plasmonic films based on colloidal lithography

Plasmonic films based on colloidal lithography

Bin Ai a, Ye Yu a, Helmuth Möhwald b, Gang Zhang a,⁎, Bai Yang a

a State Key Lab of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, PR Chinab Max Planck Institute of Colloids and Interfaces, D-14424 Potsdam, Germany

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

Available online 23 November 2013

Keywords:Plasmonic filmsSurface plasmon resonanceColloidal lithography

This paper reviews recent advances in the field of plasmonic films fabricated by colloidal lithography. Comparedwith conventional lithography techniques such as electron beam lithography and focused ion beam lithography,the unconventional colloidal lithography technique with advantages of low-cost and high-throughput has madethe fabrication processmore efficient, andmoreover brought out novel films that show remarkable surface plas-mon features. These plasmonic films include those with nanohole arrays, nanovoid arrays and nanoshell arrayswith precisely controlled shapes, sizes, and spacing. Based on these novel nanostructures, optical and sensingperformances can be greatly enhanced. The introduction of colloidal lithography provides not only efficient fab-rication processes but also plasmonic films with unique nanostructures, which are difficult to be fabricated byconventional lithography techniques.

© 2013 Elsevier B.V. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52. Plasmonic films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63. Colloidal lithography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

3.1. Preparation and modification of colloidal crystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63.2. Colloidal crystal assisted operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

4. Plasmonic films based on colloidal lithography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84.1. Plasmonic films with nanohole arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

4.1.1. Plasmonic films with conventional nanohole arrays based on colloidal lithography . . . . . . . . . . . . . . . . . . . . . . . 84.1.2. Plasmonic films with modified nanohole arrays based on colloidal lithography . . . . . . . . . . . . . . . . . . . . . . . . . 9

4.2. Plasmonic films with inverse opal arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134.3. Plasmonic films with nanoshell arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

5. Conclusions and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

1. Introduction

Surface plasmons (SPs) are coherent oscillations of conduction elec-trons on a metal surface excited by electromagnetic radiation at ametal–dielectric interface. This growing field of research on light-metalinteractions is known as “plasmonics” [1–3]. It has attracted much atten-tion and one key topic rests in fabricating various metal nanostructuresfor clarity of the fundamental mechanism and improved applications. Inview of the previous reports, two types of structures – nanoparticles

and films – are considered to cover the main plasmonic structures, ofwhich the latter one is defined as plasmonic films. Recently, more re-search has focused on the films with concave/convex nanostructures,sharp tips and adjoining nanostructures on the surface, which constituteone of the most explored platforms for miniaturized optical devices, sen-sors, and photonic circuits with special focus on medical diagnostics andtherapeutics [4–8]. These plasmonic materials offer a large range of ad-vantages compared to nanoparticles, including facile surface chemistryfor the immobilization of molecular recognition elements [9]; the possi-bility of a small (subwavelength) sensing area [10,11]; the potential formassive multiplexing (detection of several different chemical species atthe same time) [12]; easy integration with microfluidics [13], leading to

Advances in Colloid and Interface Science 206 (2014) 5–16

⁎ Corresponding author.E-mail address: [email protected] (G. Zhang).

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small device footprint; high sensitivity, excellent stability and ease of use.All these favorable properties justify the high research activity aimed atthe development of different types of SP-based films.

Plasmonic films are a highly active area due, in part, to recent ad-vances in nanofabricationmethodologies. Thesemethodologies, includ-ing electron beam lithography [14,15], focused ion beam lithography[16] and colloidal lithography (CL) [17], have led to the realization ofstructured films with nanoholes [18], slits [19], gratings [20], cones [21],and other components [22] with precisely controlled shapes, sizes, andspacing. In particular, CL, as a fast developing unconventional lithographictechnique, makes the fabrication process more efficient and furtherprovides unique advantages in fabricating three-dimensional nanostruc-tures. Monodispersemicrosphereswith sizes ranging from tens ofmicro-meters to tens of nanometers, can easily be synthesized by conventionalemulsion polymerization or sol–gel synthesis techniques. Owing to thesize and shape monodispersity, they can self-assemble into two-dimensional (2D) and three-dimensional (3D) extended periodic arrays,coined as a colloidal crystal [23–26]. The interstitial array in a colloidalcrystal was used as amask for etching or deposition to create 2D patternson a substrate. This patterning process is preferentially referred to as CL,paving a simple and low-cost route for patterning with a flexibility ofscaling down the feature size below 100 nm.

