Fabrication and Optical Spectral Characterizationof Linked Plasmonic Nanostructures and Nanodevices

Tong Zhang1,2,+, Xiao-Yang Zhang1,2, Long-De Wang1,2, Yuan-Jun Song1,2, Meng-Na Lin1,2,Lu-Ning Wang1,2, Sheng-Qing Zhu1,2 and Ruo-Zhou Li1,2

1School of Electronic Science and Engineering, Southeast University, and Key Laboratory of Micro-Inertial Instrumentand Advanced Navigation Technology, Ministry of Education, Nanjing 210096, People’s Republic of China2Suzhou Key Laboratory of Metal Nano-Optoelectronic Technology, Suzhou Research Institute of Southeast University,Suzhou 215123, People’s Republic of China

Linked plasmonic nanoparticles made of noble metal materials exhibit significant enhancement of the amplitude of electromagnetic-fieldand strongly frequency-selective response at visible ranges which are distinct from that of individual nanoparicles. We introduce recent progressin the fabrication processes to achieve linked plasmonic nanostructures with various configurations. It includes the synthesis of ultrathin goldnanosheets composed of steadily linked nanoparticles using magnetron sputtering and lift-off processes, shaped gold nanopartocles films linkedby surfactant, self-assembled silver nanoparticle films at water-organic interface assisted by phase transfer catalysts, large scale silver nanoplatefilms using self-assembly method, and silver nano-flags fabricated by chemical two-step synthesis methods. The morphologies and opticalcharacteristics of these nanostructures are shown, respectively. These plasmonic nanostructures with special optical responses show a greatpotential in the applications of optical communications, photovoltaics and biochemical sensing. [doi:10.2320/matertrans.MD201227]

(Received March 15, 2013; Accepted March 18, 2013; Published May 25, 2013)

Keywords: plasmonic, localized surface plasmon resonance, surface plasmon polaritons, nanodevices, silver nanoparticles, self-assembly,nanoplates, branched nanowires, nano-antenna, Q factor

1. Introduction

Plasmonic nanostructures constructed by adjacent metalnanoparticles have attracted great interest due to theirsignificant electromagnetic-field enhancement and specificoptical spectral responses from visible to near IR ranges. Thecontrol of the distance among the elemental nanoparticles iscrucial for the improvement of the plasmon resonanceproperty and engineering of the optical spectral signature ofthe plasmonic nanostructures.1,2) Novel fabrication routes tocreate linked or closely adjacent nanostructures are highlydesirable in practice applications. Metallic nanoparticles withdifferent shapes and sizes show frequency-dependent lightlocalization and scattering enhancement at visible rangesarising from the collective oscillation of conduction electronson the surface of metal induced by the incident light.1)

Localized surface plasmon resonance (LSPR) occurs whenthe frequency of the movement of the free electrons isapproximately the same as the incident light. Variousresearches show that the localized light intensity and lightscattering behavior at LSPR wavelength can be enhancedsignificantly when the metallic nanoparticles are closelyapproximate.2) It is because the overlapped plasmon gapmodes distribute mainly at the ultra-small gap regions of theadjacent nanoparticles. Many efforts have been paid for thedevelopment of the fabrication processes to achieve suchlinked plasmonic nanostructures. Chemical bonding is atypical method to control the anisotropic growth of nano-chains3­6) or nanoparticle aggregations7­9) constructed bysmall metal nanoparticles. Temperature or chemical inducedsintering10­14) are effective methods for the fabrication ofsteadily linked metal nanoparticles in large area. Laserinduced self-assembly of metal aggregations15­19) shows

potential application in the fabrication of nanoscaledplasmonic device. These methods provide fabrication routesto construct advanced plasmonic waveguides and thin-filmdevices with diversity of optical properties. In this paper,we introduce recent progress in the fabrication of high-density and linked plasmonic nanostructures and nano-devices having extraordinary optical spectral characteristics.These nanostructures with novel morphologies and signifi-cant light localization property are useful in many applicationfields, such as biochemical sensing, optical processing andphotovoltaics.

2. Ultrathin Gold Nanosheets Composed of SteadilyLinked Nanoparticles

First, we introduce a simple method via applying magne-tron sputtering, lift-off and ultrasonic techniques to synthe-size two-dimensional single-layer gold nanosheets in organicsolvent.20,21) These nanosheets are composed of randomdistributed gold nanoclusters with uniform size.

