Monodisperse Pattern Nanoalloying for Synergistic IntermetallicCatalysisJeong Ho Mun,†,‡ Yun Hee Chang,†,§ Dong Ok Shin,∥ Jong Moon Yoon,†,‡ Dong Sung Choi,†,‡

Kyung-Min Lee,⊥ Ju Young Kim,†,‡ Seung Keun Cha,†,‡ Jeong Yong Lee,†,‡ Jong-Ryul Jeong,⊥

Yong-Hyun Kim,*,†,§ and Sang Ouk Kim*,†,‡

†Center for Nanomaterials and Chemical Reactions, Institute for Basic Science (IBS), Daejeon 305-701, Republic of Korea‡Department of Materials Science and Engineering and §Graduate School of Nanoscience and Technology, KAIST, Daejeon 305-701,Republic of Korea∥Power Control Device Research Section, Electronics and Telecommunications Researh Institute (ETRI), Daejeon 305-700, Republicof Korea⊥Department of Materials Science and Engineering and Graduate School of Green Energy Technology, Chungnam NationalUniversity, Daejeon 305-764, Republic of Korea

*S Supporting Information

ABSTRACT: Nanoscale alloys attract enormous research atten-tions in catalysis, magnetics, plasmonics and so on. Along withmulticomponent synergy, quantum confinement and extreme largesurface area of nanoalloys offer novel material properties, preciselyand broadly tunable with chemical composition and nanoscaledimension. Despite substantial progress of nanoalloy synthesis, therandomized positional arrangement and dimensional/composi-tional inhomogeneity of nanoalloys remain significant techno-logical challenges for advanced applications. Here we present ageneralized route to synthesize single-crystalline intermetallicnanoalloy arrays with dimensional and compositional uniformityvia self-assembly. Specific electrostatic association of multiple ionicmetal complexes within self-assembled nanodomains of block copolymers generated patterned monodisperse bimetallic/trimetallic nanoalloy arrays consisting of various elements, including Au, Co, Fe, Pd, and Pt. The precise controllability of size,composition, and intermetallic crystalline structure of nanoalloys facilitated tailored synergistic properties, such as acceleratedcatalytic growth of vertical carbon nanotubes from Fe−Co nanoalloy arrays.

KEYWORDS: Alloy, block copolymer, nanopattern, catalysis

Alloys are ordered/disordered multimetallic solid solutionsextensively utilized since Bronze Age. The old idea of

alloying for synergistic material properties still holds greatpromise for current nanotechnology.1−6 Nanomaterials andnanostructures with extremely large surface areas generallysuffer from subtle reactivity and sensitivity to surroundingenvironment. Synergistic material properties of nanoalloys offerpromising opportunities to surpass the inherent weakness andstrengthen the desired properties. Meanwhile, traditionalalloying has relied on the metallurgical methods employingthe simple melt mixing of bulk metals.7 Unfortunately, thetraditional method does not ensure the size, shape, andcompositional uniformity in nanoscale and is thus inapplicablefor nanoalloying.8 Alternatively, several approaches, such asvacuum deposition of multiple metal precursors,9 liquid phasesynthesis, and electrochemical methods10−12 have beenintroduced. Those methods frequently reveal hard controll-ability over subtle nucleation and growth kinetics of differentmetal precursors.1,13 More significantly, those methods produce

randomly distributed inhomogeneous nanoparticles,14−16

whose structural/compositional distribution and spatialarrangement raise significant challenge for advanced applica-tions, such as electronics,17,18 magnetic storage media,19−21

plasmonics/metamaterials,22,23 and patterned catalysis.24

We report a straightforward synthesis of patterned single-crystalline intermetallic nanoalloys with dimensional/composi-tional homogeneity exploiting block copolymer (BCP) self-assembly. The self-assembled nanodomains in BCP thinfilms25−36 confine the simultaneous deposition of multipleionic metal precursors within nanoscale dimension andsuccessfully generated nanopatterned bimetallic/trimetallicalloys. The size/thickness of self-assembled BCP nanodomains,charge states/molar ratio of metallic precursors, and precursorloading condition precisely control the size, composition, andsingle-crystalline intermetallic atomic structures of nanoalloys.