This reviewbriefly introduces the design and fabrication of CL-basedplasmonic films, followed by placing the emphasis on the strategy ofutilizing CL to fabricate periodic structured films. These plasmonicfilms will be also reviewed with a brief update on advancements ofoptical and sensing performance, as well as novel properties based onvarious surface morphologies. Finally, challenges and outlook areoffered to inspire more exciting developments in this still young yetvery promising field in the future.

2. Plasmonic films

Conduction electrons of ametal act like a plasma and can support sur-face waves, the quantum of which is known as a surface plasmon (SP).SPs have been described as light trapped at the metal surface and, inthat sense, can be considered as two-dimensional light. Two types ofsurface plasmon resonances (SPRs) are exhibited in metal structures:(i) propagating surface plasmon polaritons and (ii) nonpropagatinglocalized SPRs. Localized SPRs are excited when the incident photon fre-quency is resonant with the collective oscillation of the conduction elec-trons confined in the volume of the nanoparticles. The spectral positionand magnitude of the localized SPRs depend on the size, shape, compo-sition, and local dielectric environment [27,28]. This property has beenexploited for label-free optical sensing where adsorbate-induced refrac-tive index changes near or on plasmonic nanostructures are used tomonitor binding events in real time [29], which has been discussed byseveral reviews and books published in the past few years [30–32].

Propagating surface plasmon polaritons are excited on metal filmsby the coupling between a light wave and a surface plasmon at ametal–dielectric interface [33]. These metal films showing SP featuresare defined as plasmonic films. However, resonance cannot be createdon a smooth metal surface by direct optical excitation [34]. The reasonis that the SP momentum is larger than that of a free photon, and directlight-to-SP conversion is forbidden. Therefore, special couplingschemes, which are used to increase the wave vector of light to matchthat of the surface plasmon, need to be devised to allow for SP genera-tion. The most common couplers used at first include a prism coupler,a waveguide coupler, and a grating coupler. Prism couplers representthe most frequently used method for optical excitation of surfaceplasmons [35–37], of which the most common configuration is theone proposed by Kretschmann and Raether [35]. In this case, the eva-nescentfield from the totally reflected light from the prism side extendsthrough a thinmetal film (about 50 nm) to launch SPs on the other sideof the film.

Grating couplers have not been used aswidely as the prism couplers.However, their compatibility withmass production (in particular, repli-cation into plastic) makes a grating coupler an attractive approach forfabrication of low-cost SPR sensing structures. Furthermore, the gratingcouplers enable a strategy of preparing the films with corrugated struc-tures to achieve the coupling between light and a surface plasmon bythe films themselves. With further research, these corrugations are ex-tended to various types, such as periodic arrays or random distributionsof subwavelength holes (nanoholes) perforated in a metal film [38],rough structured continuous films [39–41], with a few examplesshown in Fig. 1, and these novel structured plasmonic films constitutethe focus of this review.

The geometric characteristics of the structures (type of corrugation,shape, and periodicity [42]) can be tailored to control the characteristics(resonance energy) of the SPs. Not limited to one SPR mode, both SPPsand localized SPRs might play a role in a particular system, offering op-portunities to yield surprising optical effects, which result in uniquetransmission, reflection, extinction or surface enhanced Raman scatter-ing (SERS) spectra. These remarkable properties open up a colorful fu-ture for the next generation of optical devices, sensors and otherrelevant applications.

The great process in the field of plasmonic films is, in part, due to theadvances in nanofabrication methodologies. Scanning beam lithogra-phy techniques, such as electron beam lithography and focused ionbeam lithography, are the main choices to fabricate metal structuredfilms. While these conventional lithography techniques are capable ofprecise control over the size, shape, and spacing of metallic nanostruc-tures, more recent research has focused on unconventional lithographictechniques that are capable of patterning large areas in parallel at lowcost. In particular, the colloidal lithography technique, possessing theadvantages of a low-cost and flexible fabrication process, has beenmore frequently used in preparing plasmonic films. The following sec-tion will describe in more details the colloidal lithography technique.

3. Colloidal lithography

The periodical array of a close-packed microsphere monolayer wasfirst used as a mask in 1981 for the deposition of platinum by Fischerand Zingsheim [43]. After that, this new technique has been extendedby large impressive efforts, and meanwhile has diverse names such asnatural lithography, nanosphere lithography, and colloidal lithography[44–48]. Following extensive work, colloidal crystals are recognized aslow-cost, flexible, and easily adoptable masks for growing new nano-structures with diverse structural complexity. For now, colloidal lithog-raphy is known as an important unconventional fabrication techniquerelying on using colloidal crystals as masks or templates for etchingand deposition. This technique includes two types ofmasks—disorderedcolloid particles and ordered colloidal crystal. The latter one, which hasattracted much more attentions, constitutes the focus of this review.Two main procedures of preparation and modification of colloidal crys-tals, and colloidal crystal assisted operations are included in the tech-nique. By flexibly controlling these procedures, various nanostructureson planar and nonplanar substrates can be fabricated. Some examplesare shown in Fig. 2. Besides, the CL technique has unique advantagesin fabricating 3D structures due to the variable height of the spheresin vertical direction. The following sectionswill describe inmore detailsthe two main procedures.

