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Towards understanding electronic structure of cobalt and manganese doped zinc oxide quantum dots with Density Functional Theory Methods.

机译：用密度泛函理论方法了解钴和锰掺杂的氧化锌量子点的电子结构。

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摘要

The chapters in this dissertation describe investigation of the electronic structure, excitations and magnetic exchange interactions in transition metal doped zinc oxide (TM2+:ZnO) nanocrystals. A computational scheme has been developed to model and predict those characteristics with the density functional theory (DFT) methods. A set of the spherical-like wurtzite ZnO nanocrystals have been built and capped with pseudo-hydrogen atoms to remove non-physical surface-states from semiconductor bandgap. The tests on the performance of the various DFT functionals showed that inclusion of the non-local Hartree-Fock exchange is important to correctly describe the relative positions of the transition metal dopant and semiconductor host energy levels.;DFT results obtained with our scheme provide explanation to the experimentally observed charge-controlled magnetization in Mn2+-doped ZnO colloids. Injected electrons activate new ferromagnetic Mn2+-Mn2+ interactions by formation of the bound magnetic polaron. These interactions are strong enough to overcome antiferromagnetic coupling between nearest-neighbor dopants, making the full magnetic moments of all dopants observable. Analysis shows that this large effect occurs in spite of small pairwise electron-Mn 2+ exchange energies, because of competing electron-mediated ferromagnetic interactions involving distant Mn2+ ions in the same nanocrystal.;The effects of TM2+-concentration on absorption spectra and magnetic exchange parameters have been addressed. The excitonic transition maximum shifts to higher energy and decreases in intensity with increasing Mn2+ concentration. Increased Mn2+ concentration leads to broadening and increase in the intensity of the sub-bandgap charge-transfer electronic absorption band. The charge-transfer band broadening is a result of excited-state splitting arising from double-exchange-like magnetic interactions involving Mn2+ ions and the photogenerated hole. Delocalization of the photogenerated hole via the double-exchange-like mechanism leads to stabilization of the ferromagnetic configuration in the charge-transfer excited state. The strength of this ferromagnetic double-exchange interaction depends strongly on the inter-Mn2+ distance within the quantum dot. Analysis of the MLCBCT excited state wavefunctions reveals that the more stable excited state originates from the out-of-phase interaction between localized Mn d-orbitals, which is mediated by the exchange interactions between Mn spins and ZnO valence band. The charge-transfer band intensity grows linearly with the number of dopants due to the increase of excited state density and non-linearly through the delocalization of charge-transfer excited states. In Co2+ doped QDs, the double-exchange stabilization is also observed and for the dt2 → CB ML CBCT transitions and has a value comparable to the Mn2+-doped QDs. The lowest excitonic state is stabilized by strong Zener-type sp-d exchange between the TM2+ and ZnO conduction and valence band spins.;At last, the excited state relaxation of the MLCBCT and d-d transitions has been analyzed with the linear response TD-DFT approach for the Co2+-doped ZnO nanocrystals. Substantial vibronic excited state stabilization leads to the crossing between the ligand field and CT excited states potential energy surfaces. This explains the experimentally observed photocurrent arising from the localized ligand field transitions.;Overall, presented in this document is the detailed investigation of electronic structure, magnetic exchange interactions and absorption spectra of TM2+-doped ZnO QDs with the quantum chemistry methods. The methodology described below is based on the easily available quantum chemistry computational package (Gaussian); it is carefully validated, and can he used for prediction of these extremely interesting and important phenomena in other II-IV DMS quantum dots. (Abstract shortened by UMI.)
机译：本文的各章描述了过渡金属掺杂的氧化锌（TM2 +：ZnO）纳米晶体的电子结构，激发和磁交换相互作用的研究。已经开发出一种计算方案，以使用密度泛函理论（DFT）方法对那些特征进行建模和预测。已经建立了一组球形纤锌矿型ZnO纳米晶体，并用假氢原子覆盖，以从半导体带隙中去除非物理表面态。对各种DFT功能的性能测试表明，包含非局部Hartree-Fock交换对于正确描述过渡金属掺杂剂和半导体主体能级的相对位置很重要。实验观察到掺杂Mn2 +的ZnO胶体中电荷控制的磁化强度。注入的电子通过形成结合的磁极化子激活新的铁磁Mn2 + -Mn2 +相互作用。这些相互作用足够强大，可以克服最近邻掺杂剂之间的反铁磁耦合，从而可以观察到所有掺杂剂的完整磁矩。分析表明，尽管成对的电子-Mn 2+交换能量很小，但由于在同一纳米晶体中涉及远距离Mn2 +离子的竞争的电子介导的铁磁相互作用而发生了这种大效应。; TM2 +浓度对吸收光谱和磁交换的影响参数已解决。激子跃迁的最大值移至更高的能量，并随着Mn2 +浓度的增加而降低。 Mn 2+浓度的增加导致子带隙电荷转移电子吸收带的变宽和强度增加。电荷转移带变宽是由于涉及Mn2 +离子和光生空穴的双交换样磁性相互作用引起的激发态分裂的结果。经由类似双交换的机制使光生空穴离域导致在电荷转移激发态下铁磁构型的稳定。这种铁磁双交换相互作用的强度在很大程度上取决于量子点内Mn2 +之间的距离。对MLCBCT激发态波函数的分析表明，更稳定的激发态源自局部Mn d轨道之间的异相相互作用，这是由Mn自旋与ZnO价带之间的交换相互作用介导的。由于激发态密度的增加，电荷转移带的强度随掺杂剂的数量线性增长，而由于电荷转移激发态的离域而非线性地增长。在掺杂Co2 +的量子点中，也观察到双交换稳定，并且对于dt2→CB ML CBCT跃迁，其值可与掺杂Mn2 +的量子点相媲美。最低的激子态通过TM2 +与ZnO导通和价带自旋之间的强Zener型sp-d交换而稳定。最后，通过线性响应TD-DFT分析了MLCBCT和dd跃迁的激发态弛豫。 Co2 +掺杂的ZnO纳米晶体的合成方法。大量的振动激发态稳定会导致配体场和CT激发态势能面之间的交叉。这解释了实验观察到的由局部配体场跃迁产生的光电流。总体而言，本文中介绍了用量子化学方法对掺杂有TM2 +的ZnO量子点的电子结构，磁交换相互作用和吸收光谱的详细研究。下面描述的方法基于容易获得的量子化学计算软件包（Gaussian）；它经过仔细验证，可以用于预测其他II-IV DMS量子点中这些极为有趣和重要的现象。 （摘要由UMI缩短。）

著录项

  • 作者

    Badaeva, Ekaterina.;

  • 作者单位

    University of Washington.;

  • 授予单位 University of Washington.;
  • 学科 Chemistry Physical.;Physics Condensed Matter.
  • 学位 Ph.D.
  • 年度 2011
  • 页码 175 p.
  • 总页数 175
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类
  • 关键词

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