Single-crystalline Gold Nanowires Research Articles

Fabrication of single-crystalline gold nanowires on cellulose nanofibers

Cellulose nanofibers (CNF) are promising nanomaterials for functional inks and printed sensors, although the potential applications are currently limited by the available functionalization methods. This work outlines a convenient method to grow a novel and highly conductive network of single-crystalline gold nanowires (AuNW) on CNF for use in conductive inks and printed sensors. The CNF are able to reduce Au (III) precursors to Au (0) monomers and generate nucleation sites for the subsequent monomer-by-monomer growth of Au nanocrystals; sodium citrate is used to control the reduction kinetics and the crystal growth. The growth of these AuNW/CNF materials is a three-step process of redox reaction, isotropic nucleation and anisotropic crystallization: the morphology and crystal structure of Au nanocrystals on CNF can be controlled by adjusting the reaction temperature and concentrations of citrate and CNF. The AuNW/CNF materials obtained have been formulated into highly conductive and atmospherically stable inks for use in either directly writing or screen printing. We have demonstrated AuNW/CNF-printed sensors with highly controllable electrical conductivity as well as excellent stability against rinsing and immersion by water and ethanol.

Nanometer Scale Confined Growth of Single-Crystalline Gold Nanowires via Photocatalytic Reduction.

Single-crystalline gold nanowires (Au NWs) are directly synthesized by the photocatalytic reduction of an aqueous HAuCl4 solution inside high-aspect-ratio TiO2 nanotubes (NTs). Crystalline TiO2 (anatase) NTs are prepared by the template-assisted atomic layer deposition technique with a subsequent annealing. Under the irradiation of ultraviolet light, photoexcited electrons are formed on the surfaces of TiO2 NTs and could reduce Au ions to create nuclei without using any surfactant, reducing agent, and/or seed. Once nucleation occurred, high-aspect-ratio Au NWs are grown inside the TiO2 NTs in a diffusion-controlled manner. As the solution pH increased, the nucleation/growth rate decreased and twin-free (or not observed), single-crystalline Au NWs are formed. At a pH above 6, the nucleation/growth rates increased and Au nanoparticles are observed both inside and outside of the TiO2 NTs. The confined nanoscale geometries of the interior of the TiO2 NTs are found to play a key role in the controlled diffusion of Au species and in determining the crystal morphology of the resulting Au NWs.

Single-Crystalline Gold Nanowires Synthesized from Light-Driven Oriented Attachment and Plasmon-Mediated Self-Assembly of Gold Nanorods or Nanoparticles

Through the light-driven geometrically oriented attachment (OA) and self-assembly of Au nanorods (NRs) or nanoparticles (NPs), single-crystalline Au nanowires (NWs) were synthesized by the irradiation of a linearly-polarized (LP) laser. The process was conducted in a droplet of Au colloid on a glass irradiated by LP near-infrared (e.g. 1064 nm and 785 nm) laser beam of low power at room temperature and atmospheric pressure, without any additive. The FE-SEM images show that the cross sections of NWs are various: tetragonal, pentagonal or hexagonal. The EDS spectrum verifies the composition is Au, and the pattern of X-ray diffraction identifies the crystallinity of NWs with the facets of {111}, {200}, {220} and {311}. We proposed a hypothesis for the mechanism that the primary building units are aligned and coalesced by the plasmon-mediated optical torque and force to form the secondary building units. Subsequently, the secondary building units undergo the next self-assembly, and so forth the tertiary ones. The LP light guides the translational and rotational motions of these building units to perform geometrically OA in the side-by-side, end-to-end and T-shaped manners. Consequently, micron-sized ordered mesocrystals are produced. Additionally, the concomitant plasmonic heating causes the annealing for recrystallizing the mesocrystals in water.

Surface two-level state dissipation in single-crystalline gold nanomechanical resonators

We study the intrinsic dissipation in nanomechanical resonators fabricated with synthetic singlecrystalline gold nanowires. Contrary to the etched nanostructures, the measured resonance frequencies and damping rates unambiguously demonstrate temperature dependence, as predicted by the standard two-level tunneling-state (TLS) theory. We compare the data to a model assuming a thin amorphous layer on the nanowire surface. The deduced TLS parameters are consistent with bulk poly(methyl methacrylate) (PMMA) data, confirming the presence of residual PMMA on the surface as the TLS host.

Strong Hall-Petch Type Behavior in the Elastic Strain Limit of Nanotwinned Gold Nanowires.

Pushing the limits of elastic deformation in nanowires subjected to stress is important for the design and performance of nanoscale devices from elastic strain engineering. Particularly, introducing nanoscale twins has proved effective in rising the tensile strength of metals. However, attaining ideal elastic strains in nanotwinned materials remains challenging, because nonuniform twin sizes locally affect the yielding behavior. Here, using in situ high-resolution transmission electron microscopy tensile testing of nanotwinned [111]-oriented gold nanowires, we report direct lattice-strain measurements that demonstrate a strong Hall-Petch type relationship in the elastic strain limit up to 5.3%, or near the ideal theoretical limit, as the twin size is decreased below 3 nm. It is found that the largest twin in nanowires with irregular twin sizes controls the slip nucleation and yielding processes in pure tension, which is in agreement with earlier atomistic simulations. Continuous hardening behavior without loss of strength or softening is observed in nanotwinned single-crystalline gold nanowires, which differs from the behaviors of bulk nanocrystalline and nanotwinned-nanocrystalline metals. These findings are of practical value for the use of nanotwinned metallic and semiconductor nanowires in strain-engineered functional microdevices.

