7t52
Reprinted
from the Journal
of the American
Chemical Society, 1991, l.lJ.
by permission of the copyright
Copyright O 1991 by the American Chemical Society and reprinted
owner.
Comparisonof the Structuresand Wetting Propertiesof
Monolayersof n-Alkanethiolson the Coinage
Self-Assembled
Metal Surfaces,Cu, Ag, Aul
DavidL. Allara.*'t Yu-Tai Tao,l'$
Paul E. Laibinis,t'2GeorgeM. Whitesides,*'t
Atul N. Parikh,l and RalphG. Nuzzo*'#
Contributionfrom the Departmentof Chemisny,Haruard Uniuerstty,
Cambridge,Massachusetts02l,38,Departmentsof Materials Scienceand Chemisty,
16802,Departmentsof Materials
Pennsyluania
State Uniuersity,UniuersityPark, Pennsylt',ania
Scienceand Chemistry,Uniuersityof Illinois at Urbana-Champaign,Urbana,Illinois 6/,80/,,and
January14, I99l
AT&T Bell Laboratories,
Murrav Hill, NewJerser'07974.Receiued
Abstrect Long-chainalkanethiols,HS(CH2)nCHl, adsorbfrom solutiononto the surfacesof gold, silver,and copperand form
monolayers.Reflectioninfrared spectroscopy
indicatesthat monolayerson silverand on copper(when carefully prepared)
havethe chainsin welldefined molecularorientationsand in crystalline-likephasestates,as hasbeenobservedon gold. Monolayers
on silverare structurallyrelatedto thoseformedby adsorptionon gold,but differentin detailsof orientation.The monolayers
formedon copperare structurallymore complexand showa pronouncedsensitivityto the detailsof the samplepreparation.
Quantitativeanalysisof the IR data usingnumericalsimulationsbasedon an averagesinglechain modelsuggeststhat the
alkyl chainsin monolayers
on silverare all-transzig-zagand cantedby - l2o from the normalto the surface.The analysis
a twist of the planecontainingthe carbonbackboneof -45o from the planedefinedby the tilt and surfacenormal
alsosuggests
vectors.For comparison,
asjudgedby their vibrational
cn goldsurfaces,
the monolayers
that form on adsorptionof alkanethiols
. a v ea c a n ta n g l e
s p e c t r aa,r ea l s ot r a n sz i g - z a ge x t e n d ebdu t .w h e ni n t e r p r e t eidn t h ec o n t e r to f t h es a m es r n g l ec h a i nm o d e l h
of -2'7o and a twistof the planeof the carbonbackbone
of -53o The monolarcrsiormedon copper(whentheyare obtained
i n h i g h q u a l i t y )e x h i b i ti n f r a r e ds p e c t r ae f i e c t i v e l iyn d i s t i n g u r s h a bi rloen rt h o s co n : r l i e r a n d t h u sa p p e a rt o h a v et h e s a m e
andappearto containsignificant
densities
structure.Filmson @pperarealsocommonllobtainedthatarestructuraii\ill-det'ined
from wettingand XPS
of gaucheconformations.
Thesespectroscopically'based
interpretations
arecompatible*rth inferences
of the alkyl groups
measurements.
The structureof the substrate-sulfur
interfaceappearsto controlmolecularorientations
in thesefilms. An improvedstructuralmodel,incorporatinga two-chainunit cell and allowingfor the temperature-dependent
populationof gaucheconformations,is presentedand appliedto the specificcaseof the structuresformed on gold.
Introduction
This paperdescribes
the preparationand characterization
of
(SAMs) formedby the chemisorption
self-assembled
monolayers
(HS(CHz),R)on surfaces
of alkanethiols
of copperand silver.
Bothof thesemetalsurfaces
arehighlyactivein thechemisorption
of organosulfur
compounds.HTheyofferan excellent
opportunitl
to studytheinfluences
of thesubstrate
in defrningandcontrolling
the orderof self-assembled
monolayer
films. A centralobjective
of this studywasto comparethe structures
of the monolayers
formedon silverand copperwith thoseformedby chemisorption
of alkanethiols
on Bold.r-rr The molecular
orientation
andorderingof long-chain
alkyl thiolatemonolayers
on goldreflects
the epitaxialbondingof the sulfur atoms(or, perhapsmore
precisely,of gold thiolates)to the surfacelattice plane. For
long-chainalkyl disulfidesand thiolschemisorbed
on Au( I l I )
(the preferredcrystallographic
textureof typicalpolycrystalline
samples),
electrondiffraction(TEM and LEED)r0hasestablished
that the sulfuratomsin the adsorbates
form (V3 x y'3)R30'
werlaycn. Thisepitaxialoverlayer
resultsin an interchainspacing
of 4.99A andis wellsuitedto the formationof close-packed,
and
thereforeordered,organicsurfacephaseswhenthe alkyl chain
is long (n > 15)and the R grouproughlycommensurate
in size
with the polymethylene
chain.ll'12Severalrecentstudiesdiscuss
theseissues.?-13
Copper,silver,andgoldhavenearestneighborspacings
in the
( l l l ) p l a n eo f 2 . 5 6A , 2 . 8 9A , a n d2 . 8 8A , r e s p e c t i v e l y .Irfa
structures
form on copperand silverthat are analogous
to those
on gold(i.e.,(y'3 x /3)R30o), we expectthe molecular
orient HarvardUniversity.
I Pcnnsylvania
StateUnivcrsity.
I Pcrmanent
Address:Institutcof Chemistry,
Academia
Sinica,Taipei,
Taiwan,ROC.
# AT&T Bcll Laboratories;
corrcspondcncc
shouldbc addrcssed
to Departmcntof MatcrialsScience
and Engineering,
University
of lllinoisat
Urbana-Champaign,
Urbana,IL 61801.
(tona
tationsof the chainsto differ significantlysincethe averagechain
densityon copperwould haveto be considerablyhigher than on
gold or silver( - 17.03A2Tchainon copperversus21.75l*2lchain
, iven
o n s i l v e ra n d 1 1 . 6 2A 2 , t c h a i no n g o l d ) . W e w o u l d e x p e c t g
( l ) T h r sh r g h l rc o l l a b o r a t r vweo r k w a ss u p p o r t e d
b y a v a r i e t yo f s o u r c e s .
G . M W a c k n o w l e d g essu p p o r tf r o m t h e O i f i c e o f N a v a l R e s e a r c ht ,h e D e s g e n c y ( D A R P A ) , a n c it h e N a t i o n a l
f e n s eA d v a n c e dR e s e a r c hP r o . l e c tA
S c i e n c eF o u n d a t i o n( G r a n t C H E - 8 8 - 1 2 1 0 9 ) .T h e X P S s p e c t r o m e t ewr a s
obtainedthroughthe DARPA UniversityResearchInitiativeand maintained
in the Harvard Materials ResearchLaboratory. D.L.A. and A.N.P. acknowledgesupport in part from the National ScienceFoundation(Grant
DMR-9001270).
(2) Shell FoundationPre-doctoralFellow, 1987-88.
(3) Seymour,D. L.; Bao,S.; McConville,C. F.;Crapper,M. D.;Woodruff,
, . E.;
D . P . ; J o n e s ,R . G . S u y ' S c r . 1 9 8 7 , 1 8 9 - 1 9 0 , 5 2 9 - 5 3 4 .A n d e r s o nS
Nyberg, C. L. J. Electron Spectrosc.Relat. Phenom. 19X1,52,135-'146.
(4) Sandroff, C. J.; Herschbach,D. R. /. Phys. Chem. 19t2, 86,
3277-3279.
(5) Sandroff,C. J.l Garoff, S.; Leung,K. P. Chem.Phys.ktt.l9t3, 96,
547-551. Joo, T. H.; Kim, K.; Kim, M. S. "/. Phys. Chem. 19t5,90,
R. L,; Bryant,M. A.; Pemberton,
J. E. "/. Am. Chem.
58l6-58 19. Sobocinski,
Soc. 1990.I 12, 6l'17-6183.
(6) Kwon,C. K.; Kim, K.: Kim, M. S.; Lee,S. B. 8rll, KoreanChem.Scr.
19t9. /0. 254-258.
(7) Nuzzo,R. G.l Allara, D. L. J, Am. Chem.Soc. 1983, 105,4481-4483.
( 8 ) B a i n ,C . D . ; T r o u g h t o nE, . B . ; T a o ,Y . - T . ;E v a l l ,J . ; W h i t e s i d e sG,. M . ;
N u z z o ,R . G . / . A m . C h e m .S o c . 1 9 8 9 ,1 l l , 3 2 l - 3 3 5 .
( 9 ) P o r t e r ,M . D . ; B r i g h t ,T . B , ; A l l a r a , D . L . ; C h i d s e yC
, . E. D. J. Am.
C h e m .S o c . 1 9 t 7 , i 0 9 , 3 5 5 9 - 3 5 6 8 ,
( 1 0 ) T E M : S t r o n g ,L , ; W h i t e s i d e sG, . M . L a n g m u i r l 9 t t , 4 , 5 4 6 - 5 5 8 .
L E E D : D u b o i s ,L . H . ; N u z z o ,R . G . U n p u b l i s h e d
results.
( I I ) Chidsey,C. E. D.t Liu, G.-Y.; Rowntrec,P.; Scoles,G. J. Chem.Phys.
1 9 t 9 , 9 / , 4 4 2 1 - 4 4 2 3 . C a m i l l o n e ,N , , I I I ; C h i d s e y ,C . E . D . ; L i u , G . - Y . ;
Putvinski,T. M.; Scoles,G. J. Chem. Phys. 1991,94,8493-8502.
( 1 2 ) N u z z o ,R . G . ; D u b o i s ,L . H . ; A l l a r a , D . L , / . A m . C h e m .S o c .1 9 9 0 ,
t t 2, 558-564.
( 1 3 ) W h i t e s i d e sG, . M . ; L a i b i n i s , P . E .L a n g m u i r1 9 f r , 6 , 8 7 - 9 6 . B a i n ,
C . D . ; W h i t e s i d e sG, . M . A n g e w .C h e m , , l n t .E d . E n g l . 1 9 8 9 ,I 0 l , 5 2 2 - 5 2 8 .
( 1 4 ) K i t t e l , C . S o l i d S t a t e P h y s i c s5, t h e d . tW i l e y : N e w Y o r k , 1 9 7 6 ;p
32.
(f^^i^r,,
Monola.yers
of n-Alkanethiolson Cu, Ag, and Au Surfaces
thisconstraint,
that the chainson copperwouldbealignedcloser
to thesurfacenormaldirectionthanthoseon silveror gold. The
interchainspacingin the orthorhombic
yields
form of polyethylene
an average
chaindensityin theab planeof - 18.27A2lchain.r5
Th e e pit ax ialbondingo f a l k y lth i o l a teisn a (V3 x V3 )R 30"
overlayeron copperthereforemay be precludedby the steric
packingconstraints
imposedby the chains.Silverand gold,on
the otherhand.haveverysimilarlatticeconstants,
andwe expect
the monolayers
formedon themto havesimilarstructures
in the
absence
of otherperturbalions.
The studiesdescribedheredemonstrate
that the structures
formedby similaradsorbates
on polycrystalline
copper,silver.and
gold surfaceswith a preferred( I I I ) textureare relatedbut
different. The structuralcharacteristics
of monolayers
formed
on copperare extremelysensitive
to the historyof the samples
and the detailsof the experimental
techniques
usedin their
preparations.The bestfilms we haveobtainedon copperwere
comprisedof closestpackedchainsthat containedfew gauche
conformations.The poorerquality films containedsignificant
cnncentrations
of gaucheconformations.
The monolayers
obtained
on silverare highlyorganized
andcontainonly low concentrations
of gaucheconformations.The molecularorientations
deduced
from the data are compatiblewith surfacephasesfor alkanethiolateson silverand copperthat are close-packed
and have
srgnificantly
shorterinterchainspacings
thanthat foundon gold.
We presentevidence,
both from our studiesand from the literature,that the differences
in the structuresof the monolayers
fbrmedon copper,silver,and gold reflectthe differences
in the
reactivityof thesurfaceof the metal.particularly
its susceptibility
to oxidationon exposure
to laboratory
air. Thesurfaces
of copper
o xi d i z er apidly l6( and m a y a l s oa d s o rbo th e re n v i ro n mental
co n ta m inant dur
s ) ingp re p a ra ti oonf th e s a m p l e .S i l v eral so
oxidizes
on exflosure
to thelaboratory
atmosphere.
but oridation
i s su b s t ant ialli
les sex te n s i ' ,'thea n fo r c o p p e r:u n d e ra mbi ent
conditions,
oneor twomonolavers
of silveroxideformon thesilver
s u r f a c e . rT
' h ea d s o r p t i oonf a l k a n e t h i ool n
s s i l v e irs c o m p l e x
andinvolves.
we believe.
somecombination
of reduction
of Ag(l)
to Ag(0), directconversion
of Ag(l) surfaceoxidesto Ag(I)
thiolates,
andformationof an ionicsurfacephasein whichsilver
atomshavebeenremovedfrom their equilibriumlatticepositions.
We useprecedents
established
by LEED studiesto derivethis
modeland to rationalize
the orientations
determined
by infrared
sp€ctroscopy.
The data alsosuggestthat, on prolongedexposure
to theadsorbate
solution,the surfacebecomes
extensively
sulfided,
presumably
forminga thin interfacial
layerof Ag2S.rt Golddoes
not oxidizeunderambientconditions.
and the mechanism
of
conversion
of alkanethiols
to gold(l)alkanethiolates
hasstill not
beenestablished
unequivocally.re
The highactivityof silverand
copperto oxygenandto thiolsestablished
procedures
the assembly
we usedfor reproducible
formationof high-quality
monolayers.
The lengthof exposure
of the unfunctionalized
surfaceto the
atmosphere,
the solubilityof the adsorbate
in the contacring
so l ve n and
t , t het im eof e x p o s u re
to th es o l u ti o n
o f a l k a n ethi ol -variables that are generallyunimportantin the reproducible
assembiy
of alkanethiols
on gold-were veryimportanton copper
and moderately
importanton silver.
Results
Phenomenological
Differencesin the Assemblyof Alkanethiols
on Copper,Silver,and Gold. The assemblyof high-qualityalkanethiolate
monolayers
on goldis straightforward.T-13'20'21
Silver
(15) Bunn,C.W. Trans,FaradaySoc. 1939,35,482-491.Walter,E. R.;
R c d i n g ,F . P . / . P o l y m ,S c i . t 9 5 6 , 2 t , 5 6 1 - 5 6 2 .
( 1 6 ) L a w l e s sK, . R . ; G w a t h m e yA, . T . A c t a M e t a l l . 1 9 5 6 , 4 , 1 5 3 - 1 6 3 .
( 1 7 ) R o v i d a ,G . ; P r a t e s i F
, . ; M a g l i e t t a ,M . ; F e r r o n i , E , S u r f .S c i , 1 9 7 4 ,
43,23U256. Rovida,G,; Pratesi,F. Surf. Scd.1975, 52,542:5SS, Englehardt, H. A.; Menzel,D. Surf..Sci. 1976,57,591-618 and references
cited
therein.
(l.q) The promotionof silver tarnishingby sulfur-containingcompounds
i s w e l l k n o w n . S e e ,f o r e x a m p l e :G r a c d e l , T .E . ; F r a n e y J, . p , ; b a u l t i e r i , G .
J . ; K a m m l o t t ,K , W . ; M a l m , D . L . C o n o s . . S c j .l g t 5 ; 2 J , l t 6 3 * l l E 0 a n d
references
cited therein.
_ ( ! q l A n o x i d i z i n gc o r e a c t a nits n o t r e q u i r e d :D u b o i s ,L . H . : N u z z o ,R .
O. Unpublished
resulrs,
J . A m . C h e m . S o c . , ' , ' a l I. l 3 , N o . 1 9 ,I 9 9 l
7153
and copper,in contrast.readilv rorm oxidesthat adsorb polar
contaminants. We suspectthe surfacesof thesemetals becorne
rougherand more heterogeneous
*'ith ertendedexposureto air.
