Large-Scale Self-Assembly and Stretch-Induced Plasmonic Properties

Oct 21, 2014 - The frequency-domain FEM solver was used to obtain the optical ..... The work of D. Sikdar is supported by Victoria India Doctoral Scho...
0 downloads 0 Views 5MB Size
Download PDF
Article pubs.acs.org/JPCC

Large-Scale Self-Assembly and Stretch-Induced Plasmonic Properties of Core−Shell Metal Nanoparticle Superlattice Sheets Pengzhen Guo,†,‡,§ Debabrata Sikdar,∥ Xiqiang Huang,§ Kae Jye Si,†,‡ Bin Su,†,‡ Yi Chen,†,‡ Wei Xiong,†,‡ Lim Wei Yap,†,‡ Malin Premaratne,∥ and Wenlong Cheng*,†,‡ †

Department of Chemical Engineering, Monash University, Clayton, Victoria 3800, Australia The Melbourne Centre for Nanofabrication, Clayton, Victoria 3800, Australia § Center for Condensed Matter Science and Technology, Department of Physics, Harbin Institute of Technology, Harbin, Heilongjiang Province 150080, P.R. China ∥ Advanced Computing and Simulation Laboratory (AχL), Department of Electrical and Computer Systems Engineering, Monash University, Clayton, Victoria 3800, Australia ‡

S Supporting Information *

ABSTRACT: We report on a facile interfacial self-assembly approach to fabricate large-scale metal nanoparticle superlattice sheets from nonspherical core−shell nanoparticles, which exhibited reversible plasmonic responses to repeated mechanical stretching. Monodisperse Au@Ag nanocubes (NCs) and Au@Ag nanocuboids (NBs) could be induced to self-assembly at the hexane/water interface, forming uniform superlattices up to at least ∼13 cm2 and giving rise to mirrorlike reflection. Such large-area mirror-like superlattice sheets exhibited reversible plasmonic responses to external mechanical strains. Under stretching, the dominant plasmonic resonance peak for both NB and NC superlattice sheets shifted to blue, following a power-law function of the applied strain. Interestingly, the power-law exponent (or the decay rate) showed a strong shape dependence, where a faster rate was observed for NB superlattice sheets than that for NC superlattice sheets.

■

INTRODUCTION Integration of metallic nanostructures with elastomeric substrates enables fabrication of flexible/stretchable plasmonic devices that can be conformably attached to curved surfaces,1−4 which is otherwise impossible to achieve with the current predominantly rigid plasmonic nanostructure. Owing to recent technological advancements, it is possible to fabricate metallic nanostructures on polydimethylsiloxane (PDMS)5−7 and other flexible polymer surfaces.8,9 Some noteworthy examples include soft lithography techniques,10,11 photolithography,12 electron beam lithography (EBL),13,14 and nanostencil lithography.1 Nevertheless, it is known that these top-down methodologies require expensive processes and equipment in cleanroom environments and suffer from diffraction limits in highthroughput fabrication of nanoscale features.4 Bottom-up self-assembly provides an alternative strategy toward mass production of nanoplasmonic structures over a large area at low cost.15−19 In particular, two-dimensionally (2D) ordered plasmonic nanoparticle arrays (i.e., 2D nanoparticle superlattices) could be obtained via drying-mediated self-assembly,20−23 liquid/liquid interfacial self-assembly,24−28 and air/liquid interfacial self-assembly.29−31 The key parameters such as interparticle spacing in the self-assembled © 2014 American Chemical Society

superlattices could be tuned simply by adjusting the length of the capping ligands20,32,33 or by altering the shell thickness of the nanoparticles (e.g., for Poly(NIPAM) and SiO2 shell).34−37 An additional strategy is to introduce external strain, which can modulate plasmonic responses in a reversible manner simply by applying and releasing mechanical strains. Despite initial success in spherical gold nanoparticles,38 the vast majority of other complex nanoparticles “meta-atom” in the plasmonic periodic table15 have not yet been explored for stretchable plasmonics. In particular, it is expected that new lattice structures and wider tunability of stretch-modulated plasmonic coupling will arise from nonspherical core−shell bimetallic nanoparticles, which possess much richer plasmonic modes as compared to monometallic nanoparticles.39 Here, we report on large-scale interfacial self-assembly and stretchable plasmonic properties of core−shell nanoparticle superlattice sheets from monodisperse Au@Ag nanocubes (NCs) and Au@Ag nanocuboids (NBs). Scalable NC and NB superlattices could be obtained at the hexane/water interface by Received: August 10, 2014 Revised: October 21, 2014 Published: October 21, 2014 26816

dx.doi.org/10.1021/jp508108a | J. Phys. Chem. C 2014, 118, 26816−26824

The Journal of Physical Chemistry C

Article

Figure 1. General method to fabricate giant area superlattice sheets of Au@Ag core−shell nanocubes (NCs) and nanocuboids (NBs) by the liquid/ liquid interfacial (LLI) technique. (a) Schematic illustration of the process toward self-assembly of NCs and NBs at the hexane/water interface. (b,c) Optical images of (b) NC and (c) NB superlattice at the hexane/water interface (the diameter of the beaker is about 4 cm). (d) Bare PDMS substrate. (e,f) NC and NB superlattice sheets transferred onto PDMS, respectively, showing high reflectivity.

