Influence of seed layers on the vertical growth of ZnO nanowires

We synthesized vertically aligned ZnO nanowires on SiO₂ wafer <100> using the Au, ZnO and Au/ZnO seed layers through the physical vapor deposition process. The growth direction of ZnO nanowire was controlled by using the three different seed layers. From the XRD results, we observed the highest intensity of the (002) peak on the Au/ZnO seed layer among the three seed layers. The SEM images show that all of the ZnO nanowires have an average diameter of about 100 ~ 200 nm and a length of about 5 μm, and the nanowires grown on the Au/ZnO seed layer are oriented the most perpendicularly to the substrate surface. From the PL analysis, we observed that the intensity of broad emissions at 400-600 nm relating the green emission for the ZnO nanowires on the Au/ZnO seed layer was much weaker than that for the ZnO nanowires on the ZnO seed layer. The experiment results indicate that the selection of seed layers is important to grow nanowires vertically for the application of nanoscale devices.     

过渡金属(Mn2+、Ni2+、Fe3+、Cu2+)掺杂ZnO基稀磁半导体的制备及性质

利用溶液腐蚀法制备了Mn2+、Ni2+、Fe3+、Cu2+离子掺杂的ZnO基稀磁半导体。XRD表明掺杂后的ZnO仍然保持单一的纤锌矿结构,没有任何杂质相产生。由紫外-可见光反射谱可知掺杂后吸收边发生了红移。掺杂前ZnO的带隙为3.20eV,对样品分别掺入Mn、Ni、Fe和Cu后的带隙分别为3.19eV、3.15eV、3.08eV和3.17eV。掺杂后样品的室温PL谱除了紫外发射峰外,对于Mn掺杂的样品还在蓝光区域出现了2个分别位于424nm和443nm的发射峰,Fe掺杂的样品出现了一个位于468nm的微弱发射峰,Cu掺杂的样品出现了位于469nm及535nm的很宽的发射峰。室温磁滞回线显示掺杂后样品有明显的铁磁性,掺入Mn、Ni、Fe和Cu样品的剩余磁化强度(Ms)分别为0.3902×10-3emu/cm3、0.454emu/cm3、0.372emu/cm3和0.962×10-3emu/cm3,矫顽力分别为47Oe、115.92Oe、99.33Oe和23Oe。经分析室温铁磁性来源于缺陷调制的Mn2+-Mn2+长程铁磁交换相互作用。     

Investigation of indium oxide as a self-catalyst in ZnO/ZnInO heterostructure nanowires growth

We investigated the self-catalytic role of indium oxide in the growth process of ZnO/ZnInO heterostructure nanowires on Si(111). The prepared nanowires had hexagonal cross sections and were tapered with tip diameters of 90 ± 5 nm and base diameters of 230 ± 5 nm. Energy dispersive X-ray and field emission Auger spectroscopies indicated that the grown nanowires were heterostructures of ZnO and ZnInO. Analysis of the early growth process revealed that indium may play a self-catalytic role. Therefore, the vapor-liquid-solid mechanism is likely to be responsible for growth of ZnO/ZnInO nanowires. X-ray diffraction and room temperature photoluminescence (PL) data demonstrated that the presence of indium results in a decrease in nanowires' crystallinity. These wires produced a large PL emission peak in the ultraviolet (UV) region and a smaller peak in the green region of the electromagnetic spectrum. The UV peak of the ZnO/ZnInO nanowires is blue-shifted with respect to that of pure ZnO nanowires.     

Synthesis,characterization and growth mechanism of ZnO nanowires on NiCl<Subscript>2</Subscript>-coated Si substrates

Well-crystallized ZnO nanowires have been successfully synthesized on NiCl₂-coated Si substrates via a carbon thermal reduction deposition process. The pre-deposited Ni nanoparticles by dipping the substrates into NiCl₂ solution can promote the formation of ZnO nuclei. The as-synthesized nanowires were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) spectrum. The results demonstrate that the as-fabricated nanowires with about 60 nm in diameter and several tens of micrometers in length are preferentially arranged along [0001] direction with (0002) as the dominate surface. Room temperature PL spectrum illustrates that the ZnO nanowires exist a UV emission peak and a green emission peak, and the peak centers locate at 387 and 510 nm. Finally, the growth mechanism of the nanowires is briefly discussed.     

