Graphene devices on standard SiO(2) substrates are highly disordered, exhibiting characteristics that are far inferior to the expected intrinsic properties of graphene. Although suspending the ...graphene above the substrate leads to a substantial improvement in device quality, this geometry imposes severe limitations on device architecture and functionality. There is a growing need, therefore, to identify dielectrics that allow a substrate-supported geometry while retaining the quality achieved with a suspended sample. Hexagonal boron nitride (h-BN) is an appealing substrate, because it has an atomically smooth surface that is relatively free of dangling bonds and charge traps. It also has a lattice constant similar to that of graphite, and has large optical phonon modes and a large electrical bandgap. Here we report the fabrication and characterization of high-quality exfoliated mono- and bilayer graphene devices on single-crystal h-BN substrates, by using a mechanical transfer process. Graphene devices on h-BN substrates have mobilities and carrier inhomogeneities that are almost an order of magnitude better than devices on SiO(2). These devices also show reduced roughness, intrinsic doping and chemical reactivity. The ability to assemble crystalline layered materials in a controlled way permits the fabrication of graphene devices on other promising dielectrics and allows for the realization of more complex graphene heterostructures.
We report on a capacitance study of dual gated bilayer graphene. The measured capacitance allows us to probe the electronic compressibility as a function of carrier density, temperature, and applied ...perpendicular electrical displacement D. As a band gap is induced with increasing D, the compressibility minimum at charge neutrality becomes deeper but remains finite, suggesting the presence of localized states within the energy gap. Temperature dependent capacitance measurements show that compressibility is sensitive to the intrinsic band gap. For large displacements, an additional peak appears in the compressibility as a function of density, corresponding to the presence of a one-dimensional van Hove singularity (vHs) at the band edge arising from the quartic bilayer graphene band structure. For D > 0, the additional peak is observed only for electrons, while for D < 0 the peak appears only for holes. This asymmetry can be understood in terms of the finite interlayer separation and may be useful as a direct probe of the layer polarization.
This paper describes the design of an active, integrated CMOS sensor array for fluorescence applications which enables time-gated, time-resolved fluorescence spectroscopy. The 64-by-64 array is ...sensitive to photon densities as low as 8.8 times 10 6 photons/cm 2 with 64-point averaging and, through a differential pixel design, has a measured impulse response of better than 800 ps. Applications include both active microarrays and high-frame-rate imagers for fluorescence lifetime imaging microscopy.
Technological advances in fluorescent probes, solid-state imagers, and microscopy techniques have enabled biomolecular studies at the single-molecule level. Fluorescent techniques are highly specific ...but their bandwidth is fundamentally limited by the number of photons that can be collected. New electronic sensors including nanopores and nanotube field-effect transistors offer different tradeoffs between bandwidth and noise levels. Here, we discuss the performance of these direct solid-state interfaces and their potential for sensing single-molecule dynamics at shorter timescales.
Traditionally, biomolecular systems have been studied in ensemble. While much can be determined with ensemble measurements, scientific and technological interest is rapidly moving to single-molecule ...techniques, which rely primarily on fluorescent markers and advanced microscopy techniques. In this paper, we describe recent work using nanoscale transistors based on carbon nanotubes as charge-sensitive detectors. We show carbon nanotubes can be used for ensemble studies through sidewall adsorption. Sensitivity can be greatly enhanced though an engineered defect in the nanotube. Biomolecular interactions are characterized by random-telegraph-noise response, which can be analyzed to study single-molecule kinetics and thermodynamics.
This paper describes the design of an active CMOS sensor array for fluorescence applications which enables time-gated, time-resolved fluorescence spectroscopy. The 64 times 64 array is sensitive to ...photon densities as low as 8 times 10 6 photons/cm with 64-point averaging and, through a differential pixel design, has a measured impulse response of better than 800 ps. Applications include both active microarrays and high-frame-rate imagers for fluorescence lifetime imaging microscopy.
We present a label-free single-molecule based sensing platform using a carbon nanotube field-effect transistor. By point functionalizing a carbon nanotube through an electrochemical oxidation ...reaction, the conductance becomes sensitive and chemically reactive at a single point to which we can covalently attach a probe DNA molecule. Two-level fluctuations appear in the conductance of the carbon nanotube when it is immersed in a liquid buffer solution containing complementary target DNA. We show that the autocorrelation of the conductance can be used to extract DNA hybridization kinetics. The results are comparable to the one extracted through a hidden Markov model.
We have demonstrated that carbon-nanotube field-effect transistors act as highly sensitive single-molecule detectors when point functionalized. The hybridization kinetics of two complementary strands ...of DNA are associated with two-level fluctuations in the conductance of the nanotube to which the DNA is bound. We have studied the temperature dependence of the nanotube conductance and shown that the transport mechanism is consistent with Frenkel Poole emission, in which negatively charged DNA modulates a tunnel barrier at the point functionalized defect site. These transistors represent an important potential sensing platform for label-free single-molecule diagnostics.
We report on a capacitance study of dual gated bilayer graphene. The measured capacitance allows us to probe the electronic compressibility as a function of carrier density, temperature, and applied ...perpendicular electrical displacement D. As a band gap is induced with increasing D, the compressibility minimum at charge neutrality becomes deeper but remains finite, suggesting the presence of localized states within the energy gap. Temperature dependent capacitance measurements show that compressibility is sensitive to the intrinsic band gap. For large displacements, an additional peak appears in the compressibility as a function of density, corresponding to the presence of a 1-dimensional van Hove singularity (vHs) at the band edge arising from the quartic bilayer graphene band structure. For D > 0, the additional peak is observed only for electrons, while D < 0 the peak appears only for holes. This asymmetry that can be understood in terms of the finite interlayer separation and may be useful as a direct probe of the layer polarization.
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