超声喷雾热解法生长氧化锌同质p-n结及其电致发光性能研究

采用超声喷雾热解法在单晶GaAs(100) 衬底上生长ZnO同质p-n结. 以醋酸锌水溶液为前驱体, 分别以醋酸铵和硝酸铟为氮(N)源和铟(In)源, 通过氮--铟(N-In)共掺杂沉积p型ZnO薄膜, 以未故意掺杂的ZnO薄膜做为n型层获得ZnO基同质p-n结. 采用热蒸发工艺在ZnO层和GaAs衬底上分别蒸镀Zn/Au和Au/Ge/Ni电极而获得发光二极管原型器件, 在室温下发现了该器件正向电流注入下的连续发光现象.     

Rational synthesis of p-type zinc oxide nanowire arrays using simple chemical vapor deposition

We report, for the first time, the synthesis of the high-quality p-type ZnO NWs using a simple chemical vapor deposition method, where phosphorus pentoxide has been used as the dopant source. Single-crystal phosphorus doped ZnO NWs have their growth axis along the 001 direction and form perfect vertical arrays on a-sapphire. P-type doping was confirmed by photoluminescence measurements at various temperatures and by studying the electrical transport in single NWs field-effect transistors. Comparisons of the low-temperature PL of unintentionally doped ZnO (n-type), as-grown phosphorus-doped ZnO, and annealed phosphorus-doped ZnO NWs show clear differences related to the presence of intragap donor and acceptor states. The electrical transport measurements of phosphorus-doped NW FETs indicate a transition from n-type to p-type conduction upon annealing at high temperature, in good agreement with the PL results. The synthesis of p-type ZnO NWs enables novel complementary ZnO NW devices and opens up enormous opportunities for nanoscale electronics, optoelectronics, and medicines.     

Electrospun nanofibers of p-type NiO/n-type ZnO heterojunctions with enhanced photocatalytic activity

One-dimensional electrospun nanofibers of p-type NiO/n-type ZnO heterojunctions with different molar ratios of Ni to Zn were successfully synthesized using a facile electrospinning technique. X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) spectroscopy, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), UV-vis diffuse reflectance (DR) spectroscopy, resonant Raman spectroscopy, photoluminescence (PL) spectroscopy, and surface photovoltage spectroscopy (SPS) were used to characterize the as-synthesized nanofibers. The results indicated that the p-n heterojunctions formed between the cubic structure NiO and hexangular structure ZnO in the NiO/ZnO nanofibers. Furthermore, the photocatalytic activity of the as-electrospun NiO/ZnO nanofibers for the degradation of rhodamine B (RB) was much higher than that of electrospun NiO and ZnO nanofibers, which could be ascribed to the formation of p-n heterojunctions in the NiO/ZnO nanofibers. In particular, the p-type NiO/n-type ZnO heterojunction nanofibers with the original Ni/Zn molar ratio of 1 exhibited the best catalytic activity, which might be attributed to their high separation efficiency of photogenerated electrons and holes. Notably, the electrospun nanofibers of p-type NiO/n-type ZnO heterojunctions could be easily recycled without a decrease of the photocatalytic activity due to their one-dimensional nanostructural property.     

Direct observation of enhanced cathodoluminescence emissions from ZnO nanocones compared with ZnO nanowire arrays

We report, for the first time, direct observation of enhanced cathodoluminescence (CL) emissions from ZnO nanocones (NCs) compared with ZnO nanowires (NWs). For direct and unambiguous comparison of CL emissions from NWs and nanocones, periodic arrays of ZnO NW were converted to nanocone arrays by our unique HCl [aq] etching technique, enabling us to compare the CL emissions from original NWs and final nanocones at the same location. CL measurements on NW and nanocone arrays reveal that emission intensity of the nanocone at ～ 387 nm is over two times larger than that of NW arrays. The enhancement of CL emission from nanocones has been confirmed by finite-difference time-domain simulation of enhanced light extraction from ZnO nanocones compared to ZnO NWs. The enhanced CL from nanocones is attributed to its sharp morphology, resulting in more chances of photons to be extracted at the interface between ZnO and air.     

