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.     

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.     

Catalytic effects of ZnO nanorods grown by sonochemical decomposition of zinc acetate dihydrate

In this study, we prepared ZnO nanorods by a sonochemical method using a zinc acetate dihydrate as a new precursor. Well-aligned high-quality ZnO nanorods were synthesized on FTO glass by the sonochemical decomposition of zinc acetate dihydrate using a ZnO thin-film as the catalytic layer. The ZnO thin-films were deposited on the FTO glass by a sputtering method. To investigate their catalytic effects on the ZnO nanorods, catalytic ZnO thin-films of 20 nm, 40 nm, and 60 nm thickness were prepared by adjusting the sputtering time. The ZnO nanorods grown on catalytic layers with different thicknesses were characterized by SEM, XRD, and PL. The ZnO nanorods grown on the catalytic layer of 40 nm thickness show the best crystal and spatial orientation and as a result display the best optical properties. It was found that a catalytic ZnO thin-film of 40 nm in thickness yields well-aligned high-quality ZnO nanorods, due to its small surface roughness and structural strain.     

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.     

Controlled growth of ZnO nanomaterials via hydrothermal method: effect of buffer layer

We report a facile solution-based method for the controlled growth of ZnO nanomaterials on an AIN/Si substrate. A ZnO buffer layer was coated on the substrate before growing the ZnO nano-materials. The shape of the ZnO nanomaterials changed from nanosheet to nanorod as the thickness of the ZnO buffer layer increased. Doping of the buffer layer with Ga decreased the average grain size of the ZnO buffer layer, which resulted in the growth of longer and thinner ZnO nanorods on the buffer layer. The UV sensing results of the ZnO nanorod-based device revealed that the aspect ratio of the ZnO nanorods is crucial for enhancing the performance of the device.     

Effects of seed layer patterns on the structural characteristics of hydrothermally grown ZnO nanorods

We investigated the effects of surface pattern size and shape on the characteristics of hydrothermally grown ZnO nanorods. For this purpose, the structural characteristics of ZnO nanorods were examined using X-ray diffraction and scanning electron microscopy. The sputtered ZnO seed layer was patterned using photolithography techniques on a Si substrate. ZnO nanorods with a [0001] texturing structure were successfully grown on selective areas by hydrothermal processes. In our experiments, however, it was observed that the diameter and the texture of the ZnO nanorods were strongly influenced by the size of the surface pattern.     

Effects of buffer layer thickness on properties of ZnO thin films grown on porous silicon by plasma-assisted molecular beam epitaxy

ZnO thin films with different buffer layer thicknesses were grown on Si and porous silicon (PS) by plasma-assisted molecular beam epitaxy (PA-MBE). The effects of PS and buffer layer thickness on the structural and optical properties of ZnO thin films were investigated by atomic force microscopy (AFM), scanning electron microscopy (SEM), X-ray diffraction (XRD), and photoluminescence (PL). The ZnO buffer layers, the intensity of the (002) diffraction peak for the ZnO thin films and its full width at half maximum (FWHM) decreased with an increase in the thickness of the ZnO buffer layers, indicating an improvement in the crystal quality of the films. On introducing PS as a substrate, the grain sizes of the ZnO thin films became larger and their residual stress could be relaxed compared with the ZnO thin films grown on Si. The intensity ratio of the ultraviolet (UV) to visible emission peak in the PL spectra of the ZnO thin films increased with an increase in buffer layer thickness. Stronger and narrower UV emission peaks were observed for ZnO thin films grown on PS. Their structural and optical properties were enhanced by increasing the buffer layer thickness. In addition, introduction of PS as a substrate enhanced the structural and optical properties of the ZnO thin films and also suppressed Fabry-Perot interference.     

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.     

The role of the Zn buffer layers in the structural and photoluminescence properties of ZnO films on Zn buffer layers deposited by RF magnetron sputtering

Highly c-axis oriented ZnO thin films were grown on Si (100) substrates with Zn buffer layers. Effects of the Zn buffer layer thickness on the structural and optical qualities of ZnO thin films were investigated for the ZnO films with the buffer layers 90, 110, and 130 nm thick using X-ray diffraction (XRD), photoluminescence (PL) and atomic force microscopy (AFM) analysis techniques. It was confirmed that the quality of a ZnO thin film deposited by RF magnetron sputtering was substantially improved by using a Zn buffer layer. The highest ZnO film quality was obtained with a Zn buffer layer 110 nm thick. The surface roughness of the ZnO thin film increases as the Zn buffer layer thickness increases.     

