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

Paper SS+EM-WeA4
Contrasting the Assembly and Molecular Architecture of N-heteroaromatic Molecular Films on Ag(111): ACA vs. INA

Wednesday, November 2, 2005, 3:00 pm, Room 202

Session: Organic Film Growth and Characterization
Presenter: H. Li, University of Maryland at College Park
Authors: H. Li, University of Maryland at College Park
B. Xu, University of Maryland at College Park
D. Evans, University of Maryland at College Park
J. Reutt-Robey, University of Maryland at College Park
Correspondent: Click to Email
Monolayer films of isonicotinic acid (INA) and 9-acridine carboxylic acid (ACA) were prepared by physical vapor deposition and investigated with complementary scanning tunneling microscopy (STM) and spectroscopy (XPS, RAIRS) under UHV conditions. These N-heteroaromatic acids offer the same H-bonding motifs, but varied sizes and electrostatic properties, which should impact film architecture. In the monolayer regime, STM images reveal that both molecules readily assemble at room temperature into ordered "2-D" structures with typical domain sizes exceeding 100 nm for INA and 1000 nm for ACA. Differences between the domain structures (INA domains being more anisotropic and with much sharper (molecularly smooth) boundaries) are related to differences in their intermolecular forces. We propose structural models consistent with high resolution STM images and test these structural models with detailed spectroscopic measurements. XPS studies of N 1s and O 1s core level shifts identify a predominant H-bonding motif based upon N head-to- carboxyl tail interaction, in agreement with the proposed structural model. Surface infrared measurements of INA molecular films detect a pronounced out-of-plane aromatic H bending mode at 858 cm@super -1@, and an intensity analysis determines the aromatic plane to be tilted by ~20° from the substrate plane. The proposed structural model for ACA involves an arrangement of ACA molecules with greater and alternating tilt angles, which we are presently testing with infrared experiments. We will account for these distinct molecular film architectures with the differences in the intermolecular forces and discuss the generality of these effects.