Coupling of Elementary Electronic Excitations: Drawing Parallels

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Coupling of Elementary Electronic Excitations: Drawing Parallels Between Excitons and Plasmons Reshmi Thomas, Jatish Kumar, Jino George, Madhavan Shanthil, G. Narmada Naidu, Rotti Srinivasamurthy Swathi, and K George Thomas J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b01833 • Publication Date (Web): 02 Feb 2018 Downloaded from http://pubs.acs.org on February 4, 2018

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The Journal of Physical Chemistry Letters

Coupling of Elementary Electronic Excitations: Drawing Parallels Between Excitons and Plasmons Reshmi Thomas, Jatish Kumar, Jino George, M. Shanthil, G. Narmada Naidu, R. S. Swathi* and K. George Thomas* School of Chemistry, Indian Institute of Science Education and Research Thiruvananthapuram (IISER-TVM), Vithura, Thiruvananthapuram 695551, India E-mail: [email protected], [email protected]

ABSTRACT

Recent advances in understanding the theoretical and experimental properties of excitons and plasmons have led to several technological breakthroughs. Though emerging from different schools of research, the parallels they possess both in their isolated and assembled forms are indeed interesting. Employing the larger framework of the dipolar coupling model, these aspects are discussed based on the excitonic transitions in chromophores and plasmonic resonances in noble metal nanostructures. The emergence of novel optical properties in linear, parallel and helical assemblies of chromophores and nanostructures with varying separation distances, orientations and interaction strengths of interacting dipolar components is discussed. The increased dipolar strengths of plasmonic transitions over the excitonic transitions, arising due to the collective nature of the electronic excitations in nanostructures lead to the emergence of hot

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spots in plasmonically coupled assemblies. Correlations on the distance dependence of electric field with Raman signal enhancements paved way to the development of capillary tube-based plasmonic platforms for the detection of analytes.

Elementary electronic excitations in atoms, molecules, and nanostructures are at the heart of a plethora of applications in molecular and material sciences, ranging from electronics, solar cells and light harvesting, to sensing. Since various elements have their signature absorption or emission spectral patterns, the study of elementary electronic excitations in atoms has led to the birth of atomic spectroscopy, one of the simple, yet elegant techniques for the determination of elemental composition of chemical systems. The discrete spectral lines obtained upon irradiation of atoms with photons are rather sharp and well defined, particularly at low pressures where line broadening factors such as collisions are absent.1-2 In molecular systems, electronic excitations are typically associated with excitations from highest occupied molecular orbitals to lowest unoccupied molecular orbitals. Study of electronic excitations in organic chromophores has especially been of great interest to the photochemistry community as it is a fundamental phenomenon that has tremendously helped to elucidate and characterize the electronic properties


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and resulting applications of a variety of molecules.3 Unlike in atoms, the electronic spectra of such chromophores are complex. For molecules, even in the gas phase at low pressures, electronic degrees of freedom couple with the vibrational and the rotational degrees of freedom, thereby contributing to a range of energies over which transitions can happen. Hence, the sharp spectral lines that characterize atomic transitions are replaced by a set of closely spaced lines evolving into bands of finite widths in molecular systems.2 In case of electronic excitations in isolated atoms and molecules, the excited electrons eventually recombine with the holes and the original atoms or molecules are regenerated. On the other hand, the proximity of other atoms or molecules as in aggregates or in solids can bring in a lot of interesting effects arising due to charge carrier delocalization, leading to applications ranging from organic semiconductors, solar cells to other optoelectronic devices. When a large number of atoms or molecules combine to form a solid, their individual atomic or molecular orbitals hybridize to form sets of close lying states evolving into various electronic bands. Thus, in solids, electronic excitations are described in terms of the electronic band structure.4-5 An electronic excitation from a lower energy level to a higher energy level results in the generation of a hole in the lower level. The excited electron and the hole are bound to each other due to an electrostatic interaction, resulting in what is known as an exciton.6 The excitons in solid-state materials are quasiparticles that carry energy and are mobile. Depending upon the nature of interaction between the electron and the hole, excitons are classified into three types, namely Frenkel excitons, Wannier-Mott excitons and charge transfer excitons.7-8 Frenkel excitons are often encountered in organic semiconductors and molecular aggregates, possess large binding energies (~1 eV) and the charge carriers are tightly bound with a Bohr radius of < 1 nm. Wannier-Mott excitons are often associated with inorganic semiconductors, possess low binding


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energies (~0.1 eV) and the charge carriers are loosely bound (Bohr radius of < 10 nm). The optical response of solids to incident light has two distinct components of electron-hole pair formation: one arising from momentum-conserving interband transitions and the other from nonmomentum-conserving intraband transitions between states separated by the photon energy. While interband transitions are pure electronic excitations from the valence band to the conduction band, intraband transitions are phonon mediated transitions within a single band itself.5 The electronic excitations in atoms, molecules and solids that we have so far described are what are referred to as the single particle excitations. In the case of bulk metals and metal nanostructures, one often encounters a new class of electronic excitations, namely collective electronic excitations, referred to as plasmons. Plasmons are collective oscillations of conduction band electrons in metals. When metallic atoms are brought together, the valence electrons get detached and wander freely through the metal, while the metallic ions remain intact and play the role of immobile positive particles.9 Consequently, the gas of conduction electrons can be modeled classically by the application of the kinetic theory. Plasmons could be envisaged as mechanical oscillations of electron gas of a metal in the presence of an external electric field, which causes the displacement of negatively charged electrons against the fixed positive ion core.5 The excitation of these collective resonances leads to a rather strong absorption in the UVVisible region for metals such as gold and silver.10 Plasmons have been distinguished based on the dimensionality of the metallic system, (i) bulk plasmons, (ii) propagating surface plasmons, and (iii) localized surface plasmons.11,12 Bulk plasmons are associated with materials with large three-dimensional structures, and these oscillations occur at bulk plasmon frequencies. Propagating surface plasmons are optically excited surface plasmon modes of a metal at the


