ZnO Nanodisks Research Articles

Controlled growth and optical properties of Al 3+ doped ZnO nanodisks and nanorod clusters

Products consisted of Al-doped ZnO nanodisks and ZnO nanorod clusters were synthesized by chemical co-deposition process. The nanodisks with large polar surface are about 1–2 µm in diameter and 20 nm in thickness. The ZnO nanorods have an average diameter of 40 nm. The content of ZnO nanodisks can be adjusted by Al 3+ content. The growth mechanism of the ZnO nanodisks is also discussed. Room temperature photoluminescence (PL) measurements show that photoluminescence peaks of the products decrease with the increase of Al content.

Low-energy exciton states in a ZnO cylindrical nanodisk

We consider the electron-hole pair confined in a simplified infinite potential. The low-lying excition states in a ZnO cylindrical nanodisk are calculated based on effective-mass theory. To further understand the optical properties, we calculate the linear optical susceptibilities χ(w) and the radiative recombination lifetime τ of excitons in a ZnO nanodisk. The exciton radiative lifetime in a cylindrical nanodisk is of the order of tens of picoseconds, which is small compared with the lifetime of bulk ZnO material.

Morphology-Controlled Growth of ZnO Nanostructures Using Microwave Irradiation: from Basic to Complex Structures

Morphology-controlled growth of ZnO nano- and microstructures was achieved by microwave irradiation. Various basic ZnO structures, including nanorods, nanocandles, nanoneedles, nanodisks, nanonuts, microstars, microUFOs, and microballs were simply synthesized at a low temperature (90 °C) with low power microwave-assisted heating (about 50 W) and a subsequent aging process. These results could be obtained by changing the precursor chemicals, the capping agents, and the aging times. Even more complex ZnO structures, including ZnO bulky stars, cakes, and jellyfishes, were constructed by microwave irradiation to a mixture of the as-prepared basic ZnO structures and the solution I, IV, or V. This is a fast, simple, and reproducible method which does not require any template, catalyst, or surfactant but can control the morphology of ZnO crystals from simple to complex. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX), and X-ray diffraction (X...

In Vivo Detection of Superoxide Anion in Bean Sprout Based on ZnO Nanodisks with Facilitated Activity for Direct Electron Transfer of Superoxide Dismutase

Here, we report on a novel superoxide anion (O2(*-)) biosensor based on direct electron transfer of copper, zinc-superoxide dismutase (Cu, Zn-SOD) at zinc oxide nanodisks surface for in vivo tracking of O2(*-) in bean sprouts. Direct electron transfer of SOD is achieved at ZnO nanodisks film prepared by a one-step electrodeposited method, with a high heterogeneous electron rate constant of 17 +/- 2 s(-1). Spectroscopic data demonstrate that SOD strongly immobilized onto the nanostructured ZnO surfaces processes its inherent activity toward O2(*-) dismutation. A combination of the facilitated direct electron transfer and the bifunctional enzymatic catalytic activities of the SOD substantially provides a dual electrochemical approach to determination of O2(*-) with high selectivity, wide linear range, long stability, and good reproducibility. In particular, SOD adsorbed on the ZnO nanodisks film is capable of sensing O2(*-) cathodically at a very positive potential, 0 mV (vs Ag|AgCl), where the common interfering species such as hydrogen peroxide, uric acid, ascorbic acid, and 3,4-dihydroxyphenylacetic acid were effectively avoided. The excellent analytical performance of the present O2(*-) biosensor, combined with the remarkable characteristics of nanostructured ZnO films, such as biocompatibility, ease of preparation, and facile to miniaturize, paves an electrochemical way for reliable and durable in vivo determination of O2(*-) in bean sprouts.

Spatial distribution of defect in ZnO nanodisks

ZnO nanodisks were fabricated by vapor–vapor transport using zinc powder as source material. The photoluminescence (PL) and Raman scattering spectra were investigated. Based on the mapping technology, the Raman signals combined with PL were used to construct images, which revealed the spatial distribution of defect in ZnO nanodisks. It provides an intuitive method to understand the defect distribution and further helpful to growth control of crystal and design of functional materials and devices.

Solvent-controlled crystallization of zinc oxide nano(micro)disks

The effect of solvent composition on the growth of ZnO crystals has been investigated in water–ethanol solution system. Ethanol is found to effectively inhibit the crystal growth along the 〈0 0 0 1〉 direction, leading to the formation of thin disks. Hexagonal nanodisks and near-rounded microdisks have been grown in 1:1 (volume ratio) and 1:9 water–ethanol solutions, respectively. Transmission electron microscopy and selected area electron diffraction results show that the prepared hexagonal nanodisks have a single-crystal structure. In contrast, the near-rounded microdisks have a layer-by-layer structure, suggesting that crystals are grown by outward attachment. A strong sharp UV luminescence and a weak broad green-yellow emission in photoluminescence spectra indicate a high crystal quality of the prepared ZnO nano(micro)disks.

