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.     

Growth specificity of vertical ZnO nanorods on patterned seeded substrates through integrated chemical process

A simple and cost effective method has been employed for the random growth and oriented ZnO nanorod arrays over as-prepared and patterned seeded glass substrates by low temperature two step growth process and growth specificity by direct laser writing (DLW) process. Scanning electron microscopy (SEM) images and X-ray diffraction analysis confirm the growth of vertical ZnO nanorods with perfect (0 0 2) orientation along c-axis which is in conjunction with optimizing the parameters at different reaction times and temperatures. Transmission electron microscopy (TEM) images show the formation of vertical ZnO nanorods with diameter and length of ∼120 nm and ∼400 nm respectively. Photoluminescence (PL) spectroscopic studies show a narrow emission at ∼385 nm and a broad visible emission from 450 to 600 nm. Further, site-selective ZnO nanorod growth is demonstrated for its high degree of control over size, orientation, uniformity, and periodicity on a positive photoresist ZnO seed layer by simple geometrical (line, circle and ring) patterns of 10 μm and 5 μm dimensions. The demonstrated control over size, orientation and periodicity of ZnO nanorods process opens up an opportunity to develop multifunctional properties which promises their potential applications in sensor, piezoelectric, and optoelectronic devices.     

A comparative study on the NO2 gas sensing properties of ZnO thin films, nanowires and nanorods

ZnO thin films were fabricated using the spin coating method, ZnO nanowires by cathodically induced sol-gel deposition by the means of an anodic aluminum oxide (AAO) template, and ZnO nanorods with the hydrothermal technique. For thin film preparation, a clear, homogeneous and stable ZnO solution was prepared by the sol-gel method using zinc acetate (ZnAc) precursor which was then coated on a glass substrate with a spin coater. Vertically aligned ZnO nanowires which were approximately 65 nm in diameter and 10 μm in length were grown in an AAO template by applying a cathodic voltage in aqueous zinc nitrate solution at room temperature. For fabrication of the ZnO nanorods, the sol-gel ZnO solution was coated on glass substrate by spin coating as a seed layer. Then ZnO nanorods were grown in zinc nitrate and hexamthylenetetramine aqueous solution. The ZnO nanorods are approximately 30 nm in diameter and 500 nm in length. The ZnO thin film, ZnO nanowires and nanorods were characterized by X-ray diffraction (XRD) analysis and scanning electron microscope (SEM). The NO₂ gas sensing properties of ZnO thin films, nanowires and nanorods were investigated in a dark chamber at 200 °C in the concentration range of 100 ppb-10 ppm. It was found that the response times of both ZnO thin films and ZnO nanorods were approximately 30 s, and the sensor response was depended on shape and size of ZnO nanostructures and electrode configurations.     

Hydrothermal synthesis and characterization of TiO2 nanorod arrays on glass substrates

Large-scale, well-aligned single crystalline TiO₂ nanorod arrays were prepared on the pre-treated glass substrate by a hydrothermal approach. The as-prepared TiO₂ nanorod arrays were characterized by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. X-ray diffraction results show that the main phase of TiO₂ is rutile. Scanning electron microscopy and transmission electron microscopy results demonstrate that the large-scale TiO₂ nanorod arrays grown on the pre-treated glass substrate are well-aligned single crystal and grow along [0 0 1] direction. The average diameter and length of the nanorods are approximately 21 and 400 nm, respectively. The photocatalytic activity of TiO₂ nanorod arrays was investigated by measuring the photodegradation rate of methyl blue aqueous solution under UV irradiation (254 nm). And the results indicate that TiO₂ nanorod arrays exhibit relatively higher photocatalytic activity.     

