ENGINEERING RESEARCH INfTITUTE UNIVERSITY OF MICHIGAN ANN ARBOR ELECTRON DIJFRACTION INVESTIGATION OF PHOTOCONDUCTIVE CRYSTALLINE LEAD SULFIDE S[URFACES QUA1RTERLY REPORT NO. 4 5 December, 1952, to 5 abrch, 1953 J L. 0. UOCKWAY M. S. WASSERMAN Project 2031 AIR RESEARCH AND DEEOB C4lCAND,: U. S. AIR FORCE CONTRAC T AF 18(6O00)-175, E.O. NO. 355-10-2 July, 1953

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TABLE OF CONT1ENTS Page LIST OF FIGURES iii SUMMARY iv STATEMENT OF PROBLEM 1 EXPERIMENTAL'PROCEDURES 2 Electron Diffraction 2 Electron Microeopy 3 Preparation and Treatment of Specimens RESUITS 6 Chemically Precipitated Films 6 Condensed Films 8 DISCUSSION 15 BIBLIOGRAPEY 17 ii

LIST OF FIGURES Figure Page 1 Top View of Vacuum Evaporat ion Equipmesnnt. 2 Electron Micrographs (20Q,OOX) and Electron Diffraction Patterns of Chemically Prepared Photoconductive Lead Sulfide Films. 7 3 Electron Micrographs of Lead Sulfide Films Condensed in Vacuo on Glass at Various Temperatures (20,00OX) 10 4 Electron Diffraction Patterns of TIed Sulfide Films Condensed in Vacuo on Glass at Variams Tempeztues 11 5 Electron Microgrphs (20,0OOX) and Electron Diffraction Patterns of Lead Sulfide Films Condensed on Glass unlder Various Conditions 13 6 Electron Diffraction Patterns'of Iead Sulfide Filma Condensed on Mnorystallie Substrates 14 iii

SUMMARY The examination by electron diffraction and microscopy of chemically precipitated films of high and low photosensitivity has shown that both films have resolved lead sulfide crystallites of the order of Ool-micron dieameter along with some irregular agglorates of 2 to 3 microns. The agglomerates form a nearly continuous framework in the high-sensitivity films but appear in'a mach lower average concentration on the surface of -the low-sensitivity films. This is the first indication of a structural difference between fims of different.sensitivities. The characterization of the oxidized phase in these films requires the preparation of films especially mounted for transmission diffraction photographs. Condensed films of lead sulfide about 0.15 micron thick can now be controlled in particle size between 0O01 and 0.1 micron by altering the temperature of the substrate between 200* and 350~C, Preferred orientations in the lead sulfide occur to a small degree on glass under heat treatment, and are far more pronounced on selected single crystal substrates, No extra phase has been identified in films condensed and heated at 10'4 mm pressure, although extra diffraction rings pppear, under conditions not favoring the larger particle- sizes ~That they disappear on heating for three minutes at 350eC or longer at 300C -makes the formation of an oxy-sulfate on the surface of the lead sulfide an unlikely explanation of the extra rings.'Future work will include a more systematic.study of the structures of'chemical films, the oxidation of condensed films both on glass and single-crystal' substrates, the formation of agglomerates in partially oxidized' condensed films, and the testing of photoconductive sensitivity in condensed films with controlled particle size and orientation. iv

