4o(
M OLE CUI-AR SELF-AS SEMB LY AI..{DMI CROMA CI{INI}.I G
N. L. ABBOTT, H. A. BIEBIITCK S.BUCH}IOT7, J. P. FOLKERS,M. Y. }IAN,
A KUMAR, G. P. LOPEZ,C. S.WEISBECKERAND G. M. WHITESIDES
Deparrmenr of Chemisrry
Hart ard Universiry
12 Oxford Sneet
Cambridge iv{A 02138
USA
ABSTRACT. Wc are developingmolccularsclf-r.semblyas a stratcgyfor the fabricarionof nanosrructuies.
W e hav e u s e d s e l f-a s s e m b l em
d o n o l a yerfi l ms formcdby thc chcmi sorpri on
of organi cmol ecul es
onro gold
s ubs t r at e si n, c o mb i n a ti o nw i th m i c ro m a chi ni ng,
(i ) to pauerngol d surfacesw i rh w el l -defi ned
regi onsof SAM s
*' it h c ont ra s ri n gp ro o c n i e s ,(i i ) to tra n s fcrthesepatterns
to the underl yi nggol d fi l m usi ngS A Ms as nano m eler t h i c k c ; r e m i c a rl e s i s t si o r w e l c t c n i n g (, i i i ) r o c o n s t r u cm
t i c r o e l e c r r o d eb sy r a k i n ga d v a n r a goef r h ec i i e l e c : r i c
bar r ierpr o o c n i e so f SA l v ts ,a n d (i v ) ro c;e3tecontami nati on-resi stanr' t' ion
i ms
rhe sunacesof meral srharar e
o t h e r w i s ch i g h l vs u s c c c t i b l e ' tcoo n t a m i n a t i obn. va c i v e n r i r i om
u so l c c u l aar d s o r b a r easn c pi a n i c l c s .
1. Introduction
Molecularself-assembly
is the sDontaceous
seif-organizarion
of rnolecules
inroequiiibrium
supramoiecuiar
(1-3). Alrhoughmolecularself-assernbly
srrucrures
is ubiquirous
in nature
( f o r e x a m p l e ,b i o l o g i c a i l i p i d m e n b r a n e sa n d c r y s t a l l i z a r i o n )i r, i s a n e w s r r a t e g yi n
materialssciencsfor the fabrication
of naoomerer-scale
srrucrures.Usingnarurero provide
the *exjsienceirecrern",w€ havedesieneci
andsynchesized
a rangeof organicmolecules
that
sDontaneouslv
organizethemseives
into highJystnrcruredrwo-diraensional
supramolecular
arravsanciwe are currenrlycieveiopine
applicaiions
of rhesenew nanosrnrcrures.
Severaicharacteristics
of moiecularself-assenbly
make it an arrracrive
srrategy
for
making nanostluclures.First,moiecuiarself-assembiy
is characrerized
by the sponraneous
evoiurionto an equiiibriuu-r
strucrure;intervencion
wirh the
by rhe technologisc,
concurrenr
of
self-organization,
is not Decessary.
Furthernrore,
becausethesesirucrures
existat
Proess
their thermodvnamicminima, they are inherentlyself-repairing;a transienrperrurbation
ro
the strucrurew'iil be foilowedby a teudencyto reorganize
back to the equiiibriumcondition.
S e c o n d , t h e p r o c e s s o f m o l e c u i a rs e l f - a s s e m b l cy a n b e r e m a r k a b l yf r e e o f e r r o r s ;
organizacion
of as many as 1012molecules
canbe routinelyachieveci
wirh few defecsin rhe
resuitingsuperstrucrures.
by thc remarkablycomplexself-assembied
Third, asdemonsrared
structuresfouuciin narure,the useof cleanroom faciiitiesis noca prerequisite
for rheselfassenblyof moleculesincostrucrures
wichAngstrom-scale
precisioo.Finallv,wheoorganic
moieculesare useC,rireproperties
of rhesemolecules,and therefore,the funclionsof the
cau be manipulatedusing well-loown techniquesof
resulting supranolecularassemblies,
has
of self-assembiy
trg.oi"""nd'bioLogical synthesis.Recognitiooof th.s. characteristics
selfusing
as a slrategyfor nanofabrication,
prompred's to exploremolecularself-assembly
(4).
