(e)!.31

I

t .

2.

3 .

4.

i--

Chapter 52

Self-Assembled Monolayers: Models for OrganicSurface Chemistry

George M. lt'/hitesides and Christopher B. Gorman

PREPARATIONS AND TYPES OF SAMS

STRUCTU RAL CHARACTER IZATION ... . . . . .

MATEFIIALS PROPERTIES . . . . . . . . .

PATTERNING OF Snns . . . . . . . . . . . . . . . . .( I f n n r a n t H nJ . l \ - U l l L r l L L r r r u L L I l $

< ? \ , ( i ^ - ^ . . , - i r i ^ .J . - l Y u v l v - r r r r u {

< 2 r r ; ^ - ^ - ^ ^ t ; .J .J rv r IL r ULL ldLUUr lng . . . - . . .

5.4 Photol i thogr.phy . . . . . . .5.5 Electron-Beam Wrir inq . . . . . . . . . . . . .

6 . APPLICATIONS6.1 Wet t ing6.? Subsrates for Surface Soeccroscopy .. . . . . . . . . . .A ' \ E l c n r . n n h o - i c t n rv . J L l v ! q v v u v r l I J q /

6.4 Microl i thography and Nanometer Resiss6.5 AdhesionA A f n - n " i ,- - . ^ - . . J O

6.7 Tribology

!i iH:lH::: :i 3.:,:-l :: : - : :6.10 Elecrrochemicai Sensors6.1 I Noneiectrochemicai Sensors6.12 Mul t i layer Systems for Opt ica l Coaungs

7 , C O N C L U S I O N S

ACKNOWLEDGMENT

REFER ENCES

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1 . I N T R O D U C T I O N

The chemistry of organic inrertaces is important to a very wide ranee of areas of science and technology;examples include the fabricauon of microeleccronic devices, the formauoo of scrucrures by adhesion,

cell-surface chemisn-y, the use of organic thin filrns as lubncans and corrosion protective agents, and

a range of phenomena involving wertrng and spreading of l iquids on organic surfaces. The study oforsanic interthces has been substanually more difficult than rie study of metals and metal oxides, inpart because no stnrcrura.liy well-ordered organic surfaces were available hat provided substraces for

fundamentai studies analogous to those camed out on che sinele-crystal substrates used in other arers

of swface science. Self-assembled monolayers (SANfs) of surnbie organic compounds on surfaces of

metais and metal oxides have gone "

ru65tandai way toward solving this problem of the avai labi l i tyof structurai ly weil-ciet lned subsrraces. A number of hese svsrens of monolayers are norv avai iable.

In cercain cases, the organic components oI t ]rese SAMs are iugir ly ordered (at [eest quasi-crysnl l ine):in others they are disordered or t iquidl ike. They are easi iv lrepared in a wide vanery oi str. :crures( th icknesses. degree of order , cna in onencauon wi th respec i :o dre sur race) , and a ranse of funct ronaisrouDs can be incorporared in to rhem. Theyr are much [ess sr rb ie to heac rhan meta ls and mcta l o . r tdes.

F iRST P ,AGE PRCOFS

7 1 3

I l . +

althoueh thev are suif icienclv stable thac thev can be used in a vanety oistudies nef,r room temperarure.They have 6e advanraqe that, because tt ley have relacively low sol id-vapor interfacial free energiescharactenstic of organic materials, hey can often be handled rn the open atmosphere without irreversibiecontamination. They are more easi ly tabricaced and mampulated than Langmur-Blodgett GB) mono-layers and are generally more sable and more ordered. LB systems still have advancases in manyapplicacioos (such as formatioo of oprical coatings) in wb.ich muit i iayers are required, but SAMs basedon alkanephosphonares provide the basis for excei lenr mult i iayer systems.

The purpose of this chapter is to provide an inroductory survey of SfuVs, concentrating -oo

the beststudied of these system.s-monolaiers of long-chaio alkanethiolates on gold. The chapcer will serve asguide to the literarure of this area. but it is noc comprehensive. It wiil not deal wi'h the many incerestingproblems that remain to be settled in this area. nor wirh extensive comparisons with other areas oforganic surface chemistry (particulariy the areas of funcrionalized poiymer surfaces and organic fiLnsadsorbed oo the surtaces of elecrodes, where many of the phenomena observed with Sfu\4s wereforeshadowed).r:

2. PREPARATIONS AND TYPES OF SAMs

The mosr extensivelv studi id oi t-U" rypes oI SfuVs are rhose of alk"nehiolaces on gold; these suucrureswere hrsc described as ordered orsanic monolayer sysrcms and prooosed as substraces for the study oforganic surface chemrstry by Nuzzo, Allara, and co-workers.!5 Very simllar strucrures and properriescan be obcained for alkrnerhioiates oo silver.6'7 Alkanethiolates on copper are also similar in strucrure,6'Ebut are substantial lv less stable to oxidation than alk;rnethiolates on gold and si lver.e Monolayers ofalkanoic acids oo alumina were characterized and described in early worki0 l l *3 have been recenclydescribed on lhe oxide surt 'aces of si iver and copper as weil .12 These mooolayers, i lowever. are unstablein aqueous environmena. The monolayers formed by hydrolysis and cross-l i lkrng of u'rcbloro- andrialkoxyalkylsi lanes (also cal led aikylsi ioxane monolay€rs,13-16 although their Cenrled strucrure remainsi-ncomoletely derrned)17 ale the most wideiy used of organic monoiavers. Because they lack rhe de.ai ledsrrucrurai definitioo of alkanetiioiate SfuVs, and because they are subsiantiallv more difficult to maruou-late syachecically than are monolayers of alkanethiolates, they are less srarghclorward to use in modelstudies in wir ich variarion in organic functiooal i ty is imponant. A number oI other sysiems. especia. i iyaikane isooicri ies on piat inumrs and a.ikyi phosphonic acidste and hydroxamic acidsre on mera.l oides,also florm monoiayers, but these sysrems await structural characteriz-atioo.

