From the wide spectrum of potential applications of graphene, ranging from transistors and chemical sensors to nanoelectromechanical devices and composites, the field of photonics and optoelectronics is believed to be one of the most promising. Indeed, graphene's suitability for high-speed photodetection was demonstrated in an optical communication link operating at 10 Gbit s − 1 . However, the low responsivity of graphene-based photodetectors compared with traditional III-V-based ones is a potential drawback. Here we show that, by combining graphene with plasmonic nanostructures, the efficiency of graphene-based photodetectors can be increased by up to 20 times, because of efficient field concentration in the area of a p-n junction. Additionally, wavelength and polarization selectivity can be achieved by employing nanostructures of different geometries.

In this letter, a top-gated field effect device (FED) manufactured from monolayer graphene is investigated.Except for graphene deposition, a conventional top-down CMOS-compatible process flow is applied.Carrier mobilities in graphene pseudo-MOS structures are compared to those obtained from top-gated Graphene-FEDs. The extracted values exceed the universal mobility of silicon and silicon-on-insulator MOSFETs.

Using scanning tunneling microscopy in an ultrahigh vacuum and atomic force microscopy, we investigate the corrugation of graphene flakes deposited by exfoliation on a Si/SiO2 (300 nm) surface. While the corrugation on SiO2 is long range with a correlation length of about 25 nm, some of the graphene monolayers exhibit an additional corrugation with a preferential wavelength of about 15 nm. A detailed analysis shows that the long-range corrugation of the substrate is also visible on graphene, but with a reduced amplitude, leading to the conclusion that the graphene is partly freely suspended between hills of the substrate. Thus, the intrinsic rippling observed previously on artificially suspended graphene can exist as well, if graphene is deposited on SiO2.

Graphene's high mobility and Fermi velocity, combined with its constant light absorption in the visible to far-infrared range, make it an ideal material to fabricate high-speed and ultrabroadband photodetectors. However, the precise mechanism of photodetection is still debated. Here, we report wavelength and polarization-dependent measurements of metal−graphene−metal photodetectors. This allows us to quantify and control the relative contributions of both photothermo-and photoelectric effects, both adding to the overall photoresponse. This paves the way for a more efficient photodetector design for ultrafast operating speeds.KEYWORDS: Graphene, photodetectors, Raman spectroscopy, photoresponse, optoelectronics T he unique optical and electronic properties of graphene make it ideal for photonics and optoelectronics. 1 A variety of prototype devices have already been demonstrated, such as transparent electrodes in displays 2 and photovoltaic modules, 3 optical modulators, 4 plasmonic devices, 4−9 microcavities, 10,11 and ultrafast lasers. 12 Among these, a significant effort is being devoted to photodetectors (PDs). 6,10,11,13−25 Various photodetection schemes and architectures have been proposed to date. The simplest configuration is the metal− graphene−metal (MGM) PD, in which graphene is contacted with metal electrodes as the source and drain. 13−18 These PDs can be combined with metal nanostructures enabling local surface plasmons and increased absorption, realizing an enhancement in responsivity (i.e., the ratio of the lightgenerated electrical current to the incident light power). 6,26 Microcavity based PDs were also used, with increased light absorption at the cavity resonance frequency, again achieving an increase in responsivity. 10,11 Another scheme is the integration of graphene with a waveguide to increase the effective interaction length with light. 25,27 Hybrid approaches employ semiconducting nanodots as light-absorbing media. 22 In this case, light excites electron−hole (e−h) pairs in the nanodots; the electrons are trapped in the nanodot, while the holes are transferred to graphene, thus effectively gating it. 22 Under applied drain−source bias, this results in a shift in the Dirac point, thus a modulation of the drain−source current. 22 Due to the long trapping time of the electrons within the dot, the transferred holes can cycle many times through the phototransistor before relaxation and e−h recombination. This gives a photoconductive gain; i.e., one absorbed photon effectively results in an electrical current of several electrons. Responsivities >10 7 A/W were reported, 22 but with a millisecond time scale, not suitable for, e.g., high-speed optical communications. Devices were also fabricated for detection of THz light. 28,29 In this low energy range, Pauli blocking forbids the direct excitation of e−h pairs due to finite doping. Instead, an antenna coupled to source and gate of the device excites plasma waves within the channel. These are rectified, leading to a detectable dc out...

