Effect of buffer layer thickness on the growth properties of hydrothermally grown ZnO nanorods

We investigated the effect of ZnO buffer layer thickness on the growth of hydrothermally grown ZnO nanorods. A series of ZnO buffer layers with different thicknesses was deposited on a p-Si (111) substrate using a co-sputtering system. After annealing the ZnO buffer layer, ZnO nanorods grown were grown hydrothermally at 95 degrees C. Unlike ZnO nanorods grown on as-deposited ZnO buffer layer, the diameter and length of ZnO nanorods grown on annealed ZnO buffer layers can be controlled. The structural and optical properties of ZnO nanorods grown on annealed ZnO buffer layers were analyzed by field-emission scanning electron microscopy, X-ray diffraction, and photoluminescence. The influence of ZnO buffer layer thickness on ZnO nanorods growth is discussed.     

Effect of ZnO seed layer on the catalytic growth of vertically aligned ZnO nanorod arrays

We have grown vertically aligned ZnO nanorods and multipods by a seeded layer assisted vapor–liquid–solid (VLS) growth process using a muffle furnace. The effect of seed layer, substrate temperature and substrate material has been studied systematically for the growth of high quality aligned nanorods. The structural analysis on the aligned nanorods shows c-axis oriented aligned growth by homoepitaxy. High crystallinity and highly aligned ZnO nanorods are obtained for growth temperature of 850–900 °C. Depending on the thickness of the ZnO seed layer and local temperature on the substrate, some region of a substrate show ZnO tetrapod, hexapods and multipods, in addition to the vertically aligned nanorods. Raman scattering studies on the aligned nanorods show distinct mode at ∼438 cm⁻¹, confirming the hexagonal wurtzite phase of the nanorods. Room temperature photoluminescence studies show strong near band edge emission at ∼378 nm for aligned nanorods, while the non-aligned nanorods show only defect-emission band at ∼500 nm. ZnO nanorods grown without the seed layer were found to be non-aligned and are of much inferior quality. Possible growth mechanism for the seeded layer grown aligned nanorods is discussed.     

Selective growth of ZnO nanorods on SiO2/Si substrates using a graphene buffer layer

A promising strategy for the selective growth of ZnO nanorods on SiO₂/Si substrates using a graphene buffer layer in a low temperature solution process is described. High densities of ZnO nanorods were grown over a large area and most ZnO nanorods were vertically well-aligned on graphene. Furthermore, selective growth of ZnO nanorods on graphene was realized by applying a simple mechanical treatment, since ZnO nanorods formed on graphene are mechanically stable on an atomic level. These results were confirmed by first principles calculations which showed that the ZnO-graphene binding has a low destabilization energy. In addition, it was found that ZnO nanorods grown on SiO₂/Si with a graphene buffer layer have better optical properties than ZnO nanorods grown on bare SiO₂/Si. The nanostructured ZnO-graphene materials have promising applications in future flexible electronic and optical devices.     

Controllable synthesis and photoluminescence properties of ZnO nanorod and nanopin arrays

Well-aligned ZnO nanorods and nanopins are synthesized on a silicon substrate using a one-step simple thermal evaporation of a mixture of zinc and zinc acetate powder under controlled conditions. A self-assembled ZnO buffer layer was first obtained on the Si substrate. The structure and morphology of the as-synthesized ZnO nanorod and nanopin arrays are characterized using X-ray diffraction, and scanning and transmission electron microscopies, energy-dispersive X-ray spectroscopy, and photoluminescence spectroscopy. The influence of the background atmosphere on the two ZnO nanostructures has been studied. Two different growth mechanisms are mentioned to interpret the formation of ZnO nanorod and nanopin arrays in our work. The room-temperature PL features the ZnO nanorods exhibit only sharp and strong ultraviolet (UV) emission emissions, which confirms the better crystalline and optical quality than the ZnO nanopins.     

Epitaxial growth of ZnO layers using nanorods with high crystalline quality

Epitaxial ZnO films were grown on c-plane sapphire substrates by metal-organic chemical vapor deposition using a ZnO multi-dimensional structure having the sequence of ZnO film/ZnO nanorods/sapphire. The vertically well-aligned one-dimensional ZnO nanorods were grown epitaxially on the sapphire substrate with in-plane alignment under suitable growth conditions and then used as seeds for the subsequent epitaxial ZnO layer. For the transition of the ZnO structures from the nanorods to the film, the growth temperature and working pressure were controlled, while keeping the other conditions fixed. The growth of the ZnO films on the well-aligned ZnO nanorods results in homoepitaxial growth with the identical orientation relationship along the in-plane direction as well as the same c-axis orientation. The microstructural analysis of the multi-dimensional structure and analysis of the microstructural evolution from the one-dimensional nanorods to the two-dimensional film were conducted using transmission electron microscopy.     

