Surface-Enhanced Dual-Frequency Two-Dimensional Vibrational

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Cite This: J. Phys. Chem. C 2018, 122, 11015−11023

Surface-Enhanced Dual-Frequency Two-Dimensional Vibrational Spectroscopy of Thin Layers at an Interface Robert T. Mackin,† Bar Cohn,‡ Andrey Gandman,‡ Joel D. Leger,† Lev Chuntonov,*,‡ and Igor V. Rubtsov*,† †

Department of Chemistry, Tulane University, New Orleans, Louisiana 70118, United States Schulich Faculty of Chemistry and Solid State Institute, Technion, Haifa 32000, Israel

‡

S Supporting Information *

ABSTRACT: The development of spectroscopic approaches to study molecules at interfaces is important as the molecular properties often differ from those in the bulk. Implementation of surface-enhanced two-dimensional infrared (SE 2DIR) spectroscopy using lithographically fabricated plasmonic nanoarrays is demonstrated for nanometer thick films. The sample, 4-azidobutyrate-N-hydroxysuccinimide ester (azNHS), dispersed in polystyrene was deposited onto the nanoarray. Raw enhancements in the SE 2DIR spectra exceeding 5 × 104 and 1.3 × 103 fold were achieved for the CO and NN peaks, respectively. The field enhancement provided by the nanoarray was sufficient to record cross-peaks in 1 nm thick samples under dilute conditions for azNHS (∼0.1 M). Note that the cross-peaks were recorded for vibrational modes frequency separated by ∼350 cm−1 with the enhancement factor of 4.1 × 104. The effective electric field enhancement factors, measured for NN and CO modes via linear and two nonlinear IR techniques, have similar sample-thickness dependences, which permit using linear spectroscopy for enhancement evaluation. High-quality cross-peak waiting-time dependences were recorded for samples as thin as 1 nm involving several IR reporters demonstrating the applicability of an arsenal of 2DIR approaches, including spectral diffusion, chemical exchange, and relaxation-assisted 2DIR, to interrogate samples in nanometer thick films. The study opens new opportunities in analyzing structures and dynamics of molecules at interfaces.

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INTRODUCTION Electromagnetic radiation is enhanced locally near the surface of noble metal nanostructures owing to excitation of the localized plasmon resonances. Nanodevices utilizing plasmon resonances as key functional elements emerge in a variety of fields, including sensors, molecular machines, plasmon-assisted chemistry, medicine, and others.1−3 Enhancement of the near field upon excitation of the plasmon resonances leads to enhancement of the spectroscopic transitions in molecules located on the nanostructure surfacesa phenomenon that is extensively used in several sensitive spectroscopic techniques,4−6 including surface-enhanced infrared absorption (SEIRA) spectroscopy.4,7−14 SEIRA is capable of studying very thin layers of a sample and providing valuable data for molecules at interfaces.13 Rational engineering of the plasmonic fields via the design of metal nanostructure’s shape and size allows measurements with sensitivity down to a single monolayer of molecules self-assembled on metallic structures.8,15,16 It is appealing to use such plasmonic nanostructures for enhancing sensitivity in nonlinear measurements. A powerful time-resolved technique, two-dimensional infrared (2DIR) spectroscopy, reports on correlations of vibrational modes in the sample. These correlations, measured via © 2018 American Chemical Society

detection of diagonal and cross-peaks, can be linked to the molecular structures and their dynamics.17 2DIR spectroscopy of molecules at interfaces has shown that their structure and dynamics can differ from those in the bulk.18−24 Rough metal surfaces are actively used to achieve enhancements in 2DIR measurements.18−27 The raw signal enhancement of 470 fold was reported, making SE 2DIR diagonal-peak measurements of monolayers practical.22,28,29 SE 2DIR spectroscopy using plasmonic half-wavelength IR antennas has recently been demonstrated on diagonal peaks.28,30 In addition, progress has been made in theoretical approaches to quantum dynamics of molecules on plasmonic structures studied by ultrafast 2D spectroscopy.31,32 It is attractive to expand SE 2DIR into cross-peak measurements for thin-film samples, which would unleash the full power of 2DIR. Note that cross-peaks are typically several orders of magnitude weaker than the diagonal peaks. Furthermore, the typical plasmon spectrum of the IR antenna is rather narrow with a full width at half-maximum (fwhm) of Received: March 12, 2018 Revised: April 12, 2018 Published: April 27, 2018 11015

