Nanoscale

PAPER

Publ

ishe

d on

14

Febr

uary

201

3. D

ownl

oade

d by

Bro

wn

Uni

vers

ity o

n 17

/07/

2013

15:

19:3

7.

View Article OnlineView Journal | View Issue

aSchool of Materials Science and Engineering

USA. E-mail: sumandas@gatech.edu; Fax: +bWoodruff School of Mechanical Engineering

USA

† Electronic supplementary informa10.1039/c3nr33438d

Cite this: Nanoscale, 2013, 5, 3912

Received 1st November 2012Accepted 12th February 2013

DOI: 10.1039/c3nr33438d

www.rsc.org/nanoscale

3912 | Nanoscale, 2013, 5, 3912–391

Hierarchical metallic and ceramic nanostructures fromlaser interference ablation and block copolymer phaseseparation†

Taiwo R. Alabi,a Dajun Yuanb and Suman Das*ab

We report on the formation of hierarchical nanostructures of Au, PtOx, FexOy and PdOx using a hybrid

technique by combining laser interference ablation (LIA) and block copolymer phase separation (BCPS).

Different types of hierarchical arrays can be obtained including square, triangular, linear and circular

arrays by varying the loading time of the block co-polymer with metallic salts, and the laser interference

technique. The primary ordering of the as-generated nanoarrays (30–100 nm) is tunable by changing

either the composition of the block copolymer spun from solution or by changing other parameters

that affect the phase separation kinetics of the block copolymer, while the secondary ordering of the

features can be tuned from 200 nm to 2 mm, by changing the angle of convergence of the laser beams

on the patterned substrate. Such a robust method can be applied to the fabrication of other metallic

and ceramic materials including Ag, Co, and Ni (O) and has potential use in the large scale fabrication

of hierarchical arrays of catalysts that can be used to grow germanium, silicon nanowires using the

vapour–liquid–solid growth (VLS) technique. The as-generated arrays can also find use in optical as well

as sensor applications for biodetection and biosensing.

Introduction

Optical,1–3 electrical,4 thermal,5,6 magnetic,7,8 and chemical9,10

properties of nano-scaled metallic and ceramic materials differremarkably from their bulk counterparts. Harnessing suchproperties for device-oriented fabrication has led to numerousresearch into nano-materials on surfaces. The placement andordering of such nano-features on surfaces has been achievedusing several techniques including so lithography,11 dip-pennanolithography,12–14 laser interference lithography (LIL),15–19

laser interference ablation (LIA),27,28 block copolymer nano-lithography,20–24 e-beam lithography,25,26 etc. Of all the abovelisted techniques for controlled placement of nano-features onsurfaces, the two robust, inexpensive and high throughputtechniques inarguably are the LIA technique and block co-polymer phase separation (BCPS).

Features having a secondary ordering rendered by the peri-odicity of the laser interference pattern and a primary orderingrendered by the inter-domain spacing of the phase-separatedblock co-polymers can be generated by combining the twotechniques and with block co-polymers acting as a positive tone

, Georgia Institute of Technology, Atlanta,

1 4048949342; Tel: +1 4043856027

, Georgia Institute of technology, Atlanta,

tion (ESI) available. See DOI:

7

photoresist. By loading the block co-polymer with a metallic saltprior to LIA, and by selectively removing the polymer via reactiveion etching, it is possible to generate metallic or ceramic nano-domains on the substrate with a superposition of the periodicityof both the block co-polymer phase and the LIA technique.

