Impact of Plasmon-Induced Optically Rectified Electric Fields on

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C: Physical Processes in Nanomaterials and Nanostructures

Impact of Plasmon Induced Optically Rectified Electric Fields on Second Harmonic Generation Darby A Nelson, and Zachary D. Schultz J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b05685 • Publication Date (Web): 29 Jul 2019 Downloaded from pubs.acs.org on August 6, 2019

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The Journal of Physical Chemistry

Impact of Plasmon Induced Optically Rectified Electric Fields on Second Harmonic Generation Darby A. Nelson and Zachary D. Schultz* Department of Chemistry and Biochemistry, The Ohio State University, 100 W. 18th Ave, Columbus, OH, 43210 *corresponding author email: [email protected] Abstract. Plasmon associated optically rectified fields arising from electron tunneling between metal nanojunctions have been shown to impact photocatalytic reactions on the surface of plasmonic nanostructures. However, temporal differences have been shown between quantum regime electron tunneling events and optically rectified surface charge buildup. Computational work shows electron tunneling across a nanometer-sized gap between two plasmonic entities can be induced from a single ultra-fast laser pulse and occurs on a femtosecond timescale. Our group has shown charge buildup from optically rectified fields occurs over 10’s to 100’s of seconds through light induced redox events in cyclic voltammetry experiments using a CuSO4 solution. Utilizing a femtosecond laser to generate second harmonic signals along with widefield excitation from a continuous LED source, it is shown that surface potentials that average -400 mV are only generated upon continuous illumination of a gold nanoisland plasmonic surface with a widefield LED source. Negligible changes in SHG signals are seen from surface excitation with the femtosecond laser alone, showing the importance of continuous plasmon excitation for sustained surface potentials. It is also shown, through Stark spectroscopy and spectro-electrochemical measurements, that optically rectified fields impact SHG signals in a similar fashion to an externally applied bias on pristine plasmonic surfaces. This result shows the potential for utilizing SHG signals to better understand the impact surface heterogeneity has on the optically rectified DC field and enables the optimization of plasmonic surfaces for future use as photocatalysts.

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Introduction Plasmonic materials have unique optical properties and characteristics that are seeing increasing use in diverse applications. The localized surface plasmon resonances (LSPRs) of these materials can be tuned across the solar spectrum, allowing for increased light collection boosting photocurrent efficiencies when incorporated with semiconductors in solar cells.1-3 The relaxation of the LSPR can result in ‘hot’ electrons, which have been shown to facilitate photocatalytic reactions, such as water splitting and CO2 reduction, at the surfaces of plasmonic materials.2, 4-9 The electric fields generated on the surface of plasmonic materials have also been shown to drive optical processes, such as surface enhanced spectroscopies, and increasing the intensity of nonlinear phenomena, including second harmonic generation and optical rectification.10-15 Recently, the optically rectified plasmonic field has been shown to impact photocatalysis on nanostructured surfaces as well as decrease overpotential for electrochemical reductions.16-17 As the use of plasmonic materials as photocatalysts continues to grow, the impact surface heterogeneity has on the fields present at the surface of plasmonic materials needs further characterization. An increased understanding of the relationship between nanostructure morphology and the resulting electric fields could allow for surface modifications that advance plasmonic materials as photocatalysts. Localized surface plasmon resonance generation occurs through the absorption of light and creates a confined electric field at the surface of a metal nanostructure consisting of oscillating excited electrons.10, 18-19 Depending on the mechanism by which an excited electron relaxes, it can lead to different physical effects on the plasmonic surface, including optical signal enhancements, heating effects, and hot electron generation.18-21 Hot electrons have been shown to drive photocatalytic reactions on the surfaces of plasmonic materials.18, 21-23 However, it has been shown in the literature that these excited electrons can also tunnel between plasmonic entities separated by small nanometer sized gaps generating direct current (DC) electric fields across the nanojunctions.12, 14, 24 This process is known as optical rectification, in which an alternating current electric field (i.e. plasmon) is rectified to a DC electric field on the plasmonic surface.25 This rectification process has been studied in the literature across multiple experimental setups. Photocurrents have been detected through biased plasmonic nanojunctions showing strong dependence of photocurrent magnitude on irradiation power of the gap.14 Gap distance dependence on light induced photocurrent generation across nanometer sized gaps has also been performed utilizing varying sized carbon chains to control gap distance.26-27 Large decreases in photocurrents were seen when gaps reached sizes greater than 1 nm showing tunneling only occurs when gap sizes are in the quantum regime. Computational efforts have also been put forth to understand timescales associated with light induced electron tunneling events. Aguirregabiria at el. and others have shown that tunneling events across nanojunctions can be induced with ultra-fast femtosecond laser pulses.28-31 The electric fields across the junctions persist for 15 nanoseconds at most from a single pulse, showing the very short lifetimes of these excited electrons. Work in our lab indicates that optical rectification alters the surface potential and can modulate hot electron mechanisms.17

