if the growth process is only limited by the thermodynamic factors, when the nano particle size is relatively small time (for example 5 nm), but also a large part of nanoparticles symmetrical cubic shape deviations. This is because when the nanoparticle size is relatively small, it contains a very limited number of atoms, while the thermodynamic equilibrium only when a very large number of atoms when it can be achieved. Once a deviation from the cubic nanoparticles, some kinetic factors such as Perez, Juste, who proposed the electric field due to the growth of the nanoparticles can further lead to deviation from the symmetrical shape. In order to study kinetic factors, we have created a new model that contains the shape associated with nanoparticles kinetic factors, the impact of this factor as the particle aspect ratio increases. Our simulation results show that the strong dependence of the shape of the kinetic factors can lead to the formation of one-dimensional nanostructures. However, this type of kinetic factors significantly increase the aspect ratio of the last generation of nanostructures differences. Finally, based on extensive literature reported a large number of nano-structured morphology, we created another shape that contains nanoparticles kinetic factors unrelated to the model, this model does not depend on the thermodynamic factors and the kinetics of nanoparticle morphology competition factors led to this having a fixed length is formed nanorods. Our simulation results are beneficial for the growth process of nanoparticles asymmetry causes and patterns of understanding. In the fifth chapter, we made a summary of the first part. First, we simulated the solution of dendritic nanostructures growth. After the introduction of particles, and particle adhering isosceles triangle edge in a different probability later, we use a modified diffusion limited aggregation Models, and surfactants of the voltage on the dendritic morphology of nanostructures, we find , with the bias voltage increases, the branch-shaped nano-structures become more and more dense. Even more interesting is that the role of agents due to surface, when the bias is relatively small when dendritic nanostructures are linked into a sheet structure. Then, we study the solution of noble metal nano cube asymmetric growth. We have established a three phenomenological model to explain the different mechanisms for growth. Our first model can be interpreted in the synthesis of noble metal nano cube experiment, why no matter what material and experimental conditions, there will always be a certain percentage of cuboid particles. The second model can help understand the different morphologies obtained uneven product experiment. Finally, a third model explains the experimental nanoparticles obtained uniform rod experiment. In the second part of the work, we studied a number of restricted semiconductor structure spin relaxation. Spintronics is formed as an emerging interdisciplinary field, the goal is the use of the spin degrees of freedom instead of or partially replace the charge degrees of freedom, in order to develop new alternative to traditional electronics devices spintronics devices. At present, the development of spintronics can be basically divided into three stages. The first stage is spintronics initial stage of development, mainly in the study of magnetic metal. The second phase focuses on the design of the corresponding conventional electron spin transistor devices such as spin transistors, spin valve. The third stage is concerned about generating, manipulating, detecting a single or a few electron's spin. At each stage, there are a large number of experimental and theoretical work, and some even have reached the stage of practical application. In spintronics various stages of development, various systems of electron spin relaxation are very important issues, there are a lot of theoretical and experimental research work concerned about this issue, we are also done some work in this regard. Therefore, in the second part, we will first introduce developments in the field of spintronics, then introduced our own research work in several spin relaxation problem. This section details are as follows: In the first chapter, we first briefly review the development of spintronics, and then describes the three stages in the development of several major questions: magnetic resistance effect, spintronics devices, spin polarization generation and quantum dot systems. Then in the second chapter we review the various mechanisms that lead to spin relaxation as well as theoretical calculations. Then in the third chapter, we study the ferromagnetic metal - semiconductor interface Schottky junction on the impact of spin-polarized injection. We used ensemble Monte - Carlo method and the self-consistent solution of the Poisson equation to obtain the electric potential at equilibrium distribution of electron spin injection in nature. In the fourth chapter, we give a vertical-type GaAs quantum dots coupled to control electron spin relaxation and spin dephasing time of the program. We use a vertical double quantum dots added on a small gate voltage to obtain a large spin relaxation time and the dephasing time. We used the equation of motion method, and taking into account different mechanisms for spin relaxation and dephasing time effects. These mechanisms include: the spin - orbit interaction and electron - phonon scattering body binding, stress-induced spin - phonon coupling, electron spin - nuclear spin hyperfine interaction, electron - phonon scattering and electron spin - nuclear spin hyperfine interaction combined with second-order effect. We have also been given a large spin relaxation and dephasing time of changing conditions. In the fifth chapter, we study the magnetic field in different quantum well width and quantum dot diameters conditions sphalerite structure GaN quantum dot spin relaxation. Our systematic study of sphalerite structural materials resulting spin relaxation in quantum dots are two of the most important relaxation mechanisms: Electronic - phonon interaction and Dresselhaus spin - orbit coupling combined electron - nuclear spin interactions and electron - phonon interaction combined with second-order process, and compare the relative importance of the two mechanisms. Finally, in the sixth chapter of the second part of the work we have done a summary.

Solution nanostructure growth simulation system and a number of limited semiconductor spin relaxation studies

Abstract

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