We demonstrate in this report the fabrication of hierarchicalmetallic and ceramic nanostructures using a novel hybridtechnique by combining microscale periodic patterning usingLIA and nanoscaled patterning using BCPS. The hierarchicalstructures have a primary ordering domain periodicity of about30–100 nm corresponding to the primary ordering present inthe sacricial block co-polymer phase separating blocks, and atunable secondary periodicity depending on the number ofinterfering laser beams, the angle between the beams, and theintensity of the laser beam. The secondary periodicity rangesfrom 250 nm to 2 mm. By varying the loading time of the blockco-polymer with metallic salts and the laser interference tech-nique, different types of hierarchical structures can beobtained. We further show the ability to generate patternshaving Au, PtOx, FexOy, and PdOx hierarchical nano-domains.The technique is robust and can be used to generate severalother types of metallic and ceramic features including Ni, Co,etc. In comparison to other techniques discussed above forgenerating patterned nanoscale features, the technique intro-duced in this paper has reduced the processing and fabricationtime. Furthermore, other techniques work well for generatinghierarchical nanofeatures for a specic material type, but the

This journal is ª The Royal Society of Chemistry 2013

Paper Nanoscale

Publ

ishe

d on

14

Febr

uary

201

3. D

ownl

oade

d by

Bro

wn

Uni

vers

ity o

n 17

/07/

2013

15:

19:3

7.

View Article Online

technique described here can be used to generate a broad rangeof hierarchical patterns with different metallic and ceramicmaterials. The as-generated metallic features have potentialapplications in areas such as electronics where they can be usedas masks for the inexpensive fabrication of patterned arrays offree standing features on silicon and other semiconductingsubstrates. In addition, the as-generated gold features couldpotentially be used for the catalysis and growth of patternedarrays of silicon and germanium nanowires for solar applica-tions, and they also have potential use in biosensors, photonicsand optoelectronic applications.

Experimental

Asymmetric poly(styrene-block-vinylpyridine) was purchasedfrom Polymer Source Incorporated, Canada having the molec-ular weights (MWs) of 24 000 and 9500 kDa for polystyrene andpolyethylene oxide respectively. The ratio of the weight averageMw to the number average Mn molecular weights (poly-dis-persity index-PDI) is 1.01. Anhydrous toluene and anhydroustetra-hydrofuran (THF) were purchased from Sigma-Aldrich andused without further purication. Inorganic salt complexes ofiron–K3FeCN6, gold–HAuCl4, palladium–Na2PdCl6 and plat-inum–K2PtCl4 were also purchased from Sigma-Aldrich andused without further purication.

Silicon substrates with a 10 mm thermal oxide layer werepurchased from University Wafers and cleaned using standardcleaning procedures with piranha solution. Briey, substrateswere diced to about 15 mm � 15 mm square substrates andthen le overnight in a hot piranha solution containing 70 : 30vol% H2SO4 : H2O2 solution. The hydrophilic substrates werethen rigorously rinsed in deionised water and dried in a desic-cator under a nitrogen ow prior to use.

Thin lms of an 0.5–1 wt% PS-b-P4VP block copolymersolution were prepared on the cleaned silicon substrates byspin-coating at 1500 rpm for 30 s. This was followed by solventannealing in a THF : toluene vapor leading to the phase sepa-ration and generation of vertically oriented and swollen cylin-ders of polyvinyl pyridine with a periodicity of about 40 nm anda diameter of about 30 nm.

The metallic loading of the vertical cylindrical domains wasachieved by immersing the phase-separated thin lms in a 0.1wt% solution of an inorganic salt complex in hydrochloric acid.The hydrochloric acid solution serves two purposes: the rst isto activate the polyvinyl pyridine complex, by protonating thenucleophilic amine group in the heterocyclic ring, the secondfunction is to dissociate the metallic complex thus allowingcomplexation between the vinylpyridine and the metalliccomponent of the salt.

LIA of the metal-loaded block copolymer thin lm was doneusing the 266 nm wavelength (fourth harmonic) of a 10 nspulsed Nd:YAG laser. A two beam laser interference wasobtained by using a beam splitter to split the initial beam fromthe laser source followed by beam convergence on the substrateresulting in the generation of an interference fringe. For thegeneration of a square pattern, two LIA with a 90� rotationbetween successive shots were required.