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In the majority of the experiments in the literature, the gap has either had a small bias across it or has had a molecule within the gap, in some cases both, that directs the flow of electrons across the gap.14, 26-27, 32 Our previous experiments using Stark spectroscopy, which measures changes in the vibrational frequency of a chemical bond due to an applied electric bias,33 showed net changes in the surface potential and regions of positive and negative charge. Electrochemical measurements provided calibration for Stark shifts observed in air, which showed the same power dependence as the Stark shifts measured in solution.17 On heterogeneous plasmonic surfaces, it is unclear what controls the direction of the observed field on the plasmonic surfaces. How the OR field evolves on these surfaces without assistance from an external field or dipole in the nanojunction is unclear as well. Further work understanding the impact of surface heterogeneity on OR field generation will improve plasmonic surfaces for use as photocatalysts. Our group, along with others, have been attempting to understand and quantify optically rectified fields on the surfaces of heterogeneous plasmonic materials.12-13, 16-17, 34-36 In our prior work, changes in surface potential were observed on heterogenous plasmonic surfaces utilizing the vibrational Stark effect. Utilizing Raman spectroscopy, the vibrational frequency of the CN stretching mode can be monitored to determine the magnitude of the DC electric field the Stark reporter is experiencing. 13, 17, 24, 34 Stark spectroscopy studies using isolated nanogaps (individual gold nanoparticles on a gold mirror film with varied gap distances) demonstrated that rectified fields occur in gaps associated with electron tunneling.24 Recently, we demonstrated that these photoinduced changes in surface potential altered electrochemical responses,16 which suggests other methods may be able to further elucidate and characterize these effects. Optical rectification is a second order nonlinear process. Second order nonlinear processes arise from the second order nonlinear optical susceptibility term, (2), and require a lack of inversion symmetry. These second order processes are powerful techniques for studying surface effects and electric fields surrounding thin films.37-38 Second harmonic generation (SHG) is also a second order nonlinear process with (2) dependence, suggesting it could be an effective method to monitor optically rectified fields.39 SHG originates from the excitation of a nonlinear medium with two like photons followed by an emission of a single photon with double the frequency of the incident photons.39 The unique relationship between OR and SHG is seen through the expansion of the second order polarizability, Equation 1, which consists of the multiplication of a second order susceptibility term ((2)) and the two-alternating incident electric fields. Through expansion of this equation, two different terms fall out in the end, one depending on two times the incident frequency (SHG) and one without a frequency dependence (OR), Equation 3.40 ―𝑖𝜔𝑡 𝑃2𝑁𝐿 = 𝑋(2) 𝑖𝑗𝑘 ∗ 𝐸𝑖 ∗ 𝐸𝑗; 𝐸𝑖 = 𝜀𝑖 ∗ 𝑒

[1]

―𝑖𝜔𝑡 = 𝑋(2) + 𝜀𝑖∗ ∗ 𝑒𝑖𝜔𝑡)(𝜀𝑗 ∗ 𝑒 ―𝑖𝜔𝑡 + 𝜀𝑗∗ ∗ 𝑒𝑖𝜔𝑡)] 𝑖𝑗𝑘 [(𝜀𝑖 ∗ 𝑒

[2]

―2𝑖𝜔𝑡 = 𝑋(2) + 𝜀𝑖𝜀𝑗∗ + 𝜀𝑖∗ 𝜀𝑗 + 𝜀𝑖∗ 𝜀𝑗∗ 𝑒2𝑖𝜔𝑡) 𝑖𝑗𝑘 (𝜀𝑖𝜀𝑗𝑒

[3]

