摘要
微纳米材料的物理化学性质与其成分、形状和尺寸有着密切的关系。目前，对ZnO微纳材料的可控制备、表征与应用研究引起了材料、物理、化学等领域研究人员的广泛兴趣。在本文中我们选择了Cu掺杂ZnO体系作为研究对象。与其他过渡金属掺杂元相比，Cu掺杂不仅是一种实现室温铁磁性有效的掺杂元，而且具有其自身的优势：如铜替代锌的掺杂为p型，铜离子半径与锌离子离子半径差异较小，掺杂所需要的结合能低等。此外，由于Cu及其氧化物都是非磁性的，这种不会产生磁性沉淀的掺杂有利于我们研究稀磁半导体磁性的来源。
目前为止，人们已经在实验上通过各种方法实现了Cu掺杂的ZnO样品的制备，但对Cu掺杂ZnO磁性的来源是外部Cu引入还是来源于本征结构的缺陷仍然存在一些争议；而且目前的文献报道主要集中在体材料与薄膜中，只有较少关于Cu掺杂ZnO一维纳米结构报道(如一维的纳米线，纳米棒以及纳米钉等）。
而ZnO球壳结构以及三维分层结构具有特殊的结构和独特的性能，设计和可控制备球壳结构以及分层结构材料在材料制备领域受到了人们广泛的关注。如球壳结构具有独特的几何外形和大的比表面积，与一维纳米结构相比在高性能催化剂，传感器，药物传输，能量存储等方面存在潜在的规模应用。由一维结构纳米材料构筑的ZnO三维分层结构有望于应用复合功能纳米电子器件中(如气体传感器，光催化产氢等)。所以制备铜掺杂的ZnO磁性球壳结构与实现分层结构的制备是十分必要而且重要的。据我们所知，目前还没有关于铜掺杂ZnO磁性球壳结构与Z_(1-x)Cu_xO三维分层微十字架结构相关的报道。
本论文主要集中于探索Z_(1-x)Cu_xO空球壳结构与分层结构的方法，并对所制备的微纳结构进行详细表征，研究了Cu掺杂前后对样品形貌与结构的影响，并解释其生长机理；在性质方面，探讨了Cu引入对ZnO光学性质的影响；在应用方面，对所制备的Cu掺杂ZnO空球壳结构的磁性进行了研究。
实验上，在制备ZnO球壳结构与分层结构等复杂结构的过程中通常需要较高的反应温度，复杂多步的反应步骤，或者会因模板或催化剂的使用而引入一些杂质，这些都不利于所制备的纳米结构在器件上的应用。因此，用一种简单，低温，无催化，可控的方法制备出具有新颖或潜在应用价值的复杂结构仍然是一个挑战。迄今为止，在各种制备ZnO微纳结构的方法中，化学气相沉积法被证明是一种有效的易于生长出多种形貌的纳米结构的合成方法。
本文中，我们通过化学气相沉积法成功地制备出Cu掺杂ZnO空球壳结构。通过改变生长条件，研究样品的形貌，结构变化，我们对其生长机理进行了探讨。对其光学性质研究发现，Cu掺杂使得ZnO可见区发光显著增强。制备出的球壳结构具有室温铁磁性，其磁性来源于Cu引入产生的大量缺陷。缺陷会导致磁性的增强，我们可以通过改变掺杂浓度来调控磁性的强度。
接下来，我们用简单无催化的气相沉积法实现从一维纳米棒到Z_(1-x)Cu_xO三维分层的微十字架结构的制备。该方法的要点是利用了高温有利于Cu的聚集、成核、分支外延生长等。高含量的Cu的引入实现了通常ZnO六方结构向四方对称结构的转变。我们还发现本方法所制备的单个Z_(1-x)Cu_xO微十字架结构由于Cu浓度分布的不同而显示出有趣的不均匀发光现象。
此外， ZnO中高的激子束缚能量大(60meV)导致室温下高浓度的激子的产生，这将有利于ZnO在低的激发能量密度下产生激光现象，从而在短波长发光器件与激光器件中表现出巨大的发展潜力。因而在本文中，我们也选择了ZnO的激光发射性质进行研究。通过化学气相沉积法实现了纳米线环绕纳米管，纳米管组装微球壳等复杂的ZnO微纳结构的制备，从实验上观察到随机激光发射信号。在ZnO纳米塔结构中发现其具有固定模式的激光发射峰，通过理论计算，将其归因为Fabry-Perot共振增强的Whisper Gallery Mode。
The physical and chemical properties of micro/nano materials are afunction of their composition, morphology, and size. At present,controllable synthesis, characterization and application of micro/nanomaterials is of great interest to physicists, chemists, and materialscientists. On the other hand, doping is an efficient method to improve theelectrical and optical properties of semiconductors. In this dissertation,we have chosen the Cu-doped ZnO (Zn1-xCuxO) system to investigate.Compared with the other transition metal, Cu is not only an effectiveferromagnetism dopant, but also has its own advantages. Such as, thesubstitution of Zn by Cu is a p-type doping, and the size mismatchbetween Cu and Zn is very small, resulting in the low formation energy.In addition, Z_(1-x)Cu_xO is free of ferromagnetic impurities, making iteasier to investigate the origin of ferromagnetism in diluted magneticsemiconductor.
