Vapor Phase Transport Process Research Articles

ZnO Nanowires on Single-Crystalline Aluminum Film Coupled with an Insulating WO3 Interlayer Manifesting Low Threshold SPP Laser Operation.

ZnO nanowire-based surface plasmon polariton (SPP) nanolasers with metal–insulator–semiconductor hierarchical nanostructures have emerged as potential candidates for integrated photonic applications. In the present study, we demonstrated an SPP nanolaser consisting of ZnO nanowires coupled with a single-crystalline aluminum (Al) film and a WO3 dielectric interlayer. High-quality ZnO nanowires were prepared using a vapor phase transport and condensation deposition process via catalyzed growth. Subsequently, prepared ZnO nanowires were transferred onto a single-crystalline Al film grown by molecular beam epitaxy (MBE). Meanwhile, a WO3 dielectric interlayer was deposited between the ZnO nanowires and Al film, via e-beam technique, to prevent the optical loss from dominating the metallic region. The metal–oxide–semiconductor (MOS) structured SPP laser, with an optimal WO3 insulating layer thickness of 3.6 nm, demonstrated an ultra-low threshold laser operation (lasing threshold of 0.79 MW cm−2). This threshold value was nearly eight times lower than that previously reported in similar ZnO/Al2O3/Al plasmonic lasers, which were ≈2.4 and ≈3 times suppressed compared to the SPP laser, with WO3 insulating layer thicknesses of 5 nm and 8 nm, respectively. Such suppression of the lasing threshold is attributed to the WO3 insulating layer, which mediated the strong confinement of the optical field in the subwavelength regime.

One-step synthesis of high purity ZnO micro/nanostructures from pure Zn and pre-alloyed brass powders by vapor phase transport

In this research, vapor phase transport (VPT) was introduced as a facile, inexpensive method to produce ZnO micro/nanostructures from various Zn sources such as pure Zn and alpha brass pre-alloyed powders (Cu–20Zn and Cu–28Zn) at different processing temperatures of 930 °C–1050 °C. Simultaneous thermal analysis (STA) was carried out to investigate Zn evaporation and ZnO micro/nanostructure formation. STA results showed an exothermic peck at 711 °C and 728 °C for Cu–20Zn and Cu–28Zn, respectively, due to oxidation of the evaporated Zn element and formation of ZnO micro/nanostructures. X-ray diffraction results showed that high purity ZnO micro/nanostructures were successfully synthesized via VPT process and the crystallite size was increased from ~60 nm to ~100 nm with increasing processing temperature. Field emission scanning electron microscopy observations showed morphology (e.g. rods, column, tetrapods, and combs) and size of the synthesized micro/nanostructures were dependent on the Zn sources and processing temperature, in which average diameter of the synthesized ZnO structures was increased with increasing the processing temperature. The smallest (98 nm) and largest (603 nm) average diameters of synthesized ZnO micro/nanostructures were attained from the pure Zn and Cu–28Zn brass powders at 930 °C and 1050 °C, respectively.

Low threshold room-temperature UV surface plasmon polariton lasers with ZnO nanowires on single-crystal aluminum films with Al2O3 interlayers.

ZnO is one of the most promising optical gain media and allows lasing in ZnO nanowires at room temperature. Plasmonic lasers are potentially useful in applications in biosensing, photonic circuits, and high-capacity signal processing. In this work, we combine ZnO nanowires and single-crystalline aluminum films to fabricate Fabry–Perot type surface plasmon polariton (SPP) lasers to overcome the diffraction limit of conventional optics. High quality ZnO nanowires were synthesized by a vapor phase transport process via catalyzed growth. The ZnO nanowires were placed on a single-crystalline Al film grown by molecular beam epitaxy with an interlayer Al2O3 deposited by atomic layer deposition. The plasmonic laser is of metal-oxide-semiconductor (MOS) structure, compatible with silicon device processing. An optimal thickness of atomic layer deposited Al2O3 layer can lead to a low lasing threshold, 6.27 MW cm−2, which is 3 times and 12 times lower than that of previous reports for ZnO/Al and Zno/Al2O3/Al plasmonic lasers, respectively, owing to low materials loss. Both the thickness and quality of insulating layers were found to critically influence the lasing threshold of the SPP nanolasers in the subwavelength regime. The simulation results also manifest the importance of the quality of the dielectric interlayer.