Received: September 22, 2013Published: October 1, 2013

Letter

pubs.acs.org/NanoLett

© 2013 American Chemical Society 5720 dx.doi.org/10.1021/nl403542h | Nano Lett. 2013, 13, 5720−5726

The resultant synergistic properties, including catalytic proper-ties of Fe−Co nanoalloys, are precisely tunable with alloy sizeand chemical composition.Figure 1a schematically illustrates patterned monodisperse

nanoalloying procedure. Thin films of asymmetric poly-(styrene-block-4-vinylpyridine) (PS-b-P4VP) were spin-castonto a substrate and self-assembled into hexagonal verticalP4VP nanocylinder morphology by toluene/THF-(tetrahydrofuran) mixture solvent annealing (Figure 1b). Thenanodomains consisting of hydrophilic vertical P4VP nano-cylinders enclosed by a hydrophobic PS matrix was thenimmersed in acidic aqueous solutions of multiple metalprecursors. The protonated pyridinic nitrogen sites in P4VPnanocylinders attract the anionic metal complexes dissociatedfrom metal precursors. After the specific precursor associationin P4VP nanodomains, oxygen plasma treatment removes thenonmetallic elements and leaves hexagonally arranged multi-

metallic nanoclusters (Figure 1c). Subsequent thermalcalcination under Ar/H2 reductive atmosphere generatedhomogeneous single-crystalline nanoalloy arrays with inter-metallic crystalline structures (Figure 1d). The nanoalloystypically exhibit the size significantly smaller than BCPnanodomains with a narrow distribution. The precise statisticalanalysis of TEM images revealed that nanoalloys with meandiameter of 4.4 nm could be generated from the cylindricalnanodomains with mean diameter of 11.5 nm. The standarddeviation of nanoalloys was ∼0.3 nm, which corresponds tobelow 8% of mean diameter (Supporting Information FigureS1). The resultant nanoalloys shows highly ordered structureover a large surface area and can be perfectly aligned in a trench(Supporting Information Figure S2 and S3).Our patterned nanoalloying is generally compatible with

various available metal precursors to generate intermetallicalloys. Figure 1e,f shows the HRTEM images of bimetallic Co−

Figure 1. Patterned intermetallic nanoalloying. (a) Schematic illustration of nanoalloying procedure. (b) SEM image of PS-b-P4VP thin filmnanotemplate and the size distribution of P4VP nanocylinder cores. (c) TEM image of as-deposited Fe−Pt nanoclusters and their size distribution.(d) TEM image of thermal calcined intermetallic Fe−Pt nanoalloys and their size distribution. HRTEM images of single-crystalline, intermetallic (e)Co−Pt, (f) Pd−Au binary alloys, and (g) Co−Pd−Pt ternary alloy.

Nano Letters Letter

dx.doi.org/10.1021/nl403542h | Nano Lett. 2013, 13, 5720−57265721

Pt and Pd−Au alloys synthesized from K3Co(CN)6, K2PtCl4,Na2PdCl4, and HAuCl4 as the precursors for Co, Pt, Pd and Auelements, respectively (Supporting Information Figure S4).Precursor loading conditions for each alloy were 0.4 mM ofK3Co(CN)6 and 0.6 mM of K2PtCl4 for Co−Pt and 0.35 mMof Na2PdCl4 and 0.65 mM of HAuCl4 for Pd−Au in 3% HClaqueous solution. The chemical compositions measured fromenergy dispersive X-ray spectroscopy (EDS) were 43/57 forCo−Pt and 45/55 for Pd−Au, respectively. The insets in Figure1e,f illustrate the corresponding atomic crystal structures. TheCo−Pt alloy at ∼1:1 composition forms the intermetallic, L10face-centered tetragonal (fct) structure.37 The Pd−Au alloy alsoforms L10 phase at 1:1 composition38 in nanoscale particle,while it is known to form disordered alloy in bulk.39 It isnoteworthy that ternary or more component alloys could bereadily prepared by simultaneous loading of three or moremetal precursors. Figure 1g shows HRTEM image of a Co−Pd−Pt trimetallic alloy, whose atomic composition was