Oxidized surfacesof copperand sih'er are acriveto the chemisorptionof alkanethiols;the monola.vers
formed differ. howsysr,
in structure and propertiesfrom those formed on lessoridizeC
s u r f a c e s .T h e d i f f e r e n c e cs a n b e r a t i o n a l i z e db r h r p o t h c s i z r n g
that the monolayerhas formed on a roughenedmetal meral c''rrde
interphaseand may itself be structurallyheterogeneousThe most
r e p r o d u c i b l ea n d h i g h e s tq u a l i t y m o n o l a y e r s* e h a r e o b r a r n c d
on silver and copperresultedfrom proceduresin uhrch ireshly
evaporatedfilms were transferredrapidly under inert armtxpheres
i n t o a d s o r b a t e - c o n t a i n i nsgo l u t i o n s , M a n i p u l a r i o no i u n f u n c t i o n a l i z e ds u b s t r a t e si n a i r , p r i o r t o d e r i v a t i z a t i o nr.e s u l t e di n
monolayersthat exhibitedlarger hysteresesin contact anglesand
a p p e a r e dl e s sc r y s t a l l i n eb y I R a n d c o m p o s i t i o n a l l lve s sh o m o geneousby XPS.
W e b e g i n b y c o m m e n t i n go n t h e i n f l u e n c eo f t h e s o l r e n to n
the monolayersformed from thiols dissolvedin them.:: \le
exposedslidesof the three metalsto I mM octadecanethiol
solutions in a range of solvents (isooctane.chloroform. tetrahydrofuran,acetonitrile,acetone,and ethano!)for 2 to 4 h and
measuredthe advancingand recedingcontactangles(0^ and 0,)
of water and hexadecane(HD). The slideswere transferredfrom
the evaporatorinto solutionsunder flowing argon to minimize
effects due to laboratory exposure. In general, we observedno
significant differencesin wetting (or hysteresis= cos d, - cos 0r)
that could be attributed to the solvent(wetting data are available
in the supplementarymaterial). Monolayersformed on copper
exhibitedgreaterhvsteresisthan thoseformed on gold or silver.
T h e w e t t a b i l r t i c .o f -t h c n o n o l a l ' e r sf o r m e do n a l l t h r e e m e t a l s
are stmil,irtt) thr-xcrr'pxrrted
for orientedalkvl monolayers
on other
s u b s t r a t e' sl ' r ' \ P S d a t a a l s od e m o n s t r a tteh a t t h e s o i v e n t s
l i s t e da b o rc r r s r r n r c i c t ie\ t o r ia r d t h e s u b s t r a t eus n d e r t h e s e
conditions.
Copprer
and silverdid proveto be more reactiveto alkanethiols
and more sensitiveto attributesof the alkanethiolthan gold. For
e x a m p l e ,r e a c t i o no f s h o r ta l k a n e t h i o l s( n < 3 ) w i t h t h e c o p p e r
and silver surfacesoften resultedin surfacesthat appeareddiscoloredand sometimesvisibly pitted; continuedexposureseemed
to result in dissolutionof the metal. With increasedchain length,
the reproducibilityof formation of high-qualitymonolayers(as
determinedby wetting and XPS) increaseddramatically.2TWith
longer alkanethiols.the solubility of the adsorbatebecameimportant. Dcrcosanethiol.
which is solublein ethanolwith warming,
often formed multilayeredstructureson copperwhen adsorbed
f r o m e t h a n o l : 2a8d s o r p t i c nf r o m i s o o c t a n e
r e p r o d u c i b l vr e s u l t e d
i n n r o n o l e r e fri l m s
{ d s o r p t i o nt i n r e so f I t o l 2 h y r e l d e dm o n o l a y e r w
s ith the
g r e a t e s rt e p r o d u c i b i l i t ru; e u s e dt h e s ei n t e r v a l si n t h e s t u d i e s
( 2 0 ) B a r n ,C , D . ; E v a l l ,J . ; W h i t e s i d e sG, . M . J . A m . C h e m .S o c . 1 9 t 9 .
ilt,7t55-7t64.
( 2 1 ) L a i b i n i s ,P . E . l H i c k m a n .J . J . ; W r i g h t o n ,M . S . ; W h i t e s i d e sG, . M .
Science(llashington, DC) 1989, 245,845-847.
(22) The natureof the solventusedhasbeenshownto affect the wettability
of monolayerson gold derivedfrom polar-terminatedalkanethiols.mExtension
of thesesystemsto the generationof high-energyorganic interfacesrequires
the assemblybe compatiblewith polar, hydrogen-bonding
solvents.
( 2 3 ) C o h e n ,S . R . ; N a a m a n , R . ; S a g i v ,J . J . P h y s . C h e m . 1 9 8 6 , 9 0 ,
3 0 5 4 - 3 0 5 6T
. i l l m a n ,N . ; U l m a n ,A . ; S c h i l d k r a u tJ, . S . ; P e n n e rT, . C . J . A m .
Chem..Soc.l9tt, I 10,6136-4144. Wasserman,S. R.; Tao, Y.-T.; Whitesides,
G . M . L a n g m u i r1 9 t 9 , J , 1 0 7 4 - 1 0 8 7 .
(24) Allara, D. L.; Nuzzo, R. G. Iangmuir 1985, /, 45-52.
(25) Chen, S. H.; Frank, C. W. I"angmuir 19t9, J, 978-987.
( 2 6 ) T r o u g h t o n ,E . B . ; B a i n , C . D . ; W h i t e s i d e sG, . M . ; N u z z o , R , G , ;
A l l a r a , D . L . ; P o r t e r ,M , D . L a n g m u i r t 9 8 8 , 4 , 3 6 5 - 3 8 5 .
(27) We believethat the alkane providesa passivatinglayer that slows
corrosionof the copperand silver by the alkanethiols.
(28) The films formed from ethanolwere extremelyhydrophobic0fro l30o), exhibitedlarge hystereses,
and were wet by hexadecane.We bclievc
the wetting behaviorto be consistentwith formation of a porous,precipitated
layer of alkyl thiol andfor disulfide. The films preparedin ethanolcould bc
grown to the point of beingvisibleand were robustto washingwith ethanol;
washingwith hexanesresultedin dissolutionbut did not revealan underlying
monolayerfilm that was not wet by hexadecane,This problemwasmore acute
on copperthan on silver;it was also more prevalentat lower temperatures.
Laibinis et al
I99I
of
and XPS BindingEnergies
TableL WettingProperties
Adsorbedon Copper,Silver,and
Monolayersof Octadecanethiol
Gold and of RelatedMaterials
s38 53'
173
528 523
165
wetting(0", 0r),o
155
Hzo
20
10
0
(t
I
o
70 As/S(cHJ,rcH. 'i]ot"r
x
o 60
:l
P
50
J
40
o
o 30
20
.= 1 0
o
c
0
o
+t
5 70
60
50
40
30
20
10
0
zl-*^^"*-l
c
1000
'3
I E1
o].__'----]ll
lFl:z
,l o
r75
165
1ss
HD
XPS binding
(eV)D
energies
S(2p:iz) S(2s)
NA
t63.6. 217.9
NA
HSC22H4s
NA
162.6d
NA
AurSC,5HrT
t62.t
226.2
1 2 0 ,t 0 3 4 8 , 3 2
Cu/SC1sH37
t62.3 225.7
I 1 6 ,1 0 3 5 0 ,4 0
Ag/SC1sH37
l6l .9
226.4
I 1 5 ,1 0 5 4 8 ,3 6
Au/SC16H37
< 1 5 ,- '
167.4
231.6
Ag.Ag2O + HOrSCr6H33 9 6 , 6 l
oHD = hexadecane.
for all
NA = not applicable.bBindingenergies
ref
werereferenced
to C(ts; = 284.80eV. 'For comparison,
samples
to C(ts; = 284,7 eY).
3l reportsa value of 163.3eV (referenced
dReference
should
31. Gold(l) thiolatesare polymericspecies;caution
be exercisedin comparingtheir spectralpropertiesdirectly with those
obtainedon monolayers.'A dashindicatesa recedingconstantangle
for a contactingliquid that couldnot be removedfrom the surface;d, ::
00.
800
600
400
200
BindingEnergy(eV)
Figuret. XPSspectra
adsorbed
on
of monolayers
of octadecanethiol
surfaces
of copper,
silver,andgold.High-resolution
spectra
of theO(I s)
to
andS(2p)regionsarepresented
in the insetsandwerereferenced
forcomparison
of the
C(I s) = 284.80
eV. Thedashed
linesareprovided
aregivenin
S(2p)spectra.
Thebinding
energies
of S(2s)andS(2p372)
withvalues
TableI andcompared
obtained
for docosanethiol.
whosedescriptions
follow. Greaterreproducibility
on silverand
whenisooctane
copperwasobtainedwith longern-alkanethiols
wasus edast hes ol v e not w i n gto th e i n c re a s esdo l u bi l i tlof
' the
adsorbates
studiedin this solvent, Adsorptions
onto copper
surfaceswereconducted
from isooctane;
adsorptions
onto the
surfacaof goldandsilverusedethanolor isooctane,
No structural
differences
in the high-quality
on any metal
films wereobserved
that could be assignedto solvent;thosedifferencesthat were
we attributeto the solubilityof the adsorbateor to
observed,
surfaceroughening.
As indicated,
to minimizethe effectsof oxidationand conwe transferred
tamination,
the slidesof copperandsilverto nitrogen-purged
solutionsof the thiolsundera flowingstreamof
argonimmediatelyfollowingthe evaporation
of the metal. Occasionally,
this procedure
did not resultin formationof highqualitymonolayers
on copper. We believe,however,that in(background
pressure
strumentalparameters
for the evaporator
duringthe deposition,
argonflow rate,etc.)wereresponsible
for
thesefailuresand stressthe importance
of excludingoxygen(to
the extentpossiblc)
to form monolayers
on copperof the quality
we havecharacterized.
Silvercouldbe exposed
to air, to a limited
degree,withoutexhibitingthe catastrophic
failuresfoundfor
copper. We decided,basedon the insensitivityof gold to the
atmosphere,
that anaerobictransferwasunnecessary;
the films
weremanipulatedin air and transferredto solutionwithin I h
of evaporation.
The monolayers
formedon all threemetalsarestableto physical
manipulation
and washingwith polarsolvents.The monolayers
on copperand silverdo, however,
exhibitonly limitedstability
to air (changes
are observed
after - I weekof exposure),2e
but
(29)Laibinis,
P. E.;Whitesides,
C. M. Unpublished
results.
couldbe routinelymanipulatedin wayspreviouslydetailedfor
on gold.E
monolayers
assembled
alkanethiolate
of
ScanningElectronMicroscopy(SEM). The morphologies
were
after exposureto octadecanethiol
the varioussubstrates
of the
electronmicroscopy.The surfaces
examinedby scanning
whenpreoared
conditions,
are
undercomparable
threesubstrates.
appearto ha vet hesam e
si mi l ar.Thecopperandsi l versampl es
' rol l i nghrl l s-tcrturethat hasbeendescri bed
for film sof gold
w afers.s
due
Theamountof roughening
ontosrl i con
evaporated
wit h
to oxi dati onor reacti onof the copperor si l versu r f aces
morphodoesnot appearto induceany perceptible
alkanethiols
of SEM ( -20f500 A).
at the levelof resolution
logicalchanges
(XPS). Representative
XPS
Spectroscopy
X-ray Photoelectron
on
obtainedby reactionof octadecanethiol
spectraof monolayers
copper,silver,andgoldaregivenin Figurel. The mostsignificant
featurein thesespectrais the contentof impuritiesof the films,
of peaksin the O(ls) corelevel
as inferredfrom the intensities
region(bindingenergy-535 eV). Clearly,the alkanethiolate
containing
a
on an interphase
monolayer
on copperis adsorbed
copper
quantitycrfoxide.Treatment
of highlyoxidized
significant
in
w i th thi ol sresul ts,
asj udgedby X P S ,i n a decr ease
surfaces
lossof peaksdueto Cu(tl).D
anda complete
thecontentof ox1'gen
remains
high,
of thecopp€rinterface
oxlgencontent
Theresidual
of near that onl y a parti aldi spl acem ent
suggesti ng
how ever,
The
to the adsorbate.
surfaceoxlgenatomsoccurson exposure
on silveris very low,
residualoxidecontentof the monolayer
perhapsof the orderof a few percentof a monolayer.The adanyoxidation
sorptionof thiol on silverthusmusteitherreverse
that hasoccurredor displacea significantfraction
of thesubstrate
generated
of this point
by it. Furtherdiscussion
of oxygenspecies
section.On gold,the oxygen
will be deferredto the Discussion
contentof the film is belowthe detectionlimit of XPS. The
characterof the sulfur-surfacebond is indicatedby the highcoreleveldatain the S(2p)region(TableI and Figure
resolution
levelon
measured
for the 2p312core
l). The bindingenergies
copper,silver,andgold( I 62.I , 162.3,and I 6l .9 eV, respectively)
for a surfacethiolate(RS-M
areall wellwithintherangeexpected
Thesevalueswouldimplicatea formal
or RS-M+)species.3o'3r
of the S-H bondon the metalon all three
dissociative
adsorption
(eq 1;.1'o'lt
substrates
(la)
RSH + M(0) -* RS-M(I) + "Y2H2"
RSH+ M(l) * RS-M(|) + *H+'
(lb)
We can definethe metal-sulfurbondingin moredetail by exof theS(2p)corelevels.In carryingout
aminingthelineshapes
(30) Nuzzo,R. G.; Zcgarski,
B. R.; Dubois,L. H. J. Am. Chem.Soc.
't
t9t7 . t 09, 33-14A.
(31)Bain,C. D,;Biebuyck,
19t9,J,
H, A,; Whitesides,
G, M. Langmuir
723-727.
Monolayersof n-Alkanethiolson Cu, Ag, and Au Surfaces
J . A m . C h e m . S o c . ,V o l . I 1 3 , N o . 1 9 , I 9 9 l
7155
the roughnessof the two surfacesthat is not detectableby SEM.
In generalterms, however,the wetting measurementssuggestthat
s s o E!8 6
exposea low-energy
the monolayersare all oriented,and that the.v90"
and. from the perspectiveof the outermost
ogo o o o o E
methyl surfaces'23-26
few Angstroms,appear to be similar.
Investigationof the Thicknessof the Monohyer Films by El600
lipsometry and XPS. l. Ellipsometry. Ellipsometr! is an inva'
of alkanethiols
luable techniquefor measuringiilm thicknesses
300
0o
1.0
adsorbedon gold.7-e We attempted to measurethe changein
thicknesson samplesof'copper and silver that had beenexposed
-0.5
120
to air for only short intervals((15 min) prior to exposureto an
alkanethiol solution. We could, with considerableeffort (see
\r/
900
ExperimentalSection),obtain thicknesseson silver that could be
0.0
related to the length of the adsorbate;ralues *ell outsidethe
expectedrange of thicknesswere, however.also routinelv obuined.
600
o 0.5
nts. we are
Given the lack of reproducibilityin our measureme
oarsgo o
o9ogoo
to make any structural conclusionsbasedon relating
hesistant
30"
ellipsometricthicknessto the chain length of the adsorbate'
0o
1.0
Acceptable thicknessescould not be obtained on copp€r.
-0.5
120.
2. XPS Attenuation. XPS offers an independentmethod to
-;[r[ltlllt
compare thicknessesof monolayerspreparedby direct exposure
of the metal to an alkanethiol. In contrast with ellipsometry,
90"
characterizationof the unfunctionalizedinterfaceis not required'
*thin' layer by
The determinationof the absolutethicknessof a
600
XPS requires,at a minimum, precisevaluesof escapedepthsand
atomic cross sections,parameters difficult to obtain with the
30'
required accuracy. XPS does provide, however,a convenientand
0o
1.0
rapid method for comparing differences in relative thicknesses
over a homologousseriessuch as that representedby the n-aln
kanethiols. The logarithm of the intensity,/y, of photoelectrons
(CHr(CHr)SH)
of n-alkanethiols
Figure2. Wettingof monolayers
f r o m a s e m i i n f i n i t el a y e r ( > 2 0 0 A ) a t t e n u a t e db y a n n - a l k y l
(upper),
silver(center),
andgold(lower)by*ater
oncopper
adsorbed
sf
o r e r l a l e r i s p r o p o r t i o n atlo - ( n d l \ , w h e r ed i s t h e t h i c k n e s o
(circles).
(squares)
angles
are
Advancing
andreceding
andhexadecane
the monolalerper methllenegroup,n is the numberof methylene
theaverage
respectively.
