As-synthesized ZnO Nanostructures Research Articles

Growth and Characterization of Vertically Aligned Nonpolar [11̅00] Orientation ZnO Nanostructures on (100) γ-LiAlO2 Substrate

In this work, we demonstrate the capability of controlling the crystallographic orientation of ZnO nanostructure arrays grown along nonpolar [11̅00] orientation. The Au catalyst was used to initiate and guide the growth of ZnO nanostructures on (100) γ-LiAlO2 substrate. The morphology, structure, and optical properties of the as-synthesized ZnO nanostructures were characterized by field emission scanning electron microscopy, X-ray diffraction, field emission transmission electron microscope, monochromatic cathodoluminescence image, and photoluminescence spectroscopy. The ZnO nanostructure arrays exhibited special shape and were aligned vertically on the (100) γ-LiAlO2 substrate. A cooperative of Au-catalyzed VLS vertebral column growth along [11̅00] orientation, zinc-self-catalyzed VLS growth along [0001], and ZnO VS lateral expansion growth process was proposed for the special ZnO nanostructures.

Enhanced UV emission from ZnO nanoflowers synthesized by the hydrothermal process

ZnO nanoflowers were synthesized by the hydrothermal process at an optimized growth temperature of 200 °C and a growth/reaction time of 3 h. As-prepared ZnO nanoflowers were characterized by x-ray diffraction, scanning electron microscopy, UV–visible and Raman spectroscopy. X-ray diffraction and Raman studies reveal that the as-synthesized flower-like ZnO nanostructures are highly crystalline with a hexagonal wurtzite phase preferentially oriented along the plane. The average length (234–347 nm) and diameter (77–106 nm) of the nanorods constituting the flower-like structure are estimated using scanning electron microscopy studies. The band gap of ZnO nanoflowers is estimated as 3.23 eV, the lowering of band gap is attributed to the flower-like surface morphology and microstructure of ZnO. Room temperature photoluminescence spectrum shows a strong UV emission peak at 392 nm, with a suppressed visible emission related to the defect states, indicating the defect free formation of ZnO nanoflowers that can be potentially used for UV light-emitting devices. The suppressed Raman bands at 541 and 583 cm−1 related to defect states in ZnO confirms that the ZnO nanoflowers here obtained have a reduced presence of defects.

Tetrapod-Like ZnO Nanostructures Deposited on Si Substrates with AlN as Buffer Layer

In this work, radio-frequency (RF) nitrogen plasma-assisted molecular beam epitaxy (PA-MBE) technique was used to grow AlN layers on Si(111) substrate. Subsequently, the thermal evaporation technique was used to deposit the zinc films on Si(111) substrates with AlN as buffer layer. The effect of annealing temperatures ranging from 600°C to 700°C in air for 1 h on the structural and optical properties of ZnO films is evaluated using X-ray diffraction (XRD) and room temperature photoluminescence (PL) spectroscopy. Tetrapod-like ZnO nanostructures were obtained from zinc granulated (99.99%) by thermal oxidation at 600°C in air for 1 h without any catalysts. The films show a polycrystalline hexagonal (wurtzite) structure without preferred (0002) orientation. The mean grain sizes are calculated to be about 22 nm and 50 nm for the ZnO films prepared at temperatures of 600°C and 700°C. The as-synthesized ZnO nanostructures were characterized by using scanning electron microscopy (SEM). Room temperature PL spectra of the ZnO films exhibit dominant excitonic ultraviolet (UV) emission and weak deep level visible emission. It is perceived that the ZnO film annealed at 600°C shows the strongest UV emission intensity among the temperatures ranges studied. From the ZnO film annealed at 600°C, the excellent UV emission is assigned to the good crystalline quality of the ZnO film and the low rate of formation of intrinsic defects.

