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Morphological Design of Gold Nanopillar Arrays and Their Optical Properties Hanbin Zheng, Renaud A. L. Vallée, Isabelle Ly, Rui M. Almeida, Thomas Rivera, and Serge Ravaine J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b09188 • Publication Date (Web): 21 Dec 2015 Downloaded from http://pubs.acs.org on December 30, 2015

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Morphological Design of Gold Nanopillar Arrays and Their Optical Properties Hanbin Zheng, †,‡ Renaud Vallée, † Isabelle Ly, † Rui M. Almeida, ‡ Thomas Rivera∥ and Serge Ravaine*,† †

CNRS, Univ. Bordeaux, CRPP, UPR 8641, F-33600 Pessac, France.

‡

Depart. Eng. Quimica/CQE, Instituto Superior Técnico/UL, Av. Rovisco Pais, 1049-001

Lisboa, Portugal. ∥

Orange Labs, rue du Général Leclerc, 92794 Issy les Moulineaux, France.

ABSTRACT: The fabrication of large areas of gold nanopillar arrays by combining nanosphere lithography and electrodeposition of metals is demonstrated here for the first time. Both the morphology and the surface density of the nanopillars can be finely tuned by controlling the size of the colloidal beads used in the template, the sintering time of the template and the electrodeposition time of metals. The colloidal templates are removed after electrodeposition and the nanopillar arrays can be transferred onto a transparent polydimethylsiloxane (PDMS) substrate and optically characterized. Our focus here is on the ease and versatility of this technique to prepare large two dimensional arrays of plasmonic gold nanopillars and their resulting optical properties. By making use of finite difference time domain (FDTD) numerical

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simulations, the excitation of localized surface plasmon resonances of the individual nanopillar with a strong dipolar character is observed, which is characteristic of such uncoupled metallic nanostructures. Experimental optical measurements are also in good agreement with the simulations, confirming the successful engineering of the nanostructures.


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INTRODUCTION Metallic nanostructures play an important role in many applications. In the emerging fields of plasmonics and nanophotonics, the ability to engineer metals on nanometric scales allows the fabrication of new devices and the study of exciting physics. Metallic nanostructures offer the possibility of subwavelength control and the manipulation of optical energy. Two dimensional (2D) arrays of gold nanostructures are a type of interesting nanostructured material that have been shown to be applicable as surface enhanced raman spectroscopy substrates,1 as biosensors,2 and can be used in bulk heterojunction organic solar cells.3 Recently, Zhou et al. have showed that the out of plane lattice plasmon resonances can be tuned by changing the height of large gold nanoparticles (>100 nm) in 2D gold nanoparticle arrays, and this kind of strongly coupled arrays exhibit a continuously tunable Fano-like profile.4 Currently, the process to fabricate nanopillar arrays can either be the direct etching of a material via ion beam milling,5 or a two step process which involved the making of a template and the subsequent deposition of materials into the template. For the second process requiring a template, the final arrangement and morphology of the nanopillar arrays are directly dependent on the template used. Techniques such as electron-beam lithography,2,6,7 focused-ion-beam (FIB) lithography,8 photolithography9 and nanosphere lithography (NSL)10,11 can be used for fabricating well ordered templates, and techniques such as magnetron sputtering12 and electrodeposition2,7 can be used to fill up the template pores. NSL, a term first coined by Van Duyne and co-workers, makes use of the bottom-up self-assembly of colloidal particles to fabricate templates for patterned nanostructured surfaces.13,14 While NSL lacks the versatility of direct ‘writing’ methods, it can be done on a much larger scale, and at a much faster rate. The assembly of colloidal particles has been studied extensively and various assembly methods such as vertical


