Shishir Kumar

Nanoscale Mapping of Electrical Resistivity and Connectivity in Graphene Strips and Networks

In this article we map out the thickness dependence of the resistivity of individual graphene strips, from single layer graphene through to the formation of graphitic structures. We report exceptionally low resistivity values for single strips and demonstrate that the resistivity distribution for single strips is anomalously narrow when compared to bi- and trilayer graphene, consistent with the unique electronic properties of single graphene layers. In agreement with theoretical predictions, we show that the transition to bulklike resistivities occurs at seven to eight layers of graphene. Moreover, we demonstrate that the contact resistance between graphene flakes in a graphene network scales with the flake thickness and the implications for transparent conductor applications are discussed.

Site‐Specific Transfer‐Printing of Individual Graphene Microscale Patterns to Arbitrary Surfaces

massless Dirac fermions, [ 3 ] extremely high mobility, [ 4 ] special quantum Hall effect, [ 3 ] and gate voltage tunable optical transitions. [ 5 ] Those remarkable electrical and optical properties make it an attractive candidate for potential applications in integrated bipolar fi eld-effect transistors (FETs), [ 6 ] transparent electrodes for solar cells, [ 7,8 ] as well as other microscale functional devices. [ 9 ]

Simultaneous electrochemical determination of dopamine and paracetamol based on thin pyrolytic carbon films

This paper describes the determination of dopamine and paracetamol using cyclic voltammetry (CV) and differential pulse voltammetry (DPV). The selective, stable and reproducible simultaneous measurement of the two compounds is achieved using thin pyrolytic carbon (PyC) films as working electrodes. These are created via a reliable, non-catalytic chemical vapour deposition (CVD) process, and the electron transfer characteristics of the films are optimised using a simple oxygen plasma treatment. This new class of carbon electrode can detect dopamine in the range 18 to 270 μM, with a 2.3 μM limit of detection (LoD), while simultaneously sensing paracetamol in the range 15 to 225 μM (LoD 1.4 μM). A 225 mV separation between the two competing signals is realised. The accuracy of the sensor is demonstrated using human serum and commercially available pharmaceutical products. This is the first report of the application of PyC to this problem, and the performance is shown to be competitive with the leading carbon electrodes available today, particularly edge-plane pyrolytic graphite (EPPG). This work will serve as an important benchmark in the development of inexpensive, disposable, high-performance nano-structured electrodes for sensors, fuel cells and energy conversion.

Atom-Resolved Evidence of Anisotropic Growth in ZnS Nanotetrapods

ZnS nanotetrapods were investigated by atom-resolved microscopy characterization and quantitative simulation. The octahedron core enclosed with Zn- and S-terminated surfaces was verified. Four hexaprism-shaped arms were selectively grown from Zn-terminated surfaces of the core by alternately stacking zinc blende and wurtzite structures. The stacking order change at the core/arm interface is significant to activate the arm growth. The anisotropic growth mechanism was proposed and further proved by the synthesis of ZnS nanoparticles and nanobelts.

Large Magnetoresistance in Few Layer Graphene Stacks with Current Perpendicular to Plane Geometry

A large magnetoresistance (MR) effect of few-layers graphene between two non-magnetic metal electrodes with current perpendicular to graphene plane is studied. A non-saturation and anisotropic MR with the value over 60% at 14 T is observed in a two-layer graphene stack at room temperature. The resistance of the device is only tens of ohms, having the advantage of low power consumption for magnetic device applications.

Gas phase controlled deposition of high quality large-area graphene films

A gas phase controlled graphene synthesis resembling a CVD process that does not critically depend on cooling rates is reported. The controllable catalytic CVD permits high quality large-area graphene formation with deft control over the thickness from monolayers to thick graphitic structures at temperatures as low as 750 degrees C.

Graphene on Paper: A Simple, Low-Cost Chemical Sensing Platform

Graphene layers have been transferred directly on to paper without any intermediate layers to yield G-paper. Resistive gas sensors have been fabricated using strips of G-paper. These sensors achieved a remarkable lower limit of detection of ∼300 parts per trillion (ppt) for NO2, which is comparable to or better than those from other paper-based sensors. Ultraviolet exposure was found to dramatically reduce the recovery time and improve response times. G-paper sensors are also found to be robust against minor strain, which was also found to increase sensitivity. G-paper is expected to enable a simple and inexpensive low-cost flexible graphene platform.

A simultaneous perturbation stochastic approximation-based actor-critic algorithm for Markov decision processes

A two-timescale simulation-based actor-critic algorithm for solution of infinite horizon Markov decision processes with finite state and compact action spaces under the discounted cost criterion is proposed. The algorithm does gradient search on the slower timescale in the space of deterministic policies and uses simultaneous perturbation stochastic approximation-based estimates. On the faster scale, the value function corresponding to a given stationary policy is updated and averaged over a fixed number of epochs (for enhanced performance). The proof of convergence to a locally optimal policy is presented. Finally, numerical experiments using the proposed algorithm on flow control in a bottleneck link using a continuous time queueing model are shown.

Graphene field emission devices

Graphene field emission devices are fabricated using a scalable process. The field enhancement factors, determined from the Fowler-Nordheim plots, are within few hundreds and match the theoretical predictions. The devices show high emission current density of ∼10 nA μm−1 at modest voltages of tens of volts. The emission is stable with time and repeatable over long term, whereas the noise in the emission current is comparable to that from individual carbon nanotubes emitting under similar conditions. We demonstrate a power law dependence of emission current on pressure which can be utilized for sensing. The excellent characteristics and relative ease of making the devices promise their great potential for sensing and electronic applications.

Reliable processing of graphene using metal etchmasks

Graphene exhibits exciting properties which make it an appealing candidate for use in electronic devices. Reliable processes for device fabrication are crucial prerequisites for this. We developed a large area of CVD synthesis and transfer of graphene films. With patterning of these graphene layers using standard photoresist masks, we are able to produce arrays of gated graphene devices with four point contacts. The etching and lift off process poses problems because of delamination and contamination due to polymer residues when using standard resists. We introduce a metal etch mask which minimises these problems. The high quality of graphene is shown by Raman and XPS spectroscopy as well as electrical measurements. The process is of high value for applications, as it improves the processability of graphene using high-throughput lithography and etching techniques.