Letter pubs.acs.org/NanoLett
Three-Dimensional Chiral Plasmonic Oligomers Mario Hentschel,*,†,‡ Martin Schaf̈ erling,† Thomas Weiss,† Na Liu,§ and Harald Giessen† †
4th Physics Institute and Research Center SCoPE, University of Stuttgart, Pfaffenwaldring 57, D-70569 Stuttgart, Germany Max Planck Institute for Solid State Research, Heisenbergstrasse 1, D-70569 Stuttgart, Germany § Department of Electrical and Computer Engineering, Rice University, 6100 Main Street, Houston, Texas 77005, United States ‡
S Supporting Information *
ABSTRACT: The living world is chiral. Chirality or the handedness of a structure or molecule is at the heart of life itself. Recently, it has been shown that plasmonic structures exhibit unprecedented and gigantic chiral optical responses. Here we show that truly three-dimensional arrangements of plasmonic “meta-atoms” only exhibit a chiral optical response if similar plasmonic “atoms” are arranged in a handed fashion as we require resonant plasmonic coupling. Moreover, we demonstrate that such particle groupings, similarly to molecular systems, possess the capability to encode their three-dimensional arrangement in unique and well-modulated spectra making them ideal candidates for a three-dimensional chiral plasmon ruler. Our results are crucial for the future design and improvement of plasmonic chiral optical systems, for example, for ultrasensitive enantiomer sensing on the single molecule level. KEYWORDS: Plasmons, circular dichroism, oligomers, chirality, sensing, plasmon ruler
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three-dimensional chiral plasmonic structures that exhibit a strong chiral optical response remains an experimental challenge. Direct laser writing and subsequent metallization,10,13 multilayer electron-beam lithography,14−17 selfassembly18−20 as well as DNA-enabled self-assembly techniques21−29 have been utilized so far. Here, we introduce the most flexible concept of three-dimensional chiral plasmonic oligomers that consist of individual metal nanoparticles with nearly arbitrarily selectable properties in order to create artificial plasmonic molecules by means of multilayer electron-beam lithography. The electronic properties of molecules are influenced by their composition as well as by their configuration, hence by the number and kind as well as by the spatial arrangement of the consisting atoms. Both properties determine how the electronic wave functions mix and hybridize, thus giving rise to new collective orbitals. The formation of shared orbitals will lead to charge redistribution and hence causes strong interaction and binding between the atoms in molecules.30 Recently, it has been proposed and demonstrated that this concept of hybridization can be transferred to assemblies of closely packed metallic nanoparticles, termed “plasmonic molecules”.31−37 Here, the strongly enhanced local electric fields associated with the plasmonic resonances take the role of the electron wave functions. Clusters of individual nanoparticles offer the
ife is about chirality. Starting from the essential aminoacids, via carbohydrates, all the way to the nucleic acids and proteins, a huge number of biomolecules show handedness, hence being called chiral. In fact, many molecules exist in only one possible handedness, for example, all 21 essential aminoacids are L-enantiomers, indicating that the associated physiological processes show 100% stereoselectivity. Accordingly, chirality and its origin has been the subject of intense research on the route to unravel the basic foundation of life itself. Recently, plasmonic chiral systems have gathered significant scientific interest. The reason is two-fold. On the one hand chirality is of fundamental interest in biology and chemistry, yet the chiral optical response, which manifests itself as a difference in the structure's response to left- and right- handed circularly polarized light (LCP and RCP, respectively), of the studied systems is generally weak and difficult to detect. On the other hand, it has been shown that already handed planar, meaning two-dimensional, plasmonic structures show extremely strong interaction with circularly polarized light1−6 as well as with chiral molecules,7−9 despite the fact that they are not truly chiral. Combining chiral plasmonics and stereochemistry will hence open up an entirely new field of plasmonically enhanced chiral optical response in hybrid systems for the detection of chiral molecules, for the design of chiral optical modulators and devices,10 and applications in medicine and drug development.11,12 First studies have been undertaken to create three-dimensional and thus true chiral structures. Yet, the fabrication of © 2012 American Chemical Society
Received: February 24, 2012 Revised: March 21, 2012 Published: March 29, 2012 2542
