Films thin

In thin film technology there is a distinction between physical vapor deposition (PVD) and chemical vapor deposition (CVD), a combination of types is available. All methods work in vacuum. The most important physical processes are evaporation and the sputtering. The material is introduced into the system as a solid (target). With an energy introduced into the target, they resolve atoms and molecules form a layer on the substrate. The layer thickness achieved is in the micrometer range. The layer composition substantially corresponds to that of the target. It can be pure metals, alloys, or dielectrics. 

The chemical vapor deposition is characterized by at least one reaction at the surface of the work piece to be coated. In this process, reaction must have at least one 

A wide variety of techniques have been employed for the characterization of thin film samples of nonlinear polymeric materials. Many of these are similar to techniques described in the previous section for bulk material characterization, and are employed with thin film samples both to assess differences in material properties in the two physical forms and because certain measurements such as absorption or electro-optic effects may be more easily made in thin film samples. Other techniques are specific to thin film samples in which light can be guided, for which parameters can be measured having no bulk equivalent, such as waveguide scatter or nonlinear mode coupling. 

Peak power coupled into waveguide (watts) 

Australia, and scaled up by BP Solar in Spain, the heterojunction with intrinsic thin layer (HIT) cells developed by Sanyo by replacing the diffused P-doped emitter with an amorphous silicon layer and the back contact cells developed by Stanford University for use in concentrator technology and now converted to a large area for flat plate use. All three use single-crystalline silicon, while the majority of screen-printed cells use multicrystalline silicon wafers. 

Commercial module efficiency values (total outer dimensions) are in the 12-14% range for the screen-printed modules and 15-17.5% range for the best performers. 

Thin films are suitable candidates for low-cost photovoltaic modules because of reduced material consumption and predicted manufacturing advantages due to 

These features imply that the cost reduction potential for thin-film technology is very high and it is thus capable, in the longer term, of extending the P V learning curve beyond the point that can be reached by crystalline silicon technology. 

The thin-film PV industry is taking off and the challenge is now to scale up fast enough and to establish a significant presence in the PV marketplace. 

Upon drying, cellulose microfibrils form very thin, highly homogenous films, with tensile strengths far superior to those of regular print grade paper, indicating possible 

The ability of rod-like cellulose microcrystals to form self-assembling chiral-nematic phases in suspension has also led to attention for its potential applications. 

By casting films from suspensions of cellulose microcrystals, cellulose films with the optical properties of chiral nematic liquid crystals can be prepared. The films can be tailored to give different colors of reflected light by altering the salt content of the suspension for a given source of cellulose and set of hydrolysis conditions. Possible areas of application include optical variable films and ink pigments for security papers [81]. 

Spin-casting techniques have also been used to prepare imprinted thin films. Makote and Collinson recently prepared metal oxide thin films imprinted with recognition sites for dopamine [18], The dopamine template was loaded at 4 mole % in a sol with a 10 1 ratio of tetramethoxysilane and phenyltrimethoxysilane. The prepared film had a thickness of ca. 450 nm. CV analysis found that 90% of the templates could be removed by washing the films with pH 7 phosphate buffer. The opened receptor sites offered selective binding for related molecules containing catechol amines, such as dopamine, epinephrine and norepinephrine, as determined by CV. The use of phenyltrimethoxysilane turns out to be an essential ingredient for the gel matrix and is believed to provide some complementary affinity for the catechol amines via hydrophobic and/or 7i-stacking interactions. 