Optical and electrical properties of Ga-doped ZnO nanorod arrays fabricated by thermal evaporation

Well-aligned Ga-doped ZnO nanorod arrays with high optical and electrical property were fabricated by catalyst-free thermal evaporation on p-silicon substrate. As the Ga/Zn atom ratio in the source material was tuned from 0 to 0.2, wurtzite structure ZnO nanorod arrays were realized with length of -6 microm and growth direction along c-axis. With the addition of Ga, the intensity of the near-band-edge emission was enhanced and the deep-level emissions maintained neglectable. As the Ga/Zn atom ratio increased from 0 to 0.1, the red shift of the near-band-edge emission occurred due to Ga-doping induced band gap renormalization effect related with the enhancement of the carrier density, while the blue shifts of the emission were found once the Ga/Zn ratio is higher than 0.1 resulting from Burstein-Moss effect. The configuration of the vertical-aligned Ga-doped ZnO nanorod arrays on p-Si substrate makes it straightforward for the fabrication of p-n nanodiode, which shows an excellent rectifying characteristic with threshold voltage as low as -4.7 V with the Ga/Zn atomic ratio of 0.2.     

Microstructural and optical characteristics of solution-grown Ga-doped ZnO nanorod arrays

Highly oriented Ga-doped zinc oxide (ZnO) nanorod arrays have been prepared on a ZnO-buffered silicon substrate in an aqueous solution, which is a mixture of methenamine (C(6)H(12)N(4)), zinc nitrate hexahydrate (Zn(NO(3))(2)·6H(2)O), and gallium nitrate hydrate (Ga(NO(3))(3)·xH(2)O). The microstructure characteristics and optical properties of the nanorod arrays were analyzed using different characterization techniques including field-emission scanning electron microscopy (FESEM), x-ray photoelectron spectroscopy (XPS), and photoluminescence (PL). The experimental results show that the morphology, density, and surface compositions of ZnO nanorod arrays are sensitive to the concentration of gallium nitrate hydrate. The PL spectra of all ZnO nanorod arrays show three different emissions, including UV (ultraviolet), yellow, and NIR (near infrared) emissions. With the increase in the Ga doping level, the luminescence quality of ZnO nanorods has been improved. The peak of UV emission has a small redshift, which can be ascribed to the combined effect of size and Ga doping. Furthermore, Ga doping has caused defects that respond to NIR emission.     

Effect of electroless etching parameters on the growth and reflection properties of silicon nanowires

Vertically aligned silicon nanowire (Si NW) arrays have been fabricated over large areas using an electroless etching (EE) method, which involves etching of silicon wafers in a silver nitrate and hydrofluoric acid based solution. A detailed parametric study determining the relationship between nanowire morphology and time, temperature, solution concentration and starting wafer characteristics (doping type, resistivity, crystallographic orientation) is presented. The as-fabricated Si NW arrays were analyzed by field emission scanning electron microscope (FE-SEM) and a linear dependency of nanowire length to both temperature and time was obtained and the change in the growth rate of Si NWs at increased etching durations was shown. Furthermore, the effects of EE parameters on the optical reflectivity of the Si NWs were investigated in this study. Reflectivity measurements show that the 42.8% reflectivity of the starting silicon wafer drops to 1.3%, recorded for 10 μm long Si NW arrays. The remarkable decrease in optical reflectivity indicates that Si NWs have a great potential to be utilized in radial or coaxial p-n heterojunction solar cells that could provide orthogonal photon absorption and enhanced carrier collection.     

Investigation on light emission in light-emitting diodes constructed with n-ZnO and p-Si nanowires

The light emission was investigated in light-emitting diodes (LEDs) constructed with n-ZnO and p-Si nanowires (NWs). ZnO NWs were synthesized by thermal chemical vapor deposition and Si NWs were formed by crystallographic wet etching of a Si wafer. The LEDs were fabricated using the NWs via dielectrophoresis (DEP) and direct transfer methods. The DEP method enabled to align the ZnO NW at the position that led to p-n heterojunction diodes by crossing with the transferred Si NW. The I-V curve of the p-n heterojunction diode showed the well-defined current-rectifying characteristic, with a turn-on voltage of 3 V. The electroluminescence spectrum in the dark showed the strong emission at approximately 385 nm and the broad emission centered at approximately 510 nm, at a forward bias of 30 V. Under the illumination of 325-nm-wavelength light, the luminescence intensity at 385 nm was dramatically enhanced, compared to that in the dark, probably due to the electric-field-induced enhancement of luminescence.     

