Self-organization of plasmonic and excitonic nanoparticles into resonant chiral supraparticle assemblies

Article abstract of DOI:10.1021/nl502237f

Chiral nanostructures exhibit strong coupling to the spin angular momentum of incident photons. The integration of metal nanostructures with semiconductor nanoparticles (NPs) to form hybrid plasmon-exciton nanoscale assemblies can potentially lead to plasmon-induced optical activity and unusual chiroptical properties of plasmon-exciton states. Here we investigate such effects in supraparticles (SPs) spontaneously formed from gold nanorods (NRs) and chiral CdTe NPs. The geometry of this new type of self-limited nanoscale superstructures depends on the molar ratio between NRs and NPs. NR dimers surrounded by CdTe NPs were obtained for the ratio NR/NP = 1:15, whereas increasing the NP content to a ratio of NR/NP = 1:180 leads to single NRs in a shell of NPs. The SPs based on NR dimers exhibit strong optical rotatory activity associated in large part with their twisted scissor-like geometry. The preference for a specific nanoscale enantiomer is attributed to the chiral interactions between CdTe NP in the shell. The SPs based on single NRs also yield surprising chiroptical activity at the frequency of the longitudinal mode of NRs. Numerical simulations reveal that the origin of this chiroptical band is the cross talk between the longitudinal and the transverse plasmon modes, which makes both of them coupled with the NP excitonic state. The chiral SP NR-NP assemblies combine the optical properties of excitons and plasmons that are essential for chiral sensing, chiroptical memory, and chiral catalysis.

Full text of DOI:10.1021/nl502237f

Letter
pubs.acs.org/NanoLett
Self-Organization of Plasmonic and Excitonic Nanoparticles into
Resonant Chiral Supraparticle Assemblies
†
,‡
§
†,∥
⊥
†,∥
†,∥
Tao Hu, Benjamin P. Isaacoff, Joong Hwan Bahng, Changlong Hao, Yunlong Zhou, Jian Zhu,
†
,‡
‡
‡
⊥
,○
Xinyu Li, Zhenlong Wang, Shaoqin Liu, Chuanlai Xu, Julie S. Biteen,*
,
†,‡,∥,#
and Nicholas A. Kotov*
†
§
∥
#
Department of Chemical Engineering, Applied Physics Program, Biointerfaces Institute, Department of Materials Science and
Engineering, and ○Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109, United States
‡Key Laboratory of Microsystems, Micronanostructures Manufacturing, Ministry of Education, Harbin Institute of Technology,
Harbin 150080, P. R. China
⊥
State Key Lab of Food Safety and Technology, School of Science and Technology, Jiangnan University, Wuxi, JiangSu, 214122, P. R.
China
*
S Supporting Information
ABSTRACT: Chiral nanostructures exhibit strong coupling to
the spin angular momentum of incident photons. The
integration of metal nanostructures with semiconductor
nanoparticles (NPs) to form hybrid plasmon−exciton nano-
scale assemblies can potentially lead to plasmon-induced
optical activity and unusual chiroptical properties of plasmon−
exciton states. Here we investigate such effects in supra-
particles (SPs) spontaneously formed from gold nanorods
(
NRs) and chiral CdTe NPs. The geometry of this new type of
self-limited nanoscale superstructures depends on the molar ratio between NRs and NPs. NR dimers surrounded by CdTe NPs
were obtained for the ratio NR/NP = 1:15, whereas increasing the NP content to a ratio of NR/NP = 1:180 leads to single NRs
in a shell of NPs. The SPs based on NR dimers exhibit strong optical rotatory activity associated in large part with their twisted
scissor-like geometry. The preference for a specific nanoscale enantiomer is attributed to the chiral interactions between CdTe
NP in the shell. The SPs based on single NRs also yield surprising chiroptical activity at the frequency of the longitudinal mode
of NRs. Numerical simulations reveal that the origin of this chiroptical band is the cross talk between the longitudinal and the
transverse plasmon modes, which makes both of them coupled with the NP excitonic state. The chiral SP NR−NP assemblies
combine the optical properties of excitons and plasmons that are essential for chiral sensing, chiroptical memory, and chiral
catalysis.
KEYWORDS: Chiral assemblies, supraparticles, nanoscale chirality, hybrid electronic states, polarization rotation, circular dichroism
anoscale assemblies comprising both semiconductor and
metal nanoparticles (NPs) have attracted a great deal of
of chiral isomers and differential interactions with polarized
light are primary examples of distinct chemical and physical
properties related to chirality. Nanoscale particles, films, and
assemblies reported recently exhibit the strongest known
polarization rotation of visible electromagnetic radiation due
N
attention over the past decade because of the possibility of
plasmon−exciton hybridization. A variety of synthetic techni-
ques have been used to prepare such NP assemblies taking
1
−4
advantage of chemical or biological conjugation, electrostatic
to efficient coupling with the spin angular momentum of
5
−7
8,9
10−12
19−21
attraction,
templates, and solvophobic interactions.
incident photons.
The potential applications of chiral NPs
22,23
The hybridized electronic states characteristic of both types of
nanoscale materials in these diverse metal−semiconductor
assembles displayed unusual optical, catalytic, and magnetic
and their assemblies include biosensing,
enantioselective
24
25
26
separations, catalysis, memory devices, and polarized
27
light-emitting diodes. Most often, the chirality of these NP
assemblies has a biological origin and is derived from the helical
geometry of the DNA- or polymer-based “scaffolds” to which
1
3−18
properties
cance.
with both fundamental and practical signifi-
2
8,29
the NPs are attached.
Chiral geometries of NP assemblies
Concomitantly, there has been a rapid increase in the
number of studies related to the preparation and character-
ization of chiral assemblies of semiconductor and metallic NPs.