3.1. Preparation and modification of colloidal crystals

The success of using colloidal crystals asmasks for surface patterningis based on the capability of directing self-assembly of colloidal particlesand manipulating the crystal packing structures. Up to date a variety ofcolloidal crystallization techniques have successfully been developed toimplement colloidal crystallization in a controlled fashion, and eachtechnique possesses unique advantages and limitations. For example,

6 B. Ai et al. / Advances in Colloid and Interface Science 206 (2014) 5–16

spin coating allows easy and quick formation of 2D crystals over alarge area [49], however, with underlying mechanism remaining indebate; sedimentation was developed to achieve high quality colloi-dal crystals. However, time-consumption is a big drawback [50]; andvertical deposition needs much less time for colloidal crystallization[51]. Recently, colloidal crystallization at the water/air interface hasbeen extensively studied due to the advantage of precisely definedlayer thickness [52–54].

After producing 2D colloidal crystalswith high quality, as prepared 2Dcolloidal crystals can be either etched or deformed in order to increasethe structural complexity of surface patterns (micro-nanostructures).For example, annealing slightly above a glass transition temperature(Tg) of polymer spheres can cause deformation of spherical polymericbeads. Other than that, microwave radiation can much more preciselycontrol the deformation of spherical polymer particles [55]. The otherway is reactive ion etching, which has been widely used to interdepen-dently reduce the particle sizes and thus widen the interstitial space in2D colloidal crystal masks and eventually turn close-packing structuresof the crystals to non-close packing ones [56]. Furthermore anisotropicreactive ion etching paves a newway tomachine the surfaces of colloidalparticles.

3.2. Colloidal crystal assisted operations

After the preparation and modification of colloidal crystals, severalsubsequent operations can be carried out to achieve diverse structures.For example, when a 2D colloidal crystal is formed on a solid substrate,the interstices between the solid particles can be used as masks forreactive ions to create patterned bumps or pores on the substrate,which is called colloidal mask-assisted etching (Fig. 2A) [57,58].Colloidal mask-assisted chemical deposition is another significant tech-nique, which can be classified into several different specific methods.Combining microcontact printing with colloidal crystal masking, a sim-ple method – edge spreading lithography – to generate mesoscopicstructures on substrates has been developed (Fig. 2B) [59]. As thename suggests, edge spreading lithography utilizes the edges of masks– the perimeters of the footprint of particles on substrates – to define

the features of the resultant structures. Anotherway to integrate colloidalmasking and contact printing, referred to as contact area lithography, hasbeen developed to directly generate periodic surface chemical patterns atthe sub-100 nm scale [60,61].

Colloidal mask-assisted physical deposition is also known as nano-sphere lithography. In this procedure, 2D colloidal crystals are used asmasks for physical deposition. The depositingmaterials can be freely cho-sen, of which various metals such as gold and silver are commonly used.The projection of the interstices between the masks defines nanostruc-tures deposited on substrates. Furthermore, the in-plane shape of thenanostructures and the spacing of the nearest-neighboring structurescan be tunedby varying theprojection geometry of the interstices on sub-strates by titling the masks with respect to the incidence of the vaporbeam for instance. This has inspired development of angle-resolvednanosphere lithography [62]. By rotating substrates, angle-resolvednanosphere lithography cangeneratemuchmore complexmetallic nano-structures and this process is referred to as shadow nanosphere lithogra-phy [63,64].

The colloidal crystal can be used not only as mask but also as 3Dtemplate.When used as a template, various etching and deposition pro-cedures can be carried out for novel structures, especially in 3D spacing.Freely chosenmaterials can be deposited on the spheres resulting in 3Dnanoshells. Other than that, bottom-up growth of various materials canbe operated in the interstices of the colloidal template by chemicalwaysor electro-deposition. After removing the spherical template, nanovoid(nanocavity) arrays can be formed. Another facile method is that poly-mers can be spin-coated on the template resulting in nanovoid arrays.