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planar interface between a metal and the surrounding medium when it interacts with light. When the surface plasmons are confined to low-dimensional materials such as nanoparticles, they are termed as localized surface plasmon resonances (LSPRs). The LSPR oscillation frequencies in metal nanoparticles are determined by factors such as size, shape, composition and the surrounding medium.13 An excitation of an LSPR in a metal nanoparticle (henceforth referred to as plasmon) is associated with collective motion of several conduction electrons, leading to the generation of a large electric field in the vicinity of the nanoparticle.14-15 Generation of such rather large electric fields around metal particles resulted in numerous interesting advances in the area of spectroscopy, namely surface-enhanced Raman scattering,16 surface-enhanced fluorescence,17 surface-enhanced second harmonic generation18 and so forth. In contrast, in the case of atoms or molecules, the generated electronic excitations are associated with an electric field that is rather tiny when compared to the plasmonic field. This can be attributed to the single particle nature of such excitations. The current perspective draws parallels between the elementary single particle electronic excitations in organic chromophores, referred to as the excitons and elementary collective electronic excitations in metal nanostructures, referred to as the plasmons. Excitons in organic chromophores and plasmons in spherical metal nanoparticles and metal nanorods are considered as prototypes for describing the parallels. Simplest organic chromophores that one could think of are benzene, naphthalene, and anthracene. Electronic excitation of benzene involves an electronic transition from a π level to a π* level (π→π* transition), which falls in the ultraviolet region of the electromagnetic radiation. In terms of electronic states, this is also denoted as the S0→S1 transition. The spectral intensity associated with such a transition is characterized by a quantity called transition dipole moment, which represents the transient dipole resulting from the


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displacement of charges during the transition; therefore, it is strictly not a dipole moment. For electronic transition from a state characterized by Ψ to a state characterized by Ψ , the transition dipole moment is given by

= Ψ∗

Ψ , where

refers to the dipole operator. The non-zero (zero) value of the transition moment integral represents the allowed (nonallowed) nature of a given electronic transition. Higher is the magnitude of the transition dipole moment, stronger is the corresponding transition in the electronic spectrum.19 The square of this theoretical quantity is directly proportional to the experimentally determined molar extinction coefficient, which for the forbidden n→π* transitions is in the order of a few hundreds or less in units of M-1 cm-1 whereas for the symmetry-allowed π→π* transitions, it is of the order of 104105 M-1cm-1.20 For polycyclic aromatic hydrocarbons (PAHs) such as benzene, naphthalene, anthracene, pyrene, perylene etc., the absorption transition moments corresponding to the π→π* transitions are in the molecular planes. As one moves from benzene to naphthalene to anthracene, two kinds of electronic transitions, namely S0-S1 and S0-S2 transitions, with varying transition moments are possible. The direction of the transition moment with respect to the molecular axis, however, depends on the electronic state to which the molecule reaches on electronic excitation. Electronic absorption of benzene is associated with a unique transition moment for the π→π* excitation, while the absorption of PAHs like anthracene, pyrene etc. can give to different transition moments along the long axes as well as the short axes of the molecules, depending on the direction of the electric vector of a linearly polarized light. Interestingly, recent research findings have demonstrated the existence of molecular plasmons in ionized PAHs, which can be attributed to the coherent superposition of multiple electron-hole


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pair excitation.21-23 Addition or removal of a single electron to PAHs was shown to switch on/off the molecular plasmons, indicating their applicability in waveguiding, plasmonic and optoelectronic devices. Unlike in conventional metal-based plasmonics, classical description of the molecular plasmonic features in PAHs is not possible and accurate quantum mechanical approaches like time-dependent density functional theory are necessary.21-23 Studies on molecular plasmonics are still in infancy and the current perspective deals with only metal-based plasmonic systems. Parallels between the plasmonic features in metal-based systems and the excitonic features in various organic chromophores are revealed by considering the optical features in plasmonic and excitonic assemblies. Plasmons in metal nanostructures are associated with charge density fluctuations, which are typically of dipolar, quadrupolar, and octupolar nature. The leading contributor among these is the dipolar mode of excitation of the conduction electrons against the positive ionic core. Such plasmonic dipole could be thought of as analogous to the transition dipole moment created in chromophoric systems (the extinction coefficient of plasmonic dipoles is ~109-1010 M-1cm-1). However, it is important to note that, the collective nature of the excitations in plasmons makes the plasmonic dipoles much stronger in comparison with the excitonic dipoles. As a consequence, the electric fields generated due to the plasmonic dipoles are much higher in magnitude when compared to the fields generated from the excitonic dipoles. The parallels between excitons and plasmons extend well beyond this. Similar to benzene, spherical metal particles give rise to the same LSPR mode irrespective of the state of polarization of the incident light. However, metal nanorods exhibit varying response for varying states of polarization of light, analogous to the observed electronic transitions with varying transition moments along the long axes as well as the short axes in anthracene, pyrene etc.24 One refers to the plasmonic