a
Lsembled monolayersof organicmoleculesas model system
4 . 9 9 5A
A u ( 1 1 1 )A
. uSR
i3xYgRgO"
a Au(ii1)
monolayerformedby exposing
of a self-assembled
srrucrure
Figure1. Scheroaric
sunaceto CH3iCHZ)iiSi{.
bv
crys.tals
monolayers(SAjvls)are quasi-rwo-dimensional
.formed
Seif-assembiec
o-1Au'
thiois
of metals(e.g.organic
on rhesurfaces
of organicmolecules
the chemisororion
oxides
alZO:), ind semiconductor
onto
acids
Ag, Cu), meral oxides(..g..if.*t
organic
".rUo*ylic
to tbe
(e.g.organic rrichiorosii.i.s on Sio2l fil. Exposureof .thesolid substrate
of the
ordering
sPoltaDeous
molecuies,eirherin solurionor in thevaporphase,resultsin tbe
or solid-vaporintenacefFigury-1)'-Becausethese
organicmoiecuiesar the solid-soiurion
io Figure1) of the
monoiaverfrims arehigirly orderedand becausethe terminalmoietiesCX
chemistr],these
organic molecul.s can be varied using techniquepin syntbeticorgaDic
As a result'
interfaces'
sysreps offer a remarkablelevel of controlouer the propertiesof
phenomena
SAlvfs bave servedexrensivelyas modei sysrernsfoi the study of interfacial
(werting,adbesion,reactiviry)i" organicsystelns'
on gold s.ubstrates
The besr characleize,dSalvfs are probablyfte organic thiolat:s.
incluciing
to
nanofabrication,
(4).Using this systenwe iraueeiplorecifour rhemesreleianr
tbe dcvelopnent of self-assemblyas a techaiqueto complementexisringmerhods
of
microfabricatioo, as well as a future tecbnique for making nanostructures
thatcould
potentially serve as rDemoryandlogic deviccs.
2. Applications
2.1. COT.ITAT,{INATION.RESISTAIVT SURFACES
How molecuiar self-assemblycan complementthe already existing technologiesusedfor
making electronic devicescao be illustratedwith the idea of deve[opingsunlces thar are
resistantto contamination.Cunentprocesses
for microfabricationdimind stringentclean
room environ-rnents
to reducecontarnination
(by parriculates
and vapors)of high freeenergy
surfacesof electronicmaterialssuch as bare metals and silicon. Becausethe rare of
cool2minationof a surfacedependson thefree energyof is surface(marerialswirh high free
energysurfacesadsorband stickto contaminants
more quickly and tenaciously
thando low
free eaergysurfaces),and becauseit is weil esrablished
thar nanomerer-thick
organicfi6ns
formed from SA-rVscan be usedto createlow-energyinterfaceson high energysubstrates
( 4 ) , - o o ep r o m i s i n ga p p l i c a t i o no f m o l e c u l a rs e l f - a s s e m b liys t o p a s s i v a rteh e e x p o s e d
sunacesof marei-ialsduring (andafrer)fabricationwith aopropriatelychosenSAiVIs.For
sxample,lhe sunacefreeenergyof baregold,which is approximarely
400dynesla z, canbe
reducedto 20 ciynes/car
by forning a SfuV thatexposesmeihyl groupsat the sunace.Using
scanningelectronmicroscopy(SEM) to imaeefilms formecifrom molecularadsorbares,
we
havedemonstrated
that SAtr4sc3nbe usedto reducethe degreeof conraminarion
of a surface
from airborneand solurion-borne
(5). We haie aiso observedrharrheeiecrron
conta.ninants
bea'n-isduccci
of coocamination
and ciesorption
of carbonaceous
films in rheSElv{
Processes
can be influencedby the preseneand rypeof SAM that is exposeCto the elecu'onbeam(5).
