Supporting Information

Spectral screening of the energy of hot holes over a particle plasmon resonance

Evangelina Pensa a, Julian Gargiulo a, Alberto Lauri a, Sebastian Schlücker b, Emiliano Cortés a, c, Stefan A. Maier a, c

a The Blackett Laboratory, Department of Physics, Imperial College London, London SW7 2AZ, United Kingdom

b Chair of Physical Chemistry I, Department of Chemistry and Center for Nanointegration Duisburg-Essen (CENIDE), University of Duisburg-Essen, Universitätsstraße 5, 45141 Essen, Germany

c Chair in Hybrid Nanosystems, Nanoinstitute Munich, Faculty of Physics, Ludwig-Maximilians-Universität München, 80799 München, Germany

Corresponding Authors

* e.pensa@imperial.ac.uk (E.P.) and j.gargiulo@imperial.ac.uk (J.G.)

Contents______________________________________________________________________Electropolymerization of anilineS2Additional dataS3Estimation of the PANI thicknessS3Determination of the threshold potential (Eonset)………………………………………………………………..S3PANI characterization by Raman spectroscopyS5Opto-electrochemical response of AuNP in absence of aniline monomerS6Laser-assisted polymerization as function of irradianceS6 Determination of temperature increases by photoluminescence-based NanothermometryS7 Determination of temperature increases by computational calculationsS8Materials and methodsS9General….S9Applied potential profileS10AuNP characterizationS10Irradiance compensated by AuNP absorption S11 Absorption spectrum of a single AuNP on ITOS11 Determination of the beam waistS11ReferencesS12Supporting Figures______________________________________________________________Scheme S1. Polymerization mechanismS2Figure S1. Red-shift in the scattering maximum when the thickness of PANI layer increases S3Figure S2. Determination of EonsetS4Figure S3. PANI characterization by Raman spectroscopy …………………………………………………………S5Figure S4. Laser-irradiation experiments in absence of aniline precursorS6Figure S5. 561nm-laser-assisted electropolymerization at different irradiancesS6Figure S6. Photoluminescence-based Nanothermometry S7Figure S7. Computational calculations using COMSOLS8Figure S8. Applied step-potential profileS10Figure S9. AuNPs characterizationS10Figure S10 Determination of the beam waist at the focal plane (w0)S11

Electropolymierization of aniline

Scheme S1. Polymerization mechanism (adapted from ref [1]).

It is generally accepted that the electropolymerization reaction of aniline involves mainly three steps as shown in Scheme S1.1, 2 The first step has been stablished to be the oxidation of an aniline monomer into the anilinium cation radical. This step has been also found to be the rate-limiting step of the reaction.3, 4 Later, coupling of the anilinium cation radicals occurs, followed by the electrophilic substitution of a hydrogen atom in the benzene ring with an aniline molecule, resulting in an aniline dimer (middle panel in Fig. Scheme S1).5 The propagation proceeds via the same sequence - i.e. oxidation, coupling, deprotonation - until the final product is obtained. The polymer chain performs the function of mediator in the oxidation of the monomers. It has been shown that the oxidation potential of dimers, trimers and oligomers is lower than the one required for the formation of the anilinium cation radical.2

In order to ensure the polymerization of aniline in our reaction media, it is important to evaluate the stability of the solvent (water) compared to the aniline molecules. The photoexcited AuNPs could produce both, reduction and oxidation reactions involving the solvent (water). However, none of these routes seems favorable under our experimental conditions when comparing with the redox potentials for aniline electropolymerization.

1. Oxidation reactions. The electrochemical-oxidation of water on Au surfaces happens at potentials much higher than the aniline oxidation (ca 1.63 V vs SCE).6 Moreover, before the water oxidation reaction, the Au surface oxidation should take place. It has been reported that the onset potential for the oxidation of AuNPs in 0.5M H2SO4 is 1.05 V vs SCE.7 This represents a higher potential than the one required for the aniline polymerization. The later implies that the aniline molecules will be the first specie that oxidizes under these experimental conditions.