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strong chiral optical signal and that compositional plasmonic chirality is not favorable. In order for particle assemblies to be chiral, they need to be truly three-dimensional. Such clusters can be fabricated using the very versatile top-down technique of electron-beam lithography combined with layer-by-layer stacking.42 Figure 2 depicts exemplary tilted view scanning electron microscope (SEM) images of fabricated two-layered quadrumer structures (for sample and fabrication details see Methods in the Supporting Information). The first layer consists of three particles arranged in an L-shape. The fourth particle is stacked on-top of the first layer and determines the handedness of the structure. The individual clusters are arranged in a C4 symmetric lattice in order to prevent biaxiality of the superstructure and hence to suppress contributions of polarization conversion43 (compare Figures S8 and S9 in the Supporting Information for details on a biaxial structural geometry). In order to characterize chiral structures one normally examines the circular dichroism (CD) spectra. CD is defined as the difference in absorbance for left- and right-handed circularly polarized light. In contrast to transmittance, our setup does only allow measuring the reflectance for circularly polarized light in a very limited wavelength range. Thus we define the quantity ΔT as the difference in transmittance TRCP and TLCP for left and right-handed circularly polarized light, respectively. As we ensured the uniaxiality of the superstructure we can assume any contribution of polarization conversion to be negligible, hence the transmittance difference ΔT is directly correlated to CD (see Figure S1 in the Supporting Information).44 The spectroscopic results of the structures are shown in Figure 3 (the TRCP and TLCP spectra for one of the enantiomers are depicted in Figure S2 in the Supporting Information). The transmittance difference reaches values up to ∼7%. Changing the handedness of the structures is expected to flip the ΔT spectrum, which is convincingly demonstrated by the excellent mirror symmetry of the two spectra with respect to the zero line. The achiral structure shows a small remaining signal which most likely stems from fabrication tolerances, for example, the relative positions of the upper dots of the left- and right-handed structures. The remarkable sensitivity of the structure to these minute deviations will be discussed in detail below. As an important control experiment Figure 3b shows the ΔT spectra for the left and right handed enantiomers for opposite illumination directions. For a true chiral structure (as warranted
unique possibility to spectrally shift the optical response to a large range of desired wavelength regimes in the visible and infrared spectral range simply by changing the size of the basic building blocks and appropriate choice of the metal. Figure 1 depicts a simplified sketch of the two basic and most straightforward geometrical possibilities to introduce handed-
Figure 1. From a geometrical point of view two basic possibilities can be envisioned how to render an arrangement of particles chiral: (a) Identical particles are arranged in a handed fashion, termed configurational chirality. (b) Different particles are arranged in an unhanded geometry, e.g., at the corners of a tetrahedron, termed constitutional chirality.
ness and hence to create chirality in an arrangement of particles. The first possibility is to arrange identical constituents into a handed structure, which we term configurational chirality. The second one uses a nonhanded structure, in this case a tetrahedron, dressed with different particles at its corners, termed constitutional chirality. In chemistry the latter is referred to as a molecule with stereogenic center and constitutes a very prominent and hence an archetype structure. In contrast, inducing handedness due to configuration is less common.38 The premiere reason for this is its mechanical instability without adding an additional scaffold. Conceptually, both design strategies can be transferred to plasmonic molecules.22−24,39−41 Here the coupling is mediated by the plasmonic near-fields. It is crucial to note that the constituents of the plasmonic molecules do not necessarily couple despite the fact that they are in close proximity. Efficient coupling is only possible if the individual plasmonic atoms have similar resonance frequencies, as plasmonic coupling is resonant coupling. Indeed, we show that in plasmonic chiral molecules only configurational chirality will manifest itself in a
Figure 2. (a,c) Tilted overview SEM images of chiral plasmonic molecules. (b) Schematic sketch. The quadrumeric structures consist of two layers fabricated by electron-beam lithography. The first layer consists of three particles arranged in an L-shape, the second layer contains a single dot. The position of the dot in the second layer determines the handedness of the structure. The individual clusters are arranged in a C4 symmetric lattice, which prevents the superstructure from being biaxial and thus suppressing polarization conversion. The scale bar of the overview is 500 nm and for the insets is 100 nm. 2543
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large number of free electrons. Basically, this huge interaction strength is the basis of all plasmonic phenomena. Hence, once more, one benefits from the resonant excitation and generation of localized plasmons associated with huge dipole moments and flowing currents, explaining the strong circular optical response of our clusters. Figure 4 depicts the optical response of clusters arranged in a constitutional fashion, comprising of particles of different size
Figure 3. (a) Experimental ΔTLCP‑RCP spectra calculated as the difference in the transmittance of right- and left-handed circularly polarized light. Two dispersive features can be observed with a maximum ΔT of ∼7%. The spectra are nicely vertically mirrored at the zero line for interchanged handedness. The achiral structure shows small differences in the transmittances which can be explained by deviations in the relative position of the upper layer dot. (b) As expected for a true chiral structure (warranted by our C4 symmetry) the ΔT spectra do not significantly change for forward and backward illumination. The scale bars are 200 nm.
