MBE法生长ZnO纳米线阵列的结构和光学性能

在氧等离子体辅助的MBE系统中, 以1 nm厚的Au薄膜为催化剂, 基于气?液?固(VLS)机制实现了低温ZnO纳米线阵列在Si(111)衬底表面的生长. 通过场发射扫描电子显微镜(FE-SEM)可以观察到, ZnO纳米线阵列垂直生长在衬底上, 直径为20~30 nm. X射线衍射(XRD)和高分辨透射电镜(HRTEM)结果表明: ZnO纳米线为六方纤锌矿结构, 具有沿c轴方向的择优取向. 光致发光(PL)谱显示在380 nm附近有强烈ZnO本征发射峰, 475~650 nm可见光区域有较强的缺陷导致的发射峰.     

ZnO nanotips and nanorods on carbon nanotube/Si substrates: anomalous p-type like optical properties of undoped ZnO nanotips

ZnO nanotips and nanorods were grown on screen-printed multi-walled carbon nanotube (MWCNT) films via thermal chemical vapor deposition at relative low growth temperatures of 400 and 500?°C. Uniform formation of ZnO nanotips and nanorods occurred on MWCNT-printed Si substrates, but were rarely observed on bare Si substrates at the same growth temperatures. In photoluminescence (PL) measurements, it was found that ZnO nanorods exhibit typical intrinsic optical properties, while ZnO nanotips revealed p-type like luminescence behavior. Acceptor-related emission bands originating from neutral acceptor-bound exciton, free-to-acceptor and donor-acceptor pair transitions are clearly observed in temperature-dependent PL spectra of ZnO nanotips.     

Layer-structured ZnO nanowire arrays with dominant surface- and acceptor-related emissions

We report the growth of uncommon layer-structured ZnO nanowire arrays via metal-organic chemical vapor deposition (MOCVD). The morphology, microstructure, and photoluminescence (PL) of the nanowires are investigated. The nanowires grow along the [0001] direction, with periodic zig-zag edges formed by the {101?1}-type surfaces. The nanowires exhibit unique PL features. The PL spectra at low temperature are dominated by the surface exciton recombination at 3.366 eV and the controversial 3.32 eV emission. For the 3.32 eV emission, transformation from donor-acceptor pair recombination to free electron-to-acceptor transition is observed with increasing temperature. The stacking faults formed in the interface region between the layers are likely responsible for the strong emission around 3.32 eV.     

Structure, photoluminescence and wettability properties of well arrayed ZnO nanowires grown by hydrothermal method

Well aligned ZnO nanowire arrays with high crystal quality were grown on Si substrates at a low temperature (50 degrees C) by hydrothermal method using a pre-formed ZnO seed layer. ZnO seeds were prepared via radio-frequency magnetron sputtering onto Si substrates. The morphologies of the ZnO nanowire arrays were shown by field emission scanning electron microscopy. X-ray diffraction spectra showed that the full width at the half maximum of the (0002) peak of the nanowire arrays without any heat treatment was only 0.07 degrees, indicating very high crystal quality. Furthermore, the room-temperature photoluminescence spectra of the ZnO nanowire arrays exhibited excellent UV emission. The special micro/nano surface structure of the ZnO nanowire arrays can enhance the dewettability for surfaces modified via low surface energy materials such as long chain fluorinated organic compounds. The surface of the ZnO nanowire arrays is also found to be superhydrophobic with a contact angle of 165 degrees +/- 1 degrees, while the sliding angle is 3 degrees.     

高压PLD法生长p型钠掺杂氧化锌纳米线阵列

采用高压脉冲激光沉积法(HP-PLD)研究了压强、金催化层厚度对钠掺杂氧化锌纳米线(ZnO:Na)生长的影响, 并制备了ZnO:Al薄膜/ZnO:Na纳米线阵列同质pn结器件。实验发现, 当金膜厚度为4.2 nm, 生长压强为3.33×10⁴ Pa, 生长温度为875℃时, 可在单晶Si衬底上生长c轴取向性良好的ZnO纳米线阵列。X射线衍射和X射线光电子能谱综合分析证实了Na元素成功掺入ZnO纳米线晶格中。在低温(15 K)光致发光谱中, 观测到了一系列由Na掺杂ZnO产生引起的受主光谱指纹特征, 如中性受主束缚激子峰(3.356 eV, A0X)、导带电子到受主峰(3.312 eV, (e, A0))和施主受主对发光峰(3.233 eV, DAP)等。通过在ZnO:Al薄膜上生长ZnO:Na纳米线阵列形成同质结, 测得I-V曲线具有明显的整流特性, 证实了ZnO:Na纳米线具有良好的p型导电性能。     

