Comparative study of ZnO nanostructures grown on silicon (1 0 0) and oxidized porous silicon substrates with and without Au catalyst by chemical vapor transport and condensation

ZnO tetrapods and rods were grown on silicon and thermally oxidized porous silicon substrates with and without Au catalyst layer by carbothermal reduction of ZnO powder through chemical vapor transport and condensation method (CVTC). Porous silicon was fabricated by electrochemical etching of silicon in HF solution. The effect of substrates on morphology, structure and photoluminescence spectra of ZnO nanostructures has been studied. The texture coefficient (TC) of each sample was calculated from XRD data that demonstrated random orientation of ZnO nanostructures on the oxidized porous silicon substrate. Moreover, TC indicates the effect of Au catalyst layer on formation of more highly oriented ZnO nanorods. The morphology of the samples was investigated by SEM which confirms formation of ZnO nanostructures on oxidized porous silicon substrates with and without catalyst. A blue-green emission has been observed in ZnO nanostructures grown on Si and the oxidized PS substrates without Au catalyst layer by PL measurements.     

Enhanced ferroelectric, dielectric and optical behavior in Li-doped ZnO nanorods

Enhancement in the dielectric and ferroelectric properties has been observed in case of Li-doped ZnO nanorods (NR). Effect of Li-doping on ZnO structure and its optical properties has also been reported. In high resolution TEM studies, the length and diameter of as-synthesized Li-ZnO nanorods were found in the range of 100-150 nm and 20-70 nm, respectively. XRD studies, Li-doped ZnO NR exhibited wurzite structure in which lattice parameter becomes larger than the pure ZnO NR. In dielectric studies, higher dielectric constant and a ferroelectric phase transition at 72 °C were observed. In ferroelectric studies, high remnant polarization of 0.873 μC/cm² and low coercive field of 0.592 kV/cm were observed, which were better than their values in bulk Li-doped ZnO (0.044 μC/cm² and 2.0 kV/cm, respectively). In UV-Vis spectra, low absorption band edge at 352 nm was observed due to the size effect in the ZnO nanorods. In addition, PL spectra show a blue shift in both UV and visible region as a result of doping. These results are discussed in the light of the nanorods of confined geometry.     

Oxidation of etched Zn foil for the formation of ZnO nanostructure

Oxidation of Zn foil at temperatures of 100-400 °C was carried out in air to produce ZnO with various nanostructures. The final morphology of the oxidised Zn foil is largely dependent on the oxidation temperatures. At less than 300 °C, spherical oxide grains are seen. At 400 °C, 50 nm thick, porous nanosheets were formed after 30 min of oxidation. In portions of the samples, nanorods can be seen with diameters <10 nm and lengths reaching 1 μm. The nanosheets were formed in accordance to a vapour-solid mechanism whereas the nanorods were formed by diffusion of Zn through a certain path leading to the rod structure. At 450 °C, the nanorods became much more uniform. Oxidation at 500 °C resulted in ZnO nanorods. The rods are also blunt with smaller rods seen to branch out from the main rod. The luminescence properties of the ZnO were investigated as a function of the morphology of the oxide. Both green and blue emissions are seen for the samples with nanosheets whereas the nanorods ZnO has mostly green emission.     

Large-scale syntheses of uniform ZnO nanorods and ethanol gas sensors application

Uniform ZnO nanorods with a gram scale were prepared by a low temperature and solution-based method. The samples are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and photoluminescence (PL). The results showed that the sample had uniform rod-like morphology with a narrow size distribution and highly crystallinity. Room-temperature PL spectra of these nanorods show an exciton emission around 382 nm and a negligible deep level emission, indicating the nanorods have high quality. The gas-sensing properties of the materials have been investigated. The results indicate that the as-prepared nanorods show much better sensitivity and stability. The n-type semiconductor gas sensor exhibited high sensitivity and fast response to ethanol gas at a work temperature of 400 °C. ZnO nanorods are excellent potential candidates for highly sensitive gas sensors and ultraviolet laser.     

