Silver Dendrites from Galvanic Displacement on Commercial Aluminum Foil As an Effective SERS Substrate

Silver Dendrites from Galvanic Displacement on Commercial Aluminum Foil As an Effective SERS Substrat

Temperature-Induced Self-Pinning and Nanolayering of AuSi Eutectic Droplets

A process for self-pinning of AuSi eutectic alloy droplets to a Si substrate, induced by a controlled temperature annealing in ultrahigh vacuum, is presented. Surface pinning of AuSi 3D droplets to the Si substrate is found to be a consequence of the readjustment in the chemical composition of the droplets upon annealing, as required to maintain thermodynamic equilibrium at the solid−liquid interface. Structural and morphological changes leading to the pinning of the droplets to the substrate are analyzed. Phase separation is observed upon cooling of the droplets, leading to the formation of amorphous Si-rich channels within the core and the formation of crystalline Si nanoshells on the outside. The mechanism leading to the pinning and surface layering provides new insight into the role of alloying during growth of silicon nanowires and may be relevant to the engineering of nanoscale Si cavities

Single Nanowire Thermal Conductivity Measurements by Raman Thermography

A facile, rapid, and nondestructive technique for determining the thermal conductivity of individual nanowires based on Raman temperature mapping has been demonstrated. Using calculated absorption efficiencies, the thermal conductivities of single cantilevered Si nanowires grown by the vapor−liquid−solid method are measured and the results agree well with values predicted by diffuse phonon boundary scattering. As a measurement performed on the wire, thermal contact effects are avoided and ambient air convection is found to be negligible for the range of diameters measured. The method’s versatility is further exemplified in the reverse measurement of a single nanowire absorption efficiency assuming diffuse phonon boundary scattering. The results presented here outline the broad utility that Raman thermography may have for future thermoelectric and photovoltaic characterization of nanostructures

Tuning Micropillar Tapering for Optimal Friction Performance of Thermoplastic Gecko-Inspired Adhesive

We present a fabrication method and friction testing of a gecko-inspired thermoplastic micropillar array with control over the tapering angle of the pillar sidewall. A combination of deep reactive ion etching of vertical silicon pillars and subsequent maskless chemical etching produces templates with various widths and degrees of taper, which are then replicated with low-density polyethylene. As the silicon pillars on the template are chemically etched in a bath consisting of hydrofluoric acid, nitric acid, and acetic acid (HNA), the pillars are progressively thinned, then shortened. The replicated polyethylene pillar arrays exhibit a corresponding increase in friction as the stiffness is reduced with thinning and then a decrease in friction as the stiffness is again increased. The dilution of the HNA bath in water influences the tapering angle of the silicon pillars. The friction of the replicated pillars is maximized for the taper angle that maximizes the contact area at the tip which in turn is influenced by the stiffness of the tapered pillars. To provide insights on how changes in microscale geometry and contact behavior may affect friction of the pillar array, the pillars are imaged by scanning electron microscopy after friction testing, and the observed deformation behavior from shearing is related to the magnitude of the macroscale friction values. It is shown that the tapering angle critically changes the pillar compliance and the available contact area. Simple finite element modeling calculations are performed to support that the observed deformation is consistent with what is expected from a mechanical analysis. We conclude that friction can be maximized via proper pillar tapering with low stiffness that still maintains enough contact area to ensure high adhesion

Gecko-Inspired Combined Lamellar and Nanofibrillar Array for Adhesion on Nonplanar Surface

We report the fabrication from a hard polymer of lamellar structures that act as base support planes for high-aspect ratio nanofiber arrays. We experimentally show that nanofiber arrays on lamellae can adhere to both planar and nonplanar surfaces, exhibiting 5 times greater shear strength on a 100 μm peak-to-peak grating than the arrays without the lamellar support structure. The observed behavior on nonplanar surfaces is attributed to the high compliance of the lamellar flaps. The compliance of the combined lamellae and nanofiber arrays is measured to be about 160 times higher than nanofiber arrays on a flat nonlamellar backing layer

Demonstration of Hexagonal Phase Silicon Carbide Nanowire Arrays with Vertical Alignment

