Interfaces semiconductor sensors

Drawing the bottom line, this book is very useful for scientists in various disciplines and experts in domain of interface physical chemistry interested in development and application of the method of semiconductor sensors as well as for post-graduate and graduate students specialized in above domain of science. 

Particularly attractive for numerous bioanalytical applications are colloidal metal (e.g., gold) and semiconductor quantum dot nanoparticles. The conductivity and catalytic properties of such systems have been employed for developing electrochemical gas sensors, electrochemical sensors based on molecular- or polymer-functionalized nanoparticle sensing interfaces, and for the construction of different biosensors including enzyme-based electrodes, immunosensors, and DNA sensors. Advances in the application of molecular and biomolecular functionalized metal, semiconductor, and magnetic particles for electroanalytical and bio-electroanalytical applications have been reviewed by Katz et al. [142]. 

A constant bias potential is applied across the sensor in order to form a depletion layer at the insulator-semiconductor interface. The depth and capacitance of the depletion layer changes with the surface potential, which is a function of the ion concentration in the electrolytic solution. The variation of the capacitance is read out when the semiconductor substrate is illuminated with a modulated light and the generated photocurrent is measured by means of an external circuit. 

The detection principle of field-effect sensors with catalytic metal contacts is based on tbe change of the electric charge at the insulator surface caused by dissociation of the gas molecules by the catalytic material. Adsorbed gas molecules and reaction products form a polarized layer at the metal-insulator interface (Figure 2.1). This gives rise to an electric field in the insulator, which causes the concentration of mobile carriers in the semiconductor underneath the insulator to change. 

Symmetrical placement of the ion-selective membrane is typical for the conventional ISE. It helped us to define the operating principles of these sensors and most important, to highlight the importance of the interfaces. Although such electrodes are fundamentally sound and proven to be useful in practice, the future belongs to the miniaturized ion sensors. The reason for this is basic there is neither surface area nor size restriction implied in the Nernst or in the Nikolskij-Eisenman equations. Moreover, multivariate analysis (Chapter 10) enhances the information content in chemical sensing. It is predicated by the miniaturization of individual sensors. The miniaturization has led to the development of potentiometric sensors with solid internal contact. They include Coated Wire Electrodes (CWE), hybrid ion sensors, and ion-sensitive field-effect transistors. The internal contact can be a conductor, semiconductor, or even an insulator. The price to be paid for the convenience of these sensors is in the more restrictive design parameters. These must be followed in order to obtain sensors with performance comparable to the conventional symmetrical ion-selective electrodes. 

The mechanisms of detection and the functions of the conductor layer and of the semiconductor are the same in a C-I-S diode sensor as they are in a C-S diode sensor. The only difference between these two structures is the presence of the purposefully inserted interfacial layer (I-layer) between the conductor and the semiconductor in the C-I-S devices. In general, this I-layer is employed in the C-I-S sensor configuration for one of two reasons (1) either it is used to block chemical reactions between the conductor and the semiconductor or (2) it is used to augment or reduce the role of the interface in establishing the double layer or controlling transport. 

Because of the dependence of the PL intensity of TiC>2 on the nature of the gas-phase molecules introduced (alcohols) and its reversibility upon elimination of the molecules by flowing dinitrogen, there is hope that such an effect can be applied to gas sensors. With the combined use of several techniques (PL, time-resolved femtosecond diffuse reflectance spectroscopy, multiple internal reflection IR absorption), the dynamics and role of photogenerated electrons and holes in the absence or presence of metals (notably platinum) are now better understood, at both the gas-solid and liquid-solid interfaces. It is also likely that not only TiOz, but other types of semiconductors will be more thoroughly investigated in the future. 

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