Nanoscale surface chemistry directs the tunable assembly of silver octahedra into three two-dimensional plasmonic superlattices

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Abstract A major challenge in nanoparticle self-assembly is programming the large-area organization of a single type of anisotropic nanoparticle into distinct superlattices with tunable packing efficiencies. Here we utilize nanoscale surface chemistry to direct the self-assembly of silver octahedra into three distinct two-dimensional plasmonic superlattices at a liquid/liquid interface. Systematically tuning the surface wettability of silver octahedra leads to a continuous superlattice structural evolution, from close-packed to progressively open structures. Notably, silver octahedra standing on vertices arranged in a square lattice is observed using hydrophobic particles. Simulations reveal that this structural evolution arises from competing interfacial forces between the particles and both liquid phases. Structure-to-function characterizations reveal that the standing octahedra array generates plasmonic ‘hotstrips’, leading to nearly 10-fold more efficient surface-enhanced Raman scattering compared with the other more densely packed configurations. The ability to assemble these superlattices on the wafer scale over various platforms further widens their potential applications.

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### ARTICLE

Received 26 Nov 2014 | Accepted 23 Mar 2015 | Published 29 Apr 2015 DOI: 10.1038/ncomms7990 OPEN
## Nanoscale surface chemistry directs the tunable assembly of silver octahedra into three two-dimensional plasmonic superlattices

Yih Hong Lee 1, Wenxiong Shi 2, Hiang Kwee Lee 1,3, Ruibin Jiang 4, In Yee Phang 3, Yan Cui 1, Lucio Isa 5, Yijie Yang 1,
Jianfang Wang 4, Shuzhou Li 2 & Xing Yi Ling 1

A major challenge in nanoparticle self-assembly is programming the large-area organization

of a single type of anisotropic nanoparticle into distinct superlattices with tunable packing

efficiencies. Here we utilize nanoscale surface chemistry to direct the self-assembly of silver

octahedra into three distinct two-dimensional plasmonic superlattices at a liquid/liquid

interface. Systematically tuning the surface wettability of silver octahedra leads to a

continuous superlattice structural evolution, from close-packed to progressively open struc
tures. Notably, silver octahedra standing on vertices arranged in a square lattice is observed

using hydrophobic particles. Simulations reveal that this structural evolution arises from

competing interfacial forces between the particles and both liquid phases. Structure-to
function characterizations reveal that the standing octahedra array generates plasmonic

‘hotstrips’, leading to nearly 10-fold more efficient surface-enhanced Raman scattering

compared with the other more densely packed configurations. The ability to assemble

these superlattices on the wafer scale over various platforms further widens their potential

applications.

1 Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore.
2 Division of Materials Science, School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore. 3 Institute of
Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 3 Research Link, Singapore 117602, Singapore. 4 Department of
Physics, The Chinese University of Hong Kong, Shatin, Hong Kong, China. 5 Laboratory for Interfaces, Soft matter and Assembly, Department of Materials, ETH
[Zurich, Vladimir-Prelog-Weg 5, Zurich 8093, Switzerland. Correspondence and requests for materials should be addressed to X.Y.L. (email: xyling@ntu.edu.sg).](mailto:xyling@ntu.edu.sg)

ontrolled organization of nanoparticles into scalable
superlattices with tunable crystal structure and spacing is
a grand challenge in nanotechnology 1 . Nanoparticle
# C
superlattices exhibit unique crystal structure-dependent
physicochemical functionalities, playing a pivotal role in the
design of next-generation devices 2–6 . Several methods are
commonly used to form superlattices, including varying
interparticle forces among spherical particles 7–13, using
anisotropic nanoparticles with various morphologies 14–27, and
forming localized chemical patchiness on anisotropic
nanoparticles 28 . However, it remains difficult to assemble
multiple superlattices over large areas using just one
nanoparticle morphology. Overcoming this limitation will
enhance superlattice diversity, enable direct investigations of
structure-to-function variations arising solely from superlattice
structural changes and allow more specific customization of
superlattices for targeted applications 29 . In particular, anisotropic
nanoparticles possessing well-defined facets are appealing
because these facets can interact with each other in various
ways to create distinct superlattices 30 . However, such interactions
among one type of particle morphology have not yet been
extensively explored to achieve tunable self-assembled
nanoparticle superlattices, particularly in the formation of nonclose-packed superlattices.

