摘要
纳米技术是近年来随着纳米材料的合成工艺、表征技术以及微纳加工技术发展起来一门新兴的学科。当材料的尺寸进入纳米级别时,由于量子限制效应和表面效应的作用,会表现出很多新奇的物理和化学性质,在半导体器件、磁性材料、催化、光化学电池、纳米器件等领域有着重要的应用价值。为了实现纳米颗粒在实际应用中的良好性能,往往须要对纳米颗粒的某些性质进行调控,而构建核／壳结构量子点则是最近得到广泛研究的一种操纵、调控和剪裁纳米颗粒性质的有效手段。已经有广泛的研究证实,量子点的光电磁性质可以通过构成核／壳结构颗粒来连续调节。本论文采用了一种表面转化策略合成ZnO/ZnS核／壳结构量子点,并探索了其在稀磁半导体磁性操控、高性能光化学电极和纳米发电机方面的应用。结合X射线吸收谱、紫外可见吸收光谱、光致发光谱等表征手段和第一性原理密度泛函计算方法解释了核／壳结构量子点在这些应用中具有良好性能的原因,并指出了进一步进行优化的途径。
本论文主要包括以下内容
(1)利用核壳结构调控Co掺杂ZnO稀磁半导体量子点的自旋交换作用操纵稀磁半导体量子点(diluted magnetic semiconductor quantum dots, DMS QDs)中的铁磁相互作用是研发下一代自旋信息技术的关键课题。然而,由于DMS QDs中磁性离子固有的反铁磁耦合作用,这一目标的实现面临巨大的挑战。我们通过利用核壳结构改变Co掺杂ZnO中磁性离子的3d轨道相对于带边的位置,成功的实现了DMSQDs的铁磁交换作用。第一性原理计算显示,包裹在Co掺杂ZnO核外围的ZnS壳层可以有效的诱导界面以下1.2nm内的Co2+对从反铁磁耦合转变为铁磁耦合。这一方法可以为未来的自旋电子学应用中关于操纵氧化物纳米结构中交换作用的研究提供参考。
(2)高性能ZnO@ZnO:S类核／壳纳米复合材料光解水光电极
抑制光电极表面的快速空穴复合是设计和制备用于高性能光解水光电极的最核心问题之一。本章中,我们提出了通过在ZnO纳米颗粒表面层掺入S原子,产生俘获空穴的能级,以之来抑制光生电子-空穴复合,提高载流子利用效率,从而提高能量转化率。我们制备的ZnO@S掺杂ZnO类核／壳纳米复合材料在功率密度为100mW/cm2的模拟光源照射下,测得产生的光电流密度高达1.08mA/cm2,比纯ZnO纳米颗粒电极高8倍左右。实验表征和理论计算结果表明,由于S原子占据了纳米颗粒表面ZnO晶格中O原子的位置,在带隙间产生一个可以俘获光生空穴的杂质能级。杂质能级的产生导致了光解水性能的增强。这一设计为未来进一步优化ZnO光电极提供了参考。
(3)通过ZnO/ZnS核／壳结构稳定纳米颗粒中的注入电子及其在纳米发电机中的应用
在自我供能纳米技术领域,研发高效、稳定、柔性的基于ZnO纳米材料的纳米发电机具有重要的意义。本文中,我们提出一种新型的基于ZnO纳米驻极体的静电纳米发电机；通过形成具有Ⅱ型结构核/壳结构纳米颗粒,注入ZnO纳米驻极体的电荷可以被长久的保存,利用带有稳定电荷的驻极体构成电容器,环境中声波等微弱振动引发电容器中的导体对极板的共振可以在外电路中产生交流电流。为了实现这个设想,我们通过表面转化策略制备了ZnO/ZnS核／壳结构纳米颗粒,进行紫外照射向核/壳结构纳米颗粒注入电子,由于ZnS壳层的保护,注入电子可以在空气中稳定存在,因此注入电子可以长期存在于核/壳结构纳米颗粒中。随后,我们使用已经充入电荷的核/壳结构驻极体组装成一台静电纳米发电机,它可以以2.2%的能量转化率将来自环境中的声波转化为电能。实验和理论研究表明,注入核/壳结构驻极体的电子存在于ZnO晶格中的氧空位缺陷中。这项研究为制备具有良好性能和应用价值的新型ZnO基纳米发电机的发展提供了另一条可能的途径。
As the development of synthesis and characterization of nanostructures and micro/nano processing, nanotechnology has been attracted immense interests. When their sizes are in the range from1nm to100nm, materials exhibits many chemical and physical properties due to the quantum confinement effect and surface effect, which have potential application for semiconductor devices, magnetic materials, catalysis, photoelectrochemical electrodes and nanodevices. To realize practical applications, it is necessary to precisely tuning certain properties of nanoparticles and core/shell structuring is an effective way to achieve it. Extensive studies have proved that the electronic structure of nanoparticles can be continuously tuned via core/shell structure. In this thesis, we reported a surface-converting strategy for synthesize ZnO/ZnS core/shell nanoparticles and explore their application in diluted magnetic semiconductors, highly efficiency photoelectrochemical electrodes and nanogenerators. We exploited characterization techniques such as X-ray absorption spectroscopy, UV-vis absorption spectroscopy, photoluminescence spectroscopy and first-principles calculations to explain the mechanism and discover the path to further optimization.