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excitations along the length of the nanorod and along the breadth of the nanorod as longitudinal and transverse plasmonic modes of excitation, respectively. The magnitude of the electric field generated under transverse polarization of the incident light is much smaller than that obtained for longitudinal polarization.25 This can be attributed to the stronger dipole that is created on the nanorod on longitudinal excitation when compared to the transverse excitation. This is what is usually referred to as the lightning rod effect or the edge effect and can be ascribed to the tendency of charges to accumulate at the rounded corners of the long ends.26-27 Also, the absorption cross-sections for the longitudinal modes are more than an order of magnitude higher than those for the transverse modes. As one would expect, the strength of the dipolar mode can further be controlled by modulating the aspect ratios (length to breadth ratios) of the nanorods.28 Aggregation of chromophoric dyes played significant role in imaging technology, particularly in film-based photography, which is one of the major technological developments in the history of humankind. The molecular packing of dye aggregates in solution state, thin film and solid states and their photosensitization behavior thus became an important area of research. Various intermolecular forces (from weak van der Waals forces to hydrogen bonding) play a significant role in the aggregation of chromophoric dyes, substantially modifying their ground as well as excited state properties.29 Among these, a notable one is the spectral shifts observed upon aggregation leading to the occurrence of red and blue shifted transitions, compared to the monomeric transition, denoted as J- (J for Jelley, one of the researchers who first observed the transition)30-31 and H-bands (H for hypsochromic), respectively.32 Indeed, the spectral widths, oscillator strengths, emission quantum efficiencies and lifetimes of monomeric and aggregated forms of chromophoric dyes are distinctly different.33 The modified optical properties of assembled chromophoric systems are governed by the strengths, positions and the orientations of


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the interacting dipoles, and these aspects are explained within the framework of the molecular exciton coupling theory.34 In exciton coupling theory, we model the interaction between two molecular units in an aggregate by considering the interaction of their transition charge densities electrostatically. If the intermolecular electron density overlap is small, such that the molecules preserve their individuality in the aggregates, formalisms of perturbation theory can be employed, the perturbation potential being the Coulombic potential (V12). The interaction energy due to exchange of electronic excitation energy between the two units, also known as the exciton splitting term is therefore given by

= ∬ Ψ∗ Ψ Ψ Ψ∗ , which, in the dipolar approximation becomes

=

∙

−

∙ ∙

.

In the above equation, M1 and M2 are the transition dipole moments associated with molecules 1 and 2 respectively, and r is the separation vector between the centres of the two dipoles.35 The exciton splitting energy is proportional to the inverse cube of the intermolecular distance. In terms of the angle between the two molecular planes (α), and the angle between the polarization axes and the line of molecular centers (θ), the exciton splitting term for the interaction between two identical entities becomes

=

||

!"#$ − 3!"# & .

When the angle between the molecular planes, α is 0°, as in the case of H- and J- aggregates,

becomes


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=

||

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1 − 3!"# & .

As illustrated by the above equation, greater the magnitude of the transition dipole moment, larger will be the band splitting (Figure 1). In H-aggregates (θ=90°), the out-of-phase dipole interaction gives rise to an attractive interaction, which in turn leads to the lowering of energy ( is negative) whereas the in-phase dipole arrangement gives rise to a repulsive interaction raising the energy ( is positive). Since the net transition moment is given by the vector sum of the transition dipole moments of the individual components and the higher energy state of the aggregate has non-zero net transition moment, the transition from the ground state to the higher energy level of the H-aggregate is the only allowed transition. Such a transition results in a hypsochromic shift of

||

in energy. On the other hand, in J-aggregates (θ=0°), the transition to

the lower energy state having an in-phase arrangement of the transition dipoles is the only allowed transition and this results in a bathochromic shift.34 However, the magnitude of the shift ||

is higher in this case as the interaction energy is two-fold ( = -

).

Parallel to this, plasmonic nanostructures also exhibit similar spectroscopic responses on aggregation leading to the development of newer class of optical sensors.36 The extinction spectral shifts observed in assembled plasmonic systems can be explained on the basis of plasmon hybridization model, developed by Nordlander and coworkers.37 Plasmon hybridization theory is the electromagnetic analog of the molecular orbital theory and is a physically appealing model that has been used extensively for understanding the optical excitations in complex nanostructures. When the nanoparticles form a cluster, the plasmon states of the individual particles strongly interact to form the corresponding cluster states, in analogy to the building up of molecular orbitals by linear combinations of atomic orbitals. Similar to molecular orbitals, the