Here we demonstrate the concept of ‘one anisotropic particle,
multiple superlattices’ using octahedral silver (Ag) nanoparticles.
We are able to tailor the surface chemistry of Ag octahedra to
assemble three wafer-scale two-dimensional (2D) plasmonic
superlattices at the oil/water interface, one of which is a square
superlattice with Ag octahedra standing on their vertices.
Through molecular dynamics (MD) simulations, we find that
these bespoke superlattices arise from a competition of
interactions between the nanoparticles and both liquid phases.
Despite having the lowest-packing density, the square superlattice
is the most efficient surface-enhanced Raman scattering (SERS)
superlattice. This finding demonstrates the importance of
nanoscale superlattice design, where more nanoparticles do not
always generate stronger scattering signals 31, contrary to the rule
of thumb in SERS research.

Results
Assembly of three plasmonic superlattices with one particle.
We employ a chemical approach to tune the surface hydrophobicity of Ag octahedra building blocks. As-synthesized Ag
octahedra (edge lengths B356 ± 10 nm) are stabilized with the
hydrophilic polymer poly(vinylpyrrolidone) (PVP). The hydrophobicities of these particles are homogeneously increased via
ligand exchange with alkylthiols of increasing chain lengths,
including 1-propanethiol (C3SH) and 1-hexadecanethiol
(C16SH). These Ag octahedra are then assembled at the oil/water
interface and subsequently immobilized using the gel-trapping
technique 32 (Fig. 1a; Supplementary Fig. 1; Methods). Once
trapped at the interface, the superlattices can be transferred to
various solid platforms.

A hexagonal close-packed (HCP) 2D superlattice forms at the
oil/water interface using PVP-functionalized Ag octahedra
(PVP octahedra, bulk water contact angle (CA) measured on Ag
film functionalized with PVP B(39 ± 5)�; Fig. 1b,c). Neighbouring Ag octahedra contact via a face-to-face configuration (Fig. 1d;
Supplementary Fig. 2). This superlattice is planar, with one facet
of the octahedra in direct contact with the substrate (Fig. 1d).
Long-range order of this superlattice is evident from the regularly
oscillating curves in its corresponding radial distribution function
(Fig. 1e) and honeycomb network of Voronoi cells (Supplementary Fig. 3a); the nearest neighbour distance is estimated to be

188 nm (Fig. 1e). The packing efficiency of this close-packed array
is 88.9% (Supplementary Fig. 4).

The HCP superlattice changes to an open hexagonal (OH)
array using C3SH-functionalized Ag octahedra for the selfassembly (C3-octahedra, CA ¼ (83 ± 2)�; Fig. 1f,g). A comparison
of this monolayer (Fig. 1h) with the HCP array (Fig. 1d) clearly
illustrates the change from a face-to-face contact to an edge-toedge alignment. Order is also evident from the radial distribution
function and Voronoi cells of this 2D array (Fig. 1i; Supplementary Fig. 3b), with the nearest neighbour distance increased to
313 nm. The packing efficiency of this monolayer decreases to
66.7% (Supplementary Fig. 5).
An entirely different square superlattice is formed by
assembling hydrophobic C16SH-functionalized Ag octahedra
(C16-octahedra, CA ¼ (110 ± 2) o ; Fig. 1j,k). Partial submersion
of edge-to-edge aligned Ag octahedra within the poly(dimethylsiloxane) (PDMS) mold gives a pyramidal appearance (Fig. 1j;
inset). Cross-sectional scanning electron microscopy (SEM)
image of the superlattice transferred to a Si substrate definitively
shows only a single vertex of the octahedra contacting the
substrate (Fig. 1l). This superlattice of standing octahedra has the
lowest-packing efficiency among the three arrays at 33.3%
(Supplementary Fig. 6). Superlattice crystallinity is evident from
the radial distribution function (Fig. 1m) and the square Voronoi
cells (Supplementary Fig. 3c), with the nearest neighbour distance
estimated to be 362 nm. Notably, the three different superlattices
observed heretofore arise solely from variations of Ag octahedra
surface wettability.