Growth of high quality ZnO nanowires on graphene

We report growth of the ZnO nanowires on graphene/SiO2/Si substrates using a chemical vapor deposition method. The length of nanowires varies from 1 microm to 10 microm with increasing the growth time from 30 min to 90 min. X-ray diffraction and high-resolution transmission electron microscopy investigations predict the high structural quality of the c-axis grown single crystalline ZnO nanowires. Temperature dependent photoluminescence spectra from the nanowires reveal excellent optical quality and excitonic behavior in the single crystalline ZnO nanowires. A well-resolved free exciton emission at 3.375 eV, indicates high crystalline quality nanowires and a strong PL peak at 3.370 eV is assigned to neutral-donor bound excitons (D0X).     

Photoluminescence and field-emission characteristics of ZnO nanowires synthesized by two-step method

ZnO nanowires with excellent photoluminescence (PL) and field-emission properties were synthesized by a two-step method, and the ZnO nanowires grew along (0 0 2) direction. PL measurements showed that the ZnO nanowires have stronger ultraviolet emission properties at 376 nm and there is 3 nm blue shift after the nanowires were immersed in thiourea (TU) solution compared with those of without immersion. The immersed-ZnO nanowires show a turn-on field of 2.3 V/μm at a current density of 0.1 μA/cm and emission current density up to 1 mA/cm² at an applied field of 6.8 V/μm, which demonstrate that the immersed-ZnO nanowires posses efficient field-emission properties in contrast with those not immersed. The ZnO nanowires may be ideal candidates for making luminescent devices and field-emission displays.     

Fabrication of luminescent silver doped PbS nanowires in polymer

We report here fabrication of silver (0 to 1.76 mol%) doped PbS nanowires (radius r approximately 1.75 nm) in polymer by a simple wet chemical process. An X-ray photoelectron spectroscopy study clearly confirms the possibility of silver (Ag) doping in PbS nanowires. Both absorption and photoluminescence spectra reveal very strong quantum confinement effect in PbS nanowires as expected for a r/Bohr radius ratio approximately 0.0972 nm. Visible excitonic emission is observed at room temperature in the photoluminescence spectra of undoped and silver doped PbS nanowires in polymer. The excitonic emission is appreciably blue-shifted when doped by silver (1.76 mol%) indicating strong modification of the electronic states by magnetic silver ions. While Ag2+ centers at the substitutional lattice site show an emission band around 525 nm, Ag0 at the interstitial site act as nonradiative recombination centers. Effect of silver doping on the luminescence intensity is also discussed.     

Growth, photoluminescence, and field emission of hierarchical ZnO nanostructures

Hierarchical ZnO nanostructures with the morphology of sleeve-fishes have been fabricated on Si substrate through vapor phase transport at 850 degrees C. Studies find that each nansleeve-fish is composed of a screw-shaped microrod and some tapering nanowires grown on the microrod, all of which grow along the [0001] direction. Photoluminescence spectra exhibits strong UV emission around 385 nm without any green emissions detected, indicating that the high-quality ZnO nanostructures with low level of oxygen vacancies are obtained in our experiment. Field-emission measurements on the nanosleeve-fishes show a turn-on field as low as 2.2 V/microm at a current density of 0.1 microA/cm(2) with a anode-sample separation of 600 microm, and the emission current density reached 1 mA/cm(2) at an applied field of about 5.8 V/microm. The excellent field emission properties from such kind of nanostructures make them promising candidates for further applications in FE microelectronic devices.     

Thickness optimized nanocrystalline ZnO-coated silicon nanowires for cold cathode application

Silicon nanowire is an important material for the potential use as a cold cathode, but there are some bottlenecks like oxidation of the surface during field emission thereby degradation of its performance. To compete with carbon based nanostructures in this field the performance of Si nanowires as field emitter should be improved. Here, we report a simple technique for the significant improvement of field emission properties of Si nanowires by ZnO nanoparticle coating. Boron-doped p-type Si wafers were chemically etched to synthesize vertically aligned silicon nanowires and they were coated with different thicknesses of ZnO layer by radio frequency magnetron sputtering technique. The nanostructured thin films were studied by X-ray photoelectron spectroscopy for compositional and valence states information while their morphological information was obtained by a field emission scanning electron microscope and a high resolution transmission electron microscope. The field assisted electron emission performance of Si nanowires significantly improved for the thickness optimized ZnO coating. The photoluminescence spectra showed a peak at ~558 nm assigned to surface defect states of ZnO and the field emission from Si nanowires coated with ZnO for different times were correlated with the surface defect structures. The mechanism of such improvement is also discussed.     