Donor and acceptor dynamics of phosphorous doped ZnO nanorods with stable p-type conduction: photoluminescence and junction characteristics

We employed temperature-dependent photoluminescence (PL) to explain the donor and acceptor dynamics in phosphorus doped stable p-type P:ZnO nanorods. The room temperature PL revealed good crystalline and optical quality of P:ZnO nanorods. The 10 K PL spectrum exhibited a dominant acceptor bound exciton (A0X) or donor bound exciton (D0X) emission corresponding to p- and n-type P:ZnO nanorods, respectively. The donor-acceptor-pair (DAP) transitions exhibited different thermal dissociation energies for the p- and n-type P:ZnO nanorods, suggesting their different quenching channels. The quenching of the DAP transitions of the p-type ZnO:P nanorods was associated with the thermal dissociation of the DAP into free excitons, while the DAP transition of the n-type ZnO:P nanorods was quenched through the thermal dissociation of the shallow donor into free electrons. The rectifying behavior of a p-n homojunction diode formed by the p-type P:ZnO nanorods on n-type ZnO film confirmed the p-type conduction of the P:ZnO nanorods.     

Deposition of Na–N dual acceptor doped p-type ZnO thin films and fabrication of p-ZnO:(Na,N)/n-ZnO:Eu homojunction

Sodium and nitrogen dual acceptor doped p-type ZnO (ZnO:(Na, N)) films have been prepared by spray pyrolysis technique at a substrate temperature of 623 K. The ZnO:(Na, N) films are grown at a fixed N doping concentration of 2 at.% and varying the nominal Na doping concentration from 0 to 8 at.%. The XRD results show that all the ZnO:(Na, N) films exhibited (0 0 2) preferential orientation. The EDX and elemental mapping analysis shows the presence and distribution of Zn, O, Na and N in the deposited films. The Hall measurement results demonstrate that the Na–N dual acceptor doped ZnO films show excellent p-type conduction. The p-type ZnO:(Na, N) films with comparatively low resistivity of 5.60 × 10⁻² Ω cm and relatively high carrier concentration of 3.15 × 10¹⁸ cm⁻³ are obtained at 6 at.%. ZnO based homojunction is fabricated by depositing n-type layer (Eu doped ZnO) grown over the p-type layer ZnO:(Na, N). The current–voltage (I–V) characteristics measured from the two-layer structure show typical rectifying characteristics of p-n junction with a low turn on voltage of about 1.69 V. The ZnO:(Na, N) films exhibit a high transmittance (about >90%) and the average reflectance is 8.9% in the visible region. PL measurement shows near-band-edge (NBE) emission and deep-level (DL) emission in the ZnO:(Na, N) thin films.     

p-Type ZnO nanowire arrays

Well-aligned ZnO nanowire (NW) arrays with durable and reproducible p-type conductivity were synthesized on alpha-sapphire substrates by using N2O as a dopant source via vapor-liquid-solid growth. The nitrogen-doped ZnO NWs are single-crystalline and grown predominantly along the [110] direction, in contrast to the [001] direction of undoped ZnO NWs. Electrical transport measurements reveal that the nondoped ZnO NWs exhibit n-type conductivity, whereas the nitrogen-doped ZnO NWs show compensated highly resistive n-type and finally p-type conductivity upon increasing N2O ratio in the reaction atmosphere. The electrical properties of p-type ZnO NWs are stable and reproducible with a hole concentration of (1-2) x 10(18) cm(-3) and a field-effect mobility of 10-17 cm2 V(-2) s(-1). Surface adsorptions have a significant effect on the transport properties of NWs. Temperature-dependent PL spectra of N-doped ZnO NWs show acceptor-bound-exciton emission, which corroborates the p-type conductivity. The realization of p-type ZnO NWs with durable and controlled transport properties is important for fabrication of nanoscale electronic and optoelectronic devices.     