Self-assenbieC
monolaversforned from fuorocarbons(nanometer-thick
"Teflon" fiiros)
aPPearto be the most effectivein reducingthe contaminarion
of surfaces.\\'e expecrthar
issuessucaas the conrroiof sunacecontamination
and"siickiness"wiil becomeincreasingiv
wirhrhecjevelopnenr
imponanc
of ccnracr
Iichoeraphy.
2.2. i\dCR.OMACFilNi:NG
A\D MOLECUIIR SELF.ASSEMBLYTO PATTERN
SURFACES
We bave beeadevelopinga rangeof methodsthat ccmbinemolecularself-assemblv
wirh
micromachining(6), mic;owriting (7) andmasking (8) to pattern surfaceswirh regionsof
SA"rVswiih conrasring properties.The principleol regionjspecificdeposition(or rlmoval)
of materialsfrom a substrarebroadlyderlnescurrenrmiirofabricaiion pracrices.Oui
aPProaca,
however,takesadvantage
of existingcapabiiitiesin organicchemisrryro design
and synrhesize
moleculeswirh a variervof oropertiesasd to transfertheseproperries,
usiig
moiecularseif-assembly,
to weil-defined
regionsof a sunace.We havecombinedmolecular
self-assembly with micromachining, microwriting and masking to Pattern surfaceswith
fearures(a rigion of a surfaccwith a properrythat is distinguisbableftgt its surroundings)
tbat rangi io iir. from ceutinerersto 100 nanometersaod whi"!, io p!!c-tple,..9lld-extend
dowu to tbe scale of molecular dimensioos (nanometers)using AIiM and STM. The
precision cottrastswith optical
potential ability to control stnrcrurcwith nanorneter-scale
effeca) to linewidths
(by
diffraction
optics
field
iithogt.phy which is limited using far
greaterthan 0.3 to 0.5 pm.
We have developeda processof patterninga surfaceby combgrng tlq$achin-tng
and molecular self-assemblyin threesteps(Figure 2): (i) formation of an initial SAI'I of an
organic rhiolare oB goid; (ii) generationof regions of bare gold in the SAlvt by
micromachining;ana (iii) formition of a sccondSAM on tbesemicromachinedrcgions(6).
using either a surgicalscalpelbladeor the end of a
Wirh simple .ftror.iirioing techniques,
carboo fiber, we have patterneda zurfacewith micrometerand submicrometer(100 nm)
the coatrol of surfacePlopelies by using
resoiurion,respectively.We havedemoostrared
well-defined
regions9-fSAlvfswith conrrasting
this new rype of riooiabricarion ro generate
which
liquids wet thesesurfacescan
werubilirie! on goid and silversubsirates.The way in
be used as a coovenienrtechniqueto imagethe pacternsgeneratedon suchsurfaces(these
svsremsalso acras imporrantrod.ls for srudyingthe fundamenralpropertiesof wertingand
of
the patternrbg
a-dhesion
ar organicinierfaces:Figure3). Alrhoughwe have demonstrated
can
be
principle
this
that
we
beiieve
wettabiiities,
of conrras-ring
surlaces.wirh-regions
g e n e r a lzi e d , f o r - e x a m p l e u
, s i n g o r g a n i cm o l e c u l e st h a t h a v e c o n t r a s t i n ge l e c r r o a c t i v e
*.,riog properries.Indeed,SA-,Vsformedfrom molecules
iroperries,rarherrhanconrrasrin{
(9).
moietiei(forexample,ferrocene)havebeen prepared
coniainingelecrroacrive
ord-erto
techniqu.t
simple
we
used
have
In rheseexamplesof niciomachining,
il
More
rhe resuli of combininemicromacbiningand molecularself-assemblycjemonsrrare
atomic
Jut useof micrornachining_tT!"iqlesto inciude
recenrlv,w€ havebegunexrending
(AFM)
ands..nni"g tunnelingmicroscopy(STM)._UsingSTM with high
force micioscopy
bias voitages(i- iO V) we have removeciwell-definedregions of 1 SAM formecifrom
(10). We believethat the combinationof AFIvVSTM
hexaciecanerhiol
on a gold subsrrate
and molecularself-asslurblycanbe usedto make parternsin SA"rVsthat are (much)smaller
r h a a 1 0 0 n r n . T h e u l t i m a r el i r n i r t o t h e r e s o l u i i o no f t h i s t e c h n i q u ew i l l p r o b a b l yb e
on rhesurfaces.