Microwave-Assisted Insertion of Silver Nanoparticles into 3-D Mesoporous Zinc Oxide Nanocomposites and Nanorods

The nanocomposite of mesoporous ZnO and Ag was synthesized by microwave radiation in a 15 min reaction conducted under an argon atmosphere. The 3.5 ± 0.4 nm Ag particles were inserted into the pores, changing the pore structure. The particles are also present as clusters on the surface of ZnO. The hexagonal ZnO nanodisks were stacked one on top of the other for the Ag:ZnO molar ratio of 0.13, whereas for Ag:ZnO = 0.22, the nanodisks join in the form of nanorods, where Ag clusters catalyze nanorod formation. The mesoporous structure in the current study was prepared with the help of argon gas and ethylene glycol. The FTIR studies revealed the presence of the organic framework structure retained in the final products. The 14 ± 2 nm hexagonal ZnO nanodisks are bound together by this organic framework, forming pores of 3.2 ± 0.1 and 18.6 ± 0.2 nm in diameter. The BET surface area increased from 40 ± 0 (pristine ZnO) to 51 ± 1 m2/g (Ag:ZnO = 0.22) due to formation of Ag clusters on the surface of ZnO with the increase in Ag concentration. Raman spectroscopy experiments indicate the modified surface characteristics of ZnO due to the porous structure and the presence of carbon in the products. The diffuse reflection optical absorption spectra show the ZnO excitonic absorption and Ag surface plasmon bands. The room-temperature photoluminescence experiments show emission bands assigned to the band edge exciton transitions in ZnO, oxygen defects, surface-deep traps, and impurity energy levels.

Structural Characterization and Luminescence of Porous Single Crystalline ZnO Nanodisks with Sponge-like Morphology

We report the synthesis of porous single crystalline ZnO nanodisks with sponge-like morphology through a wet chemical approach. To our best knowledge, this is the first report about highly porous single crystalline nanodisks of ZnO with an average diameter of ∼100 nm. The ZnO nanodisks exhibit strong visible (blue-green) light emission on UV excitation. Scanning Transmission Electron Microcopy (STEM), High-Resolution Transmission Electron Microscopy (HRTEM), and Selected Area Electron Diffraction (SAED) were performed to confirm that the nanodisks are single crystalline and porous in nature. The porosity of the nanodisks gives them the sponge-like appearance. Energy Dispersive X-ray Spectrometry (EDS) and Electron Energy Loss Spectrometry (EELS) analysis of the nanodisks together with high-resolution electron microscopy and photoluminescence measurements were used to determine the cause of the visible emission and its relation to the sponge-like morphology and growth mechanism. The larger surface area to volume ratio of these sponge-like nanostructures makes them very attractive for applications like biochemical sensors and solar cells.

Surface Energy of In-Doped ZnO Studied by PAW+U Method

Surface energies of indium doped ZnO were calculated to explain the polarized growth of ZnO nanodisks due to indium doping. Calculation results show that the surface energy of ZnO (0001) surface is much larger than that of ZnO (10 1 0) surface, leading to a preferred growth direction of [0001] for pure ZnO. At a doping rate of 1/8, the surface energies of indium doped ZnO are greatly reduced, but the surface energy of (0001) surface is still larger than that of (10 1 0) surface. At a doping rate of 1/4, the surface energies are decreased further, and the surface energy of (0001) surface is lower than that of (10 1 0) surface. Hence, growth of ZnO along [10 1 0] direction is made possible by heavy indium doping.

Structural and lasing characteristics of ultrathin hexagonal ZnO nanodisks grown vertically on silicon-on-insulator substrates

Hexagonal ZnO nanodisks were grown on silicon-on-insulator substrates by vapor-transport method without catalysts. The resulting nanodisks possess ultrathin thickness (10–20nm) and most of them present vertical orientation on the substrate surface. Under optical excitation, these ultrathin nanodisks exhibit ultraviolet lasing emission with the excitation threshold of 205μJ∕cm2 and emission lifetime of 2ps at room temperature. The detailed lasing analysis indicates that whispering-gallery-mode lasing may occur in the ZnO nanodisks.

