Catalyst-free synthesis of vertically-aligned ZnO nanowires by?nanoparticle-assisted pulsed laser deposition

Vertically aligned ZnO nanowires have been successfully synthesized on c-cut sapphire substrates by a catalyst-free nanoparticle-assisted pulsed-laser ablation deposition (NAPLD) in Ar and N₂ background gases. In NAPLD, the nanoparticles formed in the background gas by laser ablation are used for the growth of the nanowires. The surface density of the nanowires can be controlled by varying the density of nanoparticles, which is in turn achieved by varying ablation laser parameters such as the energy and the repetition rate. When single ZnO nanowire synthesized in a N₂ background gas was excited by 355 nm laser-pulse with a pulse-width of 8 ns, stimulated emission was clearly observed, indicating high quality of the nanowire.     

Aligned growth of ZnO nanowires by NAPLD and their optical characterizations

Not only vertically aligned ZnO nanowires but also horizontally aligned ZnO nanowires have been successfully grown on the annealed (0 0 0 1) c-cut and (1 1 2 0) a-cut sapphire substrates, respectively using catalyst-free nanoparticle-assisted pulsed-laser ablation deposition (NAPLD). The as-synthesized ZnO nanowires exhibit an ultraviolet emission at around 390 nm and the absent green emission under room temperature. The single ZnO nanowire was collected in the electrode gap by dielectrophoresis (DEP). Under the optical pumping, the single ZnO nanowire exhibited UV emission at around 390 nm with several sharp peaks whose energy spacings are almost constant, which greatly differs from the broad UV emission of the film with many nanowires, suggesting ZnO nanowires as candidates for laser media. The single ZnO nanowire showed polarized photoluminescence (PL). The as-synthesized ZnO nanowires could find many interesting applications in short-wavelength light-emitting diode (LED), laser diode and gas sensor.     

单根砷掺杂氧化锌纳米线场效应晶体管的电学及光学特性

采用化学气相沉积（CVD）的方法在砷化镓基底上合成直径为20 nm左右、长约数十微米的氧化锌纳米线,然后采用热扩散的方法,将生长于砷化镓基底之上的氧化锌纳米线通过600 ℃,30 min的有氧退火处理后,获得了砷掺杂的氧化锌纳米线.将获得的掺杂后的氧化锌纳米线采用电子束曝光以及真空溅射镀膜的方法将钛/金合金作为接触电极引出,从而构建成场效应晶体管.文中研究了单根氧化锌纳米线砷掺杂前后的电学特性,证实了通过砷掺杂来获得p型的氧化锌纳米线的可行性.构建的p型砷掺杂氧化锌场效应晶体管的跨导为35 nA/V,载流 关键词： p型ZnO纳米线 砷掺杂 场效应晶体管 光致发光     

Optical and structural properties of Eu-diffused and doped ZnO nanowires

Single crystal ZnO nanowires diffused with europium (Eu) from a solid source at 900 °C for 1 h or doped with Eu during growth have been characterized. The ZnO nanowires were grown by chemical vapor deposition on Si substrates employing Au as a catalyst. The diameter of the resulting nanowires was 200 nm with a length of 1 μm. Photoluminescence spectra excited by a He–Cd laser at room temperature showed the green luminescence at 515 nm in Eu-diffused nanowires. A small red shift of near-band-edge emission of ZnO nanowires was observed in the diffused wires, but sharp emission from Eu³ ions was not present. Transmission electron microscopy shows crystalline Eu₂O₃ formation on the diffused nanowire surface, which forms a coaxial heterostructure system. When Eu was incorporated during the nanowire growth, the sharp ⁵D_O–⁷F₂ transition of the Eu³⁺ ion at around 615 nm was observed.     

Synthesis and characterization of Ag-doped p-type ZnO nanowires

P-type ZnO nanowires with silver (Ag) doping were synthesized via a chemical vapor deposition process. The incorporation of Ag was confirmed by selected-area energy-dispersive x-ray spectroscopy. The formation of acceptor states was demonstrated by temperature and excitation power-dependent photoluminescence measurements. Characterization of field-effect transistors using Ag-doped ZnO nanowires as channels showed p-type conductivity of the nanowires with a hole concentration of 4.9×10¹⁷ cm⁻³ and a carrier mobility of approximately 0.18 cm² V⁻¹ s⁻¹.     

