Revisiting thin film of glassy carbon

Glassy carbon (GC) is a chemically stable form of fully $s{p}^2$-bonded carbon with locally ordered domains. GC is the intermediate material between graphite and diamond combining various properties such as high temperature resistance, hardness, good electrical conductivity, low density, low gases and liquids permeability, and excellent resistance to a wide range of aggressive chemical environments. These characteristics make it a very promising material for many applications, but unfortunately it is not widely used because of the high temperatures required for its synthesis. In this work, synthesis of glassy carbon thin films by means of laser ablation of carbon targets under vacuum or in gaseous helium, followed by a nanosecond laser irradiation of the deposited films, is presented. In particular, it is demonstrated that the amorphous structure of a thin film can be efficiently modified to the one of glassy carbon film by nanosecond UV laser irradiation. This method is valuable to prepare thin films similar to commercial glassy carbon with a completely different route which does not require the application of temperature beyond $1000{}^{\circ }\mathrm{C}$ which is not compatible with the silicon substrate for example. This opens for glassy carbon the way to microengineering applications (mechanics, electronics,$\cdots$). Particular attention is paid to characterize the vitreous carbon. In the literature, the vitreous nature of carbon layers is often highlighted on the basis of Raman spectroscopy measurements. However, as the Raman spectrum of glassy carbon is similar to that of pyrocarbon, multiwall carbon nanotubes, or functional graphene, this technique is not sufficient to safely characterize a carbonaceous material with a high degree of allotropy. To clear up any doubts, additional characterization methods, such as x-ray spectroscopy, transmission electron microscopy, and Rutherford backscattering spectrometry, are discussed here.

Figure 1

SEM images of carbon film deposited by PLD at (a) ${10}^{-6}$ mbar and (b) 1 mbar of helium.

Figure 2

Raman spectra of the glassy carbon c.t. (a) and of carbon thin films deposited by PLD at two different pressures: (b) ${10}^{-6}$ mbar and (c) 1 mbar of helium. The peak at 950 ${\text{cm}}^{-1}$ is due to the second order Raman band from the Si substrate. The red solid line is the result of the fit.

Figure 3

Corrected Raman spectra of irradiated porous carbon film with different number of laser pulses: (a) unirradiated carbon, (b) 5 shots, (c) 10 shots, (d) 100 shots, (e) 200 shots, (f) 1000 shots (see Supplemental Material [13] Fig. S2).

Figure 4

Characteristics (position and FWHM) of the $G$ (a) and $D$ (b) bands versus the number of laser pulses delivered on porous carbon film deposited by PLD at 1 mbar of He. The $I_D/I_G$ ratio is given in (c). For the GC c.t., the two bands $D$ and $G$ are situated at around 1330 ${\text{cm}}^{-1}$ and 1599 ${\text{cm}}^{-1}$, respectively, and the $I_D/I_G$ ratio is 2.19.

Figure 5

High resolution XPS spectra of the $C_{1s}$ peak: (a) glassy carbon c.t., (b) as-deposited carbon film (1 mbar of He), and (c) irradiated film with 1000 laser pulses.

Figure 6

Electron energy loss spectra of as-deposited carbon film (1 mbar of He) and irradiated film with 200 laser pulses. Spectrum of the glassy carbon c.t. is also shown for comparison.

Figure 7

SEM images of a porous carbon film (deposited at 1 mbar of helium) before (a) and after (b) laser irradiation.

Figure 8

Low-frequency Raman spectra of the glassy carbon c.t. (a) and the nonporous carbon film before (b) and after laser irradiation with 200 pulse (c).

Figure 9

HRTEM images of porous carbon film: (a) before irradiation and (b) after irradiation with 200 pulses. (c) corresponds to the glassy carbon c.t.. The labels in (b) correspond to (1) fullerene, (2) multilayers fragment, (3) honeycomb-like fragment, (4) presence of curvature with the angle of ${120}^{\circ }$ and (5) hyperbolic structure.

Figure 10

Different steps to prepare amorphous TPMS structure compared to other methods (right bottom panel) [54].