Our approach is based on the formation of gold nanosheetson the surface of photoresist film utilizing magnetronsputtering deposition. First, photoresist thin film is spincoated on the silicon substrate and cured. Second, gold atomsare deposited on the top of the photoresist film usingmagnetron sputtering in the condition of electrical current0.4A, vacuum 0.15 Pa, Ar flux 25 sccm, discharging 1 s.Third, the sputtered gold thin film is lifted off from the siliconsubstrates and then soaked in acetone for several minutes todissolve the photoresist. Finally, the gold film breaks intoultra-thin gold nanosheets in the acetone after long-timeultrasonic irradiation at a frequency of 40 kHz.

In the fabrication process, the deposition mechanism of thesputtered metal film has been shown before.22) In the initialstep, gold atoms are deposited uniformly on the surface of the+Corresponding author, E-mail: tzhang@seu.edu.cn

Materials Transactions, Vol. 54, No. 6 (2013) pp. 947 to 952Special Issue on Nanojoining and Microjoining©2013 The Japan Institute of Metals and Materials

polymer film. Agglomeration of gold atoms occurs with theincrease of the deposition time. The gold nanoclustersproduced from the conventional routes readily aggregateand become solid film because of their high surface energy.In our case, we suspect that the existence of the polymer filmplays an important role to decrease the aggregation velocityof the gold nanoclusters. However, the accurate mechanismof the formation of the special morphology of the ultrathingold nanosheets is still unclear now.

Figure 1 shows the typical morphology of gold nanosheetsmeasured by transmission electron microscopy (TEM).From Figs. 1(a) and 1(b) we see that the nanosheets arecomposed of randomly distributed gold nanoparticles withuniform size steadily linked by ultra-thin gold film. Thesize of the gold nanoparticles is ³5 nm. They are connectedby thinner gold amorphous films which can hardly beobserved from the top view of the high-resolution TEMimage (Fig. 1(b)). However, one can see clearly thatthese small nanoparticles are linked to each other from thelateral view of the TEM images of the folded nanosheets inFig. 2. Figure 2(a) clearly shows that the nanosheets arecontinuous and flexible. The high resolution TEM image inFig. 2(b) further established that the nanosheets are continuedgold layers with a thickness of ³2 nm containing smallparticles.21)

Figure 3 shows the LSPR property of the gold nanosheetssolution.20) The difference between the gold nanosheets andthe conventional chemical synthesized gold nanospheres(inset in Fig. 3) are obviously. The extinction spectrum of thegold nanosheets shows a much wider absorption band from550 to 850 nm mainly induced by the aggregation of thesmall nanoparticles. Such ultrathin metal nanosheets withsignificant light localization have been demonstrated a usefultool in biochemical sensing.21)

3. Shaped Gold Nanoparticles Films Linked bySurfactant

Next we introduce the fabrication of gold films containingnanoparticle aggregations linked by surfactant using solventevaporation method. Gold nanoparticle solution was preparedby a two-step colloidal growth method. Cetyltri-methylam-monium bromide (CTAB) was added as a surfactant in thesynthesis routes. Details of the synthesis processes areshown.23) After finishing the grown process of the goldnanoparticles, the solution was centrifuged at 14000 rpm for10min and then redispersed into the deionized water toreduce the superfluous CTAB concentration in the solution.Next, the gold nanoparticle solution was dropped onto theglass substrate and dried naturally in the ambient condition.In the drying process, CTAB acted as a surfactant to link goldnanoparticles due to its bilayer property.24)

After solution evaporation, thin film constructed by goldnanoparticle aggregations with different structures wasobtained as shown in the scanning electron microscope(SEM) images in Fig. 4. Figures 4(b) to 4(d) show randomly

Fig. 1 The TEM micrographs of the obtained gold nanosheets.

Fig. 2 The TEM micrographs of folded gold nanosheets.

Fig. 3 Extinction spectra of gold nanosheet solution. The insert is theabsorption spectrum of the synthesized gold nanoparticles in aqueoussolution.

Fig. 4 The low (a) and high (b) to (d) resolution SEM micrographs of thegold nanoparticles film.

T. Zhang et al.948

distributed gold nanoparticle aggregations linked by CTAB.Randomly distributed dimers, trimers, long nano-chains andother multiple aggregations can be observed.

Compared to individual gold nanoparticles, the LSPR peakof linked gold nanoparticles shifts to the longer wavelength.Figure 5 displays the extinction spectra of the gold nano-particle solution and nanoparticle thin film, respectively.It is found that the LSPR peak shifts from 528 to 661 nm dueto the plasmonic coupling effect of interaction betweenindividual gold nanoparticles, as we discussed before.18)

4. Self-assembly of Silver Nanoplate Film at Water-organic Interface

In this section, we introduce a preparation method of self-organization of silver nanoplate film using a phase transfercatalyst from aqueous-organic interface.25) Self assembly ofnanoparticle film at the interface between oil and water is anewly developed nanofabrication method which attracts greatattentions in recent years.26,27) Ethanol was usually servedas inducers in previous researches.26) Here we present theshape dependent self-assembly using a newly founded phasetransfer catalyst, tetrabutyl ammonium bromide.

