A Raman spectroscopic investigation of graphite oxide derived graphene AIP Advances 2, 032183 (2012) Nanofracture in graphene under complex mechanical stresses Appl. Phys. Lett. 101, 121915 (2012) Low-energy electron transmission imaging of clusters on free-standing graphene Appl. Phys. Lett. 101, 113117 (2012) Highly tunable spin-dependent electron transport through carbon atomic chains connecting two zigzag graphene nanoribbons J. Chem. Phys. 137, 104107 (2012) Formation and control of wrinkles in graphene by the wedging transfer method

We report the thermal conductance G of Au/Ti/graphene/SiO(2) interfaces (graphene layers 1 ≤ n ≤ 10) typical of graphene transistor contacts. We find G ≈ 25 MW m(-2) K(-1) at room temperature, four times smaller than the thermal conductance of a Au/Ti/SiO(2) interface, even when n = 1. We attribute this reduction to the thermal resistance of Au/Ti/graphene and graphene/SiO(2) interfaces acting in series. The temperature dependence of G from 50 ≤ T ≤ 500 K also indicates that heat is predominantly carried by phonons through these interfaces. Our findings suggest that metal contacts can limit not only electrical transport but also thermal dissipation from submicrometer graphene devices.

Heat flow in nanomaterials is an important area of study, with both fundamental and technological implications. However, little is known about heat flow in two-dimensional devices or interconnects with dimensions comparable to the phonon mean free path. Here we find that short, quarter-micron graphene samples reach B35% of the ballistic thermal conductance limit up to room temperature, enabled by the relatively large phonon mean free path (B100 nm) in substrate-supported graphene. In contrast, patterning similar samples into nanoribbons leads to a diffusive heat-flow regime that is controlled by ribbon width and edge disorder. In the edge-controlled regime, the graphene nanoribbon thermal conductivity scales with width approximately as BW 1.8±0.3 , being about 100 W m À 1 K À 1 in 65-nm-wide graphene nanoribbons, at room temperature. These results show how manipulation of two-dimensional device dimensions and edges can be used to achieve full control of their heat-carrying properties, approaching fundamentally limited upper or lower bounds.

We report on fabrication and electrical characteristics of high-mobility field-effect transistors (FETs) using ZnO nanorods. For FET fabrications, single-crystal ZnO nanorods were prepared using catalyst-free metalorganic vapor phase epitaxy. Although typical ZnO nanorod FETs exhibited good electrical characteristics, with a transconductance of ϳ140 nS and a mobility of 75 cm 2 / V s, the device characteristics were significantly improved by coating a polyimide thin layer on the nanorod surface, exhibiting a large turn-ON/OFF ratio of 10 4-10 5 , a high transconductance of 1.9 S, and high electron mobility above 1000 cm 2 / V s. The role of the polymer coating in the enhancement of the devices is also discussed.

We directly image hot spot formation in functioning mono- and bilayer graphene field effect transistors (GFETs) using infrared thermal microscopy. Correlating with an electrical-thermal transport model provides insight into carrier distributions, fields, and GFET power dissipation. The hot spot corresponds to the location of minimum charge density along the GFET; by changing the applied bias, this can be shifted between electrodes or held in the middle of the channel in ambipolar transport. Interestingly, the hot spot shape bears the imprint of the density of states in mono- vs bilayer graphene. More broadly, we find that thermal imaging combined with self-consistent simulation provide a noninvasive approach for more deeply examining transport and energy dissipation in nanoscale devices.

The performance and scaling of graphene-based electronics is limited by the quality of contacts between the graphene and metal electrodes. However, the nature of graphene-metal contacts remains incompletely understood. Here, we use atomic force microscopy to measure the temperature distributions at the contacts of working graphene transistors with a spatial resolution of ~ 10 nm (refs 5-8), allowing us to identify the presence of Joule heating, current crowding and thermoelectric heating and cooling. Comparison with simulation enables extraction of the contact resistivity (150-200 Ω µm²) and transfer length (0.2-0.5 µm) in our devices; these generally limit performance and must be minimized. Our data indicate that thermoelectric effects account for up to one-third of the contact temperature changes, and that current crowding accounts for most of the remainder. Modelling predicts that the role of current crowding will diminish and the role of thermoelectric effects will increase as contacts improve.