SfuVs oi alkanethioiates are rypical ly formed by adsor,puon- of an alk:nethioi or dialkyldisulf ideonto gold from solut ion ff ig. 52.1); boch rypes of organosulfur compounds form similar SAMs.rc:rThe iate oi the hydrogen of alkanerhiols in fornation of aikanethiolate monolavers remains undehned.Li,nited studies of dialkylsulf ides oo gold indicate that , iese compounds form monolayers, but theyare less scable and less ordered than those oI aikanethiolates.z The goid used in most stuci ies has beenin the form of thin films-usuaily 5 ro 2000 nm thick-ieposired by eleccron-berm evaporation orsounering onto a f lat suppon of si l iccn (with a native si i icoo dioxrde surr3ce layer) or glass. Because,gold does noL wet si l icon dioxide or giass well and acihesion is poor when the gold is evaporared direct lyonto either materiai, the surface is usually coated with a thin laver of ur:nium or chroruum (aiso byevaooration) to acc as an adhesion oromoter. I t is also possible to use organic rdhesion promoters.s: 'The goid cioes noc bave to be protecred from coocaminauon aiter evaporauon; ir ceo be handled in theur and stored (ac least for peiods of days to weeks) without loss rn the quaiiry oi the SA\{. probablybecause the activity of tbe alkanerhiol in adsorption onto the surtace is sufflcient to scrub weakly boundorganic impuriries from ir.

Figure 52.1 Representation of a clos+'packed n-alkanethiolmonoiayer on a gold surrace.

FIRST PAGE PROOFS

7 1 5

Reacuon of alkanethioi wirh goid is reasonably fastx-the monoiaver is well tormed f iom a I mrVsolurion in solvens such as erhanoI and hexadecane in one hour ac room temperarure.:s-zt Monolayers

having sirmlar properues can be tormed by adsorpuon of the aikanethiots from the vaoor phase,]e-12

aithough the low vapor pressure oi high-molecuiar-weighr thiols has Limrted the ut i l i ry ot i i r is type oIprocedure. Upon beaung, the thiolates desorb slowly, probabty as dialkTl disulf ides. Looger<hain thiols

fonir films rhat are rhermally more stable than films fs16ed from sbort<bain thiols.5 Thiolares cao also

be formed by elecuochemical oridarioo of alkanethiois rn solucionls and desorbed by both eiecuocbemical

reduction! (wirh formation of thiols in solurion) and oridarioois:o (as aikans5ulfqoates). On exposure

to [fV [ght in the presence oi d.i6xygen, the rhioiate anion oxidizes to sulfonate, and the formarion o[this weakly adsorbed group provides a mechanism for photo dam:ge, the usefulness of which in imaging

is discussed below.l jJ6Wben rwo aikanerhiols or dialkyidisulfrdes are present in solucioo, so-called "mixed monolayers" form

containing borh alkanethiolate groups.le272E37J8 Tbese mrxed monoiayers have been used extensiveiy totailor the properties of surfaces.se'{o A thermodynamic analysis of a system based on A and B has

demonstrated thar these rwo-component systerns are not at thermodynamig equilibnum and that phaseseparaion in them (of tbe rype commonly observed with Langmuir monolayers at the waler-air interface)is esseorial ly impossibie ac equri ibr ium.l8ar

N{uitilayered syste ms are also of inrerest, particuiarly for the construcuon of opticd thin films (see

beiow). lni t ial ly, roures to si lane muiulayers were pursued by reectrnq a cnchlorosi lane with a hydroxy-termrnated surthce generated by hydroborauoo-oridarion of an oiehn tetminusl2 or by reducuon of anester termiaus.t3 The difficulties descnbed above when worki-og wirh silane precursors arb exacerbatedhere. As an alternauve, zuconium phosphonate mu.lt i layers have been pursued by inidal ly bondi-ng anai(vi phosphonate to a surface using a thiol (on gold) or cnchlorosi lane (on si l ica) and causing thephosphonate termrnus to react with zirconvi chioride tol lowed by a bis-phosphonate Gig. 52-3)

3. STRUCTURAL CHARACTERIZATION

A number of techniques have beeo used to characterize rhe strucrure of SfuVs of rnedium- and long-chain alkanethioiates on gold.tE Transmission electron diffracuon pronCed the [rst evideoce for crys-tai l iniry;{er0 independeoc measrrreaenrs by iow-energy hel ium ci i l f racrionit '52 and low-angie X-rayscanerings3'Jo have confi .rmed this conciusioo and provicied addiuonai evidence: rhe polymethylenechains are, oo the average, tilteC aoproximateiy 30" trom theaormal to the surlace. On the lowesr-eoergy swthce of gold, the Au(l l l ) surface, electron di l fracuon shows the sm:crure o[ e SfuV oiCi{r(CHr)21SH to be commeosurate with the underiying laruce and demonstrates that it adoots at/3 x V3R30 overiaver strucrure (Fig. 52.a). Transmission elecuon diffraction{e and low-angle X-rayscatterings5 reveal more complicated overlayers on other crystal faces oi gold. Similar techniques havebeen used with alkanerhioiates oo siiver. Strucrures aopear similar to those oo gold, although the c:nrangle ior the alkyi chain changes from ooe to the other. More interesung.iy, although shon<hain thiols(e.g., methanethiol) forrn a cori lmeosurate overiayer with Ag(11l),s a monolayer oi ocradecanethro(is i lcornmeosurate with this laruce, suggesung chat the monolayer srucrure has an rnr ' iueoce on i tsassenbly with the surt-ace.r7 These studies bave morivaced computauooal investigarions of oacking andult behavior in monolayers thar are coordinauveiy atrached to a substrate. Such investigatroos highl ighcthe imoortance of tie lardce of the subsrrare on the epicaxy in rhe 61-.:EJe

Scanning runneiing microscopy (STIO has recently been able to genera@ high-resolut ion imaees ofSfu\fs by working with very l5w runneling currents (correspooding to b.igh imp€dance, )1012 ohm,and large tip-surface seoaration;.m Working at the atomic scale (e.g., 2 nm2;, some authors claim tohave confi .rmed the f3 x V3R30 overlayer srmcrure on Au(i l l ) .6' Atomic force m-rcroscopy givessirmlar results.62'51 On a larger scaie (e.g., 200 nm:), the images that are produced suggest a complex

Figure 52.2 Representation of amonolayer on a gold surlace con-taining a mixture of long-chain andshort-chain thiols.

E o s

: fl,[:,. ll,{",,,lr t-- Ll J,,

FIRST PAGE PROOFS

7 1 6

DBP

eoisssss.sssssl Poi

Poi&Poi

III

IV

I

II

ZfiQ1; DBP

*l o*Poisr\\\\s\sr\sK\ss!