So far, considerable effort has been devoted to the fabrication ofZ_(1-x)Cu_xO materials with room temperature ferromagnetism (RTFM)through various methods. However, it remains controversial whether the observed RTFM originates from the extrinsic Cu introducing or theintrinsic structural defects. Most of the investigations have been focusedon bulk materials or thin films, whereas only a few reports onone-dimensional (1D) nanostructures (e.g., nanowires, nanonails, andnanoneedles).
In recent years, increasing attention has been paid to design andcontrollable preparation of hollow spherical structures andthree-dimensional (3D) hierarchical ZnO architectures because of theirunique geometrically shape and widely applications in many fields.Compared with the reported1D Cu-doped ZnO nanostructures, sphericalstructures could apply to catalysis, sensors, drug delivery, energy-storagemedia, and so forth because of their geometrically hollow shape and largesurface area.3D hierarchical ZnO architectures, which derived from1Dnanostructures as building blocks，show considerable promise for thedevelopment of multiple functional nanodevices (e.g., gas sensor andphoto-catalytic hydrogen generation). For these reasons, it is veryimportant and necessary to fabricate the Z_(1-x)Cu_xO hollow sphericalstructures and3D hierarchical micro/nano structures. Thus far, there havebeen no reports of such Z_(1-x)Cu_xO hollow spherical and hierarchicalstructures.
We concentrate on exploring new method to prepare the Zn1-xCuxOhollow spherical and hierarchical micro/nano structures, studying the influence of Cu doing on the morphologies and structures of theas-synthesized samples. The optical and magnetic properties are also beeninvestigated after Cu introducing.
Experimentally, the processes of prepare the Z_(1-x)Cu_xO hollowspherical and hierarchical micro/nano structures often require highertemperature, multi-step process, or introduce impurities by foreigncatalysts or the templates, which is harmful to the application in device.Therefore, it is still a challenge to find a simple and controllable methodto fabricate hollow spherical and hierarchical micro/nano structures.
In this dissertation, we firstly report a simple chemical vapordeposition method to prepare the Z_(1-x)Cu_xO hollow spherical structures.Through a series of controlled experiments by changing the growthconditions, we study on the changes of morphology and structures afterCu doping, and propose the growth mechanism of the as-preparedspherical structures. The visible luminescence is greatly enhanced afterCu introduced. We attribute the enhanced RTFM with Cu contents in ourZ_(1-x)Cu_xO hollow spherical structures to the increased intrinsic defectstriggered by the Cu doping.
Followed that, we have realized the fabrication from1D nanorod to3D hierarchical Z_(1-x)Cu_xO micro-cross structures by a simplecatalyst-free vapor-phase method. The key point is that the highertemperature is helpful to form a central core and epitaxial growth of the hierarchical structure. The introduction of abundant Cu in the core causethe usual hexagonal ZnO structures growing into four-fold hierarchicalcross-like structures. Interestingly, we have observed the distinctinhomogeneous cathodeluminescence (CL) in a single Zn1-xCuxOmicrocross structure, which is attributed to the different concentrations ofCu.
On the other hand, high concentration of excitons can be generated atroom temperature because of the large exciton binding energy (60meV)in ZnO, which will benefit to produce laser in a low excitation energy. Wealso investigate the laser emission properties of pure ZnO nanostrucures.Different hierarchical ZnO nanostructures, such as nanotube surroundednanowires, and nanotube assembled micro spherical shell structures havebeen prepared by chemical vapor deposition method. We can observe therandom action in the as-prepared ZnO hierarchical nanostructures.Moreover, we have prepared the micro-tower arrays, which can be servedas laser micro cavity. The observed fixed laser mode at room temperaturein the ultraviolet region at room temperature in micro-tower structurescan be attributed to the Whisper Gallery Mode lasing assisted byFabry-Pérot resonance.
引文
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