Based on these fundamental operations, we havemademany effortsto develop a modified CL technique for diverse nanostructures [17]. Astepwise procedure has successfully been developed to conduct angle-resolved nanosphere lithography to fabricate heterogeneous binary ar-rays and multiplex quasi-three-dimensional grids of one-dimensionalnanostructures (Fig. 2C) [65–67], which are hard to construct by con-ventional lithographic techniques. Of significance is that we havepioneered the study of decorating microspheres using the upper layersof colloidal crystals as masks for the lower layer particles during physi-cal vapor deposition (Fig. 2D) [68–70]. Based on this novel strategy,

Fig. 1. Scanning electronmicrographs (SEMs) of different types of plasmonic films. (A) AnAufilm on a quartz substrate perforatedwith a square array of holes [38]. (B) A nanodome array[39]. (C) A hybrid nanoplasmonic probe array, a nanoforest, which consists of gold nanostructures coated on top of silica nanospheres and evaporated gold film structure onto bottomthrough masks, the scale bar is 1 μm [40]. (D) A Ni-coated monolayer of quartz (a-SiO2) microspheres of diameter d ≈ 1.4 μm. The support is a 1.0 mm thick a-SiO2 platelet [41].

7B. Ai et al. / Advances in Colloid and Interface Science 206 (2014) 5–16

microspheres become the substrate where various Au patterns such asdots, triangles, squares, and bowties were embossed.

By flexibly using these operations and strategies, numerous novelplasmonic films would be able to be designed and fabricated. Detailsof the operations and nanostructures will be discussed in the followingsections in combination with instances of successful fabrication.

4. Plasmonic films based on colloidal lithography

4.1. Plasmonic films with nanohole arrays

Since Ebbesen et al. discovered extraordinary optical transmissionthrough subwavelength noble metal nanohole arrays [18], there havebeen large efforts to fabricate nanohole arrays with well-controlledelectromagnetic properties. For the most part, nanohole arrays have

been fabricated using expensive and time-consuming high-resolutionserial techniques such as electron beam lithography and focused ionbeam lithography. Recently, the low-cost and high-throughput CL hasbecome an important alternative technique. By using the CL technique,not only conventional nanohole arrays but also numerous modifiednanohole arrays can be facilely fabricated.

4.1.1. Plasmonic filmswith conventional nanohole arrays based on colloidallithography

Nanoholes using colloidal lithography as a preparationmethodologywere first introduced as disordered nanoholes [71]. Then, as the CL tech-nique developed, highly ordered colloid crystals have been prepared,resulting in extensive research on periodic nanohole arrays. Numerous2D nanohole arrays, now called conventional nanohole arrays, havebeen widely fabricated and studied for the SP performance. Fig. 3

Fig. 2. Schematic depiction of the fabrication process of (A) silicon nanowire arrays [58], (B) ring patterns [59], (C) quasi-3D grids of multiplex zigzag nanowires [17] and (D) colloidalspheres with Au-patterned surfaces [68].


shows a typical process to fabricate conventional nanohole arrays basedon CL [72]. First, the colloidal mask is self-assembled to a planar sub-strate by the ways introduced above. Then the close-packed nano-spheres can shrink by the reactive ion etching process, resulting innon-close-packed mask arrays. Subsequently, various materials can bedeposited onto the interstices and the mask by chemical or physicalmethods. The deposition angle can be varied to yield a circle or an ellip-se, as well as other hole shapes. After removing the mask, nanohole ar-rays are successfully fabricated on the substrate (Fig. 3B and C).Comparedwith the conventional lithographic technique, this developedprocess proved to have advantages of low-cost and large-area.

Based on the typical process mentioned above, numerous impres-sive works have been reported. Jiang et al. employed a specializedspin-coating technique to create 2D non-close-packed nanospheres foruse as a deposition mask, but focusing only on structural fabricationand characterization [73]. After that, Live et al. prepared microhole ar-rays by drop-coating a polymer microsphere suspension onto glassslides followed by plasma treatment to reduce the size of the spheresand then metal evaporation [74]. By using the same method, Ctistis etal. reported the fabrication of nanohole arrays but they were focusedon the size-dependent magnetic properties of nanoholes in a cobaltthin film [75]. As the CL technique was widely used in fabricatingnanohole arrays, several reviews about this field were published[76,77]. Most recently, Wang et al. have investigated the transmissionof Au films with nanohole arrays created by a similar CL process [78].