Detailed fabrication steps of the experiment have beenreported.25) Firstly, aqueous silver solution containing bothnanoplates and nanospheres was prepared. Then, cyclo-pentanone and tetrabutyl ammonium bromide were added tothe solutions in sequence. After a short time shaken, a brightlayer of silver nanoplates film can be observed at the interface

between cyclopentanone and water. The self-organizationof nanoparticles is shape selectable.25) Figure 6 shows thecompared photographs of the silver solutions with differentchemical components. When cyclopentanone and tetrabutylammonium bromide were added to the silver solutions, largesilver nanoplates self-assembled mainly at the interface.Therefore, a bright layer with strong reflection at the interfaceof the solution can be observed from Figs. 6(a) and 6(d).Only small nanospheres dispersed in the water solutions.In the absence of silver nanoplates, the color of the silversolution changed to yellow induced by the LSPR of silvernanospheres. As shown in Figs. 6(b) and 6(e), no self-assembly behavior can be observed at the interface withouttetrabutyl ammonium bromide. The color of the silversolution was violet corresponding to the LSPR of silvernanoplates,28) just like the original silver solution withoutcyclopentanone shown in Figs. 6(c) and 6(f ).

Figure 7 shows the SEM image of the self-assembly silverfilm transferred from interface of the solutions to the glasssubstrate. It can be observed that the silver nanoplates keephighly compacted in large areas. Using this phase transfermethod, one can easily fabricate dynamic linked metalnanoparticles with high density.

5. Large Scale Silver Nanoplate Films Using Self-assembly Method

In this section, we introduce a simple self-assemblymethod for the fabrication of large-scale and ultra-thin silver

Fig. 5 Extinction spectra of gold nanoparticle solution and nanoparticlethin film.

Fig. 6 Photographs of silver solutions viewed from the front side ((a) to (c)) and oblique angle by 45 degrees ((d) to (f )). (a) and (d) aresilver solution containing cyclopentanone and the phase transfer catalyst. (b) and (e) are silver solution containing cyclopentanonewithout the phase transfer catalyst. (c) and (f ) are the original silver solution without cyclopentanone and the phase transfer catalyst.

Fig. 7 The SEM image of the dense self-assembly of silver nanoplate film.

Fabrication and Optical Spectral Characterization of Linked Plasmonic Nanostructures and Nanodevices 949

nanoplate films on solid substrates without surface modifi-cation.28) Colloidal silver nanoplate solution were preparedand washed before use. Polyvinylpyrolidone (PVP) andascorbic acid were then added into the silver solutions insequence. PVP serves as a stabilizer to keep the anisotropicsilver nanoplate stable. When ascorbic acid was added intothe solution, silver nanoparticles can be trapped by thesurface of the hydrophobic substrate easily. Glass substrateswere then placed vertically into the solution. Single layer ofsilver nanoplates film were obtained after ³20 h, as shown inFig. 8. We have experimentally established that large scaleand uniform layers of silver nanospheres and gold nanocubes,nanospheres and nanoplates are also easily achieved usingthis self-assembly method.

From Fig. 8(a), one can see that the silver layer is uniformin large area. The SEM images in Figs. 8(b) and 8(c) showthat the film is single layer and constructed mainly by silvernanoplates with uniform size. We observed that the densityof the deposited nanoplates is highly dependent on theconcentration of ascorbic acid and the deposition time.28)

From Fig. 8(c) one can see diversity of silver nanoplatemultimers with different shapes. The distance between thesenanoplates is ³ few nanometers. Figure 8(d) shows thecomparison of the extinction spectra of the silver solutionand the film. It shows clearly that the special morphologies ofthe silver film induce to a broad LSPR band and significantlight scattering enhancement in the long wavelength range,especially in the near-infrared range. Our experiment resultshave proved that the light scattering property of the nanoplatelayer is obviously enhanced with the increase of thedistribution density of the nanosplates as shown in

Fig. 8(e).28) Recently, Nishijima et al. also claimed that therandom distributed metal nanostructures have much strongerscattering enhancement properties compared to periodicnanoparticle arrays.29) Considering the easy fabrication andsignificant scattering enhancement of the randomly distri-buted silver nanoplate film, we believe such self-assemblymethod may become a very attractive industrial process forthe fabrication of light trapping layer to improve the lightharvest efficiency of solar cells.30)

