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, 80539 München, Germany

ABSTRACT: Plasmonic hot carriers have been recently identified as key elements for photocatalysis at visible wavelengths. The possibility to transfer energy between metal plasmonic nanoparticles and nearby molecules depends not only on carrier generation and collection efficiencies but also on their energy at the metal-molecule interface. Here an energy screening study was performed by monitoring the aniline electro-polymerization reaction via an illuminated 80 nm gold nanoparticle (AuNP). Our results show that plasmon excitation reduces the energy required to start the polymerization reaction as much as 0.24 eV. Three possible photocatalytic mechanisms were explored: the enhanced near field of the illuminated particle, the temperature increase at the metal-liquid interface and the excited electron-hole pairs. This last phenomenon is found to be the one contributing most prominently to the observed energy reduction.

KEYWORDS: Energy of hot carriers, photocatalysis, electrochemistry, nanothermometry

1

A plasmon excitation in a metal nanoparticle decays non-radiatively by generating an electron-hole pair

which thermalizes after electron-electron scattering leading to an increase in the surface lattice temperature of the

nanoparticle.1-2 These two mechanisms (hot carriers and surface temperature) can provide a quanta of energy to

induce a chemical reaction at milder conditions compared to the non-illuminated nanostructure.3-6 At the same

time, plasmon excitation in metal nanoparticles lead to intense and confined electromagnetic fields on their

surfaces. This can enhance the probability of absorbing a photon in molecules sitting next to them, populating

molecular excited states and leading to an altered reactivity of the system. 7 Metal-to-molecule direct charge

transfer can also be assisted by the presence of intense near fields.8 All these pathways point to the fact that

energy transfer between illuminated plasmonic structures and nearby molecules can be achieved, altering the

chemical composition of the surrounding environment.9-14 This process constitutes a fundamental step in

plasmonic photocatalysis.

In the last few years, the photocatalytic effect of plasmonic nanostructures has been successfully

demonstrated for different systems. It has been found that hot electrons can either trigger or assist chemical

reactions such as photopolymerization,15-16 molecular dissociation4, 17-19 or molecular reduction.14, 20-25 Moreover,

hot holes were also exploited in photo-oxidation reactions of surrounding molecules.12, 14, 26-28 Most of these

examples report an increase in the rate of the reaction when illuminating the plasmonic particles in comparison to

a dark scenario. In some cases, plasmon-excitation even triggered chemical reactions that were blocked in

darkness.17 Whether a reaction can take place or not when illuminating a plasmonic nanoparticle is linked to the

energy of the carriers at the metal-molecule interface, while the rate of the reaction is determined by the sum of

all the elementary steps involved in the reaction. As such, despite the large number of examples showing hot-

carrier-assisted reactions, little is known about the energy of the plasmon-derived carriers at the metal-molecule

interface. Quantification of the energy of the carriers is essential to predict the reactivity of plasmonic systems

and such knowledge is one of the key aspects for moving forward in designing tailored plasmonic

photocatalysts.29

The maximum energy of a carrier after absorption of a single photon is in principle the energy of the

photon.30 However, the effective energy of the carrier at the metal-molecule interface can be significantly lower

than the expected maximum. Theoretical efforts were able to predict the energy distribution of the generated

plasmonic carriers and their energy-loss due to the transport inside the material.24,31 Calculation of the effective

energy has to account for generation, transport and injection into the molecular orbitals, making estimates

difficult from a purely theoretical view point.32 Experimentally, there are some examples estimating the energy

contribution of plasmonic carriers to a chemical reaction from the reduction in its apparent activation barrier. In

these reports, the energies of the carriers have not exceeded 50% of the incident photons energy. 4, 33-34 For

2

example, it has been found that the energy supplied by interband-excited hot electrons to the reduction of

ferricyanide anions on Au nanoparticles was 0.24 eV, corresponding to approximately 10% of the incident light

energy (514 nm or 2.4 eV).33 A similar value has been recently reported for the oxygen evolution reaction on

illuminated Au@Ni3S2 nanoparticles.35

Here, electrochemistry and single-particle dark-field microscopy and spectroscopy are combined to study

the role of a photoexcited Au nanoparticle (AuNP) in the electro-oxidation of aniline to polyaniline.

