Electrodeposition of aligned arrays of ZnO nanorods in aqueous solution

We report electrodeposition of ZnO nanorods from aqueous solution at low temperature (～80 ^°C). The substrates used were thin films of silver grown on glass by thermal evaporation. Nanorods were grown by electrodeposition from aqueous solution of Zinc Nitrate and Hexamethylenetetramine (HMT). The growth process lasts for up to 4 h and the optimum potential for growth of nanorods was ?0.8 V. The nanorods have a well-defined hexagonal morphology and have a Wurtzite structure. The nanorods were characterized by different techniques and have a band gap of 3.08 eV. The width of nanorods has a bimodal distribution.     

Selective growth of ZnO nanorods in aqueous solution

ZnO nanorod arrays find applications in solar energy conversion, light emission and other promising areas. One approach to generate ZnO nanorods is the cost efficient aqueous chemical growth (ACG). Usually the ACG process is based on a nucleation step followed by growth of ZnO nanorods in aqueous solution at temperatures below 95 C.We report on the fabrication of homogeneous, large scale arrays of nanorods on various substrate materials (Si, glass, polymer) by ACG. PL-measurements show surprisingly good optical quality although the rods were grown at low temperature.Even though we have developed patterning of these arrays with photolithographic techniques, a bottom up approach for lateral patterning is important concerning further applications especially for mass-production. The substrates with patterned metal layers were employed to realize selective growth of nanorods. The experiments were carried out on Ti-, Ag- and Pt-patterned substrates. Selective growth on metal structured glass substrates was developed and is described.     

Effect of growth time on Ti-doped ZnO nanorods prepared by low-temperature chemical bath deposition

Ti-doped ZnO nanorod arrays were grown onto Si substrate using chemical bath deposition (CBD) method at 93 °C. To investigate the effect of time deposition on the morphological, and structural properties, four Ti-doped ZnO samples were prepared at various deposition periods of time (2, 3.5, 5, and 6.5 h). FESEM images displayed high-quality and uniform nanorods with a mean length strongly dependent upon deposition time; i.e. it increases for prolonged growth time. Additionally, EFTEM images reveal a strong erosion on the lateral side for the sample prepared for 6.5 h as compared to 5 h. This might be attributed to the dissolution reaction of ZnO with for prolonged growth time. XRD analysis confirms the formation of a hexagonal wurtzite-type structure for all samples with a preferred growth orientation along the c-axis direction. The (100) peak intensity was enhanced and then quenched, which might be the result of an erosion on the lateral side of nanorods as seen in EFTEM. This study confirms the important role of growth time on the morphological features of Ti-doped ZnO nanorods prepared using CBD.     

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.     

Large hexagonal arrays of aligned ZnO nanorods

Large-scale truly periodic arrays of vertically aligned zinc oxide nanorods were grown on pre-patterned and pre-annealed gold dots on a-plane sapphire substrates via the vapor–liquid–solid mechanism. Periodic arrays of triangular gold islands were first patterned on the a-plane sapphire substrates by the nanosphere self-assembly technique. Zinc has been found to be an effective interfacial modifier between gold and sapphire to form single catalytic dots from triangular islands. The successful fabrication of zinc oxide nanowires in truly periodic arrays opens up the possibility of achieving enhanced room-temperature ultraviolet lasing and photonic crystal based devices and sensors. PACS 81.07.Bc; 81.10.-h; 81.16.Nd     

Controllable growth of well-aligned ZnO nanorod arrays by low-temperature wet chemical bath deposition method

Well-aligned ZnO nanorod arrays were synthesized by low-temperature wet chemical bath deposition (CBD) method on Si substrate under different conditions. Results illustrated that dense ZnO nanorods with hexagonal wurtzite structure were vertically well-aligned and uniformly distributed on the substrate. The effects of precursor concentration, growth temperature and time on nanorods morphology were investigated systematically. The mechanism for the effect of preparation parameters was elucidated based on the chemical process of CBD and basic nucleation theory. It is demonstrated that the controllable growth of well-aligned ZnO nanorods can be realized by readily adjusting the preparation parameters. Strong near-band edge ultraviolet (UV) emission were observed in room temperature photoluminescence (PL) spectra for the samples prepared under optimized parameters, yet the usually observed defect related deep level emissions were nearly undetectable, indicating high optical quality ZnO nanorod arrays could be achieved via this easy process chemical approach at low temperature.     

