Prasoon Joshi

n-Type Behavior of Graphene Supported on Si/SiO2 Substrates

Results are presented from an experimental and theoretical study of the electronic properties of back-gated graphene field effect transistors (FETs) on Si/SiO(2) substrates. The excess charge on the graphene was observed by sweeping the gate voltage to determine the charge neutrality point in the graphene. Devices exposed to laboratory environment for several days were always found to be initially p-type. After approximately 20 h at 200 degrees C in approximately 5 x 10(-7) Torr vacuum, the FET slowly evolved to n-type behavior with a final excess electron density on the graphene of approximately 4 x 10(12) e/cm(2). This value is in excellent agreement with our theoretical calculations on SiO(2), where we have used molecular dynamics to build the SiO(2) structure and then density functional theory to compute the electronic structure. The essential theoretical result is that the SiO(2) has a significant surface state density just below the conduction band edge that donates electrons to the graphene to balance the chemical potential at the interface. An electrostatic model for the FET is also presented that produces an expression for the gate bias dependence of the carrier density.

Intrinsic doping and gate hysteresis in graphene field effect devices fabricated on SiO2 substrates

We have studied the intrinsic doping level and gate hysteresis of graphene-based field effect transistors (FETs) fabricated over Si/SiO(2) substrates. It was found that the high p-doping level of graphene in some as-prepared devices can be reversed by vacuum degassing at room temperature or above depending on the degree of hydrophobicity and/or hydration of the underlying SiO(2) substrate. Charge neutrality point (CNP) hysteresis, consisting of the shift of the charge neutrality point (or Dirac peak) upon reversal of the gate voltage sweep direction, was also greatly reduced upon vacuum degassing. However, another type of hysteresis, consisting of the change in the transconductance upon reversal of the gate voltage sweep direction, persists even after long-term vacuum annealing at 200 °C, when SiO(2) surface-bound water is expected to be desorbed. We propose a mechanism for this transconductance hysteresis that involves water-related defects, formed during the hydration of the near-surface silanol groups in the bulk SiO(2), that can act as electron traps.

On the Possibility of a Graphene Based Chemical Sensor

The availability of 2D-nanomaterials such as graphene flakes provides a new technique for chemical sensing in which the conductivity of the graphene film serving as the sensor material can be directly modulated in the presence of chemicals as in chemistors. In this paper we investigate the potential of atomically thin graphene flakes for chemical sensing application using gated field effect response of the material.

Micromachined quartz resonator functionalized with single walled carbon nanotubes

Single walled carbon nanotubes (SWNTs) are reservoirs of gases as they can adsorb on their walls as well as retain gas molecules in their hollow cylindrical interior. This has important applications for example in fuel cell technology for hydrogen storage, as a gas sensor for realization of artificial nose, etc. Storage of gases by carbon nanotubes have been recently investigated by monitoring changes in their thermoelectric power and electrical resistivity due to their interaction with gas molecules. In this paper we present a gravimetric study of interaction of gas molecules with isolated SWNTs using a micromachined ultrasensitive quartz crystal microbalance (QCM). The adsorption and desorption of gas molecules with different molecular weights from carbon nanotubes revealed that changes in resonance frequency and quality factor of the resonating crystal scale as approximately M0.45, where M is the mass the of the gas molecule as compared to M1/3 dependence observed in case of changes in thermoelectric power and electrical resistance for thin films of the carbon nanotubes. The use of QCM enables room temperature probing of gas interaction with isolated single walled carbon nanotubes. Specific interaction of gases with carbon nanotubes on QCM provides potential application of the device as a gas sensor

Improvement of the elastic modulus of micromachined structures using carbon nanotubes

It has been shown that the addition of single walled carbon anotubes (SWNTs) cause an increase in the resonance frequency of micromachined clamped-clamped structures. This is believed to be due to an increase in the effective stiffness of the micromachined structures due to the high Youngs modulus of carbon nanotubes. These results were obtained in spite of a relatively poor control over the orientation and aerial density of the deposited SWNTs. Finite element simulations showed an increase in the resonance frequency of up to ~25% for the simulated devices. This increase in the resonance frequency of the bridges can be attributed to the high Youngs modulus (~1TPa) of the carbon nanotubes.

Electrical properties of back-gated n -layer graphene films

We present results of room temperature studies of the electrical characteristics of back-gated ultrathin graphite films prepared by mechanical transfer of thin sections of Highly Oriented Pyrolytic Graphite (HOPG) to a Si/SiO2 substrate. The films studied were quite thin, exhibiting only a few graphene layers (n). Films with thickness in the range 1 < n < 20 were studied, where n has been deduced by Atomic Force Microscopy (AFM) z-scans. The n value deduced by AFM z-scan data was correlated with the n value deduced by Raman scattering data. We discuss at some length, the issue of whether or not Raman scattering can provide a standalone measure of n. Electrical contacts were made to a few of the low n (n = 1,2,3) graphene films. Most graphene films exhibited a nearly symmetric resistance (R) anomaly vs. gate voltage (VG) in the range 25 < VG < 110 V; some films exhibited as much as a factor of ~50 decrease in R (relative to the maximum R) with changing VG. An interesting low bias shoulder on the negative side of the resistance peak anomaly was also observed. The devices were fabricated with a lithography free process.