ciererniinei
by rhelateralmobiiiryof thechemicaispec:es
2.3. DIELECTzuC
BARzuER.S
of electronicdevices.We, and ochers,have
components
Dielecrricbaniersare essential
on a gold
rhar a SAM formeC from hexadecanerhiol
showu (wirh cvclic voirammerr:v)
subsrrarecan ieduce rhe rare of tt.ntporr of electronsberween the qolg surfaceand
concaclingeiecrroiyre
(0.1M HZSO+)Ui a facrorabout 10,000. Althoughthereare many
uoanswered
ouestions
Figure 2. Schematicillustrationof
the formation of 0.1 to 1 pm scale
lines with property Y (for 6lample,
Y = COOH for a hydrophilic
surface) in a surface of X (for
= CH: for a
example, X
hydrophobic
surface) with
micromachining and SAlvIs. We
imply no asymmetryin the strucrure
of
tbe SAMs
within tbe
m i c r o m a c h i n e dg r o o v e s . A u =
evaporatedfilm of gold; Ti =
evaporatedfilm of Ti used to
p r o m o t ea d h e s i o no f t h e A u t o t h e
siliconwafer (Si).
roogoA
-zoooA
-t00A
;*
micromachi."
I
0 . 1- 1 F m
Ti
'i:l:i:]silli
Y(clLi
Y(ci1l,s
_
0
X
I
(gH2),g
I
S'
Y
I
0 (9H.).',,
I
s-
Figure 3. Drops of water on a hydrophilicSAlv{ formed from HS(CH2)1SCOOHarepinned
by micrometer-widehydrophobiclibesformedfrom [CH2(CHZ)1tS]2. The gold subsrate
was Patternedw'ith SAMs by combiningmicromachiningand molecularself-assembly
(see
F
^ .i o
5 r. r. r- e
-), ?\
surounding tbe mechanismof clecuon transportacrossa surfacecoveredwith a SAlvf (e.g.
pinhole defecs, electron tunneling,thin spos), the ability of a SAlvf to serveas nanoEeterihick dielecrric barrier is a promising one. For example, the electdcal function of metal
oxides to act as barriers to elecrroni in runnelingjunctions rnay also be achievedusing
SAlvfs. We have used the dielecrric barrier properties of SAM to coDstructband
microelectrodes,created by micromachiningbare gold grooves in the surfaceof a golq
of
subsrrate*u.t.d by a SAtvl. We bave demoniuatedmicroelectrodebehavior(the.absence
1passtransport limitation during cyclic vs[iemmetry) for-an aqueousferricyanidesysteE
(Figure 4). Becausethey have misrometer-widths,tbesemicroelecuodesare lessProneto
Eass transport limitarions tban are rnacroscopicelectrodes. The rapid resPonsetimes of
these electrodesand their easeof fabricationsuggeststhac SAlvfs may serye as useful
dielectricbarriersfor sensorsvstems.
ScanRate10 mV/s
"BareA
" u
CHs(CHe)1sSH/Au
+0.2
Micromachineci
CH3(CH2)15SH/Au
m
{
- /t
I
lllllr;';t
#
0
-0.4
0
+0.1
?\J.n
E M v s .S C E
^
(0.1M
Figure 4. Rare of elecrrontransferto an aqueoussolution of 1 mM Fe(CN)6'ra
SAM
by
covered
XaCtOq, pH 7) using (.) a macroscopicbaregold surface,(b) a gold nlm
of
formed from CHI(e HZ)15SFI,and (c) micromerer-wideband-shapedmicroelectrodes
baregold formeCby micromachiningthe surfacefrom (b).
