Surface-enhanced Infrared Absorption Enhancement Research Articles

Numerical investigation of plasmonic bowtie nanorings with embedded nanoantennas for achieving high SEIRA enhancement factors

This paper presents the numerical investigation of several complex plasmonic nanostructures — bowtie nanoring and crossed-bowtie nanoring nanoantennas with embedded bowtie nanoantennas and crossed-bowtie nanoantennas — for surface enhanced infrared absorption (SEIRA) spectroscopy-based substrates. The proposed nanostructures exhibit substantially large SEIRA enhancement factor (∼8.1 × 105) compared to previously reported enhancement factor values for bowtie nanoantennas or nanoring antennas. The plasmonic properties of the proposed nanostructures have been studied by the numerical evaluation of the near-field electromagnetic enhancement at resonant plasmon mode excitation wavelengths in the mid-IR spectral regime. The highest SEIRA enhancement of ∼8.1 × 105 occurs at a wavelength of ∼6800 nm (6.8 μm). A substantial electric field enhancement as large as ∼375, corresponding to SEIRA EF of ∼1.4 × 105 is noted even when the minimum gaps between the plasmonic nanostructures is as large as 10 nm, which can easily be fabricated using the conventional nanolithography techniques. The occurrence of several electric field hotspots due to the presence of plasmonic nanoantennas embedded inside the nanorings was observed, as the electric fields are enhanced in the vicinity of the plasmonic nanostructures being proposed. The multiple electric field hotspots in the proposed nanostructures can lead to larger average electric field enhancement as well as the average SEIRA enhancement for these substrates. Moreover, by embedding plasmonic nanoantenna structures inside the bowtie nanorings and crossed-bowtie nanorings, large spectral tunability of plasmon resonance wavelengths is achieved in the spectral regime from 4 μm to 8 μm. This is done by varying a larger number of spectral parameters that are present in these complex nanostructures. This paper also reports a novel configuration of crossed-bowtie nanoring plasmonic structure exhibiting less polarization dependence of the SEIRA enhancement factor. This structure also exhibits tunability of hotspot positions when the direction of the polarization of the incident light is rotated. The proposed structures in this paper can be fabricated by the state-of-the-art nanofabrication technologies. The proposed structures could find potential applications in chemical and biological sensing and biochemical detection of analyte molecules.

Aluminum Bowties for Plasmonic‐Enhanced Infrared Sensing

AbstractPlasmonic nanoantennas have earned outstanding credits for their powerful ability to couple light from free‐space into sub‐wavelength‐sized structures and to enhance the electric field confinement. One of the most appealing plasmonic applications is biosensing: to detect and identify many bio‐information of a molecule in a single test. Consequently, surface enhanced spectroscopy in a broad infrared (IR) range is required. In this article, a barcode‐like nanostructured surface of a metal‐insulator‐metal (MIM) platform serves as optical transducer with tunable localized surface resonances (LSPR). The conception, fabrication, and optical characterization of periodic arrays of aluminum bowties (Al‐BT) within a MIM structure compatible with Si‐technology are reported. LSPR positions are extended by controlling the side length of the tip‐to‐tip facing triangles. The reflectance measurements correlate extremely well to numerical simulations. The simulated electrical field confinement provides up to 103 of intensity relying on the tip‐to‐tip nanogap value. Surface‐enhanced infrared absorption (SEIRA) gives simultaneously spectral information on both far‐field behavior and near‐field enhancement of several vibrational lines of our trial molecule (vanillin) achieved with the barcode‐like Al‐BT. The analysis of Fano‐like profiles is concluded with SEIRA enhancement factors higher than 107 for frequency tuning ratios below 1 between the far‐field plasmonic resonances and the selected vanillin IR absorption lines.