The truly two-dimensional material graphene is an ideal candidate for nanoelectromechanics due to its large strength and mobility. Here we show that graphene flakes provide natural nanomembranes of diameter down to 3 nm within its intrinsic rippling. The membranes can be lifted either reversibly or hysteretically by the tip of a scanning tunneling microscope. The clamped-membrane model including van-der-Waals and dielectric forces explains the results quantitatively. AC-fields oscillate the membranes, which might lead to a completely novel approach to controlled quantized oscillations or single atom mass detection.

The absence of a band gap in graphene restricts its straight forward application as a channel material in field effect transistors. In this letter, we report on a new approach to engineer a band gap in graphene field effect devices (FED) by controlled structural modification of the graphene channel itself. The conductance in the FEDs is switched between a conductive "onstate" to an insulating "off-state" with more than six orders of magnitude difference in conductance. Above a critical value of an electric field applied to the FED gate under certain environmental conditions, a chemical modification takes place to form insulating graphene derivatives. The effect can be reversed by electrical fields of opposite polarity or short current pulses to recover the initial state. These reversible switches could potentially be applied to non-volatile memories and novel neuromorphic processing concepts.

The combination of plasmonic nanoparticles and graphene enhances the responsivity and spectral selectivity of graphene-based photodetectors. However, the small area of the metal-graphene junction, where the induced electron-hole pairs separate, limits the photoactive region to sub-micron length scales. Here, we couple graphene with a plasmonic grating and exploit the resulting surface plasmon polaritons to deliver the collected photons to the junction region of a metal-graphenemetal photodetector. This results into a 400% enhancement of responsivity and a 1000% increase in photoactive length, combined with tunable spectral selectivity. The interference between surface plasmon polaritons and the incident wave introduces new functionalities, such as light flux attraction or repulsion from the contact edges, enabling the tailored design of the photodetector's spectral response. This architecture can also be used for surface plasmon bio-sensing with direct-electricreadout, eliminating the need of complicated optics.Graphene-based photodetectors (PDs) [1,2] have been reported with ultra-fast operating speeds (up to 262GHz from the measured intrinsic response time of graphene carriers [3]) and broadband operation from the visible and infrared [3][4][5][6][7][8][9][10][11][12][13][14][15][16] up to the THz [17][18][19]. The simplest graphene-based photodetection scheme relies on the metal-graphene-metal (MGM) architecture [5,7,8,11,[20][21][22], where the photoresponse is due to a combination of photo-thermoelectric and photovoltaic effects [5,7,8,11,[20][21][22]. For both mechanisms, the presence of a junction is required to spatially separate excited electronhole (e-h) pairs [5,7,8,11,[20][21][22]. At the metal-graphene junction, a work-function difference causes charge transfer and a shift of the graphene Fermi level underneath the contact [4,5,7,23], compared to that of graphene away from the contact [4,5,7,23], resulting into a build-up of an internal electric field (photovoltaic mechanism) [5,7,24,25] and into a difference of Seebeck coefficients (photo-thermoelectric mechanism) [11,21,26]. An alternative way to create a junction is to use a set of gate electrodes to electrostatically dope graphene [8,11].

We report on a method for the fabrication of graphene on a silicon dioxide substrate by solid-state dissolution of an overlying stack of a silicon carbide and a nickel thin film. The carbon dissolves in the nickel by rapid thermal annealing. Upon cooling, the carbon segregates to the nickel surface forming a graphene layer over the entire nickel surface. By wet etching of the nickel layer, the graphene layer was allowed to settle on the original substrate. Scanning tunneling microscopy (STM) as well as Raman spectroscopy has been performed for characterization of the layers. Further insight into the morphology of the layers has been gained by Raman mapping indicating micrometer-size graphene grains. Devices for electrical measurement have been manufactured exhibiting a modulation of the transfer current by backgate electric fields. The presented approach allows for mass fabrication of polycrystalline graphene without transfer steps while using only CMOS compatible process steps.