Electrochemistry provides a way to control the energy of the electrons in the metal nanoparticle and at the same

time allows to compute the overpotential needed for the electrochemical reaction to proceed. This also serves as a

method to quantify the photocatalytic effect of the illuminated AuNP. Wavelength-dependence studies show that

the overall energy requirements of the electrochemical reaction can be reduced up to ~35% when exciting the NP

at it plasmon resonance. In order to understand the mechanism behind this photocatalytic effect, a single particle

nanothermometry technique based on anti-Stokes photoluminescence emission is implemented. It is shown that

even if the total absorbed energy at each excitation wavelength is the same, the reactivity of the hot-holes follow

the plasmon resonance profile. The effective energy of the reactive hot holes is in the range of 0.03eV to 0.24eV.

These results shed light on the role of the absorption processes in plasmonic photocatalysts and the maximum

energy of the reactive hot holes at the surface of a 80 nm AuNP.

3

Figure 1 (a) Scheme of the opto-electrochemical setup. The inset sketches the WE. (b) Simplified aniline polymerization mechanism highlighting that radical-cation formation is the rate-limiting step of the reaction. (c) Red-shift in the scattering observed when AuNPs are capped with PANI (thickness = 19 ± 1 nm). (d) Electropolymerization conducted in the dark (purple) and under laser excitation of AuNPs with 561nm CW laser for 30sec at I = 2.2 mW μm-2 (orange). (i) Red-shifts of the maximum scattering wavelength (Δλmax) as function of Ew. In the dark, the electropolymerization reaction takes place for both particles after E w = 0.7V is applied, as shown darkfield images and spectra in ii and iii, respectively. Moreover, the Δλmax shows the same trend than the current (black squares). Contrarily, when the polymerization is conducted under AuNPs illumination, the irradiated nanoparticles (upper particle in (iv)) catalyze the reaction, while the non-irradiated one (lower particle in (iv)) remains inactive until Ew = 0.7V is applied. In graph d-i, the symbols and the error bars correspond to the mean and standard deviation of Δλmax for more than five particles from at least two independent samples.

The oxidation of aniline molecules to generate polyaniline (PANI) on the surface of 80 nm Au nanoparticles

(AuNPs) was studied in an electrochemical cell mounted on a dark-field microscope with a water immersion

objective, as schematized in Figure 1a. This opto-electrochemical setup allows tuning the energy of electrons of

AuNPs as well as the illumination and the spectroscopic characterization of single AuNPs. It is known that the

electro-polymerization of aniline starts with the oxidation of an aniline molecule to the radical cation at the AuNP

4

surface,36-38 as sketched in Figure 1b. Then, these cations react forming the conductive polymer PANI. The first

step is considered to be the limiting rate of the reaction, as it requires higher anodic potentials than the following

propagation.39 See Scheme S1 in the supporting information (SI) for further details on PANI polymerization

reaction.

The formation of PANI is monitored optically through changes in the scattering spectra of the AuNPs. 40

Single particle scattering spectra were obtained using a WITec spectrometer based on a 600 lines/mm diffraction

grating and a cooled CCD device. A halogen lamp and a dark-field condenser were used to achieve dark-field

illumination. Super-spherical AuNPs41 were used to ensure a narrow size distribution. When immersed in aniline,

the AuNPs present a scattering peak at 561 ± 3 nm, as shown in Figure 1c (orange curve). Polymerization of

aniline generates a change in the refractive index around the AuNPs, causing a red shift and an intensity increase

in the AuNPs scattering spectra (Figure 1c light blue, more details in Fig S1). Furthermore, the formation of PANI

is confirmed by Raman spectroscopy, as discussed below.

In a first experiment, a potentiostatic-polymerization study was carried out in the absence of light.

Electrochemical control was performed by using a three-electrode configuration (see Figure 1a) and 0.2 M aniline

in 0.5 M H2SO4 solution as supporting electrolyte. A saturated calomel electrode and a Pt coil served as reference

(RE) and counter (CE) electrodes, respectively. Potentials reported here are referenced to SCE; ESCE = +0.243 V

vs. standard hydrogen electrode (SHE). The AuNPs on an ITO substrate acts as the working electrode (WE).