F:ch pointrepresents
filledandopensymbols,
g r o u p s i n e a c h c h a i n , a n d ) , i s t h e a t t e n u a t i o nl e n g t h o f t h e
of copper
Exposure
Section).
valueobtained
ona slide(seeExperimental
photoeiectronsfrom the underlying substratethrough hydrogoldwas
priorto immersionl
wasminimized
andsilverto theatmosphere
A plot of ln (intensity) versusn providesboth a
carbons.33
withinI h of exposure
to the
solution
in air to adsorbate
transferred
of the regularity of the thicknessof monqualitative assessment
of alto I mM solutions
Silvcrandgoldwereexposed
atmosphere.
under
for 4-12h at roomtemperature
isooctane
in ethanolor
olayers prepared from different length alkanethiolsand a comkanethiol
blowndry withN2,
rinsedwithethanol,
argonto formthemonolayer;
parison betweenmonolayersformed on different metals. Because
wereadsorbed
fromisooctane the escapedepth (I) is also a function of the kinetic energy of
monolayers
oncopper
andcharacterized;
usingsimilarprocedures.
we selectedspecific core level peaks based
the photoelectron,3a
section
and proximity to peak of other metals.
cross
their
on
both
in which
it is necessary
to usedatafrom analyses
thisexamination,
photoelectronsarising from the underlying
of
intensity
The
spectra
hasbeenminimized.The core-level
the X-ray exposure
substrateof kineticenergiesof - I I 25 eV (Au(4drrr) = I 134eV,
monolayers
degrade
on prolonged
indicatethat thealkanethiolate
A g ( 3 d 5z ) = 1 1 1 9e V ) a n d - 1 4 0 0 e V ( A u ( 4 f 7 , 2=) 1 4 0 2e V ,
exposure
to X-rays. The spectraon silverand gold can be fit
C u ( 3 p ) = l 4 l 2 e \ ' ; a r e p l o t t e di n F i g u r e3 f o r m o n o l a y e rpsr e well, evengiventhe noisein the spectra.as two
reasonably
of differentlengths.3sTheseplos follow
paredfrom n-alkanethiols
components
of equalwidth (1.0eV) whoseintespin-orbit-spin
I
o
g
a
r
i
t
h
m
r
cr e l a t i o n s h i p .T h e i n t e n s i t yo f p h o t o t
h
e
e
x
p
e
c
t
e
d
|
=
(i
.e
.,
g ra te dar easar ein t hera ti o2 J + |
I :2 ;J
1 2 ,3 1 ).The
o n s i l v e ra n d c o p p e ra t t e n u a t em o r e
m
o
n
o
l
a
l
e
r
s
f
o
r
e
l
e
c
t
r
o
n
s
used(1.15eV) wastakenfromtheliterature.32
spinrrbit splitting
(by - 20Vc)than for monola)'erson gold. This difference
rapidly
couldalsobe fit
on copp€roccasionally
The spectrafor samples
suggeststhat the films formed on copperand silver are each -20Vo
-0.5
rrllrrlti
r;lrrilril
1
I 20.
.c![33'i !]:AS;
';:F:.;
..5:;il:5
e
o
o
;84il:E:
lioooaoEo
g
Ss6:.gg6
r"a8ffi368
withintheseconstraints.Spectrawereroutinelyobtainedwhere
r .2 -1 .4e V ) a ndthe
th e l i newidt hswer es i g n i fi c a n tlgyre a te(1
significantlypoorerthan
resolutionbetweenthe two components
thoseobtainedon goldor silversamples.Thereis no unambiguous
that
however,
wayto interprettheselargerlinewidths;we suspect,
heterogeneity.
they reflectcompositional
WettingProperties.Static advancingand recedingcontact
(HD) weremeasured
on monoanglesof waterandhexadecane
in solutionsof
layerspreparedby the immersionof substrates
n-alkanethiols
of variouslength(Figure2). The wettingbehaviors
on the threemetalsaresimilar,with thecontactanglesof both
lowerwhenn < 8 thanwhenn 2 l l.
liquidsbeingsignificantly
formedon gold
for monolayers
Porteret al. haveshownpreviously
changes
that the inferreddegreeof crystallinityof monolayers
of the relationsbein this region.eWe deferfurtherdiscussion
tweenwettingandstructuralfeaturesto laterin the manuscript.
(A cosd = cos0, - cosd.) is smallestfor
The observedhysteresis
formedon goldandslightlylargeron silverandcopper.
monolayers
hysteresis
may reflectan underlyingdifferencein
This increased
thicker than thoseformed on gold, This observationis consistent
with the differencesin molecularorientation infened from infrared
spectroscopy(see below).
Investigation of Film Structure and Molecular Orientrtion by
IR Vibrational Spectroocopy. l. Obsened Spectra of Monolayers
IR spectraof the various monolayerswere obtained using a single
reflection geometry (-85o angle of incidence) as describedin
the ExperimentalSectionusing ppolarized light. In earlier papers,
quantitative analyses of such data have been presented for
monolayersof various substituted alkanethiols and dialkyl disulfides on gold.r2'36 The present study seeks to present a
quantitative correlation of thin-film structure as exhibited in a
homologousseriesof adsorbatesacrosstheserelated substrates.
To facilitate these analyses,the previouscomputational proto(33) Bain,C. D.;Whitesides,
93,167f 1673'
G. M, J. Phys.Chem.19t9,
(34)Seah,M. P.; Dench,W. A. Surf.InterfaceAnal. 1979,/,2-ll.
(C5-C1s)
on
(35)A similarslopewasobtained
adsorbcd
for alkanethiols
contact
significant
exhibited
copperfromethanoleventhoughthemonolayers
anglehysteresis,
(36)Nuzzo,R. G.; Fusco,
F. A.; Allara,D, L. J. Am, Chem.Soc.19t7,
t09.2358-2367.
7156 J. Am. Chem.Soc.,Vol. II3, No. 19,l99l
Laibinis et al.
- C22H45S,
Alkyl-S ChainsC16H33S
TebleII. Description
of VibrationalModesof C-H Stretchingfor Polycrystalline
mode descriptionD
peakfrequency(cm-r)'
C H 3a s y m( i p ) ,( r " - )
CH3 asym(op),(ru-)
C H 3s y m ,r + ( F R C )
C H 2a s y m d, - ( a )
C H 2a s y m d, CH2sym,d+ (FRC)
CH3sym,r+
CH2 sym,d* (o)
CH2 sym,d*
2964
2954
2935
2925
2918
2895-2907
2879
2853
2850
assigneddirection of transition dipole
moment matrix element in molecular coordinates
ip of CCC backbone,I C-CH3 bond
plane
l- CCC backbone
ll to C-CH: bond
plane
l- CCC backbone
plane
-L CCC backbone
plane,ip HCH plane
ip CCC backbone
ll to C-CHr bond
plane,ip HCH plane
ip CCC backbone
plane,ip HCH plane
ip CCC backbone
oB.scd on data takcn for
'Abbrcviationsuscd: asym = asymmctricor antisym.
samples
for
used
optical
function
determinations.
[C,H2r rS]2
mctdc, sym = symmctric,ip = in planc,op = out of plane.FRC = Fermi r€sonance
splittingcomponent.a = CHr groupslocatedat cithcr end of
=
the alkyl chain,ll psr.llcl, l- = p.rpcndicular.'Pcak frequencyis takenas that from the imaginaryopticalfunction(t) spectrum.Peakfrcqucncy
valuesfot modcsof intcnsiticstoo wcakto measurcdirectlyhavcbeenassignedusingaveragevaluestakenfrom previouslycompiledrssignments(rcf
9, 12,36,37), Thescvalueswerc suboequently
usedin the fittin8 routinesusedto generatethe components
ofthe simulatedspectra(secsupplcmcntarYmaterial).
6
5
4
F*r
Oo
!-Oc x9 1
tr
6
K,3
x5
,/a
4
v - - -
V..
AuglTpl
--tl-..-.
r'
\ot
l
r
(
cu(3p)
10
a)t
15
20
25
n
Figre 3, Intensityof photoelectrons
dueto the underlyingsubetrates
for
monolayerson gold, silver,and coppcrformedafter I h exposureto I
(CH3(CH2)/SH),The upperfigure
mM solutions
of n-alkanethiols
providescomparison
of monolayers
formedon goldand silver,the lower
on goldandcoppcr.The pointsrepresent
singlemeasurements.
The data
setshavebeenoffsetverticallyto enablecomparison;
the slopeis the
parameterthat is importanthere. The kineticenergies
of the photo= I 134eV,
clectrons
in eachgraphare similar: (upper)Au(4d372)
= 1402eV; Cu(3p)-- l4l2 eY.
Ag(3d57)= I l l9 eV; (lower)Au(4f772)
Lineswereobtainedby least-squares
analysisand gavethe following
-0.021:Ag(3drrr),-0.025;Au(4f772),
slopes(seetext): Au(4d372),
{.020: Cu(3p),4.024.
colslll6'37were modified (seebelow). Further, sincewe anticipatod
that the differences between specific adsorbatesof interest to us
might bc small, our earlier work was reexaminedwith the intention
of minimizing expcrimental errors that complicate a precise
quantitative comparison of the data obtained from different
substratematerials. The analysesthat follow are basedon multiple
sample preparationsconductedin threc different laboratoriesand
spcctral data acquired on two different instruments. The largest
variations we observed for comparable instrument optics are
compatible with the statistical variation inherent in multiple
samples. The error bars appearing in the figures reflect the scatter
dominated by effectsduc to indepcndentmeasurementof multiple
samples. lt is important to note that the data shown below reflect
(37)Allara,D. L.; Nuzzo,R. G, Langmuir1985,/, 52-66,
our specificoptical design. Large differencesin absolutespectral
intensity will result if even slightly different beam divergences
and angles of incidenceare employed.ss
The infrared data for the seriesof monolayerson copper,silver,
and gold are shown in Figure 4. The data shown for copperare
for the more well-definedstructureswe have obtained (a representativeexampleof spectraobtained for ill-defined monolayers
on copperis given in the supplementarymaterial). The setsof
data revealsubstantialvanationsin the C-H vibrational mode
i n t e n s i t i eas s a f u n c t i o no f c h a i n l e n g t hf o r g o l d a s c o m p a r e dt o
t h e o t h e r t w o : \ s t e m so f m o n o l a y e r (sc o p p e r s, i l v e r ) . G i v e n t h e
closesimilariti olthe copperdata to that of silver,we will restrict
our discussion
to the better-behaved
silversystem(althoughsimilar
would pertainfor this specificcopperdata set). We
discussions
will discussfor gold the CH3 mode intensitiesfirst and follow with
an examinationof those of the methylenes. The basisof the
assignmentof the spectralfeaturesto specificvibrational modes
has been reported previouslyl2'36'37
and will not be repeatedhere.
Details of these fairly complex fittings and assignmentsare given
in the supplementarymaterial. Table II presentsexact descriptions
of the assignments,the approximatepositionof the band, as well
as transition dipole directions for the mode as expressedin molecular coordinates. The latter information will be used for
calculationsof structure discussedin the next section. Our
quantitativeinterpretationsof molecularorientationswill be restricted to analy'ses
basedon modesappearingin this high-frequencyregion. The lower frequencymodes,while rich in information (ser below),are sufficientlyweak in intensityas to preclude
q u a n t i t a t i v ea n a l y s i s .
The strongestof the CH3 modesin this region of the spectrum
is the symmetric methyl stretch (r+), which is split by Fermi
resonanceinto two separatebands appearing at -2878 and
-2935 cm-r, and the in-planeasymmetricstretch (r.-) appearing
at2964 cm-r. The out-of-planeasymmetric stretch (16-),which
occursprominentlyat 2954cm-r in the polycrystallinebulk phase
spectra,is weak in the monolayer spectra; this mode is a weak
shoulderon the r"- band. The line shapesdo not appear to vary
significantly acrossthis seriesof monolayers,allowing band intensities to be estimated for most bands (ru- excepted) by the
intensityat the peak maximum. Figure 5 showsthe intensitydata
of the r+ (2877 cm-r) and rr- (2963 cffi-r, containsa small contribution from 16-absorption) bands for the Cru-Czofilms on gold
and silver. On gold, the data are given normalizedrelativeto the
value of the intensity of the selectedmode for the C16film. The
theoretical plots shown for gold of the modulation of these deviations in intensity are basedsolely on the changesin geometry
of the CH3 group predicted for two possible models of chain
(3E) In particular, the angleof incidencemust be correctlyset within less
than lo, thc//number of the beam must be fixed at a high value, -//15 f 120for our two instruments,and stray light must bc excludedfrom collection
of the reflectedbeamoff the sample. We (D.L.A., R.G.N.) observcdfluctuations as much as a factor of 2 bctween misaligncd, fast optics and thc
configuration above. Additional deviationscould also involvedctector nonlinearitiesfor high throughputconfigurations.
t 1 3 ,N o . j , 9 ,I 9 9 l
Monolayersof n-Alkanethiolson Cu, Ag, and Au Surfaces
AuiS(CH.)nCHs I
lgls(CH2)nCH3
Cu/S(CH2LCH3
7157
I
I
f o.oor
o
o
(g
.ct
L
o
-€
,i
n*
/1 ^-\/l
"\*-NV
^ ^./1
Q
tt
^a_r
A'/V V \*
il
n hilV
^J
IV\J
\
r-
^^A/\
v\
lvw
rll
2800
2900
3000
2800
2900
3000
Wavenumbers(cm'l)
I '
2800
2900
3000
Figure4, IR spectraof monolayers
derivedfrom exposure
(CH3(CHJ,TSH).
of surfaces
of copper,silver,and gold to n-alkanethiols
structureand not on changesin the vibrationalstates. These
modelswill be discussed
in detaillater in the manuscript.The
experimentally
observedfluctuationsin odd-ven intensitiesare
striking.It is alsoobviousthat thisexperimental
behaviorpresents
an importantcriterionfor determining
the validityof various
structuralmodels.In an earlierpublication,l2
thesevariations
werenotedqualitatively
and usedto assigna constantfixeddirectionof chaintilt with a consequent
odd-evendependence
of
the orientation
of the C-CH3 bondwith respectto the normal
to thesurface.This pointwill bediscussed
in detaillaterin the
manuscript.
Figure5 alsoshowsthat the r+ and rr- bandintensities
(absolute)on silverexhibit,to withinexperimental
error,nosignificant
modulationwith chainlength.Thisdifference
between
the spectra
obtainedon silver(andsimilarlyon copper)andon goldstrongly
suggests
a differencein structureof monolayers
on thesesupports;
thoseon silver(andcopper)indicatethat the projectionof the
C-CH3 bondon the normalto thesurfaceis invariantwith chain
length.
The C-H stretchingmodeintensities(absolute)of the CH2
groupd* andd- modesexhibita lineardependence
on chainlength
for monolayers
supportedon gold and silver(plotsare givenin
the supplementary
materials).3eNo odd-evenvariationsare
withinthe experimental
evident,suggesting,
uncertainty
of the
measurement,
that thereexistsa constantchainorientation
on
thesurfacefor thesefilms,onethat differsin magnitude
for silver
and gold.
2, Ileterminationof OpticalFunctions.Descriptions
of the
methodsusedfor determiningpreciseopticalfunctions,an overview
of relevanterroranalysis,
and representative
data usedin the
theoretical
modelling
aregivenin the supplementary
materials.
3. Calcuhtionof AverrgeMolecularOrientrtionin Monolayer
Films on Copper,Silver,rnd Gold. The theoreticalbasisfor
derivingan average
orientationof molecules
in monolayers
from
IR spectraof metalsupported
filmshasbeenpresented
in detail
elsewhere.3T
As presented
then,theessentiat
operations
involved
were( I ) thedetermination
of a setof isotropicopticalfunctions
applicable
to the monolayerfilms;{ (2) the experimental
deter(39) Linear least-squares
fits to thc intensiticsof the d+ and d- modes
vcrsuschain length for monolayensupportcdon gold and silvcr intcrsectzcro
intcnsityat chain lcngthssignificantlygreaterthan zero, Plotsand further
discussionare includcdin thc supplcmcntarymatcrials,
minationof the thicknessof the frlm from ellipsometry(or other
independent
method);(3) simulationof an experimentally
appropriatemonolayerspectrum(i.e., accountingfor reflection
geometry,
(4) assignment
etc.)usingtheaboveparameters;
of each
absorption
featureto a singleor combination
of contributing
vibrational
modes.
eachpossessing
a matrixelement
conesponding
to a transition
dipnlemomentu'itha specific
directionin molecular
(-i) asrgnment
coordinates:
of thediscrepancies
in bandintensities
b e t u e e tnh ei s o t r o p sr cr m u l a t i oann dt h ee x p e r i m etnota v e r a g e
o n e n t a t r o on fse a c ho i t h ee l e m e n ot si t h et r a n s i t i odni p o l em o ment5uslnga irameuorkof classical
electromagnetic
theory;and
(6) converti ng
thedi pol eonentati ons
i n mol ecul ar
coo r dinat es
geometr)'
vi a transi ti on
to a mol ecul ar
on thesurface
dipoledi(Tabl eII), It i s, of cour se,
recti ons
i n mol ecul ar
coordi nates
cri ti calthat thereexi stsa transferabi l i betw
ti
een
the opt ical
functions
used(derivedfroma bulkstate)andthoserelevant
to
thephasestateof themonolayer.
