Elastomeric nanoparticle composites covalently bound to Al2O3/GaAs surfaces.

Abstract

This article reports the modification of Al2O3/GaAs surfaces with multifunctional soft materials. Siloxane elastomers were covalently bound to dopamine-modified Al2O3/GaAs semiconductor surfaces using MPt (M = Fe, Ni) nanoparticles. The sizes of the monodisperse FePt and NiPt nanoparticles were less than 5 nm. The surfaces of the nanoparticles as well as the Al2O3/GaAs substrates were modified with allyl-functionalized dopamine that utilized a dihydroxy group as a strong ligand. The immobilization of the elastomers was performed via a hydrosilation reaction of the allyl-functionalized dopamines with the siloxane backbones. X-ray photoelectron spectroscopy (XPS) experiments confirmed the covalent bonding of the siloxane elastomers to the oxide layer on the semiconductor surface. Fourier transform-infrared reflection absorption spectroscopy (FT-IRRAS) measurements revealed that the allyl functional groups are bonded to the siloxane backbones. The FT-IRRAS data also showed that the density of the allyl groups on the surface was lower than that of the siloxane backbones. The mechanical properties of the surface-bound nanocomposites were tested using nanoindentation experiments. The nanoindentation data showed that the soft matrix composed of the elastomeric coating on the surfaces behaves differently from the inner, hard Al2O3/GaAs substrate.

