Electron energy-loss spectroscopy of electron states in isolated carbon nanostructures K. Suenaga, 1,2, * E. Sandre ´ , 1 C. Colliex, 1,3 C. J. Pickard, 4 H. Kataura, 5 and S. Iijima 2 1 Laboratoire de Physique des Solides, UMR CNRS 8502, Building 510, Universite ´ Paris-Sud, Orsay 91405, France 2 Japan Science and Technology Corporation, Meijo University, Nagoya 468-8502, Japan 3 Laboratoire Aime ´ Cotton, UPR CNRS 3321, Building 505, Universite ´ Paris-Sud, Orsay 91405, France 4 Institut fur Geowissenschaften, Kristallographie/Mineralogie, Olshausenstrasse 40, Kiel D-24098, Germany 5 Department of Physics, Tokyo Metropolitan University, Tokyo 192-0397, Japan Received 22 September 2000; published 3 April 2001 Electronic states of an isolated nano-object made of single graphene layer have been successfully explored by means of electron energy-loss spectroscopy with high sensitivity and high spatial resolution. In further systematic study of various carbon nanostructures, the curvature and/or the interlayer coupling between adja- cent graphene layers have been proved to govern the electronic states of carbon nanostructures. Using density functional theory, experimentally observed variations of the carbon K (1 s ) near-edge fine structure have been ascribed mostly to the increasing curvature of the graphene layers. DOI: 10.1103/PhysRevB.63.165408 PACS numbers: 73.22.-f, 71.15.Mb, 81.05.Tp, 82.80.Pv The knowledge of the distribution of electron states in isolated quantum object is of prime importance as they gov- ern their electronic properties. The family of carbon nano- structures has grown steadily over the past decade, with the discovery of variable morphologies and diversified physical properties. 1 Graphene layers two-dimensional hexagonal networks of sp 2 carbon have so far been reported with dif- ferent shapes and interactions: multiwalled or single-walled carbon layers in tubular, horn-shaped, or spherical geometry, so-called nanotubes MWNT or SWNT, nanohorns and nano-onions, respectively. And several types of aggregates made of these elementary components i.e., nanotubulites have very recently been identified, using high-resolution transmission electron microscopy HRTEM techniques. 2,3 The electron states both occupied and vacant states of these carbon nano-objects have been predicted to vary with the coupling and/or curvature of the graphene layers. 4,5 In this context, it is important to experimentally investigate the elec- tronic states in well-characterized carbon nano-objects. Electron energy-loss spectroscopy EELS, performable within a TEM, probes the unoccupied electron states over a wide energy range above the Fermi level. 6 In the small-angle experimental conditions, the information obtained is similar to the one provided by x-ray absorption technique. However, as the TEM is able to visualize and select single nano-objects with well-defined dimension, shape, and stacking of layers, the spectral and morphological information can be exactly connected at the subnanometer level. 7 In this paper, we com- pare experimental EELS spectra acquired from well-defined carbon nano-objects with results of ab initio electronic struc- ture calculations that have been performed for model struc- tures containing the correct number of carbon atoms to re- produce the relevant changes in stacking and curvature. A collection of carbon nano-objects made of single graphene layers with a variety of morphologies has been pre- pared using various techniques. Tubular or horn-shaped closed graphene structures with different inner diameters, inter-layer couplings, and/or different curvatures were pre- pared by a CO 2 laser ablation method. 3 Double-layered nanotubes, in which the inner nanotube has an extremely small diameter 0.7 nm, were obtained by coalescence of C 70 molecules under the 200 kV electron beam 8 within the 1.5-nm-diameter outer tubes selectively produced in a Nd- YAG yttrium aluminum garnet laser ablation method by controlling the furnace temperature and the catalyst. 9 The spatially resolved EELS measurements have been performed with a dedicated scanning TEM STEM, VG- HB501. The incident electron beam 100 kV was focused into a probe of about 0.5 nm diameter. For each probe posi- tion, successively addressed with 0.3-nm step, the EELS spectrum was recorded with a parallel detector Gatan PEELS 666. 10 Such a collection of carbon K-edge spectra, recorded across a nanohorn of 3 nm dimension is shown in Fig. 1. For each spectrum the acquisition time was reduced to 0.5 sec in order to prevent electron irradiation damage. Under such conditions, the jump ratio for the spectrum goes up to several hundred counts at the C K-edge onset 285 eV and allows the fine structure analysis with sufficiently good statistics. The carbon profile i.e., the integrated C K signal over a given energy window, after background subtraction across the nanohorn is shown in Fig. 1a, together with an annular dark field ADF profile that was simultaneously recorded. Both profiles exhibit two peaks with a hollow center, which is characteristic of an empty cylindrical object. The present results clearly demonstrate that chemical profiling and fine structure analysis based on spatially resolved EELS are pos- sible on a single graphene layer of carbon, in which only a few tens of carbon atoms contribute to each spectrum. Figure 2 gathers high-resolution micrographs and accom- panying schematic representations for various morphologies of interacting graphene layers. Figure 2a shows well- stacked graphene layers 10 layers in a cylindrical configu- ration with a 10–16 nm diameter in a cross-sectional view. The layers are coupled in such a graphitized MWNT. Figure 2b shows an aggregate of nanohorns of 3 nm diameter. 3 The coupled double-layer configuration is dominant in this aggregate. Such coupling of adjacent layers does not exist in bundles of SWNT’s with smaller diameter 1.5 nm, be- cause the elastic force acting on each graphene layer to keep PHYSICAL REVIEW B, VOLUME 63, 165408 0163-1829/2001/6316/1654084/$20.00 ©2001 The American Physical Society 63 165408-1