Studies of luminescence properties of ZnO and ZnO:Zn nanorods prepared by solution growth technique

Pyramidal ZnO nanorods with hexagonal structure having c-axis preferred orientation are grown over large area silica substrates by a simple aqueous solution growth technique. The as-grown nanorods were studied using XRD, SEM and UV-vis photoluminescence (PL) spectroscopy for their structural, morphological and optical properties, respectively. Further, the samples have also been annealed under different atmospheric conditions (air, O₂, N₂ and Zn) to study the defect formation in nanorods. The PL spectra of the as-grown nanorods show narrow-band excitonic emission at 3.03 eV and a broad-band deep-level emission (DLE) related to the defect centers at 2.24 eV. After some mild air annealing at 200 °C, fine structures with peaks having energy separation of ∼100 meV were observed in the DLE band and the same have been attributed to the longitudinal optical (LO) phonon-assisted transitions. However, the annealing of the samples under mild reducing atmospheres of N₂ or zinc at 550 °C resulted in significant modifications in the DLE band wherein high intensity green emission with two closely spaced peaks with maxima at 2.5 and 2.7 eV were observed which have been attributed to the V_O and Zn_i defect centers, respectively. The V-I characteristic of the ZnO:Zn nanorods shows enhancement in n-type conductivity compared to other samples. The studies thus suggest that the green emitting ZnO:Zn nanorods can be used as low voltage field emission display (FED) phosphors with nanometer scale resolution.     

Growth, properties and dye-sensitized solar cells–applications of ZnO nanorods grown by low-temperature solution process

Hexagonal-shaped small ZnO nanorods were grown in a large-quantity via simple aqueous solution process by using zinc nitrate as a source of zinc ions at low temperature under stirring. The as-grown hexagonal-shaped ZnO nanorods were characterized in detail in terms of their structural, optical and photovoltaic properties. The detailed structural investigations by HRTEM, SAED and FFT revealed that the as-synthesized ZnO nanorods are well-crystalline, possessing a perfect hexagonal ideal growth habits of wurtzite zinc oxide and grown along the [0001] direction in preference. The optical properties, composition and quality of the as-synthesized nanorods were examined by using UV-visible and FTIR spectroscopy. Moreover, films of as-grown nanorods were used as photoanode materials to fabricate the dye sensitized solar cells (DSSCs). An overall light to electricity conversion efficiency of 0.70% with a fill factor of 47.2%, short-circuit current of 1.8 mA/cm² and open-circuit voltage of 0.76 V were achieved for the solar cell based on hexagonal-shaped small ZnO nanorods.     

Fabrication of ZnO nanorods for optoelectronic device applications

Hydroxyl free zinc oxide nanorods have been synthesized by a catalyst free surfactant based one-step solid state reaction process. The powder X-ray diffraction studies reveal well defined wurtzite peaks due to crystalline ZnO, while optical absorption spectra represent prominent exciton absorption and remarkable blueshift in the onset of absorption. As predicted by transmission electron microscopy, the ZnO nanorods are ∼100 nm long and of ∼20 nm dia. Further, luminescence aspects of such nanorods are studied for possible deployment in optoelectronics devices.      

