Daohua Song

Phonon softening and crystallographic orientation of strained graphene studied by Raman spectroscopy

We present a systematic study of the Raman spectra of optical phonons in graphene monolayers under tunable uniaxial tensile stress. Both the G and 2D bands exhibit significant red shifts. The G band splits into 2 distinct subbands (G+, G−) because of the strain-induced symmetry breaking. Raman scattering from the G+ and G− bands shows a distinctive polarization dependence that reflects the angle between the axis of the stress and the underlying graphene crystal axes. Polarized Raman spectroscopy therefore constitutes a purely optical method for the determination of the crystallographic orientation of graphene.

Photocurrent spectroscopy of low-k dielectric materials: Barrier heights and trap densities

Measurements of photoinduced current have been performed on thin films of porous low-k dielectric materials comprised of carbon-doped oxides. The dielectric films were deposited on silicon surfaces and prepared with a thin gold counterelectrode. From the spectral dependence of the photoinduced current, barrier heights for the dielectric∕silicon and dielectric∕gold interface were deduced. Transient currents were also found to flow after the photoexcitation was abruptly stopped. An estimate of the density of shallow electron traps within the low-k material was obtained from the measurement of the net charge transported from this detrapping current. A density of traps in the range of 6×1016traps∕cm3 was inferred for the low-k films, far exceeding that observed by the same technique for reference dielectric films of pure SiO2. This behavior was also compatible with photocurrent I‐V measurements on the low-k dielectric films and SiO2 reference sample.

Raman Spectroscopy of Graphene under Uniaxial Strain

Polarized Raman spectroscopy was performed on single-layer graphene under uniaxial strain. The G- mode softens and splits into two components that exhibit distinct polarization properties related to the orientation of the graphene lattice.

The Lifetime of Optical Phonons in Graphite

Femtosecond pump-probe Raman spectroscopy has been applied to monitor the decay of non-equilibrium optical phonons in graphite. A lifetime of 2.2 ps is found.

Observation of the Optical Stark Effect in Semiconducting Carbon Nanotubes

An optical Stark effect has been observed in the one-dimensional system of semiconducting single-walled carbon nanotubes for non-resonant excitation.

Optical Stark effect in semiconducting single-walled carbon nanotubes

We have measured an optical Stark effect in semiconducting single-walled carbon nanotubes. Upon application of a non-resonant pump beam, we observe an instantaneous blue shift in the optical resonances that scales linearly with pump intensity.

Optical Stark Effect in Semiconducting Single-Walled Carbon Nanotubes

A strong optical Stark effect has been observed in the 1D system of semiconducting single-walled carbon nanotubes for non-resonant excitation.