measured to be 43/34.5/22.5 for Co/Pd/Pt. Precursor loadingcondition for this alloy was 0.4 mM of K3Co(CN)6, 0.3 mM ofNa2PdCl4 and 0.3 mM of K2PtCl4. Pd and Pt have very smalllattice mismatch (∼0.7%) and belong to the same elementalfamily. The compositional ratio of Co/(Pd and Pt) was 43/57,which is close to 1:1. The created Co−Pd−Pt trimetallicnanoalloy shows the L10 phase with alternating Co and (Pd andPt mixture) layers, as illustrated in the inset of Figure 1g. Thelattice spacings indicated in TEM images match with the (111)d-spacing of the suggested structures.Figure 2a−c shows the elemental mapping images of a Fe−Pt

bimetallic alloy array obtained with EDS in high-angle annulardark field scanning transmission electron microscopy (HAADF-STEM). The elemental maps for Fe-K (red color) and Pt-L(yellow-green color) edges confirm that the nanoalloys atdifferent locations have a consistent chemical composition.Interestingly, the lateral hexagonal ordering of nanoalloysbecame deteriorated with the concentration of chloroplatinate

Figure 2. Compositional tunability of Fe−Pt nanoalloys. EDS Elemental mapping of (a) Fe, (b) Pt, and (c) Fe−Pt superposition in STEM mode.(d) Atomic composition of nanoalloys vs metal precursor molar ratio in precursor solution. HRTEM images of single-crystalline intermetallicnanoalloys with the Fe/Pt compositional ratio of (e) 3/1, (f) 1/1, and (g) 1/3, respectively. Each inset shows the corresponding crystal structures.

Nano Letters Letter

dx.doi.org/10.1021/nl403542h | Nano Lett. 2013, 13, 5720−57265722

(PtCl42−) (Supporting Information Figure S5). This is

attributed to the exchange of the chloro ligands with theaqua ligands.40,41 Ligand exchange of chloroplatinate anion(PtCl4

2−) may result in complex ions with different chargestates, as follows

+ ↔ +− − −PtCl H O PtCl (H O) Cl42

2 3 2 (1)

+ ↔ +− −PtCl (H O) H O PtCl (H O) Cl3 2 2 2 2 2 (2)

The equilibrium constants for reactions 1 and 2 are 14.52and 1.09, respectively.42 The corresponding ligand exchanges ofchloroplatinate anions (PtCl4

2−) in 0.1, 0.3, and 3% HClaqueous solutions are summarized in Table 1. The complex

ions with low charge states of −1 or 0 are more populated at alow HCl concentration. The low charge anions with weakelectrostatic attractions with P4VP nanodomains may cause apoor ordering of nanoalloy array. By contrast, negligibleamount of ligand exchange occurs in the ferricyanide anion(Fe(CN)6

3−) with a relatively large crystal field stabilizationenergy. For a highly ordered hexagonal distribution ofnanoalloys, 3% HCl solution was principally used in this work.Figure 2d shows the experimental and calculated plots

presenting the nonlinear relationship between the Fe precursormolar ratio in the precursor solution and the resultantcomposition of Fe−Pt alloy. The atomic composition wasmeasured from more than 50 nanoalloys using EDS analysis.The error bar simply means the maximum and minimum valuesof measured atomic compositions and the black dot means therepresentative value of all nanoalloys we measured. Thecalculation was based on following relationship

=∑

XC F

C Fnn n

11 1

1

Where Xn is the elemental atomic composition of nthcomponent in an alloy. Cn is the specific charge and Fn is themolar fraction of the metal complex anion including nthelement in a given precursor solution. The experimental resultsfor 3% HCl aqueous solution (total precursor molarconcentration is fixed at 1 mM) match well with the calculatedline (green dash line). The specific loading rate ratio of Fe/Ptmaintained ∼1.5 over whole precursor concentrations ([specificloading rate] = [atomic composition in nanoalloy]/[precursorconcentration], Supporting Information Figure S6). It isequivalent to an anionic charge ratio of 1.5 (−3 for Fe(CN)63−and −2 for PtCl4