Effects of Metal-Organic Chemical Vapour Deposition grown seed layer on the fabrication of well aligned ZnO nanorods by Chemical Bath Deposition

Well aligned, long and uniform ZnO nanorods have been reproducibly fabricated adopting a two-steps Metal-Organic Chemical Vapour Deposition (MOCVD) and Chemical Bath Deposition (CBD) fabrication approaches. Thin (< 100 nm) ZnO buffer layers have been seeded on silicon substrates by MOCVD and ZnO layers have been subsequently grown, in form of well textured nanorods, using CBD. It has been found that the structure and thickness of the seed layer strongly influence the final morphology and the crystal texturing of ZnO nanorods as well as the CBD growth rate. There is, in addition, a strong correlation between morphologies of CBD grown ZnO nanorods and those of the seed layer underneath. Thus, nanorods deposited over low temperature MOCVD buffer layers are less homogeneous in lateral dimensions and poorly vertically oriented. On the contrary, higher temperature nano-dimensional ZnO seeds favour the CBD growth of almost mono-dimensional homologue nanorods, with an adequate control of the lateral transport of matter. The nanorod aspect ratio values decrease upon increasing the deposition temperatures of the seed layers. Moreover, the nanorods length can be tailored either by adjusting the CBD growth time or by changing concentration of the N,N,N′,N′-tetramethylethylenediamine ligand used in the CBD process. In particular, at high concentrations, the CBD process is faster with a greater global aspect ratio in agreement with a preferential one-dimensional growth of the ZnO nanostructures. Finally, these ZnO nanorod arrays possess good optical quality in accordance to the photoluminescence properties.     

Synthesis,microstructure and photoluminescence of well-aligned ZnO nanorods on Si substrate

Well-aligned zinc oxide (ZnO) nanorods were densely grown on Si substrate using ZnO thin-film seed layer without any catalysts and/ or additives by a simple solid–vapour phase thermal sublimation technique. The growth mechanism can be interpreted as self-catalyst of zinc particles based on vapour–solid (VS) mechanism. High-resolution transmission electron microscopy (HRTEM) image and selected area electron diffraction (SAED) pattern confirmed that the single-crystalline growth of the nanorods were preferentially along c-axis of hexagonal crystal system. High-crystal quality ZnO nanorods with strong near band edge emission centred at 380 nm can be achieved on Si substrate by the introduction of sufficient oxygen during the nanorod growth processing.     

Synthesis, microstructure and photoluminescence of well-aligned ZnO nanorods on Si substrate

Well-aligned zinc oxide (ZnO) nanorods were densely grown on Si substrate using ZnO thin-film seed layer without any catalysts and/or additives by a simple solid–vapour phase thermal sublimation technique. The growth mechanism can be interpreted as self-catalyst of zinc particles based on vapour–solid (VS) mechanism. High-resolution transmission electron microscopy (HRTEM) image and selected area electron diffraction (SAED) pattern confirmed that the single-crystalline growth of the nanorods were preferentially along c-axis of hexagonal crystal system. High-crystal quality ZnO nanorods with strong near band edge emission centred at 380 nm can be achieved on Si substrate by the introduction of sufficient oxygen during the nanorod growth processing.     

Effects of seed layers on structural, morphological, and optical properties of ZnO nanorods

We studied the effects of seed layers on the structural and optical properties of ZnO nanorods. ZnO and Ag-doped ZnO (ZnO:Ag) seed layers were deposited on glass substrates by magnetron co-sputtering. ZnO nanorods were grown on these seed layers by the chemical bath deposition in an aqueous solution of Zn(NO3)2 and hexamethyltetramine. SEM micrographs clearly reveal that ZnO nanorods were successfully grown on both kinds of seed layers. The XRD patterns indicate that crystallization of ZnO nanorods is along the c-axis. Meanwhile, the packing density and the vertical alignment of the ZnO nanorods on the ZnO seed layer are better than those of the ZnO nanorods on ZnO:Ag. The enhanced growth of nanorods is thought to be due to the fact that the ZnO layer exhibits a higher crystalline quality than the ZnO:Ag layer. According to the low-temperature photoluminescence spectra, the ZnO nanorods on the ZnO seed layer show a narrow strong ultraviolet emission band centered at 369 nm, while those on ZnO:Ag exhibit multiple bands. These results are thought to be related with the crystallinity of ZnO nanorods, the morphologies of ZnO nanorods, and the reflectivities of seed layers. More detailed studies for clarification of the seed layer effect on the growth of ZnO nanorods are desirable.     