High-quality vertically aligned ZnO nanorods synthesized by microwave-assisted CBD with ZnO-PVA complex seed layer on Si substrates

We successfully synthesized vertically aligned zinc oxide (ZnO) nanorods on seeded silicon substrates using chemical bath deposition assisted by microwave heating. ZnO nanorods were grown on seed layers of ZnO-polyvinyl alcohol (PVA) nanocomposites spin-coated on p-type Si (1 1 1). The nanorod's diameter was found to be dependent on the annealing temperature of the ZnO-PVA seed layer. We produced ZnO nanorods with diameters in the range of 50-300 nm from five groups of seed layers annealed at 250 °C, 350 °C, 380 °C, 450 °C, and 550 °C. The nanorods were examined with X-ray diffraction, transmission electron microscopy, and field emission scanning electron microscopy, which revealed hexagonal wurtzite structures perpendicular to the substrate along the z-axis in the direction of (0 0 2). Photoluminescence measurements revealed high UV emission at a high I_UV/I_vis ratio of 175. We also conducted Raman scattering studies on the ZnO nanorods to estimate the lattice vibration modes.     

Morphological transition of vertically aligned one-dimensional zinc oxide

Vertically aligned 1D ZnO nanostructures have been synthesized on Si (100) substrates by thermal evaporation of zinc oxide powders without using a catalyst. The morphology and size of the as-grown ZnO nanorods gradually change as the distance between the substrate and the source increases. All of the nanorods are synthesized not on the Si substrate, but on the ZnO buffer layer, which forms on Si substrate during the growth process. The as-grown ZnO nanostructures show a morphological transition from nanorods with an abrupt tip to nanorods with a blunt tip, to nanonails with a cap, and finally to ZnO quantum dots as the temperature of the Si substrate decreases.     

Synthesis and photoluminescence of wurtzite CdS and ZnS architectural structures via a facile solvothermal approach in mixed solvents

CdS and ZnS nanostructures with complex urchinlike morphology were synthesized by a facile solvothermal approach in a mixed solvent made of ethylenediamine, ethanolamine and distilled water. No extra capping agent was used in the process. The structure, morphologies and optical properties of these nanostructures were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), and photoluminescence (PL) spectroscopy. The as-synthesized urchinlike architectures were composed of nanorods with wurtzite structure. The preferred growth direction of nanorods was found to be the [0 0 1] direction. The PL spectrum of CdS nanostructures exhibited a highly intense red emission band centered at about 706 nm. On the basis of the experimental results, a possible growth process has been discussed for the formation of the CdS and ZnS urchinlike structures.     

Temperature-dependant non-catalytic growth of ultraviolet-emitting ZnO nanostructures on silicon substrate by thermal evaporation process

Needle-like nanowires, nanorods, and nanosheets containing nanowires of ZnO have been synthesized on silicon substrate by the thermal evaporation of metallic zinc powder in the presence of oxygen without the use of any catalyst or additives. It was observed that a particular type of ZnO nanostructure can be obtained in a specific temperature zone and morphology can be well controlled simply by adjusting the substrate temperature. Detailed structural analysis revealed that the formed ZnO nanostructures are single-crystalline with wurtzite hexagonal phase, grown along the [0 0 0 1] direction in preference. Raman scattering and room-temperature photoluminescence spectra showed the good crystallinity with hexagonal wurtzite phase and excellent optical properties, respectively for all the deposited ZnO nanostructures.     