Parametric Study on Dimensional Control of ZnO Nanowalls and Nanowires by Electrochemical Deposition

A simple electrochemical deposition technique is used to synthesize both two-dimensional (nanowall) and one-dimensional (nanowire) ZnO nanostructures on indium-tin-oxide-coated glass substrates at 70°C. By fine-tuning the deposition conditions, particularly the initial Zn(NO3)2·6H2O electrolyte concentration, the mean ledge thickness of the nanowalls (50–100 nm) and the average diameter of the nanowires (50–120 nm) can be easily varied. The KCl supporting electrolyte used in the electrodeposition also has a pronounced effect on the formation of the nanowalls, due to the adsorption of Cl− ions on the preferred (0001) growth plane of ZnO and thereby redirecting growth on the (100) and (20) planes. Furthermore, evolution from the formation of ZnO nanowalls to formation of nanowires is observed as the KCl concentration is reduced in the electrolyte. The crystalline properties and growth directions of the as-synthesized ZnO nanostructures are studied in details by glancing-incidence X-ray diffraction and transmission electron microscopy.

Photoluminescence and Raman analysis of novel ZnO tetrapod and multipod nanostructures

Novel ZnO tetrapod and multipod nanostructures were successfully synthesized in bulk quantity through thermal evaporation method. The morphologies and structures of the ZnO nanostructures were characterized by scanning electron microscopy, X-ray diffraction and transmission electron microscopy. The results revealed that the ZnO nanostructures consisted of tetrapods and multipods with tower-like legs. The ZnO nanostructures were of high purity and were well crystallized with wurtzite structure. The preferred growth direction of legs was found to be the [0 0 0 1] direction. Possible growth mechanisms were proposed for the formation of the ZnO nanostructures. Room temperature photoluminescence (PL) spectra showed that the as-synthesized ZnO nanostructures had a strong green emission centered at 495 nm and a weak ultraviolet emission at 383 nm. Raman spectroscopy was also adopted to explore the structural quality of the ZnO nanostructures.

Fabrication and characterization of tetrapod-like ZnO nanostructures prepared by catalyst-free thermal evaporation

Tetrapod-like ZnO nanostructures were fabricated on ZnO-coated sapphire (001) substrates by two steps: pulsed laser deposition (PLD) and catalyst-free thermal evaporation process. First, the ZnO films were pre-deposited on sapphire (001) substrates by PLD. Then the ZnO nanostructures grew on ZnO-coated sapphire (001) substrate by the simple thermal evaporation of the metallic zinc powder at 900 °C in the air without any catalysts. The pre-deposited ZnO films by PLD on the substrates can provide growing sites for the ZnO nanostructures. The as-synthesized ZnO nanostructures were characterized by using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and Fourier transform infrared spectrum (FTIR). The results show that the tetrapod-like ZnO nanostructures are highly crystalline with the wurtzite hexagonal structure. Photoluminescence (PL) spectrum of as-synthesized nanostructures exhibits a UV emission peak at ∼ 389 nm and a broad green emission peak at ∼ 513 nm. In addition, the growth mechanism of ZnO nanostructures is also briefly discussed.

Synthesis and Optical Properties of Ce-doped ZnO

ZnO nanorods were synthesized using the sol-gel method, and the effects of annealing temperature and Ce doping on the morphologies and optical properties of ZnO nanostructures were investigated in detail. The XRD measurements showed that the as-synthesized ZnO nanostructures had a hexagonal wurtzite structure. SEM images showed that uniform nanorods formed at 900 °C. Photoluminescence measurements showed an ultraviolet emission peak and a relatively broad visible light emission peak for the samples sintered at different temperatures. The UV emission peak bathochromically shifted when the annealing temperature rose from 850 to 1000 °C. Ce doping decreased the synthesized temperature of the ZnO nanorods to 500 °C, and the UV peaks hypsochromically shifted.

Preparation and photoluminescence of ZnO with nanostructure by hollow-cathode discharge

Without the use of a metal catalyst in the process, ZnO with nanostructures was successfully prepared on Si (100) substrate by simple chemical vapor-deposition method. In our work, Ar was used as the plasma forming gas, O2 was the reactive gas and metal zinc powder (99.99% purity) vaporized by cylinder hollow-cathode discharge (HCD) acted as the zinc source. The crystal structures of the as-synthesized ZnO nanostructures were characterized by X-ray diffraction (XRD); the ZnO sample growing on the wall of the crucible showed a ‘comb-like’ nanostructure, while the other one at the bottom of the crucible showed a ‘rod-like’ structure, which can be attributed to the difference of the oxygen content. The measurement on the photoluminescence (PL) performance of the ZnO nanostructures was carried out at room temperature. The results indicated that the ‘comb-shape’ ZnO nanomaterial possessed a remarkably strong ultraviolet emission peak centered at 388 nm, while ZnO nanorods, except better ultraviolet emission, also had relatively strong blue-green emission ranging from 470 to 600 nm due to the existence of oxygen vacancies. The growth mechanism of ZnO with nanostructures is also discussed in this paper.

