Smectic mesophases of functionalized silver and gold nanoparticles with anisotropic plasmonic properties

Article abstract of DOI:10.1039/c3cc43166e

Silver and gold nanoparticle superlattices with a layered structure were obtained via grafting of mesogenic molecules onto a metallic cluster surface. For the Ag superstructures unusual orientational order of the mesogenic ligands and anisotropy of the assembly afforded tunable plasmonic properties.

Full text of DOI:10.1039/c3cc43166e

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Smectic mesophases of functionalized silver and gold
nanoparticles with anisotropic plasmonic properties†
Cite this: Chem. Commun., 2013,
49, 7845
Wiktor Lewandowski,*a Doru Constantin,b Kinga Walicka,a Damian Pociecha,a
Received 29th April 2013,
Accepted 26th June 2013
a
a
´
´
Jozef Mieczkowski and Ewa Gorecka
DOI: 10.1039/c3cc43166e
www.rsc.org/chemcomm
Silver and gold nanoparticle superlattices with a layered structure assembly, holding promise for optically active aggregates formation,
were obtained via grafting of mesogenic molecules onto a metallic especially, since small Ag NPs have much stronger plasmon mode
cluster surface. For the Ag superstructures unusual orientational than Au NPs of comparable size. Such aggregates, though made
order of the mesogenic ligands and anisotropy of the assembly of isotropic (spherical) NPs, could exhibit anisotropic plasmonic
afforded tunable plasmonic properties.
properties. Anisotropic plasmonic properties are commonly observed
for oriented samples of nonspherical NPs, e.g. gold nanorods, where
Unique, collective optical properties of precisely controlled assemblies they arise primarily from the plasmon modes of individual nano-
of noble metal nanoparticles, superlattices (SL), make them an clusters. For spherical nanoparticle assemblies anisotropy must be
exciting research topic with a plethora of prospective applications.1 evoked either by an anisotropic spatial distribution of particles11
In this respect, superlattices of spherical silver NPs are particularly or by interaction of plasmons with an anisotropic organic
interesting2 since Ag exhibits the best plasmonic properties of all surrounding.10 Such aggregates are potential candidates for the
chemically stable metals,3 and thus, various approaches have been formation of metamaterials with tunable properties.12
explored to yield ordered Ag aggregates. Some examples of methods
Here, we present macroscopically ordered, anisotropic structures
used include slow sedimentation, the Langmuir–Blodgett technique, formed by spherical silver nanoparticles having mesogenic ligands
or imbedding aggregates into polymer films.4 However, from the grafted onto their surface. For comparison, properties of gold NPs
point of view of the mass production of plasmonic materials, a strategy with the same grafting layer were studied. A liquid-crystalline com-
based on self-assembly of surface grafted molecules would be more pound with stilbene-comprising molecular architecture (L, Fig. 1) was
interesting.4a Substantial work exploring the impact of the n-alkylthiol designed, based on our previous experience in the assembly of
coating variation on the self-assembly of silver nanospheres has been gold nanoparticles.13 Compound L was equipped with a mercapto-
presented by the group of Pileni.2,5 Also, charged, photoswitchable, functionalized 15-carbon-length alkyl chain that provides flexibility
and DNA-based ligands were used to prepare ordered aggregates,6 but and allows covalent bonding of the ligand to the nanoparticle surface.
surprisingly only hcp, bcc, fcc structures made of spherical silver A terminal oleylalkoxy chain was used to promote lamellar structure
nanoparticles were obtained.5
formation. L molecules form SmC and SmF phases with layer spacing
Lately, nanoparticle surface decoration with thermotropic changing with temperature in the 46–58 Å range (see the ESI†).
