Quantitative fitting of nonlinear current-voltage curves and parameter retrieval of semiconducting nanowire, nanotube and nanoribbon devices

A quantitative metal-semiconductor-metal (MSM) model and a Matlab based program have been developed and used to obtain parameters that are important for characterizing semiconductor nanowires (NWs), nanotubes (NTs) or nanoribbons (NRs). The use of the MSM model for quantitative analysis of nonlinear current-voltage curves of one-dimensional semiconducting nanostructures is illustrated by working through two examples, i.e., an amorphous carbon NT and a ZnO NW, and the obtained parameters include the carrier density, mobility, resistance of the NT(NW), and the heights of the two Schottky barriers formed at the interfaces between metal electrodes and semiconducting NT(NW).     

Measuring True Young's Modulus of a Cantilevered Nanowire: Effect of Clamping on Resonance Frequency

The effect of clamping on resonance frequency and thus measured Young's modulus of nanowires (NWs) is systematically investigated via a combined experimental and simulation approach. ZnO NWs are used in this work as an example. The resonance tests are performed in situ inside a scanning electron microscope and the NWs are cantilevered on a tungsten probe by electron-beam-induced deposition (EBID) of hydrocarbon. EBID is repeated several times to deposit more hydrocarbons at the same location. The resonance frequency increases with the increasing clamp size until approaching that under the "fixed" boundary condition. The critical clamp size is identified as a function of NW diameter and NW Young's modulus. This work: 1) exemplifies the importance of considering the effect of clamping in measurements of Young's modulus using the resonance method, and 2) demonstrates that the true Young's modulus can be measured if the critical clamp size is reached. Design guidelines on the critical clamp size are provided. Such design guidelines can be extended to other one-dimensional nanostructures such as carbon nanotubes.     

An advanced fabrication method of highly ordered ZnO nanowire arrays on silicon substrates by atomic layer deposition

In this work, the controlled fabrication of highly ordered ZnO nanowire (NW) arrays on silicon substrates is reported. Si NWs fabricated by a combination of phase shift lithography and etching are used as a template and are subsequently substituted by ZnO NWs with a dry-etching technique and atomic layer deposition. This fabrication technique allows the vertical ZnO NWs to be fabricated on 4 in Si wafers. Room temperature photoluminescence and micro-photoluminescence are used to observe the optical properties of the atomic layer deposition (ALD) based ZnO NWs. The sharp UV luminescence observed from the ALD ZnO NWs is unexpected for the polycrystalline nanostructure. Surprisingly, the defect related luminescence is much decreased compared to an ALD ZnO film deposited at the same time ona plane substrate. Electrical characterization was carried out by using nanomanipulators. With the p-type Si substrate and the n-type ZnO NWs the nanodevices represent p–n NW diodes.The nanowire diodes show a very high breakthrough potential which implies that the ALD ZnO NWs can be used for future electronic applications.     

Tuning Light Absorption in Core/Shell Silicon Nanowire Photovoltaic Devices through Morphological Design

Subwavelength diameter semiconductor nanowires can support optical resonances with anomalously large absorption cross sections, and thus tailoring these resonances to specific frequencies could enable a number of nanophotonic applications. Here, we report the design and synthesis of core/shell p-type/intrinsic/n-type (p/i/n) Si nanowires (NWs) with different sizes and cross-sectional morphologies as well as measurement and simulation of photocurrent spectra from single-NW devices fabricated from these NW building blocks. Approximately hexagonal cross-section p/i/n coaxial NWs of various diameters (170-380 nm) were controllably synthesized by changing the Au catalyst diameter, which determines core diameter, as well as shell deposition time, which determines shell thickness. Measured polarization-resolved photocurrent spectra exhibit well-defined diameter-dependent peaks. The corresponding external quantum efficiency (EQE) spectra calculated from these data show good quantitative agreement with finite-difference time-domain (FDTD) simulations and allow assignment of the observed peaks to Fabry-Perot, whispering-gallery, and complex high-order resonant absorption modes. This comparison revealed a systematic red-shift of equivalent modes as a function of increasing NW diameter and a progressive increase in the number of resonances. In addition, tuning shell synthetic conditions to enable enhanced growth on select facets yielded NWs with approximately rectangular cross sections; analysis of transmission electron microscopy and scanning electron microscopy images demonstrate that growth of the n-type shell at 860 °C in the presence of phosphine leads to enhanced relative Si growth rates on the four {113} facets. Notably, polarization-resolved photocurrent spectra demonstrate that at longer wavelengths the rectangular cross-section NWs have narrow and significantly larger amplitude peaks with respect to similar size hexagonal NWs. A rectangular NW with a diameter of 260 nm yields a dominant mode centered at 570 nm with near-unity EQE in the transverse-electric polarized spectrum. Quantitative comparisons with FDTD simulations demonstrate that these new peaks arise from cavity modes with high symmetry that conform to the cross-sectional morphology of the rectangular NW, resulting in low optical loss of the mode. The ability to modulate absorption with changes in nanoscale morphology by controlled synthesis represents a promising route for developing new photovoltaic and optoelectronic devices.     