(100 mM). The final mixture turned from colorless into red quickly, indicating the larger Au nanoparticles formed. Au seeds (11 nm) were collected by centrifugation (14.5k rpm, 30 min), washed with water once, and used for NC synthesis. For a large amount of NC synthesis, 2 mL of the CTAC-Au seeds and 18 mL of CTAC (20 mM) aqueous solution were mixed in a 100 mL vial. After the mixture was heated at 60 °C for 20 min under magnetic stirring, 20 mL of aqueous AgNO3 (2 mM) and 20 mL of aqueous solution of AA (50 mM) and CTAC (40 mM) were simultaneously injected at a rate of 0.8 mL/min using a syringe pump. During the injection, the reaction mixture turned from red to brownish-yellow. After 4 h, the vials were cooled in an ice bath for 5 min. The products were collected by centrifugation (14.5k rpm, 15 min) and redispersed in 20 mL of water for further use. The concentration of NCs is about 2.54 nM/L, as estimated from the concentration of core gold nanospheres in the solution using Beer−Lambert’s law. Synthesis of Au@Ag Core−Shell NBs. NBs were synthesized according to the procedure reported.41 Synthesis of Au nanorods: the Au seed solution was synthesized by a seed growth method, which is the same as the synthesis of the NC section. Amounts of 1 mL of AgNO3 (4 mM) solution + 25 mL of CTAB (0.2 M) + 25 mL HAuCl4 (1 mM) were mixed in a 50 mL centrifuge tube, and then 0.4 mL of L-AA (0.08 M) was added in sequence. The yellowish mixture became colorless. To grow Au nanorods, 60 μL of prepared Au seeds was added into growth solution and aged at 30 °C for 2 h. The CTAB capped nanorods were collected by centrifugation (7800 rpm for 40 min) and washed with water twice, followed by redispersion in an aqueous solution of CTAC to a total volume of 5 mL, and kept for more than 12 h to ensure total replacement of CTAB by CTAC. Synthesis of Au@Ag NBs: 1 mL of prepared CTAC-capped Au nanorods kept at 65 °C in a water bath and then 0.88 mL of AgNO3 (10 mM) were added in drops and followed by 0.44 mL of L-AA (100 mM). The reaction was continually stirred at 65 °C for 3 h. The CTAC-capped NBs were collected by centrifugation (6500 rpm for 10 min) and washed with water once and then redispersed in 10 mL of water for further use.

1-dodecanethiol-induced self-assembly of NC and NB core− shell nanoparticles. The large-area uniformity led to a mirrorlike reflection. Such large-area mirror-like superlattice sheets exhibited stretchable plasmonic property responses to external strains. Under stretching, the dominant plasmonic resonance peak for both NB and NC superlattice sheets shifted to blue and followed a power-law function of the applied strain, in consistency with our numerical simulation.

■

EXPERIMENTAL SECTION Materials and Characterization. Gold chloride trihydrate (HAuCl4·3H2O), sodium borohydride (NaBH4), hexadecyltrimethylammonium bromide (CTAB), silver nitrate (AgNO3), L -ascorbic acid (AA), cetyltrimethylammonium chloride (CTAC) (25 wt % in H2O), 1-dodecanethiol, and polyvinylpyrrolidone (PVP) were purchased from Sigma-Aldrich. nHexane and ethanol were obtained from Merch KgaA. All chemicals were used as received unless otherwise indicated. All aqueous solutions were made using demineralized water, which was further purified with the Milli-Q system (Millipore). All glassware used in the following procedures was cleaned in a bath of freshly prepared aqua regia and rinsed thoroughly in H2O prior to use. Scanning electron microscopy (SEM) images were acquired on an FEG-SEM (FEI NovaNanoSEM 430). Transmission electron microscopy (TEM) observations were conducted on a Philips CM20 TEM. The optical spectra were recorded by an Agilent 8453 UV−vis spectrophotometer. Synthesis of Au@Ag Core−Shell NCs. Large-scale synthesis of uniform 25.5 ± 1.4 nm NCs was performed according to the literature.40 The Au nanocrystal seeds were prepared using a two-step procedure. Au nanoparticles (3 nm) were first made by adding 0.6 mL of ice-cooled NaBH4 solution (10 mM) into a 10 mL aqueous solution containing HAuCl4 (0.25 mM) and CTAB (100 mM), generating a brownish solution. The seed solution was kept undisturbed for 3 h at 27 °C to ensure complete decomposition of NaBH4. Then 11 nm CTAC-capped Au seeds were synthesized by adding 0.3 mL of the 3 nm Au nanoparticles into a growth solution, containing 6 mL of aqueous HAuCl4 (0.5 mM) solution, 6 mL of aqueous CTAC solution (200 mM), and 4.5 mL of aqueous AA solution 26817

dx.doi.org/10.1021/jp508108a | J. Phys. Chem. C 2014, 118, 26816−26824

The Journal of Physical Chemistry C

Article

Figure 2. Structural components and optical properties of NC and NB superlattice sheets self-assembled by the LLI technique. (a,b) SEM images of NC and NB superlattice sheets, respectively. (c,d) Extinction spectra of nanoparticles in solution and from a superlattice sheet for (c) NCs and (d) NBs.

The concentration of NCs is about 3 nM/L, as estimated from the concentrations of the core gold nanorods obtained using the method described by Orendorff et al.42 Superlattice Fabrication and Transfer by the LLI Technique. Au@Ag NC superlattice fabrication: First, 4 mL of 25 nm NCs solution was collected by centrifugation (13k rpm, 15 min) and redispersed in 4 mL of PVP (1 mg/mL) aqueous solution in a beaker with diameter of 4 cm and then diluted to 5 mL with addition of water. An amount of 5 mL of n-hexane with 7.17 μL of DDT (0.845 μg/mL) was added into the beaker. An amount of 2 mL (a half volume of NCs solution before centrifugation) of ethanol was injected into the NC aqueous solution by a syringe pump, at 0.2 mL/min. After about 2 min, a small sheet of mirror-like membrane began to appear and become larger. After all the ethanol was injected into the NC solution, the mirror-like NC superlattice covered the whole hexane, and water-interface-like optical images in Figure 1b formed. Parafilm was used to cover the top of the beaker. The superlattice was kept undisturbed for about 1 h for NC complete self-assembly at the interface. We removed the hexane as much as possible without destroying the superlattice and transferred it to an expected area of PDMS via horizontal lifting for further optical characterization, when the hexane that was left evaporated off. Au@Ag NB superlattice fabrication: An amount of 10 mL of NB solution was centrifuged (6000 rpm, 10 min) and redispersed in 1 mL of PVP (1 mg/mL) aqueous solution and further diluted with water to 5 mL. An amount of 5 mL of n-hexane with 7.17 μL of DDT (0.845 μg/mL) was added into the beaker. An amount of 2 mL of ethanol was injected into the

