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.     

Quantum cutting effect and photoluminescence emission at about 1,000 nm from Er–Yb co-doped ZnO nanoplates prepared by wet chemical precipitation method

ZnO nanoplates with Er-doping concentrations varying in the range from 3 to 7 wt% and co-doped with (Er–Yb) (7 + 7 wt%) were successfully prepared by wet chemical precipitation method. The effects of doping on the structural and optical properties of ZnO nanostructures have been systematically investigated. The structural morphology of the prepared nanostructures was found to change with increasing Er-doping concentrations. The visible photoluminescence and infrared photoluminescence of the prepared nanostructures were measured at room temperature. The intensity of visible emission spectra was found to increase with increasing Er-doping concentrations and was further enhanced for (Er–Yb) co-doped ZnO nanoplate samples. Additionally, Er-doped (7 wt%) and Yb-doped (7 wt%) ZnO nanoplates showed an enhanced emission peak at 950 nm, whereas two enhanced emission peaks at 950 and 980 nm have been found for (Er–Yb)-co-doped (7 + 7 wt%) ZnO nanoplates samples when excited at 310, 365 and 371 nm excitation wavelengths.     

Quasi One-dimensional ZnO Nanostructures Fabricated without Catalyst at Lower Temperature

One- or quasi one-dimensional zinc oxide nanostructures possess plenty of morphologies. Only by controlling the gas flow rates, and partial pressures of argon, oxygen and zinc vapor, can various types of high-quality ZnO nanomaterials (such as wires, belts, arrays, saws or combs, tetraleg rods, nails, and pins) be synthesized through pure zinc powder evaporation without a catalyst at the temperature range of 600–700°C. In this study, deposited nanostructures were characterized by means of scanning electron microscopy, X-ray diffraction and high-resolution transmission electron microscopy. The authors propose and discuss the growth mechanisms of various ZnO. In addition, properties of room temperature photoluminescence and field emission of several typical ZnO nanostructures are measured and investigated.     

Growth and optical properties of ZnO microwells by chemical vapor deposition method

The well-like ZnO nanostructures were obtained by chemical vapor deposition method. The uniform and dense ZnO slim nano-columns were grown along the circle to form a microwell. The growth mechanisms, such as 1D linear, 2D screw dislocation and step growth are discussed. These observations provide some insight into the growth kinetics in vapor-solid growth process. The fabrication of ZnO microwell morphology provided a direct experimental evidence for explaining the 1D growth mechanism based on the axial screw dislocation. Photoluminescence (PL) microscopy showed the surface-related optical properties. The green light emission enhancement revealed that the ZnO microwells have waveguide properties. The abnormal enhancement of integrated PL intensity of deep-level emission with temperature increase showed abundant surface state existence.     

Infrared femtosecond laser-induced great enhancement of ultraviolet luminescence of ZnO two-dimensional nanostructures

Two-dimensional (2D) complex nanostructures on the surface of ZnO crystal are fabricated by the interference of three 800 nm fs laser beams. The 2D nanostructures exhibit a great enhancement of UV emission excited by infrared fs laser with central wavelengths ranging from 1,200 nm to 2,000 nm. We propose that the defect states in the band gap of 2D nanostructures induced by 800 nm fs laser ablation cause the great enhancement of UV emission. We make theoretical calculations and explain well with the experimental results.     

Growth,structure, and cathodoluminescence of Dy-doped ZnO nanowires

Dy-doped ZnO nanowires have been prepared using high-temperature and high-pressure pulsed-laser deposition. The morphology, structure, and composition of the as-prepared nanostructures are characterized by field emission scanning electron microscopy, X-ray diffraction, Raman scattering spectrometry, X-ray photoelectron spectrometry, transmission electron microscopy, and energy dispersive X-ray spectroscopy. The alloying droplets are located at the top of the as-prepared Dy-doped ZnO nanowires, which means that the growth of the Dy-doped ZnO nanowires is a typical vapor-liquid-solid process. The luminescence properties of Dy-doped ZnO nanowires are characterized by cathodoluminescence spectra and photoluminescence spectra at low temperature (8 K). Two peaks at 481 and 583 nm, respectively, are identified to be from the doped Dy³⁺ ions in the CL spectra of Dy-doped ZnO nanowires.     