Poi

Poi _-..

noisssss.ssrr.rss ng;

*i i.-roitsss.rsssss

roi

7tO(t2; DBP

ZrOCI? ; DBP

Figure 52.3 Schemat ic of f re pr+

cess emoloyed to make z i rconiumdiaiky lphosphonate monolayers.

s .01A

2 . 8 8 A

Figure 52.4 Reoresentation of the v'3 x V3R30 overlayer structure that a SAM of CH3(CHrzlSH adopts withresoec t t o a Au (111 ) sunace .

-i,_* rofss,ro,=."or=.* roi - ..eois=o...=oo.to noi

* i l- ro iso*=oos nof

_"* nolsssssso=os-.= ro I

*i' ]*l'o

;oo.o.oo...,=o rci "- eo isrss--..:{.sstqssss po i

FIHST PAGE PROOFS

7 1 7

surface 'r i th defects: especia. i ly mrssing rows and grain boundaries.s'M6 The exten[ to which cheimages thac have been obta. ined to date are representarive oi SfuV{s formed under other condit ionsremains to be establ ished. Moreover, the inrerpretacion of STM images is st i l l cootroversial. Receorwork suggesrs rhac the interprerarion of STM imaqes of mooolayers on goid (measured using a DCcurrenl at low impedance;6t'6t rr'. iargeiy the resuit of changes in the eiectronic distnbuuon on the goldsurface due to pernrrbations in the electromc srrucnre of surface sLates in the mecal 6*4.62'6a

Polanzed infrared externai reflectance sDectroscopy (PIER.S), a technique developed in this applicationby Nuzzo and Allara,6FTl 6ut been invaluable in establishing the details of strucrure of the h-ydrocarbonchains internal to the monolayeq'PIERS has become the workhorse for investigating the srnrcrure oithe organic moiecuies incorporaced into cbe monoiayers. When monolayers composed from alkanerhiois

IHS(CHjr5X, XCH3, CO2CH3, CO?H, COM{2, CH2OFIJ were studied,6e the posir ioo of tbe CH2 C-Hsretchiug modes indicated a crysullineiike packrng of the alkyl chains. The cant and wist angies ofthe chains wich respect to the surface coordinates could be determined from a model that utilized thisIR data. Mooolayers on silver are strucrurally related to those formed by adsoqption oo gold, but differenrrn derails of orientation @ig. 52.5).8 The monolayers formed on copper are slrucrurally more complexand show a pronounced sensitiviry to the details of the 5ample preoaration.s'e

A number of other techgiques are useful in the characte rizrtion of SAMs and mixed SfuVs. Opricalellipsometr.v has been userul in esrabiishing rhe mean thickness of SAMs;a527'72 measuremeots byellipsometry have been confirmed by X-ray photoe iectron speccroscopy.r'7t )CPS is useful rn estabiishingthe eiemental comoosition of mixed SAMs and thus in infemng the composition of these monolayers.Sum frequency sDectroscopy offers panicutar promise as a method of invesugaring the srrucrure ofhydrocarbon cbains of SfuV{s in cootact wirh i iquids.T5

4. MATERIALS PROPERTIES

Studies of the materials properries of SfuVs have focuseC oo alkanethiolates on gold, with the majoreffort devoted to charactenzing the wetting of rhese surfaces and to their elecrochemrcal properrles.Ln wett ing, the systems provided by mixed monolayen oi ooe hydropbobic and one hydrooir i l ic compo-nenc have demonsrared the connection between wertabi l i ry and surtace composidoo.6'E'15'10 Detariedstudies of the reiat ion between the iocation oi organic funcuonai groups with respect to the swtace oftbe monolayer rndicates that oniy runcuonaJ groups within a tew anssuoms of rhe sol id-t iquici inrenacetnfluence wetling;7'''76 Zisman bad drawn a sirorlar inference mqny yerrs previously.t Derarled srudies

Monoiayer G F

HS(CHtlsCHr 40 50

HS(CHTECO2CH3 38 55

HS(CHTECO2H 32 55

HS(CI{/15CONH2 31 55

HS(CH,15CH2OH 2A 50

Figure 52.5 Ti l t (o) and rotat jon (p) angles ( in surrace coordinates) determined for the al l- fans alkyl chainsof var ious SAMs on oo id .s

N\N

FIRST PAGE PROOFS

7 1 8

of the ionizarion of acidic groups e,rposed on SAlIs (and more [miceC studies of basic -eroups) indicaced

ihar tornarion of a charged group from a neucral ooe (that is, oi a carboxylate anLon from a carboxyi ic

acid or oi an ammonium ion from an amine) is more dif f icult (requires suonger base or sronger acid)

by ar least 2 pH unia than rhe corresponding reacrioo in soiuuoo,im and that the col lect ion oI carboxyi ic

acids in rhese monoiayers behave as polyacids. The d-ifficulty of forming charged groups oo SfuVs

undoubtedly has conrriburions borh from cbarge-charge repuisioo becween closely prorimace grougs oo

the surface and from the iower dielecrric coost2nt of the incertace compared to bulk watec the detarled

conrnbutioo of these two factors Ft oot been estabtished'

Elecuochemicat properries of SA.r\Is have been investigated in detail by a number of work€rS, rm6sg

which are Sagiv,Et Porter,33$t123L83 Chidsey,{'50'8r'8s p1nkle486'az and iWller.E6 These studies demonstrate

SA-,.Vs are excellent insuiators: a well-formed Ct8 SA-l\4 reduces the currenc thac passes across a gold

elecrrode by approximarely i06.2 The ongin of the residual curreot has not been weil estabiished:

parricularly, ir is unclear whether ir is due to pinhoies, thin spots in the mouoiayer, defeca ac grain

boundaries, or regions of damage from other causes. SAMs have proved very userul in studyrng the

dependence oi the rate oi eleccron transport across monolayers oo their ttrickness;2'tt'$r'tt'Ell related

sysrem-s have led to the inference of detarled mechanisms of elecron Eanspon in ferrocene-terminated

monolayers.s. In such sysrems. brief exchlnge of a ferrocene-terminaced thiol with a noneiectroaccive

thiol reduced tbe rare oi eieccron cransfer by at least an order of magnirude, suggesting that the majonry

oi charge passed in rhe nonexchanged syste ms is by laceral eiectron s'anst-er from detect sires Gig. 52.6).s

s. PATTERNING OF SAMs

Single+ompooeot and mixed SfuV{s of aikanethioiates on gold provide excei lent conrol over the

srructure of rhe monolayer perpendicular to the plane of tbe Sfu\f. The thick-oess of rhe SfuV. and the

posicion of funcrional groups, can be conroi led to aoproximateiy I A rn favorabie cases (rhe rncremencai

disrance of adding one merirylene group); by workine with mr-red Sfu\ls, one can generate disordered

regions of coocrol labie thickness ar the outer surt 'ace of t ie SfuVf , and the average comoosiuoo of the

monolayer can be conrol led.Addicional conrol over the composir ion of organic surfaces can be obtatned by patterning Sfu\fs

on a surlace. Tbe most usenil of rhe currenc cechniques have been developed for alkansthiolates on

gold and demonstrated for alkanethiolares on si lver and cooper. Other systems have not been extensiveiy

A Before Exchanee

B. After Exchange

Figure 52.6 Representation of a domatnboundary in a mixed monolayer consist ing ofI 'errocen+.termrnated and methyl-termtnatedalkanethiols (a) before and (b) arter exchange,arrth a soiut ion of methvl-terminated alkanethiol.

