The six articles comprising this issue of the MRS Bulletin deal with ultrathin films formed from organic molecules by a variety of techniques. In each case the component molecule forming the film is a relatively simple, single molecule which may or may not have important self-organizing properties that facilitate the formation of a film or related ordered molecular assembly. Taken together, the series of articles offer a concise look at the remarkable diversity and complexity of molecular thin films in terms of preparation and their properties.

The two-dimensional packing of organic molecules (small and large) produces ultimately thin organic films. Several possibilities exist for achieving this result from linear polymers, taking advantage of surfaces and interfaces as spatial templates. Monomolecular polymer layers can be prepared on the surface of water by polymerization of monolayers. They can be transferred onto solid supports by the Langmuir-Blodgett-Kuhn technique. Polyion complexation at the air-water interface provides a second method of two-dimensional arrangements of linear polymers. More recent approaches include polymerization-induced epitaxy (PIE) and alternate polyion adsorption. In the case of PIE, growing polymer chains in solution are adsorbed onto immersed graphite plates, most probably as single polymer layers (Figure la). Chain alignment is commensurate with the graphite lattice, as confirmed by scanning tunneling microscopy (STM). This epitaxial adsorption is independent of polymerization mechanisms. On the other hand, the alternate adsorption consists of sequential dipping of charged solid supports in aqueous solutions of oppositely charged linear polyions (Figure 1b). The regularity of the layer thickness is remarkable and, under appropriate conditions, the deposition is repeatable without limit. This method is applicable not only to pairs of linear polyions but also to combinations of linear polyions with water-soluble proteins or inorganic nanoparticles.

Recently, organic molecules and their complexes with inorganic or metallic materials have drawn many researchers' interest as candidate materials for nanoscale electronic devices of the next generation, especially since Carter's proposal on molecular electronic devices (MEDs) with the functions of gating, switching, memory, etc. in one molecule. However, in order to build such nanoscopic organic electronic devices to replace conventional silicon-based inorganic devices, one must determine how to produce such nanoscale devices and to recognize the electronic states of a single molecule.

The scanning tunneling microscope (STM) developed by G. Binning and H. Rohrer made it possible to visualize atoms and molecules in real space under various atmospheres. In addition, STMs can be used as nanoscopic tools for manipulation of individual atoms and molecules, thus realizing MEDs and nanotechnology.

In this article, we present our recent achievements concerning the STM as well as in situ x-ray diffraction studies on the molecular structure of ultrathin films prepared by vacuum evaporation. STM observations with atomic resolution reveal the mechanism of nuclei formation and the crystal-growth process in organic molecules. Computer simulations based on STM images of polar organic molecules with electronic dipoles have elucidated the role of electronic interaction for their aggregation structures.

Also, nanometer-sized molecular memory can be created by applying an electronic pulse to the evaporated organic films through the STM tip. Furthermore, we discuss the principle of a newly developed in situ total reflection x-ray diffraction (TRXD) apparatus and its application to the evaluation of crystal structure and molecular orientation in organic thin films during the evaporation process, particularly in regard to the role of the substrate, that is, epitaxial growth on organic molecular crystals.

Langmuir-Blodgett (LB) films have been applied to molecular electronics, nonlinear optics, and biosensors. Their useful properties can be accessed by using a defect-free monolayer—the precursor of LB films. In order to prepare a defect-free or defect-diminished monolayer, it is necessary to estimate the molecular arrangements and structural defects in the monolayer. The actual structure of monolayers on the surface of water have been proposed on the basis of recent morphological and structural studies. Molecular-aggregation processes or monolayer-formation processes do not always match the general concept concluded from surface pressure-area (π-A) isotherms. Therefore, a universal understanding has not been reached. Here we present a novel and systematic classification for the aggregation structure of monolayers on the water surface and also demonstrate how to prepare defect-diminished monolayers.