Errorsin theellipsometrically
determined
thickness
are alsoof concern,
although,for typical
errorsof lessthan severalAngstroms,
the influenceon the determinedorientations
is generallysmall(valuesof thickness
used
are givenin the Experimental
Section).
The procedure
usedfor numericalcomparison
in the present
studydiffersin significantdetailfrom that reportedpreviously.
The presentstudyutilizescompletespectralsimulations;
a descriptiveoverview
of the methodis givenin the supplementary
materi al ,
Simulationswereperformedfor a wide rangeof surfacegeometriesof the adsorbed
molecules.The rangeof structuresto
be considered
was narrowedsomewhatby notingthat the experimentalspectra(presented
earlier)suggest
the placement
of
thechainsin a crystalline-like
environment.
Indeed,it hasbeen
demonstrated
by severaltechniques
that the monolayers
on gold
areordered.l0'll
Therefore,
with alkylchainsin
onlystructures
an all-transconformations
wereconsidered
initially. In thelatter
portionsof the manuscript,
we will presentsimulations
in which
this all-transconstraint
hasbeenrelaxedto allowgaucheconformations
at theterminalCH2group. With regardto the present
focuson all-transchains,thecoppersystempresents
a significant
challengeto analysis.The bestsamplesgive spectravirtually
identicalwith thosefoundon silver. Their analysiswouldthus
(40) Tablesof the optical functionsfor all the alkanethiolsreportedin this
s t u d y( C , 5 - C 2 qa n d C 2 2 )a r e a v a i l a b l eo n r e q u e s (t D . L . A . ) .
j l 3 , N o . / , 9 l, 9 9 l
7158
-o-r:exp-er'exp i
te
CH.(CHr)nS/Au
- r ' r i s c m - or ' s c m
x
-.-r:lcm -a-r'tcm
\2
o
E
Laibinis et al.
(5 t-:,
d
El
or
o
qo
cE
ctl
o
'14
18
cH3(cH2)ns/As
rfl
2.O
o
o
:
..J
lo
1.5
6
E
E
ot 1,0
o
?
chainin ann-alkanethiols
diagram
of anall-trans
Figure6. Schematic
Thecoordinate
system
usedto definetheorimonolayer
ona surface.
of thechain,thecantanglea, andthechaintwist0 areshown
entations
coordinates.
to thesurface
alongwiththeirrelationship
conventionwas adoptedin which the metal-S-C geometryis
for bothtvpesclfchains.The figureasdrawndescribes
conserved
tilt for a chainwith
oi theC-CHr bondasa positive
thedirection
modes,
r* andr,-, for n-alof methylstretching
Figure
5. Intensities
an odd total numberof carbonatoms(evennumberof CH2
(CH3(CH2)"SH)
adsorbed
ongoldandsilver Theexperikanethiols
thusrequires
that a andB bespecified
values
fromtheunresolved groups).Theconvention
of r"- contain
a smallcontribution
mental
Theupperpaneldisplays
experimental
and
of ther5-modes.
intensities
for an evenchainin whichtheinitialC-CHr bondis tiltedin the
(normalized
found
to thebandintensities
intensity
deviations
calculated
(negative)
direction.
opposite
on gold)of the methyl
of hexadecanethiol
adsorbed
for a monolayer
The data abovesuggestthat all the chainlengthsexamined
adsorbed formedself-consistent
r+ (open)
andr.- (filled),for n-alkanethiols
modes,
stretching
structures. In the interestsof brevity,
models
intensities
onvarious
structural
arebased
ongold.Thecalculated
seriesof spectraare presented"
value),
scm(single-chain therefore,fits to only a selected
in thetext:exp(experimental
thatarediscussed
of monolayers
will
first
the
molecular
orientations
We
consider
(two-chain
theexperimodel).Thelowerpaneldisplays
model),
tcm
on silver.and thencomparetheseto thoseon gold. For all the
adsorbed
onsilver.The
of r+andrr- for n-alkanethiols
mentalintensities
in the simulationmodels,excellentfits to the
andasymmetric casesconsidered
in theintensities
of thesymmetric
lackof anychange
spectracouldbe foundfor theCH2 C-H stretching
experimental
modes
for oddandevenvalues
of n onsilversharply
methylstretching
(d+ and d-). In no case,however,
ongold(upperpanel).
contrasts
thatobserved
coulda
modeadsorptions
modes(r+,
convergence
of the CH3C-H stretching
simultaneous
r.-. 16-)beachieved:
thetotalerrorscouldbeminimizedto values
not
bethat sameasthat whichfollowsfor silverandis therefore
-20-30vcof thetotalmethylbandintensities
(see
approaching
materials
characterized
by'thetype
repeated.
The "disordered"
fittingerrors,best
discussion
above).On thebasisof theobserved
materialcannotbetreated
of spectrashownin thesupplementary
is definedin termsof thed+ andd- modes,which
fit convergence
initiallyimposed
on thisanalysis,
namely,
undertheconstraints
phasefrom which we can
of thechains
thetilt (a) andtwist(0) angles
areusedto determine
that theredoesnot exista reference
(seeFigure6), It is importantto notethat the symmetryof the
derivewith certaintysuitableopticalfunctions;the spectraare
the useof thed+ and
CH2groupsaboutthe chainaxisprecludes
neithercrystallinenor liquid-likein character.The datasuggest
d- modesto determine
the absolute
signof the tilt angle,a" This
trans
that thereexistsin thesematerialssomefractionof extended
whichestablishes
latterparameter,
the structuralfeaturesof the
densities
of gaucheconformations.
segments
aswellasconsiderable
maybepresent
film surface(i.e.,thedirectionat whichthe terminalC-C bond
of adsorbate
This mightimplythat lowercoverages
protrudes
into theambient),hasbeenarrivedat usingobservations
on thesesurfacesthan is to be foundon thosebearingclosest
packedchains.By assuming
of the odd-evenbehaviorof the CH3 modeintensities
and will
a coverage,
the weightingof thisfit
as domainsof liquid and solid-likematerialcouldproducea
be detailedbelow.
with the bestfits for a C1sthiolate
natureof these
The errorsassociated
simulation,
but,giventhe ill-defined
convergent
phases,
monolayer
on silverare illustratedin Figure7. The analysis
thereseemslittlejustificationfor doingso.
diagramof an all-transchainas
suggests
that thesematerialsare,in fact,well modeledby the
Figure6 showsa schematic
wellas the coordinate
systemusedto definethe orientations
of
opticalconstants
of the bulk phaseand exhibita smallbut appreciable
cantangle(a) of l 3o. The stiffness
of the fit is clearly
thechains.The chaintilt angle,a, is definedasthe cantof the
polymethylene
chainrelativeto the
demonstrated
longaxisof thetrans-zig-zag
by theobservation
that variationof a andB by even
*2o (not shown)greatlydegrades
the fit. The fits to the other
surfacenormaldirection(z). The chaintwist,p, is the twistof
the planeof thechain(*p, counterclockwise
rotation)aroundthe
silvermonolayer
datasetswereequallygoodwith a rangeof the
valuesof the cantangles(lal) varyingfrom I lo (C,e)
chainaxis. Thiscoordinate
system,basedon the typicalEuler
absolute
convention,
is sufficientto describe
the chaingeometry
of a unto l40 (Czo);the twistangles,
B, werealsosimilarrangingfrom
iaxialsystem(recallthat onlythe z component
of the transition
42oto 44o (Cre).Thesevariations
arewellwithintheaccuracy
givesriseto absorption;
dipolemoments
all x andy orientations
of this method.Inspection
of the datain Figure7 alsoreveals
and thus the systemis uniaxial), ln orderto
that the signof the cant angleis stronglyconstrained
are equivalent
by the
describeboth odd and evenchainswith this onediagram,the
simulations.Eventhoughthe absoluteintensityof the methyl
14
19
Monolayersof n- Alkanethiolson Cu, Ag, and Au Surfaces
J . A m . C h e m .S o c . V
, o l .I 1 3 , N o . 1 9 ,I 9 9 l
Ca 6H s 7S H/ A 9
Ca6H57SH
/Au
T
I
7159
I o.oor
I
I
I
a=-26
B = 5 21
=-13
=42
E
o
E
o
tr
(r
or
o
E
(tt
o
o =13
--42
B
:"\
/'
^/1
l\
it',^r,
\!w
2900
3000
W A V E N U M B(E
cm
RS
- {)
(lower)andcalculated
Figure7. Experimental
IR spectra
of octadecanethiol
adsorbed
onsilver.Calculated
spectra
having
cantangles,
c,
of -l3o (upper)and+l3o (lower),
andtwistangles,
S, of 42oare
presented.
Thedifferences
in thecalculated
sp€ctra
suggest
thata is
-l3o (asdefined
by Figure6). Specific
interpretations
arediscussed
in
thetext.Thecalculated
spectra
arebased
ona single
chainmodel(see
te xt).
2800
2900
3000
CR
MS- { )
W A V E N U M B( E
.
(lower)andcalculated
of octadeIR spectra
Figure8. Experimental
a,
spectra
havingcantangles,
ongold.Calculated
canethiol
adsorbed
p, of 52oare
of -26o (upper)and *260 (lower),andtwistangles,
provide
presented.
equallygoodfits thatdo not
Thetwosimulations
spectra
arebased
ona
assignment
of a. Thecalculated
allowabsolute
model(seetext).
single-chain
2800
the singleall-transchainstructurederivedfor n-alkanethiolate
cantangle,a, in a narrowrange
on goldhasan average
monolayers
ar e
c mul at ions
of 26-28oanda tri i st.J. of -52-55o. S peci fisi
monolayer
for both
shownin Frgure8 for an octadecanethiolate
values
data
of a aswellastheexperimental
positive
andnegative
datafor this
As before,thecalculated
for reasons
of comparison.
yieldsfits in a narrowrangeof valuesfor
particularmonolayer
a (shownherefor a = *26o) andB (52o);changingthesevalues
the qualityof the fits for the d+ and
by *,2o seriouslydegrades
d- modes.
On first glance,the data in Figure8 might be construedas
indicatingthat the propervalueof the cantangleis a = -26o,
closerfor thissignof
the fit to methyl16-mde beingsomewhat
in the two fits,
a. The weighted
errorsareactuallycomparable
whenonetakesproperaccountof the totalintensityof the r+, r.-,
and 16-modes.Thesespecificsimulationsthusare insufficient
to choosebetweenthe signsof a. The larger
in andof themselves
a basison which
thiols)doesprovide,however,
dataset(Cro-Czz
of the methylC-H
The intensities
to makesuchan assignment.
stretching
modesfluctuatewith varyingchainlength(Figure5),
revealingthat the surfaceprojectionof the methylgroupsis not
groupto thechain;
conserved
asoneaddsan additionalmethylene
although
oddandevenchainsyielddifferentmethylorientations
appearto besimilar. The
withintheselattergroupsthestructures
character
of theseintensity
simulations
revealthat thequalitative
within the confinesof the singlefluctuations
are bestdescribed
a positivesignto the chaincantangle,
chainmodelby assigning
for all chainlengths.We have
a (Figure6), whichis conserved
notedin an earlierpublicationthat thereexistsignificantperfor group located
turbationsof opticalfunctionsof the monolayer
phase.l2'al
at the ambientinterfacerelativeto the bulk reference
Theseeffectsare sufficientlylarge evenfor the fairly simple,
interactions
by dispersion
nonpolarmethylgroup(intermolecular
methyl
of an average
forces)soasto preventdefinitivecalculations
grouporientation.This perturbationcouldinvolvein principle
boththe magnitudeanddirectionof the oscillatingdipoles.On
that the directionsdo not change,a simplecalthe assumption
p[vs.
(41)Nuzzo,R, G,; Korenic,
E. M.: Dubois,
L, H. /. Chem,
t990.
dataasto thechanges
culationcanbemadefrom theexperimental
93,767-713
(42)Hautman,
J.;Klein,M.L. J. Am,Chem.
representation
Phys,19t9,9l,,4994-SCol. in intensity
requiredto allowtheexactquantitative
modessuggests
the importance
of effectsotherthan orientation
(especially
in determining
the bandintensities
r6-).the fits fail
valuesof a. Thisresult.whenviewed
mostprofoundly
for positive
in thecontextof thedatapresented
in Figure5, suggests
that the
surfaceprojection
of the methylgroupdoesnot vary in the silver-supported
monolayers
asonechanges
the lengthof thechain.
Stateddifferently,thefits, giuenthe corwentioru
adoptedin Figure
6, require negatiDesignsof a for odd numbersof methylene
groupsand positiueonesfor euennumbers.Discussion
of the
importof this observation
is deferredto laterin the manuscript.
Whereasthe spectralintensities
of the monolayers
formedon
silvercouldbe fit by the useof a single-chain
model,viz.,one
average
chaingeometryrepresenting
the structureof all the chains
in the monolayer
ensemble,
the goldsystemrequiresthe considerationof morecomplexmodels.In recentstudies,
it hasbeen
foundthat the spectraof thesemonolayers
are stronglytemperaturedependent
and revealtwo importantstructuralfeatures.al
Fi rst, t he c hains ,eve n th o u g h w e l l d e s c ri b e da s bei ng
'crystalline-like",
quantitiesof
do indeedcontainappreciable
gaucheconformations;
thesearecon@ntrated
at thechaintermini.
Second,spcctraat low temperature
exhibitfactorgroupsplittings
whichsuggest
that,at leastat T S 220K, the orderedstructure
adoptedcontainstwo chainsper unit cell. Theseobservations
are
supportedby recentmoleculardynamicssimulationson this
monolayersystem.42Thus,it is requiredfrom the outsetthat
the modelsusedincorporate
two-chaingeometries
andallowthe
populationof terminalCH2 gaucheconformations.Because
of
placedon the accuracy
thedemands
of the spectralsimulations
by suchmodelsto distinguishthe subtlenumericaldifferences
expccted,
addedemphasis
wasplacedon the measurement
and
useof improvedn, k datasets,dataunavailable
prior to this work
(seeaboveand supplementary
material).
We startwith a consideration
of thesimplestcasc,a single-chain
model. Usinga bestfit criteriabasedon the d+ and d- modcs,
Laibinis et al.
7160 J. Am. Chem.Soc.,Vol. II3, No. /,9,I99l
of CH3 C-H
of a specificdataset. The calculatedmodulations
shownin Figure5 werederivedon the
stretchingmodeintensities
basisthat the Au-S-CH2 bondangleis constantfor all chain
lengths.On this basis,the only differencebetweenodd and even
of the C-CH3 bondwith respect
chainscanbe the orientations
to the surfaceand,hence,the transitionmomentdirectionsof the
the latter are
r+, rr-, and 16-modes. With theseassumptions,
exactlydefinedfor bothoddandevenchainsfor a chosentilt/twist
(Figure6), andtheratioof the modeintensities
can
configuration
be exactlycalculatedfor an evenchainversusan oddon the basis
of the changein cos2dr, (where0r, is the angleof the transition
momentwith respectto z axisfor a givenmodeof a particular
modulishownin Figure5 were
chainlength3T).
The calculated
in r"- andr* are
changes
derivedin thismanner(thecalculated
referenced
for theevenchain). It is evidentthat
to theintensities
the discrepancy
is quitesubstantial.Most of this discrepancy
underllng thesimpleall-trans
app€ars
to bedueto theassumption
single-chain
model.
As hasbeenalreadynoted,the useof a twechainmodelis more
with currentevidence
that supportsa low-temperature
consistent
phase
structuresimilarto a bimolecular
monoclinic
hydrocarbon
with the planesof the C-C backbone
at -90o to oneanother.al
weredeveloped
From this startingpoint,two structuralclasses
structurewith
for spectralsimulations:( I ) the low-temperature
two contributingaveragechainstructures,eachof equalpopulation
with all-transconformations;
and (2) the high-temperature
structure,identicalwith the aboveall-transmodelexceptfor
allowingthepopulation
of bothtransandgaucheconformations
of the terminalCH2groupsof bothchainsin an independent.
u n c or r elat ed
f as hio n .a s
from the fittingprocedure
is that
Onedistinctresultobtained
a familyof structurcs
all givcnearlyidentical
minimumerrorfits
to theCH2 modesfor valuesof d boundcdby 0 S d < 82o'*'ith
e a c hv alueof d as s o c i a tewdi th a s p e c i fi p
c a i ro f 0 t,02 val ues.