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SHG does not require a reporter molecule on the surface, thus suggesting a label-free method to study optical rectification on unfunctionalized plasmonic surfaces. In this paper, we will use the combination of Stark spectroscopy, atomic force microscopy, electrochemistry, and SHG to determine the correlations between the OR fields detected in Stark spectroscopy and from unfunctionalized surfaces with SHG. Our goal is to build increased understanding of how surface phenomena relate to the fields that arise on these plasmonic materials. Experimental Materials and Reagents Gold pellets (Kurt J. Lesker; Pennsylvania, USA), plain glass microscope slides (25mm x 75mm; Globe Scientific; New Jersey, USA), and 4-mercaptobenzonitrile (Synfine Research, Ltd.; Ontario, CAN) were purchased and used without modification. NoChromix, sulfuric acid (95-98%), ethanol (> 99.5%), and sodium sulfate (99%) were purchased from Sigma Aldrich (Missouri, USA) and used without modification. Thermal Evaporation of Gold Nanoislands Plain glass microscope slides were soaked in a solution of NoChromix dissolved in sulfuric acid for a minimum of 24 hours to remove any organic matter on the surface of the glass slides. Cleaning the glass slides was important for nanoisland formation. Following the 24-hour soak, slides were rinsed with DI water, dried with N2 gas, plasma cleaned for 3 minutes, and finally placed in the evaporation chamber. Thermal evaporation was performed using the PVD 36 Nano Thermal Evaporation system from Kurt J. Lesker Company. Gold nanoislands with a thickness of 15 nm were created by evaporating Au at a rate of 0.03 Å/s. The extremely slow rate is necessary to maintain the small gaps between the islands, especially with films of the thickness needed for the research performed here. Monolayer Preparation A 4-mercaptobenzonitrile (MBN) monolayer was adsorbed to the Au nanoislands in order to study the plasmonic properties of the surface. Clean Au nanoislands were soaked in a 0.01 M ethanolic solution of MBN for 24 hours to ensure a complete monolayer was created. UV-Vis Measurements Absorption spectra of the Au nanoisland samples were obtained immediately after the evaporation process finished and after monolayer adsorption, to analyze the optical properties of the surface and ensure films were non-continuous as well as unchanged from monolayer creation. Increasing film thicknesses red shift the absorption peak of the sample until a continuous film is created and no absorption peak is discernible.41 Spectra were taken with a UV-1600PC Spectrophotometer from VWR using a halogen lamp light source and scanning from 350 nm to 1100 nm. The system was background corrected using a clean glass slide prior to any sample absorption measurements.

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Electrochemical Measurements Electrochemical measurements were performed with a CHI660D potentiostat for Raman spectro-electrochemical measurements and a CHI832C for SHG spectroelectrochemical measurements. A three-electrode system was utilized for all measurements. The conductive Au nanoisland surface served as the working electrode while a Ag/AgCl electrode and platinum wire served as the reference and counter electrodes, respectively. A 0.1M solution of sodium sulfate served as the electrolyte in the system. All potentials stated in the manuscript are relative to Ag/AgCl unless otherwise specified. Raman Measurements Raman spectra were acquired using a Renishaw inVia Raman microscope with a 632.8 nm HeNe laser (Thorlabs) and a 785 nm laser photodiode (Innovative Photonic Solutions). For MBN studies, scattering between 1912-2374 cm-1 was collected. All spectral analysis was performed using MATLAB with a customized peak-fitting script from Mathworks. Spectra were fit with a single Gaussian, with the optimal fit being that with the smallest percent root mean squared (RMS) error. In general, 10-15 iterations were sufficient minimize the RMS error. For experiments analyzing the changes in MBN vibrational frequency from LED exposure, a 455 nm LED (Thorlabs) was used as it provided the highest power density (4.0 mW/mm2) to the sample without exciting at wavelengths in the Raman spectrum collection region from the 632.8 nm laser. Second Harmonic Generation/Atomic Force Microscopy Measurements The SHG/AFM setup includes a Ti:Sapphire oscillator (Coherent Mira Model 900F, ~120fs, 76 MHz) pumped by a solid state diode laser (Lighthouse Photonics, SproutH, 532 nm, 10W). The oscillator generates a femtosecond pulsed beam centered at 790 nm. This femtosecond beam is used to generate SHG of the Au nanoisland sample at 395 nm. The beam was linearly polarized and brought into an upright microscope equipped with a motorized nosepiece (Scientifica, SliceScope Pro) for focus onto the sample. Second harmonic generation was collected in the forward-direction with a microscope objective (Nikon LU Plan Fluor, 10X, 0.3 NA) and the collected signal was redirected to the optical window of a photomultiplier detector (Hamamatsu, H7422P) set to a gain value of 0.925. Immediately prior to the photosensor, two filters (400 nm bandpass and 500 nm short pass, Thorlabs) were used to ensure that only the SHG signal at 395 nm reached the photosensor to be detected. The detected signal was then sent to a lock-in amplifier (Stanford Research Systems, SR844 RF) to help separate any background noise from the small microvolt SHG signal generated. The 76 MHz pulsed signal from the femtosecond cavity was sent to the lock-in amplifier and used as an external reference. The signal was recorded using an auxiliary input on the AFM system. Topographical images of the Au nanoisland surface were measured along with the optical measurements. These were performed using an MV2000 Atomic Force