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plasmonic states can be designated as bonding or antibonding, depending on the ways in which the induced charges are distributed in various configurations. In addition, depending on the magnitude of their net dipole moment, the modes of the cluster can either appear in the plasmon spectra (“bright” modes) or be invisible (“dark” modes). Like in molecular aggregates, the strength of interaction between the dipoles, which increases as the distance between the particles decreases, dictates the energetic splitting of the pairs of plasmonic modes.38 A comprehensive illustration of both excitonic and plasmonic coupling is presented in Figure 1 by considering chromophoric systems and Au nanorods as examples, based on the research work carried out in our group. Absorption spectral changes on (i) increasing the concentration of a chromophoric dye, namely bis(2,4,6-trihydroxyphenyl)squaraine39 and (ii) inducing solvent polarity dependent folding of a bichromophoric cyanine dye40 are presented as Figures 1A and 1C, respectively. Monomers of the former system possess a broad absorption band at the higher energy region (400-530 nm) followed by the formation of a sharp band at the lower energy region (565 nm) on increasing concentration. This is attributed to the formation of a head-to-tail assembly of chromophores, alternatively called as J-type aggregates. In contrast, the unfolded bichromophoric cyanine dye possesses a broad band at the lower energy region which undergoes solvent dependent conformational switching to a folded form having a transition in the higher energy region (λmax= 420 nm; H-type aggregates). These spectroscopic features are fairly understood in terms of exciton coupling theory in which the excited state of the dye splits into two energy levels (Davydov splitting), as illustrated in Figure 1B. These situations refer to the extreme cases, wherein orientations are either parallel or linear. The transition to the upper excited state is allowed in the folded (parallel) dimers and to the lower energy state is allowed for the head-to-tail (linear) dimers, characterized as H- and J-bands, respectively.


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Analogous to the excitonic interactions, plasmon resonances in noble metal nanoparticles also undergo coupling when they are brought in proximity. However, spherical Au nanoparticles often undergo uncontrolled aggregation due to their isotropic nature. In contrast, anisotropic features of Au nanorods allow their assembly in various orientations. Effect of mutual orientation of two nanorods on their plasmon resonance spectra at various distances has been investigated by Gluodenis and Foss by invoking a simple quasistatic treatment.41 Indeed, the interesting crystallographic features of Au nanorods with the end facets dominated by {111} planes and the side facets by {100} and {110} planes result in the preferential binding of thiols onto the former planes.42 By exploiting these characteristics, the linear assembly of Au nanorods was first demonstrated by Murphy and coworkers using biotin-streptavidine connectors.43 Following this, several strategies have been developed for assembling Au nanorods using electrostatic, supramolecular and covalent approaches.11, 44-45 Yet another interesting feature of Au nanorods is the existence of dipolar resonances in the form of longitudinal and transverse surface plasmon bands which permits selective coupling of various modes in linear and parallel assemblies.46-47 Plasmon coupling in Au nanorods was first experimentally verified by our group by their linear organization, and this strategy has been extensively utilized by various research groups for modulating optical properties through their controlled organization.48-49 The selection of linker molecules plays an important role in determining the ways in which Au nanorods organize: aromatic dithiol namely 1,2-phenylenedimethanethiol (PDT), which is more or less rigid in nature and an aliphatic dithiol, namely 1,6 hexanedithiol (C6DT) which possesses a flexible chain are used in the present case for achieving linear assemblies49. For assembling Au nanorods as parallel stacks, tetra(ethyleneglycol)undecanethiol is used as the linker molecule (Figures 1D and 1F). 50 The plasmon coupling of linear dimers results in the formation of a band in the lower


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energy region to that of the longitudinal plasmon mode of the monomeric Au nanorod. Spectral shifts become more prominent with time due to oligomerization of Au nanorods as higher order linear and parallel assemblies. It may be noteworthy that the assembly of Au nanorods in linear fashion is much faster than the formation of parallel stacks. The faster linear oligomerization of Au nanorods is attributed to the presence of more reactive {111} planes at their edges which are less covered with cetyl trimethylammonium bromide (CTAB). In contrast, the Au nanorods are thickly packed by the bilayer of CTAB along the lateral face and the linker molecules have less interactions to yield parallel assemblies. Plasmon hybridization model can be used to describe the hybrid states in dimeric nanostructures as bonding and antibonding modes resulting from the linear combinations of individual nanorod plasmons. According to this, organization of Au nanorods into dimers as linear assembly gives rise to four hybridized plasmon modes on exciting with longitudinal and transverse polarized light: two bonding and two antibonding modes (see, Figure IE for longitudinal polarization). Two of the modes are optically active (bright modes; dipoles add up), and the other two are optically inactive (dark modes; dipoles cancel each other). Both the bright modes can in principle be observed for linear assembly when unpolarized light is used. In the case of parallel assembly of Au nanorods, for longitudinal polarization, the higher energy mode is active (bright mode) and the lower energy mode remains as the dark mode (Figure 1H), whereas for polarization along the transverse axis, the lower energy mode is active (bright mode) and the higher energy mode is the dark mode. Hence, we observe a blue shift in the longitudinal plasmon band and a red shift in the transverse plasmon band during the parallel assembly of Au nanorods in the presence of unpolarized light. Thus, the parallel features observed in the linear and the parallel arrangements of excitonic and plasmonic systems provide a more generalized


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picture on plasmon and exciton coupling within the larger framework of the dipolar coupling model.

