Lihong H. Herman

Free-Standing Epitaxial Graphene

We report on a method to produce free-standing graphene sheets from epitaxial graphene on silicon carbide (SiC) substrate. Doubly clamped nanomechanical resonators with lengths up to 20 microm were patterned using this technique and their resonant motion was actuated and detected optically. Resonance frequencies of the order of tens of megahertz were measured for most devices, indicating that the resonators are much stiffer than expected for beams under no tension. Raman spectroscopy suggests that the graphene is not chemically modified during the release of the devices, demonstrating that the technique is a robust means of fabricating large-area suspended graphene structures.

Schottky barrier inhomogeneities at the interface of few layer epitaxial graphene and silicon carbide

In this work, we study electron transport across the heterojunction interface of epitaxial few-layer graphene grown on silicon carbide and the underlying substrate. The observed Schottky barrier is characterized using current-voltage, capacitance-voltage and photocurrent spectroscopy techniques. It is found that the graphene/SiC heterojunction cannot be characterized by a single unique barrier height because of lateral barrier inhomogeneities. A Gaussian distribution of barrier heights with a mean barrier height φBm=1.06eV and standard deviation σ=137±11meV explains the experimental data quite well.

Single-walled carbon nanotubes as excitonic optical wires

Although metallic nanostructures are useful for nanoscale optics, all of their key optical properties are determined by their geometry. This makes it difficult to adjust these properties independently, and can restrict applications. Here we use the absolute intensity of Rayleigh scattering to show that single-walled carbon nanotubes can form ideal optical wires. The spatial distribution of the radiation scattered by the nanotubes is determined by their shape, but the intensity and spectrum of the scattered radiation are determined by exciton dynamics, quantum-dot-like optical resonances and other intrinsic properties. Moreover, the nanotubes display a uniform peak optical conductivity of approximately 8 e(2)/h, which we derive using an exciton model, suggesting universal behaviour similar to that observed in nanotube conductance. We further demonstrate a radiative coupling between two distant nanotubes, with potential applications in metamaterials and optical antennas.

On-Chip Rayleigh Imaging and Spectroscopy of Carbon Nanotubes

We report a novel on-chip Rayleigh imaging technique using wide-field laser illumination to measure optical scattering from individual single-walled carbon nanotubes (SWNTs) on a solid substrate with high spatial and spectral resolution. This method in conjunction with calibrated AFM measurements accurately measures the resonance energies and diameters for a large number of SWNTs in parallel. We apply this technique for fast mapping of key SWNT parameters, including the electronic-types and chiral indices for individual SWNTs, position and frequency of chirality-changing events, and intertube interactions in both bundled and distant SWNTs.

Imaging the electrical conductance of individual carbon nanotubes with photothermal current microscopy

The one-dimensional structure of carbon nanotubes leads to a variety of remarkable optical and electrical properties that could be used to develop novel devices. Recently, the electrical conductance of nanotubes has been shown to decrease under optically induced heating by an amount proportional to the temperature change. Here, we show that this decrease is also proportional to the initial nanotube conductance, and make use of this effect to develop a new electrical characterization tool for nanotubes. By scanning the focal spot of a laser across the surface of a device through which current is simultaneously measured, we can construct spatially resolved conductance images of both single and arrayed nanotube transistors. We can also directly image the gate control of these devices. Our results establish photothermal current microscopy as an important addition to the existing suite of characterization techniques for carbon nanotubes and other linear nanostructures.

Depolarization effect in optical absorption measurements of one- and two-dimensional nanostructures

We measure absolute optical absorption cross-sections of one- (1D) and two-dimensional (2D) nanostructures using a focused laser beam while varying the numerical aperture (NA) of the focusing lens. We find the optical absorption deviates at higher NA. In the high NA regime, absorption by graphene decreases from 2.2% to below 1.8%; for Ge nanowires, it decreases from an expected value by a factor of 1.2. We explain this using the depolarization effect at the focal spot and conclude that these corrections allow for accurate quantitative measurements of optical and optoelectronic processes in 1D or 2D nanostructures.