【作者】 郭洪辉；

【导师】 林仲华； 周剑章；

【作者基本信息】 厦门大学 ， 物理化学， 2009， 博士

【摘要】 氧化锌(ZnO)，作为一种重要的宽禁带半导体材料，具有优良的光电性能，目前已被广泛地研究应用到发光二极管、光伏电池、激光等光电领域。自从发现室温下ZnO纳米线的紫外激光发射以来，一维ZnO纳米材料如纳米棒、纳米管、纳米带等由于在纳米器件和光电器件中的潜在应用前景而受到了人们极大的关注。发光二极管由于具有高效节能的特点，很有可能取代传统的白炽灯和荧光灯，成为新一代的照明光源。ZnO是继GaN之后最有可能实现商业化的光电材料。如何将一维纳米ZnO材料更好地应用到发光二极管领域，已经成为研究的热点。本论文着重应用电化学方法，通过各种途径，提高ZnO纳米棒发光二极管的发光效率，调节一维纳米ZnO材料的电致发光谱段，主要进行了以下两方面的研究工作：(1)采用电化学方法分别制备ZnO纳米棒膜、ZnO纳米棒与聚3—甲基噻吩(PMT)复合膜及ZnO纳米棒与CuSCN复合膜，构建ZnO纳米棒发光二极管、ZnO纳米棒／PMT发光二极管、ZnO纳米棒／CuSCN发光二极管，研究其电致发光性能，探讨了其电致发光的机理。(2)采用电化学方法制备ZnO纳米管膜，探讨ZnO纳米管的形成机理；构建ZnO纳米管发光二极管，研究其电致发光性能及发射白光的特性。获得如下主要的研究结果：一、ZnO纳米棒膜的电化学制备以及ITO／ZnO／Al发光二极管的构建1．优化恒电流阴极还原法在ITO基底上电沉积ZnO纳米棒膜条件，使制备的ZnO纳米棒能均匀地覆盖在ITO基底上，高度较为一致，排列紧密，直径大约为130 nm；ZnO纳米棒锌氧元素的比例接近1：1，具有六方纤锌矿型的晶体结构，在ITO基底上有很好地沿c轴取向生长的优势；在340 nm单色光照射下，控电位O V(vs SCE)时产生阳极光电流，为n型半导体；ZnO纳米棒膜的荧光光谱由强的382 nm处的紫外发光和弱的500～600 nm波段的可见发光构成，紫外发光是ZnO纳米棒的激子复合发光，而可见光谱带主要是由缺陷引起的跃迁发光。构建了ITO／ZnO／Al发光二极管，其启动电压大约为8 V，电致发光谱带形状与ZnO纳米棒膜的荧光光谱相似。二、PMT／ZnO纳米棒复合膜的制备以及ITO／ZnO／PMT／Al发光二极管的构建1．以ZnO纳米棒／ITO为工作电极，在ZnO纳米棒膜上恒电流聚合了p型PMT膜。从光电流谱和荧光光谱图都可以看出当ZnO纳米棒与PMT复合之后，缺陷态有所增加。2．主要采用电化学方法构建ITO／ZnO／PMT／Al发光二极管，其I-V特性曲线图表现出了良好的p-n异质结的整流特性。与ITO／ZnO／Al发光二极管相比，ITO／ZnO／PMT／Al发光二极管的启动电压略小，为7 V，电致发光谱带的形状相似，但是紫外的发光强度增强了约三倍。电致发光增强主要是由于在相同的偏压下ITO／ZnO／PMT／Al发光二极管的电流明显增大，以及所增加的PMT层降低了空穴注入的势垒，较好地平衡了电子和空穴注入的速率所造成的。可见发光的强度随PMT聚合时间的增加而增大，这是由于在PMT的电聚合过程中能使ZnO纳米棒的缺陷态增多引起的。三、CuSCN／ZnO纳米棒复合膜的制备以及ITO／ZnO／CuSCN／Au发光二极管的构建1．以ZnO纳米棒／ITO为工作电极，在ZnO纳米棒膜上用恒电位阴极还原法电沉积了均匀、致密的p型β-CuSCN薄膜，制备了CuSCN／ZnO纳米棒复合膜。复合膜中ZnO的含量大约为CuSCN的7．9倍，Cu与SCN的比例大约为1．0。CuSCN／ZnO纳米棒复合膜的瞬态光电流谱出现了尖锐的前后峰，说明ZnO纳米棒在电沉积了CuSCN后，表面态增多。复合膜的荧光光谱也进一步证明了这一点。2．主要采用电化学方法构建ITO／ZnO／CuSCN／Au发光二极管，其I-V特性曲线图说明n型ZnO纳米棒与p型CuSCN之间形成了p-n异质结。ITO／ZnO／CuSCN／Au发光二极管在较低的正向偏压(7 V)下开始检测到电致发光信号，电致发光谱带覆盖了从350 nm到600 nm的波段，主要是由400nm附近弱的紫外峰和530 nm附近强的可见发光构成，与ITO／ZnO／Au发光二极管相比有较大的差别，这是由于CuSCN的电沉积使ZnO纳米棒的表面态增多引起的。电致发光强度与ITO／ZnO／Au发光二极管相比明显增强，原因主要是在相同的偏压下ITO／ZnO／CuSCN／Au发光二极管的电流显著增大，以及CuSCN作为良好的空穴传输材料，较好地平衡了发光二极管中电子和空穴的注入速率。四、ZnO纳米管膜的电化学制备以及ITO／ZnO纳米管／Au发光二极管的构建1．以ZnO纳米棒／ITO为工作电极，采用了三种不同的阴极电流1．35μA／cm~2、1．65μA／cm~2、2．00μA／cm~2刻蚀制备ZnO纳米管。ZnO纳米管的形成与自身的晶体结构特点密切相关。在刻蚀过程中虽然ZnO纳米管壁受到部分的溶解，但是由于极性(001)面的亚稳定性和乙二胺(EDA)分子对非极性的侧面的保护作用，OH-更容易吸附在(001)面反应，使得ZnO纳米棒在中心的刻蚀速率远远大于边缘的刻蚀速率，最终导致了ZnO纳米管的形成。2．制备的ZnO纳米管样品为纤锌矿型，直径大约为200～300 nm，管壁大约为10～20 nm厚，长度大约为1～1．5μm。ZnO纳米管的上半部分为纳米管结构，而底部还保留了纳米棒的形态。拉曼光谱、紫外-可见吸收光谱、光电流谱、荧光光谱都表明，当ZnO纳米棒刻蚀成纳米管的过程中，缺陷都明显增多。ZnO纳米管的荧光光谱图中，400 nm处的激子发光较ZnO纳米棒明显减弱，而在500～600 nm的可见发光却大大加强。3．主要采用电化学方法构建了ITO／ZnO纳米管／Au发光二极管。二极管启动电压7 V；在较低的偏压时，ZnO纳米管的电致发光谱图与其荧光光谱图相似；当进一步增大偏压时，ZnO纳米管的可见发光波段逐渐拓宽，在约12 V时，肉眼可以观测到ZnO纳米管发射白光；在20 V时，ZnO纳米管的电致发光强度达到最大，发光谱带几乎覆盖了整个可见光波段。4．在相同的偏压下，ITO／ZnO纳米管／Au发光二极管的电流明显大于ITO／ZnO纳米棒／Au发光二极管，这也是ITO／ZnO纳米管／Au发光二极管具有较强的电致发光强度的原因。ZnO纳米棒在刻蚀成纳米管的过程中，比表面增大，产生了许多表面态和体相缺陷，而且这些缺陷的能级分布较宽，在较高的正向偏压下，更多有效辐射的复合中心被激发，导致ZnO纳米管发光二极管的电致发光谱带变宽，能够发射白光。更多还原