Hydrothermal synthesis, characterization and properties of TiO2 nanorods on boron-doped diamond film

The rutile TiO₂ nanorods have been hydrothermally synthesized on boron-doped diamond (BDD) film with a ZnO buffer layer. It is demonstrated that the ZnO buffer layer plays a key role in increasing the density and improving the morphology of synthesized TiO₂ nanorods. The heterojunction of n-TiO₂ nanorods/p-BDD shows an evident rectifying behavior with a ratio of ∼ 180 at 6 V. Experimentally, the TiO₂ nanorod-covered BDD exhibits an improved electron field-emission property over that without using a ZnO buffer layer.     

Fabrication of vertical ZnO nanowires on silicon (100) with epitaxial ZnO buffer layer

Vertical ZnO nanowires were successfully grown on epitaxial ZnO (002) buffer layer/Si (100) substrate. The nanowire growth process was controlled by surface morphology and orientation of the epitaxial ZnO buffer layer, which was deposited by radio-frequency (rf) sputtering. The copper catalyzed the vapor-liquid-solid growth of ZnO nanowires with diameter of approximately 30 nm and length of approximately 5.0 microm. The perfect wurtzite epitaxial structure (HCP structure) of the ZnO (0002) nanowires synthesized on ZnO (002) buffer layer/Si (100) substrate results in excellent optical characteristics such as strong UV emission at 380 nm with potential use in nano-optical and nano-electronic devices.     

Growth behavior and characteristics of one dimensional ZnO nanostructures by metalorganic chemical vapor deposition

The ZnO nanorods have been grown on silicon substrates by a metalorganic chemical vapor deposition (MOCVD) process with/without Au catalytic layer. The growth behavior of ZnO nanorods changed with the catalytic layer. The presence of Au catalyst complicated the growth direction of ZnO nanorods in the MOCVD methods. The ZnO nanorods had single crystalline atomic structure and pure compositions without impurities and strong and narrow excitonic emission. We investigated the growth behavior of the ZnO nanostructures which are leading candidate for optical applications.     

ZnO纳米棒/ZnCo2O4纳米片异质结的制备及其光电探测性能

ZnO纳米材料异质结是构筑高性能紫外光电探测器的有力候选之一。本工作中, 设计并制备了一种新型ZnO纳米棒/ZnCo₂O₄纳米片异质结, 研究了其电学性能及光电探测性能。使用油水界面自组装, 将ZnCo₂O₄纳米片在ITO玻璃上组装为均匀的薄膜; 通过调控ZnO种子层厚度, 在ZnCo₂O₄纳米片薄膜上水热生长了取向一致、密度适中的ZnO纳米棒阵列, 获得了高质量的ZnO纳米棒/ZnCo₂O₄纳米片异质结。该异质结具有优良的整流特性, 整流比达到673.7; 其工作在反偏状态时, 光暗电流比超过2个量级, 紫外-可见判别比为29.4, 在光电探测中有良好的波长选择特性。研究表明, 该异质结有潜力应用于构筑高性能紫外光电探测器。     

Structural and antireflective properties of ZnO nanorods synthesized using the sputtered ZnO seed layer for solar cell applications

We report the structural and antireflective properties of ZnO nanorod arrays (NRAs) on silicon (Si) substrate by wet chemical growth using the sputtered ZnO seed layer for solar cell applications. The size, height, shape, and number of ZnO nanorods depend strongly on the ZnO seed layer thickness as well as the molar zinc nitrate concentration. Clearly, the ZnO nanorods are of wurzite crystal structure from the X-ray diffraction analysis. To achieve the low reflectance over a wide wavelength range, the ZnO seed layer thickness, molar concentration, and growth time are optimized. It is found that the specular reflection spectrum of ZnO NRAs is closely related to the ZnO seed layer thickness. The solar weighted reflectance, Rw, of ZnO NRAs as antireflection coatings for Si solar cells is estimated under AM1.5 g illumination. For ZnO NRAs with 50 nm ZnO seed layer in 10 mM aqueous solution for 12 hours, the low specular reflectance (i.e., <7%) is obtained at wavelengths of 300-1200 nm, indicating a low Rw of 3.86%.     

