Removal of surface states and recovery of band-edge emission in InAs nanowires through surface passivation

Surface states in semiconductor nanowires (NWs) are detrimental to the NW optical and electronic properties and to their light emission-based applications, due to the large surface-to-volume ratio of NWs and the congregation of defects states near surfaces. In this paper, we demonstrated an effective approach to eliminate surface states in InAs NWs of zinc-blende (ZB) and wurtzite (WZ) structures and a dramatic recovery of band edge emission through surface passivation with organic sulfide octadecylthiol (ODT). Microphotoluminescence (PL) measurements were carried out before and after passivation to study the dominant recombination mechanisms and surface state densities of the NWs. For WZ-NWs, we show that the passivation removed the surface states and recovered the band-edge emission, leading to a factor of ～19 reduction of PL linewidth. For ZB-NWs, the deep surface states were removed and the PL peaks width became as narrow as ～250 nm with some remaining emission of near band-edge surface states. The passivated NWs showed excellent stability in atmosphere, water, and heat environments. In particular, no observable changes occurred in the PL features from the passivated NWs exposed in air for more than five months.     

Blueshift of electroluminescence from single n-InP nanowire/p-Si heterojunctions due to the Burstein-Moss effect

Single-crystalline n-type InP nanowires (NWs) with different electron concentrations were synthesized on Si substrates via the vapor phase transport method. The electrical properties of the InP nanowires were investigated by fabricating and measuring single NW field-effect transistors (FETs). Single InP NW/p(+)-Si heterojunctions were fabricated, and electroluminescence (EL) spectra from them were studied. It was found that both the photoluminescence (PL) spectra of the InP NWs and the EL spectra of the heterojunctions blueshift from 920 to 775?nm when the electron concentrations of the InP NWs increase from 2 × 10(17) to 1.4 × 10(19)?cm(-3). The blueshifts can be attributed to the Burstein-Moss effect rather than the quantum confinement effect in the InP NWs. The large blueshifts observed in this study indicate a potential application of InP NWs in nano-multicolour displays.     

Electrical and Optical Characterization of Surface Passivation in GaAs Nanowires

We report a systematic study of carrier dynamics in Al(x)Ga(1-x)As-passivated GaAs nanowires. With passivation, the minority carrier diffusion length (L(diff)) increases from 30 to 180 nm, as measured by electron beam induced current (EBIC) mapping, and the photoluminescence (PL) lifetime increases from sub-60 ps to 1.3 ns. A 48-fold enhancement in the continuous-wave PL intensity is observed on the same individual nanowire with and without the Al(x)Ga(1-x)As passivation layer, indicating a significant reduction in surface recombination. These results indicate that, in passivated nanowires, the minority carrier lifetime is not limited by twin stacking faults. From the PL lifetime and minority carrier diffusion length, we estimate the surface recombination velocity (SRV) to range from 1.7 × 10(3) to 1.1 × 10(4) cm·s(-1), and the minority carrier mobility μ is estimated to lie in the range from 10.3 to 67.5 cm(2) V(-1) s(-1) for the passivated nanowires.     

Sulfur passivation and contact methods for GaAs nanowire solar cells

The effect of sulfur passivation on core-shell p-n junction GaAs nanowire (NW) solar cells has been investigated. Devices of two types were investigated, consisting of indium tin oxide contact dots or opaque Au finger electrodes. Lateral carrier transport from the NWs to the contact fingers was achieved via a p-doped GaAs surface conduction layer. NWs between the opaque contact fingers had sidewall surfaces exposed for passivation by sulfur. The relative cell efficiency increased by 19% upon passivation. The contribution of the thin film grown between the NWs to the total cell efficiency was estimated by removing the NWs using a sonication procedure. Mechanisms of carrier transport and photovoltaic effects are discussed on the basis of spatially resolved laser scanning measurements.     

碱性（NH4）2对n型GaSb钝化表面的研究

利用光致发光光谱（Photoluminescence。简称PL）、光扫描系统（PLMapping）、原子力显微镜（Atomic Force Microscope,简称AFM）等测试手段,研究碱性（Nil4）2s对n型GaSbSE4L表面的影响,发现与未处理的GaSb表面相比,其光学性质明显提高。当硫化时间为180s时,光学强度提高了10倍左右,发光均匀性和表面平整度也达到了最优。同时,分析了钝化效果的时效性,发现当钝化后的样品放置于空气中48h时,钝化效果基本消失。     

Pd-nanoparticle-decorated ZnO nanowires: ultraviolet photosensitivity and photoluminescence properties

