AVS 56th International Symposium & Exhibition
    Surface Science Wednesday Sessions
       Session SS2-WeM

Paper SS2-WeM2
Effect of Surface Order and Thickness on the Adsorption Mechanism of NO on FePc

Wednesday, November 11, 2009, 8:20 am, Room N

Session: Surface Chemistry and Dynamics
Presenter: S.R. Bishop, University of California, San Diego
Authors: S.R. Bishop, University of California, San Diego
N. Tran, University of California, San Diego
A.C. Kummel, University of California, San Diego
Correspondent: Click to Email
The diverse electronic properties, chemical and thermal robustness, and ease of deposition (spin coating and organic molecular beam epitaxy) make Metallophthalocyanine (MPc) materials an attractive and economical candidate for use in chemical sensors. The purpose of this study is to obtain an atomic level understanding of the fundamental mechanisms in which analytes interact with MPc thin films. MPc thin films investigated include ordered monolayer FePc, ordered multilayer FePc, and quasi-amorphous tetra-t-butyl FePc multilayer thin films. These experiments were further supported with density functional theory (DFT) of NO adsorption on FePc via simulated potential energy diagram. Exploration of how surface order and thickness affects NO adsorption was performed via King and Wells sticking measurements. The unique sticking profile features a sharp, downward spike, representing a short saturation time. For monolayer and ordered multilayer FePc/Au(111), NO adsorption onto FePc saturates at 3% of a monolayer suggesting that the final chemisorption site is confined to the iron metal centers. Furthermore, the saturation coverage is only 2% for the quasi-amorphous multilayer. The reduced saturation coverage in comparison to ordered FePc is attributed to the lower packing density of the tert-t-butyl FePc. At low surface temperature and low incident beam energy, the initial sticking probability is as great as 40% and decreases linearly with increasing beam energy and surface temperature. This is consistent with the NO molecule sticking onto the monolayer FePc via physisorption to the organic periphery followed by diffusion to the Fe metal center, precursor-mediated chemisorption. For the multilayer ordered flat-lying film, the sticking probabilities are greater in comparison to the monolayer for the same incident beam energy and surface temperature. More efficient trapping onto the ordered multilayer film is consistent with NO having improved mass-matching with the multilayer FePc surface compared to monolayer. In addition, computations suggest that there are multiple available physisorption sites available within the organic periphery of the FePc films. The results strongly suggest the analyte adsorbs via a multiple pathway precursor-mediated chemisorption mechanism. A current study focuses on the analyte adsorption onto a totally amorphous film. MPc forms a β- polymorph on SiO2 substrates which are no longer flat-lying, and planned sticking measurements of NO interaction with amorphous FePc will complete the study of the effect of surface order on analyte adsorption.