NB aqueous solution by 0.2 mL/min. The above-mentioned procedure is the same for NC superlattice sheets. Optical Properties Study. Before doing the stretchable test, another layer of PDMS was covered by spin coating (3000 rpm, 30 s) and then cured at 65 °C for 4 h. We put the stretchable superlattice (PDMS) on a glass slide, fixed one side of the membrane, and stretched along the glass slide. We recorded the optical spectra by a UV−vis spectrophotometer at the same time under different strain strength. All the experiments have been performed in the same conditions. Simulation Model for Stretchable NC Superlattice Sheets. The numerical simulations of stretchable NC superlattice sheets were performed using the frequency-domain FEM solver of CST Microwave Studio Suite. The frequency-domain FEM solver was used to obtain the optical response (i.e., absorption, scattering, and extinction of light) of the superlattice sheet as a function of wavelength of the incident light. First, we simulated the optical response of a single Au@Ag NC in water (εsurr. = 1.7689), given in Figure S4a (Supporting Information). Each nanocube is modeled as a silver nanocube (edge length = 25.5 nm) having smooth corners and edges with an 11 nm diameter gold nanosphere in its center (see Figure S4a, Supporting Information). The permittivity of silver for the nanocube was obtained from bulk silver permittivity values,43 whereas for the gold nanosphere additional size-dependent corrections44,45 were implemented to the bulk gold permittivity values.43 Open boundaries were implemented so that incident waves can pass those boundaries with minimal reflections, thus implementing perfectly matched layer (PML) conditions. In the model, some extra space was added around the nanoparticle 26818

dx.doi.org/10.1021/jp508108a | J. Phys. Chem. C 2014, 118, 26816−26824

The Journal of Physical Chemistry C

Article

the dominating trend of the shift of the two orthogonal polarization directions. In the case of longitudinal stretching, the effective trend is a blue shift until a stretch level of 25%, and for diagonal stretching the effective blue shift can be seen for strain level up to 15%. So, the experimentally observed blueshift for strain level up to 30% (see Figure 4a) can be closely approximated by the longitudinal mode of stretching.

and the open boundary to enable far-field calculation. Tetrahedral mesh, which is more accurate at metallic material interfaces, was used in these frequency-domain simulations with automatic mesh refinement to study the optical response over the wavelength window of interest. The simulated extinction spectrum (Figure S4b, Supporting Information) closely resembles the experimental one given in Figure 2c. Then, using this NC nanoparticle as the unit cell we modeled a two-dimensional periodic array of NCs with uniform spacing, based on which we first investigated the peak of extinction resonance spectrum as a function of interparticle separation, when no strain is applied. The surrounding medium had relative permittivity of 1.96, corresponding to that of PDMS. Under no strain, the plasmon resonance (λ) peak exhibited a blue-shift with increasing interparticle spacing (d) [here, d = dx = dy] following a power-law fitting given by λ = 752.02d−0.309, with R2 = 0.9953. From this fit we obtain an estimate of the interparticle spacing d = 2.34 nm for the experimentally observed resonance peak at λ = 579 nm (Figure 2c). Figure S4b (Supporting Information) shows the simulated spectra for 2D periodic array of NCs with this interparticle spacing, where the extinction spectrum matches well with the one given in Figure 2c. The experimental spectrum has a slightly wider peak than the simulated one, as there are distributions in sizes of the synthesized NCs and nonuniformity in the interparticle spacing. Further, to qualitatively analyze the change in plasmon resonance with applied strain we calculated the extinction spectrum for different sets of dx and dy, determined by the amount of strain (stretched along the x axis). Note that Δdy = 0.5Δdx due to the Poisson’s ratio of 0.5 of the PDMS sheet supporting the NC superlattice sheet. Figure S6 (Supporting Information) depicts the models used for simulation of longitudinally stretched NC superlattice sheet. We explicitly show here the changes in dx and dy for strain changing from 0% to 5% to 15% to 25% and to 35% (Figure S7a, Supporting Information). The peak of the extinction spectrum in each case of applied strain is shown in Figure S7b (Supporting Information) when incident light is polarized parallel to the stretch axis (i.e., along the x-axis). We observed a gradual blue shift which is attributed to the gradual increase in dx, weakening the interparticle plasmonic coupling. The shift in extinction peak is shown in Figure S7c (Supporting Information) when incident light is polarized perpendicular to the stretch axis (i.e., along the y-axis). We observed a gradual red shift owing to strengthening of interparticle coupling as the nanoparticles come closer to their neighbors along the y-direction. Similarly, in Figure S8 (Supporting Information) we show the models used in simulation of diagonally stretched NC superlattice sheets. The changes in interparticle spacing dx and dy in this case are shown in Figure S9a (Supporting Information), and the corresponding shifts in the extinction peak locations are shown in Figure S9b,c (Supporting Information) for x-polarized and y-polarized incident light, respectively. It can be seen that for strain level beyond 15% interparticle spacing dy goes toward the subnanometer range, and hence, a very large red shift can be seen in Figure S9c (Supporting Information)signifying extremely high confinement of the electric field in the gaps, which leads to very strong plasmonic interactions and a large red shift. Note that, in the case of unpolarized light (used in the experiments)having electric field components in both x and y directionsthe effective trend of the spectral shift is decided by