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.     

ZnO@CdS core–shell thin film: fabrication and enhancement of exciton life time by CdS nanoparticle

In the present study photoluminescence behavior of ZnO and ZnO@CdS core–shell nanorods film has been reported. ZnO nanorods were grown on the glass coated indium tin oxide (ITO) surface by seeding ZnO particle followed with nanorods growth. These nanorods were coated with CdS by chemical bath deposition techniques to have ZnO@CdS thin film and further annealed at 200 °C for their adherence to the ITO surface. The coating was characterized for surface morphology using SEM and optical behavior using UV–visible spectrophotometer. Energy dispersive X-ray (EDX) was used for compositional analysis and time resolve photoluminescence decay for excitons life time measurement. The absorption spectrum reveals that the absorption edge of ZnO@CdS core–shell heterostructure shifted to 480 nm in the visible region whereas ZnO nanorods have absorption maxima at 360 nm. The excitons lifetime of ZnO@CdS was found to be increased with the thickness of the CdS layer on ZnO nanorod. These ZnO@CdS core–shell nanostructures will be of great use in the field of photovoltaic cell and photocatalysis in a UV–visible region.     

Improved microstructural properties of a ZnO thin film using a buffer layer in-situ annealed in argon ambient

ZnO films with improved crystallinity were grown on a Si (111) substrate by a two-step growth process using low-temperature ZnO buffer layers. The effect of the ambient gas during the temperature elevation and the in-situ thermal annealing after the growth of the low-temperature buffer layers on the optical and structural properties of the films was investigated by X-ray diffraction (XRD), photoluminescence, and transmission electron microscopy. The use of argon as the ambient gas during the thermal treatment of the buffer layer leads to the enhancement of the (0002) diffraction peak intensity at 2θ ∼ 34.4° and the reduction of the full width at half maximum value in the XRD rocking curve, which means that well-defined and c-axis oriented ZnO film was obtained. The relationship between the thickness of the SiO₂ layer between the ZnO buffer layers and Si substrates and the structural and optical properties of the ZnO films is discussed.     

The effect of seed layer thickness on alignment and morphology of ZnO nanorods

Thin films of ZnO of 20, 40,160 and 320 nm thickness were deposited on Si (100) substrates by rf-magnetron sputtering and then nanorods were grown on the seed layer at 95 °C for 2 h. The ZnO nanorods were synthesized in C₆H₁₂N₄ and Zn (NO₃)₂·6H₂O solution by a hydrothermal method and the effect of seed layer thickness on the alignment, diameter, density and growth rate of nanorods was studied.The results revealed that the alignment of nanorods depended on crystallinity, grain size and roughness frequency of the sputtered seed layer, so that, with increase of seed layer thickness, crystallinity improved. In addition the grain size increased and the roughness frequency decreased and hence alignment and diameter of nanorods increased.Finally, we present a model for the effect of seed layer thickness on the alignment and diameter of the nanorods.     

The effect of seed layer thickness on alignment and morphology of ZnO nanorods

Thin films of ZnO of 20, 40,160 and 320 nm thickness were deposited on Si (100) substrates by rf-magnetron sputtering and then nanorods were grown on the seed layer at 95 °C for 2 h. The ZnO nanorods were synthesized in C₆H₁₂N₄ and Zn (NO₃)₂·6H₂O solution by a hydrothermal method and the effect of seed layer thickness on the alignment, diameter, density and growth rate of nanorods was studied.The results revealed that the alignment of nanorods depended on crystallinity, grain size and roughness frequency of the sputtered seed layer, so that, with increase of seed layer thickness, crystallinity improved. In addition the grain size increased and the roughness frequency decreased and hence alignment and diameter of nanorods increased.Finally, we present a model for the effect of seed layer thickness on the alignment and diameter of the nanorods.     

