Hierarchical orientation of crystallinity by block-copolymer patterning and alignment in an electric field

Electron and hole conducting 10-nm-wide polymer morphologies hold great promise for organic electro-optical devices such as solar cells and light emitting diodes. The self-assembly of block-copolymers (BCPs) is often viewed as an efficient way to generate such materials. Here, a functional block copolymer that contains perylene bismide (PBI) side chains which can crystallize via π-π stacking to form an electron conducting microphase is patterned harnessing hierarchical electrohydrodynamic lithography (HEHL). HEHL film destabilization creates a hierarchical structure with three distinct length scales: (1) micrometer-sized polymer pillars, containing (2) a 10-nm BCP microphase morphology that is aligned perpendicular to the substrate surface and (3) on a molecular length scale (0.35-3 nm) PBI π-π-stacks traverse the HEHL-generated plugs in a continuous fashion. The good control over BCP and PBI alignment inside the generated vertical microstructures gives rise to liquid-crystal-like optical dichroism of the HEHL patterned films, and improves the electron conductivity across the film by 3 orders of magnitude. © 2013 American Chemical Society.

Reversible nanoscale switching of polytwin orientation in a ferroelectric thin film induced by a local electric field

The response to a local, tip-induced electric field of ferroelastic domains in thin polycrystalline lead zirconate titanate films with predominantly (110) orientation has been studied using Enhanced Piezoresponse Force Microscopy. Two types of reversible polytwin switching between well-defined orientations have been observed. When a-c domains are switched to other forms of a-c domains, the ferroelastic domain walls rotate in-plane by 109.5°, and when a-c domains are switched to c-c domains (or vice-versa), the walls rotate by 54.75°. © 2013 AIP Publishing LLC.

Analysis of SEB and SEGR in super-junction MOSFETs

The electric field distribution in the super junction power MOSFET is analyzed using analytical modeling and numerical simulations in this paper. The single-event burn-out (SEB) and single-event gate rupture (SEGR) phenomena in this device are studied in detail. It is demonstrated that the super junction device is much less sensitive to SEB and SEGR compared to the standard power MOSFET. The physical mechanism is explained.

Analysis of fields in an air-cored superconducting synchronous motor with an HTS racetrack field winding

High temperature superconducting (HTS) synchronous motors can offer significant weight and size reductions, as well as improved efficiency, over conventional copper-wound machines due to the higher current density of high temperature superconducting (HTS) materials. In order to optimise the design parameters and performance of such a machine, this paper proposes a basic physical model of an air-cored HTS synchronous motor with a copper armature winding and HTS field winding. An analytical method for the field analysis in the synchronous motor is then presented, followed by a numerical finite element analysis (FEA) model to verify the analytical solution. The model is utilised to study the influence of the geometry of the HTS coils on the magnetic field at the armature winding, and geometrical parameter optimisation is carried out using this theoretical model to obtain a more sinusoidal magnetic field at the armature, which has a major influence on the performance of the motor.

Integrating vertically aligned carbon nanotubes on micromechanical structures

The integration of high yield, uniform and preferential growth of vertically aligned carbon nanotubes (VACNT) on low stress micromechanical structures was analyzed. A combination of electron-beam crosslinked surface micromachining and direct current plasma enhanced chemical vapor deposition of electric field aligned carbon nanotubes was used for the analysis. The selective placement of high yield and uniform VACNTs on a partially suspended Ni/SiO2/Ti microstructure was also demonstrated.

Plasma enhanced chemical vapour deposited carbon nanotubes for field emission applications

Plasma Enhanced Chemical Vapour Deposition is an extremely versatile technique for directly growing multiwalled carbon nanotubes onto various substrates. We will demonstrate the deposition of vertically aligned nanotube arrays, sparsely or densely populated nanotube forests, and precisely patterned arrays of nanotubes. The high-aspect ratio nanotubes (∼50 nm in diameter and 5 microns long) produced are metallic in nature and direct contact electrical measurements reveal that each nanotube has a current carrying capacity of 107-108 A/cm2, making them excellent candidates as field emission sources. We examined the field emission characteristics of dense nanotube forests as well as sparse nanotube forests and found that the sparse forests had significantly lower turn-on fields and higher emission currents. This is due to a reduction in the field enhancement of the nanotubes due to electric field shielding from adjacent nanotubes in the dense nanotube arrays. We thus fabricated a uniform array of single nanotubes to attempt to overcome these issues and will present the field emission characteristics of this.