2. Reduction reactions. Two possible reduction reactions can occur: the water reduction (to H2) and the oxygen reduction (to H2O2). Under similar conditions than the ones employed in our experiments, the onset potential for water and oxygen reduction reactions have been reported to be more cathodic than -0.35 V and -0.05V vs SCE, respectively.8, 9 Accordingly, both reactions require higher cathodic potentials than the minimum value used in this work for our working electrode (0.2V). Moreover, it has been found that H2O2 (if formed) cannot catalyzed the aniline oxidation reaction itself, unless in the presence of a catalyst - like FeCl210 - or under a sonochemical treatment. 11

Additional data

Estimation of the PANI thickness

Figure S1. Red-shift in the scattering maximum when the thickness of PANI layer increases. The calculated values were obtained from the scattering spectra shown in the inset, where the grey arrow points the increase in the PANI thickness.

Figure S1 shows the scattering spectrum maxima of a 80 nm AuNP surrounded by different thicknesses of PANI layers (0 nm, 2 nm, 3 nm, 5 nm, 7 nm, 10 nm). Calculated using Lumerical FDTD Solutions (Au from Johnson and Christy,12 meshed with an outer layer thickness related to the PANI layer under study). The refractive index for the PANI layer was taken from literature according to the previous studies of Leroux et. al.,13 and Baba et. al.,14 and set at 2.2. The nanoparticle was then surrounded by water. Given that the refractive properties of water are relatively constant and loss-free in the studied wavelength range of 400 -1000 nm, it was modelled as homogeneous wavelength-independent material, with refractive index of 1.33. A mesh step of 1.5 nm was used for the nanoparticle and the surrounding medium, while a mesh step of 1 nm was used for the PANI layer mesh. Our results are in line with previous observations of scattering shifts due to PANI formation on Au.15

Determination of the threshold potential (Eonset)

The conventional electrochemical way to determinate the Eonset is by applying the tangent method to the current-potential curve - i.e. the intersection of the tangents between the baseline and the signal current. However, this method can only be applied for the pure electropolymerization process, as for the plasmon-assisted ones the current-potential curves does not reflect the behavior of the single illuminated AuNP. As shown in Figure 1d, the current-potential and Δλmax-potential curves are in complete agreement thus enabling to determine the Eonset for the plasmon-assisted polymerization using the tangent method in the Δλmax-potential curves. We found that the Eonset for the pure electrochemical process is 0.679 V, which corresponds to 6.32 nm red-shift in the λmax (see Fig S2a). Then, the same red-shift under illumination is reached at 0.576 V for 405 nm, 0.529 V for 532nm, 0.441 V for 561nm and 0.651 V for 633 nm (see Fig S2b).

Under our experimental conditions (i.e. all darkfield spectra recorded at the same Ew = -0.2V), the Δλmax reflects the amount of PANI generated. The later implies that the Eonset is defined for the same amount of PANI, or in other words, for the same amount of electron exchanged, allowing the direct comparison between Eonset.

Figure S2. Determination of Eonset. Exponential fitting for (a) electrochemical and (b) laser-assisted electropolymerization. The threshold potential (Eonset) was obtained from the intersection of the tangents in current-E plot (upper plot in a). This value corresponds to a red-shift of 6.32nm (as shown on lower plot in panel a). Then, the Eonset for the plasmon-assisted electropolymerization was defined as the potential at which Δλmax = 6.32nm (indicated as black dotted lines). Eonset error bars correspond to the propagation errors of the fitting parameter errors.

On the other hand, according to Bard and Faulkner16 the absolute electrochemical scale can be determined by referring the electrochemical potentials to Normal Hydrogen Electron (NHE). For this electrode the potential of the half-reactions: 2H+ + 2e- ⇌ H2 have also been assigned values of zero at all temperatures (the thermodynamic standard). More importantly, it has been found that the absolute potential of the NHE can be estimated as 4.5 ± 0.1 V, based on certain extra thermodynamic assumptions, such as about the energy involved in moving a proton from the gas phase into an aqueous solution. Thus, the amount of energy needed to remove an electron from Pt/H2/H+(a = 1) to vacuum is about 4.5 eV. Then, knowing the position of this reference level vs vacuum, the potentials of the WE can be expressed on the absolute scale. Since the potential of the SCE vs. NHE is well-known to be 0.243 V, interconversion of scales can be accomplished as follow:

[S1]

then the absolute electrochemical scale can be determined by the relationship:4.5 ± 0.1 V are 4.5 eV.

