Molecular film

Surface SHG [4.307] produces frequency-doubled radiation from a single pulsed laser beam. Intensity, polarization dependence, and rotational anisotropy of the SHG provide information about the surface concentration and orientation of adsorbed molecules and on the symmetry of surface structures. SHG has been successfully used for analysis of adsorption kinetics and ordering effects at surfaces and interfaces, reconstruction of solid surfaces and other surface phase transitions, and potential-induced phenomena at electrode surfaces. For example, orientation measurements were used to probe the intermolecular structure at air-methanol, air-water, and alkane-water interfaces and within mono- and multilayer molecular films. Time-resolved investigations have revealed the orientational dynamics at liquid-liquid, liquid-solid, liquid-air, and air-solid interfaces [4.307]. 

During the remainder of the 1930s, Langmuir and Blodgett carried out a brilliant series of studies on multilayer films of a variety of chemicals, supplemented by studies in Britain, especially at the ill-fated Department of Colloid Science in Cambridge (Section 2.1.4). Then the War came, and momentum was lost for a couple of decades. After that, L-B films came back as a major topic of research and have been so ever since (Mort 1980). It is current practice to refer to molecular films, made by various techniques (Swalen 1991), but the L-B approach remains central. 

Molecular films are of intense current concern in electronics. For instance, diacetylenes and other polymerisable monomer molecules have been incorporated into L-B films and then illuminated through a mask in such a way that the illuminated areas become polymerised, while the rest of the molecules can be dissolved away. This is one way of making a resistance for microcircuitry. L-B films have also found a major role in the making of gas-sensors (Section 11.3.3). 

Finally, there is another category of lubricants, including the laminated materials, highly ordered organic mono-layers, and various thin solid hlms, which provides effective lubrication via their properties of low shear strength or high wear resistance. Lubrication via ordered molecular films and other solid lubricants, which have been considered by some investigators as a sub-discipline of boundary lubrication, will be discussed more specifically in Section 4. 

Fig. 15—Sketch of preparation of L-B films (a) spread amphiphilic molecules on water surface, (b) compress the molecules using the barrier to get close packed and ordered molecular film, (c) transfer the film onto a substrate through the vertical immerse/retreat process, (d) transfer the film by horizontal lifting.

Readers who want more systematic information on ordered molecular films are referred to the book by Ulman [31]. 

Experimental Studies on Tribological Properties of Molecular Films... 

Phenomenal studies were made to observe the frictional behavior of L-B films and SAMs and its dependence on applied load and sliding velocity, which has been summarized in a review article by Zhang [33]. It has been confirmed that in comparison to the bare surface of the substrates, the friction on molecular films is significantly reduced, with friction coefficients in a range of 0.05-0.1. Friction forces are found... 

Perhaps, the most interesting observation on the molecular films is the dependence of friction on chain length. It has been reported from independent experiments conducted on different sorts of L-B films and SAMs that the friction decreases with an increasing number of carbon atoms, but once the chain length exceeds a certain limit the friction be-... 

Tribology performances and applications of ordered molecular films have been a long-standing research subject in SKLT, the workplace for the authors of this book. Hu and Luo [42] prepared SAMs of fluoroalkylsilane (FAS) and poly-fluorealkylmethacrylate (PFAM) on the magnetic head of computer hard disk drivers. Experiment results show that the molecular films greatly improve the performance of the... 

Study on mechanisms of ordered molecular films as a model lubricant is of particular importance in the field of micro and nano-tribology, for it would help to understand how molecules contribute to the creation of friction and wear. The understanding has been much improved in recent years through MD simulations, performed by investigators around the world, to detect interactions between the molecular films in relative motion, and to reveal the process and specific mode of energy dissipation. 

Microscale Friction and Wear of a Triacetic Acid Molecular Film... 

In order to improve the tribological properties of molecular films, molecular surface modification is the first choice to make an approach. A Diblock polymer polystyrene-poly(ethylene)oxide (PS-PEO) thin-films were studied in our previous research because of its interesting structure (one... 

Second, the properties of micro/nano friction and wear/ scratch of several representative films are introduced. These films include from organic molecular films, solid films, to multilayers. The experiments were designed reasonably to understand the behaviors of micro/nano friction and wear of the films. The sample preparation methods were also de-... 

Fig. 1—The formation and measurement of molecular films (a) dipping of magnetic head immersing in solution, and (b) the process of TOF-SIMS to measure the AWA film.

Nevertheless, the shape of the isotherm in Fignre 3 is qnite similar to those of molecular films in their solid condensed state. This reflects the strong tendency of the particles to aggregate at the air-water interface. Visnal inspection dnring spreading indicates the for-... 

Hutchison JE, Postlethweite TA, Murray RW. 1993. Molecular films of thiol-derivatized tetra-phenylporphyrins on gold Film formation and electrocatal)4ic dioxygen reduction. Langmuir 9 3277. 

Onda, M., Ariga, K. and Kunitake, T. (1999) Activity and stability of glucose oxidase in molecular films assembled alternatively with polyions. Journal of Bioscience and Bioengineering, 87, 69-75. 

Haick H, Niitsoo O, Ghabboun J, Cahen D (2007) Electrical contacts to organic molecular films by metal evaporation effect of contacting details. J Phys Chem C 111 2318-2329... 

As an example of the use of array methodology to study chemical sensor properties let us consider the thirteen molecular structures reported in Figure 5. To investigate the sensing properties of these molecules we studied the behaviour of the response of thickness shear mode resonators (TSMR) sensors, each coated with a molecular film, to different concentration of various volatile compounds (VOC). Analyte compounds were chosen in order to have different expected interaction mechanisms. 

This effect is complete in the case of porphyrins 6-8,13, which behave in the same way, independent of metal present in the inner core of the macrocycle. In conclusion, the introduction of these alkyl chains from one side gives a higher porosity to the molecular film and, as a consequence, both a speed up of the response and an increase of the sensor response. On the other hand there is a significant decrease of the importance of the selective interactions in the sensing mechanism of the organic material and, as a consequence, a lowering of the selectivity of the sensor. 

By size of pore one can mean the diameter of an equivalent cylindrical or the distance between the sides of a slit-shaped pore (i.e., in general a diameter of the largest circle that can be inscribed in a flat cross section of a pore of arbitrary form). The basis of this classification is that each of the size ranges corresponds to characteristic adsorption effect that is manifested in the isotherm of adsorption [53,115], In micropores, the interaction potential is significantly higher than in wider pores, owing to the proximity of the walls. This explains that such pores become totally full with adsorbate at low relative pressures. In mesopores, one will observe formation of mono- and then multilayer molecular film forming over the walls. After formation of a multilayer molecular film,... 

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