,rilr)

t//

FIRST PAGE PROOFS

7 1 9

develooed. Five tecirniques have been developed for tormrng patterns of SANIs wich dimensioos ranging

trom aoproximately iOO nm to I cm; I to l0 pm fearures are rourinely avai lable. These techniques

take advancage of the fast kinerics of adsorprion of thiols or disulf ides onro uncjenvacized gold compared

to reeions oi goid coordinaced by an organosulfur species. Thiolaces also do not exhibic discernable

laterai dif iusion on polycrystal l ine gold over a period oi days, ac least at scales greater than 100 nm.

A lower limrt for this diffusion has vet to be established.

5 .1 CONTACT PRINT ING

- By making an elastomeric "stamp" of potydimethyisiloxane using a photolithographically generated

master, it is possible to print patterns oi a bydrophobic Sfu\4 on a gold surface Fig. 52.7).s The spacesin between these pactems fi.lled wich another SfuV. This method is a very conveaient one (although icdoes require access to photolithographic equipment to generate the initiai master). It relies on thc abiliry

of the ndges oo the elastomeric stamp to make cootact with the surface of the gold whiie retainingsub-pm scale features, and on the fact that the solucions of alkanethiols used is 6ahng the initialimpressions do not spread on the gold.

: l ) PDMS

S iPho{orcaigt prttrrntl-l tl'm $kXntri]

CJp.tiul

[ *oo*"*",

*

** th,e nroster

m- r - Photugrrphr

PDillS. !"::

F !E | :F<< t - r t r s r< ro i r lD - . . - . . . .

'l-'T':.T,1;

Figure 52.7 Procedure for the fabncation of a pafterned elastomeric stamp and for the preparation of apafterned surface. (a) An exposed and developed photoresist padern on a silicon wafer was used as he master.Polydimethylsi loxane (PDMS) was polymerized on the master and careful ly peeled away. The stamp was inkedby exoosure to an ethanol solut ion (1 to 10 mM) of hexadecanethiol, brought into contact with the gold subsEate,and removed. The substrate was then washed with a solut jon of another alkanethiol (1 to 10 mM in ethanol),followed by washing in ethanol and drying in a strearn of nitrogen. (b) Photograph of a stamp. The region withthe feaiures are in the square section. (c) A magnified view of the features in the square section. (d) An electronmicrograph of the pafterned surface. The stamp was used to pattern the adsorption of HS(CHrrsCH3 on f iego ld ( l ight reg ions) ; the go id was then exoosed to HS(CHdroCHzOH (dark reg ions) . '6 t

I EIrnE PD.!{S to,rI solution tont.rining

i "*:tttttl5cill

FIRST P,AGE PROOFS

720

5 . 2 M I C R O W R I T I N GUsing a cao i i la ry t r l led wi rh C16 th io l , i t i s poss ib le to wnre l ines o f a Sfu\ { w i th min imum s ize o f

< I pqo.t.nl fiui merhod for parterning rhe surface of gold wirh a Sfu\I has the advancage that it does

nor requke phorotirhographic iquipmenr, buc because ir is a serial technique, rvr idng is relat ively slow.

5 . 3 M I C R O M A C H I N I N GWhen a scrarch is formed in a surtace coared with one SAM, bare gold is exposed. Dipprng this

scraccbed surface inro a soiurion conraining a second thioi permis formatioo of a second SA"Vielecuvety

in the scrarched regions.%.e3 Using rhis merbod it is possible to make lines as small as i00 nm.

5 .4 PHOTOLITHOGRAPHYThis patternrng method was originaily emptoyed by Calvert et al.%et to selectively phococleave aromatic

silanes on siiica surraces using high-energy (i93 nm) light. Monolayer films pxay be very useful in this

conrext because, although shorr-waveiength W light can permit the development of small (:400 om)

fearures, rhe deprh of focus of such light even wirh high numericai apemre leoses, is smell, and

convenlional, thick (- I mm) phororesists are not evenly dosed under these conditions. Heavy doses

of i ight (e.g.,2 J/cm2) are required, however-

Exposure of a SfuV of a rhiol on gold ro LfV [ght in an atmosphere concarni-ng dioxygen resuis in

seiecrive oxrdatioo in the i l luminared regions.l5 These oridized regions (concatning,3moog others,

alkanesuifonares)]6 exchange wirh rhiol on exposure to a soiuuoo conarilng thioi. This procedure is

capabie of producing pm-scaie fearures.r I t is, however, reladveiy siow.

5 .5 ELECTRON.BEAM WRIT ING

An elecrron beam produces damage in a SAM (perhaps combined with deposition of organic material

from rhe vacuum ihamber oo rop oi the SfuV) rhat can be used to fiorm patterns. The chemisory of

rhese elecuoo-beam-induced processes is not well established and is certainiy less deFrned tian other

merhods of forming parteros. The method bas been employed with_a subsequent chemical erching ste-p

to produce parierns with around 50-om fearures in gallium arseniderm and in goid on siiicon Fig. 52.8).r0'

These palerns can be imaged, once formed, using several techniques. Convenrional electron micros-

.ooy p.oduces images, providing thac rhe regions of patterned SAN{ differ siguihcantiy rn thickness,

inrenaciai free energy, or composicioo.lm'lm The origin of the cootrast is noc cleariy establ ished, although

borh scanenne of secondary eleccrons in the SAJV{ and scanering from impuriries adsorbed on regioos

Figure 52.8 Approximately 100-nm gold teatures (20 nm high, spaced 0.5 gm apart in one directon and spaced