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Microscope from Nanonics Imaging Ltd. All optical signals and topographical information were collected using Nanonics NWS 11. AFM height and SHG signal measurements were linearly background corrected in the Gwyddion analysis software, prior to being analyzed further in MATLAB. For experiments analyzing the changes in SHG signal from LED exposure, 565 nm and 780 nm LEDs (Thorlabs) were used as they provided the highest power density (between 1.5 mW/mm2 and 5.7 mW/mm2) to the sample with minimal overlap of SHG signal at 395 nm. However, to ensure no residual light from the LED interfered a 500 nm long pass filter (Thorlabs) was used to block lower wavelengths. LED excitation was performed by directing light back through the signal collection objective for bottom side illumination. This design allowed for much higher LED powers to reach the sample. It is important to note that all SHG experiments were performed with pristine Au nanoisland surfaces, with no MBN monolayer present. Scanning Electron Microscopy Measurements Gold nanoisland SEM images were obtained using an Apreo SEM in The Ohio State University Center for Electron Microscopy and Analysis (CEMAS). Results To assess the correlation between optical rectification and second harmonic generation, Au nanoisland films were investigated. Figure 1 shows SEM and absorbance spectra of pristine and MBN adsorbed 15 nm Au nanoislands. Experiments on lower coverage films that show more clearly defined islands were not stable under pulsed laser excitation; however, at 15 nm coverage the material was stable with respect to pulsed laser excitation and still exhibited a plasmonic response as evidenced by the SERS spectrum of an adsorbate. The SEM images show a large variance in island size and shape as well as the small nanogaps that are believed to be necessary for OR. Minimal visual and optical changes are seen in the surface from the monolayer adsorption to the nanoislands as the nanogaps are still present and only a small red shift is seen in the absorbance peak around 800 nm.41 The shift in absorbance is consistent with the change in the dielectric function on the surface of the Au nanoislands from the MBN monolayer.

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Figure 1: SEM images of pristine (a) and MBN adsorbed 15 nm Au nanoislands (b). c) Absorbance spectra of pristine (black) and MBN adsorbed 15 nm Au nanoislands (red). Visually and optically it is seen that minimal changes to the nanoisland surface occur from the adsorption of the MBN monolayer. Prior to studying the impact optically rectified DC fields have on SHG signals, the effect of an externally applied DC bias on the SHG signal was determined. Spectroelectrochemical measurements were performed to calibrate the impact of external bias on SHG signal generated from the Au nanoisland surface. Similar experiments were also performed on MBN adsorbed onto Au nanoislands using SERS in order to compare trends seen in the vibrational frequency shift of the mercaptobenzonitrile Stark reporter molecule and the SHG signal. The Stark reporter measurements show that the Au nanoisland surface acts in a similar fashion to previous surfaces used for external-bias Stark spectroscopy spectro-electrochemical measurements, where the signal in air and in solution was observed to behave similarly.16-17 Similar charging in air and water supports the shift arises from a change in surface potential and not altered double layer structure. Figure 2 shows the CN stretch frequency from an adsorbed MBN monolayer (A) and SHG intensity (B) as a function of applied electrochemical potential. The potential was swept from 0V to -1V at a scan rate of 10 mV/s while either Raman spectra, every second, or SHG signals, every 10 milliseconds, were recorded. An increasingly negative surface potential causes the CN stretch frequency to decrease to lower Raman shifts at a rate of 6 cm-1 V-1 and the SHG signal to decrease in intensity at a rate of 0.5 V V-1. A linear response is observed in the double-layer region (see Figure S1, supporting information), where no electron transfer reactions alter the oxidation state of the Au surface. The experimentally derived Stark tuning coefficient is consistent with our previous reports from MBN on Au nanowire array electrodes and the trend seen with SHG signal is consistent with the electric field induced SHG literature.1617, 42-43

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a)

b)

Figure 2: The (a) CN stretching frequency of MBN and (b) SHG intensity show a linear shift in frequency and intensity, respectively, with applied electrochemical potential (vs. Ag/AgCl). The inset of (a) shows a heatmap depicting how the center of the CN stretching vibrational mode shifts with potential. The CN stretching frequency decreases at a rate of 6 cm-1 V-1, while SHG intensity decreases at a rate of 0.5 V V-1. SERS spectra were taken using a 40x water immersion objective (NA=0.8), 1.135 mW at the sample, and 1 s acquisition times. For SHG experiments, the same water immersion objective was used for laser focus to the sample, with collection occurring in the forward direction using a 10x air objective (NA=0.3), 4.6 mW at the sample, and 10 ms acquisition times. The error bars are the standard deviation observed from 6 (SERS) and 8 (SHG) separate measurements on different spots of 15 nm Au nanoisland samples. Optical rectification on the Au nanoisland surface was demonstrated by measuring the change in SHG signal and Stark tuning of the CN stretch with increased laser power in air without an applied bias. Figure 3 shows the observed change in CN stretching frequency from increased laser power using two excitation wavelengths, 633 nm and 785 nm. Two laser wavelengths were used to provide further evidence that the Stark shift of the CN stretching frequency observed on the nanoisland surface originates from the plasmonically induced OR fields and not simply from the increased field applied by the laser itself. Theoretically, an excitation wavelength that is on-resonance (785 nm) will invoke a stronger surface plasmon, and therefore a larger Stark shift, than an excitation wavelength that is off-resonance (633 nm). The surface plasmon resonance of the Au nanoisland surface was determined from the UV-Vis extinction spectrum (Figure 1). A Gaussian was fit to each SERS spectrum to determine the vibrational energy of the detected CN frequency. A linear change in frequency is observed, as expected, for both excitation wavelengths. 16-17 However, as theorized, we observe nearly double the induced rate of

