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Collective plasmon modes in a compositionally asymmetric nanoparticledimerFuyi Chen, Negash Alemu, and Roy L. JohnstonCitation:AIP Advances 1, 032134 (2011); doi: 10.1063/1.3628346View online: http://dx.doi.org/10.1063/1.3628346View Table of Contents: http://aipadvances.aip.org/resource/1/AAIDBI/v1/i3Published by theAIP Publishing LLC.Additional information on AIP AdvancesJournal Homepage: http://aipadvances.aip.orgJournal Information: http://aipadvances.aip.org/about/journalTop downloads: http://aipadvances.aip.org/features/most_downloaded
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AIP ADVANCES 1, 032134 (2011)
Collective plasmon modes in a compositionally asymmetricnanoparticle dimer
Fuyi Chen,1,a Negash Alemu,1 and Roy L. Johnston2,b1State Key Laboratory of Solidification Processing, Northwestern Polytechnic University,
Xian, 710072, P. R. China2School of Chemistry, University of Birmingham, Birmingham, B15 2TT, U. K.
(Received 26 May 2011; accepted 28 July 2011; published online 11 August 2011)
The plasmon coupling phenomenon of heterodimers composed of silver, gold and
coppernanoparticles of 60 nm in size andspherical in shape were studied theoretically
within the scattered field formulation framework. In-phase dipole coupled -modes
were observed for the Ag-Au and Ag-Cu heterodimers, and an antiphase dipole
coupled -mode was observed for the Ag-Au heterodimer. These observations agree
well with the plasmon hybridization theory. However, quadrupole coupled modes
dominate the high energy wavelength range from 357-443 nm in the scattering
cross section of the D=60 nm Ag-Au and Ag-Cu heterodimer. We demonstrate
for the first time that collective plasmon modes in a compositionally asymmetricnanoparticle dimer have to be predicted from the dipole-dipole approximation of
plasmon hybridization theory together with the interband transition effect of the
constitutive metals and the retardation effect of the nanoparticle size. Copyright
2011 Author(s). This article is distributed under a Creative Commons Attribution 3.0
Unported License. [doi:10.1063/1.3628346]
I. INTRODUCTION
The optical properties of individual metal nanoparticles are dominated by their localized sur-
face plasmon resonances (LSPR), which are associated with the collective coherent oscillation of
conduction electrons in metallic nanostructures.1 The electromagnetic field surrounding a metal-
lic nanoparticle that has been excited near the plasmon resonance can extend for a considerable
distance, and a compelling analogy therefore exists between the plasmon resonance of individual
nanoparticles and the wave functions of simple atoms. Similar to the roles of atoms in molecules and
solids, metal nanoparticles can assemble to create a diverse range of new plasmonic nanoclusters, 2
including dimers,3 trimers,4 quadrumers5 and tetramers,6 etc.7 Each type of plasmonic nanocluster
exhibits its own unique set of collective plasmon modes.
The simplest description of collective plasmon modes is the dipoledipole interaction during
the coupling of two nearby oscillators. For two adjacent metallic spheres a lower-energy resonance
corresponds to two longitudinally aligned dipoles giving rise to a strong red-shifted absorption
peak in the optical spectrum. For the higher energy resonance the coupled dipoles cancel each other,
resulting in a resonance with essentially a zero net dipole moment that does not interact with incident
light and does not appear in the optical absorption spectrum of the particle pair. Depending on the
nanoparticle size and interparticle distance, higher-order multipoles can also play an important rolein collective plasmon modes.8
It is striking that the simplest description of collective plasmon modes, the dipoledipole
interaction has resulted in a substantial understanding of a rich array of new electromagnetic
aEmail: [email protected]: [email protected]
2158-3226/2011/1(3)/032134/16 C Author(s) 20111, 032134-1
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http://dx.doi.org/10.1063/1.3628346http://dx.doi.org/10.1063/1.3628346http://dx.doi.org/10.1063/1.3628346http://dx.doi.org/10.1063/1.3628346mailto:%[email protected]:%[email protected]:%[email protected]:%[email protected]://dx.doi.org/10.1063/1.3628346http://dx.doi.org/10.1063/1.3628346http://dx.doi.org/10.1063/1.3628346 -
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properties, such as subradiant (dark) plasmonic modes,9 Fano resonances,10 optical nanodiodes,11
electromagnetically induced transparency12 and, in particular, the magnetic response,13 which is
excited by the antisymmetric plasmonic resonance in various arrays of nanoparticle pairs such as in
metal nanostrip dimers,14, 15 nanodisk dimers,16, 17 and metal plate pairs.18, 19
Symmetry breaking in collective plasmon modes can introduce effects that are not present
in the symmetrical configurations. Recently, these effects that come from the coupling betweenthe otherwise uncoupled plasmon modes of the system were investigated using compositionally
asymmetric nanoparticle dimers.2023 For example, Fano-type absorption line profiles were ob-
served in the absorption spectra of the gold nanoparticles because of an interaction between the
discrete local plasmon resonance of silver and the interband transition of gold in a D=10 nm
Au-Ag heterodimer with an interparticle distance of 3 nm.20 The two plasmonic resonances ex-
hibit a simultaneous shift for the D=30 nm Au-Ag heterodimer, and this is in contrast to the
homodimer.21 Anomalous red-shifts of the antibonding modes were observed with respect to the
silver local plasmon resonance mode in the scattering spectra of the heterodimer composed of a
30 nm silver and a 40 nm gold nanoparticle, and this was induced by the coupling of silver
particle plasmon resonance to the continuum of the interband transitions in gold.22 The Fano
resonance has only been induced by the longitudinal component of the heterodimer surface plas-
mon resonance in a heterodimer composed of a 10 nm silver and a 30 nm gold nanoparticle. 23
Despite this intense research, neither the dipolar eigenmodes nor the induced charge configura-
tion of plasmonic resonance have been addressed in these recent heterodimers,2023 which indi-
cates that the interaction between the strongly coupled heterodimer is not as completely under-
stood as in the homodimer. Therefore, a systematic study of the collective plasmon modes in
the heterodimer is required, as it may result in a new theoretical paradigm and a host of novel
applications.
