【作者】 郭震宇；

【导师】 梁万珍；

【作者基本信息】 中国科学技术大学 ， 化学物理， 2010， 博士

【摘要】 量子点,又可称为纳米晶,通常是一种由Ⅱ族,Ⅲ-Ⅴ族元素以及过渡金属组成的纳米颗粒。量子点的粒径一般介于2-10 Nm之间,稳定、溶于水。目前研究较多的是贵金属量子点Au,Ag等,半导体量子点,如TiO2、CdS、CdSe等。近来,量子点由于其独特的性质越发受到人们的重视,其研究内容涉及到化学、物理、生物、材料等多学科,已经成为一门新兴的交叉学科。量子点独特的性质主要基于它自身的量子效应。当颗粒尺寸进入纳米量级时,尺寸的限制将引起量子限域效应、尺寸效应、宏观量子效应和表面效应。从而派生出纳米体系具有不同的低维物性,因而展现出诸多不同于宏观材料的物理与化学性质,这在非线性光学、光伏材料,磁介质、光催化、医药等方面有着极具潜力的应用价值。有机光伏电池是一种用有机材料制成的太阳能电池,主要由导电的有机聚合物或者小的有机分子组成。它具有成本低的特点,且有许多优良的特性,如柔性,异成膜等。有机分子的光吸收系数很高,因此少量的材料便可以吸收大量的光能。它主要的缺点是目前开发的产品效率较低,但它却有较高的理论光电转化率,很有研究价值与潜力。在第一章中,我们简单的介绍了密度泛函理论的框架,从最初的Thomas-Fermi模型到Hohenberg-Kohn定理,再到Khon-Sham方程。相应的含时密度泛函理论,从Runge-Gross定理到含时的Kohn-Sham方程,我们都做了系统的介绍,在此基础上,我们还介绍了时域下通过传播密度矩阵来求解含时的Kohn-Sham方程。最后,我们介绍了最小开关的Surface Hopping方法,以及线宽理论。第二章中,我们研究了银量子点的电声耦合动力学。银量子点作为一种贵金属,具有特别的局域表面等离子共振现象。其均匀共振线宽主要由去相干／去相位时间与激发态寿命(即驰豫时间)共同决定。我们首先研究了声子诱导的等离子激元去相干作用。结果表明等离子激发中电声耦合导致的去相干是总的去相干作用中重要的组成部分。较小的银纳米晶,其声子诱导的等离子激发去相干时间更短。此外,温度对去相干的影响较大,低温时的声子运动不活跃,因此其诱导的去相干较为不明显。接下来,结合最小开关的surface hopping与含时Kohn-Sham方法,我们研究了银量子点等离子激发态的电声驰豫动力学。通过模拟得到一系列等离子激发态的驰豫时间在500至1800 fs之间。其中,低能激发要快于高能激发的驰豫时间,这和超快光谱得到的实验结果基本一致。在第三章中,我们研究了染料敏化太阳能电池中染料-纳米晶界面间的电荷转移动力学。首先,运用时域中传播密度矩阵解含时Kohn-Sham方程的方法,我们研究了三种染料-TiO2纳米晶复合体系。电子动力学模拟得到的结果表明,从激发态染料分子向二氧化钛转移的过程很快,只有几个飞秒,这很好的符合实验测量的结果。同时,我们发现,当纳米晶尺寸很小时,电子转移的尺寸效应更加明显。而当尺寸增大时,其量子尺寸效应迅速的降低。第二部分,我们运用非绝热的分子动力学方法,研究了小尺寸alizarin敏化的TiO2纳米晶界面间的多指数电子转移过程。这种多指数的电子转移主要来源于纳米晶导带的能级分立,因而导致了绝热与非绝热机理共同作用于电子转移过程。此外,该体系具有很慢的电子反向转移过程。这部分理论研究为提高太阳能电池的效率提供了一定的指导意义。最后,运用量子化学方法,我们还研究了一类有机多聚体异质结,BBL-P3HT与TB1-P3HT的电子结构和光谱。研究结果表明这类异质结的吸收光谱同时覆盖了可见与红外波段,这体现了他们具有良好的采光特性。此外,两者的光活性激发态都有着显著的电荷分离现象,这表明其光激发态更倾向形成分离的电荷,而不是形成激子,这有可能会克服光伏材料中普遍存在的激子瓶颈问题。更多还原

