DTIC ADA524373: Self-Organization of FePt Nanoparticles on Photochemically Modified Diblock Copolymer Templates

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S
N
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A Inc.)undervacuumfor30mintoselectivelydegradethePMMAand Self-Organization of FePt
C crosslinkPS,leavingbehindtheporousPStemplate.
I Nanoparticles on Photochemically
N
Received:January24,2005
U Finalversion:July24,2005 Modified Diblock Copolymer
M Publishedonline:September1,2005
Templates**
M
–
O
C [1] I.W.Hamley,ThePhysicsofBlockCopolymers;OxfordUniversity BySeth B. Darling,Nataliya A. Yufa,
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[5] R.E.Segalman,Mater.Sci.Eng.Rev.2005,48,191.
[6] T. Thurn-Albrecht, J. Schotter, A. Kästle, N. Emley, T. Shibauchi, cant attention in the last decade as enabling materials for
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[7] M.Park,C.K.Harrison,M.Chaikin,R.A.Register,D.H.Adam- ty of the self-assembled domains by means of manipulating
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molecular characteristics. Recently, the potential opportu-
[8] A.M.Urbas,M.Maldovan,P.DeRege,E.L.Thomas,Adv.Mater.
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[9] H.Ma,A.K.-Y.Jen,L.R.Dalton,Adv.Mater.2002,14,1339. by new methods, which give fine control over long-range
[10] J.T.Chen,E.L.Thomas,C.G.Zimba,J.F.Rabolt,Macromolecules ordering of the microdomain structures.[4,5,7,9] Thin polymer
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myriadfunctionscanbeaddressedwithhybridhard/softmat-
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[14] E.Kretschmann,Opt.Commun.1972,6,185. nanoscale organization of inorganic materials. Of specific
[15] W. Knoll, in the Handbook of Optical Properties Vol. II, Optical interest is the interaction of surfactant-mediated colloidal
Properties of Small Particles, Interfaces and Surfaces, (Eds.: R.E. nanoparticlesanddiblockcopolymerfilmsbecausethenano-
Hummel,P.Wissmann),CRCPress,BocaRaton,FL1997. crystal capping molecules can be tailored to exhibit prefer-
[16] U. Jeong, D.Y. Ryu, D.H. Kho, J.K. Kim, J.T. Goldbach, D.H. ence for one of the polymer blocks.[10–13] This hierarchical
Kim,T.P.Russell,Adv.Mater.2004,16,533.
approachtocreateorderednanostructuresremovesthelinear
[17] W.R.Thompson,M.Cai,M.Ho,J.E.Pemberton,Langmuir1997,
13,2291. correlation of size and patterning time associated with tradi-
[18] J.W.Robertson,M.Cai,J.E.Pemberton,Adv.Mater.2001,13,662. tional lithographic techniques by self-assembling the entire
[19] P.Mansky,T.P.Russell,C.J.Hawker,M.Pitsikalis,J.Mayes,Mac- surface in parallel. Also, the spatial limits of lithography can
romolecules1997,30,6810.
be transcended and the approach can potentially be adapted
[20] P. Mansky, Y. Liu, E. Huang, T.P. Russell, C.J. Hawker, Science
toindustrial-scaleprocessing.Analternativeapproachtothe
1997,275,1458.
colloidal nanocrystal methodology is to use spherical- or cy-
[21] M.Maldovan,M.R.Bockstaller,E.L.Thomas,W.C.Carter,Appl.
Phys.B2003,76,877.
[22] C.G.Granqvist,O.Hunderi,Phys.Rev.B1978,18,2897.
[23] Adecreaseofe from2.220to2.048wascalculatedfromcompari-
D –
sonbetweenthedielectricconstantoftheinitialfilm(e =e =2.402,
x y
e =2.429)andthatoftheporoustemplate(e =e =2.377,e=2.385). [*] Prof.S.J.Sibener,A.L.Cisse
z x y z
Then, the best fractional combination of air and PMMAwas de- JamesFranckInstituteandDepartmentofChemistry
ducedtofittheeffectiveconstantof2.048,usingEquation2. TheUniversityofChicago
Chicago,IL60637(USA)
[24] Thefilmwassubjectedtotheexposureconditionsusedtoetchaway
PMMAdomainsinathinPS-b-PMMAfilmofthickness∼L (D.H. E-mail:[email protected]
0
Dr.S.B.Darling,Dr.S.D.Bader
Kim,Z.Lin,H.-C.Kim,U.Jeong,T.P.Russell,Adv.Mater.2003,15,
MaterialsScienceDivisionandCenterforNanoscaleMaterials
811),whichisabouttentimesthinnerthanthethicknessusedinthis
ArgonneNationalLaboratory
study.
