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Efficient charge transport is key to the operation of the various devices based on organic semiconductors, such as OLEDs, OPVs and OFETs. Both academia and industry are investing heavily in the development of new organic materials and processing techniques to improve device performance. An important parameter to tweak is the mobility of charge carriers. Triggered by an experimental result, here we investigate whether this parameter can be probed indirectly using UV-VIS spectroscopy. This would simplify the process of characterization and optimization of the mobility in amorphous molecular films, for example.

It is important to obtain single crystalline organic thin films for electronics and optics applications. Due to the mismatching in the crystal symmetry, it is difficult to align the crystalline grains of organic molecular films even on single crystalline surfaces. We have developed several techniques for the artificial grain alignment in organic epitaxial growth. (1) Use of nanoscale-textured surfaces prepared by step bunching of vicinally-cut single crystalline surfaces, in which the height of the steps is critically important. (2) Application of external electric field. (3) Optical excitation of the molecules which can be applied to the polar semiconducting molecules. These techniques might be applicable to other materials including ionic materials and ferroelectrics.

A microscopic theory of dielectrical properties of thin molecular films, i.e. quasi 2D systems bounded by two surfaces parallel to XY planes was formulated. Harmonic exciton states were calculated using the method of two-time, retarded, temperature dependent Green's functions. It has been shown that two types of excitations can occur: bulk and surface exciton states. Analysis of the optical properties of these crystalline systems for low exciton concentration shows that the permittivity strongly depends on boundary parameters and the thickness of the film. Conditions for the appearance of localized exciton states have been especially analyzed.

The electronic structure and highest occupied to lowest unoccupied molecular orbital gap of undoped films of the molecular icosahedra of closo-1-phospha-7-carbadodecaborane (1,7-PCB10H11, meta-phosphacarborane) are reported. For 1,7-PCB10H11 adsorbed on Au and Ag, the Fermi level is placed closer to the lowest unoccupied molecular orbital than has been observed with closo-1-phospha-2-carbadodecaborane (1,2-PCB10H11, ortho-phosphacarborane) adsorbed on Au and Ag.

The oligothiophene derivative (4-(5″″-tetradecyl-[2,2′;5′,2″;5″,2″′;5″′,2″″] pentathiophen-5-yl)-butyric acid (C14-5TBA) was synthesized and the structural and mechanical properties of self-assembled monolayers on mica have been studied by atomic force microscopy (AFM). The films were prepared by drop casting a dilute THF solution (1mM) of the oligothiphene on mica. Islands containing primarily monolayers with a very small percentage of multilayers were formed. The molecules adsorb through the carboxylic group, and expose the alkyl chain (CH2)13CH3. High resolution AFM scans reveal a well ordered structure of molecules with unit cell dimensions of 0.65 and 0.46 nm. Applying load to the tip, the molecular film was gradually compressed from an initial height of 4.1nm to a final one of 2.6 nm, corresponding to atilt of the alkyl chains. In regions covered with bilayers the molecules in the second layer were oriented opposite to those in the first layer, thus exposing the carboxylic end group to the air. These second layer was easily removed as the tip pressure increased.

Ferroelectric vinylidene fluoride (VDF) molecular films were fabricated by simple vacuum evaporation method, and the ferroelectric properties and its fatigue were investigated. Formation of ferroelectric phase in VDF oligomer with low molecular weight is favored at low substrate temperature around -150°C. The well-ordered VDF oligomer thin films exhibit a lager value of remanent polarization(130mC/m2) than that of Poly(VDF). Fatigues of polarization reversal can be performed over 10∧5 cycles. The VDF oligomer films can be one of candidates for disposable nonvolatile memory with unique features such as flexible, wide areas and low cost processing.

We contrast the interaction of mercury with adsorbed orthocarborane films and semiconducting (dehydrogenated) boron carbide. Photoemission spectra reveal small shifts in orthocarborane (C2B10H12) molecular orbital binding energies as well as the shift in mercury 5d5/2 shallow core level binding energies, suggesting only small interaction between mercury and the molecular film. Mercury does, however, interact with decomposed orthocarboranes i.e. semiconducting boron carbide.

We investigated molecular thin films fabricated using novel organic materials, such as dendrimers with a rhodamine B (Rh-B) core, by a pump-probe measurement for ultrafast time-resolved spectroscopy with a time resolution of approximately 160 fs. The photon energy transfer caused by the interaction of the dendrimer molecules was measured in terms of absorbance. The wavelength of the pump light was 550 nm and a white continuum light spanning from 450 nm to 750 nm was used as a probe light. A spin-coated molecular film made of dendrimer molecules with Rh-B core has a relatively high molecular density. We measured a strong stimulated emission at a wavelength of 612 nm, whose fast decay was assumed to be caused by intermolecular interactions between the dendrimer molecules with Rh-B core. The lifetime of the molecules in the excited state was measured to be as short as 10 ps. Several types of molecular films with different molecular densities were compared, and cases with different pump energies were also discussed.

Molecular films of amino propyl triethoxy silane (APTES) attached with cinnamoyl moieties were deposited on indium tin oxide (ITO) coated glass and quartz substrates by self-assembly technique. The deposition of the monolayer films resulted in a large change in the wettability of the substrates. X-ray photoelectron spectroscopy (XPS) and UV-spectroscopy were used to study the structure and properties of the grown films. The molecular assembly was photodimerized by irradiating it with a linearly polarized UV-light. Thus created polymer-solid interface has been shown to produce good, stable planar orientation of ferroelectric liquid crystal mixtures. The electro-optical and switching properties of the ferroelectric liquid crystal cells have been investigated and found to be quite similar to those prepared in conventional manner.

