Synthesis and optical properties of flower-like ZnO nanorods by thermal evaporation method

Flower-like ZnO nanorods have been synthesized by heating a mixture of ZnO/graphite powders using the thermal evaporation and vapor transport on Si (1 0 0) substrates without any catalyst. The structures, morphologies and optical properties of the products were characterized in detail by using X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) and Raman spectroscopy. The synthesized products consisted of large quantities of flower-like ZnO nanostructures in the form of uniform nanorods. The flower-like ZnO nanorods had high purity and well crystallized wurtzite structure, whose high crystalline quality was proved by Raman spectroscopy. The as-synthesized flower-like ZnO nanorods showed a strong ultraviolet emission at 386 nm and a weak and broad yellow-green emission in visible spectrum in its room temperature photoluminescence (PL) spectrum. In addition, the growth mechanism of the flower-like ZnO nanorods was discussed based on the reaction conditions.     

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.     

Self-assembled MnS flower-like hierarchical architectures on porous alumina membrane

MnS flower-like hierarchical architectures were self-assembled on the surface of porous alumina membrane (PAM) under hydrothermal condition. The diameter of MnS flower-like hierarchical architectures is about 2-4 μm, which are composed of single-crystal nanowires with width of 70-80 nm. X-ray diffraction and high-resolution transmission electron microscopy analysis demonstrated the nanowire have preferred orientation along [1 1 0] direction. Prolonged reaction time would result in hollow spheres. Studies show that PAM and gas bubbles formed within the nanopores of PAM under hydrothermal condition play an important role in the formation process of MnS flower-like hierarchical architectures. The room-temperature PL spectrum shows a strong emission peak at 420 nm corresponding to the MnS band edge emission.     

Fabrication and characterization of Mg-doped pencil-shaped ZnO microprisms

Two types of novel Mg-doped pencil-shaped ZnO microprisms had been successfully synthesized on Mg(NO₃)₂-coated Si (1 1 1) substrates by thermal chemical vapor deposition method. The as-prepared ZnO prisms were characterized with X-ray diffraction (XRD), scanning electron microscopy (SEM), field-emission transmission electron microscope (FETEM), selected area electron diffraction (SAED), and photoluminescence (PL) spectroscopy. The straight microprisms are made up of hexagonal pyramids tips and hexagonal prisms bodies. Both of the structures are perfect single crystal and have grown along the [0 0 0 1] direction preferentially. Photoluminescence reveals a red-shift at around 387 nm which is induced by Mg doping and a green light emission peak at around 511 nm. The pencil-shaped ZnO microstructure can provide an improvement in novel ultraviolet light-emitting devices. In addition, the growth mechanism of the special ZnO microprisms is discussed briefly.     

Synthesis, characterization and optical properties of multipod ZnO whiskers

Multipod ZnO whiskers were synthesized successfully by two steps: pulsed laser deposition (PLD) and thermal evaporation process. First, a thin layer of Zn films were deposited on Si(1 1 1) substrates by PLD. Then the whiskers grew on Zn-coated Si(1 1 1) substrate by the simple thermal evaporation oxidation of the metallic zinc powder at 900 °C in the air without any catalysts or additives. The pre-deposited Zn films by PLD on the substrate can promote the growth of ZnO multipod whiskers effectively. The as-synthesized ZnO whiskers were characterized by using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM). The results revealed that the whiskers are highly crystalline with the wurtzite hexagonal structure. Room temperature photoluminescence (PL) spectrum of the whiskers shows a UV emission peak at ∼393 nm and a broad green emission peak at ∼517 nm, which was assigned to the near band-edge emission and the deep-level emission, respectively.     

Growth and interconversion of ZnO nanostructure films on different substrates

Hexagonal c-axis oriented ZnO nanorod (NR) arrays were grown on ZnO-coated glass substrates by chemical bath deposition (CBD). By varying the concentration of the precursor solutions, NRs’ mean diameter was controlled from about 100-400 nm. With increasing the growing time, the morphology of as-synthesized ZnO NRs evolved: the top center of the ZnO NRs was eroded into volcano-like structures which developed into ZnO nanotubes (NT) and finally the ZnO NTs split to form ZnO nanosheets (NS). We have also introduced Al to the fabrication of some different ZnO nanostructures: on Al substrate, freestanding ZnO NSs formed a flower-like structure at the early growing period and these ZnO NSs reassembled into ZnO NTs as growing time increased; while on ZnO-coated glass substrate with an Al foil dipped in the growing precursor solution ZnO NR-NS composited arrays were synthesized.     