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.     

Mechanical properties of ZnO nanowires under different loading modes

A systematic experimental and theoretical investigation of the elastic and failure properties of ZnO nanowires (NWs) under different loading modes has been carried out. In situ scanning electron microscopy (SEM) tension and buckling tests on single ZnO NWs along the polar direction [0001] were conducted. Both tensile modulus (from tension) and bending modulus (from buckling) were found to increase as the NW diameter decreased from 80 to 20 nm. The bending modulus increased more rapidly than the tensile modulus, which demonstrates that the elasticity size effects in ZnO NWs are mainly due to surface stiffening. Two models based on continuum mechanics were able to fit the experimental data very well. The tension experiments showed that fracture strain and strength of ZnO NWs increased as the NW diameter decreased. The excellent resilience of ZnO NWs is advantageous for their applications in nanoscale actuation, sensing, and energy conversion.      

Highly efficient visible light photocatalysis of novel CuS/ZnO heterostructure nanowire arrays

Here, a facile approach for the fabrication of CuS nanoparticle (NP)/ZnO nanowire (NW) heterostructures on a mesh substrate through a simple two-step solution method is demonstrated. Successive ionic layer adsorption and reaction (SILAR) was employed to uniformly deposit CuS NPs on the hydrothermally grown ZnO NW array. The synthesized CuS/ZnO heterostructure NWs exhibited superior photocatalytic activity under visible light compared to bare ZnO NWs. This strong photocatalytic activity under visible light is due to the interfacial charge transfer (IFCT) from the valence band of the ZnO NW to the CuS NP, which reduces CuS to Cu(2)S. After repeated cycles of photodecolorization of Acid Orange 7 (AO7), the photocatalytic behavior of CuS/ZnO heterostructure NWs exhibited no significant loss of activity. Furthermore, our CuS/ZnO NWs/mesh photocatalyst floats in solution via partial superhydrophobic modification of the NWs.     

Heterojunction solar cells with integrated Si and ZnO nanowires and a chalcopyrite thin film

ZnO nanowires (NWs) have been successfully synthesized using a hydrothermal technique on both glass and silicon substrates initially coated with a sputtered ZnO thin film layer. Varying ZnO seed layer thicknesses were deposited to determine the effect of seed layer thickness on the quality of ZnO NW growth. The effect of growth time on the formation of ZnO NWs was also studied. Experimental results show that these two parameters have an important effect on formation, homogeneity and vertical orientation of ZnO NWs. Silicon nanowires were synthesized by a Ag-assisted electroless etching technique on an n-type Si (100) wafer. SEM observations have revealed the formation of vertically-aligned Si NWs with etching depth of ∼700 nm distributed over the surface of the Si. An electron-beam evaporated chalcopyrite thin film consisting of p-type AgGa_0.5In_0.5Se₂ with ∼800 nm thickness was deposited on the n-type ZnO and Si NWs for the construction of nanowire based heterojunction solar cells. For the Si NW based solar cell, from a partially illuminated area of the solar cell, the open-circuit voltage, short-circuit current density, fill factor and power conversion efficiency were 0.34 V, 25.38 mA cm⁻², 63% and 5.50%, respectively. On the other hand, these respective parameters were 0.26 V, 3.18 mA cm⁻², 35% and 0.37% for the ZnO NW solar cell.     