Chiralitya scale-less geometric attributetranslates into a
number of useful functional properties of molecules, individual
NPs, and their assemblies. Among them, molecular recognition
can also be obtained using careful ab ovo design of NPs
Received: June 14, 2014
Revised: October 3, 2014
©
XXXX American Chemical Society
A
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Figure 1. (A) UV−vis−NIR absorption (solid line) and photoluminescence (dashed line) spectra (λ = 380 nm) of the original chiral CdTe NPs
ex
and gold NRs; (B) CD spectra of the original chiral CdTe NPs and gold NRs; (C) UV−vis−NIR absorption (solid line) and photoluminescence
(
dashed line) spectra (λ = 380 nm) of the original NRs and chiral SP assemblies D-SP1 and L-SP1; (D) UV−vis−NIR absorption (solid line) and
ex
PL (dashed line) spectra (λ = 380 nm) of the original NRs and chiral supraparticle assemblies D-SP3 and L-SP3; (E) CD spectra of the chiral SP
ex
assemblies D-SP1 (red) and L-SP1 (blue); the inset is a schematic representation of the proposed structure of SP1; (F) CD spectra of the chiral SP
assemblies D-SP3 (red) and L-SP3 (blue); the inset is a schematic representation of the proposed structure of SP3.
assemblies by arranging NPs in space following a particular
From this perspective, self-assembling supraparticles (SPs)
offer fruitful grounds for the studies of chiral nanomaterials for
multiple reasons. First, their size typically falls in the 100−200
nm range, which is convenient for effective coupling with visible
three-dimensional (3D) pattern exemplified by tetrahe-
3
0−32
33,34
22,23,35,36
dral,
helical,
and twisted-rod patterns.
Expansion of the toolbox of NP assemblies toward simpler
47
light. Second, the assembly process of terminal SPs is simple
and versatile. It takes advantage of generic electrostatic and van
der Waals attractive forces and can be applied both to metallic
and more versatile synthetic techniques will increase the utility
3
7,38
of chiral plasmonic, excitonic, and hybridized
systems. With
the exception of two recently published studies on systems
47
31,39
and semiconductor nanocolloids. Third, the spatial proximity
synthesized by bioconjugation,
little is known about the
of the constituent NPs in SPs promotes effective resonance
chiroptical effects of combining semiconductor and metal NPs
into a chiral superstructure. Observations from organic dyes
have suggested the enhancement of chiral anisotropy results
from interactions between polarized light and chiral super-
3
,48−51
coupling between different electronic levels
versatile platform for plexcitonics
offering a
52−55
and quantum plas-
monics. Fourth, the shape of self-assembled SPs is not limited
to spheres and can potentially result in asymmetric assemblies
leading to chiral nanoscale superstructures.
Thus, we hypothesized that the preparation of chiral
exciton−plasmon superstructures could be accomplished
using SP assembly from chiral building blocks. Indeed, we
made SPs based on achiral gold nanorods (NRs) and chiral
CdTe NPs stabilized by L- and D-cysteine (CYS). Besides
proving the original hypothesis, we uncovered several
unexpected structural and optical effects in these structures.
4
0,41
structures in a plasmonic field.
The plasmonic field in these
cases were generated by a single metal NP even though the
strongest enhancements typically occur in gaps between
4
2
plasmonic NPs. Considering the similarity of the molecular
origin of chiral states in semiconductor NPs and organic
4
3
molecules, it may be possible to observe plasmonic
enhancement of optical activity of NPs experimentally and
2
5,44−46
computationally.
B
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Figure 2. TEM images of the chiral SP assemblies SP1. (A) TEM image of D-SP1; (B) HRTEM image of D-SP1; (C) TEM image of L-SP1; (D)
HRTEM image of L-SP1; (E) TEM tomography image for D-SP1. The presence of the most dense part of the CdTe coat is visible in the lower NR;
(
F) TEM tomography image for L-SP1. Visualization of the CdTe NP coat that lower e-beam contrast than gold, depends on the tomography and
image rendering settings. The red circles in B and D indicate the positions of obvious CdTe NPs. Additional video files with 3D representation of the
twisted pairs are given in the Supporting Information.
We observed two distinct modes of SP assemblieswith the
core made from either one or two NRs surrounded by CdTe
NPsdepending on the assembly conditions. SPs with a
double-NR core acquire a geometry of twisted rods. The mirror
image of this nanoscale superstructure is nonsuperimposable
with the original and leads considerable enhancement of
chiroptical activity. Unlike previous cases of similar scissor-like
by the existence of significant “cross-talk” between the
longitudinal mode and transversal plasmonic modes of gold
NRs and their hybridization with chiral excitonic states in
surrounding CdTe NPs.
Results and Discussion. Assembly of Gold NRs and
Chiral CdTe NPs. Gold NRs and CdTe NPs were chosen as SP
building blocks because their optical properties include strong
absorption and emission peaks and energy overlap between
excitonic and plasmonic states. Chiral CdTe NPs were prepared
2
2,23,36
56
NR superstructures based on DNA
or helical fibers,
self-assembly of such SPs is attributed to chiral intermolecular
interactions of constitutive CdTe NPs forming the shell around
the NRs that were recently shown to serve as structure-
43
in two enantiomeric forms using established methods,
denoted here as L-NP and D-NP, using L- or D-cysteine
(CYS) stabilizers, respectively. Figure 1A shows typical
absorption and photoluminescence (PL) spectra of L-NP and
D-NP; the sharp absorbance and PL peaks indicate the relatively
narrow size distribution of the as-prepared NP samples. As
expected, mirror image peaks are observed in the CD spectra
57
determining factor in assemblies of similar CdTe NPs.
A
description of the chiroptical properties of single- and double-
NR assemblies using electrodynamic simulations reveals
complexity of plasmon−exciton interactions in the metal−
semiconductor system. In the present study, this is exemplified
C
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Figure 3. TEM images of the chiral SP assemblies SP3. (A) TEM image of D-SP3; (B) HRTEM image of D-SP3; (C) TEM image of L-SP3; (D)
HRTEM image of L-SP3.
for L-NP and D-NP (Figure 1B), though the UV−vis−NIR
spectra are nearly identical (Figure 1A). Gold NRs were
prepared by a seed-mediated surfactant-directed approach. The
NR transverse plasmon peak is at 520 nm, and the longitudinal
plasmon peak is at 720 nm (Figure 1A), which is characteristic
of length−diameter ratio of about 3:1. The TEM images in
Figures 2 and 3 confirm this aspect ratio and show the gold
NRs to be 15 nm in diameter and 45 nm in length.
