Jan., 1959
PHOTOCHEMICAL PROCESSES IN THINSINGLE CRYSTALS OF AgBr
45
PHOTOCHEMICAL PROCESSES IN THIN SINGLE CRYSTALS OF SILVER BROMIDE : THE DISTRIBUTION AND BEHAVIOR OF LATENT-IMAGE AND OF PHOTOLYTIC SILVER IN PURE CRYSTALS AND IN CRYSTALS CONTAINING FOREIGN CATIONS BY W. WESTAND V. I. SAUNDERS Communication No. 19@ front the Kodalc Research Laboratories, Eastman Kodak Co., Rochester, N . Y Received .July 11, 1868
The distribution-in-depth of latent-image centers and of visible print-out centers has been investigated in purified thin prystals of silver bromide made from the melt and in crystals containing small additions of cadmium ion, lead ion and cuprous ion. With respect t o latent-image centers, three sensitometrically distinct regions are found in ure crystals-the nonsensitive surface, a subsurface region of low sensitivity, and the deeper interior, of relatively higi sensitivity.. Internal latent-image is found at depths much greater than those to which light can penetrate with appreciable intensity. Latent image is formed throughout the whole thickness of pure crystals exposed t o penetrating light, except for the subsurface layers of low sensitivity, but no rint-out silver is formed in the central regions. The distribution of print-out silver in pure crystals appears to be governeaby the ease of @usion of positive holes to the surface from regions at different depths within the crystal. This distribution can be modified by the introduction of traps for positive holes. The silver-ion vacancies introduced by divalent cadmium or lead ions do not act as deep traps for positive holes; on the contrary, by inhibiting diffusion, they increase the attack of positive holes on photolytic centers over that in pure crystals;, The image formed in crystals containing cadmium or lead ions contains a non-silver “complementary photolytic image, cansisting of oriented pits a t the surface, apparently formed by the escape of bromine a t localized sites. Cuprous ion acts as a deep trap for positive holes by electron exchange, increasing the yield of print-out silver on exposure to visible light by about an order of magnitude over that for pure crystals, without any increase in the bromine evolution. Penetrating light causes the appearance of print-out centers throughout the whole illuminated volume of cuprous-containing crystals. At elevated temperatures, the trapping of positive holes by cuprous ions is reversible, and the rocesses of image formation at room temperature, followed by thermal fading a t higher temperature, can be repeated a t wilf
Single crystals of silver halide, some 10 to 100 p thick and a few millimeters in the other linear dimensions, cut from sheets prepared from the melt, constitute a photographic system in a sense intermediate between massive crystals prepared by the Bridgman or Kyropoulos techniques and the grains of a normal photographic emulsion, and Mitchell and co-workers1-6 have amply shown the utility of such crystals in performing model experiments on fundamental questions of photographic sensitivity. The crystals are large enough to be handled easily, they can be subjected to controlled treatments before and after exposure, and images can be developed by the normal processes of photographic development; hence they form to some degree a readily manipulable model for the emulsion grain. Nevertheless, apart from their large linear dimensions and particularly their relatively great thickness, which permits exploration of the photographic behavior of silver halide crystals a t much greater depths from the surface than are available in normal emulsions, the mode of formation of these crystals is very different from that of emulsion grains. The large crystals have their own characteristic photographic and photochemical behavior, conditioned partly by the intrinsic properties of silver bromide, partly by the dimensions of the crystals and partly by the nature of the impurities and crystal defects, which probably are determined to a large degree by the specific mode of formation of the crystal. Not all aspects of the photographic behavior of the large crystals can be expected to apply to emulsion grains, and judgment (1) J. W. Mitchell, J . Phot. Sci., 121 1, 110 (1953). (2) J. M. Hedges and J. W. Mitchell, Phil. Mae., [7] 44, 357 (1953). (3)
T. Evans, J. M. Hedges and J. W. Mitchell, J . Phot. Sci., 121 3,
73 (1955). (4) P. V. McD. Clark and J. W. Mitchell, ibid., (21 4, 1 (1950). (5) .I. W. Mitchell, ibid., 121 5, 49 (1957).
