Chemical vapor deposition of graphene on Cu often employs polycrystalline Cu substrates with diverse facets, grain boundaries (GBs), annealing twins, and rough sites. Using scanning electron microscopy (SEM), electron-backscatter diffraction (EBSD), and Raman spectroscopy on graphene and Cu, we find that Cu substrate crystallography affects graphene growth more than facet roughness. We determine that (111) containing facets produce pristine monolayer graphene with higher growth rate than (100) containing facets, especially Cu(100). The number of graphene defects and nucleation sites appears Cu facet invariant at growth temperatures above 900 °C. Engineering Cu to have (111) surfaces will cause monolayer, uniform graphene growth.

ABSTRACT: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 cm 2 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 × 10 9 A/cm 2 , 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.

We use high pressure to reveal the dependence of interfacial heat transport on the stiffness of interfacial bonds. The combination of time-domain thermoreflectance and SiC anvil techniques is used to measure the pressure-dependent thermal conductance G(P ) of clean and modified Al/SiC interfaces at pressures as high as P = 12 GPa. We create low-stiffness, van der Waals-bonded interfaces by transferring a monolayer of graphene onto the SiC surface before depositing the Al film. For such weak interfaces, G(P ) initially increases approximately linearly with P . At high pressures, P > 8 GPa, the thermal conductance of these weak interfaces approaches the high values characteristic of strongly bonded, clean interfaces.

Heat transfer across interfaces of graphene and polar dielectrics (e.g. SiO 2 ) could be mediated by direct phonon coupling, as well as electronic coupling with remote interfacial phonons (RIPs). To understand the relative contribution of each component, we develop a new pumpprobe technique, called voltage-modulated thermoreflectance (VMTR), to accurately measure the change of interfacial thermal conductance under an electrostatic field. We employed VMTR on top gates of graphene field-effect transistors and find that the thermal conductance of SiO 2 /graphene/SiO 2 interfaces increases by up to ΔG  0.8 MW m -2 K -1 under electrostatic fields of <0.2 V nm -1 . We propose two possible explanations for the observed ΔG. First, since the applied electrostatic field induces charge carriers in graphene, our VMTR measurements could originate from heat transfer between the charge carriers in graphene and RIPs in SiO 2 . Second, the increase in heat conduction could be caused by better conformity of graphene interfaces un-

Phase change memory (PCM) devices are known to reduce in power consumption as the bit volume and contact area of their electrodes are scaled down. Here, we demonstrate two types of low-power PCM devices with lateral graphene ribbon electrodes: one in which the graphene is patterned into narrow nanoribbons and the other where the phase change material is patterned into nanoribbons. The sharp graphene "edge" contacts enable switching with threshold voltages as low as ~3 V, low programming currents (<1 μA SET, <10 μA RESET) and ON/OFF ratios >100. Large-scale fabrication with graphene grown by chemical vapor deposition also enables the study of heterogeneous integration and that of variability for such nanomaterials and devices.