We studied a D=60nm Ag-Auand a D=60 nm Ag-Cu nanoparticle heterodimer under different
interparticle separations (1-15 nm) and variable polarizer angles (0-90) as prototype systems. We
found that the longitudinal coupling between the nanoparticles shows a strong dependence on the
size of the dimer gap in the Ag-Au and Ag-Cu heterodimers. We demonstrate for the first time that a
quadrupole-coupled mode emerges in the Ag-Au heterodimers and dominates the scattering spectra
of the Ag-Cu heterodimers. An antiphase dipole-coupled mode was observed with low intensity
in the Ag-Au heterodimer; this configuration of two stacked antiphased nanoparticles is known as
magnetic resonance, and is essential for the development of negative refractive index materials in
the visible region of the spectrum.
II. METHODOLOGY
The extinction spectra and electromagnetic field distribution of the nanoparticle dimers were cal-
culated using the finite difference time domain (FDTD) method. Three-dimensional vector Maxwell
equations in the different nanostructures were solved within the scattered field formulation frame-
work. The FDTD calculations with a finer grid for the elected distances revealed the plasmon
hybridization mode in almost perfect agreement with Mie theory for spheres. The surface charge
distributions that were induced on the nanoparticle profiles were evaluated using the calculated elec-tromagnetic fields. The dimer was formed by identical spherical nanoparticles whose surfaces are
separated by a distances gap, ranging from one half of sphere radius a down to conductive touching
with gap=-3 nm. Here, we consider spheres with a diameter of 60 nm. The dielectric functions
were described using the tabulated empirical data of Johnson and Christy with a linear interpolation
for pure copper, silver and gold.24 The dimer without a substrate was embedded in an air medium
surrounded by a perfectly matched layer. The distance was a full wavelength from the nanoparticles
to avoid spurious reflections and to avoid absorbing the energy of evanescent fields. A total-field
scattered-field plane wave, which was used for excitation, was inserted on the inside of the perfectly
matched layers near the air medium interface. The studied model was discretized by a conformal
mesh method with a side length of 0.1 nm in the dimers.
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III. RESULTS AND DISCUSSION
A. Ag-Ag nanoparticle homodimer
We start with a simple homodimer and show the calculated scattering spectra of a
D=60 nm silver nanoparticle homodimer in vacuum as a function of the polarization angle in
Fig. 1(a), including parallel (black curve) and perpendicular (red curve) incident polarization withrespect to the dimer axis in Fig. 1(b). These results clearly illustrate that only the longitudinal
resonance mode (L) is present in the scattering spectrum when the polarizer is oriented along the
interparticle axis and the L-mode results from the longitudinal LSPR coupling of the particle plas-
mons and displays two distinct peaks. The lower energy peak is present at 464 nm and the higher
energy peak is at 357 nm. When the polarizer is oriented perpendicular to the interparticle axis
only the transverse resonance mode (T) contribution is observed at 364 nm, because of transverse
LSPR coupling. At all the intermediate angular orientations the scattering spectra show L- and T-
orthogonal modes and a clear isosbestic point is present near 412 nm in the spectrum. In Fig. 1(b),
a 60 nm isolated silver nanoparticle shows dipole resonance () at 378 nm. By comparison with
the single particle resonance () the lower energy L-mode has a longer wavelength and a stronger
intensity and the T-mode is slightly blue shifted and lower in intensity.