【Abstract】 Light-emitting diodes constructed by one-dimensionalnanostructural ZnO and its composite materialsZinc oxide(ZnO),as an important wide-band-gap semiconductor,hasbeen widely investigated for applications in light-emitting diodes(LEDs),photovoltaic cells and lasers,because of its excellent photoelectricproperties.Since the first report of ultraviolet lasing from ZnOnanowires,one-dimensional ZnO nanostructures such as nanorods,nanotubes,nanobelts etc.,have been especially focused on for theirpotential uses in nano devices and optoelectronics.LEDs have the potential to become the primary lighting method,replacing conventional light sources(incandescent light bulbs andfluorescent lamp) due to their low energy consumption and high efficiency.ZnO is the most promising optoelectronic material to be commercializedafter GaN.How to exploit one-dimensional nanostructural ZnO better forLED applications has become a popular topic.This dissertation focuses on improving the luminous efficiency of ZnOLED and tuning the electroluminescent(EL) spectrum of one-dimensionalnanostructural ZnO via electrochemical approaches.The work can bedivided into two parts:(1)Electrochemical preparation andcharacterization of ZnO nanorods、ZnO nanorods/poly(3-methylthiophene)(PMT) composite film、ZnO nanorods/CuSCN composite film.Fabrication ofZnO nanorod LED、ZnO nanorod/PMT LED、ZnO nanorod/CuSCN LED.Their ELmechanisms are also studied.(2)Electrochemical preparation andcharacterization of ZnO nanotubes.Fabrication of ZnO nanotube LEDs.Boththe growth and white light EL mechanisms of ZnO nanotubes are discussed.The following text is the details:(A)Electrochemical preparation of ZnO nanorods and fabrication of ZnOnanorod LED1.We electrodeposited ZnO nanorods galvanostatically on ITO substrates.During the electrodeposition process,the crystal structuralcharacteristics of ZnO play an important part in the formation of ZnOnanorod.The hexagonal ZnO nanorods can cover the entire ITO substrateuniformly with an average diameter of about 130 nm.The atomic ratioof zinc to oxygen of as-grown ZnO nanorods is about l:l.The ZnOnanorods are single crystals of wurtzite structure and orientedpreferably along c axis.When exposed to UV illumination at 340 nm undera O V bias(vs SCE),ZnO nanorods exhibit anode photocurrent,indicating that ZnO nanorods are n type semiconductor.Thephotoluminescence(PL) spectrum of ZnO nanorods is made up of two parts,strong UV peak and weak visible emission.The UV emission is centeredat 382 nm,related to exciton combination.The weak visible broad-bandluminescence over 500-600 nm region is ascribed to defect-relatedtransitions.2.We fabricated ITO/ZnO/Al LED by electrochemical approaches.Theturn-on voltage is about 8 V.The EL spectrum of ZnO nanorods showsnearly identical features with their PL spectrum.