Low energy electrons focused by the image charge interaction in carbon nanotubes Samuel A. Hevia a, * , Rodrigo Segura b , Patricio Ha ¨ berle c a Instituto de Fı ´sica, Pontificia Universidad Cato ´lica de Chile, 6904411 Santiago, Chile b Instituto de Quı ´mica y Bioquı ´mica, Facultad de Ciencias, Universidad de Valparaı ´so, Av. Gran Bretan ˜a 1111, Valparaı´so, Chile c Departamento de Fı ´sica, Universidad Te ´cnica Federico Santa Marı ´a, 2390123 Valparaı ´so, Chile ARTICLE INFO Article history: Received 6 March 2014 Accepted 12 August 2014 Available online 23 August 2014 ABSTRACT Electrons with energy in the range of a few eV are strongly affected by the interaction with the polarization charges they induce on a surface. This report shows how this effect is rel- evant for the data analysis of inverse photoemission spectroscopy (IPS) from carbon nano- tube (CNT) arrays. IPS from CNTs exhibit two main resonances, located around 2.5 eV and 12.5 eVabove the Fermi level. The intensity of the first resonance is dependent on the aver- age tube diameter and the second one has a distinctive spectral shape, which is related to the graphitization level of the CNT external walls. In order to analyze the origin of these resonances, a phenomenological reconstruction of an IPS spectrum from a CNT collection was performed. This reconstruction successfully reproduces the spectral shape of the 12.5 eV resonance. However, the intensity is lower than the actual measurements in the initial energy range of the spectrum. The analysis of these results suggests that the addi- tional intensity required to reproduce the experimental data, has its origin in an electronic focusing mechanism, induced by the CNT image charge potential. This effect is significant for low energy electrons and small diameter tubes. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction Carbon nanotubes (CNTs) have received a widespread atten- tion due to their interesting physical properties [1] and the promising new applications in a variety of systems [2,3]. Precisely, the interaction of probe particles such as electrons or photons with CNTs and other graphene based nanostruc- tures, is of current scientific interest due mainly to their potential applications in sensors and devices [4,5]. Electron spectroscopies have been widely used to examine the electronic structure of CNTs below the Fermi level (E F ) [6–12], however, there are only few experimental reports dealing with states above this energy level [2,11–13]. Some unoccupied electronic states, which are not linked to the band structure, are the image charge states [14]. These type of surface states in CNTs are generated by the interaction between an external electron and the polarization charges it induces on a nanotube [15,16]. Thus, this long range potential allows the formation of extended unoccupied electronic states around a CNT [17–19]. Additionally, as shown in sub- Section 3.1, when unbound electrons approach the vicinity of a nanotube, as they would in inverse photoemission spec- troscopy (IPS), this attractive potential bends their trajectories inducing a focusing effect around the tubes. This report presents results from a study of the unoccu- pied electronic states of single wall carbon nanotubes (SWCNTs) and multiple wall carbon nanotubes (MWCNTs) using IPS. Samples were prepared as non-oriented CNT http://dx.doi.org/10.1016/j.carbon.2014.08.023 0008-6223/Ó 2014 Elsevier Ltd. All rights reserved. * Corresponding author. E-mail address: samuel.hevia@fis.puc.cl (S.A. Hevia). CARBON 80 (2014) 50 – 58 Available at www.sciencedirect.com ScienceDirect journal homepage: www.elsevier.com/locate/carbon