Introduction

This book is the first attempt to provide a detailed description of scientific basis of the method of semiconductor chemical sensors which are presently widely applied in various domains of industry and in everyday life. The major feature of this book (which distinguishes it from the literature published up to date) is that it mainly examines the use of the method of semiconductor sensors in fine physical and chemical studies.

Arbitrary the book can be divided into two complementary parts. The first one describes the physical and chemical basics leading to description of the method of semiconductor sensors. The mechanisms of underlying processes are given. These processes involve interaction of gas with the surface of semiconductor adsorbent which brings about the change of electric and physics characteristics of the latter. Various models of absorption-induced response of electric and physics characteristics of semiconductor adsorbent are considered. Results of numerous physical and chemical experiments carried out by the authors of this book and by other scientists underlying the method of semiconductor sensors are scrupulously discussed. The possibility of qualitative measurements of ultra-small concentrations of molecules, atoms, radicals as well as excited particles in gases, liquids and on surfaces of solids (adsorbents and catalysts) is demonstrated.

The second part of the book deals with the use of above method in physical and chemical studies. In addition to illustration load, this part of the book has a separate scientific value. The matter is that as examples the book provides a detailed description of the studies of such highly interesting processes as adsorption, catalysis, pyrolysis, photolysis, radiolysis, spill-over effect as well as gives an insight to such problems as behavior of free radicals at phase interface, interaction of electron-excited particles with the surface of solid body, effect of restructuring of the surface of adsorbent on development of different heterogeneous processes.

In Chapter 1 we consider the physical and chemical basis of the method of semiconductor chemical sensors. The items dealing with mechanisms of interaction of gaseous phase with the surface of solids are considered in substantial detail. We also consider in this part the various forms of adsorption and adsorption kinetics processes as well as adsorption equilibria existing in real gas-semiconductor oxide adsorbent systems. We analyze the role of electron theory of chemisorption on development of ideas regarding the effect of adsorption on electrophysical properties of semiconductor adsorbents.

We also address the models of adsorption change in electrophysical characteristics of semiconductor adsorbent caused both by chemisorbed charging of the surface due to the charge transition between surface states and volume bands of adsorbent and by local chemical interaction of adsórbate with electrically active defects of semiconductor.

We consider the existing models of adsorption response of electrophysical characteristics of ideal monocrystalline adsorbent, monocrystal with inhomogeneous surface as well as polycrystal adsorbent characterized by an a priori barrier disorder. The role of recharging of biographic surface states in the process of adsorption charging of the surface of semiconductor is analyzed.

A detailed description is given to the role of point defects available in the volume and on the surface of oxide adsorbents on adsorption-induced change of electrophysical characteristics. We try to deduce the impact of the proper nature of adsorbent as well as the nature of adsorption centers.

In Chapter 2 we touch on theoretical models of adsorption response of electrophysical characteristics of real semiconductor adsorbents used as sensitive elements of chemical sensors.

We scrutinize issues dealing with requirements of high sensitivity and response selectivity of electrophysical parameters in reference to the gas monitored or the type of active particles under study as well as other requirements put forward to adsorbents of chemical sensors. We discuss principles underlying the basis of solving these problems. We dwell on the issue of the type of crystal of adsorbents examined, which is directly linked to the character of intracrystallite contacts.

We also pay attention impact of preparation technique of polycrystal adsorbents and applicability domains of obtained semiconductor sensors.

We consider problems related to electrophysical properties of sintered polycrystalline oxides as well as their adsorption changes. We also analyze the difference in adsorption induced changes of electrophysical characteristics of stoichiometric and non-stoichiometric partially reduced oxide adsorbents.

In Chapter 3 we briefly outline the methods of manufacturing of sensitive elements of semiconductor sensors in order to proceed with the studies of several physical and chemical processes in gases, liquids as well as on the surface of solids. Here we show the peculiarity of preparation of these elements depending on objective pursued and operation conditions. We outline the detection methods (kinetic and stationary), their peculiarities and advantages of their application in various physical and chemical systems.

The same Chapter contains results of studies of effects of adsorption of atom particles as well as simplest free radicals on electric conductivity of semiconductor zinc oxide films.

Using the studies on adsorption of H-atoms as examples (as well as atoms of various metals) by volumetric and beam methods we prove a rigorous proportionality between the values of adsorption and the change in the number of carriers in adsorbent. Similar dependencies are given for the case of adsorption of acceptor particles, as well. These are such particles as methyl and ethyl radicals, hydroxyls and nitrogen-containing radicals. Using examples of adsorption of above particles we have shown the applicability of simplest relationships between their concentration and the change of dope electroconductivity of adsorbent.

We outline experimental results and provide theoretical interpretation of effect of adsorption of molecular oxygen and alkyl radicals in condensed media (water, proton-donor and aproton solvents) having different values of dielectric constant on electric conductivity of sensors. We have established that above parameter substantially affects the reversible changes of electric conductivity of a sensor in above media which are rigorously dependent on concentration of dissolved oxygen.

It is shown that similar effects are observed in case of gas or vapor phase media under condition of availability of saturated vapor in the liquid forming a thin layer on the surface of a semiconductor adsorbent.

Chapter 4 deals with several physical and chemical processes featuring various types of active particles to be detected by semiconductor sensors. The most important of them are recombination of atoms and radicals, pyrolysis of simple molecules on hot filaments, photolysis in gaseous phase and in absorbed layer as well as separate stages of several catalytic heterogeneous processes developing on oxides. In this case semiconductor adsorbents play a two-fold role: they are acting both as catalysts and as sensitive elements, i.e. sensors in respect to intermediate active particles appearing on the surface of catalyst in the course of development of catalytic process.

Apart from above studies we dwell on experimental results of evaporation of superstoichiometric metal atoms from the surface of various oxides as well as from the surface of metal foils in order to determine their evaporation heats. The results obtained are consistent with tabulated values obtained by other experimental techniques.

“Plane” oxide sensors were used to study lateral diffusion of H-atoms and spill-over effect.

The combined use of the method of semiconductor sensors and that of molecular beams enabled us to investigate adsorption of atom, molecular and cluster particles of metals on metal oxides.

We indicate a possible use of semiconductor sensors in the studies of interaction of hydrogen ions and electrons with various energy with a surface of oxides and hydrogen layers absorbed to them. These sensors can also be used to study γ-radiolysis of hydrocarbons in gases and liquid media.

Chapter 5 outlines problems of the use of semiconductor sensors to study heterogeneous processes developing in presence of electrically excited atoms and molecules. We give a short description of physical and chemical properties of electrically excited particles, methods of their detection as well as items dealing with their deactivation on the surface of semiconductor oxides. We provide a scientific basis to apply sensor method for detection of electronically excited particles. The method of application of sensors in the studies of such type is described using specific examples of interaction of singlet oxygen and metastable atoms of inert gases with the surface of semiconductors and dielectrics. It is shown in what manner the sensors can be used to determine the efficiency of heterogeneous quenching of electrically excited particles and to elucidate the channels of dissipation of excitation energy on surfaces of various nature. The mechanisms of the change of electric conductivity in semiconductor oxides with pure and dope modified surface due to effect of electrically excited particles are discussed.

Chapter 6 provides experimental results on detection and studies of a wide scope of phenomena dealing with emission of active particles from the surface of solids due to energy released during...