■

RESULTS AND DISCUSSION To grow large-area superlattice sheets, monodispersed Au@Ag NCs and NBs were first synthesized following the two-step procedure protocols reported,40,41,46,47 namely, for NCs: (i) synthesis of gold seeds, followed by (ii) coating of the gold seeds with silver,40 and for NBs: (i) synthesis of gold nanorods,46 followed by (ii) coating of the gold nanorods with silver.41 As shown in the typical transmission electron microscopy (TEM) images (Figure S1, Supporting Information), highly monodispersed NCs and NBs were obtained. The edge length of NCs is 25.5 ± 1.4 nm, whereas, NBs had a typical width of 35 nm ± 1.2 nm and a typical length of 61 ± 3.2 nm. Then, liquid/liquid interfacial (LLI) self-assembly was used to grow superlattice sheets of these nanoparticles. The liquid/ liquid interface is an ideal 2D plane for confining self-assembly of nanoparticles to produce 2D assemblies with long-range orders.24,48 A range of nanoparticles have been investigated using LLI self-assembly so far,24,49−54 albeit mainly limited to sphere-like nanoparticles as building blocks resulting in sheets with mirror-like reflection54−56 despite the exception for nanorods.27,57 The complex interactions between nonspherical nanoparticles often render it challenging to obtain large-area ordered nanoassemblies.24,50,58,59 We show that it is possible to obtain large-area superlattice sheets from both NCs and NBs by using LLI self-assembly (Figure 1a). The as-prepared nanoparticles were first spun down and redispersed into PVP aqueous solution. PVP acted as a stabilizer for NCs or NBs.60 Then 5 mL of 8 μM 1dodecanethiol (DDT) hexane solution was added into 5 mL of the aqueous solution containing nanoparticles. Upon addition of 2 mL of ethanol gradually, nanoparticles spontaneously selfassembled at the water/hexane interface with metal mirror-like reflection (Figure 1b). In this process ethanol reduced the interfacial surface tension, promoting ligand exchanges between strong-binding DDT ligands with weak-binding cetyltrimethylammonium chloride (CTAC) surfactants. Note that thiol moieties of DDT molecules can spontaneously form strong covalent bonds with silver surface, enabling the formation selfassembled monolayers of DDT on nanoparticl surfaces. Thus, exposed alkyl chains rendered nanoparticles hydrophobic, leading to deposition of nanoparticles at the interfaces. The upper-phase hexane was then removed carefully without destroying nanoparticle membranes. Remarkably, the nanoparticle layers could be transferred to elastomeric PDMS substrate with high transfer fidelity as evidenced by maintaining the high mirror-like reflection property (Figure 1d−f). The internal structures of these nanoparticle superlattice mirrors were examined using a scanning electron microscope (SEM). It is observed that in both NC and NB superlattice sheets the nanoparticles are mostly assembled in face-to-face orientations (Figure 2a,b). Due to structural symmetry, NCs assembled into many local islands of perfectly ordered nanoparticles within the giant superlattice sheet (Figure 2a), whereas NBsbeing elongated nanoparticlesare mostly 26819

dx.doi.org/10.1021/jp508108a | J. Phys. Chem. C 2014, 118, 26816−26824

The Journal of Physical Chemistry C

Article

Figure 3. Spectral variation of the plasmon resonance characteristics of NC and NB stretchable superlattice sheets with increasing strain. (a) Schematic showing the process of obtaining stretchable NC superlattice mats sandwiched between two layers of PDMS, as used in the experiments. (b,d) Extinction spectra of (b) NC and (d) NB superlattice sheets showing the optical responses of the sheets under increasing strain. (c,e) Twodimensional plot of the extinction spectra in (b) and (d) showing the relative variation in extinction intensity in the parametric domain of the applied strain and incident light wavelength.

2c). However, in the case of the NC superlattice sheet, the dominant peak in the extinction spectrum appears at 579 ± 1.0 nm, which arises from intense asymmetrical dipole−dipole coupling between the nanocubes. Along with this dominant peak, a low-energy shoulder peak is also observed at around 720 nm, which can be attributed to symmetrical dipole−dipole coupling (Figure S5, Supporting Information). For NBs, an individual NB is found to possess four distinct peaks at 348, 388, 451, and 550 nm, corresponding to facet-associated octupolar mode, edge-associated octupolar mode, transverse dipolar mode, and longitudinal dipolar resonance mode, respectively41 (Figure 2d). However, in the case of a NB superlattice sheet, only one strong peak corresponding to dipole−dipole coupling mode governs the overall plasmonic properties, as seen from the extinction spectrum in Figure 2d. We further investigated how plasmonic response changes when external strains were applied on a superlattice sheet. Using the aforementioned horizontal lifting, large-area NC and NB superlattice sheets could be transferred onto PDMS surfaces, followed by an additional layer of PDMS to fully embed the superlattices into PDMS (Figure 3a). The embedded superlattice sheets were then uniaxially stretched,

oriented in a random manner both locally and within the giant sheet (Figure 2b). The degree of ordering for a NB superlattice mirror could be quantified using the following ordering parameter61 S2D =

1 NNB

NNB

∑ cos 2θi i=1

(1)

where θi is the angle between ith NB and the average orientation of NBs in a region of radius r around it, and NNB is the total number of NBs in that region. The ordering parameter, calculated with increasing radius (plotted in Figure S2b, Supporting Information), is about 0.06 over a region with radius of ∼1.12 μm (as shown in Figure S2a, Supporting Information), which enumerates the high randomness in the orientations of NBs in the NB superlattice sheet. The optical responses of the superlattice sheets and their constituent nanoparticles were further examined, and the measured extinction spectra are given in Figure 2c,d. An individual NC possesses two distinct plasmon modes at ∼350 and ∼424 nm, corresponding to edge-associated-plane octupolar mode and corner dipole mode, respectively62 (Figure 26820

dx.doi.org/10.1021/jp508108a | J. Phys. Chem. C 2014, 118, 26816−26824

The Journal of Physical Chemistry C

Article

Figure 4. Variation in the plasmon resonance features of a stretchable superlattice sheet subjected to increasing strain. (a) Experimentally measured plasmon resonance peak locations of NC and NB superlattice sheets with increase in strain. (b) Interparticle gap of the real NC superlattice sheet estimated using a fitted line to the plasmon peaks obtained from simulation by varying interparticle separation of an unstretched sheet. (c) Schematic of an NC superlattice sheet being longitudinally stretched. (d) Variation in the numerically calculated plasmon peak location with applied strain for different polarization characteristics of the incident light. (e) Schematic of an NC superlattice sheet being diagonally stretched. (f) Variation in the numerically calculated plasmon peak location with applied strain for different polarization characteristics of the incident light. Dashed lines in each case are fitted with power-law function: y = axb.

