Rapid formation of single crystalline Ge nanowires by anodic metal assisted etching

Germanium nanowires are produced by a novel approach, combining two well known electrochemical and metal assisted chemical etching. The metal assisted etching procedure is enhanced by incorporation of HF in the catalytic solution and application of a constant bias to the substrate. Fast etching, caused by metal nanoparticles, facilitate pore nucleation for further pore growth. The improved current transport de- veloped in the vicinity of the metal nanoparticles maintains a concentrated current density at the pore tip which results in an elongation of the pores in one direction and formation of long nanowires. With this new approach it is possible to fabricate nanowires with diameter below 100 nm and tens of micrometers long

Memristive devices based on single ZnO nanowires-from material synthesis to neuromorphic functionalities

Memristive and resistive switching devices are considered promising building blocks for the realization of artificial neural networks and neuromorphic systems. Besides conventional top-down memristive devices based on thin films, resistive switching devices based on nanowires (NWs) have attracted great attention, not only for the possibility of going beyond current scaling limitations of the top-down approach, but also as model systems for the localization and investigation of the physical mechanism of switching. This work reports on the fabrication of memristive devices based on ZnO NWs, from NW synthesis to single NW-based memristive cell fabrication and characterization. The bottom-up synthesis of ZnO NWs was performed by low-pressure chemical vapor deposition according to a self-seeding vapor-solid (VS) mechanism on a Pt substrate over large scale (∼cm2), without the requirement of previous seed deposition. The grown ZnO NWs are single crystalline with wurtzite crystal structure and are vertically aligned respect to the growth substrate. Single NWs were then contacted by means of asymmetric contacts, with an electrochemically active and an electrochemically inert electrode, to form NW-based electrochemical metallization memory cells that show reproducible resistive switching behaviour and neuromorphic functionalities including short-term synaptic plasticity and paired pulse facilitation. Besides representing building blocks for NW-based memristive and neuromorphic systems, these single crystalline devices can be exploited as model systems to study physicochemical processing underlaying memristive functionalities thanks to the high localization of switching events on the ZnO crystalline surface

Recommended implementation of electrical resistance tomography for conductivity mapping of metallic nanowire networks using voltage excitation

The knowledge of the spatial distribution of the electrical conductivity of metallic nanowire networks (NWN) is important for tailoring the performance in applications. This work focuses on Electrical Resistance Tomography (ERT), a technique that maps the electrical conductivity of a sample from several resistance measurements performed on its border. We show that ERT can be successfully employed for NWN characterisation if a dedicated measurement protocol is employed. When applied to other materials, ERT measurements are typically performed with a constant current excitation; we show that, because of the peculiar microscopic structure and behaviour of metallic NWN, a constant voltage excitation protocols is preferable. This protocol maximises the signal to noise ratio in the resistance measurements—and thus the accuracy of ERT maps—while preventing the onset of sample alterations

Structure-Dependent Influence of Moisture on Resistive Switching Behavior of ZnO Thin Films

Resistive switching mechanisms underlying memristive devices are widely investigated, and the importance as well as influence of ambient conditions on the electrical performances of memristive cells are already recognized. However, detailed understanding of the ambient effect on the switching mechanism still remains a challenge. This work presents an experimental investigation on the effect of moisture on resistive switching performances of ZnO-based electrochemical metallization memory cells. ZnO thin films are grown by chemical vapor deposition (CVD) and radio frequency sputtering. Water molecules are observed to influence electrical resistance of ZnO by affecting the electronic conduction mechanism and by providing additional species for ionic conduction. By influencing dissolution and migration of ionic species underlying resistive switching events, moisture is reported to tune resistive switching parameters. In particular, the presence of H2O is responsible for a decrease of the forming and SET voltages and an increase of the ON/OFF resistance ratio in both CVD and sputtered films. The effect of moisture on resistive switching performance is found to be more pronounced in case of sputtered films where the reduced grain size is responsible for an increased adsorption of water molecules and an increased amount of possible pathways for ion migration

Preparation and Properties of PTFE-PMMA Core-Shell Nanoparticles and Nanocomposites

he preparation of polytetrafluoroethylene-poly(methyl methacrylate) (PTFE-PMMA) core-shell particles was described, featuring controlled size and narrow size distribution over a wide compositional range, through a seeded emulsion polymerization starting from a PTFE seed of 26 nanometers. Over the entire MMA/PTFE range, the particle size increases as the MMA/PTFE ratio increases. A very precise control over the particle size can be exerted by properly adjusting the ratio between the monomer and the PTFE seed. Particles in the 80240 nm range can be prepared with uniformity indexes suited to build 2D and 3D colloidal crystals. These core-shell particles were employed to prepare nanocomposites with different compositions, through an annealing procedure at a temperature higher than the glass transition temperature of the shell forming polymer. A perfect dispersion of the PTFE particles within the PMMA matrix was obtained and optically transparent nanocomposites were prepared containing a very high PTFE amount

Electrochemical nanolithography on silicon: An easy and scalable method to control pore formation at the nanoscale

Lithography on a sub-100 nm scale is beyond the diffraction limits of standard optical lithography but is nonetheless a key step in many modern technological applications. At this length scale, there are several possible approaches that require either the preliminary surface deposition of materials or the use of expensive and time-consuming techniques. In our approach, we demonstrate a simple process, easily scalable to large surfaces, where the surface patterning that controls pore formation on highly doped silicon wafers is obtained by an electrochemical process. This method joins the advantages of the low cost of an electrochemical approach with its immediate scalability to large wafers