Full text

Page 1

Elastomeric Nanoparticle Composites Covalently Bound to Al2O3/GaAs
Surfaces
Hyon Min Song,†Peide D. Ye,‡,§and Albena Ivanisevic*,†,|
Department of Chemistry, Birck Nanotechnology Center, School of Electrical and Computer Engineering,
and Weldon School of Biomedical Engineering, Purdue UniVersity, West Lafayette, Indiana 47907
ReceiVed April 5, 2007. In Final Form: June 14, 2007
ThisarticlereportsthemodificationofAl2O3/GaAssurfaceswithmultifunctionalsoftmaterials.Siloxaneelastomers
werecovalentlyboundtodopamine-modifiedAl2O3/GaAssemiconductorsurfacesusingMPt(M)Fe,Ni)nanoparticles.
The sizes of the monodisperse FePt and NiPt nanoparticles were less than 5 nm. The surfaces of the nanoparticles
aswellastheAl2O3/GaAssubstratesweremodifiedwithallyl-functionalizeddopaminethatutilizedadihydroxygroup
as a strong ligand. The immobilization of the elastomers was performed via a hydrosilation reaction of the allyl-
functionalizeddopamineswiththesiloxanebackbones.X-rayphotoelectronspectroscopy(XPS)experimentsconfirmed
the covalent bonding of the siloxane elastomers to the oxide layer on the semiconductor surface. Fourier transform-
infrared reflection absorption spectroscopy (FT-IRRAS) measurements revealed that the allyl functional groups are
bonded to the siloxane backbones. The FT-IRRAS data also showed that the density of the allyl groups on the surface
was lower than that of the siloxane backbones. The mechanical properties of the surface-bound nanocomposites were
tested using nanoindentation experiments. The nanoindentation data showed that the soft matrix composed of the
elastomeric coating on the surfaces behaves differently from the inner, hard Al2O3/GaAs substrate.
Introduction
The “grafting-from” method provides a powerful route to the
functionalizationofsubstrateswithpolymersbyusingpreformed
tethered initiators and has been widely used to produce dense,
homogeneous molecular layers on surfaces.1-4Studies in the
literature have shown that one can utilize living radical
polymerization,5-10plasmapolymerization,11-13andring-opening
polymerization14-16to form self-assembled monolayers of the
initiators and avoid local aggregation or condensation. The
resulting hydrophobic interface is compatible with a variety of
vinyl monomers and can be used to generate covalently bound
networks with specific functionalities. Some examples include
block copolymer-functionalized silicate,17GaAs-PMMA (poly-
(methyl methacrylate)) hybrids,18and chromophore-attached
conducting polymers on TiO2surfaces.19However, relatively
fewstudieshavelookedatusingthismethodologytofunctionalize
semiconductor surfaces with polymer/nanoparticle composites.
Such composites can increase the utility of the semiconductor
surface and can be used to fabricate devices with multiple
desirable, tunable properties.
Inthisreportweusethegrafting-frompolymerizationmethod
to prepare multifunctional siloxane elastomers on Al2O3-
terminated GaAs surfaces. The elastomers were cross linked by
dopamine-treated superparamagnetic MPt (M ) Fe, Ni) nano-
particles. The motiviation for this work was to immobilize
nanoparticlesonsemiconductorsurfacesinacontrolledstepwise
fashion. Self-assembly methods depending on physical forces
can lead to poor film formation and local aggregation of the
nanoparticles.Functionalizationofthenanoparticlesurfacescan
enablecovalentbondingwithdifferententities.Siloxane-amine
conjugation was chosen to make elastomeric siloxane polymers
that can act as a stabilizer and prevent the nanoparticles from
aggregating. The MPt (M ) Fe, Ni) nanoparticles were chosen
for their superparamagnetism at room temperature. Polymer/
GaAscompositeshavebeenwidelystudiedforapplicationssuch
as flexible inorganic electronics.20-22GaAs doped with Al2O3
has been used to fabricate metal oxide semiconductor field-
* Author to whom correspondence should be addressed. E-mail:
albena@purdue.edu. Tel: 765-496-3676. Fax: 765-496-1459.
†Department of Chemistry.
‡Birck Nanotechnology Center.
§School of Electrical and Computer Engineering.
|Weldon School of Biomedical Engineering.
(1) Zhao, B.; Brittain, W. J. Prog. Polym. Sci. 2000, 25, 677-710.
(2) Stewart, M. P.; Maya, F.; Kosynkin, D. V.; Dirk, S. M.; Stapleton, J. J.;
McGuiness, C. L.; Allara, D. L.; Tour, J. M. J. Am. Chem. Soc. 2004, 126,
370-378.
(3) Wang, Y.; Hu, S.; Brittain, W. J. Macromolecules 2006, 39, 5675-5678.
(4) Fellah, S.; Ozanam, F.; Chazalviel, J. N.; Vigneron, J.; Etcheberry, A.;
Stchakovsky, M. J. Phys. Chem. B 2006, 110, 1665-1672.
(5) Kim, J. B.; Bruening, M. L.; Baker, G. L. J. Am. Chem. Soc. 2000, 122,
7616-7617.
(6) Matyjaszewski, K.; Miller, P. J.; Shukla, N.; Immaraporn, B.; Gelman, A.;
Luokala, B. B.; Siclovan, T. M.; Kickelbick, G.; Vallant, T.; Hoffmann, H.;
Pakula, T. Macromolecules 1999, 32, 8716-8724.
(7) Ejaz,M.;Yamamoto,S.;Ohno,K.;Tsujii,Y.;Fukuda,T.Macromolecules
1998, 31, 5934-5936.
(8) Xu, F. J.; Cai, Q. J.; Kang, E. T.; Neoh, K. G. Langmuir 2005, 21, 3221-
3225.
(9) Zhang, M.; Russell, T. P. Macromolecules 2006, 39, 3531-3539.
(10) Mori,H.;Boker,A.;Krausch,G.;Muller,A.H.E.Macromolecules2001,
34, 6871-6882.
(11) Tarducci, C.; Badyal, J. P. S.; Brewer, S. A.; Willis, C. Chem. Commun.
2005, 406-408.
(12) Tarducci, C.; Schofield, W. C. E.; Badyal, J. P. S. Chem. Mater. 2002,
14, 2541-2545.
(13) Barton, D.; Shard, A. G.; Short, R. D.; Bradley, J. W. J. Phys. Chem. B
2005, 109, 3207-3211.
(14) Juang, A.; Scherman, O. A.; Grubbs, R. H.; Lewis, N. S. Langmuir 2001,
17, 1321-1323.
(15) Harada, Y.; Girolami, G. S.; Nuzzo, R. G. Langmuir 2003, 19, 5104-
5114.
(16) Buchmeiser, M. R.; Sinner, F.; Mupa, M.; Wurst, K. Macromolecules
2000, 33, 32-39.
(17) Zhao, B.; Brittain, W. J. J. Am. Chem. Soc. 1999, 121, 3557-3558.
(18) Cai, Q. J.; Fu, G. D.; Zhu, F. R.; Kang, E. T.; Noah, K. G. Angew. Chem.,
Int. Ed. 2005, 44, 1104-1107.
(19) Senevirathna, M. K. I.; Pitigala, P. K. D. D. P.; Tennakone, K. J. Phys.
Chem. B 2005, 109, 16030-16033.
(20) Tantraporn, W. Appl. Phys. Lett. 1979, 6, 449-451.
(21) Salata, O. V.; Dobson, P. J.; Hull, P. J.; Hutchison, J. L. Appl. Phys. Lett.
1994, 65, 189-191.
(22) Manorama,V.;Bhoraskar,S.V.;Rao,V.J.;Kshirsagar,S.T.Appl.Phys.
Lett. 1989, 55, 1641-1643.
9472
Langmuir 2007, 23, 9472-9480
10.1021/la700979r CCC: $37.00© 2007 American Chemical Society
Published on Web 07/26/2007

