No abstract

This report introduces a type of plasmonic crystal that consists of metal coated nanostructures of relief molded on a polymer film as a substrate for surface-enhanced Raman scattering ͑SERS͒. Such crystals exhibit SERS enhancement factors of ϳ10 5 , over large areas and with sufficiently high levels of uniformity for precise two-dimensional Raman mapping of surface bound monolayers. The ease of fabrication together with the high sensitivities and spatial resolution that can be achieved suggests an attractive route to SERS substrates for portable chemical warfare agent detection, environmental monitors, noninvasive imaging of biomolecules, and other applications.

Environment-induced effects on the E 2G G-band and A 1 ′ 2D-band Raman spectral features of single-layer graphene provide insights on the intrinsic and extrinsic dependences of the phonon energy and line width on temperature. Graphene prepared via mechanical exfoliation in air exhibits a G-band line width that increases with temperature between 298 and 573 K but shows an opposite trend after annealing under Ar. The opposing temperature dependences are considered within the context of Kohn anomaly induced phonon softening and broadening. The primary cause of the changes in the E 2G phonon energy and the electron-phonon coupling is attributed to ambient O 2 shifting the Fermi level away from the Dirac point. Our results emphasize the need to carefully consider the sample environment when investigating electronic and vibrational properties of graphene.

Raman spectra of electrostatically gated single-layer graphene are measured from room temperature to 560 K to sort out doping and thermally induced effects. Repeated heating cycles under Ar led to convergent first-order temperature coefficients of the G-band (χ(G) = -0.03 cm(-1)/K) and the 2D-band (χ(2D) = -0.05 cm(-1)/K) frequencies, which are independent of doping level as long as the Fermi level does not shift with temperature. While the intrinsic behavior may be different (e.g., χ(G) ∼ -0.02 cm(-1)/K near room temperature), these values appear more appropriate in describing responses of most graphene samples on SiO(2) substrates. The more negative χ(G) value than theoretical expectations may be explained by interactions with the substrate reducing the lattice thermal expansion contribution to the temperature dependence of G-band frequency. Enhanced interactions with the substrate may also be responsible for zero-charge, room-temperature G-band line width increase and 2D-band frequency downshift.

Two terminal abrupt junction diodes are fabricated from single semiconducting carbon nanotubes with simple photopatterned polymer layers defining air-stable p- and n-regions. These intratube diodes show nearly ideal behavior with relatively low series resistance and no sign of Zener breakdown at room temperature. Spatial doping profiles measured by micro-Raman spectroscopy and selective electrochemical gating of the n-region indicate that diode performance depends strongly on relative doping levels. A short circuit current of 1.4 nA with an open circuit voltage of 205 mV are measured when illuminated to saturation.