Vertically-aligned ZnO Nanorods Research Articles

Compact and vertically-aligned ZnO nanorod thin films by the low-temperature solution method

Highly c-axis-oriented ZnO nanorod thin films were obtained on silica glass substrates by a simple solution-growth technique. The most compact and vertically-aligned ZnO nanorod thin film with the thickness of ∼ 800 nm and average hexagonal grain size of ∼ 200 nm exhibits the average visible transmittance 85%, refractive index 1.74, packing density 0.84, and energy band gap 3.31 eV, and it was fabricated under the optimum parameters: 0.05 M, 75 °C, 6 h, multiple-stepwise, and ZnO seed layer with an average grain size of ∼ 20 nm. The photoluminescence spectrum indicates that the densest ZnO nanorod thin film possesses lots of oxygen vacancies and interstitials. As we demonstrate here, the solution-growth technique was used to produce high-quality and dense ZnO nanorod thin films, and is an easily controlled, low-temperature, low-cost, and large-scale process for the fabrication of optical-grade thin films.

Buckling and elastic stability of vertical ZnO nanotubes and nanorods

Buckling and elastic stability study of vertical well aligned ZnO nanorods grown on Si substrate and ZnO nanotubes etched from the same nanorods was done quantitatively by nanoindentation technique. The critical load, modulus of elasticity, and flexibility of the ZnO nanorods and nanotubes were observed and we compared these properties for the two nanostructures. It was observed that critical load of nanorods (2890 μN) was approximately five times larger than the critical load of the nanotubes (687 μN). It was also observed that ZnO nanotubes were approximately five times more flexible (0.32 nm/μN) than the nanorods (0.064 nm/μN). We also calculated the buckling energies of the ZnO nanotubes and nanorods from the force displacement curves. The ratio of the buckling energies was also close to unity due to the increase/decrease of five times for one parameter (critical load) and increase/decrease of five times for the other parameter (displacement) of the two samples. We calculated critical load, critical stress, strain, and Young modulus of elasticity of single ZnO nanorod and nanotube. The high flexibility of the nanotubes and high elasticity of the ZnO nanorods can be used to enhance the efficiency of piezoelectric nanodevices. We used the Euler buckling model and shell cylindrical model for the analysis of the mechanical properties of ZnO nanotubes and nanorods.

Growth of high transmittance vertical aligned ZnO nanorod arrays with polyvinyl alcohol by hydrothermal method

Well-aligned ZnO nanorod arrays were synthesized on SnO 2:F (FTO) glass substrates by hydrothermal method with the addition of polyvinyl alcohol. Morphology and optical properties of the well-aligned ZnO nanorod arrays were studied. The transmittance of the as grown samples was dramatically increased by the addition of polyvinyl alcohol. PVA was regarded to play an important role in improving the optical transmittance and its role in hydrothermal growth process is also discussed in this work.

Selective Growth of Vertical-aligned ZnO Nanorod Arrays on Si Substrate by Catalyst-free Thermal Evaporation

By thermal evaporation of pure ZnO powders, high-density vertical-aligned ZnO nanorod arrays with diameter ranged in 80–250 nm were successfully synthesized on Si substrates covered with ZnO seed layers. It was revealed that the morphology, orientation, crystal, and optical quality of the ZnO nanorod arrays highly depend on the crystal quality of ZnO seed layers, which was confirmed by the characterizations of field-emission scanning electron microscopy, X-ray diffraction, transmission electron microscopy, and photoluminescence measurements. For ZnO seed layer with wurtzite structure, the ZnO nanorods grew exactly normal to the substrate with perfect wurtzite structure, strong near-band-edge emission, and neglectable deep-level emission. The nanorods synthesized on the polycrystalline ZnO seed layer presented random orientation, wide diameter, and weak deep-level emission. This article provides a C-free and Au-free method for large-scale synthesis of vertical-aligned ZnO nanorod arrays by controlling the crystal quality of the seed layer.

Structural and Optical Properties of Nitrogen-Ion-Implanted ZnO Nanorods

We present a study of the structural and the optical properties of nitrogen-ion-implanted ZnO nanorods. Vertically-aligned ZnO nanorods were synthesized on Al2O3 substrates by using a catalyst-free metal-organic chemical vapor deposition (MOCVD) procedure. Nitrogen ions with an energy of 120 keV and an ion ux of 1 1016 particles/cm2 were implanted on the verticallyaligned ZnO nanorods. The structural properties were studied by using various techniques, including eld emission transmission electron microscopy (FE-TEM), X-ray di raction (XRD), and X-ray absorption ne structure (XAFS) analyses. The XRD measurements revealed that the crystalline properties of the nitrogen-ion-implanted ZnO nanorods were comparable with those of the as-grown ZnO nanorods. However, the XAFS analysis demonstrated that the bonding lengths of atomic pairs on the c-axis were slightly elongated and that the ion implantation caused an extra structural disorder of Zn-O pairs. The FE-TEM revealed that the nanorods had structurally-damaged spots along the entire length of the nanorods. From the photoluminescence (PL) spectra, we observed a considerably weak main donor-acceptor transition peak at 3.02 eV in the temperature range of 5 300 K. The blue shift and the intensity decrease of the main transition peak strongly suggest that the implanted nitrogen ions play a role in the optical property changes of the nanorods.