In fact a liquid surface in the absence of external perturbations such as mechanical vibrations is perhaps the most smooth (disordered) surface that can be achieved by the action of gravity. Clearly, LB films could not be grown in the absence of gravity, e.g., in a Space Shuttle. In the case of water, the flat water/air interface has a roughness of 0.3 nm, a value determined by X-ray reflectivity measurements (Braslau et al, 1985). A further advantage of water is its relatively high surface tension yo. when compared to other liquids, which amounts to 72.8 mN m for pure water at RT. This value originates in the formation of a network of weak 

0-H 0 hydrogen bonds at the water/air interface. In general, the state of the ML on the water surface is monitored by measuring the surface pressure fl, dehned as the difference between yo and that of the hlm-covered surface y. 0 is of the order of a few mN m  

Different film architectures can result upon deposition as depicted in Fig. 3.9(e). Y-type multilayers are the most common and can be prepared on either hydrophilic or hydrophobic substrates, and are typically the most stable due to the strength of the head-head and tail-tail interactions. The X-type and Z-type films, with head-tail interactions, are less common. 

Finally, although this book has been intentionally not focused on polymers, there is an example worth being discussed, since it illustrates the importance of some 

It is also possible to produce covalently bonded alkyl MLs on Si(l 11) surfaces using a variety of chemical reactions with passivated H-terminated Si(l 11), but the preparation methods are more complex than the immersion strategy in part due to the higher reactivity of silicon. This is a major achievement because it allows direct coupling between organic and bio-organic materials and silicon-based semiconductors. Both pyrolysis of diacyl peroxides (Linford Chidsey, 1993) and Lewis acid-catalyzed hydrosilylation of alkenes and direct reaction of alkylmagnesium bromide (Boukherroub et al, 1999) on freshly prepared Si(lll)-H produce surfaces with similar characterishcs. These surfaces are chemically stable and can be stored for several weeks without measurable deterioration. Thienyl MLs covalently bonded to Si(l 11) surfaces have also been obtained, in which a Si(l 11)-H surface becomes brominated, Si(lll)-Br, and is further reacted with lithiated thiophenes (He etal, 1998). 

Of the many properties of films in their successive stages, those most commonly studied nowadays are the magnetic, electrical and mechanical ones. The magnetic properties and uses of thin films, especially multilayers, have been outlined in Section 7.4 and need not be repeated here however, it is worth pointing out an excellent survey of magnetic multilayers (Grunberg 2000). Electrical properties have been covered by Coutts (1974). 

The mechanical properties, especially the internal stresses set up by interaction of substrate and deposit, have a close bearing on the behavior of metallic interconnects (electrical conductors) in integrated circuits. Such interconnects suffer from more diseases than does a drink-sodden and tobacco-crazed invalid, and stress-states play roughly the role of nicotine poisoning. A very good review specifically of stresses in films is by Nix (1989). 

On the broad subject of thin films generally, a well-regarded early text is by an Indian physicist, Chopra (1969), while a very broad, didactic treatment of thin films in all their aspects is by Ohring (1992). A recent survey of the effect of structure on properties of thin films relevant to microelectronics is by Machlin (1998). 

Many bulk polymer properties have been well defined in the recent literature. In addition, theories that offer reasoning for changes in these bulk properties with temperature, pressure, etc. have been developed. Focus has been shifted in the past decade to polymers in thin film geometries. These systems remain obscure to polymer scientists. 

Thin films have become ubiquitous in the polymer industry. Fueled by the microelectronics industry and its need for miniature parts, polymer applications have become more demanding. Polymers in these environments have been tested and modeled in laboratories around the world from universities to automobile companies, research surrounding thin films is quickly growing.  

Two main areas of research have emerged that cover thin polymer films. The first, free-standing films, investigates thin films unattached to a substrate. The main concern in this field is how the air-polymer interface affects the behavior of the polymer as the film decreases in thickness. The second, polymer films on substrates, focuses on the interactions at the polymer-substrate interface. Both research fields have reported data and offered explanations for what has been observed. Although the second area is more relevant to the current research, the two interrelate, so the literature for each appears below. 

Although useful for modeling, most thin film applications are not free-standing, but confined to a substrate. These confined geometries present new challenges as the effects of more than one interface must be considered. 