DOI: 10.1021/acs.jpcc.8b02436 J. Phys. Chem. C 2018, 122, 11015−11023

Article

The Journal of Physical Chemistry C 200 cm−1, which adds to the challenge of cross-peak measurements. Therefore, the signal enhancement depends strongly on the frequency mismatch between the molecular and plasmonic transitions. In addition, because the strength of the plasmon-enhanced near field is known to drop fast with the distance from the nanoantenna surface,10,33 the signal enhancement also depends on the sample thickness. In this work, we demonstrate that in spite of such unfavorable settings, robust systematic measurements of the enhanced signals are possible even with moderate enhancements by mid-IR antenna, which are significantly lower than those routinely obtained in the visible region.4−6 A range of SE 2DIR techniques were performed, including diagonal and crosspeak measurements on dilute samples of various thicknesses (1−85 nm). Cross-peaks between vibrational labels spatially separated by over 10 Å and frequency separated by over 350 cm−1 were recorded. In addition, relaxation-assisted 2DIR (RA 2DIR) measurements reporting on the energy transport within molecules and between the sample film and the gold nanoantenna were successfully performed. The magnitudes of signal enhancements that made such measurements possible were determined using heterodyned three-pulse photon echo, vibrational pump−probe, and linear Fourier transform infrared (FTIR) techniques as well as by numerical simulations such as the finite difference time domain (FDTD) approach. This work demonstrates that a large arsenal of 2DIR approaches, including RA 2DIR, spectral diffusion, and other waiting time-dependent methods, is now applicable to samples as thin as 1 nm, opening an avenue for measuring structural and dynamics properties of the molecules at interfaces.

each beam. The third (probe) beam was further suppressed for both 2DIR and pump−probe experiments, resulting in ca. 8 nJ pulse energy. The signals in the 2DIR measurements on CaF2 and gold under perpendicular polarization were scaled down to correspond to the same pulse energies used in the measurements on the gold array under parallel polarization. Note that the noncollinear laser beam geometry used in 2DIR measurements permits the collection of only the third-order signals in a given direction/measurement. FDTD Simulations. Numerical electromagnetic FDTD calculations were done using Lumerical software. The transmission spectrum was calculated by integration of the flux of the Poynting vector of the light passing through the sample. We used a plane-wave excitation and a nonuniform mesh to increase sampling in the vicinity of the antenna; the grid size down to 0.25 nm was used for thin layers. The simulation space involved a three-dimensional box with dimensions matching those of the array’s unit cell and extending to 10 μm along the light propagation dimension. The 2D array was modeled using a periodic boundary condition. Absorbing boundary conditions were imposed for the propagating light. All the calculations were performed with an effective refractive index of the environment of n = 1.21, which represented a combined effect of the CaF2 substrate and Cr adhesion layer under the gold structures. The gold dielectric constants from Johnson and Christy were used,36 while the vibrational transitions were modeled by a collection of Lorentz oscillators with the permittivity function ε(ω) = n0 2 + ∑k

fk ω0 2 ω0, k 2 − ω2 − iωγk

, where

n0 = 1.58 represents the refractive index of the polystyrene (PS) matrix (assumed to be constant across the relevant frequency range), and the summation is over three CO and one NN transitions of 4-azidobutyrate-N-hydroxysuccinimide ester (azNHS), such that ω0,k is the corresponding transition frequency, f k is the oscillator strength, and γk is the associated line width. We used ω0,1 = 1742 cm−1, ω0,2 = 1788 cm−1, ω0,3 = 1818 cm−1, and ω0,4 = 2103 cm−1. The oscillator strengths were calibrated in a series of linear absorption experiments with thick polymer layers, resulting in f1 = 0.0205, f 2 = 0.0033, f 3 = 0.0025, and f4 = 0.0089; the same width, γ = 25 cm−1, was used for all transitions.