A ZnO nanowire vacuum pressure sensor

In this study, we report the growth and characterization of lateral ZnO nanowires (NWs) on ZnO:Ga/glass templates. Using x-ray diffraction and micro-Raman spectroscopy, it was found that crystal quality of the as-grown ZnO NWs is good. It was also found that the average length and average diameter of the laterally grown ZnO NWs were 5?μm and 30?nm, respectively. A vacuum pressure sensor was then fabricated using a single NW bridging across two electrodes. By measuring the current-voltage characteristics of the samples at low pressure, we found that the currents were of 17, 34.28, 57.37 and 96.06?nA for the ZnO NW measured at 1 × 10(-3)?Torr, 1 × 10(-4)?Torr, 3 × 10(-5)?Torr and 5 × 10(-6)?Torr, respectively. These values suggest that the laterally grown ZnO NWs prepared in this study are potentially useful for vacuum pressure sensing.     

一步掩膜法制备ZnO纳米线忆阻器

本文基于单根ZnO纳米线(NW),采用一步掩膜的方法制备了Au/ZnO NW/Au忆阻器。器件表现出无极性忆阻行为,开关比可达10~5以上。低阻态具有半导体导电特性,推测忆阻行为可能来源于ZnO NW表面氧空位形成的不连续导电丝的通断。一步掩膜法工艺简单,制备过程对器件污染少,因此是制备纳米线器件的有效方法。     

Partial conversion reaction of ZnO nanowires to ZnSe by a simple selenization method and their photocatalytic activities

A simple novel synthetic method for preparing ZnSe/ZnO heterostructured nanowire (NW) arrays via the selenization of ZnO NWs is reported. A hydrothermally grown ZnO NWs array on a glass substrate was reacted with selenium vapor to generate a 20–30 nm of zincblend ZnSe nanoparticles (NPs) on wurtzite ZnO NWs. A growth mechanism was proposed based on SEM, XRD, and TEM analysis to explain the partial chemical conversion of ZnO NW surfaces into ZnSe NPs. This mechanism is applicable to the synthesis of other chalcogenide compounds. The as-synthesized ZnSe/ZnO heterojunctions showed enhanced UV–visible light absorption properties. The materials exhibited excellent photocatalytic activity toward the decomposition of an organic dye compared to the bare ZnO due to enhanced light absorption and the type-II cascade band structure.     

Integrated multilayer nanogenerator fabricated using paired nanotip-to-nanowire brushes

We present a new approach to a nanogenerator (NG) that is composed of integrated, paired nanobrushes made of pyramid-shaped metal-coated ZnO nanotip (NTP) arrays and hexagonal-prism-shaped ZnO nanowire (NW) arrays, which were synthesized using a chemical approach at <100 degrees C on the two surfaces of a common substrate, respectively. The operation of the NGs relies on mechanical deflection/bending of the NWs, in which resonance of NWs is not required to activate the NG. This largely expands the application of the NGs from low frequency (approximately the hertz range) to a relatively high frequency (approximately the megahertz range) for effectively harvesting mechanical energies in our living environment. With one piece of such a structure stacked in close proximity over another to form a layer-by-layer matched brush architecture, direct current is generated by exciting the architecture using ultrasonic waves. A four-layer integrated NG is demonstrated to generate an output power density of 0.11 microW/cm(2) at 62 mV. The layer-by-layer assembly provides a feasible technology for building three-dimensional NGs for applications where force or pressure variations are available, such as a shoe pad, an underskin layer for airplanes, and next to a vibration source such as a car engine or tire.     

The growth of porous ZnO nanowires by thermal oxidation of ZnS nanowires

The growth of porous ZnO nanowires (NWs) via phase transformation of ZnS NWs at 500-850 degrees C in air was studied. The ZnS NWs were first synthesized by thermal evaporation of ZnS powder at 1100 degrees C in Ar. On subsequent annealing at 500 degrees C in air, discrete ZnO epilayers formed on the surface of ZnS NWs. At 600 degrees C, polycrystalline ZnO and the crack along the (0001) interface between the ZnO epilayer and ZnS NW were observed. At 700-750 degrees C ZnS NWs transformed to ZnO NWs, meanwhile nanopores and interfacial cracks were observed in the ZnO NWs. Two factors, the evaporation of SO2 and SO3 and the stress induced by the incompatible structure at the interface of ZnO epilayer and ZnS NW, can be responsible for the formation of porous ZnO NWs from ZnS NW templates on annealing at 700-750 degrees C in air. Rapid growth of ZnO at 850 degrees C could heal the pores and cracks and thus resulted in the well-crystallized ZnO NWs.     