A facile growth process of highly single crystalline Ir1−xVxO2 mixed metal oxide nanorods and their electrochemical properties

We introduce a facile growth process for mixed metal oxide nanorods of Ir1−xVxO2 in the broad range of metal contents between iridium and vanadium via a vapor phase transport process and their electrochemical performance. The growth process of Ir1−xVxO2 nanorods was performed by modulating the ratios of two precursors of VO2 and IrO2, the growth temperature, the position of metal oxide precursors in a furnace, and the flow rate of carrier gases. The results indicated that quasi one-dimensional nanostructures of Ir1−xVxO2 can be obtained of a single crystalline nature with the tetragonal rutile crystal structure by randomly incorporating Ir4+ and V4+ metal ions into the same lattice unit of the tetragonal structure owing to similar ionic sizes. In addition, the fundamental electrochemical performances for Ir1−xVxO2 nanorods grown directly on an Au microelectrode of 25 μm in diameter were carefully tested in order to explore the nonenzymatic amperometric sensing capacity of ascorbic acid as a biologically important molecule.

Synthesis and Photoluminescence Studies of ZnO Films Using Vapor Phase Transport Combined With Rapid Heating

The vapor phase transport (VPT) process is usually used for the growth of one-dimensional nanostructures rather than for film deposition. In this study, for the first time, we report on the fabrication and optical propeties of ZnO film produced by VPT combined with rapid heating. The X-ray diffraction results showed that the intensity of the ZnO (002) peak was the largest, which indicated that the c-axis orientation of the ZnO grains was perpendicular to the substrate. In the photoluminescence (PL) spectra of the ZnO film at 12 K, PL peaks were observed at 3.374, 3.362, 3.319, and 3.242 eV, which were attributed to free excitons (FXs), excitons bound to neutral donors, donor-acceptor pairs (DAPs), and first-order longitudinal optical phonon replicas of DAPs (DAPs-1LO), respectively. The energy interval between DAPs and DAPs-1LO was almost the same as the longitudinal optical (LO) phonon energy of bulk ZnO of ~76 meV. The PL spectra in the temperature range from 12 to 300 K shifted to lower energy with an increasing temperature. The activation energy of FX was estimated to be 61 meV. This value is similar to the exciton binding energy of 60 meV. (Received September 4, 2015; Accepted December 11, 2015)

Direct resonant coupling of Al surface plasmon for ultraviolet photoluminescence enhancement of ZnO microrods.

More than 170-fold ultraviolet emission enhancement was obtained from the hexagonal ZnO microrods synthesized by a simple vapor-phase transport process and decorated by Al nanoparticles (NPs). Based on the stable and transient photoluminescence (PL) spectra of the ZnO microrods sputtered with Al NPs for different times, the underlying mechanism was deduced and can be attributed to the metal surface plasmon resonance (SPR) coupling with ZnO. Interestingly, with increasing of the sputtering time, the ratio of the band gap emission to the defect-related emission was increased from 0.1 to 42.7. Our results demonstrated that ZnO microrods decorated with Al NPs shed light on developing stable and high-efficiency excitonic optoelectronic devices such as light-emitting diodes and lasers.