Page 2

effect transistors (MOSFETs).23,24Such devices show great
promise because of the intrinsic low noise and high mobility
properties of GaAs, and the excellent dielectric insulating
properties of Al2O3.25We chose to explore the functionalization
route in Scheme 1 because siloxane polymers on semiconductor
surfaces can act as etch masks and lithographic coatings. The
incorporation of nanoparticle elastomers on the GaAs surface
provides additional characteristics such as magnetic properties
andtheabilitytotunethemechanicalpropertiesofthetopsurface
layer.
The main goal of this project is to pattern magnetically active
nanoparticlesonsemiconductorsurfacesbyabottom-upapproach
inordertofabricatedense,homogeneousfilmsonthenanometer
scale. Three dimensionally confined magnetic nanoparticles
embedded in insulating layers have shown promise for use in
nonvolatileflashmemorydevices,26andtheresultsofthisreport
can be utilized in the construction of such devices. The bonding
within the composite network is covalent and leads to better
(23) Ye, P. D.; Wilk, G. D.; Yang, B.; Kwo, J.; Chu, S. N. G.; Nakahara, S.;
Gossmann, H. J. L.; Mannaerts, J. P.; Hong, M.; Ng, K. K.; Bude, J. Appl. Phys.
Lett. 2003, 83, 180-182.
(24) Ye, P. D.; Wilk, G. D.; Yang, B.; Kwo, J.; Gossmann, H. J. L.; Hong,
M.; Ng, K. K.; Bude, J. Appl. Phys. Lett. 2004, 84, 434-436.
(25) Ye, P. D.; Wilk, G. D.; Tois, E. E.; Wang, J. J. Appl. Phys. Lett. 2005,
87, 013501-1-013501-3.
(26) Kim, J. H.; Jin, J. Y.; Jung, J. H.; Lee, I.; Kim, T. W.; Lim, S. K.; Yoon,
C. S.; Kim, Y. H. Appl. Phys. Lett. 2005, 86, 032904.
Scheme 1.Stepwise Modification of the Al2O3/GaAs Surfaces
Elastomeric Composites on Al2O3/GaAs SurfacesLangmuir, Vol. 23, No. 18, 2007 9473