Optical and magnetic properties of Cu-doped ZnO nanoparticles

Cu-doped ZnO nanoparticles were synthesized by a simple chemical method at low temperature with Cu:Zn atomic ratio from 0 to 5 %. The synthesis process was based on the hydrolysis of zinc acetate dehydrate and copper acetate tetrahydrate heated under reflux to 65 °C using methanol as a solvent. X-ray diffraction (XRD) analysis reveals that the Cu-doped ZnO crystallize in a wurtzite structure with a change of crystal size from 12 nm for undoped ZnO to 5 nm for Cu-doped ZnO. These nano size crystallites of Cu doped ZnO self-organized into microspheres. The XRD patterns, Scanning electron microscopy and transmission electron microscopy micrographs of doping of Cu in ZnO confirmed the formation of microspheres and indicated that the Cu²⁺ is successfully substituted into the ZnO host structure of the Zn²⁺ site. Cu doping shifts the absorption onset to blue from 373 to 350 nm, indicating an increase in the band gap from 3.33 to 3.55 eV. A relative increase in the intensity of the deep trap emission of Cu-doped ZnO is observed when increasing the concentration of Cu. Magnetic measurements indicate that Cu-doped ZnO samples are ferromagnetic at room temperature except pure ZnO.     

Fabrication and characterization of flower-like CuO-ZnO heterostructure nanowire arrays by photochemical deposition

A simple two step solution-based method was applied to fabricate CuO-ZnO heterostructured nanowire (NW) arrays. First, ZnO nanowires were grown on a Si substrate using the ammonia solution hydrothermal reaction. Afterwards, flower-like CuO crystals were photochemically deposited on the tip of the ZnO NWs, using ultraviolet (UV) light (312 nm wavelength) irradiation at room temperature. The morphology of the CuO was controlled by reaction time, density of ZnO NWs, and concentration of the solution. Because the deposited CuO is p-type and has narrow band gap properties, CuO-ZnO heterostructured NWs exhibited a stable p-n junction property and good ability to absorb visible light. Through investigation of UV light-triggered reaction phenomena, we found that the production of OH(-) from the photocatalytic process on the surface of ZnO NWs plays a critical role in the CuO deposition mechanism.     

Highly sensitive and selective trimethylamine sensor using one-dimensional ZnO-Cr2O3 hetero-nanostructures

Highly selective and sensitive detection of trimethylamine (TMA) was achieved by the decoration of discrete p-type Cr(2)O(3) nanoparticles on n-type ZnO nanowire (NW) networks. Semielliptical Cr(2)O(3) nanoparticles with lateral widths of 3-8 nm were deposited on ZnO NWs by the thermal evaporation of CrCl(2) at 630 °C, while a continuous Cr(2)O(3) shell layer with a thickness of 30-40 nm was uniformly coated on ZnO NWs at 670 °C. The response (R(a)/R(g): R(a), resistance in air; R(g), resistance in gas) to 5 ppm TMA of Cr(2)O(3)-decorated ZnO NWs was 17.8 at 400 °C, which was 2.4 times higher than that to 5 ppm C(2)H(5)OH and 4.3-8.4 times higher than those to 5 ppm p-xylene, NH(3), benzene, C(3)H(8), toluene, CO, and H(2). In contrast, both pristine ZnO and ZnO (core)-Cr(2)O(3) (shell) nanocables (NCs) showed comparable responses to the different gases. The highly selective and sensitive detection of TMA that was achieved by the deposition of semielliptical Cr(2)O(3) nanoparticles on ZnO NW networks was explained by the catalytic effect of Cr(2)O(3) and the extension of the electron depletion layer via the formation of p-n junctions.     

可见光诱导TiO2光催化及其机理研究进展

简述了二氧化钛的光催化机理。针对其禁带宽度较大,只能被小于387nm的紫外光所激发的缺点,综述了近年来国内外针对纳米TiO2可见光催化的改性方法和改性机理研究进展,包括离子掺杂、半导体复合、表面光敏化等方法。最后展望了提高纳米TiO2可见光光催化活性研究的前景。     

A ZnO nanorod inorganic/organic heterostructure light-emitting diode emitting at 342 nm

An inorganic/organic heterostructure light-emitting diode consisting of the hole-transporting layer N, N'-di(naphth-2-yl)- N, N'-diphenylbenzidine (NPB) and n-type ZnO nanorods fabricated by hydrothermal decomposition is reported. Poly(methyl methacrylate) was used to form a smooth surface on top of ZnO nanorod array with ZnO nanorod tops exposed for subsequent NPB deposition. An unusual ultraviolet emission at 342 nm was observed in the electroluminescence spectrum. Compared to band gap energy of ZnO (3.37 eV), the excitonic emission is blue-shifted and broadened. The mechanism of the blue shift is discussed in terms of the energy band diagram of the heterostructure.     

