1

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, ₤ Wenlong Cheng † ‡ *

†Department of Chemical Engineering, Monash University, Clayton, Victoria 3800, Australia.

E-mail: wenlong.cheng@monash.edu

‡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 3800, Victoria, Australia.

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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

hexane/water interface, forming uniform superlattices up to at least ~13 cm2 and giving rise to

mirror-like 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.

Keywords: surface plasmon resonance, interfacial self-assembly, superlattice mirrors,

nanocubes, nanocuboids

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

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that these top–down methodologies require expensive processes and equipment in cleanroom

environments, as well as suffers from diffraction limit in high throughput fabrication of

nanoscale features. 4

Bottom–up self-assembly provides an alternative strategy towards mass production of

nanoplasmonic structures over 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 superlattices could be tuned simply by adjusting the length of the capping ligands 20, 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 table 15 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

hexane/water interface by 1-dodecanethiol induced self-assembly of NC and NB core–shell

nanoparticles. The large-area uniformity led to a mirror-like reflection. Such large-area mirror-

like superlattice sheets exhibited stretchable plasmonic properties responses to external strains.

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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, polyvinylpyrrolidone (PVP) were purchased from Sigma Aldrich. n-Hexane 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 Mill-Q system (Millipore). All glassware used in 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 a FEG-SEM (FEI NovaNanoSEM 430).

Transmission electron microscopy (TEM) observations were conducted on a Philips CM20 TEM.

The optical spectra were recorded by 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. 3 nm Au nanoparticles was 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 oC 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,

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containing 6 mL aqueous HAuCl4 (0.5 mM) solution, 6 mL aqueous CTAC solution (200 mM),

and 4.5 mL of aqueous AA solution (100 mM). The final mixture turned from colorless into red

quickly, indicating the larger Au nanoparticles formed. 11 nm Au seeds were collected by

centrifugation (14.5k rpm, 30 min), washed with water once, and used for NCs synthesis. For

large amount of NCs 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 oC for 20 min

under magnetic stirring, 20 mL of aqueous AgNO3 (2 mM) and 20 mL 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 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 same with the synthesis of

NCs section. 1 mL AgNO3 (4mM) solution + 25 mL CTAB (0.2 M) + 25 mL HAuCl4 (1mM)

were mixed in a 50 ml centrifuge tube, and then 0.4 mL 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 oC for two hours. The CTAB capped nanorods were

collected by centrifugation (7800 rpm for 40min) and washed with water twice, followed by

redispersion in an aqueous solution of CTAC to a total volume of 5 mL, and kept or more than

12 h to ensure total replacement of CTAB by CTAC.

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Synthesis of Au@Ag NBs: 1 mL prepared CTAC capped Au nanorods kept at 65 o C in

water bath and then 0.88 mL AgNO3 (10 mM) were added in drops, and followed by 0.44 mL L-

AA (100mM). The reaction was continually stirred at 65 oC 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 water for further use. 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 LLI technique:

Au@Ag NCs superlattices fabrication: Firstly, 4 mL 25 nm NCs solution were collected by

centrifugation (13k rpm, 15 min), and redispersed in 4 mL PVP (1mg/mL) aqueous solution in a

beaker with diameter of 4 cm, and then diluted to 5 mL with addition of water. 5 mL n-Hexane

with 7.17 μL DDT (0.845 μg/mL) was added into beaker. 2 mL (a half volume of NCs solution

before centrifugation) of ethanol was injected into NCs aqueous solution by a syringe pump, 0.2

mL/min. About 2 min, small sheet of mirror like membrane began to appear, and become larger.

After all the ethanol was injected into NCs solution, mirror like NCs superlattice covered whole

hexane and water interface like optical images in Figure 1b formed. Parafilm was used to cover

the top of beaker. The superlattice was kept undisturbed for about 1 h for NCs complete self-

assemble at interface. Removed the Hexane as much as possible without destroy the superlattice,

and transferred it to an expected area of PDMS via horizontal lifting for further optical

characterization, when Hexane left evaporated off.