A template-free, sonochemical route to porous ZnO nano-disks

A template-free, sonochemical aqueous route was used to synthesize hexagonal-shaped ZnO nanocrystals with a combined micro- and mesoporous structure. The products are much more porous when the sonohydrolysis is carried out under argon than their sonication under air. This has been attributed to the higher average specific heat ratio γ (= C p / C v ) of argon gas, leading to higher bubble collapse temperatures. Small-angle XRD (SAXRD) studies show that the microporosity is lost at 250 °C, while the mesoporous structure persists till a very high temperature (550 °C). The BET surface area of the products synthesized under argon and air are 35 and 13 m 2/g, respectively. The pore size is distributed from 1 nm (micropore) to 3.1–3.4 nm (mesopore), while the ZnO nanoparticles are 6.3 ± 1.2 nm. The possible mechanisms of the self-assembled pore formation are attributed to the organic porous framework of basic zinc acetate. The excitonic absorption of the ZnO occurs at 349 nm. The photoluminescence (PL) spectra of the ZnO nano-disks show the red-shifted band edge exciton transitions and the presence of deep levels due to oxygen vacancies or surface-deep traps, because of the porous structure.

Fabrication and Characterization of In-Doped Zinc Oxide Nanodisks

In-doped ZnO nanodisks were successfully fabricated by thermal evaporation Zn, In2O3 and graphite powder mixture without catalyst. Morphology, structures and components of ZnO nanodisks were investigated by SEM, HRTEM, EDS and X-Ray diffraction. ZnO nanodisks have perfect hexagonal shape, with 1~3μm size and 40~100 nm in thickness. The nanodisks are single-crystalline ZnO with wurtzite structure and In content of nanodisks reaches 2.2%. The growth along [0001] is suppressed leading to the formation of ZnO nanodisks. Room temperature photoluminescence spectra of the nanodisks shows that the UV emission peak blueshifts and becomes broader after doping.

Doping and defects in the formation of single-crystal ZnO nanodisks

High purity growth of polar surface dominated ZnO nanodisks was fabricated by introducing In ions in the raw material by thermal evaporation process without a catalyst. The nature of the sharp-contrast lines in the disks was investigated. The results suggested that the existence of sharp-contrast lines is due to the local segregation of In. Defects were initiated by segregation of the doping element of indium, which reduced the surface energy of ZnO (0001) leading to the fastest growth of the nanodisks along ⟨011¯0⟩. The preferred growth along ⟨011¯0⟩ is considered to maximize the effect of the piezoelectricity.

Controlled growth and properties of Pb 2+ doped ZnO nanodisks

Pb 2+ doped zinc oxide nanodisks have been grown through a facile solvothermal method. The nanodisks have perfect hexagonal shape with about 1 μm in diagonal and 100 nm in thickness. The existence of Pb 2+ is vital to the formation of the disk morphology. Room temperature photoluminescence measurements show two photoluminescence peaks centered at 518 and 648 nm. The mechanism of the nanodisk growth is also discussed.

Effects of processing parameters on ultraviolet emission of In-doped ZnO nanodisks grown by carbothermal reduction

Effects of cooling rate and oxygen partial pressure in flowing Ar on ultraviolet (UV) emission of In-doped ZnO nanodisks grown by carbothermal reduction at 1000 °C were studied. The In doping favored the growth of ZnO nanodisks instead of ZnO nanowires. Air-cooled ZnO nanodisks showed a strong green emission, while furnace cooling in conjunction with introducing O 2, around 1.0%, into flowing Ar during growth significantly enhanced the growth and UV emission of ZnO nanodisks. The causes can be attributed to the reduction of oxygen vacancies and surface defects in ZnO nanodisks. However, higher oxygen partial pressure in flowing Ar resulted in a decrease in the Zn vapor and thus suppressed the growth and UV emission of ZnO nanodisks.

ZnO nanosheet networks and hexagonal nanodiscs grown on silicon substrate: growthmechanism and structural and optical properties

Two-dimensional ZnO nanosheet networks composed of many thin anduniform hexagonal-shaped ZnO nanosheets and ZnO nanodiscs were preparedin a large scale on silicon substrate through thermal evaporation usingZnCl2 and O2 as source materials for Zn and oxygen, respectively, without the use of metal catalysts oradditives. Detailed structural studies indicated that the synthesized products aresingle crystalline with wurtzite hexagonal structure. Raman scattering of thesynthesized products confirmed that the as-grown structures have good crystalquality with a hexagonal wurtzite phase. Room temperature photoluminescencespectra showed a strong green band with a suppressed UV emission from the ZnOnanosheet networks, but on the other hand a dominant and strong near bandedge emission with a much suppressed deep level emission was observed in thenanodiscs. The growth mechanism of these structures is also discussed in detail.

Low Temperature Hydrothermal Synthesis of ZnO Nanodisk Arrays Utilizing Self-assembly of Surfactant Molecules at Solid–liquid Interfaces

ZnO nanodisk arrays have been prepared by utilizing cooperative surface assembly of surfactants and inorganic species at the solid–liquid interfaces under the mild condition. Photoluminescence measurement showed that the obtained ZnO nanodisk arrays exhibit new optical property comparing to the previous results of diverse ZnO nanostructures. Such nanostructures are expected to find more applications employing their optical and electrical properties.