ZnO nanowires (NWs) have been decorated with Pd nanoparticles of sizes less than 10 nm (Pd-ZnO NWs) via a chemical solution route. The microstructural characterizations have been done using field emission scanning electron and high-resolution transmission electron microscopes. The effects of attaching Pd nanoparticles to the walls of ZnO NWs have been investigated by studying the ultraviolet (UV) photosensitivity and photoluminescence (PL) properties. The surface-modified NWs show a UV photosensitivity more than double and a response seven times faster compared to the bare NWs. The photocarrier relaxation under the steady UV illumination condition is quite different in Pd-ZnO NWs. The higher and faster photosensitivity has been explained on the basis of photocarrier transfer from the conduction band of ZnO to the Fermi level of Pd and subsequent electron trapping by the adsorbed O(2) molecules on the NWs' surface, which have been presented through a proposed model. The PL spectrum of Pd-ZnO NWs shows that the intensities of the band-edge and defect-related emissions decrease and increase, respectively, due to Pd anchoring, the effect being pronounced as the density of Pd nanoparticles increases.     

A generic approach for vertical integration of nanowires

We report on the collective integration technology of vertically aligned nanowires (NWs). Si?and ZnO NWs have been used in order to develop a generic technological process. Both mineral and organic planarizations of the as-grown nanowires have been achieved. Chemical vapour deposition (CVD) oxides, spin on glass (SOG), and polymer have been investigated as filling materials. Polishing and/or etching of the composite structures have been set up so as to obtain a suitable morphology for the top and bottom electrical contacts. Electrical and optical characterizations of the integrated NWs have been performed. Contacts ohmicity has been demonstrated and specific contact resistances have been reported. The photoconducting properties of polymer-integrated ZnO NWs have also been investigated in the UV-visible range through collective electrical contacts. A small increase of the resistivity in the ZnO NWs under sub-bandgap illumination has been observed and discussed. A comparison of the photoluminescence (PL) spectra at 300?K of the as-grown and SOG-integrated ZnO nanowires has shown no significant impact of the integration process on the crystal quality of the NWs.     

Low-temperature synthesis of indium tin oxide nanowires as the transparent electrodes for organic light emitting devices

Low-temperature growth of indium tin oxide (ITO) nanowires (NWs) was obtained on catalyst-free amorphous glass substrates at 250?°C by Nd:YAG pulsed-laser deposition. These ITO NWs have branching morphology as grown in Ar ambient. As suggested by scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM), our ITO NWs have the tendency to grow vertically outward from the substrate surface, with the (400) plane parallel to the longitudinal axis of the nanowires. These NWs are low in electrical resistivity (1.6×10?? Ω cm) and high in visible transmittance (~90–96%), and were tested as the electrode for organic light emitting devices (OLEDs). An enhanced current density of ~30 mA cm?2 was detected at bias voltages of ~19–21 V with uniform and bright emission. We found that the Hall mobility of these NWs is 2.2–2.7 times higher than that of ITO film, which can be explained by the reduction of Coulomb scattering loss. These results suggested that ITO nanowires are promising for applications in optoelectronic devices including OLED, touch screen displays, and photovoltaic solar cells.     

Self-assembly of conductive Cu nanowires on Si(111)'5 × 5 '-Cu surface

Upon room-temperature deposition onto a Cu/Si(111)'5 × 5' surface in ultra-high vacuum, Cu?atoms migrate over extended distances to become trapped at the step edges, where they form Cu?nanowires (NWs). The formed NWs are 20-80?nm wide, 1-3?nm high and characterized by a resistivity of ～8?μΩ?cm. The surface conductance of the NW array is anisotropic, with the conductivity along the NWs being about three times greater than that in the perpendicular direction. Using a similar growth technique, not only the straight NWs but also other types of NW-based structures (e.g. nanorings) can be fabricated.     

Fabry–Perot Microcavity Modes in Single GaP/GaNP Core/Shell Nanowires

Semiconductor nanowires (NWs) are attracting increasing interest as nanobuilding blocks for optoelectronics and photonics. A novel material system that is highly suitable for these applications are GaNP NWs. In this article, we show that individual GaP/GaNP core/shell nanowires (NWs) grown by molecular beam epitaxy on Si substrates can act as Fabry‐Perot (FP) microcavities. This conclusion is based on results of microphotoluminescence (μ‐PL) measurements performed on individual NWs, which reveal periodic undulations of the PL intensity that follow an expected pattern of FP cavity modes. The cavity is concluded to be formed along the NW axis with the end facets acting as reflecting mirrors. The formation of the FP modes is shown to be facilitated by an increasing index contrast with the surrounding media. Spectral dependence of the group refractive index is also determined for the studied NWs. The observation of the FP microcavity modes in the GaP/GaNP core/shell NWs can be considered as a first step toward achieving lasing in this quasidirect bandgap semiconductor in the NW geometry.     

