California Association of Chemistry Teachers
William F. Sheehan
of Santa Clara Santa Clara, California
University
I
Experiments with Photoconductive
M a n y well-planned and well-tested experiments that illustrate thermodynamic principles are available. Undergraduate experiments in chemical physics are, however, less common and frequently require expensive and specialized apparatus. One way of including modern experiments on X-ray or molecular spectra, reaction mechanisms, and magnetic properties is to present to the student a real experimental record.' A well-chosen system will furnish a record which is simple enough for an undergraduate to interpret and understand. "Dry-lab" or not, the thrill of determining a unit cell length, an interatomic distance, or a dipole moment is there for each student. I n this way even the college with a modest budget for equipment and maintenance can offer the most modern experiment, and this with a minimum of laboratory supervision and time. Solid state chemistry, because it generally seems to demand ultra-pure reagents and refined measurements, would a t first thought appear to be amenable only to dry-lab. Presenting students with a commercially prepared photoconductor or a record of its properties would indeed be simple. Fortunately, it is easy to prepare photoconductive CdS, and students find the actual preparation most satisfying. Characterizing t,heir own photoconductors by directions which at the end leave room for individuality and discovery is even more satisfying. The two procedures for preparing photoconductive CdS as set forth here call for only simple apparatus, and chemicals of the usual reagent grade. I n Procedure A, suitably doped CdS is precipitated slowly by thioacetamide in water containing CdCll and traces of CnC12. It has been used with good success by students of various ability at the University of Santa Clara. Readily detected photoconductivity occurred in every fired preparation. Whether using a formate buffer2 would interfere with electrical behavior in the fired product has not been investigated. Procedure B involves treating already prepared CdS (particle size about 10 microns) with aqueous solutions of CdCL 1
See, for example, CUPP, L E A L L ~B., N
AND
WILLEFORD,
BENNET R., ''Report of the Bucknell Conference on the Undergraduate Training of Chemistry Majors," Bucknell University, Lewisburg, Pennsylvania, June 12-18,1960, pp 40-41. B o w ~ ~ s oD. x , F., AND SWI*T,E. H., Anal. C h a . . 30, 12x8 (1958).
540
/
Journal of Chemical Education
and C U C I ~ .The ~ author has used it to explore systematic trends in dependence of photoconductivity in CdS on chlorine and copper content. This procedure has not been tested by students, but it is clearly simpler than A. Inasmuch as students who undertake this kind of experiment will have at hand in their textbook of physical chemistry much of the theory required, there is no need to discuss theory here. The best survey of photoconductors and their physical and electrical properties is "Photoconductivity of Solids" by R. H. Bube.4 Cadmium sulfide itself is discussed on about one-fourth of its pages, and it contains primary references to the literature of photoconduct~ivity. There are other references which undergraduates may find very u s e f ~ l . ~ The Experiment
The objects of this experiment are (1) to prepare photoconductive CdS containing deliberately added traces of chlorine and copper, (2) to determine the influence of copper content upon electrical properties of the photoconductor, and (3) to characterize the photocondoctor by measuring certain physical properties (e.g., light and dark currents, speed of response to light). Preparation Procedure A. Clean all glassware with aqueous detergent and rinse well with distilled water. If traces of CdS remain from previous use, rinse with concentrated HCI under the hood and then rise well with distilled water. With a motordriven glass stirrer, stir 100 ml 0.50 M CdCL ~olutionat 60% Dissolve 2g thioacetamide in 15 ml distilled H1O and add 1 ml of this solution to the hot stirred solution of CdCb. After a. colloidal precipitate of CdS has begun to form (about 30 min), add 2 or 3 rnl of thiaacetnmide solution every 15 min until all is added. After each such addition of 2 or 3 ml, add from a pipet 3 or 4 drops ol 0.0005 M CuCb solution until 20 drops (about 1 ml) are added. After two or more hours of such additions and continuous stirring at 60°C, allow the doped precipitate of CdS to settle. (If time or inclination permits, decant the hot supernatant
*THOMSEN, S. M., AND BUBE,R. H., RLQ.Sci. Instr. 26, 664 (1955). 'Bum, RICHARDH., "Photoconductivity of Solids," John Wiley $ Sons, Inc., New York, 1960. "ee, for example, (a) December, 1955 issue of P~oeeedingsof the Institute of Radio Engineers; (b) HONIG, J. M., J. CHEUI. EDUC., 34,224,343 (1957); (c) May, 1961 issue of THIS JOURNAL [J. CHEnl. EDUC..38.224-250 (1961)l.
