Bhupesh Chandra

Tunable infrared plasmonic devices using graphene/insulator stacks

Superlattices are artificial periodic nanostructures which can control the flow of electrons. Their operation typically relies on the periodic modulation of the electric potential in the direction of electron wave propagation. Here we demonstrate transparent graphene superlattices which can manipulate infrared photons utilizing the collective oscillations of carriers, i.e., plasmons of the ensemble of multiple graphene layers. The superlattice is formed by depositing alternating wafer-scale graphene sheets and thin insulating layers, followed by patterning them all together into 3-dimensional photonic-crystal-like structures. We demonstrate experimentally that the collective oscillation of Dirac fermions in such graphene superlattices is unambiguously nonclassical: compared to doping single layer graphene, distributing carriers into multiple graphene layers strongly enhances the plasmonic resonance frequency and magnitude, which is fundamentally different from that in a conventional semiconductor superlattice. This property allows us to construct widely tunable far-infrared notch filters with 8.2 dB rejection ratio and terahertz linear polarizers with 9.5 dB extinction ratio, using a superlattice with merely five graphene atomic layers. Moreover, an unpatterned superlattice shields up to 97.5% of the electromagnetic radiations below 1.2 terahertz. This demonstration also opens an avenue for the realization of other transparent mid- and far-infrared photonic devices such as detectors, modulators, and 3-dimensional meta-material systems.The collective oscillation of carriers--the plasmon--in graphene has many desirable properties, including tunability and low loss. However, in single-layer graphene, the dependence on carrier concentration of both the plasmonic resonance frequency and magnitude is relatively weak, limiting its applications in photonics. Here, we demonstrate transparent photonic devices based on graphene/insulator stacks, which are formed by depositing alternating wafer-scale graphene sheets and thin insulating layers, then patterning them together into photonic-crystal-like structures. We show experimentally that the plasmon in such stacks is unambiguously non-classical. Compared with doping in single-layer graphene, distributing carriers into multiple graphene layers effectively enhances the plasmonic resonance frequency and magnitude, which is different from the effect in a conventional semiconductor superlattice and is a direct consequence of the unique carrier density scaling law of the plasmonic resonance of Dirac fermions. Using patterned graphene/insulator stacks, we demonstrate widely tunable far-infrared notch filters with 8.2 dB rejection ratios and terahertz linear polarizers with 9.5 dB extinction ratios. An unpatterned stack consisting of five graphene layers shields 97.5% of electromagnetic radiation at frequencies below 1.2 THz. This work could lead to the development of transparent mid- and far-infrared photonic devices such as detectors, modulators and three-dimensional metamaterial systems.

Scaling of resistance and electron mean free path of single-walled carbon nanotubes.

We present an experimental investigation on the scaling of resistance in individual single-walled carbon nanotube devices with channel lengths that vary 4 orders of magnitude on the same sample. The electron mean free path is obtained from the linear scaling of resistance with length at various temperatures. The low temperature mean free path is determined by impurity scattering, while at high temperature, the mean free path decreases with increasing temperature, indicating that it is limited by electron-phonon scattering. An unusually long mean free path at room temperature has been experimentally confirmed. Exponentially increasing resistance with length at extremely long length scales suggests anomalous localization effects.

Variable Electron-Phonon Coupling in Isolated Metallic Carbon Nanotubes Observed by Raman Scattering

We report the existence of broad and weakly asymmetric features in the high-energy (G) Raman modes of freely suspended metallic carbon nanotubes of defined chiral index. A significant variation in peak width (from 12 cm(-1) to 110 cm(-1)) is observed as a function of the nanotubes chiral structure. When the nanotubes are electrostatically gated, the peak widths decrease. The broadness of the Raman features is understood as the consequence of coupling of the phonon to electron-hole pairs, the strength of which varies with the nanotube chiral index and the position of the Fermi energy.

Mediated Enzyme Electrodes with Combined Micro- and Nanoscale Supports

We report the electrochemical performance of a redox polymer-mediated glucose anode catalyzed by glucose oxidase and sup-ported on a multiscale carbon material. The support is composed of carbon paper upon which is grown multiwall nanotubes bychemical vapor deposition combined with ohmic heating of the carbon paper. The material possessed 100-fold higher surface areaand demonstrated tenfold higher electrochemical performance, compared to bare carbon paper. Maximum performance is limitedby biocatalyst and reactant transport in the micro- and nanoscale pores of the support material.© 2007 The Electrochemical Society. DOI: 10.1149/1.2712049 All rights reserved.Manuscript submitted November 8, 2006; revised manuscript received January 4, 2007. Available electronically March 6, 2007.

Excitons and high-order optical transitions in individual carbon nanotubes: A Rayleigh scattering spectroscopy study

We examine the excitonic nature of high-lying optical transitions in single-walled carbon nanotubes by means of Rayleigh scattering spectroscopy. A careful analysis of the principal transitions of individual semiconducting and metallic nanotubes reveals that in both cases the line shape is consistent with an excitonic model, but not one of free carriers. For semiconducting species, sidebands are observed at

Carbon nanotube thin film transistors on flexible substrates

\ensuremath{\sim}200\text{ }\text{meV}

Molecular-Scale Quantum Dots from Carbon Nanotube Heterojunctions

above the third and fourth optical transitions. These features are ascribed to exciton-phonon bound states. Such sidebands are not apparent for metallic nanotubes, as expected from the reduced strength of excitonic interactions in these systems.

Engineering of Contact Resistance between Transparent Single-Walled Carbon Nanotube Films and a-Si:H Single Junction Solar Cells by Gold Nanodots

Carbon nanotube thin film transistors (CNT-TFTs) are fabricated on flexible substrates using purified, surfactant-based CNT suspensions, with >95% semiconducting CNT fraction. The TFTs are made up of local bottom-gated structures with aluminum oxide as the gate dielectric. The devices exhibit high ON current densities (0.1 μA/μm) and on-off ratios (∼105) with mobility values ranging from 10-35 cm2/Vs. A detailed numerical model is used to understand the TFT performance and its dependence on device parameters such as TFT channel length, CNT density, and purity.

Growth of Nanotubes and Chemical Sensor Applications

Carbon nanotube heterojunctions (HJs), which seamlessly connect nanotubes of different chiral structure using a small number of atomic-scale defects, represent the ultimate scaling of electronic interfaces. Here we report the first electrical transport measurements on a HJ formed between semiconducting and metallic nanotubes of known chiralities. These measurements reveal asymmetric IV-characteristics and the presence of a quantum dot (QD) with approximately 60 meV charging energy and approximately 75 meV level spacing. A detailed atomistic and electronic model of the HJ enables the identification of specific defect arrangements that lead to the QD behavior consistent with the experiment.

Raman scattering from individual, isolated metallic carbon nanotubes

The viability of single-walled carbon nanotubes (SWCNTs) as a transparent conducting electrode on a-Si:H based single junction solar cells was explored. A Schottky barrier formed at a SWCNT/a-Si:H interface was removed by introducing high work function gold nanodots at the SWCNT/a-Si:H interface. This allows comparable device performance from SWCNT-electrode-based a-Si:H solar cells to that obtained by using conventional transparent conducting oxides.