Supporting Information) reveal that there is a trend of minor blueshift of the plasmon resonance peak for stretching cycle up to ∼20−30 and that it exhibits no further noticeable shift for additional cycles. Note that the trend is more prominent in the case of NBs as compared to NC superlattice sheets. Nevertheless, the shift in the plasmonic coupling peak for NB superlattices was found to be more sensitive to strain than that for NC superlattices (Figure 4a). For example, under 35 ± 1.5% strain an ∼36 ± 5.0 nm blue-shift was observed for NB superlattice sheets, whereas only an ∼12 ± 1.0 nm blue-shift was measured for NC superlattice sheets. In contrast, around 20 nm blue-shift was reported for the spherical gold nanoparticle superlattice sheets under similar strain.38 These results clearly demonstrate the shape dependence of stretchable plasmonic properties of superlattice sheets. From the experimental results, it appears that the dominant plasmon resonance peak in the extinction spectrum of a

and their plasmonic properties were measured at various strain levels. For both NC and NB superlattice sheets, clear trends of blue shift were observed in the resonance wavelength of the coupled dipole−dipole plasmons with increasing strains (Figure 3b,d). Stretching also induces an evident decrease in the plasmon resonance intensity and broadening of the resonance peak, as can be seen by comparing the peaks of the extinction spectra (Figure 3c,e and Figure S11, Supporting Information). Such plasmonic responses, obtained under observation with a fixed beam spot size, could be attributed to the decay of nearfield coupling strength between nanoparticles due to the increased interparticle separations under strain and also reduction in number of interacting nanoparticles over the beam spot area. Remarkably, all these spectral changes could be fully reverted after releasing the applied stress (Figure S3a,b). Long-term stability analyses upon multiple reversible stretching cycles of NC and NB superlattice sheets (Figure S3c,d, 26821

dx.doi.org/10.1021/jp508108a | J. Phys. Chem. C 2014, 118, 26816−26824

The Journal of Physical Chemistry C

Article

direction. However, in case of diagonal stretching, an overall trend of blue shift is seen until a strain level of 15%, beyond which the plasmon peak drastically shifts toward red (Figure 4f). A very large red shift can be seen for strain level increasing from 20% to 25%, where the NCs get very close to each other along the y-direction, even to a subnanometer range (see Figure S9, Supporting Information).64 The spectral trend of the plasmon resonance peak wavelength with increasing strain for both longitudinal and diagonal (up to 15%) stretching, obtained from the simulation results, can also be fitted by the power-law function given as

superlattice sheet blueshifts by following a power-law decay function of applied strain (Figure 4a). Numerical fittings to the experimental data lead to the following two empirical equations for NC and NB superlattice sheets λNC = 561.79 × ε−0.01

(2)

λNB = 587.26 × ε−0.02

(3)

where λNC and λNB are wavelengths of the extinction resonance peak for NC and NB superlattice sheets, respectively, and ε is the strain applied. Note that all the fitting parameters reported in this work are rounded up to two decimal places. From eqs 2 and 3, it can be seen that the power-law exponent for the NB empirical equation is twice that for NC, signifying the fact that stretch-induced plasmonic mode changes are more prominent in the case of NB superlattice sheets. In order to provide a simple qualitative understanding, we performed full-wave electromagnetic simulations for NC superlattice sheets to investigate the stretch-induced changes in plasmonic resonances of stretchable superlattices. First of all, the optical responses of an NC superlattice sheetmodeled as an infinite two-dimensional periodic array of NCswere simulated with different interparticle separations to estimate the average interparticle gap in the initial unstretched NC superlattice sheet (Figure 4b). The variation of the plasmon peak wavelength (λ0% NC) as a function of interparticle spacing (d) exhibits a power-law dependency (Figure 4b)38 given as 0% λNC = 752.02 × d −0.31

simL λNC = 582.16 × ε−0.01

(5)

simD λNC = 446.33 × ε−0.01

(6)

λsimL NC

where ε is the applied strain, and and are the extinction peak wavelength of longitudinally and diagonally stretched NC superlattice sheets, respectively. Note that these fitted functions have similar power-law exponents as obtained from the fit to the experimental data in eq 2. In experiments, a stretched NC superlattice sheet (Figure 2a) may undergo more complicated scenarios of stretching. Nevertheless, using simulations we observe that stretch-induced plasmonic responses can be well described mostly by the longitudinal mode of stretching. For instance, under 35 ± 1.5% strain the experimentally measured blue shift of ∼12 ± 1.0 nm is found to be in excellent agreement with the shift of ∼12 nm from simulations of longitudinal stretching (Figure 4a,d and Figure S12, Supporting Information). This indicates that, within the level of strain measured, the longitudinal mode of stretching is dominant in our giant NC superlattice sheet.

(4)

This allows us to estimate the average interparticle spacing as ∼2.34 nm corresponding to the experimentally measured extinction peak at 579 nm (marked in Figure 4b), which is less than double the fully stretched length of DDT molecule (∼3.4 nm), demonstrating ligand interdigitation.51 An NC superlattice sheet, supported by PDMS (Poisson’s ratio of 0.563), exhibits stretching-induced anisotropy in the trends of shifts in plasmon resonance peak for increasing strain, depending on the directions of polarization of the incident light and applied stretch (Figure 4c−f). Under applied stress, interparticle separation increases along the stretch axis, whereas it decreases along its orthogonal axis (Figures S6 and S8, Supporting Information). This allows for weakening (strengthening) of plasmonic coupling in the direction parallel (perpendicular) to the stretch axis, resulting in a blue (red) shift in extinction resonance peak (Figures S7 and S9, Supporting Information). Therefore, for unpolarized light having electric field components along both the orthogonal polarization directionsthe trend of red/blue shift in plasmon resonance peak is decided by whichever shift dominates over the other. We considered two probable scenarios of stretching, where an NC superlattice sheet can be stretched either longitudinally (Figure 4c) or diagonally (Figure 4e). In the particular scenario of longitudinal stretching, an effective blue shift is obtained for unpolarized light after the shift toward blue for x-polarized light gets compensated by a relatively smaller shift toward the red wavelength for y-polarized light (Figure 4d). Detailed analyses of the variation of different spectral features (e.g., resonance peak, peak intensity, and width) of the extinction spectra for longitudinal stretching are depicted in Figure S10 (Supporting Information), which show strain-induced changes in spectral characteristics and their variation trends with light polarization