The formation of SPs was achieved by adding chiral NPs to
NRs in solution. To realize different assembly configurations,
various quantities of chiral NPs were added to 2 mL of NR
solution at room temperature. Three NR−NP molar ratios
were used, 1:15, 1:60, and 1:180, and the resulting assemblies
will be referred to as SP1, SP2, and SP3, respectively, with D-
and L-prefixes indicating SPs assembled from D-NPs and L-NPs,
respectively. After adding NPs to the NR solution, the self-
assembly process was monitored by circular dichroism (CD)
spectroscopy. The CD signal was recorded every 10 min, and
the signal increased with time until the chiral SP assemblies of
NRs and chiral CdTe NPs reached a steady state after 20 min
where the NRs are assembled by, for instance, ionic
interactions, the chiral geometries associated with L-ascorbic
22
acid ligands and their optical shifts might be essential. The
SP3 CD peaks at 520 and 720 nm (Figure 1F) appear at the
same positions as the transverse and longitudinal plasmon
modes of the NRs, respectively (Figure 1A). Importantly, the
CD spectra of L-SP3 and D-SP3 are mirror images of each other
(Figure 1F) as expected for corresponding enantiomers.
In the course of the study, we observed a distinct difference
in spectral features between the CD spectra of SP assemblies
prepared with different molar ratios (Figures 1E and F, Figure
S4A). While only peaks at 520 and 720 nm appear for SP3
(Figure 1F), the CD peaks for SP1 are much stronger and
appear at 520, 678, and 801 nm. The CD spectra of SP2, which
was made with an intermediate NR−NP ratio, reveal
similarities with both SP1 and SP3 although revealing its own
chiroptical features. All of these facts indicate that the origin of
chiroptical bands in SP1 and SP3 are different and should be
associated with different nanoscale geometries.
We also calculated the anisotropic factor (g-factor) values for
the relative amounts of NRs and NPs (Figure S2). The g-factors
are defined as Δε/ε where Δε = ε − ε and ε = ε + ε ,
(Figure S1). Subsequently, all samples characterized by CD,
UV−vis−NIR, PL, and TEM measurements were prepared
with a 20 min incubation following the initial mixing of the NPs
and NRs.
L
R
L
R
respectively, and ε is the molar extinction coefficient using
58
nonpolarized light. The range of g-factors for SP1 are from
1.45 × 10− to −1.54 × 10 , almost 15 times greater than
3
−3
While the CYS-capped D-NPs and L-NPs are optically active
below ∼510 nm (Figure 1B), the SP3 assemblies instead show
two pronounced CD peaks at 520 and 720 nm (Figure 1F). It
should be noted that, in Figure 1B, the small peak in the
original NR CD spectrum at around 750 nm is associated with
the likely presence of L-ascorbic acid ligands on the NRs before
SP assembly; we do not consider this peak in our interpretation
of the CD spectra of the SPs, although in some other systems
those of SP3, which exhibited g-factors ranging from 0.09 ×
10 to −0.09 × 10 .
−3
−3
Optical and Structural Characterization. To better under-
stand the structure of different NR−NP ratios, the SP
assemblies were examined by UV−vis−NIR spectroscopy,
photoluminescence (PL) spectroscopy, dynamic light scattering
(DLS), zeta-potential characterization, and transmission
D
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electron microscopy (TEM). The representative UV−vis−NIR
spectra demonstrate clear differences between SP1 and SP3
remains unchanged for SP3 (Figure S3G,H). The bigger size
peak, which is attributed to the NR length, was not changed
markedly from the bare NRs to SP1 or SP3. Taken together,
this indicates that SP1 is composed predominantly of SBS NR
dimers while SP3 is composed mainly of monomeric NRs. For
both SP1 and SP3, a third, even larger DLS size distribution
peak appears, indicating the presence of larger NR aggregates in
the mixture. This peak is particularly intense for SP2 (Figure
S3G and S3H), indicating that a significant number of NRs are
in large agglomerates. At the same time, the charge of the SP1
is decreased from that of the original NRs, presumably by the
adsorbed, negatively charged NPs, though SP1 is still positive
(Table 1). Meanwhile, the charge of the SP3 assemblies is
(Figure 1C and D, respectively). Relative to the longitudinal
plasmon absorption band of bare NRs at 725 nm, the SP1 UV−
vis−NIR peak is decreased and blue-shifted (Figure 1C). Such a
change is characteristic of side-by-side (SBS) assemblies of
6
,59,60
NRs.
On the other hand, the UV−vis−NIR spectrum of
SP3 shows a similar decrease but a red-shift of the longitudinal
plasmon absorption band (Figure 1D).
The nanoscale SP assemblies were investigated by TEM,
which revealed that, for SP1, the majority of NRs do indeed
assemble into SBS dimers (Figure 2) when the chiral CdTe
NPs are electrostatically attracted to two NRs at the same time.
This is consistent for both D-SP1 and L-SP1 (Figures 2A and C,
respectively). In these TEM images, we found a large number
Table 1. Electrokinetic Zeta-Potential Values for NRs, NPs,
and Their Assemblies
(70%) of NRs in dimers surrounded by NPs, that agrees with
the blue-shifted UV−vis−NIR spectra of SP1; some SPs based
on single NR cores were observed as well. Though TEM was
performed on dried samples, here we found that most of the
NR dimers self-assemble in SP1 solution and drying effects
contribute little to the formation of NR dimers. High-
resolution TEM (HRTEM) further confirmed that the SP
assemblies of NR dimers were induced by the chiral NPs
original
NPs
NPs at
pH 7
enantiomer
NRs
SP1
SP2
SP3
levorotatory
+55.3
+55.3
−21.6
−24.5
−12.4
−13.6
30.9
30.7
−26.3
−29.2
−45.3
−45.4
dextrorotatory
61
negative (Table 1), which means that more negatively charged
NPs surround each NR in SP3 relative to SP1. For SP2, the size
distribution peaks became much broader (Figure S3E,F), in
accordance with the large aggregates in the TEM image (Figure
S5). Although the assembly size was large, the charge on SP2
was negative and smaller than that of SP3 (Table 1), which fit
with our experiment observation that the SP2 was not stable
and precipitated after 6 h.