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deposition,15 dip-coating,16 self assembly at the air-water interface,17 Langmuir-Blodgett technique,18 and shearing,19 have been reported. Using NSL combined with a top down deposition or etching process, periodic patterned nanostructures such as inverted silicon nanocones,20 silicon nanopillars,21,22 gold nanowells,23 and gold/silver nanopyramids11 have been fabricated. Recently, Tabatabaei et al. reported the fabrication of tetrahedral plasmonic nanopyramids of silver and gold that exhibit near field enhancement by combining NSL with electron beam evaporation. They predicted that such nanopyramid arrays can be integrated into solar cells with the goal to improve the photovoltaic conversion efficiency.11 By combining electrochemical deposition with self assembled colloidal templates,24-25 porous periodic metallic surfaces with interesting optical and plasmonic properties26,27 have been previously reported. In this work, we report the use of NSL with the electrochemical deposition of gold through the modified pores to demonstrate for the first time, the fabrication of optically tunable plasmonic gold nanopillar arrays with different morphologies by an entirely bottom-up fabrication route. The entire bottom-up gold nanopillar array fabrication process is shown in Figure 1. The control of several experimental parameters, i.e. the size of the polystyrene spheres used as template, their sintering time and the electrodeposition time of the metals through the template allows us to finely tune the height, the shape, as well as the surface density of the nanopillars. We have also successfully transferred these nanopillar arrays onto transparent PDMS substrates and checked the influence of the above parameters on their resultant optical properties. Comparison between the optical properties simulated by FDTD and our measured experimental data shows a very good correlation between the results. The simple process to fabricate these easily modifiable gold nanopillar arrays allows the possibility of designing


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nanopillars with a specific configuration for applications such as SERs, biosensors, field emission antennae, or for use in solar cells.

Figure 1. Schematic illustration of the process to fabricate 2D gold nanopillar arrays.

EXPERIMENTAL SECTION Sample Preparation. Polystyrene (PS) spheres with a diameter, D, equal to 260 nm, 400 nm or 520 nm were synthesized by emulsion polymerization according to a reported procedure.28 Close-packed monolayers of the PS spheres on nickel-plated gold-coated glass slides were fabricated by direct assembly of the particles at the air–water interface followed by their transfer onto the substrates.17 A short sintering step was performed whereby each substrate coated with a monolayer of close packed PS spheres is placed in an oven set at 110°C from several seconds to several minutes depending on the requirements. After sintering the templates, the substrates were used as working electrodes in a typical three-electrode cell with a carbon plate as counter


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electrode and an Ag/AgCl electrode as reference. A cyanide-free gold plating bath purchased from Metalor (ECF-60; gold concentration 10g.L-1) and a nickel plating solution purchased from Alfa Aesar were used as received for the metal deposition. The potentiostatic electrodeposition experiments were made in a water bath set at 25±1°C. The intensity of the faradaic current generated from the Ni-ion and Au-ion reduction was measured using an Autolab PGSTAT 20 potentiostat (EcoChemie) system monitored by a PC running the GPES 4.9 software. The dissolution of PS spheres was done by immersing the samples into tetrahydrofuran at room temperature for 2 hours. Preparation of polydimethylsiloxane (PDMS) was done by mixing the monomer and curing agent together in a 10:1 ratio. The resulting mixture was poured over the sample (after removal of the PS template) and left in a dessicator for one to two hours to completely remove the bubbles. Once there are no bubbles, the sample with the PDMS is left in an oven at 75°C for one hour. Once the PDMS had solidified, the sacrificial nickel layer was then dissolved in 10 vol% nitric acid to leave the gold pillars embedded in the transparent PDMS matrix, without any observable effect on the gold nanopillars or the PDMS matrix. Sample Characterization. The UV–vis–NIR spectra were recorded under normal incidence using a CRAIC 2020 microspectrophotometer. The SEM experiments were carried out using a JEOL 6700F microscope operating at 5 kV. FDTD Simulations. The spectroscopic properties (UV-visible-NIR spectra) of the designed pillar structures have been simulated by Finite Difference Time Domain (FDTD) methods, as implemented in the freely available MEEP package software.29 The parameters that have been adjusted are the relative refractive index of the PDMS filling and the resolution of the grid size (a 5 nm grid size was used for the simulations for the structures elaborated from 260 nm and 400


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nm PS beads while a 10 nm grid size was used for the simulations for the structures elaborated from 520 nm PS beads).

