Work Function Sensors

Work Function (WF) plays a key role in the physics and chemistry of materials. Phenomena such as the semiconductor field effect, photo- and thermionic electron emission (Allen and Gobelli, 1962), catalysis (Vayenas et al 1996), and the like are dominated by the WF. This fundamental property of electronic materials is defined as the minimum work required to extract an electron from the Fermi level Ep of a conducting phase, through the surface and place it in vacuum just outside the reach of the electrostatic forces of that phase (Trasatti and Parsons, 1986). The reference level for this transfer is thus called the vacuum reference level. Because even a clean surface is a physical discontinuity, a surface dipole t] with its associated electric field always appears at the surface of the condensed phase. Thus, the work of extracting the electron can be conceptually divided between the work required to 

The energy with which electrons are bound in conducting materials is known as the electron affinity of the material. Materials with a high electron affinity bind electrons strongly and exhibit noble behavior (i.e., are relatively inert and do not oxidize spontaneously in air). Gold is an example. On the other hand, metals such as aluminum or copper are less noble and their surfaces, once exposed to air, are readily oxidized. When two dissimilar electronic conductors are placed in contact with each other, electrons flow from the material that is less noble (e.g., copper) to the more noble material (e.g., palladium) until an equilibrium is reached and the contact potential is formed at their junction. Because of the multitude of possible combinations of conductors in the real world, contact potential is the most ubiquitous of all junction potentials. 

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VOC (volatile organic compound Work-function sensors, micro-spectrometer, IR-sources, IR-detectors, I R-filters Hybrid or integrated, surface micromachining... 

Karthigeyan A., Gupta R. R, Schamagl K., Burgmair M., Zimmer M., Sharma S. K., and Eisele L, Low temperature NOj sensitivity of nano-particulate Sn02 film for work function sensors. Sens. Actuators B, 78, 69-72, 2001. 

Electrodes catalysts filters for all types of sensors surface plasmon resonance sensors sensors work function sensors... 

Doll T, Lechner J, Eisele I, Schierbaum KD, Gopel W (1996) Ozone detection in the ppb range with work function sensors operating at room temperature. Sens Actuators B 34 506-510 Drake C, Deshpande S, Bera D, Seal S (2007) Metallic nanostructured materials based sensors. Intern Mater Rev 52(5) 289-317... 

During the past few decades, many inorganic materials have been tested. Experiment has shown that ionic films like hydrated potassium halogenides provide excellent work-function sensor signals for ozone detection at room temperature (Doll et al. 1996). Figure 17.5a shows the work-function changes at RT of various materials exposed to O3 (-300 ppb in air). It is seen that KI has maximum response. 

Starting with this definition the semiconductor diemical sensors can be arbitrary classified with respect to following features the type of electrophysical characteristics diosen for monitoring, such as electric conductivity, thermal-electromotive force, work function of electron, etc. type and nature of semiconductor adsorbent used as an operational element of the sensor and, finally, the detection method used for monitoring the adsorption response of electrophysical characteristics of die sensor. 

Derivation of simple and unambiguous quantitative relations between the signal amplitude of a sensor, i.e., the value of the change of electric conductivity, work function, etc. and concentration of detected traces of admixture in the medium under study is also important for successful development of the sensor measuring technique. Theoretical considerations given in this book show that such relations exist in most simple form. The purpose of experiment consists in statistical substantiation that these dependencies rigorously hold at proper conditions. 

Of all existing methods to monitor electrical properties while using semiconductor sensors, only two [5] have become widely implemented both in experimental practice and in industrial conditions. These are kinetic method, i.e. measurement of various electrical parameters under kinetic conditions, and stationary (equilibrium) method based on the measurement of steady-state parameters (conductivity, work function. Hall s electromotive force, etc.). 

N. Barsan, A. HeUig, J. Kappler, U. Weimar, and W. Gopel. CO-water interaction with Pd-doped Sn02 gas sensors simultaneous monitoring of resistances and work functions , Proc. Eurosensors XIII, The Hague, Netherlands (1999), 183-184. 

Another possible effect of PdAu deposits on PdAu/SnOx sensors is through the formation of a Schottky barrier between PdAu and SnOx, as in the case of the Pd/CdS hydrogen sensor. If such a barrier is formed, then a depletion layer is created inside the semiconductor tin oxide. Since the Pd work function can be reduced by hydrogen absorption through dipole or hydride formation (14,15), the width of the depletion layer in tin oxide may be reduced. The reduction of the depletion layer width causes the sample resistance to decrease. Such a possibility was checked and was ruled out, because a good ohmic contact was obtained between Pd (-50 nm thick) and SnOx- It is also commonly known that gold forms an ohmic contact with tin oxide. 

Recently, Yamazoe et al. observed an extremely high hydrogen sensitivity for Ag-Sn02 sensors ([16). They attributed this to the Fermi level of SnOx being pinned at the redox potential of Ag+/Ag° when the sensor was in air, and at the work function of Ag°... 

If a conductor material undergoes a work function change when exposed to a certain chemical species, then clearly one has the foundations of a C-S diode sensor. However, this sensor cannot be made to function if the conductor chemically reacts with the semiconductor. This loss of sensitivity occurs because the new material resulting from the reaction, in general, will not have the same work function sensitivity to the chemical species as the conductor has. The C-I-S configuration solves this problem since a properly chosen I-layer, capable of supporting an electrical current, can be inserted between the conductor and semiconductor to prevent their reaction. The resultant C-I-S structure is able to respond to the effects of the gas species on the conductor. 

Mode 2 devices which rely on a different detection principle are the Kelvin probe sensor and the CHEMFET. In the first case, a vibrating capacitor measures the change of the work function (see Figure 2), while in the second case the interaction is detected in the field-effect transistor mode.29 31... 

Janata J, Josowicz M. Chemical modulation of work function as a transduction mechanism for chemical sensors. Acc Chem Res 1998 31 241-8. 

Various recent papers have discussed possible applications of dendrimers [3, 4, 11, 14, 20, 23]. From the point of view of electron transfer, the interest in highly branched structures such as dendrimers is related not so much to their size, but rather to the presence of different components. In fact, an ordered array of different components can generate valuable properties, such as the presence of cavities with different size, surfaces with specific functions, gradients for photoinduced energy and electron transfer, and sites for multielectron transfer catalysis. At this point it is perhaps worth summarizing the practical applications of dendrimers encountered in this chapter. Dendrimers have already been used (i) in catalytic processes [122, 149, 172] (ii) in the functionalization of electrodes [48, 63, 172-175] which can also work as sensors [44, 48, 63] (iii) in the fabrication of nanoparticles [170-172] and (iv) in other devices such as LEDs [140-148] and molecular gates [173, 174], Moreover, as many different exciting applications will certainly be added to this list in the future, it is easy to foresee a remarkable expansion in dendrimer related research. 

Correlation for the zirconia gas sensor function List of the influential factors and the parameters of their work functions Dependence of arguments on the influential factors involving into the zirconia sensor function... 

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