This work details a method to make efficacious field-effect transistors from monolayers of polycyclic aromatic hydrocarbons that are able to sense and respond to their chemical environment. The molecules used in this study are functionalized so that they assemble laterally into columns and attach themselves to the silicon oxide surface of a silicon wafer. To measure the electrical properties of these monolayers, we use ultrasmall point contacts that are separated by only a few nanometers as the source and drain electrodes. These contacts are formed through an oxidative cutting of an individual metallic single-walled carbon nanotube that is held between macroscopic metal leads. The molecules assemble in the gap and form transistors with large current modulation and high gate efficiency. Because these devices are formed from an individual stack of molecules, their electrical properties change significantly when exposed to electron-deficient molecules such as tetracyanoquinodimethane (TCNQ), forming the basis for new types of environmental and molecular sensors.chemistry ͉ electronic materials ͉ nanoscience ͉ self-assembly T his work details a method to make chemoresponsive transistors by making devices out of a monolayer of polycyclic aromatic hydrocarbons that are chemically attached to surfaces. The devices are formed through a self-assembly process of organic semiconductors on the oxide surface of a silicon wafer (Fig. 1A) (1, 2). Previous studies on organic field-effect transistors (OFETs) (3, 4) have shown that the path for electrical current is through at most the first few layers of molecules at the oxide interface (5-7). In general, when the semiconducting layers of typical OFETs are scaled down to a monolayer, their properties become poor, presumably due to discontinuities or defects in the films (8-11). The strategy used here circumvents this problem by a chemical functionalization of the molecular semiconductors ( Fig. 1B) so that they both assemble laterally and chemically attach themselves to the substrate (Fig. 1C). The important result is that when ultrasmall point contacts separated by molecular length-scales are used as the source and drain (S͞D) electrodes, transistors can be made that have high gate efficiency and large ON͞OFF ratios from only a monolayer of molecules. The electrical properties of these monolayers are responsive to electron acceptors such as tetracyanoquinodimethane (TCNQ). Results and DiscussionDevice Fabrication. We first describe the devices used to measure the properties of the monolayers and then the structural and electrical characterization of these monolayers. Fig. 2 shows a schematic and micrograph of the devices used. Au (50 nm) on Cr (5 nm) pads, which are separated by 20 m, form the contact to an individual single-walled carbon nanotube (SWNT). The nanotubes were grown by a chemical vapor deposition (CVD) process described elsewhere (12, 13). The nanotube is then oxidatively cut by using an ultrafine lithographic process that produces a very small gap between the nan...

The anomalous glass-like thermal conductivity of crystalline clathrates has been suggested to be the result of the scattering of thermal phonons of the framework by 'rattling' motions of the guests in the clathrate cages. Using the site-specific (83)Kr nuclear resonant inelastic scattering spectroscopy in combination with conventional incoherent inelastic neutron scattering and molecular-dynamics simulations, we provide unambiguous evidence and characterization of the effects on these guest-host interactions in a structure-II Kr clathrate hydrate. The resonant scattering of phonons led to unprecedented large anharmonic motions of the guest atoms. The anharmonic interaction underlies the anomalous thermal transport in this system. Clathrates are prototypical models for a class of crystalline framework materials with glass-like thermal conductivity. The explanation of the unusual molecular dynamics has a wide implication for the understanding of the thermal properties of disordered solids and structural glasses.

The surface-normal electron density profile rhos(z) of concentrated aqueous salt solutions of RbBr, CsCl, LiBr, RbCl, and SrCl2 was determined by x-ray reflectivity (XR). For all but RbBr and SrCl2 rhos(z) increases monotonically with depth z from rhos(z)=0 in the vapor (z<0) to rhos(z)=rhob of the bulk (z>0) over a width of a few angstroms. The width is commensurate with the expected interface broadening by thermally excited capillary waves. Anomalous (resonant) XR of RbBr reveals a depletion at the surface of Br- ions to a depth of approximately 10 A. For SrCl2, the observed rhos(z)>rhob may imply a similar surface depletion of Cl- ions to a depth of a few angstorms. However, as the deviations of the XRs of RbBr and SrCl2 from those of the other solutions are small, the evidence for a different ion composition in the surface and the bulk is not strongly conclusive. Overall, these results contrast earlier theoretical and simulational results and nonstructural measurements, where significant surface layering of alternate, oppositely charged, ions is concluded.

The collective dynamics of methane and xenon hydrate in the energy range of Ϯ20 meV and the Q range of 1.5-11 nm Ϫ1 has been investigated by inelastic x-ray scattering and compared to results from inelastic neutron scattering experiments and lattice dynamical calculations. The experiment focused on the low-frequency phonon dispersion curves that were found to point towards the existence of an avoided crossing between the acoustic lattice phonons and the localized guest modes. The calculations reproduce the experimental spectra and show that the localized vibrations of the guest molecules or atoms are mixed with the collective host lattice vibrations, leading to a damping of the intensity of the acoustic host lattice phonons. This observation supports the idea of a resonant scattering mechanism for the strong guest-host phonon interactions in clathrate hydrates.

A study of the vibrational and thermoelectric properties of silicon type I and II clathrates J. Appl. Phys. 105, 043503 (2009); 10.1063/1.3078157 Experimental and computational studies on collective hydrogen dynamics in ammonia borane: Incoherent inelastic neutron scatteringWe report results from a high-resolution, incoherent inelastic neutron scattering ͑IINS͒ study of xenon hydrate. This study extends previous work in which the existence of a strong coupling between localized guest vibrations and the lattice modes was shown 1 for the first time ͓Tse et al., Europhys. Lett., 54, 354 ͑2001͔͒. This guest-host coupling might be responsible for the glass-like temperature dependence of the thermal conductivity of the crystalline gas hydrates. Our experiment focused on the low-frequency phonon density of states of the ice-like water lattice of xenon hydrate. We found two broad maxima in the density of states ͑DOS͒ at energy transfers of 7.3 and 10.3 meV. The first peak is assigned to the transverse acoustic ͑TA͒ phonons near the zone boundary and the second to the fold-back of the TA modes towards the zone center. The guest-host coupling could be confirmed by finding three distinct low energy peaks in the DOS at energy transfers of 2.05, 2.87, and 3.94 meV. In addition, another broad inelastic scattering component extending from 0 to 1.5 meV has been found, which may also be important for the low-temperature behavior of the thermal conductivity. The line positions of the coupled modes shift to higher frequencies with increasing temperature, pointing towards the importance of the repulsive part of the host-guest interaction which is responsible for the stability of gas hydrate structures.