Thin films on substrates provide a characterization challenge the whole system must be measured at once and substrate properties can easily mask those of the thin film. Other techniques have been employed to study these complex systems. Fryer et al. used local thermal analysis to probe polystyrene (PS) and PMMA on two different substrates. These results were comparable to ellipsometry values establishing local thermal analysis as an effective technique. Again, PS did not have a favorable interaction with the polar or nonpolar silicon surface and both PS systems showed a decrease in Tg with decreasing film thickness. PMMA showed similar behavior on the nonpolar surface. However, on the polar substrate the Tg increased as the PMMA films became thinner. Porter saw similar effects measuring PMMA on silica with a differential scanning 

The balance between the ordering forces involved in various magnetic alignments is very delicately poised, and one form can be destabilised with respect to one another in many ways. The situation is modified in thin films and will depend upon the film thickness, the substrate carrying the film, the interfacial region between film and substrate and, in multilayer films, the number and spacing of the layers. In instances where these interfaces can be controlled to give sharp boundaries, the interfaces can 

In superlattices of differing perovskite oxides, these effects are frequently magnified and, dependent upon layer thickness, can produce a variety of magnetic ordering patterns. 

Kirk-Othmer Encyclopedia of Chemical Technology (4th Edition) 

For their preparation, a drop of a solution of the desired material in a volatile organic solvent, immiscible with water, is put onto the surface of highly pure water 

Blodgett films which are free from defects or impurities over large areas. If this is intended, extreme care must be taken with the purity of the substances used, especially the water. It must in any case be free of surface-active impurities. Other disadvantages of LB films are their lack of mechanical strength and their limited thermal stability. 

The most flexible methods, which can be applied in principle to nearly all molecules and which provide the most important techniques for the preparation of organic molecular films, are vacuum evaporation and molecular-beam epitaxy (cf [14]). In both methods, the substance required is evaporated or sublimed and captured on a substrate which is cooled as necessary. In molecular-beam epitaxy, the substrate must be an (inorganic) single crystal. The nature of the substrate s surface, its temperature and the velocity of evaporation or sublimation, adjustable 

When the substrate is so cold that practically no motion of the arriving molecules on its surface can take place, then the molecules remain at the positions where they land. Since these positions are statistically distributed without any ordering, one in general obtains amorphous layers without measurable crystalline order at low temperatures. It is, however, surprising to observe how mobile even larger molecules such as anthracene can be on substrates even only a little above 4 K. 

If the temperature of the film of molecules deposited onto a substrate at low temperature is slowly increased, their mobility also increases. The molecules can then move across the surface and can form an ordered structure which is energetically more favourable than the disordered amorphous phase. When the interactions of the evaporated molecules with each other are stronger than those with the substrate material, then the molecules can collect together to form island stmctures. The size and spacing of the islands depend particularly on the amount of the substance which was evaporated. With increasing temperature, a collection of microcrystals is formed on the substrate. 

In a nano-indentation test, the load applied to the indenter and the resultant displacement are measured as a function of time. In this sense, the test 

If the indentation does not exceed approximately 20% of the film thickness the substrate does not 

The slope is related to the elastic modulus of the indenter, the film and the substrate, and when the substrate is ignored the initial slope is given by  

Note how the holder slides into the mount in the FTIR sample compartment, and how the windows rest on the two posts protruding from the metal plate. This type of device can also be used to support KBr pellet holders, mulls, and cast hhns. The background spectrum is run on the same two windows that will be used to prepare the sample. 

In addition to being useful with liquids, the capillary thin film technique can be used with soft solids or viscous liquids using the elegantly named smear technique. 

FIG U RE 4.19 A sample holder for transmission analysis shown installed in the sample slide mount of an FTIR. The protruding metal arms can support the KBr windows, KBr pellets, mulls, cast films, and capillary thin films. 

FIGURE 4.20 The infrared spectrum of henzonitrile obtained using the capillary thin film method. 

FIGURE 4.21 The capillary thin film spectrum of a smear of chunky peanut butter. 

