We have achieved mobilities in excess of 200,000 cm 2 V −1 s −1 at electron densities of ∼2×10 11 cm −2 by suspending single layer graphene. Suspension ∼150 nm above a Si/SiO2 gate electrode and electrical contacts to the graphene was achieved by a combination of electron beam lithography and etching. The specimens were cleaned in situ by employing current-induced heating, directly resulting in a significant improvement of electrical transport. Concomitant with large mobility enhancement, the widths of the characteristic Dirac peaks are reduced by a factor of 10 compared to traditional, non-suspended devices. This advance should allow for accessing the intrinsic transport properties of graphene. Graphene, the latest addition to the family of twodimensional (2D) materials, is distinguished from its cousins by its unusual band structure, rendering the quasiparticles in it formally identical to massless, chiral fermions. The experimental realization of graphene thus presents tantalizing opportunities to study phenomena ranging from the topological phase resulting in exotic quantum Hall states [1,2] to the famous Klein paradox -the anomalous tunneling of relativistic particles [3]. However, despite tremendous interest and concerted experimental efforts , the presence of strong impurity scattering -which limits the electron mean free path to less than a micron -has been a major barrier to progress. At the same time, there is strong evidence that graphene is a nearly perfect crystal free of the structural defects [4,5] that characterize most conductors. As a result, it has been put forth that the scattering of charge carriers stems from extrinsic sources [7,8,9,10].Although the exact nature of the scattering that limits the mobility of graphene devices remains unclear, evidence has mounted that interactions with the underlying substrate are largely responsible. Surface charge traps [6,7,8,9], interfacial phonons [11], substrate stabilized ripples [5,10,12], and fabrication residues on or under the graphene sheet may all contribute. Consequently, improving substrate quality or eliminating the substrate altogether by suspending graphene over a trench seems a promising strategy towards higher quality samples. While devices suspended over the substrate were achieved in the past [12,13], they lacked multiple electrical contacts thus precluding transport measurements.In this Letter we report the fabrication of electrically contacted suspended graphene and achieve a tenfold improvement in mobility as compared to the best values reported in the literature for traditional devices fabricated on a substrate. Besides opening new avenues for studying the intrinsic physics of Dirac fermions, this improvement demonstrates the dominant role played by extrinsic scattering in limiting the transport properties of unsuspended graphene samples.The fabrication of a suspended graphene device starts with optically locating a single-layer mechanically exfoliated graphene flake on top of a silicon substrate covered with 300 nm of SiO 2 . Singl...

The quantum Hall effect (QHE), one example of a quantum phenomenon that occurs on a truly macroscopic scale, has been attracting intense interest since its discovery in 1980 (1). The QHE is exclusive to two-dimensional (2D) metals and has elucidated many important aspects of quantum physics and deepened our understanding of interacting systems. It has also led to the establishment of a new metrological standard, the resistance quantum h/e 2 that contains only fundamental constants of the electron charge e and the Planck constant h (2). As many other quantum phenomena, the observation of the QHE usually requires low temperatures T, typically below the boiling point of liquid helium (1). Efforts to extend the QHE temperature range by, for example, using semiconductors with small effective masses of charge carriers have so far failed to reach T above 30K (3,4). These efforts are driven by both innate desire to observe apparently fragile quantum phenomena under ambient conditions and the pragmatic need to perform metrology at room or, at least, liquid-nitrogen temperatures. More robust quantum states, implied by their persistence to higher T, would also provide added freedom to investigate finer features of the QHE and, possibly, allow higher quantization accuracy (2). Here, we show that in graphene -a single layer of carbon atoms tightly packed in a honeycomb crystal lattice -the QHE can be observed even at room temperature. This is due to the highly unusual nature of charge carriers in graphene, which behave as massless relativistic particles (Dirac fermions) and move with little scattering under ambient conditions (5). Figure 1 shows the room-T QHE in graphene. The Hall conductivity σxy reveals clear plateaux at 2e 2 /h for both electrons and holes, while the longitudinal conductivity ρxx approaches zero (<10Ω) exhibiting an activation energy ∆E ≈600K (Fig. 1B). The quantization in σxy is exact within an experimental accuracy of ≈0.2% (see Fig. 1C Fig. 1B). This implies that, in our experiments at room temperature, ω h exceeded the thermal energy kBT by a factor of 10. Importantly, in addition to the large cyclotron gap, there are a number of other factors that help the QHE in graphene to survive to so high temperatures. First, graphene devices allow for very high carrier concentrations (up to 10 13 cm -2 ) with only a single 2D subband occupied, which is essential to fully populate the lowest LL even in ultra-high B. This is in contrast to traditional 2D systems (for example, GaAs heterostructures) which are either depopulated already in moderate B or exhibit multiple subband occupation leading to the reduction of the effective energy gap to values well below ω h . Second, the mobility µ of Dirac fermions in our samples does not change appreciably from liquid-helium to room temperature. It remains at ≈10,000 cm 2 /Vs, which yields a scattering time of 13 10 − τ sec so that the high field limit 1 >> ⋅ = B µ ωτ is reached in fields of several T. These characteristics of graphene foster hopes for the room-T QHE obs...