For d > 82o,the fits worsen.Sincethe structureof a orthorhombiccrystalline
subcellwouldrequired = 90o,theupperlimit
valueof Q = 82owasselected
asthe mostphysicallyrealvalue.{
For thisconstraint,
the all-trans,two-chainmodelyieldschain
twistsof 50 and-48o. The corresponding
simulatedspectrum,
(a3) To dcscribcthe geometryof thesemodels,the following conventions
wcre adoptcd. For a givcn pair of chains,an anglc betwccnplanesof thc chain,
d, was selectedand thc common axis betweenthe planesallowed to rotate
(equivalentto twistingthc chainsin conccrtby thc sameamount)and tilt away
from the surfaccnormal. It followsthat, for a pair of chainsindexedas I and
2, 6 = l1r - p2l and at = d2 = o. Sincc the planc of the chainscan bc rotated
by l80o with no effect on the optical responseof the CH, modes,one can
cquivalentlyspecifyd as l80o - l0r - dzl. As a conventionwe choosethe
smallestvalueof d, The stratcgyfor thc bcst fit-simulationproccduresfor
both modelsinvolved: (l) fixed a to bc that determinedfrom the bcst fit of
the singlechain model (given above)and (2) variationof p1 and p2 indcpcndently between*90o and -90o (one ncgative value of p is rcquired to
producethe proper methyl surfacecorrugation;sce below). In addition, for
the gauche-allowedmodel, 0r and p2 for thc bcst fit all-trans model were
sclcctcdand fixcd while a largc variety ( - I d) of conformational populations
were scarchedindcpcndcntlyfor cach chain in a collcctionof 20 indcpcndent
unit subcclls(2 chainspcr unit subccll). Thc monolaycrthus was reprcscntcd
by 40 chains (20 with p; and 20 with pr), with each chain indepcndcntly
populatingone of three conformationalgeometries,trans, gauche(l), and
gauche(2), with cach conformationexhibitinga different contributionto
spcctral intcnsity.
(44) Thc numerical prccisionin thesccalculationsis such that a rangeof
{ valucsbctween-75 to t5o (or equivalentlyl80o - (76 to 85o) = 104to
95o) will give a rcasonablcfit to the intensiticsof thc CH2 modes. Thcrc is
an intcrestingand simplcgcometricrclationshipthat can bc dcrivcdrclating
thc twist angleof a singlechain model,p,, to thc twist anglcsof the two chain
model, p1 and 02, for thc condition that thc overall geometric factors (scc
supplementarymaterial, footnote7) for thc d+ and d- modcs,Gd' (a. d) and
Go (o. d), cach bc idcntical, rcspcctivcly,in both models:
l9 r- B z=l c s l o (d ,\-2 p .)
(2)
In linewithour earlierconvention
of choosing
d, Q(p,)= 0o whenQoS 0,
( 45oando(0,) - l80owhcn45o3 P. < 90o, Thccondition
requircd
by
thisincquality
is thatthc modclsgivcidcnticalsimulatcd
CH2spcctra.Thus,
for an orthorhombic
unit subccllwith d - 90o,it followsthat p. - 45oand
all two-chainmodelscan bc rcplaccdby oncsinglc-chain
modcl. Anothcr
rcsultis that for 9r= 52o,theaveragc
rcsultdcrivcdfor a singlc-chain
twist
on goldin thetcxt,thc incqualityrcquires6 S 760,cl6c to thc rangcfound
numerically
of -76-85o.
/Au
CasH3TSH
ALL - TRANS
T W O C H A I NM O D E L
a=26
F = 50 AND-48
o
(l
(r
o
o
G A U C H E. A L L O W E D
T W O C H A I NM O D E L
Q=26
I = 5O AND-48
I
Io oor
EXP.
2800
3000
2900
W A V E N U M B E R(Sc m - { )
Figure 9. Comparisonof experimental(lower) and calculatedspectraof
octadecanethioladsorbedon gold, The calculated spectraare basedon
a two'chain model The charnswere assumedto be all-transin the upper
s i m u i a t i o na n d a l l o w e dt o a d o p t g a u c h ec o n f o r m a t i o n sa t t h e t e r m i n a l
m e t h l l e n e g r o u p r n t h e l o w e r . S p e c i f i cd i s c u s s i o nas r e p r e s e n t e di n t h e
l e xl .
AcFd)
,,,
>o
5rE
!1,
Ac(3p)
Erh
1 1h
th
a
1n 1Ca 101
Blndlng En rgy
(d)
6m5mtm3m2mlm0
BlndlngEnergy
(ev)
Figure10. XPS spectraof silversamplesexposedto I mM solutionsof
for l, ll, and 37 h each. With continued
octadecanethiol
in isooctane
exposure,
the sulfursignalsbecomemoreintensewith "growth"of peaks
(seeinset);no otherchanges
in the spcctrum
at lowerbindingenergies
to solution
wereobserved.The S(2p)spectrumfor the sampleexposed
components
of
for 37 h couldnot be fit with the two spin-orbital-split
easily
species.This spectrumwas,however,
a singlesulfur-containing
onepair resembled
thc
fit by two pairsof spin-orbital-splitcomponents;
spectrumgivenfor l-h exposuresuggesting
that the thiolateremains
presentwhile an additionalspecies(oneat lowerbindingenergy)had
been incorporatcd. We bclicvc the ncw sulfur-containingspeciesto
correspondto the formationof a thin film of Ag2S at thc silver/
of the sampleswereessentially
monolayerinterface.The wettabilities
filmson themare
indistinguishable
andconfirmedthat the hydrocarbon
oriented.
shown in Figure 8, while producing an excellentfit for the CH2
modcs,clearly yields a poor fit to the intensity profiles of CH3
J. ,-1trt t ht'nt
Monoiayers ai n-Alkanethiois on Cu, ,4g, und Au SurJaces
modes(indecd.oncsno betterthan the single-chainmodeli. Using
t h e s ef i x e d v a l u e so f a , 0 r , a n d B 2 w h i c h p r o v i d eq u a n t i t a t i v e
simulation of the CH2 modes,the fitting proc€ssrevealeda second
distinct result,namely,that a large number of bestfits to the CH3
m o d e se x i s t w i t h n e a r l y a r a n d o m m i x t u r e o f a l l o w e dg a u c h e
populations. ln the above,the worst fit to the total intensityof
the CH, modesis obtainedfor zero gauchestates,viz., the all-trans
model above. The calculatedspectrum shown in Figure 9 is
derived from a model with a total conformer population of 45Vc
gauche and 55Votrans in the collectionof 40 chainssampled. In
general,evena small percentof gauchestatescan decreasethe
fitting error associatedwith the CH3 modesby a t'actorof -5.
It shouldbe noted that. of course.certain combinationsof conformational statesgive poor fits but none worse than the all-trans
model. Thus, randomizationof terminal conformationscan only
lead to improvedfits. The final point to note is that no combination of conformationalstatessampledcould providea lessthan
- ljVo error betweenthe spectralintensitiesof the calculatedand
observedtotal CH3 modes.
I t m u s t b e e m p h a s i z e dt h, e r e f o r et,h a t p r e c r s eq u a n l i t a t i ( ) nt o
better than *10% requiresconsiderationof perturbiitionsirf the
opticalconstants
of the CH3 group at the airlmonolayerinterlace
relativeto the CH3 group in a crystallinereferencematerral.Such
to be in thc range
effectsprobablyexistand shouldbe considered
of - i A7oof ihe t<ltalvaluesof k 1assuggested
by,the fittrng errcrrs
above). The significantpoint remains,however.that the bestfits
a r e c o n s i s t e nwt i t h a p o p u l a t i o no f g a u c h ec o n f o r m a t i o n as t t h e
c h a i n e n d s ,a s i n d e p e n d e net x p e r i m e n t s u g g e s e
t xrst.a5
Influence of Extended Immersion Times or Exposure to Air on
the Nature of Self-AssembledMonolayers on Silver and Copper.
We haveobservedthat, while the nature of the monolayersformed
on gold are insensitiveto changesin the immersiontimes in the
adsorbatesolution over periodsof severalweeks,those formed on
silver and coppercan differ in considerabledetail. We have begun
to examinethesefeaturesin greaterdetail and can report some
initial findings, Extendedexposureof copperand silversurfaces
t o s o l u t i o n sc o n t a i n i n ga l k a n e t h i o l sr e s u l t si n f i l m s t h a t a p p e a r
t o o t h i c k b y e l l i p s o m e t r yt;h e t h i c k n e s s eisn f e r r e cai r e t o o l a r g e
t o b e e x p l a i n e db y a d s o r p t i o no f a m o n o l a l e r ( s e ea b o v e ; . \ \ ' e
f o u n d , b y X P S , t h a t t h e f i l m s f o r m e d o n s i l v e r .a f t e r e x t e n d e d
exposureto the adsorbatesolution,containedno ox)'genand the
amount of carbonwas similar to that found on samplesexposed
for shorterimmersiontimes (Figure l0). The major difference
betweenthe XPS spectrais the increasedamount of sulfur present
with continuedexpoaureto the alkanethiolsolution.6 The amount
of sulfur (relative to the amount of carbon) is too large to be
explainedif sulfur is only presentas alkanethiolate. We believe
formal C-S bond cleavagehas occurred to generatea thin film
it may be this reaction
of Ag2S at the silver/monolayerinterface;a7
that is responsiblein part for the anomalous ellipsometric
thicknessesmentionedearlier. Surprisingly,the wettabilitiesby
water and hexadecaneof the hydrocarbonfilms remainedsimilar
to thoseobtained after more limited exposuresand were consistent
(45) There is a further subtlecorrectionin the simulatedspectralintensities
of the d+ and d- bandswhich shouldbe made wheregaucheconformations
are introducedinto the chainend. Becausethesemodesare delocalized(see
supplementarymaterial),the lossof one trans conformerin an all trans chain
by conversionto a gauchestatecan diminishthe overallband intensitiesby
an amountgreaterthan the fraction of the total CH2 group which the altered
group represents,For the caseof an originally all-trans-(CH2),7- chain
converted
t o a I g a u c h ek i n k c h a i n ,t h u s w e e x p e c tI t f I t S 1 7 l 1 8 = 0 , 9 4o r
a more than 6Vodrop in intensity. A reasonableupper bound is probably
-l0Vo (SnyderR
, . G . ; M a r o n c e l l iM
, . ; S t r a u s s ,H . L . ; H a l l m a r k ,V . M . " / .
Phys. Chem, 1985, 90,5623-5631). Considcringthat probablyonly half
(-45Vo for the fit givenin Figure 9) thc chainshavc gauchckinks, the cffect
of the drop in intensityin the simulationis to increasethe chain tilt a and
a changein the tilt from 260 to -29o will accomplishthis. Although such
a correctioncannot be made rigorouslybecauseof the lack of actual gauche
statcpopulationsand the exactintcnsityrelationship,the sizcofthe correction
is reasonablein terms of physicallyacceptablevalucsof a.
(46) No differenceswerenotcd in the characterof the Ag(3d572)
core level
upon extendedexposureto the alkanethiolsolution;however,the binding
c n e r g i c so f A g ( 3 d 5 7 2f )o r A g 2 Sa n d b u l k A g a r e s i m i l a r( A B E S 0 . 1 c V ) . ' r
(4?) Cleavageof C-S bondsby silvcr of thiols,disulfides,and sulfidcshas
been reported.a
\''.,.
1 e .1 9 9 1 7 l 6 l
I
o
o
c
o
ll
L
o
a
ll
J o.oor
b
a
I
__T____r-]ffi
800
1000
1200
'1400
1600
Wavenumbers(cm'')
prepared
to
on silverby exposure
ra of monolayers
FigureI l. I R specl
exporn ethanol,afterdifferentintervalsof subsequent
octadecanethioi
(upper
(a, -l h; b, -2b). The high-frequency
sureto theatmosphere
(lowerpanel)regionsof the IR spectradisplay
panel)andlow-frequency
regionshows
the changesthat developwith time. The high-frequency
chainshavebecomeorientedlessperpendicularly
that the hydrocarbons
region
The low-frequency
environment.
but remainin a crystalline-like
and the XPS spectra(Figure!2) showthat someof the thiolatehead
to sulfonates.
groupshavebeenoxidizeci
packed.meth)'lsurface. We believe
with the presenceci a ,Jensely
t h a t d u r i n gt h e f o r m a t r o no f . r nt n t e r p h a soef A g 2 S ,a n o r g a n i z e d ,
o r r e n t e da l l l r l t h r o i a t ea s s e m b l a gceo n t i n u e tso e x i s t( s e eb e l o w ) .
On copper. similar lncreasesin sulfur content were observed,
suggestiveoi formation of copper sulfide; however,the surfaces
routinely becamepitted and did not allow characterizationby
wetting,
Upon extendedexpcsureof an alkanethiolatemonolayeron
silver or copper to the atmosphere.the samplechanges;a representativeexampleof the IR spectrumof such a material is shown
in Figure I L This sample was obtained after a few days of
t eo n o l a l e r o n s i l v e r t o a i r ,
e x p o s u r eo f a n o c t a d e c a n e t h i o l a m
Three featuresof the data are of particular significance.First,
the peak positionsof the CH2 C-H stretchingmodesare consistent
phase(d+, 2850cm-l;
with the presence
of a densecrystalline-like
d-,2917 cm-l). Second,the correspondingintensitiessuggestthat
the chains are more highly cantecithan thosedescribedabove.
Third, the low-frequencyportionof the spectrumexhibitsa very
intenseband centeredat - 1200cm-r. Using the opticalconstants
of an n-alkyl disulfideas a basisset, we roughll estimatethat the
chaincant, a, for this new phaseis -l8o with the twist E = 45o.
We urge cautionin interpretingthis result.as the headgroup on
the chain is now presumablynot thiolate (seeDiscussionbelow)
and perturbationsof n and & are expectedto result. It doesseem
safe to conclude,however,that the orientationsof the methyl
groupsat the ambient surfacediffer in significantdeuil from thooe
supportedby a thiolate head group,
The XPS data shown in Figure 12 for a monolayeron copp€r
exposedto the atmospheresuggestthat someof the sulfur atoms
in this monolayerare oxidizedasjudged by the additionalS(2pyz)
core levelat a binding energyof - 167.5eV, An increasein the
intensityof the O( ls) core level region is alsoobserved.We believe
the speciespresenton the surfaceto be an alkyl sulfonic acid,
presumablypresentas a sulfonateanion. The binding energyof
S(2pyz) is comparablein positionto that obtained by treating
an oxidized silver or copper surface with an alkyl sulfonic acid
(seeTable I). We have observedon silver similar oxidationof
Laibinis et al.
7162 J . A m . C h e m .S o c .,Vo l , I1 3 , N o . 1 9 ,I9 9 l
1is
o
'x
o4
tr
=
o
93
.=
o
52
s
173
168
163
158
Binding Energy(eV)
Figure t2. Effect of exposure to atmosphere on XPS spectra of octadecanethiolatemonolayerson copper (bottom. (l h; middle, -2 wk).
The upper spectrum is of a silver surface derivatized in an isooctane
solution of hexadecanesulfonic
acid, presumablyadsorbedas the sulfonate
a n i o n , a n d i s p r o v i d e d f o r c o m p a r i s o n . A l l s p e c t r a( 4 0 s c a n s )* ' e r e
r e f e r e n c e tdo C ( l s ) = 2 8 4 . 8 0e V . T h e l o w e rs p e c t r u me x h i b i t sa S l l p t
e n v e l o p ec o n s i s t e n*ti t h a t h i o l a t ea t t a c h m e n t t; h e s a m p l ee r p o s e dt o
a t m o s p h e r ef o r I u e e k s d i s p l a y sp e a k sa s s o c i a t e *dr t h t h e p a r t i a l a t t a c h m e n to f t h e m o n o l a y e r
a s u l f o n a t em o i e t l ( s e eT a b l e I ) a n d
"'ia
c o n f i r m sa s s i g n m e n ti sn F i g u r e I l . X P S * a s l e s ss e n s i t i v teo t h e p r e s e n c e
o f o x i d i z e d h e a d g r o u p s t h a t I R ; l o n g e r e x p o s u r e st o a t m o s p h e r e\ \ e r e
required to confirm the productsof oxidation by XPS. Similar changes
are observedin the S(2p) spectraof alkanethiolatemonolayerson silver
although they occur at a slower rate.
thiolatesto sulfonates.2e
The appearance
of a strong
the adsorbed
peakat - 1200cm-r in the IR spectraof silversamples(Figure
with a sulfonate
I I ) is entirelyconsistent
headgroup.