Absorption-emission study of hydrothermally grown Al:ZnO nanostructures

The preparation, structural characterization and optical properties of aluminum doped ZnO (Al:ZnO) nanostructures grown under hydrothermal method are reported. One-dimensional (1-D) growth is achieved by the controlled addition of metal nitrate as precursors in the presence of long chain surfactant, poly-ethylene glycol (PEG) at 160 °C for 20 h. The as-synthesized ZnO rods are single crystalline, exhibiting an oriented growth along [001] direction. The Al6 rod has an aspect ratio of 3.2, which can be effectively applied in optoelectronic devices. Comprehensive structural analysis using X-ray diffraction method (XRD) and Energy dispersive X-ray analysis (EDX) indicate that the dopant Al atom occupies Zn sites in ZnO and the elemental composition of Al is consistent with the amount utilized in the hydrothermal synthesis. XRD shows that the Al:ZnO nanostructures from 1 to 9 atomic percent (at.%) has hexagonal wurtzite structure of ZnO. The Al dopant effects on lattice vibration and electronic transitions of the ZnO nanostructures have been investigated by Fourier transform Infrared spectroscopy (FT-IR), Ultraviolet-visible (UV-vis) absorption spectroscopy and photoluminescence (PL) emission recorded at room temperature. The correlation existing between absorption and emission study tell that their characteristic band edge peak of doped ZnO shifts towards higher wavelength side for 3-9 at.% with respect to Al0 thus, exhibiting a red shift phenomenon with decrease in optical bandgap. The observed PL reveals two emission peaks centered at 374 nm and 530 nm. The near band edge (NBE) to defect emission ratio increases with dopant concentration indicating the linear enhancement in crystal quality and declination in zinc vacancies from 3 to 9 at.% of Al.     

Defect mediated photocatalytic activity in shape-controlled ZnO nanostructures

Shape-controlled ZnO nanostructures were synthesized through a facile soft-chemical approach by varying the concentration of OH⁻ ions. X-ray diffraction and Raman spectra reveal the formation of highly crystalline single-phase hexagonal wurtzite nanostructure. It has been observed that the concentration of OH⁻ ions plays an important role in controlling the shape of ZnO nanostructures. TEM micrographs indicate that well-spherical particles of size about 8 nm were formed at lower concentration of OH⁻ ions whereas the higher concentration of OH⁻ ions favor the formation of nanorods of length 30-40 nm. The optical studies confirmed that the band gap and near band edge emission of ZnO nanostructures are strongly dependent on the shape of particles. Furthermore, the decrease in the intensity of green emission as shape of particles changes from sphere to rod indicates the suppressing of defect density, which in turn influences the photocatalytic activity and ferromagnetic-like behavior of ZnO nanostructures.     

Preparation of porous flower-like ZnO nanostructures and their gas-sensing property

Porous flower-like ZnO nanostructures have been synthesized by a template-free, economical hydrothermal method combined with subsequent calcination. Calcination of the precursors produced flower-like ZnO nanostructures, composed of interconnected porous ZnO nanosheets with high porosity resulting from the thermal decomposition of the as-prepared precursors, i.e., flower-like basic zinc carbonate (BZC). Moreover, the nanostructures have been characterized through X-ray diffraction, thermogravimetric-differential thermalgravimetric analysis, scanning electron microscopy, transmission electron microscopy, and Brunauer-Emmett-Teller N₂ adsorption-desorption analyses. Compared with ZnO nanorods, the as-prepared porous flower-like ZnO nanostructures exhibit a good response and reversibility to some organic gas, such as ethanol and acetone. The sensor responses to 100 ppm ethanol and acetone are 21.8 and 16.8, respectively, at a working temperature of 320 °C. In addition, the sensors also exhibited a good response to 2-propanol and methanol, which indicate that these porous flower-like ZnO nanostructures are highly promising for applications of gas sensors.     

Influence of growth temperature on the optical and structural properties of ultrathin ZnO films

This study investigates the effect of growth temperature on the optical and structural properties of ultrathin ZnO films on the polished Si substrate. Thickness of the ultrathin ZnO films deposited by atomic layer deposition (ALD) method was about 10 nm. Photoluminescence (PL), X-ray diffraction (XRD), transmission electron microscopy (TEM) and atomic force microscopy (AFM) techniques were used to measure the properties of ultrathin ZnO films. Experimental results showed that the ultrathin ZnO film deposited at 200 °C had excellent ultraviolet emission intensity, and the average roughness of the film surface was about 0.26 nm.     