Observation of tunable surface plasmon resonances and surface enhanced infrared absorption (SEIRA) based on indium tin oxide (ITO) nanoparticle substrates

The application of surface enhanced infrared absorption (SEIRA) is severely restricted in many fields due to the SEIRA substrates are constructed mainly from expensive noble metals. Therefore, the development of new SEIRA substrates other than the noble metallic ones is very valuable. Here we introduced a new semiconductor SEIRA substrate, the indium tin oxide (ITO) nanoparticles (NPs), to study the SEIRA property. The results demonstrate that the ITO NPs show the SEIRA property and the enhancement is dependent to the doping ratio of the heteroatoms of tin. The ITO NPs with the 5% atomic doping ratio show the highest SEIRA enhancement factor (EF), which is about 24. The limit of detection (LOD) of the 1,1′-dicarboxyferrocene (dcFc) molecule was as low as 10−5 mol/L. The present study proves that the tin-doped indium oxide can be used as a new and inexpensive semiconductor SEIRA substrate. It also proves that the doped semiconductor NPs have strong potentials for being used as emerging SEIRA substrates.

Computer simulation of key structural parameters for enhanced infrared absorption with Au nano-array

The optical constants of thin films with nanoscale structures, such as metamaterials and nanostructured films used in surface enhanced infrared absorption (SEIRA), are represented by effective medium approximation (EMA). In a representative EMA, the effective permittivity is determined by parameters such as the volume filling factor (F). In a metal square column array modeled on an evaporated film, the effective permittivity is determined by the gap size/particle size ratio (f). However, f is also expressed as a function of F. For this study, we simulated the SEIRA spectrum using a model in which f and F were varied independently to investigate which was the most important parameter in the square column array. The results showed all the simulations performed, results showed that the gap size/particle size ratio (f) is an important parameter for the increase in SEIRA in the square column array. This study has led to new discoveries that support elucidation of the SEIRA enhancement mechanism.

Multiple-resonant pad-rod nanoantennas for surface-enhanced infrared absorption spectroscopy

Due to the ability to tightly confine electromagnetic energy, plasmonic nanoantennas have been widely studied for surface-enhanced infrared absorption (SEIRA) spectroscopy, surface-enhanced Raman spectroscopy as well as refractive index sensing. However, most of the nanoantennas are limited by narrow resonant band and it is rather challenging to detect multiple molecular fingerprints. In this work, we report dual and triple- resonant pad-rod plasmonic nanoantennas which are nanorods with large pads at their ends placed above gold (Au) mirror separated by a spacer layer. By adjusting the geometries, the nanoantennas have demonstrated dual and triple resonant bands enabling detection of molecular fingerprints at different wavelength. The calculated maximum SEIRA enhancement factor is around 1.8 × 106, which is among the highest reported so far. The pad-rod plasmonic nanoantennas have been used for the detection of molecules of polymethyl methacrylate (PMMA) by SEIRA and fingerprints of C=O and C–H bands are clearly identified. This work has shown that the multiple-resonant pad-rod plasmonic nanoantennas are promising for chemical and biomolecular sensing by the detection of vibrational fingerprints with SEIRA.

Metasurface based on cross-shaped plasmonic nanoantennas as chemical sensor for surface-enhanced infrared absorption spectroscopy

Infrared spectroscopy is an effective technique extensively used in research and industry for the label-free and unambiguous identification of molecular species. However, the sensitivity of this technique is severely limited as a result of Beer’s law and, the small infrared absorption cross-section that make prohibitively weak the absorption signals, of minute amounts of analyte as those present in monolayers. This limitation can be overcome by enhancing the infrared vibration of molecules through the enhancement of the electromagnetic (EM) field. Surface Enhanced InfraRed Absorption (SEIRA) using resonant metal Nano-scale Antennas (NAs) can provide huge electromagnetic fields on the nanometer scale featuring localized collective oscillations of electrons, an effect named Localized Surface Plasmonic Resonances (LSPRsWe here report on a series of 2D arrays of cross-shaped NAs having several mm2 area coverage (metasurface) as SEIRA optimized antennas, which can be used in practical applications such as the vibrational sensing of chemical and biological analytes. Cross-shape designed NAs are insensitive to the polarization of the electromagnetic radiation impinging the active area. Due to the random orientation of the dipole moments of molecules they are particularly suitable for the construction of bio-molecular sensors. At the same time, the 2D-array configuration ensures a good near-field signal enhancement arising from the coupling between neighbour NAs Moreover, SEIRA NAs can be easily integrated with micrometre-sized channels and be suitable for the high sensitivity, real time analysis of IR emitting samples, in matrices where IR spectroscopy is severely limited due to absorption bands of liquid water. We present the design, fabrication and experimental characterization of large-area metasurfaces based on cross-shaped plasmonic NAs for the spectroscopic characterization of various types of compounds and for sensing applications in the mid-infrared range. The cross-shaped NAs we have designed exhibit SEIRA phenomena which are very sensitive to both refractive index changes in the surrounding medium and to the specific molecular vibration band emerging from surface adsorbed molecules. To test this effect on our device, we have used as model compounds small molecules (molecular weight (MW) < 500 g/mol) containing triple bond groups resonating at about 2100 cm−1 and a large polymer (MW ˜ 950,000 g/mol) containing carbonyl groups resonating at wavenumbers of about 1700 cm−1. We show a sensitivity of 600 nm/RIU at different wavelengths at a maximum amount of immobilized small molecule of 0.7 fmoles and a SEIRA enhancement factor of 48,000. We also show the device potential to reveal chemical reactions, occurring on the sensor surface at the same scale, where the nitrile group is converted to a triazole ring.

