Controlling inelastic light scattering quantum pathways in graphene.

Inelastic light scattering spectroscopy has, since its first discovery, been an indispensable tool in physical science for probing elementary excitations, such as phonons, magnons and plasmons in both bulk and nanoscale materials. In the quantum mechanical picture of inelastic light scattering, incident photons first excite a set of intermediate electronic states, which then generate crystal elementary excitations and radiate energy-shifted photons. The intermediate electronic excitations therefore have a crucial role as quantum pathways in inelastic light scattering, and this is exemplified by resonant Raman scattering and Raman interference. The ability to control these excitation pathways can open up new opportunities to probe, manipulate and utilize inelastic light scattering. Here we achieve excitation pathway control in graphene with electrostatic doping. Our study reveals quantum interference between different Raman pathways in graphene: when some of the pathways are blocked, the one-phonon Raman intensity does not diminish, as commonly expected, but increases dramatically. This discovery sheds new light on the understanding of resonance Raman scattering in graphene. In addition, we demonstrate hot-electron luminescence in graphene as the Fermi energy approaches half the laser excitation energy. This hot luminescence, which is another form of inelastic light scattering, results from excited-state relaxation channels that become available only in heavily doped graphene.

Direct observation of a widely tunable bandgap in bilayer graphene.

The electronic bandgap is an intrinsic property of semiconductors and insulators that largely determines their transport and optical properties. As such, it has a central role in modern device physics and technology and governs the operation of semiconductor devices such as p-n junctions, transistors, photodiodes and lasers. A tunable bandgap would be highly desirable because it would allow great flexibility in design and optimization of such devices, in particular if it could be tuned by applying a variable external electric field. However, in conventional materials, the bandgap is fixed by their crystalline structure, preventing such bandgap control. Here we demonstrate the realization of a widely tunable electronic bandgap in electrically gated bilayer graphene. Using a dual-gate bilayer graphene field-effect transistor (FET) and infrared microspectroscopy, we demonstrate a gate-controlled, continuously tunable bandgap of up to 250 meV. Our technique avoids uncontrolled chemical doping and provides direct evidence of a widely tunable bandgap-spanning a spectral range from zero to mid-infrared-that has eluded previous attempts. Combined with the remarkable electrical transport properties of such systems, this electrostatic bandgap control suggests novel nanoelectronic and nanophotonic device applications based on graphene.

Observation of carrier-density-dependent many-body effects in graphene via tunneling spectroscopy.

We find the scanning tunneling spectra of backgated graphene monolayers to be significantly altered by many-body excitations. Experimental features in the spectra arising from electron-plasmon interactions show carrier density dependence, distinguishing them from density-independent electron-phonon features. Using a straightforward model, we are able to calculate theoretical tunneling spectra that agree well with our data, providing insight into the effects of many-body interactions on the lifetime of graphene quasiparticles.

Graphene plasmonics for tunable terahertz metamaterials.

Plasmons describe collective oscillations of electrons. They have a fundamental role in the dynamic responses of electron systems and form the basis of research into optical metamaterials. Plasmons of two-dimensional massless electrons, as present in graphene, show unusual behaviour that enables new tunable plasmonic metamaterials and, potentially, optoelectronic applications in the terahertz frequency range. Here we explore plasmon excitations in engineered graphene micro-ribbon arrays. We demonstrate that graphene plasmon resonances can be tuned over a broad terahertz frequency range by changing micro-ribbon width and in situ electrostatic doping. The ribbon width and carrier doping dependences of graphene plasmon frequency demonstrate power-law behaviour characteristic of two-dimensional massless Dirac electrons. The plasmon resonances have remarkably large oscillator strengths, resulting in prominent room-temperature optical absorption peaks. In comparison, plasmon absorption in a conventional two-dimensional electron gas was observed only at 4.2 K (refs 13, 14). The results represent a first look at light-plasmon coupling in graphene and point to potential graphene-based terahertz metamaterials.