1.0 pm apart in the other direct ion). The features were fashioned by electron-beam writ ing on a hexadecanethiol

monoiayer on gold followed by chemical etching. lmaging of the features is performed using atomic force

mrci 'oscogv. 'ot

FIRST PAGE PROOFS

7 2 1

of rhe SAI,I having high inrerracie.t free enersies seem to cootr ibute. SII4S also imases patterned

SAN{s.lo2.l}r lnorher uietul rechnique is to condense water vapor onto the surface of rhe SAM a-od

image the partern of snal l droples of warer that lorm. This technique-imaging using so-cai ied

"coodensarion f igures" (CFs!-- is surprisingiy versaule and bas resolut ion char seems comoarable to

more comoiex techniques.e'Parrcrned SfuVs can be used in combination with etches seiecrive for bare gold or gold covered

wirh a thin monoiayer ro geoerate pactems of gold; thick, hydrophobic monolayers (e.g., those from

hexadecanerhiol) provide prorecrion of rhe uoderlyrng goid trom the etchant, wbjJe the baie goid (or

gold covered wirh a rhin monolay-er, e.g., p-mercaptoethanoi) etches smoothly. The most exteosively

demoosuared etcbanr is aqueous cyanide ioo in the presence of an oxidant such as dioxygen or

ferricyanide;m ocher sysrems that do noc conrain cyanide also work, however. This ability to make

arrays of microfabricared gold t'earures with coocrol of fearures at the pm level is directiy useful in

making microelecrrode arrays for sensors and other electrochemical devices trig. 52.9).

By empioying patterns of rwo metals with different characterisrics as adsorbeots it is also possible

to geoerare surfaces havrng patterns. Form.ation of an array of pm-scale wires of gold and aiuminum

(covereC with aiuminum oxide), followed by exposure of the system to a solution containing a mixture

of aikanecarboxyiic acids and {k;rnethiols, resulted in formacion of aikaoe carboxyiate monolayers on

the alumina and aikanerhioiace mooolayers oo rhe goid.l05'106 This rype of process is called "orthogooal

seif-assembly," and ic has been used to seiectiveiy bind redox acrive molecuies on specitrc elecu'ode

sur taces Gig. 52.10) . 'o '

6. APPLICATIONS

The systems of Sfu\4s have been used for a wide variery of appl icat ions that depend on the abi l i ty to

cootrol the charactenstics of organic surthces.

6.1 WETTINGSA*VIs are especially well suired for fundamental studies of wetring and of the interacuon of organic

functronal groups wirh liquids. Inreraccions of functional grouDs on SfuVs with tiquids can be easrly

detecced by measuring cootact angles or &op shapes.6'8'{6e76107'108 The abi l i ty lo use tbese surfaces

pacterueci with acidic or basic funcrional Eoups ro study deoroconadon or protonacion reacdoos at

incerlaces indicared how complicared these phenomena are.7s's0'toe' l l0 A number of systems provide

examples of rhe complexiry oi these simpie protoo-transt 'er reacuons. In the reacrion oIcarboxytic acids

with hydroxide ion in aqueous soluuoo, the rmponance of reacuve spreading--that is, the advance oi

rhe edge of a drop driven by the free energy of ionization of the acid-in determining the advancing

cooracr angle remai65 undeilned.l l t nr a second example, the wettabi l i ty of a suriace presenttng

carboxyi ic acicis oiren shows a min-imum in werabi l i ry at a value of solut ioo pFi that con-esponds

approximarely ro haif- ionrzauon oi the carboxytic acid groups. The impticacioo of this resuit is tbar

there is an rnrermediace surface s[ate corresponding to -CO?H-O1C- that has a unique wettabi l i ry.7E'

Gradiens in rhe hydrophil iciry oi a surtace detined by the assembly ot alkaisi ianes on giass generaced

rhe movemenr of drops rn rhe direcrion of increasing wetabi l i ry (Fig. 52.1 1).rr: The kinetics oi advanceni:r drnn edoe nn surthces are Sti l l largeiy unexplored tor SfuVs on gold, although the subject hasv t q u v v v u 5 w v r r

been carerui ly analyzed using alkylsi ioxane SAMst5'rr l *6 the viscous t iquid poiydimethylsi loxane by

Leger , l la Brochard, l ts ' t l6 Q:rza[3qt l7 ' l lE *6 the i r co-workers .

6 .2 SUBSTRATES FOR SURFACE SPECTROSCOPYBecause SAMs can be parterned and used to display a variery of funcrional groups, they are exceilent

subsrrares for a variery of specrroscopic studies. The abiiity to form patterns on the micrometer scale

is panicularly useful when ir is necessary to use one region of the surface as an internal standard for

examrnauon of another (for example, in e.ramimng conrast betweeo regioos wirh dif terenr funcuooal

grouos) 1Fig. 52.12).rt . tm-tor They have been used as subsuaces in examrnacion of the mechanism oi

damaee during exposure to X-rays, wirh the conclusion that photoelecrons are the pnncipal mediacors

oI damage in tbese sorrs of experimeos.l te'120 This result is usefui in suggesung that resists designed

for X-rav and e-beam tirhoeraphycan incor.oorate the same pnnciples of design in concrol l ing sensit iviw.

The abri irv to imase SAMs using STM (see above) sussests their use in scudying the mechanism of

contrast tormacion in oreanic matenals.

FIRST P,AGE PROOFS

722

lrnprku a gt.rtttrn olFEiCiiiJl5CHl r,n goid

SA. \ l i - l - l nm)

, \u r lXl nm)

Ti t5 nm)

I

I etcir in a KOH/, l ,*t*pannl solution

If-

Etch in a cl'anirlerolqtion

..; t

1ip2. .iI{ ,

r-- Thinfitm o(TIo1I

SET(-...-.>

' 'r )-)1

_/ -si- \-

tYesh inrqu.r rtaia

( 1 0 0 |

( r r r r I t\ . tk

sElr;- j . , -*

Figure 52.9 Formation of features produced using microstructures of gold as masks for the etching of si l icon. '(a) Schemaric representation of the procedure. A stamp consist ing of paral lel l ines of 2-mm width was used inccmoination with wet etching in a cyanrde solutron to produce fre gold mask. (b) Grooves of si l icon wereformeC using an anisotropic etch for sr l icon. The (100) face etches faster than the (1 1 1) tace. (c) Washing hestructure with aqua regia removed the f lm and exposed the morphology of f te grooves. 'n

