One-step plasmonic welding and photolithographic patterning of silver nanowire network by UV-programable surface atom diffusion

Silver nanowire(AgNW)based transparent electrode(TE)plays a pivotal role in optoelectronics where TE is generally required to have fine pattern and high perform...     

Synthesis of single-crystalline silver sulfide nanowires by diffusion in porous anodic aluminium oxide template

Highly ordered single-crystalline silver sulfide (Ag₂S) nanowires have been successfully achieved directly using silver nitrate and thioacetamide (TAA) as the reactants, by diffusion in the channels of anodic aluminium oxide (AAO) membrane. The products have been characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), selected area electron diffraction (SAED) and high-resolution transmission electron microscopy (HRTEM). The results of the research show that the as-prepared Ag₂S nanowires are monodisperse with sizes of about 50 nm in diameter, closely corresponding to the pore size of the AAO membrane. Furthermore, its photoluminescence properties and the growth mechanism are also discussed.     

Strain-induced structural transformation of a silver nanowire

We have investigated the structural characteristics of the experimentally observed phase transition of a silver nanowire into a tube under tensile strain. In the simulations, atoms are allowed to interact via a model potential extracted from the modified embedded atom method. Our calculations demonstrate that the formation of the hollow structure is governed by the nature of the applied strain, the length of the wire, and the initial cross-sectional shape. The results further offer insights into the atomistic nature of this specific structural transformation into a nanotube with the smallest possible cross-section.     

A chemically-responsive nanojunction within a silver nanowire

The formation of a nanometer-scale chemically responsive junction (CRJ) within a silver nanowire is described. A silver nanowire was first prepared on glass using the lithographically patterned nanowire electrodeposition method. A 1-5 nm gap was formed in this wire by electromigration. Finally, this gap was reconnected by applying a voltage ramp to the nanowire resulting in the formation of a resistive, ohmic CRJ. Exposure of this CRJ-containing nanowire to ammonia (NH(3)) induced a rapid (<30 s) and reversible resistance change that was as large as ΔR/R(0) = (+)138% in 7% NH(3) and observable down to 500 ppm NH(3). Exposure to water vapor produced a weaker resistance increase of ΔR/R(0,H(2)O) = (+)10-15% (for 2.3% water) while nitrogen dioxide (NO(2)) exposure induced a stronger concentration-normalized resistance decrease of ΔR/R(0,NO(2)) = (-)10-15% (for 500 ppm NO(2)). The proposed mechanism of the resistance response for a CRJ, supported by temperature-dependent measurements of the conductivity for CRJs and density functional theory calculations, is that semiconducting p-type Ag(x)O is formed within the CRJ and the binding of molecules to this Ag(x)O modulates its electrical resistance.     

Synthesis and structural characterization of single-crystalline branched nanowire heterostructures

We report the synthesis of three-dimensional single-crystalline branched nanowire heterostructures, where the backbones and branches are assembled with ZnS and CdS, respectively. Growth of branch and backbones with control over the compositions was enabled via sequential seeding of gold nanocluster catalysts. Elemental mapping data confirmed that branched nanowire heterostructures were synthesized with the intended chemical modulation, CdS branches on ZnS backbones. Transmission electron microscopy studies showed that the growth of heterostructure branches occurs epitaxially from the backbone while maintaining single-crystalline structure. This unique class of heterostructures holds great potential in assembling electronics and photonics in three dimensions.     

Fabrication and optical property of single-crystalline InSb nanowire arrays

Semiconductor InSb nanowire arrays have been synthesized by the pulsed electrochemical deposition from citric acid aqueous solution into anodic alumina membranes. It was found that the InSb nanowires are of zinc-blende structure, and high filling rate and single-crystalline InSb nanowire array with right stoichiometric composition can be fabricated by proper controlling of the concentration of In and Sb ions and the PH value in the electrolyte and the pulse voltage, and the nanowires have [100] orientation with structure defects such as twins. The optical band gap has a strong blue shift with decreasing the diameter of the nanowire due to the quantum confinement effect.     

The modulation of surface texture for single-crystalline Si solar cells using calibrated silver nanoparticles as a catalyst

We have employed Ag nanoparticles with calibrated size as catalysts to modulate the surface texture of single-crystalline Si surfaces for reducing sunlight reflectivity. Both experiments and theoretical analysis have proved that a well-organized microporous structure on the pyramids can be obtained by optimizing the size of Ag nanoparticles and the texturing time, and the Si wafer with such structures can effectively reduce the reflectivity of sunlight. However, based on the conventional cell fabrication process, the performance of silicon solar cells with such microporous structures gets degraded. It is closely associated with the strong surface recombination and the high phosphorus diffusion barrier induced by the microporous textures. These results are interesting for us to understand the application of nanotechnology on the silicon solar cell.     