Au@Ag NBs superlattices fabrication:

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10 mL NBs solution was centrifuged (6000 rpm, 10 min), and redispersed in 1 mL PVP

(1mg/mL) aqueous solution, further diluted with water to 5 mL. 5 mL n-Hexane with 7.17 μL

DDT (0.845 μg/mL) was added into beaker. 2 mL of ethanol was injected into NBs aqueous

solution by 0.2 mL/min. The abovementioned procedure is same for NC superlattice sheets.

Optical properties study: Before doing the stretchable test, another layer of PDMS was

covered by spin coating (3000 rpm, 30s), and then cured at 65 o C for 4 h. We put the stretchable

superlattice (PDMS) on glass slide, fixed one side of the membrane, and stretched along the

glass slide. Record the optical spectra by UV–vis spectrophotometer at the same time under

different strain strength. All the experiments have been performed in the same condition.

Simulation model for stretchable NC superlattice sheets:

The numerical simulations of stretchable NC superlattice sheets were performed using

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 (𝜖"#$$. = 1.7689), given in Figure S4a. Each

nanocube is modelled 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). The permittivity of

silver for the nanocube was obtained from bulk silver permittivity values; 43 whereas for gold

nanosphere additional size-dependent corrections 44, 45 were implemented to the bulk gold

permittivity values. 43 Open boundaries with implemented so that incident waves can pass those

boundaries with minimal reflections, thus implementing perfectly matched layer (PML)

condition. In the model, some extra space was added around the nanoparticle and the open

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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) closely resembles the experimental one given in Figure 2c.

Then, using this NC nanoparticle as the unit cell we modelled 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 by𝜆 = 752.02𝑑23.435, with𝑅7 =

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 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 slightly wider peak than the simulated one, as there are distributions in sizes of the

synthesized NCs and non-uniformity 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 x axis). Note that Δdy = 0.5 Δdx due to the Poisson’s ratio of 0.5 of the

PDMS sheet supporting the NC superlattice sheet. Figure S6 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).

The peak of the extinction spectrum in each case of applied strain is shown in Figure S7b when

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incident light is polarized parallel to the stretch axis (i.e., along 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 when incident light is polarized

perpendicular to the stretch axis (i.e., along y-axis). We observed a gradual red shift owing to

strengthening of interparticle coupling as the nanoparticles come closer to their neighbours along

y-direction.

Similarly, in Figure S8 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, and the corresponding shift in the extinction peak locations are shown in Figure S9b,

c for x-polarized and y-polarized incident light, respectively. It can be seen that for strain level

beyond 15 %, interparticle spacing dy goes towards sub-nanometer range and hence, a very large

red shift can be seen in Figure S9c—signifying extremely high confinement of electric field in

the gaps, which leads to very strong plasmonic interactions and large red shift.

Note that, in case of unpolarised 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 the

dominating trend of the shift of the two orthogonal polarization directions. In case of

longitudinal stretching, the effective trend is a blue shift till 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 blue-shift for strain level up to 30% (see Figure 4a) can be closely

approximated by longitudinal mode of stretching.

RESULTS AND DISCUSSION

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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), 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. 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 stabiliser for NCs or NBs.60 Then 5 ml

of 8 μM 1-dodecanethiol (DDT) hexane solution was added into 5 ml of the aqueous solution

containing nanoparticles. Upon addition of 2 ml ethanol gradually, nanoparticles spontaneously

self-assembled 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

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bonds with silver surface, enabling the formation self-assembled 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 (Fig. 1d–f).

The internal structures of these nanoparticle superlattice mirrors were examined using

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 oriented in a random manner both locally and within the giant sheet (Figure 2b). The

degree of ordering for an NB superlattice mirror could be quantified using the following ordering

parameter:61

𝑆7; =<

=>?cos 2𝜃D

=>?DE< , (1)

where 𝜃D is the angle between ith NB and the average orientation of NBs in a region of radius r

around it, and 𝑁GH is the total number of NBs in that region. The ordering parameter, calculated

with increasing radius (plotted in Figure S2b), is about 0.06 over a region with radius of ~1.12

μm (as shown in Figure S2a), 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 Figures 2c,d. Individual NC

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possesses two distinct plasmon modes at ~350 and ~ 424 nm, corresponding to edge-associated-

plane octupolar mode and corner dipole mode, respectively 62 (Figure 2c). However, in case of

