Fast synthesis of ZnO nanostructures by laser-induced chemical liquid deposition

ZnO nanostructures were obtained by directly irradiating a small volume of a solution of precursor on a fused-quartz substrate using an unfocused continuous wave CO₂ laser for 2-30 s at laser powers ranging from 20 to 40 W. The laser-based thermochemistry approach allows rapid non-isothermal heating and convection enhanced mass transport which opens new growth mechanisms for the rapid deposition of nanomaterials at predetermined locations on a substrate. The deposits consist of a variety of ZnO nanostructure morphologies, including aggregated nanoparticles, nanorods, faceted nanocrystals and nanowires. The samples were characterized by scanning and transmission electron microscopy, X-ray diffraction and photoluminescence spectroscopy. They were found to exhibit an intense room-temperature photoluminescence, which is characterized by the presence of a strong UV peak around 390 nm and no visible emission. The relationship between the PL signal characteristics and specific ZnO nanostructures was investigated in order to point up optimal nanostructures for possible luminescent devices.     

Strong violet luminescence from ZnO nanocrystals grown by the low-temperature chemical solution deposition

The luminescence properties of zinc oxide (ZnO) nanocrystals grown from solution are reported. The ZnO nanocrystals were characterized by scanning electron microscopy, X-ray diffraction, cathodo- and photoluminescence (PL) spectroscopy. The ZnO nanocrystals have the same regular cone form with the average sizes of 100-500 nm. Apart from the near-band-edge emission around 381 nm and a weak yellow-orange band around 560-580 nm at 300 K, the PL spectra of the as-prepared ZnO nanocrystals under high-power laser excitation also showed a strong defect-induced violet emission peak in the range of 400 nm. The violet band intensity exhibits superlinear excitation power dependence while the UV emission intensity is saturated at high excitation laser power. With temperature raising the violet peak redshifts and its intensity increases displaying unconventional negative thermal quenching behavior, whereas intensity of the UV and yellow-orange bands decreases. The origin of the observed emission bands is discussed.     

Effect of mesh patterning with UV pulsed-laser on optical and electrical properties of ZnO/Ag-Ti thin films

In this study, the ZnO/Ag-Ti structure for transparence conducting oxide (TCO) is investigated by optimizing the thickness of the Ag-Ti alloy and ZnO layers. The Ag-Ti thin film is deposited by DC magnetron sputtering and its thicknesses is well controlled. The ZnO thin film is prepared by sol-gel method using zinc acetate as cation source, 2-methoxiethanol as solvent and monoethanolamine as solution stabilizer. The ZnO film deposition is performed by spin-coating technique and dried at 150 °C on Corning 1737 glass. Due to the conductivity of ZnO/Ag-Ti is dominated by Ag-Ti, the sheet resistance of ZnO/Ag-Ti decrease dramatically as the thickness of Ag-Ti layer increases. However, the transmittances of ZnO/Ag-Ti become unacceptable for TCO application after the thickness of Ag-Ti layer beyond 6 nm. The as-deposited ZnO/Ag-Ti structure has the optical transmittance of 83% @ 500 nm and the low resistivity of 1.2 × 10⁻⁵ Ω-cm. Furthermore, for improving the optical and electrical properties of ZnO/Ag-Ti, the thermal treatment using laser is adopted. Experimental results indicate that the transmittance of ZnO/Ag-Ti is improved from 83% to 89% @ 500 nm with resistivity of 1.02 × 10⁻⁵ Ω-cm after laser drilling. The optical spectrum, the resistance, and the morphology of the ZnO/Ag-Ti will be reported in the study.     