6 . 3 E L E C T R O C H E M I S T R Y ,Because the thickness of rhe SA-rVs can be concrolled with high precision by conu'olling the strucnrres

of the organic molecules making rhem up, and electrochemically active funcuoual grouos ca.o be

incroduced at wi l l , they have been shown to be usetul substrates for studies of mechanisms of elecsoo

tunoeling and Eanspon.io'72'st 'ss'84 16iols can be desorbed from r sold surface by means of oxidaciouir i6

and reduction.l t 's3 This process has be en used in reverse to conu'ol the extent of deposit ion of a monolaver

on goidsl and to conrol rhe wettabi i i ry of a rhioi-covered gold suface.tzt The abi l i ty to use monolayers

in che fabncauon oi microsrructures has been used in preparanoos oI several kinds oi microelecu'odes,

including simpie a.rays of scratches in a surtace otherwise rnsulated by a SAM. and SA-,Vs have been

used. in conjunction wirh erching, to make smail wires wirh gaos.ne3 On. such system had an eiectrode

surrace area oI approximareiy 10,000 nmr (0.1 mm:). The ease with whrch simple microstructures

FIRST PAGE PROOFS

723

cqH

Figure 52.10 Schemabc o{ th+formatron of orthogonal SAMs by adsoqction from a soluton containing a

mirfure of alkanethiol [HS(CHr)-Xl and alkyl carboxyl ic acid [HOzC(CH2)nt onto a pafterned gold-aluminum

oxide surface.106

can 'ce

prepared using SfuVs makes them parricularly atractive for preparing microelecrodes and

microeiecu.ode ar,:zrys, although these r,vpes of electrodes can be fabricated by more cooventional

microiabricarion rechniques. SAN{s on mrcrofabncated eiectrodes have be used to prepare regions of

eiecuochemical ly acrive organic materials, and these have been used in several protorype devices.l8' t?2-t2{

!{ost recenrly, SAMs have been applied to elecuocaralysis as surface-modifred electrodes to cat". lyze

cl ioxygen reduction using cobalt teriphenylporphyrins rmmobil ized oo a gotd surthce (Fig.52.13;.tzs' t :o

6 . 4 M I C R O L I T H O G R A P H Y A N D N A N O M E T E R R E S I S T S

The exrraordinary abi i i ty of SfuVs to prorect a metal slr iace agalnst etciants bas already been establ ished

to be valuable in r ieu appiicauon as resists.F'rm Because they are so thin ( l to 2 nm), eCge effecrs

ihar mighr limrr the eCge-iesolurion in thicker materiais seem uglikely to Lim.ic theLr pertorrnance.reE

The smallesr fearures rhat have been made usiog SAMs as resists have dimeosions of less than 100 nm

(prepared by e-beam wrir ing).10o' l0t Jt is not clear whac the lower l imit to che resolurioo obtainable

using these materials as resists wil l be.

6 . 5 A D H E S I O NThe cbaracrer of rhe funcrionai group exposed at the surtace oIa Sfu\ l is clearly ref lected in the streogth-

oi adhesion ro the SfuV; they are, thererore, excel lent subsuates wirh which to study the physical-

orqanic chemistry of adbesion.lzT-l ir On iracrure, fai lure can occur eicher at the interface between the

SfuV and rhe adhesive, or between rhe SfuM and u\e gold subsrrate. Ln the latter case, the soecies leit

oo t-he tracture surt 'ace aDoe3rs to be a eold(l) thiolate.:a

6.6 coRRosloNSAMs provide a very high degree of protection of the underiyrng menl against certain rypes of

corrosion.l35'136 16ir prorecuon has been examined in some deta-r l tor alkanethiolates on copper, with

rle inrerence rhar rhe rare of dir iusion of dioxygen through the monoiayer is important in deterrnimng

rhe k ioer ics o i formacion of copper ox ide f rom copper meta l .e Exeminauon of t ie cor ros ioo o[go id in

rhe solurion of cyanide ion wirh oxidanc thac is used as an etchanc in patterning, and relaced studies of

elecrochemical corrosion in cyanide-conraining solut ion, have oot defined the origin of ihe pinholes

rhar appear even in SfuVs rhac have no( been exposed or patterned. These incrinsic defects deserve

subsranriaily more eifort, because minimizing them is an importenc ccmponent in developing SfuVs as

potenciai resists in microl i thography. The best values that have been obtained so far are approxrmatelyone defecr . , " r . - l chatcan be dececred by erch ing in CN-/O1 (obcained underc ieen-roomconci i t ions) . t37v r l v s w l ! ! ! y v t v r r t

1,.,,--T-- zo A

| . ,oo o

I|

- 2oo0

_l_

FIRST PAGE PHOOFS

724

Figure 52.11 The edge of a drop of hexadecane in conLact wi th a l ternat ing str ipes of hydrophi l ic

[ -S (CH?) l .COOH] and hyd rophob ic [ -S (CHJ 'sCH3] SAMs on go ld : t he h re+ 'phase con tac t l i ne de fo rms a long

the more weftable (carboxyi ic acid- terminated) l ines. The per iod of these Ceiormat ions is ident ical to f ie spacing

of ihe pat tern of funcUonal groups in the SAM. The s l ip in the l ine of contact befween adiacent regions determines

the maci 'oscopic shape of the drop of hexadecane on the pat terned surrace: the macroscopic drop is e l l ipsoidal

wi th i ts major axis onented in the di recl ion of the str ipes. The eccen[ icr fy of the el l iosoidal regron of contact

befween the drop and sur lace increases wi th a decrease in the per iod of the paf tern in the wettabi l i fy of he surrace.

6 . 7 T R I B O L O G YStudies oi tnboiogy'" oi systems consist ing of relauvely thick (200 nm) goid fr l-cos supporo.ng SfuVs

using conventional si ider or pin-and-disk apoararusesl le have not been successful, because the gold is

easily deformed, and the wear rhac is measured is pnmarily that due to deformacion of the gold, rather

than to inceractions involving rhe SfuV. Srudying friction with a scanning probe nicroscope in lateral

force mocie has, however, given promrsing results. These rypes oI experiments were f irst camed oucwith LB f ikns using phase-separated rwo<omponent monolayers by FrommgJ,lao' lcl but the sarne rypesof experiments aiso work wich patterned SAMs on goid.

