Anisotropic molecular dynamics in the vicinity of order-disorder transitions in organic monolayers

Abstract

Variable-temperature scanning tunneling microscopy has been used to observe molecular dynamics in the vicinity of order-disorder transitions in alkane monolayers at the interface between graphite and organic fluids. In crystalline lamellae adsorbed from solution a reversible increase in longitudinal molecular motion with increasing temperature is observed, until the amplitude becomes comparable to the lamella width and the order disappears. A two-dimensional smectic and a columnar phase are observed at the interface with a neat melt and a solution of a particularly long-chain alkane, respectively. PACS numbers: 68.55Jk, 61.16.Di, 81.15.Lm Order-disorder transitions at interfaces, like melting, roughening, and shape fluctuations, occur in various different condensed-matter systems, including metals, semiconductors, polymers, colloids, or biological membranes. Statistical mechanical models and scaling theories have been employed to investigate the underlying theoretical physics (see, e.g., [1,2]). Scattering techniques like ion scattering [3] and low-energy electron diffraction [4] allowed observation of surface premelting, holographic interferograms have been used to follow roughening transitions at the 4 He solid-superfluid interface [5], and optical microscopy was employed to study shape fluctuations in lipid vesicles as a function of temperature [6]. Scanning tunneling microscopy (STM) has been used before to investigate crystalline monomolecular layers adsorbed from liquid-crystalline ambients [7] or the frozen-in roughness of steps on a vicinal silicon surface [8]. However, for a molecular system a microscopical observation of orderdisorder transitions directly on the molecular level has not yet been reported.Monomolecular layers of long-chain w-alkanes, adsorbed at the interface between organic solutions or melts and the basal plane of graphite, are a simple model system for physisorbed organic monolayers. Their adsorption isotherms have been measured [9], and the molecular structure [10-12] and dynamics [13] of crystalline monolayers adsorbed from solution have been investigated recently by STM, indicating that long-chain alkanes may adsorb as epitaxial monolayers at room temperature. In the following we describe order-disorder transitions with increasing temperature and molecular weight and attribute them to anisotropic molecular dynamics within the monolayer.The STM was homebuilt [14]. It allows scan rates up to 1 ms per line, corresponding to an image recording rate of up to 10 frames (100x100 points each) per second. Real-time video recordings can be used to directly observe sufficiently slow molecular dynamics, like molecular reorientations associated with annealing processes [11,15]. They also offer a convenient way to identify thermal drifts. For variable-temperature STM studies between room temperature and 120°C the sample was mounted on a Peltier element, which was heated by a constant current. The sample temperature was measured with a thermoresistor attached with silver paint to the side of th...

Effects of conformational isomerism on the desorption kinetics of n-alkanes from graphite

“…A set of n-alkanes (C N H 2Nϩ2 , Nϭ5, 6,7,8,10,12,14,16,18,20,22,24,26,44, 60͒ were purchased having purities of Ͼ98% from Aldrich Chemicals. Acros Chemicals supplied n-alkanes (Nϭ28, 32, 36, 40͒ in purities of Ͼ97%.…”

“…These phenomena are similar to those observed by Askadskaya and Rabe in their studies. 26 All the variable temperature STM images of long chain n-alkanes adsorbed on graphite suggest that heating causes displacement of methylene groups from the surface and might thereby decrease ⌬E des ‡ relative to that expected for n-alkanes in an all-trans conformation.…”

“…Such disorder has been observed in variable temperature STM images of long chain n-alkanes adsorbed on graphite which indicate that a multitude of conformations may be adopted at elevated surface temperatures. Askadskaya and Rabe 26 used a variable temperature STM to image monolayer coverages of C 24 H 50 and C 32 H 66 on graphite in air. High-resolution images of each monolayer were obtained at 297, 303, 313, and 318 K. At 297 K, highly ordered lamellae of the n-alkanes were observed with their carbon atoms in an all-trans conformation and their molecular backbone parallel to the surface.…”

Structure and phase transitions of monolayers of intermediate-length n-alkanes on graphite studied by neutron diffraction and molecular dynamics simulation

“…3,24,25 Of these systems, probably the best characterized are monolayers of C32 and C24 on high-quality graphite basal-plane surfaces that have been investigated by scanning tunneling microscopy ͑STM͒, [10][11][12][13][14] x-ray diffraction, 15 and neutron scattering. [16][17][18][19][20][21] Graphite substrates also have the advantage that relatively reliable empirical atom-atom potentials are available for representing the molecule/graphite basal-plane interaction.…”

“…Previous experimental studies of the monolayer structure of intermediate-length alkanes have focused on high-quality solid surfaces of graphite, [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] Ag͑111͒, 22,23 and SiO 2 . 3,24,25 Of these systems, probably the best characterized are monolayers of C32 and C24 on high-quality graphite basal-plane surfaces that have been investigated by scanning tunneling microscopy ͑STM͒, [10][11][12][13][14] x-ray diffraction, 15 and neutron scattering.…”