Synthesis and luminescence studies of CdSrS nanostructures

Cadmium strontium sulfide nanostructures were synthesized by solid state diffusion method in the presence of sodium thiosulfate. XRD confirmed the presence of CdS and SrS structures in the synthesized samples. TEM micrographs revealed the formation of nearly spherical nanoparticles with grain size in the range 30-40 nm. The PL emission was centred around 532 nm with a shoulder at 468 nm. The PL emission of CdSrS shows a blue shift in comparison to that of CdS. Substantially enhanced photoluminescence emission was observed with the addition of Bi³⁺ as a dopant. The effects of different excitation wavelengths on the PL spectra have also been investigated. It is suggested that the emission processes are linked to divalent Cd ions with broadening resulting due to these ions in being differing ion environments.     

Synthesis of cadmium oxide doped ZnO nanostructures using electrochemical deposition

Ternary ZnCdO alloy semiconductor nanostructures were grown using electrochemical deposition. Crystalline nanostructures/nanorods with cadmium concentration ranging from 4 to 16 at% in the initial solution were electrodeposited on tin doped indium oxide (ITO) conducting glass substrates at a constant cathodic potential −0.9 V and subsequently annealed in air at 300 °C. X-ray diffraction measurements showed that the nanostructures were of wurtzite structure and possessed a compressive stress along the c-axis direction. The elemental composition of nanostructures was confirmed by energy dispersive spectroscopy (EDS). ZnO nanostructures were found to be highly transparent and had an average transmittance of 85% in the visible range of the spectrum. After the incorporation of Cd content into ZnO the average transmittance decreased and the bandgap tuning was also achieved.     

Growth of ZnO nanorod arrays on soft substrates by hydrothermal process

ZnO nanorod arrays with quite homogeneous size and shape were fabricated by introducing ZnO seed-layer as nucleation centers on the soft ITO substrates prior to the hydrothermal reaction. The samples were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction and photoluminescence method. After the ZnO seed-layer is introduced, the resulting deposits on the substrates develop into nanorods, and the diameter decreases obviously to about 100 nm. Influences of the coated nanocrystal seed nuclei on the morphology of ZnO nanorod arrays were discussed. The results show that each nanorod is monocrystalline with wurtzite-type structure and oriented in c-axis direction. The increase of the intensity ratio of ultraviolet to visible emissions in room-temperature photoluminescence spectra and the decrease of the ultraviolet PL linewidths show the improvement of the quality of ZnO nanorods. A simple and effective method to synthesize ZnO nanorod arrays with fairly uniform size and shape on soft substrates is dip-coating ZnO nanocrystals prior to hydrothermal reaction, and it may be also feasible for the fabrication of other small-size metal oxide nanostructures on soft substrates.     

Nanosphericals and nanobundles of ZnO: Synthesis and characterization

ZnO nanosphericals and nanobundles well dispersion have been synthesized using [(N,N′-bis(salicylaldehydo)ethylenediamine)zinc(II)]; [Zn(salen)] as precursor via two methods. Nanosphericals of ZnO has been prepared via thermal decomposition of [Zn(salen)] in the presence of oleylamine at 290 °C for 90 min. Also nanobundle of ZnO has been synthesized via thermolysis of [Zn(salen)] in the air at 500 °C for 5 h. The as-synthesized products were characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and scanning electronic microscopy (SEM). The room temperature photoluminescence (PL) spectra of both nanostructures are dominated by the green emission attributed to the oxygen vacancy (V_O) related donor-acceptor transition. Presence of several infrared (IR) inactive vibrational modes in the Fourier transform infrared (FT-IR) absorption spectra of the samples indicates a breakdown of translational symmetry in the nanostructures induced by native defects.     

Oriented growth of ZnO nanostructures on Si and Al substrates

Aligned ZnO nanorod arrays and oriented ZnO nanoplates were synthesized on Si and Al substrates, respectively, using a hydrothermal method without any surfactant. The process involved the deposition of ZnO seeds on the substrate and the oriented growth of ZnO nanostructure in aqueous solution. The ZnO seeds were indispensable for the alignment of ZnO nanorods and TEM and XRD analysis confirmed that the ZnO rods are single crystalline grown along [001] direction. Al substrate caused formation of (001) surface dominated ZnO nanoplates, in which ZnO preferential growth direction was suppressed. The photoluminescence spectra of the as-grown ZnO products were measured to indicate their structural and optical quality. These oriented ZnO nanostructures are expected to be prospectively applied in nanodevices fabrication.     