ZnO nanorods/plates on Si substrate grown by low-temperature hydrothermal reaction

The zinc oxide (ZnO) nanorods/plates are obtained via hydrothermal method assisted by etched porous Al film on Si substrate. The products consist of nanorods with average diameter of 100 nm and nanoplates with thickness of 200-300 nm, which are uniformly distributed widely and grown perpendicularly to the substrate. The ZnO nanoplates with thickness of 150-300 nm were grown on Si substrate coated with a thin continuous Al film (without etching) in the same aqueous solution. The growth mechanism and room temperature photoluminescence (PL) properties of ZnO nanorods/plates and nanoplates were investigated. It is found that the introduction of the etched Al film plays a key role in the formation of ZnO nanorods/plates. The annealing process is favorable to enhance the UV PL emissions of the ZnO nanorods/plates.     

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.     

Vapour transport growth of ZnO nanorods

The fabrication of low-dimensional ZnO structures has attracted enormous attention as such nanostructures are expected to pave the way for many interesting applications in optoelectronics, spin electronics gas sensor technology and biomedicine. Many reported fabrication methods, especially for ZnO nanorods are mostly based on catalyst-assisted growth techniques that employ metal-organic sources and other contaminating agents like graphite to grow ZnO nanorods at relatively high temperatures. We report on catalyst-free vapour-phase epitaxy growth of ZnO nanorods on 6H-SiC and (11-20)Al₂O₃ using purely elemental sources at relatively low temperatures and growth pressure. ZnO nanorods with widths of 80–900 nm and lengths of up to 12 μm were obtained. Nanorod density on the order of 10⁹ cm^-2 with homogenous luminescence and high purity was also noted. PACS 81.10.Bk; 81.15.Kk; 81.16.Hc; 78.55.Et; 81.05.Dz; 81.07.Bc     

Studies on structural and magnetic properties of Co-doped pyramidal ZnO nanorods synthesized by solution growth technique

Large-area arrays of highly oriented Co-doped ZnO nanorods with pyramidal hexagonal structure are grown on silica substrates by wet chemical decomposition of zinc–amino complex in an aqueous medium. In case of undoped ZnO with an equi-molar ratio of Zn²⁺/hexamethylenetetramine (HMT), highly crystalline nanorods were obtained, whereas for Co-doped ZnO, good quality nanorods were formed at a higher Zn²⁺/HMT molar ratio of 4:1. Scanning electron microscope (SEM) studies show the growth of hexagonal-shaped nanorods in a direction nearly perpendicular to the substrate surface with a tip size of ～50 nm and aspect ratio around 10. The XRD studies show the formation of hexagonal phase pure ZnO with c-axis preferred orientation. The doping of Co ions in ZnO nanorods was confirmed by observation of absorption bands at 658, 617 and 566 nm in the UV–vis spectra of the samples. The optical studies also suggest Co ions to be present both in +2 and +3 oxidation states. From the photoluminescence studies, a defect-related emission is observed in an undoped sample of ZnO at 567 nm. This emission is significantly quenched in Co-doped ZnO samples. Further, the Co-doped nanorods have been found to show ferromagnetic behavior at room temperature from vibrating sample magnetometer (VSM) studies.     

Fabrication of ZnO nanoparticles by laser ablation of sintered ZnO in aqueous solution

Fabrication of ZnO nanoparticles by laser ablation in liquid medium is reported. The possibility of using a sintered ZnO target for the ablation as well as a Zn plate is demonstrated. The appropriate aqueous solution of sodium dodecyl sulfate is found to be 1 mM for ZnO growing. The shape of ZnO nanoparticles is sphere and its diameter is 30∼60 nm. Fourier transform infrared spectra, Raman scattering spectra, and photoluminescence spectra reveal the optical properties of ZnO nanoparticles. Nanoparticles obtained by using ZnO targets show a smaller defect density compared with those by using Zn targets.     

Origin of the surface recombination centers in ZnO nanorods arrays by X-ray photoelectron spectroscopy

The surface composition of as-grown and annealed ZnO nanorods arrays (ZNAs) grown by a two-step chemical bath deposition method has been investigated by X-ray photoelectron spectroscopy (XPS). XPS confirms the presence of OH bonds and specific chemisorbed oxygen on the surface of ZNAs, as well as H bonds on surfaces which has been first time observed in the XPS spectra. The experimental results indicated that the OH and H bonds play the dominant role in facilitating surface recombination but specific chemisorbed oxygen also likely affect the surface recombination. Annealing can largely remove the OH and H bonds and transform the composition of the other chemisorbed oxygen at the surface to more closely resemble that of high temperature grown ZNAs, all of which suppresses surface recombination according to time-resolved photoluminescence measurements.     