This journal is ª The Royal Society of Chemistry 2013

Phase and height images of the asymmetric block copoly-mers were obtained using a Digital Instruments (DI) atomicforce microscope (AFM) operating in the light tapping modewith a commercial silicon micro-cantilever tip operating at aresonance frequency of 360 kHz. The silicon micro-cantileverhas an aluminum coating on the backside. To obtain highresolution phase and height images, the scan rate of the AFMwas set at about 13 kHz.

Scanning electron Microscope (SEM) images were obtainedfrom a Zeiss Ultra 60 SEM working at an acceleration voltage of5 kV with an SE detector. Thin lm samples prior to SEMimaging were coated with gold using a Hummer gold sputterer.

XPS binding energy spectra and data for the ablated blockcopolymer thin lms, metal-loaded block copolymer thin lmsand reactive ion etched substrates were obtained from a FisherScientic Thermo Ka XPS. The X-ray source is an aluminum foilwith photon energy of 1486 eV. The X-ray beam size wasmaintained at 100 mm while the step size for the X-ray scan was0.5 eV. The detector pass energy was varied for the differentmetals under consideration.

Etching of the metal-loaded block copolymer thin lms wasdone using a plasma thermal reactive ion etching (RIE) tool. TheRIE etching tool operates at a radio frequency of 13.56 MHz.Etching of the organic components was done under a pressureof 350 mTorr and a temperature of 27 �C. Oxygen ow into theRIE chamber was maintained at 50 cm3 min�1 and the powerwas 120 W while the etch time was set for 30 min.

Results and discussion

Scheme 1 shows the steps employed to generate the hierarchicalnano-features on a silicon and silicon oxide substrate. Thesubstrate is rst coated with a thin lm of a block copolymer.The block copolymer used here is an asymmetric PS-b-P4VPblock copolymer having a polyvinyl pyridine minor component.Such a block copolymer under the SSL29 (strong segregationlimit) and using the SCFT30 (self consistent eld theory) isexpected to form phase-separated domains31 with a periodicityof �30 to 50 nm.32 Substrates with the block copolymer lm arelater solvent annealed in an enclosed chamber (Scheme 1(A)) toproduce phase-separated domains having P4VP vertical cylin-ders in a PS matrix. The solvent annealing process was done in atoluene : THF (20 : 80) environment to reduce the evaporationrate from the lm and thus allow for the formation of verticallyoriented cylinders.33,34 The phase separation process thus servesas a template for producing the primary chemical ordering. Thephase-separated thin lms can either be loaded with metallicsalts followed by laser ablation (Scheme 1(B–D)) or a laserablation process could be carried out before the thin lms areloaded with metallic salts (Scheme 1(B, E and F)).

Either of the two processes can be used to generate hierar-chical features because the metallic loading step is independentof the laser interference ablation. However, the metallic loadingstep is dependent on the material composition of the block co-polymer as well as on the phase separation, aer solventannealing, between the PS and P4VP blocks of the PS-b-P4VPblock co-polymer. The laser ablation patterning process gives

Nanoscale, 2013, 5, 3912–3917 | 3913

Scheme 1 Scheme showing the steps followed for the generation of hierar-chical functional ceramic and metallic nanoparticles using block copolymer phaseseparation and laser interference ablation.

Nanoscale Paper

Publ

ishe

d on

14

Febr

uary

201

3. D

ownl

oade

d by

Bro

wn

Uni

vers

ity o

n 17

/07/

2013

15:

19:3

7.

View Article Online

the secondary physical ordering to the thin lms with or withoutmetallic loading. Aer the laser ablation patterning, a reactiveion etch step is performed to remove all the organic compounds,polystyrene and polyvinyl pyridine, on the substrate and subse-quently generate the corresponding metallic and ceramic hier-archical nanoparticles on the substrate.