6 .8 ATTACHMENT OF PROTEINSBy conuoll ing the functional group rhat is displayed ar rhe surtace oi a SAM, i t is possible ro control

the extent of adsorpuon of proceins from solurion onco the surtace of the Sfu\{. The functional groups

FIHST PAGE PROOFS

t z 3

Figure 52.12 lmages of a paf terned SAMobtained by di f ferent techniques. The sur-face consists of a l ine of a SAM obtainedby the adsorphon o f HS(CD. r ) , .CD, w i th thei 'emainder of the sunace ccvered by a SAM Fobtarned f rom HS(CHrrsCC2H. (A) Opt icaimicrograph of a condensat ion f igure formedfrom water. (B) Opt ical micrograph of thesample snown in (A) af ter fur ther water con-densat ion. (C) Opt ical micrograph of ihesample wi thout formabon of a condensat ionf igure. (0) SIMS ion map (mlz = 50). (E) : :

SIMS ion map (m/z = 55). (F)Scanning elec-t ron mic i 'ograph of the paf terned SAM.ei

u\at best resist adsomuon of proteins are those based on the oi igoethylene glycoi (EG) groupi eitherh ish suface dens iues of shon o l igomers @G1 or EG.) , ep lewer dens ic ies o f longer cha ins (e .g . ,EGrr) provide surthces showing excel lenc resistance to adsomcioo of proterns. Certain surt-ace-artachedcarbohydrates also resist acisomrion of prorelns .t42't41 CharseC and hydrophobic surfaces promote theadhesion oi proteins.r* The molecuiar basis tbr the selecuvirv of these processes has noc yet been

... ,::,:i.:).:::: '^::...r.;i:1l^i{.)r1,.::/a.i:;i::::i.rri.r.::. .:i::l:;ii$:.iy::$:;

FIRST PAGE PROOFS

t z o

Figure 52.13 Thiol-functional ized porphyrins that catalyze the reduction of diorygen when ptaced on a gotde lect rode. r2s.126

examrned. SfuVfs bnng to the studv of protein attacbment on surthces, and to rhe study of arrached cel lculture. the abi i i ty to control the moiecular derarl oi the surface sgtf icienrly rhac ir is possible ro displavcompiex organic functional groups-ligand,polymers, carbohydrares---on rhe surface r,,,irh a high deereeof concroi over attachment and surface densiry.

Funcuonalized SfuVs have also beeo used to immobii ize proteins on a surface.l l5 Using a carbodijmidecouoling, bovine l iver cataiase was covalenciy bound to a carboxyl-terminared surthce to avoid up-induced s;rmple movement during scanning-probe microscopy studies.ro6 Alrhough rhis mechod did noryield the best images (compared to immobil izat ion by Pt/C coacing), such covalenr;oupl ing techniquesseem promrsi lg. Adhesion co an etecrrode surface has faci l i taced studies of elecron ransfer protei-ns 'Lncluding rumarate reductase,loT glutathione reductase,luE and cytochrome c: loe the covalently boundproteins ci ici not lose their electroactivirv after coupling to the surface. Scbemes for sire- and oriencacion-speclf lc coupling of such proteins wii l be useful in more definir ive eleciroo-rransfer srudies.

6 .9 ATTACHMENT OF CELLSAnachmenr of ancborage-dependenr cel ls to surtaces involves, in molecuiar detai l , attachment of cel l-surface receptors to proteins adsorbed oo the surfaces. The ability to regulare protein aruchment thusmakes i t possible to regulale the actachment of cel ls. The use of SAMs ro study the arLachmenr of cel lsis just beginning,% l-.0 *6 the experimencs that have been carr ied out esrabl ish tbar rhe stabi i i t ies ofthe SAMs under the condit ions required for cei l culture-that is, in conract with a cuirure mediumcontaining a complex variety of organic materials including organosuifur compounds-is more rhanadequate (Fig.52.14). SAMs thus otfer f lexibi i i ty in surface cheuusrry. The use oi rhin, opcical lytransparent gold substrata makes i t practical to observe the cei ls as they erow using normal prorocolswith invened optical mrcroscopy:r5r the conductivi ty of rhe gold otfers rhe possibi l , i ry to srimulate theatcached cel ls electrochemrcal ly. Pacterning makes i t possible ro regulare the area(s) oi rhe surthce overwhich the cel ls can spread. Because the patterns can inciude areas smaller than the normal area oietncirment of a single cei l , i t is possible to examine the inf iuence oi conscraining cei l soreading on

f(

)(

S H

H

o

N

I(

)

)HS

o

,Co.N ,

. N :

=^ -4t

FIRST PAGE PROOFS

727

I (s,n-i l. -

''"r!" ,t-J . :rik:nerhiol

SA-el(-1-2 nm)

L-*=--: J 1oopry

Sevl

sL\{

Figure 52.14 (A) Scheme for creat ino pat ierned subsbates. (B) Scanning e lect ron micrograDn (SEM) of thesubsfatum after patlerning the suriace wrth hexadecanethioi anC polyethyiene glycol-terrninated thiol (PEG-th io l ) . Adhes ive hexadecaneth io la te-coated reo ions appear l iont aEainst a darker nonadhes ive PEG- ih io ia te 'coated background. (C) SEM of a lamrnrn+oated patterned substr i te. Protein coattng resuits in a reversalo f the l iqht and dark s ta in ing pat terns. (D) SEM of a s imi lar lamin in-coated subst ra te ccnta in ing adheredrat hepatocytes.tto

cel luiar met-abol ism. Parterned surlaces also of ier t-be possibi i i ty of indexing cel ls on tbe basis of t-beirpositron on a surface patterDed rn rwo ci imensions; t i r is capabrl i ty may be useful i l drug test ing

or tox ico loey.The resu ls r t rar have been obra ined * ' i th SAMs are compauble wi t i resu l ls obte ined us ing a lka is i iox- '

a.ne monolavers on si l icon/si i icon dioxjde r-bar had been patten-red photol i thograoical ly ' .reE Tbe surface

functionai izauon possibie wirh the aikane$iolares on goicl is much more converuent than that fora lk l ' l s i ioxa-oes: the sub i i i r - " - o f t -he laner is probabiy somewhat h igber . For s tud ies o f meci :an isms ofce l l a t rachment SAMs on go ld seem rbe besr system now avai lab le ; a l tbough more I r ru ted m scope,methods based on aik-visi ioxane monolavers on si l icon/si l icon diox-rde mav be supenor wben larse areasof suppon are required.

