Sami Rosenblatt

Performance of monolayer graphene nanomechanical resonators with electrical readout

The enormous stiffness and low density of graphene make it an ideal material for nanoelectromechanical applications. Here, we demonstrate the fabrication and electrical readout of monolayer graphene resonators, and test their response to changes in mass and temperature. The devices show resonances in the megahertz range, and the strong dependence of resonant frequency on applied gate voltage can be fitted to a membrane model to yield the mass density and built-in strain of the graphene. Following the removal and addition of mass, changes in both density and strain are observed, indicating that adsorbates impart tension to the graphene. On cooling, the frequency increases, and the shift rate can be used to measure the unusual negative thermal expansion coefficient of graphene. The quality factor increases with decreasing temperature, reaching approximately 1 x 10(4) at 5 K. By establishing many of the basic attributes of monolayer graphene resonators, the groundwork for applications of these devices, including high-sensitivity mass detectors, is put in place.

Electron−Phonon Scattering in Metallic Single-Walled Carbon Nanotubes

Electron scattering rates in metallic single-walled carbon nanotubes are studied using an atomic force microscope as an electrical probe. From the scaling of the resistance of the same nanotube with length in the low- and high-bias regimes, the mean-free paths for both regimes are inferred. The observed scattering rates are consistent with calculations for acoustic-phonon scattering at low biases and zone boundary/optical phonon scattering at high biases.

Mixing at 50GHz using a single-walled carbon nanotube transistor

We have probed the electrical properties of top-gated single-walled carbon nanotube transistors at frequencies up to 50GHz by using the device as a microwave mixer. We find that the amplitude of the mixing signal decays as a function of frequency with a characteristic time constant that is limited by the setup. Despite the setup-limited cutoff frequency of ∼10GHz, we show that the devices still operate faster than 50GHz.

Electrical Nanoprobing of Semiconducting Carbon Nanotubes Using an Atomic Force Microscope

We use an atomic force microscope (AFM) tip to locally probe the electronic properties of semiconducting carbon nanotube transistors. A gold-coated AFM tip serves as a voltage or current probe in three-probe measurement setup. Using the tip as a movable current probe, we investigate the scaling of the device properties with channel length. Using the tip as a voltage probe, we study the properties of the contacts. We find that Au makes an excellent contact in the p region, with no Schottky barrier. In the n region, large contact resistances were found which dominate the transport properties.

Electrical cutting and nicking of carbon nanotubes using an atomic force microscope

In this letter, we demonstrate that voltage pulses from a metal-coated AFM tip can be used to permanently modify the electrical properties of NT devices. By adjusting the properties of the voltage pulses, we can either electrically break ~‘‘cut’’ ! NTs or create tunneling barriers ~‘‘nick’’ ! at any point along them. We demonstrate the utility of these techniques by creating single NT devices through the cutting of unwanted extra NTs, and making ultrasmall NT quantum dots can be created by nicking a NT at two places along its length. The NT devices used in this work were prepared following an approach similar to that of Kong et al. 21 First, catalyst islands containing Fe(NO 3 ) 3 i9H 2 O, MoO 2 (acac) 2 and alumina nanoparticles were defined on a degenerately doped silicon wafer with 200-nm-thick thermally grown oxide. Photolithography and etching were used to pattern a poly~methylmethacrylate! layer, which was subsequently used as a lift-off mask for the catalyst. NTs were then grown by chemical vapor deposition. 21 Metal electrodes consisting of Cr~ 5n m! and Au~50 nm! were patterned over the catalyst islands using photolithography and a lift-off process, with a spacing between source and drain electrodes between 1 and 3 mm. This process allowed the parallel production of hundreds of NT devices using only optical lithography. The samples were annealed at 600 °C for 45 min in an Ar environment to decrease the contact resistance between the NTs and the electrodes. Typical NT conductances obtained were 0.2‐ 2e 2 /h.

Determination of the Young's Modulus of Structurally Defined Carbon Nanotubes

We have combined optical characterization with a magnetic actuation technique to measure the stiffness of single-walled carbon nanotubes of defined crystal structure. The measured stiffnesses correspond to an average Youngs modulus of E = 0.97 +/- 0.16 TPa. For the structures investigated, no dependence on the nanotube chiral index was observed within the indicated experimental accuracy.

Controlled dielectrophoretic assembly of carbon nanotubes using real-time electrical detection

We investigate dielectrophoretic deposition of single-walled carbon nanotubes using an in situ detection system. Pairs of electrodes are stimulated with a small-amplitude, low-frequency voltage superimposed on a large-amplitude, high-frequency dielectrophoretic voltage. Measuring the magnitude of the current both at dc (Idc) and at the low frequency (Iac) through a digital lock-in technique allows us to determine when a nanotube has made electrical contact and to halt the dielectrophoretic process. Because Idc is determined by nonlinearities in the device current-voltage characteristic, measurement of the Idc/Iac ratio allows the real-time determination of whether the deposited nanotube is metallic or semiconducting.

NEMS applications of graphene

Graphene, which consists of a single atomic sheet of graphene, possesses high electronic mobility, and excellent mechanical properties, making it an ideal candidate for nanoelectromechanical (NEMS) applications, including sensing and signal processing. Toward these applications, we have measured the mechanical properties of graphene sheets and demonstrated fabrication and electrical readout of monolayer graphene NEMS that show vibrational resonances in the MHz range. The dependence of the resonant frequency on applied gate voltage yields the mass density and built-in strain. Upon addition of mass, we observe changes in both the density and the strain, indicating that adsorbates impart tension to the graphene. The quality factor increases monotonically with decreasing temperature and reaches ∼104 at 5 K.

X-ray optics fabricated by deep reactive ion etching (invited)

When hard x-ray focusing optics require segmented bending, the focal spot can be minimized by matching segment size to the synchrotron source size and focal ratio. At modern sources this implies features etched or cut into silicon on the 100 μm scale. We describe our experience using photolithography and high aspect ratio reactive ion etching (RIE) to produce sagittal focusing crystals. The work was carried out at the Cornell Nanofabrication Facility. RIE has been of great utility for the manufacture of microelectromechanical systems (MEMS) which contain features typically a few microns in size. We have learned that because of the very large dielectric constant of silicon, a number of important considerations must go into a successful design when features are scaled up for x-ray optics applications. Nevertheless, we have designed, built and tested x-ray optical devices produced using MEMS techniques, and in fact they are in regular use at CHESS. We describe our results from the point of view of mechanical...