Furthermore, the conventional nanohole arrays can be improved byfurther operations. By combining colloidal lithographywith subsequentisotropic chemical etching of an underlying glass substrate, Zhang et al.have reported the fabrication of Ag nanohole arrays behaving as though

floating above the substrate with only small support area (Fig. 4) [79].After the preparation of conventional nanohole arrays, HF was used toetch the glass substrate to elevate the nanoholes. The SP energy is wellmatched due to the elevated structures, and thus the refractive indexsensitivity has been greatly increased. They further found that bymaking the Ag nanohole arrays from embedment in silicon oxide toelevation from the glass substrates, the white light could be filteredinto individual colors across the entire visible band (Fig. 4C–F) [80].The method provides a new approach for the fabrication of compactcolor filters.

In summary, the conventional nanohole arrays can be easily fabricat-ed based on the CL technique due to the circular feature of the sphericalmask, which proved to possess advantages of low-cost and large-areacomparedwith conventional lithographic techniques. The structural pa-rameters, such as shape, period, hole diameter and thickness, as well asmaterials, can be precisely controlled in the process. Moreover, thetransmission efficiency and sensing performance of nanohole arrayscan be enhanced by further operations.

4.1.2. Plasmonic films with modified nanohole arrays based on colloidallithography

Not limited to the conventional nanohole arrays, novel types ofnanohole arrays with attractive properties, especially 3D ones, havebeen fabricated bymodified CL techniques. Lee et al. reported a promis-ing new strategy for the fabrication of large-area gold nanowell arrayswith novel geometric features that makes use of the trapping of self-assembled colloidal particles on a polymer surface (Fig. 5A and B)[81]. The plasmon coupling between the brims and the disks of thenanowellsmakes the plasmon resonancemore sensitive to surrounding

Fig. 3. (A) Schematic representation of process steps for fabricating nanohole arrays using CL. SEM images of (B) a single crystalline hexagonal nanohole array and (C) an elliptical nanoholearray [72].


materials. With a similar focus on the inverted hemispherical colloidallithography, Shen et al. have developed amethod to fabricate a periodicarray of 3D crescent-like holes (Fig. 5C and D) [82]. Extraordinary opti-cal peak based on this unique nanohole array is insensitive to the inci-dence angles and sensitive to the angle between the electric field ofthe incident light and the cross-line of the 3D crescent-like holes.Based on the similar strategy, they further developed amethod to fabri-cate a silver nano-mouth array as shown in Fig. 6A [83]. Of significancein the process are the embedment and two inclined deposition proce-dures.When the nano-mouth array was transferred onto a polydimeth-ylsiloxane substrate, the localized surface plasmon resonance of thenovel structures can be tuned via the swelling and recovery of the poly-dimethylsiloxane in ethyl acetate solvent (Fig. 6B–D). This tuningmeth-od could provide a new degree of freedom in molecular sensing anddetection, which is very useful for sensors, SERS, and enhanced fluores-cence measurements.

By using the spheres themselves as templates and subsequent etch-ing processes, we reported the fabrication of novel 3D Au nanoholearrays with greatly enhanced transmission efficiency and sensitivity(Fig. 7) [84]. In the process, the top of the gold semishells could be ex-posed with well controlled size. Then the exposed Au was selectivelyetched away resulting in opening-nanodome arrays which were called3D nanohole arrays (Fig. 7B). Based on the lifted nanoholes, the SP ener-gy on both surfaces of 3D nanohole arrays is excellently matched(Fig. 7C). The exciting result leads to a strong enhancement in extraor-dinary optical transmission, wavelength selection and refractive indexresponsiveness. Besides improved optical properties, good transferabil-ity of nanohole arrays without substrate dependencewas predicted and

Fig. 4. (A) A schematic illustration of the process for the fabrication of elevated Ag nanohole arrays. (B) Typical cross-sectional SEM image of the array. The nanohole array behaves asthough floating above the substrate with only small support area [79]. (C) A schematic illustration of the etching process from embedded nanohole arrays (EMANAs) in silica to exposedAg nanohole arrays. (D) Apparent color changes for embedded Ag nanohole arrays with 120 nm silica coating at various HF etching durations. (E) A schematic illustration of the etchingprocess from Ag nanohole arrays on the glass substrate to elevated Ag nanohole arrays (ELANAs). (F) Apparent color changes at various HF etching durations [80].

Fig. 5. (A) SEM image of fabricated gold nanowell arrays. (B) Magnified cross-sectionalSEM image of the gold nanowell shown in (A). The scale bars in (A) and (B) are 1 μmand 200 nm, respectively [81]. (C) Cross-section view of the unit cell cutting along a linewhich passes through the center of the circle and perpendicular to the cross-line.(D) SEM image of 3D crescent-like holes, where black area is not covered with silver.The red arrow indicates the cross-line [82].


realized experimentally the first time due to the unique protuberantholes of 3D nanohole arrays.

