Smectic-layer alignments of surface-modified gold nanoparticles in the nanocomposite induced by a hydrogen-bonded bent-core liquid crystalline host under electric fields

Article abstract of DOI:10.1002/chem.201102607

Packing tips: The layer spacing of 5.5 nm (see TEM image) of the nanocomposite VPy-SiA/AuNPs-S (5 wt %) under DC/AC electric fields perfectly matches the d spacing of 5.5 nm obtained by in situ XRD measurements under a DC electric field. TEM images revealed that the well-organized packing of layers of surface-modified gold NPs could be induced in the nanocomposite under electric fields. Copyright

Full text of DOI:10.1002/chem.201102607

DOI: 10.1002/chem.201102607
Smectic-Layer Alignments of Surface-Modified Gold Nanoparticles in the
Nanocomposite Induced by a Hydrogen-Bonded Bent-Core Liquid
Crystalline Host under Electric Fields
Wei-Hong Chen,[a] Yi-Ting Chang,[a] Jey-Jau Lee,[b] Wei-Tsung Chuang,[b] and
Hong-Cheu Lin*[a]
There has been great interest
recently in the fabrication of
highly ordered metal nanoparti-
cle (NP) assemblies.[1] Large
scale periodical arrangements
of NPs are fascinating systems
for the development of nano-
structural materials exhibiting
new electronic,[2] magnetic,[3]
optical,[4] and photonic proper-
ties.[5] Many examples of the
application of templates have
appeared for the generation of
ordered assemblies of NPs, by
using, for example, surface-
modified polymers,[6] self-as-
sembled monolayers,[7] lipid
films,[8] and DNA.[9] In addition,
the self-organization behavior
of gold NPs surface-modified
with rod-like,[10] bent-core,[11]
and discotic[12] liquid crystal
Scheme 1. Chemical structures of the hydrogen-bonded bent-core LC host VPy-SiA (containing the hydrogen
(LC) surfactants has been re-
ported. Moreover, several strat-
egies have been proposed to
template arrays of NPs by using
bond acceptor VPy and the hydrogen bond donor SiA) and the surface-modified gold NPs AuNPs-S present-
ing the covalently bonded bent-core surfactant S.
block copolymer systems, in which the spatial arrangements
are directed through self-assembly of the block copoly-
mers.[13] LC materials possessing a variety of mesomorphic
structures can be aligned in a straightforward manner
through the application of electric fields,[14] magnetic
fields,[15] and polymer networks.[16] The self-organization of
LC molecules is determined by the characteristic arrange-
ments of the mesophasic types, including the nematic, cho-
lesteric, smectic, and columnar phases.
Herein, we describe the synthesis of the novel hydrogen-
bonded bent-core LC host VPy-SiA (bearing a flexible car-
bosilane chain) and the gold nanocomposite (VPy-SiA/
AuNPs-S) by adding 5 wt% the surface-modified gold NPs
(AuNPs-S) to the self-assembled LC host (VPy-SiA;
Scheme 1). We developed a novel in situ technique for
alignment of the nanocomposite VIPy-SiA/AuNPs-S under
(DC/AC) electric fields, and examined the orientation be-
havior of the smectic layer structures using two-dimensional
X-ray diffraction patterns and frozen transmission electron
microscopy (FTEM). To the best of our knowledge, no pre-
vious reports have described the ordered layer arrangements
of NPs in a nanocomposite—induced by compatibility of the
NPs in the phase of the LC hosts—that could then be fur-
ther aligned under electric fields.
[a] W.-H. Chen, Y.-T. Chang, Prof. H.-C. Lin
Department of Materials Science and Engineering
National Chiao Tung University, Hsinchu 300 (Taiwan)
Fax : (+8863)5724727
E-mail: linhc@cc.nctu.edu.tw
[b] J.-J. Lee, W.-T. Chuang
National Synchrotron Radiation Research Center
Hsinchu, 30076 (Taiwan)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201102607.