Figure 1. (A) Concentration dependent aggregation of a squaraine dye (1.2-13.3 µM) and absorption spectral changes. The broad band (400-530 nm) and the sharp band at 565 nm


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correspond to monomeric forms (blue traces) and J-aggregates (red traces), respectively. (B) Schematic illustration of the energy levels of the ground and the excited states of the monomeric form, as well as the J- and H-aggregates of a chromophoric dye. (C) Solvent-dependent folding of a bichromophoric cyanine dye in toluene-CH2Cl2 mixtures and absorption spectral changes. The broad band in the long wavelength region and the sharp band at 420 nm correspond to monomeric form (unfolded) and H-aggregates (folded), respectively. (D-I) Extinction spectral changes followed using unpolarized light on addition of various linker molecules (PDT/C6DT/TEGU), plasmon hybridization schemes based on linearly polarized light for the linear and the parallel assemblies of Au nanorods and representative TEM images: (D) extinction spectral changes during linear assembly on addition of C6DT, (F) TEM image of linear Au dimers in the presence of PDT. (Note: extinction spectral changes on addition of PDT and TEM images in the presence of C6DT are presented in ref. 49), (G) extinction spectral changes during parallel assembly on addition of TEGU (0-45 min), and (I) a representative TEM images during the early stage (15 min) of parallel assembly, and the plasmon hybridization schemes for (E) the linear and (H) the parallel assemblies of nanorods. Panel A adapted from ref. 39 with permission from American Chemical Society copyright 1993; Panel C adapted from ref. 40 with permission American Chemical Society, copyright 2001; Panels D and F adapted from ref. 49 with permission from Wiley, copyright 2008; Panels G and I adapted from ref. 50 with permission from The Royal Society of Chemistry, copyright 2014. Mechanistic investigations on the linear (end-to-end) assembly of Au nanorods to nanochains, in the presence of various dithiols, were reported by our group.48-49, 51 All the extinction spectral studies presented (for e.g., Figures 1D,G) are carried out in solution using unpolarized light. A decrease in the extinction maximum of the longitudinal plasmon mode is observed along with the


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concomitant formation of a new red-shifted band, ascribed to the formation of linear dimers of Au nanorods. The spectral changes due to dimerization are marked through a clear isosbestic point (Figure 1D), further to which the band gradually shifts to a longer wavelength with time due to oligomerization. This red-shifted peak corresponds to the bonding mode due to longitudinal polarization for nanorods assembled in the linear fashion (Figure 1E). In principle, a blue shift corresponding to transverse polarization is also expected during dimer formation; however, this anibonding mode is too weak to be observed. No isosbestic point is observed in the extinction spectrum during the parallel assembly of Au nanorods (Figure 1G), due to simultaneous blue shift in the longitudinal plasmon band and a red shift in the transverse plasmon band when unpolarized light is used (vide supra). The angle between the nanorods in dimers has pronounced influence on plasmon coupling and this aspect was investigated using Au nanorod dimers that are linked via rigid (PDT) as well as flexible (C6DT) linkers. An increase in the angle between the Au nanorods resulted in larger bathochromic shift in the coupled plasmon band confirming the role of effective dipolar coupling.49 The shape anisotropy permits the existence of numerous orientations for Au nanorods in assemblies leading to various coupling modes and optical properties arising from these aspects are noteworthy.52-53 However, the plasmon coupling investigations presented in the above section are carried out on ensembles in solution using unpolarized light. As a result, the observed optical properties of these assembled structures invariably arise from a statistical average of the overall nanoparticle sizes and shapes present in the ensemble. These studies do not provide quantitative information on the coupled plasmon resonances in higher order assemblies such as trimers and tetramers. This limits the exact correlation of various structure-optical property relationships of discrete assemblies. Single particle spectroscopy is ideal for understanding these aspects. In


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collaboration with the research group of Mulvaney, we have investigated the optical properties of assembled nanostructures at the single particle level by combining dark field microscopy and high resolution scanning electron microscopy.54 Investigations are focused on two different aspects of plasmon coupling: (i) the effect of number of nanorods (Figure 2A and corresponding SEM images) and (ii) the dependency of the angle between the nanorods from parallel (0°) to linear (180°) assemblies. Single particle investigations showed that the wavelength of the coupled plasmon band red-shifts significantly as the number of rods in the chain increases from one to four. However, the magnitude of the red shift of the plasmon wavelength decreases as the chain number increases. Nanorod assemblies with larger aspect ratios showed larger red shifts and by using an exponential fit,55 it was found that the red shift asymptotes at assemblies with 6– 8 nanorods for rods with an aspect ratio of 1.7 and at 10–12 nanorods for rods with an aspect ratio of 2.0.


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Figure 2. (A) Normalized experimental scattering spectra and corresponding SEM images of linearly assembled Au nanorods (aspect ratio of 1.7). (B-E) Non-polarized scattering spectra of nanorod dimers with angles of (B) 176±3°, (C) 117±3°, (D) 93±3° and (E) 0±0.5°. Insets show the corresponding SEM images of the dimers. Adapted from ref. 54 with permission from The Royal Society of Chemistry, copyright 2013.