Figure 1(c) shows the calculated scattering spectra of a D=60 nm silver nanoparticle homod-
imer in vacuum as a function of interparticle separations under longitudinal polarization. For an
interparticle separation larger than 9 nm, only lower energy L-modes are present and observed to
red-shift significantly from 411 nm at a 15 nm gap to 505 nm at a 1 nm gap. For an interparticle
separation smaller than 9 nm the scattering peak splits into two well-defined peaks and the higher
energy L-mode (at 357 nm in Fig. 1(b)) is spectrally separated from the lower energy L-mode by
a pronounced dip. This phenomenon is the result of strong longitudinal SPR coupling, and indi-
cates that the electromagnetic interaction between the nanoparticles is far more significant for an
interparticle separation smaller than 9 nm. These higher energy L-modes have been observed to
red-shift slightly with a decrease in the interparticle separation, which agrees with previous studies
on nanoparticle dimer arrays.25, 26
Figure 2 shows the electric-field intensity and the surface charge distribution profile for the L-
mode and the T-mode in a D=60 nm silver nanoparticle homodimer. As expected, the electric field
intensity is enhanced at the junction of the Ag-Ag homodimer for both the lower energy and the high
energy L-mode; however, the local charges exhibit a dipole-dipole (DD) pattern for the lower energy
L-mode (464 nm) and a quadrupole-quadrupole (QQ) pattern for the high energy L-mode (357 nm).
For the T-mode at 364 nm the electric field intensity is localized around the silver nanoparticle,
i.e., the nonjunction ends of the Ag-Ag homodimer and the local charges exhibit a dipole-dipole
(DD) pattern. For the dipole-dipole L-mode and the dipole-dipole T-mode the charges confined to the
same edge of the nanoparticle have the same sign, which indicates that these dipole-dipole modes are
in-phase collective plasmon modes. Therefore, the bonding-mode () and antibonding-mode (*)
can be assigned as for the dipole-dipole L-mode and the dipole-dipole T-mode, respectively, from
the classification frame of the plasmon hybridization model.27 The quadrupole-quadrupole L-mode
is associated with the spectral superposition of higher order longitudinal resonance modes, and its
main contribution comes from the quadrupole plasmon resonance of an individual silver particle.25, 26
The collective plasmon modes can also be classified as bright and dark modes for the complex
nanostructures that consist of multiple closely spaced nanoparticles or patterned nanoholes. Asshown in Fig. 2, the quadrupole-quadrupole L-mode, the dipole-dipole L-mode and the dipole-
dipole T-mode possess finite net dipole moments, and these plasmonic modes are bright plasmonic
modes, which are efficiently excited by incident light and are clearly visible in the optical spectrum.
The quadrupole-quadrupole L-mode is not bright for an interparticle separation bigger than 9 nm in
the homodimer.
B. Ag-Au nanoparticle heterodimer
Dark plasmonic modes possess a zero net dipole moment and do not couple to light, and are
therefore not very prominent in optical spectra. To enhance the manifestation of dark plasmonic
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FIG. 1. Scattering spectra for the D=60 nm silver nanoparticle homodimer as a function of the polarization angle with
interparticle separations of 3 nm (a). Scattering spectra under longitudinal and transverse polarization with respect to a
60 nm isolated silver nanoparticle (b), and as a function of interparticle separations under a polarizer angle of 0 degrees (c).
modes we introduced compositional asymmetry in the collective plasmon modes, and can therefore
discuss the optical properties of a D=60 nm silver and gold nanoparticle heterodimer. In Fig. 3(a)
and 3(b) we show the calculated scattering spectra of the Ag-Au nanoparticle heterodimer as a
function of the polarization angle (0-90) and the two extreme cases for orthogonal polarization,
respectively. Following the identification of the L- and T- plasmon modes in homodimers similar
mode can be assigned as the Ag-Au heterodimer. Figure 3(a) and 3(b) shows that the Ag-Au
heterodimer supports the higher energy L-mode at 410 nm, the lower energy L-mode at 545 nm
under longitudinal polarization (0), the higher energy T-modes at 373 nm and the lower energy
T-mode at 527 nm under transverse polarization (90). The primitive or unhybridized mode of
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FIG. 2. Electric near-field profiles and surface charge distributions in a D=60 nm silver nanoparticle homodimer for
longitudinal and transverse resonance modes.
the isolated 60 nm silver nanoparticle (1) and the isolated 60 nm gold nanoparticle (2) are
at 379 and 531 nm, respectively. This result is quite different from the D=60 nm Ag-Ag homodimer,
where only two or three bright modes exist. The scattering spectra in Fig. 3(a) and 3(b) show four
bright modes at 410 nm (L-mode), 545 nm (L-mode), 373 nm (T-mode) and 527 nm (T-mode).
Figure 3(c) shows calculated scattering spectra as a function of interparticle separation under
longitudinal polarization for a D=60 nm silver and gold nanoparticle heterodimer. Both the lower
energy L-mode at 545 nm and the high energy L-mode at 410 nm were observed to red-shift
significantly with a decrease in the interparticle separation although the high energy L-mode changes
from a peak to a flat shoulder in the scattering spectra for interparticle separations smaller than 9 nm.