Probing the Growth Habit of Highly Single Crystalline Twinned V-Shape RuO2 Nanowires by Polarized Raman Scattering

Polarized Raman scattering measurements have been carefully employed to explore the growth orientation and direction of a highly single crystalline twinned V-shaped ruthenium dioxide nanowire which were directly grown on a Si wafer with the thin SiO2 layer at 750 °C without any catalyst via a vapor phase transport process. Interestingly, the morphology of most of nanowires represents a well-defined twinned V-shape with a specific angle of approximately 52° between the two branches under a specific growth condition. We measured polarized Raman spectra of a single V-shaped nanowire and calculated the Raman intensity of RuO2 nanowire by considering a crystal coordinate. We observed that each branch of a V-shaped RuO2 nanowire grows into [001] direction, exactly consistent with the prediction of the growth direction by high resolution transmission electron microscopy (HRTEM).

Highly Efficient Electrochemical Responses on Single Crystalline Ruthenium–Vanadium Mixed Metal Oxide Nanowires

Highly efficient single crystalline ruthenium-vanadium mixed metal oxide (Ru1-xVxO2, 0≤x≤1) nanowires were prepared on a SiO2 substrate and a commercial Au microelectrode for the first time through a vapor-phase transport process by adjusting the mixing ratios of RuO2 and VO2 precursors. Single crystalline Ru1-xVxO2 nanowires show homogeneous solid-solution characteristics as well as the distinct feature of having remarkably narrow dimensional distributions. The electrochemical observations of a Ru1-xVxO2 (x=0.28 and 0.66)-decorated Au microelectrode using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) demonstrate favorable charge-transfer kinetics of [Fe(CN)6]3-/4- and Ru(NH3)6(3+/2+) couples compared to that of a bare Au microelectrode. The catalytic activity of Ru1-xVxO2 for oxygen and H2O2 reduction at neutral pH increases as the fraction of vanadium increases within our experimental conditions, which might be useful in the area of biofuel cells and biosensors.

Hierarchically assembled 1-dimensional hetero-nanostructures: single crystalline RuO2 nanowires on electrospun IrO2 nanofibres

We report a facile growth route to synthesize hierarchically grown single crystalline metallic RuO2 nanowires on electrospun IrO2 nanofibres via a combination of a simple vapour phase transport process with an electrospinning process. This synthetic strategy could be extended to design a variety of highly branched network architectures of functional hetero-nanostructures with possible future applications.

Synthesis of aligned ZnO nanocolumn arrays by a vapor phase transport method

Vertically large-scale aligned ZnO nanocolumn arrays have been synthesized on Si(100) substrate at 550°C via a simple vapor phase transport process in a horizontal chemical vapor depositon (CVD) reactor. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations show that the ZnO nanocolumns have a uniform length of about 3 μm with an average diameter of 80 nm. The clear lattice fringes in HRTEM image indicate the synthesis of good quality hexagonal single crystal ZnO. The X-ray diffraction (XRD) result shows the nanocolumns are c-axis preferentially oriented.

Growth of Highly Single Crystalline IrO2 Nanowires and Their Electrochemical Applications

We present the facile growth of highly single crystalline iridium dioxide (IrO2) nanowires on SiO2/Si and Au substrates via a simple vapor phase transport process under atmospheric pressure without any catalyst. Particularly, high-density needle-like IrO2 nanowires were readily obtained on a single Au microwire, suggesting that the melted surface layer of Au might effectively enhance the nucleation of gaseous IrO3 precursors at the growth temperature. In addition, all the electrochemical observations of the directly grown IrO2 nanowires on a single Au microwire support favorable electron-transfer kinetics of [Fe(CN6)]4–/3– as well as Ru(NH3)63+/2+ at the highly oriented crystalline IrO2 nanowire surface. Furthermore, stable pH response is shown, revealing potential for use as a miniaturized pH sensor, confirmed by the calibration curve exhibiting super-Nernstian behavior with a slope of 71.6 mV pH–1.

Highly Single Crystalline IrxRu1–xO2 Mixed Metal Oxide Nanowires

The nanostructured iridium–ruthenium mixed metal oxide (IrxRu1–xO2, 0 ≤ x ≤ 1) is the most promising candidate as a highly efficient electrode material for the electrolysis of water. We present a facile synthesis and structural characterization of highly single crystalline IrxRu1–xO2 mixed metal oxide nanowires for the first time via a simple vapor phase transport process by carefully controlling the relative ratios of IrO2 and RuO2 precursors. We also observed that an Eg mode of an IrxRu1–xO2 nanowire measured from Raman scattering is being linearly blue shifted and its line width is asymmetrically broadened with respect to the increase of the Ir contents, indicating that we could quantitatively determine stoichiometry information as well as crystal structure for mixed metal oxide nanowires.

