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Epitaxial Electrodeposition of Copper(I)Oxide on Single-Crystal Gold(100)

Eric W. Bo h a n n a n , Ma rk G . Sh u msky, a n dJ ay A. S wit zer*

U n i ver si ty o f M i ssou r i sRolla, Department of Ch emi str y a n d G r a d u a te Cen ter for M a ter i a l s

Research, Rolla, M issour i 65409-1170

Receiv ed M ay 18, 1999Revised M anu scri pt Received J ul y 14, 1999

Introduction

Single-crysta l films a re importa nt for device applica -t i ons , as t he i nt r i nsi c propert i es of t he m at eri a l o finterest can be exploi ted rather than i ts grain bound-aries. The ability to simply and inexpensively prepareepitaxial f i lms with a desired orientat ion al lows theselection of the film orienta tion tha t most enha nces theproperty of interest . For example, it would be desirableto select t he orientat ion of a mat erial tha t exhibits t hehighest ionic mobili ty 1 or t o s e le ct t h e p la n e of am at eri a l t hat show s t he hi ghest degree of c at a l y t i cactivity.2 Recently, w e reported t ha t epitaxia l thin filmsof -Bi2O3 can be electrodeposited from an alkal ineta rtra te solution onto single-crystal Au at a tempera tureof 65 C .3 This cubic polymorph of B i2O3has been shownto have the highest oxide ion mobil i ty of any knownmaterial . 4 In the present work w e use electrodepositionto prepa re epita xial films of copper(I) oxide (Cu 2O) ona single-crysta l Au(100) substr a te. C u 2O i s a p- t ypesemiconductor tha t ha s been electr odeposited previouslyon a va riety of substrat es.5-8 We ha ve previously sh ownthat the orientation of electrodeposited Cu2O films is afunction of th e pH of the a lkal ine lacta te solution andthat the deposition current is limited by a Schottky-likeb arri er t hat form s b et w een C u 2O and the deposit ionsolution.9 We h a v e a l so s h ow n t h a t , u n de r ce rt a i nga lvan ostat ic conditions, the w orking electrode potent ialspontaneously oscillates,10,11 formin g a Cu /Cu 2O layerednan ostructure tha t shows a negative di fferential resis-tance (NDR) feature during perpendicular transport

measurements.12 The N D R feat ure shif t s w i t h a 1/L 2

dependence on th e Cu 2O layer thickness, suggestive ofquant um conf inem ent of carr i ers i n t he nanoscal eC u 2O. 13 These layered nanostructures were depositedon polycrysta l line substrat es. However, i t w ould bedesirable for device applica tions t o deposit t hese str uc-tures as epitaxial films on single-crystal substrates. Inthe present work, we explore the epitaxial growth of

pure Cu 2O onto single-crystal gold. In subsequent work,we will st udy the deposition of Cu/Cu 2O layered nan o-structures on single-crystal substrates.

The un derpotential deposition of monolayers of meta lson s i ngl e-cryst a l subst rat es has b een s t udied qui t eextensively. 14 Litt le work, however, has been done onthe electrodeposition of epitaxial semiconductor filmswith thicknesses in t he micrometer ra nge. Electrochemi-cal at omic layer epita xy ha s been used for t he formationof CdS e monolay ers on Au(111).15 Epita xial Cd Te filmscan be electrodeposited from aqueous solution onto InP-(111) single crystals.16 Large area epitaxial thin fi lmsof C dSe17 and quantum dots of Cd(Se,Te) have beenprepared on evaporated Au(111) films.18 Epitaxial CdS

nanocryst a l s hav e b een prepared on graphit e b y acombined electr ochemica l/chemical met hod.19,20

We show in t he present w ork tha t electrodepositedfilms of Cu2O gr ow epita xially on Au(100) single crys-tals. The crystal structures of Au and Cu 2O are shownin Figure 1. Cuprous oxide is primitive cubic, spacegroup Pn3m, with a lattice parameter of 0.427 nm. Auis fcc, space group F m3m, w i t h a l a t t i ce param et er of0.4079 nm. The lat t ice mismatch ((afilm - as ub s t r at e)/as ub s t r at e) betw een th e t wo m a teria ls is 4.7%. The epi-taxial Cu2O film grown on Au(100) show s a high d egreeof structural perfection, with no rotat ion of the Cu 2Owith respect to Au.