Highly efficient avalanche multiphoton luminescence from coupled Au nanowires in the visible region

We demonstrate highly efficient avalanche multiphoton luminescence (MPL) from ordered-arrayed gold nanowires (NWs) with low time-average excitation intensity, Iexc (5.0-9.1 kW/cm2). The intensity of avalanche MPL, IMPL, is about 10(4) times larger than that of three-photon luminescence, the slope partial differential log IMPL/ partial differential log Iexc of avalanche MPL reaches as high as 18.3, and the corresponding polarization dependence of IMPL has a form of cos50 phip. The emission dynamics of avalanche MPL and three-photon luminescence are also studied comparatively. These observations indicate that the highly efficient avalanche MPL is attributed to the giant enhancement and coupling of longitudinal surface plasmon resonance of ordered-arrayed gold NWs.     

Structural and optical properties of glancing angle deposited TiO2 nanowires array

TiO2 nanowires (NWs) have been synthesized by glancing angle deposition technique using e-beam evaporator. The average length 490 nm and diameter 80 nm of NWs were examined by field emission-scanning electron microscopy. Transmission electron microscopy emphasized that the NWs were widely dispersed at the top. X-ray diffraction has been carried out on the TiO2 thin film (TF) and NW array. A small blue shift of 0.03 eV was observed in Photoluminescence (PL) main band emission for TiO2 NW as compared to TiO2 TF. The high temperature annealing at 980 degrees C partially removed the oxygen vacancy from the sample, which was investigated by PL and optical absorption measurements.     

Large-area self-catalysed and selective growth of ZnO nanowires

A simple procedure to selectively grow zinc oxide nanowires (ZnO NWs) on a large scale without any catalyst is reported. The process is based on the use of a zinc metal layer deposited onto substrates before the NW growth. The zinc layer, which becomes liquid at the synthesis temperature, favours the correct local conditions for a selective growth of pure ZnO NWs in an effective area of several square centimetres (up to 20?cm(2) in our laboratory-scale reactor). The?proposed method is suitable for patterned ZnO NW synthesis.     

Broadband photodetector of high quality Sb2S3 nanowire grown by chemical vapor deposition

Low dimensional semiconductors can be used for various electronic and optoelectronic devices because of their unique structure and property.In this work,one-dimensional Sb2S3 nanowires(NWs)with high crystallinity were grown via chemical vapor deposition(CVD)technique on SiO2/Si substrates.The Sb2S 3 NWs exhibited needle-like structures with inclined cross-sections.The lengths of Sb2S3 nanowires changed from 7 to 13 pm.The photodetection properties of Sb2S3 nanowires were comprehensively and systematically characterized.The Sb2S3 photodetectors show a broadband photoresponse ranging from ultraviolet(360 nm)to near-infrared(785 nm).An excellent specific detectivity of 2.1×1014 Jones,high external quantum efficiency of 1.5×104％,sensitivity of 2.2×104 cm2W-1 and short response time of less than 100 ms was achieved for the Sb2S3 NW photodetectors.Moreover,the Sb2S3 NWs showed out-standing switch cycling stability that was beneficial to the practical applications.The high-quality Sb2S3 nanowires fabricated by CVD have great application potential in semiconductor and optoelectronic fields.     

Tunable light emission from quantum-confined excitons in TiSi2-catalyzed silicon nanowires

Visible and near-infrared photoluminescence (PL) at room temperature is reported from Si nanowires (NWs) grown by chemical vapor deposition from TiSi2 catalyst sites. NWs grown with average diameter of 20 nm were etched and oxidized to thin and passivate the wires. The PL emission blue shifted continuously with decreasing nanowire diameter. Slowed oxidation was observed for small nanowire diameters and provides a high degree of control over the emission wavelength. Transmission electron microscopy, PL, and time-resolved PL data are fully consistent with quantum confinement of charge carriers in the Si nanowire core being the source of luminescence. These light emitting nanowires could find application in future CMOS-compatible photonic devices.     