ZnO–SnO2 branch–stem nanowires based on a two-step process: Synthesis and sensing capability

ZnO–SnO₂ branch–stem nanostructures were realized on a basis of a two-step process. In step 1, SnO₂-stem nanowires were synthesized. In step 2, ZnO-branch nanowires were successfully grown on the SnO₂-stem nanowires through a simple evaporation technique. We have pre-deposited thin Au layers on the surface of SnO₂ nanowire stems and subsequently evaporated Zn powders on the nanowires. The ZnO branches, which sprouted from the SnO₂ stems, had diameters in a range of 30–35 nm. As-synthesized branches were of single crystalline hexagonal ZnO structures. Since the branch tips were comprised of Au-containing nanoparticles, the Au-catalyzed vapor–liquid–solid growth mechanism was more likely to control the growth process of the ZnO branches. To test a potential use of ZnO–SnO₂ branch–stem nanostructures in chemical gas sensors, their sensing performances with respect to NO₂ gas were investigated, showing the promising potential in chemical gas sensors.     

Pulsed laser deposition of Zr–N codoped <Emphasis Type="Italic">p</Emphasis>-type ZnO thin films

Present p-type ZnO films tend to exhibit high resistivity and low carrier concentration, and they revert to their natural n-type state within days after deposition. One approach to grow higher quality p-type ZnO is by codoping the ZnO during growth. This article describes recent results from the growth and characterization of Zr–N codoped p-type ZnO thin films by pulsed laser deposition (PLD) on (0001) sapphire substrates. For this work, both N-doped and Zr–N codoped p-type ZnO films were grown for comparison purposes at substrate temperatures ranging between 400 to 700 °C and N₂O background pressures between 10⁻⁵ to 10⁻² Torr. The carrier type and conduction were found to be very sensitive to substrate temperature and N₂O deposition pressure. P-type conduction was observed for films grown at pressures between 10⁻³ to 10⁻² Torr. The Zr–N codoped ZnO films grown at 550 °C in 1×10⁻³ Torr of N₂O show p-type conduction behavior with a very low resistivity of 0.89 Ω-cm, a carrier concentration of 5.0×10¹⁸ cm⁻³, and a Hall mobility of 1.4 cm² V⁻¹ s⁻¹. The structure, morphology and optical properties were also evaluated for both N-doped and Zr–N codoped ZnO films.     

Hydrothermal fabrication of well-ordered ZnO nanowire arrays on Zn foil: room temperature ultraviolet nanolasers

Well-ordered nanowires of the hexagonal wurtzite ZnO having an average diameter of 80 nm, a typical length of 12 μm, and a mean packing density of 7.5 nanowires μm⁻² have been directly grown on Zn foil in a preferred [0001] direction by a hydrothermal process and employed for room temperature ultraviolet nanolasers. The lasing action of arrayed ZnO nanowires has been observed from 370 to 400 nm with threshold irradiance of 25 kW cm⁻². Photoluminescence decays biexponentially: the fast component is attributed to free-exciton decay, and the slow one is to bound-exciton decay. The amplitude of the fast component increases whereas its lifetime decreases with the increment of threshold irradiance, suggesting that ZnO nanowire arrays undergo a change in the lasing mechanism from exciton–exciton scattering to electron–hole plasma recombination.     

Structure and electrical properties of p-type twin ZnTe nanowires

Resonant tunneling is firstly found in twin p-type ZnTe nanowire field-effect transistors. The twin ZnTe nanowires are synthesized via the thermal evaporation process. X-ray diffraction and high-resolution transmission electron microscopy characterization indicate that the as-grown twin nanowire has a zinc-blende crystal structure with an integrated growth direction of [11-1]. The twin plane is (11-1) and the angle between the mirror symmetrical planes is 141°. The formation of twins is attributed to the surface tension from the eutectic liquid droplet. Field-effect transistors based on single ZnTe twin nanowire are constructed, the corresponding electrical measurements demonstrate that the twin nanowires have a p-type conductivity with a mobility (μ _h ) of 0.11 cm² V⁻¹ S⁻¹, and a carrier concentration (n _h ) of 1.1×10¹⁷ cm⁻³. Significantly, the negative differential resistance with a peak-to-valley current ratio of about 1.3 is observed in p-type twin ZnTe nanowire field-effect transistors at room temperature. As the periodic barriers produced in the periodic twin interfaces can form multi-barrier and multi-well along one-dimensional direction. The multibarrier can be modulated under external electrical field. When the resonant condition is met, the space charge will be enhanced with the inherent feedback mechanism, and the resonant tunneling will occur.     

