Coadsorption of two different carboxylic acids, benzenetribenzoic acid and trimesic acid, was studied at the liquid-solid interface in two different solvents (heptanoic and nonanoic acid). Independent alteration of both concentrations in binary solutions resulted in six nondensely packed monolayer phases with different structures and stoichiometries, as revealed by means of scanning tunneling microscopy (STM). All of these structures are stabilized by intermolecular hydrogen bonding between the carboxylic acid functional groups. Moreover, phase transitions of the monolayer structures, accompanied by an alteration of the size and shape of cavity voids in the 2D molecular assembly, could be achieved by in situ dilution. The emergence of the various phases could be described by a simple thermodynamic model.

A simple model system for the 2D self-assembly of functionalized organic molecules on surfaces was examined in a concerted experimental and theoretical effort. Monolayers of 1-halohexanes were formed through vapor deposition onto graphite surfaces in ultrahigh vacuum. Low-temperature scanning tunneling microscopy allowed the molecular conformation, orientation, and monolayer crystallographic parameters to be determined. Essentially identical noncommensurate monolayer structures were found for all 1-halohexanes, with differences in image contrast ascribed mainly to electronic factors. Energy minimizations and molecular dynamics simulations reproduced structural parameters of 1-bromohexane monolayers quantitatively. An analysis of interactions driving the self-assembly process revealed the crucial role played by small but anisotropic electrostatic forces associated with the halogen substituent. While alkyl chain dispersion interactions drive the formation of a close-packed adsorbate monolayer, electrostatic headgroup forces are found to compete successfully in the control of both the angle between lamella and backbone axes and the angle between surface and backbone planes. This competition is consistent with energetic tradeoffs apparent in adsorption energies measured in earlier temperature-programmed desorption studies. In accordance with the higher degree of disorder observed in scanning tunneling microscopy images of 1-fluorohexane, theoretical simulations show that electrostatic forces associated with the fluorine substituent are sufficiently strong to upset the delicate balance of interactions required for the formation of an ordered monolayer. The detailed dissection of the driving forces for selfassembly of these simple model systems is expected to aid in the understanding of the more complex self-assembly processes taking place in the presence of solvent.haloalkanes ͉ conformation ͉ simulations F uture advances in nanoscale science and engineering are expected to rely increasingly on the controlled bottom-up assembly of molecular arrays. The successful creation of targeted molecular device structures demands a fundamental understanding of the interactions governing 2D self-organization. Simple functionalized hydrocarbon molecules are known to form self-ordered structures at a variety of surfaces and interfaces (1-32) and can serve as ideal model systems to study the underlying forces driving the selfassembly process.Numerous experimental (1-10, 13, 14, 16-19, 21, 22, 24, 27, 28, 30-41) and theoretical (39, 42, 43) studies have investigated the self-assembled monolayers formed when a melt or solution containing alkanes or their derivatives is brought in contact with the basal plane of a graphite substrate. Scanning tunneling microscopy (STM) (2-6, 8-10, 13, 14, 16-19, 24, 27, 28, 31-38, 40, 41) and diffraction-based probes (7,39,44,45) have been used to characterize the crystallographic monolayer parameters as well as the orientation and conformation of the constituent molecular species. For many alkane...

The structural properties of self-assembled monolayers of short 1-bromoalkanes and n-alkanes on graphite were investigated by a combination of ultrahigh vacuum scanning tunneling microscopy (UHV-STM) at 80 K and theoretical methods. STM images of 1-bromohexane reveal a lamellar packing structure in which the molecules form a 57° ± 3° lamella-molecular backbone angle and a head-to-head assembly of the bromine atoms (Müller, et al. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 5315). STM images of 1-bromoheptane also show a head-to-head 60° ± 3° lamella-molecular backbone pattern; however, the molecules pack in a herringbone structure. The odd/even chain-length alternation in the monolayer morphologies of 1-bromoalkanes is similar to that observed for the self-assembly of short n-alkanes on graphite, suggesting that the bromine atom acts effectively as an extension of the carbon backbone. The analogy, however, is incomplete. Odd and even short n-alkanes (hexane, heptane, octane) display 60° herringbone and rectangular (not 60°) lamella-molecular backbone configurations, respectively. The balance of intermolecular forces and packing considerations responsible for this odd/even alternation in monolayer morphology for short 1-bromoalkanes on graphite is examined here according to classical molecular dynamics simulations and in light of the structural properties of analogous n-alkane assemblies.