Fabrication of ZnO nanowire arrays by cycle growth in surfactantless aqueous solution and their applications on dye-sensitized solar cells

Large-scale ZnO nanowire arrays vertically aligned on the substrates were achieved from cycle growth without surfactants. The 8 μm long ZnO nanowire arrays were prepared by 20 cycles. The aspect ratio of ZnO nanowire can be increased with increasing the growth cycle. As displayed by microstructures and photoluminescence (PL) analysis, the ZnO nanowire was good single crystal and the defects in the as-prepared ZnO nanowire arrays were controlled at a low concentration. By increasing the length and aspect ratio of ZnO nanowire, the performances of dye-sensitized solar cells based on the ZnO nanowire arrays were improved. As-prepared ZnO nanowire arrays have potential applications in fabricating next generation nanodevices.     

Exciton scattering mechanism in a single semiconducting MgZnO nanorod

Excitonic phenomena, such as excitonic absorption and emission, have been used in many photonic and optoelectronic semiconductor device applications. As the sizes of these nanoscale materials have approached to exciton diffusion lengths in semiconductors, a fundamental understanding of exciton transport in semiconductors has become imperative. We present exciton transport in a single MgZnO nanorod in the spatiotemporal regime with several nanometer-scale spatial resolution and several tens of picosecond temporal resolution. This study was performed using temperature-dependent cathodoluminescence and time-resolved photoluminescence spectroscopies. The exciton diffusion length in the MgZnO nanorod decreased from 100 to 70 nm with increasing temperature in the range of 5 and 80 K. The results obtained for the temperature dependence of exciton diffusion length and luminescence lifetime revealed that the dominant exciton scattering mechanism in MgZnO nanorod is exciton-phonon assisted piezoelectric field scattering.     

Enhanced surface-excitonic emission in ZnO/Al(2)O(3) core-shell nanowires

We report the influence of an Al(2)O(3) shell on the photoluminescence emission of ZnO nanowires. At room temperature, the spectrum of the core-shell nanowires shows a strong reduction of the relative intensity of the green defect emission with respect to the near-band-edge emission. At 5?K an increase of the relative intensity of the surface exciton band with respect to the donor-bound exciton emission is observed. Annealing the core-shell nanowires at 500?°C does not increase the green defect luminescence at 5?K. We propose a model explaining the spectral changes.     

Nature of sub-band gap luminescent eigenmodes in a ZnO nanowire

The emission spectrum of individual high-quality ZnO nanowires consists of a series of Fabry-Pérot-like eigenmodes that extend far below the band gap of ZnO. Spatially resolved luminescence spectroscopy shows that light is emitted predominantly at both wire ends, with identical spectra reflecting standing wave polariton eigenmodes. The intensity of the modes increases supralinearly with the excitation intensity, indicating that the mode population is governed by scattering among polaritons. Due to strong light-matter interaction, light emission from a ZnO nanowire is not dictated by the electronic band diagram of ZnO but depends also on the wire geometry and the excitation intensity. Delocalized polaritons provide a natural explanation for the pronounced subwavelength guiding in ZnO wires that has been reported previously.     

Direct synthesis of ZnO nanowire arrays on Zn foil by a simple thermal evaporation process

ZnO nanowire arrays were synthesized on zinc foil by a simple thermal evaporation process at relatively low temperature. Morphology and size controlled synthesis of the ZnO nanostructures was achieved by variation of the synthesis temperature, reaction time and the surface roughness of the substrate. A gas-solid and self-catalytic liquid-solid mechanism is proposed for the growth of nanowires at different temperatures. High-resolution transmission electron microscopy (HRTEM) showed that the as-grown nanowires were of single crystal hexagonal wurtzite structure, growing along the [101] direction. Photoluminescence exhibited strong UV emission at ～382?nm and a broad green emission at ～513?nm with 325?nm excitation. Raman spectroscopy revealed a phonon confinement effect when compared with results from bulk ZnO. The nanowire arrays also exhibited a field emission property.     

反应源温度对ZnO纳米线阵列定向性及发光性能的影响

以Au薄膜为催化剂、ZnO与碳混合粉末为反应源,采用碳热还原法在单晶Si衬底上制备了ZnO纳米线阵列.通过扫描电子显微镜( SEM)、X射线衍射仪(XRD)、荧光分光光度计对样品的表征,研究了反应源温度对ZnO纳米线阵列的定向性和光致发光性能的影响.样品在源温度920℃条件下沿(002)方向择优生长,定向性最好,温度过低不利于ZnO纳米线阵列密集生长,而温度过高导致Zn原子二次蒸发,因而也不利于纳米线阵列的定向和择优生长;样品在源温度880℃有最强的近紫外带边发射,表明温度过高和过低都不利于ZnO晶体结构的优化;由于ZnO纳米线在缺氧氛围下生长,氧空位是缺陷存在的主要形式,因此所有样品都有较强的绿光发射.温度升高导致纳米线生长速度提高而增加了氧空位缺陷数量,从而使样品绿峰强度增强并在源温度920℃时达最大值,但温度的进一步升高可导致ZnO纳米线表面Zn元素的蒸发而降低氧空位缺陷的数量,从而抑制绿峰强度.     