must be exercised as to the validity of the large crystal model for the study of any given photographic phenomenon. The present communication is concerned with aspects of the photochemical behavior of the large crystals in themselves, in the endeavor to secure background for the further application of the system to problems of photographic sensitivity. I. Preparation of the Crystals and General Procedure.Pure crystals have been studied as well as crystals containing small amounts of divalent cations and cuprous ions. The starting mat,erial was the purest available silver bromide precipitate made from silver nitrate and hydrobromic acid, for which we are indebted to F. Urbach and co-workers, of these Laboratories.6 The cr stalline sheets were prepared following the procedures o? Mitchell and co-workers.294 The essential steps in this preparation are: the removal of a small quantity of solid siliceous material from the bromide precipitate by filtering the melt through Pyrex-glass capillary tubes; a further purification by melting the bromide under vacuum, which removes some volatile materials; the formation of a film of silver halide by melting a pellet of the purified halide between Vycor plates; solidification by slowly drawing the plates through a horizontal temperature gradient; and separation of the crystalline film from the plates by immersion in water. The resulting crystalline sheets, about 2 inches in diameter and usually between 50 and 100 p thick (determined by Pyrex glass or platinum spacers), consisted of a few single crystals, from which small pieces of single crystal could be cut. With the ohject of minimizing silver or silver oxide production in the making of the sheets, a halogen atmosphere was maintained during all heating operations except when it was desired to incorporate cuprous ion. I n this latter case, the sheets were prepared under an atmosphere of nitrogen. Usually, the crystals were not annealed after separation from the plates. Bromide films were examined at the wave lengths 365, 405 and 436 mp, isolated from the radiation of an AH-4 mercury lamp by filters. (For 365 mp, Corning Filter No. 3860 aqueous copper sulfate 0.75 M , 3.2 cm. thick; for 405 mp, Corning Filters No. 3060 5970 aqueous copper sulfate as before; for 436 mp, one thickness of Kodak Wratten Filter No. 2A two thicknesses of Kodak Wratten Filter No. 47 aqueous copper sulfate as before.)
+
+
+
+
+
(6) N. R. Na-il, F. Moser, P. E. Goddard and F. Urbach, Rev. Sci. Inatr., 28, 275 (1957).
46
W. WESTAND V. I. SAUNDERS
Light of wave lengths 365 and 405 mp is completely absorbed within a few microns of the surface of incidence, while 90% absorption of light of wave length 436 mp takes place in penetrat,ing about 50 p . In addition, experiments were performed using exposures to light which penetrated the crystal with only about 30% absorption by a layer 100 p thick. A tungsten source, filtered by means of a Kodak Wratten Filter No. 4, sometimes combined with the copper sulfate filter and sometimes without this filter, as described later, was used for this purpose. The effective wave lengths in these exposures to penetrating light comprised a band between 470 and 480 mp. Crystals carefully prepared by these procedures show little or no surface sensitivity; no reduced silver image appears on the surface after exposure and development.283 Latent image is produced in the interior, however, which can be developed after the application of suitable etching procedures..2,3 The observations to be described in this communication are concerned with the formation of this internal latent-image and of the visible print-out or photolytic image formed by long exposures. As was f i s t shown by Hedges, Evans and Mitchell, latent-image centers and photolytic centers are produced mostly a t sites in the dislocation networks which constitute the polyhedral substructure of the ~ r y s t a l . ~ * ~ J A method of “graded etching” which reveals the distribution of internal latent-image throughout the depth of the exposed crystal has been described.* After exposure, the crystal, with one side protected by mounting on glass, is gvadually immersed in an etching bath of 4.2 M potassium bromide solution containing 0.1 M potassium ferricyanide, so that, at the end of the operation, the crystal is wedgeshaped. The etched surface then cuts through the original distribution of latent image and, on development, the distribution in depth of the centers is revealed. The function of the ferricyanide is to oxidize the latent-image centers as soon as they are uncovered by the etchant. Without the oxidizing agent, these centers accumulate because of a tendency to adhere to the newly formed etched surface, and, on development, the distribution of developed centers does not match that of the original latent-image centers. When the etch bath contains ferricyanide, the surface formed is completely free from latent image, but an additional shallow etch of a fraction of a micron by means of potassium