Nanowire-based Devices Research Articles

Size-dependent fracture behavior of silver nanowires

Silver (Ag) nanowires have great potential to be used in the flexible electronics industry for their applications in flexible, transparent conductors due to high conductivity and light reflectivity. Those applications always involve mechanical loading and deformations, which requires an in-depth understanding of their mechanical behavior and performance under loadings. However, current understanding on the mechanical properties of Ag nanowires is limited, especially on their size-dependent fracture behavior. In this work, mechanical properties of Ag nanowires with diameters ranging from 50 to 300 nm were systematically studied by in situ TEM tensile testing for the first time. The size effect was clearly found, with the increasing of the diameter of Ag nanowires, the ultimate tensile stress decreased. More importantly, the fracture behavior of Ag nanowire was studied and a brittle-to-ductile transition in fracture behavior was observed at the diameters around 100 nm which could be attributed to the dislocation activities within the geometry confinement. This work could give insights for understanding nanosized Ag wires and the design of Ag nanowire-based flexible devices and touchable panels.

Controlling the Growth of Single Nanowires in a Nanowire Forest for near-Infrared Photodetection

Controlling one-dimension nanowire growth at single nanowire level is vital for building up multifunctional nanowire-based devices and getting insights into nanowire growth mechanisms. In this lett...

The effect of V/III ratio on the morphology and structure of GaAs nanowires by MOCVD

In this paper, GaAs nanowires with different V/III ratios (70, 140, 280 and 560) were vertically grown from bottom to top on GaAs substrates by using metal organic chemical vapor deposition based on gold assisted vapor-liquid-solid mechanism. It is found that the growth rate of nanowires is inversely proportional to their V/III ratio. And the V/III ratio can also change nanowire growth type. For the nanowire with small V/III ratios (≤280), the reactants are most from those atoms merged in the catalyst. But, for the nanowire with V/III ratio 560, the contribution mainly comes from the diffusions of atoms pyrolyzed on the surface of the nanowire and the substrate. A shrunken neck under the catalyst is observed in TEM characterizations. These results will provide a theoretical basis for potential practical applications of nanowire-based devices.

Optimizing Photovoltaic Charge Generation of Hybrid Heterojunction Core-Shell Silicon Nanowire Arrays: An FDTD Analysis.

Development of highly efficient nanowire-based photovoltaic devices requires an accurate modeling of light scattering from interfaces and optical carrier generation inside the cell. A comprehensive study of optical absorption and carrier generation enables us to tap the full potential of nanowire arrays (NWAs). In this study, we have done a systematic study to optimize the core–shell structure of vertically aligned silicon nanowire (Si NW) arrays coated with PTB7:PC71BM by means of finite difference time domain optical simulations to maximize the photon absorption. Initially, the core thickness of hybrid Si NWs has been optimized for the most efficient light absorption. The further improvement of light absorption has been studied by varying the coating thickness of low-band gap organic polymer PTB7:PC71BM on Si NWAs. A delineative analysis shows that NWs with a 150 nm thick silicon core and 60 nm thick coating of PTB7:PC71BM exhibit broad band absorption and the optimum ideal current density of about 34.95 mA/cm2, which are larger than those of their planar counterpart with the same amount of absorbing material and also better than those previously reported for NWs. The basic principle and the physical process taking place during absorption and current generation have been also discussed. The optimization of the hybrid heterojunction Si NW arrays and understanding of their optical characteristics may contribute to the development of economical and highly efficient hybrid solar cells.

In situ TEM observation of preferential amorphization in single crystal Si nanowire

The nanoinstability of a single crystal Si nanowire under electron beam irradiation was in situ investigated at room temperature by the transmission electron microscopy technique. It was observed that the Si nanowire amorphized preferentially from the surface towards the center, with the increasing of the electron dose. In contrast, in the center of the Si nanowire the amorphization seemed much more difficult, being accompanied by the rotation of crystal grains and the compression of d-spacing. Such a preferential amorphization, which is athermally induced by the electron beam irradiation, can be well accounted for by our proposed concepts of the nanocurvature effect and the energetic beam-induced athermal activation effect, while the classical knock-on mechanism and the electron beam heating effect seem inadequate to explain these processes. Furthermore, the findings revealed the difference of amorphization between a Si nanowire and a Si film under electron beam irradiation. Also, the findings have important implications for the nanoinstability and nanoprocessing of future Si nanowire-based devices.