Argonne,IL60439(USA)
[25] D.E.Aspnes,ThinSolidFilms1982,89,249.
N.A.Yufa
[26] Notethatthevaluesofe ande are1.774and1.845,respec-
PBS EtOH JamesFranckInstituteandDepartmentofPhysics
tively. TheUniversityofChicago
[27] K.H.A.Lau,L.-S.Tan,K.Tamada,M.S.Sander,W.Knoll,J.Phys. Chicago,IL60637(USA)
Chem.B.2004,108,10812. [**] TheauthorsthankWardLopesforusefuldiscussionsandA.C.Sa-
[28] J.L. Wang, T. Grimaud, K. Matyjaszewski, Macromolecules 1997, mia,J.Schleuter,andX.M.LinforprovidingtheFePtnanoparticles
30,6507. used in this study. The work was supported by the University of
Chicago–ArgonneNationalLaboratoryConsortiumforNanoscience
Research, the NSF-Materials Research Science and Engineering
CenteratTheUniversityofChicago(NSF-DMR-0213745),andthe
______________________ AFOSR sponsored MURI Center for Materials Chemistry in the
Space Environment. Work at Argonne is supported by the U.S.
Department of Energy, Basic Energy Sciences-Materials Sciences,
underContract#W-31-109-ENG-38.
2446 ©2005WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim DOI:10.1002/adma.200500960 Adv.Mater.2005,17,2446–2450
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4. TITLE AND SUBTITLE 5a. CONTRACT NUMBER
Self-Organization of FePt Nanoparticles on Photochemically Modified
5b. GRANT NUMBER
Diblock Copolymer Templates
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lindrical-phase diblock copolymers as etch masks to produce solution concentration, but higher concentrations typically U
self-assembledarrays of nanoscale dots, pillars, or lines com- lead to excessive unwanted aggregation of the nanoparticles. N
posed of various materials.[3,14–16] The advantage of our ap- Anotherroutetoobtainhighercoverageistoperformmulti- I
C
proachisthatthelength-scaleofthefinalfunctionalmaterial pledepositionsusingtheprocedureoutlinedabove.Following A
is an order of magnitude smaller than the original polymer threesuccessivecastings,weobtainedclusterdensitiesonthe T
I
domains. surfaceof∼300/lm2. O
N
WehaveselectedFePtnanoparticlesbecausetheyarecen- StructuralcharacterizationoftheFePt/PS-b-PMMAsystem
S
traltomagneticapplicationsowingtotheirchemicalstability can be achieved using scanning or transmission electron mi-
andthereporteduniaxialmagnetocrystallineanisotropyofK croscopy(SEMorTEM)oratomicforcemicroscopy(AFM).
u
∼ 7×106Jm–3.[17] A large anisotropy correspondsto a stable High-energyelectronsfromSEMandTEMhaveahighpro-
magnetic moment. The probability, P, of thermal activation pensitytodamageboththepolymerfilmandtheorganicsur-
reversing the magnetic axis of an individual nanoparticle factant molecules surrounding the FePt cores, so AFM was
scalesas: usedin theseexperiments. Imaging with AFM requirescare-
(cid:1) (cid:2)
fultuningofthetappingconditions.Theselectionofthetap-
K V
P(cid:1)exp (cid:2) u (1) ping mode AFM imaging parameters, in particular the tip
k T
B driving amplitude, has a significant effect, not only on what
whereVistheparticlevolume,k isBoltzman’sconstant,and one observes but also on the structure of the system itself.