RESULT AND DISCUSSIONS Modification of self-assembled colloidal monolayer of polystyrene particles. Closed packed monolayers of polystyrene particles with diameters of 260 nm, 400 nm and 520 nm were assembled via direct assembly at the air-water interface17 and transferred onto nickel coated conductive substrates. Figure S1 (in supporting information) shows the nicely ordered arrangement of the 260 nm particles in a single monolayer transferred onto the substrate. Similar results were obtained with the monolayers made using the larger 400 nm and 520 nm polystyrene particles. The monolayers were then sintered at 110°C to adjust the pore sizes between three adjacent polystyrene particles in the close packed monolayer. There is a clear reduction in the pore size between three adjacent polystyrene particles with increased sintering time (Supporting information Figure S2). The spherical polystyrene particles lose their shape over time and appear to become hexagonal with prolonged sintering, until they eventually fuse together completely and there are no more pores between them. This final pore size of the templates can hence be adjusted by controlling the sintering time of the polystyrene particle monolayers. Controlled electrodeposition of nickel and gold into the monolayer templates. The sintered monolayers of polystyrene particles were used as templates for the electrodeposition of nickel and gold using commercially available plating solutions. We have previously reported that it is


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particularly easy to accurately control the thickness of a metallic deposit in a colloidal template due to the current density variations.24,27 The local minima/maxima of the current density can be attributed to the periodic variation of the electroactive surface area of the growth front during the gold deposition. By monitoring the current profiles during potentiostatic electrodeposition, it is then possible to estimate the thickness of the metal electrodeposited. For instance, we can deduce that the thickness of the electrodeposited metal is half the diameter of the particles when the current magnitude is at the minimum. By using this estimation, and a simple assumption that the rate of electrodeposition is constant as the metal was electrodeposited in the template (i.e. thickness of metal varied linearly with time), we are then able to electrodeposit sequentially nickel followed by gold to desired depths, with the electrodeposited gold vertically centered at the middle of the pore (i.e. half the diameter of the beads). Figure 2 shows the electrodeposition profile of nickel followed by gold and the resulting SEM of the desired structure electrodeposited. From Figure 2, we can see that the electrodeposited nickel and gold replicates perfectly the pores of the sintered polystyrene template. Furthermore, the position of the electrodeposited gold layer is vertically centered at half the bead diameter (in the middle of the pores), resulting in the hexagonally arranged standing gold nanopillar array. We can observe a very distinct separation of the underlying nickel layer (darker region) and the gold layer (brighter region) on top of it.


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Figure 2. (a) Current profile of nickel and gold electrodeposited into a sintered monolayer of 400 nm polystyrene particles. (b) Backscattered cross-sectional SEM image of the electrodeposited metallic structure without the template. Tilted (c) and top down (d) SEM images of the electrodeposited structure. To further illustrate the control we can achieve in electrodepositing these nanostructures, Figure 3 shows how it is possible to determine the point at which gold is electrodeposited onto the first nickel layer by monitoring the electrodeposition current profile of nickel and gold. When less nickel is electrodeposited, the gold nanopillars can be grown in such a way that they are connected to each other at the bottom (Figure 3a). When sufficient nickel is first deposited in the colloidal template, the resulting gold nanopillars are disconnected and individually standing (Figure 3b). When an excess of nickel is electrodeposited first, followed by gold, the resulting gold nanostructures can be made to be connected at the top (Figure 3c). Some further examples of the different possible positions and morphologies of the electrodeposited gold nanostructures (in different sizes of template beads) that can be obtained by varying the electrodeposition


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thickness of the nickel and gold layers across the different colloidal templates are shown in Figure 3 d)-i). From Figure 3, we show that it is possible to alter the shape of the resulting nanopillar by carefully adjusting the deposition time of the nickel and gold layers. An interesting morphology that was observed can be seen in Figure 3h and 3i, where the resulting gold nanostructure looks like little triangular mushrooms, and the inter-distance between the tips of these triangular mushrooms can be controlled by adjusting the amount of gold electrodeposited. We believe that such structures could have interesting “hot spot” properties when the distance between the tips is small enough but still not touching. In addition, by making use of polystyrene templates that have been sintered for different times, we can create narrower channels in the templates which result in the electrodeposition of narrower gold nanopillars (Supporting information Figure S3).