Correlated quantum-chemical calculations performed on supermolecular structures, i.e., on clusters made of several oligomer chains in interaction, provide insight into structure-property relationships in well-organized molecular films. This supermolecular approach is applied to crystalline phases of two prototypical conjugated oligomers and shows that variations in chain packing can lead to dramatically different optical (Davydov) splittings and carrier mobilities. Optimal chain organizations for various types of devices are discussed.

Hydrogen-terminated silicon surfaces exposing a molecular film of covalently grafted pyrrole units were formed by a solution-phase reaction using the organolithium reagent 5-(N-pyrrolyl)pentyllithium. Electrochemical polymerization of pyrrole onto native hydrogen-terminated silicon surfaces and these chemically modified supports produced silicon/polypyrrole junctions that showed diode-like characteristics, with those formed on the latter substrate exhibiting higher current densities and better ideality factors. The presence of the grafted pyrrole films on silicon provided a better electrochemical control over the polymerization process and yielded smoother polypyrrole films than on the native hydrogen-terminated silicon supports. Impedance measurements revealed that the electrical improvements to the silicon/polypyrrole junctions were a consequence of incorporating sites on the silicon surface for direct contact between the conducting polymer and the semiconductor, as the barrier height of the junction appeared to be unaffected by the chemical modification.

This works addresses the question of the direct attachment of organic molecules on Si(111) by an electrochemical method. Anodic grafting of -OR group is demonstrated by in-situ STM and the LDOS characterized. The grafting of aryl groups, by reduction of aryl diazonium salts in aqueous solution, is also described. This approach leads to well ordered and close-packed thin molecular films with various functionality. Different chemical and structural characterizations conclude to a Si-C binding, between the Si surface and aryl groups. The stability of films is also investigated.

We introduce a model to calculate the static equilibrium configuration between two monolayers of large, planar organic molecules bonded by van der Waals (vdW) forces. The Model significantly simplifies analysis by replacing the conventional atom-atom potential summation with a single ellipsoidal potential centered in the molecular plane. Our results indicate that recent observations of crystalline “quasi-epitaxial” growth of these planar molecular films in ultrahigh vacuum results from the relatively large intralayer elasticity as compared to the small interlayer stress. Good agreement between calculated and observed structures is achieved using no adjustable parameters. The Model is used to predict molecular structures which are likely to form quasi-epitaxial layers. Quasi-epitaxy is a general feature of large planar molecules of similar shape and size in adlayers and substrates where there is comparatively weak vdW bonding between layers.

Time-of-flight (TOF) experiments were performed to study the desorption of trans-l,2-dichlorocyclohexane molecules from molecular films condensed on a quartz-crystal microbalance at 77 K. The wavelength dependence of the desorption signal was investigated by resonant excitation of the internal vibrational mode of the molecule in the 10 μm region with a pulsed line-tunable TEA CO2 laser. The TOF temperatures showed the same spectral dependence as the optical absorption coefficient. The TOF distributions were narrower than Maxwellian distributions for multilayer desorption yields and approached Maxwellian distributions for submonolayer desorption yields. A saturation behavior was found at high laser intensities, indicating that the kinetic energy of the desorbing molecules is limited to a maximum value.

Certain molecules when placed in contact with a surface can bind to form well-ordered two dimensional arrays. This process is called molecular selfassembly (MSA), and it is emerging as an important method to control the interactions between a surface and its environment (interfacial properties). Molecular self-assembly uses the fundamental forces between molecules (Van der Waals interactions, hydrophobic effects and hydrogen bonding) to form highly ordered macromolecular systems. This process has been used to synthesize molecular films on a variety of surfaces including glass, gold, alumina, platinum, and silicon. The objective of this paper is to describe the application of molecular self-assembled films to the study of biological systems. Specifically, we will describe methods to synthesize and characterize surfaces that contain molecules that bind to bacteria and viruses. We will also evaluate the use of surface science techniques to characterize the physical and chemical properties of these surfaces, and we will attempt to correlate these properties with the adhesion of biological molecules.

Molecular electronics requires materials of molecular size exhibiting switching or memory properties. The Langmuir-Blodgett method (L.B.) and its variants provide controlled fabrication of molecular films one molecule thick and their transfer onto solid supports. In these 2D monolayers, molecules are highly organized, so that complex organizates aimed at specific properties can be built. One of these, a semi-amphiphilic complex made of two NCS and two amphiphilic phenantrolines in a cis position, exhibits a spin transition around - 20 °C. It switches from high spin (S = 2) at room temperature to low spin (S = 0) at low temperature. The transition does not follow the expected thermal Boltzman statistics, but is rather steep, pointing out molecular cooperation. The L.B. film exhibits a hysteresis about 4 °C in width, while the parent 3D compound exhibits a hysteresis of less than 0.2 °C, but a much sharper transition. This apparently contradictory behaviour is ascribed to the high impurity content and the multicristallinity of the L.B. film in association with high intermolecular coupling. Because of its intrisic properties this new material is a breakthrough on the way to magnetic memories at the molecular scale for molecular electronics.