ZnO nanowire co-growth on SiO2 and C by carbothermal reduction and vapour advection

Vertically aligned ZnO nanowires (NWs) were grown on Au-nanocluster-seeded amorphous SiO(2) films by the advective transport and deposition of Zn vapours obtained from the carbothermal reaction of graphite and ZnO powders. Both the NW volume and visible-to-UV photoluminescence ratio were found to be strong functions of, and hence could be tailored by, the (ZnO+C) source-SiO(2) substrate distance. We observe C flakes on the ZnO NWs/SiO(2) substrates which exhibit short NWs that developed on both sides. The SiO(2) and C substrates/NW interfaces were studied in detail to determine growth mechanisms. NWs on Au-seeded SiO(2) were promoted by a rough ZnO seed layer whose formation was catalysed by the Au clusters. In contrast, NWs grew without any seed on C. A correlation comprising three orders of magnitude between the visible-to-UV photoluminescence intensity ratio and the NW volume is found, which results from a characteristic Zn partial pressure profile that fixes both O deficiency defect concentration and growth rate.     

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.     

Localized joule heating as a mask-free technique for the local synthesis of ZnO nanowires on silicon nanodevices

We report a mask-free technique for the local synthesis of ZnO nanowires (NWs) on polysilicon nanobelts and polysilicon NW devices. First, we used localized joule heating to generate a poly(methyl methacrylate) (PMMA) nanotemplate, allowing the rapid and self-aligned ablation of PMMA within a short period of time (ca. 5 μs). Next, we used ion-beam sputtering to prepare an ultrathin Au film and a ZnO seed layer; a subsequent lift-off process left the seed layers selectively within the PMMA nanotemplate. Gold nanoparticles and ZnO NWs were formed selectively in the localized joule heating region.     

Synthesis of high crystallinity ZnO nanowire array on polymer substrate and flexible fiber-based sensor

Well aligned ZnO nanowire (NW) arrays are grown on Kevlar fiber and Kapton film via the chemical vapor deposition (CVD) method. These NWs have better crystallinity than those synthesized through the low-temperature hydrothermal method. The average length and diameter of ZnO NWs grown on Kevlar fiber can be controlled from 0.5 to 2.76 μm and 30 to 300 nm, respectively. A flexible ultraviolet (UV) sensor based on Kevlar fiber/ZnO NWs hybrid structure is made to detect UV illumination quantificationally.     

ZnO nanowires hydrothermally grown on PET polymer substrates and their characteristics

Zinc oxide nanowires (ZnO NWs) were successfully synthesized on the ITO/PET polymer substrates by a hydrothermal method. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy investigations were carried out to characterize the crystallinity, surface morphologies, and orientations of these NWs, respectively. The influence of NW surface morphologies on the optical and electrical properties of ZnO NWs was studied. The hydrothermally grown ZnO NWs with direct band gap of 3.21 eV emitted ultraviolet photoluminescence of 406 nm at room temperature. Field emission measurements revealed that the threshold electric fields (Eth, current density of 1 mA/cm2) of ZnO NWs/ITO/PET and ZnO NWs/ZnO/ITO/PET are 1.6 and 2.2 V/microm with the enhancement factors, beta values, of 3275 and 4502, respectively. Furthermore, the field emission performance of ZnO NWs deposited on the ITO/PET substrate can be enhanced by illumination with Eth of 1.3 V/microm and displays a maximum emission current density of 18 mA/cm2. The ZnO NWs successfully grown on polymer substrate with high transmittance, low threshold electric field, and high emission current density may be applied to a flexible field emission display in the future.     

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.     

Structural and optical properties of individual GaP/ZnO core–shell nanowires

Structural and optical properties of ZnO–GaP core–shell nanowires were studied by means of electron microscopy and microphotoluminescence. A thin ZnO shell layer was deposited by RF sputtering on GaP nanowires, which were grown on GaP (111)B substrates under vapour–liquid–solid mode by MOVPE. The SEM and TEM characterization showed that the ZnO shells fully covered the surface of the NWs from top to bottom. Each GaP NW core is composed of many well-defined twinned segments with the planes of twinning oriented in perpendicular to the growth direction. This was contradicted in kinked GaP NWs: their growth direction was initially perpendicular to the twinning planes, but once the NW had kinked, it changed to lie within the twinning planes. The ZnO shell deposited on the GaP core has a columnar morphology. The columns are inclined at a positive angle close to 70° with respect to the GaP growth axis. All observed columns were tilted at this angle to the growth direction. Micro-photoluminescence study showed that thermal annealing improved the quality of the ZnO crystallographic structure; the annealing made observable the photoluminescence peak related to the band-to-band transition in ZnO.