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change in CN stretching frequency from 785 nm excitation (-1.5 cm-1 mW-1) compared to that of the 633 nm excitation (-0.8 cm-1 mW-1), over the same power range of 0.1 mW to 3.6 mW. The increased response from excitation coinciding with the plasmon resonance supports OR of the localized plasmon induced electric field on the surface of the Au nanoislands. Furthermore, the results in Figures 2 and 3 suggest that this induced OR field from the plasmonic structures gives rise to a predominantly negative surface charge. The inset of Figure 3 shows the average spectra observed at 0.1 mW and 3.6 mW excitation power at 785 nm. A clear shift is seen in the peaks, however minimal broadening is observed. This negligible broadening supports our assertion that temperature does not having a prominent impact at these low powers. Temperature should cause a heterogeneous line broadening but is not expected to change the vibrational frequency of the CN stretching mode.

Figure 3: CN frequency as a function of laser excitation power on 15 nm Au nanoislands with a MBN monolayer in air. Two laser wavelengths are shown (Black: 633 nm, Off-resonance; Red: 785 nm, On-resonance) to give rise to Stark shifts in the CN stretch frequency. For both wavelengths, the CN stretch is observed to decrease with increasing laser power. However, the power dependence of the Stark shift is nearly double with on-resonance excitation of the plasmon mode (633 nm: -0.8 cm-1 mW-1; 785 nm: -1.5 cm-1 mW-1). This decrease in Raman shift with power indicates a negatively charged optically rectified field is formed on the plasmonic surface. The inset shows average spectra from excitation with 785 nm at 0.1 mW (black) and 3.6 mw (red). The spectra were corrected for acquisition time and laser power and then normalized for comparison. Spectra were obtained using a 50x air objective (NA=0.75). The error bars are the standard deviation observed from 5 different spots on the same surface. The laser power dependence of SHG intensity was measured to illustrate the relationship between excitation power and SHG generation at the surface of the Au nanoislands, as shown in Figure 4. The pulsed lasers provide energy very near the

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damage threshold of the Au nanoisland films, requiring low laser intensities, even at the high repetition rate of 76 MHz, to avoid damaging the surface. The low laser fluence resulted in a noisy signal, which required signal averaging to assess the power dependence. The average SHG intensity across a 7.5 m2 area (65,536 data points) was used at each laser power to ensure signal was detectable above the background noise. By averaging over a sufficiently large area (7.5 m2), the heterogeneous nature of the island formation on the surface is negligible, enabling comparisons between different spots and surfaces. The black trace in Figure 4 shows the observed change in SHG intensity with increased laser power. The data points follow a quadratic power dependence, which is expected for the second order nonlinear SHG phenomenon.37-38 The laser power experiment was repeated at the same 5 laser powers, on 5 new spots on the surface, but this time with constant surface illumination with a 565 nm LED (filtered to transmit only from 500 nm to 800 nm) with a power density of 1.5 mW/mm2. The red trace in Figure 4 shows the observed change in SHG intensity with increased laser power and constant LED excitation of the surface. A nonlinear trend is again observed, however as laser power increases, the SHG signal is observed to decrease relative to the SHG intensity in the absence of the LED. The change is particularly evident at SHG laser powers greater than 3 mW. A significantly decreased SHG intensity was observed from continuous Au films (Figure S2, supporting information). The relationship between surface bias and SHG intensity is not as straight forward as with Stark spectroscopy. Previously, in the CN stretching frequency experiment, the observable being studied was not SERS intensity, rather the vibrational energy of the CN stretching mode. By measuring the frequency rather than the intensity, when excitation power is increased to induce a stronger plasmonic field, which increases SERS intensity, the change in intensity does not impact the measurement. This avoids complications associated with changes in the number of molecules present. For SHG experiments, the signal intensity is the only observable available. For an isolated nanoparticle, the increased electric field should increase the SHG signal; however, in the quantum regime where electrons tunneling between nanoparticles alters the electric field and where optical rectification has been observed the SHG response is more complicated.24, 32, 44-45 Figure 2 shows that a negatively charged surface causes a decrease in the observed SHG signal and therefore the negatively charged OR field will decrease the SHG signal, as seen in Figure 4. An interesting point is the difference in the lifetime of the plasmon and excited hot charge carriers relative to the optical excitation. Previous work indicates that the lifetime of the plasmon resonance is on the order of picoseconds to femtoseconds.46-50 Prior work on the optically rectified fields indicate that the surface charge develops on a slower time scale, possibly as slow as seconds, resulting from cumulative asymmetric tunneling rates of excited electrons.16 Thus, the plasmon resonances excited by the femtosecond laser running at 76 MHz (13 nanoseconds between pulses) likely relax to the ground state before the next pulse arrives. Thus, the SHG signal in the dark does not experience the change in surface charge. This provides an explanation for the changes in the SHG in the presence of LED illumination, where the surface charge can accumulate from the continuous excitation and alter the SHG response. This observation also correlates with the trends in the Stark shifts induced by OR fields. It should be noted that the CW lasers used in the Raman Stark spectroscopy results will