Discussion
Thedatareportedheresupporta generalunderstanding
of the
structureof the alkanethiolate
monolayers
on copper,silver,and
gold. Theanswerto thesimplest
question
wecanask-*Are they'
thesame?"-isclearly"no". How theydifferis easilydescribed
in generalterms,
Monolayers
on Copper.All the evidence
suggests
that the films
preparedon copperunderthe conditionswe used(that is, by
excluding,
exposure
to laboratory
minimizing,but notcompletely
metalunder
atmosphere
by manipulating
the unfunctionalized
a flow of argon)canvary widelyin character.The significant
variationsof spectralbandintensities
all suggest
and lineshapes
that somefactorrelatedto the structureof the monolayercomplicatesthis system.The clearinferencefrom the XPS data is
due
that the variationsin the surfaceof the startingsubstrate,
with atmospheric
oxidationand otherreactions
to adventitious
constituents
in the morphology
of the
and,perhaps,to differences
surface,are likelyunderlyingfactors.
is clearly
The coppersurfacethat supports
thesemonolayers
not metallic.The O(ls) coreleveldataestablish
that thecopper
containsoxygen.No Cu(ll)(2r1,Dsatellitestructurecouldbe
detected
by XPS;thus,the mostlikelyphasepresentis a hydrous
surfaceoxiderelatedto Cu2O.This oxidesurfaceis alsoknown
contaminants
to behighlyactivein thesorptionof environmental
of CO2as carbonate.The
as well as in the strongadsorption
of this surfaceis unclear. It is thus
microscopic
topography
possiblethat any one or all of thesefactorscould conspireto
or in
precludetheadsorption
of thiolsto the veryhighcoverages
the geometriesnecessaryto form a structurecomprisedof
and,perhap,orderedalkylchains.It is notsurprising
close-packed
prepared
usingapparentlyindistinguishable
that coppersamples
protocolsyielded monolayersexhibiting various degreesof conformational disorder by IR and complementary differences in
hysteresisof contactanglesof water (seesupplementarymaterials).
of Cu( I I I ) under UHV conditions,
On clean elementalsurface,s
s i m p l et h i o l s a n d d i s u l f i d e s( C H 3 S H a n d C H 3 S S C H 3 )a d s o r b
readily.a8 The surfacespeciesgeneratedfrom either adsorbate
is a surfacethiolate.CH3SCul.Cuo,.The structureof this organic
surfacephaseon Cu(l I l) has not yet beendeterminedby LEED,
but the evidenceobtained from angle-resolvedphotoemission
measurementssupports a highly defined structural model containing a S-C bond projecting along the direction of the surface
normal. Such a surfacephaseis qualitativelyconsistentwith the
character of the best monolayerswe have prepared on copper
although the values of a found suggest a somewhat different
a l i g n m e n to f t h e a l l - t r a n sc h a i n ( a = l 2 o ) . I n m o s t i n s t a n c e s ,
however,the data obtained in this study do not support a structure
resemblingthe well-definedstructure obtained in UHV. We
therefore suggestthat the oxide surfacespresentin experiments
involvingadsorptionfrom solution must seriouslycomplicatethe
formation of denseorganic phases.Under certain conditionsthis
oxidation might be reversibleand/or the Cu atoms removableas
ions (Cu(*l)) to allow the formation of the densephasesseen
on occasion. As noted, though, the experimental variable(s)
involvedremainsundefined. Further oxidation of the surfaceafter
formation of the monolayer,and, possibly,oxidationof the thiolates
as well. may also contributeto the disorderin the monolayer.ae
The XPS data clearly suggestthat the major sulfur species
p r e s e n it- .a t h i o l a t c( R S C u ) : t h e S ( 2 p ) c o r e l e v e l s( T a b l e I ) a r e
s h i f t e db r , ) r e r I e V t o i o w e r b i n d i n ge n e r g yf r o m t h e p o s i t i o n
e r p c c t c dl \ ) r l t h l o i t u i t h t h c -S - H b o n di n t a c t ) . T h e s a m es o r t
trf :p€clc: it)fms ,]n elementalsurfacesin UHV.47 The microscopic
m o r p h o l o g ro f t h e : u r f a c e sm a y b e v e r l ' d i f f e r e n ta, n d t h e i n f l u e n c eo n s t r u c t u r eo f s u r f a c es i t e so c c u p i e db y o x i d e i o n sa n d
of alkyl sulfinicor sulfonicacidsis presently
bi small cluantitres
unclear. The XPS data also strongly suggestthat the bonding
at the surface can be heterogeneous(a broad S(2p) envelopeis
sometimesseen). Whether this heterogeneityis attributable to
different sulfur valencestatesor metal bonding sites is unclear.
Monolayers on Silver. In contrast to the copper-supported
monolayer, the monolayers formed on silver are remarkably
well-defined,and the assemblyof monolayerson this metal better
behavedthan on copper. We would expectthat silver,which also
forms an oxide in air, might show complexitiessimilar to those
found for adsorptionon copper. The core-leveldata clearly show,
however.that nearlv all of this oxide must have been removed
on the adsorptionof the alkanethiolsand that oxidationsubsequent
t o f o r m a t i o no f t h e m o n o l a y e rm u s t b e s l o w . W e d o n o t k n o w
at presentwhere the residualoxygendetectedby XPS remains,
but suggestthat grain boundariesare at leastone likely candidate
site. The data also show that this surfacephase,when exposed
to the atmosphere,gradually converts to what we believe is a
material attachedas a sulfonate;the O(ls) core level intensity
seen at short immersions may, in fact, correspondto a partial
present
surfacecoverageof this latter ligand. The signal-to-noise
in the XPS measurementmakesthe S(2p) core levelsof this latter
group hard to detect. The IR data require a structural model for
fresh (not aged) monolayerson silverin which the chainsare very
denselypacked. The line widths, the peak positions,and the shapes
of the d- and d+ modesresemblethoseexpectedfor a crystalline
hydrocarbon solid to a remarkable extent. The structure we
propose is thus one based on an all-trans chain with very few
gaucheconformations. The quality of the methyl band fits suggeststhat gauche conformationsare probably lessimportant in
the phasediagram of thesephasesat 300 K than they are in the
correspondingstructureson gold (seebelow). The simulations
in Figure 7 also definea structurein which the main chain axis
(4E)Bao,S,; McConville,
C. F.; Woodruff,
D.P. Surf. Sci.19t7,187,
N. P.;Seymour,
D. L.; WoodruflD. P.;Jones,
133-143,Prince,
R.G.;
Walter,W. Suy' Scd,1989,215,566-576.
(49)Theboundthiolateandunderlying
copper
surface
eachoxidizewith
to atmosphere:
exposure
of themonolayer
Laibinis,P. E.;Whitcextended
results.
sides,G. M. Unpublished
Monolayersof n- Alkanethiolson Cu, Ag, and Au Surfaces
Figure 13. Space-filling model of an octadecanethiolatemonolayer on
silver as derived from infrared data interpreted in terms of a single-chain
model.
is only veryslightlycantedfrom the surfacenormaldirection.A
graphicaldepictionof thisstructureis shownin Figurel3 for an
octadecanethiolate
monolayer.Theseresultsimplicatea significant
reductionof theinterchainspacingin thesephases
in comparison
to the analogous
onesthat havebeenextensively
characterized
o n Au (l I l) in t hisando th e rp a p e rsOn
. th i sl a tte rs u rfa c e.
we
nowknowwith greatcertaintythat cantedphases
(a = 28owith
thechainrotationp = 52o,seebelow)areformedwhosecharacter
is wellsuitedto the verystrongtendency
of thissystemto form
ordered(J3 x V3)R30" sulfuroverlayers.
We believethat the
cantedsurfacephases
seenon goldreflectsthe spacingof sulfur
in this overlayerstructure.The differingcharacterof similar
monolayers
on silver,despitethe similarmetal-metalspacingfor
silverand gold,suggests
that the sameorderedoverlayeris not
formedon silver.5o
(50) Portcr et al. have also characterizedthe structure and propertiesof
a-alkancthiolsadsorbedon evaporatedsilver films by wetting, ellipsometry,
, . M . ; C h u n g ,C . ; S t o l e ,S . M . ; W i d r i g , C . A . ; P o r t e r ,M .
and IR (WalczakM
D. J. Am. Chem.,Soc.
t99l, I I 3,237F2378). A comparison
with the results
reportedhere for silver indicatesgeneralagre€mentsbetweenthe setsof data,
but showsdiffercnccsfor monolayersderivedfrom shortern-alkanethiols.For
elqr_nRlcr_1vc.do
not observean odd-vcn effect in the wettability of Ag/S(CH2)"CH3by hexadecane
(n = 4-8); Porteret al. do. They concludethat
the hydrocarbonchain is cantedat -*l3o for all ualuesof n;we contend
that the signof the cant angle,a (Figure6), alternatesbctweenpositiveand
negativcvalucs for evenand odd valuesof n, respectively. We bclievetheir
analysisto bc in error. In both works,no variationin the intensitiesof methyl
stretchingmodeswasobscrvedfor n 2 5. For shorterchain length (n = 4-7),
Porter et al. obscrveas much as a 4Mo increascin thc intensity ol thesemodes.
Our spectralsimulations(Figure 7) suggestthis variationii too small to bc
due to odd-cvcn variations in chain lcngth; we cstimate that an -l00%o
increasein intensity is required. They concludean odd-cven effect is responsiblefor this variation but do not offer satisfactoryexplanationfor its
disappcarancc
at n > 8, Wc rationalizcthis constantintcnsityby the existcncc
of a singlemcthyl orientationon silveras n is varieddue to-an'alternation
in
thc-signof thc cant anglc,a (Figure l4). Portcr ct al. also usc corresponding
odd-even variations in wettability for n = 4-8 (that we do not obsirvc) ai
complementaryevidence. We suspcctmost (if not all) of the differences
bctweenour work and that of Porter et al. to bc due to adventitious(and
pcrhapscontaminantpromotcd)oxidationof their evaporatedfilms of silver
prior to adsorptionof the alkanethiol,and to the sensi[ivityof this systemto
surfaccoxidationwhen onc uscsshorter(n s 6) n-alkancthiols.we notc that
both papershave had difficulty preparing high-quality monolayersfrom
shorterchain alkanethiols.we cannot,howcvei,eicludi somecontribution
to differcncesin the proceduresused to preparethe sitver films, and to re.
sultingdifferencesin grain size,preferredorientation,and surfacetopology,
J. Am. Chem. Soc., Vol. II3, No. /,9,l99l
7163
ComparisonsbetweenMonolayers on Gold, Silver, and Copper.
The differencesin geometrv (a and p) of the hydrocarbon portion
of the assemblies
on the coinagemetal surfacescan be considered
to arise from the balanceof free energiesfrom three sources:(l)
t h e a d s o r b a t e / s u b s t r a itnet e r f a c e (, 2 ) t h e c h a i n - c h a i np a c k i n g
o f t h e i n t e r i o ro f t h e a s s e m b l ya, n d ( 3 ) t h e m e t h y l i n t e r f a c e( o r
solvent/methylinterface). The energeticsof the first contribution
abovehave beenexaminedin the caseof gold3o(the energetics
on silver and copper are no doubt similar) and are likely to be
larger in magnitudethan is possiblefrom the energeticsof hydrocarbon chain interactionsslor hydrocarbonsurfaceenergies.
Therefore,it is likely that head group/substrateenergeticsdrive
the major structural featuresof the entire assembly. After this
consideration,one could ask the questionof what structure of the
hydrocarbonchain (tilt angle and packing) and what arrangement
of surfacemethyl groups (orientation relative to the surfaceand
to other methyl group) representminimum free energystructures
given no constraintof surface/adsorbatebonding,viz., no prefened
lattice spacingor valenceangle. This questionis impossibleto
answerexperimentallyat present,but, asjudged by the infrared
d a t a . t h e t w o d i f f e r e n t c l a s s e so f r l e l l - d e f i n e dm o n o l a y e r sw e
o b s e n ' e( g o l d :r i i r e r a n d c o p p er ) b o t h e x h i b i td e n s e c, r y s t a l l i n e
p a c k i n ga
. n d r n t h c c a s eo i g o l d .s o m ec o m p o s i t i o n adli s o r d e r i n g
at the alr (or solvent)interface.The energydifferencesbetween
thesetuo classes
of films.althoughindeterminate
experimentally,
may be as small as differences between orthorhombic and
monoclinic phasesof hydrocarbons.aaGiven this negligibledifferenceand also given such a large discrepancybetweenthe total
free energiesof the adsorbate/substrateinterface and of the
interior and the surface of the film for a particular system, it
appearsthat the most likely factor determining the differences
betweentwo films formed on different substratesmust be primarily
the substratesthemselves.On this basis,we considerthe head
group/substratestructure in detail first.
As has beennoted.the XPS data indicatethat the bondingto
the surfaceis in the form of surfacethiolatesfor the threemetals.
T h e l i t e r a t u r ec o n t a r n sc o n s r d e r a b lper e c e d e nfto r t h e b o n d i n g
species
of sulfur-containing
on silverin this manner. On Ag(l I l)
(the preferred crystallographictexture of the polycrystalline
samplesusedin this study; seeExperimentalSection),elemental
sulfur and H25 have beenfound by LEED to form a number of
stable,orderedsurfacephases.s2The preferredhabit is a ({7
x \/1)Rl0.9o overlayer. This denseand structurally complex
phase is believedto correspondto an ordered, lattice-matched
interfacebetweenAg(l I l) and (l ll)-oriented 7-Ag2S. The silver
in the first layer is believedto be ionic. Dimethyl disulfidedissociativelychemisorb,s
on Ag( I I I ), and the surfacethiolate formed
also adopts the (t/1 x V7)Rl0,9o structure.s3 The S-S nearest
neighbordistancein sucha structureis -4.41 A. It would thus
seemthat the bondingof sulfur in this sort of ionic surfacephase
(one that significantlyreconstructsthe (l I I ) lattice plane) is
consistentwith the characterof the phasesinferred by infrared
spectroscopy.Molecular dynamicscalcuIat i onss suggest,however,
that this lattice may be too dense(16.87 AulnS) to accommodate
the all-transchain sterically. The other phasesseenin the H2S
adsorption
studies,s2
mostnotablythe r'39R I 6. I o x /39R16.1 o
overlayer(a structurecorrespondingto an ordered(100) layer
of 7-Ag2S),could alsosupportthe adsorptionof the sulfur ligands
a t d e n s i t i e s- 8 V o h i g h e r( - 2 0 , 3 A 2 , ' R S ; t h a n t h o s ei n t h e ( V 3
x V3)R30o overlayersfound on gold. The small angle of cant
derived from IR data. we believe.reflectsthe short interchain
spacingsimposedby the silve175u1;ur
lattice. LEED or other
( 5 1 ) T h e g r o u pc o n s t i t u e nht e a to f s u b l i m a t i o no f n o r m a lh y d r o c a r b o n s
i s 1 . 8k c a l / m o lC H 2 a t 0 K : S a l e m ,L . J . C h e m .P h y s . 1 9 6 23, 7 , 2 1 C G 2 1 1 3 .
For very long hydrocarbontails (n I 20), thc enthalpies
due to chainpacking
and the metal-headgroup interactionmay well bc of comparablemagnitudcs.
( 5 2 ) S c h w a h aK
, . : S p e n c c rN
, . D . ; L a m b e r t ,R . M . S r l . S c d1. 9 7 9 ,8 / ,
2 7 3 - 2 8 4 . R o v i d a ,G . : P r a t e s i F
, . S u r f . . S c i .l 9 t l , 1 0 4 , 6 0 9 4 2 4 .
( 5 3 ) H a r r i s ,A . L . : R o t h b c r g L, , ; D h a r , L . ; L e v i n o sN
, . J . ; D u b o i s ,L . H .
Phys. Reu.Ittt . l9XJ. 64, 2086-2089. Harris, A. L.; Rothbcrg, L.; Dhar, L.;
Levinos,N. J.; Dubois,L. H. J. Chem. Phys. 1991,94,2438-2448.
( 5 4 ) B a r e m a nJ. . P . ; K l e i n , M . L . J . P h y s .C h e m . l 9 { n , 9 4 , 5 2 0 2 - 5 2 0 5 .