Synthesis and characterization of amorphous SiOx nanowires prepared by vapor phase transport process

Large quantities of amorphous SiOx nanowires have been synthesized by vapor phase transport process under atmospheric pressure in a horizontal furnace. The fabricated materials have been fully characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), energy dispersive X-ray analysis (EDX) and X-ray photoelectron spectroscopy (XPS). The nanowires have a typical length up to several tens of micrometers and a diameter in the range of 100nm to 300nm. The formation mechanism of the nanowires has also been discussed. The covering boats and the weight ratio of the raw materials were regarded to play an important role during the growth of the nanowires.

Influence of Shell Thickness on the Raman Properties of ZnO/ZnS Core/Shell Nanowires

Well aligned ZnO nanowire arrays are fabricated by a simple vapor phase transport process. Field-emission scanning electron microscopy shows the nanorods have a uniform length of about 1 um with diameters of 100 nm. After modified by ZnS, ZnO/ZnS core/shell nanowire arrays are formed and the thickness of ZnS layer increase with the increase of treatment time. X-ray diffraction analysis confirms that the as-synthesized ZnO nanorods are c-axis orientated, the modification of ZnS shell induces weak ZnS (100) diffraction peak for the treatment time of 180 min. The effects of shell thickness as a function of ZnS treatment time on the Raman scattering properties are studied. The results suggest that the coating of ZnS shell will change the Raman energy position and intensity of the ZnO nanowires, crystal lattice expansion and reconstruction of the ZnO/ZnS interface are responsible for the observed changes.

Preparation of a Superhydrophobic Carbon Dilm by a Novel Vapor Phase Transport Process

Extreme water repellency is greatly desired for anticontamination and self-cleaning applications in new building walling materials. A superhydrophobic carbon film was fabricated on the pure nickel substrates by a novel vapor phase transport method. The surface of morphologies of the pure nickel surface was characterized using a non-contact mode of atomic force microscope (AFM) and the structural properties of as-grown carbon film was characterized using scanning electron micrograph (SEM). The contact angles and the rolling angles on such films were measured through an optical contact angle meter. Wettability studies revealed the films exhibited a superhydrophobic behaviour with a higher contact angle of 160.7°±2.1° and lower rolling angle of less than 4°-a water droplet moved easily on the surface. SEM showed that compared with pure smooth nickel surface, a lotus-like structrure with micro-nano hierarchical papillae was obviously observed on the superhydrobic carbon film surface. Such a speccial surface microstructure may result in the superhydrophobicity.

Low temperature deposition of zinc oxide nanoparticles via zinc-rich vapor phase transport and condensation

ZnO nanoparticles as small as 80 nm were successfully synthesized using a modified vapor phase transport (VPT) process at substrate temperatures as low as 222 °C. Particle size distribution and morphology were characterized by scanning electron microscopy and atomic force microscopy. Energy dispersive X-ray spectroscopy and X-ray diffraction indicate the synthesis of high quality crystalline ZnO structures. Low temperature (4.2 K) photoluminescence (PL) spectroscopy was used to characterize the optical quality of the nanoparticles. Ultraviolet emission and a nanostructure specific feature at 3.366 eV are strong in the PL spectra. The 3.366 eV feature is observed to predominate the spectrum with decrease in particle size. This size effect corroborates the luminescence as a nanostructure-specific surface related exciton feature as previously speculated in the literature. In addition, self-assembled ZnO mesoparticles (>100 nm) were realized by increasing the growth time. Low growth temperatures of the particles allow for their potential utilization in flexible organic hybrid optoelectronics. However, this work focuses mainly on the modified synthesis and optical characterization of nanoparticles.