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Figure 3. a) to c): Comparison of the electrodeposition profile and the corresponding electrodeposited structure (from a) to c)) with increased nickel and decreased gold electrodeposition into colloidal templates made from 260 nm polystyrene beads that have been sintered for two minutes. d) to i): SEM images of electrodeposited gold nanostructures by varying the electrodeposition times of nickel and gold. (d) 34 s of Ni followed by 80 s of Au in a PS 260 nm template. (e) 44 s of Ni followed by 42 s of Au in a PS 260 nm template. (f) 83 s of


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Ni followed by 53s of Au in a PS 400 nm template. (g) 37 s of Ni followed by 36s of Au in a PS 260 nm template. (h) 40 s of Ni followed by 97s of Au in a PS 520 nm template. (i) 64 s of Ni followed by 58s of Au in a PS 520 nm template. (PS particles have been removed to allow for better viewing of the electrodeposited metal layers). Having established the various parameters that affect the final morphology of the electrodeposited gold nanopillars, we proceeded to fabricate self standing gold nanopillars that are symmetrically centered vertically about the middle of the template beads (i.e. in the middle of the pores, vertically) with controllable inter-pillar spacing (which is directly dependent on the template particle size). A series of samples were made for each monolayer template according to the final dimensions we desired as listed in Table 1. The lengths of the electrodeposited gold nanopillars were designed to vary as a fraction of PS particle diameter used as template. Figure 4 shows examples of the gold nanopillar arrays grown to a height of 1/5 of the respective bead diameters in different sintered monolayer templates made by 260 nm, 400 nm and 520 nm polystyrene beads (the samples were mounted at a 45° angle for SEM viewing). Table 1. List of samples prepared such that the gold nanopillars are symmetrically centered vertically about the middle of the template beads.

Template size, D (nm)

260

400

520

Thickness of nickel layer (D)

1ൗ 3

3ൗ 8

2ൗ 5

5ൗ 12

1ൗ 3

3ൗ 8

2ൗ 5

5ൗ 12

1ൗ 3

3ൗ 8

2ൗ 5

5ൗ 12

Electrodeposition time of Ni (s)

25

27

28

30

26

28

30

31

30

32

35

36

Length of gold nanopillars (D)

1ൗ 3

1ൗ 4

1ൗ 5

1ൗ 6

1ൗ 3

1ൗ 4

1ൗ 5

1ൗ 6

1ൗ 3

1ൗ 4

1ൗ 5

1ൗ 6

Electrodeposition time of Au (s)

54

43

36

31

80

62

55

47

91

71

58

50


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Figure 4. Tilted backscattered SEM images (at low (left) and high (right) magnification) of gold nanopillars vertically centered about the middle of the template beads (after template removal). Top row: Electrodeposition of 28 s of Ni followed by 36 s of Au in a PS 260 nm template. Middle row: Electrodeposition of 30 s of Ni followed by 55 s of Au in a PS 400 nm template. Bottom row: Electrodeposition of 35 s of Ni followed by 58 s of Au in a PS 520 nm template. The lighter colored regions correspond to the electrodeposited gold and the darker regions correspond to the electrodeposited nickel.'

Transfer of electrodeposited gold nanopillars onto a transparent substrate and optical characterizations. The fabricated gold nanopillar arrays (as listed in Table 1) with examples shown in Figure 4 were subsequently transferred from the conductive substrate onto


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polydimethylsiloxane (PDMS), which is a transparent substrate, for optical characterisation. Figure 5 shows the images of one gold nanopillar array on the conductive substrate (a); after transfer onto PDMS (b) and the corresponding SEM image of the gold nanopillars in PDMS (c).