Fig. 1A and B show the Atomic Force Microscopy (AFM)phase contrast and topographical images, of a phase-separatedthin lm of PS-b-P4VP block copolymer on a silicon oxidesubstrate. From the phase and height images, the harder

Fig. 1 AFM phase (A) and topographical (B) images of a phase-separated PS-b-P4VP block copolymer on a silicon oxide substrate.

3914 | Nanoscale, 2013, 5, 3912–3917

material i.e. the vinylpyridine component appears to be swollenwhile the soer material (polystyrene) appears to line theswollen domains formed by the much harder poly (vinyl-pyridine). AFM images were also taken of thin lms before andaer metallic loading to observe the degree of swelling of thelms (the AFM images are provided in the ESI†). Metallicloading was enabled by soaking the thin lms in a 9 : 1 vol% ofmetallic salt solution and hydrochloric acid solution – 1.0 wt%.The hydrochloric acid activates the vinylpyridine component ofthe block copolymer thus allowing the formation of complexesbetween the heterocyclic nitrogen containing compound andthe metallic salt complex.35,36 Hydrochloric acid also dissociatesthe complex inorganic salt into its respective anions andcations. The anionic complex formed in the hydrochloric acidsolution is stabilized by the amino group donating electrons tothe complex.37 Because the bond between the metallic complexand amino group is mainly electrostatic in nature, by varyingthe loading time, it is possible to vary the metallic loading of theblock and thus the diameter of each of the metallic domainsobtained aer the RIE step shown in Scheme 1.

To conrm metallic loading directly into the pyridinecomponent of the diblock, X-ray photoelectron spectra (XPS) ofthe as-annealed andmetal-loaded thin lms were also obtained.

As shown in Fig. 2, the XPS spectra obtained for the nitrogencomponent in the heterocyclic aromatic compound that forms acomplex with the metallic salt solution indicates an upwardshi in the binding energy of nitrogen from about 398.75 eV to400 and 402.5 eV. The shi in the binding energy can directly belinked to the complexation of the vinylpyridine component.Since nitrogen in P4VP has two free electrons that do notcontribute to the aromatic ring structure, upon complexation,the lone pair of electrons are donated to the metallic anioncomplex and increase the valency of nitrogen from 0 to +1 whichresults in the higher binding energy as seen in the XPS spectra.

Depending on the route taken in Scheme 1, laser interferenceablation with a 266 nm wavelength Nd:YAG frequency doubled10 ns pulsed laser can be done before or aer themetallic loading.Using a two-beam laser interference patterning technique,

Fig. 2 XPS N 1s spectra of nitrogen in PS-b-P4VP and metal-loaded PS-b-P4VPdiblock copolymer.

This journal is ª The Royal Society of Chemistry 2013

Fig. 3 SEM images of (A) linear periodic arrays, (B) periodic square arrays, (C)periodic triangular arrays and (D) periodic hexagonally packed holes of a PS-b-P4VP thin film processed by LIA. The scale bars are all 4 mm.

Fig. 4 AFM (A) topographical image of a laser patterned PS-b-P4VP showingperiodic line arrays and (B) topographical and phase contrast image of a laserpatterned PS-b-P4VP showing periodic square arrays.

Fig. 5 AFM topographical image of hierarchical metallic and ceramic (A) linearperiodic arrays and (B) periodic square arrays.

Paper Nanoscale

Publ

ishe

d on

14

Febr

uary

201

3. D

ownl

oade

d by

Bro

wn

Uni

vers

ity o

n 17

/07/

2013

15:

19:3

7.

View Article Online

periodic line, square, triangular, and circular arrays can begenerated. Fig. 3A shows the SEM image of a laser patterned PS-b-P4VP block co-polymer showing linear patterns with a periodicityof �2 mm. Square periodic arrays could also be generated via twosuccessive laser interference ablations with 90� rotation betweensuccessive shots. The square periodic arrays are shown in the SEMimage in Fig. 3B. Furthermore, three successive shots at 60�

rotation between successive shots can also result in the genera-tion of triangular arrays on the substrate as shown in Fig. 3C,while hexagonally packed holes or islands can be generated usinga 3-beam laser interference pattern, as shown in Fig. 3D.