Figure 4. (a) Spectra and SEM close-up micrographs of compositionally chiral clusters. The constituting dots in the tetrahedronal structure are of different size (200, 150, 90, and 60 nm, respectively) and consequentially do not couple efficiently via their plasmonic nearfields to one another. The clusters hence comprise four nearly uncoupled and thus individual particles and therefore only show a very weak chiral optical response. Hence, compositional plasmonic chirality with different-sized nanoparticles is at best very weak. The scale bar is 200 nm. (b) Artist impression of the compositionally chiral clusters and tilted view SEM micrographs of the clusters. The scale bar is 100 nm for the close-up images and 200 nm for the overview.
by the C4 symmetry of our superstructure arrangement), the handedness does not depend on the direction of light propagation. Indeed, we find nearly perfectly identical spectra for both structures under forward and backward illumination. This finding additionally underlines that we do not observe polarization conversion as this contribution would change sign for different illumination directions (cf. Figure S6 in the Supporting Information). Naturally occurring chiral optical materials show weak CD, showing values of the dissymmetry factor g of about ∼10−7 to ∼10−5.45,46 Our structures, being about 100 nm in thickness, show enormously high values of around ∼7% transmittance difference for left and right-handed circularly polarized light, which roughly corresponds to a maximum dissymmetry factor g ∼0.14 and an ellipticity of ∼1000 mdeg for the structures of Figure 3a. This behavior can be explained as follows: If circularly polarized light interacts, for example, with a chiral molecule, it displaces the electron clouds of the molecule from their equilibrium position forcing them to move on helical paths. The helical movement of these displacement currents (as generally no real current can flow) gives rise to an induced magnetic moment component parallel to the usual electric dipole moment and in the direction of propagation of the light, resulting in a unique interaction of the molecule with circularly polarized light. In our system, however, we can excite real currents flowing in the disks together with displacement currents between the elements. Overall, these real currents and the displacement currents in the plasmonic molecule are much stronger compared to a molecule, as the gold particles possess a
(200, 150, 90, and 60 nm, respectively, exemplary TRCP and TLCP spectra are shown in Figure S3 in the Supporting Information). In contrast to the configurational case shown in Figure 3, we only observe a very weak chiral optical response. On the one hand there is a geometrical explanation for this behavior as the tetrahedral frame of the cluster is highly symmetric and achiral.40 On the other hand there is insufficient plasmonic coupling between the particles. From a coupling point of view, the structure consists of four nearly or completely uncoupled plasmonic particles and hence does not possess a strong chiral optical response. The remaining response most likely stems from a weak resonant coupling of the second biggest particle in the upper layer to the biggest particle of the lower layer. Figure S4 in the Supporting Information depicts spectra and SEM images of structures showing a combination of compositional and constitutional chirality, which exhibit no chiral optical response at all, despite the fact that these structures possess a geometrical handedness. Overall we can thus deduce that resonant plasmonic coupling between the constituents of the clusters is of utmost importance and an indispensable prerequisite for a pronounced 2544
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The nonvanishing chiral optical response of the achiral structure in Figure 3 is an intriguing phenomenon that has to be studied in greater detail. Inspection of the SEM images of these structures suggests small deviations of the position of the upper dot from its symmetric position. Yet, these small deviations cause a significant change in chiral transmittance ΔT. In biology and chemistry, circular dichroism is a powerful tool for structural investigations. Likewise, we can expect that structural changes will have a significant impact on the chiral optical spectra of the plasmonic cluster hence rendering them ideal candidates for a three-dimensional plasmon ruler.24,47−49 In order to test the capabilities of the chiral plasmonic clusters as three-dimensional plasmon ruler, we fabricated a series of clusters deliberately displacing the upper dot from the symmetric position of the achiral structure. Figure 6 depicts a
chiral optical response. An efficient coupling can only be achieved between particles of similar resonance frequencies and dipole moments within a geometrically handed arrangement. The major importance of resonant coupling of the cluster particles is even more strikingly visible in Figure 5, depicting
Figure 5. Spectra and SEM micrographs of chiral oligomer clusters with different-sized upper layer dots. When decreasing the size of the dot in the upper layer the plasmonic coupling between the lower layer and the upper layer vanishes. From a plasmonic coupling point of view, this effectively collapses the system to a two-dimensional one that is not capable of supporting chiral optical properties. The scale bar is 200 nm.
the experimental proof of vanishing chiral optical response upon decreasing resonant coupling (compare Figure S5 in the Supporting Information for exemplary TRCP and TLCP spectra). The quadrumer structure becomes truly three-dimensional and thus chiral due to the coupling of the L-shaped nanoparticle arrangement to the single dot in the upper layer. Reducing this coupling should render a chiral optical response impossible. In order to prove this behavior, we successively decrease the size of the dot in the upper layer. Both enantiomers with four equally sized particles in Figure 5a show strong chiral optical response. Reducing the diameter of the upper dot in Figure 5b from 200 to 140 nm significantly weakens the coupling strength of the upper dot to the lower layer. Yet, there is still a chiral optical response observable, although strongly reduced in magnitude. The chiral optical response vanishes completely for an upper dot diameter of 80 nm (cf. Figure 4c). The resonance energies of the upper and lower layer dots are too different to still facilitate an efficient coupling. Although the structure still shows a structural handedness, it effectively collapses into a two-dimensional system that cannot exhibit chirality. In order for the system to be “plasmonically” three-dimensional all the particles need to couple efficiently. In a plasmonic system a mere structural handedness does not suffice for a chiral optical response.
