We study the intrinsic transport properties of suspended graphene devices at high fields (≥1 V/μm) and high temperatures (≥1000 K). Across 15 samples, we find peak (average) saturation velocity of ...3.6 × 107 cm/s (1.7 × 107 cm/s) and peak (average) thermal conductivity of 530 W m–1 K–1 (310 W m–1 K–1) at 1000 K. The saturation velocity is 2–4 times and the thermal conductivity 10–17 times greater than in silicon at such elevated temperatures. However, the thermal conductivity shows a steeper decrease at high temperature than in graphite, consistent with stronger effects of second-order three-phonon scattering. Our analysis of sample-to-sample variation suggests the behavior of “cleaner” devices most closely approaches the intrinsic high-field properties of graphene. This study reveals key features of charge and heat flow in graphene up to device breakdown at ∼2230 K in vacuum, highlighting remaining unknowns under extreme operating conditions.
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We examine the transfer of graphene grown by chemical vapor deposition (CVD) with polymer scaffolds of poly(methyl methacrylate) (PMMA), poly(lactic acid) (PLA), poly(phthalaldehyde) (PPA), and ...poly(bisphenol A carbonate) (PC). We find that optimally reactive PC scaffolds provide the cleanest graphene transfers without any annealing, after extensive comparison with optical microscopy, x-ray photoelectron spectroscopy, atomic force microscopy, and scanning tunneling microscopy. Comparatively, films transferred with PLA, PPA, PMMA PC, and PMMA have a two-fold higher roughness and a five-fold higher chemical doping. Using PC scaffolds, we demonstrate the clean transfer of CVD multilayer graphene, fluorinated graphene, and hexagonal boron nitride. Our annealing free, PC transfers enable the use of atomically-clean nanomaterials in biomolecule encapsulation and flexible electronic applications.
Ring oscillators (ROs) are the most important class of circuits used to evaluate the performance limits of any digital technology. However, ROs based on low-dimensional nanomaterials (e.g., 1-D ...nanotubes, nanowires, 2-D MoS2) have so far exhibited limited performance due to low current drive or large parasitics. Here we demonstrate integrated ROs fabricated from wafer-scale graphene grown by chemical vapor deposition. The highest oscillation frequency was 1.28 GHz, while the largest output voltage swing was 0.57 V. Both values remain limited by parasitic capacitances in the circuit rather than intrinsic properties of the graphene transistor components, suggesting further improvements are possible. The fabricated ROs are the fastest realized in any low-dimensional nanomaterial to date and also the least sensitive to fluctuations in the supply voltage. They represent the first integrated graphene oscillators of any kind and can also be used in a wide range of applications in analog electronics. As a demonstration, we also realized the first stand-alone graphene mixers that do not require external oscillators for frequency conversion. The first gigahertz multitransistor graphene integrated circuits demonstrated here pave the way for application of graphene in high-speed digital and analog circuits in which high operating speed could be traded off against power consumption.
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The growth of high-density arrays of vertically oriented, single crystalline InAs NWs on graphene surfaces are realized through the van der Waals (vdW) epitaxy mechanism by metalorganic chemical ...vapor deposition (MOCVD). However, the growth of InGaAs NWs on graphene results in spontaneous phase separation starting from the beginning of growth, yielding a well-defined InAs–InxGa1–xAs (0.2 < x < 1) core–shell structure. The core–shell structure then terminates abruptly after about 2 μm in height, and axial growth of uniform composition InxGa1–xAs takes place without a change in the NW diameter. The InxGa1–xAs shell composition changes as a function of indium flow, but the core and shell thicknesses and the onset of nonsegregated InxGa1–xAs axial segment are independent of indium composition. In contrast, no InGaAs phase segregation has been observed when growing on MoS2, another two-dimensional (2D) layered material, or via the Au-assisted vapor–liquid–solid (VLS) mechanism on graphene. This spontaneous phase segregation phenomenon is elucidated as a special case of van der Waals epitaxy on 2D sheets. Here, considering the near lattice matched registry between InAs and graphene, InGaAs is forced to self-organize into InAs core and InGaAs shell segments since the lack of dangling bonds on graphene does not allow strain sharing through elastic deformation between InGaAs and graphene.
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We study graphene nanoribbon (GNR) interconnects obtained from graphene grown by chemical vapor deposition (CVD). We report low- and high-field electrical measurements over a wide temperature range, ...from 1.7 to 900 K. Room temperature mobilities range from 100 to 500 cm2·V–1·s–1, comparable to GNRs from exfoliated graphene, suggesting that bulk defects or grain boundaries play little role in devices smaller than the CVD graphene crystallite size. At high-field, peak current densities are limited by Joule heating, but a small amount of thermal engineering allows us to reach ∼2 × 109 A/cm2, the highest reported for nanoscale CVD graphene interconnects. At temperatures below ∼5 K, short GNRs act as quantum dots with dimensions comparable to their lengths, highlighting the role of metal contacts in limiting transport. Our study illustrates opportunities for CVD-grown GNRs, while revealing variability and contacts as remaining future challenges.
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We numerically design and experimentally test a SERS-active substrate for enhancing the SERS signal of a single layer of graphene (SLG) in water. The SLG is placed on top of an array of ...silver-covered nanoholes in a polymer and is covered with water. Here we report a large enhancement of up to 2 × 10
in the SERS signal of the SLG on the patterned plasmonic nanostructure for a 532 nm excitation laser wavelength. We provide a detailed study of the light-graphene interactions by investigating the optical absorption in the SLG, the density of optical states at the location of the SLG, and the extraction efficiency of the SERS signal of the SLG. Our numerical calculations of both the excitation field and the emission rate enhancements support the experimental results. We find that the enhancement is due to the increase in the confinement of electromagnetic fields on the location of the SLG that results in enhanced light absorption in the graphene at the excitation wavelength. We also find that water droplets increase the density of optical radiative states at the location of the SLG, leading to enhanced spontaneous emission rate of graphene at its Raman emission wavelengths.
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The high-frequency performance of transistors is usually assessed by speed and gain figures of merit, such as the maximum oscillation frequency f
, cutoff frequency f
, ratio f
/f
, forward ...transmission coefficient S
, and open-circuit voltage gain A
. All these figures of merit must be as large as possible for transistors to be useful in practical electronics applications. Here we demonstrate high-performance graphene field-effect transistors (GFETs) with a thin AlOx gate dielectric which outperform previous state-of-the-art GFETs: we obtained f
/f
> 3, A
> 30 dB, and S
= 12.5 dB (at 10 MHz and depending on the transistor geometry) from S-parameter measurements. A dc characterization of GFETs in ambient conditions reveals good current saturation and relatively large transconductance ~600 S/m. The realized GFETs offer the prospect of using graphene in a much wider range of electronic applications which require substantial gain.
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