Characterization and optical property of ZnO nano-, submicro- and microrods synthesized by hydrothermal method on a large-scale

In the present paper, well-dispersed ZnO nano-, submicro- and microrods with hexagonal structure were synthesized by a simple low temperature hydrothermal process from zinc nitrate hexahydrate without using any additional surfactant, organic solvent or catalytic agent. The phase and structural analysis were carried out by X-ray diffraction (XRD), the morphological analysis was carried out by field emission scanning electron microscopy (FESEM) and the optical property was characterized by room-temperature photoluminescence (PL) spectroscopy. The results revealed the high crystal quality of ZnO powder with hexagonal (wurtzite-type) crystal structure and the formation of well-dispersed ZnO nano-, submicro- and microrods with diameters of about 50, 200 and 500 nm, and lengths of 300 nm, 1 μm and 2 μm, respectively, on a large-scale just using the different temperatures. Room-temperature PL spectrum from the ZnO nanorods reveals a strong UV emission peak at about 360 nm and no green emission band at ∼530 nm. The strong UV photoluminescence indicates the good crystallization quality of the ZnO nanorods. Room-temperature PL spectra from the ZnO submicro- and microrods reveal a weak UV emission peak at ∼400 nm and a very strong visible green emission at 530 nm, that is ascribed to the transition between V_oZn_i and valence band.     

Synthesis of Cu-ZnO and C-ZnO nanoneedle arrays on zinc foil by low temperature oxidation route: Effect of buffer layers on growth, optical and field emission properties

Different densities of ZnO nanoneedle films have been prepared by pre-coated zinc foils with thin layer of copper and carbon followed by thermal oxidation at 400 °C in air. The X-ray diffraction patterns show well defined peaks, which could be indexed to the wurtzite hexagonal phase of ZnO. The scanning electron microscope images clearly reveal formation of ZnO needles on the entire substrate surface. The X-ray photoelectron spectroscopy studies indicate that Cu and C ions are incorporated into the ZnO lattice. Photoluminescence studies evaluate different emission bands originated from different defect mechanism. From the field emission studies, the threshold field, required to draw emission current density of ∼100 μA/cm², is observed to be 2.25 V/μm and 1.57 V/μm for annealed zinc foil pre-coated with copper and carbon, respectively. The annealed film with copper layer exhibits good emission current stability at the pre-set value of ∼100 μA over a duration of 4 h. The results show that buffer layer is an important factor to control the growth rate, resulting in different density of ZnO needles, which leads to field emission properties. This method may have potential in fabrication of electron sources for high current density applications.     

Structural and optical properties of ZnO and Al-doped ZnO microrods obtained by spray pyrolysis method using different solvents

ZnO and Al-doped ZnO microrods were obtained by spray pyrolysis method using different solvents such as methanol and propanol. The effect of the type of solvent in the starting solution on the structural, morphological and optical properties of the samples was investigated. X-ray diffraction patterns showed that the undoped and Al-doped ZnO microrods exhibited hexagonal crystal structure with a preferred orientation along (0 0 2) direction. Surface morphology of the samples obtained by scanning electron microscopy revealed that undoped and Al-doped ZnO microrods grew as quasi-aligned hexagonal shaped microrods with diameters varying between 0.7 and 1.3 μm irrespective of solvents used. Optical studies indicated that microrods had a low transmittance (≅30%) and the band gap increased from 3.24 to 3.26 eV upon Al doping. Photoluminescence measurements indicated the existence of two emission bands in the spectra: one sharp ultraviolet luminescence at ∼383 nm and one broad visible emission ranging from 420 to 580 nm.     