6 . 1 O E L E C T R O C H E M I C , A L S E N S O R SSAMs of alkanerhioiates on goid have been used in a number of studies directed toward prototype

sensors. A number of rhe mer-hods of parrernins are useful in bui lding an elecuode (see above). Themost i i lusrrauve SAM-based device is an inrernal ly referenced pH sensor based on lhe relarive sensir ivi tyof the fencene/ferrocinium and p-quinone/hydroquinone couples.lx To date. most monolayer-basedsensors bave been constructed using Langmur-Blodgert techniques [e.g., nickel-phtbajocyantne NO2sensorsls2 -6 avidin-bioun (giucose oxidase) glucose senscrsl531, although covalenr immobil izat ionhas precedence [e.e., diaphorase on a un-oxide elecuode for a nicotrnamide adenine dinucleotide [NAD*/NADH) sensor ] . r50 Molecu lar recosni r ion, an imponant componeDt o f se iecuve sens ing, has beeni l lus t rarcd us ing b is(acetoacerate) r -<-< ' r r6 and ca l ixarene-based l igands l5T and by creat ine "ho ies" in the

:&i#8,. . . ' : t ' ; ,319i

:s .-sErr r .

I , r . - a: . : : : : : - . r ! r : e .

- . . I

,j$-:.'$['

Adhcsive to nrofein I t.t*" surfao: ro a s<riurton ofi\on-adbesi"i I j nstcnzlr;(oCH:ClblsoHt o p r o t g n 1 l I :

II

I{cpatocyte I S"*a ".ttt

\ I L)n surtace

\1€

F iRST PAGE PROOFS

N

728

N

\_

N

\_

l_"| ):.,."O.",l-, IloL.\|

")\'',.n,;'/).n,

_l-'

)o -s i

\o-F i

o-s i(.

so-s i

5)

o-J-o-zr_o_

Io

N

N

nv

I ro. f

O-P-I

\ ^ r ,\ J N

r'\\J

\ ,or-o-P-,{. -oH

TJ

t ro- - D -

, / ! '

N\otu

O ^ o .- J-o- zr-o-p>" ( )-*| -\

\J 'i*/1-

o \ ^' \_y'-

RePeat Un i t

Figure 52.1S po iar or ienta t ions o f cnromophores us ing a ikoxysr iane ls and zr rcon ium phosphonates

muih laver aooroaches.

monolaver during formauon by means of a templating scheme-l5E-l6l As in the electrochemicai experi-

nents descnbed above, eveo a -smail number of detects can complicare a-nd cooceivably domlnate the

respoose in such eiecuochemical sensors. Embedding holes or unusualty shaped species rn the monolayer

is a hindrance ro rhe close oackins required for elecuochem.ical isolation oi the elecuode from the soluuon.

6 . 1 1 N O N E L E C T R O C H E M I C A L S E N S O R S,{l though rhe ma;onrv of efforr based on deveiopmenc of sensors hls been focused on eiectrochemrcal

sysrems, SAMs have also been demonsuated as the basis tor oonelectrochemicai sensors. A combinauon

of panerned Sfu\{s and condensarion t-rgures provide an approacir to a group of optical sensors. in

which dit fracrion provides rhe basis for signai decection. '62 In this system, a paltern of hvdrophil ic and

hydrophobic l ines wirh micromerer rvidths are formed on a ref lect ive gold subsrate. When the relattve

humrci i tv of the envr.ronment and the remoerature oI the subsuate are suff icienr tor condensacion on the

suriace. rhe l iquici warer i i im is iniciat lv cont-rned ro hydrophil ic regions. When these resions are

Repeat Un i t

F IRST PAGE PROOFS

729

prererenrrai lv wer. ihe surtace 3crs as a di i fract ion eraung; when the surface is dr-v, i t does not di i fract.

The oucleacion anci prosression of condensatioo can thus be [ol lowed using the deveLopmeoc oidif i raci ion

in this system.

6 .12 MULT ILAYER SYSTEMS FOR OPTICAL COATINGS

A number of systens have been surveyed for their abit iry to torm muit i layer coaungs. Alkanethiolates

on gold are obviously nor easi ly appi icable for rhis purpose, but alkanephosphonates and svs', .r :ms mixing

aikanephosphonares wirh orher rypes of surface functionai groues (siloxanes, for example) iorm good

mult i layer slsrems.]g7.r63 Si loxan6-based systems, although more dif f icult to deal with because of the

water sensiciviry oi the components, also can be used to form multilayers.a2'{3'16't'l6J Polar orieotacion

of molecules in a mulr i layered sysrem is oi panicular interest for even-ordered noolinear oPticai effects

such as second harmoorc generarion- Sucb schemes are showo below. Recently, the defecuveness o[

such multilayers has come under scrutiny. Parricuiarly, the zirconium phosphonale systems, although

userul, are hardiy pcrfecr.t6 The next task in the design of multrlayered systems will be to compensate

for detects.

7 . C O N C L U S I O N S

Self-assembled monolayers have assumed an Lnporranc place i-n surtace science as the best model

sysrcms with which ro srudy organic surface phenomena. The best-characterized system, and one wirh

many ar t racr ive characcenst ics , is thar o [ se l f -assembled mooolayers o [ funct iona l ized a lkaneth io la tes

on evaporated goid fr lms. This system is easi ly and f ler ibly assembled, modif,red, and patterned, end

ir has orher aru'acuve properries such as opticai transparency (for thin, < 200 A, gold coaungs),rnnr t , ,nr i r r i r r r . - ,J ?ood s tab i l i ty a t room lenaperarure and in concact wi th so lu f ions. One of he mostl v L t u u L L t Y t L ) , e t \ 1 :

acrracive characierisr ics is rhar the gold used as the subsuace does not require manipulat ioo in lugh

vacuum berween irs iormarion and i ts exposure to the alkanettrrol; rhe abi l i ty of the thiol to remove

welkiy adsorbeC species dunng tormarion of the Sfu\f makes this rype of chemisuv accessible to

laborarories rhar do not have access to high-vacuum systems. Most recentiy, the abi l i ty lo pattern these

matenals conven-rently ooens up nerv aoproaches in the design and nampuiauon of surface cbemlstnes.

Some of the eariy successes of rhis control have been i l lusiraceC here. More mature tech-nologies wil l

certarnly ioi low.

ACKNOWLEDG MENT

We wouid l ike ro chank Flans Biebuyck f lor many helpful discussiocs and references.

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l l . D . L . A l l a ra and R . G . N" "s . Langmutn l . 52 (1985) .

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