2−). This confirms that electrostatic attractionplays a principal role in the ionic precursor loading. The chargestate as well as compositional ratio of ionic metal precursorsdetermines the precursor loading ratio.Figure 2e−g shows the high-resolution TEM (HRTEM)

images of single-crystalline intermetallic Fe−Pt nanoalloys,synthesized from different precursor molar ratios: 0.65 mM ofK3Fe(CN)6 and 0.35 mM of Na2PtCl4 for Figure 2e), 0.35 mMof K3Fe(CN)6 and 0.65 mM of Na2PtCl4 for Figure 2f, and 0.2

mM of K3Fe(CN)6/0.8 mM of Na2PtCl4 for Figure 2g,respectively. After thermal calcination, the normalized atomicFe/Pt ratios measured by EDS were 72/28 for Figure 2e, 47/53for Figure 2f, and, 27/73 for Figure 2g, respectively. Thecalcinated nanoalloys have projected shapes of hexagon alongits [110] direction, and flat {100} and {111} facets with lowsurface energies were observed. The measured (111) d-spacingindicated in Figure 2e−g increases with Pt composition, whoseatomic size is larger than Fe. Each inset illustrates the crystalstructure of the Fe−Pt bimetallic phase. Fe3Pt L12 phase(Figure 2e) and FePt3 L12 phase (Figure 2g) show face-centered-cubic (fcc) crystal structures. The FePt L10 phase(Figure 2f) corresponds to fct crystal structure.43 In the XRDanalysis, the (111) peak at 2θ = 41.05°, the (001) and (110)peaks at 2θ = 23.97° and 32.84° confirmed the fct L10 phase(Supporting Information Figure S7).44 The c-axis of fct isshorter than the other axes, as the atomic layers of Fe and Ptare alternate along the c-axis. This variation of bimetallic phasedemonstrates that the chemical composition of nanoalloys isprecisely controllable with the metal precursor molar ratio.In addition to the fine controllability of chemical

composition, our patterned nanoalloying also offers subnan-ometer-level tunability of alloy size. Several different processingparameters, including BCP molecular weight, BCP filmthickness, precursor solution concentration, and precursorloading time could be manipulated for the wide range of sizetunability in nanoscale (Supporting Information Figure S8).Figure 3 shows the average diameters of Fe−Pt nanoclusters

and nanoalloys controlled by precursor loading time. Typically,the mean diameter of as-deposited nanoclusters employing 25nm thick BCP film was controllable from 6 to 17 nm withloading time. The reason for larger diameter of nanoclustersthan P4VP nanocylinder cores is a swelling of P4VP blockduring precursor loading in HCl aqueous solution. After

Table 1. Calculated Molar Composition of Various MetalComplex Anions As a Result of Aqua Ligand ExchangeReaction of Chloroplatinate

HCl conc. (wt %) PtCl42− PtCl3(H2O)

− PtCl2(H2O)2

3% ∼98.3% ∼1.7% ∼0%0.3% ∼84.8% ∼15% ∼0.2%0.1% ∼65% ∼33.6% ∼1.4%

Figure 3. Tunability of Fe−Pt nanoalloy size. Nanocluster andnanoalloy sizes vs precursor loading time in 0.4 mM Fe, 0.6 mM Pt,and 3% HCl aqueous solution.

Nano Letters Letter

dx.doi.org/10.1021/nl403542h | Nano Lett. 2013, 13, 5720−57265723

thermal calcination, the corresponding diameter of single-crystalline intermetallic nanoalloy ranged from 3.5 to 8 nm.As an immediate application of monodisperse patterned

nanoalloys, synergistic catalysis of bimetallic Fe−Co arrays wasinvestigated for the vertical carbon nanotubes (CNTs) growthby plasma-enhanced chemical vapor deposition (PECVD). Fe−Co alloy arrays were prepared and thermally reduced under aslow stream of a H2/NH3 mixture. Afterward, acetylene (C2H2)feed gas was slowly streamed over nanoalloy arrays at 973 K togrowth vertical CNTs under plasma environment. The uniformnanoalloy size minimizes the catalyst deactivation by Ostwaldripening. In addition, the fine controllability of nanoalloy sizeenables the tunability of CNT diameter and correspondingcarbon wall number.25 More significantly, optimized nanoalloycomposition demonstrated the synergistic enhancement ofCNT growth yield. Figure 4a shows the relative growth rateplotted against alloy composition. Fe4Co1 alloy demonstratesthe highest CNT growth rate, which is ∼25% enhancementfrom pure Fe catalyst. The growth rate gradually decreases with