SiC nanowire based electronics hold promise for data collection in harsh environments wherein conventional semiconductor platforms would fail. However, the full adaptation of SiC nanowires as a material platform necessitates strict control of nanowire crystal structure and orientation for reliable performance. Toward such efforts, we report the growth of hexagonal phase SiC nanowire arrays grown with vertical alignment on commercially available single crystalline SiC substrates. The nanowire hexagonality, confirmed with Raman spectroscopy and atomic resolution microscopy, displays a polytypic distribution of predominantly 2H and 4H. Employing a theoretical growth model, the polytypic distribution of hexagonal phase nanowires is accurately predicted in the regime of high supersaturation. Additionally, the reduction of disorder-induced phonon density of states is achieved while maintaining nanowire morphology through a postgrowth anneal. The results of this work expand the repertoire of SiC nanowires by implementing a low-temperature method that promotes polytypes outside the well-studied cubic phase and introduces uniform, vertical alignment on industry standard SiC substrates

Two-Fluid Wetting Behavior of a Hydrophobic Silicon Nanowire Array

The two-fluid wetting behavior of surfaces textured by an array of silicon nanowires is investigated systematically. The Si nanowire array is produced by a combination of colloidal patterning and metal-catalyzed etching, with control over its roughness depending upon the wire length. The nanowires are made hydrophobic and oleophobic by treatment with hydrocarbon and fluorinated self-assembled monolayers, respectively. Static, advancing, and receding contact angles are measured with water, hexadecane, and perfluorotripentylamine in both single-fluid (droplet on solid in an air environment) and two-fluid (droplet on solid in a liquid environment) configurations. The single-fluid measurements show wetting behavior similar to that expected by the Wenzel and Cassie–Baxter models, where the wetting or non-wetting behaviors are amplified with increasing roughness. The two-fluid systems on the rough surface exhibit more complex configurations because either the droplet or the environment fluid can penetrate the asperities depending upon the wettability of each fluid. It is observed that, when the Young contact angles are significantly increased or reduced from single-liquid to two-liquid systems, the effect of roughness is relatively minimal. However, when the Young contact angles are similar, roughness has almost identical influence on apparent contact angles in single- and two-liquid systems. The Wenzel and Cassie–Baxter models are modified to describe various two-fluid wetting states. In cases where metastable behavior is observed for the droplet, advancing and receding measurements are performed to suggest the equilibrium state of the droplet

Atomic-Scale Electronic Characterization of Defects in Silicon Carbide Nanowires by Electron Energy-Loss Spectroscopy

The atomic-level resolution of scanning transmission electron microscopy (TEM) is used for structural characterization of nanomaterials, but the resolution afforded by TEM also enables electronic characterization of defects in these materials through electron energy-loss spectroscopy (EELS). Here, the power of EELS is harnessed to characterize the local band gap of inclusion defects in hexagonal silicon carbide nanowires with a high density of stacking faults. The band gaps we extract from the EELS data align within 0.1 eV of expected values for hexagonal silicon carbide and stacking faults within hexagonal silicon carbide. These experiments show that individual cubic phase inclusions in hexagonal silicon carbide significantly alter the local electronic structure, in particular, the band gap, in contrast to the polarizability tensor that retains the characteristic signature of the global hexagonal crystal structure

Selective Growth of Si Nanowire Arrays via Galvanic Displacement Processes in Water-in-Oil Microemulsions

Galvanic displacement processes are employed in water-in-oil microemulsions to deposit gold nanoclusters selectively on Si surfaces and sidewalls. The gold clusters then serve as catalysts to achieve selective growth of vertically and laterally aligned Si nanowire arrays by chemical vapor deposition via the vapor−liquid−solid growth mechanism. The size of the gold clusters is shown to have a good correlation with the microemulsion parameters, which in turn controls the size of the synthesized nanowires

Tuning the Friction Characteristics of Gecko-Inspired Polydimethylsiloxane Micropillar Arrays by Embedding Fe₃O₄ and SiO₂ Particles

In order to improve stiffness of polydimethylsiloxane (PDMS) pillars while maintaining high friction, the effects of embedding Fe₃O₄ and SiO₂ particles on the friction behavior of PDMS micropillars are studied. Both types of added particles increase the stiffness of the PDMS composite, but affect the friction behavior differently. The frictional force of the fibrillar array fabricated with Fe₃O₄/PDMS composite decreases initially, then increases as the particle content increases. For silica/PDMS composite pillars, the frictional force is independent of the particle density. Characterization by scanning electron microscopy shows that Fe₃O₄ particles are distributed uniformly in the PDMS matrix at low concentration, but heterogeneous distribution is observed at high particle loading, with particles being hindered from penetrating into the pillars. For silica/PDMS composite pillars, the particles distribute homogeneously inside the pillars, which is attributed to the formation of hydrogen bonding between silica particles and PDMS. The difference in particle distribution behavior is used to explain the observed difference in the friction response of these two composite systems