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In掺杂ZnO超晶格纳米材料的光学性质
中文摘要

具有超晶格结构的InMO₃(ZnO)〓(M=In,Ga和Al等)同系化合物,由于具有优良的光电性能,一直受到人们的关注.近年来,大量的InMO₃(ZnO)〓准一维超晶格纳米结构被成功制备.然而已有的工作主要集中在此类化合物的制备和结构表征上,对该同系物超晶格纳米结构的物性研究特别是光学性质、电学性质研究还很缺乏.对其物性深入研究将是下一步InMO₃(ZnO)〓同系超晶格纳米结构的研究重点.本论文研究工作以In₂O₃(ZnO)〓纳米结构为主要研究对象,从In₂O₃(ZnO)〓超晶格纳米带的光学、电学性质到In₂O₃(ZnO)〓超晶格纳米棒的缺陷发光性质开展了一系列工作.论文包括五部分内容:第一章为绪论部分,介绍了准一维纳米材料的制备方法、光学性质和当前应用面临的挑战;第二章介绍了In₂O₃(ZnO)〓平面向超晶格纳米带的光致发光性质及其电学性质.第三章介绍了In₂O₃(ZnO)〓超晶格纳米棒的制备和缺陷性质.第四章分两个小节分别介绍了ZnO纳米带-In₂O₃纳米颗粒复合结构和In掺杂层状ZnO超晶格纳米棒的制备和物性的初步研究.具体来说包括以下几个方面: 1.利用化学气相传输法制备了小m值(m=3,5)的In₂O₃(ZnO)〓平面向超晶格纳米带.这种小m值纳米带的低温PL谱只显示一个很强的非对称宽激子发射峰,同时首次观察到该激子发射峰相对于纯ZnO纳米带存在14 meV的轻微蓝移.我们发现小m值时,纳米带的超晶格层-In/Zn-O层厚度略小于ZnO的激子波尔半径,由此带来较强的量子限域效应,超过了In重掺杂导致的红移效应,最终导致激子峰蓝移.随后我们研究了这种纳米带的变温发光性质,得到了束缚到In施主的束缚激子(D⁰X→In)的激活能大约在18.3-25.7 meV范围,并通过带隙收缩经验公式得到相应的参数E₀=3.370-3.373 eV,α=1.0-1.2 meV/K和β=500-520 K。同时我们也分析和解释了低温到高温激子峰的展宽机制,发现低温下超晶格结构引起的势阱、势垒起伏导致的非均匀展宽占主导,高温下为温度相关的均匀展宽占主导.室温下的PL谱表明In₂O₃(ZnO)〓纳米带的可见光区域发光峰被明显抑制,可能和In重掺杂形成的中性深能级复合缺陷有关.最后我们初步研究了其室温下的电学输运性质,发现欧姆接触的I-V曲线随着偏置电压增加,在3.7 eV左右开始出现了从线性到非线性转变的现象,并测得这种超晶格纳米带的电阻率(ρ)约为3.0Ω·㎝,优于已报道的Tn掺杂ZnO纳米带. 2.利用化学气相传输的方法合成了In掺杂In₂O₃(ZnO)〓(m=17,19)超晶格纳米棒.通过XRD、Raman、TEM、EDS确认了产物为沿着纳米棒生长方向(c轴)分层堆垛的超晶格纳米棒.研究了纳米棒低温下的变温PL发光,观察到一个3.325 eV的未知峰A,可能和类受主型的缺陷有关.室温下分别在H₂气和空气下对样品进行退火处理,发现样品的深能级缺陷发光同V〓或V〓相关的复合缺陷(用V〓-R表示)、O〓相关的复合缺陷(O〓-R)关联.用XPS进一步分析也证明纳米棒内部处于氧过量状态,说明样品存在较多的V〓/V〓-R缺陷,还存在O〓/O〓-R缺陷.我们用电子顺磁共振技术测试了样品,发现g~1.96处存在两个信号,1.9524和1.9443,分别来自于V〓和In〓缺陷。同时测量了样品在H₂气和空气下退火后的EPR变化,联合PL和EPR的退火变化关系可以推断样品的深能级发光主要来源于V〓或者V〓-R,而O〓相关的缺陷发光出现在更低能红光区域。值得注意的是用氙灯产生的白光照射样品后,g~1.96信号逐渐减弱,撤去光照后又逐渐恢复,具有很好的重复性,说明样品初始状态(无光照)下V〓和In〓缺陷主要以顺磁中心价态存在,而V〓相关的缺陷可能是具有顺磁态的复合。缺陷(In〓-V〓)⁰.通过二价态跃迁模型进一步讨论了缺陷的光致电离动力学过程.另外超晶格纳米棒在室温下表现出明显的铁磁性,可能来源于自补偿效应产生大量的V〓. 3.(1)利用化学气相传输方法,1480℃下蒸发ZnO和In₂O₃混合物粉末首次成功制备出ZnO纳米带-In₂O₃八面体颗粒的复合结构.这种复合结构的主体部分为ZnO纳米带,而纳米带的头部则附着一颗In₂O₃八面体纳米颗粒.我们根据已有的文献报道提出了这种复合结构的生长机制,其中In₂O₃八面体纳米颗粒在生长过程中起到自催化的作用.另外通过PL发光揭示In₂O₃颗粒还能使得纳米带的表面部分钝化,从而导致近带边发射增强,深能级发射减弱.最后联合EPR结果,初步确认PL谱中的深能级发光可能来自于氧空位.(2)通过改变In掺人量,用气相法制备了In掺杂ZnO层状纳米棒.这种纳米棒在SEM观察下为六棱柱状结构,六棱柱表面具有粗糙层状结构.用微区PL聚焦单根纳米棒,结果在可见光区域出现8条很强的尖锐峰.通过计算我们发现这些共振峰来源于纳米棒正六边形微谐振腔的回音壁谐振腔模式,而非普通的F-P谐振腔模式.层状结构可能有利于形成回音壁式微谐振腔共振.这些结果可能有利于未来把In摻杂ZnO层状纳米棒应用于微型可见光激光. 关键词:In₂O₃(ZnO)〓,纳米结构,超晶格,光学性质,缺陷性质

英文摘要

Due to the advantageous optical and electrical properties, homogenous compounds of InMO₃(ZnO)〓 (M=In,Ga,Al) have attracted a tremendous amount of attention in recent years. Various quasi-ID InMO₃(ZnO)〓 nanomaterials have been successfully synthesized by many groups. However, previous works largely focus on the preparation and characterization of these homogenous compounds. The research on the physical properties, such as optical and electrical properties, is rather rare. Thus, the key point of the next-stage research of the homogenous compounds of InMO₃(ZnO)〓 is to deeply study their physical properties. As described in this thesis, a series of studies were performed mainly targeted at the In₂O₃(ZnO)〓 (IZO) nanostructures, focusing on the optical and electrical properties of IZO planar superlattice nanoribbons, and deep defect emissions of IZO superlattice nanorods. This thesis consists of five parts, which have different themes. The