Site-Specific Growth and in Situ Integration of Different Nanowire Material Networks on a Single Chip: Toward a Nanowire-Based Electronic Nose for Gas Detection.

A new method for the site-selective synthesis of nanowires has been developed to enable material growth with defined morphology and, at the same time, different composition on the same chip surface. The chemical vapor deposition approach for the growth of these nanowire-based resistive devices using micromembranes can be easily modified and represents a simple, adjustable fabrication process for the direct integration of nanowire meshes in multifunctional devices. This proof-of-concept study includes the deposition of SnO2, WO3, and Ge nanowires on the same chip. The individual resistors exhibit adequate gas sensing responses toward changing gas concentrations of CO, NO2, and humidity diluted in synthetic air. The data have been processed by principal component analysis with cluster responses that can be easily separated, and thus, the devices described herein are in principle suitable for environmental monitoring.

Theoretical study on electronic properties of p-type GaN nanowire surface covered with Cs

By using density functional theory, we have investigated the Cs adsorption mechanism of Mg-doped GaN nanowire photocathode. The results show that in the GaN nanowire, Mg atoms tend to replace the Ga atoms in the central position, because the position has the lowest formation energy, and the carrier concentration is highest, which shows better conductivity. Besides, Mg doping leads to the Fermi level close to the valence band, exhibiting p-type conductivity. After Cs activation, the most stable adsorption site of Mg-doped nanowires is B N site. After Cs adsorption, the conduction band minimum and the valence band maximum both move to the lower energy, which is favorable for forming the n-type surface state and improving the photoelectron escape probability. This study can be used to guide the Cs adsorption process of Mg-doped GaN nanowires, which is helpful to improve the performance of GaN nanowire-based optoelectronic devices.

Electrical transport properties and related mechanism of single SnO<sub>2</sub> nanowire device

Defect engineering in a semiconductor nanowire-based device has aroused intensive attention due to its fascinating properties and the potential applications in nanoelectronics. Here in this work, in order to investigate the effect of oxygen defects on the electrical transport properties in a SnO<sub>2</sub>-nanowire-based device under normal environment, we synthesize an individual SnO<sub>2</sub> nanowire, by a thermal chemical vapor deposition method and further construct a two-terminal Au/SnO<sub>2</sub> nanowire/Au device by using optical lithography. The electrical transport properties of a single SnO<sub>2</sub> nanowire device are measured under the condition of air and vacuum after hydrogen reduction. It is found that the transport performances in air are unusually different from those in vacuum. Strikingly, the reduction of electric current through the device and the increment of contact barrier of the Au/SnO<sub>2</sub> interface in air can be observed with the <i>I-V</i> scan times increasing. While in vacuum, the current increases and a change from Schottky contact to ohmic contact at the interface between Au and SnO<sub>2</sub> can be obtained by performing more scans. Our results demonstrate that the oxygen vacancy concentrations caused by the oxygen atom adsorption and desorption on the surface of nanowires play the key role in the transport properties. Furthermore, we calculate the relevant electronic properties, including energy band structure, density of states, as well as <i>I-V</i> characters and transmission spectrum at the interface of Au/SnO<sub>2</sub> within the framework of density functional theory. We find that the bandgap of SnO<sub>2</sub> nanowires decreases with oxygen vacancy concentration increasing. Also, the existence of oxygen defects enlarges the electron transmission at the interface of Au/SnO<sub>2</sub> and enhances electrical transport. Therefore, our results provide a new strategy for designing the integrated nano-functional SnO<sub>2</sub>-based devices.