liquor into a clean beaker and continue stirring a t 6 0 T with gradual addition of 0.005 M CuCb solution so as to produce a second preoipitrtte containing more copper.) Decant and wash the precipitate three times with distilled water, each time decanting the supernatant liquid completely, perhaps with a few of the finer particles. With a camel's hair brush, paint a bit of the thick CdS slurry on a cbip (about 10 mm by 10 mm) of poraua porcelain streak plate or anodized aluminum. Allow to dry in air. (If a photoconductive powder is desired instead of a sintered layer, the slurry can be dried in a beaker in a vacuum oven at 80°C for about one hour. This can then be fired in a closed or open Vycor tube.) Preparation Procedure B. Clean several 5 0 4 s c r e ~ c a pglass bottles with aqueous detergent and rinse well with distilled water. If traces of CdS remain from previous use, rinse with concentrated HCI under the hood and then rinse well with distilled water. Into each bottle weigh 2.9 g (20 millimoles) of CdS of suitable purity. Add about 5 millimales of CdCL as an aqueous solution from a buret; then add enough distilled water so that, after subsequent addition of CuCl? solution belnw, the final volume will be 40 ml. After shaking the mixture of H1O, CdS, and CdCI?, add from a. buret a dilute solution of CuClz containing about 0.01 millimoles of CuCla and shake well a t once. The quantities of CdCb and CuClz can be varied from bottle to bottle in a systematic way to determine the effects of doping on behavior. Shake each bottle several times over a period of an hour, or shake well and let stand over night. Decant the supernatant liquor from each bottle until the finest particles just begin to come off. With a camd's hair brush, paint a bit of the thick CdS slurry on a chip (about 10 mm by 10 mm) of porous porcelain streak plate or anodieed aluminum. Allow theseveral painted chips to dry in the air.
Firing. Place one dried preparation from either A or B in a 20-mm diameter Vycor or silica tube about 2 f t long and open to the air a t each end. Insert the tube containing the one sample into a preheated furnace at about 600°C and fire for 3 to 5 minutes. This firing should be done in a hood to avoid fumes of CdCI,. Withdraw the tube and its contents, allow them to cool a few minutes, and remove the sample from the tube for complete cooling in laboratory air. Several samples should be fired one a t a time in the order of increasing chloride content and in the same region of the Vycor tube to avoid the deposits of CdCL which form in the cold zones of the tube. Electrodes. With conductive silver paint, add electrodes to the top of the fired CdS. Suitable distances are about 1.2 mm between parallel electrodes about 10 mm long. While the painted electrodes dry, prepare for the electrical testing. Testing. In a light-tight box containing a variable light source (e.g., a frosted tungsten bulb with variable diaphragm and/or voltage control, and pwhaps various optical filters for determining the effects of changes in wavelength), measure the current passed by the specimens in the light and in the dark. It is desirable to place a 10-ma fuse and a 1000-ohm resistance in series with the ammeters and the variable 500-v dc source, for occasionally a sample may arc. The ammeters should be capable of measuring currents from less than one microampere to several milliamperes. Because time and facilities vary in different laboratories, detailed testing instmctions are not given here. If an oscilloscope and propeller-like beam chopper of variable speed are available, the times for certain fract,ions of the stationary light current to be reached and for certain fractions of it to decay (as the chopper interrupts and passes the light) can be measured. The extent to which light and dark conductivity depend on temperature and previous exposure to light may be
of considerable interest. also sensitive to X-rays.
CdS photoconductors are
Results
Procedure B, which involves treating already prepared CdS with aqueous solutions of CdC1, and CuC12, was used by the author to explore some of the variables involved: The dependence of light and dark currents and of rise and decay times on time and temperature of firing, and on the amounts of chlorine and copper added. The standard amount doped was 20 purified CdS. The copper (11) ion reacts immediately with yellow CdS to produce brown particles. While the reaction with copper ion is probably essentially complete, the amount of chlorine held by a sample depends on several variables, including the exact technique of decanting and firing. Because of such uncertainties and because a limited statistical analysis of results from similar. samples suggests that fourfold variations in current for supposedly identical sintered layers are not uncommon, the trends observed through use of Procedure B are set forth here only graphically. At almost any level of sophistication, the preparation of photoconductors is an art. The data reported in these figures is based on a study of 35 samples of various impurity content, each fired in several ways. Some chloride impurity was lost in decanting and firing, but the systematic trend should reappear, perhaps with some distortion, in other preparative methods such as Procedure A. Losses of copper are less likely so that the abscissa is a reliable index of total copper present, but not necessarily of homogeneity. The region of chemical parameter space in which there could be less than 2 C1 per Cu (lower right-hand part of figures) was not investigated, for Cu was added only as CuC12. I n the figures the abscissa, log X, is the logarithm of the number of millimoles X of CuCL added in water to the 20 millimoles of CdS. The ordinate log Y is the logarithm of the number of millimoles of C l added as CdC1, and CuCL in water, again to the standard 20 millimoles of CdS. The numbers on the lines indicate the power of ten of the light current expressed in microamas. (Fia. - 1) . or the power of ten of the ratio (Fig. 2). The data reported in Figures 1 and 2 were observed a t constant light intensity with painted silver electrodes 9-16 mm long and 0.9-1.6 mm apart,. I n lieu of a better method, all currents were corrected linearly to those expected of a standard having electrodes 1.0
Figure 1 (left). Lighl current (samples prepored by Procedure 8. doped CdSfired 5 min a t 600°C). Figure 2 (right): Ratioof light current to dark current (samples prepared by Procedure 8: doped CdSfired 5 min a t 600°CI.