Effect of Co-doping content on hydrothermal derived ZnO array films

Cobalt doped ZnO films are synthesised using a hydrothermal process. The effect of Co²⁺ concentration on morphology, phase composition, crystallisation and spectroscopic characteristics of ZnO films is investigated. The results indicate that both the structure and morphology of the ZnO films evolve with the concentration of cobalt ions incorporated into the lattice. In the presence of a small amount of Co²⁺ ions, films are formed that comprise hexagonal ZnO nanorods, oriented with the c-axis perpendicular to the substrate. With increasing amount of Co²⁺, cracks in the ZnO nanorods can be observed and growth in the [0 0 1] direction is significantly inhibited. When the Co²⁺ concentration exceeds 0.010 M, ZnO rods with the typical hexagonal structure are no longer observed and instead, ZnO films comprising close-packed grains with an irregular polygonal structure are formed. The epitaxial growth of ZnO films is nearly completely inhibited when the concentration of Co²⁺ is increased above 0.050 M. This behaviour can be explained by the selective adsorption of the organic substances in the solution onto the (0 0 1) ZnO crystal face, thus inhibiting growth in the [0 0 1] direction and disrupting the crystallisation of ZnO films. Increasing the Co content deteriorates the crystallisation of ZnO rods and increases tensile stresses present in the ZnO films.     

Effect of source temperature on the morphology and photoluminescence properties of ZnO nanostructures

ZnO nanostructures have been synthesized by heating a mixture of ZnO/graphite powders using the thermal evaporation and vapor transport on Si(1 0 0) substrates without any catalyst and at atmospheric argon pressure. The influence of the source temperature on the morphology and luminescence properties of ZnO nanostructures has been investigated. ZnO nanowires, nanoflowres and nanotetrapods have been formed upon the Si(1 0 0) substrates at different source temperatures ranging from 1100 to 1200 °C. Room temperature photoluminescence (PL) spectra showed increase green emission intensity as the source temperature was decreased and ZnO nanowires had the strongest intensity of UV emission compared with other nanostructures. In addition, the growth mechanism of the ZnO nanostructures is discussed based on the reaction conditions.     

Dual mode emission and harmonic generation in ZnO-CaO-Al2O3: Er nano-composite

Er³⁺ doped ZnO-CaO-Al₂O₃ nano-composite phosphor has been synthesized through combustion method and its emission and harmonic generation properties have been studied. The X-ray diffraction and thermal analysis techniques have been used to prove the dual phase (ZnO and CaO-Al₂O₃) nature of the phosphor. The phosphor has shown up-conversion emission on near-infra-red (976 nm) excitation and down-conversion emission on 355 nm excitation in presence of Er³⁺ and thus behaves as a dual mode phosphor. On excitation with 976 nm diode laser, material shows color tunability (calcination of composite material at different temperatures). Formation of ZnO nanocrystals on heat treatment of as-synthesized sample has shown its characteristic emission at 388 nm and also the energy transfer from ZnO to Er³⁺ ions. The low temperature emission measurements have been carried out and the results have been discussed. Phosphor has shown strong second harmonic generation (SHG) at 532 nm on 1064 nm and at 266 nm on 532 nm excitation.     

Sol-gel synthesis of sub-50 nm ZnO nanowires on pulse laser deposited ZnO thin films

A simple synthesis route to high-quality sub-50 nm ZnO nanowires is reported, utilizing ZnO thin films grown by pulse laser deposition (PLD) as seed layers. Depending upon the PLD growth conditions, the surface morphology of the ZnO nanowires on ZnO film was distinctively different whereas the diameters were almost the same. With the increase of the concentration of zinc nitrate/methenamine solution from 0.002 to 0.02 M, the average diameter of the ZnO nanowire increased but remained sub-50 nm. The grown ZnO nanowires showed a high crystallinity with a low defect density confirmed by a sharp photoluminescence spectrum.     

ZnO nanoparticles produced by reactive laser ablation

The paper presents the results of theoretical and experimental researches of the analysis of nanopowder ZnO and ZnO-based structures formation mechanisms by means of pulse laser reactive technology (λ = 1.06 μm, τ = 10⁻⁷ to 10⁻⁵ s). The developed 2D model combines non-stationary heat transfer and fluid motion along with the calculated profile of surface deformation. The characteristics of the dispersive and chemical compositions and structural parameters of the synthesized nanopowder together with the influence of the energy of laser impulse evaporation, its duration and gas pressure in the reaction chamber have been studied using X-ray diffractrometry (XRD), energy-dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM). Particle size distribution analysis of ZnO has shown that the majority of them range from 5 to 60 nm in size. The photoluminescence emission spectra of the initial ZnO nanopowder at room temperature have been identified.     