Nanoantenna structures for the detection of phonons in nanocrystals.

We report a study of the infrared response by localized surface plasmon resonance (LSPR) modes in gold micro- and nanoantenna arrays with various morphologies and surface-enhanced infrared absorption (SEIRA) by optical phonons of semiconductor nanocrystals (NCs) deposited on the arrays. The arrays of nano- and microantennas fabricated with nano- and photolithography reveal infrared-active LSPR modes of energy ranging from the mid to far-infrared that allow the IR response from very low concentrations of organic and inorganic materials deposited onto the arrays to be analyzed. The Langmuir–Blodgett technology was used for homogeneous deposition of CdSe, CdS, and PbS NC monolayers on the antenna arrays. The structural parameters of the arrays were confirmed by scanning electron microscopy. 3D full-wave electromagnetic simulations of the electromagnetic field distribution around the micro- and nanoantennas were employed to realize the maximal SEIRA enhancement for structural parameters of the arrays whereby the LSPR and the NC optical phonon energies coincide. The SEIRA experiments quantitatively confirmed the computational results. The maximum SEIRA enhancement was observed for linear nanoantennas with optimized structural parameters determined from the electromagnetic simulations. The frequency position of the feature’s absorption seen in the SEIRA response evidences that the NC surface and transverse optical phonons are activated in the infrared spectra.

Graphene-assisted multilayer structure employing hybrid surface plasmon and magnetic plasmon for surface-enhanced vibrational spectroscopy.

A graphene-assisted vertical multilayer structure is proposed for high performance surface-enhanced Raman scattering (SERS) and surface-enhanced infrared absorption (SEIRA) spectroscopies on a single substrate, employing simultaneous localized surface plasmon in the visible region and magnetic plasmon resonance in the mid-infrared region. Such multilayer structure consists of a monolayer graphene sandwiched between Ag nanoparticles (NPs) and a metal-insulator-metal (MIM) microstructure, which can be easily fabricated by a standard surface micromachining process. Benefiting from the large near field enhancement by the hybrid plasmons in both visible and mid-infrared regions, a high enhancement factor of up to 107 for SERS and 105 for SEIRA can be achieved. Additionally, the strong magnetic resonance of the MIM microstructure can be tuned in broadband to selectively enhance the desired vibration modes of molecules. The strong SERS and SEIRA enhancement together with easy fabrication provides new opportunities for developing integrated plasmonic devices for multispectral detection of molecules on the same substrate.

