Electronic Structure and Optical Limiting Behavior of Carbon Nanotubes  
  P. Chen, X. Wu, X. Sun, J. Lin, W. Ji, and K. L. Tan  
  PHYS REV LETT 82: (12) 2548-2551 MAR 22 1999  
 
 
     
  The unique properties of carbon nanotubes have for the past decade inspired entire communities of researchers and generated what looks to be an inexorable slew of potential applications. With regards to their electronic and optical properties, we compliment the many theoretical predictions with several experimental verifications.  
 
Multiwalled carbon nanotubes of diameter 15-20nm prepared by catalytic CO disproportionation were studied wusing TEM, XRD, UPS, XPS, and nonlinear optical spectroscopies. By using XRD, carbon nanotubes are shown to retain the hexagonal ring structure of graphite, with the exception of greater interplanar distance as seen from the shift in the (002) reflection. The valance band structure of carbon nanotubes is basically the same as that of graphite, with a noticeably lower intensity in the binding energy region of 2-7 eV as seen in the UPS He II spectra obtained from the annealed carbon nanotubes and graphite (Figure 1).
Fig 1. UPS He II valence band spectra of the carbon nanotubes (solid line) and graphite (dotted line)
 
  This corresponds to a reduction in the pp electron density for carbon nanotubes which happens to be a corollary of the curvature of the graphene sheets.  
  XPS studies has also revealed the difference between graphite and the carbon nanotubes in the C 1s core level energy position, full width at half maximum (FWHM), and its energy loss fine structures. The negative shift of 0.3eV in the binding energy for the nanotubes can be explained by the weaker C-C bonds resulting from the curvature of the graphene layers, which also accounts for the stronger plasma excitations in carbon nanotubes. Figure 2 compares the optical limiting behavior of the carbon nanotubes with that of carbon black and C60. It can be seen that the carbon nanotubes exhibits a stronger optical limiting effect and have broadband limiting capabilities up to 1064nm. Since no ground state absorption exists at 532 and 1064nm, the limiting property of the carbon nanotubes may mainly result from other mechanisms like non-linear scattering, and may possibly be related to the electronic properties discussed earlier.  
   
 
Fig 2. Nonlinear transmission of the carbon nanotubes in ethanol (O); C60 in touline (+); and carbon black in distilled water.