Growth of ZnO nanorods by aqueous solution method with electrodeposited ZnO seed layers

ZnO nanorod arrays on ZnO-coated seed layers were fabricated by aqueous solution method using zinc nitrate and hexamethylenetetramine at low temperature. The seed layers were coated on ITO substrates by electrochemical deposition technique, and their textures were dominated by controlling the deposition parameters, such as deposition potential and electrolyte concentration. The effects of the electrodeposited seed layers and the growing parameters on the structures and properties of ZnO nanorod arrays were primarily discussed. The orientation and morphology of both the seed layer and successive nanorods were analyzed by using X-ray diffraction (XRD), SEM and TEM. The results show that the seed layer deposited at −700 mV has evenly distributed crystallites and (0 0 2) preferred orientation; the density of resultant nanorods is high and ZnO nanorods stand completely perpendicular onto substrates. Meanwhile, the size of nanorods quite also depends on the growth solution, and the higher concentration of growth solution primary leads to a large diameter of the ZnO nanorods.     

Photoluminescence from ZnO-SiO2 opals with different sphere diameters and thicknesses

We systematically investigated the photoluminescence (PL) and transmittance characteristics of ZnO-SiO₂ opals with varied positions of the stop-band and film thicknesses. An improved ultraviolet (UV) luminescence was observed from ZnO-SiO₂ composites over pure ZnO nanocrystals under 325 nm He-Cd laser excitation at room temperature. The UV PL of ZnO nanocrystals in SiO₂ opals with stop-bands center of 410 nm is sensitive to the thickness of opal films, and the UV PL intensity increases with the film thickness increasing. The PL spectra of ZnO nanocrystals in SiO₂ opals with stop-bands center of 570 nm show a suppression of the weak visible band. The experimental results are discussed based on the scattering and/or absorbance in opal crystals.     

Surface morphology and optical properties of ZnO epilayers grown on Si(1 1 1) by metal organic chemical vapor deposition

Heteroepitaxial ZnO epilayers were grown on Si(1 1 1) substrates using a vertical geometry atmospheric pressure metal organic chemical vapor deposition (AP-MOCVD) system. The growth temperature was varied from 550 °C to 650 °C in steps of 25 °C. The ZnO growth rate and surface morphology were strong functions of the growth temperature and ranged from ∼0.16 μm/h to 1.36 μm/h. The surface morphology of the ZnO films changed from granular to sharp tips as the growth temperature increased. The effect of buffer thickness was also examined, and was found to have a strong effect on the optical properties of the ZnO. An optimized growth condition for ZnO epilayers was found at 625 °C, producing a FWHM in the room temperature photoluminescence (PL) spectrum of 4.5 nm and a preferred growth orientation along the (0 0 2) direction.Transmission electron microscopy images and selected area diffraction patterns showed excellent crystalline quality of both the buffer and ZnO overlayer. When non-optimized growth temperatures were employed, post-growth annealing was found to greatly enhance the ratio of band-edge to deep level emission.     

Particulate assisted growth of ZnO nanorods and microrods by pulsed laser deposition

Zinc oxide (ZnO) thin films were deposited on the gallium nitride (GaN) and sapphire (Al₂O₃) substrates by pulsed laser deposition (PLD) without using any metal catalyst. The experiment was carried out at three different laser wavelengths of Nd:YAG laser (λ = 1064 nm, λ = 532 nm) and KrF excimer laser (λ = 248 nm). The ZnO films grown at λ = 532 nm revealed the presence of ZnO nanorods and microrods. The diameter of the rods varies from 250 nm to 2 μm and the length varies between 9 and 22 μm. The scanning electron microscopy (SEM) images of the rods revealed the absence of frozen balls at the tip of the ZnO rods. The growth of ZnO rods has been explained by vapor-solid (V-S) mechanism. The origin of growth of ZnO rods has been attributed to the ejection of micrometric and sub-micrometric sized particulates from the ZnO target. The ZnO films grown at λ = 1064 nm and λ = 248 nm do not show the rod like morphology. X-ray photoelectron spectroscopy (XPS) has not shown the presence of any impurity except zinc and oxygen.     