Hydrothermal in-situ growth of Tris(8-hydroxyquinolate) aluminum nanorods thin films

Tris(8-hydroxyquinolate) aluminum(Alq₃) thin films assembled with large-scaled nanorods have been fabricated on Al substrates through hydrothermal in-situ growth method assisted by the surfactant of sodium dodecylbenzenesulfonate. The obtained Alq₃ thin film is composed of uniformly sized (500-800nm × 4-10 μm) nanorods with regular hexagonal cross section, which are assembled to form dense nanorod arrays perpendicularly to the Al substrate. X-ray diffraction revealed that the prepared Alq₃ nanorods were the α-phase. Photoluminescence spectra showed that the Alq₃ nanorods thin film possessed a spectral blue-shift (10 nm) compared with the Alq₃ solution. The hydrothermal growth mechanism of nanorods was studied, which implied that the hydrothermal in-situ growth process on the metal substrate played an important role in the formation of the Alq₃ nanorods thin film. This simple hydrothermal method provides a convenient fabrication approach for nanocrystalline functional organic/metal interface.     

A template-free alcoholthermal route to Ti(Sn)-doped ZnO nanorods

Ti(Sn)-doped single-crystalline ZnO nanorods with an average diameter of 20 nm and length up to nearly 1 μm were synthesized by a facile ultrasonic irradiation-assisted alcoholthermal method without involving any templates. Photoluminescence spectra of the Ti-doped ZnO nanorods were measured at room temperature and three emitting bands, being a violet emission at 400-415 nm, a blue band at 450-470 nm and a green band at around 550 nm, were detected. The emission intensities of the Ti-doped ZnO nanorods enhance gradually with increasing the doping concentrations. As to the Sn-doped ZnO nanorods, the green emission shifts to 540 nm and the emission intensities increase first but decrease later with increasing the doping concentrations.     

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.     

Fast response ultraviolet Ga-doped ZnO based photoconductive detector

A metal-semiconductor-metal photoconductive detector was fabricated using high quality Ga-doped ZnO film epitaxially grown onto alumina substrate by spray pyrolysis. The photocurrent increases linearly with incident power density for more than two orders of magnitude. Reflectance and photocurrent measurements were carried out to study optoelectronic properties of Ga-doped ZnO thin film. Both spectra are consistent with each other showing good response in UV than visible region. Peak responsivity of about 1187 A/W at 5 V bias for 365 nm light was obtained in UV region.     

Structural,optical, and electrical properties of Schottky diodes based on undoped and cobalt-doped ZnO nanorods prepared by RF-magnetron sputtering

Cobalt-doped ZnO nanorods were successfully synthesized on Si/SiO₂ substrate using RF-magnetron sputtering at room temperature. The undoped and Co-doped ZnO nanostructures were characterized by XRD, FE-SEM, AFM, and PL spectra. The results showed that Co²⁺ replaced Zn²⁺ in the ZnO lattice without changing the wurtzite structure. The ZnO structure became high crystallite and was gradually converted into nanorods without extra phases as increased cobalt doping levels to 3 at.% and 4 at.%. The as-synthesized nanorod arrays were dense and vertically grew on the substrate with lengths of approximately 341 and 382.3 nm for 3 at.% and 4 at.% CO, respectively. PL analysis revealed that the ultraviolet (UV) emission intensity decreased and exhibited a blue shift with increased Co atomic percentage. This result was consistent with the energy bandgap values (3.26–3.3 eV) obtained from UV–vis spectra. The I–V characteristics revealed that the Shottky diodes based on Co-doped ZnO nanostructure with Pd electrodes have high barrier height (0.715–0.797 eV) and low saturation current (0.035–0.841 μA). The barrier height decreased after annealing the diodes at 500 °C for 2 h. To the best of our knowledge, Schottky diodes based on Co-doped ZnO nanorods prepared by RF-magnetron sputtering have not yet been reported.     