2.1. C}IEMICAL RESISTS
can serveas nanometer-thick
resists. In acjditionto beine good dielectricbarriers,
SA-lr4s
SAlv{scan aiso be good barriersro cerrainions in aqueoussoiution,includingionic species
that act as wet chemical etchans (11). We have demonstratedthat SAlvfsformedfrom
hexadecanethiolon a gold film (200 nm thick) can protect the gold film from atr aqueous
solutioo of alkaline potassiumcyanide(a well hown etchaotof gold) for a sufficienttine
that unprotectedgold areascan be etchedto tbe siiicon substrate(11). Thc abiiiryof a SAlv{
to act as a barrier to etchantsof metaishas allowed us to dcvelop simple tecbniquesto
transfer patterDscreated in SA-ttfson a gold subsrrateinto the underlying gold film. For
example, using micromachining to expose a bare gold line ia a SAIVIformed from
hexdecanethioland then immersingthe entire5:mple in an aching solutionof 0.1M KCN (in
1M KOFI), *. bave createdmicrometerwide trenchesin a gold film (Figure4). With an
alteruative procedure,we have usedneatliquid hexadecanethiolas atr "i.D,k"to write (using
micropens)micrometer-widelinesof hexadecanethiol
onto a goid nin. By etchingtheentire
by the SAM
film we have for:nedconductingmicrowiresof gold from the regionsprotected
(Figure 4).
Figure 4.- Scanningelectronmicrograpirs
of a 10 Fm gold wire (a and b) anda i prmtrench
(c and d) in a gold fiim. Borh microsrn:crures
were madeusing a SAIvI of CHZ(CH2)15S'
as
a resislto an erchingsolurionof 0.1M KCN and 1M KOH (seetext and ( 1 1 ) f o rd e t a i i s ) .
3. Future Prospectsand Conclusions
If molecular sclf-assemblyis to serveas a usefulstrategyfor nanofabrication,we belicvethat
some of the important, .od ]€t unrnsweredquestions,are (i) wbat degreeof perfectionis
igshnique?;(ii) how do we measurethe degrecof imperfecrion
required of a gif-.r."nbling
in a SAlv{?; and (iii) how I"r u. we from what is required? To addresstheser1ryesof
questions,we .r. b".rr.loping asTyl for the detectionof defectsin SAlvIs. In one approach
we usc wet etching to dei..itb. alfects becauseetch pits (y$ch can be observedby SEM)
form ar defecrssites. The resuls of this assayindicaic tldi (io. wet chepstry laboratory)
we are able ro produce sAlv{s on gold wirh fewer than 1 defect per mmz @t 1 defectper
l3lz molecul es)(12).
Many oih.t interesringchallengeswill be faced before molecular self-assembly
becomes a rourine rool for- nanofablication. For example, our curreDtforms of
and therefore,thesetechniquesInay
micromachining and microwriting are serialprocesses,
be
be too slow for"industriaipro."rl.s. It is possible,however,tbat thesetechniques.can
our
made ro be parailel, for example,by stanpiog-typeprocesses.Finaily, we note that
recenrdemonsrrationsof forming SAlvfson colloidalgold suggestexcitingpossibilitiesfor
colloidscoveredwith SAlr4s(dielectricbarriers)and
nanofabricarion(i3). For exam-ple,
surfaiecouldform thebasisof singieelectronrransistors.
assembiedonro a parrerned
is
stntcrures
as a principlefor the fabricationof small-scale
Moleculai seif-assembly
a new srraregythacis at the srrg. of "'concipC'ratherthao "developmen!".UsingSAJv{sas
to seryebochas a
model ryt,.i-ti, w€ are explo.ingthe porenriaiof molecularself-assembiy
as
the
furure, the foundarionof
ind, perhaos,furtherin
suoplemenrro exiscingtechnolJgies
new typesof eiectronicsdevices.
ACIC{OWLEDGMENTS
Projects
and the DefenseAcivancedResearch
Supportedby rhe Office of Navai R.eseerch
Agency.
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