Subcellular neural probes from single-crystal gold nanowires.

Size reduction of neural electrodes is essential for improving the functionality of neuroprosthetic devices, developing potent therapies for neurological and neurodegenerative diseases, and long-term brain–computer interfaces. Typical neural electrodes are micromanufactured devices with dimensions ranging from tens to hundreds of micrometers. Their further miniaturization is necessary to reduce local tissue damage and chronic immunological reactions of the brain. Here we report the neural electrode with subcellular dimensions based on single-crystalline gold nanowires (NWs) with a diameter of ∼100 nm. Unique mechanical and electrical properties of defect-free gold NWs enabled their implantation and recording of single neuron-activities in a live mouse brain despite a ∼50× reduction of the size compared to the closest analogues. Reduction of electrode dimensions enabled recording of neural activity with improved spatial resolution and differentiation of brain activity in response to different social situations for mice. The successful localization of the epileptic seizure center was also achieved using a multielectrode probe as a demonstration of the diagnostics potential of NW electrodes. This study demonstrated the realism of single-neuron recording using subcellular-sized electrodes that may be considered a pivotal point for use in diverse studies of chronic brain diseases.

Cold welding of ultrathin gold nanowires

The welding of metals at the nanoscale is likely to have an important role in the bottom-up fabrication of electrical and mechanical nanodevices. Existing welding techniques use local heating, requiring precise control of the heating mechanism and introducing the possibility of damage. The welding of metals without heating (or cold welding) has been demonstrated, but only at macroscopic length scales and under large applied pressures. Here, we demonstrate that single-crystalline gold nanowires with diameters between 3 and 10 nm can be cold-welded together within seconds by mechanical contact alone, and under relatively low applied pressures. High-resolution transmission electron microscopy and in situ measurements reveal that the welds are nearly perfect, with the same crystal orientation, strength and electrical conductivity as the rest of the nanowire. The high quality of the welds is attributed to the nanoscale sample dimensions, oriented-attachment mechanisms and mechanically assisted fast surface-atom diffusion. Welds are also demonstrated between gold and silver, and silver and silver, indicating that the technique may be generally applicable.

A novel method for preparing vertically grown single-crystalline gold nanowires

A surfactant-free, template-less and seed-less method, namely the thermal-assistedphotoreduction (TAP) process, has been developed to synthesize vertically grown Au nanowires(30–80 nm in diameterand about 2 µm in length) on the surface of thin film titanium dioxide(TiO2), which is locally excited by blackbody radiation. The Au nanowires thus produced aresingle-crystalline with a preferred growth direction. The electrical behavior investigated using a nanomanipulation deviceindicates that the Au nanowires possess an excellent electrical resistivity of about3.49 × 10−8 Ω m.

Seed-mediated shape evolution of gold nanomaterials: from spherical nanoparticles to polycrystalline nanochains and single-crystalline nanowires

We studied the kinetics of the reduction of a gold precursor (HAuCl4) and the effect of the molar ratio (R) of sodium citrate, which was introduced from a seed solution, and the gold precursor on the shape evolution of gold nanomaterials in the presence of preformed 13 nm gold nanoparticles as seeds. The reduction of the gold precursor by sodium citrate was accelerated due to the presence of gold seeds. Nearly single-crystalline gold nanowires were formed at a very low R value (R = 0.16) in the presence of the seeds as a result of the oriented attachment of the growing gold nanoparticles. At a higher R value (R = 0.33), gold nanochains were formed due to the non-oriented attachment of gold nanoparticles. At a much higher R value (R = 1.32), only larger spherical gold nanoparticles grown from the seeds were found. In the absence of gold seeds, no single-crystalline nanowires were formed at the same R value. Our results indicate that the formation of the 1D nanostructures (nanochains and nanowires) at low R values is due to the attachment of gold nanoparticles along one direction, which is driven by the surface energy reduction, nanoparticle attraction, and dipole–dipole interaction between adjacent nanoparticles.

Ultrafine Single‐Crystalline Gold Nanowire Arrays by Oriented Attachment

Ultrafine single-crystalline gold nanowires (see figure) are synthesized by oriented attachment of nanoparticles in the solution phase. An amine capping agent is preferentially removed from the {111} planes, which causes the wires to form by fusion along these planes. Such nanowires can be used as the fundamental building blocks for nanoelectronics, and will enable basic studies on electric transport in one-dimensional structures to be carried out.

Gold/Carbon Composite Tubes and Gold Nanowires by Impregnating Templates with Hydrogen Tetrachloroaurate/Acetone Solutions

Carbon/gold composite nanotubes as well as gold nanorods were synthesized by the impregnation of porous alumina with diluted hydrogen tetrachloroaurate (HAuCl4)/acetone solutions and subsequent thermolysis at moderate temperatures. Gold(III) is reduced, whereas carbon provided by the solvent acetone is oxidized. No oxygen and chlorine remain in the tube walls, which consist of gold nanocrystals supported by amorphous carbon. If the template pores have a diameter below a certain threshold value, which is of the order of 180 nm, single-crystalline gold nanowires are obtained. The methodology presented here may allow for synthesizing carbon tubes from ordinary acetone just by annealing.