7164 J . A m . C h e m.S o c .,V o l .l l 3 , N o .i ,9 , 1 9 91
to definethe structureof this
diffractionstudieswill be necessary
underlyinglatticeunambiguously.
of the adAgain,as is the casewith copper,the mechanism
on silveris unclear.The oxidationof one
sorptionof alkanethiol
of silverat the solid-airinterfaceshouldbe
or two monolayers
facileunderthe conditionsusedin the preparationof thesesamples.
boththat
arguments
It is alsoclearfrom simplethermodynamic
the S-H bondand
an Ag(0) surfaceshouldbe ableto dissociate
shouldbe reducibleby thiols. Giventhat
that AgO, species
of oxygenatomson somesilversurfaces
replacements
metathetical
leading
are alsofacile,it is easyto envisiona numberof processes
This latterchemistryalso
to the materialsseenexperimentally.
which
densities
anotherreasonfor the higheradsorbate
suggests
to gold. The initialoxidationof the
formon silverascompared
of oxygenatomson the
by Oz yieldsa high coverage
substrate
by sulfurwouldthusdirectlyyield
Theirsubstitution
surface.lT
esa surfacephaseof densechains.55Evenso.the precedent
that
suggests
of dimethyldisulfide
by the UHV studies
tablished
is intrinsicto
in highsurfacedensities
the formationof thiolates
t he A g( l I l) latti c ep l a n e .
As for bothcopperandsilver,the bondingto goldalsoappears
to be asa surfacethiolateasjudgedby the corelevelshiftsseen
of the sulfuratomsin a (r/3
in the XPS data. The placement
x /3)R30o overlayer,as mentionedabove,is now well estabis oneinvolvinga morecovalent
lished.l0'llThe pictureweenvision
structurethan that on silver. On gold,thereare no largedisplacements
of the metalatomsfrom their normal( I I I ) lattice
sites. The constantchaincant angleseenin the gold system
presumably
of themetal-sulfur
reflectsthedominantenergetics
s e e nre fl e c ts
th e i n te rp l a o;-f t hedomi nant
bond.T he or de ri n g
c ont ri b u ti o nosf th e me ta l -s u l fubr o ndi ngand the
ener get ic
of thepackingof thealkylchains.In the
second-order
energetics
bondingresults
caseof gold,thecovalentcharaclerof the substrate
in the adoptionof a fixedAu-S-CHt valenceangle,providing
an interestingcontrastto the behaviorsseenon copperand silver.
interaction,
thedominantadsorbate/
substrate
Having discussed
we nowturn to the issueof film structure.An intriguingpoint
is that the methylmodespectraof
arisingfrom the IR analysis
in the
the filmsformedon silverdistinctlydo not reflectchanges
signof the chaincantangle,a (Figure6), with changing
absolute
carbonchainlength. If the absolute
directionof the chaintilt
with odd-even
variationof length,thecharacter
wereconserved
of the spectrain the regionof the methylmodeswouldvarl
(seeFigure5). Thepresence
considerably
andbequiteobservable
d e th y ls u rfa c es, u c ha s e x i s tso n gol d,w oul d
of a dis or der em
diminishthiseffectsomewhat,
but thequalityof thesimulations
in the regionof the r+ and rs- modesis sufficientlygood(Figure
7) to indicatethat gaucheconformers
arelessimportanton silver
of orientationgivenin Figure
thanon gold. Usingthe conventions
on silver(and likewiseon copper)are
6, the chainstructures
described
in termsof onesin whichthe chainis requiredto cant
in oppositedirectionsand the Ag-S-CH2 ualenceangleto alteruutebetweentwo different sigtuof a for odd and euennumbers
groups(Figurel4). The methylsurfaceprojection
of methylene
selected
by eachis the samedensephase,one which we can
characterize
asbeingmorelike a surfaceethylgroup(Figurel3)
maximizingnearestneighborinteractions
at the chaintermini.
with this methyl surface
Althoughthe energetics
associated
structureshouldbe small,theyseemto besufficientto outweigh
any preferences
dueto the sulfur, We believethis pictureto be
with the hypothesis
consistent
that the bondingof the sulfurat
in
thissurfaceis largelyionic.53With the aboveconsiderations
(55)Scc,for cxamplc:Wachs,I. E.; Madix,R. J, Surf. Sci.191t,76,
Stuve,
B. A.;Madix,R. J. Surl Sci.t9tl, i,05,177-195.
531-558.
Sexton,
E, M.; Madix,R. J.;Sexton,
B. A. Chem.Phys.ktt.19t2,89,48-53,Stuvc,
Canning,
E. M.; Madix,R. J.:Scxton,
B. A, Surf. Sci.19t2,I 19,279-290.
N. D. S.; Madix,R. J, /, Phys.Chem.1984,
88,2437-2446.Fcltcr,T. E.;
Wcinbcrg,
W. H.; Lastushkina,
G, K.; Hrbck,
G. Y.; Zhdan,P.A.; Borcskov,
S.;Robcrts,
J, Appl.Surf, Sci.t983,/6. 351-364.Au, C. T.; Singh-Bopari,
Capote,
M. W,; Joyner,R, W. "/.Chem.Soc.,Faraday19t3,79,1779-1791,
J. Am. Chem.Soc.19t9,
A. J,; Madix,R, J, Sry' Sci.19t9,214,276-288;
Soc.1989,/ / /,
I I I ,357G3577.Brainard,
R. L.; Madix,R. J, /, Am.Chem.
andrcfercnccs
citcdthcrein.
3826-3835
n = even
"f" )'r'
\
,J,;,X;,
\
\t
45"
'HH
\(
o=-'2"(
),=,,
)
(r:;:;;,
t/
))
H
H
o - -27'
(notformodlor
n - odd or even)
formeduponadsorption
Figure14. Illustrationof the cantedstructures
on copper,silver,and gold. The
CH3(CH2),'SH,
of n-alkanethiols.
that formon copperandsilverdo not exhibitany changein
structures
in the number
increase
of methylmodeswith incremental
rherntensrtres
is consistent
groups(Figures4 and 5). Thisobservation
of methylene
only *rth the formationof a structurern whichthe cant angle,a, is
groupsandnegative
for an odd
positive
for an evennumberof methylene
groups;the geometryof the metal-thiolateinternumberoi methylene
actionon thesemetalsis not fixed as n is varied(upperpanel). The
formedon goldexhibitan odd-evenmodulationin the inten'
structures
chainlength(Figures4 and
sitiesof the methylmodeswith increasing
5), This modulationreflectsthe formationof a structurein whichvardo not perturbthe geometry
iationsin the chainlengthof the adsorbate
(lowerpanel).
interaction
of the gold-thiolate
m i n d . w e c o n c l u d et h a t a s s e m b l yo f a g i v e nf i l m p r o c e e d s o a s
to satisfl the adsorbatmubstrateenergeticsfirst and then those
riith the methyl (ambient)surface(in the form of the
associated
a m b i e n tf i l m s u r f a c et e n s i o n ) .
The structuresformed on gold are understoodquite consistently
with the aboveprinciple. The placementof the sulfur atoms in
a (J3 x v/3)R30o overlayerwith a fixed Au-S-CH, valence
angle forces the chains into the same absolute orientation regardlessof chain length. As a consequence,the orientation of
the methyl group modulateswith carbon number (Figure 5).
There are, however,further subtletiesto these structures. We
also know, both from experiment and theory, that the lowest energy
structure for this systemdoesnot involvea simple hexagonallatticc
of the chains but, rather, is one reminiscent of the bimolecular
monoclinicphasesof the normal hydrocarbons.s6We also know
the symmetry of the methylenesubcell cannot be simply orthorhombic as evidencedby the optical anisotropiesin the infrared
data (an orthorhombicsubcell,at any valueof a, would yield an
average0 = 45ou and be accompaniedby d+ and d- intensity
ratios approximately a factor of 2 different from those seenexperimentally). To this point, both experimentand theory have
failed to establishhow important this two-chainunit cell is to the
structure of the monolayerat temperaturesgreater than 220 K.
The simulationspresentedin Figure 9 suggestthat the structure
presentat 300 K may contain still significant signaturesof this
two-chainsubcell. Our resultsalso show that the IR data are
consistentwith, but do not independentlyestablish,significant
(perhapsas high as 45Vo)conformationaldisorderingof the end
(56)Shearer,
9, 379-384.
H, M. M.; Vand,U, Acta Crystollogr.l95S,
Monolayersof n- Alkanethiolson Cu, Ag, and Au Surfaces
. l A m . C h e m . S o c . ,V o l . I l 3 . N o . 1 9 , l g g l
7165
stricted and thereforehomogeneousmethyl surface. It seemsmost
interesting.given rhcsevery different surfacestructures,that a
d i f f e r e n c er n u e r t a b i l i t y i s n o t o b s e r v e d .
It followsbr rnferencefrom the lattice argumentsabovethat
the structuresf()rmcdon gold can accommodatestructuralperturbationsthat arc preciudedon silver. The reconstructionof the
m e t h y l s u r f a c c .u h c t h e r d r r v e nt h e r m a l l y o r i n r e s p o n s et o a
contactingexternalphase.r. aifectedmechanisticallyboth by the
p o p u l a t i o no f g a u c h cc o n f o r m e r sa n d r h e u n l o c k i n go f t h e c h a i n
t i l t d i r e c t i o na n d m a g n i r u d et :
T h e c a n t e ds t r u c t u r e so n g o l d . i n c o n t r a s t o t h o s ef o r m e do n
c o p p e ra n d s i l v e r .m i g h t p r e s u n t a b l ar c c o m m o d a t seo m ep a r t i a l
i n t e r c a l a t i o no f h e x a d e c a nbe1 u n c a n t i n gc h a i n s e g m e n t s .T h e
a m o u n t o f v o i d s p a c eg e n e r a t e db 1 u n c a n r i n gr h e h r d r o c a r b o n
s t r u c t u r ew o u l d b e m u c h g r e a t e ro n g o l d t h a n s i l v e r :f u r t h e r .
n e g l i g i b l es p a c ei s p r o b a b l ya v a i l a b l eo n s i h ' e rs i n c et h e c h a i n s
are, prior to contactwith a liquid, orientednear the surfacenormal
(a = l2o). Intercalation
o f h e x a d e c a nien t o a m o n o l a v e rw o u l d
resultin a loweringof the advancingand recedingcontactangles
of hexadecane
and occur.basedon structure.to the greatestextent
possibleon rhe materialsformedon gold. The contactanglesand
F i g u r e 1 5 , I l l u s t r a t i o n o f t h e s t r u c t u r e o f a n : o n o r a ' v eor i a n ( i c r a d e h r s t e r e s ios f h e r a d e c a noen t h e m o n o l a y e r fso r m e do n s i l v e ra n d
c a n e t h i o l a t em o n o l a y e ro n g o l d i n f e r r e df r o m a n o - c h a i n; i n r u l a t i o n
g o l d a r e .r n i a c r .i n d i s r i n g u i s h a bul ee: t h u sc o n c l u d et h a t i n t e r b a s e do n a l l - t r a n sc h a i n s ,T h e i n s e ts h o w st h e r e r a t i o n s h iopf a n o r r h o calaticrn
trf heradecane
r n r om o n o l a l e r so n g o l d d o e sn o t o c c u r .
r h o m b i c c h a i n s u b l a t t i c e( u n c a n t e d )r e l a t i v er o t h e h e r a g o n a ls u l f u r
Bain ct al found rhar alkanethiolate
monolayers
on gold prepared
s u b l a t t i c ea n d t h e d i r e c t i o no f t h e c a n t o f t h e h y d r o c a r b o nc h a i n i o r t h i s
i n h e x a d e c a n c\ { e r e n o t d i f f e r e n t ( b y e l l i p s o m e t r yX
, PS, and
a r r a n g e m e n ta s i n f e r r e d f r o m I R s p e c t r o s c o p ) ' .
wetting) from monolayerspreparedfrom other solvents. This
observationsuggeststhat, even in the presenceof a large excess
chain segments at 300 K. These'quantitative'values cannot be
of hexadecane,intercalation of solvent had not occurred,20
taken too literally. however, given the significant perturbations
On gold, the orientation of the methyl groups at the monoof the methyl group transition moments noted in the data. A
layer-air interface is highly dependenton the number (odd versus
schematic depiction of an idealized two-chain structure on gold
even) of methylenegroupsin the adsorbate:monolayerson silver
is shownin Figure15. The readershouldnotethat thisstructure
do not exhibit any orientationalvariationwith chain length (Figure
derivesfrom onlyonefit to the data,howbeitonewe believeto
5 ) . T h e s t r u c t u r a ld i f f " e r e n c ebse t u e e n t h e s et w o m o n o l a y e r
beplausible
on the basisof stericarguments.Diffractionstudies
s \ s t e m sa i l o u ' c o n t p r ; i s 6 n6 f a n " o d d - e v e n "s e n s i t i v es t r u c t u f e
will berequired
to settletheimportance
of thetwo-chain
unitceil
r . n g o l d u i i h t h e ' t , d d -e ' , ' c n ' n e u t r aslt r u c t u r eo n s i l v e ra s t h e y
at roomtemperature.
r e l a t et o a \ u r f r c c - s e n s i r i rper o p e r t ) ,w e t t i n g . T h e w e t t a b i l i t i e s
StructuralEffectson wetting. The strucrures
on the three
c f r r s l s i a n d h c r a d e c a n eb o t h m o n o t o n i c a l l yi n c r e a s ew i t h i n metalsdifferin significant
waysthat mayaffecttheirrespective c r e a s i n gc h a i n l e n g t h .n o p e r i o d i c
t r e n d i s o b s e r v e do n g o l d ( o r
we tta bilit iesA. s ignif ica ni st s u eth a t n o we x i s tsfo r o rs a ni zed
s i l v e r ) . I t i s w o r t h w h i l et o a s k , i n v i e w o f t h e m a g n i t u d eo f t h e
films is what are the detaitedrelationships
betweenrnJlecular
differencesin methyl orientationon gold, why there appearsto
featuresat surfaces,
suchas orientationand corrugation,
and
be no influenceon wetting. It is conceivablethat the number of
wetting.,Wettingis influenced
by a numberof factors,among
gaucheconformerson gold, particularlyin the region in contact
themmolecular-scale
andmacroscopic
roughness.5T
The monwith solvent,is sufficientto lessenthe magnitudeof the corruolayerswe prepared
on copperwereof variablequality,in part
gation. There is, of course,no requirementthat the structural
due to morphological
changesthat occurredduiing oxidation
characteristicsof the monolayerwhen in contact with air (or
andfor subsequent
adsorption
of the alkanethiol;
monolayers
on
vacuum) are preservedwhen the monolayeris placedin contact
silver,in contrast,weremorereproducible.Structurafly,
our
with a liquid. It is alsoentirelypossiblethat hexadecane
and water
highestqualitymonolayers
prepared
on copperweresimilarto
are sensitiveto more than the -0.5 A that would be requiredto
monolayers
prepared
on silver.Giventhissimilarity.we restrict
observethe small differencesin structure that arise from conour analysis
to a contrasting
of thestructure-property
relationship
formational differencesin the methyl groups.
wetting
in
of monolayers
formedon silverand gold.
In general,measurements
of wetting on theseinterfaceswere
Thewettabilities
of monolayers
formedon silverandgoldexhibit
extremelyusefulin defining monolayersthat were disorderedor
a commonincrease
in hydro-andoleophobicity
with increasing
rough. They did not, however,allow discriminationbetweenthe
chainlength(Figure2). Similarprogressions
havebeenobserved
two types of ordered structuresobservedhere, even when the
with alkanoicacidsadsorbed
on alumina.3T
The lowerhydrostructuressubstantiallyaffect (on the molecular level) the structure
phobicities
of monolayers
prepared
fromshorteralkanethiols
(n
of the monolayer-vaporinterface. whatever differencesexist
( 9) havebeenrationalized
previously
in similarsystemsby
betweengold and silver in such structural details find little exassuming
disordering
of the shorterchainadsorbates
and interpressionin wetting. Other probesof the structureof thin films
actionof the probeliquidwith the underlyingmetalsub,strate.8,e,37 (for
example,helium diffractionr') ma) proveto be more sensitive
Thelimitingcontactanglesfor monolayers
derivedfrom long(n
to thesesorts of perturbations.
>
l5) alkanethiols
are,withinexperimental
error,the sameon
thethreemetalsexamined
andarein accordwith varues
reported
for orientedalkylmonolayers
on othersubstrates.E.zl-zs
16i 61r,
on silver,whichwe believe
containa smailernumberof gauche
conformersthan thoseon gold,might be expectedto prisenta
lowerenergyinterfaceexpressing
a moreconformationally
re_ .(57)Wenzel,R. N,.lnd,Eng.Chem.lggfi,28,9Eg-994.Dettre,R. H.;
Johnson,
R, E. Adu.Chem.Set.19d4,No.43,136-144.