Page 3

stabilitycomparedtothatachievedinLangmuir-Blodgettfilms
orinthinfilmspreparedbyphysisorption.Moreover,inaddition
totheirsuperparamagnetism,theMPt(M)Fe,Ni)nanoparticles
can be used to enforce the overall mechanical properties of the
polymer coatings. To the best of our knowledge, this is the first
report that details the construction of elastomeric FePt and NiPt
nanoparticlethinfilmsonthistypeofsurface.Covalentbonding
in the composite network is achieved in a chemical way, which
is different from traditionally used self-assembly routes.
Experimental Section
General Methods. All chemicals and solvents were used as
received without further purification. Air- and water-sensitive
reactions employed standard Schlenk techniques under an argon
atmosphere. Al2O3(160 Å) on a GaAs substrate was grown via the
atomic layer deposition (ALD) method as previously described.25
Monodisperse FePt and NiPt nanoparticles were prepared using a
modified literature procedure where the starting organometallic
precursors were M(acac)2(acac ) acetylacetonate) and PtCl4.27
PowderX-raydiffraction(XRD)oftheas-synthesizednanoparticles
was measured with a Bruker AXS diffractometer (20 kW) using a
graphite monochromator (Cu KR radiation; λ ) 1.54056 Å). We
used the θ-2θ scanning method from 20 to 100° (2θ, wide angle).
Transmission electron microscopy (TEM) samples were prepared
bycastingFePtandNiPtnanoparticlesinhexanesolventoncarbon-
(27) Sun,S.;Anders,S.;Thomson,T.;Baglin,J.E.E.;Toney,M.F.;Hamann,
H. F.; Murray, C. B.; Terris, B. D. J. Phys. Chem. B 2003, 107, 5419-5425.
Figure 1. (a) Height AFM image of the as-synthesized NiPt
nanoparticles,zscale)15nm.(b)Phaseimageoftheas-synthesized
NiPt nanoparticles, z scale ) 15°. (c) Height image and (d) phase
image of the as-synthesized FePt nanoparticles. The z scale is 20
nmforpartcand20°forpartd.(e)TEMimageoftheas-synthesized
NiPt nanoparticles and (f) TEM image of the as-synthesized FePt
nanoparticles. (g) X-ray powder diffraction patterns of the as-
synthesized NiPt (black) and FePt (purple) nanoparticles. Face-
centered cubic phases (Fm-3m) with cell parameters of a ) 3.843
Å (FePt) and 3.755 Å (NiPt) were identified.
Figure 2. (a) Height and (b) phase AFM images of dopamine-
modified Al2O3/GaAs surfaces. The z scales are 10 nm and 10° in
parts a and b, respectively. (c) Height and (d) phase images of the
elastomer-immobilized surfaces using NiPt nanoparticles as part of
the film. The z scales are 10 nm in height and 10° in phase. (e)
Height and (f) phase images of the elastomer-bound surfaces using
FePt nanoparticles as part of the film. The z scales are 20 nm in
height and 20° in phase.
9474 Langmuir, Vol. 23, No. 18, 2007Song et al.

