Mingyuan Huang

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.

Probing Strain-Induced Electronic Structure Change in Graphene by Raman Spectroscopy

Two-phonon Raman scattering in graphitic materials provides a distinctive approach to probing the materials electronic structure through the spectroscopy of phonons. Here we report studies of Raman scattering of the two-dimensional mode of single-layer graphene under uniaxial stress and which implicates two types of modification of the low-energy electronic structure of graphene: a deformation of the Dirac cone and its displacement away from the K point.

Determination of the Young's Modulus of Structurally Defined Carbon Nanotubes

We have combined optical characterization with a magnetic actuation technique to measure the stiffness of single-walled carbon nanotubes of defined crystal structure. The measured stiffnesses correspond to an average Youngs modulus of E = 0.97 +/- 0.16 TPa. For the structures investigated, no dependence on the nanotube chiral index was observed within the indicated experimental accuracy.

Variable Electron-Phonon Coupling in Isolated Metallic Carbon Nanotubes Observed by Raman Scattering

We report the existence of broad and weakly asymmetric features in the high-energy (G) Raman modes of freely suspended metallic carbon nanotubes of defined chiral index. A significant variation in peak width (from 12 cm(-1) to 110 cm(-1)) is observed as a function of the nanotubes chiral structure. When the nanotubes are electrostatically gated, the peak widths decrease. The broadness of the Raman features is understood as the consequence of coupling of the phonon to electron-hole pairs, the strength of which varies with the nanotube chiral index and the position of the Fermi energy.

Single-walled carbon nanotubes as shadow masks for nanogap fabrication

We describe a technique for fabricating nanometer-scale gaps in Pt wires on insulating substrates, using individual single-walled carbon nanotubes as shadow masks during metal deposition. More than 80% of the devices display current-voltage dependencies characteristic of direct electron tunneling. Fits to the current-voltage data yield gap widths in the 0.8–2.3nm range for these devices, dimensions that are well suited for single-molecule transport measurements.

Molecular-Scale Quantum Dots from Carbon Nanotube Heterojunctions

Carbon nanotube heterojunctions (HJs), which seamlessly connect nanotubes of different chiral structure using a small number of atomic-scale defects, represent the ultimate scaling of electronic interfaces. Here we report the first electrical transport measurements on a HJ formed between semiconducting and metallic nanotubes of known chiralities. These measurements reveal asymmetric IV-characteristics and the presence of a quantum dot (QD) with approximately 60 meV charging energy and approximately 75 meV level spacing. A detailed atomistic and electronic model of the HJ enables the identification of specific defect arrangements that lead to the QD behavior consistent with the experiment.

1- to 2-nm-wide nanogaps fabricated with single-walled carbon nanotube shadow masks

The authors present a method for producing nanometer-scale gaps, based on metal evaporation through a suspended single-walled carbon nanotube acting as a shadow mask. 83% of the nanogap devices display current-voltage dependencies characteristic of direct electron tunneling. Fits to the current-voltage data yield gap widths in the 0.8–2.3nm range for these devices, dimensions that are well suited for single-molecule transport measurements.

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.

Raman scattering from individual, isolated metallic carbon nanotubes

We have obtained Raman spectra of the high-energy (G) modes from individual metallic carbon nanotubes. The Raman lines are broadened to widths up to 100 cm¿1, indicating strong phonon damping by electron-hole pairs.

Optical studies of individual single-walled carbon nanotubes under axial strain

The effect of axial strain on individual single-walled carbon nanotubes has been investigated experimentally. Changes in the optical transition energies reflect both the nature of the transition being probed and crystallographic structure of the nanotube.