Pyramidal nanostructures of zinc oxide

Zinc oxide (ZnO) nanostructures have been prepared by pulsed laser deposition of the oxide onto Si(100) substrate at 600 degrees C. An examination of the morphology using atomic force microscopy and scanning electron microscopy reveals well formed pyramidal structures consistent with the growth habit of ZnO. A domain matched epitaxy across the interface makes the ZnO pyramids orient along the axes of Si(100) surface. The pyramidal nanostructures signify an intermediate state in the growth of hexagonal nanorods of ZnO. The hardness of the nanostructures as well as their response to oxygen gas have been investigated using nanoindentation and conducting probe methods respectively. ZnO nanostructures are much harder than their bulk. The hardness of ZnO pyramids obtained by nanoindentation is 70 +/- 10 GPa which is about one order more that of bulk ZnO. Besides, the nanostructures exhibit high sensitivity towards oxygen. A 70% increase in the resistance of ZnO nanostructures is observed when exposed to oxygen atmosphere.     

Fabrication and characterization of tetrapod-like ZnO nanostructures prepared by catalyst-free thermal evaporation

Tetrapod-like ZnO nanostructures were fabricated on ZnO-coated sapphire (001) substrates by two steps: pulsed laser deposition (PLD) and catalyst-free thermal evaporation process. First, the ZnO films were pre-deposited on sapphire (001) substrates by PLD. Then the ZnO nanostructures grew on ZnO-coated sapphire (001) substrate by the simple thermal evaporation of the metallic zinc powder at 900 °C in the air without any catalysts. The pre-deposited ZnO films by PLD on the substrates can provide growing sites for the ZnO nanostructures. The as-synthesized ZnO nanostructures were characterized by using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and Fourier transform infrared spectrum (FTIR). The results show that the tetrapod-like ZnO nanostructures are highly crystalline with the wurtzite hexagonal structure. Photoluminescence (PL) spectrum of as-synthesized nanostructures exhibits a UV emission peak at ～ 389 nm and a broad green emission peak at ～ 513 nm. In addition, the growth mechanism of ZnO nanostructures is also briefly discussed.     

Characterization of pulsed laser deposited zinc oxide

The pulsed laser deposition of zinc oxide films (ZnO) has been studied as a function of laser wavelength, and substrate temperature. Optical emission spectroscopy of the laser produced plume was used to characterize the deposition process. The deposited films were characterized by X-ray diffractometry, Auger electron spectroscopy, and scanning electron microscopy. Highly textured (002) ZnO films deposited at substrate temperatures of 300 °C with laser wavelengths of 532 nm and 248 nm. However, the energy fluence of 248 nm radiation controls the degree of texturing, allowing highly textured films to be deposited at room temperature.     

Effect of substrate temperature on (<Emphasis Type="Italic">00l</Emphasis>) oriented growth of ZnO nanostructures on fused quartz substrate by PLD

A simple and novel catalyst free (00l) oriented zinc oxide (ZnO) nano-structures were synthesized on quartz substrate by pulsed laser deposition (PLD). The effects of substrate temperature on structural and optical properties of these nanostructures were investigated using X-ray diffraction (XRD), atomic force microscopy (AFM), photoluminescence (PL) and spectroscopic ellipsometry. XRD showed that the ZnO nanostructures had c-axis oriented hexagonal wurtzite crystal structure. Crystallite sizes were found to increase as substrate temperature increases. An AFM measurement confirms the grain formation and increase in surface roughness at higher substrate temperature. Optical band gap of these ZnO nanostructures was calculated using transmittance spectra in UV–Vis region and found to decrease from 3.24 to 3.21 eV as substrate temperature is increased from 500 to 800?°C. PL spectra show that all the peaks in UV region around 389 nm; 3.18 eV. The decrease in band gap may be attributed to decrease in oxygen vacancies at higher substrate temperature and may be useful for different applications.     