Impact of Metal-Optical Properties on Surface-Enhanced Infrared Absorption

Surface-enhanced infrared absorption (SEIRA) spectroscopy using resonant metallic nanostructures is increasingly attracting interest during the last decade. Nevertheless, the impact of the metals’ intrinsic properties on SEIRA is still little studied. We present an experimental work on this topic, examining the infrared-optical resonance spectra of linear nanoantennas made of five of the most common metals (gold, silver, copper, aluminum, and iron) with respect to the intrinsic and radiation damping. Highly material- and size-dependent ratios of the two damping contributions were found and discussed. Using layers of organic probe molecules, we obtained SEIRA enhancement factors for the different nanoantennas and experimentally verified the predicted relationship between the plasmonic damping mechanisms and the SEIRA enhancement. The multitude of our experimental data for the ratio between the intrinsic electronic damping and the radiation damping is compared with the measured SEIRA enhancement of the various nanoantennas and therefore deliver the proof that the best SEIRA enhancement is achieved when both damping mechanisms equally contribute. Furthermore, it is shown that for a given nanoantenna geometry, the red-shift away from the plasmonic extinction maximum is strongly dependent on material parameters.

Exploiting Anisotropy of Plasmonic Nanostructures with Polarization Modulation Infrared Linear Dichroism Microscopy (µPM‐IRLD)

AbstractMetallic nanostructures that exhibit plasmon resonances in the mid‐infrared range are of particular interest for a variety of optical processes where the infrared excitation and/or emission can be enhanced. This plasmon‐mediated enhancement can potentially be used toward highly sensitive detection of an analyte(s) by techniques such as surface‐enhanced infrared absorption (SEIRA). To maximize the SEIRA enhancement, it is necessary to prepare highly tuned plasmonic resonances over a defined spectral range that can span over several microns. Noteworthy, nanostructures with anisotropic shapes exhibit multiple resonances that can be exploited by controlling the polarization of the input light. This study demonstrates the role of polarization‐modulation infrared linear dichroism coupled to microscopy measurements (µPM‐IRLD) as a powerful means to explore the optical properties of anisotropic nanostructures. Quantitative µPM‐IRLD measurements are conducted on a series of dendritic fractals as model structures to explore the role of structural anisotropy on the resulting surface‐enhanced infrared absorption and sensing application. Once functionalized with an analyte, the µPM‐IRLD SEIRA results highlight that it is possible to selectively enhance further vibrational modes of analytes making use of the structural anisotropy of the metallic nanostructure.

Nanogapped Au Antennas for Ultrasensitive Surface-Enhanced Infrared Absorption Spectroscopy.

Surface-enhanced infrared absorption (SEIRA) spectroscopy has outstanding potential in chemical detection as a complement to surface-enhanced Raman spectroscopy (SERS), yet it has historically lagged well behind SERS in detection sensitivity. Here we report a new ultrasensitive infrared antenna designed to bring SEIRA spectroscopy into the few-molecule detection range. Our antenna consists of a bowtie-shaped Au structure with a sub-3 nm gap, positioned to create a cavity above a reflective substrate. This three-dimensional geometry tightly confines incident mid-infrared radiation into its ultrasmall junction, yielding a hot spot with a theoretical SEIRA enhancement factor of more than 107, which can be designed to span the range of frequencies useful for SEIRA. We quantitatively evaluated the IR detection limit of this antenna design using mixed monolayers of 4-nitrothiophenol (4-NTP) and 4-methoxythiolphenol (4-MTP). The optimized antenna structure allows the detection of as few as ∼500 molecules of 4-NTP and ∼600 molecules of 4-MTP with a standard commercial FTIR spectrometer. This strategy offers a new platform for analyzing the IR vibrations of minute quantities of molecules and lends insight into the ultimate limit of single-molecule SEIRA detection.

Surface-Enhanced Infrared Absorption by Optical Phonons in Nanocrystal Monolayers on Au Nanoantenna Arrays

We report on a study of surface-enhanced infrared absorption (SEIRA) by optical phonons in monolayers (MLs) of CdSe, CdS, and PbS nanocrystals (NCs) deposited on arrays of linear nanoantennas the optimized structural parameters of which allow coupling between the localized surface plasmon resonance (LSPR) and diffraction modes in the far-infrared spectral region. The Langmuir–Blodgett technique was used for homogeneous deposition of the NCs. The structural parameters of the arrays and the NC MLs were determined by scanning electron microscopy. According to the three-dimensional electrodynamic simulations of the electromagnetic field distribution around the antennas, the maximal SEIRA enhancement is realized for an array period of about 15 μm when the energy of a diffraction mode coincides with that of the LSPR mode. SEIRA experimental results are in perfect quantitative agreement with the simulation. The maximal SEIRA enhancement is observed for the nanoantenna length and transverse periodicity predicted ...

