| AVS 54th International Symposium | |
| Surface Science | Thursday Sessions |
| Session SS-ThP |
| Session: | Surface Science Poster Session |
| Presenter: | J.A. Leraas, Washington State University |
| Authors: | J.A. Leraas, Washington State University S.R. John, Washington State University S.C. Langford, Washington State University J.T. Dickinson, Washington State University |
| Correspondent: | Click to Email |
Polyvinylidene fluoride [PVDF—(CH 2CF2)N] is a piezoelectric polymer employed in a variety of sensors. PVDF also possesses excellent thermal and chemical stability, which complicate the manufacture of parts with complex geometries. Structure formation by energetic radiation is potentially important to the broader application of this material. In this work, we characterize the products produced by exposing PVDF to 157-nm radiation from an F2 excimer laser. In the early stages of irradiation, the dominant neutral product is HF, produced by photochemical excitation of side chain bonds. HF production is accompanied by the conjugation of C-C bonds and cross-linking. Given sufficient laser fluences, prolonged exposure produces bursts of positive and negative charge, neutral particles, and light. We attribute burst formation to the accumulation of conjugated carbon bonds in the near surface region. When the peak temperatures generated by successive laser pulses reach a critical level, a burst results. This unusual behavior is associated with the chemistry of the PVDF molecule. In polytetrafluorethylene, for instance, 157-nm radiation cleaves the polymer backbone to produce small fragments that are continuously removed from the target. Side chain cleavage in PVDF allows damaged material to accumulate until conditions produce a large burst of emission. The charged products accompanying a burst are strongly coupled to form a tenuous plasma, or charge cloud. The positive ions in the charge cloud have kinetic energies of 7-10 eV, consistent with electrostatic ejection from the polymer surface. We show evidence for high electron densities in the charge cloud—sufficient to neutralize most (but not all) of its positive charge. These electrons are largely confined to the charge cloud, and play an important role in generation of negative ions by dissociative electron attachment to neutral species. The relatively high velocity of the charge cloud is matched only by the fastest neutrals. Therefore only the fastest neutral particles are vulnerable to attachment. The spatial and temporal distribution of the product species have important consequences on product evolution prior to deposition. The unsteady emission intensities, with occasional large bursts, provide challenges and opportunities for the growth of PVDF films by laser ablation at 157 nm.
1 This work was supported by the US Department of Energy under Grant DE-FG02-04ER-15618.