(Figure 2B and D). Several hundreds of NPs are adsorbed
around the NR dimers. In a mechanism similar to previous
observations of NR dimers formed in the presence of sodium
5
9
citrate, here NR assembly results from electrostatic attraction
between positively charged NR surfaces and negatively charged
chiral NP surfaces. The separated peak in the CD spectrum of
SP1 (Figure 1E) supports the existence of dimers since exciton
theory predicts that excited-state levels will split into two levels
Assembly Mechanism of Hybrid Metal−Demiconductor
SPs. Consistent with the previously proposed SP assembly
(
one symmetric and one antisymmetric) upon dimerization.
On the basis of TEM, the structure of SP3 is different from
64
mechanism, the NRs have a net positively charged surface due
+
that of SP1 (Figure 3). In particular, SP3 does not contain NR
dimers, other SBS assembled structures (e.g., ladders), or any
to strongly adsorbed CTA ions along the side surface, and at
the pH of almost 7, the chiral CYS-capped CdTe NPs have a
negative surface charges as confirmed by zeta-potential
measurements (Table 1).
For higher NR−NP ratios (e.g., 1:15 for SP1), the presence
of NPs induces SBS assembly of NRs, and the negatively
charged chiral NPs adsorb along the side surfaces of the NRs,
leading to negatively charged NR surface that are electrostati-
cally attracted to positively charged adjacent NRs, resulting in
SP assembly in the geometry of SBS dimers. “Overcharging” of
the surfaces upon adsorption of macromolecular species is well-
6
,62
other end-to-end (ETE) assemblies.
Though a number of
NRs were aggregated, the majority of the SP3 assemblies
contain a single NR in the core (Figure 3A and C).
Furthermore, the high-resolution TEM (HRTEM) image
revealed that a large number of NPs surround each NR (Figure
3B and D). This observation of isolated NRs surrounded by
NPs in SP3 indicates that the red-shift of the plasmon
absorption band (Figure 1F) can be attributed to an increased
index of refraction about the NRs and/or to exciton−plasmon
3
,48−51
65
hybridization.
known in the field of layer-by-layer assembly. Being attracted
The TEM images for the SP2 sample reveal complex
agglomerates that can be described as a combination of NR
monomers and NR dimers (Figure S5). Both L-SP2 and D-SP2
gave assemblies similar in organization and geometry.
to both adjacent gold surfaces, the CdTe NPs positioned in the
middle serve as “bridges” between the two NRs in the
assemblies.
At a NR−NP ratio of 1:60 (SP2), a greater number of
negatively charged NPs are adsorbed along the NR side
surfaces. The electrostatic repulsion between the NRs
surrounded by the CdTe NPs begins to increase, as confirmed
by the increasingly negative zeta-potential (Table 1). SPs with
double-NR cores start reassembling into SPs with a single-NR
core conformation (Figure S3E,F), as supported by TEM
images (Figure S5). We also see a wider size distribution of the
aggregates.
Upon further decreasing the NR−NP ratio to 1:180 (SP3), a
large number of NPs adsorb on the side surfaces of each NR,
separating the NPs into monomeric SPs. This agrees well with
the TEM images (Figure 3A and C).
Photoluminescence measurements of the solution phase
provide a characterization of the assemblies complementary to
6
3
UV−vis−NIR. As previously reported, when NPs are
attached to NRs, and NP emission is quenched due to charge
transfer to the metal. Relative to the photoluminescence spectra
of the original NPs (Figure 1A), the SP1 and SP3 assemblies
demonstrate strong emission quenching (Figure 1C and D,
respectively). This quenching is attributed to the great majority
of NPs being adsorbed to NRs.
The solutions were further characterized by dynamic light
scattering (DLS) and electrokinetic zeta-potentials (ζ). For the
original NRs, two DLS peaks are observed (Figures S3A,B).
The smaller size peak is attributed to the NR diameter: this
doubles between bare NRs and SP1 (Figure S3C,D) but
Chiroptical Activity of Different SPs. To better understand
the origin of the optical activity of NR-based SPs (Figure 1E−
E
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Figure 4. (A) CD spectra of the chiral SP assemblies D-SP4 (red) and L-SP4 (blue), (B) CD spectra of the chiral SP assemblies D-SP5 (red) and L-
SP5 (blue), (C) UV−vis−NIR absorption of the original NRs and chiral SP assemblies D-SP4 and L-SP4; (D) UV−vis−NIR absorption of the
original NRs and chiral SP assemblies D-SP5 and L-SP5.
F), new SPs were prepared from the same L- and D-NPs, but
with shorter NRs (aspect ratio 2:1). The transverse plasmon
peak of these shorter NRs is located at 520 nm, and the
longitudinal plasmon peak is at 660 nm (Figure 4C). Two
different NR−NP molar ratios were used, 1:10 and 1:120, and
the resulting assemblies will be referred to as SP4 and SP5,
respectively. Similar to SP1, TEM indicates that SP4 is a
mixture of NR SBS dimer assemblies (60%) and NR monomer
SPs (40%), while like SP3, SP5 is composed entirely of
assemblies based on single-NR cores.
when the two NRs are not exactly parallel, and the bonding
mode is therefore not totally “dark” (i.e., the dipole moment is
22
finite). Thus, for the SP1 and SP4 geometries, the CD
spectrum is predicted to be the sum of the extinction band of
this hybrid mode, and the rotational strength and direction of
this CD spectrum can be described as R = Im(p·m), where p
and m are the electric and magnetic dipole moments of the
56
plasmonic states, respectively. Here, we only consider the
electric dipole moments, so R only depends on the vector
56
product of the two electric dipole moments, and the
SP5 shows two CD peaks at 520 and 660 nm (Figure 4B),
and like SP3, these two peaks agree well with the transverse and
longitudinal SPR peaks, respectively, for the corresponding NR
expression can be reduced to R = ± (π/2λ)r ·(p × p ),
12
1
2
where λ is the wavelength of the uncoupled plasmon and r is
1
2
the vector joining the two dipoles. If the two oscillating dipoles
of the plasmonic states are parallel, r12 is zero when the two
oscillating dipoles have opposite directions (antisymmetric
hybrid modes). However, the two dipoles are not parallel for
the SP1 and SP4 assemblies, so r12 is not zero for both of the
symmetric and the antisymmetric hybrid modes, and so R
changes sign between the two modes. This sign change means
that the incident light polarization is rotated in different
directions for the two modes and explains the bisignated line
shape of the SBS dimer CD spectra (Figures 1E and 4A).
