Screening in nanowires and nanocontacts: field emission, adhesion force, and contact resistance

Shape- and size-dependent electronic capacitance in nanostructured materials

The shape and size are the two important geometrical factors that affect the electronic screening in nano-materials. Here, we develop an analytical theory for electronic capacitance based on Thomas–Fermi screening in conjunction with ‘multiple scattering method’ for arbitrary-shaped nanostructures including electronic spillover correction. We relate the electronic capacitance of the material to the curvature correction expressed in terms of ratio of electronic screening length to principal radii of curvature. Electronic capacitance of various nanostructures is obtained showing geometrical shape- and size-dependent electronic screening in nanostructures that manifest important consequences in charge storage enhancement or reduction.

BioFET-SIM web interface: implementation and two applications

We present a web interface which allows us to conveniently set up calculations based on the BioFET-SIM model. With the interface, the signal of a BioFET sensor can be calculated depending on its parameters, as well as the signal dependence on pH. As an illustration, two case studies are presented. In the first case, a generic peptide with opposite charges on both ends is inverted in orientation on a semiconducting nanowire surface leading to a corresponding change in sign of the computed sensitivity of the device. In the second case, the binding of an antibody/antigen complex on the nanowire surface is studied in terms of orientation and analyte/nanowire surface distance. We demonstrate how the BioFET-SIM web interface can aid in the understanding of experimental data and postulate alternative ways of antibody/antigen orientation on the nanowire surface.

Positive and negative Coulomb drag in vertically integrated one-dimensional quantum wires

Electron interactions in and between wires become increasingly complex and important as circuits are scaled to nanometre sizes, or use reduced-dimensional conductors such as carbon nanotubes, nanowires and gated high-mobility two-dimensional electron systems. This is because the screening of the long-range Coulomb potential of individual carriers is weakened in these systems, which can lead to phenomena such as Coulomb drag, where a current in one wire induces a voltage in a second wire through Coulomb interactions alone. Previous experiments have demonstrated Coulomb electron drag in wires separated by a soft electrostatic barrier of width ≳80 nm (ref. 12), which was interpreted as resulting entirely from momentum transfer. Here, we measure both positive and negative drag between adjacent vertical quantum wires that are separated by ∼15 nm and have independent contacts, which allows their electron densities to be tuned independently. We map out the drag signal versus the number of electron sub-bands occupied in each wire, and interpret the results both in terms of momentum-transfer and charge-fluctuation induced transport models. For wires of significantly different sub-band occupancies, the positive drag effect can be as large as 25%.

Electrodeposition growth of nanowire arrays with height gradient profiles for microwave device applications

A simple and nonexpensive adapted dip-coating technique is presented and used to fabricate arrays of magnetic nanowires with a linear varying height profile. This approach allows controlling the wire height from tenths of nanometers up to several micrometers. Furthermore, the main parameters of this height gradient can be controlled, such as the maximum wire height and the lateral span of the wire array, which can be predicted with excellent accuracy using a proposed analytical model. Moreover, we show that by sequential electrodeposition with dip-coating, arrays of these height varying wires can be grown. This technique represents a novel method to fill porous templates with controlled spatial growth, leading to the fabrication of novel structures and providing control over structural features on the nanoscale level. In particular, the use of these asymmetrically loaded magnetic nanowired substrates to obtain improved microwave nonreciprocal behavior is shown for a microwave phase shifter.

Large field enhancement at electrochemically grown quasi-1D Ni nanostructures with low-threshold cold-field electron emission

Ni nanorod arrays have been vertically grown on a Ta-coated Si substrate via an electrodeposition process through the nanopores of a porous alumina membrane. Field emission studies of the samples are performed which show a considerable low-threshold field around 5 V µm(-1). The field emission mechanism followed Fowler-Nordheim tunneling due to large field enhancement at the emitter tips. Low-dimensional structures of the nanorod tips provided the large geometrical field enhancement and thus produce a high enough local or barrier field for low-threshold cold-field electron emission. The cost-effective synthesis of vertically aligned Ni nanorods on an Si substrate and low-threshold field emission properties can provide a potential alternative to conventional carbon-based field emitters for low power panel applications.