Substrate effects on ZnO nanostructure growth via nanoparticle-assisted pulsed-laser deposition

The effects of various substrate conditions on the morphology, crystal structure and photoluminescence of ZnO nanostructures synthesized by nanoparticle-assisted pulsed-laser ablation deposition were investigated. It is concluded that the sapphire substrate with a 1 h anneal at 1000 °C is the most favorable to the vertical growth of ZnO nanostructures. SEM analysis indicates that the well-aligned diameter-modulated ZnO nanonails with unique shape were successfully synthesized on the annealed sapphire substrate. The as-synthesized ZnO nanostructures exhibit an ultraviolet emission at around 390 nm and the absent green emission under room temperature, indicating that there is a very low concentration of deep-level defects inside ZnO lattices. The novel ZnO nanostructures could offer novel opportunities for both fundamental research and technological applications.

Controllable growth of ZnO nanoarrays in aqueous solution and their opticalproperties

ZnO sheet arrays were vertically aligned onto an aluminium flake after pyrolysis ofZn5(OH)8Cl2H2O sheets which were fabricated in aqueous zinc chloride andhexamethylenetetraamine solution. The ZnO sheets, with diameter of0.5–1 µm and thickness of 30–50 nm, incline to have satisfactory configuration of verticalalignment on the aluminium flake. Vertically aligned ZnO rod arrays withdiameters of 50 nm were successfully grown on a silicon wafer coated with ZnO sol(Sicoated) inthe same aqueous solution. Furthermore, from the photoluminescence (PL) and the energy dispersivex-ray (EDX) analysis, the dominant intrinsic defect of the ZnO sheet arrays was vacancies of zinc(VZn). As-synthesized ZnO nanostructures have potential applications in fabricatingnext-generation nanodevices.

Tailor of ZnO morphology by heterogeneous nucleation in the aqueous solution

The umbrella-like ZnO nanostructures have been prepared by the morphological tailoring in the aqueous solution at 95 °C in the addition of heterogeneous seeds such as MnO 2 and CdS nanoparticles. The morphology and structure of as-synthesized umbrella-like ZnO nanostructures have been characterized by transmission electron microscopy (TEM), high-resolution transmission electron microscope (HRTEM), electron energy loss spectroscopy (EELS), field emission scanning electron microscopy (FESEM) and X-ray diffraction (XRD). The heterogeneous seeds play the critical role for the formation of umbrella-like ZnO nanostructures. Furthermore, the formation mechanism of the umbrella-like nanostructures has been phenomenally presented.

Precursor induced synthesis of hierarchical nanostructured ZnO

As-synthesized ZnO nanostructures with a bladed bundle-likearchitecture have been fabricated from a flower-like precursorZnO·0.33ZnBr2·1.74H2O via a mechanism of dissolution–recrystallization. Experimental conditions, such as initial reactantsand reaction time, are examined. The results show that no bladed bundle-like ZnO hierarchicalnanostructures can be obtained by using the same molar amount of other zinc salts, such asZnBr2, instead ofthe flower-like ZnO·0.33ZnBr2·1.74H2O precursor, and keeping other conditions unchanged. The products werecharacterized by field emission scanning electron microscopy (FE-SEM) andtransmission electron microscopy (TEM). The ZnO nanostructures are mainlycomposed of nanowires with a diameter around 40–50 nm and length up to1.5–2.5 µm. Meanwhile, ZnO nanoflakes with a thickness of about 4–5 nm attached to the surface ofZnO nanowires with a preferred radially aligned orientation. Furthermore, thephotoluminescence (PL) measurements exhibited the unique white-light-emittingcharacteristic of hierarchical ZnO nanostructures. The emission spectra cover the wholevisible region from 380 to 700 nm.