Self-assembled monolayers of chrysene and indene on graphite have been observed and characterized individually with scanning tunneling microscopy (STM) at 80 K under low-temperature, ultrahigh vacuum conditions. These molecules are small, polycyclic aromatic hydrocarbons (PAHs) containing no alkyl chains or functional groups that are known to promote two-dimensional self-assembly. Energy minimization and molecular dynamics simulations performed for small groups of the molecules physisorbed on graphite provide insight into the monolayer structure and forces that drive the self-assembly. The adsorption energy for a single chrysene molecule on a model graphite substrate is calculated to be 32 kcal/mol, while that for indene is 17 kcal/mol. Two distinct monolayer structures have been observed for chrysene, corresponding to highand low-density assemblies. High-resolution STM images taken of chrysene with different bias polarities reveal distinct nodal structure that is characteristic of the molecular electronic state(s) mediating the tunneling process. Density functional theory calculations are utilized in the assignment of the observed electronic states and possible tunneling mechanism. These results are discussed within the context of PAH and soot particle formation, because both chrysene and indene are known reaction products from the combustion of small hydrocarbons. They are also of fundamental interest in the fields of nanotechnology and molecular electronics.

The phase ordering of 1-bromoeicosane (C20H41Br) at the liquid-graphite and vacuum-graphite interfaces is examined through a joint experimental (part I) and theoretical effort (part II). The stable morphologies under solvent and ultrahigh vacuum conditions are revealed by STM experiments to be the head-to-head structures with 90° and 60° lamella−backbone angles, respectively. At 90° and 60° close packing is attained, independent of the corrugation of the graphite lattice. The potential energy of the minimized 60° structure is slightly lower than that of the 90° structure under vacuum conditions. In addition, the basin of the potential energy surface about the 90° form is very narrow. All-atom molecular dynamics simulations depict a 90°-to-60° phase transition in vacuum. Both morphologies are stable when an explicit solvent model is included. We speculate that the choice of the 90° form under solvent is driven by symmetry considerations and the self-assembly pathway. For example, the 90° structure may serve as a superior template for solvent capping. An implicit solvent model fails to stabilize the 90° form; however, it does lower the potential energy of this structure relative to the 60° geometry.

The self-assembly of cyanuric acid into ordered nanostructures on a crystalline substrate, highly ordered pyrolytic graphite (HOPG), has been investigated at low temperature under ultrahigh vacuum (UHV) conditions by means of scanning tunneling microscopy in conjunction with theoretical simulations. Many domains with different self-assembly patterns were observed. One such domain represents the formation of an open 2D rosette (cyclic) structure from the self-assembly process, the first observation of this type of structure for pure cyanuric acid on a graphite substrate. Each self-assembled domain exhibits characteristic superstructures formed through different hydrogen bond networks at low coverage and low deposition rate. Experimental observation of coexistent, two-dimensional crystalline structures with distinct hydrogen bond patterns is supported by energy minimizations and molecular dynamics calculations, which show multiple stable structures for this molecule when self-assembled on graphite.

Self-assembled monolayers of 1-bromoeicosane (BrC20H41) have been investigated at the vacuum−graphite and liquid−graphite interfaces using scanning tunneling microscopy (STM) (Part I) and theory (Part II). Under ultrahigh vacuum conditions at 80 K, STM images show 1-bromoeicosane in a lamellar assembly structure where individual molecules are predominantly arranged with their bromine groups pointed head-to-head with a 66 ± 3° angle between the lamella direction and the molecular backbone axis. A significant degree of disorder is observed under vacuum conditions, in which head-to-tail defects are interspersed throughout the film. When 1-bromoeicosane monolayers are formed and imaged in equilibrium with solution at ∼290 K, the molecules again pack with their bromines oriented head-to-head but shift to form a nearly rectangular array. Subtle changes are observed depending on the solvent utilized, with the lamella−backbone angle varying between 81 ± 3° and 90 ± 2°, and the corresponding intermolecular spacing ranging from 0.36 ± 0.02 to 0.46 ± 0.05 nm. The differences between the vacuum- and solution-based packing, as well as the possibility of two different packing structures occurring in solution, are discussed in light of the joint experimental and theoretical analysis.

This study details a scanning tunneling microscopy investigation into the mechanism of chiral grain growth in highly ordered, self-assembled monolayer films composed of cruciform pi-systems. Although the molecules themselves are achiral, when they adsorb from solution onto graphite, they adopt a gear-like conformation that, by virtue of the surface, is chiral. These handed subunits arrange themselves into enantiomeric two-dimensional domains. The unique finding from this study is that Ostwald ripening is frustrated between domain boundaries that are of opposite chirality because direct interconversion between the chiral units on the surface is energetically inhibited.