Dynamics of confined plumes during short and ultrashort pulsed laser ablation of graphite

Abstract

The optical emission from electronically excited C species in the ablation plume following the short ͑ns͒ and ultrashort ͑fs͒ UV pulsed laser ablation of graphite is studied. Wavelength, time and spatially resolved imaging of the plume, in background pressures of inert gases such as argon and helium, is performed. Analysis of images of optical emission from C +* ions and C 2 * radicals, yielded estimates of the apparent velocity of emitting species, which appear to arise both from the initial ablation event and, in the presence of background gas, mainly from impact excitation. At elevated background pressures of argon ͑P Ar ͒, the formation and propagation of a shock wave is observed for ns pulses, whereas for fs pulses, the propagation of two shock waves is observed. During fs ablation, the first shock wave we associate with an initial burst of highly energetic/electronically excited ablated components, indicative of an enhanced fraction of non-thermal ejection mechanisms when compared with ns ablation. The second shock wave we associate with subsequently ejected, slower moving, material. Concurrent with the plume dynamics investigations, nanostructured amorphous carbon materials were deposited by collecting the ablated material. By varying P Ar from 5 to 340 mTorr, the film morphology could be changed from mirror smooth, through a rough nanostructured phase and, at the highest background pressures for ns pulses, to a low density cluster-assembled material. The evident correlations between the film structure, the mean velocities of the emitting C species, and their respective dependences upon P Ar are discussed for both pulse durations. In addition, we comment on the effect of observed initial plume dynamics on the subsequent C cluster formation in the expanding plume.

Highly conductive nanoclustered carbon:nickel films grown by pulsed laser deposition

“…1, panels (a) -(d). The undoped nanostructures prepared using the pyrolytic graphite target display similar morphologies to those reported previously in [18] and [19]. Except for the presence of a low density of Ni nanodroplets, the as-deposited thin films formed from either target at any given p(Ar) show few obvious differences.…”

“…Except for the presence of a low density of Ni nanodroplets, the as-deposited thin films formed from either target at any given p(Ar) show few obvious differences. As Figure 1 shows, and as discussed previously [18][19], the deposited film varies from a scratch-resistant diamond-like coating at low p(Ar) to self-assembled clusters at higher pressures. This cluster assembly has been attributed to the presence of Ar, which presents a higher collision cross-section than lighter background gases such as He [18].…”

“…As Figure 1 shows, and as discussed previously [18][19], the deposited film varies from a scratch-resistant diamond-like coating at low p(Ar) to self-assembled clusters at higher pressures. This cluster assembly has been attributed to the presence of Ar, which presents a higher collision cross-section than lighter background gases such as He [18]. Figure 1 panels (e) -(f) display the microstructure of the film prepared by PLA of the mixed C:Ni target at 248 nm, in p(Ar) = 340 mTorr, before and after annealing, respectively.…”

“…Recently, we reported the preparation of nanostructured carbon thin films of differing morphologies by PLD, with morphological control achieved simply by varying the background gas pressure during laser ablation of a pyrolytic graphite target [18][19]. Here we report the electrical properties of such nanoclustered films and demonstrate how their conduction properties can be enhanced due to metal incorporation, presently achieved through PLA of a C:Ni composite target.…”

“…Furthermore, a simple scratch-resistance test suggests that the material deposited by 193 nm PLA is significantly harder than that formed by 248 nm PLA at all p(Ar), suggesting that the former films are more sp 3 -like in structure. Although argon was used as the background gas during film deposition at both ablation wavelengths, the gas flow within the chamber differed between the two experiments and so the non-observation of discrete clusters in the resulting films may reflect a consequent difference in cluster diffusion dynamics [18]. Alternatively, or in addition, if it were found that 193 nm PLA produces more energetic material, then this might account for the difference in observed morphologies and could also explain the fractured appearance, as presented in Fig.…”