Polarized infrared optical emission was observed from a carbon nanotube ambipolar field-effect transistor (FET). An effective forward-biased p-n junction, without chemical dopants, was created in the nanotube by appropriately biasing the nanotube device. Electrical measurements show that the observed optical emission originates from radiative recombination of electrons and holes that are simultaneously injected into the undoped nanotube. These observations are consistent with a nanotube FET model in which thin Schottky barriers form at the source and drain contacts. This arrangement is a novel optical recombination radiation source in which the electrons and holes are injected into a nearly field-free region. Sucha source may form the basis for ultrasmall integrated photonic devices.

We measure the channel potential of a graphene transistor using a scanning photocurrent imaging technique. We show that at a certain gate bias, the impact of the metal on the channel potential profile extends into the channel for more than one-third of the total channel length from both source and drain sides; hence, most of the channel is affected by the metal. The potential barrier between the metal-controlled graphene and bulk graphene channel is also measured at various gate biases. As the gate bias exceeds the Dirac point voltage, VDirac, the original p-type graphene channel turns into a p-n-p channel. When light is focused on the p-n junctions, an impressive external responsivity of 0.001 A/W is achieved, given that only a single layer of atoms are involved in photon detection.

 These authors contributed equally to this work. * avouris@us.ibm.com, 914-945-2722 We investigate polyethylene imine and diazonium salts as stable, complementary dopants on graphene.Transport in graphene devices doped with these molecules exhibits asymmetry in electron and hole conductance. The conductance of one carrier is preserved, while the conductance of the other carrier decreases. Simulations based on nonequilibrium Green's function formalism suggest that the origin of this asymmetry is imbalanced carrier injection from the graphene electrodes caused by misalignment of the electrode and channel neutrality points.

The infrared absorption from molecular monolayers is enhanced a factor of 20 by thin metal overlayers or underlayers with use of the attenuated-total-reflection technique.The total enhancement, including contributions from the attenuated-total-reflection geometry, is almost 10 . This effect is consistent with an electric field enhancement due to collective electron resonances associated with the island nature of the thin metal films. PACS numbers: 78.30.-j The vibration@ states of molecular monolayers can be studied experimentally with inelastic electron-tunneling spectroscopy,~Raman scattering, 2 ' inelastic electron scattering, ' and infrared absorption. " In the inelastic electron-tunneling experiments it is necessary for the molecules to be sandwiched between two conducting media.Inelastic electr on-scattering experiments must be performed in vacuum and have relatively low energy resolution. Both conventional Raman scattering' and conventional infrared absorption' from monolayers must be done with high surfacearea samples, with consequent difficulties in sample char acterization, substr ate absorption and Quorescence. Baman scattering from surfaces can become quite strong with thin overlayers' or underlayers'~of metal, but so far only Ag and Au with a few molecular monolayers have shown a lar ge enhancement. The attenuatedtotal-reflectance (ATR) geometry in infrared absorption' achieves a large effective surface area without the need for powder samples. In this Letter, we report the first observation of the enhancement of infrared absorption from molecular monolayers due to thin metal overlayers and underlayers in the ATB geometry and show that this technique has application to a number of interesting current problems in surface science.The samples used in this study consisted of molecular monolayers of organic acids deposited on silicon substrates, either followed by or preceded by the evaporation of a thin metal layer. The sample preparation technique has been described by Hansma and Kirtley. ' The molecular monolayers used in this study were: 4-nitrobenzoic acid, benzoic acid, and 4-pyridine-COOH. The metal overlayers and underlayers studied were either Ag or Au and were evaporated at room temperature to an average thickness, d 100 I no Ag 80 I-LLI O 60 Z 0 I-40 0 M CO 20 I

We used the high local electric fields at the junction between the suspended and supported parts of a single carbon nanotube molecule to produce unusually bright infrared emission under unipolar operation. Carriers were accelerated by band-bending at the suspension interface, and they created excitons that radiatively recombined. This excitation mechanism is approximately 1000 times more efficient than recombination of independently injected electrons and holes, and it results from weak electron-phonon scattering and strong electron-hole binding caused by one-dimensional confinement. The ensuing high excitation density allows us to observe emission from higher excited states not seen by photoexcitation. The excitation mechanism of these states was analyzed.

Abstract.A near-field scanning optical microscope is used to locally induce photocurrent in a graphene transistor with high spatial resolution. By analyzing the spatially resolved photo-response, we find that in the n-type conduction regime a p-n-p structure forms

We measure the temperature distribution in a biased single-layer graphene transistor using Raman scattering microscopy of the 2D-phonon band. Peak operating temperatures of 1050 K are reached in the middle of the graphene sheet at 210 KW cm -2 of dissipated electric power. The metallic contacts act as heat sinks, but not in a dominant fashion. To explain the observed temperature profile and heating rate, we have to include heat-flow from the graphene to the gate oxide underneath, especially at elevated temperatures, where the graphene thermal conductivity is lowered due to umklapp scattering. Velocity saturation due to phonons with about 50 meV energy is inferred from the measured charge density via shifts in the Raman G-phonon band, suggesting that remote scattering (through field coupling) by substrate polar surface phonons increases the energy transfer to the substrate and at the same time limits the high-bias electronic conduction of graphene.

We show that the Raman frequency associated with the vibrational mode at approximately 1,580 cm(-1) (the G mode) in both metallic and semiconducting carbon nanotubes shifts in response to changes in the charge density induced by an external gate field. These changes in the Raman spectra provide us with a powerful tool for probing local doping in carbon nanotubes in electronic device structures, or charge carrier densities induced by environmental interactions, on a length scale determined by the light diffraction limit. The G mode shifts to higher frequency and narrows in linewidth in metallic carbon nanotubes at large fields. This behaviour is analogous to that observed recently in graphene. In semiconducting carbon nanotubes, on the other hand, induced changes in the charge density only shift the phonon frequency, but do not affect its linewidth. These spectral changes are quantitatively explained by a model that involves the renormalization of the carbon nanotube phonon energy by the electron-phonon interaction as the carrier density in the carbon nanotube is changed.