Co composition and eventually, pure Co particles generatebarely grown CNTs (Supporting Information Figure S9).The synergistic enhancement of CNT growth from Fe4Co1

nanoalloys was analyzed with density functional theory (DFT)calculations. CNT growth from Fe−Co nanoalloys underplasma activation consists of four distinct steps: (1) adsorptionand (2) dissociation of the CH radical at Fe−Co alloy surface,and (3) solution and (4) diffusion of C atom in Fe−Co alloyvolume, as schematically illustrated in Figure 4b.45−47 Owing tothe highly reactive nanoalloy surface and plasma activation ofcarbon sources,45 the reaction steps of (1) and (2) arespontaneous without energy barrier. By contrast, either step (3)or (4) can be the principal rate-determining step for thisreaction. Figure 4c,d and Supporting Information Table S1summarizes the DFT dissolution energy and diffusion barrierfor interstitial C atom in Fe−Co alloys. While C dissolutionenergy increases from 0.75 eV for pristine Fe to 1.28−1.87 eVfor Fe−Co alloys, the diffusion barrier reduces from 0.86 eV forpristine Fe to 0.80−0.85 eV for Fe−Co alloys. In particular, the

Figure 4. Synergistic catalysis of Co−Fe nanoalloys for vertical CNT growth. (a) CNT growth length vs Co compositional ratio in Co−Fenanoalloy. Three insets are tilted SEM images of vertical CNTs grown from pure Fe, Fe4Co1, and Fe3Co2 nanocatalyst arrays. Scale bars correspondto 10 μm. (b) Schematic illustration of PECVD CNT growth from Fe−Co catalysts. Green, blue, yellow, and white spheres indicate the Fe, Co, C,and H atoms, respectively. The “1” indicates the adsorption of CH radical. The “2” indicates the dissociation of CH radical. The “3” and “4” indicatethe dissolution and diffusion of C interstitial atom within catalyst volume, respectively. (c) & (d) Minimum-energy paths for C diffusion in (c) Feand (d) Fe3Co1. The insets illustrate the atomic structures of octahedral-to-octahedral C diffusion. In the transition state, C atom is tetrahedralcoordinated.

Nano Letters Letter

dx.doi.org/10.1021/nl403542h | Nano Lett. 2013, 13, 5720−57265724

diffusion barrier was minimum (0.80 eV) for Fe3Co1 alloys.These DFT results indicate that the carbon diffusion in catalystparticle is noticeably enhanced when Co is alloyed with Fe (i.e.,positive contribution to fast CNT growth), while the carbonsolubility decreases with Co alloying (i.e., negative contributionto fast CNT growth). Consequently, growth rate is optimizedwhen a small amount of Co is alloyed with Fe. This isconsistent with our experimental result and the previouslyreported reduced activation energy for Fe0.96-Co0.04 alloys.48

DFT electronic structure analyses (Supporting InformationFigures S11 and S12) show that the alloying of Co into Feincreases Fermi energy and thus weakens C binding energy.Also, the impurity state at the Fermi energy is noticeablyreduced for Fe3Co1 due to the strong orbital hybridization,which is responsible for the reduced diffusion energy barrier forFe3Co1.How to control the positional ordering and dimensional/

compositional inhomogeneity has been a principal techno-logical issue for the advanced applications of intermetallicnanoalloys in plasmonics, magnetics, catalysis, and so on. Thehighly specific, simultaneous deposition of multiple metalprecursors within uniform self-assembled nanodomains in BCPthin films addresses these issues along with the facile andprecise controllability of alloy size, composition, intermetalliccrystalline structure and corresponding synergistic physicalproperties. We anticipate that this straightforward nanoalloyingmay offer a valuable route to novel intermetallic nanoalloys, ascertain elements immiscible in bulk can be miscible in thenanoscale confinement with reduced atomic strain energy.49

Besides, this solution phase deposition in conjunction withBCP self-assembly is genuinely scalable to an arbitrary largearea process, potentially required for device-oriented nano-fabrication.