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). For NBs, an individual NB

is found to possess four distinct peaks at 348 nm, 388 nm, 451 nm, and 550 nm, corresponding to

facet-associated octupolar mode, edge-associated octupolar mode, transverse dipolar mode, and

longitudinal dipolar resonance mode, respectively 41 (Figure 2d). However, in 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 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 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 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). Such plasmonic responses, obtained under observation

with a fixed beam spot-size, could be attributed to the decay of near-field coupling strength

between nanoparticles due to the increased interparticle separations under strain and also

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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) reveal that there is a trend of minor blueshift of the plasmon resonance

peak for stretching cycle up to ~20-30 and exhibits no further noticeable shift for additional

cycles. Note that, the trend is more prominent in case of NB 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 ~36 ± 5.0 nm blue-shift was observed for NB superlattice sheets, whereas only ~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 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:

𝜆GI = 561.79×𝜀23.3< (2)

𝜆GH = 587.26×𝜀23.37 (3)

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where λNC and λNB are wavelength 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 Equations (2) and (3), it can be seen that the

power-law exponent for NB empirical equation is twice as that for NC, signifying the fact that

stretch induced plasmonic mode changes are more prominent in 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—modelled as an infinite two-dimensional periodic array of NCs—were

simulated with different interparticle separations to estimate average interparticle gap in the

initial unstretched NC superlattice sheet (Figure 4b). The variation of the plasmon peak

wavelength (𝜆GI3%) as a function of interparticle spacing (d) exhibits a power-law dependency

(Figure 4b) 38 given as:

𝜆GI3% = 752.02×𝑑23.4<. (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 the

double fully stretched length of DDT molecule (~3.4 nm), demonstrating ligand interdigitation.51

An NC superlattice sheet, supported by PDMS (Poisson’s ratio of 0.5 63), 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 (Figure S6, 8). This allows for weakening

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(strengthening) of plasmonic coupling in the direction parallel (perpendicular) to the stretch axis,

resulting in blue (red) shift in extinction resonance peak (Figure S7,9). 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

towards blue for x-polarized light gets compensated by relatively smaller shift towards 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, which show strain-induced changes in

spectral characteristics and their variation trends with light polarization direction. However, in

case of diagonal stretching, an overall trend of blue shift is seen till strain level of 15%, beyond

which the plasmon peak drastically shifts towards 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 y-direction, even to sub-nanometer range (see Figure S9).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:

𝜆GI"MNO = 582.16×𝜀23.3<, (5)

𝜆GI"MN; = 446.33×𝜀23.3<, (6)

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where ε is the applied strain, and 𝜆GI"MNO and 𝜆GI"MN; 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 Equation (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 shift of ~12 nm from simulations of

longitudinal stretching (Figure 4a,d and Figure S12). This indicates that, within the level of

strain measured, longitudinal mode of stretching is dominant in our giant NC superlattice sheet.

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 top-down lithography such as EBL13, 14, our

fabrication approach remained yet to control plasmonic structures with high uniformity. But it

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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 ‘meta-atoms’, thus, potentially leading to novel two dimensional

plasmonics with a wide range of applications in nanophotonics and biosensing.

AUTHOR INFORMATION

Corresponding Author

*Tel: +61 3 9905 3147. Fax: +61 3 9905 5686. E-mail: wenlong.cheng@monash.edu. Website:

http://users.monash.edu.au/∼wenlongc/

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENT

This work is financially supported 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.

ASSOCIATED CONTENT

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Supporting Information. 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 optical

responses of single NC and NC superlattice, simulation results of NC superlattice under various

strain. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 1. A general method to fabricate giant area superlattice sheets of Au@Ag core–shell nanocubes (NCs)

and nanocuboids (NBs) by liquid/liquid interfacial (LLI) technique. a) Schematic illustration of the process

towards self-assembly of NCs and NBs at hexane/water interface. b,c) Optical images of b) NC and c) NB

superlattice at hexane/water interface (the diameter of beaker is about 4 cm). d) Bare PDMS substrate. e,f) NC

and NB superlattice sheets transferred onto PDMS, respectively, showing high reflectivity.

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Figure 2. Structural components and optical properties of NC and NB superlattice sheets self-assembled by

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.

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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) Two

dimensional 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.

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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

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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 = a xb.

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