The effect of Al doping on the morphology and optical property of ZnO nanostructures prepared by hydrothermal process

The undoped and Al-doped ZnO nanostructures were fabricated on the ITO substrates pre-coated with ZnO seed layers using the hydrothermal method. The undoped well-aligned ZnO nanorods were synthesized. When introducing the Al dopant, ZnO shows various morphologies. The morphology of ZnO changes from aligned nanorods, tilted nanorods, nanotubes/nanorods to the nanosheets when the Al doping concentrations increase. The ZnO nanostructures were characterized by X-ray diffraction, field emission scanning electron microscopy, X-ray photoelectron spectroscopy, photoluminescence and Raman technology. The Al doping concentrations play an important role on the morphology and optical properties of ZnO nanostructures. The possible growth mechanism of the ZnO nanostructures was discussed.     

Effect of sputtered films on morphology of vertical aligned ZnO nanowires

Vertical aligned ZnO nanowires were grown by MOCVD technique on silicon substrate using ZnO and AlN thin films as seed layers. The shape of nanostructures was greatly influenced by the under laying surface. Vertical nanopencils were observed on ZnO/Si, whereas the nanowires on both sapphire and AlN/Si substrate have the similar aspect ratio. XRD patterns suggest that the nanostructures have good crystallinity. High-resolution transmission electron microscopy (HRTEM) confirmed the single crystalline growth of the ZnO nanowires along [0 0 1] direction. Room-temperature photoluminescence (PL) spectra of ZnO nanowires on AlN/Si clearly show a band-edge luminescence accompanied with a visible emission. More interestingly, no visible emission for the nanopencils on ZnO/Si substrates, were observed.     

The effect of morphology and doping on photoluminescence of ZnO nanostructures

In this work, optical properties of ZnO nanostructures prepared by chemical vapor deposition under different conditions were investigated. ZnO nanostructures were characterized by electron microscopy and photoluminescence. A high intensity green emission and a narrow UV emission band are observed in photoluminescence spectra of ZnO nanostructures related to the below-band-gap and band-edge that their intensities depend on the morphology of the nanostructures. It is considered that the green emission is originated from structural defects. In addition, the influence of thermal treatment and dopants such as iron and copper, on the photoluminescence (PL) properties of the ZnO nanostructures was investigated.     

Ultrasonic induced photoluminescence decay in sonochemically obtained cauliflower-like ZnO nanostructures with surface 1D nanoarrays

Cauliflower-like ZnO nanostructures with average crystallite size of about 55 nm which have surface one dimensional (1D) nanoarrays with 10 nm diameter were successfully fabricated through a simple sonochemical route. X-ray diffractometry (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and room temperature photoluminescence (PL) characterizations were performed to investigate the morphological and structural properties of the obtained nanostructures. It has been shown that the synthesized cauliflower-like ZnO nanostructures irradiated UV luminescence and a green peak in visible band. Ultrasonic post-treatment of the particles for about 2 h increased the density of surface defects resulted in an increase in the green emission intensity.     

Synthesis and ultraviolet emission of aligned ZnO rod-on-rod nanostructures

Aligned ZnO rod-on-rod nanostructures were synthesized on silicon substrate via a simple thermal evaporation process at low temperature without catalysts. Pictures taken with the use of the scanning electron microscope demonstrate that the well-ordered ZnO rod-on-rod nanostructures grow on the Si substrate, and the single nanostructure consists of two parts. Transmission electron microscopy image and the selected area electron diffraction pattern indicate that the single-crystal nanorod grows along [0001] direction. The X-ray diffraction pattern proves that the samples have good crystal quality. The detailed nanorod growth mechanism is proposed and discussed. The room-temperature photoluminescence (PL) spectrum shows the dominant ultraviolet emission, which indicates their potential application in ultraviolet optoelectronic devices. The temperature-dependent PL spectra reveal that the strong ultraviolet emission should originate from the longitudinal optical phonon replicas of free exciton.     