Transfer-free batch fabrication of large-area suspended graphene membranes.

We demonstrate a process for batch production of large-area (100-3000 microm(2)) patterned free-standing graphene membranes on Cu scaffolds using chemical vapor deposition (CVD)-grown graphene. This technique avoids the use of silicon and transfers of graphene. As one application of this technique, we fabricate transmission electron microscopy (TEM) sample supports. TEM characterization of the graphene membranes reveals relatively clean, highly TEM-transparent, single-layer graphene regions ( approximately 50% by area) and, despite the polycrystalline nature of CVD graphene, membrane yields as high as 75-100%. This high yield verifies that the intrinsic strength and integrity of CVD-grown graphene films is sufficient for sub-100 microm width membrane applications. Elemental analysis (electron energy loss spectroscopy (EELS) and X-ray energy-dispersive spectroscopy (EDS)) of the graphene membranes reveals some nanoscaled contamination left over from the etching process, and we suggest several ways to reduce this contamination and improve the quality of the graphene for electronic device applications. This large-scale production of suspended graphene membranes facilitates access to the two-dimensional physics of graphene that are suppressed by substrate interactions and enables the widespread use of graphene-based sample supports for electron and optical microscopy.

A tunable phonon-exciton Fano system in bilayer graphene.

Fano resonances are features in absorption, scattering or transport spectra resulting from the interaction of discrete and continuum states. They have been observed in a variety of systems. Here, we report a many-body Fano resonance in bilayer graphene that is continuously tunable by means of electrical gating. Discrete phonons and continuous exciton (electron-hole pair) transitions are coupled by electron-phonon interactions, yielding a new hybrid phonon-exciton excited state. It may also be possible to control the phonon-exciton coupling with an optical field. This tunable phonon-exciton system could allow novel applications such as phonon lasers.

Tunable graphene dc superconducting quantum interference device.

Graphene exhibits unique electrical properties on account of its reduced dimensionality and "relativistic" band structure. When contacted with two superconducting electrodes, graphene can support Cooper pair transport, resulting in the well-known Josephson effect. We report here the fabrication and operation of a two junction dc superconducting quantum interference device (SQUID) formed by a single graphene sheet contacted with aluminum/palladium electrodes in the geometry of a loop. The supercurrent in this device can be modulated not only via an electrostatic gate but also by an applied magnetic fielda potentially powerful probe of electronic transport in graphene and an ultrasensitive platform for nanomagnetometry.

Graphene at the edge: stability and dynamics.

Although the physics of materials at surfaces and edges has been extensively studied, the movement of individual atoms at an isolated edge has not been directly observed in real time. With a transmission electron aberration-corrected microscope capable of simultaneous atomic spatial resolution and 1-second temporal resolution, we produced movies of the dynamics of carbon atoms at the edge of a hole in a suspended, single atomic layer of graphene. The rearrangement of bonds and beam-induced ejection of carbon atoms are recorded as the hole grows. We investigated the mechanism of edge reconstruction and demonstrated the stability of the "zigzag" edge configuration. This study of an ideal low-dimensional interface, a hole in graphene, exhibits the complex behavior of atoms at a boundary.

Gate-variable optical transitions in graphene.

Two-dimensional graphene monolayers and bilayers exhibit fascinating electrical transport behaviors. Using infrared spectroscopy, we find that they also have strong interband transitions and that their optical transitions can be substantially modified through electrical gating, much like electrical transport in field-effect transistors. This gate dependence of interband transitions adds a valuable dimension for optically probing graphene band structure. For a graphene monolayer, it yields directly the linear band dispersion of Dirac fermions, whereas in a bilayer, it reveals a dominating van Hove singularity arising from interlayer coupling. The strong and layer-dependent optical transitions of graphene and the tunability by simple electrical gating hold promise for new applications in infrared optics and optoelectronics.