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN.EL10RON DIEFaAXCTION INVEISTIION OF'PHOTOCONDUCTIVE CRYSTALLEA LID.SULFIDE SURFACES The use of micrcrystalline led "sulfide prtin the preparation of sensitive photoconductive surfaces has been devreloped by empirical methods,.l,2 3 Various recipes have been devied for d epositing lead sulfide fims on noneon-| ducting substrates either by chemical precipitation or by- condensation from the vpr followed by partial oxidation. Variations in the preparative proeeires hbre a strong influence on the optical prpertie:s of the films * While most of the enpirieal infornation on film prertion h not been published, ~Soem rep_orts ar eavyalable4,5,6 on studies of photosensitivity, timer of respon8e and noise lerel in relation to the -iethod of preparing the film, the frequeney of the it-idenSt riaion, tfhe- te te f the' film, and the com — positiqn'and pressure of:theb s in contact.with.the film. -On the other band, kncmlJdgt of the achiyl ad physial Ipoxiti0n of thbe flms in relation t o -either th eth, d of trbpart-ion or the optitll properties is very meager, even though -an nw t ing-'of tthe natre of: pbotoeanuctivity certainly depenia on information (oaernng the coposition of the phases involved, the relative, distribution of the phaes,, the effeet of crystallite size and orienttion in veach pha,bj the possible- variationsa from one crystallographic snrfaie plan*e to anotherj, -and other sCh qestions* With'.uch inf.rtion it should. b-e possible to demelp.a nuem= satisfactoy control of the: proprties of lead sulfide undaeeB and-to udistan soethingof the fndmentwl nte of their:pottconductivi,ty. An investilgtion of sixrfces to detemine p.hase positions and distributions as ell s particle size and orientation requires electron diffraction an.micros.copy..'The low penetra ting poer.of electrons makes them well adapted for d-ecting "and i'eXntifying crystallne films. as thin as 3O lt when used by the reflecPition itechnique, so thate it is poasible to stuty oxidtion products on a surcfae in the very early stage of the raeation.'CollodisOn

ENGINEERING RESEARCH INSTITUTE *~ UNIVERSITY OF MICHIGAN replics of the surface exgmined in anelectron mi;eroseope contribute inrmation on particle size, s1zpe. and distribution. These two uses of electrons are an indispensable part of the attack- on the problem of photoconductive surfaces. Electron diffraction has been used previusly718 to identify one of the possible -oxidatian products of lead sulfide, namely axrkite (PbO PbSO.), but no sy3terjti0e study. haa been reparted of the oxidized phases in relation to variations in the method of preparation. The electron microscope examination8 of'repli of a PbS film shoied particles ranging in diameter from 0,1 to 1.0 microns. For a systematic application of electron diffraction and microscopy to the study of photo sitive fims it vai decided to examine such chomically pres'd fils "s efzsuld be readily ra-ilable and to oettrate at first on a inoe itens satudy of the f~etxrs affcting the struetue and composition of ev ted fil. The program for evpate films includes the use of vari-' r Zsubstrxtes (gl/s:d single Crystals of mica, siat quartz -and sodiaum chloride)and investigatiom of the effect of cbanging t he-temperature of the substrate during evapo-rtionof the lead sulfide in the range from 200~C to c000C, the effcrt of different rtes of condensation and different total film thickness, tb effeet-of faging the films at various tet*ratvres Subsequent to condenration, the eff-ect of Oxidation at viarious oxygen pressures and tenperatures, and filly the effect of exposure to norl air at atmspheric psBUe* Lm-ited tasts of Varition in photosensitivity with these factors ae also to be Mma* Ire e oplete optical examinmtions of Appropriate films will be -d* under the diretiOn of Proofessor r rian O'B'ien at the University of Electron Diffraction The electron diffEraction technique directts a well-defined monoenergetic bem of electns -.onto spcim' ad t Eri pttern of diffraeted electrons is e- or dd phbtog'P,t ifally. The pattern yields two kinds of informtion: (a) the identity of the crystallne pbases contributing to the pLttern "a reogniszed fo te Bragg d vAlue and. tbe r1ativ, intansities of the diffraction ma ina, amd (b) the e of nonreaxidn orientation in polycrystallin* specimens or reignition of the orientation of single crystals. L eadsulfide films on gl'as are mx.t..ly-d atsudied by the "reflection" technique' Of elet ron di.ffrmact'ion in which the- el.etron beam is directed,across the. film at an agl.of b *out i cr. 2., Beciase of the low penetrating

