Effect of seed layer on the growth of well-aligned ZnO nanowires

We demonstrate that vertical well-aligned crystalline ZnO nanowire arrays were grown on ZnO/glass substrates by a low-temperature solution method. Different thicknesses of ZnO seed layers on glass substrates were prepared by radio-frequency sputtering. In this work it was found that the morphology of ZnO nanowires strongly depends on the thickness of ZnO seed layers. The average diameter of nanowires is increased from 50 to 130 nm and the nanowire density is decreased from 110 to 60 μm⁻² while the seed layer thickness is varied from 20 to 1000 nm. The improved control of the morphology of ZnO nanowire arrays may lead to an enhanced carrier collection of hybrid polymer photovoltaic devices based on ZnO.     

Aligned growth of ZnO nanowires by NAPLD and their optical characterizations

Not only vertically aligned ZnO nanowires but also horizontally aligned ZnO nanowires have been successfully grown on the annealed (0 0 0 1) c-cut and (1 1 2 0) a-cut sapphire substrates, respectively using catalyst-free nanoparticle-assisted pulsed-laser ablation deposition (NAPLD). The as-synthesized ZnO nanowires exhibit an ultraviolet emission at around 390 nm and the absent green emission under room temperature. The single ZnO nanowire was collected in the electrode gap by dielectrophoresis (DEP). Under the optical pumping, the single ZnO nanowire exhibited UV emission at around 390 nm with several sharp peaks whose energy spacings are almost constant, which greatly differs from the broad UV emission of the film with many nanowires, suggesting ZnO nanowires as candidates for laser media. The single ZnO nanowire showed polarized photoluminescence (PL). The as-synthesized ZnO nanowires could find many interesting applications in short-wavelength light-emitting diode (LED), laser diode and gas sensor.     

Auger and photoluminescence analysis of ZnO nanowires grown on AlN thin film

ZnO nanowires were grown on AlN thin film deposited on the glass substrates using a physical vapor deposition method in a conventional tube furnace without introducing any catalysts. The temperature of the substrates was maintained between 500 and 600 °C during the growth process. The typical average diameters of the obtained nanowires on substrate at 600 and 500 °C were about 57 and 22 nm respectively with several micrometers in length. X-ray diffraction and Auger spectroscopy results showed Al diffused from AlN thin film into the ZnO nanowires for the sample grown at 600 °C. Photoluminescence of the nanowires exhibits appearance of two emission bands, one related to ultraviolet emission with a strong peak at 380-382 nm, and the other related to deep level emission with a weak peak at 503-505 nm. The ultraviolet peak of the nanowires grown at 500 °C was blue shifted by 2 nm compared to those grown at 600 °C. This shift could be attributed to surface effect.     

Morphology engineering of ZnO nanostructures

Nanosized ZnO structures were grown by atmospheric pressure metalorganic chemical vapor deposition (APMOCVD) in the temperature range 200–500 °C at variable precursor pressure. Temperature induced evolution of the ZnO microstructure was observed, resulting in regular transformation of the material from conventional polycrystalline layers to hierarchically arranged sheaves of ZnO nanowires. The structures obtained were uniformly planarly located over the substrate and possessed as low nanowires diameter as 30–45 nm at the tips. The observed growth evolution is explained in terms of ZnO crystal planes free energy difference and growth kinetics. For comparison, the convenient growth at constant precursor pressure on Si and SiC substrates has been performed, resulting in island-type grown ZnO nanostructures. The demonstrated nanosized ZnO structures may have unique possible areas of application, which are listed here.     

Catalyst-free synthesis of novel brush-shaped ZnO particles by a simple oxidation in air

Brush-shaped ZnO particles were synthesized by controlling the growth time in the direct melt oxidation process of Al-Zn mixture in air at atmospheric pressure. Particles with two kinds of structures were formed. One was consisted of nanowires grown along [0 0 0 1] direction at the six corners and the center of (0 0 0 1) basal plane on hexagonal ZnO microrod. The other was constructed by nanobelts between the corner-nanowires as well as nanowires at the corners on ZnO microrod. The structural configuration that the nanowires and the nanobelts have a well coherent orientation alignment with the base microrod implies that the brush-shaped ZnO is single crystal. Room temperature PL spectrum of the brush-shaped ZnO particles displayed predominant green emission with a wavelength of 510 nm.     