Interfacial behaviour of Ag octahedra. MD simulations show
that Ag octahedra move spontaneously to the interface to attain
thermodynamic equilibrium 33 (Supplementary Notes 1 and 2;
Supplementary Movies 1–3; Supplementary Fig. 7). PVP
octahedron rotates from a tilted configuration in the aqueous
phase to become planar on breaching the oil/water interface
(Fig. 2a). For C3-octahedron, the particle rotates from a nearly
upright configuration to attain a stable planar equilibrium
configuration. In contrast, C16-octahedron adopts an upright
configuration from the beginning and this configuration is
maintained at equilibrium. Notably, the interfacial configurations
of these Ag octahedra match well with the experimental topological
characterization using atomic force microscopy (AFM; Fig. 2e).
The experimental height percentages of the particle in contact
with the oil phase are 11%, 18% and 57% for PVP, C3- and
C16-octahedra, respectively (Fig. 2f; Supplementary Fig. 8;
Supplementary Table 1). These values are different from those
observed in the simulations (Supplementary Fig. 9), likely arising
from different magnitudes of capillary forces due to the smaller
particle sizes used in the simulations 33 .

The interfacial behaviour of Ag octahedra is dictated by the
competition between the particle–water and particle–oil interactions, defined as the hydrophobic/hydrophilic potential energy
ratio. This ratio increases from 0.3 to 6, and to 52 as the ligands
change from PVP to C3SH, and to C16SH, respectively (Fig. 2d;
Supplementary Fig. 10). Particle–oil interaction becomes increasingly favourable with increasing hydrophobicity of the ligands.
Consequently, PVP octahedron remains in contact with the
aqueous phase due to a favourable ligand–water interaction,
forming the HCP superlattice. C3-octahedron moves further into
the oil phase with a stronger particle–oil interaction, suggesting
that an OH lattice is desirable to achieve an optimal interaction
with both phases. In contrast, dominant particle–oil interaction
for C16-octahedron drives it into a standing configuration to
minimize contact with the aqueous phase, giving rise to a square
superlattice of standing octahedra aligned edge to edge.

80 °C
Add

Decane

PDMS

Decant decane

Gellan Gum

at interface Replace with PDMS

Lift off

PDMS

2.0

1.5

1.0

0.5

3.0

2.0

1.0

3.0

2.0

1.0

0 400 800 1,200

Distance (nm)

0 400 800 1,200
Distance (nm)

Figure 1 | Controlling nanoscale surface wettability of Ag octahedra to assemble three superlattices. (a) Schematic of interfacial self-assembly protocol
at the oil/water interface. (b–d,f–h,j–l) SEM characterization and corresponding fast-Fourier transforms (FFTs) of the various Ag octahedra superlattices.
(b) HCP monolayer of PVP octahedra is confirmed with FFT (c), adopting a planar configuration (d). (f,g) The planar OH superlattice is formed using
C3-octahedra, with the octahedra aligned edge to edge (h). (j,k) A square superlattice with a pyramidal appearance is observed using C16-octahedra. The
octahedra are aligned edge to edge. (l) Cross-sectional SEM shows only the vertices of Ag octahedra contacting the substrate. (b,f,j) Scale bars, 2 mm;
(b,f,j) inset scale bars, 200 nm; (d,h,l) scale bar, 100 nm. (e–m) Radial distribution functions of the three superlattices highlight excellent long-range order.
The nearest neighbour distance increases from 188 nm (e) to 313 nm (i) and to 362 nm (m) for the HCP, OH and square superlattices, respectively.