λsimD NC

■

CONCLUSION We have successfully fabricated large-area anisotropic core− shell nanoparticle superlattice sheets using liquid/liquid interfacial self-assembly. Plasmonic coupling of dipole−dipole modes dominated the overall optical responses for both nanocube and nanocuboid superlattice sheets. The dominant plasmonic coupling peak in the extinction spectrum shifted to blue wavelength as the superlattice sheets were stretched uniaxially, which followed a power-law decay function of the applied strain, in agreement with the results obtained from numerical simulations. However, the power-law exponent (or the decay rate) showed a strong shape dependence, where a faster rate was observed for nanocuboid superlattice sheets than that for superlattice sheets of nanocubes. Compared to topdown lithography such as EBL,13,14 our fabrication approach remained yet to control plasmonic structures with high uniformity, but it offers advantages of rapid large-scale fabrication at low cost. The technique of obtaining large-area stretchable plasmonic superlattice mirrors demonstrated here can be extended to obtain similar sheets from any other “metaatoms”, thus potentially leading to novel two-dimensional plasmonics with a wide range of applications in nanophotonics and biosensing.

■

ASSOCIATED CONTENT

* Supporting Information S

TEM images of NCs and NBs, analysis of orientational order parameter of NB superlattice mirror, reversibility of plasmon resonance of the NC and NB superlattice sheets upon stretching, spectra obtained from numerical simulations of the 26822

dx.doi.org/10.1021/jp508108a | J. Phys. Chem. C 2014, 118, 26816−26824

The Journal of Physical Chemistry C

Article

(14) Ueno, K.; Misawa, H. Spectral properties and electromagnetic field enhancement effects on nano-engineered metallic nanoparticles. Phys. Chem. Chem. Phys. 2013, 15, 4093−4099. (15) Tan, S. J.; Campolongo, M. J.; Luo, D.; Cheng, W. Building plasmonic nanostructures with DNA. Nat. Nanotechnol. 2011, 6, 268− 276. (16) Gong, J.; Li, G.; Tang, Z. Self-assembly of noble metal nanocrystals: Fabrication, optical property, and application. Nano Today 2012, 7, 564−585. (17) Zhu, Z.; Liu, W.; Li, Z.; Han, B.; Zhou, Y.; Gao, Y.; Tang, Z. Manipulation of Collective Optical Activity in One-Dimensional Plasmonic Assembly. ACS Nano 2012, 6, 2326−2332. (18) Li, Z.; Zhu, Z.; Liu, W.; Zhou, Y.; Han, B.; Gao, Y.; Tang, Z. Reversible Plasmonic Circular Dichroism of Au Nanorod and DNA Assemblies. J. Am. Chem. Soc. 2012, 134, 3322−3325. (19) Si, K. J.; Sikdar, D.; Chen, Y.; Xu, Z.; Tang, Y.; Xiong, W.; Guo, P.; Zhang, S.; Lu, Y.; Bao, Q.; et al. Giant Plasmene Nanosheets, Nanoribbons, and Origami. ACS Nano 2014, DOI: 10.1021/ nn504615a. (20) Cheng, W.; Campolongo, M. J.; Cha, J. J.; Tan, S. J.; Umbach, C. C.; Muller, D. A.; Luo, D. Free-standing nanoparticle superlattice sheets controlled by DNA. Nat. Mater. 2009, 8, 519−525. (21) Ng, K. C.; Udagedara, I. B.; Rukhlenko, I. D.; Chen, Y.; Tang, Y.; Premaratne, M.; Cheng, W. Free-Standing Plasmonic-Nanorod Superlattice Sheets. ACS Nano 2011, 6, 925−934. (22) Cheng, W.; Park, N.; Walter, M. T.; Hartman, M. R.; Luo, D. Nanopatterning self-assembled nanoparticle superlattices by moulding microdroplets. Nat. Nanotechnol. 2008, 3, 682−690. (23) Chen, Y.; Fu, J.; Ng, K. C.; Tang, Y.; Cheng, W. Free-Standing Polymer−Nanoparticle Superlattice Sheets Self-Assembled at the Air− Liquid Interface. Cryst. Growth Des. 2011, 11, 4742−4746. (24) Reincke, F.; Hickey, S. G.; Kegel, W. K.; Vanmaekelbergh, D. Spontaneous Assembly of a Monolayer of Charged Gold Nanocrystals at the Water/Oil Interface. Angew. Chem., Int. Ed. 2004, 43, 458−462. (25) Duan, H.; Wang, D.; Kurth, D. G.; Möhwald, H. Directing SelfAssembly of Nanoparticles at Water/Oil Interfaces. Angew. Chem., Int. Ed. 2004, 43, 5639−5642. (26) Wang, D.; Duan, H.; Mohwald, H. The water/oil interface: the emerging horizon for self-assembly of nanoparticles. Soft Matter 2005, 1, 412−416. (27) Cheng, L.; Liu, A.; Peng, S.; Duan, H. Responsive Plasmonic Assemblies of Amphiphilic Nanocrystals at Oil−Water Interfaces. ACS Nano 2010, 4, 6098−6104. (28) Turek, V. A.; Cecchini, M. P.; Paget, J.; Kucernak, A. R.; Kornyshev, A. A.; Edel, J. B. Plasmonic Ruler at the Liquid−Liquid Interface. ACS Nano 2012, 6, 7789−7799. (29) Tao, A.; Sinsermsuksakul, P.; Yang, P. Tunable plasmonic lattices of silver nanocrystals. Nat. Nanotechnol. 2007, 2, 435−440. (30) Mueggenburg, K. E.; Lin, X.-M.; Goldsmith, R. H.; Jaeger, H. M. Elastic membranes of close-packed nanoparticle arrays. Nat. Mater. 2007, 6, 656−660. (31) Dong, A.; Chen, J.; Vora, P. M.; Kikkawa, J. M.; Murray, C. B. Binary nanocrystal superlattice membranes self-assembled at the liquid-air interface. Nature 2010, 466, 474−477. (32) Malynych, S.; Chumanov, G. Light-Induced Coherent Interactions between Silver Nanoparticles in Two-Dimensional Arrays. J. Am. Chem. Soc. 2003, 125, 2896−2898. (33) Chen, C.-F.; Tzeng, S.-D.; Chen, H.-Y.; Lin, K.-J.; Gwo, S. Tunable Plasmonic Response from Alkanethiolate-Stabilized Gold Nanoparticle Superlattices: Evidence of Near-Field Coupling. J. Am. Chem. Soc. 2007, 130, 824−826. (34) Ung, T.; Liz-Marzán, L. M.; Mulvaney, P. Gold nanoparticle thin films. Colloids Surf., A 2002, 202, 119−126. (35) Viktor, M.; Jessica, R.-F.; Isabel, P.-S.; Alison, M. F.; Carolina, N.; Paul, M.; Luis, M. L.-M.; Abajo, F. J. G. d. Modelling the optical response of gold nanoparticles. Chem. Soc. Rev. 2008, 37, 1792−1805. (36) Karg, M.; Hellweg, T.; Mulvaney, P. Self-Assembly of Tunable Nanocrystal Superlattices Using Poly-(NIPAM) Spacers. Adv. Funct. Mater. 2011, 21, 4668−4676.