Fabrication and characterization of ZnO nanoneedle array using metal organic chemical vapor deposition

High density and vertically well-aligned ZnO nanoneedle arrays were fabricated on the ZnO thin film deposited on silicon substrates. The ZnO buffer layer and nanoneedles were synthesized by metal organic chemical vapor deposition using diethylzinc and oxygen gas. The ZnO buffer film was grown at 250 degrees C and the growth temperature of nanoneedles was in the range of 480-500 degrees C. As-grown ZnO nanoneedles showed single crystalline structure of ZnO (002). The crystalline properties of three samples (A: as-deposited ZnO buffer layer, B: annealed buffer film, C: ZnO nanoneedles) were compared using XRD and Raman spectroscopy. The synthesized ZnO nanoneedles (sample C) showed highest crystalline quality among three samples. The field emission properties of ZnO nanoneedles were investigated, which showed low turn on field of 4.8 Vmicrom(-1) and high field enhancement factor of 3.2 x 103.     

Preparation and optimization of epitaxial growth of transparent ZnO nanotip thin films by hydrothermal method

Zinc oxide (ZnO) nanotip thin films were prepared on ZnO coated nanocrystalline ITO/glass substrates by hydrothermal method. In order to obtain the ZnO nanotip arrays with high aspect ratio, the experimental conditions were optimized. The scanning electron microscope images showed that the surface morphology of ZnO thin films could be easily manipulated by changing the seed layer thickness and growth time. The ZnO nanotip thin films were grown epitaxially on ZnO seed layer coated ITO/glass substrates. The surface morphology of ZnO thin films on ITO/glass substrate changed from nanorods with a flat-top end to nanotips as the growth time was increased from 3 to 15 h. The ZnO thin films prepared under these deposition conditions were highly oriented along (002) direction. The as-prepared sample (15 h) was annealed at different temperatures (30, 100, 150, and 270 degrees C). The surface morphologies of annealed ZnO thin films did not show any remarkable change and the best crystallinity was observed at 100 degrees C. The photoluminescence spectra showed that the near band edge emission shifted to shorter wavelength as the annealing temperature was increased from 30 to 270 degrees C, it was due to the intrinsic stress in the films. This was confirmed by X-ray diffraction analyses. NPB thin films were prepared on ITO/clay and ITO/glass substrates by thermal evaporation method. The electrical properties of Ag/NPB/ITO/Clay showed the Ohmic characteristics (J proportional V(1.0)). The J-V characteristic of Ag/NPB/PMMA/ZnO/ITO/Glass showed good rectification behaviour with a diode-ideality factor of 1.36.     

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.     

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.     

Sputter deposition of ZnO nanorods/thin-film structures on Si

Using a recently developed sputter deposition technique, ZnO deposits were grown at the room temperature on silicon wafers with various kinds of copper surface layers. The copper layers were prepared using sputter deposition, thermal evaporation, or electroless plating technique. It was found that the surface copper prepared using both sputter deposition and thermal evaporation technique grew only ZnO thin films, while the surface copper prepared using sputter deposition technique grew ZnO nanorods/thin-film deposits. The relation between the copper characteristics and the growth of ZnO nanorods/thin-film deposits was investigated. The growth kinetics of the ZnO nanorods/thin film structure is also discussed.     

Effects of thermal annealing in oxygen plasma for buffer layers on properties of ZnO thin films

ZnO thin films with ZnO buffer layers were grown by plasma-assisted molecular beam epitaxy (PA-MBE) on p-type Si(100) substrates. Before the growth of the ZnO thin films, the ZnO buffer layers were deposited on the Si substrates for 20 minutes and then annealed at the different substrate temperature ranging from 600 to 800 degrees C in oxygen plasma. The structural and optical properties of the ZnO thin films have been investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and room-temperature (RT) photoluminescence (PL). A narrower full width at half maximum (FWHM) of the XRD spectra for ZnO(002) and a larger grain are observed in the samples with the thermal annealed buffer layers in oxygen plasma, compared to those of the as-grown sample. The surface morphology of the samples is changed from rugged to flat surface. In the PL spectra, near-band edge emission (NBEE) at 3.2 eV (380 nm) and deep-level emission (DLE) around 1.77 to 2.75 eV (700 to 450 nm) are observed. By increasing the annealing temperatures up to 800 degrees C, the PL intensity of the NBEE peak is higher than that of the as-grown sample. These results imply that the structural and optical properties of ZnO thin films are improved by the annealing process.