■ ASSOCIATED CONTENT*S Supporting InformationExperimental, Methods, additional SEM and TEM images, andadditional measurements. This material is available free ofcharge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: (S.O.K.) sangouk.kim@kaist.ac.kr. Phone: +82-42-350-3339. Fax: +82-42-350-3310.*E-mail: (Y.-H.K.) yong.hyun.kim@kaist.ac.kr.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was financially supported by Institute for BasicScience (IBS). Y.H.C. and Y.-H.K. were supported by theGlobal Frontier R&D (Center for Multiscale Energy Systems:No. 2011-0031566) program through the NRF of Korea.

■ REFERENCES(1) Ferrando, R.; Jellinek, J.; Johnston, R. L. Chem. Rev. 2008, 108,845.(2) Chen, M.; Kumar, D.; Yi, C.-W.; Goodman, D. W. Science 2005,310, 291.(3) Stamenkovic, V. R.; Mun, B. S.; Arenz, M.; Mayrhofer, K. J. J.;Lucas, C. A.; Wang, G.; Ross, P. N.; Markovic, N. M. Nat. Mater. 2007,6, 241.

(4) Schrinner, M.; Ballauff, M.; Talmon, Y.; Kauffmann, Y.; Thun, J.;Moller, M.; Breu, J. Science 2009, 323, 617.(5) Cortie, M. B.; McDonagh, A. M. Chem. Rev. 2011, 111, 3713.(6) Wang, D.; Xin, H. L.; Hovden, R.; Wang, H.; Yu, Y.; Muller, D.A.; DiSalvo, F. J.; Abruna, H. D. Nat. Mater. 2013, 12, 81.(7) Pollack, H. W. Materials Science and Metallurgy; Prentice Hall:New York, 1988.(8) Clouet, E.; Lae, L.; Epicier, T.; Lefebvre, W.; Nastar, M.;Deschamps, A. Nat. Mater. 2005, 5, 482.(9) Binns, C. Surf. Sci. Rep 2001, 44, 1.(10) Wang, D.; Li, Y. Adv. Mater. 2011, 23, 1044.(11) Lu, X.; Tuan, H.−Y.; Chen, J.; Li, Z.−Y.; Korgel, B. A.; Xia, Y. J.Am. Chem. Soc. 2007, 129, 1733.(12) Mallin, M. P.; Murphy, C. J. Nano Lett. 2002, 2, 1235.(13) Peng, Z. M.; Yang, H. Nano Today 2009, 4, 143.(14) Pileni, M.−P. Nat. Mater. 2003, 2, 145.(15) Alloyeau, D.; Ricolleau, C.; Mottet, C.; Oikawa, T.; Langlois, C.;Le Bouar, Y.; Braidy, N.; Loiseau, A. Nat. Mater. 2009, 8, 940.(16) Yu, A. C. C.; Mizuno, M.; Ssaki, Y.; Kondo, H. Appl. Phys. Lett.2004, 85, 6242.(17) Bettini, J.; Sato, F.; Coura, P. Z.; Dantas, S. O.; Galvao, D. S.;Ugarte, D. Nat. Nanotechnol. 2006, 1, 182.(18) Dong, A.; Chen, J.; Vora, P. M.; Kikkawa, J. M.; Murray, C. B.Nature 2010, 466, 474.(19) Sun, S. H. Adv. Mater. 2006, 18, 393−403 (2006).(20) Abes, J. I.; Cohen, R. E.; Ross, C. A. Mater. Sci. Eng., C 2003, 23,641.(21) Oh, Y. −J.; Kim, J. −H.; Thoompson, C. V.; Ross, C. A.Nanoscale 2013, 5, 401.(22) Tassin, P.; Koschny, T.; Kafesaki, M.; Soukoulis, C. M. Nat.Photonics 2012, 6, 259.(23) Blaber, M. G.; Arnold, M. D.; Ford, M. J. J. Phys.: Condens.Matter 2010, 22, 143201.(24) Yamada, Y.; Tsung, C. −K.; Huang, W.; Huo, Z.; Habas, S. E.;Soejima, T.; Aliaga, C. E.; Somorjai, G. A.; Yang, P. Nature Chem.2011, 3, 372.(25) Shin, D. O.; Lee, D. H.; Moon, H. S.; Jeong, S. J.; Kim, J. Y.;Mun, J. H.; Cho, H.; Park, S.; Kim, S. O. Adv. Funct. Mater. 2011, 21,250.(26) Hamley, I. W. Angew. Chem., Int. Ed. 2003, 42, 1692.(27) Thurn-Albrecht, T.; Schotter, J.; Kastle, C. A.; Emley, N.;Shibauchi, T.; Krusin-Elbaum, L.; Guarini, K.; Black, C. T.; Tuominen,M. T.; Russell, T. P. Science 2000, 290, 2126.(28) Stoykovich, M. P.; Muller, M.; Kim, S. O.; Solak, H. H.;Edwards, E. W.; de Pablo, J. J.; Nealey, P. F. Science 2005, 308, 1442.(29) Chai, J.; Wang, D.; Fan, X. N.; Buriak, J. M. Nat. Nanotechnol.2007, 2, 500.(30) Arora, H.; Du, P.; Tan, K. W.; Hyun, J. K.; Grazul, J.; Xin, H. L.;Muller, D. A.; Thompson, M. O.; Wiesner, U. Science 2010, 330, 214.(31) Kruss, S.; Srot, V.; Aken, P. A.; Spatz, J. P. Langmuir 2012, 28,1562−1568.(32) Ethirajan, A.; Wiedwald, U.; Boyen, H. −G.; Kern, B.; Han, L.;Klimmer, A.; Weigl, F.; Kastle, G.; Ziemann, P.; Fauth, K.; Cai, J.;Behm, J.; Romanyuk, A.; Oelhafen, P.; Walther, P.; Biskupek, J.;Kaiser, U. Adv. Mater. 2007, 19, 406−410.(33) Gao, Y.; Zhang, X. W.; Yin, Z. G.; Si, F. T.; Bai, Y. M.; Zhang, X.L.; Qu, S.; Wang, Z. G. J. Appl. Phys. 2011, 109, 11307.(34) Menezes, W. G.; Zielasek, V.; Thiel, K.; Hartwig, A.; Baumer, M.J. Catal. 2013, 299, 222−231.(35) Yun, S. −H.; Yoo, S. I.; Jung, J. C.; Zin, W. −C.; Sohn, B. −H.Chem. Mater. 2006, 18, 5646−5648.(36) Kim, T. H.; Huh, J.; Hwang, J.; Kim, H. −C.; Kim, S. H.; Sohn,B. −H.; Park, C. Macromolecules 2009, 42, 6688−6697.(37) Liou, S. H.; Huang, S.; Klimek, E.; Kirby, R. D.; Yao, Y. D. J.Appl. Phys. 1999, 85, 4334.(38) Nagasawa, A. J. Phys. Soc. Jpn. 1964, 19, 2344.(39) Hayes, F. H.; Kubaschewski, O. Met. Sci. J. 1971, 5, 37.(40) Burns, R. G. Mineralogical applications of crystal field theory;Cambridge University Press: New York, 1993.