Using a potentiostat the potential of the WE (Ew) is varied relative to RE while any potential-induced change on

the WE is followed by measuring the current between the WE and CE. With this configuration, it is possible to

control the potential of the WE relative to the RE, which is equivalent to controlling the energy of the electrons

within the WE (i.e. the energy of electrons of the AuNPs and ITO). Imposing a more positive Ew will lower the

energy of electrons and vice-versa.42

Figure 1d-i (black curve) shows measured current vs Ew. The drastic increase in the current for high

potentials Ew indicates the generation of the conductive PANI. Fitting an exponential growth to the curve, the

threshold potential to start the reaction was found to be Eonset =0.68 V (details provided in SI-Fig S2). Polymer

generation is also evident by observing the sample turning pale white using a digital color photo camera, as

shown in Figure d-ii. As expected, polymerization takes place on the entire WE – i.e. on both ITO and AuNPs

surfaces.39 An analysis using Raman spectroscopy (see Fig S3) confirms that the generated polymer is indeed

PANI.

Alternatively, insight on the PANI generation can be obtained by analyzing the scattering spectra of the

individual AuNPs at different applied potentials Ew (see Fig 1d).43-46 Figure 1d-i (purple curve) shows the spectral

shift of the scattering maximum vs applied potential Ew. A strong red shift is observed at potentials higher than the

5

aniline polymerization threshold, indicating the equivalence between the macroscopic current measurement and

the single-particle spectroscopic measurement.

The effect of plasmon excitation in the aniline polymerization was then studied. Individual AuNPs were

illuminated at their plasmon resonance by a 561 nm CW laser focused to its diffraction limit using a water

immersion objective with a 1.0 NA. Positioning of the AuNPs in the center of the beam was achieved with a XYZ

piezo-electric stage. A potential was then applied to the illuminated AuNP (30 seconds of illumination with an

irradiance of 2.2 mW μm-2). Then, a scattering spectrum was acquired in the absence of laser illumination at Ew =-

0.2V. Figure 1d-i (orange curve) shows the spectral shift of the scattering maximum vs applied potential Ew.

Remarkably, the polymerization reaction occurs at lower applied potentials if the particle is illuminated, meaning

that plasmon excitation catalyzes the reaction. At 0.5V, PANI is formed selectively only on the surface of the

illuminated AuNPs, and not on the non-illuminated ones or on the ITO surface, as shown in figure 1d-iv. If the

potential is further increased to 0.7 V, the polymer is formed on the entire sample, as already described. It is

important to note that if the illumination is conducted in an aniline-free electrolyte, no changes are observed in

the scattering spectra of the AuNPs (Fig S4), confirming that spectral shifts are due to PANI formation and not to

changes in the AuNPs geometry. Moreover, the formation of PANI was further confirmed by single-particle

surface enhanced Raman spectroscopy, SERS (Fig S3). The voltage threshold to start the reaction was estimated

to be Eonset =0.44 V (see Fig S3 for details). It is important to note that the value for this threshold is independent

of the excitation power. For voltages below the threshold Eonset, the reaction does not start, even if the power is 3-

fold increased. Increasing the power may influence the polymerization rate once the reaction has started but does

not decrease the value for the starting threshold, as shown in Figure S5.

6

Figure 2 Aniline polymerization onto AuNPs assisted by laser-illumination at different wavelength. (a)

Experimental extinction spectra of the WE as function of Ew showing that no photo-active species are generated

at Ew lower than 0.7V. Aniline spectrum is also included at the bottom. (b) Simulated absorption spectrum of

AuNPs on ITO immersed in aqueous media. Vertical lines show the wavelengths of the different lasers employed:

405 nm (blue), 532nm (green), 561 nm (orange) and 633 nm (red). (c) Increase of the AuNPs’ surface temperature

obtained from simulation (solid lines) and from single-particle thermometry (dotted symbols) at 532nm. The

different symbols correspond to 5 different particles. The determined slope from simulations were: 25.3°C

μm2/mW for 405 nm; 32.7 °C μm2/mW for 532 nm, 36.6 °C μm2/mW for 561 nm and 8.7 °C μm2/mW for 633

nm. Slope from single-particle thermometry was 32.7 ± 0.9 °C μm2/mW for 532 nm. (d) Δλmax as a function of the

applied potential for different illumination wavelengths with the intensity normalized by absorption: (orange) 405

nm (I405 = 3.2 mW μm-2), (red) 532 nm (I532 = 2.4 mW μm-2), (green) 561nm (I561 = 2.2 mW μm-2) and, (blue) 633

nm (I633 = 9.3 mW μm-2). For comparison, Δλmax in-dark is included in violet. Solid lines correspond to the

7

exponential growing fit. The symbols and the error bars correspond to the mean and standard deviation of Δλ max

for more than 5 particles from at least 2 independent samples.