After that, we further fabricated novel 3D nanohole arrays with vol-cano shaped holes (truncated cones) by a much simpler CL method(Fig. 8) [85]. A polystyrene sphere monolayer was first prepared on asubstrate coated with photoresin film by the interfacemethod. Then re-active ion etchingwas carried out to etch the photoresinfilmandmasks,making the photoresin to show the shape of circular truncated cones.After removing polystyrene spheres andphotoresin, Ag nanovolcano ar-rays were formed (Fig. 8A). The nanovolcano arrays were considered tobe composed of two types of holes. Based on the novel structures, purecolors were achieved by the interactions of two different SPRs excited ineach substrate-supporting nanovolcano while needing no index-matching layers (Fig. 8B and C), which is a great progress for fabricatingstructural color based on nanohole arrays. Moreover, the pure colorcould be tuned facilely and inexpensively across the whole visiblerange with excellent purity, showing an efficient smart pure colordisplay (Fig. 8D).

Peng et al. introduced another way to fabricate nanohole arraysbased on CL. After polystyrene beads were spin-coated onto the surfaceof a silicon wafer, nanoholes were formed by scanning probemicrosco-py tip machining, where the tip is used to expose the gold-coated poly-styrene bead, and then wet chemistry is used to remove the remainingpolystyrene [86]. With a similar focus on the fabrication methodology,close-packed nanosphere lithography assembled nanospheres were re-cently used byWuet al. as nanolenses for forming a nanohole array [87].Huang et al. used colloidal templating to create a double-layer colloidalcrystal mask with two different nanosphere diameters and performedan oxygen etch, an Au deposition, and removal of the top colloidal crys-tal layer to create gold nanohole substrates [88].

In view of the above impressive work, we conclude that CL not onlycan be efficiently used to fabricate conventional nanohole arrays butalso has unique advantages in fabricating nanohole arrays with novelstructures, especially by the3D spacingdue to the3D feature of the tem-plate. In addition the 3D nanohole arrays would make SP energy wellmatched, leading to greatly enhanced transmission efficiency and

Fig. 6. (A) Schematic diagram for the fabrication of a periodic array of nano-mouths. (B) SEM image of the nano-mouth array on polydimethylsiloxane (PDMS) from the upturned nano-arc-gap array and dissolution of the polystyrene film. (C) SEM image of the nano-mouth array on PDMS after swelling in ethyl acetate for 3 min. The nano-mouth becomes larger as thePDMS swells in ethyl acetate. (D) Optical transmission spectra of the nano-mouth array on PDMS before and after swelling in ethyl acetate for 3 min [83].


Fig. 7. (A) Schematic diagram of the fabrication of 3D nanohole arrays and side views of the arrays with different hole sizes. (B) SEM image of the 3D nanohole array. The scale bar in theinset image is 1 μm. The hole diameter is 500 nm. Near-field electric field profile of (C) the 3D nanohole array on a supporting substrate (n = 1:51) and surrounded by a homogeneousdielectric medium (n = 1). The upper sides of the glass substrates in (C) are at z = 0 [84].

Fig. 8. (A) Schematic diagram for the fabrication of nanovolcano arrays. (B) SEM images and measured (full line)/simulated (dotted line) transmission spectra of Ag nanovolcano arrayswith different periods. Scale bar: 200 nm. (C) The macroscopic optical images of the ~1.5 × 1.5 cm2 Ag nanovolcano array with different periods corresponding to (B). Scale bar: 1 cm.(D) Macroscopic optical images of the process where a blue Ag nanovolcano array substrate is immersed in ethanol to be changed to green [85].


sensing performance. The nanohole arrays with certain heightsmay be-come a significant development direction for the field of nanoholearrays.

4.2. Plasmonic films with inverse opal arrays

Besides the plasmonic films penetrated with nanohole arrays, in-verse opal arrays (porous metallic films) that show SP features canalso be fabricated by the CL technique. Tessier et al. reported the assem-bly of gold nanostructured films templated by colloidal crystals andtheir use in surface-enhanced Raman spectroscopy [89]. They furtherdeveloped two methods for forming thin porous metallic films by di-rectly coating a substrate with a mixture of latex and gold particles,which upon drying forms an ordered gold structure templated by thecolloidal crystal [90]. Conceptually, their procedures of “outside-in”templating of the overall shape by restricting and crystallizing the com-ponents within a film can be viewed as complementary to the “inside-out” templating of the internal structure of thematerial by the colloidalcrystal.