The L-mode is a bonding-mode where a large portion of the electric field experiences a modified
dielectric environment and increases with a decrease in interparticle separation.
In Figure 4, we calculated the surface charge distribution for four bright modes in a D=60 nm
silver and gold nanoparticle heterodimer. The high energy L-mode at 410 nm and the lower energy
L-mode at 545 nm, similar to that of the D=60 nm Ag-Ag homodimer, gave an enhanced electric
field intensity at the junction of the dimer and revealed a quadrupole-quadrupole (QQ) mode and a
dipole-dipole (DD) mode in the surface charge pattern, respectively. The dipole-dipole L-mode at
545 nm is a bonding-mode () in the plasmon hybridization model like that in the D=60 nm Ag-Ag
homodimer.
The high energy T-mode at 373 nm and the lower energy T-mode at 527 nm are new collectiveplasmonic modes for the Au-Ag heterodimer, which exhibits a dipole-quadrupole (DQ) mode and
a dipole-dipole (DD) mode in the surface charge pattern. Judging from the signs of the charges
confined to the nanoparticle edge the dipole-dipole T-mode at 527 nm is an out-of-phase collective
plasmon mode of the individual silver plasmons (1) and the gold plasmons (2). It is important to
note that the lower energy T-mode at 527 nm, which is an otherwise dark mode in the symmetric
dimer, is clearly visible in the optical spectrum in the Au-Ag heterodimer, and can be assigned as a
bonding-mode () from the classification frame of the plasmon hybridization model.
For the higher energy T-mode at 373 nm the electric field intensity is localized around the silver
nanoparticle of the D=60 nm Ag-Au heterodimer and the surface charge in the Ag nanoparticle
exhibits a dipolar pattern while the surface charge in the Au nanoparticle exhibits a quadrupolar
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FIG. 3. Scattering spectra for a heterodimer of a D=60 nm silver and gold nanoparticle heterodimer as a function of the
polarization angle with interparticle separations of 3 nm (a). Scattering spectra under longitudinal and transverse polarization
with respect to the normalized scattering spectra for a 60 nm isolated silver and gold nanoparticle (b), and as a function of
interparticle separations at a polarizer angle of 0 degrees (c).
pattern. The charges confined to the Au nanoparticle have a + sign in the left half-sphere and this
is opposite to the Ag neighbor nanoparticle and the - sign in the right half-sphere indicating that
the quadrupolar charge pattern in the Au nanoparticle is induced by the electric field from the nearby
Ag nanoparticle.
As shown in Figure 4, the quadrupole-quadrupole (QQ) L-mode at 410 nm, the dipole-dipole
(DD) L-mode at 545 nm, the dipole-quadrupole (DQ) T-mode at 373 nm and the dipole-dipole (DD)
T-mode at 527 nm have a nonzero electric dipole moment and can be excited by an incident plane
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FIG. 4. Electric near-field profiles and surface charge distributions in a D=60 nm sliver and gold nanoparticle heterodimer
for longitudinal and transverse resonance modes.
wave. The dipole-dipole (DD) T-mode or the -mode at 527 nm dipole are active, because of the
incomplete cancellation of the dipoles of the silver and gold particles in the Au-Ag heterodimer.
C. Ag-Cu nanoparticle heterodimer
As another example of a nanoparticle heterodimer, we obtained the optical properties of a D=
60 nm silver and copper nanoparticle heterodimer. The calculated scattering spectra of the het-
erodimer are shown in Fig. 5(a) as a function of the polarization angle. The extreme cases for the
longitudinal and transverse polarization are plotted in Fig. 5(b). Similar to the 60 nm silver particle
homodimer, scattering from the longitudinal polarization has a higher energy L-mode at 443 nm and
a lower energy L-mode at 575 nm. Scattering from transverse polarization has a T-mode at 373 nm.
Figure 5(c) shows the calculated scattering spectra as a function of interparticle separation
under longitudinal polarization for a D=60 nm silver and copper nanoparticle heterodimer. The
higher energy L-mode gave a significant red-shift with a decrease in interparticle separation and it
splits into two well-defined scattering peaks for interparticle separations smaller than 2 nm. Lower
energy L-modes were clearly observed for interparticle separations smaller than 5 nm and these red-
shift slightly with a decrease in interparticle separation and they have a lower scattering intensity
compared to the higher energy L-mode.
Figure 6 shows the electric field intensity and the surface charge distribution profile for the
higher energy L-mode at 443 nm, the lower energy L-mode at 575 nm and the T-mode at 373 nm
in a D=60 nm silver and copper nanoparticle heterodimer. As expected, the electric field intensity
is enhanced at the junction of the Ag-Cu heterodimer for the higher energy L-mode and the lower
energy L-mode. The surface charge configurations exhibit a dipole-dipole (DD) pattern for thelower energy L-mode and a quadrupole-quadrupole (QQ) pattern for the higher energy L-mode.