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A bitumen sample is oxidized at high temperature under well defined conditions and its physical characteristics are measured before and after this artificiai ageing process. The method is defined in France as AFNOR T 66-032 and in the USA by ASTM D 2872 (Rolling Thin-Film Oven Test). 

Hardening per RTFOT (Rolling thin-film ouen test) NF T 66-032/ASTM D 2872... 

RTFOT rolling thin film oven test... 

The topic of capillarity concerns interfaces that are sufficiently mobile to assume an equilibrium shape. The most common examples are meniscuses, thin films, and drops formed by liquids in air or in another liquid. Since it deals with equilibrium configurations, capillarity occupies a place in the general framework of thermodynamics in the context of the macroscopic and statistical behavior of interfaces rather than the details of their molectdar structure. In this chapter we describe the measurement of surface tension and present some fundamental results. In Chapter III we discuss the thermodynamics of liquid surfaces. 

A belief that solid interfaces are easier to understand than liquid ones shifted emphasis to the former but the subjects are not really separable, and the advances in the one are giving impetus to the other. There is increasing interest in films of biological and of liquid crystalline materials because of the importance of thin films in microcircuitry (computer chips ), there has been in recent years a surge of activity in the study of deposited mono- and multilayers. These Langmuir-Blodgett films are discussed in Section XV-7. 

In the case of Langmuir monolayers, film thickness and index of refraction have not been given much attention. While several groups have measured A versus a, [143-145], calculations by Knoll and co-workers [146] call into question the ability of ellipsometry to unambiguously determine thickness and refractive index of a Langmuir monolayer. A small error in the chosen index of refraction produces a large error in thickness. A new microscopic imaging technique described in section IV-3E uses ellipsometric contrast but does not require absolute determination of thickness and refractive index. Ellipsometry is routinely used to successfully characterize thin films on solid supports as described in Sections X-7, XI-2, and XV-7. 

In the context of the structural perturbations at fluid-solid interfaces, it is interesting to investigate the viscosity of thin liquid films. Eaily work on thin-film viscosity by Deijaguin and co-workers used a blow off technique to cause a liquid film to thin. This work showed elevated viscosities for some materials [98] and thin film viscosities lower than the bulk for others [99, 100]. Some controversial issues were raised particularly regarding surface roughness and contact angles in the experiments [101-103]. Entirely different types of data on clays caused Low [104] to conclude that the viscosity of interlayer water in clays is greater than that of bulk water. 

The long-range van der Waals interaction provides a cohesive pressure for a thin film that is equal to the mutual attractive force per square centimeter of two slabs of the same material as the film and separated by a thickness equal to that of the film. Consider a long column of the material of unit cross section. Let it be cut in the middle and the two halves separated by d, the film thickness. Then, from one outside end of one of each half, slice off a layer of thickness d insert one of these into the gap. The system now differs from the starting point by the presence of an isolated thin layer. Show by suitable analysis of this sequence that the opening statement is correct. Note About the only assumptions needed are that interactions are superimposable and that they are finite in range. 

A thin film of hydrocarbon spread on a horizontal surface of quartz will experience a negative dispersion interaction. Treating these as 1 = quartz, 2 = n-decane, 3 = vacuum, determine the Hamaker constant A123 for the interaction. Balance the negative dispersion force (nonretarded) against the gravitational force to find the equilibrium film thickness. 

D. D. Eley, ed., Adhesion, The Clarendon Press, Oxford, 1961. E. Passaglia, R. R. Stromberg, and J. Kruger, eds., Ellipsometry in the Measurement of Surfaces and Thin Films, National Bureau of Standards Miscellaneous Publication 256, Washington, DC, 1964. 

Transmission electron microscopy (TEM) can resolve features down to about 1 nm and allows the use of electron diffraction to characterize the structure. Since electrons must pass through the sample however, the technique is limited to thin films. One cryoelectron microscopic study of fatty-acid Langmuir films on vitrified water [13] showed faceted crystals. The application of TEM to Langmuir-Blodgett films is discussed in Chapter XV. 