脉冲激光沉积方法生长硅基ZnO薄膜的特性

用脉冲激光沉积法(PLD)在n型硅(111)平面上生长ZnO薄膜.X射线衍射(XRD)在2θ=34°处出现了唯一的衍射峰,半高宽为0.75°;傅里叶红外吸收(FTIR)在414.92cm-1附近出现了对应Zn-O键的红外光谱的特征吸收峰;光致发光(PL)测量发现了位于370和460nm处的室温光致发光峰;扫描电子显微镜(SEM)和选区电子衍射(SAED)显示了薄膜的表面形貌以及晶格结构.利用PLD法制备了具有c轴取向高度一致的六方纤锌矿结构ZnO薄膜.     

Electron field emission characteristics of different surface morphologies of ZnO nanostructures coated on carbon nanotubes

The optimal carbon nanotube (CNT) bundles with a hexagonal arrangement were synthesized using thermal chemical vapor deposition (TCVD). To enhance the electron field emission characteristics of the pristine CNTs, the zinc oxide (ZnO) nanostructures coated on CNT bundles using another TCVD technique. Transmission electron microscopy (TEM) images showed that the ZnO nanostructures were grown onto the CNT surface uniformly, and the surface morphology of ZnO nanostructures varied with the distance between the CNT bundle and the zinc acetate. The results of field emissions showed that the ZnO nanostructures grown onto the CNTs could improve the electron field emission characteristics. The enhancement of field emission characteristics was attributed to the increase of emission sites formed by the nanostructures of ZnO grown onto the CNT surface, and each ZnO nanostructure could be regarded as an individual field emission site. In addition, ZnO-coated CNT bundles exhibited a good emission uniformity and stable current density. These results demonstrated that ZnO-coated CNTs is a promising field emitter material.     

Study of ZnO layers growth by pulsed laser deposition from Zn and ZnO targets

The article deals with structural properties of ZnO thin layers prepared on Si (111) by pulsed laser deposition at different pressures (1-35 Pa) of ambient oxygen in the deposition chamber. The growth temperature was 400 °C and a pulsed Nd:YAG laser was used at a wavelength of 355 nm. Two parallel sets of samples deposited by ablation of different targets (a sintered ceramic pellet of ZnO and a pure metallic Zn target) were examined. The samples were characterized by different analytical methods: scanning electron microscope (SEM), secondary ion mass spectroscopy (SIMS), and X-ray diffraction (XRD). The prepared layers exhibited columnar structure and uniform preferred c-axis orientation. The results showed that deposition of the high quality of ZnO films fabricated from both targets is comparable, except for those obtained at low (1 Pa) pressures.     

Annealing and recrystallization of amorphous ZnO thin films deposited under cryogenic conditions by pulsed laser deposition

This article deals with the annealing of amorphous ZnO thin films prepared by pulsed laser deposition (PLD) under cryogenic conditions. The substrate holder was cooled by liquid nitrogen. X-ray diffraction analysis evidenced that as-deposited films had amorphous structures: analysis by scanning electron microscopy (SEM) revealed their fine grained surface and inner structure. Annealing at temperatures in the range of 200-800 °C resulted in a transition in the thin film crystal structure from amorphous to polycrystalline. Various properties of the ZnO films were found depending on the recrystallization temperature. In depth investigations employing SEM, X-ray diffraction, atomic force microscopy and secondary ion mass spectroscopy provided comparisons of the recrystallizations of undoped ZnO thin films during the phase transition processes from amorphous to hexagonal wurtzite structures.     

