Terahertz imaging and spectroscopy of large-area single-layer graphene

We demonstrate terahertz (THz) imaging and spectroscopy of a 15x15-mm^2 single-layer graphene film on Si using broadband THz pulses. The THz images clearly map out the THz carrier dynamics of the graphene-on-Si sample, allowing us to measure sheet conductivity with sub-mm resolution without fabricating electrodes. The THz carrier dynamics are dominated by intraband transitions and the THz-induced electron motion is characterized by a flat spectral response. A theoretical analysis based on the Fresnel coefficients for a metallic thin film shows that the local sheet conductivity varies across the sample from {\sigma}s = 1.7x10^-3 to 2.4x10^-3 {\Omega}^-1 (sheet resistance, {\rho}s = 420 - 590 {\Omega}/sq).Comment: 6 pages, 5 figure

Terahertz imaging of inhomogeneous electrodynamics in single-layer graphene embedded in dielectrics

We investigate electron transport properties in large-area, single-layer graphene embedded in dielectric media, using free-space terahertz (THz) imaging and time-domain spectroscopy. Sandwiched between a thin polymethyl methacrylate (PMMA) layer and a Si substrate, graphene layers of different growth recipes exhibit distinctive spatial inhomogeneity of sheet conductivity. The non-contacting, non-destructive THz probe reveals that the PMMA layer induces a small, yet noticeable reduction in conductivity. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4749280]Keywords: Gas, SIO

High-Contrast Imaging of Graphene via Time-Domain Terahertz Spectroscopy

We demonstrate terahertz (THz) imaging and spectroscopy of single-layer graphene deposited on an intrinsic Si substrate using THz time-domain spectroscopy. A single-cycle THz pulse undergoes multiple internal reflections within the Si substrate, and the THz absorption by the graphene layer accumulates through the multiple interactions with the graphene/Si interface. We exploit the large absorption of the multiply reflected THz pulses to acquire high-contrast THz images of graphene. We obtain local sheet conductivity of the graphene layer analyzing the transmission data with thin-film Fresnel formula based on the Drude model.Keywords: Graphene, Time-domain spectroscopy, Multiple internal reflections, Terahertz imagin

Determining the Chiral Index of Semiconducting Carbon Nanotubes Using Photoconductivity Resonances

We utilize photoconductivity spectroscopy to identify the unique chiral structure of individual carbon nanotubes (CNTs). Peaks in photoconductivity are measured throughout the visible and near-IR wavelength ranges. Photoconductivity peaks associated with individual CNTs are referenced against existing Rayleigh scattering measurements to uniquely identify chiral indices. We find close agreement between our assigned exciton resonances and the previously published exciton resonances. The typical net energy mismatch is ≤20 meV. By enabling chiral identification of CNTs after the completion of device fabrication, the technique offers a facile method for investigating relationships between CNT structure and electronic/optoelectronic properties

Small Molecule Injection into Single-Cell <i>C. elegans</i> Embryos via Carbon-Reinforced Nanopipettes

<div><p>The introduction of chemical inhibitors into living cells at specific times in development is a useful method for investigating the roles of specific proteins or cytoskeletal components in developmental processes. Some embryos, such as those of <i>Caenorhabditis elegans</i>, however, possess a tough eggshell that makes introducing drugs and other molecules into embryonic cells challenging. We have developed a procedure using carbon-reinforced nanopipettes (CRNPs) to deliver molecules into <i>C. elegans</i> embryos with high temporal control. The use of CRNPs allows for cellular manipulation to occur just subsequent to meiosis II with minimal damage to the embryo. We have used our technique to replicate classical experiments using latrunculin A to inhibit microfilaments and assess its effects on early polarity establishment. Our injections of latrunculin A confirm the necessity of microfilaments in establishing anterior-posterior polarity at this early stage, even when microtubules remain intact. Further, we find that latrunculin A treatment does not prevent association of PAR-2 or PAR-6 with the cell cortex. Our experiments demonstrate the application of carbon-reinforced nanopipettes to the study of one temporally-confined developmental event. The use of CRNPs to introduce molecules into the embryo should be applicable to investigations at later developmental stages as well as other cells with tough outer coverings.</p> </div

Effect of Latrunculin A on PAR-6 localization.

<p>(<b>a</b>) Control with injection buffer containing 3.75% DMSO (<i>N</i> = 9). (<b>b</b>) Injection of 60 µM (<i>N</i> = 10) and (<b>c</b>) 90 µM (<i>N</i> = 5) LatA. In each panel, DIC is on the left and PAR-6::mCherry is on the right.</p

Carbon-reinforced nanopipette (CRNP) fabrication.

<p>(<b>a</b>) CRNP fabrication procedure. Quartz capillaries were filled with Fe(NO₃)₃ catalyst and left to dry, then pulled into pipettes of desired geometry. Carbon was grown within the pipette using chemical vapor deposition (CVD). (<b>b</b>) A SEM image of the tip of a CRNP, scale bar is 20 nm.</p

CRNP injection characterization and experimental configuration.

<p>(<b>a</b>) Characterization of injection volume. A differential interference contrast (DIC) image of a CRNP and fluorescence images before injection of dextran-TxRed into a droplet of glycerol and immediately after the injection. (<b>b</b>) Histogram of injection volume from multiple injections using one representative CRNP. (<b>c</b>) A cartoon and a (<b>d</b>) DIC image of the experimental configuration for embryo injection. A quartz holding pipette was used to immobilize the embryo during injection. Light suction applied through the holding pipette also allowed for the withdrawal of the CRNP. In all figures, unless otherwise stated, <i>t</i> = 0 is defined as the beginning of meiosis II, injection occurs at t ^≈ 0: 15, anterior is to the left, and scale bars are 10 µm.</p

Injection of YOYO-1 into multi-cell embryos.

<p>(a) Two-cell embryo in which the P1 blastomere has been injected with 1 µM YOYO-1. (<b>b</b>) Four-cell embryo in which the ABa blastomere has been injected with 1 µM YOYO-1.</p