Induction of zinc particles on the morphology and photoluminescent property of globular Zn/ZnO core/shell nanorod heterojunction array architectures

Zn/ZnO metal/semiconductor nanostructures were successfully synthesised by a facile zinc-rich chemistry liquid-phase approach with zinc microspheres as sacrificial templates at ambient temperature. A series of globular Zn/ZnO core/shell structures and hollow microsphere architectures self-assembled by Zn/ZnO nanorod heterojunction arrays were obtained by controlling the amount of zinc particles. The structure, morphology, composition and optical properties of the products have been characterised by X-ray diffraction, scanning electron microscopy, Raman spectroscopy and photoluminescent spectroscopy. A possible growth mechanism of the Zn/ZnO nanostructures has been proposed based on the structural analysis. The growth mechanism of Zn/ZnO hollow microspheres is ascribed to Kirkendall effect. A new strong blue emission at 440 nm and a green emission around 500 nm with an enhancement over one order of magnitude compared with the pure ZnO sample have been observed. These emission bands are attributed to two kinds of mechanisms that have been discussed in detail.     

Preparation and Photoluminescence of Ordered ZnO Nanowire Arrays

Ordered ZnO nanowire arrays embedded in anodic aluminum oxide (AAO) membranes were fabricated by electrochemical deposition of Zn(NO3)2 H3BO3 solution in a boiling bath. Scanning electron microscope (SEM) and transmission electron microscope (TEM) observation results show that the polycrystalline ZnO nanowires with diameters around 100 nm were uniformly assembled into the ordered nanochannels of the AAO. The results of the investigation into photoluminescence (PL) and electronic paramagnetic resonance (EPR) measurements reveal that the interfaces between the ZnO nanowires and the pore walls of the AAO create a lot of oxygen vacancies, which are responsible for the green light emission (peaking around 512 nm) and the huge enhancement of the PL emission.     

Galvanic deposition of ZnO using mixed electrolyte and their photoluminescence properties

The effects of Zn(OAc)₂ concentrations and chemical nature of supporting electrolytes on the galvanic deposition of ZnO have been investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy-dispersive X-ray (EDX) microanalysis. The results show that the taper-like ZnO crystals are apt to be produced at lower Zn(OAc)₂ concentrations, while the rod-like ZnO crystals tend to be grown at higher Zn(OAc)₂ concentrations. The photoluminescence of as-prepared ZnO nanorods shows that there exist a strong UV emission band, a broad blue band at 468 nm, and a very weak green band at 550 nm. The blue-shift of UV emission is attributed to the Cl doping of ZnO in chloride electrolyte.     

High density Si/ZnO core/shell nanowire arrays for photoelectrochemical water splitting

Si/ZnO core/shell nanowire (NW) arrays were fabricated using atomic layer deposition of ZnO shell on n-Si NW arrays prepared by metal assisted electroless etching method. Scanning electron microscopy, transmission electron microscopy and X-ray diffraction were utilized to characterize the core/shell structures. Water splitting performance of the core/shell structures was preliminarily studied. The Si/ZnO core/shell NW arrays yielded significantly higher photocurrent density than the planar Si/ZnO structure due to their low reflectance and high surface area. The photoelectrochemical efficiency was found to be 0.035 and 0.002 % for 10 μm-long Si/ZnO NW array and planar Si/ZnO sample, respectively. These results suggested that core/shell structure is superior to planar heterojunction for PEC electrode design. We demonstrated the dependence of photocurrent density on the length of the core/shell array, and analyzed the reasons why longer NW arrays could produce higher photocurrent density. The relationship between the thickness of ZnO shell and the photoconversion efficiency of Si/ZnO NW arrays was also discussed. By applying the core/shell structure in electrode design, one may be able to improve the photoelectrochemical efficiency and photovoltaic device performance.