Figure 6. Chiral plasmonic nanoparticle clusters can serve as a threedimensional chiral plasmon ruler. ΔTRCP‑LCP spectra for a set of deliberate shifts of the upper dot from the symmetric achiral position (blue) and their corresponding SEM images. The spectra are shifted upward for clarity. The gray spectra depict the response of the preceding structure to facilitate comparison of the spectra. The spectra show significant changes of the spectral features as well as in the magnitude of the signal. The scale bar is 200 nm.
selection of ΔT spectra (in blue) for five different displacements and the corresponding normal view SEM images (the complete series with displacements in 10 nm steps can be found in Figure S6 in the Supporting Information). One observes huge changes in the ΔT spectra for comparably small relative sifts. This is particularly obvious when comparing the 2545
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spectra before and after an additional shift. The gray spectra correspond to the spectra of the preceding displacement. For each step changes in the spectral shape as well as in the magnitude are clearly observable. Strikingly, this behavior will be visible for the analogous shift in the other direction as well, accompanied by a sign flip of the ΔT spectra. Hence, the position of the upper dot can be traced in 2D above the structure. Figure S7 in the Supporting Information shows that this is still feasible when the dot is no longer on-top of the Lshape. It is noteworthy that even the position of the dot in the vertical direction can be traced. An increased (decreased) distance will lead to a decreased (increased) coupling strength of the upper dot to the L-shape. Overall, we hence have experimentally demonstrated that the three-dimensional chiral quadrumer cluster is capable to encode its arrangement in unique ΔTRCP‑LCP spectra, rendering the retrieval of the 3D structural information possible. Three-dimensional chiral plasmon rulers could utilize a lookup database, as in the case of nuclear magnetic resonance (NMR), where the optical spectra corresponding to all possible structural configurations are documented. Spectral features could then be associated uniquely to certain distortions. In conclusion, we have shown that three-dimensional plasmonic oligomers can form artificial plasmonic molecules with a strong chiral optical response. Being composed of individual metallic nanoparticles whose properties can be nearly arbitrarily changed these clusters are spectrally highly tunable. We have shown that resonant plasmonic near-field coupling is an indispensable prerequisite for the emergence of a chiral optical response. Hence, only configurational chirality is possible in a plasmonic molecule. Inducing a handedness by different particle sizes in a symmetric configuration causes no chiral optical response as the particles are not resonantly coupled. Moreover, we have shown that a chiral particle cluster can serve as a three-dimensional chiral plasmon ruler with the capability to encode its three-dimensional arrangement in strongly modulated optical spectra. We believe that our proof-of-concept studies on structures fabricated by electron-beam lithography as a top-down technique will be realizable in larger quantities using selfassembly strategies50 with progressing advances in DNA enabled self-assembly.51−58 These clusters will pave the road for new devices possessing unprecedented strong chiral optical responses. Being three-dimensional and hence showing true chirality, these clusters are expected to show an even higher response and coupling to chiral biomolecules for enantiomer sensors with single molecule sensitivity for applications in medicine, biology, drug development, and biosensing.59−63 An important application afforded by our oligomer clusters is a 3D chiral plasmon ruler. Such 3D plasmon rulers would enable a variety of multidisciplinary experiments, especially for monitoring structural dynamics in macromolecules in real time.64 For example, in a 3D chiral plasmon ruler conjugated by a DNA scaffold the bending or cleavage of a specific strand can perturb the symmetry of the chiral oligomer, and consequently its CD spectrum can be very sensitive to the action of enzymes. Importantly, 3D chiral plasmon rulers will allow for examining dynamic macromolecular processes under physiological conditions, making it an important adjunct to other structural determination methods including macromolecular crystallography or NMR spectroscopy.
Letter
ASSOCIATED CONTENT
S Supporting Information *
A description of methods and Figures S1−S9. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge financial support by Deutsche Forschungsgemeinschaft (SPP1391, FOR730, and GI 269/111), by BMBF (13N9048 and 13N10146), by BadenWürttemberg Stiftung, by German-Israeli Foundation, and by DFH/UFA. The authors would like to thank H.-G. Kuball, Timothy J. Davis, and Alexander O. Govorov for insightful discussion. Sebastian Pricking, Bernd Metzger, and Christina Bauer are acknowledged for experimental aid in an early stage of the experiment, as is Sven Hein for his visualisations.
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