liquid-crystalline molecules (LCs), a new strategy for the self-assembly
of nanoparticles, has attracted considerable attention.7 The use of
mesogenic surface ligands was shown to yield a wide spectrum of 1D,
2D liquid-crystalline and 3D crystalline gold NP structures.8 Nematic
and cubic arrangements of surface-plasmon active Au NPs (sizes in
6–11 nm range) covered with mesogenic ligands were reported.9
However, the optical properties10 of such particle assemblies have
been scarcely studied. Successful application of the LC-based strategy
to silver nanoclusters would open the way to new modes of Ag NPs
a Faculty of Chemistry, University of Warsaw, ul. Pasteura 1, 02-093 Warsaw,
Poland. E-mail: wlewandowski@chem.uw.edu.pl
b
´
Laboratoire de Physique des Solides, Universite Paris-Sud, CNRS UMR 8502,
Fig. 1 Scheme of hybrid nanoparticle synthesis (using the example of Ag NPs);
silver nanoparticles (A6), liquid crystalline ligand (L) and hybrid nanoparticles
(A6L); sizes are not to scale.
Orsay, France
† Electronic supplementary information (ESI) available: Synthetic procedures and
analytical data. See DOI: 10.1039/c3cc43166e
c
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In our studies, we pursued a two-step approach (Fig. 1) for
the synthesis of grafted nanoparticles. First, nanoclusters of
gold and silver, covered with n-alkyl thiol molecules, were
obtained. Then, a ligand exchange reaction was performed to
equip the organic coronas of NPs with the mesogenic ligands L.
For the preparation of silver nanoparticles a modified literature
method was used,14 yielding Ag NPs covered with hexylthiol
(denoted as A6). An X-ray photoelectron spectroscopy (XPS) survey
analysis of A6 revealed that silver, carbon and sulfur atoms are
present in the sample (see the ESI†) confirming successful applica-
tion of the modified synthetic method. Small angle X-ray scattering
(SAXS) and transmission electron microscopy (TEM) studies of A6
Fig. 3 SAXRD patterns with main signals indexed to (a) A6L (at 30 1C) (b) G8L
(at 150 1C); integrated profiles are given in the ESI.†
estimated the NP diameter to be 4.4 Æ 0.3 nm. Gold nanoparticles the shearing direction, as apparently the torque is smaller for such
were prepared according to a slightly modified Brust–Schiffrin an orientation of nanocluster layers. The gap between the metallic
procedure affording 2.4 Æ 0.3 nm diameter Au NPs (denoted as G8) layers, B8.7 nm, is filled mainly with mesogenic molecules. Within
covered with C8H17SH (see the ESI†).15 In the second step, we the layer, the metal cores have short-range positional order, as
introduced the liquid-crystalline ligand L to the surface of A6 and evidenced by the diffused X-ray signal, with an average interparticle
G8 (Fig. S1, ESI†) yielding A6L and G8L hybrid NPs, respectively, distance only slightly higher than that observed for A6. This suggests
without affecting the metal core diameter.
only a small contribution of L molecules to the in-plane distance
To evaluate the exact composition of organic coronas of the between NPs. Scans at different temperatures were collected for the
studied nanoparticles, thermogravimetric analysis (TGA) and ordered A6L sample, but no structural changes were observed until
XPS were used (see the ESI†). The numbers of alkyl (Nalkyl) and the decomposition of the sample at B180 1C.
mesogenic (NL) molecules on the surface of a single NP are given
For the material G8L we have obtained X-ray patterns with Bragg
in Table 1. The TEM pictures (Fig 2) confirmed different arrange- peaks corresponding to the layer thickness slightly split in the
ments of NPs in the samples before and after ligand exchange. azimuthal direction (Fig. 3). Such patterns are typical for the
The self-assembly properties of hybrid nanoparticles were inves- modulated lamellar phase.7b For G8L hybrids the unit cell para-
tigated by SAXRD measurements. To obtain homogeneous mono- meters were temperature-dependent. At 90 1C the interlayer distance
domains, dropcast films of A6L and G8L were sheared at 150 1C and was B11.6 nm, the modulation period along the layers B29.4 nm
subsequently cooled to room temperature. The XRD patterns of A6L and the in-layer distance between NPs B4.3 nm. At 170 1C those
showed a series of commensurate, sharp Bragg peaks along the values were B11.2, 42.3 and 5.5 nm, respectively, but noticeably the
direction perpendicular to shearing, corresponding to B13.1 nm period of modulation along layers corresponds to B7–8 in-plane
periodicity and a diffused reflection in the direction parallel to nanoparticle distances for all temperatures.