Formation of ZnO nanorod arrays on polytetraflouroethylene (PTFE) via a seeded growth low temperature hydrothermal reaction

ZnO nanorod arrays were formed by a low temperature hydrothermal process on seeded polytetraflouroethylene (PTFE) sheets. The seed layer was formed using thermal oxidation of a thin evaporated Zn film on the PTFE sheet at 300 °C in air for 10 min. The formation of ZnO nanorod arrays in the hydrothermal reactive bath consisting of hexamethylamine (HMT) and Zn ions occurred via the reaction of hydroxyl ions released during the thermal degradation of HMT with the Zn ions. The seed layer provided a template for the nucleation of the ZnO and HMT which also acted as a chelating agent that promoted growth of the ZnO along the c-axis, leading to the formation of exclusively (0 0 2) ZnO nanorods. The effect of exposure time of the seeded PTFE to the reactive solution on the formation of the nanorods was investigated. Well aligned, relatively uniform tapered 300 nm long nanorods can be formed after 8 h of exposure. Longer exposure times to 24 h resulted in the formation of more uniform nanorods with base diameter averaged of ∼100 nm and the tip diameter of ∼50 nm. XRD analysis showed that the ZnO nanorod array had a hexagonal wurtzite structure. This result is in agreement with HR-TEM observations and Raman scattering analysis. Photoluminescence study showed that a strong UV emission peak was obtained at 380 nm and a small peak at 560 nm, which is associated with green emission. The optical band gap measured from these plots was at 3.2 eV on average.     

Structural and optical properties of electrodeposited ZnO thin films on conductive RuO2 oxides

ZnO thin films were grown on the 150 nm-thick RuO₂-coated SiO₂/Si substrates by electrochemical deposition in zinc nitrate aqueous solution with various electrolyte concentrations and deposition currents. Crystal orientation and surface structure of the electrodeposited ZnO thin films were characterized by X-ray diffraction (XRD) and scanning electron microscopy, respectively. The XRD results show the as-electrodeposited ZnO thin films on the RuO₂/SiO₂/Si substrates have mixed crystallographic orientations. The higher electrolyte concentration results in the ZnO thin films with a higher degree of c-axis orientation. Moreover, the use of an ultra-thin 5 nm-thick ZnO buffer layer on the RuO₂/SiO₂/Si substrate markedly improves the degree of preferential c-axis orientation of the electrodeposited ZnO crystalline. The subsequent annealing in vacuum at a low temperature of 300 °C reduces the possible hydrate species in the electrodeposited films. The electrodeposited ZnO thin films on the 5 nm-thick ZnO buffered RuO₂/SiO₂/Si substrates grown in 0.02 M electrolyte at −1.5 mA with a subsequent annealing in vacuum at 300 °C had the best structural and optical properties. The UV to visible emission intensity ratio of the film can reach 7.62.     

Sb doping behavior and its effect on crystal structure, conductivity and photoluminescence of ZnO film in depositing and annealing processes

The Sb-doped ZnO (ZnO:Sb) and undoped ZnO films with wurtzite structure and (0 0 2) preferred orientation were deposited on Si(1 0 0) substrate at 550 °C. It is deduced from XRD and XPS measurements that the Sb in the as-grown ZnO:Sb has high oxidation state and dopes in the form of oxygen-rich Sb-O clusters, which results in a large inner stress and a great increase of the c-axis lattice constant. After annealing at 750 °C under vacuum, the c-axis lattice constant of the ZnO:Sb decreases sharply to near the value of ZnO bulk, the electrical properties change from n-type to p-type and the PL intensity ratio of the visible to ultraviolet emission band goes down greatly, as the Sb content increases from 0 to 2.1 at.%. EDS and XRD measurements indicate that some of Sb dopants escape from the ZnO:Sb films and the oxygen-rich Sb-O clusters vanished after the annealing process. The effect of the change in Sb doping behavior on crystal structure, conductivity and PL is discussed in detail.