Selective growth of silica nanowires using an Au catalyst for optical recognition of interleukin-10

The vapor-liquid-solid (VLS) growth procedure has been extended for the selective growth of silica nanowires on SiO(2) layer by using Au as a catalyst. The nanowires were grown in an open tube furnace at 1100?°C for 60?min using Ar as a carrier gas. The average diameter of these bottom-up nucleated wires was found to be 200?nm. Transmission electron microscopy analysis indicates the amorphous nature of these nanoscale wires and suggests an Si-silica heterostructure. The localized silica nanowires have been used as an immunoassay template in the detection of interleukin-10 which is a lung cancer biomarker. Such a nanostructured platform offered a tenfold enhancement in the optical response, aiding the recognition of IL-10 in comparison to a bare silica substrate. The role of nanowires in the immunoassay was verified through the quenching behavior in the photoluminescence (PL) spectra. Two orders of reduction in PL intensity have been observed after completion of the immunoassay with significant quenching after executing every step of the protocol. The potential of this site-specific growth of silica nanowires on SiO(2) as a multi-modal biosensing platform has been discussed.     

Room temperature photoluminescence properties of CuO nanowire arrays

Cupric oxide (CuO) nanowire arrays were synthesized by a simple and cost-effective thermal oxidation method. Optical properties of CuO nanowires (NWs) have been investigated through photoluminescence (PL) measurements at room temperature. CuO NWs appear several defects and vacancies related emission bands which usually appear only at low temperatures. Blue–green light emission with peaks at around 403, 474 and 489 nm is obtained from CuO NWs. In addition, the PL studies of CuO at room temperature reveal three emission peaks at around 713, 735 and 758 nm in the red and near infrared region. The study is attributed to understand the optical properties of CuO NWs and the design of CuO-based photo-electronic devices.     

Controlled dielectrophoretic nanowire self-assembly using atomic layer deposition and suspended microfabricated electrodes

Effects of design and materials on the dielectrophoretic self-assembly of individual gallium nitride nanowires (GaN NWs) onto microfabricated electrodes have been experimentally investigated. The use of TiO(2) surface coating generated by atomic layer deposition (ALD) improves dielectrophoretic assembly yield of individual GaN nanowires on microfabricated structures by as much as 67%. With a titanium dioxide coating, individual nanowires were placed across suspended electrode pairs in 46% of tests (147 out of 320 total), versus 28% of tests (88 out of 320 total tests) that used uncoated GaN NWs. An additional result from these tests was that suspending the electrodes 2.75 μm above the substrate corresponded with up to 15.8% improvement in overall assembly yield over that of electrodes fabricated directly on the substrate.     

Massive assembly of ZnO nanowire-based integrated devices

Although a directed assembly strategy has been utilized for the massive assembly of various nanowires and nanotubes (NWs/NTs), its application has usually been limited to rather small-diameter NWs/NTs prepared in solution. We report two complementary methods for the massive assembly of large-size ZnO nanowires (NWs). In the solution-phase method, ZnO NWs were assembled and aligned selectively onto negatively charged surface patterns in solution. In addition, the substrate bias voltage and capillary forces can be used to further enhance the adsorption rate and degree of alignment of ZnO NWs, respectively. In the direct-transfer method, a NW film grown on a solid substrate was placed in close proximity to?a?molecule-patterned substrate, and ultrasonic vibration was applied so that the NWs were directly transferred and aligned onto the patterned substrate. The solution-phase and direct-transfer methods are complementary to each other and suitable for the assembly of NWs?prepared in solution and on solid substrates, respectively.     

Electric conductivity-tunable transparent flexible nanowire-filled polymer composites: orientation control of nanowires in a magnetic field

Cobalt compound nanowires were dispersed in a transparent nonconductive polymer film by merely stirring, and the film's transparency and electrical conductivity were examined. This composite film is a unique system in which the average length of the nanowires exceeds the film's thickness. Even in such a system, a percolation threshold existed for the electric conductivity in the direction of the film thickness, and the value was 0.18 vol%. The electric conductivity value changed from ～1 × 10(-12) S/cm to ～1 × 10(-3) S/cm when the volume fraction exceeded the threshold. The electric conductivity apparently followed the percolation model until the volume fraction of the nanowires was about 0.45 vol %. The visible light transmission and electric conductivity of the composite film of about 1 vol % nanowires were 92% and 5 × 10(-3) S/cm, respectively. Moreover, the electric conductivity in the direction parallel to the film surface did not depend on the amount of the dispersed nanowires, and its value was about 1 × 10(-14) S/cm. Even in a weak magnetic field of about 100 mT, the nanowires were aligned in a vertical and parallel direction to the film surface, and the electric conductivity of each aligned composite film was 2.0 × 10(-2) S/cm and 2.1 × 10(-12) S/cm. The relation between the average wire length and the electric conductivity was examined, and the effect of the magnetic alignment on that relation was also examined.