Phase slips are topological fluctuation events that carry the superconducting order-parameter field between distinct current carrying states 1 . Owing to these phase slips low-dimensional superconductors acquire electrical resistance 2 . In quasi-one-dimensional nanowires it is well known that at higher temperatures phase slips occur via the process of thermal barrier-crossing by the orderparameter field. At low temperatures, the general expectation is that phase slips should proceed via quantum tunnelling events, which are known as quantum phase slips (QPS). However, resistive measurements have produced evidence both pro 3-6 and con [7][8][9] and hence the precise requirements for the observation of QPS are yet to be established firmly. Here we report strong evidence for individual quantum tunnelling events undergone by the superconducting order-parameter field in homogeneous nanowires. We accomplish this via measurements of the distribution of switching currents-the high-bias currents at which superconductivity gives way to resistive behaviour-whose width exhibits a rather counter-intuitive, monotonic increase with decreasing temperature. We outline a Quantum phenomena involving systems far larger than individual atoms are one of the most exciting fields of modern physics. Initiated by Leggett more than twentyfive years ago 14,15 , the field has seen widespread development, important realizations being furnished, e. g., by macroscopic quantum tunnelling (MQT) of the phase in Josephson junctions, and of the magnetization in magnetic nanoparticles [16][17][18][19] . More recently, the breakthrough recognition of the potential advantages of quantum-based computational methods has initiated the search for viable implementations of qubits 20 , several of which are rooted in MQT in superconducting systems. In particular, it has been recently proposed that superconducting nanowires (SCNWs) could provide a valuable setting for realizing qubits 12 . In this case, the essential behaviour needed of SCNWs that they undergo QPS, i.e., topological quantum fluctuations of the superconducting order-parameter field via which tunnelling occurs between currentcarrying states. It has also been proposed that QPS in nanowires could allow one to build a current standard, and thus could play a useful role in aspects of metrology 13 .Additionally, QPS are believed to provide the pivotal processes underpinning the 3 superconductor-insulator transition observed in nanowires 21-25, Observations of QPS have been reported previously on wires having high normal resistance (i.e., R N > R Q , where R Q = h/4e 2 ≈ 6,450 Ω) via low-bias resistance (R) vs. temperature (T) measurements 3,4 . Yet, low-bias measurements on short wires with normal resistance R N < R Q have been unable to reveal QPS 7,8 . Also, it has been suggested that some results ascribed to QPS could in fact have originated in inhomogeneity of the nanowires.Thus, no consensus exists about the conditions under which QPS occur, and qualitatively new evidence for QPS remains highl...

2Graphene and related two-dimensional materials are promising candidates for atomically thin, flexible, and transparent optoelectronics 1,2 . In particular, the strong light-matter interaction in graphene 3 has allowed for the development of state-of-the-art photodetectors 4,5 , optical modulators 6 , and plasmonic devices 7 .In addition, electrically biased graphene on SiO 2 substrates can be used as a low-efficiency emitter in the mid-infrared range 8,9 . However, emission in the visible range has remained elusive. Here we report the observation of bright visible-light emission from electrically biased suspended graphenes. In these devices, heat transport is greatly minimised 10 ; thus hot electrons (~ 2800 K) become spatially localised at the centre of graphene layer, resulting in a 1000-fold enhancement in the thermal radiation efficiency 8,9 . Moreover, strong optical interference between the suspended graphene and substrate can be utilized to tune the emission spectrum. We also demonstrate the scalability of this For the realisation of graphene-based bright and broadband light-emitters, a radiative electron-hole recombination process in gapless graphene is not efficient because of the rapid energy relaxation that occurs through electron-electron and electron-phonon interactions [11][12][13] .Alternatively, graphene's superior strength 14 and high-temperature stability may enable efficient thermal light emission. However, the thermal radiation from electrically biased graphene supported on a substrate 8,9,[15][16][17] has been found to be limited to the infrared range and 3 to be inefficient as an extremely small fraction of the applied energy (~ 10 -6 ) 8,9 is converted into light radiation. Such limitations are the direct result of heat dissipation through the underlying substrate 18 and significant hot electron relaxation from dominant extrinsic scattering effects such as charged impurities 19 and surface polar optical phonon interaction 20 , limiting the maximum operating temperatures.On the other hand, a freely suspended graphene is mostly immune to such undesirable vertical heat dissipation 10 and extrinsic scattering effects 21,22 , promising much more efficient and brighter radiation in the infrared-to-visible range. Fortuitously, due to the strong Umklapp phonon-phonon scattering 23 , we find that the thermal conductivity of graphene at high lattice temperatures (1800 ± 300 K) is greatly reduced (~ 65 Wm -1 K -1 ), which additionally suppresses lateral heat dissipation; therefore, hot electrons (~ 2800 K) become spatially localised at the centre of the suspended graphene under modest electric fields (~ 0.4 V/µm), greatly increasing the efficiency and brightness of the light emission. The bright visible thermally emitted light interacts with the reflected light from the separated substrate surface, allowing interference effects that can be utilized to tune the wavelength of the emitted light.We fabricate the freely suspended graphene devices with mechanically exfoliated graphene flakes, and for d...