first chapter is the introduction, which presents the methods of synthesis, basic optical properties of qusi-lD nanostructures, and their challenges in device applications nowadays are also discussed. The second chapter discusses the optical and electrical properties of IZO planar superlattice nanoribbons. The third chapter describes the synthesis of the IZO superlattice nanorods, and studying of their deep defects through photoluminescence (PL) and electron paramagnetic resonance (EPR). The fourth chapter includes two subsections, which are about the growth and optical properties of ZnO nanobelt-In₂O₃ octahedron complex structures, as well as the synthesis and whispering gallery mode lasing of In-doped ZnO superlattice nanorods with a layer-structured surface, respectively. The specific contents of chapters two through four are summarized as: 1.In₂O₃(ZnO)〓 planar superlattice nanoribbons with small m (m=3,5) were synthesized by a chemical vapor transport method. The low-temperature PL spectrum of the planar superlattice nanoribbons shows only one dominant broad asymmetric exci-tonic emission with a peak at 3.37 eV, which is about 14 meV higher than the bound exciton emission in pure ZnO. For small m, the thickness of an In/Zn-O slab is slightly less than the effective Bohr radius for exciton in bulk ZnO, thus the blueshift due to quantum confinement effect exceeds the redshift resulting from the heavy doping of In, leading to the observed blueshift for our samples. From the temperature dependence of the PL intensity, the activation energy of donor bound exciton bound to In donors in our sample was derived to be around 18.3-25.7 meV. We also obtained the shrinkage parameters of the band gap of the nanoribbons, alpha=1.0-1.2 meV/K and beta=500-520K, by using the Varshni empirical formula. Meanwhile, an interpretation of the broadening mechanism of the excitonic peak was given that an inhomogeneous broadening mechanism was dominant at low temperature, while a homogeneous broadening mechanism was dominant at high temperature. Compared with the pure ZnO nanoribbons, the deep level emission (DLE) from native defects in IZO superlattice nanoribbons was greatly reduced, which may be related to the formation of the deep level complex due to heavy doping of In. Finally, the electrical I-V properties of the IZO nanoribbons were preliminarily studied. As the bias voltage increased, the I-V curve deviated from a linear zone at around 3.7 eV and exhibited a nonlinear characteristic even under the ohmic contact measurement condition. The measured resistivity of the IZO superlattice nanoribbons was about 3.0 Ω.㎝, which was lower than the reported In-doped ZnO nanoribbons. 2.In₂O₃(ZnO)〓 superlattice nanorods (m=17,19) were synthesized via a chemical vapor transport method, and their superlattice structure that the alternate layers stacked along the growth direction (c-axis) was determined by XRD, Raman, TEM and EDS. An unknown peak at around 3.325 eV marked as A was observed in the low-temperature PL spectrum of the IZO nanorods, which may be related to acceptor-like defects. It was found that the deep level emissions may originate from V〓/V〓-R (V〓 related acceptor