Physical Origin of Chiroptical Activity in NR-Based SPs.
Finite difference time domain (FDTD) electromagnetic
simulations confirmed the physical origins of the CD spectra
for the SPs made from 3:1 NRs (SP1 and SP3) and the SPs
made from 2:1 NRs (SP4 and SP5). The sizes and geometries
of NRs, SBS dimer assemblies, and single-NR assemblies were
taken from TEM (Figures 2, 3, and S6; Table S1). To account
for heterogeneous SP mixtures in the experiments, appropri-
ately weighted averages of the monomer and dimer spectra
were used: SP1 was modeled as 70% SBS dimer assemblies and
30% NR monomer assemblies; SP3 was modeled as 100% NR
monomer assemblies; SP4 was modeled as 60% SBS dimer
assemblies and 40% NR monomer assemblies; and SP5 was
(Figure 4C). As was reported previously, chiral materials in a
plasmonic field will have strongly enhanced CD at the plasmon
4
4,66
frequency.
By comparing the CD spectra of SP3 and SP5
(Figures 1F and 4B, respectively) to the measured absorbance
of 3:1 and 2:1 NRs (Figures 1A and 4D, respectively), it is clear
that the CD peaks are related to the NR localized surface
plasmon modes, especially as the long-wavelength peak changes
with the NR longitudinal plasmon resonance.
The CD peaks for SP4, which appear at 520, 640, and 758
nm, are much stronger than those of SP5 (Figure 4A). This is
consistent with the trend for the 3:1 aspect ratio NRs: the CD
spectrum of SP1 is also stronger than that of SP3. To discover
the source of the strong optical activity that we measured for
the SP1 and SP4 assemblies, TEM tomography was used to
visualize the detailed 3D geometry of the NR dimers found in
SP1 and SP4. By imaging these SBS dimers at multiple angles
by TEM tomography (Figure 2E, F and Figure S6), we found
that the two NRs of the assembly are not parallel. Rather,
between the NR axes there is a clear dihedral angle, which we
measured to be ca. 10°. Though parallel NRs will have a
forbidden bonding mode for the coupled plasmonic states of
individual NPs, this symmetry and selection rule is relaxed
F
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Figure 5. Simulated CD spectra for SP1 and SP3 assemblies. (A,B) Schematics of (A) SBS dimer and (B) NR monomer supraparticle models used
in simulations. Yellow: 45 × 15 nm gold NR; blue: 0.5 nm spacer layer; red: 2.5 nm thick layer of adsorbed CdTe NPs. SP1 is modeled as 70% SBS
dimer assemblies and 30% NR monomer assemblies; SP3 is modeled as 100% NR monomer assemblies. (C,D) SPs modeled with the measured
refractive index and chirality of CdTe NPs (Figure S7B). (C) Simulated CD spectra of D-SP1 (red) and L-SP1 (blue); (D) simulated CD spectra of
D-SP3 and L-SP3. (E,F) SPs modeled with the measured refractive index of CdTe NPs but no chiral absorption (Figure S7A). (E) Simulated CD
spectra of D-SP1 and L-SP1; (F) simulated CD spectra of D-SP3 and L-SP3. (G) Simulated CD spectra of 3:1 D- and L-SBS dimer assemblies with
CdTe layer removed. (H) Simulated CD spectra of D-SP3 and L-SP3 with CdTe layer replaced by a chiral, but otherwise wavelength-independent,
material (Figure S7C).
modeled as 100% NR monomer assemblies. The simulated
geometries are depicted in Figures 5A and 6A. The monomer
assemblies (Figures 5B and 6B for 3:1 and 2:1 NRs,
respectively) are composed of a gold NR (yellow) surrounded
by a 0.5 nm thick shell of water representing the surface ligands
The simulated CD spectra are presented in Figures 5 (SP1
and SP3) and 6 (SP4 and SP5). Here, CD is the differential
extinction of left-handed and right-handed circularly polarized
light, i.e., CD = extLHCP − extRHCP. To calculate the CD
spectrum of D-SP1 (red curve in Figure 5C), the experimentally
measured D-NP CD peak from 435 to 565 nm (Figure 1B) was
added to the imaginary part of the refractive index for the CdTe
NPs for excitation by LHCP light only, producing a chiral NP
shell (Figure S7B). The simulated CD spectrum of L-SP1 (blue
curve in Figure 5C) is provided for consistency with
experiments; this is the negative of the CD spectrum of D-
SP1. In excellent agreement with the experimental CD
spectrum for SP1 (Figure 1E), the simulated spectrum for
SP1 shows three peaks at λ ≈ 540, 690, and 810 nm. Similarly,
(blue) and a 2.5 nm thick shell of CdTe NPs (red). The SBS
dimer assemblies (Figures 5A and 6A for 3:1 and 2:1 NRs,
respectively) are composed of two monomer assemblies with a
small dihedral angle relative to one another. Notably, both SP1
and SP4 revealed a dihedral angle of ∼10−15°. The reasons for
consistency of the dihendral angle between two scissor-like
assemblis investigated here as well as other chiroplasmonic NR
assemblies studied before, is not quite clear at the moment.