shearing, centred at B6.6 nm (Fig. 3). The pattern is characteristic
In contrast to previous studies, we did not observe melting of the
of a lamellar structure in which layers made of metal cores are superstructures upon raising the temperature; both the G8L and A6L
separated by organic sublayers.7 The layer normal is perpendicular to materials decompose before phase transition to the isotropic liquid
takes place. The difference in ordering of G8L and A6L samples is in
line with our previous research13 in which we have shown that the
Table 1 The average number of ligands per nanoparticle
larger is the ratio of the mesogenic ligand length to the nanoparticle
diameter, the more ordered structures are formed.
NP
Nalkyl
NL
%L
To investigate orientational order of the mesogenic ligands
placed between the metallic layers we have performed polarized
optical microscopy (POM) studies. The annealed samples were
optically birefringent (see the ESI†), showing that the mesogens in
the organic sublayer are orientationally ordered. In contrast, the G8L
samples were not birefringent; it should be noted that many
previously studied lamellar structures made of hybrid Au NPs were
also optically isotropic.7 Thus, for gold nanoparticles the mesogenic
units in the organic sublayer are not orientationally ordered, while in
A6L particle aggregates the orientational order is present. Further
details of L molecules ordering in the aligned A6L material were
derived from polarized IR studies, which have shown an anisotropy
of the absorption bands related to the phenyl ring stretching
(at 1600 cmÀ1) and stretching of alkyl CH2 groups (2850 and
2920 cmÀ1). The absorption for the phenyl stretching is strongest
in the direction perpendicular to the layer normal, while absorption
maximum for the CH2 group is observed in the direction parallel to
the layer normal (Fig. 4, see the ESI† for details). This clearly
A6
G8
A6L
G8L
410
140
190
70
—
—
260
80
—
—
58
52
Fig. 2 TEM images of silver nanoparticles: (a) A6, (b) A6L after thermal treatment;
6 NPs exhibit 2D hexagonal packing with an average distance of B5.2 nm, while
A
A6L NPs show packing with a characteristic distance of B13 nm – red lines are
used to show the tendency of NPs towards packing into lamellar structures.
c
7846 Chem. Commun., 2013, 49, 7845--7847
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Noticeably, this is the first report on spherical Ag nanoparticle
superlattice formation having symmetry other than hcp, fcc or
bcc without the use of external matrices. Moreover, in contrast
to the regular smectics made of purely organic materials,
for the hybrid materials studied here, mesogenic units are
perpendicular to the layer normal, being aligned parallel to
the planes formed by metal spheres. The assembly strategy
presented here resulted in tunability of the plasmon resonance
wavelength of the ordered silver nanoparticle aggregate when
polarization of the incident light was changed. Future prospects
envisage the design of reconfigurable aggregates, shown to be
available within the LC-based approach, with temperature-varied
optical properties.
Fig. 4 (a) Intensity of the IR absorption band at 1600 cmÀ1 vs. angle between
the polarization plane of the incident IR beam and the shearing direction. (b)
Schematic drawing of the A6L particles assembled into a lamellar mesophase
based on SAXRD and IR measurements.
This work was partially supported by the Polish National
Science Center (Project 2011/01/N/ST5/03322). Authors wish to
acknowledge the TEAM program from Foundation for Polish
Science (Project TEAM/2010-5/4).
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´
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Chem. Commun., 2013, 49, 7845--7847 7847
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