Page 4

coatedTEMgrids.TheTEMimagesweretakenwithanFEI/Philips
CM-100 instrument operated at 100 keV.
Synthesis of N-Allyl-3,4-dihydroxyphenylethylamine.28Dial-
lycarbonate (5.2 mL, 0.036 mol) was added dropwise for 5 min
under an argon atmosphere to stirring reaction mixtures of 1,4-
dioxane (50 mL), H2O (25 mL), dopamine hydrochloride (6.9 g,
0.036 mol), and NaOH (1 M, 25 mL). Stirring was continued for
12h,andthecompletionofthereactionwasconfirmed.Subsequently,
the reaction mixture was diluted with 50 mL of ethyl acetate and
acidified with dilute HCl solution (1 M) in order to adjust the pH
value to 3. The crude mixture was analyzed via chromatography
withaneluantofhexane/ethylacetate/MeOH(3:2:0.5)togiveN-allyl-
3,4-dihydroxyphenylethylamine in 85% yield.1H NMR (300 MHz,
CDCl3): δ 6.75 (m, 2H), 6.56 (m, 1H), 5.90 (m, 1H), 5.25 (m, 2H),
4.97 (s, 1H), 4.55 (m, 2H), 3.36 (t, 2H), 2.73 (t, 2H).13C NMR (75
MHz, CDCl3): δ 156.07, 145.62, 144.07, 130.81, 119.82, 116.62,
116.10, 78.17, 42.77, 35.85, 29.11.
SurfaceModificationofAl2O3/GaAswithAllyl-Functionalized
Dopamine.SmallpiecesofAl2O3/GaAs(0.5×0.5cm2)weredipped
into a small glass vial containing a mixture of 2 mL of methanol,
5 mL of H2O, and 30 mg (0.155 mmol) of N-ally-3,4-dihydrox-
yphenylethylamine.HCl(2dropsofa1Msolution)wasadded,and
thesubsequentmixturewassonicatedfor30min.Aftertheaqueous
solutionwasremoved,thesemiconductorsubstratewaswashedwith
methanol several times to remove excess unbound dopamine
molecules.
PreparationofSurface-ModifiedFePtandNiPtNanoparticles.
FePt or NiPt nanoparticles (40 mg) were dispersed in 10 mL of
hexane solution and mixed with 20 mg (0.103 mmol) of N-ally-
3,4-dihydroxyphenylethylamine, 5 mL of methanol, and 10 mL of
H2O. The mixture was sonicated for 1 h after the pH value was
adjusted to approximately 4. Subsequently, the crude product was
centrifuged to remove the aqueous phase. The centrifuging cycle
was repeated several times with the addition of methanol. After
centrifugation, all solvents were evaporated off under vacuum to
leave behind the dopamine-modified FePt or NiPt nanoparticles.
HydrosilationReactionofSiloxaneElastomersonAl2O3/GaAs.
The small vial with the dopamine-treated Al2O3/GaAs substrate on
thebottomwasused,and3mLoftoluene,1mLofhexanedispersed
and surface-treated FePt or NiPt nanoparticles (10 mg), and poly-
(methyl hydrosiloxane) (0.02 mL) were mixed with dichloro(1,5-
cyclooctadiene)platinum (1 mg). The vial was placed on a vacuum
rotaryevaporatorwiththetemperatureofthewaterbathsetto80°C.
The vial was slowly rotated for 20 min under vacuum. The gel-like
toluene solution was removed, and the Al2O3/GaAs substrate was
washed with toluene several times.
Atomic Force Microscopy (AFM) Characterization. AFM
images were taken in tapping mode with a silicon nitride tip (model
OTESPA7fromVeeco)usingaDigitalInstrumentsNanoscopeIIIa.
The morphology of the as-synthesized nanoparticles was examined
firstbydroppingnanoparticle-dispersedhexanesolutionontomica.
Additionally,wecharacterizedthetopographyandsurfaceroughness
of the following samples: (i) GaAs with 160 Å of Al2O3, (ii) Al2O3/
GaAs modified by allyl-functionalized dopamine, and (iii) Al2O3/
GaAs terminated on siloxane elastomers cross linked by modified
FePt or NiPt nanoparticles.
Fourier Transform-Infrared Reflection Absorption Spec-
troscopy (FT-IRRAS) and X-ray Photoelectron Spectroscopy
(XPS)Measurements.FT-IRRASspectraofthedopamine-modified
(28) Xu, C.; Xu, K.; Gu, H.; Zheng, R.; Liu, H.; Zhang, X.; Guo, Z.; Xu, B.
J. Am. Chem. Soc. 2004, 126, 9938-9939.
Figure 3. X-ray photoelectron spectroscopy measurement of the Al2O3/GaAs substrate (black line), dopamine-treated Al2O3/GaAs surface
(red line), and surface functionalized with siloxane elastomers (green line). High-resolutions scans of the following regions: (a) C 1s, (b)
O 1s, (c) N 1s, (d) Ga 3d, (e) As 3d, and (f) Al 2p.
Elastomeric Composites on Al2O3/GaAs SurfacesLangmuir, Vol. 23, No. 18, 2007 9475