A localized surface plasmon can photocatalyze chemical reactions by three mechanisms, namely: intense

electromagnetic fields, local heating, or the generation of highly energetic electron–hole pairs (hot-carriers). 5, 7, 10,

24, 47 In the following, each mechanism will be addressed separately.

AuNPs illuminated at their plasmon resonance produce significant enhancement of the electromagnetic

field close to their surface.48 These intense near fields can drive chemical reactions by intramolecular excitation of

molecules surrounding the AuNPs.7 However, this cannot occur in the present case because aniline – which is the

precursor of the reaction - is transparent to the excitation wavelength. Figure 2a shows extinction spectra of

aniline on the working electrode at different potentials Ew. No absorption peak is present in the visible range

before the formation of PANI. Furthermore, if it were possible to polymerize aniline using visible light, PANI

formation should be observed for illuminated AuNPs at any potential. This is not the case, as shown in figure 1 d,

thus ruling out the enhanced near field as the driving mechanism.

Plasmonic nanoparticles are efficient converters of light into heat.49 Upon illumination, the temperature of

their surface can increase by tens or even hundreds of degrees, which can lead to thermal catalysis of chemical

reactions.23, 50-53 This happens simultaneously to the generation of hot carriers, rendering it difficult to

distinguishing between both catalytic phenomena.4, 54 In order to disentangle these effects, the reactivity of

illuminated particles was studied at different wavelengths and constant temperature. Figure 2b shows the

absorption spectrum of the 80 nm AuNPs, calculated using Lumerical (cf. SI-‘material and methods’). Vertical

lines indicate the excitation wavelengths used in these experiments (405 nm, 532 nm, 561 nm and 633 nm). All

CW lasers were focused to their diffraction-limit and their powers were carefully chosen to produce the same

surface temperature increase on the AuNPs. In order to properly estimate the temperature, a nanothermometry

technique based on Au photoluminescence was implemented.55-58 This technique allows measurement of the

surface temperature increase on individual AuNPs, as explained in SI. Figure 2c (scatter dots) shows measured

temperature increases vs irradiance for five individual AuNPs illuminated at 532 nm. Temperature increases were

found to be 32.7 ± 0.9 °C mW-1 μm2 at 532 nm and 36 ± 2 °C mW-1 μm2 at 561 nm (see Fig. S6 for further

details). Using input from these measurements, a temperature calculation was performed in COMSOL for every

wavelength (details provided in SI- Fig. S7). Figure 2c (solid lines) shows the estimated temperature increases

versus irradiance for the four wavelengths used in the experiments.

The role of the excitation wavelength in the aniline polymerization process was then studied at four

different wavelengths, keeping the temperature constant. Irradiance was chosen for each laser so that the

temperature increase of the AuNPs was always ΔT = 77 ± 2 °C, as indicated in the vertical dashed lines in figure

8

2c. Figure 2d shows the scattering maximum shift Δλmax as function of Ew when exciting the AuNP at 405 nm, 532

nm, 561 nm and 633 nm. Interestingly, the threshold potential Eonset is different for each wavelength, meaning that

the temperature increase – which is the same for all cases – is not the key phenomenon catalyzing the reaction.

Having ruled out near field and temperature increases as the dominant drivers of the observed photocatalytic

process, it can be concluded that aniline oxidation is assisted by photo-excited holes at the AuNP-aniline

interface. Moreover, at 633 nm (red squares in figure 2d), the threshold potential is very similar to the dark – or

pure-electrochemical – process (violet rhomboids in figure 2d), pointing out that: (1) local heating alone does not

catalyze the aniline polymerization, (2) changes in the redox potential ascribed to the local temperature increase 5,

42 are negligible in our system and (3) the generated hot-carriers are the main responsible for the observed

behavior.