PANI characterization by Raman spectroscopy

Figure S3. PANI characterization by Raman spectroscopy. (a) Spectrum of an electrochemical generated PANI layer on Au. (b-d) Three single-particle SERS spectra of PANI obtained after each single AuNP were illuminated with (b) 561 nm CW laser and (c-d) 532 nm CW laser. Lighter-colored curves correspond to the raw data and dark-colored ones to the filtered-raw data with the Savitzky–Golay method (order 2, 10 points). (e) Position and relative intensity of the observed Raman bands. Assignments are based mainly on the known data.17-19 The numbers given in parentheses refer to the relative intensity of the band. Abbreviations used: B: benzene type ring, Q: quinone type ring, ∼: bond intermediate between a single and a double bond. Data for PANI on AuNP in the table corresponds to the average of the three single-particle spectra shown in Figures S3b-d.

Figure S3a shows the Raman spectrum of an electrochemical generated PANI layer on Au substrate. The sample was obtained by applying Ew = 0.7 V for 5 min. The spectrum recorded in H2SO4 0,5 M shows similar features than the ones obtained by other authors.17, 18 The characteristic strong band at 1570 cm−1 has been assigned to the C=C vibration in quinonoid rings, while the band at 1093 cm-1 and 572 cm-1 to the C-H bending and out-of-plane deformations of the benzene ring, respectively. The band at 1341 cm−1 is usually link to the C∼N+ stretching vibrations, corresponding to a charge delocalization. The C-N stretching and C-N-C bending modes were also detected at 1169 cm-1 and 806 cm-1, respectively.

The PANI layer obtained by plasmon-assisted electropolymerization of aniline on AuNPs was confirmed by surface enhanced Raman spectroscopy (SERS) at the single-particle level. To this end, after the polymer was generated, the aniline solution was removed and prior rinsed with ultra-pure water, replaced by the free-aniline electrolyte, i.e. 0.5M H2SO4. Figure S3b-d show the SERS spectra of three different illuminated single AuNP. The spectra c-d were obtained after illumination with the 532 nm CW laser while the spectrum b with 561 nm. The excellent agreement between them point out the reproducibility in the generated product and its independence on the illumination wavelength.

In order to make a proper assignment of the peaks, the raw spectra (light colored curves) were filtered with the Savitzky–Golay method (order 2, 10 points). The relevant peaks of the polyaniline film were clearly observed in the AuNPs spectra with small changes in the position and almost the same relative intensity (see Figure 3e). Importantly, the fact that the relative intensity and positions of all the peaks are similar to the PANI film and that any new peak is observed - particularly at 1630 cm-1 and 1430 cm-1 - discard the complete or partial decomposition of PANI into carbon-like compounds or crosslinked units, respectively.18, 20, 21 In conclusion, Raman spectra of illuminated single AuNP are similar to the PANI electrochemical generated film, confirming that the plasmon-assisted electropolymerization conducts to the same product than by electrochemistry.

Opto-electrochemical response of AuNP in absence of aniline monomer

Figure S4. Laser-irradiation experiments in absence of aniline precursor. Negligible changes in the maxima scattering wavelength (Δλmax) are observed when the step-potential profile shown in Fig. S9 is applied on AuNPs in absence of aniline precursor and under 532nm-laser irradiation.

Laser-assisted polymerization as function of irradiance.

Figure S5. 561nm-laser-assisted electropolymerization at different irradiances. The laser irradiation was performed for 60 s at two Ew: 0.4 V (open circles) and 0.5V (filled squares). After 0.5 V was applied, the Δλmax increases as a function of irradiance, pointing out that the polymerization rate increases. However, any evidence of polymerization was observed after 0.4V was applied, even if the irradiance was increased 3-fold.

Determination of temperature increases by photoluminescence-based Nanothermometry

Figure S6. Photoluminescence-based Nanothermometry. (a) Photoluminiscence spectra of a single Au NP at four different increasing irradiances. Green vertical line indicates the excitation wavelength at 532 nm. (b) Ratio between anti-Stoke photoluminescence spectra of figure a versus the energy of the emitted light (solid lines). Dashed lines indicate the fitting using equation [S3]. (c-d) Estimated surface temperature increase versus laser irradiance for 532 nm(c) and 561nm (d) laser excitation. Each color indicates a different NP. The dashed line is the mean slope calculated from the measurement of 15 different particles (c) and 5 particles (d).