Fabrication and low-temperature photoluminescence spectra of Mn-doped ZnO nanowires

Mn-doped ZnO nanowires have been fabricated through a high temperature vapor-solid deposition process. The low-temperature photoluminescence spectra of the samples show that there are multipeak emissions at the ultraviolet (UV) region (about 3.4?C3.0?eV). The excitonic and phonon-assisted transitions in Mn-doped ZnO nanowires were investigated. The results show that there is an obvious oscillatory structure emission at the UV region under low temperature from 12?C125?K. The oscillatory structure has an energy periodicity about 70?meV and the oscillatory structure is mainly attributed to longitudinal optical (LO) phonon replicas of free excitons?(FX). The multipeak emissions at 12?K are attributed to a donor-bound exciton (DBX, 3.3617?eV), 1LO-phonon replicas of a free exciton (FX-1LO, 3.3105?eV), 2LO-phonon replicas of a free exciton (FX-2LO, 3.2396?eV), and 3LO-phonon replicas of a free exciton (FX-3LO, 3.1692?eV), respectively. The intensity of UV emission and the efficiency of emission from the Mn-doped ZnO nanowires are improved.     

Optical properties of ZnO nanorods realised by aqueous chemical growth

Photoluminescence investigations of ZnO nanorods realised by an advanced two-step aqueous chemical growth process have been carried out revealing well-resolved near-band-edge emission accompanied by phonon replicas. The optical properties of nanorods with different lengths and diameters are quite similar indicating a good control of the growth process without influencing the optical properties even on plastic substrate. The near-band-edge emission has a very broad line-width of 10 meV. Annealing in Ar atmosphere reduces the deep-level emission with a corresponding increase of the near-band-edge emission.     

High-quality ZnO nanorods grown on graphite substrates by chemical solution method

ZnO nanorods (NRs) were grown perpendicularly on graphite substrates using low-temperature wet chemical bath deposition (CBD) with sputtered ZnO film as seed layer. The individual ZnO NRs exhibit single-crystalline feature with well defined hexagonal prism shape and smooth side facets. The high optical qualities of ZnO NRs on graphite substrates were demonstrated by the dominant near-band edge emission and nearly undetectable deep level emissions in room-temperature photoluminescence spectra. The extremely low average reflectance of 0.45 % was obtained for the ZnO NRs/graphite structure in the spectra range from 200 to 1100 nm, indicating that the reported ZnO NRs/graphite structures have significant opportunity for potential application in high-performance photovoltaic devices. Considering the excellent material characteristics of ZnO NRs and the versatile and fascinating features of graphite substrates, the achievements make it possible for the development of high-performance ZnO-based nano-devices even in transferable, flexible, or stretchable forms.     

Density-controlled growth of ZnO nanorods using ZnO nanocrystals-embedded polymer composite

This work focuses on novel synthesis of ZnO nanorods for their potential applications to optoelectronic and electronic nanodevices. The growth density of ZnO nanorods was modulated through controlling of the density of ZnO nanocrystals dispersed on Si substrate. For this, ZnO nanocrystals synthesized via a polyol process were blended with a polymer matrix. ZnO nanocrystals-embedded polymer composite film was generated by spin-coating the mixed solution. Subsequent heat treatment of composite film removed a polymer matrix and left ZnO nanocrystals on the substrate, serving as seeds for the following ZnO nanorod growth. The density of grown ZnO nanorods was well controllable, depending on the density of dispersed ZnO nanocrystals on the substrate, which was varied by the concentration of ZnO nanocrystal-polymer solution.