Tuning superlattice structure using surface hydrophobicity. By
systematically using Ag octahedra functionalized with different
alkylthiols (to impart different hydrophobicity to the particles) for
self-assembly (Fig. 3a), we show that the superlattice structures
observed are related to each other (Fig. 3b; Supplementary
Fig. 11). The hydrophobicity of the alkylthiols we use increases in
the order of PVPo12-mercaptododecanoic acid (MDA)oC3SHo1-hexanethiol (C6SH)o1-dodecanethiol (C12SH)oC16SH
(Fig. 3a). The HCP superlattice quickly loses its crystallinity as the

PVP octahedra is changed to the slightly more hydrophobic
MDA-octahedra (CA ¼ (69 ± 5)�). Changing MDA-octahedra to
C3-octahedra leads to the formation of the OH array; a mixture
of planar and standing octahedra clusters forms when C3-octahedra is replaced by C6-octahedra (CA ¼ (95 ± 3)�). The population of the standing octahedra increases markedly with the use
of C12-octahedra (CA ¼ (101 ± 4)�). A schematic illustrating the
relationship between the superlattices is shown in Fig. 3c. While
the oil/water interface is quintessential for the formation of the

PVP

_R_ = 0.3 C3SH _R_ = 6 C16SH _R_ = 52

0 0.2 1.5 4.0

Time (ns)

_h_
_h_

0 0.5 1.5 4.0

Time (ns)

C16SH

C3SH

PVP

0 1.0 2.0 4.0

Time (ns)

PVP

C3SH

C16SH

0 1.0 2.0 3.0 4.0 PVP C3SH C16SH

PVP

C3SH
Ligand type

Time (ns)

Figure 2 | Using molecular dynamics simulations to unravel the interfacial behaviours of Ag octahedra. (a–c) Evolution of the interfacial position and
configuration of Ag octahedra over simulation times of 4 ns, during which thermodynamic equilibrium is achieved. Both PVP octahedron (a) and C3octahedron (b) attain a stable planar configuration over time. However, PVP octahedron remains mostly in contact with the aqueous phase, whereas C3octahedron moves into the oil phase. (c) C16-octahedron moves across the oil/water interface, maintaining a stable standing configuration. (d) Plot of
hydrophobic/hydrophilic potential energy ratios over time. Hydrophobic interactions become dominant as the ligands change from PVP to C16SH. (e) AFM
topological characterization of the variously functionalized Ag octahedra on PDMS. All scale bars, 500 nm. (f) From AFM measurements, the height
percentage of the Ag octahedra in contact with the oil phase can be derived (Supplementary Fig. 8). Increasing surface hydrophobicity of Ag octahedra
increases the height percentage in contact with the oil phase in the experiments. Error bars are s.d. collected from sampling at least 50 particles.

120

100

PVP MDA C3SH C6SH C12SH C16SH

Ligand type

Figure 3 | Systematic tuning of surface wettabilities creates continuous superlattice structural change. (a) Increase in the bulk water contact angles of a
Ag film functionalized with various thiols. Error bars are s.d. of five measurements for each ligand. (b) Using increasingly hydrophobic Ag octahedra for
self-assembly gives rise to a continuous superlattice evolution, with the HCP superlattice changing into the OH structure and finally into the square
superlattice. All scale bars, 200 nm. (c) Schematic illustration of the relationship between the three superlattices.

standing octahedra superlattice (Supplementary Figs 12,13), the
formation of the standing octahedra superlattice is insensitive to
the locations at which the C16-octahedra are added in the presence of both phases (Supplementary Fig. 14). Our self-assembly
protocol enables versatile superlattice transfer onto various
polymeric platforms (Supplementary Fig. 15), Si substrates
(Supplementary Fig. 16), and scaling up to B60 cm 2 (Fig. 4a).
Bending this large mold leads to distinct colour changes (Fig. 4b),
likely arising from the angle-dependent Bragg reflection of incident light off the periodic pyramidal superlattice.

SERS behaviours of the plasmonic superlattices. A structure-tofunction characterization of the SERS capabilities of these three

superlattices highlights the superior performance of the square
superlattice over the hexagonally tiled ones. Constructing different superlattices without changing nanoparticle morphology
allows us to single out the influence of superlattice structure on
the corresponding SERS performance. Using 4-methylbenzenthiol as the probe molecule, fingerprint vibrational modes at
1,080 and 1,600 cm � 1 are observed (Fig. 4c; Supplementary
Table 2). The SERS enhancement factor for the 1,080 cm � 1 mode
of the standing array is estimated to be 9.9 � 10 4, 7.5-fold higher
than both the hexagonal arrays (Fig. 4d; Supplementary Note 3).