Wavelength Scaling in Antenna-Enhanced Infrared Spectroscopy: Toward the Far-IR and THz Region

Weak vibrational signals in the infrared and terahertz spectral region can be enhanced by orders of magnitude when employing the electromagnetic near fields of plasmonic nanostructures. This approach is known as antenna-assisted surface-enhanced infrared absorption (SEIRA) and allows for a broad range of possible sensing applications. In the present work, we investigate the scaling of the SEIRA enhancement with wavelength, particularly toward the molecular fingerprint region (500–1500 cm–1). We apply the concept of SEIRA to perform resonant antenna-enhanced spectroscopy of molecules in a spectral range from 4.5 to 45 THz (6.7–67 μm wavelength, 150–1500 cm–1) using a standard Fourier transform infrared spectrometer. We fabricate arrays of rectangular gold antennas by electron-beam lithography and coat them with 30 nm thick layers of the fullerenes C60 and C70, respectively. For the single digit THz measurements, we utilize spin-coated amino acids, particularly threonine. The resonances of the structures are tailored to spectrally match the molecular absorption features. An increased SEIRA enhancement of 2 orders of magnitude is found for antennas resonant at 6.7 THz when compared to 45 THz, corresponding to a λ3 scaling over a frequency range of 1 order of magnitude. This scaling behavior is in excellent agreement with both numerical simulations and classical antenna theory. Further increase toward the single-digit THz region will yield the potential for ultrasensitive THz spectroscopy.

Plasmonic Enhancement of Infrared Vibrational Signals: Nanoslits versus Nanorods

We report on systematic investigations of plasmonically active nanoslits as a beneficial substrate for surface-enhanced infrared absorption (SEIRA). Arranged in arrays, nanoslits with the proper geometry feature strong nanorod-like resonances in the infrared spectral range, as predicted by Babinet’s principle for the same geometrical dimensions. SEIRA enhancement as studied with a thin self-assembled molecular layer of octadecanethiol reaches the values obtained with nanorods if the slit geometry is optimized. We show by systematically examining the important parameters that the slit width has an especially strong influence on the near-field intensity and therefore on the SEIRA signal. Furthermore, the transversal and longitudinal couplings of nanoslits are studied. Compared to nanorod arrays, a stronger influence of the array periodicities on the plasmonic excitations is observed, which indicates coupling via surface plasmon polaritons. So the array periodicity could be further optimized toward higher SE...

Fabrication of Square-Centimeter Plasmonic Nanoantenna Arrays by Femtosecond Direct Laser Writing Lithography: Effects of Collective Excitations on SEIRA Enhancement

We demonstrate the use of femtosecond direct laser writing lithography for a fast and homogeneous large-area fabrication of plasmonic nanoantennas on a substrate by creating a patterned polymer as an etch mask on a metal layer. Subsequent argon ion beam etching provides plasmonic nanoantennas with feature sizes below the diffraction limit of the laser light. They exhibit tunable high-quality plasmon resonances in the mid-infrared spectral range, which are ideally suited for surface-enhanced infrared absorption (SEIRA). In the present work, we demonstrate reliable, fast, and low-cost fabrication of a wide variety of antenna arrays and examine particularly the influence of plasmonic coupling between neighboring antennas on the SEIRA enhancement effect. Specifically, we measure the enhanced infrared vibrational bands of a 5 nm thick 4,4′-bis(N-carbazolyl)-1,1′-biphenyl layer evaporated on arrays with different longitudinal and transversal spacings between antennas. An optimum SEIRA enhancement per antenna of...

Fan-shaped gold nanoantennas above reflective substrates for surface-enhanced infrared absorption (SEIRA).