The number of generated hot carriers is proportional to the light intensity.10 On the other hand, the energy

distribution of the generated carriers depends only on the excitation wavelengths and not on its intensity. For this

reason, the threshold potentials Eonset are independent of light intensity (Figure S5). The intensity can only affect

the polymerization rate once the reaction has started (i.e. above the threshold E onset). The different slopes above

the threshold potential in Figure 2d are related to the different rates of the polymerization reaction (i.e. faster

growth of the PANI shell). In the following, energetic aspects of the hot carriers (not yields) will be discussed

using the information of the threshold potentials.

Figure 3 Schemes for the electro-oxidation reaction of aniline under dark or under laser illumination conditions.

(a) The oxidation of aniline – which triggers the polymerization reaction and the formation of PANI – does not

take place at open circuit potential (OCP) in darkness on the surface of the 80 nm AuNP. When the applied

potential reaches Ew = 0.68 V (violet dashed line), the reaction takes place due to the alignment of the Fermi level

and the HOMO level of aniline. b) Under 561 nm laser illumination – efficiently exciting the plasmon resonance

of the 80 nm AuNP – the oxidation reaction is triggered at Ew = 0.44 V (orange dashed line). By changing the

illumination to 633 nm – off-plasmon resonance – and increasing the laser power in order to reach the same

9

surface temperature on the AuNP, the reaction takes place at Ew = 0.65 V (red dashed line). Note that this last

value is very similar to the dark scenario, highlighting that temperature increase in not the main contribution for

the observed photocatalytic effect at 561 nm.

The photocatalytic process taking place in this study can be understood as sketched in Figure 3. In dark and

open circuit potential (OCP) conditions, aniline oxidation does not take place because the Fermi level is higher

than the aniline Highest Occupied Molecular Orbital (HOMO), as shown Figure 3a. When the electrode potential

is increased, the energy of electrons is reduced. At the threshold potential to start the reaction E onset = 0.68 V

(violet dashed-line, Figure 3a), the Fermi Level is low enough to allow the flow of electrons from the aniline

molecules into the metal. Upon illumination, hot holes are generated, as shown in Figure 3b, and electrons

coming from aniline molecules can be transferred more easily, reducing the Eonset (red and orange dashed-lines).

Accordingly to the above mentioned, the variation in the threshold potential can give energetic information

of the hot holes generated during AuNP illumination- i.e. by deducting the E onset under illumination at each

wavelength to the one corresponding to the pure electrochemical process, where hot carriers do not catalyze the

reaction. The Eonset under each illumination wavelength were determined to be ca. 0.58 V for 405 nm, ca. 0.53 V

for 532nm, ca. 0.44 V for 561nm and ca. 0.65 V for 633nm (see Figure S2, for further details). The determined

values indicate that hot holes provide an extra energy that ranges from 4% to 35% with the excitation wavelength.

Ascribing the Eonset found at 633 nm – which is the smallest one compared to the electrochemical process – to a

pure thermal contribution, would lead to a lower-bound threshold of ca. 31% (35%-4% = 31%), occurring at 561

nm. It must be recalled that these wavelength-dependent measurements where performed at the same absorbed

power, leading in all cases to the same lattice temperature increase on the AuNP.

10

Figure 4. Hot holes energy contribution as a function of the incident wavelength (red squares, left axis),

estimated from the reduction of Eonset as schematized in Figure 3 and expressed in the absolute electrochemical

potential scale (i. e. in eV, see Fig S2). Energy of the incident photons as a function of the wavelength (blue solid

curve, left axis). Note that the data has been divided by a factor ten for proper comparison. Normalized integral of

the square of the electric field |E|2 inside the NP (green curve, right axis). Vertical dashed line indicates the

threshold wavelength for interband excitations.

Figure 4 shows the hot holes energy contribution to the reaction (square dots) and the energy of the

incident photons (blue curve). It should be noted that the maximum effective energy of the holes (i.e., the energy

that can be used to trigger a chemical transformation at the nanoparticle surface) corresponds to excitation at 561

nm and is at least a factor of 10 smaller than the energy of the incident photons (blue curve). This indicates that

for a 80 nm AuNP transport and extraction of carriers play an important damping role that lowers the final

effective energy transferred to the molecule. In order to further rationalize the results from Figure 4, it is

important to discuss the wavelength dependence of the different absorption pathways that could lead to hot

carriers with different energies.

