Steve Cronin

Stephen B. Cronin
Assistant Professor
Department of Electrical Engineering - Electrophysics
University of Southern California
Los Angeles, CA 90089
scronin@usc.edu ______________________________________________________

Optical and Electrical Measurements of Individual Carbon Nanotubes

Carbon Nanotubes are tubes of graphitic carbon typically 1-2nm in diameter. They possess electrical, optical, thermal, and mechanical properties which far exceed those of any known material, making them suitable for many potential applications.

Using the techniques of electron-beam lithography and nano-fabrication we perform electrical and optical measurements on the same individual nanotube. The figure below shows an atomic force microscope image of a single carbon nanotube molecule contacted by two metal electrodes. The white circle in the figure indicates the approximate size and location of the laser spot when the spectrum on the right was taken. By performing these two measurements on the same nanotube we learn a wealth of information about the nanotube including: bandgap, electronic energy levels, nanotube diameter, defect concentration, semiconducting/metallic nature, temperature, strain, and precise atomic structure. Using techniques in nano-fabrication and nano-manipulation, we measure the response of a nanotube to strain[1][2], temperature, changes in the environment, and electrochemical potential[3]. The main objective of my work is to study the exceptional properties of carbon nanotubes using novel optical techniques, and to identify and evaluate potential applications of the nanotubes that exploit their exceptional properties.

Atomic force microscope (AFM) image of a carbon nanotube contacted by two metal electrodes. The white circle indicates the size and location of the laser spot when the Raman spectrum on the right was taken.

Raman Spectroscopy of Individual Carbon Nanotubes Under Strain

Raman spectroscopy can be used to measure the vibrational frequencies of individual carbon nanotubes at various degrees of strain. The strain is induced by kinking the nanotubes with an atomic force microscope (AFM) tip. The ends of the nanotubes are held fixed by metal electrodes thereby translating the transverse kink in to an elongation of the nanotube. Under strains varying from 0.06%–1.65%, the in-plane vibrational mode (G-band) frequencies are lowered by as much as 1.5% (40cm^-1). We understand the downshift in vibrational frequency due to the elongation of the carbon-carbon bond, which weakens the bond and therefore lowers it vibrational frequency. [1]

G and D band Raman spectra (top) and atomic force microscope (AFM) image (bottom) of an individual carbon nanotube strained with an AFM tip [1].

Near-Field Raman Spectroscopy of Individual Carbon Nanotubes

In micro-Raman spectroscopy, the incident laser beam is focused using a conventional light microscope to an approximately 0.5mm spot size, which is roughly the wavelength of the laser light (633nm). The microscope stage can be controlled very precisely in the x and y directions, allowing us to spatially map the Raman spectra of a nanotube. The figure below shows the spatial mapping image of the G-band Raman intensity of a nanotube taken with a conventional light microscope, demonstrating 0.5mm resolution. The figure also shows the spatial mapping image taken with a near-field microscope, demonstrating 14nm resolution [4]. With the near-field microscope, we bring a sharp metal tip, like an atomic force microscope (AFM) tip, very close to the surface of the sample, in the focused laser spot. The sharp tip serves to enhance the electric field at the tip, providing a 10-20nm light source. Now when we spatially map out the Raman spectra we get 14nm resolution, with 633nm light, which is quite remarkable. Each point on these plots represents the G-band intensity taken from a different Raman spectrum taken at a slightly different location on the sample.

Spatial mapping images of the G-band Raman intensity of a single nanotube using (a.) a conventional light microscope demonstrating 0.5mm resolution and (b.) a near-field microscope demonstrating 14nm resolution. [4]

Electrochemical Gating of Individual Carbon Nanotubes Observed by Raman Spectroscopy

Because of nanotubes' large surface to volume ratio they are very sensitive to the presence of any chemical species on their surface. By applying a voltage between a nanotube and a reference electrode in a small drop of electrolytic solution, ions accumulate on the surface of the nanotube and serve to gate it very effectively. Using resonant confocal micro-Raman spectroscopy, we observe a 9cm^-1 upshift of the tangential mode (G-band) vibrational frequency, as well as a 90% decrease in intensity by applying 1V between an individual nanotube and a silver reference electrode in a dilute sulfuric acid solution. This upshift indicates a change in the carbon-carbon bond strength and hence the vibrational frequency. The decrease in intensity indicates a shifting of the electronic energy levels in the nanotube off of resonance with the laser energy. By applying an electrochemical gate voltage we are essentially able to tune the electronic and vibrational energies of an individual carbon nanotube [3].

G-band Raman mode of an individual carbon nanotube under electrochemical gating conditions.

Resonant Raman Scattering of Individual Carbon Nanotubes

Despite the extremely small geometric cross-section of a nanotube the Raman signal from a single isolated nanotube can be observed. This is due to a 10⁵ factor of enhancement in the scattering cross-section due to a resonance between the electronic states in the nanotube and the laser energy. The figure below shows the density of electronic states of a nanotube. The sharp spikes correspond to the electronic energy levels in the nanotube. When the laser energy (E_laser) is equal to a transition from one of the valence energy levels to the corresponding conduction level (E_ii) then a resonance occurs, and we can observe the signal from a single nanotube. As a consequence of this we only observe nanotubes that are in resonance with the laser energy.

Electron density of states of a semiconducting carbon nanotube.

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