Fabrication of a Horizontal and a Vertical Large Surface Area Nanogap Electrochemical Sensor

Nanogap sensors have a wide range of applications as they can provide accurate direct detection of biomolecules through impedimetric or amperometric signals. Signal response from nanogap sensors is dependent on both the electrode spacing and surface area. However, creating large surface area nanogap sensors presents several challenges during fabrication. We show two different approaches to achieve both horizontal and vertical coplanar nanogap geometries. In the first method we use electron-beam lithography (EBL) to pattern an 11 mm long serpentine nanogap (215 nm) between two electrodes. For the second method we use inductively-coupled plasma (ICP) reactive ion etching (RIE) to create a channel in a silicon substrate, optically pattern a buried 1.0 mm × 1.5 mm electrode before anodically bonding a second identical electrode, patterned on glass, directly above. The devices have a wide range of applicability in different sensing techniques with the large area nanogaps presenting advantages over other devices of the same family. As a case study we explore the detection of peptide nucleic acid (PNA)−DNA binding events using dielectric spectroscopy with the horizontal coplanar device

Porous PDMS force sensitive resistors

Here we present an elastomeric force sensitive resistor (FSR) made from a porous matrix of polydimethylsiloxane (PDMS) filled with carbon black. The fabrication process is based on the use of a low cost sacrificial sugar cube scaffold which leads to a highly porous and compressible material. By filling this porous matrix with carbon black we can achieve typical resistance changes from 20 kW to 100 W for an applied 95% compressive strain. This material is suitable for a wide variety of sensing applications which include tactile artificial skin for robotics and solvent detection

Field-enhanced direct tunneling in ultrathin atomic-layer-deposition-grown Au-Al2O3-Cr metal-insulator-metal structures

Metal-insulator-metal structures based on ultrathin high-k dielectric films are underpinning a rapidly increasing number of devices and applications. Here, we report detailed electrical characterizations of asymmetric metal-insulator-metal devices featuring atomic layer deposited 2-nm-thick Al2O3 films. We find a high consistency in the current density as a function of applied electric field between devices with very different surface areas and significant asymmetries in the IV characteristics. We show by TEM that the thickness of the dielectric film and the quality of the metal-insulator interfaces are highly uniform and of high quality, respectively. In addition, we develop a model which accounts for the field enhancement due to the small sharp features on the electrode surface and show that this can very accurately describe the observed asymmetry in the current-voltage characteristic, which cannot be explained by the difference in work function alone

Spatially Resolved On-Chip Picosecond Pulse Detection Using Graphene

We present an on-chip time domain terahertz (TD-THz) system in which picosecond pulses are generated in low-temperature-grown gallium arsenide (LT-GaAs) and detected in graphene. The detected pulses were found to vary in amplitude, full width at half maximum (FWHM), and DC offset when sampled optically at different locations along a 50-μm-long graphene photoconductive (PC) detector. The results demonstrate the importance of detection location and switch design in graphene-based on-chip PC detectors

Brillouin light scattering study of magnetic-element normal modes in a square artificial spin ice geometry

We report the results, from experimental and micromagnetic studies, of the magnetic normal modes in artificial square spin ice systems consisting of ferromagnetic-monodomain islands. Spin wave properties are measured by Brillouin light scattering. The mode spectra contain several branches whose frequencies are sensitive to the magnitude and in-plane orientation of an applied magnetic field. We also identify soft modes that exhibit different behaviour depending on the direction of the applied magnetic field. The obtained results are well described with micromagnetic simulations of independent magnetic elements arranged along two sublattices

Thickness dependence of spin wave excitations in an artificial square spin ice-like geometry

We present a comparative study of the spin wave properties in two magnetic films patterned into an artificial square spin ice-like geometry. The array elements are rectangular islands with the same lateral dimensions but with different thicknesses: 10 nm and 30 nm. Using Brillouin light scattering, the frequencies of spin wave excitations were measured as a function of the magnetic field going from positive to negative saturation. We find substantial changes with thickness to spin wave mode frequencies and the number of detected modes. Frequencies of spin waves localized at element edges are observed to evolve non-monotonically with magnetic fields and soften at critical fields. These critical fields enable us to extract information of the magnetization reversal of individual islands within the array. Finally, we discuss the effects of separation between islands and examine the possibilities for dynamic coupling through the overlap of collective edge modes

Terahertz emission mechanism and laser excitation position dependence of nano-grating electrode photomixers

The emission mechanism of continuous wave (CW) terahertz (THz) photomixers that make use of nanostructured gratings (NSGs) is studied. Two different photomixer designs, based on a single-sided NSG and a double-sided NSG, embedded in the same antenna design and fabricated on an Fe doped InGaAsP substrate, are characterized with ∼1550 nm excitation. They are shown to exhibit similar performance in terms of spectral bandwidth and emitted power. The emission is mapped in terms of the laser excitation position, from which the emission mechanism is assigned to an enhanced optical electric field at the tips of the NSGs

Spin relaxation through Kondo scattering in Cu/Py lateral spin valves

The temperature dependence of the spin diffusion length typically reflects the scattering mechanism responsible for spin relaxation. Within nonmagnetic metals it is reasonable to expect the Elliot-Yafet mechanism to play a role and thus the temperature dependence of the spin diffusion length might be inversely proportional to resistivity. In lateral spin valves, measurements have found that at low temperatures the spin diffusion length unexpectedly decreases. By measuring the transport properties of lateral Py/Cu/Py spin valves, fabricated from Cu with magnetic impurities of <1 ppm and ∼4 ppm, we extract a spin diffusion length which shows this suppression below 30 K only in the presence of the Kondo effect. We have calculated the spin-relaxation rate and isolated the contribution from magnetic impurities. We find the spin-flip probability of a magnetic impurity to be 34%. Our analysis demonstrates the dominant role of Kondo scattering in spin relaxation, even in low concentrations of order 1 ppm, and hence illustrates its importance to the reduction in spin diffusion length observed by ourselves and others

Spin-valve Josephson junctions with perpendicular magnetic anisotropy for cryogenic memory

We demonstrate a Josephson junction with a weak link containing two ferromagnets with perpendicular magnetic anisotropy and independent switching fields in which the critical current can be set by the mutual orientation of the two layers. Such pseudospin-valve Josephson junctions are a candidate cryogenic memory in an all superconducting computational scheme. Here, we use Pt/Co/Pt/CoB/Pt as the weak link of the junction with dCo=0.6 nm, dCoB=0.3 nm, and dPt=5 nm and obtain a 60% change in the critical current for the two magnetization configurations of the pseudospin-valve. Ferromagnets with perpendicular magnetic anisotropy have advantages over magnetization in-plane systems, which have been exclusively considered at this point, as, in principle, the magnetization and magnetic switching of layers in the junction should not affect the in-plane magnetic flux