Volume
39, Number 10, Ocfober 1962
/
541
mm apart and 10 mm long. When measurements were made at voltages less than 500, several voltage-current observations and the relation I = kVn were used to estimate by extrapolation a value for 500 volts. Here n is an empirical exponent (Fig. 3) which best fits the observed light currents I a t voltages V of about 100 v. Samples containing more Cu than those of log X = -1 deteriorated after several days of exposure to the laboratory atmosphere. Rise and decay times, measured with an oscilloscope whose record was synchronized with the current changes
With these remarks and figures as guide, students can explore the effectsof the variables. Analogous behavior, a t least in gross trends, may exist for other systems having similar substrates such as CdSe. Student Results with Procedure A . Table 1 lists some Table I. Light and Dark Currents of Typical Student Preparations of Photoconductive CdS (Procedure A)
Dimensions of photoconductor Preu- Distance Length Firing Variables between of a& elec- Furnace Current at 100 v. tion elecnum- trodes trodes tem . Time (microamperes) (min) Light Dark (mm) her (mm)
cod;
la. lb 2a 2h 3a 3b 4a. 4h
Figure 3. Dependence of light current on voltage (numbers are the power n in I = kV"1.
caused by a propeller-like beam chopper of variable speed, are given in Figures 4 and 5. These time intervals increase by an order of magnitude as the light intensity is decreased, and the time intervals of decay t o 16% of the stationary light current when the light beam is interrupted by the chopper are about six t i e s as great as those reported in Figure 5. The rate of decay varies as a power of the current, where that power is 2 a t log X = -2 and rises to about 3 a t log X = -4. I n the region of maximum light-to-dark ratio, rise and decay times decrease as light intensity increases. They are generally of the same order of magnitude, and decrease or remain the same as firing temperature increases. However, current rise time a t low light intensity may increase as firing temperature increases. Rise and decay times depend strongly on copper content, and firing temperature influences decay times more than rise times. The ratio of decay time for 84% decay to decay time for 50% decay decreases as light intensity or copper content increases, but is almost independent of firing temperature and apparent chlorine content.
>
S rn
-"
::m +i
0
-1
SLOW
Acknowledgments
-2 -4
-3
-2
-1
0
+i
-4
loglo X
-3
-1 losla X
-2
0
+i
Figure 4 (lefll. Time (nillirecondr) for light current to rise holfwoy to its rbtionory value lrampler prepared by Procedure B: doped CdS flred 5 min a t 600°CI. Figure 5 (right). Time (millirecondr) for light current to fall hdfwoy to ib stationary valve in dark lrampler prepared b y Procedure 0: doped CdS flmd 5 min a t 6OO'C).
542
/
student results with Procedure A, which involves precipitation by thioacetamide. The measurements were made in an improvised light-tight box containing a 25watt frosted bulb about 50 mm from the sample. Light from the bulb passed through a diaphragm of variable size near the bulb and fell vertically on a horizontal sample. The data of Table 1 are for an opening 20 mm in diameter nit,h the bulb at 110 volts ac. This intensity is about double that used in Figures 1-5. Preparations l a and l b are differeilt regions of the same ceramic chip, the CdS doped with 20 drops (1 ml) of 0.005 M CuC12. Preparations 2, 3, and 1 m.ere doped wit,h 1 ml of 0.0005 M CuC12. Although students were given complete freedom to perform 7-arious tests in various ways, without detailed inst,ructions they established that light current mas ohmic and responded almost linearly with the area of the diaphragm opening. The most sensitive galvanometer used (full scale = 15 microamps; one division = 0.5 microamp) was barely able to detect dark currents at 100 r. Many preparations can be tested without arcing at 500 r, holyever, even in the light of the laboratory. The dark cul~ents of Table 1 and Figures 1-5 are of the order of the magnitude found for bare ceramic chips of the kind used to support the CdS. Hence, they reflect the resistance of the substrate chip or poor insulation in the apparatus rather than true dark currents of the sintered CdS when their value is about 0.1 microamp. Some students noticed that photoconductive behavior depended on previous periods of illumination (heating and perhaps redistribution of electrons in traps).
lournal o f Chemical Education
The Hewlett-Packard Company of Palo Alto provided a research grant to the University of Santa Clara (1956-1957 academic year). The help of Mr. Charles Reis especially is acknowledged. The students who tested Procedure A were R. L. Armanasco, Jorge R. Castelazo, Thomas K. Hall, Timothy H. Johnston, Richard Larrabee, Raphael A. Legorreta, Terence Miraglia, and John A. Oberholzer.