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Dipolar coupling of plasmonic modes has a profound dependence on the angle between the dipoles and these aspects are investigated using Au nanorod dimers possessing different orientations. Only one scattering peak is observed for both linear (180°) and parallel dimers (0°). The linear dimer exhibited a bathochromically shifted scattering peak at 785 nm corresponding to the attractive coupling of the longitudinal plasmon modes. For parallel dimer, the transverse plasmon modes interact attractively undergoing a red shift, and the longitudinal plasmon modes interact repulsively undergoing a blue shift.52, 54 The result is a slight hypsochromic shift in the scattering peak (to 605 nm) compared to the monomer due to the dominance of the longitudinal plasmons in the polarization-averaged spectra. Nanorod dimers with angles between 180° and 0° showed two scattering peaks due to the varying contributions of the bonding and the antibonding modes to the total scattering intensities. As the dimer angle decreases, the lower energy band due to the longitudinal bonding mode becomes less intense and the higher energy band, due to the anti-bonding mode, becomes prominent. When dimer angles are close to 900, the peak intensities of the two modes become nearly equal. Single particle investigations thus confirm that Au nanoassemblies exhibit polarization-dependent optical properties due to the selective excitation of collective bonding and antibonding modes. More importantly, these results point to the fact that the size, shape and orientation of plasmonic elements in assembled nanostructures have a significant influence on their optical properties and single particle investigations are essential for understanding these aspects. Parallel to this, angle-dependent dipolar coupling is also observed in chromophoric dyes, resulting in the formation of J- and H-aggregates and aggregates with other intermediate orientations.56 Asymmetric dipolar coupling in excitonic and plasmonic systems is another topic of discussion of the current perspective. Circular dichroism (CD) spectroscopy is one of the most widely used


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tools for investigating the electronic transitions of chiral molecules and supramolecular and nanoscopic structures.57 A chiral system differently absorbs the right and the left circularly polarized light resulting in an elliptically polarized beam. Depending on the relative absorption of each of the circularly polarized components, CD signal possesses either positive or negative sign. According to quantum theory of CD, the CD spectroscopic signal is expressed in terms of the rotational strength for a given electronic excitation from the ground state to the jth excited state as

/0 = 12[450 ∙ 605 ], where 450 and 605 are the electric and the magnetic transition dipole moments, respectively. In order for the CD signal to be observed, both the transition moments should be non-zero and therefore the selection rules in CD are much more stringent than the selection rules in UV-Vis spectroscopy.57-58 Asymmetric organization of plasmons and excitons yields chiral plasmons and excitons, respectively showing bisignated CD signals and their discrete handedness is indeed interesting. The sign of the bisignated signals originating from the excitonic as well as the plasmonic CD spectra of supramolecular/nanoscopic assemblies is the thumb-rule for assigning the handedness of the assemblies, rather than microscopic images such as AFM and TEM. The positive couplet (otherwise termed as positive chirality) is usually used for a bisignated CD signal when the positive CD signal in the long wavelength region is followed by a negative CD signal in the short wavelength region (with zero-crossing matching the absorption maximum). Such spectroscopic signal is attributed to the formation of right handed helical conformation in the supramolecular assembly (P-form). In contrary, a negative couplet (otherwise termed as negative chirality) is usually used for a bisignated CD signal when the negative CD signal in the long wavelength region is followed by a positive CD signal in the short wavelength region. Such


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spectroscopic signal is attributed to the formation of left handed helical conformation in supramolecular assembly (M-form). While chirality of molecular systems is a subject of investigation for decades, plasmonic chirality is a relatively new topic. Substantial research activities have been focused in recent years on understanding the fundamental aspects of plasmonic chirality. In contrast to molecular systems wherein the degree of interaction is limited, the rather large-scale dipolar and multipolar interactions possible in plasmonic nanomaterials render intense chiroptical responses. Strong interaction of achiral plasmonic nanoparticles with chiral molecules can lead to the induction of optical activity in the electronic states of isolated nanoparticles.59-60 However, plasmon coupling between plasmonic elements organized in a chiral geometry, with the aid of chiral molecules or templates, is considered to be a more efficient approach to induce chirality in achiral plasmonic nanoparticles.61-62 One of the initial observations on plasmonic chirality were made in our group, the findings of which are presented in Figure 3A: spherical Au nanoparticles form chiral assemblies when adsorbed on diphenylalanine nanotubes giving rise to bisignated CD signals at their surface plasmon frequency.63 Specifically, these assemblies show positive and negative couplets when adsorbed on D- and L-peptide nanotubes, respectively, indicating the controlled organization of Au nanoparticles in clockwise and anti-clockwise directions, driven by the chiral molecules on the nanotubes. A comprehensive report on nanoscale chirality in metal and semiconductor nanoparticles is presented in recent reviews.64-66 Asymmetric organization of dipoles is the key factor that leads to symmetry breaking and intense chiroptical signals in helical plasmonic assemblies originate through the electromagnetic dipole–dipole interactions between large plasmonic dipoles. In an elegant theoretical demonstration, Govorov and coworkers have reported CD responses of various helical gold


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nanoparticle assemblies using electromagnetic calculations carried out within the dipolar framework.67 CD response originates from the absence of any mirror symmetry in the nanoparticle assembly, and its strength is dependent on the electric and the magnetic dipole moments. The observed CD signals are highly sensitive to the geometry, number and composition of nanoparticle assembly. For example, plasmonic CD band got inverted with slight variation in the number of nanoparticles in the assembly: positive CD couplets for helices with five and six particles and negative couplets for helices with four and seven particles. Later, it was demonstrated that, for long helices, CD signal remained almost unchanged when the number of nanoparticles increased. Detailed investigations of the influence of nanoparticle radius, helical pitch, helix radius and defects on the CD signals were carried out to obtain optimized conditions that will enable the design of plasmonic nanostructures exhibiting strong chiroptical activity in the visible range. Uniqueness of Au nanorods for generation of large surface plasmon CD was first reported by Liz-Marzán and co-workers: a large bisignated Cotton effect was observed at the plasmon coupled frequency of Au nanorods when adsorbed on chiral supramolecular fibres.68 Various experimental investigations and numerical calculations on anisotropic plasmonic nanostructures revealed that the strength of optical activity is largely dependent on the strength of the dipoles, distance and orientation between the dipoles as well as the size of the assembly. Fundamental understanding of the chiroptical properties of plasmonic nanostructures has opened up numerous possibilities in the design of nanoscale devices for chiral synthesis, sensing and catalysis. Some of the initial efforts in this direction include attomolar sensing of DNA by Kotov and co-workers,69 design of molecular switches and machines using DNA origami based Au nanorod assemblies by Liu and co-workers70 and fabrication of materials with negative refractive indices (metamaterials).71