B
T is temperature. Even room temperature is sufficient to Verylighttapping(∼50mV)allowsonetoimagethenanopar-
inducefluctuationsinthemagneticmomentsinsmallcrystals ticles resting on the polymer film, but not the fingerprint
of most materials. For this reason, highly anisotropic nanos- domains of the polymer. The PS-b-PMMA cylindrical struc-
cale FePt particles, which are unusually well protected from ture becomes apparent along with the nanoparticles as the
unwantedmagnetizationreversals,havebecomelikelycandi- driving amplitude is increased to moderate levels (100–
dates for future ultrahigh-density recording media.[18] Synth- 200mV).Thisistheimagingregimeusedtoanalyzetheselec-
esisofFePtnanoparticlesisbasedonthepolyolprocessasde- tivity of nanoparticle adsorption. When the amplitude is
scribedbySunetal.[19]Asprepared,thesenanocrystalsarein intensifiedbyafurther200%(∼300mV),manyofthenano-
a chemically disordered face-centered cubic (fcc) phase and crystalsarepushedintothepolymermatrixordraggedacross
require annealing above 803K to achieve the high-K L1 the surface by the tip. While a precise determination of the
u 0
face-centered tetragonal (fct) phase. This annealing process minimumAFMtipforcenecessarytomovethenanoparticles
hasprovenproblematicinthatthenanoparticlestendtocoa- ismadedifficultbycomplexinteractionsbetweenthetipand
lesceathightemperatures,therebydefeatingtheadvantageof the soft substrate,[28] various theoretical analyses suggest the
small, dispersed FePt particles for recording applications.[20] imaging conditions discussed above produce peak forces of
Recentworkhasshown,however,thatthecoalescencecanbe theorderof40nN.[29–32]Theseeffectsarereadilyobservedby
prevented by using suitable linker molecules or oxide coat- repeatedlyimagingthesameregionunderhardtappingcondi-
ings, and there are reports that the fct phase can even be tions and watching individual nanoparticles disappearing or
directlysynthesized.[21–24]Forthepresentstudy,wehavecho- plowingtroughsthroughthepolymerastheyarepulledalong.
sentoworkwithfccFePttoestablishtheeffectivenessofthe Carewastakenduringthisstudytopreventtipinfluenceson
polymeric-templatingmethodology.Sincetheassemblyisdri- thenanoparticlepositions.
venbyinteractionsbetweenthesurfactantcappingmolecules Nanoparticleadsorptionontheas-preparedfilmsexhibited
and the polymer substrate, the magnetization of the core is a moderate preference for the PS domains with 50–80% of
assumedtoplayanegligiblerole.Creatingorderednanostruc- theFePtresidingthere(notshown),guidedbythehydropho-
tureswiththeL1 phaseremainsthelong-termgoal. bic interactions between styrene and the alkane tails of the
0
Theasymmetricpoly(styrene)-block-poly(methylmethacry- oleic acid capping molecules. The distribution of nanoparti-
late)(PS-b-PMMA)diblockcopolymerusedinthisstudyhas cles between the two domains displayed significant variation
29wt.-%PMMA,sointhebulkitformshexagonallypacked between different samples despite similar preparation condi-
PMMAcylindersinaPSmatrixwithanaturallayerthickness tions.Inordertomaximizetheselectivityandstrengthenthe
(L)of30nm.Usingtheprocedureoutlinedintheexperimen- nanoparticle–substrate communication, one must modify the
talsection,weachievedfilmswithanaveragethicknessofL, chemistryofthepolymerand/orthecappingmolecules.Dual-
whichhaveanunusualdualpresentationofPSandPMMAat surfacepresentationofthesePS-b-PMMAfilmsisararechar-
the air interface due to the half-cylinder structure resulting acteristicthatisconvenientfortemplatingapplications,soitis
fromsimilarsurfaceenergiesfortheblocks.[25]Theselaterally more straightforward to alter the chemistry post-casting
alternating domains have significant short-range order but ratherthantobeginwithadifferentdiblockcopolymermate-
form “fingerprint” patterns with no long-range order unless rial.
