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Surface Enhanced Infrared Absorption Spectroscopy Using Charge Transfer Plasmons Tao Wang, Zhaogang Dong, Eleen Huey Hong Koay, and Joel K.W. Yang ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.9b00229 • Publication Date (Web): 25 Apr 2019 Downloaded from http://pubs.acs.org on April 25, 2019
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ACS Photonics
Surface Enhanced Infrared Absorption Spectroscopy Using Charge Transfer Plasmons Tao Wang†, Zhaogang Dong†, Eleen Huey Hong Koay†, Joel K. W. Yang*,§,†
†Institute
of Materials Research and Engineering, A*STAR (Agency for Science,
Technology and Research), 2 Fusionopolis Way, #08-03, Innovis, Singapore 138634, Singapore. §Singapore
University of Technology and Design, 8 Somapah Road, Singapore 487372,
Singapore. *Email:
[email protected].
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ABSTRACT Plasmonic antennas have been widely used to enhance the infrared absorption of molecular moieties, owing to their ability to localize mid-infrared light into nanometer spaces at desired wavelengths. However, at mid-infrared wavelengths, these metallic plasmonic antennas do not benefit from the wavelength-scaling that allows for the smaller sizes of antennas operating at shorter wavelengths. Here, we show that charge transfer plasmons (CTPs) enables a smaller antenna footprint with resonances in the mid-infrared that are tunable with nanometer-scale variation in the width of a conductive bridge. The CTP resonance is readily tuned to match the vibrational modes and realize surface enhanced infrared absorption (SEIRA) of a self-assembled monolayer of hexadecanethiol molecules. Moreover, with their small footprint and the width tuning ability, the CTP antennas are arranged in a grating array to further increase the plasmonic hot-spot density as well as the SEIRA signal. Our results show that CTPs provide a new approach for the mid-infrared antenna design for SEIRA with the nanometer-scale resonance tuning ability.
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KEYWORDS: plasmonic antenna, charge transfer plasmons, infrared absorption, surface enhanced infrared absorption, SEIRA
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Infrared absorption provides unique fingerprint information about the chemical bonds and is widely used to identify molecular moieties.1,2 However, due to the intrinsically small absorption cross-sections (10-20 − 10-19 cm2) of the chemical bonds, the sensitivity of infrared absorption spectroscopy of molecules is extremely low.1-3 In order to improve the sensitivity, molecules are usually adsorbed onto metallic surfaces that drastically enhance their infrared absorption, thus achieving surface enhanced infrared absorption (SEIRA).1-3 In practice, the enhancement is achieved using plasmonic antennas that concentrate infrared light of the desired frequency into nanometer spaces where the molecules reside.321
These plasmonic antennas are often arranged in the form of randomly distributed
assembles or well-defined grating arrays5-10,12,14-16,18-21 to improve the SEIRA signal. It is also possible to obtain SEIRA signals from individual antennas3,4,11,13,17 by carefully optimizing the local field enhancements of the plasmonic antennas. Recently, in the realm of ultracompact plasmonic devices, there is a need to decrease the size of antennas and enable precise resonance tuning of the plasmonic antennas through nanometer-scale size variation. For example, by using the Fabry-Pérot modes in nanoscale metal-insulator-metal resonators, the plasmonic resonances were readily tuned with the sub-10 nm insulator gaps for SEIRA.18,21,22 However, such resonance tuning is difficult for the dipolar plasmonic antennas, which typically have micrometer-scale footprints to support resonances at midinfrared frequencies.1-3 Several more experiments have demonstrated the ability to tune mid-infrared plasmonic resonances. For example, by continuously shrinking the gap of the dimer antenna via photochemical deposition, the bonding dipolar plasmon (BDP) resonance shifted to the merged dimer condition to double the resonant wavelength.23 However,
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photochemical deposition is inhomogeneous and grainy, reducing the ability to precisely control the resulting structures. By stretching the flexible polydimethylsiloxane (PDMS) substrate, the coupled plasmonic resonance of two split-ring shaped antennas was nicely controlled by their gap (~100 nm).6 However, as PDMS has its own infrared vibrational features, it may be difficult to distinguish from target molecules in SEIRA applications. Furthermore, by changing the symmetry of a single cross-bar antenna8, the gap size of a single split-ring shaped antenna24, or just the antenna size4,5,7,10,12,14,20, the plasmonic resonance was tuned, but it required a large geometric variation of hundreds of nm. In contrast to BDP, the charge transfer plasmon (CTP) mode has arguably remained an academic curiosity and unexploited for use in actual applications. CTP appears when the plasmonic dimers are brought close together such that charge transfer occurs across the dimer through either quantum tunneling or classical conduction.25-33 The CTP resonance is largely determined by the inductance L of the conductive bridge, and the capacitance coupling C of the dimer.28 Under illumination, an induced current I flows through the bridge and the energy will be stored in two forms34: the conventional magnetic energy
U M = LM I 2 2 and the kinetic energy of the electrons U K = LK I 2 2 , where the conventional magnetic
inductance35,36 LM : 0l l n l wh 2
and
the
kinetic
inductance34
LK l wh 0 p2 , with l as the bridge length, w as the bridge width, h as the bridge height, ε0 as the permittivity of free space, μ0 as the permeability of free space, and ωp as the plasma frequency of the metal. At the CTP resonance, the total energy oscillates between the electro-static potential from the charges in the dimer and the energy (UM+UK) from the current flow in the bridge. Thus, the CTP antenna can be considered as a LC circuit with the resonant frequency
with the total inductance LT = LM + LK. Note that
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both LM and LK are dependent on w, which means the CTP resonance can be also tuned by w. As the bridge width is typically