The charges confined to the same edge of the nanoparticle show the same sign for the lower energy
L-mode and the opposite sign for the higher energy L-mode. This indicates that the lower energy
L-mode is an in-phase collective plasmonic mode and the higher energy L-mode is an out-of-phase
collective plasmonic mode. The lower energy dipole-dipole L-mode for these modes at 575 nm is a
bonding-mode () in the classification frame of the plasmon hybridization model.
Compared to collective plasmonic modes in the Ag-Au heterodimer the higher energy
quadrupole-quadrupole (QQ) L-mode is more prominent in the scattering spectra of the Ag-Cu
heterodimer. The Ag-Cu heterodimer supports only one T-mode at 373 nm, which is almost the
same as the T-mode at 373 nm in the Ag-Au heterodimer, which is a dipole-quadrupole (DQ)
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FIG. 5. Scattering spectra for a D=60 nm sliver and copper nanoparticle heterodimer as a function of the polarization angle
with interparticle separations of 3 nm (a). Scattering spectra under longitudinal and transverse polarization with respect to
the normalized scattering spectra for a 60 nm isolated copper nanoparticle (b), and as a function of interparticle separations
from 1 nm to 15 nm under a polarizer angle of 0 degrees (c).
collective plasmonic mode. The quadrupole charge pattern in the copper nanoparticle is due to phase
retardation effects of the Ag nanoparticles and not from the LSPR coupling of the gold particle
plasmons.
D. The dipole-dipole approximation of the plasmon hybridization model
According to the well-established model of plasmon hybridization introduced by Nordlander and
Halas,27, 28 it is sufficient to consider a longitudinal or transverse interaction for simple geometries.
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FIG. 6. Electric near-field profiles and surface charge distributions in a D=60 nm silver and copper nanoparticle heterodimer
for longitudinal and transverse resonance modes.
FIG. 7. Dipole-dipole interaction model from the first approximation of the hybridization model. A level scheme of two
coupled dipoles are shown where the bright mode is red and the dark mode is black (middle) while the surface charge
distributions in the dimer under longitudinal coupling (left) and transverse coupling (right) are also shown.
We set up a dipoledipole interaction model as a first approximation of the plasmon hybridization
model, as shown in Fig. 7, and the collective plasmon modes can be viewed as bonding and antibond-
ing linear combinations of primitive plasmons in molecular orbital theory. The individual nanosphere
plasmon modes (1 and 2) of two interacting metal nanoparticles hybridize either in-phase (1 +
2) or out-of-phase (1 2). Under longitudinal polarization the in-phase combination reflects
a bonding mode and the out-of-phase mode represents an antibonding configuration, respectively
denoted and *.29 When the polarization is transverse the in-phase combination is an antibonding
mode (*) and the out-of-phase mode is a bond mode ().29
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1. D=60 nm Ag-Ag homodimer
I n a D=60 nm Ag-Ag homodimer (Figs. 1 and 2) the low-energy L-mode at 464 nm is a -mode
with its electric field strongly localized at the junction of the dimer and the only T-mode at 364 nm is
a *-mode. Both of these modes are bright in-phase modes. However, the antiphase mode was not
observed in the Ag-Ag homodimer. These results agree with the dipole-dipole approximation of theplasmon hybridization model wherethe antiphasemode is spectrally dark, because of the cancellation
of the equal but oppositely oriented dipoles on the two particles. Therefore, the homodimer structure
under longitudinal polarization supports only a bonding plasmon mode () and the homodimer
under transverse polarization supports only an antibonding *-mode. It is important to note that the
high-energy L-mode at 357 nm, as observed in the D=60 nm Ag-Ag homodimer, is a quadrupole-
quadrupole (QQ) mode.
2. D=60 nm Ag-Au heterodimer
In the D=60 nm Ag-Au heterodimer (Figs. 3 and 4), the lower energy L-mode at 545 nm is a
bonding -mode and a bright in-phase mode, while the lower energy T-mode at 527 nm is a bonding
-mode and an otherwise dark antiphase mode. The high energy L-mode at 410 nm is a bonding
quadrupole-quadrupole (QQ) mode and the high energy T-mode at 373 nm is a dipole-quadrupole
(DQ) mode. The Ag-Au heterodimer does not support the antibonding *-mode and the *-mode.
These are not the expected results, because the heterodimer is expected to support all the bonding
and antibonding modes at the same time because of its broken symmetry. This is in striking contrast
to previous work on a D=40 nm Ag-Au heterodimer, which show that it is possible to excite all four
bonding and antibonding modes through the preparation of compositionally asymmetric dimers. 22
While the in-phase *-mode is a bright mode as observed in the D=60 nm Ag-Ag homodimer,
the absence of* i n t h e D=60 nm Ag-Au heterodimer is not just due to its antibonding property. The
magnitude of the coupling of the anti-bonding mode to external light will depend on the net dimer
dipole. The anti-bonding mode has zero dipole moment in homodimer, which does not give rise to
a sufficiently large cross section to be seen in the extinction spectra. By reducing the symmetry, the
anti-bonding mode for a heterodimer has a non-zero dipole moment because of the asymmetry in
composition, as in this study. Though it interacts more weakly with light than the bonding dimer
mode, the anti-bonding heterodimer mode gains intensity and eventually is seen in the extinction
spectra. In this sense, both antiphase * and modes have similar magnitudes of coupling to light.
