Bond-Breaking Efficiency of High-Energy Ions in Ultrathin Polymer Films

Thin films of poly(methyl methacrylate) and poly(vinyl chloride) of different thickness are used to investigate the effect of spatial confinement on the efficiency of bond breaking induced by 2 MeV ${\mathrm{H}}^{+}$ and 2.1 GeV Bi ions. Effective cross sections for oxygen and chlorine loss are extracted for films down to a thickness of about 5 nm and are compared to theoretical estimations based on radial energy density profiles simulated with geant-dna. The cross sections are to a large extent thickness independent, indicating that bond breaking is dominated by short-range processes. This is in contrast to the strongly reduced efficiencies found recently for cratering induced by high-energy ions in similar ultrathin polymer films [Phys. Rev. Lett. 114, 118302 (2015)].

Figure 1

$\mathrm{C}1s$ XPS spectra irradiated thin films of PVC (a)–(c) and PMMA (d)–(f). For PVC, spectra from (a) 100, (b) 25, and (c) 5 nm films irradiated with 2 MeV ${\mathrm{H}}^{+}$ are shown. For PMMA, spectra from (d) 100, (e) 20, and (f) 13 nm films irradiated with 2.1 GeV Bi ions are depicted. The black lines are the spectra of Si substrates irradiated either with $2.5\times {10}^{15}\mathrm{H}/{\mathrm{cm}}^2$ or $7\times {10}^{11}\mathrm{Bi}/{\mathrm{cm}}^2$.

Figure 2

Relative intensity of deconvoluted $\mathrm{C}1s$ XPS peaks as a function of fluence for various thicknesses. (a) ${\mathrm{C}}_3$ (${\mathrm{CH}}_3─\mathrm{O}$) and (b) ${\mathrm{C}}_4$ ($\mathrm{C}=\mathrm{O}$) peaks of PMMA irradiated by 2 MeV ${\mathrm{H}}^{+}$. (c) ${\mathrm{C}}_3$ and (d) ${\mathrm{C}}_4$ peaks of PMMA irradiated by 2.1 GeV ions. (e) Chlorine peak of PVC irradiated by 2 MeV ${\mathrm{H}}^{+}$ using the ratios $\mathrm{Cl}:({\mathrm{C}}_1+{\mathrm{C}}_3+{\mathrm{C}}_4)$ and (f) $\mathrm{Cl}:{\mathrm{C}}_{\text{total}}$.

Figure 3

Chemical damage cross sections $\sigma$ of PVC and PMMA films of different thicknesses $h$. (a) Cross sections of chlorine loss ($\mathrm{C}─\mathrm{Cl}$ bonds) in PVC irradiated by 2 MeV ${\mathrm{H}}^{+}$ deduced from the ratio of the total chlorine to the total carbon peaks $\mathrm{Cl}:{\mathrm{C}}_{\text{total}}$ (blue) or from the ratio $\mathrm{Cl}:({\mathrm{C}}_1+{\mathrm{C}}_3+{\mathrm{C}}_4)$ (green). Carbon-oxygen bonds from PMMA irradiated by (b) 2 MeV ${\mathrm{H}}^{+}$ and (c) 2.1 GeV ions. The uncertainties given are fitting errors.

Figure 4

Results from MC simulations of 2 MeV ${\mathrm{H}}^{+}$ in water films. (a) Linear energy transfer [$\mathrm{\Delta }E(z)/\mathrm{\Delta }z$] as a function of depth $z$; (b) deposited energy density as a function of radial distance $r$ from the track center; (c) damage cross sections calculated from the simulated $\epsilon (r)$ curves of (b) using different laws of bond-breaking probability. Experimental cross sections for the polymer films are also shown for comparison. The ${\epsilon }_0$ values for Cl loss are 73.6 (hit model, $m=1$), 6.0 (hit model, $m=2$), and $4.8\mathrm{eV}/{\mathrm{nm}}^3$ (activation model), and for $\mathrm{O}─{\mathrm{CH}}_3$ bonds are 344.3 (hit, $m=1$), 13.7 (hit, $m=2$), and $7.6\mathrm{eV}/{\mathrm{nm}}^3$ (activation).