In this book, we will designate ultrathin films, or, as they are also called in the literature, nanolayers, to mean layers ranging from submonolayers to several monolayers; these may be formed from a wide range of organic and inorganic substances or present adsorbed atoms, molecules, biological species, on a substrate or at the interface of two media. These films
play an important role in many current areas of research in science and technology, such as submicroelectronics, optoelectronics, optics, bioscience, flotation, materials science of catalysts, sorbents, pigments, protective and passivating coatings, and sensors. It could even be argued that the rapid advances in thin-film technology has necessitated the development of special approaches in the synthesis and investigation of nanolayers and superlattices. Nowadays, these approaches are generally applicable in so-called nanotechnology, which includes the synthesis–deposition and characterization of ultrathin films with a prescribed composition, morphology–architecture and thicknesses on the order of 1 nm.
Common features in all studies in the field of nanotechnology arise from problems connected with the physicochemical investigation of ultrathin films, which originate in general from their extremely small thickness. To solve these problems, a number of technically complicated physical methods that operate under UHV conditions, such as AES, XPS, LEED, HREELS are used.
Infrared (IR) spectroscopy and in particular Fourier transform IR (FTIR) spectroscopy— a method that enables the determination of molecular composition and structure—offers important advantages in that the measurements can be carried out for nanolayers located not only on a solid substrate but also at solid–gaseous, solid–liquid, liquid–gaseous, and solid–solid interfaces, including semiconductor–semiconductor, semiconductor–dielectric, or semiconductor–metal, with no destruction of either medium. Thus IR spectroscopy is one of a few physical methods that can be used for both in situ studies of various processes on surface and at interfaces and technological monitoring of thin-film structures in fields such as microelectronics or optoelectronics under serial production conditions.
The versatility of modern FTIR spectroscopy provides means to characterize
ultrathin coatings on both oversized objects (e.g., works of art) and small (10–20- μm) single particles, substrates with unusual shapes (e.g., electronic boards), and recessed areas (e.g., internal surfaces of tubes). It should be emphasized that IR spectroscopy can be highly sensitive to ultrathin films: Depending on the system, the sensitivity is 10−5 –10% monolayer.
However, the various IR spectroscopy techniques must be adapted to measure
spectra of very small amounts of substance in the form of ultrathin films. While for analyses of bulk materials it is possible to select the optimum mass of substance to record its spectrum, in the case of ultrathin films it is only possible to vary the conditions under which the spectra are recorded (measurement technique, polarization, angle of incidence, immersion media, number of radiation passages through the sample). For this purpose, it is necessary first to theoretically assess the effect of the recording conditions on the intensity of the absorption bands.
By understanding optical theory for stratified media, it is also possible to distinguish optical effects (artifacts), which present in each IR spectrum of an ultrathin film, and, hence, to avoid misinterpretation of the experimental data. Although the optimum conditions for a number of simple systems are known (e.g., for ultrathin films on metals reflection–absorption (IRRAS) at grazing angles of incidence is commonly used, and for ultrathin films on transparent substrates, multiple internal reflection (MIR) is most suitable in many cases), the spectral contrast can be further enhanced by employing additional special technical approaches.
DOWNLOAD
No hay comentarios:
Publicar un comentario