Experimental Section

The deposition solution was prepared by adding 450 mL of5 M NaOH t o 60 g of Cu(II) sulfate penta hydra te an d 150 mLof 85+%lactic acid, resulting in a dar k blue solution. The finalp H of t he solut ion w as ad ju st ed t o p H 9.0 wit h 5 M N a O H .H P L C -g r ad e wat e r (Ald r ic h) was u se d t o p r e p ar e t he 5 MN aO H . All ot he r c hemicals we r e r e ag e nt g r ad e , p u r chase dfrom Aldrich. The working electrode consisted of an electropol-ished Au(100) single-crysta l purcha sed from Monocrysta ls Co.The Au crystal h ad a diameter of 10 mm a nd a thickness of 1mm. A g old wir e w as f i t t ed a r ound t he e d ge of t he c ry st a l t o

* To w h om cor r es pon d en ce s h ou ld b e a d d r es s ed . E -m a i l:jswitzer@umr.edu.

(1) G arcia, M. E. ; Ga rofal ini , S . H. J . El ectrochem. Soc.1999, 1 46,840.

(2) J ones, P . M.; May , J . A.; Reitz, B .; Solomon, E. I . J . Am . C h em .Soc. 1998, 12 0, 1506.

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284, 293.(4) Shu k, P .; Wiemhofer, H.-D.; G oth, U .; G opel, W.; Gr eenblatt ,M. Solid State Ionics1996, 89, 179.

(5) Mukhopadhyay, A. K. ; Chakraborty, A. K. ; Chat terjee, A. P. ;Lahiri , S . K. T h i n S o l i d F i l m s 1992, 20 9, 92.

(6) Chat terjee, A. P. ; Mukhopadhyay, A. K. ; Chakraborty, A. K. ;Sasmal , R. N.; Lahiri , S . K. Mater. Lett. 1991, 11, 358.

(7) Ra khsha ni, A. E.; Varghese, J . T h i n S ol i d F i l m s 1988, 1 57, 87.(8) Tench, D.; Warren, L. F. J. Electrochem. Soc. 1983, 130, 869.(9) G olden, T. D. ; Shumsky, M. G. ; Zhou, Y. ; Vanderwerf , R. A.;

Van Leeuwen, R. A.; Switzer, J . A. Chem. Mater. 1996, 8, 2499.(10) Boha nna n, E. W.; Hua ng, L.-Y.; Miller, F. S.; Shum sky, M. G.;

Sw itzer, J . A.L a n g m u i r 1999, 15, 813.(11) Sw itzer, J . A.; Hun g, C.-J .; Hua ng, L.-Y.; Miller, F. S .; Zhou,

Y.; Raub, E. R.; Shumsky, M. G.; Bohannan, E. W. J . M ater. Res. 1998,13, 909.

(12) Sw itzer, J . A. ; Hung, C.-J . ; Huan g, L .-Y.; S wi tzer, E. R. ;Kammler, D. R. ; Golden, T. D. ; B ohannan , E. W. J. Am. Chem. Soc.1998, 120, 3530.

(13) Switzer, J . A. ; Maune, B . M.; Raub, E. R. ; Bohanna n, E. W. J .Phys. Chem. B 1999, 10 3, 395.(14) Bu devski, E.; Sta ikov, G.; Lorenz, W. J . Electrochemical Phase

Formati on and Growth: An I ntr oduction to the In it i al Stages of M etal

Deposition; Wiley-VCH: Weinheim, Germ an y, 1996.(15) Lister, T. E.; Stickney, J . L. Ap p l . Su r f . Sc i . 1996, 107, 153.(16) Lincot , D. ; Kampma nn, B. ; Mokili , B . ; Vedel , J . ; Cortes, R. ;

Froment , M. Appl. Phys. Lett. 1995, 67, 2355.(17) Gola n, Y.; Alperson, B.; Hut chison, J . L.; Hodes, G.; Rubinst ein,

I . Ad v . M a t er . 1997, 9, 236.(18) G olan, Y.; Huchison, L.; Rubinstein, I.; Hodes, G. Ad v . M a t er .

1996, 8, 631.(19) Gorer, S . ; G anske, J . A. ; Hemminger, J . C. ; Penner, R. M. J .

Am. Chem. Soc. 1998, 120, 9584.(20) Hsiao, G. S . ; Anderson, M. G. ; G orer, S . ; H arris, D. ; Penner,

R. M. J. Am. Chem. Soc. 1997, 119, 1439.

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serve as the electrical contact during deposition. The counterelectrode was a large area copper wire. A constant cathodiccurrent densit y of 0.01 mA/cm2 was ap p lie d t o t he wor kingelectrode with an EG&G Princeton Applied Research model273A potent iosta t/ga lva nosta t for a period of 40 000 s, givinga nominal Cu 2O film t hickness of 0.5m. This current densitycorresponds to a deposition rate for Cu 2O of approximately 0.12/s, a ssum ing a 100% current efficiency for deposition. Ap-plicat ion of current densit ies higher tha n about 0.1 mA/cm2

in t his solu t ion g ive s r ise t o oscil la t ions in t he p ot e nt ia lcoincident wit h the deposit ion of a Cu/Cu 2O lay e r e d nano-st r u ct u r e on t he wor king e le ct r od e.10-12 To deposit purecuprous oxide, it is necessary to a pply current dens ities belowt his t hr e shold valu e . The s t ir r ed d e posit ion solu t ion wa smaint a ined a t a c onst ant t e mp er at u r e of 30 C w it h a F ishermodel 9100 circulator. X-ra y diffraction (XRD) experimentswere performed w ith a Scinta g 2000 diffractometer using C uKRr ad iat ion. Azimu t hal a nd t i l t scans we r e obt aine d b y t h euse of a texture goniometer accessory that had been fashionedin-house for the Scintag 2000.