Growth mechanism studies of ZnO nanowire arrays via hydrothermal method

Well-controlled ZnO nanowire arrays have been synthesized using the hydrothermal method, a low temperature and low cost synthesis method. The process consists of two steps: the ZnO buffer layer deposition on the substrate by spin-coating and the growth of the ZnO nanowire array on the seed layer. We demonstrated that the microstructure and the morphology of the ZnO nanowire arrays can be significantly influenced by the main parameters of the hydrothermal method, such as pH value of the aqueous solution, growth time, and solution temperature during the ZnO nanowire growth. Scanning electron microscopy observations showed that the well oriented and homogeneous ZnO nanowire arrays can be obtained with the optimized synthesis parameters. Both x-ray diffraction spectra and high-resolution transmission electron microscopy (HRTEM) observations revealed a preferred orientation of ZnO nanowires toward the c-axis of the hexagonal Wurtzite structure, and HRTEM images also showed an excellent monocrystallinity of the as-grown ZnO nanowires. For a deposition temperature of 90 °C, two growth stages have been identified during the growth process with the rates of 10 and 3 nm/min, respectively, at the beginning and the end of the nanowire growth. The ZnO nanowires obtained with the optimized growth parameters owning a high aspect ratio about 20. We noticed that the starting temperature of seed layer can seriously influence the nanowire growth morphology; two possible growth mechanisms have been proposed for the seed layer dipped in the solution at room temperature and at a high temperature, respectively.     

Selective growth of ZnO nanowires on substrates patterned by photolithography and inkjet printing

Zinc oxide nanowires (ZnO NWs) were grown by a two-step growth method, involving the deposition of a patterned ZnO thin seeding layer and the chemical vapor deposition (CVD) of ZnO NWs. Two ways of patterning the seed layer were performed. The seeding solution containing ZnO precursors was deposited by sol–gel/spin-coating technique and patterned by photolithography. In the other case, the seeding solution was directly printed by inkjet printing only on selected portion of the substrate areas. In both cases, crystallization of the seed layer was achieved by thermal annealing in ambient air. Vertically aligned ZnO NWs were then grown by CVD on patterned, seeded substrates. The structure and morphology of ZnO NWs was analyzed by means of X-ray diffraction and field emission scanning electron microscopy measurements, respectively, while the vibrational properties were evaluated through Raman spectroscopy. Results showed that less-defective, vertically aligned, c-axis oriented ZnO NWs were grown on substrates patterned by photolithography while more defective nanostructures were grown on printed seed layer. A feature size of 30 µm was transferred into the patterned seed layer, and a good selectivity in growing ZnO NWs was obtained.     

Ion beam synthesis and carrier dynamics of ZnO nanoparticles embedded in a SiO2 matrix

Zinc oxide (ZnO) nanostructures have been synthesized by the implantation of ZnO molecular ions into SiO₂ followed by high temperature thermal annealing. 35 keV ZnO^? ions were implanted to a fluence of 5×10¹⁶ ions/cm² into SiO₂ at room temperature (RT). The implanted sample was annealed in an oxygen environment to allow the growth of ZnO precipitates. In the as-implanted sample, Zn nanoparticles up to 4.5 nm in diameter were observed and were distributed throughout the implanted depth in the SiO₂. The highest concentration of Zn from the implantation was at a depth of 25 nm. During annealing, Zn diffused into the substrate and combined with oxygen to form ZnO. ZnO nanostructures thus formed had diameters up to 8 nm, embedded in SiO₂. Donor-bound exciton (D, X), acceptor-bound exciton (A, X), and donor–acceptor-pair (DAP) transitions were observed in low temperature photoluminescence (PL) measurements on an annealed sample. RT-PL measurement showed band-edge emission in the ultraviolet region with a full width at half maximum of 121 meV. Time-resolved PL measurements performed at 4 K revealed an excitonic lifetime of 160 ps.     

Structures of planar defects in ZnO nanobelts and nanowires

Quasi-one-dimensional (1D) nanostructures, such as nanowires, nanobelts and nanorods, are the forefront materials for nanotechnology. To date, such nanostructures have been synthesized for a wide range of semiconductors and oxides, and they are potential building blocks for fabricating numerous nano-scale devices. 1D ZnO nanostructures, due to its unique semiconducting, piezoelectric, and bio-safe properties, have received wide attention. From structure point of view, a common characteristic of ZnO nanostructures is that they are mostly dislocation-free. However, planar and point defects do frequently exist in such nanostructures. The objective of this paper is to present detailed electron microscopy study about the structures of planar defects, such as stacking faults, twins, inversion domain walls that existed in 1D ZnO nanostructures. These planar defects are important for understanding the growth mechanism and relevant physical and possibly chemical properties of 1D ZnO nanostructures.     

