Effects of heat treatment on the electronic structure of nanometric Si/C/Npowders by x-ray photoelectron spectroscopy

The electronic structure of ultrafine laser synthesized powders with contro lled composition have been investigated by using x-ray photoelectron spectr oscopy. The Si 2p, C 1s, and N 1s core levels as well as the valence band d istributions have been analyzed. The experimental analysis is focused on in termediate composition samples with C/N equal to 0.58 and 0.67; in this ran ge the C/N value remains approximately the same under heat treatments. In a s-formed samples, the nanopowders exhibit a high degree of chemical disorde r; mixed tetrahedra with both C and N atoms around each Si atom are likely to occur. After a 1500 degrees C treatment, the powders remain amorphous an d the presence of N-C and C-C bonds is clearly shown. After a 1600 degrees C annealing the samples are crystallized; the measured core levels and vale nce spectra reveal the presence of C-Si and N-Si bonds in Si-C-4 and Si-N-4 groups. It is noteworthy that in this case the C 2s and N 2s valence band states appear as two well separated peaks in the valence band. Our results show that the Si/C/N nanopowders, in the composition range close to C/N=0.6 , are not a simple mixture of carbide and nitride groups at an atomic scale , but exhibit a specific atomic structure including C-N bonds. The evolutio n of the atomic structure of these systems, as a function of heat treatment s, depends noticeably on the initial composition of the nanopowder. (C) 199 8 American Institute of Physics. [S0021-8979(98)02615-2].