A single particle nanothermometry technique based on anti-Stokes photoluminescence emission was implemented. NPs were excited with a 532 nm laser at different irradiances, while recording the photoluminescence emission for 90 seconds. Example photoluminescence spectra are shown in figure S9a. The anti-Stokes emission should follow:

[S2]

where is the laser irradiance, is the NP photoluminescence spectra, is the absolute temperature, is the energy of the emitted photon and is the excitation laser energy. In addition, temperature increases are assumed to be linear with the laser irradiance.

[S3]

where is the room temperature and a proportionality factor. Using [S2] and [S3], it can be shown that the ratio between two anti-Stokes photoluminescence spectra measured at two different excitation irradiances and is

[S4]

where is the only free parameter. Figure S6b shows the ratio between anti-Stokes spectra from figure S6a (solid lines) including fittings using equation S4. Figure S6c shows extracted temperature versus excitation irradiance for five different 80 nm Au NPs on ITO (same data as figure 2c in the main text). Each color represents a different single NP. The dashed line indicates the mean slope calculated from the measurement of 15 different particles at 10 different irradiances each.

Determination of temperature increases by computational calculations

Figure S7. Computational calculations using COMSOL. (a) Scheme of the model used for Comsol simulations. and indicates the interfacial thermal resistances for water/AuNP and ITO/AuNP, respectively (b) Temperature increase distribution when AuNP is laser illuminated with 532 nm CW laser and irrandiance = 1mW μm-2.

Thermal simulations were performed using the Heat Transfer in Solids module from COMSOL Multiphysics. The steady state temperature field T around a NP is given by the heat diffusion equation22

[S5]

where is the thermal conductivity and is the heat density. Convective heat transfer has been neglected, considering the typically very low Rayleigh number at the nanoscale (see ref. [22] for details). The function represents an energy (heat) source coming from light absorption in the materials. Here, we assumed the electromagnetic losses from AuNP to be the only heat source. Considering that the thermal conductivity of Au is much larger than the ones of the surrounding media, it was assumed that the temperature and the heat density are homogeneous inside the AuNPs,23 and therefore the heat source inside the NP can be approximated by

[S6]

where is the irradiance at the particle position and is the particle volume. It is important to note that interfacial thermal resistances can strongly affect the estimated temperature values.24 To account for this fact, a boundary condition was implemented

[S7]

where is the heat flux, is the temperature gradient and is the unitary normal vector. The used values of thermal resistance were for the Au- Glass interface and for the Au – Water interface, as taken from Jones et. al.25 All other material properties were taken from the COMSOL library.

Simulating heat transfer between a sphere and a planar substrate is not an easy task. It’s been shown that the results can present important variations if the contact area is changed.26 If the sphere and the plane are assumed to be touching in a single point, an unphysical very hot spot is predicted. In reality, the sphere is not perfectly spherical, and the plane has a non-negligible roughness, leading to a non-zero contact area. This fact is modeled by embedding the NP a certain distance “d” inside the substrate, as schematized in Figure S7.

In order to find to best model for the experimental conditions, a parameter sweep was performed for the value of “d” to get the best fit with the experimental data under 532 nm laser illumination (see Figure S6c). The best match was found for d=1.5 nm. Using this value, the surface temperature increase of was reproduced (see Fig. S6b).

Then, temperature increases for the different excitation wavelengths were simulated. It is important to note that all thermal () and geometrical (d) parameters remain constant when the wavelength is changed. A wavelength change implies a different absorbed caused by a different absorption cross section and a different irradiance and can be estimated using equation S2. Using this procedure, surface temperature increases of 24.9, 36.0 and 8.6

were predicted for 405, 561 and 633 nm respectively (solid lines of figure 2c in the main text).

Materials and methods

General

All reagents are high-purity and were used as received. For aqueous solutions, ultra-pure water was employed (18.2 MΩ·cm, Milli-Q®, Millipore, France).

Electrochemical measurements were performed at room temperature (24.0 ± 0.2 ºC) in a three-electrode configuration cell and with a CHI760C potentiostat (CH Instruments, United States). 0.5M H2SO4 (99.99%, Sigma-Aldrich) aqueous solution was employed as supporting electrolyte. Aniline (99%, Sigma Aldrich) was dissolved in the supporting electrolyte until final concentration of 0.2M. A Pt coil (99.99%, Goodfellow, UK) and saturated calomel electrode serve as counter and reference electrode, respectively. Before every experiment Pt coil was cleaned by butane flame annealing following by the quenching in ultrapure water. The Ew was step-increased by 0.1V from 0.2V up to 0.7V (cf. Fig S8).