Figure 5. Images of an array of gold nanopillars grown in a monolayer template of 400 nm PS beads by electrodepositing Ni for 30 s and Au for 55 s on (a) a conductive substrate with a droplet of water on top of the gold nanopillar array, (b) in PDMS, and (c) the corresponding SEM image of the gold nanopillars after transferring onto PDMS. Briefly, the procedure to do this was to first dissolve the polystyrene template. The resulting structure was then covered with PDMS which was cured. Next, the sacrificial nickel layer was dissolved. At this point, the gold nanopillars are embedded in the PDMS (see Figure 5b). The final PDMS samples with the gold nanopillars embedded are not only transparent and flexible, but also exhibit a good adhesion to smooth surfaces. These transparent samples were then optically characterized. Figure 6 shows the measured transmission spectra for each set of fabricated samples. We can observe that for each template particle size, as the gold nanopillars become longer, there is a blue shift in the main transmission dips. This blue shift is due to the decrease in the effective aspect ratio of the nanopillars, because the ends of the pillars get wider as the length is increased (observed in Figure 4). For gold nanopillars fabricated from templates based on the 260 nm PS beads, the corresponding absorption peaks for the 1/6 D, 1/5 D, 1/4 D


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and 1/3 D samples are around 715 nm, 680 nm, 647 nm and 635 nm respectively. For gold nanopillars fabricated from templates based on the 400 nm PS beads, the corresponding absorption peaks for the 1/6 D, 1/5 D, 1/4 D and 1/3 D samples are around 930 nm, 846 nm, 799 nm and 662 nm respectively. For gold nanopillars fabricated from templates based on the 520 nm PS beads, the corresponding absorption peaks for the 1/6 D, 1/5 D, 1/4 D and 1/3 D samples are around 775 nm, 692 nm, 597 nm and 557 nm respectively.

Figure 6. Experimental transmission spectra (left) and simulated spectra (right) of gold nanopillar arrays with different lengths fabricated using (a) 260 nm, (b) 400 nm and (c) 520 nm PS particles.


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The spectroscopic properties (UV-visible-NIR spectra) of the designed pillar structures have also been simulated by Finite Difference Time Domain (FDTD). The structures were designed by adjusting the size parameters of the spheres, sintering parameters and refractive index of the infiltrated PDMS to best match the experimental results. The relative permittivity of bare PDMS was taken to be 1.96. The dielectric permittivity of gold was specified by using a sum of Drude and Drude-Lorentz terms, according to the work performed by Rakic et al..30 In all simulations, light propagates along the +Z direction (i.e. wave-vector kz) and is polarized along the +X direction (i.e. exciting electric field, Ex). Periodic boundary conditions are used in the X and Y directions while perfectly matched layers (PMLs) were implemented in the Z direction. By Fourier transforming the response to a short, broadband, spatially extended Gaussian pulse in the far-field of the structures and normalizing with the response of a reference (the substrate alone) for the same excitation conditions, a single simulation yielded the transmission, reflection, and absorption spectra over a wide spectrum of frequencies. Figure 6 also shows the simulated spectra. A very good correlation between the experimental and simulated spectra can be seen, confirming the successful engineering of the nanopillar arrays. There is a similar trend in the variation of the intensity as well as the positions of the peaks as the length of the gold nanopillars decrease from 1/3 D to 1/6 D. We observe that there is a broadening of the experimental peaks as well as a reduction in the magnitude of the intensity of these peaks as compared to the simulated spectra. This can be attributed to the fact that our fabricated samples have inherent structural defects in them such as cracks which results in disruptions in the overall organization of the gold nanopillars, and this irregularity is absent in the ideally simulated structures. Hence, it is expected that the experimental spectra will not show transmission peaks that are as defined as those predicted by simulations.