Ordered ZnO/AZO/PAM nanowire arrays prepared by seed-layer-assisted electrochemical deposition

An Al-doped ZnO (AZO) seed layer is prepared on the back side of a porous alumina membrane (PAM) substrate by spin coating followed by annealing in a vacuum at 400 °C. Zinc oxide in ordered arrays mediated by a high aspect ratio and an ordered pore array of AZO/PAM is synthesized. The ZnO nanowire array is prepared via a 3-electrode electrochemical deposition process using ZnSO₄ and H₂O₂ solutions at a potential of − 1 V (versus saturated calomel electrode) and temperatures of 65 and 80 °C. The microstructure and chemical composition of the AZO seed layer and ZnO/AZO/PAM nanowire arrays are characterized by field emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HR-TEM), and energy-dispersive X-ray spectroscopy (EDS). Results indicate that the ZnO/AZO/PAM nanowire arrays were assembled in the nanochannel of the porous alumina template with diameters of 110–140 nm. The crystallinity of the ZnO nanowires depends on the AZO seed layer during the annealing process. The nucleation and growth process of ZnO/AZO/PAM nanowires are interpreted by the seed-layer-assisted growth mechanism.     

Comparative study of structural and optical properties of ZnO nanostructures prepared by three different aqueous solution methods

In the present work, zinc oxide (ZnO) nanoparticles with different morphologies and sizes were successfully synthesized via three different aqueous solution routes named proteic sol–gel, PVA-assisted sol–gel and microwave-assisted hydrothermal method. Sol–gel samples were crystallized into hexagonal structure after calcination at 350 °C, presenting uniform growth and predominantly spherical particles. On the other hand, the sample produced via hydrothermal method assumed nanorod morphology, probably due to the adsorption of ammonium on the surface of ZnO nuclei, which affect the growth orientation of the crystals. All the samples exhibited a sharp UV emission peak, related to excitonic recombination, and a broad emission band in the visible region, attributed to internal transitions in color centers. Sol–gel samples calcined at the lowest temperatures presented an UV emission intensity that was 44 and 89 times higher than the visible emission, which can be related to the passivation of the defects by hydrogen ions. As-prepared hydrothermal sample presented a broad emission band centered at approximately 596 nm, which is possibly related to OH groups adsorbed on the particle surfaces. Nevertheless, the emission band of samples calcined above 800 °C was shifted to 540 nm, which is probably related to oxygen vacancy according to the results from chemical analysis.     

Excitonic luminescence in ZnO nanopowders and ceramics

Fast photoluminescence spectra in the spectral region of 3.1–3.45 eV in ZnO and ZnO:Al ceramics were studied at 14 and 300 K. Ceramics with grains smaller than 100 nm were sintered from nanopowders by high pressure (8 GPa) and low temperature (350 °C). Ceramics with grain sizes 1–5 μm were sintered at 1400 °C. It is shown that excitonic luminescence spectra depend on the ceramics grain size, post preparing annealing and doping. The excitonic luminescence decay time was faster than 2 ns and the afterglow at 30 ns was 0.05%.     

Growth of high quality ZnO nanowires on graphene

We report growth of the ZnO nanowires on graphene/SiO2/Si substrates using a chemical vapor deposition method. The length of nanowires varies from 1 microm to 10 microm with increasing the growth time from 30 min to 90 min. X-ray diffraction and high-resolution transmission electron microscopy investigations predict the high structural quality of the c-axis grown single crystalline ZnO nanowires. Temperature dependent photoluminescence spectra from the nanowires reveal excellent optical quality and excitonic behavior in the single crystalline ZnO nanowires. A well-resolved free exciton emission at 3.375 eV, indicates high crystalline quality nanowires and a strong PL peak at 3.370 eV is assigned to neutral-donor bound excitons (D0X).     