ZnO nanoparticles produced by novel reactive physical deposition process

This paper reports the deposition of ZnO nanoparticles with controlled sizes and different particle densities and their structural, composition and optical properties. They were deposited by means of a DC magnetron based vacuum nanoparticle source onto different substrates (GaAs, Si and Ti/SiO₂/Si). We believe that this is the first time that such nanoparticles have been produced using this unique technique. Zinc was used as sputtering target to produce zinc nanoparticles which were oxidized in-line using molecular oxygen. The structural properties and chemistry of the ZnO were studied by transmission electron microscopy. An average particle size of 6(±2) nm was produced with uniform size distribution. The particle density was controlled using a quartz crystal monitor. Surface densities of 2.3 × 10¹¹/cm², 1.1 × 10¹³/cm² and 3.9 × 10¹³/cm² were measured for three different deposition runs. The ZnO particles were found to be single crystalline having hexagonal structure. Photoluminescence measurements of all samples were performed at room temperature using a cw He-Cd laser at 325 nm excitation. The UV emission around 375 nm at room temperature is due to excitonic recombination and the broad emission centered at 520 nm may be attributed to intrinsic point defects such as oxygen interstitials.     

Fabrication of p-type Li-doped ZnO films by pulsed laser deposition

p-Type ZnO thin films have been realized via doping Li as acceptor by using pulsed laser deposition. In our experiment, Li₂CO₃ was used as Li precursor, and the growth temperature was varied from 400 to 600 °C in pure O₂ ambient. The Li-doped ZnO film prepared at 450 °C possessed the lowest resistivity of 34 Ω cm with a Hall mobility of 0.134 cm² V⁻¹ s⁻¹ and hole concentration of 1.37 × 10¹⁸ cm⁻³. X-ray diffraction (XRD) measurements showed that the Li-doped ZnO films grown at different substrate temperatures were of completely (0 0 2)-preferred orientation.     

Excimer laser processing of ZnO thin films prepared by the sol-gel process

ZnO thin films were prepared on soda-lime glass from a single spin-coating deposition of a sol-gel prepared with anhydrous zinc acetate [Zn(C₂H₃O₂)₂], monoethanolamine [H₂NC₂H₄OH] and isopropanol. The deposited films were dried at 50 and 300 °C. X-ray analysis showed that the films were amorphous. Laser annealing was performed using an excimer laser. The laser pulse repetition rate was 25 Hz with a pulse energy of 5.9 mJ, giving a fluence of 225 mJ cm⁻² on the ZnO film. Typically, five laser pulses per unit area of the film were used. After laser processing, the hexagonal wurtzite phase of zinc oxide was observed from X-ray diffraction pattern analysis. The thin films had a transparency of greater than 70% in the visible region. The optical band-gap energy was 3.454 eV. Scanning electron microscopy and profilometry analysis highlighted the change in morphology that occurred as a result of laser processing. This comparative study shows that our sol-gel processing route differs significantly from ZnO sol-gel films prepared by conventional furnace annealing which requires temperatures above 450 °C for the formation of crystalline ZnO.     

Shape alterations of ZnO nanocrystal arrays fabricated from NH3·H2O solutions

Well-aligned crystalline ZnO nanorod arrays were synthesized via an aqueous solution route with ammonia and zinc nitrate as inorganic precursors. ZnO crystalline seed films were firstly coated on ITO substrates for epitaxial growth of rods through sol-gel processing and heat treatment. SEM, TEM, SAED and XRD were utilized to characterize morphologies and structures of ZnO crystals. Heterogeneous nucleation is crucial for rod growth. A broad scope of pH favorable for heterogeneous nucleation was disclosed at zinc concentration from 0.04 to 0.1 M in the inorganic system due to the complex reaction of ammonia with Zn²⁺. Elevation of initial zinc concentration or pH promoted growth rate of rods and enlarged rod size. ZnO nanorods were transformed to nanotubes, nanosheets and rods with blanket-like shaped surface mainly by secondary pH adjustment. All ZnO nanocrystals are wurtzite structure preferentially oriented in c-axis direction.     