ENGINEERING RESEARCH INSTITUTE ~ UNIVERSITY OF MICHIGAN pover of eledtrons,m the resulting difs'ration patte"rn is characteristic of an outer layter of the sutrface only about 30 to 50 i thick the surface is, vysx~ th. On rougher surfces, ridges as thiek as about 1000 A my be penetrated by tht elctros Since the roughness of the surface is a principal tector in detirminng -whether all aas Ot f the surfae tcan be reached by the beam, the emiinatin of surfce replias by electron microsdfpy is atn iolr- tarrt aid, in pevaloting the ifraci ptterns btaid m y surface The teraamisaion tchnique can be ud an lead sulfide films which can be from the nsubstrate wian d supported on'A e t h Specimens imxunth e in this way aDrt turned noxLnl to the bzam soft thate electrons penchtrate the, film in areia with thickness iot -exceediz gtheir max range Whenevere possible, both tanrmission and reffleCtion techniques are used to detect variatios in co.u:sition between the surface nd the bulrk of the f lm. The eletmn hdiffraction -equipmint sthed in this study is a standard RC'A unit, Model Et.-e1 pure -rth0 i ndrestigation'The electron beam is accelatthed by 50 kiloftsea and as usedr t bas a dtiametr at the spcimen positions of. about Q*5 mm,. The electrom rtiaselpt e iprovide infortiot n on surface contours by recording an -enla.rs ged patterndi of the variations of electron transalmiaion over a specmcen cisting of a thin film. In certain pbases of this study it was not feasible to eE the a. lfide from the s"bstrate; theirefore surface replicjs were uSed., The general field. of electron mieroscpy, as well as the techniques of surfacr e replication, has been discussed extensively.9o10 lTe microscop studie wre nan.e with an RCA Mil tMf.2A, made'available throtgh the crrtesy dof Dr. T'Fracis and Miss Hilda Iwtz -of the Unliveresity of MichigaSn School -of Public Elealthiv Replica of the lead sulfide surfaes vere prepared as follows. A thin layer of a solutiOn of *llodion in ayl acetate (1:9 by volum) was flowed ovr the surface. eOn drying, the collion film is'approxiately 1000 X thick. The usual "dxy stripping" method of separkting the collodion with cellophane tape10 is not very uesfEi bese of the strong adhesion of the aollodion to the lead sulfide. Therefore a ivoet stripping" mthe using dilute nitri4 aeid to xloiosn the coldn frm the lead sulfide vas developed. One end of the glass substrate is dipped into, the acid in ueh a way that the a~id reachesb the edgeof th-e -loi led sulfide boundary while keeping the upper surface Of the collodion dry, As the lad sulfide dissolves, the coolloi~n graylly easla free andm on frther slow immersion of thle glass the| o11odion floats fiee on the surface of the acid.. It is trZaerred to t he surface of distilld water and allowed to soak foraew seod Several specimen imunts (l/8-inch-diameter wire gaus) are then laid on top of the

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN collodion replica, and it is lifted from the surfae of the water with a strip of wet paper (newsprint). After drying, the specimen supports and the adhering replicas are trimmed away from the remaining portirns of the collodion. The mouted replica has a smooth surface next to the wire su-pport and the detail-of the lead Sulfide sur-face is reproduced in negative on the sidee of the collodion avy from the support Two. fUrther steps are used in preparing the specimens, An internal scale. fro c:alibration it provided by spherical polystyrene latex particlesllA42 of 2580 + 30 dia; these are deposited on the specimen from a drop of a very dilute atemr suspensicn,Just prior to shladwing., The replicas are shadowcast with palladium to inerease the contrat in electron transmission between -different areas and to show the relief on the surface; the paladium is evapora.td onto' the colldion'replica at an angle of 30 3and the shadows of the latex particles providE a s of dist es norl to the siMaae and ad in distingitShin g between depressions and projections on the original lead sulfide: ir'ace.,hq and t, _of_ Spec~ni The present report describes the emination'of'certain chemically prepared specir ns supplied by the atstman Kodak Company and of condensed lead sulfide films pFepared in this lboratory., The latter have been evaporated in vacuum onto me.oth glass'under various condition of'evaporation'rate, glass temperature., -and subsequent heat tretn, Thisa provided a istudy of the means of cEntrolling the rystallite size and:orientation of the lead sulfide before: Oxidation. The glass ubstrates were 1-inch by 1/2-inch pieces cut from standarl micros/eOpe slt4es. They were cleaned by rimmJ sing fr fiftn minutes in cncentratd chromie-sulfrie cid, rinsing in tap water an in hot distilled ater.directly fran the still. ad -drying at 110C int a.greae-free oven, The glass pieces were d only with lean etal tvwezers. The -lead sulfide used was the B and A reaglt grade The v poratiion unit pwas pi L haaed fromOptical Film Engineering Co,, and.modified by the insertion of a liquid nitrogen trap between the vacuum chamber and the high-vacuw.um valve. The set-up for Pevaporation is shown in Figare 1,'The specimen heater is a strip of Q005-inch by 3/4-inch Nichrome Ehet bent as shown; it has the advantage of low het capacity for rapid cooling "after the specimen has been condensed. The glass plate. iS held.nto the heater by two. smll screw cla ms, one of which alsa holds a themceouple junction in position on the fce of the glass, The iron-xeonstantan junction is brazed to a thin stainless steel plat, 3/16 inch by 7/16 inch, which is caught under the clanqp to ins-ure good the ml contact between the junction and the glass,