Fracture of a silicon nanowire at ultra-large elastic strain

Understanding the fracture behavior of one-dimensional (1-D) nanomaterials is critical for their functional device applications and maximizing their service life. At the nanoscale, solid materials’ fracture properties could significantly deviate from their bulk counterparts. Our recent study (Zhang et al. in Sci. Adv. 2(8):e1501382, 2016. https://doi.org/10.1126/sciadv.1501382 ) showed that silicon (Si) nanowires, one of the most important functional 1-D nanomaterials for nanoelectronics and nano-electro-mechanical systems, demonstrated distinctly different mechanical properties than microscale and bulk Si crystals with the elastic strain up to 10% or even more, approaching their theoretical elastic limit. It is therefore intriguing to understand the fracture behavior of a Si nanowire under such deep ultra-strength, as well as their failure mechanisms. In this work, we will experimentally study the fracture behavior of ultrahigh elastic Si nanowires in situ and quantitatively understand their fracture mechanics with the assist of molecular dynamics simulations. The insights obtained in this nanomechanical study may be of help on the development of robust Si nanowire-based mechatronic devices.

Morphologies and electrical properties of multiple CuO nanowire-based device controlled by electroplating and thermal oxidation process

Metal oxide nanowire-based devices have been widely studied for chemical sensors. Based on the knowledge of the work functions of copper oxide nanowires and metal electrodes, the electrical junction properties such as Schottky barrier height could be modulated by changing the electrode material or decorating the nanowires with another material. Especially for the nanowire decoration process, nanoparticle sputtering or wet chemical synthesis has been used as a post processing method, which is complex and costly. These drawbacks have greatly limited the actual applications of the nanowire-based devices. In this work, a facile one-step thermal oxidation of electroplated interdigitated Cu patterns is proposed for multiple copper oxide nanowire-based device fabrication and a modulation mechanism based on adjusting nanowire density and total area of CuO/Cu–Au interface is discussed. The initial thickness of Cu and the electrode configuration play a deterministic role in the final CuO nanowire morphologies and their junction properties, which is a new finding.

Dielectrophoresis-based multi-step nanowire assembly on a flexible superstrate

Nanowire assembly based on dielectrophoresis (DEP) could be a useful and efficient tool for fabricating nanowire-based devices. Although there have been extensive reports on the DEP nanowire assembly, the new approaches that make DEP more facile and affordable are still desirable. Herein, we present an approach using the reusable electrodes to assemble silver nanowires onto a removable, independent polyethylene terephthalate (PET) film. The PET film is placed on the reusable electrodes, and a sinusoidal AC voltage is applied to the electrodes to induce DEP force for nanowire assembly upon the flexible film. We explore the influences of voltage, frequency and film thickness on nanowire assembly and further realize the assembly of silver nanowire arrays. In addition, the induced electric field is rotated in two consecutive steps to assemble the rectangular mesh-like nanowire networks. This reusable and facile approach for DEP nanowire assembly could provide a low-cost, precise, rapid and convenient tool for applications in the fields of flexible electronics.

Au nanowire junction breakup through surface atom diffusion

Metallic nanowires are known to break into shorter fragments due to the Rayleigh instability mechanism. This process is strongly accelerated at elevated temperatures and can completely hinder the functioning of nanowire-based devices like e.g. transparent conductive and flexible coatings. At the same time, arranged gold nanodots have important applications in electrochemical sensors. In this paper we perform a series of annealing experiments of gold and silver nanowires and nanowire junctions at fixed temperatures 473, 673, 873 and 973 K (200 °C, 400 °C, 600 °C and 700 °C) during a time period of 10 min. We show that nanowires are especially prone to fragmentation around junctions and crossing points even at comparatively low temperatures. The fragmentation process is highly temperature dependent and the junction region breaks up at a lower temperature than a single nanowire. We develop a gold parametrization for kinetic Monte Carlo simulations and demonstrate the surface diffusion origin of the nanowire junction fragmentation. We show that nanowire fragmentation starts at the junctions with high reliability and propose that aligning nanowires in a regular grid could be used as a technique for fabricating arrays of nanodots.

