Photoconductivity of single-walled carbon nanotubes under CW illumination. I. A. Levitsky Emitech, Inc., Fall River, MA 02720, P. T. Kanelos, W. B. Euler Department of Chemistry, 51 Lower College Road, University of Rhode Island, Kingston, RI 02881 Abstract -The photoconductive response of single-walled carbon nanotubes (SWNTs) has been studied under CW illumination in the NIR (900 – 1800 nm) and far-IR (9-10 μm) range. SWNTs were coated onto patterned gold electrodes with spacing which was varied from 5 to 300 nm. SWNT samples exhibit absorption NIR bands that are typical for semiconducting nanotubes with a diameter distribution of 0.8 – 1.1 nm and band at ~10 μm in the far-IR range. A clear correlation between current and IR exposure has been observed. The detection of photoconductivity requires instrumentation with a high dynamic range due to a significant dark current. The photocurrent exhibits a linear response with light intensity and with bias voltage. Possible mechanisms of photoconductivity in SWNT films and applications of this new photosensitive material are discussed. I. INRODUCTION The large amount of attention placed on the electronic and optical properties of single-walled carbon nanotubes (SWNTs) is associated with both the nanotube well-defined 1D structure and the possibility of employing them as building blocks for various nanotechnology applications [1]. In particular, the interaction of light with the nanotube structures is a matter of special interest due to the opportunity to observe new photophysical effects related to the nanotube low dimensionality and quantum confinement. For the past several years, numerous studies have been performed in the field of nanotube light absorption [2-4], Raman scattering [1, 5], fluorescence [4, 6], photoinduced molecular desorption [7], and electroluminescence [8]. Nevertheless, the SWNT photoconductivity remains a practically non-explored area: a temporal photocurrent was detected in the first reported observation of SWNT mat photoconductivity using pulsed laser excitation [9]. Such scarce information hints that observation of steady-state SWNT photoconductivity can be hampered by obstacles related to low light absorption of the nanotubes and/or a high level of dark current due to the metal conductivity masking the semiconductor properties [1]. Thus, the SWNT photoconductive response under steady state or continuous wave (CW) illumination presents interest not only for a fundamental understanding of SWNT photoinduced processes, but also for the development of a new photosensitive material with unique optical and conductive features. In this paper we report the first observation of CW photoconductivity of single-walled carbon nanotubes in the NIR and far-IR spectral range. II. EXPERIMENT SWNTs were synthesized by the arc discharge method and purified (85%) using air oxidation, acid treatment and thermal annealing, as purchased from BuckyUSA, Inc. The average diameter of the most nanotubes was in the range of 0.8 – 1.1 nm according to NIR spectroscopy and TEM observation. However, as it was found from FTIR spectra, that some SWNTs have significantly larger diameter (7-8 nm) with the absorbance band at ~ 10 µm. For the photoconductivity study, a nanotube suspension in methanol was sonicated followed by dropping a small amount (2-3 μl) onto a patterned gold substrate with an insulating gap between the opposite electrodes. The typical film thickness was 0.2-0.5 μm, except for specially prepared dense and thick films (~ 10-15 μm) made from a highly concentrated suspension. The gap width between electrodes was varied from 5 μm to 300 μm, and the sample resistance was in the range of 80 –1500 Ω. The NIR source was a tungsten-halogen lamp Figure 1. UV-NIR absorption spectrum of the SWNT film. For absorption measurements, the film was prepared by airbrush spraying of the SWNT suspension in methanol onto preheated (70 o C) glass substrate. Inset shows the spectrum after π-plasmon background was substracted. 800 1200 1600 0.5 1.0 Absorbance, a. u. nm 800 1200 1600 0.0 0.5