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COMMUNICATION
Figure 1. 1H NMR spectra of: a) surfactant S (signals marked a, b, and c correspond to specific protons of the disulfide unit), and b) the gold NPs
AuNPs-S (signals marked aꢁ, bꢁ, and cꢁ represent the corresponding protons after ring opening and attachment of the disulfide unit to the gold NPs).
1
Table 1. Mesomorphic properties (phase transition temperatures, enthal-
pies) of the hydrogen-bonded bent-core complex VPy-SiA and its gold
nanocomposite VPy-SiA/AuNPs-S incorporating 5 wt% surface-modified
gold NPs (AuNPs-S).[a]
Figure 1 presents the H NMR spectra of the covalently
bonded bent-core surfactant S and the surface modified gold
1
NPs, AuNPs-S. The broad signals in the H NMR spectrum
of AuNPs-S were caused by the dipolar spin relaxation of
the surfactant S units on the surface of the gold NPs.[17] The
up-field shift and broadness of the methine proton (proton
aꢁ) of the attached surfactant S unit were probably due to
restricted ligand rotation in the proximity of the gold sur-
face. Two new signals (protons bꢁ and cꢁ) appeared for the
disulfide units of AuNPs-S after ring opening and attach-
ment of disulfides to the gold NPs. The TEM image in Fig-
ure S1 in the Supporting Information indicates that the sur-
face stabilized gold NPs (AuNPs-S) had an average diame-
ter of (2.5ꢀ0.8) nm. The inflammable part of the ligands
was 48% AuNPs-S and a 9 wt% char yield of S (>8008C;
Figure S2 in the Supporting Information). The number of
Au atoms in one cluster of AuNPs-S was calculated with
Equation (1), where n is the number of atoms per cluster, R
is the NP radius ((2.5ꢀ0.8) nm, TEM), and ng is the molar
volume of Au (ng =10.2 cm3 molꢁ1). The number of ligands
in one cluster of AuNPs-S was estimated by using Equa-
tion (2), where N is the number of ligands per cluster, Aw is
the atomic weight of Au, Mw is the molecular weight of li-
gand S [Eq. (2) is a rearrangement of the equation, wt% of
ligand=(NꢂMw)/(NꢂMw+nꢂAw]:
Complex
Phase transition temperature [8C]
and enthalpy [Jgꢁ1
]
VPy-SiA
VPy-SiA/AuNPs-S (5 wt%)
[a] Iso: isotropic state; Cr: crystalline state; SmCPA: “nonclassical” AF
switching polar smectic phase; SmCPF: FE switching polar smectic phase.
The phase transitions were measured by DSC at the second heating and
cooling scans with a rate of 58Cminꢁ1
.
Table 1 lists the mesomorphic properties (phase transition
temperatures, enthalpies) of the hydrogen-bonded bent-core
complex VPy-SiA and its gold nanocomposite VPy-SiA/
AuNPs-S (5 wt%). Relative to the hydrogen bonded com-
plex VPy-SiA, we observed broader mesophasic ranges for
the nanocomposite VPy-SiA/AuNPs-S (5 wt%), which ex-
hibited a polar smectic (B2 or SmCP) phase[19] during both
the heating and cooling processes (Figure S3 in the Support-
ing Information displays the fan-like textures of these sam-
ples).
The results of the switching current experiments (Fig-
ure 2a and b) at a temperature (T) of 1108C reveal a single
well-resolved repolarization current peak in each half
period of the applied triangular wave at Vpp values of 300
and 150 V (both reaching saturated polarization) for the
complex VPy-SiA and the nanocomposite VPy-SiA/AuNPs-
S (5 wt%), respectively. The modified triangular wave
method can also be used to understand the actual switching
current behavior. To determine the polarity of the SmCP
4pR3
3ng
ð1Þ
ð2Þ
nAu ¼
n ꢂ Aw ꢂ wt % of ligand
Mw ꢂ wt % of Au
N ¼
Accordingly, we could roughly calculate[18] an average of 481
gold atoms and 99 ligand molecules on the surface-modified
gold NPs AuNPs-S by combining the TGA data with the
average particle size obtained from the TEM image.
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H.-C. Lin et al.