Optical properties of zigzag twinned geometry of Zn2SnO4 nanowires

High-density single-crystalline Zn2SnO4 nanowires have been successfully synthesized by using a simple thermal evaporation method by heating a mixture of ZnO and SnO2 nano powders. The products in general contain various geometries of wires, with an average diameter of 80-100 nm. These nanowires are ultra-long, up to 100 microns. The transmission electron microscopy study showed that these nanowires exhibited zigzag twinned geometry, and grow along the (111) direction. Low-temperature photoluminescence properties of the nanowires were measured, showing a strong green emission band at about 515 nm and a weak peak corresponding to UV emission at about 378 nm, which have not been reported before.     

Fabrication of vertical ZnO nanowires on silicon (100) with epitaxial ZnO buffer layer

Vertical ZnO nanowires were successfully grown on epitaxial ZnO (002) buffer layer/Si (100) substrate. The nanowire growth process was controlled by surface morphology and orientation of the epitaxial ZnO buffer layer, which was deposited by radio-frequency (rf) sputtering. The copper catalyzed the vapor-liquid-solid growth of ZnO nanowires with diameter of approximately 30 nm and length of approximately 5.0 microm. The perfect wurtzite epitaxial structure (HCP structure) of the ZnO (0002) nanowires synthesized on ZnO (002) buffer layer/Si (100) substrate results in excellent optical characteristics such as strong UV emission at 380 nm with potential use in nano-optical and nano-electronic devices.     

Ultraviolet lasing action in ZnO nanosheets

We report room-temperature ultraviolet lasing action in large quantities of uniform multilayer ZnO nanosheets grown by a vapor-transport method via thermal evaporation of Zn powder. An excellent multimode lasing emission at a center wavelength of 390 nm with a mode linewidth less than 0.33 nm occurs above an excitation threshold of 8 mJ pulse(-1) cm(-2). The observed multimode lasing action may be attributed to microcavity effect and low concentration of defects in the nanosheets. We believe that the single-mode lasing emission can be obtained by growing completely uniform nanosheets. ZnO nanosheet is an attractive candidate as gain medium to realize ultraviolet semiconductor diode lasers.     

Photophysics of resonantly and non-resonantly excited erbium doped Ge nanowires

We have fabricated Er doped germanium nanowires of different diameters by pulsed laser deposition and chemical methods. Er induced photoluminescence emission due to the intra-4f (4)I(13/2)→(4)I(15/2) transition of Er energy levels at 1.53 μm has been achieved at room temperature using both resonant (980 nm) and non-resonant (325 nm) excitation of Er ions. The observed 1.53 μm photoluminescence signal upon non-resonant 325 nm excitation is attributed to the Ge related oxygen deficiency centers surrounding the Ge core. For direct excitation, the infrared photoluminescence characteristics have been studied as a function of Er concentration, photon flux, and diameter of the nanowires. The Er related emission signal is found to be enhanced with increase in Er concentration, pump flux of 980 nm, and the nanowire diameter. The time resolved characteristics of the Er induced emission peak have been studied as a function of the pump flux as well as the diameter of the Ge nanowires.     

Characterization of ZnO nanowires grown on Si (100) with and without Au catalyst

ZnO nanowires were grown on Si (100) substrates with and without Au catalyst by chemical vapor deposition employing the vapor-liquid-solid (VLS) and vapor-solid (VS) mechanisms, respectively. The diameters of the resulting nanowires were in the range 80-150 nm with typical length about 10 μm. The near-band-edge (NBE) emission of ZnO nanowires grown with and without catalyst was observed at 382 nm and 386 nm, respectively. The intensity of the NBE emission of ZnO nanowires grown without the catalyst was higher than that of the green luminescence. By sharp contrast, the intensity of the NBE emission of ZnO nanowires grown with catalyst was lower than that of green luminescence. The X-ray diffraction (XRD) spectrum of the ZnO nanowires grown without catalyst exhibited a peak intensity of c-plane 5 times higher than that of m-plane and 10 times higher than that of a-plane. However, the XRD spectrum of the ZnO nanowires grown with catalyst exhibited a peak intensity of the c-plane about 1.5 times higher than that of the m-plane and 4 times higher than that of a-plane intensity. Thus, the ZnO nanowires grown without catalyst have a preferential orientation along the c-axis direction. Our results show that the catalyst strongly effects optical and structural properties of the ZnO nanowires.     