Low-temperature growth and ethanol-sensing characteristics of quasi-one-dimensional ZnO nanostructures

ZnO nanorods, nanobelts, nanowires, and tetrapod nanowires were synthesized via thermal evaporation of Zn powder at temperatures in the range 550-600 °C under flow of Ar or Ar/O₂ as carrier gas. Uniform ZnO nanowires with diameter 15-25 nm and tetrapod nanowires with diameter 30-50 nm were obtained by strictly controlling the evaporation process. Our experimental results revealed that the concentration of O₂ in the carrier gas was a key factor to control the morphology of ZnO nanostructures. The gas sensors fabricated from quasi-one-dimensional (Q1D) ZnO nanostructures exhibited a good performance. The sensor response to 500 ppm ethanol was up to about 5.3 at the operating temperature 300 °C. Both response and recovery times were less than 20 s. The gas-sensing mechanism of the ZnO nanostructures is also discussed and their potential application is indicated accordingly.     

Synthesis and optical characterization of vertically grown ZnO nanowires in high crystallinity through vapor-liquid-solid growth mechanism

The gas-phase growth and optical characteristics of 1-dimensional ZnO nanostructure have been investigated. The ZnO nanowires (NWs) were grown vertically on Au coated silicon substrates by vapor-liquid-solid (VLS) growth mechanism using chemical vapor deposition (CVD). The ZnO NWs were grown in the crystal direction of [0 0 0 1]. The ZnO NWs exhibit the uniform size of less than 100 nm in diameter and up to 5 μm in length. Photoluminescence (PL) spectrum of ZnO NWs shows the strong band-edge emission at ∼380 nm (∼3.27 eV) without significant deep-level defect emission. The exciton lifetime of ZnO NWs was measured to be approximately 150 ± 10 ps.     

Synthesis of ZnO nanowires by pulsed laser deposition in furnace

ZnO nanowires were fabricated on Au coated (0 0 0 1) sapphire substrates by using a pulsed Nd:YAG laser with a ZnO target in furnace. ZnO nanowires have various sizes and shapes with a different substrate position inside a furnace. The length and the diameter of these ZnO nanowires were around 3-4 μm and 120-200 nm, respectively, confirmed by scanning electron microscopy (SEM). The diameter control of the nanowires was achieved by varying the position of substrates. The ultraviolet emission of nanowires from the near band-edge emission (NBE) was observed at room temperature. The formation mechanism and the effect of different position of substrates on the structural and optical properties of ZnO nanowires are discussed.     

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.     

Thickness influence on surface morphology and ozone sensing properties of nanostructured ZnO transparent thin films grown by PLD

Transparent zinc oxide (ZnO) thin films with a thickness from 10 to 200 nm were prepared by the PLD technique onto silicon and Corning glass substrates at 350 °C, using an Excimer Laser XeCl (308 nm). Surface investigations carried out by atomic force microscopy (AFM) and X-ray diffraction (XRD) revealed a strong influence of thickness on film surface topography. Film roughness (RMS), grain shape and dimensions correlate with film thickness. For the 200 nm thick film, the RMS shows a maximum (13.9 nm) due to the presence of hexagonal shaped nanorods on the surface. XRD measurements proved that the films grown by PLD are c-axis textured. It was demonstrated that the gas sensing characteristics of ZnO films are strongly influenced and may be enhanced significantly by the control of film deposition parameters and surface characteristics, i.e. thickness and RMS, grain shape and dimension.     

The effect of substrate surface roughness on ZnO nanostructures growth

The ZnO nanowires have been synthesized using vapor-liquid-solid (VLS) process on Au catalyst thin film deposited on different substrates including Si(1 0 0), epi-Si(1 0 0), quartz and alumina. The influence of surface roughness of different substrates and two different environments (Ar + H₂ and N₂) on formation of ZnO nanostructures was investigated. According to AFM observations, the degree of surface roughness of the different substrates is an important factor to form Au islands for growing ZnO nanostructures (nanowires and nanobelts) with different diameters and lengths. Si substrate (without epi-taxy layer) was found that is the best substrate among Si (with epi-taxy layer), alumina and quartz, for the growth of ZnO nanowires with the uniformly small diameter. Scanning electron microscopy (SEM) reveals that different nanostructures including nanobelts, nanowires and microplates have been synthesized depending on types of substrates and gas flow. Observation by transmission electron microscopy (TEM) reveals that the nanostructures are grown by VLS mechanism. The field emission properties of ZnO nanowires grown on the Si(1 0 0) substrate, in various vacuum gaps, were characterized in a UHV chamber at room temperature. Field emission (FE) characterization shows that the turn-on field and the field enhancement factor (β) decrease and increases, respectively, when the vacuum gap (d) increase from 100 to 300 μm. The turn-on emission field and the enhancement factor of ZnO nanowires are found 10 V/μm and 1183 at the vacuum gap of 300 μm.     