Transparent conducting silver nanowire networks

We present a transparent conducting electrode composed of a periodic two-dimensional network of silver nanowires. Networks of Ag nanowires are made with wire diameters of 45-110 nm and a pitch of 500, 700, and 1000 nm. Anomalous optical transmission is observed, with an averaged transmission up to 91% for the best transmitting network and sheet resistances as low as 6.5 Ω/sq for the best conducting network. Our most dilute networks show lower sheet resistance and higher optical transmittance than an 80 nm thick layer of ITO sputtered on glass. By comparing measurements and simulations, we identify four distinct physical phenomena that govern the transmission of light through the networks: all related to the excitation of localized surface plasmons and surface plasmon polaritons on the wires. The insights given in this paper provide the key guidelines for designing high-transmittance and low-resistance nanowire electrodes for optoelectronic devices, including thin-film solar cells. For the latter, we discuss the general design principles to use the nanowire electrodes also as a light trapping scheme.     

On slippage induced by surface diffusion

The mechanism of surface diffusion is taken at the basis of the phenomenon of slippage of the contact line of a liquid film. With the aid of the condition of continuity of the traction vectors at the solid-liquid interface, we obtained an evolution equation for the velocity of the fluid particles at the wall which shows a marked resemblance with Millikan's equation for the slippage coefficient of gases and reduces, in the limit of small surface diffusivity, to the classical Stokes-Einstein model. The influence of surface roughness is explicitely taken into account and, among other results, cases of absence of slip caused by the attachment of the liquid film to the solid surface and of slippage solely induced by surface roughness are found. Finally, the effect of the surface deformation upon the surface velocity of the fluid particles is examined in some detail.     

Flexible conductor fabrication via silver nanowire deposition on a polydopamine-modified pre-strained substrate

We fabricated a flexible conductor by depositing silver nanowires (AgNWs) on the surface of pre-strained polydopamine-modified polyurethane (PU) film. The polydopamine layer on the PU film created a highly hydrophilic surface that enhanced AgNWs adhesion to the PU film. When the pre-strained film was released, the PU film surfaces formed a uniformly buckled conductive layer. After embedding this substrate in polydimethylsiloxane (PDMS), a PU/AgNWs/PDMS flexible conductor was obtained. This flexible conductor can adapt to stretching (up to 20 % strain) and bending (3 mm bending radius) with slight changes in resistance, which was maintained at ~0.95 Ω after 1000 cycle tests. The resistance of the flexible conductor was 5.4 Ω under mechanical elongation of up to ~50 %, further demonstrating its obvious stretchable characteristics. Wavy AgNWs film-based stretchable conductors fabricated using a simple buckling approach could play an important role in the future development of flexible electronics.     

Polarization-dependent surface-enhanced Raman scattering from a silver-nanoparticle-decorated single silver nanowire

Single silver nanowires produced by DC electrodeposition in porous anodic alumina templates were surface-functionalized with the bifunctional molecule 4-aminobenzenthiol (ABT) then exposed to aqueous silver nanoparticles resulting in a silver nanoparticle-decorated silver nanowire. The polarization dependent surface-enhanced Raman (SERS) signal from this system showed significant intensity anisotropy when measured at a midsection of the nanowire, where the largest SERS intensities were observed when the incident light was polarized perpendicular to the nanowire's long axis but was almost isotropic near the tip of the nanowire. The observed effects were accounted for in terms of the electromagnetic fields concentrated in the collection of hot spots created through the ABT-linker-driven nanoparticle-nanowire self-assembly process.     

The dependence of the optoelectrical properties of silver nanowire networks on nanowire length and diameter

We have characterized the optoelectrical properties of networks of silver nanowires as a function of nanowire dimension by measuring transmittance (T) and sheet resistance (R(s)) for a large number of networks of different thicknesses fabricated from wires of different diameters (D) and lengths (L). We have analysed these data using both bulk-like and percolative models. We find the network DC conductivity to scale linearly with wire length while the optical conductivity is approximately invariant with nanowire length. The ratio of DC to optical conductivity, often taken as a figure of merit for transparent conductors, scales approximately as L/D. Interestingly, the percolative exponent, n, scales empirically as D2, while the percolative figure of merit, Π, displays large values at low D. As high T and low R(s) are associated with low n and high Π, these data are consistent with improved optoelectrical performance for networks of low-D wires. We predict that networks of wires with D = 25 nm could give sheet resistance as low as 25 Ω/□ for T = 90%.     

