Molecular solid films

EXPERIMENTAL STUDIES OF RARE GAS AND MOLECULAR SOLID FILMS... 

By scattering within molecular solids and at their surfaces, LEE can excite with considerable cross sections not only phonon modes of the lattice [35,36,83,84,87,90,98,99], but also individual vibrational levels of the molecular constituents [36,90,98-119] of the solid. These modes can be excited either by nonresonant or by resonant scattering prevailing at specific energies, but as will be seen, resonances can enhance this energy-loss process by orders of magnitude. We provide in the next two subsections specific examples of vibrational excitation induced by LEE in molecular solid films. The HREEL spectra of solid N2 illustrate well the enhancement of vibrational excitation due to a shape resonance. The other example with solid O2 and 02-doped Ar further shows the effect of the density of states on vibrational excitation. 

In the UPS or XPS of solid films, the front of the sample (the polymer or condensed molecular solid film) can be maintained in electrical contact (equilibrium) with the metallic substrate, because the films employed are so thin that electron tunneling from the substrate, or numerous other effects, prevent any positive surface electronic charge from building up during the course of the measurements. In this way, electronic sample charging effects23 40 41 are avoided in works reported herein, and thus will not be covered. 

Consider the polymer-on-metal interface, which might be prepared by coating a thin metal film with polymer in a polymer-based LED. The case of the counter electrode, formed by vapor-deposition, is discussed subsequently. First, assume that the substrates have clean surfaces hydrocarbon and oxide free, or naturally oxidized but still hydrocarbon free (pointed out as necessary). Typically, in connection with polymer-based LEDs, the metallic substrate could be gold, ITO (indium tin oxide) coated glass, the clean natural oxide of aluminum ( 20 A in thickness), the natural oxide which forms upon freshly etched Si( 110) wafers ( 10 A), or possibly even a polyaniline film. Dirt , which may be either a problem or an advantage, will not be taken up here. Discussions will alternate between coated polymer films and condensed model molecular solid films, as necessary to illustrate points. 

A word of caution the case of conjugated polymer films is generally identical to the case of condensed molecular solid films, as described above, but with occasional small differences. In general, the Fermi level is found to lie very near the centre of the energy gap, Eg. Small amounts of impurities, defects, or other charge donation (acceptor) species, however, can move the position of the Fermi... 

Homogeneous and Inhomogeneous Contributions. Two other contributions to UPS linewidths for molecular solids have been articulated in a study of isopropyl benzene films at low temperatures ( ). The shape and size of the isopropyl benzene molecule prohibited the explicit observation of the surface effect discussed for anthracene. Isopropyl benzene was of interest as a model molecule for polystyrene, however. The measurements were carried out on condensed molecular-solid films in the temperature range 15°K < T < 150°K. 

Finally, it has been possible to obtain LEED patterns from films of molecular solids deposited on a metal-backing. Examples include ice and naphthalene [80] and various phthalocyanines [81]. (The metal backing helps to prevent surface charging.)... 

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-... 

Recent publications on the thermal stability of proteins organized in dense solid films, deposited by LB (Nicolini et al. 1993, Facci et al. 1994, Erokhin et al. 1995) and by self-assembling (Shen et al. 1993), leave several questions unanswered, hi particular, it is still not completely clear which parameter is responsible for this phenomenon. Two main factors are discussed when speaking about induced thermal stabihty, namely, decreased water content and molecular close packing (Nicolini et al. 1993). It seems that both of them work in parallel, and unfortunately it is difficult to settle directly which one plays the dominant role. 

Maruyama, N., Koito, T Nishida, J Cieren, X., Ijrro, K, Karthaus, O. and Shimomura, M. (1998) Mesoscopic patterns of molecular aggregates on solid substrates. Thin Solid Films, 327—329, 854-856. 

The relative concentration of the various butadiene microstructures, (1,4 cis, 1,4 trans, and 1,2 vinyl), were determined from the infrared spectra of solid films cast on KC1.(26) The 1,2 microstructure content of all the polymers considered in this paper were between 5-8 mole percent as determined from the IR spectra. Number average and the weight average molecular weight of the polymers were obtained via osmotic pressure and HPLC. The molecular weight of all polymers is around 200,000 g/mole while the polydispersities were about 1.1 thus, all of these polymers have a relatively narrow molecular weight distribution. Note, that both the precursor diene blocks and hydrogenated copolymers... 

Mattoussi H, Murata H, Merritt CD, Iizumi Y, Kido J, Kafafi ZH (1999) Photoluminescence quantum yield of pure and molecularly doped organic solid films. J Appl Phys 86 2642-2650... 

Some transition metal complexes are excellent conductors. Thin films of cyto-chrome-C3, which contains four heme moieties coordinated by protein, exhibited a high conductivity with mixed valence state (Fe /Fe ) and showed an increase in conductivity as the temperature was decreased (2 x 10 S cm at 268 K) [68-70]. The temperature dependence of conductivity in the highly conductive region is the opposite of that of semiconductors and may preclude the ionic conduction as a dominant contribution. However, since the high conductivity is realized in the presence of hydrogenase and hydrogen, the system is not strictly a single but rather a multicomponent molecular solid. 

Wang G, Yoo H, Na SI, Kim TW, Cho B, Kim DY, Lee T (2009) Electrical conduction through self-assembled monolayers in molecular junctions Au/molecules/Au versus Au/ molecule/PEDOT PSS/Au. Thin Solid Films 518(2) 824-828... 

Electron-Induced Reactions—HREELS Measurements. Novel LEE-induced chemistry has also been observed in HREEL measurements of molecular solids and molecules physisorbed on the surface of RGS. For example, Lepage et ah, building on the initial observations of Jay-Gerin et al. [141], have employed HREELS to measure in situ, neutral dissociation products arising from the impact of low-energy electrons on thin multilayer films of methanol [37] and acetone [38]. The technique is similar to that developed earlier by Martel et al. [258] for chemisorbed systems, in that the same electron beam is used for both the production and the detection of the neutral fragments. However, in the work of Lepage... 

From comparison with the results of charge trapping and anion BSD experiments on pure and doped -hexane films [32] (see as an example Fig. 13), it has been suggested that negative charging between 7 and 15 eV in XPLE is dominated by DBA to alkane chains. In contrast, charging below 5 eV is likely to be associated with molecular impurities (water, O2, and antioxidant additives) since pure alkane molecular solids are unable to trap electrons at these energies (Fig. 13). 

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