(B) Electrochemical preparation of ZnO nanorods/PMT composite film andfabrication of ZnO nanorod/PMT LED1.We electropolymerized p-PMT film galvanostatically on the surface ofZnO nanorods.Both photocurrent and PL spectra indicate that ZnO nanorods in composite films contain more defects after the growth ofPMT.2.We fabricated ITO/ZnO/PMT/A1 LED by electrochemical approaches.TheI-V curve of ITO/ZnO/PMT/Al LED shows a good rectificationcharacteristic of p-n heterojunction.Compared with ITO/ZnO/Al LED,ITO/ZnO/PMT/A1 LED has a lower turn-on voltage of 7 V.Its EL spectrumshows similar features with ITO/ZnO/M LED.However,the emissionintensity is greatly enhanced and the UV emission of ITO/ZnO/PMT/AlLED is about three times stronger than ITO/ZnO/Al LED.When increasingthe deposition time of PMT,the visible emission can be improved morethan the UV peak.That is because the growth process of PMT can generatemore defects of ZnO nanorods.The EL enhancement phenomenon can beexplained by the quicker growth rate of forward current and thepresence of PMT layer.The PMT layer can lower the barrier of holeinjection and balance the electrons and holes injection rates.(C) Electrochemical preparation of ZnO nanorods/CuSCN composite film andfabrication of ZnO nanorod/CuSCN LED1.We electrodeposited a compact and uniform layer of p typeβ-CuSCNon the surface of ZnO nanorods.In the composite film,the contentof ZnO is about 7.9 times of CuSCN and the molar ratio of Cu to SCNis about 1.0.The transient photocurrents of ZnO nanorods/CuSCNcomposite film exibit sharp peaks,indicating that ZnO nanorods incomposite films contain more surface states after the growth of CuSCN,which is also certified by their PL spectrum.2.We fabricated ITO/ZnO/CuSCN/Au LED by electrochemical approaches.TheI-V curve of the ITO/ZnO/CuSCN/Au LED shows a good rectificationcharacteristic of p-n heterojunction.When applied a forward direct voltage of 7 V,the EL signal of ITO/ZnO/CuSCN/Au LED becomesdetectable.The EL spectrum consists of a weak UV peak centered at400 nm and a strong visible broad-band emission centered at 530 nm,which shows quite different features from the ITO/ZnO/Au LED.Thatis because the growth process of CuSCN can generate more suface statesof ZnO nanorods.The EL enhancement phenomenon can be explained bythe quicker growth rate of forward current and the presence of CuSCNlayer.As a good hole transport material,the CuSCN layer can balancethe electrons and holes injection rates and increase the probabilityof their efficient recombination.