ZnO nanostructure thin films by continuous spray pyrolysis using doped precursor for Si solar cell application

This work presents deposition of Zn solution seed layer assisted growth of zinc oxide (ZnO) nanostructure layers by continuous spray pyrolysis reactor using lanthanides (Er and Eu) and metal (Al) influenced zinc acetate precursor solution. Dopants in precursors have influenced structural property, surface morphology and optical reflectance of resulting ZnO thin films which are supported by X-ray diffractometer, scanning electron microscope and reflectance measurements. Enhanced dispersion amongst nanorods is observed under the influence of Er and Al dopant in ZnO thin film. The change of precursor from Zinc acetate to Titanium tetraisopropoxide for Er doped precursor is helping to achieve better crystalline ZnO nanorods arrangement with increased homogenous growth, which results into improved light reflectance reduction of thin film. The experimental evidences of light reflectance from ZnO nanorods on Si surface is studied with the help of FDTD based Lumerical software package which can be a useful study for designing ZnO nanorods thin film in device purposes. The utility of ZnO layer by this reactor on low efficiency Si solar cell is also explored in improving device efficiency via increase of photocurrent.     

Impact of layer thickness and light transmission of ZnO nanomaterials on GaN-based light emitting diodes

Highly transparent ZnO nanomaterials have been successfully dispersed in the form of nanoparticles and nanorods on InGaN/GaN-based surface mounted light emitting diodes (SM-LEDs). An effortless spin-coating technique is employed to disperse the ZnO nanoparticle layers, and a well-known hydrothermal technique is used for growing the ZnO nanorods. The layer thickness and the light transmission at a specific wavelength are the major factors in improving the light output power of the devices. Field emission scanning electron microscope (FESEM) images are used to confirm the uniform dispersion of the ZnO nanostructures on the top of the SM-LEDs. The layer thickness and the level of light transmission at 460 nm are examined from the cross-sectional FESEM images and UV absorption spectra, respectively.     

Charge influence and growth mechanism of ZnO nanorods

We demonstrate the influence of charges near the substrate surface on vertically aligned ZnO nanorod growth. ZnO nanorods were fabricated on n-type GaN with and without H+ treatments by catalyst-free metal-organic chemical vapor deposition. The ZnO nanorods grown on n-GaN films were vertically well-aligned and had a well-ordered wurtzite structure. However, the ZnO did not form into nanorods and the crystal quality was very degraded as they were deposited on the H+ treated n-GaN films. The charge influence was also observed in the ZnO nanorod growth on sapphire substrates. These results implied that the charges near the substrate surface dominantly affected on the crystalization and formation of ZnO nanorods.     

Synthesis of Ga-doped ZnO nanorods using an aqueous solution method for a piezoelectric nanogenerator

Mechanical energy is a potential energy source for self-powered electronic devices. Due to their unique semiconducting and piezoelectric properties, wurtzite-structured nanomaterials have been considered as potential candidates for piezoelectric nanogenerators that convert mechanical energy into electricity. In the present work, we report on the growth of Ga-doped ZnO (GZO) nanorods and investigate the performance of nanogenerators fabricated from undoped ZnO (UZO) nanorods, low Ga-doped ZnO (LGZO) nanorods, and high Ga-doped ZnO (HGZO) nanorods. A nanogenerator integrated with LGZO nanorods exhibited a current density of 1.2 microA/cm2, an enhancement over the 0.4 microA/cm2 and 0.7 microA/cm2 current densities of nanogenerators integrated with UZO and HGZO nanorods, respectively.     

Directed spatial organization of zinc oxide nanorods

The ability to precisely place nanomaterials at predetermined locations is necessary for realizing applications using these new materials. Using an organic template, we demonstrate directed growth of zinc oxide (ZnO) nanorods on silver films from aqueous solution. Spatial organization of ZnO nanorods in prescribed arbitrary patterns was achieved, with unprecedented control in selectivity, crystal orientation, and nucleation density. Surprisingly, we found that caboxylate endgroups of omega-alkanethiol molecules strongly inhibit ZnO nucleation. The mechanism for this observed selectivity is discussed.     

Charge influence and growth mechanism of ZnO nanorods

We demonstrate the influence of charges near the substrate surface on vertically aligned ZnO nanorod growth. ZnO nanorods were fabricated on n-type GaN with and without H+ treatments by catalyst-free metal-organic chemical vapor deposition. The ZnO nanorods grown on n-GaN films were vertically well-aligned and had a well-ordered wurtzite structure. However, the ZnO did not form into nanorods and the crystal quality was very degraded as they were deposited on the H+ treated n-GaN films. The charge influence was also observed in the ZnO nanorod growth on sapphire substrates. These results implied that the charges near the substrate surface dominantly affected on the crystallization and formation of ZnO nanorods.