Pacific Rim Symposium on Surfaces, Coatings and Interfaces (PacSurf 2016)
    Thin Films Wednesday Sessions
       Session TF-WeE

Paper TF-WeE7
Study of Structural Phase Transitions during Growth of Tetracene and Pentacene Films on H/Si(001) via NEXAFS and AFM

Wednesday, December 14, 2016, 7:40 pm, Room Makai

Session: Microstructure & Surface Morphological Evolution in Organic & Inorganic Films
Presenter: Xiaorong Qin, University of Guelph, Canada
Authors: X.R. Qin, University of Guelph, Canada
J. Shi, University of Guelph, Canada
T. Regier, Canadian Light Source, Canada
D.T. Jiang, University of Guelph, Canada
Correspondent: Click to Email
To understand the formation of organic thin films and substrate influences to the molecular structures, we investigated the orientation of polyacene molecules (i.e., tetracene, pentacene) on hydrogen-passivated Si(001) as a function of coverage, using polarization-dependent C1s near-edge x-ray absorption fine structure spectroscopy (NEXAFS). Molecular films were prepared via vacuum molecular beam deposition, and in situ NEXAFS measurements were performed on the films of different coverage (from 0.2 ML to 12 ML). We report that under the film growth condition, at the early stages of the film growth the average orientation of the molecules is largely disordered. Clear upwards tilting of the molecules start when coverage exceeds a significant portion of a monolayer. With further increasing the film coverage to around 3-4 ML, a structural phase transition occurs, leading to the average molecule tilting angle around the bulk value. We conclude that, in both tetracene and pentacene growth, after the coverage of 3-4 ML, the molecules adopt the organization similar to that in the bulk-phase structure due to strong the self-assembly effect and less influence from the molecule-substrate interface. Atomic force microscopy (AFM) imaging of the films was carried out ex situ to show the morphology of the surfaces at different coverage. The film formation mechanisms and their impact to the charge transport properties of the film will be discussed.