Photoelectronic Properties of C-based Nanostructures C . Zhang, J.C. Chao, X.G. Guo and Feng Liu

1. Photoelectronic Properties of C-based Nanostructures C. Zhang, J.C. Chao, X.G. Guo and Feng Liu Dept. of Materials Science and Engineering, University of Utah, Salt Lake City, UT We have recently developed a tight-binding method [1] that enabled us to calculate the photo adsorption rate of carbon-based nanostructures, such as nanotubes and graphene nanoribbons, due to impurity and defect scattering. We formulated a quantum transport equation in the presence of both electron-electron and electron-impurity scattering, to calculate photon absorption rate and photoconductivity of nanostructures. The electron-impurity scattering is treated in the most general manner by a random potential within the tight-binding formalism. Physically, the random potential will induce indirect and intraband transitions that are forbidden in an ideal system, by adding an arbitrary momentum to the electrons Figure 1 shows our calculations [1] of the optical properties for three (10,0), (15,0), and (20,0) zigzag SWNTs. In Fig. 1a, we plot the imaginary part (eI) of the dielectric functions of the three SWNTs, which are essentially their photon adsorption spectra without impurity scattering. For semiconductor (10,0) and (20,0) tubes, electrons are excited from the valence to the conduction bands of the same momentum. The peaks in the eI (Fig. 1a) correspond to various transition energies. There exists a threshold excitation energy, which is defined by the band gap for the two semiconductor tubes. For the metallic (15,0) tube, the intraband transition channel is also open and the threshold energy is much lower. The eI changes drastically with the frequency, reflecting both resonant and non-resonant coupling of electrons with photons.

2. In Fig. 1b, we show the photo absorption rates of the same three SWNTs as a function of photon energy, taking into account the impurity scattering. They are plotted against the background of the solar energy spectrum to indicate their frequency range of adsorption. Common to all the SWNTs is the unusually high value of absorption rate induced by impurity scattering, reflected by peaks followed by a large additional continuum in the spectra. In contrast, without impurity scattering, the absorption spectra contain only discrete peaks separated by small tails (Fig. 1a). There is a striking difference between the absorption rate of semiconducting (10,0) and (20,0) tubes and that of the metallic (15,0) tube. The semiconductor tubes [(10,0) and (20,0) tubes] have a threshold absorption energy below which no absorption takes place, in accordance with their respective band gaps. In general, the smaller semiconductor tubes have a higher absorption rate than larger tubes. So, the smaller tube has a high threshold but also a larger output voltage. The absorption of the metallic (15,0) tube shows an additional absorption peak in the far IR range that is due to the intraband plasmon excitation. (a) (b) Fig. 1. The imaginary part (eI) of the dielectric functions of the three zigzag SWNTs, as a function of photon energy. [1] “Impurity mediated absorption continuum in single-walled carbon nanotubes”, C. Zhang, J.C. Chao, X.G. Guo and Feng Liu, Appl. Phys. Lett.90, 023106 (2007).