Joanny,
J. F.;de
Gcnnes,
P. G. /. Chem.Phys.1984,81,552-562.De Gcnncr,p. C. Rrr.
Mod. Phys,19t5,57, E27-863.
Conclusions
n-Alkanethiolsspontaneously'
assembleonto surfacesof copper,
silver,and gold and form denselypacked,orientedmonolayerfilms.
The films formed on silver and copper differ in structural detail
from those formed on gold: the hydrocarbon chain is oriented
more perpendicularlywith respectto the surface;the films (when
carefully prepared)exhibit a lower populationof gauche conformationsat room temperature;the orientationof the carbonsulfur bond relativeto the surfaceplanevarieswith chain length.
The formation of reproduciblefilms on copperand silver is com-
7 1 66 J . A m . Che m.
Vol. I I 3,
IA,I99I
plicated by the sensitivityof thesesystemsto exposureof the
unfunctionllized or functionalizedsub,strateto air, and to extended
exposureto the solution containing the thiol. Films of high quality
ari, ho*euer, readily obtainedwith careful attention to detail.
Extendedexposureto alkanethiolsolutionresultsin reactionof
the alkanethiol with the surfacesof silver and copper to form an
interphaseof metal sulfide on which an organized,oriented
monolayeris supported. Although the systemsof alkanethiols
assembledon copperand silver are more difficult to work with
than the correspondingsystemson gold. this set of systemsoffers
the opportunity to explorethe effect of structural differenceson
a number of propertiesincludingwetting, intercalation,and capacitance.
Experimental Section
as received
Materials.Materialswereobtainedfrom Aldrichand'.tsed
(Pfritz & Bauer).tet13denl
below. Nonyl,undec.v-l
unlessspecified
thiolsweredistilledunderreduceci
(Pfaltz & Bauer).and octadec-vl
anddocosllthiolsuc're
nonadecli,
priorto use. Heptadecyl.
pressure
wassupplied
by our
Pentadecanethiol
studies.E'i3
fromprevious
available
c o l l e a g uKe e v i nL . P r i m e( H a r v a r d ) .H e x a d e c a n e s u l f oanci cdw a s
with sulfuric
of sodiumhexadecanesulfonate
obtainedby acidification
from hexanes.Octadecanethiol-du
acidand purifiedby recrystallization
perdeuterated
bromide(Cambridge
waspreparedfrom the corresponding
andsubby thioacetate
displacement
98VoD) by nucleophilic
Isotopes,
TLC
the productexhibited
with K2CO3/MeOH;
sequintmethanolysis
and formedmonolayers
behavioridenticalwith that of octadecanethiol
properties(wetting,XPS. and ellipsometric
with indistinguishable
(for monolayers
on gold))from thosefcr:medfr6m octadecanthickness
bv
wereobtainedfrom Mallinckrodtanddeoxvgenated
ethiol. Solvents
belor'gr in the tett.
specified
otherwise
bubb!ingN2 for 30 min unless
a i r . I s o o c t a nte\ l d r i c h ) u a s
n o f u r t h e re f f o r t sw e r eu s e dt o e x c l u d e
percolated
twicethroughneutralalumina;ethanol(abs)*as obtained
fromQuantumChemicalCorp. Thesilverandcopp€r(Aldrich)andgold
wereof
sampies
(MaterialsResearch
Corp.)usedto prepareeyaporated
purity. Silicon(100)waiers(100mm) r,lereobtainedirom
>99.99Vo
SiticonSenseand washedwith absoluteethanol. PrepurifiedAr
<5 ppm 02) wasobtainedfrom Med-tech.
(>99.998Vo,
Samplenepandon. Filtt of the variousmetals( 1200-2000A) were
to depositionof 200A of chromium
subsequent
preparedby evaporation
ontosiliconwafers:evaporation
or titanium(the latterbeingpreferred)
of silverandcopperwereontoprecutI X 3 cm2slidesof siliconaswell
ontosiliconwafersand,in some
asontosiliconwafers;Au wasevaporated
in
wereconducted
cut into slides.The evaporations
subsequently
cases,
pumpedelectronbeamchamber(basepressure-8
eithera cryogenically
boat chamber(base
x l0-ETorr). a cryogenically
oumpedresistive
*2 x l0-8Torr), or a thermalevaporator
with a
evacuated
pressure
d i f f u s i o np u m p( b a s ep r e s s u roef ( l X l 0 { T o r r ) ;o n l i ' m o n o l a r e r s
used.For e!'aporato the evaporator
prepared
on copperweresensitive
with prepurifieC
argonimthechamberwasbackfilled
tionsof copper.
(prolyethylene
bags*'ereplacearound
followingevaporation
mediately
to minimizediffusionof air intc the
of the evaporator
the openings
undera flow of argonto
weretransferred
chamber),andthe substrates
vials. Filmsof silverwereprepareci
scintillation
in disposable
solutions
for copperor by backfitlwith N2 and transferof the
eitherasdescribed
in air to solutionsin Teflon containers.The backfill and
substrates
typicallytookat the most2 and5 min.respectively.
transferprocedures
Slideswereremovedfrom solutionsafter the stateddurationof ex'
posure.washedwith absoluteethanol,blowndry with N2, and characterized. Samplesusedfor IR were preparedon 2-in. Si wafersand
in Petridishes.
functionalized
ireshly
werepreparedby transferring
samples
The alkyl sulfonate
of hexade'
samples
of copperandsilver,in air, to solutions
evaporated
After - l2 h of immersion,
acid(- I mM) in isooctane.
canesulfonic
blowndry with N2, andchar'
werewashedwith hexanes,
the samples
acterized.
texturesof the
Morphologyof the Substrate.The crystallographic
on Si(100)usedin
polycrystalline
supportcd
substrates
highlyreflective
by X-ray diffraction.Two independent
this studyweredetermined
wereroutinelymade,The first (a socalledd - 2dscan),involved
analyses
of an X-ray powderpatternto thoseof
of the intensities
the comparison
simpler
authenticpowdersample. The secondmethod,a somewhat
of thc ( I I I ) rockingcurvefor each
procedure,
involvedthe determination
substrate.Bothcopperandsilverwerefoundto be heavily( I I I ) textured
underpoor
(althoughlesssothan gold). Copperdepositions
conducted
yieldedrandompolycystalline
surfaces.Charactervacuumconditions
hasbeenreportedpreviously,36
izationof the goldsubstrates
(XPS). Spcctrawerc obtained
Spectroscopy
X-ray Photoelectron
that
spectrometer
SSX-100X-rayphotoclcctron
usinga SurfaceScience
Laibinis et al.
has previouslybeen described.33High-resolutionspectrawere obtained
at a passen.tgy of 50 eV using a 600-pm spot size: survey spectra (2
tcant; were obtainedat a passenergyof 100 eV and a 1000-pmspot size.
Use of peaks associatedwith the underlying substrate as refe-rencest
= 368.3
.vt9u!?.plr ) . =932'7eYl
eV ,A g(3d5rz)
l A ut4f,i ,)= 84.00
ieadsto'the unreasonabledifferdnces(range = 0.6 eV) in the positionof
C ( l s ) a s s o c i a t e dw i t h t h e m o n o l a y e r w h i c h w e b e l i e v ew e r e d u e t o
problemsof calibration of our detector;spectra were insteadreferenced
i o C { l s ) = 2 8 4 8 0 e V . A t t e n u a t i o nd a t a w e r e c o l l e c t e da t a p r e s s u r eo f
-3 x 10-eTorr and a passenergyof 100 eV using a 1000-rrmspot size;
the slideswere characterizedin randonnorder b-Vobtaining a single scan
( * 2 m i n ) a n d i i t t i n g t h e r e s u l t i n gs p e c t r aw i t h a n 8 0 7 oG a u s s i a n f2 0 V a
Lorentzian peak. Soectra of polycrystallinedocosanethiolrequired use
of a 2-eV flood gun t0 overcomecharging; spectra were referencedto
C { l s ) = 2 8 4 . 8 0e V .
Contact Angles. Static advancingand recedingcontact angles were
measureil{jn \tallc drttpsusing a Ram€-Hart goniometerunder ambient
) ith a Matrix
c o n d r t r o q , .( o r i . : i l r 0 g i r q u i d .ur e r e a p p l i e d( a n d r e r n o v e dw
'L
f c c h n r . i o s r * i c r r r ' ; . r p i p e t toep e r a t e da t i t s i o w e s t s p e e d( - I p l i s ) '
C o n t l c t : r n g i t ' \ r ' l i : r r r e . i s t t r eodn b o t h s i d e so f t h e d r o p o n a t l e a s t 3
s f t h c s l i d ew h i i e t h e t i p o f t h e p i p e t
c l r , r r rar p p i ; c C . rdt ; t f c r t ' n lp o s i t i o n o
r e n t a i n e da t t a c h e d
optics' and analytica!-PIg:
Infrared Spectroscopy.Thc -\.Decirometer.
tocolsusedin this pup.riruu. beendescribedin earlier publications.s'3?'5e
F o r t h e p u r p o s e so f t h i s s t u d y , g i v e n t h e d i f f i c u l t y o f o b t a i n i n g , ac l e a n
referenci spectrum on either silver or copper, we have used two different
references:- ( I ) monolayers prepared from octadecanethiol-d37on the
metal of interestas a referencemirror and (2) gold cleanedby oxidative
treatment in either peroxysulfuric acid or ozone under UV irradiation
(ABTEChI). (Caution: PeroxysulJuric acid reacts uiolently with orEanicmaterial.s and should be used with great care.ffi) This procedure
v i e t d sa s p c c r r u mf o r p r o t o n a t e ds a m p l e si n t h e 3 0 0 0 - c m - r r e g i o nw h o s e
i n t e n s , t i c e. r l r f l c r e l i a b l vu s e dl n t h e q u a n t i t a t i v ea n a l y s i sf r o r n w h i c h
-!og
t h e n t r ) r e L u i . ,l ri i - r , J n t a t i oanrse ' d e d u c c c iA. l i S p € c t r aa r e r e p o r t e da s
w
i
t
h
m
o
n
o
l
a
y
e
ra n d
t
h
e
,
'
s
u
b
s
t
r
a
t
e
o
f
l
h
c
:
c
f
l
e
c
t
i
v
j
t
v
R R , .u h r r c F l l h c
s t i i i z e df i l m
R , .r : t h e i c i l c c t t v t t i, r l t n e r e f ' e r e n c eS. p e c t r a sl i m u l a t i o n u
i h r c k n e s s eds e t e r r n r n e df o r t h e m o s t p a r t b y s i n g l e w a v e l e n g t he l l i p s o m e t r \ , . S e v e r a lr n d e p e n d e ndt e t e r m i n a t i o n sw e r e a l s o m a d e u s i n g s p e c troscopiccllipsometry'over the wavelength340-850 nm. The valuesused
f o r c a l c u l a t i o nasr e a s f o l l o w s :t l ) A u : C , 6 ( 1 9 . 0A ) , C , , ( 2 0 . 3A ) , C r t
( 2 1 . 6A ) , C , q ( 2 2 . 9A ) , C r ( 2 4 . 0A ) , a n d c r , ( 2 5 . 5A ) : ( 2 ) 4 9 : C 1 6( 2 1 . 5
A ) , C , ' t( 2 2 . 8A ) . C r E( 2 4 . 1A ) , C r e ( 2 5 . 3A ) , a n d c 2 e ( 2 6 . 6A ) ' W e h a v e
noted-earlierthat ellipsometric studiesof silver samplesare somewhat
problematicgiven the difficulty of obtaining reliable substrateconstants.
The values reported above were obtained from replicate measurements
shrelderifrom the iaboratory ambient. The valuesselected
on sub-ctrates
w e ; " e . ; i ir r e n i i r i n e di n t l i e R e s u i t s .f ' r o mi h e l o w e r r a n g eo f e x p e r i m e n t a l
rrcnsibtytt,the sizeSof the adsOrbatemOleCUleS
veitr,:s.lnr,\ -(...-:esixrndrng
u 3 r r J \ , , , , c , r r r :i r : ' g e rt h a n r h e a l i - t r a n se x t e n s i o no f t h e c h a i n . W e
A . T ' h ew o r s t p r o b e i i c y et h e \ e y a l u e si r - -b,c a c c u r a t ct o w i t h i n - * l
g 1 ' ; r -rr11- 1 cl ga l c u l a t l o n o
O
r i e n t a t i Oins s m a l l
t
h
e
c
h
a
i
n
i
s
o
l
'
.
.
h
r
s
lection
r p e r h a p si e s si h a n l ' i n r i o n s i l v e ra n d g o l d ) . T h e m o s t i m p o r t a n tp o i n t
t o n o t e l s t h a t t h e n u m b e r su s e da r e b u t o n e p a r a m e t e ru s e dt o s i m u l a t e
the optical anisotroptesevidencedin the experimentaldata. Selectionof
thicknessvalues correspondingto an all-trans extension of the chains
would yield very similar quantitative fits to that shown above.
NoteAddedin Proof. Recentdiffractionstudies(X-ray and
helium)haveshownthat the latticeconstantof n-alkanethiolate
on silveris shorterthanthat on gold. The
supported
monolayers
neighborspacing
unit ceiiof the formeris -107o less(nearest
with that inferredfrom
of 4.77A). a resultin verygoodagreement
formedis an incommensurate
Theoverlayer
infraredspectroscopy.
of an expanded1{1 x
phase.
reminiscent
one
hexagonal
P'; Li, J.;
P.; Eisenberger,
Fenter,
See:
t1)ntO.9o structure.
T.
G.; Ramanarayanan'
S.;Scoles,
S., Ill: Bernasek,
Camillone.
A.: Liang,K. S. Langmuir,in Press.
chaintilts on Au( I I I ) are alsofoundto be in
The measured
with the valuesinferredfrom vibrationalspecgoodagreement
communication).
troscopy:G. Scoles(personal
An,
Ag, 7440-22'4;
Cu, 7440-50-8;
RegistryNo. HD, 544-76-3;
(5E) Briggs. D.; Seah, M' P. Practical Surface Analysis; John Wiley:
cited therein.
Chichester,-i983;nppendix 4 and references
(59) Dubois,L. Fi.l Zegarski,B. R'; Nuzzo, R. G. Proc. Natl. Acad' Sci.
u.s.A, 1981, 84, 4739-4742.
, H--Qhey..F!g'Nrnt
, . G . ; T h e o p o l dK
( 6 0 ) D o b b s ,D . A . l B e r g m a n R
1990.6S ( I 7), 2. Erickson,C. Y . Chem' Eng. Newst990' 68 (32), 2'
7 t67
7 4 4 0 - 5 7 -C
5 ;H 3 ( C H ) , . S H( n = 3 ) , 5 1 3 - 5 3 - lC; H 3 ( C H 2 ) " S(Hn = 4 ) ,
l l 0 - 6 6 - 7 ;C H 3 ( C H 2 ) S H( n = 5 ) , l l l - 3 1 - 9 ; C H 3 ( C H 2 ) ' S H
(n = 6),
(n = 7), I I l-88-6;CH3(CH2),,SH
1639-09-4:
(n = 8),
CH3(CH2),rSH
(n = 9), 143-10-2;
1455-21-6;CH3(CH2),.SH
(n = l0),
CH3(CHz),SH
(n = I l), I l2-55-0;CH3(CHJFH (n = l2\,
5332-52-5;
CH3(CH2),.SH
(n = l3),2079-95-0;
19484-26-5;
CH3(CH2),SH
CHr(CH2)'SH(n =
l4), 25276-7G4;
CHr(CHz),SH(n = I 5), 2917-26-2:CH3(CH2)'SH
(n
= l6), 53193-22-9:
CH3(CH2)'SH(n = l7), 2885-00-9;
CH3(CH,),SH
(n = l8), 53193-23-0;
(n = l9), 13373-97-2:
CH3(CH2),.SH
CHr
(n = 20),66326-t7-8;
(CH2),SH
(n -- 2l ), 7773-83-3.
CHr(CHz)SH
Supplementery
Materiel Aveileble:Tableof wettingproperties
preparedfrom differentsolvents,IR spectraof
of monolayers
low-qualitymonolayers
on copper,plot of intensityof CH2 modes
with chainlength,anddiscussion
of themethodsusedin simulating
IR spectra(l I pages).Orderinginformationis givenon any
currentmastheadpage.

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