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EXPERIMENTAL METHODS Electron Beam Lithography. The gold antennas were fabricated on solvent-cleaned CaF2 windows. A double layer poly(methyl methacrylate) (PMMA) coating (495-A2 and 950A3 MicroChem) was used to make a negative slope template by the electron beam. A 20 nm layer of chromium (Cr) was deposited to have good conductivity for the electron beam lithography (EBL). The CaF2/PMMA/Cr samples were exposed to a predesigned pattern using an EBL instrument (RAITH, EBPG 5200). The total area of the array was 5 mm × 5 mm. During the development, the Cr layer was removed by an etchant (Micro Chemicals); the exposed PMMA was removed by methyl isobutyl ketone−isopropanol solution (MicroChem). A 5 nm Cr adhesion layer and an 80 nm layer of gold were deposited, and the lift-off was done with acetone. Linear Spectroscopy. Linear FTIR measurements were performed in a transmission mode with a Tensor 27 spectrometer (Bruker). A wire-grid polarizer was placed in the sample cell compartment to measure polarized FTIR spectra without having to move the sample. Additionally, an aperture of ca. 3 mm in diameter was placed in front of the sample to limit the sampled area to the location of the array. Dual-Frequency 2DIR and Pump−Probe IR Spectroscopies. A fully-automated dual-frequency three-pulse 2DIR spectrometer with heterodyned detection is described in detail elsewhere.34,35 To prevent nanoarray degradation, a neutral density (ND) filter, OD = 1, was placed on all three beams before the sample. To increase the signal, this filter was removed during perpendicular polarization measurements of nanoarray and in off-gold measurements. An additional ND filter was placed on the first and second beams (OD = 0.3), resulting in a pulse energy at the sample of ca. 40 nJ/pulse for

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RESULTS AND DISCUSSION The magnitude of the signal enhancement is the key quantity in assessing the feasibility of surface-enhanced spectroscopy. We evaluated the signal enhancement in a series of experiments, where we have systematically varied the amount of molecules contributing to the signal and their location within the enhanced near field. Our studies were conducted with azNHS compound (Figure 1b, inset) dispersed in a PS polymer film. The linear and 2DIR measurements were focused predominantly on two vibrational modes of azNHS: an asymmetric CO stretching mode on the succinimide (νCO) centered at 1742 cm−1 and a stretching mode of the azido moiety (νNN) centered at 2103 cm−1. Figure 1b (green line) shows the linear absorption (FTIR) spectrum of azNHS in a PS film measured under light polarized perpendicular to the long axis of the gold bars, which matches the absorption spectrum of azNHS in solution. The samples were made by spin-coating an azNHS/PS solution onto the gold nanoarray (see scanning electron microscopy (SEM) image in Figure 1a) and on a bare CaF2 window. The extinction spectrum of the bare plasmonic array 11016

DOI: 10.1021/acs.jpcc.8b02436 J. Phys. Chem. C 2018, 122, 11015−11023

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The Journal of Physical Chemistry C

enhancement provided by the nanoantenna, the DFTIR values Au were compared to the signals generated from the sample of the same thickness but lacking the plasmon enhancement. Three reference samples were tested to ensure that the sample thickness on the array is represented well by the reference sample. These included a bare CaF2 region of the CaF2 window bearing the gold array (off the gold), on a different CaF2 window spin-coated under the same conditions as for the nanoarray, and on the nanoarray with light polarization perpendicular to the gold bars. The film thickness was evaluated by the plasmon shift (see the Supporting Information) as well as by the absorption peak of PS in the CH stretching region (2924 cm−1). The analysis suggests that the thickness of the spin-coated layer on CaF2 and off the gold array is essentially the same, in agreement with the conclusion of our previous study.30 Note that the spin-coated layer essentially conforms to the surface, covering the gold bars from all directions.30 Comparison of the νCO absorption peaks for the film on the array measured with perpendicular polarization with that of the film off the array showed slight enhancement for the former by an average factor of 1.69. An enhancement was also predicted by the modeling (see the Supporting Information). This experimental factor was applied to the reference data sets measured with perpendicular polarization on gold resulting in a good match between different series. FTIR The resulting ratios, DFTIR Au /DRef , are shown in Table S3 along with the effective field enhancement ξlin = (DFTIR Au / 1/2 DFTIR Ref ) . Figure 2a shows the electric field enhancement as a function of film thickness in the FTIR measurements for the νCO and νNN transitions. Both curves show a drastic increase of the enhancement toward very small thicknesses with field enhancements exceeding 22 fold. The enhancement for νCO is essentially constant between 10 and 25 nm thicknesses at ca. 8.5; it decreases slowly for thicker films. The enhancement curve for νNN does not show a plateau but decreases monotonically toward larger thicknesses. Two effects play an important role for these dependences. First, the near field decays away from the gold bar surface (Figure S4), resulting in a reduction of the effective field enhancement experienced by molecules in thicker films. Because thicker films have a greater contribution to the signal generated from the sample located farther away from the antennas,30,33 as the film thickness increases, the average electric field felt by the sample decreases, which is most apparent at small thicknesses ( 40 ps, is caused by residual heating of the sample impinged by the excitation pulses, which results in a thermal grating diffracting the third beam in the phase-matched direction, producing cross-peaks.48 Note that at time delays exceeding 40 ps, the local vibrational energy equilibration (thermalization) is mostly complete. The transient grating can vanish because of cooling, which can occur in two directions, along the substrate and perpendicular to the grating grooves. Under the experimental conditions, the period of the transient grating is ca. 17 μm; thermalization in this direction takes microseconds. Thermalization into the substrate (gold) can occur rapidly for nanometer thick films. As the signals are generated mostly in the vicinity of the gold bars, the cooling into the gold bars should dominate the thermalization dynamics. Therefore, the waiting time dynamics at delays >40 ps can report on the cooling of the transient grating into the substrate, providing data on the thermal conductivity at the interface.

