Molecular thin films

The growth of a well ordered fullerene monolayer, by means of molecular beam epitaxy, has been used for the controlled nucleation of single crystalline thin films. The quality and stability of molecular thin films has been shown... 

M. L. Gee, P. M. McGuiggan, J. N. Israelachvili, A. M. Homola. Liquid to solidlike transitions of molecularly thin films under shear. J Chem Phys 92 1895-1906, 1990. 

J. Israelachvili, P. McGuiggan, M. Gee, A. Homola, M. Robbins, P. Thompson. Liquid dynamics in molecularly thin films. J Phys Condens Matter 2 SA89-98, 1990. 

M. Schoen, C. L. Rhykerd Jr, D. J. Diestler, J. H. Cushman. Shear forces in molecularly thin films. Science 245 1223-1225, 1989. 

Experiments on transport, injection, electroluminescence, and fluorescence probe the spatial correlation within the film, therefore we expect that their response will be sensitive to the self-affinity of the film. This approach, which we proved useful in the analysis of AFM data of conjugated molecular thin films grown in high vacuum, has never been applied to optical and electrical techniques on these systems and might be an interesting route to explore. We have started to assess the influence of different spatial correlations in thin films on the optical and the electro-optical properties, as it will be described in the next section. 

Liquid lubricant confined in molecularly thin films would experience dramatic changes in its physical properties, such as increased viscosity, slow relaxation, and solidihcation. Progress in studies of thin film rheology has greatly improved our understanding of boundary lubrication, which is the subject to be discussed in this section and in the next. 

The shear thinning is also found for the bulk fluid if it is sheared at a very high rate, but in thin films the shear thinning occurs at much lower shear rates. In other words, a Newtonian lubricant in the bulk would exhibit the non-Newtonian shear response—the shear thinning, when it is confined in molecularly thin films. The observations of the... 

Another remarkable feature of thin film rheology to be discussed here is the quantized" property of molecularly thin films. It has been reported [8,24] that measured normal forces between two mica surfaces across molecularly thin films exhibit oscillations between attraction and repulsion with an amplitude in exponential growth and a periodicity approximately equal to the dimension of the confined molecules. Thus, the normal force is quantized, depending on the thickness of the confined films. The quantized property in normal force results from an ordering structure of the confined liquid, known as the layering, that molecules are packed in thin films layer by layer, as revealed by computer simulations (see Fig. 12 in Section 3.4). The quantized property appears also in friction measurements. Friction forces between smooth mica surfaces separated by three layers of the liquid octamethylcyclotetrasiloxane (OMCTS), for example, were measured as a function of time [24]. Results show that friction increased to higher values in a quantized way when the number of layers falls from n = 3 to n = 2 and then to M = 1. 

MD simulations also provide an opportunity to detect the structure of molecularly thin films. The most commonly known ordering structure induced by the confinement, the layering, has been revealed that the molecules are packed layer by layer within the film and the atoms would concentrate on several discrete positions. This has been confirmed in the simulations of liquid decane [29]. The density profile of unite atoms obtained from the simulations is given in Fig. 12 where two sharp density peaks appear at the locations near the walls, as a result of adsorption, while in the middle of the film smaller but obvious peaks can be observed on the density profile. The distance between the layers is largely identical to the thickness of the linear chain of decane molecules, which manifests the layered packing of molecules. 

A thick is deposited on top. This is then covered with a molecularly thin film of lubricant to minimize wear during start-stop contacts and to passivate the disc surface against contamination and corrosion. High-molecular-weight perfluoropolyalkylether (PFPE) polymers are widely used for this purpose. In order to improve surface bonding, the PFPEs are modified with specific functional end groups. All these molecules have similar backbone structures, namely ... 

Tal O, Epstein I, Snir O, Roichman Y, Ganot Y, Chan CK, Kahn A, Tessler N, Rosenwaks Y (2008) Measurements of the Einstein relation in doped and undoped molecular thin films. Phys Rev B 77 201201... 

When analyzing wetting it is important to know whether we are in a saturated vapor or if we are working under dry conditions. In the first case the solid surface is usually covered with a molecularly thin film of adsorbed liquid molecules and the drop spreads on this adsorbed film. In the dry case the solid surface is usually not covered by molecules of the liquid and is not in equilibrium. 