Computer simulation of surface diffusion of copper, silver and gold

The binding energies to copper, silver and gold (111) surfaces of self-atom clusters have been calculated. The activation energies of motion of these ad-atom clusters, vacancies and divacancies on copper, silver and gold (111) surface, and of the conversion of ad-atom clusters on (111) and (100) have been calculated by use ofn-body embedded atom potentials and molecular dynamics.     

Morphological evolution driven by strain induced surface diffusion

We have developed a three-dimensional finite element method to simulate the morphological evolution of a strained surface via surface diffusion, with a view to understanding the self-assembly, shape transitions and stability of low-dimensional quantum structures. We model deposition of an elastic film on a large lattice mismatched substrate. The film surface evolves by surface diffusion, driven by a gradient of the surface chemical potential, which includes the elastic strain energy, elastic anisotropy, surface energy, surface energy anisotropy and the interaction between the film and the substrate. Our simulations reveal that surface energy anisotropy and elastic anisotropy have a strong effect on the morphological evolution and shape transitions of the self-assembled islands. Our simulation results show a good qualitative agreement with experimental results.     

In situ TEM observation of heterogeneous phase transition of a constrained single-crystalline Ag2Te nanowire

Laterally epitaxial single crystalline Ag2Te nanowires (NWs) are synthesized on sapphire substrates by the vapor transport method. We observed the phase transitions of these Ag2Te NWs via in situ transmission electron microscopy (TEM) after covering them with Pt layers. The constrained NW shows phase transition from monoclinic to a body-centered cubic (bcc) structure near the interfaces, which is ascribed to the thermal stress caused by differences in the thermal expansion coefficients. Furthermore, we observed the nucleation and growth of bcc phase penetrating into the face-centered cubic matrix at 200 °C by high-resolution TEM in real time. Our results would provide valuable insight into how compressive stresses imposed by overlayers affect behaviors of nanodevices.     

Carrier depletion and exciton diffusion in a single ZnO nanowire

Carrier depletion and transport in a single ZnO nanowire Schottky device have been investigated at 5?K, using cathodoluminescence measurements. An exciton diffusion length of 200?nm has been determined along the nanowire axis. The depletion width is found to increase linearly with the reverse bias. The origin of this unusual dependence in semiconductor material is discussed in terms of charge location and dimensional effects on the screening of the junction electric field.     

Light propagation in curved silver nanowire plasmonic waveguides

Plasmonic waveguides made of metal nanowires (NWs) possess significant potential for applications in integrated photonic and electronic devices. Energy loss induced by bending of a NW during light propagation is critical in affecting its performance as a plasmonic waveguide. We report the characterization of the pure bending loss in curved crystalline silver NW plasmonic waveguides by decoupling the energy loss caused by bending and propagation. The energy attenuation coefficiency due purely to bending was also determined, which exhibited an exponential relationship with the bending radius. Finite-difference-time-domain (FDTD) methods were utilized for theoretical simulations, which matched the experimental results well.     

Integration of photonic and silver nanowire plasmonic waveguides

Future optical data transmission modules will require the integration of more than 10,000 x 10,000 input and output channels to increase data transmission rates and capacity. This level of integration, which greatly exceeds that of a conventional diffraction-limited photonic integrated circuit, will require the use of waveguides with a mode confinement below the diffraction limit, and also the integration of these waveguides with diffraction-limited components. We propose to integrate multiple silver nanowire plasmonic waveguides with polymer optical waveguides for the nanoscale confinement and guiding of light on a chip. In our device, the nanowires lay perpendicular to the polymer waveguide with one end inside the polymer. We theoretically predict and experimentally demonstrate coupling of light into multiple nanowires from the same waveguide, and also demonstrate control over the degree of coupling by changing the light polarization.     

Solid-phase diffusion mechanism for GaAs nanowire growth

Controllable production of nanometre-sized structures is an important field of research, and synthesis of one-dimensional objects, such as nanowires, is a rapidly expanding area with numerous applications, for example, in electronics, photonics, biology and medicine. Nanoscale electronic devices created inside nanowires, such as p-n junctions, were reported ten years ago. More recently, hetero-structure devices with clear quantum-mechanical behaviour have been reported, for example the double-barrier resonant tunnelling diode and the single-electron transistor. The generally accepted theory of semiconductor nanowire growth is the vapour-liquid-solid (VLS) growth mechanism, based on growth from a liquid metal seed particle. In this letter we suggest the existence of a growth regime quite different from VLS. We show that this new growth regime is based on a solid-phase diffusion mechanism of a single component through a gold seed particle, as shown by in situ heating experiments of GaAs nanowires in a transmission electron microscope, and supported by highly resolved chemical analysis and finite element calculations of the mass transport and composition profiles.