Physics Letters A 375 (2011) 3012–3016

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Physics Letters A

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Effect of chromium interlayer deposition on periodic silver nanoparticle arraystructure fabricated by nanosphere lithography

Shengli Huang a,b,c,∗,1, Lingqi Kong a,b, Chunjing Zhang a,b, Yan Wu a,b,d, Xianfang Zhu a,b,∗,2

a Department of Physics, Xiamen University, Xiamen, Fujian 361005, PR Chinab China–Australia Joint Laboratory for Functional Nanomaterials, Xiamen University, Xiamen, Fujian 361005, PR Chinac State Key Lab of Silicon Materials, Zhejiang University, Hangzhou 310027, PR Chinad Department of Physics, Jimei University, Xiamen, Fujian 361021, PR China

a r t i c l e i n f o a b s t r a c t

Article history:Received 22 April 2011Received in revised form 21 June 2011Accepted 23 June 2011Available online 30 June 2011Communicated by R. Wu

Keywords:Nanoparticle arrayNanosphere lithographyInterlayerAdhesive ability

Effect of chromium interlayer deposition on 2-dimensional, periodic silver nanoparticle array structurewas systematically investigated. The silver nanoparticle array was fabricated by nanosphere lithographywith assembled polystyrene nanospheres being as a deposition mask. The chromium interlayer was de-posited by thermal evaporation either on the nanosphere mask or directly on the silicon substrate. Thestructures of the achieved silver nanoparticle arrays were characterized by scanning electron microscopeand were compared with that of silver nanoparticle array without the interlayer. With analysis of theanomalies among the structures the critical role of the interlayer in the periodic nanoparticle array fab-rication was revealed.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Arrays of noble metal nanoparticles, which can exhibit a strongUV-vis absorption band with localized surface plasmon resonance(LSPR) that does not present in the spectrum of the bulk metal,have been extensively studied for their potential convenient sens-ing capability in nanoscale chemosensors and biosensors [1–5].The LSPR refers to the excitation of surface plasmons by lightfrom nanometer-sized metallic particles. The position and inten-sity of it are sensitively dependent on composition, size, shape, andinter-particle spacing of the nanoparticles as well as the dielectricproperties of their local environments [3]. Many methods, includ-ing photolithography [6], electron beam lithography [7], dip pennanolithography [8], nanosphere lithography (NSL) [9], etc., wereattempted for fabrication of such nanoparticle arrays, among whichthe NSL is the most low-cost, high-throughput method for produc-ing periodic, geometrically tunable nanostructure arrays. The NSLmakes use of a template formed by the self-assembly of monodis-perse nanospheres on flat surface acting as a deposition/etchingmask. Nevertheless, even with the NSL, complicated physical and

* Corresponding authors at: Department of Physics, Xiamen University, Xiamen,Fujian 361005, PR China.

E-mail addresses: huangsl@xmu.edu.cn (S. Huang), zhux@xmu.edu.cn (X. Zhu).1 Tel.: +86 592 2180437; fax: +86 592 2189426.2 Tel./fax: +86 592 2180436.

0375-9601/$ – see front matter © 2011 Elsevier B.V. All rights reserved.doi:10.1016/j.physleta.2011.06.052

chemical processes are normally involved and thus render it diffi-cult to achieve a designed structure. One of the major problems isthe adhesive ability of the deposited metallic particles to the sub-strate. The cohesive force and wettability of a noble metal, silveror gold, for example, are normally very limited on a glass or sil-icon substrate. The noble metals are inclined to be lifted off dueto the abrupt change of physical and chemical properties acrossthe interface between the metal and the substrate. Both theoriesand experiments suggested that an interlayer could be depositedto promote the adhesion of metallic nanoparticles to the substrates[1,3,10–15]. For example, A.J. Haes et al. [3] applied chromiumbuffer thin film to increase adhesion of silver nanoparticles to theglass substrate. X.F. Zhu et al. [11] suggested a gradient film to op-timize or grade the abrupt change of microstructure and internalstress across the interface between the metal and the substrate.However, except the article reported by X.L. Sun and J.D. Shao[10] that focused on the study of the optical properties and ad-hesion of silver films with chromium interlayer of different thick-ness on glass substrates, no articles were found to explore theimportant role of the interlayer on the nanoparticle arrays fabri-cation. The interlayer was deposited either underneath or on thedeposition mask as normally described in the experimental sec-tion but the difference between the two procedures was rarelyexplored.