Page 5

Al2O3/GaAs substrate and the surface functionalized with siloxane
elastomers were collected in single-reflection mode with an FTIR
spectrometer (Thermo Nicolet). A Continuum IR microscope was
coupled to the instrument, which enabled us to focus the IR beam
onthesemiconductorsurfaces.Anarrow-bandmercurycadmium-
telluride (MCT) detector was used to detect the reflected light. All
spectra were collected using 512 scans at a resolution of 2 cm-1.
XPSdatawascollectedwithanAxisUltraDLD(KratosAnalytical)
instrument equipped with a monochromatic Al KR X-ray source
(1486.7 eV). The pass energy was 160 eV. All XPS spectra were
curve fitted with commercially available software (CasaXPS from
Casa Software Ltd).
Measurement of Mechanical Properties Using Nanoinden-
tation.NanoindentationexperimentswereconductedwithaHysitron
Ubi 1 scanning quasistatic Nanomechanical Test Instrument (Min-
neapolis, MN). The samples tested were (i) GaAs with 160 Å of
Al2O3, (ii) Al2O3/GaAs modified by allyl-functionalized dopamine,
and(iii)Al2O3/GaAsterminatedonsiloxaneelastomerscrosslinked
bymodifiedFePtorNiPtnanoparticles.Twotypesoftipswereused
to measure the mechanical properties: a three-sided pyramidal
Berkovich tip (142.3°) and a conical 90° tip. The testing was done
in open loop mode with loading and unloading rates of 100 µN/s.
Before the indentation measurements on the GaAs surfaces, a tip
calibrationwasperformedonaquartzsamplewithaknownreduced
elastic modulus (69.6 MPa). The calibration was performed with
loading forces between 100 and 7000 µN, and a total of 25
indentations were carried out. The indent area (A) was fitted with
the indent depth (hc) as the variable in the polynomial equation A
) C0hc2+ C1hc+ C2hc1/2+ C3hc1/4+ C4hc1/8+ C5hc1/16.29The C0
value was set to 24.5 for the Berkovich tip and to 2.598 for the
conical90°tip.TheC1-C5termswerefittothecurve.Theunloading
curveswereusedtoanalyzethereducedelasticmodulusandhardness
of the samples using a power law fitting.
Results and Discussion
In this section, we present our detailed characterization of the
modified Al2O3/GaAs substrates. We utilized surface-sensitive
techniquesinordertounderstandthemorphologyandcomposition
of the elastomeric composite on the semiconductor surface. The
techniquesthatwechosealsoallowedustocollectevidencethat
supports the formation of covalent bonds within the surface-
anchored composite network as depicted in Scheme 1. We also
report the mechanical properties of the composites and compare
the measured hardness and elastic modulus values.
MPt (M ) Fe, Ni) Nanoparticle Characterization. Both
typesofnanoparticlesweresynthesizedonthebasisofaliterature
method where the starting organometallic precursors were
M(acac)2(acac)acetylacetonate)andPtCl4.27Theas-synthesized
particles were initially characterized by AFM after they were
castonmicafromhexanedispersions(Figure1a-d).Thezscale
in the height images for the NiPt nanoparticles is 15 nm, and the
phase images’ lag scale is 15° (Figure 1a,b). The FePt
nanoparticles are displayed in Figure 1c,d, where the z scale is
20 nm for the height data and 20° for the phase image. The
average diameter was determined from TEM images to be 3.2
nmfortheNiPtnanoparticles(Figure1e)and3.6nmfortheFePt
nanoparticles (Figure 1f). We note that others have reported that
such size distributions extracted from TEM images can be
unreliablebecauseofdifficulties(i.e.,lackofadequatecontrast)
associated with counting small particles.30From the X-ray
diffraction pattern, the face-centered cubic (fcc, Fm-3m) phases
were identified with cell parameters of a ) 3.843 Å (FePt) and
3.755 Å (NiPt) (Figure 1g).
Surface Modification and Immobilization of the FePt and
NiPt Nanoparticles on Al2O3/GaAs Surfaces. The siloxane
elastomers are used as an interface between the nanoparticles
and the surface. A common way of synthesizing nanoparticle
polymercompositesistheextractionofpreformednanoparticles
into the polymer matrix, which can cause aggregation and phase
separation of nanoparticles from the polymers. Controlled
stepwisefunctionalizationofnanoparticlesandthereactionwith
the polymer backbones significantly reduce nanoparticle ag-
gregationandevenpreventitoververylargeareas,asseenfrom
the AFM data. Surface functionalization was carried out on the
basis of Scheme 1. Allyl-functionalized N-ally-3,4-dihydrox-
yphenylethylamine was synthesized at room temperature under
an argon atmosphere through a reaction of 3,4-dihydroxyphe-
nylethylamine hydrochloride with diallyl carbonate in dioxane/
H2O solvent (step 1). This modified dopamine was covalently
attached to the Al2O3/GaAs substrate and also to the surfaces of
the FePt or NiPt nanoparticles (step 2). The final hydrosilation
reaction was performed in a toluene solvent at 80 °C with a
platinum catalyst. This reaction was between the surface-
modifiednanoparticlesandthepolymethylhydrosiloxanemoieties
(29) Ubi 1 User’s Manual; Hysitron Inc.: Minneapolis, MN.
(30) Weibel,A.;Bouchet,R.;Boulc’h,F.;Knauch,P.Chem.Mater.2005,17,
2378-2385.
Figure 4. High-resolution XPS scans of the regions corresponding
to (a) Pt 4d, (b) Pt 4f, and (c) Si 2p.
9476 Langmuir, Vol. 23, No. 18, 2007 Song et al.