Preparation,characterization, and photoluminescence study of PVA/ZnO nanocomposite films

ZnO nanoparticles with average diameter of 25 nm were synthesized by a modified sol–gel method and used in the preparation of (in wt.%) (100 − x) poly(vinyl alcohol) (PVA)/x ZnO nanocomposite films, with x = 0, 1, 2, 3, 4, and 5. The PVA/ZnO films were exposed to UV radiation for 96 h and their thermal, morphological, and spectroscopic properties were investigated. In inert atmosphere, the nanocomposite films showed lower thermal stability than the pure PVA film, and the calorimetric data suggest an interaction between PVA and ZnO in the nanocomposite films. Some crystalline phases could be seen in the films with ZnO, and a direct dependence on the ZnO concentration was also observed. The original structure of ZnO nanoparticles remained unaltered in the PVA matrix and they were uniformly distributed on the film surface. The roughness of the PVA film was not modified by the addition of ZnO; however, it increased after 96 h of UV irradiation, more significantly in the nanocomposite films. The films showed an absorption band centered at 370 nm and a broad emission band in the UV–vis region when excited at 325 nm.     

Performance of polymer/ZnO hybrid photovoltaic devices determined by reaction time for oriented ZnO nanorod growth

The performance of hybrid polymer/metal oxide photovoltaic devices based on poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene) and oriented ZnO nanorods is studied. The ZnO nanorods on indium tin oxide-coated glass were prepared by hydrothermal method, where the length and the defect concentration of ZnO nanorods were controlled by the reaction time (T_r) for nanorod growth. Increasing T_r results in longer ZnO nanorods and higher defect concentration. Results show that both photocurrent and electron lifetime have strong dependence on the nanorod length (i.e., growth time) due to the exponential attenuation of incident light intensity in the device, offering a peak conversion efficiency of 0.337% under 1.5 AM illumination for T_r = 120 min. Combinational analyses of the data in this experiment and the previous data for the electrodeposited ZnO nanorods provide the insights into the dependence of the device performance on the intrinsic property of the ZnO nanorods.     

Growth and characterizations of ZnO nanorod/film structures on copper coated Si substrates

ZnO deposits were obtained on electroless copper coated Si substrates using a conventional RF magnetron sputter deposition technique at room temperature. The deposition pressure was varied from 6.67 Pa to 0.667 Pa. The RF powers were from 100 to 200 W and the electrode distance was fixed at 5 cm. The ZnO deposition time was varied from 1 to 30 min. The deposits consist of ZnO nanorods and a ZnO film, with the roots of the nanorods embedded in the film. The growth of the nanorods far exceeds the growth of the film in the beginning of the deposition process. The nanorod lengthening rate then slows down and becomes lower than the film growth rate. Effects of sputter deposition parameters on the growth of ZnO nanorods/film structures were also investigated.     

The effects of substrate temperature on the structure and properties of ZnO films prepared by pulsed laser deposition

ZnO thin films were prepared by pulsed laser deposition (PLD) on glass substrates with growth temperature from room temperature (RT) to 500 °C. The effects of substrate temperature on the structural and optical properties of ZnO films have been investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission spectra, and RT photoluminescence (PL) measurements. The results showed that crystalline and (0 0 2)-oriented ZnO films were obtained at all substrate temperatures. As the substrate temperature increased from RT to 500 °C, the ratio of grain size in height direction to that in the lateral direction gradually decreased. The same grain size in two directions was obtained at 200 °C, and the size was smallest in all samples, which may result in maximum E_g and E₀ of the films. UV emission was observed only in the films grown at 200 °C, which is probably because the stoichiometry of ZnO films was improved at a suitable substrate temperature. It was suggested that the UV emission might be related to the stoichiometry in the ZnO film rather than the grain size of the thin film.     