G
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Figure 6. Simulated CD spectra for SP4 and SP5 assemblies. (A,B) Schematics of (A) SBS dimer and (B) NR monomer supraparticle models used
in simulations. Yellow: 47 × 22 nm gold NR; blue: 0.5 nm spacer layer; red: 2.5 nm thick layer of adsorbed CdTe NPs. SP4 is modeled as 60% SBS
dimer assemblies and 40% NR monomer assemblies; SP5 is modeled as 100% NR monomer assemblies. (C,D) SPs modeled with the measured
refractive index and chirality of CdTe NPs (Figure S7B). (C) Simulated CD spectra of D-SP4 (red) and L-SP4 (blue); (D) simulated CD spectra of
D-SP5 and L-SP5. (E,F) SPs modeled with the measured refractive index of CdTe NPs but no chiral absorption (Figure S7A). (E) Simulated CD
spectra of D-SP4 and L-SP4; (F) simulated CD spectra of D-SP5 and L-SP5. (G) Simulated CD spectra of 2:1 D- and L-SBS dimer assemblies with
CdTe layer removed. (H) Simulated CD spectra of D-SP5 and L-SP5 with CdTe layer replaced by a chiral, but otherwise wavelength-independent,
material (Figure S7C).
the simulated CD spectrum for SP4 (Figure 6C) also
successfully reproduces the corresponding experimental CD
spectrum (Figure 4A). The CD spectra for SP1 and SP4 were
both weighted averages of the CD spectra for monomer SPs
and SBS dimer SPs. The extinction curves for these simulations
are shown in Figure S8; these also correspond well to
experiments.
To determine the source of the CD of SP1 and SP4, the
simulations were repeated with the chiral NP shell replaced by
an achiral NP shell (Materials and Methods; Figure S7A). The
main features of the resulting CD spectra (Figures 5E and 6E
for SP1 and SP4, respectively) are essentially unchanged from
the CD spectra for SPs with chiral NP shells (Figures 5C and
6C). The small peaks at λ ≈ 520 nm corresponding primarily to
the chiroptical activity of NPs, predictably decrease. This
indicates that the circular dichroism of the NPs does not
produce the strong CD spectra of SP1 and SP4. To further
probe the role of the NPs and to uncover any possible plasmon-
exciton interaction, SBS NR dimer pairs were simulated
without any CdTe layer at all, i.e., the SP geometries were
left unchanged, but the NP shell was replaced by water.
Remarkably, the shape of the CD spectra are preserved, though
with the peaks slightly blue-shifted, as is expected due to the
decreased local refractive index around the NRs (Figures 5G
and 6G for 3:1 and 2:1 SBS NR dimer pairs, respectively). This
indicates that the strong CD spectra of the SP1 and SP4
H
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Letter
assemblies are not caused by the chiroptical activity of CdTe
NPs per se. Also note that SP1 and SP4 contained a large
number of SBS dimer assemblies, while SP3 and SP5 were
composed nearly exclusively of single NR SPs. Therefore, the
experimental CD spectra of these SPs (Figures 1F and 4B)
cannot be explained simply with geometry as the monomeric
NR is not geometrically chiral in isolation in the framework of
this model.
(LM) of the NR. Though the excitonic states of CdTe NPs and
their CD peaks overlap spectrally only with the TM mode of
the NRs, the TM and LM modes are not strictly orthogonal;
there is some amount of intermodal “cross talk”. The presence
of the NP coat can further enhance the cross-talk due to
scattering of the plasmons on the field inhomogeneities
produced by the proximal NPs. It should be noted that our
computations involve only classical effects and are linear (no
nonlinearities, e.g., saturation effects, are included). Thus, we
account for the absorption of a given wavelength of light by an
SP as the sum of three linear processes: (1) absorption by the
CdTe layer, (2) direct absorption into the plasmon mode that
is spectrally resonant with that light, and (3) indirect excitation
of the second mode via “cross talk”. For example, in the case of
SP3, 520 nm light can be absorbed “directly” to excite the TM,
or the energy from the 520 nm excitation can be transferred
from the TM to “indirectly” excite the 730 nm LM, while 730
nm light, which directly excites the LM, can also be absorbed
“indirectly” into the TM.
The origin of the CD bands in SP1 and SP4 should therefore
be attributed to the twisted cross-rod geometry of the SBS
dimer SPs and is not to the “hot spot” enhancement of the
chiroptical properties of chiral CdTe NPs. In other words, the
difference in chiroptical activity of D-SP1 vs L-SP1 (and for D-
SP4 vs L-SP4) comes not from the differences in chiroptical
activity of individual D- and L-NPs, but rather from the chirality
of the SBS assemblies as a whole. In turn, the latter originates
from the chiral intermolecular interactions of NPs coated by L-
and D-CYS stabilizers, which favor a NR twist in a particular
direction. The evidence of the structure-determining role of the
chiral interactions between NPs can be seen in the assemblies
of CYS-stabilized CdTe NPs without the gold NRs. They form
mesoscale helices with specific handedness depending on L- and
D-form of CYS used to stabilize NPs (Supporting Information,
Figure S10). A similarly strong indication that chirality of the
NPs affects the geometry of the nanoscale assemblies is given in
the recent study of the photoinduced assembly of CdTe
The increased k for LHCP light incident on the CdTe NP
shell (Figure S7B) has two effects on D-SP3: (a) CdTe absorbs
LHCP excitation more strongly at λ ≈ 500 nm and (b) the
2
dielectric constant, ε = (n + ik) , is increased. Increasing ε
outside a metal NP decreases the skin depth and thus the
amplitude of the plasmon oscillation. Therefore, under LHCP
polarized light, the TM is weakened and thus absorbs less
strongly. This second effect, (b), is expected to be more subtle
than the first effect, (a).
Let us consider now the peak in the D-SP3 CD at 540 nm
(Figure 1F). From (a) above, SP3 should absorb LHCP light
more strongly than RHCP light because of increased k in the
NP absorbance at that wavelength (Figure S7B). However,
there is some decreased TM absorbance by the gold as
suggested by (b) above. Furthermore, some of the 540 nm light
is absorbed indirectly into the LM via the TM, and this
absorption pathway is also slightly weakened because there is
less TM absorbance. Still, because the effect of increased NP
absorbance dominates, overall, LHCP 540 nm light is absorbed
more strongly than RHCP 540 nm light, leading to a peak in
the CD spectrum (CD = extLHCP − extRHCP).
