Carbon nanotube transistors for biosensing applications.

Electronic detection of biomolecules, although still in its early stages, is gradually emerging as an effective alternative to optical detection methods. We describe field effect transistor devices with carbon nanotube conducting channels that have been developed and used for biosensing and biodetection. Both transistors with single carbon nanotube conducting channels and devices with nanotube network conducting channels have been fabricated and their electronic characteristics examined. The devices readily respond to changes in the environment, and such effects have been examined using gas molecules and coatings with specific properties. Device operation in (conducting) buffer and in a dry environment--after buffer removal--is also discussed. Applications in the biosensing area are illustrated with three examples: the investigation of the interaction between devices and biomolecules, the electronic monitoring of biomolecular processes, and attempts to integrate cell membranes with active electronic devices.

Integration of cell membranes and nanotube transistors.

We report the integration of a complex biological system and a nanoelectronic device, demonstrating that both components retain their functionality while interacting with each other. As the biological system, we use the cell membrane of Halobacterium salinarum. As the nanoelectronic device, we use a nanotube network transistor, which incorporates many individual nanotubes in such a way that entire patches of cell membrane are contacted by nanotubes. We demonstrate that the biophysical properties of the membrane are preserved, that the nanoelectronic devices still function as transistors, and that the two systems interact. Further, we use the interaction to study the charge distribution in the biological system, finding that the electric dipole of the membrane protein bacteriorhodopsin is located 2/3 of the way from the extracellular to the cytoplasmic side.

Transparent and flexible carbon nanotube transistors.

We report the fabrication of transparent and flexible transistors where both the bottom gate and the conducting channel are carbon nanotube networks of different densities and Parylene N is the gate insulator. Device mobilities of 1 cm(2) V(-1) s(-1) and on/off ratios of 100 are obtained, with the latter influenced by the properties of the insulating layer. Repetitive bending has minor influence on the characteristics, with full recovery after repeated bending. The operation is insensitive to visible light and the gating does not influence the transmission in the visible spectral range.

Electrodynamics of a Coulomb glass in n-type silicon.

Measurements of the complex frequency dependent conductivity of uncompensated n-type silicon are reported. The experiments are done in the quantum limit, variant Planck's over 2pi omega>k(B)T, across a broad doping range on the insulating side of the metal-insulator transition. The low energy linear frequency dependence is consistent with theories of a Coulomb glass, but discrepancies exist in the relative magnitudes of the complex components. At higher energies we observe a crossover to a quadratic frequency dependence that is sharper than expected. The concentration dependence gives evidence that the Coulomb interaction energy is the energy scale that determines this crossover.

Nature of heavy quasiparticles in magnetically ordered heavy fermions UPd(2)Al(3) and UPt(3).

The optical conductivity of the heavy fermions UPd(2)Al(3) and UPt(3) has been measured in the energy range from 0.04 to 5 meV. In both compounds a well pronounced pseudogap of less than 1 meV develops in the optical response at low temperatures; we relate this to the antiferromagnetic ordering. From the energy dependence of the effective mass and scattering rate we conclude that the enhancement of the mass mainly occurs below the energy which is related to magnetic correlations between the local magnetic moments and the itinerant electrons. This implies that the magnetic order in these compounds is the prerequisite to the formation of the heavy quasiparticles and eventually to superconductivity.

Measurements of the complex conductivity of NbxSi(1-x) alloys on the insulating side of the metal-insulator transition.

We have conducted temperature and frequency dependent transport measurements in amorphous NbxSi1-x samples in the insulating regime. We find a temperature dependent dc conductivity consistent with variable range hopping in a Coulomb glass. The frequency dependent response in the millimeter-wave frequency range can be described by the expression sigma(omega) varies with (-iota omega)(alpha) with the exponent somewhat smaller than 1. Our ac results are not consistent with extant theories for the hopping transport.

Fluctuation-facilitated charge migration along DNA.

We propose a model Hamiltonian for charge transfer along the DNA double helix with temperature-driven fluctuations in the base pair positions acting as the rate limiting factor for charge transfer between neighboring base pairs. We compare the predictions of the model with the recent work of Barton and Zewail on the unusual two-stage charge transfer of DNA.

Dynamical properties of the one-dimensional band insulator (NbSe4)3I

Optical and photoemission experiments reveal unexpected spectral signatures of one-dimensional band insulators. In the model compound (NbSe (4))3I the optical conductivity decays as a power law sigma(1)(omega) approximately omega(-4.25) above a sharp gap edge. Photoemission observes both the valence and a shadow band, produced by a commensurate superstructure. We identify an optical and photoemission band gap consistent with other measurements but much smaller than the energy scale defined by the dispersion of the band peak in the photoemission spectra.

Electronic structure of solids with competing periodic potentials.

When electrons are subject to a potential with two incommensurate periods, translational invariance is lost, and no periodic band structure is expected. However, model calculations based on nearly free one-dimensional electrons and experimental results from high-resolution photoemission spectroscopy on a quasi-one-dimensional material do show dispersing band states with signatures of both periodicities. Apparent band structures are generated by the nonuniform distribution of electronic spectral weight over the complex eigenvalue spectrum.

Charge transport along the lambda-DNA double helix.

We have measured the conductivity sigma along the lambda phage DNA (lambda-DNA) double helix at microwave frequencies using lyophilized DNA in and also without a buffer. The conductivity is strongly temperature dependent around room temperature with a crossover to a weakly temperature dependent conductivity at low temperatures. Removal of the water mantle around the double helix leads to reduced conductivity.

Quantum-critical conductivity scaling for a metal-insulator transition

Temperature (T)- and frequency (omega)-dependent conductivity measurements are reported here in amorphous niobium-silicon alloys with compositions (x) near the zero-temperature metal-insulator transition. There is a one-to-one correspondence between the frequency- and temperature-dependent conductivity on both sides of the critical concentration, thus establishing the quantum-critical nature of the transition. The analysis of the conductivity leads to a universal scaling function and establishes the critical exponents. This scaling can be described by an x-, T-, and omega-dependent characteristic length, the form of which is derived by experiment.

Dimensionality-driven insulator-to-metal transition in the bechgaard salts

Optical experiments were conducted on a series of organic linear chain conductors with different values of the interchain single-electron transfer integral tb, which quantifies the degree of anisotropy. Electron-electron interactions together with Umklapp scattering resulted in a correlation gap and an insulating state for small tb. An insulator-to-metal transition was observed when tb exceeded a critical value, on the order of the correlation gap Egap. The absence of a plasma edge on the insulator side of the transition for polarization perpendicular to the chains suggests that the electrons are confined to the chains. The optical features of the metallic state, when contrasted with the magnetic properties, are suggestive of spin-charge separation.