Investigation of ZnO nanorods synthesized by a solvothermal method, using Al-doped ZnO seed films

ZnO nanorods were successfully grown on common glass substrates using a simple solvothermal method via the precursors of zinc acetate dihydrate (Zn(CH₃COO)₂·2H₂O) and Hexamethylenetetramine (C₆H₁₂N₄) with equal molar concentration at 0.01 mol/l, 0.025 mol/l, 0.05 mol/l, and 0.075 mol/l. The ZnO nanorods were characterized by X-ray diffraction (XRD), Scanning electron microscopy, UV-Vis absorption spectrophotometer and photoluminescence (PL) spectrometer. XRD results indicated that all the ZnO nanorods were preferentially grown along [0 0 0 1] direction (c-axis). With an increase of Zn(CH₃COO)₂·2H₂O and C₆H₁₂N₄ concentration, the diffraction intensity of ZnO nanorod along c-axis also increased. Scanning electron microscopy images showed that the well-faceted hexagonal ZnO nanorods were grown vertically from the common glass substrates. In addition, with the increase of Zn(CH₃COO)₂·2H₂O and C₆H₁₂N₄ concentration, the exciton band of ZnO nanorods determined by UV-Vis absorption spectra gradually became narrow and the intensity of exciton band also remarkably augmented. Photoluminescence spectra showed that with the increase of Zn(CH₃COO)₂·2H₂O and C₆H₁₂N₄ concentration, the position of emission peak of ZnO nanorod blue-shifts towards shorter wavelength in UV region and the luminescence intensity remarkably enhances in visible emission range (470-630 nm).     

Growth of compact arrays of optical quality single crystalline ZnO nanorods by low temperature method

We report the synthesis and optical properties of compact and aligned ZnO nanorod arrays (dia, ∼ 50–200 nm) grown on a glass substrate with varying seed particle density. The suspension of ZnO nanoparticles (size, ∼ 15 nm) of various concentrations are used as seed layer for the growth of nanorod arrays via selfassembly of ZnO from solution. We studied the effect of various growth parameters (such as seeding density, microstructure of the seed layer) as well as the growth time on the growth and alignment of the nanorods. We find that the growth, areal density and alignment of the nanorods depend on the density of seed particles which can be controlled. It is observed that there is a critical density of the seed particles at which nanorod arrays show maximum preferred orientation along [002] direction. The minimum and maximum radius of the aligned nanorods synthesized by this method lie in the range 50–220 nm which depend on the seeding density and time of growth. These nanorods have a bandgap of 3.3 eV as in the case of bulk crystals and show emission in the UV region of the spectrum (∼ 400 nm) due to excitonic recombination and defect related emission in the visible region.     

Investigations on structural, optical and electrical properties of p-type ZnO nanorods using hydrothermal method

Undoped and doped ZnO nanorods were grown from an aqueous solution at low temperature (90 °C) on sapphire (100) substrates coated with ZnO thin film annealed in air at 550 °C for 1 h. X-ray diffraction results show that these nanorods have wurtzite type structure, and they are oriented in the c-axis direction. The optical properties are examined by room temperature micro photoluminescence and Raman scattering analysis which confirm that the nanorods exhibit good optical and electrical properties. A strong enhancement of multiple-phonon Raman scattering process with longitudinal optical phonon overtone up to fifth order was observed. It is found that the thin film coating of ZnO plays an important role in the c-axis oriented growth of undoped and doped ZnO nanorods due to good lattice match between the thin film and nanorods.     

Enhanced light to electricity conversion efficiency of CdS–ZnO composite nanorod based electrochemical solar cell

Cadmium sulfide–zinc oxide composite nanorods having at least 100 nm diameters were synthesized by a two-step chemical deposition technique. Polycrystalline nanorods of ZnO were grown on indium tin oxide coated quartz substrate by aqueous chemical growth technique. Cadmium sulfide was deposited on the surface of the ZnO nanorod thin film by chemical bath deposition. The X-ray diffraction results revealed the co-existence of polycrystalline CdS and ZnO, both having hexagonal structures. Neither any phase mixing nor any surface diffusion induced alloying was observed. Micro-Raman study detected a pair of optical phonons at 301 cm⁻¹ and 438 cm⁻¹ corresponding to hexagonal CdS and ZnO, respectively. An enhanced light to electricity conversion efficiency of 2.52% was recorded from CdS–ZnO photoanode based electrochemical solar cell under 0.5 sun illumination condition (50 mW cm⁻²). We observed a significant enhancement of short circuit current of the electrochemical solar cells due to addition of ionic salt solution to the electrolyte.     