guided by external constraints.[26,27] The FePt nanoparticle We modified the polymer templates with vacuum ultravio-
deposition procedure described in the Experimental section let (VUV) radiation prior to FePt deposition. Fourier-trans-
leadstonanocrystalclustersurfacedensitiesofapproximately form infrared(FTIR) spectroscopy, ultraviolet photoelectron
100/lm2. One can adjust the coverage by manipulating the spectroscopy, mass spectrometry, and NMR studies with
Adv.Mater.2005,17,2446–2450 www.advmat.de ©2005WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim 2447
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A PMMA homopolymer reported in the
C
literature have been used to character-
I
N ize its response to high-energy irradia-
U
tion, including VUV as used here-
M
in.[33–35] PMMA reacts through several
M
routes: severing the C–C main-chain
O
bonds, cleaving ester side-chain link-
C ages, andgeneratingC(cid:1)Cbondsin the
backbone. PS is believed to crosslink
under VUV exposure resulting in a
chemical presentation at the interface
similar to that present before irradia-
tion.Therefore,ourprocessisexpected
to selectively etch away the PMMA
half-cylinders leaving a highly corru-
gated PS surface; thishas been verified
bymeansofAFM(Fig.1).Figure2de-
picts FTIR spectra of the PS-b-PMMA
film before and after VUV irradiation.
Spectra of PS and PMMA homopoly-
mers (notshown)reveal that the decay
of the peak centered at ∼1730cm–1,
characteristic of a C(cid:1)O stretch (see
inset), is indicative of PMMA degrada-
tion. The chemical moieties displayed
within the etched regions are inferred
to be a combination of methyl methac-
Figure1. Effect of VUV irradiation on a PS-b-PMMA ultrathin film depicted in schematic form
rylate fragments and oxidized radical
(a,d),withtappingmodeAFMphaseimages(2lm×2lm)(b,e),andwithlinescansshowingsur-
bonds. These more reactive species facecorrugation(c,f).Pre-exposuresurfacecorrugationis∼0.6nm,withthePMMAphaseappear-
attract the nanoparticles to the etched ingbrighter(higher).VUVirradiationcausesselectiveetchingofthesurfacePMMAhalf-cylinders
domains. This results in two notable resultinginacorrugationof∼8nm.Theetchedsurfaceisahighlyselectivetemplateforthead-
sorptionofFePtcolloidalnanoparticles.
changes: 1)a reversal of the FePt do-
main selectivity when compared with
theoriginalfilm,i.e.,fromPStoetched
PMMA; and 2)a significantly higher h
c
degreeofselectivity(Fig.3).Apossible et
str
adwsemeoldteiteclt-geuhhnwerteitneinadooatrtnttfiidhilnvyoetdgehmrtepiwreaeloxaihsdunpyerg,sdlonahtupanhsrl,areeiinbotttriegeloembcenaisynanpuyciaisstonelhhul-,toecenhawasdtassieietttntraiphsngstoeoegsm,lFeivmtaeoteiohcPonrhneet-t PS OOP ring bends PS IP bends &PMMA C-O-C C-C stretches A C=O stretch
g M
ttihmeesufrofracneainnotphaersteiclreesgitoonsd.[e36p]oWsiet hoanvtoe PS rin PM
tested this latter, kinetic hypothesis by
heating the FePt/VUV-PS-b-PMMA
system to 367K, just below the PS-b-
PMMA glass-transition temperature
(T =378K), and have determined that
g
the adsorption selectivity presented in
Figure2.FTIRspectraofPS-b-PMMAbefore(black)andafter(red)VUVirradiation.Decayofthe
Figure3 is thermodynamically stable C(cid:1)Ostretchingvibrationpeakcenteredat∼1730cm–1indicatesetchingofthePMMAdomains.It
and hence likely driven by chemical doesnotfullydisappearbecausethereisstillanintactPMMAlayerincontactwiththesiliconni-
interactions rather than de-wetting be- tridesubstrate.IPandOOPstandforin-planeandout-of-plane,respectively.
2448 ©2005WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim www.advmat.de Adv.Mater.2005,17,2446–2450
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tionwhileincreasingtheoverallsurfacecoverageandobtain- U
ing the high anisotropy fct-FePt phase without inducing N
interparticle sintering. A related issue important for future I
C
applicationsandfundamentalinvestigationsofone-andtwo- A
dimensional nanoparticle interactions is the spatial ordering T
I
of the nanocrystals within the channels. We are investigating O
N
the driving forces that govern this ordering with the aim of
S
ultimately gaining control of both the interparticle spacing
andthelong-rangeorder.