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN Thermocouple Substrate on tHeater Platinum Evaporating Cup in Tungsten Coil Metal Shields Wall of Vacuum Chamber Figure,1 Tp VieWt.of Vaeuwm EYau' ti.en Equiipment TbE uoS'1Vl etP1y in g t:aldia4 fuiA film on gltss is the follcrwing. Th pl*tin c:ip is eh'with a mt' su0nt of lead Msulfide pindr.and mte im the. tngen fi..;nt (t-e 1), sd a glass ali.d is. m:onted On the heater.* iThe et is eaete to a pressuree G 1Q-of i I mmg an tbe of the g m t.a prieted| alue. The tungsten fil't 1a t is then b;hel f:r a.t 4.finite tim eat'a definite:euaent setting to e the 1a sulfe at a reibe xt. A pressure rise for ~a f* BzaeOOn is ntd -at t begimg of thb ertrpotion, aprently du to diEaIng of the- fianet. A ubi.z:talle. ing film of ted to tthe ch amer tt the speerm* ia.tr*an:: iekly to the:eleetrn dirtin unit, Th fkr ma-si t-i e eip)4tzp to br the vaewMan is $eA toed v.ir the apein in the difF t unit as'eJry litt!l oxidlzing eff~ct cn the specimen atr.. r t 1e t.; e.n.tbhelesa the patterns -are alvays':exmin feor ev.f -aeih ati4tt An effect of this air:expoure an the particle size::-or ~mriesttie nt:, //ec In the dif.cx!ti4on oamera, the -:n- ntig *bttrxateof the led Sufide film al:Iw same ae...t lti.on of ~ re f' the'e1et'n bea5wm whieh| 5...

ENGINEERING RESEARCH INSTITUTE ~ UNIVERSITY OF MICHIGAN.ast be n alir& l by an-d'el.etrn spray fran an at*iary gmi. B.eane of ppsib-le the l ffects:on tht surfa'e from the, axdlary- spra thb dif-'fraction -exposes are n: in as sihsrt a time as posib2*. The, ccllcin ztpli are lly prqanted fwom th speeicnn upon re Wi from the dif frat tion -mit. llwss 1Tr series fe n hmtaly p cipitatd lea slfid. flms of knwn i phtenc sesitivity wtrZW prf/pafl&.. supplie by t.matr at tran.Keak G. R s, New Yrk, pi. ly 1/& ineh by 3/4 inch with a 1/8-inch-wi-d* b. f g1. - ted on aeh eind'r nkLng elec.itricfl -contAct wit.the ~.Iite theth metphd'-rt n-: t ny subsequtnt treta,nts zt the z.srfeu wse specified. Seriet Acnd six pisece showing gMain in photssitivity; Strits B'ctindt. th-.t pi.eces, all of yry. high sensitivity; Seri es C c.ntai s' ix piecs:o lw nn..sitirity. El.ctron dWiffata patt:Erswere obtained by eti f ro all of th pices; then't'tt. with onl mn v.'artis was o,tainede frmm "erystait*# -- spts.showing in Figure 2ea br ight spe be the. psiti ns qf the ledt saulf i d l'dfr ton rings-s'Tte /.specks'a f& r few in umxber t.all an idntifictaio. lhil. th psenc of an oxidid p is prnsmtd to b> squired. frq itvty the filmase awple 4* no ~rt shpvtfii a ble A oaftSof a eend b ying 4n thbe srf in ps n tht can b rhx d by t he.tle. in the fe:ltt* tebiq. An attcpttt t r p th film frem the glas f Oar -a tfilr ai s nat s esstl.'. "e.:~ —:. O,m.": _a-:,rpU~.- da sahw diff-reces e..n aing vith p ititity.:iT'The d ry-s.triping teehniqfl was not CCg4Cl.et*el -afalt_.: tn thearplets. ceuld be obtained m fgr ly tO spe Cies in filf sere. A typ iel f.rom Series B is sh0m in Figure. t2' and froa Series c in Figr 2:b.'e. white. cir tele in ~.eatof thes figu. re is a later. baL-.' Q58-ai ti,nr eyr. Both figres shotw nl crystalgized p.ctert'raging f'orm abt.i Q_Q5 to n S miz, In salid. i V "Teg.,9 k a-f. -OX t"Al6