Mn-doped ZnO nanocrystals embedded in Al2O3: structural and electrical properties

We report on the structural and electrical properties of Mn-doped ZnO/Al(2)O(3) nanostructures produced by the pulsed laser deposition technique. Grazing incidence small angle x-ray scattering (GISAXS) and Rutherford backscattering spectrometry revealed the multilayered structure in as-deposited samples. Annealing of the nanostructures was shown to promote the formation of nanocrystals embedded in the Al(2)O(3) matrix, as was evidenced by GISAXS and high resolution transmission microscopy. Particle-induced x-ray emission analysis showed a doping of 8 at.% Mn in ZnO. Grazing incidence x-ray diffraction and Raman spectroscopy demonstrated that the nanocrystals have the pure wurtzite ZnMnO crystalline phase. Resonant Raman scattering displayed an increase of intensity of the 1LO mode as well as broadening of the 2LO mode related to the size effect. Capacitance-voltage measurements showed carrier retention with a voltage shift higher than those reported for similar systems.     

A study on electrodeposited zinc oxide nanostructures

Zinc oxide (ZnO) nanostructures prepared by electrochemical deposition method from aqueous zinc nitrate solution at 65 °C onto fluorine doped tin oxide coated glass substrates were investigated. Characterization of ZnO nanostructures was realized using conventional electrochemical techniques, scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques. Cyclic voltammetry experiments were performed to elucidate the electrodic processes that occurred when potentials were applied and the optimum potential for electrodeposition were determined. From the Mott-Schottky measurements, the flat-band potential and the donor density for the ZnO nanostructure are determined. From single-step potential experiment in the potential ranges from ?1.1 to ?1.4 V, the formation of ZnO nuclei in the early deposition stages was proceeded according to the three dimensional (3D) instantaneous nucleation followed by diffusion-limited growth rather than a progressive one. SEM images demonstrated that the morphology of ZnO nanostructures depend greatly on the potential depositions. XRD studies revealed that the deposited films were polycrystalline in nature with wurtzite phase.     

Effect of ammonia water on the morphology of monoethanolamine-assisted sonochemicaly synthesized ZnO nanostructures

In the present work, ZnO nanostructures were synthesized by monoethanolamine (MEA)-assisted ultrasonic method at low temperature. Structural analysis was carried out by X-ray diffraction (XRD) confirmed the formation of hexagonal wurtzite structure of ZnO. The effect of ammonia water on the molecular structure of MEA, and its effect on the morphology of ZnO nanostructures were monitored by electron microscopy. Scanning electron microscopy (SEM) results suggest that ZnO nanoparticles with 100 nm in diameter were produced in case of MEA-assisted ultrasonic method. However, as ammonia water was added into the reaction system the morphology of ZnO nanoparticles changed into nanorods, flower-like nanostructures and finally microrods. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) studies showed that as prepared ZnO nanostructures were single crystalline in nature and grew in different directions resulted in the formation of various structures. The growth mechanism of as prepared ZnO nanostructures was discussed in detail. It was proposed that the addition of ammonia water into the reaction system resulted into the formation of ethylene diamine (EDA) which directed the growth of ZnO. The optical property was studied by photoluminescence (PL) spectroscopy showed only UV emission and no defects mediated visible emission.     

Position-, size-, and shape-controlled highly crystalline ZnO nanostructures

Highly ordered ZnO nanoboxes and nanowire structures with a width of ～ 20 nm have been successfully fabricated by the combination of nanoimprint lithography and pulsed laser deposition utilizing a glancing angle deposition (GLAD) technique. The periodicity, size, and shape of the ZnO nanoboxes and nanobelts can be easily controlled over a large area by changing the molds and deposition conditions. At the initial stage of growth by GLAD, nanonucleation led to nanopillar structures, which agglomerated to form nanobox and nanobelt structures at room temperature (RT). The ZnO nanostructures have a c-axis orientation along the nanopillar direction after postannealing and exhibit an intense cathodoluminescence peak around 380 nm at RT.     