With the above considerations, in the present article, effect ofchromium interlayer deposition on 2-dimensional periodic silvernanoparticle structure was systematically investigated. The silver

S. Huang et al. / Physics Letters A 375 (2011) 3012–3016 3013

Fig. 1. Schematic illustration of silver nanoparticle array fabrication with differentprocesses: (a) Process I: a single-layer of size-monodisperse PS nanospheres wasdrop-coated onto the substrate followed by the deposition of a silver film; (b) Pro-cess II: a thin chromium layer was sandwiched between the PS mask and thesubstrate as an adhesive layer; (c) Process III: a thin chromium layer was sand-wiched between the silver film and the PS mask as an adhesive layer.

nanoparticles were fabricated by NSL with polystyrene (PS) nano-spheres as a deposition mask. Thermal evaporation was applied todeposit silver and chromium films. The chromium interlayer wasdeposited either on the PS mask or directly on the silicon sub-strates. The structures of the achieved silver nanoparticle arrayswere characterized by scanning electron microscope (SEM) andthey all were compared with that of silver array without the inter-layer. The results suggested that the chromium interlayer playedan important role in the improvement of the adhesive ability ofthe hexagonally arranged triangular silver nanoparticles to siliconsubstrate. Moreover, the interlayer should be deposited after theformation of the sphere mask, otherwise a pre-deposited interlayerwould limit the dispersion of the PS spheres over the substrate toform a large-scale monolayer array mask with hexagonally close-packed structure.

2. Experimental

The PS nanospheres with a mean diameter of 360 nm and aconcentration of 10 wt% in solution were purchased from SuzhouNano–Micro Bio-Tech Co. Ltd. To achieve a large-area hexagonalclose-packed nanosphere monolayer as a deposition mask, the PSnanosphere solution was diluted to be 3 wt% with deionized water,and the silicon substrates ((100) orientation, n-type) were thor-oughly cleaned, first by sonication in toluene, acetone, ethanol for10 min respectively and in piranha solution (H2SO4 and H2O2 inthe ratio of 3 : 1) for 2 hours to remove organic residues, then bysonication in NH4OH, H2O2, and H2O in the ratio of 1 : 1 : 5 for2 hours to produce a hydrophilic surface. After each sonication thesubstrates were rinsed with copious amounts of deionized water,then they ware stored in water until used.

To explore the effect of chromium interlayer deposition on thesilver nanostructure, three different NSL processes were carriedout before removing PS spheres from the substrates. In Process I,a single-layer of the PS nanospheres was directly drop-coated ontothe substrate, and a silver film was then perpendicularly depositedon the nanosphere mask, as shown in Fig. 1(a). In Process II,a chromium thin film was deposited on the substrate prior to thedrop-coating of the PS nanospheres, and the silver layer was de-posited in the end, as shown in Fig. 1(b). In Process III, followingdrop-coating of the PS nanospheres on the substrate, a chromiumfilm and a silver layer, respectively, were deposited on the PS masksequentially, as shown in Fig. 1(c).

The deposition of silver (99.9%) and chromium (99.9%) wasperformed in a home-built thermal evaporator at a pressure of5.0 × 10−4 Pa. The metallic granulate source materials were placedin molybdenum boats (of size 1 cm2) that faced up directly to-ward the substrate in a distance of 16.0 cm. During the depositionthe substrates were rotated at a frequency of 16.5 rmp. The powerfor heating up of the source materials was increased carefully toachieve homogeneous deposition rates of ∼ 2.5 nm/s for silver and∼ 4.0 nm/s for chromium. The thickness of the films was mea-

sured by a Dektak 3 Series surface profiler and was controlled tobe 150 nm for silver film and 10 nm for chromium film so asto achieve an identical depth for a low reflectance [10]. After thesilver deposition, the PS spheres were lifted off with dipping inabsolute ethanol for about 5 s. To examine the adhesive ability ofthe silver nanoparticles to the silicon substrates, the PS sphereswere also removed by sonication (B3500S-MT, Branson, 140 W,42 kHz) in absolute ethanol for different periods. Nanostructuresof the achieved PS mask and silver particle array were character-ized by a LEO-1530 SEM.