Catalyst-free growth of well-aligned arsenic-doped ZnO nanowires by chemical vapor deposition method

ZnO nanowires with different arsenic concentration were grown on Si (1 0 0) substrates by chemical vapor deposition method without using catalyst. Zn/GaAs mixed powders were used as Zn and As source, respectively. Oxygen was used as oxidant. The images of scanning electron microscope show that the arsenic-doped ZnO nanowires with preferred c-axial orientation were obtained, which is in well accordance with the X-ray diffraction analysis. The arsenic related acceptor emission was observed in the photoluminescence spectra at 11 K for all arsenic-doped ZnO samples. This method for the preparation of arsenic-doped ZnO nanowires may open the way to realize the ZnO nanowires based light-emitting diode and laser diode.     

Synthesis, characterization and optical properties of multipod ZnO whiskers

Multipod ZnO whiskers were synthesized successfully by two steps: pulsed laser deposition (PLD) and thermal evaporation process. First, a thin layer of Zn films were deposited on Si(1 1 1) substrates by PLD. Then the whiskers grew on Zn-coated Si(1 1 1) substrate by the simple thermal evaporation oxidation of the metallic zinc powder at 900 °C in the air without any catalysts or additives. The pre-deposited Zn films by PLD on the substrate can promote the growth of ZnO multipod whiskers effectively. The as-synthesized ZnO whiskers were characterized by using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM). The results revealed that the whiskers are highly crystalline with the wurtzite hexagonal structure. Room temperature photoluminescence (PL) spectrum of the whiskers shows a UV emission peak at ∼393 nm and a broad green emission peak at ∼517 nm, which was assigned to the near band-edge emission and the deep-level emission, respectively.     

Effect of sputtered films on morphology of vertical aligned ZnO nanowires

Vertical aligned ZnO nanowires were grown by MOCVD technique on silicon substrate using ZnO and AlN thin films as seed layers. The shape of nanostructures was greatly influenced by the under laying surface. Vertical nanopencils were observed on ZnO/Si, whereas the nanowires on both sapphire and AlN/Si substrate have the similar aspect ratio. XRD patterns suggest that the nanostructures have good crystallinity. High-resolution transmission electron microscopy (HRTEM) confirmed the single crystalline growth of the ZnO nanowires along [0 0 1] direction. Room-temperature photoluminescence (PL) spectra of ZnO nanowires on AlN/Si clearly show a band-edge luminescence accompanied with a visible emission. More interestingly, no visible emission for the nanopencils on ZnO/Si substrates, were observed.     

Tuning the growth of ZnO nanowires

ZnO nanowires (NWs) with different diameters were obtained by controlling the particles of ZnO sub-layer (SL) exploring hydrothermal method; the diameter of the epitaxial NWs could be tuned from 60 to 146 nm when using SL with a thickness of 70 nm. The thickness of the SL would influence the orientation of the NWs. The top agglomerate NWs could be formed on the SL with a thickness of 10 nm, and the NWs with better orientation were obtained using SL with a thickness of 70 nm. Well aligned ZnO NWs grew perpendicular to the completely stress released SL. The diameter of the NWs was also greatly influenced by the solution concentration; thus ultra fine (diameter∼11 nm) ZnO NWs were obtained through adjusting the solution concentration to 0.001 mol/L. Through our research, we also found that the growth rate of the NWs could also be influenced by the different polarity surface of the SL. In other words, the size of the ZnO NWs could be tuned exactly under optimal conditions.     