Surface Trap-Induced Conductivity Type Switching in Semiconductor Nanowires: Analytical and Numerical Analyses

Due to high surface-to-volume ratio, surface traps play crucial role in electronic properties of semiconductor nanowires and nanowire-based devices. Recently, it was experimentally demonstrated that surface traps even can induce conductivity type inversion in semiconductor nanowires. Here, we have performed theoretical and numerical analyses to investigate the influence of surface traps on semiconductor nanowire conductivity including possible inversion due to surface traps. The Poisson equation with consideration of both type of mobile charges and ionized donors was solved having surface trapped charges in Neumann boundary conditions. Different semiconductor materials and surface trap densities have been considered. Calculations were done considering single energy level of surface traps and then the results were generalized to exponential energetic distribution of surface states density. The effect of surface traps was analyzed both for low-doped and highly doped nanowires. It was shown that for moderate or lightly doped NWs, the conductivity type switching from n- to p-type can occur at small radii. The developed potential-based analytical model allows calculating the inversion charge raised due to surface traps. The accuracy of analytical calculations has been validated with TCAD simulations and qualitatively compared with published experimental data. This paper is useful for design optimization of nanowire-based photodetectors, sensors, and reconfigurable FETs.

Self-Assembly Growth of In-Rich InGaAs Core-Shell Structured Nanowires with Remarkable Near-Infrared Photoresponsivity.

Understanding the compositional distribution of ternary nanowires is essential to build the connection between nanowire structures and their potential applications. In this study, we grew epitaxial ternary InGaAs nanowires with high In concentration on GaAs {111}B substrates. Our detailed electron microscopy characterizations suggest that the grown ternary InGaAs nanowires have an extraordinary core-shell structure with In-rich cores and Ga-enriched shells, in which both nanowire cores and shells showed compositional gradient. It was found that In-rich nanowire cores are formed due to the Ga-limited growth environment, caused by the competition with the spontaneous InGaAs planar layer growth on the substrate that consumes more Ga than the nominal Ga concentration during the growth. Moreover, the composition gradient in the nanowires cores and shells is a result of strain relaxation between them. Our optoelectronic property measurements from prototype nanowire devices show a remarkable photoresponsivity under the near-infrared illumination. This study provides a new approach for designing and realizing complex nanowire heterostructures for high-efficiency nanowire-based systems and devices.

I–V curve hysteresis induced by gate-free charging of GaAs nanowires' surface oxide

The control of nanowire-based device performance requires knowledge about the transport of charge carriers and its limiting factors. We present the experimental and modeled results of a study of electrical properties of GaAs nanowires (NWs), considering their native oxide cover. Measurements of individual vertical NWs were performed by conductive atomic force microscopy (C-AFM). Experimental C-AFM observations with numerical simulations revealed the complex resistive behavior of NWs. A hysteresis of current-voltage characteristics of the p-doped NWs as-grown on substrates with different types of doping was registered. The emergence of hysteresis was explained by the trapping of majority carriers in the surface oxide layer near the reverse-biased barriers under the source-drain current. It was found that the accumulation of charge increases the current for highly doped p+-NWs on n+-substrates, while for moderately doped p-NWs on p+-substrates, charge accumulation decreases the current due to blocking of the conductive channel of NWs.

Molecular Beam Epitaxy of GaN Nanowires on Epitaxial Graphene.

We demonstrate an all-epitaxial and scalable growth approach to fabricate single-crystalline GaN nanowires on graphene by plasma-assisted molecular beam epitaxy. As substrate, we explore several types of epitaxial graphene layer structures synthesized on SiC. The different structures differ mainly in their total number of graphene layers. Because graphene is found to be etched under active N exposure, the direct growth of GaN nanowires on graphene is only achieved on multilayer graphene structures. The analysis of the nanowire ensembles prepared on multilayer graphene by Raman spectroscopy and transmission electron microscopy reveals the presence of graphene underneath as well as in between nanowires, as desired for the use of this material as contact layer in nanowire-based devices. The nanowires nucleate preferentially at step edges, are vertical, well aligned, epitaxial, and of comparable structural quality as similar structures fabricated on conventional substrates.

Proposal for a Nanoscale Superconductive Memory

Energy efficiency is a key issue for modern high performance computing. Superconductive digital electronics has already demonstrated superior performances in terms of speed and energy dissipations. However, there is still the open issue of the realization of effective submicron scale superconductive memories. Superconducting nanowires represent the state-of-the-art of single-photon detectors. Their technology has also been used to realize three-terminal active devices, where the output response is triggered by a current pulse. The combination of the electrothermal mechanism of the nanowire and the magnetic coupling with a suitable material can be used for the realization of a nanowire-based memory device scalable to nanoscale. The principle of operation and material requirements are presented here. In particular, the feasibility of the proposed device using EuS as magnetic material and NbN as nanowire is discussed. By using a physical model of the nanowire dynamics, in terms of the spatial distribution of electron and phonon temperatures, the feasibility of the proposed device has been verified, through numerical simulations. The device configurations considered have the specific goal of realizing reliable and high-speed read and write operations, with the possibility of scalability to nanoscale.