【Abstract】 Quantum dots (QDs), also known as nanocrystals, are a special class of materials known as noble metal and semiconductors, which are crystals composed of periodic groups of II-VI, III-V, or IV-VI materials. Quantum dots are very small, ranging from 2-10 nanometers in diameter. Recently noble metal and semiconductor quantum dots, such as Au, Ag, TiO2, CdSe, CdS, are extensively investigated. The size of the dot becomes small enough that it approaches the size of the material’s Exciton Bohr Radius, then the electron energy levels can no longer be treated as continuous-they must be treated as discrete, meaning that there is a small an finite separation between energy levels. This situation of discrete energy levels is called quantum confinement. Due to its quantum size effects, quantum dots behave lots of unique characters, which are very different from bulk systems. Researchers have studied quantum dots in nonlinear optics, photovoltaic material, magnetic medium, photocatalyst, biological sensor, etc.An organic photovoltaic cell (OPVC) is a photovoltaic cell that uses organic electronics-a branch of electronics that deals with conductive organic polymers or small organic molecules for light absorption and charge transport. The plastic itself has low production costs in high volumes. Combined with the flexibiity of organic molecules, this makes it potentially lucrative for photovoltaic applications. Molecular engineering like changing the length and functional group of polymers can change the energy gap, which allows chemical change in these materials. The optical absorption coefficient of organic molecules is high, so a large amount of light can be absorbed with a small amount of materials. The main disadvantages associated with organic photovoltaic cells are low efficiency, low stability and low strength compared to inorganic photovoltaic cells.In the first part of Chapter 1, we introduced the basic idea of density functional theory, from original Thomas-Fermi model, Hohenberg-Kohn theorem, to Kohn Sham equation. Similarly, we also introduced time dependent functional theory, from Runge-Gross to time dependent Kohn-Sham (TDKS) equation. Based on the above content, we illustrated how to solve TDKS equation by propagating density matrix in time domain. Finally, we introduced fewest switches surface hopping (FSSH) method based on the TDKS formalism, together with line width theory. In the second Chapter, we studied phonon induced plasmon dephasing for silver quantum dots. Silver quantum dots, known as noble metal, is unique due to its localized surface plasmon resonance. The homogeneous linewidth for plasmon resonance is determined by plasmon dephasing and relaxation. In the first part, the electron-phonon coupled plasmon dephasing is investigated. The results indicate that the electron-phonon dephasing mechanism is an important part of the overall dephasing process, and that it creates a note worthy contribution to the plasmon linewidth. The dephasing time shows weak dependence on QD size but changes significantly with temperature. In the second part, we investigated electron-phonon relaxation for silver QDs by FSSH-TDKS method. The relaxation times range from 500 to 1800 fs, relaxation for high energy plasmon states is slower than low energy states, which agree well with experimental results.In the first part of Chapter 3, the ultrafast electron transfer processes in three dye-sensitized TiO2 nanocrystals are studied by using the real-time TDDFT. We predict an electron injection time of a few femtoseconds for the present finite systems, which is slightly longer than the experimental measurements and other theoretical predictions for the ET time on the same dye-sensitized bulk TiO2 systems due to the small clusters used in our simulation. We find that the ET time is appreciably dependent on the QD size when the QD is quite small. However, the size effects on ET time reduce dramatically as the cluster size reaches to a moderate middle size. In the second part, multi-exponential electron transfer processes across ultrasmall dye-TiO2 nanocrystal are studied by ab initio nonadiabatic molecular dynamics. The multi-exponential electron transfer occurs from the finite separation between energy levels of conduction band, which leads to both adiabatic and nonadiabatic electron transfer. Furthermore, slow back electron transfer exists in alizarin-TiO2 system, which increases the possibility of efficiency of solar cell.In the last chapter, a novel class of compounds aimed at improving the efficiency of organic photovoltaic devices is investigated by ab initio electronic structure theory. Two heterojunctions composed of chemically bound donor and acceptor species shows little charge transfer in the ground electronic state. In contrast, photoexcitation results in substantial charge separation between the two species, suggesting that the optically excited states present a separated charge pair rather than a strongly interacting pair of charges forming an exciton. The optical cross-section of this charge separated state is quite high due to a good overlap of the tails of the ground and excited states wave-functions. The absorption spectrum of the systems covers visible spectrum and extends to infrared, suggesting good prospects of light harvesting. The calculations results indicate that the proposed class of semiconducting heteroj unctions may be able to overcome the exciton bottleneck problem in organic photovoltaic materials.更多还原