Effect of seed layer on the growth of well-aligned ZnO nanowires

We demonstrate that vertical well-aligned crystalline ZnO nanowire arrays were grown on ZnO/glass substrates by a low-temperature solution method. Different thicknesses of ZnO seed layers on glass substrates were prepared by radio-frequency sputtering. In this work it was found that the morphology of ZnO nanowires strongly depends on the thickness of ZnO seed layers. The average diameter of nanowires is increased from 50 to 130 nm and the nanowire density is decreased from 110 to 60 μm⁻² while the seed layer thickness is varied from 20 to 1000 nm. The improved control of the morphology of ZnO nanowire arrays may lead to an enhanced carrier collection of hybrid polymer photovoltaic devices based on ZnO.     

Growth of self-assembled ZnO nanowire arrays

This paper reports on ZnO nanowires arrays synthesized using Sn as a catalyst. The Sn particles were produced from the reduction of SnO₂ powders via a vapour-solid growth process. Control of growth conditions led to the formation of ZnO nanowire arrays, radial nanowire ‘flowers’ and uniaxial fuzzy nanowires. ZnO nanowire–nanobelt junctions were also grown by changing the growth direction. As-grown nanowire arrays could be fundamental materials for investigating physical and chemical properties at nano-scale dimensions.     

Influence of Sb in synthesize of ZnO nanowire using sandwich type substrate in carbothermal evaporation method

The paper deals with synthesis of Sb doped ZnO nanowire by considering Si coated with Sb and Au as substrate using carbothermal evaporation method. The horizontally oriented Sb doped ZnO nanowires with a diameter of 1 μm synthesized at 900 °C, which is quite high as compared to the Pure ZnO nanowires generated without the influence of Sb at 900 °C. The nanowire synthesized at 900 °C showed a measurable lower angle of about 0.06° from XRD and suppression of A₁T and E₁(L0) modes in Raman spectroscopic, this confirms the incorporation of Sb in ZnO lattice. The strong exciton emission and weak deep-level emission from room temperature PL and Strong emission attributed to the radiant recombination from neutral-acceptor-bound exciton (A⁰X) peak accompanied by two strong and broad emission of donor acceptor pair (DAP) from low temperature PL, this confirms the use of Sb as an acceptor for ZnO.     

UV-blocking ZnO nanostructure anti-reflective coatings

In this paper, we apply finite difference time domain simulation to determine the absorptance and reflectance of ZnO nanowire and nanohole array structures for an efficient UV-blocking anti-reflective coating. Comparing to ZnO thin films, both nanowires and nanoholes have much improved performance. ZnO nanowires and nanoholes have similar absorptions in the UV range. However, ZnO nanowires have lower absorptance than nanoholes in the visible range. Influences of different parameters including lattice constant a, ZnO filling ratio f and nanowire heights h are analyzed. The optical properties of the nanostructures are less dependent on the incident angle of light, which enables them to be used as wide angle anti-reflective coatings with UV blocking.     

Structural characterization of ZnO/ Er2O3 core/shell nanowires

Zinc oxide/erbium oxide core/shell nanowires are of great potential value to optoelectronics because of the possible demonstration of laser emission in the 1.5 μm range. In this paper we present a convenient technique to obtain structures of this composition. ZnO core nanowires were first obtained by a vapor–liquid–solid (VLS) method using gold as a catalyst. ZnO nanowires ranging from 50 to 100 nm in width were grown on the substrates. Erbium was incorporated into these nanowires by their exposure to Er(tmhd)₃ at elevated temperatures. After annealing at 700 C in air, the nanowires presented 1.54 μm emission when excited by any of the lines of an Ar⁺ laser. An investigation of nanowire structure by HRTEM indicates that indeed the cores consist of hexagonal ZnO grown in the 001 direction while the surface contains randomly oriented Er₂O₃ nanoparticles. EXAFS analysis reveals that the Er atoms possess a sixfold oxygen coordination environment, almost identical to that of Er₂O₃. Taken collectively, these data suggest that the overall architectures of these nanowires are discrete layered ZnO/ Er₂O₃ core/shell structures whereby erbium atoms are not incorporated into the ZnO core geometry.     

Effect of growth temperature on the ZnO nanowires prepared by thermal heating of Zn powders

ZnO nanowires have been synthesized by heating Zn powders under nitrogen (N₂) gas atmosphere. The influence of the growth temperature on the morphology, structure, and photoluminescence (PL) properties of ZnO nanowires has been investigated. At the higher-temperature growth process, thinner nanowires have been obtained. Interestingly, it is observed that the variation of growth temperature has significantly affected the photoluminescence spectra of the ZnO nanowires, showing an enhancement in the relative intensity of the green to UV emission bands with the increase of the growth temperature. In addition, the oxygen sensing properties of the as-synthesized ZnO nanowires were tested.     

