We have employed a chip-bending method to exert continuous and reversible control over the tensile stress in doubly clamped nanomechanical beam resonators. Tensile stress is shown to increase the quality factor of both silicon nitride and single-crystal silicon resonators, implying that added tension can be used as a general, material-independent route to increased quality factor. With this direct stretching technique, we demonstrate beam resonators with unprecedented tunability of both frequency and quality factor. Devices can be tuned back and forth between a high and low stress state, with frequency tunability as large as several hundred percent demonstrated. Over this wide range of frequency, quality factor is also tuned by as much as several hundred percent, providing insights into the loss mechanisms in these materials and this class of nanoresonator. Devices with frequencies in the 1-100 MHz range are studied, with quality factor as high as 390,000 achieved at room temperature, for a silicon nitride device with cross-sectional dimensions below 1 microm, operating in a high stress state. This direct stretching technique may prove useful for the identification of loss mechanisms that contribute to the energy balance in nanomechanical resonators, allowing for the development of new designs that would display higher quality factors. Such devices would have the ability to resolve smaller addendum masses and thus allow more sensitive detection and offer the potential for providing access to previously inaccessible dissipation regimes at low temperatures. This technique provides the ability to dramatically tune both frequency and quality factor, enabling future mechanical resonators to be used as variable frequency references as well as variable band-pass filters in signal-processing applications.

A minimal model is exactly solved for electron spin transport on a helix. Electron transport is assumed to be supported by well oriented p(z) type orbitals on base molecules forming a staircase of definite chirality. In a tight binding interpretation, the spin-orbit coupling (SOC) opens up an effective π(z) - π(z) coupling via interbase p(x,y) - p(z) hopping, introducing spin coupled transport. The resulting continuum model spectrum shows two Kramers doublet transport channels with a gap proportional to the SOC. Each doubly degenerate channel satisfies time reversal symmetry; nevertheless, a bias chooses a transport direction and thus selects for spin orientation. The model predicts (i) which spin orientation is selected depending on chirality and bias, (ii) changes in spin preference as a function of input Fermi level and (iii) back-scattering suppression protected by the SO gap. We compute the spin current with a definite helicity and find it to be proportional to the torsion of the chiral structure and the non-adiabatic Aharonov-Anandan phase. To describe room temperature transport, we assume that the total transmission is the result of a product of coherent steps.

Semiconducting oxide nanoparticles have proven to be excellent in detecting extremely low-concentrations of molecules through surface-enhanced Raman scattering (SERS) effects. While the enhancement of the Raman activities arises from a large increase in polarizability due to charge transfer from the molecule to the semiconducting nanoparticle, little is known about how the oxide composition, nanoparticle size, solvent, or pH affects the observed Raman activities. In the current study, we examine these effects by carrying out extensive computational investigations of semiconducting TiO2, SnO2 and Fe2O3 nanoparticles and their complexes with both catechol and dopamine. An increase in the size of the oxide cluster or a decrease in the pH of the system under observation leads to enhanced Raman activities; the variation of the activities in different solvents is very much dependent on the nature of the vibrational modes. The marked increase in the Raman activities of molecules adsorbed on SnO2 or Fe2O3 over that of molecules adsorbed on TiO2 seems to indicate that these oxide nanoparticles would be useful substrates for SERS sensors. Our results also indicate that the Raman activities of some of the TiO2 modes are magnified upon adsorption of molecules, which concurs with some very recent experimental observations. All these results are consistent with a recently proposed theoretical model of SERS on semiconducting substrates. Further, this work has implications on the development of molecular sensing, dye-sensitized solar cells, and photocatalysis.

A titania nanotube/single-wall carbon nanotube composite was prepared by a simple hydration dehydration process. These composites were characterized using X-ray diffraction and spectroscopic techniques (UV−visible diffuse reflectance, Raman, photoluminescence, and EPR) as well as electron microscopy (SEM, TEM). SEM and TEM images indicated that single-wall carbon nanotubes (SWCNTs) were interwoven with the titania nanotubes. Raman spectra further confirmed the chemical interaction between the titania nanotube and the SWCNT in the composites. The photoactivity of these composites was tested by the photooxidation of acetaldehyde. The composites showed enhanced photoactivity under both visible and UV light in comparison with conventional titania (P25) and controls. The composite having a mass ratio of 1:50 (SWCNT/TiNT) showed the maximum photocatalytic activity for acetaldehyde decay under visible light. XPS and EPR spectra indicated the creation of Ti−O−C bonds between the titania nanotube and the SWCNTs during the hydration dehydration process, which explains the visible light photoactivity.