a. High Sensitivity b. Low Sersitivity c. Typical Electron Diffraction Pattern d. Special Film of High Sensitivity Figure 2. Electron Micrographs (20,000 X) and Electron Diffraction Patterns of Chemically Prepared Photoconductive Lead Sulfide Films.

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN the high-sensitivity films large, well compacted agglomerates appear, reaching 2.0 or 3.0 microns in size (as shown in Figure 2a to the upper right of the center). From the study of the available micrographs it appears that these agglomerates are distributed over the surface forming an almost continuous framework. The low-sensitivity films in Series C show a much lower average concentration of these agglomerates and have a much more nearly uniform distribution of the small crystallites. Both series show that the surfaces are rough, with high and low spots differing by several tenths of a micron. If any second phase occurring in the high spots has been removed by accidental rubbing of the surface, the failure of the diffraction pattern by the reflection technique to show the second phase is understandable. Figure 2d is included to emphasize the fact that variations occur among sensitive films. This specimen was prepared by chemical precipitation at a different time from the three series mentioned above and while it was reported to have a good photosensitivity, it differs in three respects from the Series B: (a) the individual crystallites are larger (averaging nearly 0.5 micron), (b) the crystallites appear to be separated by a different phase showing nearly white in the figure, and (c) a high degree of preferred orientation of the crystallites is indicated by the diffraction patterns. Condensed Films The work to date includes the exanLination of lead sulfide films condensed on glass at temperatures of 250C and 2000 to 3500C followed in some cases by subsequent heating at 300~ to 3500C, as well as preliminary experiments using as substrates single crystals of mica, diamond, magnesia and spinel. The films on glass were prepared by subliming 4.5 mg of reagentgrade lead sulfide powder during a three-minute period. The average film thickness is estimated at 0.15 micron from direct weighing on a microbalance. The films required twenty to thirty minutes to cool in the vacuum of the evaporation chamber. The results are summarized in Table I, and Figure 3 and 4 show micrographs and diffraction patterns. The principal observation from the micrographs in Figure 3 is that the film shows a graininess of dimensions of about 0.01 micron but no distinct crystal faces at 2000C, with a gradual change to clearly defined crystals of about 0.1 micron size at 350~C. Higher temperatures might lead to still larger crystal sizes, but the present experiments in vacuum (i.e., residual pressure of 10-4 mm) lead to re-evaporation of the film at higher temperatures.

TABLE I CHARACTERISTICS OF LEAD SULFIDE FIMS EVAPORATED ON GLASS SUBSTRATES Substrate Nature of Natturre of Temslpet ures ~,,Dif tion Pttez~n EleCtron.Microgpw iaes 200~C Somewhat blurred. Uniform, grainy* 5a, 4a Fairly intense extra No definite crystal rings, faces at 20,DtOOX 225e Little chanage Little change 250~ Sharper PbS rings, Coarser. Still no 3b Sam extra rings crystal faces. 275* Similar to 250~ Still. aarser 3e, 4b Extra rings slightly than 250~. fainter. 300~ Relative intensities of Crystal faces 3d rings. different. Pre- begin to appear, ferred orientation of PbS. Extra rings rmch fainter. 325~ e. i-*ntati. Crystals about 4 Extr.rings matth 0,05 micron. Pfainter, 350: bMore orientation, Crystals up 3c, 4d'Practically nO extra to: 0.*1 mieron. rings., 1_ —'.... ".,'.~''.''...........9.