Multi-photon-pumped stimulated emission from ZnO nanowires: A time-resolved study

In this work, we study temporal evolution of multi-photon-pumped stimulated emission from ZnO nanowires. In addition to second harmonic generation, ultraviolet stimulated emission is observed in ZnO nanowires under femtosecond pulse excitation at 800 nm. Sharp emission peaks appear when excitation flux reaches a threshold of 80 mJ/cm², which can be interpreted as lasing action in self-formed nanowire microcavities. Temporal evolution of the emission captured by Kerr shutter technique shows strong excitation-power dependence. The dynamic trace of stimulated emission exhibits a fast decay with a lifetime about 4.5 ps at intermediate excitation (∼100 mJ/cm²) and a lifetime about 2 ps at high excitation (>160 mJ/cm²). The difference in the lifetime can be attributed to different gain mechanisms related to excitonic interaction and electron-hole plasma, respectively.     

Effect of surface states on electron transport in individual ZnO nanowires

The current–voltage (I–V$I\text{–}V$) characteristics of single ZnO nanowires have been studied in the humid air, dry air, vacuum, and under ultraviolet (UV) irradiation. A model of a single ZnO nanowire connected with two opposite diodes was proposed to calculate the observed I–V$I\text{–}V$ behaviors. The results show that the electrical characteristics are dominated by the reverse barrier height, and the barrier height can be adjusted by surface adsorption, which is ascribed to the effect of surface states on surface band bending and Fermi level pining. Furthermore, the nanowire exhibited an enormous increase of conductance upon UV irradiation and a considerable persistent photocurrent after withdraw of the UV excitation, which further confirms the surface states have a pronounced effect on the electronic transport in single ZnO nanowires.     

Photoluminescence of ZnO nanowires dependent on O2 and Ar annealing

High-purity ZnO nanowires have been synthesized on Si substrates without the presence of a catalyst at 600 °C by a simple thermal vapor technique. Photoluminescence (PL) spectra of the annealed samples at 900 °C under oxygen and argon gases have been investigated. After O₂ or Ar annealing, the PL visible-emission intensity that is related to intrinsic defects (oxygen vacancies) is greatly reduced compared with as-grown ZnO nanowires because the oxygen-gas ions or oxygen interstitials diffuse into the oxygen vacancies during annealing process. The blue-band peak of the O₂- or Ar-annealed ZnO naonowires is also smaller than the green-band peak in the visible broadband because of the reduction of oxygen vacancies. Therefore, the main intrinsic defects (oxygen vacancies) of as-grown ZnO nanowires can be reduced by O₂ or Ar annealing, which is an important procedure for the development of advanced optoelectronic ZnO nanowire devices.     

Detection of CO using bulk ZnO Schottky rectifiers

Bulk Pt/ZnO Schottky rectifiers show gas sensitivities for CO of 4% and 8%, respectively, at 150 °C for 1 and 10% CO in N₂. The current or voltage changes are time dependent, with an activation energy of 40.7 kJmol^-1. Over a limited range of partial pressures of CO (P_CO) in the ambient gas, the on-state resistance R of the sensor at fixed bias voltage decreased according to R=(R_O+A(P_CO)^0.5)^-1, where A is a constant and R_O is the resistance in N₂. Since these devices are also sensitive to H₂ and C₂H₄, the ZnO rectifiers appear promising for a range of gas-sensing applications. PACS 81.05.Dz; 73.61.Ga; 72.80.Ey     

Pulsed laser deposition of ZnO in N2O atmosphere

The contribution deals with ZnO thin layers doped by nitrogen which were prepared by pulsed laser deposition in N₂O ambient atmosphere. Our approach is based on ablation of undoped ZnO target in active atmosphere containing N₂O gas without any supporting excitation equipment in parallel. Ablation of ZnO target was performed at different pressures (1–32 Pa) of N₂O ambient atmosphere by pulsed Nd:YAG laser (at 355 nm). Layers of ZnO were grown on different substrates (Si, sapphire, fused silica) and their properties were investigated by various analytical methods: scanning electron microscopy (SEM), secondary ion mass spectroscopy (SIMS), X-ray diffraction (XRD), and optical transmission spectroscopy. The results confirmed incorporation of nitrogen into ZnO layers and its concentration was pressure dependent. According to SIMS analysis, there is a certain pressure level (above 10 Pa) when the presence of N becomes negligible. Transmittance spectra showed increasing of the optical band gap (E _g) according to the pressure of N₂O.