The effect of substrate surface roughness on ZnO nanostructures growth

The ZnO nanowires have been synthesized using vapor-liquid-solid (VLS) process on Au catalyst thin film deposited on different substrates including Si(1 0 0), epi-Si(1 0 0), quartz and alumina. The influence of surface roughness of different substrates and two different environments (Ar + H₂ and N₂) on formation of ZnO nanostructures was investigated. According to AFM observations, the degree of surface roughness of the different substrates is an important factor to form Au islands for growing ZnO nanostructures (nanowires and nanobelts) with different diameters and lengths. Si substrate (without epi-taxy layer) was found that is the best substrate among Si (with epi-taxy layer), alumina and quartz, for the growth of ZnO nanowires with the uniformly small diameter. Scanning electron microscopy (SEM) reveals that different nanostructures including nanobelts, nanowires and microplates have been synthesized depending on types of substrates and gas flow. Observation by transmission electron microscopy (TEM) reveals that the nanostructures are grown by VLS mechanism. The field emission properties of ZnO nanowires grown on the Si(1 0 0) substrate, in various vacuum gaps, were characterized in a UHV chamber at room temperature. Field emission (FE) characterization shows that the turn-on field and the field enhancement factor (β) decrease and increases, respectively, when the vacuum gap (d) increase from 100 to 300 μm. The turn-on emission field and the enhancement factor of ZnO nanowires are found 10 V/μm and 1183 at the vacuum gap of 300 μm.     

Low-temperature synthesis of ZnO nanonails

Wurtzite ZnO nanonails on silicon substrate were successfully synthesized by thermal vapor transport and condensation method at a low temperature without a metal catalyst. Pure Zn powders were used as raw material and O₂/Ar powders as source gas. The products were characterized by scanning electron microscopy, transmission electron microscopy and X-ray diffraction. The results show that the deposited nanostructures include aligned ZnO nanonails. The ZnO nanonails, with crystalline cap and small-diameter shafts, grow along the c-axis. The optical properties have been revealed by photoluminescence spectra. We considered that the ZnO nanonails growth is a vapor-solid process.     

Effect of annealing temperature on optical and electrochemical properties of chitosan–ZnO nanostructure

Chitosan–ZnO nanostructures were prepared by chemical precipitation method using different concentration of zinc chloride and sodium hydroxide solutions. Nanorod-shaped grains with hexagonal structure for samples annealed at 300 °C and porous structure with amorphous morphology for samples annealed at 600 °C were revealed in SEM analysis. X-ray diffraction patterns confirmed the hexagonal phase ZnO with crystallite size found to be in the range of ~24.15–34.83 nm. Blue shift of UV–Vis absorption shows formation of nanocrystals/nanorods of ZnO with marginal increase in band gap. Photoluminescence spectra show that blue–green emission band at 380–580 nm. The chitosan–ZnO nanostructures used on surface of a glassy carbon electrode gives the oxidation peak potential at ~0.6 V. The electrical conductivity of chitosan–ZnO composites were observed at 2.1?×?10^?5 to 2.85?×?10^?5?S/m. The nanorods with high surface area and nontoxicity nature of chitosan–ZnO nanostructures observed in samples annealed at 300 °C were suitable as a potential material for biosensing.     

Fabrication of three-dimensional ZnO with hierarchical structure via an electrodeposition process

A novel three-dimensional (3D) hierarchical structured ZnO was prepared on TiO₂ nanoparticles film by electrodeposition process from aqueous ZnCl₂ solution. The hierarchical structured ZnO was observed by scanning electron microscopy. The results showed that the deposition time had an obvious effect on the morphology of the ZnO structures. Accordingly, a possible growth mechanism was proposed. Furthermore, the room-temperature optical properties of hierarchical structured ZnO were investigated by photoluminescence spectrum, indicating that a strong green emission peak centered at 542 nm.     

A comparative study of conventional type II and inverted core–shell nanostructures based on CdSe and ZnS

The objective of this work is to investigate structural, morphological and optical properties of conventional CdSe/ZnS core–shell and inverted ZnS/CdSe core–shell nanostructures for opto-electronic device applications. For this purpose both nanostructures were synthesized using chemical bath deposition technique in thin film form. The structural properties were studied using X-ray diffraction technique with Rietveld refinement and transmission electron microscopy (TEM). The surface morphology of synthesized thin film was illustrated in the form 2D and 3D images using atomic force microscopy (AFM). The optical properties were explained using UV–Vis absorption spectroscopy and photo luminescence (PL) spectroscopy in in situ monitoring process. A comparison of estimated particle size from XRD, high resolution AFM and TEM images was resulted in good agreement as 2.1, 2.4 and 2.1 nm respectively for conventional CdSe/ZnS core–shell and as 2.5, 2.5 and 2.2 nm respectively for inverted ZnS/CdSe core–shell nanostructures.