Figure 2. Switching current responses at 1108C by using the triangular wave method (100 Hz) for: a) the hydrogen-bonded complex VPy-SiA (Vpp
=
300 V), and b) the nanocomposite VPy-SiA/AuNPs-S (5 wt%; Vpp =150 V); and the modified triangular wave method for: c) VPy-SiA (Vpp =200 V), and
&
d) VPy-SiA/AuNPs-S (5 wt%; Vpp =200 V). e) Values of Ps plotted with respect to the applied voltages for VPy-SiA ( ) and its nanocomposite VPy-SiA/
*
AuNPs-S (5 wt%; ).
phase in the complex VPy-SiA and the composite VPy-SiA/
AuNPs-S (5 wt%), the switching current phenomena of the
modified triangular wave (f=100 Hz) clearly revealed “non-
classical” antiferroelectric (AF) switching[20] with two peaks
for the SmCPAꢁ phase and ferroelectric (F) switching with
one peak for the SmCPF phase. Hence, the complex VPy-
SiA exhibited the SmCPAꢁ phase, as verified by the fact that
the current response was divided into two repolarization
peaks under the modified triangular wave at a single pulsed
triangular wave with a Vpp value of 200 V (Figure 2c pro-
vides one example).
In contrast, Figure 2d reveals that only one repolarization
peak has the SmCPF phase under the modified triangular
wave when supplying the nanocomposite VPy-SiA/AuNPs-S
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Smectic-Layer Alignments of Surface-Modified Gold NPs
COMMUNICATION
(5 wt%) with a single pulsed triangular wave with a Vpp
value of 200 V. As illustrated in Figures S4 and S5 in the
Supporting Information, rotations of the circular domains
occurred in the hydrogen-bonded complex VPy-SiA. Fig-
ure 2e displays the Ps values with respect to the applied vol-
tages. The values of saturated Ps of the SmCP phases
(SmCPF and SmCPA) were all reached at specific saturated
voltages (Vs, where it reached the saturated Ps), with the
values of saturated Ps and Vs both correlated to the content
of AuNPs-S. The value of saturated Ps decreased from 410
to 190 nCcmꢁ2 and the value of Vs decreased from approxi-
mately 180 to 70 V upon increasing the gold NP concentra-
tion from 0 to 5 wt%. Therefore, doping with the gold NPs
decreased the value of Ps of the nanocomposite VPy-SiA/
AuNPs-S as a result of the dilution (and/or dispersion)
effect of the polar arrangement, but the surface modified
gold NPs also reduced the operating voltage of the device.
To understand molecular stacking behavior, alignment
techniques can be performed while applying external elec-
tric or magnetic fields. Here, we recorded in situ electric
field XRD and FTEM data by using the same apparatus
(Figure S6 in the Supporting Information). Figure S7 in the
Supporting Information presents 1D XRD profiles of the
hydrogen bonded complex VPy-SiA and the nanocomposite
VPy-SiA/AuNPs-S (5 wt%). Upon increasing the content of
AuNPs-S from 0 to 5 wt%, the value of d1 increased propor-
tionally from 5.34 to 5.5 nm, due to insertion of the surface-
modified AuNPs-S into the smectic layer of VPy-SiA in the
nanocomposite VPy-SiA/AuNPs-S.
Furthermore, the hydrogen-bonded complex and all of
the nanocomposite exhibited comparable wide-angle diffuse
halos, corresponding to d spacings of 0.44 nm; this indicates
that similar liquid-like, in-plane orders with analogous aver-
age intermolecular distances were prevalent within the
smectic layers of these studied compounds. To evaluate the
detailed orientational relationship between the layer struc-
tures (small-angle region) and the intermolecular packing
(wide-angle region), we recorded the 2D diffraction patterns
(at 1108C during the cooling process) of the complex VPy-
SiA and the nanocomposite VPy-SiA/AuNPs-S (5 wt%), re-
spectively. Based on the results of the small-angle arcs (Fig-
ure 3a and c), we deduced that the smectic layer normal
(small-angle region), which is also parallel to the equator,
was perpendicular to the direction of the electric fields. Fig-
ure 3b and d reveal that the direction of the smectic layer
was aligned along the equator direction, with a highly or-
dered smectic structure.