o 0 0 0 Cu o Co ) CO 0 Pd CC;.. 10 rd aS nH r..) b0 0I rH 1(~~ijrr0 I~~C O ~~D 31 Ln U~~~~~~~~~~~~r-. ~~L' v -o

a. 2000C b. 275~C c. 3250C d. 3500C Figure 4. Electron Diffraction Patterns of Lead Sulfide Films Condensed in Vacuo on Glass at Various Temperatures. 11

ENGINEERING RESEARCH INSTITUTE ~ UNIVERSITY OF MICHIGAN The electron diffraction patterns also show the effect'of increasing crystallite size, as indicated by increased sha ss of ea. sulfide diffraction lines, as well as the tendency with increasing temperature toward-orien-'tation of same crystallites relative to the glass surfree. The diffraction patterns show rings in addition to those of lead'ulfide at the lower temperatures but not at'temperattres near 350eC, Although four or five of these "extra" rings could be meued in any of the pattierns, it has not yet been possible to mateh then -satisfactorily with any compound which might conceiv- ably be fc aed and for which cmparison patter, ns are available Additicsl experients were.carried out on the effect of heating subsequent to condensation'and of a more rapid condesation rate..Figure 5a shows the micrograph and diffraction pattern for a film eondensed at 3000C and heated for two hours at the same temperature, - The faint extra rings apparent, at the end of the condensation (Figures lib and 4ec) have almost entirely disapeared here, while the heating has increased the particle size and has producd. sou* preferred orientation of the lead sulfide with the (111) plane parallel to the glass, Another film heated'to 350C for'one hour shows simniatr effects (Figure 5e) with partiele sizes'now in excess of 0il, micron and ssome.-orientation ~of'the (100) plane parallel to the glass, The diffraction pattern in Figur 5b shows the effect of ondention ofhe at|ut of lead sulfide in one- mimte instead of the usual three minutes, followed by immediat-e coling. In copison with Figure 1d it is.observed that the oneminute -condensation leas intensae extra rings sim lar to those produced at lower temperatures, whereas none appear in the three-miute condensation, Preli.minary experiments have been started to study the effect of a more proncameed preferred Orientation -of the lead sulfide,'This is'achieved by. using:sinle-rystal substrattes- in place of glas.* Different substrates should lead to different cIrysallographic planes app ing on the surface of the lead sulfide film with-possIble variations in pbotosensitivity following Eprtial. oxidation, The crystal faees used. as substrates include: the cleavage face of mica, the (111) and (l10) faces of diamond, the eleaTage (100) face of -iagai.a and the (100) face of spirnl (magnesium "almnate). The -dif.fraction patterns from lead sulfide condened on three — f these are'illustrated in Figure 6 A strong prefernee for the (10') plane of lead sulfide parallel to the surfaee is shown in Ficgure 6b tid fort (ll) in Figure 6co In each ease there is random orientation around the nsrmal to the plane. Consideration of -the Strong orienting effect of these'substrates suggests the use of sodium chloride-, wbhose stractue is the same as that of lead sulfide and whose interionic spaeings are nearly the s In eneral it is found that the greater the sim'larity between substrate and overgrowth, the stronger the tendency for preferred orientation.

a. 300~C - Heated 2 hr in Vacuo b. 3500C - Higher Evaporation Rate c. 3500C - Heated 1 hr in Vacuo Figure 5. Electron Micrographs (20,000X) and Electron Diffraction Patterns 13

a. On Cleaved Mica - 3000C b. On Cleaved MgO - 2000C c. On (111) Diamond Face - 200~C Figure 6. Electron Diffraction Patterns of Lead Sulfide Films Condensed on Monocrystalline Substrates. 14