Next, we consider the dip in D-SP3 CD at 750 nm (Figure
1F). At this longer wavelength, the CdTe absorbance is the
same for LHCP and RHCP excitation light (Figure S7B), and
because the dielectric constant of the NP shell is unchanged,
the direct absorbance of the LM is also independent of
polarization. Still, because some of the 750 nm light is indirectly
absorbed into the TM via “cross talk” with the LM, and since
this TM mode is weakened for LHCP light absorbance due to
the increased dielectric constant at 540 nm, there is an overall
decrease in absorption of 750 nm LHCP polarized light, leading
to a dip in the CD spectrum (CD = extLHCP − extRHCP). This
phenomenological description of the chiroptical activity of SP 3
can also be applied to the CD spectra of SP5 (Figure 4B).
Conclusions. In summary, we have investigated the
formation and optical activity of terminal SP assemblies
composed of gold NRs and chiral semiconductor NPs. When
the molar excess of NPs is small (NR: NP = 1:10−1:15), the
SPs are based on NR dimers with a scissor-like conformation.
When the molar excess of NPs is large (NR: NP = 1:120−
1:180), we observed a separation of the NR dimers into SPs
based on single NRs due to increased electrostatic repulsion
from a denser layer of adsorbed NPs. The chiral optical activity
of the SPs based on NR dimers is attributed to the twisting of
57
stabilized by thioglycolic acid. Note, in the latter case the
stabilizer is achiral, and the enantioselectivity of the product
mesoscale twisted nanoribbonsoriginates from the chiral
interactions characteristic of the inorganic core of the NPs.
Both the molecular scale chirality of the stabilizer and the
nanoscale chirality of the CdTe core are likely to play a role in
chiral discrimination of the NR-based SPs with SBS geometry
described in this study.
To further substantiate this attribution of chiroptical
properties of SPs and to better understand light−matter
interactions in these nanoscale assemblies, SP3 and SP5 were
modeled using the optical properties for the CdTe NP shell
about monomeric NRs (Figure S7B). On the basis of this
material definition, the extinction spectra and corresponding
CD spectrum were calculated for SP3 (Figure S8A and 5D,
respectively) and for SP5 (Figure S8C and 6D, respectively). In
the extinction spectra for SP3 (Figure S8B), we identify the
peak at λ ≈ 520 nm as the transverse plasmon mode (TM) and
the peak at λ ≈ 730 nm as the longitudinal plasmon mode
(LM). In the corresponding calculated CD spectrum (Figure
5
D), the main features are a broad dip at λ ≈ 540 nm and a
peak at λ ≈ 760 nm. These match very well with the
experimentally measured CD features shown in Figure 1F.
Similarly, the experimental and simulated CD spectra for SP5
(Figures 4B and 6D, respectively) agree well. Unlike the case of
SP1 and SP4, which contain SBS dimer assemblies, here, the
simulated CD for SP3 and SP5 requires the chiroptically active
CdTe NP shell: when these simulations of SP3 and SP5 were
repeated with the chiral NP shell replaced by an achiral NP
shell (Materials and Methods), no CD response was observed
(
Figures 5F and 6F, respectively).
Remarkably, despite the fact that the SP3 and SP5 shells
reveal CD bands only from 435 to 535 nm, these monomeric
SPs exhibit CD also at wavelengths strongly shifted to the red
part of the spectrum. In order to explain this unanticipated
result, we attribute some of these red CD signals to interactions
between the transverse mode (TM) and the longitudinal modes
I
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NRs in respect to one other, leading to concomitant
polarization rotation. Enantiomeric preference is associated in
this case with chiral interactions between the NPs situated
between the NRs. The optical activity of SPs with single-NR
cores occurs due to the cross-talk between the longitudinal and
transverse plasmonic modes of the NRs, with the latter being in
resonance with the chiral exciton in the NPs. Note that this
finding was possible due to the use of chiroplasmonic
spectroscopy that enabled registration of distinct peaks
corresponding to different excited states and therefore
discrimination of different resonance phenomena. This
experimental method can be further applied to other studies
of electronic and energy transfer phenomena in complex
plasmon-exciton systems. Besides the academic significance of
identifying chiral exciton−plasmon states, the simplicity of
preparation of SPs and similar terminal assemblies opens the
door for further studies of chiral sensing, new polarization-
based optical memory devices, and enantioselective catalysis.
Materials and Methods. Materials. Cadmium perchlorate
hexahydrate (CdClO ·6H O) was purchased from Alfa-Aesar.
for 2 min. Then the gold seed solution was kept in a water bath
maintained at 30 °C for 2 h. A 4 mL aliquot of 0.01 M HAuCl4,
(0.04 mmol), 0.48 mL of 0.01 M AgNO (0.0048 mmol),
3
freshly prepared 0.48 mL of 0.10 M L-ascorbic acid (0.048
mmol), and 0.096 mL of CTAB-capped gold seed solution was
synthesized beforehand and then were all added to 80 mL of
0.1 M CTAB, in that order, one by one, followed by thorough
mixing after every addition. Finally, the gold NR suspension
was left undisturbed at 30 °C overnight. The samples were
purified by centrifugation (8000 rpm, 20 min) twice and
redispersed in water before use.
FDTD Simulations. Electromagnetic simulations were
performed using the commercial software package Lumerical
FDTD Solutions. Gold NRs were modeled as cylinders with
hemispherical end-caps. CdTe NP-conjugated NRs were
approximated as a gold NR core surrounded by a 0.5 nm
water layer representing the NR surface ligands, covered by a
2.5 nm thick CdTe NP film. GNR and SP geometries were
taken from TEM (Table S1). The simulations were performed
3
with a SP at the center of a simulation volume of 0.685 μm ,
4
2
Aluminum telluride powder (Al Te ) was purchased from
where near the SP a fine-mesh grid with a cell volume of 0.091
nm3 was used. Perfectly matched layer (PML) absorbing
boundary conditions were used. The simulation was excited by
a circularly polarized plane wave source from 400−900 nm, and
scattering and absorption cross sections were measured in the
particle near-field.
To simulate isotropically oriented particles in solution, all
spectra were averaged over perpendicular excitation directions.
UV−vis−NIR absorption measurements, which used incoher-
ent light, were simulated by averaging the extinction spectra
from right-hand and left-hand circular polarization excitation
light (RHCP and LHCP, respectively). Circular dichroism was
calculated as the difference between the extinction spectra for
RHCP and LHCP excitation. To account for heterogeneous SP
mixtures in the experiments, simulated spectra are presented as
appropriately weighted averages of the monomer, and dimer
spectra were used.