bromide solution alone forms a new surface containing latent-image centers in the amounts corresponding to the true distribution in depth. The general procedure was to expose through a narrow slit a piece of single crystal mounted on glass, carry out a graded etch as just described, apply a thin layer of gelatin by momentary immersion of the crystal in 2% aqueous gelatin, followed by draining and drying, then to develop by immersing about 30 seconds in a p-methylaminophenolascorbic acid developer .s A slit-like image appeared along the inclined surface formed by the etching operation, the distribution of density along the length of which reflected the distribution in depth of latent-image centers from t,he original surface of incidence. The depths were determined by microscopic observations of the thicknesses of the original and etched crystal. The distribution in depth of photolytic silver was determined by direct microscopic observation of the discrete particles of silver in the unetched crystal. I n the subsequent discussion, the term “latent-image sensitivity” will be used to denote the sensitivity associated with the production of developable silver from microscopically invisible latent-image centers, while “photolytic sensitivity” will denote the sensitivity associated with the production of directly visible deposits of silver. It is not the object of this communication to discuss, other than incidentally, the details of the formation of latentimage centers or of photolytic centers. Surveys of these topics have been made recently by Mitchell, and mechanisms have been proposed.6J0,11 In the following discussions, it is assumed that the ab(7) J. M. Hedges and J. W. Mitchell, Phil. Mag., 171 4 4 , 223 (1953). (8) W. West and V. I. Saunders, “Proc. Internat. Congress for Scientific Phot.,” Cologne, 1956, in press. (9) T. H. James, W. Vanselow and R. F. Quirk, P S A J . (Phot. Sei. a n d Tech.), 19B,170 (1953). (10) J. W. Mitchell, Phot. Row., 93, I. Sonderlieft (1957). (11) J. \V. Mitclrell and N. I?. hfott, Phzl. .Mag,, [SI2, I149 ( l Q 5 7 ) .
Vol. 63
sorption of light by silver halide crystals causes the appearance of electrons in the conduction band and of positive holes in the valence band. I n chemically pure and highly perfect crystals, most of the electrons and positive holes recombine, and photographic sensitivity depends on the prevention of this process. This may be accomplished by the introduction of suitable traps which will capture the electrons and positive holes separately, and, according to Mitchell,6J1 it is of special importance that the tra ped electron be at a site which will repel a positive hole. $he holes may eventually escape as free halogen, or they may be dee ly trapped chemically. The trapped electrons combine wit! silver ions, forming silver atoms and aggregates of silver atoms. The latent image is assumed to be a stable aggregate containing a small number of silver atoms, which, on the addition of developer, catalyzes the reduction of the silver halide so as to yield a visible image of reduced sjlver. Prolonged exposure results in the formation of deposits of colloidal silver which are visible without development, the so-called “print-out’’ or “photolytic” silver. Latent-image centers and photolytic centers are formed at the same type of site in sheet crystals, namely, at dislocation^,^ and it seems probable that under certain circumstances the latentimage center may, on prolonged exposure, grow into a particle of photolytic silver. One of the results of the Observations to be reported here is that the conditions for the growth of latent-image centers t o photolytic centers are severely limited and much of the following discussion is concerned with the origin of the peculiarities which are found in the distribution of latent image and of photolytic centers in pure and impure sheet crystals of silver bromide. 11. Surface, Subsurface and Internal Latent-image Sensitivity. “a-Three distinct regions, exhibiting characteristic 1atent:image sensitivity, can-be recognized-in the sheet crystals: the insensitive surface layer already alluded to, a subsurface region extending a few tenths of a micron from the surface and the deep interior. Latent image is found a t depths slightly below the surface and extending to depths within the crystal considerably below those to which light can penetrate .4,a,12 For example, radiation of wave length 365 mp is attenuated to 1%of the incident value after penetrating about 8 p into the crystal, but latent image can be found at depths up to 50 p from the surface in unannealed crystals and to 90 p in lightly annealed crystals. The immediate subsurface region differs sharply in sensitometric behavior from regions deeper in the interior of t h e crystal: (a) the general sensitivity in the subsurface layer is lower than in the deeper layers; (b) the subsurface layer shows a greater reciprocity failure at low intensities than the interior; and (c) the image produced in the subsurface region by