(D) Electrochemical preparation of ZnO nanotubes and fabrication of ZnOnanotube LEDs1.The ZnO nanorods were selectively dissolved to form nanotubes byapplying a constant cathodic current density.In our experiments,wechosel.35μA/cm~2、1.65μA/cm~2、2.00μA/cm~2 as the etching cathodiccurrents.The structural characteristics of ZnO play an important partin the formation of ZnO nanotube.Although the tube wall is partiallydissolved,the protective function of the EDA molecules on the lateralsurfaces and the metastability of the(O001) plane make the etchingrate in the [0001] direction much faster than at the lateral surfaces,resulting in the formation of the hollow-structured ZnO nanotubes.2.As-prepared ZnO nanotubes show the hexagonally faceted morphology,maily 200-300 nm in diameter.The wall thickness of ZnO nanotubes isabout 10-20 nm and the length is about 1-1.5μm.The ZnO nanotubescontain sections that are tubular and sections that maintain their rodform.The raman spectra,absorption spectra,transient photocurrentsand PL spectra indicate that ZnO nanotubes contain more surface states and bulk defects,which are generated during the etching process.Compared with the ZnO nanorods,the excitonic emission of ZnO nanotubesis much weaker and centered at 400 nm.Meanwhile,the broad-bandvisible PL of ZnO nanotubes in the range of 500-600 nm,which originatesfrom defects,becomes much stronger.3.We fabricated ITO/ZnO nanotube/Au LEDs by electrochemical approaches.Their turn-on voltages is about 7 V.At low bias values,the EL spectraconsist of an excitonic emission centered at 400 nm and a broadluminescence from 450 to 600 nm.The characteristics of the EL spectraare nearly identical to those of the PL spectra.Further increasingthe bias causes the EL intensity to increase drastically,and thevisible emission region also broadens gradually.A white light ELemission is visible by the naked eyes above a voltage of ca.12 V.Ata high bias value of 20 V,the EL spectrum almost covers the entirevisible spectrum from 400 to 600 nm.4.ZnO nanotube LEDs show a much higher current growth rate than ZnOnanorod LEDs.Due to the large surface area,ZnO nanotube LEDs offera superior total electric current with the same forward bias,whichmight also account for the greatly enhanced EL.The high defect densityof ZnOnanotubes is generated during the etching process.In addition,the generation of surface states is favored by the large specificsurface of ZnO nanotubes.The energy levels of the defects may bedistributed over a relatively wide range.By increasing the appliedvoltage,more radiative recombination centers can be activated,resulting in a white light emission from the ZnO nanotubes.更多还原