Magnetic and optical properties of Mn doped ZnO nanocrystalline films

We have investigated the properties of Mn-doped ZnO nanocrystalline film growing on zinc foil by the hydrothermal method. X-ray photoelectron spectroscopy shows that the manganese ions exist as Mn2+ in the film. From UV-vis spectra, we observe a red shift in wavelength of absorption and greater reflectivity due to the Mn ion incorporation in ZnO lattices. The photoluminescence spectrum of the Mn-doped ZnO film shows two strong new blue peaks centered at 424 nm and 443 nm, besides the UV emission peak owing to the band gap of ZnO semiconductor. The magnetic property of the Mn-doped ZnO exhibits a room temperature ferromagnetic characteristic with a saturation magnetization (Ms) of 0.3902 x 10(-3) emu/cm3 and a coercive field of 47 Oe. We suggest that the blue emission of the Mn-doped ZnO film corresponds to the electron transition from the level of interstitial Zn and Mn to the valence band. The defects brought about by Mn ion incorporation are the main cause of the room temperature ferromagnetic property.     

ZnO nanoparticles embedded in sapphire fabricated by ion implantation and annealing

ZnO nanoparticles were fabricated in sapphire (α-Al(2)O(3) single crystal) by Zn ion implantation (48?keV) at an ion fluence of 1 × 10(17)?cm(-2) and subsequent thermal annealing in a flowing oxygen atmosphere. Transmission electron microscopy (TEM) analysis revealed that metallic Zn nanoparticles of 3-10?nm in dimensions formed in the as-implanted sample and that ZnO nanoparticles of 10-12?nm in dimensions formed after annealing at 600?°C. A broad absorption band, peaked at 280?nm, appeared in the as-implanted crystal, due to surface plasma resonance (SPR) absorption of metallic Zn nanoparticles. After annealing at 600?°C, ZnO nanoparticles resulted in an exciton absorption peak at 360?nm. The photoluminescence (PL) of the as-implanted sample was very weak when using a He-Cd 325?nm line as the excitation source. However, two emission peaks appeared in the PL spectrum of ZnO nanopraticles, i.e., one ultraviolet (UV) peak at 370?nm and the other a green peak at 500?nm. The emission at 500?nm is stronger and has potential applications in green/blue light-emitting devices.     

Growth of Zn(1_x)Mg(x)O and Zn(1_x)Cd(x)O nanowires and the application in light emitting devices

Magnesium and Cadmium doped ZnO nanowires were successfully grown by Chemical Vapor deposition method in a tube furnance. Photoluminescence spectra show that the band gap of ZnO nanowire has been tuned from 4.00 eV to 2.08 eV by Magnesium and Cadmium doping. Transmission Electron Microscopy and X-ray diffraction characterization analysis indicate that most of the formed nanowires are single crystalline with good quality. Zn(1-x)Cd(x)O nanowire sample was used for heterojunctional light emitting diode fabrication. Electroluminescence measurement yields a strong emission peak at 553 nm from the Zn(1-x)Cd(x)O nanowire.     

Crabwise ZnO nanowires: growth and field emission properties

Crabwise ZnO nanowires with an average length of 5 microm and an average diameter of 30 nm were selectively grown on ZnO:Ga/glass templates. Cathodoluminescence measurement indicated that the crystal quality of the crabwise ZnO nanowires was good. With an applied voltage of 120 V, the crabwise ZnO nanowire field emitters gave an emission current of 0.1 mA/cm2. Moreover, the field enhancement factor, beta, of the crabwise ZnO nanowires was approximately 980.     

Temperature-dependent controlled preparation of ZnO nanostructures and their photoluminescence properties

High purity and yield of ZnO nanobelts, nanocombs and nanowires have been synthesized using a simple catalyst-free vapor transport process and their growth mechanism and photoluminescence properties have been investigated. Transmission electron microscopy and selected-area electron diffraction analyses reveal that the intrinsic growth behaviors of ZnO vary with the growth temperature, leading to the formation of nanostructures with different morphologies. These ZnO nanostructures show a UV emission at about 385 nm and a broad green emission around 500 nm in their photoluminescence spectra, and the green to UV emission intensity ratio is determined by their microstructures.     

Vertical single-crystal ZnO nanowires grown on ZnO:Ga/glass templates

Vertical single-crystal ZnO nanowires with uniform diameter and uniform length were selectively grown on ZnO:Ga/glass templates at 600/spl deg/C by a self-catalyzed vapor-liquid-solid process without any metal catalyst. It was found that the ZnO nanowires are grown preferred oriented in the [002] direction with a small X-ray diffraction full-width half-maximum. Photoluminescence, field-emission scanning electron microscopy, and high-resolution transmission electron microscopy measurements also confirmed good crystal quality of our ZnO nanowires. Field emitters using these ZnO nanowires were also fabricated. It was found that threshold field of the fabricated field emitters was 14 V//spl mu/m. With an applied electric field of 24 V//spl mu/m, it was found that the emission current density was around 0.1 mA/cm/sup 2/.