The * and *-modes are absence in the D=60 nm Ag-Au heterodimer. The proposed reason
is that the high-energy antibonding *-mode and the *-mode are blue-shifted with respect to the
LSPR of the isolated gold nanoparticles (2), and then they are typically located near the ultraviolet
(UV) region, are damped by the interband transitions of the gold nanoparticles, and appear as dark
modes in the heterodimer.
Theoretically, noble metal nanoparticles (Cu, Ag & Au) attract significant attention because
they can support localized surface plasmon resonances in the ultraviolet, visible, and near-infrared
regions of the spectrum depending on internal and external effects. The optical properties of these
metal nanoparticles originate not only from the coherent oscillation of conduction electrons but also
from optical excitation of an electron from an occupied state in the valence band to an unoccupied
state in the conduction band, which is known as an interband transition. In view of this electronicstructure theory,1 it is very important to consider the interaction of electronic interband transitions
with individual plasmon resonances or coupled plasmon modes.
According to the model of plasmon hybridization, there are two plasmon modes under longi-
tudinal polarization, namely a low energy in-phase bonding mode and a high energy out of phase
anti-bonding mode. Generally, the low energy bonding mode is located at visible or near infrared
wavelengths, while the higher energy anti-bonding mode is positioned near the ultraviolet region.
Since the anti-bonding mode shows small net dipole moment, the strength of its interaction with
light is also weak and it may be obscured by the interband transitions in the metal.
The antiphase -mode in a D=60 nm Ag-Au heterodimer is an important result in this study,
because its antisymmetric plasmonic resonance may be used to excite the magnetic response 13
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in various arrays of nanoparticle pairs. We confirm that this antiphase mode occurs in Ag-Au
heterodimers for a diameter range of 40-60 nm and a separation range of 1-3 nm.
3. D=60 nm Ag-Cu heterodimer
In the D=60 nm Ag-Cu heterodimer (Figs. 5 and 6) the lower energy L-mode at 575 nm is
a bonding -mode and a bright in-phase mode, the high energy L-mode at 443 nm is a bonding
quadrupole-quadrupole (QQ) mode and the high energy T-mode at 373 nm is a dipole-quadrupole
(DQ) mode. The Ag-Cu heterodimer does not support a *-mode, a *-mode or even a -mode
as this is not expected for the heterodimer. Similar to the Ag-Au heterodimer, a possible reason is
that both the high energy */*-mode and the low energy -mode are diminished by the interband
transitions of copper nanoparticles and appear as dark modes in the heterodimer. The electronic
interband transitions of the metals are electronic transitions from the energetically deeper valence
band into the conduction band, which forms a steep absorption edge starting at the threshold
frequency, where all plasmon resonance diminishes dramatically. The threshold energies are 3.8 eV
(326 nm), 2.5 eV (500 nm) and 2.1 eV (590 nm) for pure Ag, pure Au and pure Cu, respectively. By
comparing the collective plasmon modes of the 60 nm Ag-Au heterodimer with that of the 60 nm
Ag-Cu heterodimer there is tendency that the low interband transition energies of the constitutivemetals causes the low energy -mode and the two high energy antibonding modes to be diminished
by the interband transitions of the copper nanoparticles, and this causes the lowest energy -mode
to be visible in the Ag-Cu heterodimers.
Compared to previous studies into theD=40 nm Ag-Auheterodimer,22 the high energy collective
plasmon modes are governed by the quadrupole moment in the two 60 nm Ag-Au (Cu) heterodimers
from 357-443 nm. Although resonant excitation of larger spherical particles gives rise to broad
extinction spectra with significant contributions from high-order multipole modes, for the silver
particle of size D=60 nm in this study the quadrupole absorption components were not able to be
isolated form the dipole scattering for Ag-Au (Cu) heterodimers.
The proposed reason is a retardation (finite size) effect, which is also found in the larger
homodimer, as shown in the quadrupole-quadrupole L-mode in the D=60 nm Ag-Ag homodimer.
The retardation effect becomes more apparent in larger nanostructures with respect to the spatial
wavelengthof thelight at theplasmonresonance andthe onset of higherordered multipolar resonance
modes strongly influences and even shadows the position of the anti-bonding resonance. In this
study, a quadrupole-coupled mode emerges for the D=60 nm Ag-Au nanoparticle heterodimer at
interparticle separations smaller than 9 nm, and it is prominent in the D=60 nm Ag-Cu nanoparticle
heterodimer while it has the highest scattering intensity for all the investigated interparticle gap
separations and polarizer angles.