Zinc oxide epitaxial thin film deposited over carbon on various substrate by pulsed laser deposition technique

Zinc Oxide (ZnO) is a promising candidate material for optical and electronic devices due to its direct wide band gap (3.37 eV) and high exciton binding energy (60 meV). For applications in various fields such as light emitting diode (LED) and laser diodes, growth of p-type ZnO is a prerequisite. ZnO is an intrinsically n-type semiconductor. In this paper we report on the synthesis of Zinc Oxide-Carbon (ZnO:C) thin films using pulsed laser deposition technique (PLD). The deposition parameters were optimized to obtain high quality epitaxial ZnO films over a carbon layer. The structural and optical properties were studied by glazing index X-ray diffraction (GIXRD), photoluminescence (PL), optical absorption (OA), and Raman spectroscopy. Rutherford backscattering spectroscopy (RBS), scanning electron microscopy with energy dispersive spectroscopy (SEMEDS) and atomic force microscopy (AFM) were employed to determine the composition and surface morphology of these thin films. The GIXRD pattern of the synthesized films exhibited hexagonal wurtzite crystal structure with a preferred (002) orientation. PL spectroscopy results showed that the emission intensity was maximum at -380 nm at a deposition temperature of 573 K. In the Raman spectra, the E2 phonon frequency around at 438 cm(-1) is a characteristic peak of the wurtzite lattice and could be seen in all samples. Furthermore, the optical direct band gap of ZnO films was found to be in the visible region. The growth of the epitaxial layer is discussed in the light of carbon atoms from the buffer layer. Our work demonstrates that the carbon is a novel dopant in the group of doped ZnO semiconductor materials. The introduction of carbon impurities enhanced the visible emission of red-green luminescence. It is concluded that the carbon impurities promote the zinc related native defect in ZnO.     

The evolution of well-aligned amorphous carbon nanotubes and porous ZnO/C core-shell nanorod arrays for photosensor applications

Well-aligned amorphous carbon nanotube (a-CNT) and porous ZnO/C core-shell nanorod (NR) arrays were fabricated for the first time by a proposed deposition-etching-evaporation (DEE) route. The arrays were prepared by deposition of carbon on the surface of well-aligned ZnO NR arrays by thermal decomposition of acetone followed by spontaneous etching and evaporation of core-ZnO. By utilizing the decomposition of acetone as well as distinct degrees of interaction between intermediate products and ZnO, well-aligned nonporous ZnO/C core-shell NR, porous ZnO/C core-shell NR, and a-CNT arrays were separately prepared by varying the working temperature from 400 to 700?°C. Scanning electron microscopy and high-resolution transmission electron microscopy show that the thickness of carbon shells increases from 3 to 10 nm with the increase in working temperature. Raman spectra demonstrate slight sp(2) bonds of carbon, indicating small graphite regions embedded in amorphous carbon nanoshells. The E(2) peaks of ZnO reduce with the increase in substrate temperature. Photoresponse measurements of ZnO/C NR arrays shows enhancement of both photoresponsivity and response velocity, and the interference of humidity with regard to photosensing is effectively reduced by the capping of carbon nanoshells. The work not only provides an effective route to improve the photosensing of semiconductor nanomaterials for practical applications, but also sheds light on preparing various hollow carbon and porous ZnO/C core-shell nanostructures with distinct morphologies by employing the routes presented in the paper on diverse ZnO nanostructures for optoelectrochemical applications.     

Low-temperature growth of well-aligned ZnO nanorods/nanowires on flexible graphite sheet and their photoluminescence properties

We have grown large-scale well-aligned ZnO nanorods/nanowires on commercial flexible graphite sheet (FGS) at low temperature via chemical vapor deposition method. The products were characterized by X-ray diffraction, scanning electron microscopy, and high-resolution transmission electron microscopy. The effects of the growth temperature and oxygen flow rate on the morphology of ZnO nanostructures have been investigated. The growth mechanism of ZnO is found to be a self-catalytic vapor–solid process assisted by the immiscibility of ZnO with graphite. The as-grown ZnO/FGS products show strong green emission and their photoluminescence properties can be tuned by changing growth condition or annealing treatment.     

