The Buried Interface

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 investigation of the interface of a real device contact with siuface sensitive techniques is generally not possible, since the interface is buried and these techniques penetrate only a few atomic layers. However, an interface analysis with, e.g., electron spectroscopic techniques would be highly desirable since these methods provide valuable (semi-) quantitative information on the electronic structure and ehemical interaction in the contact regime. To tackle this problem, i.e. to aecess the buried interface, we have applied a lift-off technique, whieh is illustrated in Figure 14.8a and was previously developed by our group [30]. Therefore, we prepared a sample sandwich which consists of the aetual model OFET, i.e. a gold film deposited on a DIP film that itself... 

Electrodeposition of copper was performed on p-GaAs. XPS Studies of the buried interfaces show that an interfacial chemical reaction happens. A copper- arsenic compound is detected. 

In this work we have shown that p-GaAs coated by a copper layer undergoes a complex chemical transformation. The phenomenon is only located at the interface. The Study of the Asjd, Cu2p and Auger signals coming from the buried interface shows that Cu-As bonds are present in the interfacial layer. Over this interfacial layer a pure copper layer can grow. The... 

Kline, R.J., McGehee, M.D. and Toney, M.F., Highly oriented crystals at the buried interface in polythiophene thin-film transistors, Nat Mater, 5, 222-228, 2006. 

While the macroscopic concepts of hardness, adhesion, friction, and slide have evolved over the last two centuries, atomic level understanding of the mechanical properties of surfaces eluded researchers. The discovery of the atomic force microscope in recent years promises to change this state of affairs. Being able to measure forces as small as 10 newton or as large as 10 newton [5] over a very small surface area (few atoms) and by simultaneously providing atomic spatial resolution, this technique permits the study of deformation (elastic and plastic), hardness, and friction on the atomic scale. The buried interface between moving solid surfaces can be studied with spectroscopic techniques on the molecular level. Study of the mechanical properties of interfaces is, again, a frontier research area of surface chemistry. 

Figure H.A. Scheme of molecules at the buried interface when they arv sandwiched between solids. Studies of iheir mulecular slniciin and oriental ion by eleciron emission is possible ns lung as one ol" the solids is a rhin layer.

The surface force apparatus (SFA) operates in a manner similar to that of the atomic force microscope (AFM). However, it provides an atomically smooth interface over a large ( 100 m ) area that permits the spectroscopic scrutiny of molecules adsorbed at the buried interface. Discuss one study that was recently reported in the literature [56]. 

Fig. 1.1 The problem that exists in the analysis of the buried interface a region responsible for adhesion that is nanometers thick buried between thick layers of adhesive or coating and substrate materials.

SFG can provide considerable information regarding the buried interface that is of central importance to corrosion inhibition processes. A model system in this respect is the monolayer of benzotriazole (BTA) that forms beneath a thick multilayer of the same molecule on Cu. Two SFG studies have examined this system thus far [125, 126]. In the study by Schultz et al., SFG showed that BTA forms a relatively well-ordered monolayer on Cu(lOO) between -0.7 and -tO.2 V, while on Cu(lll) this order is only present at high potential. Titration with Cl showed that the monolayer was destabilized at lower Cl concentrations than those needed to destabilize the polymeric and somewhat more inaccessible multilayer. Work performed by Romero et al. using 5-methylbenzotriazole on Cu(poly) show that the 5-methylbenzotriazole is stable on the surface with no orientation changes with potential [125]. Similarly to the system studied by Schultz, the degree of preferential ordering of BTA on Cu(lll) seems to be less than that on the Cu(poly) surface. 

No consensus has been reached on the roles of physical absorption and chemical bonding when investigating the surface chemistry of carbon fibers and made more difficult by the buried interface. Jones [47] claims that the electrolytic surface treatment process produces a surface on which the known concentration of chemical functionalities cannot be accommodated on the surface of a smooth cylinder. Absorption studies [48] support the fact that erosion could occur and active species can be deposited in the vicinity of intercrystallite voids. Types A and HT fibers have more basal planes that emerge directly to the surface than is the case with HM fiber and hence are more readily surface treated. Hence, it was suggested [49,50] that HM fiber would require an active epoxy group of smaller dimensions that could be accommodated within the micropore. 

Infrared synchrotron micro-spectroscopy is also an appropriate method for identifying and visualizing the existence of localized water at buried interfaces, particularly between multilayers of polymers. It was recently shown that water inclusions can be imaged at the buried interface of solid-contact-ion-selective electrodes (SC-ISEs) [22]. In this study a poly(methyl metha-crylate)-poly(decyl methacyrlate) [PMMA-PDMA] copolymer was used. Since the PMMA-PDMA copolymer is known to be water repellent and unsuitable for water sorption at measurable levels in the bulk membrane, the detection (or non-detection) of water by reflectance SR-FTIR is symbolic of the presence (or absence) of localized zones of water at the buried interface of a solid-contact ISE employing PMMA-PDMA as the sensing membrane. In fact, SR-FTIR revealed the presence of micrometer-sized inclusions of water at the gold-to-membrane interface, whereas coupling a hydrophobic solid contact of poly(3-octylthiophene 2,5-diyl) (POT) prevented the accumulation of water at the buried interface (Fig. 2) [22]. 

Some electronic equilibrium between the buried interface and the polymer surface is necessary. Usually, this equilibrium is obtained for swollen polymer films which contain small amounts of water. If... 

It has to be taken into account that the Kelvinprobe is strictly an in situ tool, which detects the actual electrode potential of the buried interface being defined by the actual electrochemical reactions. It is not useful for investigating a dry sample which had been corroded before, as the link between the electrochemical situation of the dry interface and the previous corrosion is not obvious. 

In a typical experiment therefore, a polymer-coated substrate is used with a well-defined defect prepared such that the electrolyte will not wet the polymer surface. The sample is fixed inside the Kelvinprobe chamber and a humid atmosphere is established with a water activity of nearly one. Then the Volta potential distribution is measured at the buried interface as a function of the delamination time, the electrolyte composition, the oxygen partial pressure, etc. It should be noted, however, that the rate of delamination depends on the electrochemical condition of the defect. As active and passive sites are usually situated close together, the delamination rate will differ for both sites if the scratch is not homogeneously activated by a special surface treatment. 

Speetrum of the fiber coated with a very thin layer of matrix material = S. It is e.vsential that the. spectrum of the underlying fiber can be seen in the XPS data, to make sure that the buried interface is within the sampling depth. 

Figure 19. Illustration of the method of extracting the valence-band spectrum of the buried interface between an oxidized carbon fiber and a phenolic resin, with an intermediate titanium alkoxide agent, (a) Valence-band spectrum of oxidized carbon fiber alone, (b) valence-band spectrum of phenolic matrix alone, (c) valoncc-band spectrum of the composite, and (d) difference. spectrum—the result of subtracting spectra (a) and (b) from (c). Spectrum (d) represents the desired valence-band spectrum of the interface. (From Ref. 51.)...

The electrical and electronic characteristics of heterojunctions, e.g., GaAs/Ga Ali-xAs, are of fundamental importance in electronic and opto-electronic devices. Experimentally it is quite difficult to gain detailed atomistic information on the properties of the buried interfaces of a heterojunction. Therefore, first-principles electronic structure calculations play an important role to provide insight into the quantities such as the charge density distribution and the electrostatic potential across heterojunctions. Calculations thus give an understanding of the relationship between chemical composition, crystallographic structures, and the electronic properties. 

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