Effects of growth duration on the structural and optical properties of ZnO nanorods grown on seed-layer ZnO/polyethylene terephthalate substrates

Well-aligned single crystalline zinc oxide (ZnO) nanorods were successfully grown, by hydrothermal synthesis at a low temperature, on flexible polyethylene terephthalate (PET) substrates with a seed layer. Photoluminescence (PL), field-emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) measurements were used to analyze the optical and structural properties of ZnO nanorods grown for various durations from 0.5 h to 10 h. Regular and well-aligned ZnO nanorods with diameters ranging from 62 nm to 127 nm and lengths from 0.3 μm to 1.65 μm were formed after almost 5 h of growth. The growth rate of ZnO grown on PET substrates is lower than that grown on Si (1 0 0) substrates. Enlarged TEM images show that the tips of the ZnO nanorods grown for 6 h have a round shape, whereas the tips grown for 10 h are sharpened. The crystal properties of ZnO nanorods can be tuned by using the growth duration as a growth condition. The XRD and PL results indicate that the structural and optical properties of the ZnO nanorods are most improved after 5 h and 6 h of growth, respectively.     

Microstructural and optical properties of europium-doped zinc oxide nanowires

Single-crystal Eu³⁺-doped wurtzite ZnO micro- and nanowires were synthesized by chemical vapor deposition. The nanostructures grew via a self-catalytic mechanism on the walls of an alumina boat. The structure and properties of the doped ZnO were characterized using X-ray diffraction, energy-dispersive X-ray spectroscopy, scanning and transmission electron microscopy, and photoluminescence (PL) methods. A 10-min synthesis yielded vertically grown nanowires of 50–400 nm in diameter and several micrometers long. The nanowires grew along the ±[0001] direction. The Eu³⁺ concentration in the nanowires was 0.8 at.%. The crystal structure and microstructure of were compared for Eu³⁺-doped and undoped ZnO. PL spectra showed a red shift in emission for Eu³⁺-doped (2.02 eV) compared to undoped ZnO nanowires (2.37 eV) due to Eu³⁺ intraionic transitions. Diffuse reflectance spectra revealed widening of the optical bandgap by 0.12 eV for Eu³⁺-doped compared to undoped ZnO to yield a value of 3.31 eV. Fourier-transform infrared spectra confirmed the presence of europium in the ZnO nanowires.     

Engineering of nearly strain-free ZnO films on Si(1 1 1) by tuning AlN buffer thickness

ZnO properties were investigated as a function of AlN buffer layer thickness (0–100 nm) in ZnO/AlN/Si(1 1 1) structures grown by metal organic vapor phase epitaxy. A significant improvement of ZnO film crystallinity by tuning AlN buffer thickness was confirmed by x-ray diffraction, topography and photoluminescence measurements. An optimal AlN buffer layer thickness of 50 nm is defined, which allows for growth of nearly strain-free ZnO films. The presence of free excitons at 10 K suggests high crystal quality for all ZnO samples grown on AlN/Si(1 1 1) templates. The intensities of neutral and ionized donor bound exciton lines are found to correlate with the in-plane and out-of-plane strain in the films, respectively.     

Synthesis and photoluminescence properties of SnO2/ZnO hierarchical nanostructures

SnO₂/ZnO hierarchical nanostructures were synthesized by a two-step carbon assisted thermal evaporation method. SnO₂ nanowires were synthesized in the first step and were then used as substrates for the following growth of ZnO nanowires in the second step. Sn metal droplets were formed at the surfaces of the SnO₂ nanowires during the second step and were acted as catalyst to facilitate the growth of ZnO nanowires via vapor-liquid-solid mechanism. Room temperature photoluminescence measurements showed that the SnO₂/ZnO hierarchical nanostructures exhibited a strong green emission centered at about 520 nm and a weak emission centered at about 380 nm. The emissions from the SnO₂ were drastically constrained due to screen effect caused by the ZnO layer.     

Fabrication of ZnO nanostructures of various dimensions using patterned substrates

ZnO nanostructures were grown on silicon, porous silicon, ZnO/Si and AlN/Si substrates by low-temperature aqueous synthesis method. The shape of nanostructures greatly depends on the underlying surface. Scattered ZnO nanorods were observed on silicon substrate, whereas aligned ZnO nanowires were obtained by introducing sputtered ZnO film as a seed layer. Furthermore, both the combination of nanorods and the bunch of nanowires were found on porous silicon substrates, whereas platelet-like morphology was observed on AlN/Si substrates. XRD patterns suggest the crystalline nature of aqueous-grown ZnO nanostructures and high-resolution transmission electron microscopy images confirm the single-crystalline growth of the ZnO nanorods along [0 0 1] direction. Room-temperature photoluminescence characterization clearly shows a band-edge luminescence along with a visible luminescence in the yellow spectral range.