Surface plasmons are collective oscillations of electrons in metals or semiconductors that enable confinement and control of electromagnetic energy at subwavelength scales. Rapid progress in plasmonics has largely relied on advances in device nano-fabrication, whereas less attention has been paid to the tunable properties of plasmonic media. One such medium--graphene--is amenable to convenient tuning of its electronic and optical properties by varying the applied voltage. Here, using infrared nano-imaging, we show that common graphene/SiO(2)/Si back-gated structures support propagating surface plasmons. The wavelength of graphene plasmons is of the order of 200 nanometres at technologically relevant infrared frequencies, and they can propagate several times this distance. We have succeeded in altering both the amplitude and the wavelength of these plasmons by varying the gate voltage. Using plasmon interferometry, we investigated losses in graphene by exploring real-space profiles of plasmon standing waves formed between the tip of our nano-probe and the edges of the samples. Plasmon dissipation quantified through this analysis is linked to the exotic electrodynamics of graphene. Standard plasmonic figures of merit of our tunable graphene devices surpass those of common metal-based structures.

We report on infrared (IR) nanoscopy of 2D plasmon excitations of Dirac fermions in graphene. This is achieved by confining mid-IR radiation at the apex of a nanoscale tip: an approach yielding 2 orders of magnitude increase in the value of in-plane component of incident wavevector q compared to free space propagation. At these high wavevectors, the Dirac plasmon is found to dramatically enhance the near-field interaction with mid-IR surface phonons of SiO(2) substrate. Our data augmented by detailed modeling establish graphene as a new medium supporting plasmonic effects that can be controlled by gate voltage.

Pump-probe spectroscopy is central for exploring ultrafast dynamics of fundamental excitations, collective modes, and energy transfer processes. Typically carried out using conventional diffraction-limited optics, pump-probe experiments inherently average over local chemical, compositional, and electronic inhomogeneities. Here, we circumvent this deficiency and introduce pump-probe infrared spectroscopy with ∼ 20 nm spatial resolution, far below the diffraction limit, which is accomplished using a scattering scanning near-field optical microscope (s-SNOM). This technique allows us to investigate exfoliated graphene single-layers on SiO2 at technologically significant mid-infrared (MIR) frequencies where the local optical conductivity becomes experimentally accessible through the excitation of surface plasmons via the s-SNOM tip. Optical pumping at near-infrared (NIR) frequencies prompts distinct changes in the plasmonic behavior on 200 fs time scales. The origin of the pump-induced, enhanced plasmonic response is identified as an increase in the effective electron temperature up to several thousand Kelvin, as deduced directly from the Drude weight associated with the plasmonic resonances.

Chemical functionalization is a promising route to band gap engineering of graphene. We chemically grafted nitrophenyl groups onto exfoliated single-layer graphene sheets in the form of substrate-supported or free-standing films. Our transport measurements demonstrate that nonsuspended functionalized graphene behaves as a granular metal, with variable range hopping transport and a mobility gap ~ 0.1 eV at low temperature. For suspended graphene that allows functionalization on both surfaces, we demonstrate tuning of its electronic properties from a granular metal to a gapped semiconductor, in which charge transport occurs via thermal activation over a gap ~ 80 meV. This non-invasive and scalable functionalization technique paves the way for CMOS-compatible band gap engineering of graphene electronic devices.

We report pronounced magnetoconductance oscillations observed on suspended bilayer and trilayer graphene devices with mobilities up to 270,000 cm²/V s. For bilayer devices, we observe conductance minima at all integer filling factors ν between 0 and -8, as well as a small plateau at ν=1/3. For trilayer devices, we observe features at ν=-1, -2, -3, and -4, and at ν∼0.5 that persist to 4.5 K at B=8 T. All of these features persist for all accessible values of Vg and B, and could suggest the onset of symmetry breaking of the first few Landau levels and fractional quantum Hall states.

We present a lithography-free technique for fabrication of clean, high quality graphene devices. This technique is based on evaporation through hard Si shadow masks, and eliminates contaminants introduced by lithographical processes. We demonstrate that devices fabricated by this technique have significantly higher mobility values than those obtained by standard electron beam lithography. To obtain ultra-high mobility devices, we extend this technique to fabricate suspended graphene samples with mobilities as high as 120 000 cm 2 /(V·s).

We have performed low temperature scanning tunneling spectroscopy measurements on exfoliated bilayer graphene on SiO 2 . By varying the back gate voltage we observed a linear shift of the Dirac point and an opening of a band gap due to the perpendicular electric field. In addition to observing a shift in the Dirac point, we also measured its spatial dependence using spatially resolved scanning tunneling spectroscopy. The spatial variation of the Dirac point was not correlated with topographic features and therefore we attribute its shift to random charged impurities.Monolayer graphene (MLG), which is just a single sheet of carbon atoms thick, has novel electronic properties as a consequence of its linear band structure. Stacking one more layer on top of the monolayer gives rise to bilayer graphene (BLG) that is an exciting system with a different set of tunable properties 1,2 . The bilayer structure is characterized by a quadratic dispersion relation E = ± 2 k 2 /2m with the conduction band and valence bands touching making BLG a zero band gap semiconductor. When an electric field is applied perpendicular to the plane of carbon atoms, it is possible to open up a band gap between the conduction band and valence band [3][4][5] . Recent experiments with techniques like angle resolved photoemission spectroscopy 6 , infrared spectroscopy 7-9 and transport measurements with a double-gate 10 have confirmed this band gap opening. These techniques are non-local and only provide information about the average properties of the BLG. However, from a device application perspective it is important to get details about how the spatial extent and morphology of the layers affect the electronic properties. Scanning tunneling microscopy (STM) is a powerful tool for this purpose. Previous STM studies have shown that impurites in MLG 11,12 and phonons 13 influence the chargecarrier scattering mechanisms in graphene. In this letter, we present scanning tunneling spectroscopy results for BLG on a SiO 2 substrate. These results show the spatial variation of the Dirac point as well as the control of the Dirac point and band gap due to the application of an electric field from the back gate.The BLG was prepared using the mechanical exfoliation technique 14,15 . Degenerately doped Si with 300 nm thick SiO 2 on top was used as a back gate. Bilayer areas were identified using an optical microscope and then Ti/Au electrodes were deposited using a shadow mask technique described elsewhere 16 . The device was then cooled to 4.6 K using an Omicron low temperature STM operating in ultrahigh vacuum (p ≤ 10 −11 mbar). Electrochemically etched tungsten tips that exhibited a constant density of states on a Au surface were used for a) Electronic mail: leroy@physics.arizona.edu imaging and spectroscopy to avoid unwanted tip effects. Due to the cleaner fabrication procedure, no PMMA is used, it is possible to obtain atomic resolution images over large areas of the BLG without any additional cleaning procedure unlike in previous STM measurements on exfol...

Graphene is nature's thinnest elastic membrane, and its morphology has important impacts on its electrical, mechanical, and electromechanical properties. Here we report manipulation of the morphology of suspended graphene via electrostatic and thermal control. By measuring the out-of-plane deflection as a function of applied gate voltage and number of layers, we show that graphene adopts a parabolic profile at large gate voltages with inhomogeneous distribution of charge density and strain. Unclamped graphene sheets slide into the trench under tension; for doubly clamped devices, the results are well-accounted for by membrane deflection with effective Young's modulus E = 1.1 TPa. Upon cooling to 100 K, we observe buckling-induced ripples in the central portion and large upward buckling of the free edges, which arises from graphene's large negative thermal expansion coefficient.