AVS 52nd International Symposium
    Surface Science Wednesday Sessions
       Session SS+EM-WeA

Paper SS+EM-WeA2
Energetics of Metals Adsorption on Polymers: Calorimetric Studies

Wednesday, November 2, 2005, 2:20 pm, Room 202

Session: Organic Film Growth and Characterization
Presenter: J.F. Zhu, University of Washington
Authors: J.F. Zhu, University of Washington
S.F. Diaz, University of Washington
P. Goetsch, University of Washington
J.J.W. Harris, University of Washington
C.T. Campbell, University of Washington
Correspondent: Click to Email
We report here the first calorimetric measurement of any metal adsorption energy onto any polymer surfaces wherein the sticking probability of the metal also was measured. The heat of adsorption and sticking probability were measured for Pb and Ca gas atoms adsorbing onto clean poly(methyl methacrylate) (PMMA) and electron-irradiated PMMA. The PMMA film was spin coated directly onto the heat detector, a pyroelectric polymer foil (polyvinylidene fluoride - PVDF) precoated on both sides with metal thin films to serve as electrodes. This provides a detector sensitivity of ~450 V/Jabs with a pulse-to-pulse standard deviation of 1.2 kJ/mol and an absolute accuracy within 2%. The Pb atoms interact very weakly with the outgassed pristine PMMA surface, with a sticking probability of 0.02 ± 0.02. They deposit a heat into the PMMA of 12.7 ± 0.7 kJ per mole of dosed Pb, independent of Pb exposure up to 10 ML. This is slightly less that would be expected even if no Pb atoms stuck to the PMMA, but if they completely thermally accommodated to the substrate temperature during their collisions with the surface. This proves that thermal accommodation is incomplete, highlighting the weakness of the Pb - PMMA interaction. Calcium interacts with pristine PMMA much more strongly, with an initial sticking probability of ~0.5 and an initial heat of adsorption above 240 kJ/mol. Damaging the PMMA surface with electrons causes an increase in reactivity with Pb, as evidenced by increases in the initial heat of adsorption up to 134.0 ± 0.7 kJ/mol and the initial sticking probability up to 0.51 ± 0.01. Both increase with increasing coverage toward values expected for Pb adsorption onto bulk Pb.