Thin films spectroscopy

Ultraviolet, visible, infrared and EPR s[ tra are u ful in studying pyridinyl radicals. The major recent developments have come from experiments using new apparatus for known techniques. The developments, especially that for thin film spectroscopy, will be useful for other chemical problems. Therefore, the techniques are described in detail. 

The blue film, formed by distillation of l-alkyl-2-carbomethoxypyridinyl radical, 2, onto a 77 K surface, disappears on warming. The thin film spectroscopy apparatus depicted in Fig. 3 allowed a) direct cooling of the condensing surface so that tern-... 

Fig. 3. Exploded view of the thin film spectroscopy apparatus, showing the relative positions of the optical cooling surface, the interferometer, the optical pathway for measurement and the material source for the film. W, window (polished quartz or sapphire), O, ground surface onto which windows are glued, LED, light emitting diode, PD, photodiode...

The infrared spectra of pyridinyls are measured conveniently with the thin film spectroscopy apparatus and the changes which ensue on dimerization have been examined. A complete inO red spectrum for 4 at 77 K has been reported External sapphire windows on the thin film apparatus allow spectra to be measured... 

Irradiation of the matrix (420-500 nm, a range not corresponding to the pimer charge-transfer transition) produced an additional triplet (ti j 10 hours) with D = 0.0175 cm and E = O, r = 5.4 A. Careful examination of the spectra of concentrated solutions of 4 revealed absorption in the range of 420-500 nm Thin film spectroscopy of 4 revealed that covalent dimers (rather than singlet pairs) gave rise to the new triplet via a no ncr transition Formation of a triplet [D = 0.0146 cm" and E = O, r = 5.5 A] from the dimer of 2 , 2-2, has been accomplished by irradiation in an MTHF glass. The equilibria describe the relationship of the triplets and radicals in Eq. 13. The two triplet pairs (A and B) are not readily interconvertible. 

Pyridinyl radical studies have stimultated the development of useful equipment, such as those for thin film spectroscopy, for variable i th measurements under oxygen-free conditions (the VV-cell ) and a spectr< lectrochemical cell. In addition, a new technique for measuring the photodissodation spectra of unstable species which can be generated electrochemically was introduced. 

For bulk structural detemiination (see chapter B 1.9). the main teclmique used has been x-ray diffraction (XRD). Several other teclmiques are also available for more specialized applications, including electron diffraction (ED) for thin film structures and gas-phase molecules neutron diffraction (ND) and nuclear magnetic resonance (NMR) for magnetic studies (see chapter B1.12 and chapter B1.13) x-ray absorption fine structure (XAFS) for local structures in small or unstable samples and other spectroscopies to examine local structures in molecules. Electron microscopy also plays an important role, primarily tlirough unaging (see chapter B1.17). 

Atr—ftir can be readily performed on most commercial ftir spectrometers through the use of an attachment for atr spectroscopy. These devices provide ir-transparent internal reflection elements that are typically made of Ge, KRS-5, ZnSe, or ZnS. These internal reflection elements are made of materials that are of extremely high purity to avoid losses from absorption by impurities in these devices. Coupling of a thin film or surface sample to one of these reflection elements is accompHshed by pressing the sample against the element while acquiring the spectmm. 

Physical Properties. Raman spectroscopy is an excellent tool for investigating stress and strain in many different materials (see Materlals reliability). Lattice strain distribution measurements in siUcon are a classic case. More recent examples of this include the characterization of thin films (56), and measurements of stress and relaxation in silicon—germanium layers (57). 

Auger electron spectroscopy is the most frequently used surface, thin-film, or interface compositional analysis technique. This is because of its very versatile combination of attributes. It has surface specificity—a sampling depth that varies... 

For applied work, an optical characterization technique should be as simple, rapid, and informative as possible. Other valuable aspects are the ability to perform measurements in a contactless manner at (or even above) room temperature. Modulation Spectroscopy is one of the most usehil techniques for studying the optical proponents of the bulk (semiconductors or metals) and surface (semiconductors) of technologically important materials. It is relatively simple, inexpensive, compact, and easy to use. Although photoluminescence is the most widely used technique for characterizing bulk and thin-film semiconductors. Modulation Spectroscopy is gainii in popularity as new applications are found and the database is increased. There are about 100 laboratories (university, industry, and government) around the world that use Modulation Spectroscopy for semiconductor characterization. 

For characterization purposes of bulk or thin-film semiconductors the features at Eq and E] are the most useflil. In a number of technologically important semiconductors (e.g., Hgi j d Te, and In Gai j ) the value of. ) is so small that it is not in a convenient spectral range for Modulation Spectroscopy, due to the limitations of light sources and detectors. In such cases the peak at E can be used. The features at Eq and are not useflil since they occur too far into the near-ultraviolet and are too broad. 

Some of the techniques included apply more broadly than just to surfaces, interfaces, or thin films for example X-Ray Diffraction and Infrared Spectroscopy, which have been used for half a century in bulk solid and liquid analysis, respectively. They are included here because they have by now been developed to also apply to surfaces. A few techniques that are applied almost entirely to bulk materials (e.g.. Neutron Diffraction) are included because they give complementary information to other methods or because they are referred to significantly in the 10 materials volumes in the Series. Some techniques were left out because they were considered to be too restricted to specific applications or materials. 

RAIRS spectra contain absorption band structures related to electronic transitions and vibrations of the bulk, the surface, or adsorbed molecules. In reflectance spectroscopy the ahsorhance is usually determined hy calculating -log(Rs/Ro), where Rs represents the reflectance from the adsorhate-covered substrate and Rq is the reflectance from the bare substrate. For thin films with strong dipole oscillators, the Berre-man effect, which can lead to an additional feature in the reflectance spectrum, must also be considered (Sect. 4.9 Ellipsometry). The frequencies, intensities, full widths at half maximum, and band line-shapes in the absorption spectrum yield information about adsorption states, chemical environment, ordering effects, and vibrational coupling. 

Infrared spectroscopy, including Fourier-transform infrared (FTIR) spectroscopy, is one of the oldest techniques used for surface analysis. ATR has been used for many years to probe the surface composition of polymers that have been surface-modified by an etching process or by deposition of a film. RAIR has been widely used to characterize thin films on the surfaces of specular reflecting substrates. FTIR has numerous characteristics that make it an appropriate technique for... 

In order to understand RAIR spectroscopy, it is convenient to model the experiment (see Fig. 4). Consider a thin film with refractive index n =n ik and thickness d supported by a reflecting substrate with refractive index ni = ri2 — iki- The refractive index of the ambient atmosphere is o- Infrared radiation impinges on the film at an angle of incidence of 6 . The incident radiation can be polarized parallel to or perpendicular to the plane of incidence. 

As discussed above, it is desirable to use large, almost grazing angles of incidence in RAIR spectroscopy in order to maximize the sensitivity of the technique to ultra-thin films. Using such large angles of incidence usually requires the substrates to be at least a few centimeters in length. This severely limits the spatial... 

Impedance spectroscopy This technique is essentially the extension of polarization resistance measurements into low-conductivity environments, including those listed above. The technique can also be used to monitor atmospheric corrosion, corrosion under thin films of condensed liquid and the breakdown of protective paint coatings. Additionally, the method provides mechanistic data concerning the corrosion processes, which are taking place. 

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