This difference in SERS enhancement factors of the three
superlattices arises from the stark contrast in their plasmonic hot
spot distribution. The local electromagnetic field distribution for
the Ag octahedra superlattices along the surface of the PDMS

10 counts

Square

HCP

1,000 1,200 1,400 1,600 1,800

Raman shift (cm –1 )

1.2

1.0

0.8

0.6

0.4

0.2

HCP OH Square

Superlattice structure

| _E_ 2 |/| _E_ |

–2

–4

–6

–8

HCP OH Square

Figure 4 | Structure-to-function optical characterization of the three 2D plasmonic superlattices. (a,b) Bending the B60 cm 2 flexible PDMS film of
standing octahedra superlattice (a) leads to unique colour changes (b). (c,d) SERS spectra (c) and EFs (d) of 4-MBT from the three superlattices. Error bars
in (d) are s.d. of 10 measurements (area of each measurement is 60 � 3 mm 2 ). The low-packing density square superlattice of standing octahedra is an
order of magnitude more efficient than the hexagonal lattices. (e) Localized electromagnetic field distributions of the three superlattices. All scale bars,
100 nm. Large-area plasmonic ‘hotstrips’ in the square superlattice is observed. 4-MBT, 4-methylbenzenthiol.

molds is simulated using finite-difference time-domain (FDTD)
method. At the incident laser excitation wavelength of 532 nm,
plasmonic hot spots are highly localized in both hexagonal
superlattices to regions of contacting points between neighbouring Ag octahedra (Fig. 4e). On the other hand, strips of plasmonic
hot spots are formed in the standing array: hot spots spread over
the entire edge length of the exposed Ag octahedra and are almost
continuous along the entire array across neighbouring Ag
octahedra. An estimate of the hot spot areas in these arrays
indicates that a single hot spot is almost 130-fold larger in the
standing array compared with the hexagonal lattices (B22,650
versus B360 nm 2 ). Furthermore, the pyramidal structure of the
standing octahedra array enables efficient scattering of SERS
signals 34 . These factors collectively lead to much stronger SERS
enhancement in the square standing superlattice. Notably, this
SERS study indicates that ‘more is not always better’ for SERS,
since the lowest-packing density superlattice is the most efficient
SERS substrate. Our finding shows that structural design on the
nanoscale can significantly impact the resulting macroscopic
optical behaviours, owing to the nanoscale unique and structurespecific light–matter interactions. We believe this finding will

inspire future work to factor in the relevance of nanoscale
organization in designing functional and efficient materials.

Discussion
In this work, we demonstrate the ability to assemble three
bespoke 2D plasmonic superlattices by tailoring the surface
chemistries of Ag octahedra. By controlling the hydrophobic/
hydrophilic interactions of the octahedral particles at the oil/
water interface, we demonstrate the relationship between the
three superlattices. The homogeneity of surface functionalities
also enables the superlattices to be assembled on the wafer scale,
bridging the gap between nanoscopic materials and macroscopic
applications. To this end, we further highlight the importance of
superlattice design in tailoring nanoscale light–matter interactions for efficient macroscopic sensing applications.

Methods
Nanocrystal synthesis. The preparation of Ag octahedra was carried out using
the polyol reduction method 35, starting first with the synthesis of Ag nanocubes. In
a typical nanocube synthesis, 10 ml of CuCl2 (8 mg ml � 1 ), PVP (20 mg ml � 1 ) and
AgNO3 (20 mg ml � 1 ) were separately dissolved in 1,5-pentanediol (PD). 35 ml