Effect of molecular layers on charge transport in nanowires

Stable ultrathin gold nanowires (Au NWs, 2nm diameter) were produced using wet chemical synthesis, which implied oleylamine molecules, and subsequently assembled onto gold electrodes. Transport properties of fabricated structures were studied using the noise spectroscopy technique. Lorentzian-shaped components were revealed in the noise spectra. The characteristic frequencies of these components were independent of the bias applied, which thus excludes self-heating of the nanowires. Noise peculiarities were shown to be the result of the presence of oleylamine molecules on the NWs. The evidence of oleylamine on the surface of the fabricated structures was confirmed by high-resolution transmission electron microscopy. Organic monolayers can strongly influence charge properties in nanosystems. These results should be taken into account in designing nanowire-based devices for molecular electronics.

Semiconducting Nanowire-Based Optoelectronic Fibers.

The recent ability to integrate semiconductor-based optoelectronic functionalities within thin fibers is opening intriguing opportunities for flexible electronics and advanced textiles. The scalable integration of high-quality semiconducting devices within functional fibers however remains a challenge. It is difficult with current strategies to combine high light absorption, good microstructure and efficient electrical contact. The growth of semiconducting nanowires is a great tool to control crystal orientation and ensure a combination of light absorption and charge extraction for efficient photodetection. Thus far, however, leveraging the attributes of nanowires has remained seemingly incompatible with fiber materials, geometry, and processing approaches. Here, the integration of semiconducting nanowire-based devices at the tip and along the length of polymer fibers is demonstrated for the first time. The scalable thermal drawing process is combined with a simple sonochemical treatment to grow nanowires out of electrically addressed amorphous selenium domains. First principles density-functional theory calculations show that this approach enables to tailor the surface energy of crystal facets and favors nanowire growth along a preferred orientation, resulting in fiber-integrated devices of unprecedented performance. This novel platform is exploited to demonstrate an all-fiber-integrated fluorescence imaging system, highlighting novel opportunities in sensing, advanced optical probes, and smart textiles.

The influence of surfaces on the transient terahertz conductivity and electron mobility of GaAs nanowires

Bare unpassivated GaAs nanowires feature relatively high electron mobilities (400–2100 cm2 V−1 s−1) and ultrashort charge carrier lifetimes (1–5 ps) at room temperature. These two properties are highly desirable for high speed optoelectronic devices, including photoreceivers, modulators and switches operating at microwave and terahertz frequencies. When engineering these GaAs nanowire-based devices, it is important to have a quantitative understanding of how the charge carrier mobility and lifetime can be tuned. Here we use optical-pump–terahertz-probe spectroscopy to quantify how mobility and lifetime depend on the nanowire surfaces and on carrier density in unpassivated GaAs nanowires. We also present two alternative frameworks for the analysis of nanowire photoconductivity: one based on plasmon resonance and the other based on Maxwell–Garnett effective medium theory with the nanowires modelled as prolate ellipsoids. We find the electron mobility decreases significantly with decreasing nanowire diameter, as charge carriers experience increased scattering at nanowire surfaces. Reducing the diameter from 50 nm to 30 nm degrades the electron mobility by up to 47%. Photoconductivity dynamics were dominated by trapping at saturable states existing at the nanowire surface, and the trapping rate was highest for the nanowires of narrowest diameter. The maximum surface recombination velocity, which occurs in the limit of all traps being empty, was calculated as 1.3 × 106 cm s−1. We note that when selecting the optimum nanowire diameter for an ultrafast device, there is a trade-off between achieving a short lifetime and a high carrier mobility. To achieve high speed GaAs nanowire devices featuring the highest charge carrier mobilities and shortest lifetimes, we recommend operating the devices at low charge carrier densities.