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provide continuous excitation to the plasmon resonance and can drive OR independently.

Figure 4: SHG intensity as a function of excitation power on Au nanoislands in air. Intensities from a 7.5 m2 area (65,536 individual data points) were averaged for each laser power. Different spots were used for each laser power to ensure sample photodamage did not impact results from one laser power to the next. To show the impact of optically rectified fields on SHG intensity, a set of increasing laser power experiments were performed without LED excitation (black) and with LED excitation (red). Data points for both experiments follow a quadratic fit (solid line) with increasing laser power, which is expected for a second order nonlinear process. The red curve used a 565 nm LED, which generated 1.5 mW/mm2 at the sample. A 50x air objective (NA=0.5) was used to focus both the laser and LED onto the sample. The SHG signal was collected in the forward direction using a 10x air objective (NA=0.3) and 10 ms integration time. Error bars are the standard deviation observed across the 7.5 m2 area for each respective spot on the surface. Figure 5 provides further evidence that OR induced DC fields have an impact on SHG intensities from the Au nanoisland surface. SHG signal was collected over the same 12.5 m2 area in back to back scans. In the first experiment, the second scan was repeated without the LED excitation to generate the distribution in black. In the second experiment, the second scan was obtained while the surface was illuminated with a 780 nm LED (5.7 mW/mm2) giving rise to the distribution in red. The values plotted in each of the histograms were determined through subtraction of the first scan intensities from the second scan intensities and those values were converted to volts using the relationship determined in Figure 2, 0.5 V V-1. The mean intensity change of the no LED excitation histogram is around -30 mV, whereas the mean intensity change of the histogram generated from LED excitation is around -400 mV. It should be noted that this change in surface potential is larger than was observed in prior work that did not 11



illuminate at the plasmon resonance frequency and is consistent with the plasmonic effects noted in Figure 3.16 The change in SHG intensity in Figure 5 further indicates optical rectification generates a more negatively charged surface when illuminated with the LED. It is important to note that the very small mean shift in SHG intensities in the black histogram provides evidence of little to no damage to the Au nanoislands from the pulsed laser that gives rise to the SHG signal. This supports that the larger decrease in SHG signal seen in the red histogram is from the OR induced negative DC field rather than a reduction in signal from damage to the surface.

Figure 5: LED induced changes in SHG signal intensity in air. SHG signal was collected over the same area of the Au nanoisland film (12.5 um2) in back to back scans. Black bars: no widefield LED excitation during second scan; Red bars: Au nanoislands optically excited by a 780 nm LED (5.7 mW/mm2) during the second scan. Solid lines are a Gaussian fit to each of the respective data sets. Both histograms show the difference in SHG signal determined through subtracting the first scan intensities from the second scan intensities and converting to volts through 0.5 V V-1 relationship determined in Figure 2. The black histogram has a mean value of -30 mV, while the red histogram has a mean value of -400 mV. A 50x air objective (NA=0.5) was used to focus the laser onto the sample (4 mW at sample), and the SHG signal was collected in the forward direction using a 10x air objective (NA=0.3) and 10 ms integration time. Four trials of different areas of the surface were averaged together for the red histogram (see supporting information Figure S3 for the individual distributions), while two trials of different areas of the surface were averaged together for the black histogram. The net negative surface charge is an interesting observation, given the connection to tunneling junctions previously associated with the optical rectification process.24 To correlate the impact of topography and surface heterogeneity on the OR fields, correlated AFM and LED impacted SHG images were measured. The spatial distribution of the surface charge is evident through correlation of the changes in SHG signal from LED surface excitation and topography of the Au nano-islands. Similar to 12


