Electronic States of Nanocrystalline Carbon
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Electron energy loss spectroscopy (EELS) has been used to investigate the electronic states of isolated, nanocrystalline carbon particles. Small carbon nanocrystals were prepared via sputter deposition onto Si0 2 substrates, followed by annealing to 700C. The structure and size distribution of the particles have been characterized by Raman scattering, Auger electron spectroscopy and electron microscopy. EELS observations indicate that a semimetal to semiconductor transition occurs for particles smaller than lnm. In addition, hydrogen adsorption is found to significantly affect the electronic states of these particles, indicating that both finite size and dangling bond effects modify the properties of small carbon nanocrystaflites.
Introduction The structure and properties of small carbon particles vary widely with the conditions of formation, yielding molecular clusters, amorphous networks and disordered nanocrystallites. Nanocrystalline carbon (nc-C) particles, consisting of two-dimensional graphitelike fragments, have been considered as important constituents of diverse systems, ranging from interstellar dust to arc generated soots and vacuum deposited thin films. These objects present an unique opportunity to observe finite size effects in a ir electron system, in contrast to nanostructures based on other group IV elements such as Si and Ge which only exhibit a-bonding interactions. Theoretical calculations for carbon have indicated that the electronic structure is quite sensitive to medium range correlations [1, 2, 3]. For two dimensional graphitic fragments, tight binding calculations [1, 2] predict a loss of semimetallic character as the fragment size is reduced although such a transition has not yet been observed experimentally. Small nanocrystalline particles will also have a significant number of undercoordinated edge atoms. Passivation of these dangling bond sites by adsorption of atomic hydrogen is expected to strongly influence the properties of the nc-C particles. For example, planar molecules consisting of several fused hexagonal carbon rings with edges terminated by hydrogen, known as PAHs (polycyclic aromatic hydrocarbons), exhibit substantial gaps of order 2.5-4eV[4, 5]. In this paper we report size dependent modifications of the electronic states of isolated nc-C particles supported on Si0 2 substrates. Electron energy loss spectroscopy has been used to observe the predicted semimetal-semiconductor transition as the particle size is reduced. Hydrogenation of the dangling bonds is also found to strongly affect the electronic states. These results demonstrate that both finite size and the increasing
215 Mat. Res. Soc. Symp. Proc. Vol. 452 01997 Materials Research Society
importance of dangling bonds influence the properties of nanoscale carbon particles. Furthermore the present work also demonstrates the utility of EELS as a powerful probe of the properties of nanocrystalline solids.
Sample Preparation and Characterization In order to study the properties of isolated nanocrystalline carbon part
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