defects), or O〓/O〓-R defects, by the comparison of the room temperature PL spectra of the IZO nanorods annealed respectively under H₂ and air atmosphere. The conclusion was further confirmed by the result of XPS which revealed the O-rich condition in the IZO nanorods, The EPR measurements of the IZO nanorods exhibited two EPR signals, g = 1.9524 and 1.9443, coming from V〓 and In〓 defects, respectively. The EPR signals of the IZO nanorods annealed respectively under H₂ and air atmosphere were also obtained. Combining EPR and PL investigations of the IZO nanorods annealed under different atmospheres, it can be concluded that the DLE is mainly derived from the V〓 or V〓-R complexes, and the emission relating to O〓/O〓-R defects emerged as a red luminescence band. It is noteworthy that the intensities of the g~ 1.96 lines were found gradually decreasing after continuous illumination of our sample by white light, and to increase to their starting value after the light was switched off, which shows a reversible process. The intensity behavior under white light excitation suggests that initially the defects of V〓 and In〓 are in paramagnetic charge state before illumination, while the defect of V〓 -R may be in the (In〓- V〓)⁰ paramagnetic charge state. A two-charge-state model was proposed to interpret the photoionization kinetic process of V〓. Besides, room temperature ferromagnetism in the IZO superlattice nanorods was obviously observed, which may be related to V〓 induced by self-compensation. 3.(1). ZnO nanobelt-In₂O₃ octahedron complex structures were first synthesized at 1480℃ via a chemical vapor transport method. ZnO nanobelt are the body part of the complex structure, and its top tip is capped by an In₂O₃ octahedron particle. As per previous reports, a growth mechanism was therefore proposed that the In₂O₃ particles played a self-catalytic role in the growth process. The PL result revealed that the In₂O₃ octahedron particle could partially passivate the nanobelt surfaces, bringing about an improved near band edge emission and the suppression of the deep level defect emission. Lastly, based on the EPR result, the DLE may derive from the V〓 defects. (2). In-doped ZnO superlattice nanorods with a layer-structured surface were synthesized by a chemical vapor transport method. The morphology of the nanorod shows a hexagonal cross-section with a layer-structured surface in the SEM image. There were eight sharp peaks that could be observed in the visible region of the μ-PL spectrum from individual layer-structured nanorod. By calculating the effective optical path length, we found that these peaks were attributed to a whispering gallery mode (WGM) formed in the hexagonal shaped microcavity of the individual layer-structured nanorod, rather than a normal F-P resonant mode. However, the formation of WGM may benefit from the layer structure in the nanorod. The findings mentioned above could pave the way towards the integration of the In-doped ZnO layer-structured nanorods for future visible micro-laser applications. Keywords: In₂O₃(ZnO)〓, nanostructurc, superlattice, optical properties, defect properties

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