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Figure 5, back to back SHG scans with LED excitation during the second scan were performed, however prior to any SHG measurements, an AFM image was taken of the surface. Contour plots of island topographies measured by AFM enable visualization of changes in island height across the surface. Figure 6 shows a side by side comparison of surface topography and SHG signal changes. From the changes in the SHG signal with and without LED illumination, the spatial distribution of plasmon induced OR fields were determined. Changes in SHG signal were once again calculated by subtracting the signals of the first scan from the second scan, during which the surface was optically excited by a 780 nm LED at 5.7 mW/mm2. The change in SHG intensity was converted to volts using the relationship determined from Figure 2. The domains in Figure 6 were not observed on a continuous Au film (see supporting information Figure S4). To better visualize the correlation between topography and change in surface potential, two smaller areas on the surface representing the two largest SHG signal changes are shown in the panels to the right of the large SHG image. SHG signal changes are seen in (b) and (d) along with their corresponding Au nanoisland contour plots shown in panels (c) and (e) respectively. The spacing between the nanoislands is smaller than can be interrogated using standard AFM tips. Thus, the nanojunctions evident in the SEM are not resolved in the AFM measurements. However, there is a qualitative agreement between slight changes in the nanoisland height and LED induced SHG signal change. Decreases in SHG signal upon LED excitation of the surface seem to correlate to areas of slight increases in the height of the surface features.

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Figure 6: Correlated AFM and SHG difference images of Au nanoisland topography and LED induced SHG signal changes in air. (a) Large scale image of LED induced SHG signal changes over the AuNI surface. (b) and (d) Zoomed in images on the largest LED induced SHG signal changes from the full-scale image. The blue and red boxes seen in (a) depict the locations on the surface of (b) and (d) respectively. (c) and (e) Contour plots, correlating to (b) and (d) respectively, generated from the AFM image taken prior to SHG collection. SHG difference images were taken in the same fashion and with same parameters as those in Figure 5. Figure 7 shows images of the surface charge detected from the MBN reporter molecule on the Au nanoisland surface in the Stark spectroscopy experiments. Changes in CN stretch frequency upon LED excitation were performed in the same fashion as the LED induced SHG signal experiments above. The change in the CN stretch frequency was calculated by subtracting the signals of the first scan from the second scan, during which the surface was illuminated with a 455 nm LED at power of 4.0 mW/mm2. In order to measure the Raman while illuminating the surface, a lower wavelength LED was needed. The change in vibrational frequency was converted to volts using the 6 cm-1 V-1 Stark tuning coefficient determined from Figure 2. Figure 7a shows a map of LED induced surface charge on the Au nanoisland film. The results are consistent with previous reports, showing areas of positive and negative charge.16 The Raman and SHG results are obtained with comparable spatial resolution and show similar patterns in negative and positive potentials localized on the surface of both. It is worth noting that SHG signals are oversampled and collected on smaller step sizes and further do not depend on the probe molecule SERS intensity, both of which may provide a clearer spatial picture of the surface charge domains. Figure 7b shows the CN stretch frequency histograms with and without LED illumination. Figure 7 shows that the same trend for the change in surface charge with LED illumination, an overall more negative 14


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surface, is seen in the CN vibrational frequencies as with SHG signals. The LED illuminated distribution in Figure 7 shows a change in mean value that equates to an approximately -100 mV more negative surface. This lower change in surface potential relative to Figure 5 is consistent with the off-resonance LED used.

Figure 7. a) The LED induced surface charge is plotted from the change in the CN stretching frequency from MBN monolayer adsorbed on Au nanoisland surface in air. The same area (5 um2) was mapped with LED illumination of the surface occurring during the second scan to see the changes in CN vibrational frequency. (b) Histograms of first scan frequencies (black bars; no LED excitation) and second scan frequencies (red bars; with Au nanoislands optically excited by a 455 nm LED, 4.0 mw/mm2). Solid lines are a Gaussian fit to each distribution. Spectra were obtained using 632.8 nm laser excitation with 2.5 mW of power at the sample. A 50x air objective (NA=0.75) was used to focus the laser and collect the scattering from 0.25 sec acquisitions raster scanned across the surface. Discussion The results presented provide new perspectives on how the flow of electrons between nanostructures alters the optical, chemical, and physical properties of the system. Prior work indicates that photo-induced tunneling of electrons between nanostructures gives rise to fields that alter the observed properties.12, 17, 24, 26, 28 In addition to the change in optical processes, rectification of the optical response on plasmonic materials changes the surface potential and can drive chemical reactions. 1617 The OR fields are observed as steady-state phenomena that are associated with electron tunneling between junctions associated with coupled nanostructures in the quantum regime.24 In light of these results, questions remain regarding the origin of the optically rectified fields: Do plasmonically induced OR DC fields have an impact on 15