optical responses of single NC and NC superlattice, and simulation results of the NC superlattice under various strain. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: +61 3 9905 3147. Fax: +61 3 9905 5686. E-mail: [email protected]. Web site: http://users.monash. edu.au/∼wenlongc/. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work is financially supported by ARC discovery projects DP120100170 and DP140100052. This work was performed in part at the Melbourne Centre for Nanofabrication (MCN) in the Victorian Node of the Australian National Fabrication Facility (ANFF). P. Z. Guo acknowledges the scholarship from China Scholarship Council. The work of D. Sikdar is supported by Victoria India Doctoral Scholarship. The work of M. Premaratne is supported by ARC Discovery Grants DP110100713 and DP140100883.

■

REFERENCES

(1) Aksu, S.; Huang, M.; Artar, A.; Yanik, A. A.; Selvarasah, S.; Dokmeci, M. R.; Altug, H. Flexible plasmonics on unconventional and nonplanar substrates. Adv. Mater. 2011, 23, 4422−4430. (2) Heber, J. Around the bend. Nat. Mater. 2011, 10, 902−902. (3) Zheludev, N. I.; Kivshar, Y. S. From metamaterials to metadevices. Nat. Mater. 2012, 11, 917−924. (4) Polavarapu, L.; Liz-Marzan, L. M. Towards low-cost flexible substrates for nanoplasmonic sensing. Phys. Chem. Chem. Phys. 2013, 15, 5288−5300. (5) Pryce, I. M.; Aydin, K.; Kelaita, Y. A.; Briggs, R. M.; Atwater, H. A. Highly Strained Compliant Optical Metamaterials with Large Frequency Tunability. Nano Lett. 2010, 10, 4222−4227. (6) Lee, S.; Kim, S.; Kim, T.-T.; Kim, Y.; Choi, M.; Lee, S. H.; Kim, J.-Y.; Min, B. Reversibly Stretchable and Tunable Terahertz Metamaterials with Wrinkled Layouts. Adv. Mater. 2012, 24, 3491− 3497. (7) Vazquez-Mena, O.; Sannomiya, T.; Tosun, M.; Villanueva, L. G.; Savu, V.; Voros, J.; Brugger, J. High-Resolution Resistless Nanopatterning on Polymer and Flexible Substrates for Plasmonic Biosensing Using Stencil Masks. ACS Nano 2012, 6, 5474−5481. (8) Hossain, M. K.; Willmott, G. R.; Etchegoin, P. G.; Blaikie, R. J.; Tallon, J. L. Tunable SERS using gold nanoaggregates on an elastomeric substrate. Nanoscale 2013, 5, 8945−8950. (9) Huang, M.; Galarreta, B. C.; Artar, A.; Adato, R.; Aksu, S.; Altug, H. Reusable Nanostencils for Creating Multiple Biofunctional Molecular Nanopatterns on Polymer Substrate. Nano Lett. 2012, 12, 4817−4822. (10) Xia, Y.; Kim, E.; Zhao, X.-M.; Rogers, J. A.; Prentiss, M.; Whitesides, G. M. Complex Optical Surfaces Formed by Replica Molding Against Elastomeric Masters. Science 1996, 273, 347−349. (11) Childs, W. R.; Nuzzo, R. G. Decal Transfer Microlithography: A New Soft-Lithographic Patterning Method. J. Am. Chem. Soc. 2002, 124, 13583−13596. (12) Bowen, A. M.; Nuzzo, R. G. Fabrication of Flexible Binary Amplitude Masks for Patterning on Highly Curved Surfaces. Adv. Funct. Mater. 2009, 19, 3243−3253. (13) Wang, L.; Xiong, W.; Nishijima, Y.; Yokota, Y.; Ueno, K.; Misawa, H.; Qiu, J.; Bi, G. Spectral properties of nanoengineered Ag/ Au bilayer rods fabricated by electron beam lithography. Appl. Opt. 2011, 50, 5600−5605. 26823

dx.doi.org/10.1021/jp508108a | J. Phys. Chem. C 2014, 118, 26816−26824

The Journal of Physical Chemistry C

Article

charged nanoparticles at the water/oil interface. Phys. Chem. Chem. Phys. 2006, 8, 3828−3835. (59) Bresme, F.; Oettel, M. Nanoparticles at fluid interfaces. J. Phys.: Condens. Matter 2007, 19, 413101. (60) Ali, M. S.; Suhail, M.; Ghosh, G.; Kamil, M. Interactions between cationic gemini/conventional surfactants with polyvinylpyrrolidone: Specific conductivity and dynamic light scattering studies. Colloids Surf., A 2009, 350, 51−56. (61) Hore, M. J. A.; Composto, R. J. Nanorod Self-Assembly for Tuning Optical Absorption. ACS Nano 2010, 4, 6941−6949. (62) Gong, J.; Zhou, F.; Li, Z.; Tang, Z. Synthesis of Au@Ag Core− Shell Nanocubes Containing Varying Shaped Cores and Their Localized Surface Plasmon Resonances. Langmuir 2012, 28, 8959− 8964. (63) Brandrup, J.; Immergut, E. H.; Grulke, E. A.; Abe, A.; Bloch, D. R. Polymer handbook; Wiley: New York, 1999; Vol. 89. (64) Jain, P. K.; Huang, W.; El-Sayed, M. A. On the Universal Scaling Behavior of the Distance Decay of Plasmon Coupling in Metal Nanoparticle Pairs: A Plasmon Ruler Equation. Nano Lett. 2007, 7, 2080−2088.