Nano Letters Letter

dx.doi.org/10.1021/nl403542h | Nano Lett. 2013, 13, 5720−57265725

(41) Rodgers, G. E. Descriptive inorganic, coordination, and solid-statechemistry; McGraw-Hill: New York, 1994.(42) Ciacchi, L. C.; Pompe, W.; Vita, A. D. J. Am. Chem. Soc. 2001,123, 7371.(43) Gutfleisch, O.; Lyubina, J.; Muller, K. −H.; Schultz, L. Adv. Eng.Mater. 2005, 7, 208.(44) Sun, S. H.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Science2000, 287, 1989.(45) Meyyappan, M.; Delzeit, L.; Cassell, A.; Hash, D. Plasma SourcesSci. Technol. 2003, 12, 205.(46) Jiang, D. E.; Emily, A. C. Phys. Rev. B 2005, 71, 045402.(47) Denysenko, I.; Ostrikov, K. J. Phys. D: Appl. Phys. 2009, 42,015208.(48) Hofmann, S.; Csanyi, G.; Ferrari, A. C.; Payne, M. C.;Robertson, J. Phys. Rev. Lett. 2005, 95, 036101.(49) Tersoff, J. Phys. Rev. Lett. 1995, 74, 434.

Nano Letters Letter

dx.doi.org/10.1021/nl403542h | Nano Lett. 2013, 13, 5720−57265726