Efficient carbon nanotube field emitter using electrospun carbon nanofibers as a flexible electrode

Thermal-stable, conductive, and flexible carbon fabric (CF), which is composed of thin carbon fibers prepared by electrospinning, was used for the substrate of carbon nanotube (CNT) field emitter arrays. The field emitter arrays were prepared by chemical vapor deposition (CVD). The current density-electric field characteristics revealed that the CNT field emitter arrays on CF produced a higher current density at a lower turn-on voltage compared to ones on a Si substrate. This emitter integrated with a gate electrode based on hierarchy-structured carbon materials, CNTs on CF, can be used for light sources, displays, and other electronic devices. © 2009 Materials Research Society.

Tuning of the electronic characteristics of ZnO nanowire field effect transistors by proton irradiation.

We demonstrated a controllable tuning of the electronic characteristics of ZnO nanowire field effect transistors (FETs) using a high-energy proton beam. After a short proton irradiation time, the threshold voltage shifted to the negative gate bias direction with an increase in the electrical conductance, whereas the threshold voltage shifted to the positive gate bias direction with a decrease in the electrical conductance after a long proton irradiation time. The electrical characteristics of two different types of ZnO nanowires FET device structures in which the ZnO nanowires are placed on the substrate or suspended above the substrate and photoluminescence (PL) studies of the ZnO nanowires provide substantial evidence that the experimental observations result from the irradiation-induced charges in the bulk SiO(2) and at the SiO(2)/ZnO nanowire interface, which can be explained by a surface-band-bending model in terms of gate electric field modulation. Our study on the proton-irradiation-mediated functionalization can be potentially interesting not only for understanding the proton irradiation effects on nanoscale devices, but also for creating the property-tailored nanoscale devices.

Size-dependent trajectories of DNA macromolecules due to insulative dielectrophoresis in submicrometer-deep fluidic channels.

In this paper, we demonstrate for the first time that insulative dielectrophoresis can induce size-dependent trajectories of DNA macromolecules. We experimentally use lambda (48.5 kbp) and T4GT7 (165.6 kbp) DNA molecules flowing continuously around a sharp corner inside fluidic channels with a depth of 0.4 mum. Numerical simulation of the electrokinetic force distribution inside the channels is in qualitative agreement with our experimentally observed trajectories. We discuss a possible physical mechanism for the DNA polarization and dielectrophoresis inside confining channels, based on the observed dielectrophoresis responses due to different DNA sizes and various electric fields applied between the inlet and the outlet. The proposed physical mechanism indicates that further extensive investigations, both theoretically and experimentally, would be very useful to better elucidate the forces involved at DNA dielectrophoresis. When applied for size-based sorting of DNA molecules, our sorting method offers two major advantages compared to earlier attempts with insulative dielectrophoresis: Its continuous operation allows for high-throughput analysis, and it only requires electric field strengths as low as approximately 10 Vcm.

Plasma enhancement of current from carbon cathodes for field emission displays

This paper examines the possibility of using a background gas medium to enhance the current available from low threshold carbon cathodes. The field emission current is used to initiate a plasma in the gas medium, and thereby achieve a current multiplication effect. Results on the variation of anode current as a function of electric field and gas pressure are presented. These are compared with model calculations to verify the principles of operation. The influence of ion bombardment on the long term performance thin film carbon cathodes is examined for He and Ar multiplication plasmas. A measure of the influence of current multiplication on display quality is presented by examining light output from two standard low voltage phosphors. Also studied are the influence of doping the carbon with N to lower the threshold voltage for emission as well as the consequent impact on anode current from the plasma.

Hot electron field emission via individually transistor-ballasted carbon nanotube arrays.