Surface morphology and optical properties of ZnO epilayers grown on Si(1 1 1) by metal organic chemical vapor deposition

Heteroepitaxial ZnO epilayers were grown on Si(1 1 1) substrates using a vertical geometry atmospheric pressure metal organic chemical vapor deposition (AP-MOCVD) system. The growth temperature was varied from 550 °C to 650 °C in steps of 25 °C. The ZnO growth rate and surface morphology were strong functions of the growth temperature and ranged from ∼0.16 μm/h to 1.36 μm/h. The surface morphology of the ZnO films changed from granular to sharp tips as the growth temperature increased. The effect of buffer thickness was also examined, and was found to have a strong effect on the optical properties of the ZnO. An optimized growth condition for ZnO epilayers was found at 625 °C, producing a FWHM in the room temperature photoluminescence (PL) spectrum of 4.5 nm and a preferred growth orientation along the (0 0 2) direction.Transmission electron microscopy images and selected area diffraction patterns showed excellent crystalline quality of both the buffer and ZnO overlayer. When non-optimized growth temperatures were employed, post-growth annealing was found to greatly enhance the ratio of band-edge to deep level emission.     

Porous ZnO nanobelts evolved from layered basic zinc acetate nanobelts

Novel porous ZnO nanobelts were successfully synthesized by heating layered basic zinc acetate (LBZA) nanobelts in the air. The precursor of LBZA nanobelts consisted of a lamellar structure with two interlayer distances of 1.325 and 0.99 nm were prepared using a low-temperature, solution-based method. X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and infrared spectroscopy are used to characterize the as-products. PL measurements show that the porous ZnO nanobelts have strong ultraviolet emission properties at 380 nm, while no defect-related visible emission is detected. The good performance for photoluminescence emission makes the porous ZnO nanobelts promising candidates for photonic and electronic device applications.     

Two-dimensional zinc oxide nanostructure

Transparent two-dimensional ultralong and ultrabroad single crystal zinc oxide (ZnO) nanosheets were directly synthesized by a simple solid vapor deposition process under lead oxide (PbO) atmosphere. The nanosheets are well grown single crystals with thickness of about 50-70 nm, breadth of 50-100 μm and length of 4-6 mm. The growth mode of the ultrabroad nanosheets displays a unique aspect that (001) planes form the narrowest facets of the nanosheets, which is completely different from other belt-like nanostructures of ZnO. Control experiments show that PbO play an important role in the vapor-solid growth process of ZnO nanosheets.     

Synthesis and optical characterization of vertically grown ZnO nanowires in high crystallinity through vapor-liquid-solid growth mechanism

The gas-phase growth and optical characteristics of 1-dimensional ZnO nanostructure have been investigated. The ZnO nanowires (NWs) were grown vertically on Au coated silicon substrates by vapor-liquid-solid (VLS) growth mechanism using chemical vapor deposition (CVD). The ZnO NWs were grown in the crystal direction of [0 0 0 1]. The ZnO NWs exhibit the uniform size of less than 100 nm in diameter and up to 5 μm in length. Photoluminescence (PL) spectrum of ZnO NWs shows the strong band-edge emission at ∼380 nm (∼3.27 eV) without significant deep-level defect emission. The exciton lifetime of ZnO NWs was measured to be approximately 150 ± 10 ps.     

A template-free sol-gel technique for controlled growth of ZnO nanorod arrays

The growth of ZnO nanorod arrays via a template-free sol-gel process was investigated. The nanorod is single-crystalline wurtzite structure with [0 0 0 1] growth direction determined by the transmission electron microscope. The aligned ZnO arrays were obtained directly on the glass substrates by adjusting the temperatures and the withdrawal speeds, without seed-layer or template assistant. A thicker oriented ZnO nanorod arrays was obtained at proper experimental conditions by adding dip-coating layers. Room temperature photoluminescence spectrum exhibits an intensive UV emission with a weak broad green emission as well as a blue double-peak emission located at 451 and 468 nm, respectively. Further investigation results show that the difference in the alignment of nanorods ascribes to the different orientations of the nanoparticles-packed film formed prior to nanorods on the substrate. Well ordered ZnO nanorods are formed from this film with good c-axis orientation. Our study is expected to pave a way for direct growth of oriented nanorods by low-cost solution approaches.     

