Fabrication of flexible silicon nanowires by self-assembled metal assisted chemical etching for surface enhanced Raman spectroscopy

A homogenous array of flexible gold coated silicon nanowires was fabricated by the combination of nano spheres lithography and metal assisted chemical etching to obtain highly effective Surface Enhanced Raman Spectroscopy (SERS) substrates. 3D nanostructures with different aspect ratios and well-defined geometries were produced by adjusting the fabrication parameters in order to select the best configuration for SERS analysis. The optimum flexible nanowires with an aspect ratio of 1 : 10 can self-close driven by the microcapillary force under exposure to liquid and trap the molecules at their metallic coated ``fingertips'', thus generating hot spots with ultrahigh field enhancement. The performance of these SERS substrates was evaluated using melamine as the analyte probe with various concentrations from the millimolar to the picomolar range. Flexible gold coated SiNWs demonstrated high uniformity of the Raman signal over large area with a variability of only 10% and high sensitivity with a limit of detection of 3.20 x 10(-7) mg L-1 (picomolar) which promotes its application in several fields such food safety, diagnostic and pharmaceutical. Such an approach represents a low-cost alternative to the traditional nanofabrication processes to obtain well ordered silicon nanostructures, offering multiple degrees of freedom in the design of different geometries such as inter-wire distance, density of the wires on the surface as well as their length, thus showing a great potential for the fabrication of SERS substrates

Thermally activated tunneling in porous silicon nanowires with embedded Si quantum dots

Electronic transport properties of porous Si nanowires either with embedded Si quantum dots or with a percolative crystalline path are studied as a function of the temperature for the first time. We show that unlike bulk porous Si, the predesigned structure of the wires results in a single distinct conduction mechanism such as tunneling in the former case and variable range hopping in the latter case. We demonstrate that the geometry of the systems with a large internal surface area and high density of the Si quantum dots have a significant conduction enhancement compared to bulk porous silicon. These results can also improve the understanding of the basis of the different electronic transport mechanisms reported in bulk porous silicon

Metal-insulator transition in single crystalline ZnO nanowires

In this work, we report on the metal–insulator transition and electronic transport properties of single crystalline ZnO nanowires synthetized by means of Chemical Vapor Deposition. After evaluating the effect of adsorbed species on transport properties, the thermally activated conduction mechanism was investigated by temperature-dependent measurements in the range 81.7–250 K revealing that the electronic transport mechanism in these nanostructures is in good agreement with the presence of two thermally activated conduction channels. More importantly, it was observed that the electrical properties of ZnO NWs can be tuned from semiconducting to metallic-like as a function of temperature with a metal-to-insulator transition (MIT) observed at a critical temperature above room temperature (Tc ∼ 365 K). Charge density and mobility were investigated by means of field effect measurements in NW field-effect transistor configuration. Results evidenced that the peculiar electronic transport properties of ZnO NWs are related to the high intrinsic n-type doping of these nanostructures that is responsible, at room temperature, of a charge carrier density that lays just below the critical concentration for the MIT. This work shows that native defects, Coulomb interactions and surface states influenced by adsorbed species can significantly influence charge transport in NWs

Metal-insulator transition in single crystalline ZnO nanowires

In this work, we report on the metal-insulator transition and electronic transport properties of single crystalline ZnO nanowires synthetized by means of Chemical Vapor Deposition. After evaluating the effect of adsorbed species on transport properties, the thermally activated conduction mechanism was investigated by temperature-dependent measurements in the range 81.7-250 K revealing that the electronic transport mechanism in these nanostructures is in good agreement with the presence of two thermally activated conduction channels. More importantly, it was observed that the electrical properties of ZnO NWs can be tuned from semiconducting to metallic-like as a function of temperature with a metal-to-insulator transition (MIT) observed at a critical temperature above room temperature (Tc ∼ 365 K). Charge density and mobility were investigated by means of field effect measurements in NW field-effect transistor configuration. Results evidenced that the peculiar electronic transport properties of ZnO NWs are related to the high intrinsic n-type doping of these nanostructures that is responsible, at room temperature, of a charge carrier density that lays just below the critical concentration for the MIT. This work shows that native defects, Coulomb interactions and surface states influenced by adsorbed species can significantly influence charge transport in NWs

Electrical Contacts on Silicon Nanowires Produced by Metal-Assisted Etching: a Comparative Approach

Silicon nanowires fabricated by metal-assisted chemical etching can present low porosity and a rough surface depending on the doping level of the original silicon wafer. In this case, wiring of silicon nanowires may represent a challenging task. We investigated two different approaches to realize the electrical contacts in order to enable electrical measurement on a rough silicon nanowire device: we compared FIB-assisted platinum deposition for the fabrication of electrical contact with EBL technique

Niobium NanoSQUIDs Based on Sandwich Nanojunctions: Performance as a Function of the Temperature

In this paper, an experimental investigation of the main characteristics of a niobium nano Superconducting QUantum Interference Device (nanoSQUID) as a function of the temperature (9-0.3 K) is presented. The nanosensor consists of a niobium superconducting loop (0.4 × 1.0μm2) interrupted by two sandwich nanojunctions (Nb/Al-AlOx/Nb) having an area of about (300 × 300) nm2. These nanodevices have been fabricated by means of a focused ion beam sculpting method, which is used as a lithographic technique to define the various elements of the SQUID. We have performed measurements of current-voltage, critical current-magnetic flux characteristics, and switching current distributions from the zero voltage state for different temperatures. The high critical current modulation depths and the low intrinsic dissipation exhibited by these devices ensure a suitable sensitivity for nanoscale applications in the whole temperature range investigated

Impact of pore anisotropy on the thermal conductivity of porous Si nanowires (vol 8, 12796, 2018)

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Impact of pore anisotropy on the thermal conductivity of porous Si nanowires

Porous materials display enhanced scattering mechanisms that greatly infuence their transport properties. Metal-assisted chemical etching (MACE) enables fabrication of porous silicon nanowires starting from a doped Si wafer by using a metal template that catalyzes the etching process. Here, we report on the low thermal conductivity (κ) of individual porous Si nanowires (NWs) prepared from MACE, with values as low as 0.87W·m−1·K−1 for 90nm diameter wires with 35-40% porosity. Despite the strong suppression of long mean free path phonons in porous materials, we fnd a linear correlation of κ with the NW diameter. We ascribe this dependence to the anisotropic porous structure that arises during chemical etching and modifes the phonon percolation pathway in the center and outer regions of the nanowire. The inner microstructure of the NWs is visualized by means of electron tomography. In addition, we have used molecular dynamics simulations to provide guidance for how a porosity gradient infuences phonon transport along the axis of the NW. Our fndings are important towards the rational design of porous materials with tailored thermal and electronic properties for improved thermoelectric devices