Transparent conductive ZnO:B films deposited by magnetron sputtering

This paper focuses on the preparation of boron doped ZnO (ZnO:B) films prepared by nonreactive mid-frequency magnetron sputtering from ceramic target with 2 wt.% doping source. Adjusting power density, ZnO:B film with low resistivity (1.54 × 10^− 3 Ω cm) and high transparency (average transparency from 400 to 1100 nm over 85%) was obtained. Different deposition conditions were introduced as substrate fixed in the target center and hydrogen mediation. Hall mobility increased from 11 to above 26 cm²/V·s, while carrier concentration maintained almost the same, leading to low resistivity of 6.45 × 10^− 4 Ω cm. Transmission spectra of ZnO:B films grown at various growth conditions were determined using a UV-visible-NIR spectrophotometer. An obvious blue-shift of absorption edge was obtained while transmittances between 600 nm and 1100 nm remained almost the same. Optical band baps extracted from transmission spectra showed irregular enhancement due to the Burstein-Moss effect and band gap renormalization. Photoluminescence spectra also showed a gradual increase at UV emission peak due to free exciton transition near band gap. We contributed this enhancement in both optical band gap and UV photoluminescence emission to the lattice structure quality melioration.     

A facile and green approach for the fabrication of ZnO nanorods using bamboo charcoal as the template

A green synthetic approach was presented for the fabrication of ZnO nanorods via the bamboo charcoal-assisted impregnation route with ZnC₂O₄ colloid in ethanol as the inorganic precursor, followed by calcination at 800 °C for 7 h in air. These ZnO samples were characterized by means of X-ray diffraction (XRD) and transmission electron microscopy (TEM). It is shown that wurtzite hexagonal structured ZnO nanorods were fabricated, with an average diameter of about 300 nm and a length up to several micrometers. Bamboo charcoal played a key role in the formation of ZnO nanorods. The possible formation mechanism for ZnO nanorods was proposed.     

Nanostructures and sensing properties of ZnO prepared using normal and oblique angle deposition techniques

Nanostructured zinc oxide (ZnO) for gas sensing application has been prepared by using normal and oblique angle sputtering deposition techniques under different substrate temperatures. Oblique angle plasma beam deposition is demonstrated effectively growing large-area uniformly aligned and inclined ZnO nanorod arrays on catalyst-free silicon substrate due to a self-shadowing mechanism, whereas normal radio frequency sputtering deposition yields nanoparticles as island growth mode. Furthermore, the density of the nanorod arrays is dependent on the incident angle of ZnO plasma beam. With an increase of the incident flux angle, large inter spacing was induced, resulting in sparser nanorod arrays. The nanorod arrays grown with an incident angle of 70° have an average diameter of ∼ 150-300 nm and length of ∼ 700-750 nm. The experimental data from characterization of the samples indicates that the obtained samples at different substrate temperatures and incident angles have wurtzite structure with a c-axis orientation.Sensing characterization reveals that the nanorod-based sensor shows higher sensitivity, faster response and recovery time, as well as better reproducibility than that of nanoparticle-based gas sensor to 100 ppm hydrogen and methane at low operating temperature below 150 °C due to the porosity and large grain boundaries of the nanorod arrays. It demonstrates that oblique angle of sputtering deposition is a simple, inexpensive synthesis process to get high-porosity nanostructures and as a result, improves the sensing properties of fabricated ZnO sensors, which permits us to obtain sensors with high sensitivity, low operating temperature and stability.