Here, we report a new nanoantenna for surface-enhanced infrared absorption (SEIRA) detection, consisting of a fan-shaped Au structure positioned at a well-specified distance above a reflective plane with an intervening silica spacer layer. We examine how to optimize both the antenna dimensions and the spacer layer for optimal SEIRA enhancement of the C-H stretching mode. This tunable 3D geometry yields a theoretical SEIRA enhancement factor of 10(5), corresponding to the experimental detection of 20-200 zeptomoles of octadecanethiol, using a standard commercial FTIR spectrometer. Experimental studies illustrate the sensitivity of the observed SEIRA signal to the gap dimensions. The optimized antenna structure exhibits an order of magnitude greater SEIRA sensitivity than previous record-setting designs.

Nanogap-enhanced infrared spectroscopy with template-stripped wafer-scale arrays of buried plasmonic cavities.

We have combined atomic layer lithography and template stripping to produce a new class of substrates for surface-enhanced infrared absorption (SEIRA) spectroscopy. Our structure consists of a buried and U-shaped metal-insulator-metal waveguide whose folded vertical arms efficiently couple normally incident light. The insulator is formed by atomic layer deposition (ALD) of Al2O3 and precisely defines the gap size. The buried nanocavities are protected from contamination by a silicon template until ready for use and exposed by template stripping on demand. The exposed nanocavity generates strong infrared resonances, tightly confines infrared radiation into a gap that is as small as 3 nm (λ/3300), and creates a dense array of millimeter-long hotspots. After partial removal of the insulators, the gaps are backfilled with benzenethiol molecules, generating distinct Fano resonances due to strong coupling with gap plasmons, and a SEIRA enhancement factor of 10(5) is observed for a 3 nm gap. Because of the wafer-scale manufacturability, single-digit-nanometer control of the gap size via ALD, and long-term storage enabled by template stripping, our buried plasmonic nanocavity substrates will benefit broad applications in sensing and spectroscopy.

Surface-Enhanced Infrared Absorption of o-Nitroaniline on Nickel Nanoparticles Synthesized by Electrochemical Deposition

Nickel nanoparticles were electrochemically deposited on indium-tin oxide (ITO) coated glass plate in a modified Watt’s electrolyte. The surface-enhanced infrared absorption (SEIRA) effect of the nanoparticles was evaluated by attenuated total reflection spectroscopy (ATR-FTIR) using o-nitroaniline as a probe molecule. Electrodeposition parameters such as deposition time, pH value, and the type of surfactants were investigated. The morphology and the microstructure of the deposits were characterized by the field emission scanning electron microscope (FESEM) and the atomic force microscope (AFM), respectively. The results indicate that the optimum parameters were potential of 1.3 V, time of 30 s, and pH of 8.92 in the solution of 0.3756 mol/L diethanolamine, 0.1 mol/L nickel sulfate, 0.01 mol/L nickel chloride, and 0.05 mol/L boric acid. The FESEM observation shows that the morphology of nickel nanoparticles with best enhancement effect is spherical and narrowly distributed particles with the average size of 50 nm. SEIRA enhancement factor is about 68.

Surface-Enhanced Infrared Absorption on Elongated Nickel Nanostructures

There is a need for increasing the number of transition metals that can be used as substrates for surface-enhanced infrared absorption (SEIRA). We present here microscopy and infrared experiments that show oblique-angle deposition of Ni onto CaF2 or BaF2, which result in elongated Ni nanostructures (ENiNSs) that are partially aggregated and exhibit surface plasmon resonances in the mid-infrared. SEIRA enhancement factors in the range of 10–20-fold were observed for a monolayer of the p-nitrobenzoate ion adsorbed onto the ENiNS. Extending SEIRA to a metal such as Ni would yield different ways of studying Ni thin film and catalysis chemistry. This work also suggests that oblique-angle deposition might be used to create new SEIRA substrates from other metals.

Surface enhanced infrared spectroscopy with gold strip gratings

We investigate surface enhanced infrared absorption (SEIRA) spectroscopy with gold strip gratings made by standard optical lithography. By exciting surface plasmon polaritons on both air-gold and gold-substrate interfaces, the resonance of the 1D gratings is linearly tunable with the grating period. With the field enhancement at the edge of the gold strips, a SEIRA enhancement factor more than 6000 for PMMA molecules is achieved. The strong SEIRA enhancement together with the easy fabrication makes the gold strip grating a promising candidate for SEIRA experiments.