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In order to better understand the physics behind the far-field spectral features exhibited especially by the largest nanostructures (based on spheres of 520 nm and 100 nm height of the pillars), a near-field optical picture is required. Figure 7 shows a side-by-side comparison of the simulated far-field and near-field optical properties of pillar arrays (e-h) and a single pillar (a-d).

Figure 7. FDTD-simulated extinction (black line), scattering (red line) and absorption (blue line) cross-sections and electric field intensity distribution maps for a single pillar at λ = 700 nm (a–d) and a two-dimensional pillar array at λ = 700 nm (e–h); parameters D= 520 nm, h=100 nm, grid resolution = 10nm; slight sintering effect taken into account in order to avoid direct metallic connections between pillars.(i) : sketch of the detection plane relative to the gold nanopillar.


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Figure 7a shows that a single pillar exhibits a relatively broad asymmetric plasmon resonance extending from 600 to 700 nm with a shallow dip in between two peaks. The extinction peak results from scattering and absorption contributions, with the latter dominating as expected for relatively small particles. The calculated electric field intensity maps (Fig. 7b–d) reveal that the broad resonance is a complex combination of radiative in-plane and out-of-plane dipole moments driven by the incident field polarized along x and propagating along z. The interference of these two modes, intrinsically inherent to the geometry of these structures, results in the complex asymmetric (Fano) pattern observed in Fig. 7a. Figure 7e depicts how a two-dimensional array of pillars exhibits different optical properties compared to a single pillar of the same size and shape. Notably, the dip and peaks in the 600-700 nm range deepen drastically, featuring a more efficient interference mechanism. In fact, in contrast with the electric field intensity displayed by a single pillar (Fig. 7d), the one for an array of nanopillars (Fig. 7h) clearly shows maxima on the external border of the nanopillars, i.e. extending to the neighbors to which they couple. This coupling is a result of both an in-plane dipole oscillation (Fig. 7f) and an out-of-plane dipole oscillation (Fig. 7g) both extending towards their neighbors. As a comparison of Figs. 7 b-d and 7 f-h reveals, the strongly coupled pillar arrays can effectively trap the incident light and induce a large field enhancement both in the plane of the array as well as surrounding the plane.

CONCLUSION Two dimensional arrays of hexagonally arranged gold nanopillars with controllable spacing and shapes have been successfully fabricated for the first time by using a combination of nanosphere


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lithography with the sequential potentiostatic electrodeposition of nickel and gold. A simple sintering step has been incorporated in the process to allow a fine control of the pore size of the monolayer templates, and thus the narrowness of the nanopillars. By monitoring the current variation during potentiostatic electrodeposition of the metals, we can accurately estimate the thickness of the electrodeposited metals. Many other interesting nanostructured gold architectures can also be obtained by varying either the pore size and/or the deposition time of each metal. The self standing gold nanopillar arrays on conductive substrates have been successfully transferred onto PDMS, a transparent and durable substrate. The resulting optical spectra have been measured and the corresponding simulations show a very good and consistent correlation between the two sets of data. We show that the plasmon absorption bands of these types of structures can be tuned by varying the height of the nanopillars as well as the inter-pillar spacing. This result is similar to what Zhou et al. previously demonstrated for 2D nanoparticle arrays.4 The use of PDMS to transfer these structures could also allow them to be used in applications where a thin coating of these architectures is desired, such as in solar cells or certain optoelectronic devices. Further work can also be done to investigate in detail the effects of stretching and bending on the optical/plasmonic response of this kind of material under stress.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]


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ASSOCIATED CONTENT Supporting Information. Preparation of close-packed PS monolayer, modification of PS monolayer via sintering and arrays of gold nanopillars fabricated from different templates. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENTS The authors thank the Région Aquitaine in the frame of the Erasmus Mundus International Doctoral School in Functional Materials (IDS-FunMat) for the Ph.D scholarship of H. Zheng. This work was supported by Orange Labs Networks contract no. 0050012310-A10141.


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Morphological Design of Gold Nanopillar Arrays ... - ACS Publications

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