In general, the optical properties of coupled plasmonic particles can be systematically modeled
and studied in to two regimes depending on the overall size of the system. When the size of the
hybridized system is smaller than the plasmon wavelength, the coupled plasmonic system can be
modeled under the quasi-static (dipole) limit, thus retardation effect can be neglected. However,
when the dimension of the coupled system is greater than the incident wavelength the quasi-
static approximation is no longer applicable (results in significant error) because of retardation
effects. Therefore, it is desirable to consider retardation, which is the effect of the phase differencebetween the fields propagating from two different regions of the nanoparticle, during the study
of large size coupled plasmons to obtain reasonable and accurate optical properties for the given
system.
In the quasi-static regime, we get a constant resonant frequency independent of particle dimen-
sions or retardation effects, if the bulk dielectric is used the size effect in the quasi-static regime
comes from the dependence of the permittivity on the nanoparticle size because of quantum con-
finement. For nanoparticles which are large in size, as in our case, the quantum confinement or size
dependence of dielectric coefficients becomes negligible and the role of retardation effect comes to
play and higher order multipolar resonance modes begin to be observed. Therefore, the appearance
of higher order multipolar modes is due to retardation or finite size effect.
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FIG. 8. Scattering spectra calculated for varying dimer separation and conductive overlap for the silver-gold heterodimer.
E. Effects of conductive contact on the collective plasmon modes
In real experiments, a molecular layer of surfactant molecules used in the self-assemble may
actually wet the particle surface. The gap between the heterodimers Au-Ag and Ag-Cu is probably
governed by the size of the molecular species and is expected to keep the nanoparticles 2-3 nm apart,
however, conductive contact can still occur at localized points, especially if the coating is uneven and
considering that the molecules on the nanoparticle surface can rearrange. These issues are especially
significant for correlating simulations with experiments while even high resolution TEM only yields
a 2-D projection of 3-D objects, thereby obscuring important topographical and structural details.
Figure 8 shows the scattering spectra calculated for varying dimer separation and conductive
overlap for the silver-gold heterodimer. The accompanying near-field plots and surface charge
distributions corresponding to selected separation and wavelength points are shown in Figure 9. As
the gap decreases from 3 to 0 nm, the spherical nanoparticles begin to make conductive contact at
a single point: the 545 nm peak corresponding to the dipole-dipole (DD) coupled plasmon mode
splits into two modes, i.e., a DD coupled mode at 554 nm and a new infrared dipole-dipole (DD)
coupled mode at the 1128 nm peak, respectively. The 410 nm peak corresponding to the quadrupole-
quadrupole (QQ) coupled plasmon mode splits into two modes, one red shifts from 410 nm toa quadrupole-quadrupole (QQ) coupled mode at 414 nm, the other blue shifts from 410 nm to a
quadrupole-dipole (QD) coupled mode at 384 nm. With increasing conductive overlap from 0 to
-3 nm, the infrared dipole-dipole (DD) coupled mode at 1128 nm blue shifts to a dipole nanorod
mode at 772 nm and a new coupled mode appears at 364 nm due to retardation effects. However, the
dipole-dipole (DD) coupled mode at 554 nm, the high-order multipolar quadrupole -dipole (QD) at
384 nm and quadrupole-quadrupole (QQ) at 414 nm coupled modes disappear in the new scattering
spectrum with gap=-3 nm, in part, due to mode decoupling.
Figure 9 shows the electric near-field profiles and surface charge distributions in a D=
60 nm sliver-gold nanoparticle heterodimer at selected wavelengths with gap = 3nm, 0 nm and
-3 nm. Obviously, oscillations driven by external light in electrically separated particles must respect
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FIG. 9. Electric near-field profiles and the surface charge distributions for neighboring silver and gold spheres with their
separation gap=-3 nm, 0 nm and 3 nm, the wavelength corresponding to selected points of Fig. 8. For the dimer with gap=
3 nm, the near-field maps at = 1288 nm is also calculated before the single point contact to check conductive effects, the
near-field maps at = 410 nm is not plotted here and it is same to the profile in Fig. 4.
charge neutrality within each particle. In the near-field maps of the dipole-dipole (DD) coupled mode
of the Ag-Au dimer with gap= 3 nm at = 1288 nm, the charge pileup in each particle near the gap
region has a spatial width along the sphere surface that is compensated by a smooth distribution of
the opposite charge along the rest of the sphere surface.