illumination through a narrow slit shows no broadening outside the geometrical image of the slit, while in the interior, the image spreads laterally in a manner suggesting that it is formed in this region by the diffusion of electrons from the light-absorption zone near the surface. When the surface is chemical1 sensitized, e.g., by means of sodium aurothiosulfate wit1 ammonium c h l ~ r o i r i d i t e , the ~ ~ ~ developed image in the surface does not spread beyond the illuminated area.1v3 The differences in the sensitivity and in the reciprocity behavior of the subsurface and of the deeper regions within the crystal are illustrated by the data in Table I. A sheet crystal was exposed to 436 mp radiation through a slit 100 p wide so as to receive the same energy (intensity X exposure time) at three different intensity levels. After graded etching and development, the densities of the images were measured at various levels from the original surface by means of a recording microdensitometer, which scanned the slit-like image from side to side. As already mentioned, the internal image occupies a width greater than the geometrical image of the slit; hence as a measure of the total photographic effects at various levels, the densities were integrated over the width of the image. These quantities are listed in the table as relative integrated densities. Data are reported for exposures in air and under a layer of 50% aqueous sodium ni(11s) Examination of surface and of subsurface regions formed by etching well into the interior of the original crystal shows that t h e properties described in the following section pertain to surface and subsurface regions per 86, and are not restricted to the original surface prepared in contact with the hot T’ycor plates. (I?) JV. \Vest and V. 1. Sa,indprs, I’kgs. Reo., [Z]98, 1567 (1966).
I N THINSINGLE PHOTOCHEMICAL PROCESSES
Jail., 1959
CRYPTALS OF
AgBr
47
Loening16 in sensitizing sols of silver bromide. The inefficiency of nitrite as a surface sensitizer, contrasted with its sensitizing effect on the subsurface region with which it has no immediate contact, is at first sight surprising, but is conTABLE I sistent with the distinction which Mitchell has made beINTEGRATED DENSITIESOF DEVELOPED SUBSURFACE AND tween a surface sensitizer, functioning by capturing positive DEEPER INTERNAL LATENT-IMAGES IN PURE SILVER holes which approach the surface,6 and a halogen acceptor like nitrite ion waich is unable to transfer an electron to a BROMIDE positive hole in silver bromide, although able to do so to Intensity, quanta cm.-’ 8ec. -1 1.6 X 1.6 X 1.6 X 1018 1014 101s free bromine in aqueous medium. Time, 880. 10 1 0.1 The deep internal latent-image has already been deExppsure Position scribed in some detail.* I n addition to its appearance a t In of image Relstive integrated densities depths greater than those to which light can penetrate with 0 3 15 Air Subsurface significant intensity, important characteristics of this image are: ( a ) it persists without much decrease in maxi. 6 p below 20 20 20 Air mum depth when the exposure is carried out a t - 5 0 ” , 23 24 18 Air 10 p below followed by etching and development a t room temperature; 15 p below 22 26 12 Air and (b) when fo!med at intermediate depths by prolonged 30 u below 10 1 0 Air exposure at -50 ,followed by normal etching and development, it is strongly solarized, while no solarization of the 10 17 36 Nitrite Subsurface internal image in pure crystals is observed when exposures 22 20 19 Nitrite 6 p below are made at room temperature: The existence of the deep 10 u below 22 20 11 Nitrite internal latent-image can scarcely leave any doubt that 20 16 0 Nitrite 15 p below electrons or silver atoms diffuse into the interior, possibly along dislocations, from the absorption layer near the sur7 0 0 Nitrite 30 u below face wherein the primary photoelectrons and positive holes The images produced in the subsurface region at low in- are produced. If the bromination hypothesis of solarizatensity are much feebler than the images produced by the tion is adopted,‘? bromine also in some form must diffuse same exposures some distance within the crystal and also into the interior and attack latent image a t low temperature. It does not seem possible at present to say decisively than the subsurface images produced by the same exposure (intensity X time) at the higher intensities; Le., the sub- whether the deep internal image in these crystals is produced surface image shows a marked reciprocity failure at low in- by diffusion of electrons or of silver atoms from the absorptensities, greater than that of the internal image. The tion zone. The deep image might conceivably be formed by deeper internal images, in fact, show distinct high-intensity a redistribution of latent image near the surface on absorption of radiation, but since it is found to be produced with reciprocity failure. The distinction between the subsurface layer and the in- no less efficiency by short exposures (0.01 sec.) than by terior of the crystal is especially marked for exposures to longer exposures a t the same intensity, this mechanism appears to be excluded. The relatively high rate at which weakly absorbed penetrating radiation of wave length 470480 mp. The incident intensity of this radiation was com- the equilibrium distribution of the deep latent image is parable to that of the blue and ultraviolet mercury lines attained is consistent with its formation by diffusion of used for exposures at shorter wave lengths, but the low ab- primary electrons. The operations of etching and developsorption coefficient of the light a t the longer wave lengths ment impose a time interval of a few minutes from the end causes the volume concentration of the absorbed quanta to of the exposure before the distribution can be ascertained, be only about l / 6 0 of that of the radiation of wave length 365 but it is certainly set up within that interval, since no inmp. Under these conditions, very little subsurface imrtge crease in the maximum depth of the internal latent-image was formed by exposures which produced strong latent image is observed if the interval between exposure and deveiopin the interior. The relatively insensitive subsurface layer ment is increased. It is true that surface films of silver dewas found under both the surfaces of incidence and of exit of posited on silver bromide by evaporation in vacuum have the light, showing that the behavior of this region is definitely been shown by Evans, Hedges and Mitchell3 to lose silver associated with some condition prevailing near the surface. by migration along the surface and into the interior, probSurface states are well known in germanium, and it has been ably by an electron-silver-ion process, but this migration recognized that the surfaces of semi-conductors in general seems to be a slower process than that involved in the formamay be regions of intense trapping and recombination of tion of the deep internal latent-image. On the whole, the known facts regarding the formation of the deep internal charge carriers. The sensitivity of the subsurface layer was found to be latent-image seem most consistent with its formation by the considerably increased when the sheet was exposed to light migration of primary electrons from the absorption layer. Similarly, although the observed solarization of the deep of wave length 436 mpt through a thin layer of concentrated aqueous sodium nitrite solution (Table I). Since the best- internal image at low temperatures might be caused by the known photographic action of nitrite is halogen acceptance, diffusion of atomic or molecular bromine along internal surthis observation suggests that the low sensitivity of the faces, the conclusive evidence for the motion of positive subsurface layer is connected with inefficiency of removal of holes in sheet crystals a t room temperature afforded by the photobromine from the surface in the normal mode of ex- increase in the electrical conductivity of the crystals when * ? ~ ~it highly probable that posure in an atmosphere of air. Various lines of evidence placed in bromine v a p ~ r ~makes indicate that bromine may dissolve in silver bromide at room the solarization is caused by the migration of positive holes temperature as positive holes.*+-16 If, as a result of expo- into the interior from the absorption zone of the crystal. sure, an adsorbed layer of bromine accumulates on the surIII. Distribution of Internal Latent-Image and face of a silver bromide crystal, the concentration of positive holes in the subsurface region will be greater than when the Photolytic Silver throughout the Depth of Sheet has been shown by Hedges and bromine is immediately removed, and since positive holes Crystals.-It at sufficiently high concentrations can attack trapped elec- Mitchell2 that the photolytic silver formed by protrons and silver atoms, the prevention of this attack by a layer of nitrite will increase the subsurface sensitivity. It longed exposure of unsensitized sheet crystals is is to be noted that exposure under nitrite induced no more situated below the surface. Our measurements than a trace of latent-image sensitivity at the extreme sur- show that the photolytic centers produced by ultraface, a n effect paralleling the inefficiency of halogen acceptors violet radiation are confined to a shallow layer just such as semicarbazides or phenol observed by Sutherns and below the surface, few particles being more than trite solution, 40 cidence.
p
thick, in contact with the surface of in-
(13) G. W. W. Stevens, “F.undamenta1 Mechanisms of Photographic Sensitivity,” J. W. Mitchell (editor), Butterworths Soi. Publications Ltd., London, 1951, p. 227. (14) G. W. Luckey a n d W. West, J . Chem. Phys., 34, 879 (1956). (15) L. M.Shamovskii, A. A. Dunina and M. L. Gosteva, J . Bzpt. Theorst. Phys. U S S R , SO, 640 (1956).
about 2 p from the surface (Fig. 2a). (16) E. A. Sutherns and E. E. Loening, J. Phot. Sci., 121 4, 154 (1956). (17) C. E. I
