Synthesis and Characterization of Carbynoid Structures in Cluster-Assembled Carbon Films - Polyynes: Synthesis, Properties, and Applications - page 23

FIGURE 2.6 In situ Raman spectra of cluster assembled carbon films obtained
with different excitation wavelengths: a frequency-doubled Nd-Yag (532 nm) and a
He-Ne (632.8 nm). In the inset: dispersion of polycumulene peak (solid circles) and
polyyne peak (solid triangles).

wavelength of 632.8 nm we observe a G peak shift towards a lower
frequency (roughly 30 cm 1 ) and an I D /I G increase with respect to the case
in which a 532 nm wavelength is used. This reflects the typical dispersion
behavior for G and D features in mainly sp 2

rel is
smaller at 632.8 nm in agreement with the observation on polymeric carbon

amorphous carbon [37]. I

C

reported in Reference 9 . By varying the laser excitation energy the relative

polycumulene/polyyne intensity ratio also varies and it is larger at 632.8 nm
than at 532 nm.

2.3.3 G AS E XPOSURE

The sp chains are metastable and have the tendency to undergo cross-linking
reactions to form sp 2 phase [7]. In order to characterize the sp metastable
decay we have monitored the sp chain stability through the evolution of the
C peak intensity, either keeping the sample under UHV for several days (at
a pressure of about 2 10 9 Torr) or exposing it to different atmospheres

(H 2 , He, N 2 and dry air) [40]. In the case of UHV conditions, we have
observed a slow decrease of the intensity of both the two components of the
C peak and small changes in the shape of the G and D bands. The temporal
evolution of I C rel

is well described by an exponential decay plus a constant: