Surfaces and interfaces

In general, the properties of surfaces and interfaces differ from those of the bulk. In particular, this phenomenon is important for polymer blends, as they consist of at least two constituents with different surface energies. To obtain materials which combine good mechanical properties of the bulk polymer with specific surface properties, such as hydrophobicity, wettability, adhesion, or biocompatibility, is a challenging issue. 

Unfortunately, due to the low energy used (1.5 MeV beam of He+ ions), the accessible depth was insufficient to analyze a full thickness of the film, despite the configuration of the experiment which maximized the investigated depth (the beam for the FRES experiment was incident at an angle of 19°, and the scattering angle was 30° [167]). 

The phase behaviors of miscible, partially miscible, and immiscible polymers were investigated in the case of sulfonated poly(styrene-ran-styrenesulfonate) 

F re 23.38 LE-FRES volume fraction profile of d-PBroo S in a PSid-PBro.oeS blend annealed at for 3 days (open circles). The sub- 

Using FRES and others techniques, Wang and Composto investigated wetting and phase separation in polymer blend films composed of deuterated poly(methyl methacrylate) (d-PMMA) and poly(styrene-raw-acrylonitrile) (h-SAN) at the critical concentration [176-180]. This blend is characterized by a lower critical solution temperature (LCST) behavior with Tlcst 160 °C and 0d-PMMA 0.5 [180]. In this 

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H. Yu, in Physics of Polymer Surfaces and Interfaces, I. Sanchez ed., Butterworth-Heinemann, 1992, Chapter 12. 

B. W. Rossiter and R. C. Baetzold, Physical Methods of Chemistry, 2nd ed., Vol. IXA, Investigations of Surfaces and Interfaces, Wiley-Interscience, New York, 1993. 

Below are brief descriptions of some of the particle-surface interactions important in surface science. The descriptions are intended to provide a basic understanding of how surfaces are probed, as most of the infonuation that we have about surfaces was obtained tluough the use of techniques that are based on such interactions. The section is divided into some general categories, and the important physics of the interactions used for analysis are emphasized. All of these teclmiques are described in greater detail in subsequent sections of the encyclopaedia. Also, note that there are many more teclmiques than just those discussed here. These particular teclmiques were chosen not to be comprehensive, but instead to illustrate the kind of infonuation that can be obtained from surfaces and interfaces. 

Flimpsel F J, McFeely F R, Morar J F, Taleb-lbrahimi A and Yarmoff J A 1990 Core level spectroscopy at silicon surfaces and interfaces Proc. Enrico Fermi School on Photoemission and Adsorption Spectroscopy and Interfaces with Synchrotron Radiation vo course CVIII, eds M Campagna and R Rose (Amsterdam Elsevier) p 203... 

Luth H 1995 Surfaces and Interfaces of Solid Materials 3rd edn (Beriin Springer)... 

A system of interest may be macroscopically homogeneous or inliomogeneous. The inliomogeneity may arise on account of interfaces between coexisting phases in a system or due to the system s finite size and proximity to its external surface. Near the surfaces and interfaces, the system s translational synnnetry is broken this has important consequences. The spatial structure of an inliomogeneous system is its average equilibrium property and has to be incorporated in the overall theoretical stnicture, in order to study spatio-temporal correlations due to themial fluctuations around an inliomogeneous spatial profile. This is also illustrated in section A3.3.2. 

Because of the generality of the symmetry principle that underlies the nonlinear optical spectroscopy of surfaces and interfaces, the approach has found application to a remarkably wide range of material systems. These include not only the conventional case of solid surfaces in ultrahigh vacuum, but also gas/solid, liquid/solid, gas/liquid and liquid/liquid interfaces. The infonnation attainable from the measurements ranges from adsorbate coverage and orientation to interface vibrational and electronic spectroscopy to surface dynamics on the femtosecond time scale. 

The focus of the present chapter is the application of second-order nonlinear optics to probe surfaces and interfaces. In this section, we outline the phenomenological or macroscopic theory of SHG and SFG at the interface of centrosymmetric media. This situation corresponds, as discussed previously, to one in which the relevant nonlinear response is forbidden in the bulk media, but allowed at the interface. 

As we have discussed earlier in the context of surfaces and interfaces, the breaking of the inversion synnnetry strongly alters the SFIG from a centrosynnnetric medium. Surfaces and interfaces are not the only means of breaking the inversion synnnetry of a centrosynnnetric material. Another important perturbation is diat induced by (static) electric fields. Such electric fields may be applied externally or may arise internally from a depletion layer at the interface of a semiconductor or from a double-charge layer at the interface of a liquid. 