The working electrode consists in super-spherical Au nanoparticles (AuNPs) drop-casted on ITO (Nanoscribe, Germany). AuNPs capped with CTAB (80 nm in diameter, OD solution: 0.05, Fig S9a) were synthetized and purify accordingly to ref (27). AuNPs were deliberately selected as their low polydisperse and smoother surface reduce drastically the optical data variability ascribed to the crystallinity and size of the AuNPs (see Figure S9b).27 Lower concentrations of AuNPs on ITO are employed to guaranty high amount of isolated AuNPs on the surface (surface coverage ~0.2%, Fig S9c).

Optical measurements were performed with a confocal microscope (Witec, Germany) and a water immersion objective (NA= 1.0, x63 magnification, Zeiss, Germany) to provide the incident linearly polarized illumination and to collect the scattered light from single particles. For AuNPs laser illumination experiments the following CW laser were employed: 405 nm (PicoQuant, Germany), 532 nm (WITec, Germany), 561 nm (Cobolt AB, Sweden) and 633 nm (Witec, Germany). The CW-lasers were focused to their diffraction limit on the substrate plane (see below further details). For darkfield microscopy and imaging, the sample was illuminated from the bottom with white light (BLO2, Thorlabs, United States) through an oil-immersion dark-field ultracondenser (NA = 1.2−1.4, Zeiss, Germany). Scattered light was collected with the water immersion objective and directed to the color CCD camera. Scattering spectra were recorded at -0.2V vs SCE, before and after the Ew potentials were applied to avoid any potential dependence of the refractive index of either PANI and AuNP surrounding. 28, 29

In solution and 2D films absorption spectra were recorded UV-Vis at room temperature (24.0 ± 0.2 ºC) and with a resolution of 0.5 nm in an Agilent Cary 60 Spectrophotometer (Agilent Technologies, Inc., United States).

Applied potential profile

Figure S8. Applied step-potential profile. Scheme showing the step-potential profile. Black line indicates the different Ew and the red lines the potential at which darkfield spectra were taken. The laser is kept ON at the indicated shadow areas, otherwise is OFF.

AuNP characterization

Figure S9. AuNPs characterization. (a) Histogram of the size distribution of supper-spherical AuNPs from analysis of SEM images. (b) Histograms of the maximum scattering for super-spherical AuNPs (orange) and non-super-spherical AuNPs (violet) drop-casted on ITO. Data was obtained from scattering spectra recorded in water. (c) Histogram of the coverage of ssAuNPs on ITO employed as WE.

Irradiance compensated by AuNP absorption

Absorption spectrum of a single AuNP on ITO

The absorption spectrum of a single 80 nm AuNP on ITO was calculated using Lumerical FDTD Solutions according to the following model: a nanoparticle of 80nm radius, material Au from Johnson and Christy12 was placed on top of a substrate of material ITO by Konig et al.30 The nanoparticle was then surrounded by water. Given that the refractive properties of water are relatively constant and loss-free in the studied wavelength range of 400 -800 nm, it was modelled as homogeneous wavelength-independent material, with refractive index of 1.33. A mesh step of 1.5nm was used and test were done to assure convergence.

Table S1. Absorption cross-section coefficient of AuNP on ITO for the four excitation wavelengths employed in this work.

Determination of the beam waist

Figure S10. Determination of the beam waist at the focal plane, w0, for the lasers employed in this study. (a) Image of the light scattered by a single AuNP under CW 532 nm laser-irradiation. Gaussian fitting of the cross-section.

The irradiance of a focused Gaussian beam propagating along the z direction is given by

[S8]

where P is the total power intensity of the beam and w is defined such at the focal plane (z=0) is equal to the beam waist (w0), i.e. the radius at which the intensity is 1/e2.

The value of w0 was determined experimentally by detecting the light dispersed as the focused lasers were scanned over a single AuNP (Fig. S10a). This measurement is proportional to I(r). As shown in Fig S10b, by fitting a Gaussian function to detected signal we obtained the values of w0 shown in Fig S10c.

Table S2 w0 obtained for all the employed laser. Last column shows the absorption-irradiance used for AuNP excitation experiments.

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