Results and Discussion

Figure 2 shows t he 2X-ra y pa tt ern for th e electrode-posited Cu 2O film on single-crystal Au(100). Only the(200) an d (400) peaks for t he film and the subst ra te a reobserved. The (200) C u 2O pea k is g rea t er t h a n at h o u s a n d t i m e s m o r e i n t e n s e t h a n a n y o t h e r C u 2Oreflection, indicative of a strong out-of-plane texture.Rocking curves performed on the (200) reflections ofb ot h A u and C u 2O gave ful l-width at hal f-maximum(fwhm) values of 0.6 and 0.7, respectively, indicatingtha t t he film had only a sl ightly greater mosaic spreadtha n t he substrat e. An epita xial relat ionship might beinferred from Figure 2, as t he Cu 2O appears to followthe orienta t ion of t he Au substra te. However, there isa complicating fa ctor. U nder t he presently used deposi-

tion conditions t he kinetically preferred gr owth directionfor Cu2O is [100]. Hence, even a Cu 2O film grown on apolycrystalline substrate will show a strong (100) tex-t u r e. To s h ow t h a t t h e C u 2O f i l m t r u l y h a s g r o w n

epitaxially on the Au substrate, it is necessary to showboth in-plane as well as out-of-plane orientation.To det erm i ne t he i n-plane epi t axial rela t i onship

between the film and substrate, it is necessary to bringother reflections, besides the (100), into t he B ra ggcondition. This is accomplished by tilting the sample,through the use of a goniometer, to the t i l t angle ()b et w een t he planes of i nt erest . G i ven t ha t t he a nglebetween t he (100) and (110) plane is 45 a nd the (100)plane is oriented parallel to the surface in our sample, must be 45 in order to bring the (110) reflection intothe Bragg condit ion. Sett ing 2 equal to the reflectionof interest, in this case the (220) reflections of Au andC u 2O, and rotat ing the sample provides an azimuthal

() scan. Azimuthal scans run at a succession of differ-en t t il t a n g le s c a n b e u s ed t o g e n er a t e a t h r ee-dimensional pole figure for th e cryst a l of interest. Figur e3 shows the (220) pole figures for both Au and Cu 2O.The pole figures were obtained by setting 2 equal t ot h e a n g le of m a x im u m d if fr a c t ed i nt e ns it y f or t h ematerial of interest (2 )64.58 and 61.34 for Au a ndC u 2O, respectively) and obtaining azimuthal scans attilt a ngles from 0 to 60 . Four-fold sym metry is observedi n b ot h c ases , w i t h a m axi m um i nt ensi t y a t ) 45,and w i t h no rot at i on of t he C u 2O with respect to Au.The pole figure shows t ha t t he film has a str ong in-planeorientation in addition to the out-of-plane orientationt h a t w a s m ea s u r ed i n t h e 2 sc an. I f t he f i l m had a

random in-plane orientation but a strong out-of-planeorientation (i.e. , a fiber texture), the 4-fold symmetryw ou ld n ot b e s e en . I n s t ea d , a r in g w i t h m a x im u mintensity a t a tilt a ngle of 45 would be observed in thepole figure.

Figure 4 shows a plot of the logarith m of intensit y vsaz i m ut hal angl e, , for the (220) reflections of Au andC u 2O both obtained at a t i lt a ngle, , of 45 . Consistentw i t h t he rocking curv e m easurement s , t he av eragefwhm va lues for the azimutha l sca ns were 1.1 a nd 1.4for Au and Cu 2O, respectively. The ba seline of the Cu 2Oazimut ha l sca n is essentia lly identical to tha t of the Au,indicating a minimal presence of polycrystalline Cu 2Oin the film.

Figure1. C rysta l structures of Au an d Cu 2O. Au is fcc, spacegroup F m3m, with a lat t ice parameter of 0.4079 nm. Cu 2O isprimitive cubic, space group Pn3m, wit h a la t t ic e p ar ame t e rof 0. 427 nm. I n t he C u 2O st r u c t u r e , t he lar g e sp he r e s ar ecopper a nd th e sma ll spheres are oxygen. The lat tice mismat chbetween th e tw o ma terials is 4.7%.