Ultrafast third-order optical nonlinearities of vertically-aligned ZnO nanorods

Vertically-aligned ZnO nanorods with different rod diameters of 50 nm, 110 nm and 240 nm were synthesized by a low-temperature wet-chemical method. Morphologies and structures of ZnO nanorods were examined by scanning electron microscopy and X-ray diffraction. Third-order optical nonlinearities such as nonlinear refraction and absorption were determined by closed- and open-aperture z-scan measurements at 815 nm using femtosecond laser pulses. Enhanced optical nonlinearities were observed with increasing rod diameters and aligned degrees of ZnO nanorods along the c-axis. Nonlinear response time, measured by using a time-resolved optical Kerr-gate technique, was less than 95 fs.

Surface states dominative Au Schottky contact on vertical aligned ZnO nanorod arrays synthesized by low-temperature growth

Vertical aligned ZnO nanorod (diameter = 50 nm) arrays on indium tin oxide (ITO) substrates have been grown using a new two-step soft chemical procedure. The resulting current–voltage (I–V) characteristics of the ZnO nanorods exhibited a clear rectifying behaviour. This rectifying behaviour was attributed to the formation of a Schottky contact between the Au coated atomic force microscopy (AFM) tip and ZnO nanorod (nano-M/SC) which was dominated by the surface states in ZnO itself. Photo-assisted conductive AFM (PC-AFM) was used to demonstrate how the I–V characteristics are influenced by the surface states. Our I–V results also showed that the nano-M/SCs had a good photo-electric switching effect at reverse bias.

Low-temperature (∼270 °C) growth of vertically aligned ZnO nanorods using photoinduced metal organic vapourphase epitaxy

We successfully produced a drastic decrease in the required growth temperatureof single-crystalline ZnO nanorods, and enabled successful growth of verticallyaligned ZnO nanorods on a Si(100) substrate using photoinduced metal organicvapour phase epitaxy (MOVPE). We introduced 325 nm light during the MOVPEgrowth, and achieved vertical growth of single-crystalline ZnO nanorodswith a hexagonal crystal structure on Si(100) at a growth temperature of270 °C. The successful low-temperature growth of ZnO nanorods on the Si(100) substratedescribed here is a promising step toward designing nanoscale photonic and electronicdevices required by future systems.

Hydrothermally grown oriented ZnO nanorod arrays for gas sensing applications

C-axis vertically aligned ZnO nanorod arrays were synthesized on a ZnO thinfilm through a simple hydrothermal route. The nanorods have a diameterof 30–100 nm and a length of about several hundred nanometres. The gassensor fabricated from ZnO nanorod arrays showed a high sensitivity toH2 from room temperature to a maximum sensitivity at250 °C and a detection limit of 20 ppm. In addition, the ZnO gas sensor also exhibited excellent responsesto NH3 and CO exposure. Our results demonstrate that the hydrothermally grown verticallyaligned ZnO nanorod arrays are very promising for the fabrication of cost effective and highperformance gas sensors.

Buffer layer effect on ZnO nanorods growth alignment

Vertical aligned ZnO nanorods array was fabricated on Si with introducing a ZnO thin film as a buffer layer. Two different nucleation mechanisms were found in growth process. With using Au catalyst, Zn vapor could diffuse into Au nanoclusters with forming a solid solution. Then the ZnO nucleation site is mainly on the catalyst by oxidation of Au/Zn alloy. Without catalyst, nucleation could occur directly on the surface of buffer layer by homoepitaxy. The density and the size of ZnO nanorods could be governed by morphological character of catalyst and buffer layer. The nanorods growth is followed by vapor–solid mechanism.

The structural and optical properties of ZnO nanorod arrays

High quality vertical-aligned ZnO nanorod arrays were synthesized by a simple vapor transport process on Si (111) substrate at a low temperature of 520 °C. Field-emission scanning electron microscopy (FESEM) showed the nanorods have a uniform length of about 1 μm with diameters of 40–120 nm. X-ray diffraction (XRD) analysis confirmed that the nanorods are c-axis orientated. Selected area electron diffraction (SAED) analysis demonstrated the individual nanorod is single crystal. Photoluminescence (PL) measurements were adopted to analyze the optical properties of the nanorods both a strong UV emission and a weak deep-level emission were observed. The optical properties of the samples were also tested after annealing in oxygen atmosphere under different temperatures, deep-level related emission was found disappeared at 600 °C. The dependence of the optical properties on the annealing temperatures was also discussed.