Inverse opal arrays based on colloid monolayers are also callednanovoid arrays. Nanostructured films composed of periodic sphericalvoids have been formed using self-assembled polystyrene colloidalcrystal masks by Cole et al. (Fig. 9) [91–94]. These “nanovoid arrays”support delocalized Bragg and localizedMie plasmons andwere formedby electrochemical deposition of Au through a single layer self-assembled colloidal template. After that, the PS spheres were dissolvedin tetrahydrofuran to yield a “nanovoid array”. These structures havebeen used with some success as substrates for SERS.

Based on a similar strategy, Jose et al. have reported that sphericalcap gold nanocavity arrays with varied internal diameters could be fab-ricated on smooth gold films (Fig. 10) [95]. Selective modification of thetop surface and interior walls of the gold nanocavity arrays with[Ru(bpy)2(Qbpy)]2+ was accomplished using a two step adsorptionprocess exploiting the assembled polystyrene spheres asmasks. This se-lective modification approach permitted direct quantitative compari-son, for the first time, of plasmonic enhancement of Raman signal and

luminescence signal from a monolayer adsorbed at the top surfaceversus interior walls of all-gold nanocavity arrays. Significantly greaterRaman and luminescence signal enhancement was observed from[Ru(bpy)2(Qbpy)]2+ monolayers adsorbed at the top surface of thearray compared with the cavity walls.

The fabrication of nanovoid arraysmakes full use of the 3D feature ofthe spherical template, showing the unique advantages of the CLtechnique compared with the conventional lithographic techniques.And the unique concave structures prove to be very advantageous forSERS substrates.

4.3. Plasmonic films with nanoshell arrays

Besides for nanovoid arrays, the spherical mask can be used to fabri-cate another type of plasmonic film—nanoshell array. Li et al. reported astudy of the infrared transmission properties of gold films patterned on2D colloidal crystals (Fig. 11A) [96]. The ordered metallic microstruc-tures were prepared by sputtering a thin gold layer onto a monolayerof dielectric microspheres self-assembled onto a quartz chip. Themicrobeads were hemispherically covered with metal and the resultinggold film consists of a hexagonally close-packed array of gold half-shells

Fig. 9. Color online scanning electronmicroscope images of a samplewith a void diameterof 600 nm at three thicknesses. Sample is tilted by 45° for clarity of viewing. Schematicstructures of the surface at each thickness are also shown (right column) [94].

Fig. 10. Schematic representation of selective adsorption of surface active species at thetop surface and interior walls of gold nanocavity arrays. (A) Selective modification ofedges of arrays using [Ru(bpy)2(Qbpy)]2+ dye. (B) Selective modification of the interiorwalls of nanocavity arrays after functionalizing the edges with thiol [95].


with a size dictated by the template spheres. The fabricatedmetallodielectric structures have a strong surface corrugation as wellas a 2D periodic pore array. The surface plasmon polaritons on thesecurved surfaces display unusual dispersion properties, compared withthose of metal films on flat substrates studied before. The dielectricproperty of the template spheres is also found to have a substantial ef-fect on the transmission. The resultswill be useful for designing and fab-ricating new optical devices based on surface plasmon polaritonexcitation. They further fabricated a quasi-three dimensional metallicnanohole array (Fig. 11B) [97]. The periodicmetallic structures are com-posed of interlinkedmetallic half-shells supported on a planar dielectricsubstrate, which were prepared by metal deposition on a sacrificial twodimensional colloidal crystal template. The Au shell reduces the

substrate effect, resulting in a pronounced surface plasmon couplingthe strength of which is independent of the dielectric environment, acharacteristic absent in conventional planar metallic subwavelengthhole arrays. The sensitivity, a value of 1192 nm per refractiveindex unit, shows a five-fold increase as compared with the metallicstructures supported on the template. Moreover, interconnected Aunanobowl arrays could be prepared via a transferring process(Fig. 11C) [98]. It is further observed that the Raman signal enhance-ment shows a noticeable difference when reversing the orientations ofthe Au nanobowls in relation to the underlying flat dielectric substrate.As the pump laser wavelength is tuned in the vicinity of the resonantplasmonic mode of the structure, the enhancement on an upward Aunanobowl array can be five-fold compared with that on a downwardone.