other second order nonlinear phenomena such as second harmonic generation, and if so, can SHG signals be used as label free method to monitor the surface charge that develops on nanostructured surfaces? Does the SHG signal provide new insight into the origins and magnitudes of the evolved OR fields. As shown above the second order susceptibility gives rise to both optical processes (e.g. SHG) and rectification of the fields. The SHG signal derives from the time dependent response at the asymmetric interface, while the rectification is the time independent offset associated with electrons not recovering before the next optical excitation. Electric field induced second harmonic experiments show that the SHG signal is perturbed by an applied electric field, which has also been modeled as a (3) process incorporating the DC field as the third field. Our results indicate that OR of the plasmonically induced fields impact the SHG signal in a manner similar to an applied external bias. This result appears somewhat at odds with previous reports that show the second harmonic signal increases with the plasmon induced local electric fields.44, 51 One possibility is a difference in the time dependence of the second order nonlinear optical process relative to the optical rectification. Density functional calculations indicate that tunneling between nanostructures upon excitation with a short (femtosecond) pulse will polarize the gap junction.28 In these simulations, this polarization increases when the optical pulse matches the plasmon resonance and gives rise to an increased optical signal. These simulations, similar to prior results in our lab,24 demonstrate the importance of narrow gaps in the quantum regime for an observed effect. In the results above, the increased SHG signal can be attenuated with CW excitation from an LED, suggesting a second effect that can impact these structures. As noted above, the lifetime of the plasmon resonance is short lived (fs to ps) relative to the time between pulses (ns). The charge transferred in this pulse has sufficient time to decay before the next pulse arrives. This is a different condition than one would obtain from a photocatalyst under constant illumination. When continuously excited, our results indicate the electrons tunneling across the gap junction give rise to domains of predominantly negative charge. From the energy of the single femtosecond pulse, calculations have shown electrons have the ability to tunnel across a nanometer sized gap on a femtosecond time scale and induce an electric field across the gap. The gap is created between two parallel 10 nm diameter gold nanorods.29 However, the field induced from the single femtosecond pulse only lasts for a single optical cycle of a few fs. Previous results in our lab suggest that surface DC charge increases for as much as 10’s to 100’s of seconds, a clearly different time scale than a fs laser pulse.16 Calculations show that electron tunneling occurs within femtoseconds of initial excitation; suggesting that OR and SHG do act on the same timescales initially, but that OR can be continuously pumped with an LED or other CW illumination to produce a larger effect.29-30 This longer time scale effect gives rise to the DC bias that then modulates the SHG signal. This longer time scale event is still difficult to predict. The SHG and Stark shift maps suggest the charge spans micron sized areas. Qualitatively, the domains observed in the two experiments are comparable (Figures 6 and 7). Second order phenomena require a break in inversion symmetry to be observed. In simulations a phase shift observed in the time dependent signals alters the electron distribution after a

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junction is polarized. This effect is expected to be small, but with continuous pumping could build into a sizeable effect. In the Stark shift experiments, the dipole of the Stark reporter can add directionality to the gap junction, though the results here indicate this is not a significant effect. The SHG experiments show similar trends without the dipole adsorbed to the interface. Thus, SHG appears to be a viable approach for monitoring the evolved surface charge, although the power of pulsed lasers presents challenges for small nanoparticles. An alternate explanation for the evolved surface charge comes from surface heterogeneity. It is known that the Fermi level of nanoparticles is dependent on the concentration of electrons in the conduction band, resulting in a lower Fermi level energy for larger particles.52 Thus in a heterogeneous nanostructure, electrons that tunnel across junctions may face an activation barrier in the reverse direction. Unfortunately, the films used in this experiment have features that are not adequately resolved by the imaging methods available. The simulations noted above saw polarization along gaps of identically sized particles.28-29 If this polarization can be continuously pumped, it would provide an explanation independent of heterogeneity. In the case of asymmetric nanoparticles, one may expect an even larger polarization and optically rectified field. The net negative charge on the surface of the Au nanoisland film remains a curiosity. This suggests that either the interior of the structures in the film adopt a positive charge or something else is directing the net polarization. While the origin remains elusive, its utility may enable future research in photocatalysts. We have previously shown Ag and Au structures to alter the rates and thermodynamics of reactions on their surfaces. SHG may help optimize these materials for future use as photocatalysts. Conclusion. Optically rectified fields have been shown to impact SHG signals in a similar fashion to an externally applied bias. Negligible change in SHG signals were seen from surface excitation with the femtosecond laser alone. However, a shift in SHG signal corresponding to as much as a -400 mV change in the surface potential was detected when illuminating the surface with a continuous widefield light source. This suggests that although electron tunneling events have been shown to occur on femtosecond timescales, sustained charge buildup on plasmonic surfaces requires continuous plasmon excitation and occurs over much longer timescales. Pristine, unfunctionalized plasmonic surfaces were shown to generate optically rectified fields impacting SHG signals indicating electron tunneling occurs and can be monitored on heterogeneous surfaces. This work shows the potential to use SHG signals to study optical rectification on plasmonic nanostructures and possibilities to optimize these processes and materials for future use as photocatalysts. Acknowledgment. This work was supported by National Science Foundation award CHE-1830994 and funding from the Ohio State University. Electron microscopy was performed at the Center for Electron Microscopy and Analysis (CEMAS) at The Ohio State University.

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