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The roots of the dipolar coupling models applied in helical plasmonic assemblies go back to the exciton coupling observed in chiral chromophoric systems72 and in this section, we discuss the emergence of parallels between chiral plasmons and chiral excitons. Asymmetric dipolar interactions in chromophoric systems lead to the formation of chiral excitons, as is evident from the emergence of bisignated CD signals. These aspects are well established and documented for molecular systems.73 Hence, a proper understanding of the origin of plasmonic chirality can be envisaged by tracking the mechanism of organic self-assembly. In the process of understanding the origin of macroscopic chirality in organic soft and nanomaterials, a large variety of chiral organic chromophoric systems have been subjects of investigation.74-76 The introduction of stereocenters into the molecular building blocks results in the formation of helical structures with a preferred handedness. To unravel the structural pathways of molecular self-assembly, Meijer and co-workers carried out extensive research on the fibrillation of a pair of oligo(pphenylenevinylene) (OPV) enantiomers possessing chiral side chains.77-78 One end of the molecule was functionalized with a tridodecyloxybenzene and the other end by ureidotriazine, in order to attain self-complementary fourfold hydrogen bonding. The R- and S-OPVs under selfassembling conditions exhibited mirror image CD signals confirming the formation of aggregates possessing opposite helicity. Mechanistic investigations revealed that the hierarchical self-assembly involves different intermediates that determine the resultant helicities of the structures. Initial step involves the quadruple hydrogen bonding between the monomers of OPVs to form the dimers. Few dimers (10-15) combine to form disordered stack (isodesmic pathway), which under suitable conditions become more restricted in relative position (cooperative process) and undergo a coil-helix transition to form a chiral nucleus (~28 dimers). At this point, elongation-growth pathway sets in finally leading to lengthening of stacks and clustering of the


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assemblies. From these studies, it is evident that the formation of a chiral nucleus is the critical step that influences the growth of monomers to fibers with the desired helicity. Analogously, the initial symmetry breaking through the asymmetric arrangement of dipoles of the plasmonic elements will have a determining influence on the emergence of chiroptical properties in assembled plasmonic nanostructures.


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Figure 3. Helical organization of Au nanoparticles and chromophoric dyes and the formation of chiral plasmons and chiral excitons: (A) Bisignated CD signals with symmetrical mirror images at the surface plasmon frequency of Au nanoparticle assemblies with opposite chirality (note the direction of arrows). The growth is directed by chiral molecules on the D- and L-isomers of peptide nanotubes. (B) Calculated CD spectra of helical assemblies with 4 and 5 Au nanoparticles and the models with 4-7 nanoparticles. (C) Molecular structures and CD spectra of S- and R-chiral OPVs. Both the enantiomers assemble via a nucleated growth mechanism into helical stacks with left-handed (S-OPV) and right-handed (R-OPV) helicities. Panel A adapted from ref. 63 with permission, American Chemical Society copyright 2010; Panel B adapted from ref. 67 with permission, American Chemical Society copyright 2010; Panel C adapted from ref. 78 with permission from PNAS, copyright 2013. Further, we have investigated how the hybridization of plasmons influences the electric field prevailing on the surfaces of plasmonic substrates by following surface-enhanced Raman scattering (SERS). Hybridized plasmons generate hot spots, which are essentially regions of enhanced electric field. Selective functionalization of the edges of Au nanorods using thiol derivatives is a convenient way for placing the molecules at the isolated edges using monothiols and at the junctions of dimeric Au nanorods using dithiols. Such nanohybrid systems are useful for investigating the effect of the field at the Au nanorod edges and at the hot spots in the junctions. These aspects were investigated by linking a bipyridine moiety onto the edges of Au nanorods using monothiol derivative (bipyridinemonomethanethiol; bipy-MT), by placing a dithiol derivative (bipyridinedimethanethiol; bipy-DT) at the junctions of two nanorods and the extinction and Raman spectral changes were further probed (Figure 4).79 The extinction spectra of Au nanorods remain unaffected even at higher concentration of monothiols indicating that the


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nanorods remain stable and isolated in solution without any aggregation (Figure 4A, B). The Raman signals corresponding to the various vibrational modes of the monothiol showed a gradual increase with increase in concentration and leveled off at higher concentrations (Figure 4C). Interestingly, no Raman signals corresponding to the monothiol could be observed in the absence of Au nanorods even upon increasing the concentration by 104 fold. The extinction spectral changes observed on addition of dithiol derivative indicated dimerization, which was further confirmed through TEM imaging (Figure 4D,E). Interestingly, the dimerization step is marked with a spontaneous enhancement in Raman signal intensity (Figure 4F). The bipyridine molecules at the junctions experience high electric field resulting in enhanced Raman signal intensities (enhancement factor of ~105) through electromagnetic enhancement mechanism. Further, by combining the methodologies of linear and parallel assemblies, quartets of Au nanorods were designed. The large enhancement in the Raman signals of molecules was attributed to the high electric field existing at the quartet junctions. Finite-difference timedomain (FDTD) simulations showed better electric field distribution in slipped quartets compared to linear ones and these studies provide newer insights into the design of plasmonic platforms.50