Because intraparticle charge neutrality is no longer required when the two particles overlap (i.e.each half of the overlapping dimer can have a net charge), it is expected that the charge buildup at the
physical gap of separated particles would neutralize as soon as the gap is diminished. Surprisingly,
the charge remains on the edges of the narrow conductive neck, as shown in the Ag-Au dimer with
gap= 0 nm at = 1288 nm. The large pileup of induced charge in the interparticle gap gives rise to
a physical mode that corresponds to the infrared peak that emerges right after the particles touch at
a single point in Fig. 8. For the infrared dipole-dipole (DD) coupled mode at 1128 nm, there is a net
electrical charge in each half of thedimer, that is, eithersilver or gold particle is charged, as illustrated
in the lower inset of Fig. 9. This shows that the dominant low-energy modes of non-touching and
touching dimers are distinctly different. For non-touching spheres, the lowest-energy dipole-dipole
(DD) mode, which arises from coupling between dipole modes of each sphere, is charge neutral in
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FIG. 10. Scattering spectra calculated for varying dimer separation and conductive overlap for the silver-copper heterodimer.
each sphere. For touching spheres, the lowest-energy mode is a true dipolar mode with a net charge
on each particle.
As shown in the Ag-Au dimer at = 772 nm, when the overlap grows to gap= - 3 nm, the
charge in each particle has a smooth distribution along the sphere surface, the charge pileup effect at
the narrow conductive neck eventually disappears, and the response again becomes that expected for
a single particle. The 1128 nm lowest-energy mode of the single-point-touching dimer transitions
into the 772 nm higher energy mode of the overlapping dimer. The charge density plot shows the
conductive 772 nm mode resembles the dipole mode of a continuous nanorod having twice the
length of the constituent nanospheres, and the near-field map and charge distribution of the 364 nm
mode resembles the dipole mode of an individual silver nanosphere.30
Figure 10 shows the scattering spectra calculated for varying dimer separation and conductive
overlap in the silver-copper heterodimer, and their accompanying near-field maps and the surface
charge distributions are similar to the corresponding modes in the silver-gold heterodimer. As the
spherical nanoparticles begin to make conductive contact at a single point with gap=0 nm, the
575 nm peak corresponding to the dipole-dipole (DD) coupled plasmon mode splits into two modes,
i.e., a dipole-dipole (DD) coupled mode at 438 nm and a new infrared dipole-dipole (DD) coupled
mode at the 1123 nm. The 443 nm peak corresponding to the quadrupole-quadrupole (QQ) coupledplasmon mode disappears in thenew scattering spectrum with gap=0 nm. With increasing conductive
overlap from 0 to -3 nm, the infrared dipole-dipole (DD) coupled mode at 1123 nm blue shifts to a
dipolenanorod mode at 762nm anda new coupled mode emerges at 365nm dueto retardation effects,
and the dipole-dipole (DD) coupled mode at 438 nm disappears in the new scattering spectrum with
gap=-3 nm.
The presence of new plasmon modes due to the conductive contact has been observed in both
silver-gold and silver-copper systems approaching toward a single point of contact and the plas-
mon resonance mode has a similar wavelength around 1223 -1228 nm. As the particles approach
closer with gap = -3 nm, the dominant dipolar feature in both metallic system becomes simi-
lar with the plasmon resonance around 762-772 nm. However, at a single point of contact, with
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gap = 0 nm, the silver-gold heterodimer exhibits a richer structure of higher-energy modes than
the silver-copper heterodimer. The silver-gold heterodimer exhibits three resonance peaks and the
silver-copper heterodimer exhibits one resonance peak with two shoulders in the wavelength range
300-650 nm of the scattering spectra.
IV. CONCLUSIONS
In summary, the plasmon-coupling phenomenon of heterodimers composed of silver, gold and
copper nanoparticles with a size of 60 nm were studied theoretically using the finite difference
time domain method within the outline of the scattered field formulation. The energies and the
line widths of the collective plasmon modes strongly depend on interparticle gap separations and
polarization angles. In-phase bonding-modes were observed forthe Ag-Au andAg-Cu heterodimer
and an antiphase bonding -mode was observed for the Ag-Au heterodimer, and these are in good
agreement with that expected from plasmon hybridization theory. However, the quadrupole-coupled
mode dominates the high energy wavelength range from 357-443 nm in the scattering cross section
of the D=60 nm Ag-Au and Ag-Cu heterodimer. We demonstrate for the first time that collective
plasmon modes in a compositionally asymmetric nanoparticle dimer like the D=60 nm Ag-Au and
Ag-Cu heterodimer cannot simply be predicted using plasmon hybridization theory. The interband
transition energies of the constitutive metals and nanoparticle size retardation can overlap spectrally
with the antibonding mode from plasmon hybridization, which results in new collective plasmon
modes, such as a bonding quadrupole-quadrupole (QQ) mode and a dipole-quadrupole (DQ) mode
in the Ag-Au heterodimer. This is a unique phenomenon and has not previously been observed in a
compositionally asymmetric nanoparticle dimer.
ACKNOWLEDGMENTS
This work was supported by the National Natural Science Foundation of China (Nos. 50971100
and 50671082), the Research Fund of State Key Laboratory of Solidification Processing (NWPU),
China (Grant No. 30-TP-2009), the NPU Foundation for Fundamental Research (No. NPU-FFR-
ZC200931), and the Graduate Starting Seed Fund of Northwestern Polytechnical University (Nos.
Z2011002, Z2010011, and Z200915).
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