Growth Mechanisms and Characteristics of ZnO Nanostructures Doped with In and Ga

In this paper we present the crystallization, photoluminescence (PL), and field-emission (FE) properties of ZnO nanostructures doped with In and Ga cationic substituents and grown by the vapor-phase transport process. During the growth, Zn/ZnOx was adsorbed on the surface of Ag nanograins and self-catalyzed to form ZnO nanoparticles. Hexagonal-faced nanobricks and nanorods were grown by increasing the ZnO vapor concentration. However, nanodisks rather than nanobricks were grown when In2O3 was doped. Furthermore, the nanodisks aggregated to form nanoballs when the synthesis was carried out at high In2O3 doping concentrations. In contrast, nanostructures with a sea-urchin-like morphology were grown when Ga2O3 was doped; individual nanorods with a screw-dislocation structure grew from the same root. We present the growth mechanisms for the ZnO, ZnO:In, and ZnO:Ga nanostructures. ZnO:Ga nanorods exhibited better PL intensity and FE properties than ZnO nanorods and ZnO:In nanoballs.

A Novel Method to Synthesize a Large Area of Single Crystalline LiAl5O8 Nanorods

A large area of single crystalline LiAl5O8 nanorods was synthesized by a vapor phase transport process via Au-catalyzed crystal growth on LiAlO2 substrate. The nanorods were grown at high density, with the [111] axis maintained perpendicular to the [100] substrate surface by the direct reaction of CO gas and LiAlO2 substrate. The dependence of the nanorods’ diameters and lengths on the growth conditions was systematically investigated. The nanorods were characterized by X-ray diffraction, scanning electron microscopy, and high resolution transmission electron microscopy. These results indicated that the crystallized LiAl5O8 nanorods are free of segregations of a second phase and extended defects. The vapor−liquid−solid (VLS) crystal growth mechanism of the single crystalline LiAl5O8 is also discussed.

Indium-doped zinc oxide nanometre thick disks synthesised by a vapour-phase transport process

Indium-doped ZnO nanometre thick disks were successfully fabricated by thermal evaporation of a powder mixture of ZnO, In2O3, and graphite without catalyst. Incorporation of indium metal in the nanodisks during the synthesis was not successful due to the formation of In2O3 phase. SEM images show that some ZnO disks have perfect hexagonal shape. These disks are about 1–5 µm in size and 40–100 nm in thickness. X-ray diffraction, Raman, transmission electron microscopy, and energy dispersive spectroscopy observations show that the disks are single crystalline ZnO with wurtzite structure. Effect of indium doping on the structure, morphology, optical absorption, and photoluminescence of the nanodisks are studied.

Fine structure investigations on seeded dry gel and FER zeolite in a novel VPT process

An unusual crystallization kinetic curve composed of four different stages was determined for FER zeolite, which was transformed from a seeded Na 2O–SiO 2–Al 2O 3 dry gel (SDG) in tetrahydrofuran (THF)/H 2O vapor with a method of vapor phase transport (VPT) at 433 K for 72 h. The evolution of morphology and fine structures of the intermediate samples were investigated with FT–IR, SEM, TEM, HRTEM, SAED and N 2 adsorption to reveal the crystallization mechanism of SDG by VPT. The initial large globoids of SDG, which are composed of nano-scale amorphous globules, are found to diminish and disperse into smaller particles. Some mesoporous voids with the diameter of 10 nm are observed in SDG, which provide the channels for THF/H 2O vapor to enter and then react with the gel. The mesopores in the SDG gradually decrease to form dense flake-like dry gel, which is thought to be the primary part of gel crystallization in the VPT process. FER zeolite crystallites appear at 10.5 h, which are found to grow on the interface of dense dry gel. Most of the untransformed dry gel further becomes to the flake-like crystallites for another 25 h. The dry gel disappears at 36 h, and a re-crystallization stage occurs from 36 h to 72 h.