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CONCLUSIONS Successful implementation of SE 2DIR spectroscopy using plasmonic arrays is demonstrated for nanometer thick films. The sample, azNHS compound dispersed in a dilute fashion in the PS film, was deposited onto a plasmonic nanostructure, fabricated lithographically on a CaF2 window. Raw enhancements of the SE 2DIR diagonal spectra exceeding 5 × 104 and 1.3 × 103 fold were achieved for the CO and NN peaks, respectively. The latter value is smaller because of the frequency mismatch between the molecular transition and the plasmon maximum, ΔΩ. Selig et al. recently reported SE 2D pump− probe measurements for the CO stretching mode of PMMA enhanced by a randomized gold nanoarray.28 A raw signal enhancement of 6 × 103 was obtained for a 5 nm thick film deposited on an array under conditions of ΔΩ ≈ 0.28 The enhancement of 5 × 104 obtained in our work for the CO peak suffers from the frequency mismatch and would be several-fold larger under ΔΩ ≈ 0 conditions. However, one of the objectives of this work was to quantify the enhancement for cross-peaks, where the ΔΩ ≈ 0 condition cannot be satisfied simultaneously for both vibrational modes involved. Nevertheless, the field enhancement was sufficient to record the cross-peaks in a 1 nm thick sample at ca. 0.1 M azNHS concentration. Note that under these conditions, there are only ca. 1250 molecules of azNHS per side of a single bar (w × h = 0.021 μm2) with a rather low density with one molecule per ∼1700 Å2 (∼40 Å × 40 Å) area. The corresponding surface density of 0.06 molecules per nm2 is ca. 100-fold smaller than the surface density of small molecules in a monolayer. High-quality waiting-time dependences for the NN/CO cross-peaks were demonstrated. It was confirmed that the observed dependences are characteristic of the compound (azNHS) and were not noticeably affected by the plasmon

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b02436. Determination of the film thickness, signal referencing with raw nanoarray and reference signal values; near-field intensity profile; transient pump-probe spectra for different sample thicknesses; CO diagonal kinetics for on- and off-gold; FDTD spectral simulations with scattering and absorption contributions (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (L.C.). *E-mail: [email protected] (I.V.R.). 11021

DOI: 10.1021/acs.jpcc.8b02436 J. Phys. Chem. C 2018, 122, 11015−11023

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The Journal of Physical Chemistry C ORCID

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Lev Chuntonov: 0000-0002-2316-4708 Igor V. Rubtsov: 0000-0002-3010-6207 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by a grant 2016167 from the United StatesIsrael Binational Science Foundation (BSF). Plasmonic arrays were prepared at the Micro- and Nano-Fabrication Unit, Technion, with support from the Russell Berrie Nanotechnology Institute. Support by the United States National Science Foundation (CHE-1462075) and by Israel Science Foundation (1118/15) is gratefully acknowledged.

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DOI: 10.1021/acs.jpcc.8b02436 J. Phys. Chem. C 2018, 122, 11015−11023