In 1988 a modified surface forces apparatus (SFA) was introduced [470,471] to analyze friction. The principle of operation of the SFA has already been introduced in Section 6.4. The modified version allowed a relative shearing of the two mica surfaces. In the SFA, the substrate has to have an atomically flat, transparent surface. In most cases mica is used to fulfill these requirements. Although there is a strong limitation in the choice of materials, due to the high resolution in the vertical direction, the SFA has become an important tool to study the friction and lubrication properties of molecularly thin films. 

The [Ru(P)]2 systems are extremely air-sensitive (see Sect. 3.22). The first sample alleged to be [Ru(OEP)]2 [32] was probably one of its autoxidation products, [RuOH(OEP)]20 (see Sect. 3.22). A molecular thin film claimed to contain porphyrin dimers prepared from RuCO(Meso-IX-DME)Py by irradiation [221] showed UV/Vis spectra similar to those described for the first such product [32]. Therefore, it probably contained also a p-oxobis(ruthenium(IV)) complex. 

T.E. Karis and G.W. Tyndall, Calculation of spreading profiles for molecularly-thin films from surface energy gradients, J. Non-Newtonian Fluid Mechanics 82, 287-302 (1999). 

However, in an attempt to integrate the SFA and spectroscopic techniques, the use of silver for optical interferometry has been seen as a drawback due to the fact that it precluded sufficient excitation source intensity to illuminate the buried interface. In order to circumvent this problem Mukhopadhyay and co-workers in an experimental set-up where the SFA was combined with fluorescence correlation spectroscopy (FCS) used, instead of silver, multilayer dielectric coatings that allowed simultaneous interferometry and fluorescence measurements in different regions of the optical spectrum [75]. Using this set-up they succeeded in measuring diffusion in molecularly thin films with singlemolecule sensitivity. 

The surface force apparatus is now being used routinely to study the equation of state of solutions confined between opposed, molecularly thin solid films. The apparatus is also used in one laboratory to study electrochemistry of thin films at electrodes a few nanometers thick and in a few other laboratories to study the behavior of molecularly thin films subjected to shear and flow [7]. 

Broadband Spectroelectrochemical Interrogation of Molecular Thin Films by Single-Mode Electro-Active Integrated Optical Waveguides... 

Figure 3. UPS spectra of the clean Pd substrate of a molecular thin-film ( 15 A) sample of isopropyl benzene in the gas phase (41).

Tappura, K. IVikholm-Lundin, I. Albers, W. M., Lipoate-based imprinted self-assembled molecular thin films for biosensor applications, Biosens. Bioelectron. 2007, 22, 912-919... 

Nano-technology will play a prominent role in the future synthesis of molecular thin films and devices. Nano-technology is defined as the study and manufacture of structures and devices with dimensions about the size of a molecule. Nano-scale physics and chemistry might lead directly to the smallest and fastest transistors and the strongest and lightest materials ever made [2], Likewise, bio-catalysts such as proteins will be increasingly used to facilitate relevant chemical reactions at ambient conditions. Natural macromolecules will be explored to provide selectivity similar to inorganic chemicals such as zeolites. 

Hu, J. Xiao, X. Ogletree, D. F. and Salmeron, M. (1995). Imaging the Condensation and Evaporation of Molecularly Thin Films of Water with Nanometer Resolution. Science 268 267. 

In summary, the position of the frontier electron energy levels (HOMO and LUMO) of porphyrins and phthalocyanines can be finely tuned by the appropriate combination of central metal and substituted ligand as detected in photoelectron spectroscopy or in the redox potentials. A rich redox chemistry in interplay with a number of reactants and counterions has been estabhshed. A systematic consideration of the electrochemical and photoelectrochemical characteristics of porphyrins and phthalocyanines as individual molecules in solution, as molecules adsorbed at surfaces, and as molecular thin films serving to mimic the characteristics of molecular aggregates is of much relevance to the choosing or designing of optimized porphyrin or phthalocyanine sensitizers for DSSCs. 

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