Insummary,wereportanovelapproachforobtainingself-
organizedassembliesofmagneticnanoparticlesusingamodi-
fied polymeric template. A PS-b-PMMA diblock copolymer
thin film was irradiated with VUV light to selectively etch
the surface PMMA domains thereby producing a corrugated
organic substrate. The exposed film was used as a template
for the self-assembly of oleic acid-capped FePt nanocrystals
that adsorb with nearly 100% selectivity within the photo-
chemicallycreatednanoscopicchannels.Ramificationsofthis
work extend to potential future bit-patterned magnetic-stor-
agemedia.Themethoddoesnotrelyonthemagneticcharac-
teroftheFePtcore.Itis,therefore,generaltoall surfactant-
mediatednanoparticlessuggestingpossibleapplicationsinthe
templatingofelectronic,photonic,andcatalyticsystems.
Figure3.Representative2lm×2lmtappingmodeAFMphaseimages
showing∼99%selectiveadsorptionofsingleFePtnanoparticlesontothe
Experimental
photochemically modified polymer domains (dark stripes) with a)low
coverage (∼100clusters/lm2) and b)high coverage (∼225clusters/
lm2).Increasingthecoverageresultsinasomewhatlargerpopulationof AsymmetricPS-b-PMMAwithamolecularweightof77000gmol–1
multiparticle clusters but does not reduce the overall selectivity of the and a polydispersity of 1.09 was obtained from Polymer Source.
singleparticles. ExcessPShomopolymerwasremovedviaSoxhletextractionincyclo-
hexane. Thin films were then spin-cast at 5000rpm from 1.5wt.-%
toluenesolutionontocleansiliconnitridesubstratesandannealedfor
six hours at 518K in an inert atmosphere. FePt nanoparticles were
havior. Indeed, further indication of the attractive interac-
depositedontopreparedpolymerfilmsviaspin-coatinga5lldropof
tionsisthatthenanoparticlesdonotevenmigratewithinthe dilute octane solution (∼0.5wt.-%) at 3000rpm. Imaging was
etcheddomainsduringannealing. performedonaDigitalInstrumentsNanoscopeIVusingVeecotap-
While the as-synthesized nanoparticles exist primarily as ping-mode etched silicon tips (radius of curvature ∼ 10nm, spring
constant∼40Nm–1).VUVirradiationwascarriedoutatpressuresof
isolatedsingleparticlesindilutesolutionmediatedbythesol-
∼2×10–7torr (1torr=133.32Pa) using a Hamamatsu L2793 deuter-
vent, they have a tendency to aggregate into small clusters
iumlampwithaMgF window.Thisconfigurationprovidescontinu-
2
duringdepositionduetoattractivevanderWaals’interactions umspectraloutputintherange115–400nm,withanaverageintensity
betweenthesurfactantcappingmolecules.Asthesolventeva- of8×10–6Jcm–2s–1.Typicalexposurestimeswere7–15min.
poratesduringspin-castingtheconcentrationofthedeposited
nanoparticle solution increases, thereby inhibiting the ability Received:May11,2005
Finalversion:June17,2005
ofthesolventtopreventaggregation.Clusterscontainingtwo
Publishedonline:August22,2005
particles represent20%ofthe surfacepopulation,andthose
containing three or more particles represent 30%. The re- –
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[*] Dr.I.L.Medintz
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Code6900,U.S.NavalResearchLaboratory
Washington,DC20375-5320(USA)
E-mail:[email protected]
Dr.H.Mattoussi,Dr.A.R.Clapp
______________________ DivisionofOpticalSciences
Code5611,U.S.NavalResearchLaboratory
Washington,DC20375-5320(USA)
E-mail:[email protected]
Dr.J.S.Melinger
ElectronicsScienceandTechnologyDivision
Code6800,U.S.NavalResearchLaboratory
Washington,DC20375-5320(USA)
Dr.J.R.Deschamps
LaboratoryfortheStructureofMatter
Code6812,U.S.NavalResearchLaboratory
Washington,DC20375-5320(USA)
[**] The authors acknowledge NRL and A.Ervin and L.Chrisey at the
Office of Naval Research (ONRgrant no. N001404WX20270) and
A.KrishnanatDARPAforsupport.A.R.C.is supportedby a Na-
tionalResearchCouncilFellowshipthroughNRL.SupportingInfor-
mation is available online from Wiley InterScience or from the
author.
2450 ©2005WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim DOI:10.1002/adma.200500722 Adv.Mater.2005,17,2450–2455