We present electronically controlled field emission characteristics of arrays of individually ballasted carbon nanotubes synthesized by plasma-enhanced chemical vapor deposition on silicon-on-insulator substrates. By adjusting the source-drain potential we have demonstrated the ability to controllable limit the emission current density by more than 1 order of magnitude. Dynamic control over both the turn-on electric field and field enhancement factor have been noted. A hot electron model is presented. The ballasted nanotubes are populated with hot electrons due to the highly crystalline Si channel and the high local electric field at the nanotube base. This positively shifts the Fermi level and results in a broad energy distribution about this mean, compared to the narrow spread, lower energy thermalized electron population in standard metallic emitters. The proposed vertically aligned carbon nanotube field-emitting electron source offers a viable platform for X-ray emitters and displays applications that require accurate and highly stable control over the emission characteristics.

Design and optimization of planar mesa termination for diamond Schottky barrier diodes

In this paper, we present planar mesa termination structure with high k dielectric Al2O3 for high-voltage diamond Schottky barrier diode. Analysis, design, and optimization are carried out by simulations using finite element technology computer-aided design (TCAD) Sentaurus Device software. The performances of planar mesa termination structure are compared to those of conventional field plate termination structure. It is found that optimum geometry of planar mesa terminated diode requires shorter metal plate extension (1/3 of the field plate terminated diode). Consequently, planar mesa terminated diode can be designed with bigger Schottky contact to increase its current carrying capability. Breakdown performance of field plate termination structure is limited at 1480 V due to peak electric field at the corner of Schottky contact (no oxide breakdown occurs). In contrast, peak electric field in planar mesa termination structure only occurs in the field oxide such that its breakdown performance is highly dependent on the oxide material. Due to Al2O3 breakdown, planar mesa termination structure suffers premature breakdown at 1440 V. Considering no oxide breakdown occurs, planar mesa termination structure can realize higher breakdown voltage of 1751 V. Therefore, to fully realize the potential of planar mesa terminated diode, it is important to choose suitable high k dielectric material with sufficient breakdown electric field for the field oxide. © 2013 Elsevier B.V.

Novel field emission from graphene sheets supported by CNTs arrays

A good quality graphene is transferred onto honeycomb-like CNTs arrays with inner supporting CNTs. The efficient field emission is demonstrated due to a high aspect ratio protrusions and graphene crack edges. A high efficient current density about 1.2 mA/cm2 at threshold electric field of 7.8 V/μm with a turn-on electric field of 1.8 V/μm at the current density of 10 μA/cm2 is observed due to high localized electric field. Stable field emission is tested in a vacuum chamber. The results are of significance to the development of Graphene based field emitters. © 2013 IEEE.

Tunable Rashba effect in two-dimensional LaOBiS2 films: Ultrathin candidates for spin field effect transistors

Rashba spin splitting is a two-dimensional (2D) relativistic effect closely related to spintronics. However, so far there is no pristine 2D material to exhibit enough Rashba splitting for the fabrication of ultrathin spintronic devices, such as spin field effect transistors (SFET). On the basis of first-principles calculations, we predict that the stable 2D LaOBiS2 with only 1 nm of thickness can produce remarkable Rashba spin splitting with a magnitude of 100 meV. Because the medium La2O2 layer produces a strong polar field and acts as a blocking barrier, two counter-helical Rashba spin polarizations are localized at different BiS 2 layers. The Rashba parameter can be effectively tuned by the intrinsic strain, while the bandgap and the helical direction of spin states sensitively depends on the external electric field. We propose an advanced Datta-Das SFET model that consists of dual gates and 2D LaOBiS2 channels by selecting different Rashba states to achieve the on-off switch via electric fields. © 2013 American Chemical Society.

Characterization of a liquid crystal microlens array using multiwalled carbon nanotube electrodes.

Reconfigurable liquid crystal microlenses employing arrays of multiwalled carbon nanotubes (MWNTs) have been designed and fabricated. The cells consist of arrays of 2 microm high MWNTs grown by plasma-enhanced chemical vapor deposition on silicon with a top electrode of indium tin oxide coated glass positioned 20 microm above the silicon and the gap filled with the nematic liquid crystal BLO48. Simulations have found that, while its nematic liquid crystal aligns with MWNTs within a distance of 10nm, this distance is greatly enhanced by the application of an external electric field. Polarized light experiments show that light is focused with focal lengths ranging from approximately 7 microm to 12 microm.