The main content in this dissertation is as follows:
(1) Realizing ferromagnetic coupling in diluted magnetic semiconductor quantum dots
Manipulating the ferromagnetic interactions in diluted magnetic semiconductor quantum dots (DMSQDs) is a central theme to the development of the next-generation spin-based information technologies, but remains a great challenge because of the intrinsic antiferromagnetic coupling between impurity ions therein. Here, we propose an effective approach capable of activating ferromagnetic exchange in ZnO-based DMSQDs, by virtue of a core/shell structure that engineers the energy level of the magnetic impurity3d levels relative to the band edge. This idea has been successfully applied to Zn0.96CO0.04O DMSQDs covered by a shell of ZnS or Ag2S. First-principles calculations further indicate that covering a ZnS shell around the Co-doped ZnO core drives a transition of antiferromagnetic-to-ferromagnetic interaction, which occurs within an effective depth of1.2nm underneath the surface in the core. This design opens up new possibility for effective manipulation of exchange interactions in doped oxide nanostructures for future spintronics applications.
(2) ZnO@S-doped ZnO Core/Shell Nanocomposites for Highly Efficient Solar Water Splitting
Prohibiting the rapid holes recombination is a key issue for designing highly efficient photoelectrodes for solar water splitting. Here, we propose a strategy incorporating S atoms in the surface shell of ZnO nanoparticles to generate holes trapper for restraining the electron-hole recombination. The obtained ZnO@S-doped ZnO core/shell-like nanocomposite exhibits a high photocurrent density of1.08mA/cm2with conversion efficiency up to0.74%,8times larger than that of pristine ZnO nanoparticles. Comprehensively, the results from experimental and computational aspects reveal that the S dopants substituting the surface O sites narrow the band gap and introduce a mid-gap impurity band, contributing to the enhanced water splitting activity. This design provides some guides for future optimization of ZnO-based photoelectrodes.
(3) A Novel Nanogenerator based on ZnO/ZnS core/shell electrets with Stabilized Quasi-permanent Charge
Flexible, efficient and robust ZnO-based nanogenerators with convenient integration process are particularly desirable for self-powered technology, which is however difficult to achieve simultaneously in traditional piezoelectric ZnO nanogenerators due to the complexity of the function unit for generating piezopotential. Here, we proposed a novel flexible electrostatic ZnO nanogenerator based on charged ZnO nanoscale electrets, in which extra charge is stabilized with a carefully devised type-Ⅱcore/shell structure. Via a previously reported surface-converting strategy, we successfully covered ZnS shells around ZnO nanoparticles, preventing UV-induced electron carriers form decay in the atmosphere; and then the charged electrets were assembled with a simple process into a nanogenerator, which is enabled to convert vibrational energy from the environment into electric power with a conversion efficiency of2.2%. Photoluminescence, X-ray absorption spectroscopy and first-principles studies revealed that in the charged ZnO/ZnS core/shell electrets, the injected electrons occupy the site of oxygen vacancies. This design opens up an alternative path for fabricating flexible ZnO-based nanogenerator for future nanotechnology application.
引文
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