H. Keyzer California State College LOS Angeles, 90032
A Short Experiment in Xerography
The technological advances generated by solid state chemistry, witnessed this century by countless semiconductor devices, have been nothing short of a miracle. The applicat,ion of solid state concepts t o biochemical systems has met with considerable success, e.g., the phenomenon of olfactory transduction ( I ) , the primary event of vision ( 2 ) , the transmission of nerve impulses across the synapse (S), to name but several. Evcn thc explosive dissemination of knowledge in the last decade or so has been enormously aided by the revolutionary solid state copying process known as xerography. Although solid state chemistry is steadily gaining ground in the undergraduate curriculum very few student experiments are available in this field. Many solid state experiments require extensive preparation, highly purified materials, and unremitting care. However, the following experiment requires simple equipment and readily available materials and should serve as a pleas-
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ant introduction to the fascinating field of the solid state. Xerography is coined from the greek tapor, dry, and y p a + ~ i v , to write. It is ironic to reflect that the ancient Greeks could have invented this process since they had the required ingredients in abundance, e.g., sulfur, soot, sunlight, resin, and papyrus. Further, they were acquainted with the phenomenon of static electricity even though they were ignorant of its nature; for instance, Thales of Miletus (about 600 BC) entertained his visitors by attracting straws to an amber rod which had been rubbed with a cloth. I n fairness it must he noted that although their hypothetical xerographic experiment would demonstrate the principle it would be so inefficient as to be practically useless. Even today xerography as a solid state phenomenon is not fully understood. The process is thought to involve excitons and trapping centers as follows. When a crystal absorbs a photon a valence electron may be excited into the conduction band and a hole left
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Figure 1 . Eiporure of sulfur plate and opaque templote to ultraviolet light.
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hgure 2. Sulfur surface offer dusting with carbon block and removal of eicerr.
Figure 3.
Fixing of the image on the p a p e r l i t h
a hot air gun.
behind in the valence band. Since the hole is a location of missing negative charge, it has an effective positive charge and hence attracts the electron but may not recombine with it. This association between an electron and a positive hole is known as an exciton. The exciton migrates through the crystal until it meets a trapping center T which may he a crystal defect, an impurity or a surface, in short, an anisotropic region. The exciton then dissociates in one of two ways (e-. . . h + ) T = (h+. . .T) + e-
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Irrespective of which reaction occurs charge carriers are released. Sulfur has a resistivity of 2 X 1017ohm cm. When it is illuminated with light corresponding to one of its absorption bands its conductivity is greatly increased. Therefore, where light strikes the sulfur charge carriers are released and driven into shielded areas. Finely divided, solid dielectrics, e.g., carbon black, can then he attracted by the shielded area which makes possible the xerography process. Commercial xerography employs selenium which is a far more efficient photoconductor. Our experiment uses sulfur because of its lesser toxicity. Further, unlike the hypothetical ancient Greek, we will use ultraviolet light from a George W. Gates & Co., 420-U1 nv source instead of sunlight to reduce exposure times. Experimental Melt sufficient sulfur into a metal tray and let it wol slowly to form a solid layer approximately 1 em thick. Smooth the surface wibh sand paper snd charge the plate electrostatically by vigorous friction with cat's fur. (The commercial process does this by corona discharge.) Place an opaque template, say a pasteboard cut-out, and expose the system for about 10 min to ultraviolet light. (See Fig. 1). NOTE: The irradiation should take place in s fume hood which has, say, brown paper taped to the window because uv radiation is harmful to vision and, moreover considerable qoantities of ozone are produced from the air. When the light is switched off remove the template and dust cwhon black onto the sulfur surface. Dusting is most conveniently done by means of a plastic squeeze bottle. Remove excess carbon by a gentle stream of air and the image will become elearlv visible (see Fie. 2). Then mess a. clean sheet of oaoer onto
particles (see Fig. 3). (Commercially, resin is incorporated with the pigment.) Since sulfur is a high resistivity material the charge is difficult ta remove and the unexposed ares retains much of the pigment. This wav it is oossiblo to obtain ur, to half a dozen faint hut discernible copies of the image from the one exposure. Best resulk are obtained with the sulfur heated to about 80°C during exposure. This temperatore is readily achieved by plaeing the uv lamp about 9 in. from the plate. The experiment takes about one t o one and one-half hours.
Discussion
This experiment should pose the following questions to the student. What is the concept of a valence and conduction band? How are these concepts used to explain the difference between metals and semiconductors? How does heating cause sulfur to increase its couduction if metals exhibit decreased conductivity upon being heated? The relationship between resistivity (ohm cm) and absolute temperature T should he discussed in terms of the energy gap equation P = po exp(-E/2kT)
where E (in eV) is the Fermi energy and k is Boltzmann's constant. If both electron and positive hole traps can occur in sulfur why don't the released charge carriers recombine and thereby produce no photocurrent effect at all? Acknowledgment
The author expresses gratitude to the physical chemistry students of this college who determined the optimum conditions for this experiment. Literature Cited (1) ROSENBERO, B., A N D POSTOW,E., "Sernicondu~tionin Proteins and Lipids and Its Possible Biological Import," Biophysics Department, Michigan State University, East Lansing, Michigan, 1967. (2) ROSENBERG, B., Pholochern. and Pholobiol., 1, 117 (1962). "Chemical aud Molecular Basis of Nerve (3) NACHMATISON,D., Activity,"Academic Press, New York, 1959.
General References
GUTMANN, F. G., AND LYONS, L. E., "Organic Semiconductors," John Wiley & Sons, Inc., New York, 1967, rhi. 1 , l l . MOORI:,W. J., "Seven Solid States," W. A. Benjamin, Inc., New York, Amsterdam, 1967. "Xerography," Australian Photography, February, p. 40 (196.5).
Volume 46, Number 8, August
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