6. Two-step Synthesis of Plasmonic Nanodevices

Plasmonic nanodevices supporting surface plasmon polar-iton modes beyond optical diffraction limit are buildingblocks in next generation integrated optoelectronic cir-cuits.31­36) Integrated plasmonic functional devices areusually designed and fabricated using traditional lithographyprocess.31­33) Chemical synthesis methods have been proveda better way for the fabrication of elemental opticalwaveguides using one-dimensional nanowires.34­36) How-ever, to achieve nanodevices with different configurations,complicated micromanipulation and assembly method withhigh accuracy are usually needed as reported previously.35,36)

Here we show that the concept of two-step synthesis ofplasmonic nanodevices may become an alternative strategyfor the fabrication of complicated nanostructures. Recently,Tsuji reported a two-step chemical synthesis process tofabricate silver nano-flags37,38) by changing the growthenvironment of the nanostructures. In the first step, theysynthesized silver nanowire seeds using PVP as cappingagent in heated ethylene glycol. In ethylene glycol, the

Fig. 8 (a) Photograph of the silver film, (b) and (c) show the low and high resolution SEM images of the silver films. (d) shows thecomparison of the extinction spectra of the silver solution and film. (e) shows the photograph of the scattering image of the silver film.

T. Zhang et al.950

reduced Ag atoms mainly grow along [100] facets to formone-dimensional nanowires. In the second step, the silvernanowire seeds were added in heated N, N-dimethylform-amide. As the reducted Ag atoms grow along [111] facets inthis solution environment, triangular nanoplates regrowedfrom the lateral facet of the nanowire seeds to form flag typenanostructures. The final configuration of the nanostructurescan be tuned by the careful control of the concentration ofthe chemicals.

We exhibited that such silver nano-flags support compet-itive LSPR mode and Fabry-Perot (FP) cavity modesimultaneously by numerical simulation.39) The uniqueresonant mechanism results in a remarkable frequency-dependent electromagnetic enhancement. Figure 9(a) showsthe comparison of the optical spectra for a silver nanoflag anda silver nanoplate with a same width. The length and thediameter of the nanowire integrated in the nano-flag are 2 µmand 30 nm, respectively. The width of the integrated triangleis 210 nm. The simulation images of the electric-fielddistribution of the nanostructures are shown in the insetsof Fig. 9(a). Figures 9(b) to 9(d) show the electric-fielddistribution of the silver nanoplate at resonance wavelength(1085 nm) and off resonance (900 and 1500 nm), respec-tively. It is found that light mainly distributes at one corner ofthe nanoplate. Figures 9(e) to 9(g) show the electric-fielddistribution of the silver nanoflag at resonance wavelength(1094.3 nm) and off resonance (900 and 1500 nm), respec-tively. Different from the images in Figs. 9(b) to 9(d), theelectric-field distribution of silver nanoflag depend heavily onthe incident wavelength induced by the mode competitionbetween the LSPR resonance mode and FP cavity mode.The simulation result indicated that such silver nano-flagsare very attractive plasmonic nano-antennas39) with ultra-highquality factor and tunable optical spectral signatures whichare distinct from conventional chemical synthesized metalnanostructures. Such complicated nanostructures show po-tential applications in light trapping and scattering enhance-ment over broad-bandwidth wavelength range.

7. Conclusion

In this paper, we present several fabrication routes for thefabrication of linked plasmonic nanostructures with differentsizes and shapes. These methods can be used to obtain largescale and high compacted metal nanoparticles with stronginteractions. The gold nanosheets composed of steadilylinked nanoparticles are newly developed plasmonic nano-structures with unique shape. Such advanced metal nano-structures with large specific surface area may be useful inthe applications of frequency-selectable light trapping, opticalprocessing and chemical catalysis. The other three fabricationmethods introduced can be used to fabricate metal filmswith diverse of optical spectral signatures dependent on theshapes and the sizes of the elemental nanoparticles used.The two-step synthesis method provided new strategy for thefabrication of functional nanodevices directly using chemicalmethod. These new technologies provide various fabricationprocesses useful in the development of plasmonic thin filmsdevices and nanoscale plasmonic devices.

Acknowledgments

This work is supported by NSFC under grant number60977038, Doctoral Fund of Ministry of Education of Chinaunder grant number 20110092110016, the National BasicResearch Program of China (973 Program) under grantnumber 2011CB302004, the Scientific Research Foundationof Graduate School of Southeast University under grantnumber YBPY1104, and the Foundation of Key Laboratoryof Micro-Inertial Instrument and Advanced NavigationTechnology, Ministry of Education, China under grantnumber 201204.

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