54 research outputs found

    Raman spectroscopy on etched graphene nanoribbons

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    We investigate etched single-layer graphene nanoribbons with different widths ranging from 30 to 130 nm by confocal Raman spectroscopy. We show that the D-line intensity only depends on the edge-region of the nanoribbon and that consequently the fabrication process does not introduce bulk defects. In contrast, the G- and the 2D-lines scale linearly with the irradiated area and therefore with the width of the ribbons. We further give indications that the D- to G-line ratio can be used to gain information about the crystallographic orientation of the underlying graphene. Finally, we perform polarization angle dependent measurements to analyze the nanoribbon edge-regions

    Edge chirality determination of graphene by Raman spectroscopy

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    Raman imaging on the edges of single layer micromechanical cleavage graphene (MCG) was carried out. The intensity of disorder-induced Raman feature (D band at ~1350 cm-1) was found to be correlated to the edge chirality: it is stronger at the armchair edge and weaker at the zigzag edge. This shows that Raman spectroscopy is a reliable and practical method to identify the chirality of graphene edge and to help in determination of the crystal orientation. The determination of graphene chirality is critically important for fundamental study as well as for applications.Comment: 14 pages, 3 figures, 1 tabl

    Direct Imaging of Graphene Edges: Atomic Structure and Electronic Scattering

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    We report an atomically-resolved scanning tunneling microscopy (STM) investigation of the edges of graphene grains synthesized on Cu foils by chemical vapor deposition (CVD). Most of the edges are macroscopically parallel to the zigzag directions of graphene lattice. These edges have microscopic roughness that is found to also follow zigzag directions at atomic scale, displaying many ~120 degree turns. A prominent standing wave pattern with periodicity ~3a/4 (a being the graphene lattice constant) is observed near a rare-occurring armchair-oriented edge. Observed features of this wave pattern are consistent with the electronic intervalley backscattering predicted to occur at armchair edges but not at zigzag edges

    Study on Electrolyte-gated Graphene Nanoelectronic Biosensors for Biomarker Detection

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    Biosensors are called upon to provide valuable benefits for human society in vital fields such as disease diagnosis, food inspection, environment monitoring, etc. Among the various biosensor architectures, the field effect transistor (FET) biosensors are promising as the next generation nanoelectronic biosensors, particularly attractive for point-of-care biomedical applications. The FET biosensors typically operate by measuring the conductance change of the semiconducting channel induced by the adsorption of the target biomolecules on it. The superior properties of graphene, including the unique electronic characteristics, facile functionalization and good biocompatibility, etc., make it an ideal building block for the FET biosensors. In this dissertation, we present studies on the electrolyte-gated graphene field effect transistor (EGGFET) biosensor and its application for the label-free detection of biomarkers. Poly(methyl methacrylate) (PMMA) residues have long been a critical challenge for the transfer of the chemical vapor deposited (CVD) graphene, which is critical to obtain reliable devices. To address this issue, we first studied the degradation of the PMMA residues upon thermal annealing using Raman spectroscopy. An electrolytic cleaning method is shown to be effective to remove these post-annealing residues, resulting in a clean, residue-free graphene surface. The performance of the EGGFET biosensor is demonstrated by the successful detection of human immunoglobulin G (IgG) using IgG-aptamer as the bioreceptor. The gate voltage with the minimum conductivity (Dirac) in the transfer curve of the EGGFET biosensor is used for the quantitative measurement of IgG concentration. In EGGFET biosensors, the graphene channels are directly exposed to the electrolytes, of which the composition, concentration and pH may vary during the testing. The response of the EGGFET biosensors is found to be susceptible to these variations which might lead to high uncertainty or even false results. We present an EGGFET immunoassay which allows well regulation over the matrix effect. The performance is demonstrated by the detection of human IgG from serum. The detection range of the EGGFET immunoassay for IgG detection is estimated to be around 2-50 nM with a coefficient of variation (CV) of less than 20%. The limit of detection (LOD) is around 0.7 nM. Different from the metal-oxide-semiconductor field effect transistors (MOSFET), the gate voltage is applied on the electrolyte and the electrical double layer (EDL) at the electrolyte-graphene interface serves as the gate dielectric in EGGFET. We studied the capacitance behavior of the electrolyte-graphene interface; the results suggest that the electrolyte-graphene interface exhibits a complex constant phase element (CPE) behavior (1 = 0 () ) with both 0 and varying as functions of the gate voltage. The EDL capacitance and the quantum capacitance are determined which allows us to extract the carrier density and mobility in graphene. This study give insight into the device physics of the EGGFET biosensor and is instructive for the design of the EGGFET biosensors on the device level
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