Four different mechanisms of carrier generation through plasmon decay have been identified – interband

absorption, phonon (or defect) assisted absorption, electron-electron scattering assisted absorption, and Landau

damping (or surface collision assisted absorption). The excitation wavelength determines the relative contribution

of each mechanism and therefore the energy distribution of the generated carriers. At 405 nm, where interband

absorption dominates, the effective energy of the generated d-holes is less than half than its maximum at 561 nm.

This fact is also in line with recent experimental work.59 Theoretically, Sundararaman et. al. predicted the

generation of holes with initial energies close to the photon excitation from plasmon decay in thin Au films

excited above the interband energy threshold.31 However, it must be noted that d-holes have a small mobility,

rendering them difficult to access for the analyte.30 Figure 4 (green curve) shows the integral of the electric field

inside the particle, normalized to its maximum value. This magnitude is a good indicator of plasmon-induced

electric field confinement, which is the physical phenomenon allowing the generation of hot-carriers via Landau

damping. This damping mechanism has been identified as the responsible for the generation of highly energetic

carriers.60 Indeed, Figure 4 shows that the highest effective holes energy is found at 561 nm, corresponding to the

plasmon resonance of the NPs. This would indicate that Laundau damping is the mechanism that contributes the

most to the production of hot holes with high effective energy. These holes are generated close to the surface and

are therefore more reactive. At 633 nm, the effective energy of the generated holes is almost negligible. At that

wavelength, neither Landau damping nor interband excitations are significant and carrier generation is assisted by

either phonon absorption or electron-electron scattering. It has been predicted that these two mechanisms not

11

effective for photocatalysis applications.61 Furthermore, at 633 nm absorption might be dominated by resistive

losses without carrier generation.30, 59

This simple analysis could provide pathways to engineer highly reactive or even energy-selective

plasmonic photocatalysts, i.e. by favoring Landau-damping over the other absorption mechanisms and/or by

reducing the size of the photocatalyst so as to increase the carrier energy at the surface. However, the yield of

production and collection of hot-carriers should not be overlooked if both selectivity and efficiency are desired. 29

It has been recently shown that plasmon excitation does not influence the collection yield in photoexcited gold

(Au)–gallium nitride (GaN) Schottky diode.63 Instead, the metal band structure and carrier transport processes

dictate the number of collected carriers. Putting together the energy and yield results could assist in the future

design of plasmonic photocatalysts.

In summary, opto-electrochemical experiments at the single AuNP level revealed the effect of light

illumination in the electro-polymerization of aniline. A reduction of 35% in the applied potential can be achieved

when exciting the system at it plasmon resonance frequency. A single particle nanothermometry technique based

on anti-Stokes photoluminescence emission was implemented in order to determine the role of local lattice

heating. By rigorous experimental work, it was found that besides all the possible mechanisms for plasmon decay,

the generated hot holes are the main contribution of such a photocatalytic effect. The energy of hot carriers that

effectively triggers the electro-polymerization reaction was determined along the plasmon excitation profile,

accounting for energies ranging from 0.03 eV to 0.24 eV. Plasmon absorption through Landau damping was

found to be a key step to produce highly energetic carriers that cannot be generated by direct interband excitation,

even when the incident photon’s energy is higher. This highlights the role of Landau damping in photocatalysis

besides other absorption mechanisms that lead to less energetic carriers.

ASSOCIATED CONTENT

Supporting Information. Methods and additional relevant experimental data. Simulation details. Experimental

conditions for AuNP photoluminescence experiments. “This material is available free of charge via the Internet at

http://pubs.acs.org.”

AUTHOR INFORMATION

Corresponding Authors

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

12

ACKNOWLEDGMENT

This work has been supported by the EPSRC through the Reactive Plasmonics Programme (EP/M013812/1), the

Royal Society, AFOSR/EOARD and the Lee-Lucas Chair in Physics. J.G. and E.C. acknowledge financial

support from the European Commission through a Marie Curie fellowship (J.G.) and ERC starting grant 802989

CATALIGHT (E.C.). S.S. acknowledges financial support from the Deutsche Forschungsgemeinschaft (DFG,

German Research Foundation, Projektnummer 278162697 - SFB 1242 Non-equilibrium dynamics of condensed

matter in the time domain). S.A.M. acknowledges financial support from the Deutsche Forschungsgemeinschaft

(DFG, German Research Foundation) under Germany's Excellence Strategy - EXC 2089/1 - 390776260".

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