We then used FTEM to evaluate the distribution of the
gold NPs (AuNPs-S) and their self-assembly behavior under
DC/AC electric fields in the nanocomposite VPy-SiA/
AuNPs-S. The electrode setup (Figure S6 in the Supporting
Information) for these FTEM experiments was the same as
that used for the in situ XRD diffraction experiments.
Figure 4 displays FTEM images of the quenched LC struc-
tures of the nanocomposite VPy-SiA/AuNPs-S (5 wt%),
which had been quenched through rapid immersion of the
LC phase in liquid N2 and removed from the electrodes (in
Figure 3. In situ XRD measurements under DC electric fields (200 V,
0.33 Vmmꢁ1) of: a) the hydrogen-bonded complex VPy-SiA, b) its expan-
sion in the layer region at 1108C, c) the nanocomposite VPy-SiA/AuNPs-
S (5 wt%; arrangement under a DC electric field of 200 V, 0.33 Vmmꢁ1),
and d) its expansion in the layer region at 1108C (arrangement in the ab-
sence of an electric field).
the presence and absence of DC/AC electric fields). The
gold NPs (AuNPs-S) were randomly distributed in the nano-
composite VPy-SiA/AuNPs-S (5 wt%) in the absence of an
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H.-C. Lin et al.
the AuNPs-S was generally aligned into layers (layer spac-
ing: 5.5 nm), in which the slice direction of TEM was paral-
lel to the electric field.
This TEM result perfectly matches the d spacing of
5.5 nm obtained through in situ XRD measurements under
a DC electric field (Figure 3c). However, curved and dis-
torted directions of the layer structure composed of gold
NPs (AuNPs-S; 5 wt%; Figure 4b) would be induced by the
turbulent flow of the continuous DC electric field applied
between electrodes (0.33 Vmmꢁ1). Therefore, to eliminate
the effect of the turbulent flow under DC electric fields, the
nanocomposite VPy-SiA/AuNPs-S (5 wt%) was further ap-
plied by a stronger AC electric field (0.66 Vmmꢁ1, 1 Hz) for
1 h to stabilize the layer structure of AuNPs-S in the nano-
composite. Compared with Figure 4b, under a DC electric
field, the TEM image of a more ordered and large-scaled
layer structure (with a d spacing of 5.5 nm) of the nanocom-
posite VPy-SiA/AuNPs-S (5 wt%) was obtained under an
AC electric field (Figure 4c). However, as illustrated in Fig-
ure S8 in the Supporting Information, the layer structure in
the nanocomposite VPy-SiA/AuNPs-S (1 wt%) with a lower
wt% of gold NPs (AuNPs-S) under the same AC electric
field (i.e., 0.66 Vmmꢁ1, 1 Hz) was not well aligned as that in
the nanocomposite VPy-SiA/AuNPs-S (5 wt%). Figure 4d
presents a possible cartoon model of the nanocomposite ar-
rangements in the presence of the electric fields. As a conse-
quence, the direct evidence from these TEM images reveals
that well-organized packing of surface-modified gold NPs
can be induced by the bent-core LC host in the nanocompo-
site subjected to DC/AC electric fields.
In summary, we have prepared a gold nanocomposite
(VPy-SiA/AuNPs-S) incorporating 5 wt% surface-modified
gold NPs (AuNPs-S) doped within a hydrogen-bonded bent-
core LC complex VPy-SiA. Doping VPy-SiA with an appro-
priate amount of decorated AuNPs-S led to a broader meso-
phasic range and lower transition temperatures for the
nanocomposite VPy-SiA/AuNPs-S. The altered molecular
packing induced by raising the concentration of gold NPs
(AuNPs-S) in the SmCP phase resulted in the AF switching
behavior of the complex VPy-SiA being changed to the F
switching behavior of the nanocomposite VPy-SiA/AuNPs-S
(5 wt%). Using our technique for alignment of the bent-
core molecules under (DC/AC) electric fields, we could con-
trol, in a straightforward manner, the orientations of the
molecular stacks (bent-core apexes were aligned along the
electric fields) and layer arrangements (layer normals were
aligned perpendicular to the electric fields) of the LC mole-
cules (induced by intra- and intermolecular interactions), as
confirmed from associated X-ray diffraction patterns. TEM
images revealed directly that well-organized packing of
layers of surface-modified gold NPs could be induced in the
nanocomposite under electric fields, due to the interaction
and alignment of the bent-core LC host molecules.
