Optical Gap Measurements on Individual Boron Nitride Nanotubes by Electron Energy Loss Spectroscopy Raul Arenal, 1, * Odile Stéphan, 2 Mathieu Kociak, 2 Dario Taverna, 2,3 Annick Loiseau, 1 and Christian Colliex 2 1 Laboratoire d’Etude des Microstructures, ONERA-CNRS UMR 104, 92322 Châtillon, France 2 Laboratoire de Physique des Solides, UMR CNRS 8502, Université Paris-Sud, 91405 Orsay, France 3 Université Paris VI, Paris, France Abstract: Electromagnetic response of individual boron nitride nanotubes ~BNNTs! has been studied by spatially resolved electron energy loss spectroscopy ~EELS!. We demonstrate how dedicated EELS methods using subnanometer electron probes permit the analysis of local dielectric properties of a material on a nanometer scale. The continuum dielectric model has been used to analyze the low-loss EEL spectra recorded from these tubes. Using this model, we demonstrate the weak influence of the out-of-plane contribution to the dielectric response of BNNTs. The optical gap, which can be deduced from the measurements, is found to be equal to 5.8 6 0.2 eV, which is close to that of the hexagonal boron nitride. This value is found to be independent of the nanotubes configuration ~diameter, helicity, number of walls, and interaction between the different walls!. Key words: electron energy loss spectroscopy ~EELS!, low loss, optical gap, boron nitride, nanotubes INTRODUCTION Nanocrystalline systems have attracted much interest for their novel properties, which differ remarkably from bulk crystals ~Dresselhaus et al., 2001; Cao, 2004!. With the growing technology ~e.g., high-density magnetic recording, high-speed optical computing, solar energy, biomaterials, biosensors! of these materials, it is increasingly necessary to understand the detailed basis for these new properties. The electronic structure of materials can be investigated with several experimental techniques as optical spectroscopy ~emission or absorption! with high-energy resolution ~,0.1 eV!~Fox, 2002!. However, these methods are limited by the spatial resolution. In such cases, the study of these nanomaterials individually at the nanometer scale is not possible, and this fact limits the complete information that can be extracted. In this way, electron energy loss spectros- copy ~EELS! in a scanning transmission electron micro- scope ~STEM! is a key technique that permits simultaneously the study of the composition and the electronic structure of these materials individually ~Stéphan et al., 1994, 2002; Egerton, 1996; Suenaga et al., 1997, 2000; Zhang et al., 1998; Kociak et al., 2001; Arenal et al., 2005!. Among these new nanomaterials boron nitride nano- tubes ~BNNTs! have recently received great interest due to the development of their synthesis methods and the study of their remarkable properties ~Lee et al., 2001; Arenal, 2005; Arenal et al., 2005!. In particular these BNNTs are insulators with a constant band gap ~5.8 eV! independent of their helicity and diameter ~Arenal et al., 2005!. This leads to interesting optical properties, which make these inorganic tubes a possible alternative to their carbon nano- tube counterparts in regard to possible applications. The physical phenomena associated with EELS in the low-loss energy region ~below 50 eV! involve excitations of valence electrons ~either collective plasma oscillations or individual interband transitions! that may reveal the struc- ture of the band gap in the case of semiconducting or insulating materials and have some major features of their optical properties. For a bulk material, the optical gap is generally mea- sured by optical absorption spectroscopy that probes the frequency dependence of the imaginary part of the dielec- tric function ~e~v!! ~Fox, 2002!. On the other hand, for a bulk material, an electron energy loss ~EEL! spectrum is proportional to the energy loss function ~Im~ 1/e~v!!! ~Egerton, 1996!, which can be described as the response function of the specimen to the electron beam. This energy loss function in a penetrating geometry, and neglecting in a first approximation the surface loss contributions, is de- Received October 26, 2007; accepted March 3, 2008 *Corresponding author. E-mail: raul.arenal@onera.fr Microsc. Microanal. 14, 274–282, 2008 doi: 10.1017/S1431927608080331 Microscopy AND Microanalysis © MICROSCOPY SOCIETY OF AMERICA 2008