2
3
Materion Advanced Chemicals. Hydrogen tetrachloroaurate
trihydrate (HAuCl ·3H O), L-ascorbic acid, silver nitrate
4
2
(
AgNO ), sodium borohydride (NaBH ), hexadecyltrimethy-
3
4
lammonium bromide (CTAB), L-cysteine hydrochloride, and D-
cysteine hydrochloride were purchased from Sigma-Aldrich. All
chemicals are used as received. All chemicals used here were
analytical grade or the highest purity available.
Measurements. CD spectra were recorded in aqueous
solution by a JASCO J-810 spectropolarimeter. A portion of 2
mL of each sample was measured in a 1 cm quartz cell at a scan
speed of 500 nm/min with a bandwidth of 10 nm, and the
results were averaged over two consecutive scans. UV−vis−
NIR absorption measurements were carried out using an
Agilent 8453 spectrometer (300−900 nm), emission spectra
were measured with a Fluoromax-4 spectrometer (Horiba Jobin
Yvon) with excitation wavelength at 420 nm, and transmission
electron microscopy (TEM) images were taken by JEOL-3011
transmission electron microscope. The hydrodynamic diame-
ters of the supraparticles were determined by Malvern Zetasizer
Nano ZS. The zeta-potentials were performed by the same
instrument at room temperature.
All SPs were simulated surrounded by water (constant
refractive index of n = 1.33, k = 0). The same refractive index
72
for Au was used for all simulations. Experimental ellipsometry
data obtained from Prof. N. Gaponik for achiral CdTe NPs
were used for the refractive index of achiral CdTe NP (SI,
Figure S7A). CdTe optical activity was artificially introduced in
our simulations of SPs by adding the experimentally measured
D-NP CD peak from 435−565 nm (Figure 1B) to the imaginary
part of the refractive index for the CdTe NPs for excitation by
LHCP light, while the original CdTe refractive index was used
for SPs excited by RHCP light (SI Figure S7B). The CdTe
exciton was artificially removed by replacing the spectrally
dependent n and k with a constant n and k equal to the average
values. Here again, CdTe optical activity was artificially
introduced by adding the experimentally measured D-NP CD
peak from 435 to 565 nm (Figure 1B) to the imaginary part of
the wavelength-independent refractive index for the CdTe NPs
for excitation by LHCP light, while the original wavelength-
independent CdTe refractive index was used for SPs excited by
RHCP light (SI, Figure S7C).
Synthesis of Cysteine (CYS)-Capped CdTe NPs. CdTe NPs
2
+
were prepared by reacting Cd and H Te gas in the presence
2
of chiral molecules as stabilizers according to the Rogach−
6
7
68
Weller method which we later modified. A 0.498 g (0.59
mmol) sample of Cd(ClO ) ·6H O and 0.498 g (0.14 mmol)
4
2
2
of (D,L)-cysteine was dissolved in 62 mL of distilled water. The
pH was then adjusted to 11.2 by adding 1 M NaOH. The
solution was placed in the three-necked flask and deaerated by
N gas for about 30 min. H Te gas generated by the mixture of
2
2
0
.2 g of Al Te , and 15 mL of 0.5 M H SO in another three-
2 3 2 4
necked flask was directed into the prepared solution under a N2
atmosphere. The precursors were converted to CdTe nano-
crystals by refluxing the reaction mixture at 100 °C for ∼6−8 h
in N gas.
2
Synthesis of Gold Nanorods (NRs). Gold NRs were
prepared from a seed-mediated, surfactant-directed approach
69−71
ASSOCIATED CONTENT
Supporting Information
Time dependence of CD spectra for SP1 and SP3 (Figure S1);
g-factor spectra of SP1 and SP3 (Figure S2); DLS spectra of the
original gold NRs and of three SP assemblies (Figure S3); CD
described previously.
Briefly, 0.25 mL of 0.01 M HAuCl4
■
(
(
0.0025 mmol) solution was added to 7.5 mL of a 0.1 M CTAB
0.75 mmol) solution, and the solutions were gently mixed.
*
S
This was followed by addition of an ice-cold, 0.6 mL of aqueous
0
.01 M NaBH solution all at once, followed by rapid mixing
4
J
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Letter
and UV−vis−NIR absorption spectra of SP2 (Figure S4); TEM
images of SP2 (Figure S5); TEM tomography image of D-SP1
at various tilt angles (Figure S6). Optical parameters of
simulated CdTe NPs (Figure S7). Calculated extinction spectra
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3
(
(
(
Figure S8). Sizes and geometries of simulated SP assemblies
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Internet at http://pubs.acs.org.
(
AUTHOR INFORMATION
Corresponding Authors
E-mail: jsbiteen@umich.edu.
E-mail: kotov@umich.edu.
■
2013, 12, 802−807.
(21) Xia, Y. S.; Zhou, Y. L.; Tang, Z. Y. Nanoscale 2011, 3, 1374−
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1382.
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22) Ma, W.; Kuang, H.; Xu, L. G.; Ding, L.; Xu, C. L.; Wang, L. B.;
Kotov, N. A. Nat. Commun. 2013, 4, 2689.
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Notes
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The authors declare no competing financial interest.
1
(
1
(
ACKNOWLEDGMENTS
■
This work was largely supported by the Center for Photonic
and Multiscale Nanomaterials (C-PHOM) funded by the
National Science Foundation Materials Research Science and
Engineering Center program DMR 1120923. The computa-
tional work was funded in part by a National Science
Foundation CAREER award (CHE-1252322) to J.S.B. The
funding for TEM studies came from the Center for Solar and
Thermal Energy Conversion, an Energy Frontier Research
Center funded by the U.S. Department of Energy, Office of
Science, Office of Basic Energy Sciences under Award Number
No. DE-SC0000957. We are thankful to Dr. Nikolai Gaponik
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from Technische Universitat Dresden for spectral data for
CdTe NPs. We appreciate the great help with TEM imaging
given to us by Dr. Kai Sun and acknowledge the support of
NSF Grant No. DMR-0315633 for the purchase of 3011 JEOL
HRREM.
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