Formation of aggregates has been observed as a general phenomenon for a wide variety of organic molecules, especially aromatic compounds and dyes. Aggregation is most commonly encountered in crystals or in thin films. However, it has been increasingly observed in microheterogenous media or in other situations where high local concentrations occur or where specific orientation is favored. Two limiting types of aggregation have been defined based on the orientation of transition dipoles and their absorption spectral characteristics. These are the “J” aggregate, in which head-to-tail arrangements of transitiondipole moments are characterized by a sharp, intense, red-shifted transition compared to the isolated (solvated) monomer, and the “H” aggregate, where head-to-head (card-pack) orientations are characterized by a blue shift of the prominent transition compared with the monomer. Several treatments have been proposed to correlate the observed spectral shifts with the aggregate structure. For a number of compounds, the association of known x-ray-determined structures with spectral features has supported the theoretical predictions developed by Kasha and Hochstrasser or by Czikkely, Försterling, and Kuhn. The focus of the studies described here has been on aggregation occurring in Langmuir-Blodgett (LB) films and related media, such as bilayer vesicles, which are characterized by an assembly of molecules in an interfacial situation where a polar-nonpolar or water-hydrocarbon boundary should provide a strong organizing influence. In early cases where aggregates were encountered as prominent components of mixed LB films (even when relatively dilute mixtures were used), the phenomenon was usually dismissed as “microcrystallization,” which was considered an unavoidable nuisance and not really due to fundamental intermolecular interactions. More recent studies have shown that aggregation in these media is really a significant molecular phenomenon that shows dependence both on the specific molecules and the topology of the film-forming surfactant. Although some previous investigations have been carried out with different results for various substrates, we have embarked on a study to correlate aggregation behavior for a number of different chromophores incorporated into amphiphilic structures to obtain a general picture of the relative importance of different factors that can control aggregation phenomena.

When the first paper describing the formation of self-assembled monolayers (SAMs) of octadecyltrichlorosilane [CH3(CH2)17SCl3, or OTS] by adsorption on SiO2 was published, it could not have been predicted that this area of research would become so important in only one decade. Although Zisman was the first to discover that monolayers can be prepared by adsorption of a surfactant onto a clean metal surface, the real revolution in the field occurred when Nuzzo and Allara showed that SAMs of alkanethiolates on gold can be prepared by adsorption of di-n-alkyl disulfides from dilute solutions. A decrease in the use of moisture-sensitive alkyl trichlorosilanes and the increased use of crystalline gold surfaces were two important reasons for the success of these SAMs. Indeed, monolayers of alkanethiolates on gold are the most studied SAMs to date and thus deserve the most detailed discussion.

SAMs have been intensively studied in the past few years because of their relevance to science and technology. Due to their dense and stable structure, SAMs have potential applications in corrosion prevention, wear protection, and biosensing, for example. The ability to tailor both head and tail groups of the constituent molecules makes them ideal for gaining a more fundamental understanding of phenomena affected by competing intermolecular, molecular-substrate, and molecule-solvent interactions like ordering and growth, wetting adhesion, lubrication, and corrosion.

Interactions of proteins with cell membranes are of great interest in studies such as molecular recognition at cell surfaces. A monolayer lipid film at the airwater interface is useful in cell-surface modeling. Studies in binding behavior of proteins from a solution with a lipid monolayer have been reported by using various in situ techniques: surface-tension measurements, fluorescent-labeling techniques, radio-labeling techniques, fluorescence reflection methods, surface plasmon resonance, and surface-force measurements. These methods have potential applications in the observation of protein bindings. However, they require large, expensive equipment for in situ measurements and present difficulties for quantitatively obtaining the amount of protein adsorbed and the time frame for both binding and dissociation processes.

In this article, we discuss a new, easy in situ technique to detect interactions of adsorbed proteins with a phospholipids monolayer through the use of a Langmuir film balance on which a quartz-crystal microbalance (QCM) is horizontally attached to the lipid monolayer from the air phase (see Figure 1). QCMs are known to be very sensitive mass-measuring devices because their resonance frequency decreases with an increase of a given mass on a QCM at the nanogram level. Adsorption and penetration behavior of proteins can be observed quantitatively from the frequency changes (ΔF) of the QCM on the monolayer and the surface-pressure changes (Δπ) of the monolayer responding to the addition of proteins. The amount of adsorbed proteins (Δm) wasalso obtained from the frequency changes after lifting and drying the sample in air.

Recently, QCMs have become popular tools for detection of bioactive compounds such as odorous and bitter substances, and for measurement of protein adsorption, immuno-assays, DNA hybridization, enzyme reactions, and cell growth.