Figure 4. TEM morphologies of the nanocomposite VPy-SiA/AuNPs-S
(5 wt%) upon quenching from the LC phase: a) in the absence of a DC
electric field, b) under a DC electric field of 200 V (0.33 Vmmꢁ1), and
c) under an AC electric field of 400 V, f=1 Hz (0.66 Vmmꢁ1, f=1 Hz).
d) Cartoon model of the nanocompositeꢁs arrangement under electric
fields.
electric field (Figure 4a). As shown in Figure 4b, the TEM
image of the nanocomposite VPy-SiA/AuNPs-S (5 wt%)
prepared in the presence of a DC electric field revealed that
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Smectic-Layer Alignments of Surface-Modified Gold NPs
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Synthesis and sample preparation: The detailed synthesis procedures of
the receptor monomer (M1) are described in the Supporting Information.
The synthesis procedures are shown in Scheme S1–S4 in the Supporting
Information. Hydrosilylation of the compounds was mediated by Kar-
stedtꢁs catalyst to provide the corresponding siloxane-substituted materi-
als. All compounds were purified through column chromatography and
recrystallization; all synthesis details are described in the Supporting In-
formation. The hydrogen-bonded complex VPy-SiA (bent-core LC host)
was prepared by dissolving the hydrogen bond acceptor VPy and the hy-
drogen bond donor SiA (1:1 molar ratio) in anhydrous THF, evaporating
the solvent gradually at 408C, and then drying it under vacuum, over-
night. By using the same procedure, the hydrogen-bonded nanocomposite
VPy-SiA/AuNPs-S was prepared from 1 wt%, 5 wt% of AuNPs-S in the
LC host VPy-SiA.
Measurements and characterization: 1H NMR spectra were recorded by
using a Varian Unity 300 MHz spectrometer with [D6]DMSO and CDCl3
as solvents. Mass spectra were recorded by using a Micromass TRIO-
2000 GC/MS instrument. Elemental analyses were performed by using a
Heraeus CHN-OS RAPID elemental analyzer. Mesophasic textures were
characterized by using a Leica DMLP polarizing optical microscope
equipped with a hot stage. FTIR spectra were recorded by using a
Perkin–Elmer Spectrum 100 spectrometer. A Linkam TSTE350 stage, an
MDS 600 and CI 94 controller, and Linksys 32 temperature/motor con-
trol software were used as the temperature controlling system. The phase
transition temperatures and corresponding enthalpies were determined
through differential scanning calorimetry (DSC, model: Perkin–Elmer
Pyris 7) under N2 at heating and cooling rates of 58Cminꢁ1. Thermogravi-
metric analysis (TGA) was performed by using a TA-TGA Q-500 instru-
ment (Thermal Analysis) operated at a heating rate of 108Cminꢁ1 under
an N2 atmosphere. TEM was conducted by using JEOL JEM-2011 and
Philips Tecnai G2F20 microscopes operated at 200 kV. The samples were
cryosectioned by using a Leica Reichert Ultracut E ultramicromote, and
the section thicknesses of 30–70 nm were collected on 400-mesh gold
grids. In situ electric wide-angle X-ray scattering (WAXS) was performed
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the National Synchrotron Radiation Research Center (NSRRC), Taiwan.
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Acknowledgements
We thank the financial support from the National Science Council of
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Keywords: nanocomposites · nanomaterials · nanoparticles ·
nanostructures · self-assembly
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Received: July 13, 2011
Published online: October 18, 2011
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