Figure 2. 2 X-r ay p at t e r n for a 0 . 5 m t h i c k C u 2O filmelectrodeposited on single-crystal Au (100). Only the (200) and(400) reflections for the film and substrate are observed. Theminor p e ak mar ke d wit h an ast e r isk is f r om t he (200) Aureflection and is due to Cu K r ad iat ion.

2290 Ch em . M at er ., V ol . 11, N o. 9, 1999 Com m u n i cat i on s

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Another measure of the st ructural perfection of theepitaxial C u 2O film is a tilt scan. These were obtainedby sett ing the azimutha l angle,, equal to the a ngle ofthe first of the four maxima observed for Cu 2O an d Auin Figures 3 and 4. Leaving fixed, wa s varied from0 to 70 and the di ffracted intensi ty was observed forthe (220) reflections of Au a nd C u2O, independently. Thefwhm va lues for th e tilt sca ns were identical for Au andC u 2O, with a value of 1.4 , indicat ing that , within theresolution of our instrument, the Cu 2O f i l m and A usubstrate are of comparable quality, despite the lack ofstrict commensurism between fi lm a nd substrat e.

This study h as shown tha t epitaxial films of Cu 2O canbe electrodeposited on s ingle-cryst a l Au(100). The epi-ta xial f ilms are of high qual i ty and a re not rotat ed withrespect to the Au substrate. A question that remains tobe answ ered is how th e significant lat t ice mismatch of4.7% is accommodated in this epitaxial system. Theexperi m ent al l y det erm i ned l a t t i ce param et er of t heC u 2O film (0.426 nm) is within experimental error ofthe literature value for Cu 2O (0.427 nm),9 showing thatthe bulk of the film ha s not been str ained int o commen-surism wit h th e substra te. These results do not ru le outthe possibility of a strained pseudomorphic Cu 2O filmgrowing to a very thin critical th ickness before relaxing

t o t he b ul k s t ruc t ure. Moni t ori ng t he grow t h of t heepitaxial f i lm in real t ime with a scanning tunnelingmicroscope and using cross-sectional transmission elec-tron microscopy should provide important details aboutthe interface between the two materials.

P reliminary results for th e electrodeposition of Cu 2Oon the other low-index faces of Au reveal that ini t ialgrowth of the film is epitaxial with no rotation relativeto the substr a te. However, a s the film becomes thicker,

the kinetical ly preferred [100] growth direction pre-dominat es. This should provide a convenient mea ns t ost udy t he i nt erpl ay b et w een t he t herm odynam i c s ofepitaxial growth and the kinetical ly preferred growthdirection of Cu 2O. Epitaxial electrodeposition of otheroxides, such as ZnO,22 Tl2O3,23,24 P bO 2,25 and A gO26

should also be possible. Electrodeposition provides asimple and inexpensive means for the d eposition of high-quali ty epitaxial f i lms that could be used for deviceapplicat ions, with our specific interest being the Cu/C u 2O layered na nostructure system.13

Acknowledgment. Thi s w ork w as support ed b yOffice of Nava l Research G ra nt N00014-96-1-0984,

Nat iona l Science Founda tion Gr a nts CHE -9816484 andDMR -9704288, a nd t he U niversity of Missouri Resea rchBoard.

CM990304O

(21) Dakkouri, A. S. Solid State Ionics1997, 94, 99.(22) Peulon, S.; Lincot, D. Ad v . M a t er . 1996, 8, 166.(23) Ph illips, R. J . ; Sha ne, M. J . ; Swit zer, J . A. J. M ater. Res.1989,

4, 923.(24) S witzer, J . A.; Hung, C.-J .; Br eyfogle, B. E .; Shum sky, M. G .;

Van Leeuwen, R.; Golden, T. D. Science1994, 264, 1573.(25) Mindt, W. J. Electrochem. Soc. 1969, 11 6, 1076.(26) Breyfogle, B. E.; Hung, C.-J .; Shumsky, M. G.; Switzer, J . A.

J. Electrochem. Soc. 1996, 143, 2741.

Figure 3. (220) pole figures for (A) Au (2 ) 64.58) a nd (B )C u 2O (2 ) 61.34 ). No other ma xima are observed in eitherpole figure except for those occurrin g a t ) 45. The pole figure

extends to )

55 .

Figure 4. Azimuthal scans for the (220) reflection of (A) Au(2 ) 64.58) an d (B) Cu 2O (2 ) 61.34) both obtained at )

45 . The e xp ec t ed 4-fold sy mme t r y is obse r ve d , w it h nor ot at ion of t he C u 2O film with respect to the substrate. Theaverage fwhm for the Au peaks is 1.1 , while the average fort he C u 2O peaks is 1.4 .

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