AFM images of laser patterned samples are also shown inFig. 4A and B. Fig. 4A shows the laser ablation pattern formedfrom a single laser ablation resulting in the generation of linearfeatures on the block co-polymer thin lm. The depth of thegrooves formed (shown in the ESI†) is found to be �150 nmthrough AFM analysis. Periodic square arrays with �150 nmgroove depth and periodicity of 2 mm are shown in the AFMimage of Fig. 4B.

Periodic island arrays obtained from a 3-beam laser inter-ference patterning were also analyzed as described in the ESI.†Aer the LIA of the metal-loaded sample, the sample is etchedusing a RIE tool. The RIE tool uses radio frequency reactive ionplasma to etch through a sample. The oxygen plasma etchremoves all the organic materials from the substrate andreduces the metallic salt to their corresponding elementalcomponent and oxides. Alternatively, the block copolymer thinlm can be patterned and then loaded with metallic salts fol-lowed by oxygen reactive ion etching to produce the periodicarrays of metallic and ceramic nano-particles on the surface.

The AFM image of the etched samples is shown in Fig. 5Aand B. In Fig. 5A, the periodic line arrays produced from the LIA(shown in Fig. 4A) translate into a hierarchical line array ofmetallic nano-particles. Such arrays of metallic gold andceramic FexOy, PtOx and PdOx have been produced as indicatedin the XPS spectra of the reactive ion etched samples shown inFig. 6A and B. Fig. 5B is the AFM image of samples obtained

This journal is ª The Royal Society of Chemistry 2013

from two successive laser interference patterns with 90� rota-tion between successive shots. The secondary periodicity of thegenerated hierarchical features appears to correlate with the 2mm periodicity obtained for the laser interference ablation,while the primary periodicity correlates with the periodicity ofthe phase-separated block copolymer thin lms.

The XPS spectra in Fig. 6A shows the binding energies for theFe 2P3/2 and the Fe 2P1/2 for the complexed iron salt and theoxide of iron obtained aer the reactive ion etching step. Aerreactive ion etching there is a shi to a higher binding energyfor the doublet peak indicating the formation of a state ofhigher valency i.e. the oxides in this case. Furthermore, the peakappears to broaden indicating that iron may exist in severalvalence states aer etching.

On the other hand from the XPS spectra shown in Fig. 6B,gold has two doublets for the Au 4F7/2 and Au 4F5/2 indicating

Nanoscale, 2013, 5, 3912–3917 | 3915

Fig. 6 XPS spectra of (A) metallic iron salt complexed with P4VP before and after reactive ion etching (B) metallic gold salt complexed with P4VP before and afterreactive ion etching (C) metallic palladium salt complexed with P4VP before and after reactive ion etching (D) metallic platinum salt complexed with P4VP before andafter reactive ion etching.

Fig. 7 SEM images of (A) hierarchical periodic linear array of metallic andceramic nanoparticles and (B) hierarchical periodic square arrays of metallic andceramic nanoparticles.

Nanoscale Paper

Publ

ishe

d on

14

Febr

uary

201

3. D

ownl

oade

d by

Bro

wn

Uni

vers

ity o

n 17

/07/

2013

15:

19:3

7.

View Article Online

that gold may be present in two different oxidation states orchemical bonding states in the complexed metallic salt solu-tion. Upon etching there is a shi in the binding energy of goldto a lower binding energy indicating that elemental gold isbeing deposited on the sample upon etching. The XPS spectrafor palladium indicates a single doublet for Pd 3d5/2 and Pd3d3/2 before and aer reactive ion etching of the complexed salt.Aer reactive ion etching there is a shi in the binding energy ofpalladium to higher binding energies indicating the potentialformation of oxides of palladium. Platinum also follows thesame general trend like palladium, although the shi in thebinding energy for platinum's Pt 4F7/2 and Pt 4F5/2 is �5 eVcompared to the 1 eV change in binding energy for palladium.Elemental palladium, iron and platinum hierarchical nano-particles can be obtained from their oxides via argon reactiveion etching for a period of about 30 min but the elementalfeatures are themselves unstable in air and rapidly oxidize backto the oxide state upon exposure to ambient environments.