Im et al. reported a novel method for templated, high-throughputfabrication of a periodic array of ringshaped nanocavities with 10 nmgap size by combining CL with straightforward batch processing steps,namely, atomic layer deposition and ion milling (Fig. 12) [99]. First, anAg film over a nanosphere substrate was prepared by depositing anAg film on polystyrene nanospheres assembled on a glass substrate.For nanoring cavity formation, a thin Al2O3 layer was conformally de-posited on the Ag surface via atomic layer deposition, followed by depo-sition of another Ag film. Subsequent anisotropic etching of the top Aglayer reveals the Al2O3 underlying layer as well as bowl-shaped Agnanostructures formed along the curvature of the Ag film over a nano-sphere substrate. Partially removing the Al2O3 layer reveals Ag–air–Agnanoring cavities that act as sites for localization of electromagneticfields and thus, sensitive detection sites for SERS. Compared with con-ventional Ag film over a nanosphere substrate, the addition of nanoringcavities increases the SERS enhancement factor by at least an order ofmagnitude, and the resonance wavelength can be readily tuned bychanging the size of nanospheres used in the original templating step.

5. Conclusions and outlook

The vast literature summarized in this review develops a compellingstory about both the development of and the prospect for plasmonicfilms fabricated by colloidal lithography and greatly enhanced SP fea-tures based on these novel nanostructures. Conventional 2D nanoholearrays with excellently controlled period, hole size, thickness, as wellas materials can be efficiently fabricated by a simple CL method whichis capable of patterning large areas in parallel at low cost. Furthermoreskillful strategies based on the CL technique have been used to fabricatea number of plasmonic films with novel nanostructures (especially theones in 3D spacing) which are hard to be implemented, or cannot bein some cases, by conventional lithographic techniques. By using 3Dspheres as mask or templates, and combing the subsequent etching ordepositing process, plasmonic films with 3D nanohole arrays, nanovoidarrays or nanoshell arrays can be fabricated. Further research revealsthat these undulant plasmonic films provide great enhancement in SPfield strength and have higher performance in optical and sensing appli-cations. This indicates that plasmonic filmswith 3D nanostructuresmaybecome a significant development direction for the field of plasmonicfilms.

Although the CL technique has these intrinsic advantages which canpotentially fulfill the requirements for the development of plasmonicfilms, there are still some challenges in the field of using CL in fabricatingplasmonic films. The presence of unavoidable defects in colloidal crystalreduces the patterning precision of the nanostructures of plasmonicfilms, which may reduce the performance of the plasmonic films basedon CL. However, few researches have been made about effect of defectconcentration. Colloidal crystal prepared perfectly over dozens of micro-meters is always used for fabricating plasmonic films, which proves tohave little effect on spectra. Besides, the CL technique is low-cost in fab-ricating the structural model. Cost of following and further operationsmay be varied. Furthermore monodisperse microspheres readily self-

Fig. 11. (A) SEM image (tilted view) of a highly corrugated goldfilmdeposited on a 2D col-loidal crystal assembled from 1.58 μmdiameter silica spheres on a planar quartz chip [96].(B) Templated quasi-3D Au nanohole array composed of ordered interconnected Au half-shell array on a clean quartz substrate [97]. (C) Au nanobowl arrays. The silica templatespheres have a diameter of 1.0 μm [98].


assemble to single layers andmultilayerswith a hexagonal close-packingarray. To achieve diverse arrangements of arrays for the CL-based plas-monic films, further operations should be carried out to first obtain var-ious formations of the crystal lattice. Of most importance is that CL is an“indirect” fabrication technique relying on flexibly using the colloidalcrystal and subsequent operations, needing more ingenious ideas onthe design strategy than “direct” techniques, such as electron beamlithography, focused ion beam lithography.

Further research on using CL in fabricating plasmonic films is be-lieved to mainly focus on three aspects: First, further improvement ofthe control of preparing colloid crystals, including quality and crystalformation; second, ingenious strategy on using colloid crystals asmask or template and combination with other fabrication techniques;and third, further studies on the SP performance based on novel plas-monic films and optimized applications. We envision that the perfor-mance of CL used in plasmonic films will continue to evolve and thatan increasing number of novel plasmonic films with surprising effectswill be fabricated by flexibly using the CL techniques. These plasmonicfilmswill greatly develop the field of plasmonics and further benefit nu-merous important sectors such as medical diagnostics, environmentalmonitoring, and food safety and security.

Acknowledgments

This work was supported by the National Natural Science Foundationof China (51073070, 51173068, 51373066).

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Fig. 12. (A) Scheme of the fabrication process of plasmonic nanoring cavities using nanosphere lithography and atomic layer deposition. (B) Cross-sectional schematics of the fabricationprocess. SEM images of (C) the nanoring cavities on the film over a nanosphere substrates and (D) the nanoring cavity array formed over a 16 μm × 10 μm area. Scale bar: 200 nm.(E) Photograph of the nanoring cavity sample [99].


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