CuCl2 solution was added to the AgNO3 solution. 20 ml PD was then heated to
190 �C for 10 min. 250 ml PVP precursor was added to a round-bottom flask
dropwise every 30 s, while 500 ml AgNO3 precursor was injected into the flask every
minute in one go. The reaction was allowed to proceed for B20 min following
which, the injection was continued using 30 ml of PVP (20 mg ml � 1 ) and AgNO3
(40 mg ml � 1 with 120 ml CuCl2). Both precursors were separately prepared in PD.
The reaction was allowed to proceed until the precursors were used up. The Ag
octahedra solution was redispersed in 20 ml ethanol after removing the PD via
multiple centrifugation rounds, and diluted, then vacuum filtered multiple times
using polyvinylidene difluoride filter membranes (Durapore) with pore sizes
ranging from 5,000, 650, 450 to 220 nm to remove impurities before finally
dispersing in ethanol. Following purification, the PVP-functionalized Ag octahedra
were subjected to ligand exchange reactions to tailor their surface chemistry for
subsequent self-assembly experiments.

Ligand exchange reactions. The purified Ag octahedra dispersion was generally
allowed to sediment. Sediment (0.5 ml) was dispersed in ethanol and centrifuged once
more before dispersing in 1.5ml of ethanol/isopropyl alcohol (1:1). 100 ml of 10mM
thiol solution (MDA, C3SH, C6SH, C12SH and C16SH) was then added dropwise to
this dispersion under stirring. Ligand exchange was allowed to take place for 4 h,
followed by two rounds of centrifugation, redispersal in 1.5 ml of ethanol/isopropyl
alcohol (1:1) and addition of fresh thiol solution under stirring. This step was allowed
to continue for another 3 h, followed by three rounds of centrifugation and washing
with isopropyl alcohol/water (1:1). Subjecting Ag octahedra to two rounds of ligand
exchange removed the replaced PVP and facilitated subsequent ligand exchange. For
ligand exchange with thermally evaporated Ag film, the substrate was immersed in a
10 mM thiol solution for at least 12 h to allow the self-assembled monolayers to form
before rinsing with ethanol to remove the excess unbound thiols.

Interfacial self-assembly and gel-trapping experiments. The gel-trapping
experiments are illustrated in Fig. 1a. Gellan gum aqueous solution (2wt%) was used
as the water phase and n-decane was used as the oil phase instead. The gel-trapping
experiments were slightly modified from the procedure reported by Paunov and
co-workers 32 . The gellan gum solution was first heated to B80 �C in an oil bath to
ensure that the gel was fully hydrated. Subsequently, pre-heated n-decane was added
to the top of the gellan gum solution to create the oil phase. Ag octahedra were then
added to the oil/water interface and the entire mixture was left at 80 �C for B15min
before being allowed to cool slowly to room temperature. This ensured that the
nanoparticles had sufficient time to spread and achieve thermodynamic equilibrium
at the oil/water interface. Once the gel had set in the aqueous phase, the oil phase was
then decanted gently. In place of n-decane, a layer of premixed PDMS precursor
mixture (5:1; elastomer:curing agent) was poured over the nanoparticle monolayer
and the container was left at room temperature for the PDMS to cure. After the
PDMS had hardened, it was lifted off from the gel and washed in hot water. For
transfer to other polymeric films, 1 ml of either poly(methyl methacrylate) (PMMA)/
toluene solution or poly(3-hexylthiophene-2,5-diyl) (P3HT)/toluene/chloroform was
added in place of PDMS and allowed to evaporate before lift-off. For interfacial selfassembly using hexane as the oil phase, Si substrates were immersed within the water
phase before the assembly process. Ag octahedra was typically dispersed in a
isopropyl alcohol/water (1:1) mix and added dropwise to the interface. The Ag
octahedra monolayer was allowed to stabilize for around 15min before film transfer
to the substrates. The substrates were slowly dipped up into the hexane phase using
the motorized dipper of a Langmuir–Blodgett machine (KSV Nima, KN1002) at a
rate of 25 mm s � 1 for the transfer of Ag octahedra monolayer onto the substrates.
After emerging from the hexane phase, the substrates were allowed to dry under
ambient conditions.

Contact angle measurements. Contact angles of the functionalized 114-nm-thick
Ag films were measured on a Theta Lite tensiometer equipped with a Firewire digital
camera. Static contact angle was measured with a 4-ml ultrapure sessile water droplet.
A total of five readings were taken at different spots on the same substrate for each of
the thiol-functionalized Ag film and averaged to obtain the bulk contact angles.

