W. S. Koh

Highly sensitive graphene biosensors based on surface plasmon resonance

A surface plasmon resonance (SPR) based graphene biosensor is presented. It consists of a graphene sheet coated above a gold thin film, which has been proposed and experimentally fabricated recently [ChemPhysChem 11, 585 (2010)]. The biosensor uses attenuated total reflection (ATR) method to detect the refractive index change near the sensor surface, which is due to the adsorption of biomolecules. Our calculations show that the proposed graphene-on-gold SPR biosensor (with L graphene layers) is (1 + 0.025 L) x gamma (where gamma > 1) times more sensitive than the conventional gold thin film SPR biosensor. The improved sensitivity is due to increased adsorption of biomolecules on graphene (represented by the factor gamma) and the optical property of graphene.

Remarkable influence of the number of nanowires on plasmonic behaviors of the coupled metallic nanowire chain

We investigate the plasmonic properties in terms of plasmonic resonances, near field intensity, and charge distribution of interacting nanowires chain which consists of small and large numbers of coupled silver nanowires. We show that the dominant resonance wavelength increases monotonically as the number of nanowires increases. On the other hand, the near field intensity is not only dependent on the chain length but also on the plasmonic resonances in the same chain length. The charge distribution is also demonstrated to fully understand the dependence of plasmonic properties on the chain length.

The Potential of Graphene as an ITO Replacement in Organic Solar Cells: An Optical Perspective

Graphene possesses innate potential to replace indium tin oxide (ITO) as the transparent electrode in an organic solar cell device. From our transmittance study weighted with the air mass 1.5 global (AM1.5G) solar spectrum, it is found that a higher transparency may be obtained for up to four layers of graphene in comparison to ITO. Our findings suggest that replacing ITO with monolayer graphene in organic solar cells yields comparable performance. Due to the increased optical absorption, organic solar cells with four-layer graphene (with the same sheet resistance as ITO at 30 Ω/□) are capable of attaining at least 92% of the same organic photovoltaic device with an optimized ITO electrode for both normal and angular AM1.5G illumination.

Three-Dimensional Optoelectronic Model for Organic Bulk Heterojunction Solar Cells

This paper describes a 3-D optoelectronic device model for organic bulk heterojunction solar cells. Three-dimensional full-wave optical simulation enables us to incorporate different modern light trapping techniques, such as subwavelength nanostructures, in a typical organic bulk heterojunction solar cell, while 3-D electrical simulation allows us to handle localized enhancement or reduction of polaron/charge generation, recombination, and transport induced by modern light trapping techniques in the device. We calibrate our model with an experimental poly (3-hexylthiophene) (P3HT):phenyl-C60-butyric acid methyl ester (PCBM) organic bulk heterojunction solar cell by tuning only one free parameter as compared with other device models, which have multiple fitting parameters. A 3-D example of a silver nanoparticle array in a typical P3HT:PCBM organic bulk heterojunction cell is also demonstrated, and the current density-voltage relation is predicted with our model.

Carbon nanotube Schottky diode: an atomic perspective.

The electron transport properties of semiconducting carbon nanotube (SCNT) Schottky diodes are investigated with atomic models using density functional theory and the non-equilibrium Greens function method. We model the SCNT Schottky diode as a SCNT embedded in the metal electrode, which resembles the experimental set-up. Our study reveals that the rectification behaviour of the diode is mainly due to the asymmetric electron transmission function distribution in the conduction and valence bands and can be improved by changing metal-SCNT contact geometries. The threshold voltage of the diode depends on the electron Schottky barrier height which can be tuned by altering the diameter of the SCNT. Contrary to the traditional perception, the metal-SCNT contact region exhibits better conductivity than the other parts of the diode.

High-field half-cycle terahertz radiation from relativistic laser interaction with thin solid targets

It is found that half-cycle terahertz (THz) pulses with the peak field over 100 MV/cm can be produced in ultrashort intense laser interactions with thin solid targets. These THz pulses are shown to emit from both the front and rear sides of the solid target and are attributed to the coherent transition radiation by laser-produced ultrashort fast electron bunches. After the primary THz pulses, subsequent secondary half-cycle pulses are generated while some refluxing electrons cross the vacuum-target interfaces. Since such strong THz radiation is well synchronized with the driving lasers, it is particularly suitable for applications in various pump-probe experiments.

Two-dimensional model of space charge limited electron injection into a diode with Schottky contact

We present an analytical two-dimensional (2D) model of a high current space charge limited (SCL) electron injection into a solid through a Schottky contact for a n + ‐i‐n + diode with constant mobility and no velocity overshoot effect. The amount of electron injection is limited by the barrier at the contact, which includes the effect of electron tunnelling through the Schottky barrier. For a given barrier height, the SCL current condition can be reached if the tunnelling current becomes dominant over the drift‐diffusion current. An analytical scaling is present to show the enhancement of the SCL current due to finite contact size, which is verified by a 2D device simulator over a wide range of parameters. (Some figures in this article are in colour only in the electronic version)

Efficiencies of Aloof-Scattered Electron Beam Excitation of Metal and Graphene Plasmons

We assessed the efficiencies of surface plasmon excitation by an aloof-scattered electron beam on metals and graphene. Graphene is shown to exhibit high energy transfer efficiencies at very low electron kinetic energy requirements. We show that the exceptional performance of graphene is due to its unique plasmon dispersion, low electronic density, and thin-film structure. The potential applications of these aloof-scattered graphene plasmons are discussed in the aspects of coherent radiation.

Two-dimensional electromagnetic Child–Langmuir law of a short-pulse electron flow

Two-dimensional electromagnetic particle-in-cell simulations were performed to study the effect of the displacement current and the self-magnetic field on the space charge limited current density or the Child–Langmuir law of a short-pulse electron flow with a propagation distance of ζ and an emitting width of W from the classical regime to the relativistic regime. Numerical scaling of the two-dimensional electromagnetic Child–Langmuir law was constructed and it scales with (ζ/W) and (ζ/W)2 at the classical and relativistic regimes, respectively. Our findings reveal that the displacement current can considerably enhance the space charge limited current density as compared to the well-known two-dimensional electrostatic Child–Langmuir law even at the classical regime.

Short-pulse space-charge-limited electron flows in a drift space

In this paper, the space-charge-limited (SCL) electron flows in a drift space is studied by including the effect of finite electron pulse length, which is smaller than the gap transit time. Analytical formulas are derived to calculate the maximum SCL current density that can be transported across a drift space under the short-pulse injection condition. For a given voltage or injection energy, the maximum current density that can be transported is enhanced by a large factor (as compared to the long-pulse or steady-state case), and the enhancement is inversely proportional to the electron pulse length. In drift space, the effect of pulse expansion is important at very short-pulse length, and the short-pulse enhancement factor is smaller as compared to a diode. The enhancement factor will be suppressed when the injection energy is larger than the electron rest mass, and effect of pulse expansion is less critical at relativistic energy. The analytical formulas have been verified by performing a particle-in-cell simulation in the electrostatic mode.