ENGINEERING RESEARCH INSTITUTE ~ UNIVERSITY OF MICHIGAN DISCUSSION The chemically prepared films in Series B and C have. led to the first indication of any ensistent difference in physical stri*cture between leads sulfide films of high and low photosensitivity. A systematic study of chemical fil ms wuld show whether the particlar.structure observed' in Series B films is essentialto very high.sensitivity in chemial films, The question f the nature and istribution of the oxidized phase can be follawed by three different experixrerts: (a) the- e'minir tion of clemicasl films carefully protected to ayoid the Sec-idental removal of any naterial from the surface of the film, (b) tshe irunting of cemial films on a thin ctiuus support such -as aollodion to allow trnsission difractin patterns for the detection of second phasets in the body of the lead zlfide film, and (e) the exmination by diffraction of very thin deposits of lead sulfide from qleous solution ontQ a wire gauze in an -attetpt to Atudy se partieles of the chemni l precipitate before aggl!erati.on A thorvgh study ofs chemical films will be undertabkn, as additional p lersoel becones available because of the interesting contrasts betwen the Lchemially precipitated and the,evaported films, One exam~ple of this dif-erenee is the great decrease in photosensitivity on exposure to air, shown by tevportd films but not by those precipitated from solution. The xperlments onevaported films he prescrid the conditions for controlling thei partile size of the lead sulfide crystals between about Q*01 an0d Ol micron. T'reAiults. on rthe Series B chemical films suggest an attempt to produce.agg,lorates of. the 0*1 micron condened particles either during or following a parti~al 4oidation, This aggl meration might be accomplished by hiating to temperatures of 500C, in which ease an appreciable ptesture -of an inert rt will pr ly have to be present in the -evapoation ehamber to suppress the o.ertiom of the l. n./slfide, Extended heating of -the l.'d films without oxidation apparently lads only to an increase in the size of the individual erystals; an increae beyond the'Ol-micron size without providingTa eoattin of'an oxidized phase is probably' nt, desirable. This:suggests that the extended heating shld be d itone with sme.residual pre'ssure of oxygen, The o curren:e of extra rings in the diffraction patterns from speeinens condensed at l1 am preasure raises the question of oxidation:een at this pressure (although the extra-ring pattern has not yet been identified), Another possibility is a def"iciency structure accompanying a deviation from the stoichicmetri, ratio.betwen lead and.sulf a e the- result of the process of'evapoation and ~cn&ensation. In either case, the interesting dependenee of -theSe rings on the t ietwre and -rte of cndensation and subsequent heating,ast be explained. It mry be, for e.:eampte that the higher temperatue for'a longer tfime prelferfentially favors the migration and crystal graowth 15

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN of the lead sulfide with'a.covering up of the.aeon1 p"base.'The answer to this suggestion depends on obtaining tranmi ssion difftaction patterns through the heat-treated films. In addition to a better bcharacterization of the lead sulfide film empoarted onto glass, further work will inclu1 sAtudies of films on sodium -chloride and of the oxidation of films, bothn glass and soitum chloride, to produce a detectable photosensitivity. It is noted that the photosensitivity will have to be tested on films while mounted in the evaporation chamber to arvoid the loss associated with -exposure to the atmosphere The chemical effeet of the e'xpe e to air is a subject for later study.

BIBLIOGRAPE ftsshmsnt R,J J w S= *. ta* &; 356A (1946) 2. nSoanawki, L,* Starkiieitt, Ji, -and Sip0son, 0., st, 15, 818 (1947). 3. Kieinaki, F., CFheix* C ad mda n54-7 (1948). 4. Elliot, A., Chapter in Electroncs, Pilot Press, Iondan, 1947. 5. Sutherland, G.B.B.M,, an d ee, E Rept*.,, 1, 144 (1947). 6* S4ithy R1 A., Semi-Co duectigg I,t BLxtterworths lond, 1951, p. 198. 7. Wilnm,, Proe. _oy. S_60. (Lodon), 60, 177 (1948). 8. Doughty, J,,Iark-k —rovitz, Ki., Roth, L. M,, and Shapiro B., Ps, R., 79, 203 (1950'). 9. Zorykin, V+, Hillier, J, Electron -ptics and t lctron Eicroscape, Wiley, New Yrk, 1945.. 10. Wyckoff, R, WF- G Miccopy, Interseiene Publications Inc., New York, 1949. 11. Willians R. C., }ckus, R, C., J plied Eta.,,0 224 (1949). 12;. tGerould, C. H., J. Appled adF * 21; 183 (1950). o~ ~ ~~~~~~~1

I1 | | fl Of MICHIGAN 3 9015 02526 1408