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Figure 4. TEM images (A,D), extinction (B,E) and Raman spectra (C,F) on addition of two analytes (bipy-MT and bipy-DT) to Au nanorods (aspect ratio of 2.5; 0.12 µM). Extinction spectra of Au nanorods in a mixture of water and acetonitrile (1:4) in the absence and presence of analytes: traces a in B and E correspond to absence of analyte, traces (b-e) in B and E correspond to bipy-MT of varying concentrations (0.5-3.0 µM) and bipy-DT (1.0 µM) recorded successively in a time interval of 3 min. Corresponding Raman spectra are presented as C and F. Raman spectra are recorded using He-Ne laser (633 nm, acquisition time of 10 s). Adapted from ref. 79 with permission, American Chemical Society copyright 2011.


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As discussed in the earlier section, assembled nanostructures create enhanced electric fields at the nanogaps (hot spots) due to surface plasmon coupling, which are significantly higher compared to the monomeric nanostructures.80-81 Owing to the enhanced electric fields, analyte molecules placed at the nanogaps of assembled nanostructures showed considerably large enhancement in the spectroscopic signals.82-85 However, electric field has a strong distance dependence: the field decays as we move away from the surface of an isolated plasmonic nanoparticle and on varying the distance between the plasmonic nanoparticles in assemblies.86-87 These aspects were investigated by taking silica-coated silver nanoparticles (Ag@SiO2) of varying silica shell thickness (t = 3, 6, 10, 15, and 25 nm) as the plasmonic elements.88 The zeta potential (ζ) studies indicate that Ag@SiO2 nanoparticles possess negative surface charge and by adopting an electrostatic binding approach, SERS probe molecules (a pyrene molecule bearing ammonium ion; Py-A) were bound onto the surface of silica. The variation of electric field on the surface of silica forms the basis of SERS studies. Pyrene on silica surface experiences 88, 55, 33, 20, and 8 % of the total field on the Ag surface for t = 3, 6, 10, 15, and 25 nm, respectively. HRTEM images of representative Ag@SiO2 nanoparticles and corresponding FDTD simulation results showing the intensities of the electric fields in the vicinity of the corresponding nanoparticle systems are presented as Figure 5A-C and Figure 5A’-C’, respectively. The measured Raman spectra of Py-A obtained by varying the silica shell thickness are presented as Figure 5D. Maximum enhancement was observed for Ag@SiO2 nanoparticles with t = 3 nm (EF ∼ 103), followed by a decay with increase in shell thickness and distance from the particle centre (Figure 5E). Dimeric nanostructures having well-defined gaps between two Ag nanoparticles were prepared by varying the gap size (d) from 1.5 to 40 nm. The intensities of the electric field in the vicinity


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of the dimeric Ag@SiO2 nanoparticles, calculated from the FDTD method, were found to be two orders of magnitude higher than that of the incident field and an order of magnitude higher than those obtained for the corresponding monomers. HRTEM images of representative dimeric Ag@SiO2 nanoparticles and the corresponding FDTD simulation results showing distance dependent formation of hot spots are presented as Figure 5F-H and Figure 5F’-H’, respectively. The dimers with gap distance ≤15 nm showed significant Raman signal enhancement compared to the corresponding monomers (Figure 5I). Further increase in the gap size gave rise to rather negligible SERS enhancements. The enhancement factors for each of the dimeric systems plotted with respect to the gap size are shown in Figure 5J. The experimental Raman signal enhancement factors at the hot spots followed a 1/dn dependence, with n = 1.5, in agreement with the theoretical studies by Schatz and co-workers.89 Thus, by precisely controlling the gap distance, we could achieve enhancement factors as high as 1.5 x 105 in Ag@SiO2 dimeric systems. Distance dependent investigation of Raman signal enhancement at the hot spots provided insights on the optimal nanogaps that can be used for the design of plasmonic platforms.


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Figure 5. Dimerization of Ag@SiO2 nanostructures, generation of hot spots and Raman signal enhancement. (A-E) correspond to monomeric Ag@SiO2 nanoparticles having varying thickness of silica shell (t):

90

HRTEM images (A−C) of Ag@SiO2, FDTD results (A−C) showing the

intensity of the electric field in the vicinity of the corresponding nanoparticle system, Raman spectra (D), the plot of the SERS enhancement factor of the Raman signal at 1239 cm−1 as a function of t and inset shows the variation of the enhancement factors with the distance from the center of the Ag nanoparticle and the fit to the expected 1/R12 dependence (E). HRTEM images (F−H) of dimeric Ag@SiO2 having varying Ag surface to Ag surface distances (d), the FDTD results (F′−H′) showing distance dependent formation of hot spots, Raman spectra (I) and the plot of the enhancement factor of the Raman signal at 1239 cm−1 as a function of d (J). Note:


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dashed circles in A′−C′ represent the periphery of the silica shell surface; the scale bars in electric field profiles are kept uniform for comparison; Raman spectra were recorded immediately after the (

Coupling of Elementary Electronic Excitations: Drawing Parallels

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