Effects of growth duration on the structural and optical properties of ZnO nanorods grown on seed-layer ZnO/polyethylene terephthalate substrates

Well-aligned single crystalline zinc oxide (ZnO) nanorods were successfully grown, by hydrothermal synthesis at a low temperature, on flexible polyethylene terephthalate (PET) substrates with a seed layer. Photoluminescence (PL), field-emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) measurements were used to analyze the optical and structural properties of ZnO nanorods grown for various durations from 0.5 h to 10 h. Regular and well-aligned ZnO nanorods with diameters ranging from 62 nm to 127 nm and lengths from 0.3 μm to 1.65 μm were formed after almost 5 h of growth. The growth rate of ZnO grown on PET substrates is lower than that grown on Si (1 0 0) substrates. Enlarged TEM images show that the tips of the ZnO nanorods grown for 6 h have a round shape, whereas the tips grown for 10 h are sharpened. The crystal properties of ZnO nanorods can be tuned by using the growth duration as a growth condition. The XRD and PL results indicate that the structural and optical properties of the ZnO nanorods are most improved after 5 h and 6 h of growth, respectively.     

Modified thermal evaporation process using GeO2 for growing ZnO structures

In the current work, zinc oxide (ZnO) nano/microstructures are synthesized using a modified thermal-evaporation process by introducing germanium oxide (GeO₂) powder mixed with metallic Zn powder as the raw material. Without the use of any catalyst and oxygen flow in the furnace system, GeO₂ is utilized to provide an oxygen source for the growth of ZnO structure. The samples are treated by different temperatures ranging from 500 to 900 °C. Morphology, phase structure, and photoluminescence properties are investigated by scanning electron microscopy (SEM), X-ray diffractometer (XRD) and photoluminescence (PL) spectrometer. The structures and morphologies of the samples were found to vary with growth temperature. The XRD diffraction peaks show that the films grown at temperature from 600 to 800 °C consist of hexagonal wurtzite ZnO structures. Room-temperature PL measurement revealed ZnO spectra representing two bands: near-band-edge emission in the ultraviolet (UV) region and broad deep-level emission centered at about 500 nm. The strong UV emission in the PL spectra indicates that the GeO₂ supplies sufficient oxygen for formation of ZnO structures with few oxygen vacancies. The growth mechanism and the roles of GeO₂ for formation of ZnO structures are discussed in detail.     

Synthesis of ZnO nanowires by pulsed laser deposition in furnace

ZnO nanowires were fabricated on Au coated (0 0 0 1) sapphire substrates by using a pulsed Nd:YAG laser with a ZnO target in furnace. ZnO nanowires have various sizes and shapes with a different substrate position inside a furnace. The length and the diameter of these ZnO nanowires were around 3-4 μm and 120-200 nm, respectively, confirmed by scanning electron microscopy (SEM). The diameter control of the nanowires was achieved by varying the position of substrates. The ultraviolet emission of nanowires from the near band-edge emission (NBE) was observed at room temperature. The formation mechanism and the effect of different position of substrates on the structural and optical properties of ZnO nanowires are discussed.     

Rapid synthesis of novel flowerlike ZnO structures by thermolysis of zinc acetate

Novel flowerlike ZnO structures have been rapidly synthesized on (1 0 0)-Si substrates via thermolysis of zinc acetate in air ambient without any catalyst. The obtained ZnO products exhibit well-defined flowerlike morphologies consisting of multilayer petal crystals with tapering feature. High-resolution transmission electron microscope (HRTEM) and corresponding selected area electron diffraction pattern (SAED) reveal that these petal crystals are single crystal in nature and preferentially oriented in the c-axis direction. Room-temperature photoluminescence (PL) spectra show that all the samples exhibit prominent UV emissions around 376.8 nm and very weak visible emission peaks, which demonstrates that there are few deep-level defects in the single crystal petals of the flowerlike ZnO structures. The growth mechanism of the as-synthesized flowerlike ZnO structures was also discussed.     

Carbon nanosheets by microwave plasma enhanced chemical vapor deposition in CH4-Ar system

We employ a new gas mixture of CH₄-Ar to fabricate carbon nanosheets by microwave plasma enhanced chemical vapor deposition at the growth temperature of less than 500 °C. The catalyst-free nanosheets possess flower-like structures with a large amount of sharp edges, which consist of a few layers of graphene sheets according to the observation by transmission electron microscopy. These high-quality carbon nanosheets demonstrated a faster electron transfer between the electrolyte and the nanosheet surface, due to their edge defects and graphene structures.     

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.