Tailor of ZnO morphology by heterogeneous nucleation in the aqueous solution

The umbrella-like ZnO nanostructures have been prepared by the morphological tailoring in the aqueous solution at 95 °C in the addition of heterogeneous seeds such as MnO₂ and CdS nanoparticles. The morphology and structure of as-synthesized umbrella-like ZnO nanostructures have been characterized by transmission electron microscopy (TEM), high-resolution transmission electron microscope (HRTEM), electron energy loss spectroscopy (EELS), field emission scanning electron microscopy (FESEM) and X-ray diffraction (XRD). The heterogeneous seeds play the critical role for the formation of umbrella-like ZnO nanostructures. Furthermore, the formation mechanism of the umbrella-like nanostructures has been phenomenally presented.     

Role of self-assembled gold nanodots in improving the electrical and optical characteristics of zinc oxide films

We have studied the effect of embedding nanocrystalline Au particles on the electrical and optical characteristics of ZnO films. Au-embedded epitaxial ZnO films were deposited on (0001) sapphire substrates with a pulsed laser deposition technique. The crystalline quality of both the ZnO matrix and Au nanoparticles was investigated by X-ray diffraction and transmission electron microscopy. Composite films were characterized by photoluminescence, optical absorption, and low-temperature electrical resistivity measurements. Photoluminescence spectra of theses films showed a sharp excitonic peak at 3.22 +/- 0.05 eV without any signature of green band emission. Electrical resistivity measurements showed these films to be highly conducting, with a room-temperature resistivity of 3.4 +/- 0.2 m omega-cm.     

Spatial fluctuations of optical emission from single ZnO/MgZnO nanowire quantum wells

MgZnO/ZnO quantum wells on top of ZnO nanowires were grown by pulsed laser deposition. Ensembles of spatially fluctuating and narrow cathodoluminescence peaks with single widths down to 1?meV were found at the spectral position of the quantum well emission at 4?K. In addition, the number of these narrow QW peaks increases with increasing excitation power in micro-photoluminescence, thus pointing to quantum-dot-like emission centers. Indeed, laterally strained areas of about 5?nm diameter were identified at the quantum well positions on top of the nanowires by high-resolution transmission electron microscopy.     

Field emission characteristics from tapered ZnO nanostructures grown onto vertically aligned carbon nanotubes

Zinc oxide (ZnO) nanostructures were grown on vertically aligned carbon nanotubes (CNTs) using thermal chemical vapor deposition (CVD) to enhance the field emission characteristics. The shape of ZnO nanostructure was tapered. Scanning electron microscopy (SEM) image showed the ZnO nanostructures were grown onto CNT surface uniformly. The field electron emission of pristine CNTs and ZnO-coated CNTs were measured. The results showed that ZnO nanostructures grown onto CNTs could improve the field emission characteristics. The ZnO-coated CNTs had a threshold electric field at about 3.1 V/μm at 1.0 mA/cm². The results demonstrated that the ZnO-coated CNT is an ideal field emitter candidate material. The stability of the field emission current was also tested.     

Fabrication of ZnO nanorods using metal nanoparticles as growth nuclei

ZnO nanorods were produced by pulsed laser deposition (PLD). Drops of nanoparticle colloid (gold or silver) were placed on silica substrates to form growth nuclei. All nanoparticles were monocrystalline, with well-defined crystal surfaces and a negative electrical charge. The ZnO nanorods were produced in an off-axis PLD configuration at oxygen pressure of 5 Pa. The growth of the nanorods started from the nanoparticles in different directions, as one nanoparticle could become a nucleus for more than one nanorod. The low substrate temperature used indicates the absence of a catalyst during the growth of the nanorods. The diameters of the fabricated 1-D ZnO nanostructures were in the range of 50-120 nm and their length was determined by the deposition time.