AFM experiments. AFM was used to investigate the topological changes in the
self-assembled Ag octahedra superlattices on the PDMS molds using a Bruker
Dimension ICON with NanoScope V controller from Bruker. Tapping mode (noncontact mode) image was acquired using silicon probes (Tap300Al-G with 30-nm
aluminium reflex coating) from BudgetSensor. Data analysis was carried out using
WSxM Scanning Probe Microscopy Software, a free program from Nanotec
Electro´nica S.L 36 .

Quantitative image analyses. Long-range order of the various 2D octahedra
arrays was characterized using the freeware ImageJ. The radial distribution function
profiles were analysed by converting the SEM images into a binary format, followed
by the automatic identification of the centres of the Ag octahedra. Voronoi–
Delaunay cells were constructed using custom-written IDL (Exelis) algorithms. The
centre of each particle (or each facet for the hexagonal superlattices) was found by

locating local intensity maxima after filtering the SEM images with a band-pass filter,
followed by constructing the Voronoi–Delaunay cells.

MD simulations. All MD simulations were conducted on single Ag octahedron
placed at the oil/water interface using the GROMACS 4.07 simulation package 37

and GROMOS96 force fields 38 . See Supplementary Fig. 17, Supplementary Note 1
and Supplementary Tables 3–5 for detailed discussion.

SERS experiments. SERS measurements were performed using the x–y imaging
mode of the Laser Raman Microscope RAMANtouch system with an excitation
wavelength of 532 nm (power ¼ 3 mW). A - 100 (numerical aperture 0.9) objective
lens with 5 s accumulation time was used for data collection between 200 to
1,800 cm � 1 . X–y imaging measurements were collected from 10 different spots
with the area of each spot spanning 60 � 3 mm 2 . The probe molecule used was
4-methylbenzenethiol and was place exchanged onto the superlattices.

FDTD calculations. FDTD simulations were performed using FDTD Solution 7.5,
which was developed by Lumerical Solutions, Inc. The periodic boundary was used
to simulate the nanostructure arrays. During the simulations, an electromagnetic
pulse in the wavelength range 300 � 1,200 nm was launched to simulate a propagating plane wave interacting with the nanostructure. The unit cells of nanostructure arrays and its surrounding medium were divided into meshes of 0.5 nm in
size. All nanostructure arrays were built according to the experimental parameters.
The refractive index of PDMS was taken to be 1.4 (ref. 39) and the refractive index
of air was set to be 1. The dielectric function of silver was taken from previously
reported values 40 . The electric field intensity at wavelength of 532 nm was
monitored at the surface of PDMS.

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Acknowledgements
X.Y.L. thanks the support from National Research Foundation, Singapore (NRFNRFF2012-04), Nanyang Technological University’s start-up grant (M4080758). H.K.L.
acknowledges A*STAR Graduate Scholarship support from A*STAR, Singapore. J.W.
thanks the financial support from Hong Kong RGC CRF (CUHK1/CRF/12G). L.I.
acknowledges financial support from the Swiss National Science Foundation grant
PP00P2_144646/1.

Author contributions
Y.H.L. and X.Y.L. designed research; Y.H.L., H.K.L., I.Y.P., Y.C. and Y.Y.J. performed
research; W.X.S. and S.Z.L. performed MD simulations; R.B.J. and J.F.W. performed
FDTD simulations; L.I. performed image analyses; Y.H.L., W.X.S., S.Z.L. and X.Y.L.
wrote the manuscript. All authors read and commented on the manuscript.

Additional information
Supplementary Information accompanies this paper at [http://www.nature.com/](http://www.nature.com/naturecommunications)
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Competing financial interests: The authors declare no competing financial interests.

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How to cite this article: Lee, Y. H. et al. Nanoscale surface chemistry directs the tunable
assembly of silver octahedra into three two-dimensional plasmonic superlattices. Nat.
Commun. 6:6990 doi: 10.1038/ncomms7990 (2015).

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