The integral A/, while expressible in terms of surface free energy differences, is defined independently of such individual quantities. A contact angle situation may thus be viewed as a consequence of the ability of two states to coexist bulk liquid and thin film. 

The coefficient of friction between two unlubricated solids is generally in the range of 0.5-1.0, and it has therefore been a matter of considerable interest that very low values, around 0.03, pertain to objects sliding on ice or snow. The first explanation, proposed by Reynolds in 1901, was that the local pressure caused melting, so that a thin film of water was present. Qualitatively, this explanation is supported by the observation that the coefficient of friction rises rapidly as the remperarure falls, especially below about -10°C, if the sliding speed is small. Moreover, there is little doubt that formation of a water film is actually involved [3,4]. 

The traditional, essentially phenomenological modeling of boundary lubrication should retain its value. It seems clear, however, that newer results such as those discussed here will lead to spectacular modification of explanations at the molecular level. Note, incidentally, that the tenor of recent results was anticipated in much earlier work using the blow-off method for estimating the viscosity of thin films [68]. 

The speed of wetting has been measured by running a tape of material that is wetted either downward through the liquid-air interface, or upward through the interface. For a polyester tape and a glycerol-water mixture, a wetting speed of about 20 cm/sec and a dewetting speed of about 0.6 cm/sec has been reported [37]. Conversely, the time of rupture of thin films can be important (see Ref. 38). 

The importance of the thin film between the mineral particle and the air bubble has been discussed in a review by Pugh and Manev [74]. In this paper, modem studies of thin films via SFA and interferometry are discussed. These film effects come into play in the stability of foams and froths. Johansson and Pugh have studied the stability of a froth with particles. Small (30-/ m), moderately hydrophobic 6c = 65°) quartz particles stabilized a froth, while more hydrophobic particles destabilized it and larger particles had less influence [75]. 

Exerowa and co-workers [201] suggest that surfactant association initiates black film formation the growth of a black film is discussed theoretically by de Gennes [202]. A characteristic of thin films important for foam stability, their permeability to gas, has been studied in some depth by Platikanov and co-workers [203, 204]. A review of the stability and permeability of amphiphile films is available [205]. 

The rupture process of a soap film is of some interest. In the case of a film spanning a frame, as in Fig. XIV-15, it is known that rupture tends to originate at the margin, as shown in the classic studies of Mysels [207, 211]. Rupture away from a border may occur spontaneously but is usually studied by using a spark [212] as a trigger (a-radia-tion will also initiate rupture [213]). An aureole or ridge of accumulated material may be seen on the rim of the growing hole [212, 214] (see also Refs. 215, 216). Theoretical analysis has been in the form of nucleation [217, 218] or thin-film instability [219]. 

As a point of interest, it is possible to form very thin films or membranes in water, that is, to have the water-film-water system. Thus a solution of lipid can be stretched on an underwater wire frame and, on thinning, the film goes through a succession of interference colors and may end up as a black film of 60-90 A thickness [109]. The situation is reminiscent of soap films in air (see Section XIV-9) it also represents a potentially important modeling of biological membranes. A theoretical model has been discussed by Good [110]. 

Figure Al.7.10. STM image (1000 A x 1000 A) of the (111) surface of a tungsten single crystal, after it had been coated with a very thin film of palladium and heated to about 800 K (courtesy of Ted Madey).

In moist enviromnents, water is present either at the metal interface in the fonn of a thin film (perhaps due to condensation) or as a bulk phase. Figure A3.10.1 schematically illustrates another example of anodic dissolution where a droplet of slightly acidic water (for instance, due to H2SO4) is in contact with an Fe surface in air [4]. Because Fe is a conductor, electrons are available to reduce O2 at the edges of the droplets. 

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