Visible and near ultraviolet photocurrent generation in carbon nanotubes M. De Crescenzi a , F. Tombolini a , M. Scarselli a , S. Del Gobbo a , E. Speiser a , P. Castrucci a, * , M. Diociaiuti b , S. Casciardi c , E. Gatto d , M. Venanzi d a Dipartimento di Fisica, CNISM, Universita’ di Roma Tor Vergata, 00133 Roma, Italy b Dipartimento di Tecnologie e Salute, Istituto Superiore di Sanita’, 00161 Roma, Italy c Dipartimento di Igiene del Lavoro, ISPSEL, 00040 Monte Porzio Catone, Italy d Dipartimento di Scienze e Tecnologie Chimiche, Universita’ di Roma Tor Vergata, Roma, Italy Available online 22 December 2006 Abstract Multiwall carbon nanotubes are found to generate photocurrent in the visible and near ultra violet spectral range using a photoelect- rochemical technique. Peaks in the photocurrent are observed at excitation energies in the visible region. Their electron energy loss spec- tra exhibit the p plasmon feature, typical of graphite layers, and a peak at lower energy. Features at energies between 0 and 4 eV have been already observed for single wall carbon nanotubes and ascribed to interband electronic transitions due to the reduced dimension- ality of these systems. The present measurements suggest that the usual identification of multiwall carbon nanotubes electronic density of states with that of graphite layers is not sufficient and more theoretical investigations are necessary to shed light on this point. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Carbon nanotubes; Electron energy loss spectroscopy (EELS); Raman scattering spectroscopy; Photocurrent measurements 1. Introduction Multiwall carbon nanotubes (MWCNTs) consist of multiple layers of graphite forming concentric cylinders. The number of concentric cylinders in a MWCNT can vary between 2 and many tens. These systems are closely related to graphite layers and are generally considered to exhibit similar electronic properties. On the other hand, the MWCNTs electronic density of states is difficult to model, due to the high number of shells and their mutual interac- tions and, therefore, only few theoretical predictions of their electronic properties are available [1]. Theoretical and experimental efforts have been dedicated to double wall carbon nanotubes (DWCNTs), being the simplest form of multiwall carbon nanotube [2–4]. In particular, DWCNTs have been found to generate photocurrent in the infrared and visible regions and the peaks in the ob- served photocurrent spectra have been related to the elec- tronic transitions between van Hove singularities in their electron density of states [3]. This is consistent with energy loss spectra calculations performed for single and multiwall carbon nanotubes, predicting the presence of interband electronic transitions due to the reduced dimensionality of these systems [1]. In the present paper, we show that (i) MWCNTs are able to generate a photocurrent in the visible and near ultraviolet energy regions (ii) features are present in the photocurrent spectra at excitation photon energy in the visible range; (iii) peaks at energies between 2.2 and 4 eV are detected in the electron energy loss spec- tra, in addition to the typical graphite p plasmon . These results indicate that the simplified description of the multi- wall carbon nanotubes electronic density of states with that of graphite is not sufficient to describe the electronic prop- erties of these systems. 0039-6028/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2006.12.046 * Corresponding author. Tel.: +39 06 72594532; fax: +39 06 2023507. E-mail address: Paola.Castrucci@roma2.infn.it (P. Castrucci). www.elsevier.com/locate/susc Surface Science 601 (2007) 2810–2813