The high carrier mobility of graphene has been exploited in field-effect transistors that operate at high frequencies. Transistors were fabricated on epitaxial graphene synthesized on the silicon face of a silicon carbide wafer, achieving a cutoff frequency of 100 gigahertz for a gate length of 240 nanometers. The high-frequency performance of these epitaxial graphene transistors exceeds that of state-of-the-art silicon transistors of the same gate length.

We observe subpicosecond charge separation and formation of indirect excitons a van der Waals heterostructure formed by molybdenum disulfide and molybdenum diselenide monolayers. The sample is fabricated by manually stacking monolayer MoS2 and MoSe2 flakes prepared by mechanical exfoliation. Photoluminescence measurements confirm the formation of an effective heterojunction. In the transient absorption measurements, an ultrafast laser pulse resonantly injects excitons in the MoSe2 layer of the heterostructure. Differential reflection of a probe pulse tuned to the MoS2 exciton resonance is immediately observed following the pump excitation. This proves ultrafast transfer of electrons from MoSe2 to MoS2 layers, despite the strong Coulomb attraction from the holes in the resonantly excited excitons. Conversely, when excitons are selectively injected in MoS2, holes transfer to MoSe2 on an ultrafast time scale, too, as observed by measuring the differential reflection of a probe tuned to the MoSe2 resonance. The ultrafast charge transfer process is followed by the formation of spatially indirect excitons with electrons and holes residing in different layers. The lifetime of these indirect excitons are found to be longer than that of the direct excitons in individual MoS2 and MoSe2 monolayers.

Ultraminiaturized mass spectrometers are highly sought-after tools, with numerous applications in areas such as environmental protection, exploration, and drug development. We realize atomic scale mass sensing using doubly clamped suspended carbon nanotube nanomechanical resonators, in which their single-electron transistor properties allows self-detection of the nanotube vibration. We use the detection of shifts in the resonance frequency of the nanotubes to sense and determine the inertial mass of atoms as well as the mass of the nanotube. This highly sensitive mass detection capability may eventually enable applications such as on-chip detection, analysis, and identification of compounds.Because of their small size, nanoscale systems are highly sensitive to their environment, affording great potential for sensing applications. Chemical and biological charge-based sensors using carbon nanotubes 1,2 and nanowires 3 have already been demonstrated. Nanoelectromechanical systems have been proposed for highly sensitive mass detection of neutral species, 4,5 and significant progress has been made in using nanofabricated resonators 6-8 and carbon nanotubes 9-12 for mass sensing. However, the ultimate limit of atomic resolution mass sensitivity remains unrealized. Here we demonstrate that individual double-clamped single-walled carbon nanotube resonators are capable of atomic-scale mass sensing and determining the inertial mass of atomic species. Analysis of our data also yields the nanotube mass, giving a general approach to determining the mass of molecular nanostructures. Our results open the door to ultraminiaturized mass spectroscopy or arrays, potentially enabling on-chip detection and identification of unknown analytes, as well as the study of single atom adsorption and desorption.We fabricate our devices by first growing nanotubes from catalyst islands 13 under either pure CH 4 13 or a CH 4 /H 2 mixture 14 on an oxidized Si wafer. We then attach Pd/Au source/drain electrodes and a side gate using electron beam lithography (EBL). In a second step, a layer of poly(methyl methacrylate) (PMMA) is spun on the chip, and a ∼450 nm window is opened over a segment of the nanotube between the electrodes, again using EBL. The SiO 2 is etched using buffered HF to suspend the carbon nanotube within the window. Following a critical point drying step, the samples are electrically tested at room temperature; we then load selected devices into a custom-built cryostat, which is maintained at a high vacuum. Figure 1a shows a scanning electron microscope (SEM) image of a completed suspended nanotube transistor device.At the temperature T ≈ 6 K of our experiment, the nanotubes act as single-electron transistors (SET) in which the charge on the nanotube is an integer multiple of the electron charge e. 15 As the gate voltage V g is swept, electrons enter the SET one at a time, with each transition between discrete charge states producing a Coulomb peak in the source-drain conductance (see e.g. ref 16). Figure 1b shows this b...

We utilize an organic polymer buffer layer between graphene and conventional gate dielectrics in top-gated graphene transistors. Unlike other insulators, this dielectric stack does not significantly degrade carrier mobility, allowing for high field-effect mobilities to be retained in top-gate operation. This is demonstrated in both two-point and four-point analysis and in the high-frequency operation of a graphene transistor. Temperature dependence of the carrier mobility suggests that phonons are the dominant scatterers in these devices.

The high carrier mobility and thermal conductivity of graphene make it a candidate material for future high-speed electronic devices. Although the thermal behaviour of high-speed devices can limit their performance, the thermal properties of graphene devices remain incompletely understood. Here, we show that spatially resolved thermal radiation from biased graphene transistors can be used to extract the temperature distribution, carrier densities and spatial location of the Dirac point in the graphene channel. The graphene exhibits a temperature maximum with a location that can be controlled by the gate voltage. Stationary hot spots are also observed. Infrared emission represents a convenient and non-invasive characterization tool for graphene devices.