Optical emission and absorption spectra of Zn–ZnO core-shell nanostructures

The core-shell Zn–ZnO nanostructures were fabricated from Zn-powder embedded in graphite (i.e. carbon matrix) in a thin-films form by an inexpensive vacuum arc technique followed by laser ablation. The grazing incidence X-ray diffraction pattern shows that intensity of Zn-peak decreases, and subtle ZnO-peak increasing with the increase in laser power. The high resolution transmission electron microscopic study clearly exhibits the formation of a core-shell nanostructure as fabricated by laser ablation. The emission characteristics of laser ablated (with different powers) samples show a strong exciton peak at 388 nm, and a few more weak peaks (due to weak defect states in the visible range). The optical absorption spectra were obtained from the excitonic peaks (from 344 nm to 317 nm) on decreasing laser power. These peaks occur due to the coupling of exciton absorption (from ZnO shell layer) and core metal interband absorption. The Zn–ZnO core-shell nanostructure is useful for nanophotonic applications.     

Structure and Optical Behavior of Nanocomposite Hybrid Films of Well Monodispersed ZnO Nanoparticles into Poly （vinylpyrrolidone）

We fabricated an inorganic-polymeric photoluminescent thin film based on ZnO nanoparticles, which were grown directly in the poly（vinylpyrrolidone） （PVP） matrix. The microstructure, composition, thermal stability, and the temperature-dependent photoluminescence of the thin film were investigated. X-ray diffraction （XRD） and transmission electron microscopy （TEM） results indicated that all the ZnO nanoparticles with a polycrys talline hexagonal wurzite structure were well separated from each other and were dispersed in the polymeric matrix homogeneously and randomly. Raman spectrum （Raman） showed a typical resonant multi-phonon process within the hybrid thin film. The shifts of infrared bands for PVP in the hybrid film should be attributed to strong coulombic interaction between ZnO and polymeric matrix. The stability of the hybrid film and the effect of the perturbation of ZnO on the stability were determined by means of the thermogravimetric analysis （TG） and differential thermal analysis （DTA）. The ultraviolet-visible adsorption （UV-vis） showed distinct excitonic features. The photoluminescent spectrum （PL） of the ZnO nanoparticles modified by PVP molecules showed markedly enhanced ultraviolet emission and significantly reduced green emission, which was due to the Perfect surface passivation of ZnO nanoparticles. Temperature dependent photoluminescent spectrum studies suggested that the ultraviolet emission was associated with bound exciton recombination.     

Fluorescence spectroscopy of electrochemically self-assembled ZnSe and Mn:ZnSe nanowires

We report room temperature fluorescence spectroscopy (FL) studies of ZnSe and Mn-doped ZnSe nanowires of different diameters (10, 25, 50?nm) produced by an electrochemical self-assembly technique. All samples exhibit increasing blue-shift in the band edge fluorescence with decreasing wire diameter because of quantum confinement. The 10?nm ZnSe nanowires show four distinct emission peaks due to band-to-band recombination, exciton recombination, recombination via surface states and via band gap (trap) states. The exciton binding energy in these nanowires exhibits a giant increase (～10-fold) over the bulk value due to quantum confinement, since the effective wire radius (taking into account side depletion) is smaller than the exciton Bohr radius in bulk ZnSe. The 25 and 50?nm diameter wires show only a single FL peak due to band-to-band electron-hole recombination. In the case of Mn-doped ZnSe nanowires, the band edge luminescence in 10?nm samples is significantly quenched by Mn doping but not the exciton luminescence, which remains relatively unaffected. We observe additional features due to Mn(2+) ions. The spectra also reveal that the emission from Mn(2+) states increases in intensity and is progressively red-shifted with increasing Mn concentration.     

Physical mechanism of blue-shift of UV luminescence of a single pencil-like ZnO nanowire

Cathodoluminescence spectroscopy is used to address the ultraviolet (UV) luminescence of a single pencil-like ZnO nanowire whose diameter gradually reduces from bottom to top in the range of 700-50 nm. It is found that the UV emission energy evidently shifts to the high energy with the ZnO nanowire's diameter decreasing and the blue-shift of 90 meV is observed when the nanowire diameter reduces to 50 from 700 nm. The physical mechanism of the UV blue-shift of the ZnO nanowire is attributed to the Burstein-Moss effect under the high carrier concentration.     

ZnO纳米线的制备及场发射特性的研究进展

从ZnO纳米线的生长机制出发,重点讨论了催化剂在制备过程中的作用,比较了采用VLS和VS不同机制生长ZnO纳米线的优缺点,并结合二者特点发现采用金属自催化将是制备高质量ZnO纳米线阵列的一种有效方法.分析了几种有利于提高其场发射性能的后处理方法,经过适当的后处理ZnO纳米线晶体的结构将更加完善,场发射开启场、阈值场将进一步降低,电流密度和场增强因子也将随之大大提高.