All of the symmetry classes compatible with the long-range periodic arrangement of atoms comprising crystalline surfaces and interfaces have been enumerated in table Bl.5,1. For each of these syimnetries, we indicate the corresponding fonn of the surface nonlinear susceptibility With the exception of surfaces... 

An important distinction among surfaces and interfaces is whether or not they exliibit mirror synnnetry about a plane nonnal to the surface. This synnnetry is particularly relevant for the case of isotropic surfaces (co-synnnetry), i.e. ones that are equivalent in every azunuthal direction. Those surfaces that fail to exliibit mirror synnnetry may be tenned chiral surfaces. They would be expected, for example, at the boundary of a liquid comprised of chiral molecules. Magnetized surfaces of isotropic media may also exliibit this synnnetry. (For a review of SFIG studies of chiral interfaces, the reader is referred to [68]. ... 

Given the interest and importance of chiral molecules, there has been considerable activity in investigating die corresponding chiral surfaces [, and 70]. From the point of view of perfomiing surface and interface spectroscopy with nonlinear optics, we must first examhie the nonlinear response of tlie bulk liquid. Clearly, a chiral liquid lacks inversion synnnetry. As such, it may be expected to have a strong (dipole-allowed) second-order nonlinear response. This is indeed true in the general case of SFG [71]. For SHG, however, the pemiutation synnnetry for the last two indices of the nonlinear susceptibility tensor combined with the... 

The nonlinear response of an individual molecule depends on die orientation of the molecule with respect to the polarization of the applied and detected electric fields. The same situation prevails for an ensemble of molecules at an interface. It follows that we may gamer infonnation about molecular orientation at surfaces and interfaces by appropriate measurements of the polarization dependence of the nonlinear response, taken together with a model for the nonlinear response of the relevant molecule in a standard orientation. 

Many of the fiindamental physical and chemical processes at surfaces and interfaces occur on extremely fast time scales. For example, atomic and molecular motions take place on time scales as short as 100 fs, while surface electronic states may have lifetimes as short as 10 fs. With the dramatic recent advances in laser tecluiology, however, such time scales have become increasingly accessible. Surface nonlinear optics provides an attractive approach to capture such events directly in the time domain. Some examples of application of the method include probing the dynamics of melting on the time scale of phonon vibrations [82], photoisomerization of molecules [88], molecular dynamics of adsorbates [89, 90], interfacial solvent dynamics [91], transient band-flattening in semiconductors [92] and laser-induced desorption [93]. A review article discussing such time-resolved studies in metals can be found in... 

The influence of an applied magnetic field, as introduced in section Bl.5.2.2. is quite different from that of an applied electric field. A magnetic field may perturb the interfacial nonlinear response (and that of the weak bulk tenns), but it does not lead to any dipole-allowed bulk nonlmear response. Thus, in the presence of magnetic fields and magnetization, SHG remains a probe that is highly specific to surfaces and interfaces. It... 

Dumas P, Weldon M K, Chabal Y J and Williams G P 1999 Molecules at surfaces and interfaces studied using vibrational spectroscopies and related techniques Surf. Rev. Lett. 6 225-55... 

Heinz T F 1991 Second-order nonlinear optical effects at surfaces and interfaces Noniinear Surfaoe... 

McGilp J F 1995 Optical characterisation of semiconductor surfaces and interfaces Prog. Surf. Sc/. 49 1-106... 

Reider G A and Heinz T F 1995 Second-order nonlinear optical effects at surfaces and interfaces recent advances Photonio Probes of Surfaoes ed P Halevi (Amsterdam Elsevier) pp 413-78... 

DiNardo N J 994 Nanoscale Characterization of Surfaces and Interfaces ( Nembe m VCH)... 

McGlip J F 1990 Epioptics linear and non-linear optical spectroscopy of surfaces and interfaces J. Phys. Condens Matter 2 7985-8006... 

Stoohr J, Ederer D L, Perera R C C, Tong W and Shunli D K 1995 Surface and interface analysis at 3rd generation light sources Prog. Surf Sci. 50 37-51... 

Harding J H 1997. Defects, Surfaces and Interfaces. In Catlow C R A (Editor) Inorganic Crystallography, pp. 185-199. 

SURFACE AND INTERFACE ANALYSIS] (Supplement) [SURFACE AND INTERFACE ANALYSIS] (Supplement)... 

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