3. Results and discussion

Fig. 2(a) shows a representative SEM image of PS mask pre-pared by Process I. The nanospheres over a large area (a typicallateral size of such domains is on the scale of several microme-ters) form a regular monolayer array of a hexagonally close-packedstructure. Nevertheless, a few array structural defects are observeddue to the sloping nature and hydrophilic properties of the siliconsubstrate or the small difference (CV = 2.80%) in PS nanospherediameters, the environmental humidity and temperature, and thedrying speed. Figs. 2(b) and 2(c) present typical SEM images ofPS mask prepared by Process II. In this case, the nanospherestend to form multilayer islands or tend to disperse randomly overthe silicon substrate. Thus, the desired monolayer array with thehexagonally close-packed structure is hardly obtained. In this pro-cess, the chromium deposition introduced a thin film on the siliconsubstrate, which might cause some protuberance on the surfaceof the silicon substrate and should not be as smooth as the par-ent surface. Thus, the dispersion of the PS nanospheres would beblocked by the rough surface because otherwise they would beassembled in a relatively straightforward manner with a capillary-driven method [16]. Fig. 2(d) shows a typical SEM image of the PSmask prepared by Process III. In this case, except a darker imagefor the presence of chromium interlayer, the structure of the nano-sphere mask is analogous to that of Process I. The chromium filmwas deposited after the assembly of the PS mask, which showedless negative effect on the dispersion of PS nanospheres on thesubstrate.

Fig. 3 illustrates plan view SEM images of silver nanoparticlearray formed within gap interspaces of the PS spheres mono-layer. The PS spheres were all lifted off with dipping in ethanolfor 5 s. For Process I, as shown in Fig. 3(a), the produced silvernanoparticles exhibit a hexagonally arranged disc structure withincreased thickness from edges to the center. In addition, somesilver nanoparticles nearby merge together due to the grain bound-ary defects within the PS mask and exhibit a larger scale rhombicstructure. For Process II, as shown in Fig. 3(b), the silver nano-particles present a triangular structure in the hexagonal array, butthe structure can be only acquired within a limited area near theboundary of the PS mask. The most of the surface area is coveredwith a layer of silver film or silver particles with irregular features.For Process III, the nanoparticles still exhibit a triangular structurebut that is more uniform than the other cases: they are arrangedas a hexagonal array over a large scale area (on the scale of severalmicrometers in lateral size).

It is well known that the nanoparticle arrays achieved from NSLare due to the metallic deposition in the gap interspaces amongthe spheres as well as the shadowing effect of the sphere mask.As shown in Fig. 4(a), when the radius of the evaporating particlesis less than r (the value of r is dominated by the sphere radius R

with a relation of r = ( 2√

33 − 1)R), the particles in certain deposi-

tion directions will pass through the shadowing mask and depositonto the surfaces over the gap interspaces. In the thermal evapo-ration, the evaporating materials are in an atomic state for metals.

3014 S. Huang et al. / Physics Letters A 375 (2011) 3012–3016

Fig. 2. Representative SEM images of the PS mask: (a) in Process I; (b, c) in Process II; (d) in Process III. The scale bar is 1 μm.

Fig. 3. Representative SEM images of the silver nanoparticle array: (a) in Process I;(b) in Process II; (c) in Process III. The PS spheres were lifted off with dipping inethanol for 5 s. The scale bar is 1 μm.

Fig. 4. Schematic illustration of: (a) the gap among hexagonally close-packed PSnanospheres and (b) the corresponding hexagonally arranged triangular nano-particle array formed within the gap interspaces as shown in (a).

The radius of the metallic atoms is only 0.175 nm for silver and0.185 nm for chromium, which is far less than the limited dimen-sion of r = 28 nm for the PS nanosphere mask with R = 180 nm.The evaporating metals would thus conveniently pass through theshadowing mask and deposit onto the surface of the gap to forma hexagonally arranged triangular array, as shown in Fig. 4(b). Forthe silver nanoparticles fabricated without an adhesive medium,the cohesive force between the particles and the substrate is rel-atively weak due to the abrupt change of microstructure and in-ternal stress across the interface between the silver and the siliconsubstrate, especially for the particles at the tips and edges withlarger positive nanocurvatures. During the removal of the PS mask,the particles in these areas would be washed away from the sili-con substrate. Only those in the middle area left, resulting in thehexagonally arranged disc structure. However, for the nanoparticles

S. Huang et al. / Physics Letters A 375 (2011) 3012–3016 3015

Fig. 5. Representative SEM images of the silver nanoparticle array: (a, b) The PS spheres were lifted off, respectively, with dipping in ethanol for 5 s and with sonication inethanol for 3 s in Process I; (c, d) The PS spheres were lifted off, respectively, with dipping in ethanol for 5 s and with sonication in ethanol for 3 s in Process III. The scalebar is 500 nm.

fabricated with the chromium interlayer, the chromium would actas an adhesive medium to improve the cohesive force between thesilver particles and the silicon substrate. According to the Cr–Siphase diagram [17], there is a chemical reaction between thechromium and silicon at about 1610 ◦C. The melting point of thechromium is as high as 2180 ◦C. The evaporating chromium atomswould thus immediately react with the silicon to form the CrSi2,which gives rise to a strong cohesive force and wettability to thesilicon substrate. The following evaporating chromium deposits onthe CrSr2 and shapes the designed hexagonally arranged triangularstructure in the gap interspaces among hexagonally close-packedPS nanospheres. As the thickness of the chromium film is only10 nm, the chromium merely provides a thin interlayer in thebottom of the gap interspaces. The evaporating silver atoms passthrough the PS mask and deposit on the chromium interlayer. Thesilver and chromium adhere together easily with a strong cohesiveforce because both are bonded with metallic bonds.