(37) Barrow, S. J.; Funston, A. M.; Wei, X.; Mulvaney, P. DNAdirected self-assembly and optical properties of discrete 1D, 2D and 3D plasmonic structures. Nano Today 2013, 8, 138−167. (38) Millyard, M. G.; Min Huang, F.; White, R.; Spigone, E.; Kivioja, J.; Baumberg, J. J. Stretch-induced plasmonic anisotropy of selfassembled gold nanoparticle mats. Appl. Phys. Lett. 2012, 100, 073101−073101−3. (39) Cortie, M. B.; McDonagh, A. M. Synthesis and Optical Properties of Hybrid and Alloy Plasmonic Nanoparticles. Chem. Rev. 2011, 111, 3713−3735. (40) Ma, Y.; Li, W.; Cho, E. C.; Li, Z.; Yu, T.; Zeng, J.; Xie, Z.; Xia, Y. Au@Ag Core−Shell Nanocubes with Finely Tuned and WellControlled Sizes, Shell Thicknesses, and Optical Properties. ACS Nano 2010, 4, 6725−6734. (41) Jiang, R.; Chen, H.; Shao, L.; Li, Q.; Wang, J. Unraveling the evolution and nature of the plasmons in (Au core)-(Ag shell) nanorods. Adv. Mater. 2012, 24, OP200−OP207. (42) Orendorff, C. J.; Murphy, C. J. Quantitation of Metal Content in the Silver-Assisted Growth of Gold Nanorods. J. Phys. Chem. B 2006, 110, 3990−3994. (43) Johnson, P. B.; Christy, R. W. Optical Constants of the Noble Metals. Phys. Rev. B 1972, 6, 4370−4379. (44) Sikdar, D.; Rukhlenko, I. D.; Cheng, W.; Premaratne, M. Effect of number density on optimal design of gold nanoshells for plasmonic photothermal therapy. Biomed. Opt. Express 2013, 4, 15−31. (45) Sikdar, D.; Rukhlenko, I.; Cheng, W.; Premaratne, M. Tunable Broadband Optical Responses of Substrate-Supported Metal/Dielectric/Metal Nanospheres. Plasmonics 2014, 9, 659−672. (46) Xiong, W.; Sikdar, D.; Walsh, M.; Si, K. J.; Tang, Y.; Chen, Y.; Mazid, R.; Weyland, M.; Rukhlenko, I. D.; Etheridge, J.; et al. Singlecrystal caged gold nanorods with tunable broadband plasmon resonances. Chem. Commun. 2013, 49, 9630−9632. (47) Okuno, Y.; Nishioka, K.; Kiya, A.; Nakashima, N.; Ishibashi, A.; Niidome, Y. Uniform and controllable preparation of Au-Ag core-shell nanorods using anisotropic silver shell formation on gold nanorods. Nanoscale 2010, 2, 1489−1493. (48) Pieranski, P. Two-Dimensional Interfacial Colloidal Crystals. Phys. Rev. Lett. 1980, 45, 569−572. (49) Lin, Y.; Skaff, H.; Emrick, T.; Dinsmore, A. D.; Russell, T. P. Nanoparticle Assembly and Transport at Liquid-Liquid Interfaces. Science 2003, 299, 226−229. (50) Boker, A.; He, J.; Emrick, T.; Russell, T. P. Self-assembly of nanoparticles at interfaces. Soft Matter 2007, 3, 1231−1248. (51) Park, Y.-K.; Yoo, S.-H.; Park, S. Assembly of Highly Ordered Nanoparticle Monolayers at a Water/Hexane Interface. Langmuir 2007, 23, 10505−10510. (52) Park, Y.-K.; Park, S. Directing Close-Packing of Midnanosized Gold Nanoparticles at a Water/Hexane Interface. Chem. Mater. 2008, 20, 2388−2393. (53) Hu, L.; Chen, M.; Fang, X.; Wu, L. Oil-water interfacial selfassembly: a novel strategy for nanofilm and nanodevice fabrication. Chem. Soc. Rev. 2012, 41, 1350−1362. (54) Fang, P.-P.; Chen, S.; Deng, H.; Scanlon, M. D.; Gumy, F.; Lee, H. J.; Momotenko, D.; Amstutz, V.; Cortés-Salazar, F.; Pereira, C. M.; et al. Conductive Gold Nanoparticle Mirrors at Liquid/Liquid Interfaces. ACS Nano 2013, 7, 9241−9248. (55) Edel, J. B.; Kornyshev, A. A.; Urbakh, M. Self-Assembly of Nanoparticle Arrays for Use as Mirrors, Sensors, and Antennas. ACS Nano 2013, 7, 9526−9532. (56) Yen, Y.-T.; Lu, T.-Y.; Lee, Y.-C.; Yu, C.-C.; Tsai, Y.-C.; Tseng, Y.-C.; Chen, H.-L. Highly Reflective Liquid Mirrors: Exploring the Effects of Localized Surface Plasmon Resonance and the Arrangement of Nanoparticles on Metal Liquid-like Films. ACS Appl. Mater. Interfaces 2014, 6, 4292−4300. (57) Kim, K.; Han, H. S.; Choi, I.; Lee, C.; Hong, S.; Suh, S.-H.; Lee, L. P.; Kang, T. Interfacial liquid-state surface-enhanced Raman spectroscopy. Nat. Commun. 2013, 4. (58) Reincke, F.; Kegel, W. K.; Zhang, H.; Nolte, M.; Wang, D.; Vanmaekelbergh, D.; Mohwald, H. Understanding the self-assembly of 26824

dx.doi.org/10.1021/jp508108a | J. Phys. Chem. C 2014, 118, 26816−26824