SEM images of the hierarchical metallic and ceramic nano-particles are shown in Fig. 7A and B. Fig. 7A shows the linear

3916 | Nanoscale, 2013, 5, 3912–3917

arrays of hierarchical gold, palladium, iron and platinumnanoparticles for LIA that leads to the generation of lineararrays on a substrate. The SEM image shows that the size ofeach metallic domain is �30 nm which conforms to the size ofthe P4VP microdomains formed aer phase separation. Fig. 7Bshows hierarchical metallic and ceramic nanoparticles for LIAthat generate square arrays on a substrate.

This journal is ª The Royal Society of Chemistry 2013

Paper Nanoscale

Publ

ishe

d on

14

Febr

uary

201

3. D

ownl

oade

d by

Bro

wn

Uni

vers

ity o

n 17

/07/

2013

15:

19:3

7.

View Article Online

Conclusions

In conclusion, we have demonstrated the ability to fabricatehierarchical arrays of gold, platinum, palladium and ironnanofeatures on silicon and silicon oxide substrates using anovel approach that combines the primary ordering inherent inphase separating block copolymers and the secondary orderingobtainable from a laser interference ablation technique. Wehave also shown the possibility to tune the size of the nano-features by varying the loading time of the thin lms and haveshown that the nanofeatures observed appear to be of singledomain and possibly crystalline with potential catalytic prop-erties. The use of such hierarchical metallic and ceramicfeatures is immense as they can not only be used for growth ofpatterned carbon nanotubes, germanium and silicon nanowiresas well as zinc oxide nanowires, but can also nd use as metallicmasks for the generation of sub-40 nm features on varioussemiconducting substrates for use in next-generation low-costmicrochip fabrication.

Notes and references

1 K. L. Kelly, E. Coronado, L. L. Zhao and G. C. Schatz, J. Phys.Chem. B, 2002, 107(3), 668–677.

2 F. Vallee, in Nanomaterials and Nanochemistry, ed. C.Brechignac, P. Houdy and M. Lahmani, Springer, Berlin,Heidelberg, 2007, pp. 197–227.

3 H. H. Huang, F. Q. Yan, Y. M. Kek, C. H. Chew, G. Q. Xu,W. Ji, P. S. Oh and S. H. Tang, Langmuir, 1997, 13(2), 172–175.