To further confirm that the difference in the above structurestems from the variation in the cohesive force, the PS spheres werealso removed by sonication in ethanol for different periods. Fig. 5shows SEM images of silver nanoparticle array achieved by sonica-tion in ethanol for 3 s. For comparison, the SEM images of nano-particle array by dipping method for the mask removal are alsopresented herein. For the nanoparticle array fabricated by Process I,as shown in Figs. 5(a) and 5(b), the tiny debris and small brokenparts around the nanoparticles are also removed during the soni-

cation, resulting in a greater deformation of the nanoparticle con-tour profile. However, for the nanoparticle array fabricated by Pro-cess III, the silver nanoparticles morphology in Fig. 5(c) is identicalto that in Fig. 5(d). Little influence is observed on the structureby sonication for the limited duration. Fig. 6 shows a representa-tive SEM image of silver nanoparticle array achieved by Process IIIwith sonication in ethanol for 20 s. There are much less silvernanoparticles remained on the substrate. The particles are in alarge variable number and normally tend to accumulate as a singlecluster in the middle of chromium islands. Moreover, some nano-particles are almost removed entirely for the prolonged sonication,resulting in a nude chromium island (as indicated by arrows) ora nude substrate surface (as indicated by a circle). Even for thechromium islands, their tips are not as sharp as those predictedin Fig. 4(b). There are two possibilities for this phenomenon. First,the cohesive force between the chromium particles and the siliconsubstrate is not strong enough. The weak tips are washed awayin ethanol by sonication for the long duration. Second, except thecohesive force, there is a surface tension for the chromium nano-particles, which would be enhanced with the increased nanocurva-ture [18]. Such sharp tips would tend to vanish on the chromiumislands in order to decrease the surface energy.

Therefore, the pre-deposition of chromium interlayer on thenanosphere mask plays an important role to improve the adhesiveability of the hexagonally arranged triangular silver nanoparticlesformed by NSL. The chromium interlayer, with a stronger cohesive

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Fig. 6. Representative SEM image of the silver nanoparticle array in Process III. ThePS spheres were lifted off with sonication in ethanol for 20 s. The scale bar is100 nm.

force and better wettability to the silicon substrate, can producea desired triangular island for the silver deposition. As chromiumand silver can be bonded with each other with metallic bonds,they adhere together easily with a strong cohesive force. However,the adhesion does not imply that the silver nanoparticles can resistany disturbance. Sonication in ethanol with a long duration maycause lots of defects in the array nanostructure. Further thermalannealing may be alternatively applied to improve the situation [1,14,19,20].

4. Conclusions

Effect of chromium interlayer deposition on 2-dimensional pe-riodic silver nanoparticle array structure was systematically inves-tigated. The silver nanoparticles were fabricated by NSL with PSnanospheres as a deposition mask, and the chromium interlayerwas deposited either on the PS mask or directly on the silicon sub-strates. By comparison of silver arrays formed with and without achromium interlayer, the deposition of a chromium interlayer wasrevealed to play an important role in improvement of the adhesiveability of the hexagonally arranged triangular silver nanoparticlesto the silicon substrate. In addition, the experiments suggested aswell that the interlayer should be deposited after the formationof the sphere mask because the interlayer might compromise thedispersion of the PS spheres on the substrate to form a large-scalemonolayer array with the hexagonally close-packed structure.

Acknowledgements

This work was supported by China–MOST International Sci& Tech Cooperation and Exchange Project under grant No.2008DFA51230, National Key Basic Science Research Program(973 Project) under Grant No. 2007CB936603, NSFC projects un-der Grant No. 60776007 and No. 11074207, China Ministry ofEducation Special Scientific Research Fund for Doctor Discipline ofInstitution of Higher of Learning under Grant No. 20100121110023,the SRF for ROCS, SEM, Research Fund from Fujian Key Laboratoryof Semiconductors and Applications at Xiamen University.

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