Morphology-dependent upconversion luminescence of ZnO:Er nanocrystals

Morphology impact on the upconverted luminescence of ZnO:Er³⁺ nanocrystals was studied with controllable morphology of nanorod, prickly sphere-like, column-like, branch rod, prism-like, and grain-like, prepared via the cetyltrimethylammonium bromide (CTAB)-assisted hydrothermal process. The upconversion emission of Er³⁺ with 980 nm excitation demonstrated morphology sensitivity which was related with the local environments of Er³⁺ ions in ZnO and doping efficiency. Under ultraviolet (UV) direct excitation, where exciton and defect emissions of ZnO appeared, morphology sensitivity was discussed in terms of surface-to-volume ratios.     

Cathodoluminescent and nonlinear optical properties of undoped and erbium doped nanostructured ZnO films deposited by spray pyrolysis

We have deposited zinc oxide (ZnO) and erbium doped zinc oxide (ZnO:Er) thin films on heated glass substrates using spray pyrolysis technique. The effect of erbium dopant on structural, morphological, luminescent and nonlinear optical properties was studied. The deposited films have been analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM), ex situ compositional analysis (ESCA), profilometry, cathodoluminescence (CL) and third harmonic generation (THG) measurements. All films were polycrystalline, having a preferential growth orientation along the ZnO (0 0 2) plane, with a corresponding average crystallite size of less than 41 nm. Addition of erbium can effectively control the film surface morphology and its cathodoluminescent properties. The films containing low erbium concentration show a uniform surface covered with hexagonal shaped grains and a strong UV light emission intensity as well as TH response. In contrast, when the erbium doping ratio exceeds 3%, a porous surface with columnar textural growth becomes more pronounced, and a substantial reduction of the cathodoluminescent and TH response. A strong TH signal was obtained for the film with good crystalline quality at the concentration of 2%. Third order nonlinear optical susceptibility (χ^〈3〉) values of the studied materials were in the remarkable range of 10⁻¹² esu.     

Substrate temperature influence on the properties of nanostructured ZnO transparent ultrathin films grown by PLD

Zinc oxide films of 40 nm thickness have been deposited on glass substrates by pulsed laser deposition using an excimer XeCl laser (308 nm) at different substrate temperatures ranging from room temperature to 650 °C. Surface investigations carried out by using atomic force microscopy have shown a strong influence of temperature on the films surface topography. UV-VIS transmittance measurements have shown that our ZnO films are highly transparent in the visible wavelength region, having an average transmittance of ∼90%. The optical band gap of the films was found to be 3.26 eV, which is lower than the theoretical value of 3.37 eV. Besides the normal absorption edge related to the transition between the valence and the conduction band, an additional absorption band was also recorded in the wavelength region around 364 nm (∼3.4 eV). This additional absorption band may be due to excitonic, impurity, and/or quantum size effects. Photoreduction/oxidation in ozone of the ZnO films lead to larger conductivity changes for higher deposition temperature. In conclusion, the ozone sensing characteristics as well as the optical properties of the ZnO thin films deposited by pulsed laser deposition are strongly influenced by the substrate temperature during growth. The sensitivity of the films towards ozone might be enhanced significantly by the control of the films deposition parameters and surface characteristics.     

Effect of temperature on pulsed laser deposition of ZnO films

ZnO thin films have been deposited on Si(1 1 1) substrates at different substrate temperature by pulsed laser deposition (PLD) of ZnO target in oxygen atmosphere. An Nd:YAG pulsed laser with a wavelength of 1064 nm was used as laser source. The influences of the deposition temperature on the thickness, crystallinity, surface morphology and optical properties of ZnO films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), selected area electron diffraction (SAED), photoluminescence (PL) spectrum and infrared spectrum. The results show that in our experimental conditions, the ZnO thin films deposited at 400 °C have the best surface morphology and crystalline quality. And the PL spectrum with the strongest ultraviolet (UV) peak and blue peak is observed in this condition.