4 M.-C. Daniel and D. Astruc, Chem. Rev., 2003, 104(1), 293–346.

5 Q. Jiang, S. Zhang and M. Zhao, Mater. Chem. Phys., 2003,82(1), 225–227.

6 T. Bachels, H.-J. Guntherodt and R. Schafer, Phys. Rev. Lett.,2000, 85(6), 1250.

7 A.-H. Lu, E. L. Salabas and F. Schuth, Angew. Chem., Int. Ed.,2007, 46(8), 1222–1244.

8 A. K. Gupta and M. Gupta, Biomaterials, 2005, 26(18), 3995–4021.

9 H. Ohde, C. M. Wai, H. Kim, J. Kim and M. Ohde, J. Am.Chem. Soc., 2002, 124(17), 4540–4541.

10 X. Luo, A. Morrin, A. J. Killard and M. R. Smyth,Electroanalysis, 2006, 18(4), 319–326.

11 B. D. Gates, Q. Xu, M. Stewart, D. Ryan, C. G. Willson andG. M. Whitesides, Chem. Rev., 2005, 105(4), 1171–1196.

12 X. N. Xie, H. J. Chung, C. H. Sow and A. T. S. Wee,Mater. Sci.Eng., R, 2006, 54(1–2), 1–48.

13 R. D. Piner, J. Zhu, F. Xu, S. Hong and C. A. Mirkin, Science,1999, 283(5402), 661–663.

This journal is ª The Royal Society of Chemistry 2013

14 K.-B. Lee, S.-J. Park, C. A. Mirkin, J. C. Smith andM. Mrksich,Science, 2002, 295(5560), 1702–1705.

15 J. H. Moon, J. Ford and S. Yang, Polym. Adv. Technol., 2006,17(2), 83–93.

16 W. K. Choi, T. H. Liew, M. K. Dawood, H. I. Smith,C. V. Thompson and M. H. Hong, Nano Lett., 2008, 8(11),3799–3802.

17 (a) H. Segawa, N. Abrams, T. E. Mallouk, I. Divliansky andT. S. Mayer, J. Am. Ceram. Soc., 2006, 89(11), 3507–3510; (b)A. F. Lasagni, D. Yuan and S. Das, Adv. Eng. Mater., 2009,11(5), 408–411.

18 C. S. Lim, M. H. Hong, Y. Lin, Q. Xie, B. S. Luk'yanchuk,A. S. Kumar and M. Rahman, Appl. Phys. Lett., 2006, 89(19),191125.

19 C. J. M. van Rijn, J. Microlithogr., Microfabr., Microsyst., 2006,5(1), 011012.

20 G. Roman, et al., Nanotechnology, 2003, 14(10), 1153.21 J. Y. Cheng, C. A. Ross, V. Z. H. Chan, E. L. Thomas,

R. G. H. Lammertink and G. J. Vancso, Adv. Mater., 2001,13(15), 1174–1178.

22 M. Lazzari and M. Lopez-Quintela, Adv. Mater., 2003, 15(19),1583–1594.

23 I. W. Hamley, Nanotechnology, 2003, 14(10), R39.24 F. K. Thomas and J. M. Luke, J. Phys. D: Appl. Phys., 2010,

43(18), 183001.25 A. S. Eppler, G. Rupprechter, L. Guczi and G. A. Somorjai, J.

Phys. Chem. B, 1997, 101(48), 9973–9977.26 I. Baek, J. Vac. Sci. Technol., B: Microelectron. Nanometer

Struct.–Process., Meas., Phenom., 2005, 23(6), 3120.27 D. Yuan, R. Guo, Y. Wei, W. Wu, Y. Ding, Z. L. Wang and

S. Das, Adv. Funct. Mater., 2010, 20(20), 3484–3489.28 A. F. Lasagni, D. Yuan, P. Shao and S. Das, Adv. Eng. Mater.,

2009, 11(3), B20–B24.29 F. S. Bates, Science, 1991, 251(4996), 898–905.30 M. W. Matsen and F. S. Bates, Macromolecules, 1996, 29(4),

1091–1098.31 K.-V. Peinemann, V. Abetz and P. F. W. Simon, Nat. Mater.,

2007, 6(12), 992–996.32 D. O. Shin, D. H. Lee, H.-S. Moon, S.-J. Jeong, J. Y. Kim,

J. H. Mun, H. Cho, S. Park and S. O. Kim, Adv. Funct.Mater., 2011, 21(2), 250–254.

33 S. Park, J.-Y. Wang, B. Kim, W. Chen and T. P. Russell,Macromolecules, 2007, 40(25), 9059–9063.

34 S. Park, J.-Y. Wang, B. Kim, J. Xu and T. P. Russell, ACS Nano,2008, 2(4), 766–772.

35 M. Aizawa and J. M. Buriak, J. Am. Chem. Soc., 2005, 127(25),8932–8933.

36 M. Aizawa and J. M. Buriak, J. Am. Chem. Soc., 2006, 128(17),5877–5886.

37 J. Q. Lu and S. S. Yi, Langmuir, 2006, 22(9), 3951–3954.

Nanoscale, 2013, 5, 3912–3917 | 3917