ELECTRON-ENERGY-LOSS SPECTROSCOPY OF CARBON NANOMETER-SIZE TUBES

Electron-energy-loss spectra from individual carbon nanometer-size tubes are presented. The bulk plasmon-loss (sigma+pi) peak seen at approximately 27 Ev in graphite undergoes systematic shift towards lower energy, as the tube size is reduced. The low-energy-loss plasmon (approximately 7 eV) due to the pi electrons is weak and not well defined for very small tubes. It is seen that the peak shift depends on the number of layers that constitute the tubes and the tube diameter. The carbon K edge from nanometer-size tubes shows similar energy-loss features as in graphite. Thin amorphouslike glassy structures formed in the carbon soot also show shifts of peaks compared to graphite and graphitelike glassy structures. We conclude that the pi electrons tend to behave less delocalized in small tubes, due to the curvature-induced strain, and contribute less to the collective excitations in structures with poorly developed c-plane ordering.