Chemical Interface Damping Depends on Electrons Reaching the

Chemical Interface Damping Depends on Electrons Reaching the Surface Benjamin Foerster,†,‡ Anneli Joplin,§,∥ Katharina Kaefer,‡,# Sirin Celiksoy,‡ Stephan Link,*,§,⊥,∥ and Carsten Sönnichsen*,‡ †

Graduate School for Excellence Materials Science in Mainz, Johannes Gutenberg University Mainz, Staudinger Weg 9, D-55128 Mainz, Germany ‡ Institute of Physical Chemistry, Johannes Gutenberg University Mainz, Duesbergweg 10-14, D-55128 Mainz, Germany § Department of Chemistry, ⊥Department of Electrical and Computer Engineering, ∥Laboratory for Nanophotonics, Rice University, Houston, Texas 77005, United States # Max Planck Graduate Center, Johannes Gutenberg University Mainz, Staudinger Weg 9, D-55128 Mainz, Germany S Supporting Information *

ABSTRACT: Metallic nanoparticles show extraordinary strong light absorption near their plasmon resonance, orders of magnitude larger compared to nonmetallic nanoparticles. This “antenna” effect has recently been exploited to transfer electrons into empty states of an attached material, for example to create electric currents in photovoltaic devices or to induce chemical reactions. It is generally assumed that plasmons decay into hot electrons, which then transfer to the attached material. Ultrafast electron−electron scattering reduces the lifetime of hot electrons drastically in metals and therefore strongly limits the efficiency of plasmon induced hot electron transfer. However, recent work has revived the concept of plasmons decaying directly into an interfacial charge transfer state, thus avoiding the intermediate creation of hot electrons. This direct decay mechanism has mostly been neglected, and has been termed chemical interface damping (CID). CID manifests itself as an additional damping contribution to the homogeneous plasmon line width. In this study, we investigate the size dependence of CID by following the plasmon line width of gold nanorods during the adsorption process of thiols on the gold surface with single particle spectroscopy. We show that CID scales inversely with the effective path length of electrons, i.e., the average distance of electrons to the surface. Moreover, we compare the contribution of CID to other competing plasmon decay channels and predict that CID becomes the dominating plasmon energy decay mechanism for very small gold nanorods. KEYWORDS: surface plasmons, energy transfer, single particle spectroscopy, gold nanorods, plasmon damping, thiols

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Unfortunately, this process is very inefficient, because hot electrons relax quickly via electron−electron scattering back to the Fermi level before they can transfer to molecules at the nanoparticle surface. However, recent work by Wu et al. shows that plasmon excitation can also lead to direct transfer of an electron into empty orbitals of an attached material, thereby creating interfacial electron−hole pairs.23 Such a process should make plasmon induced interfacial electron transfer much more efficient. However, this direct electron transfer pathway is still in competition with the decay of plasmons into heat and radiation. It is therefore of central importance to determine

old nanoparticles absorb light very efficiently near their plasmon resonance frequency with a remarkable, centuries long photostability.1−3 Like a nanoscopic “antenna”, gold nanoparticles absorb and concentrate light energy into a very small volume.4−6 After this primary light absorption process that excites a plasmon oscillation, the energy is mostly re-emitted or converted to heat by electron−electron and electron−lattice scattering. In some circumstances, the energy of the plasmon oscillation is converted into an electron transfer to an attached material, potentially useful for the creation of electric currents,4,7−12 dissociation of gases,13,14 water-splitting reactions,15,16 upconversion of light,17,18 or selective catalysis of organic reactions.19 This plasmon induced electron transfer is believed to occur when a plasmon decays into electron−hole pairs that consist of excess energy electrons, which transfer into empty states of the attached material.20−22 © 2017 American Chemical Society

Received: November 29, 2016 Accepted: March 16, 2017 Published: March 16, 2017 2886

DOI: 10.1021/acsnano.6b08010 ACS Nano 2017, 11, 2886−2893

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transfer from a gold sphere to an attached CdSe nanorod.23 While plasmon assisted charge transfer has been observed in many recent studies on plasmonic photodetectors and photocatalysis,4,7,9,10 any difference between CID and electron transfer following plasmon decay into a metal localized electron−hole pair remains unclear. Furthermore, it is unclear if CID and electron-surface scattering are two distinct phenomena or not as both involve the interface between the metal and its molecular environment. Ensemble studies by Schulze and co-workers provided primary evidence that CID is size dependent as well.35 Later, Persson described CID in a classical model as inelastic scattering at the nanoparticle surface.38 Persson’s theory predicts that CID depends on the effective path length of electrons to the surface in the same way as electron-surface scattering. However, none of these early studies measured the homogeneous plasmon line width. Although Zijlstra et al. recently investigated CID on the single particle level by adsorption of thiols on gold nanorods,39 they did not investigate the detailed dependence of CID on the dimensions of nanoparticles. To better understand CID, it is therefore necessary to examine the size dependent broadening of the plasmon resonance for single nanoparticles while changing their chemical environment. In this study we employ single particle spectroscopy to investigate the dependence of CID on the overall size of gold nanorods. Similar to Zijlstra et al., the adsorption of dodecanethiol (DDT) on gold nanorods was used as a model system to change the chemical interface. By using several batches of gold nanorods with different average dimensions but similar aspect ratios, hence similar resonance energies, and by carefully following the homogeneous plasmon line width and scattering intensity of the same single gold nanorods during the adsorption process, we are able to extract the contribution of CID to the overall line width. The intensity and line width change exactly as predicted by the damped harmonic oscillator model and confirm the predictions by Persson. Using the established size dependence of the competing plasmon energy dissipation channels allowed us to predict how much energy the plasmon oscillation is able to transfer to surface molecules, an important step in potential applications using plasmonic nanoparticles for light energy conversion.

quantitatively the relative contributions of those damping mechanisms in order to evaluate and optimize the plasmonic properties of gold nanoparticles for light harvesting and energy conversion applications. It is well-known that plasmons decay on the order of 5−20 fs. This fast relaxation makes it challenging to measure it directly in time-resolved experiments.1,24 An alternative to timeresolved measurements is to determine the homogeneous plasmon line width Γ of a single nanoparticle,25,26 as the line width is directly proportional to the plasmon decay rate. The current picture is that four main decay channels contribute to the homogeneous plasmon line width Γ(or plasmon decay rate):27−29 bulk damping γb, electron-surface scattering Γsurf, radiation damping Γrad, and chemical interface damping (CID) ΓCID. Γ = Γb + Γrad + Γsurf + ΓCID

(1)

Bulk damping describes the scattering of electrons with thermal phonons, impurities, electrons etc. and converts the absorbed light into heat.30 Usually it is assumed that these scattering efficiencies are the same as in the bulk material. Radiation damping describes plasmon damping by secondary light emission and electron surface scattering describes the scattering of electrons at the surface of the nanoparticle.31 Finally, CID describes additional damping by changing the nanoparticle’s chemical interface.3,27,32 The above-mentioned damping mechanisms make the plasmon line width a function of nanoparticle size and shape,26,28,34 material,3 and environment.35 Any nanoparticle fabrication method, especially wet chemical synthesis, leads to significant size and shape distributions,36 which broaden the (heterogeneous) line width obtained in ensemble studies. Only the homogeneous line width obtained in single particle spectroscopy quantitatively relates to plasmon damping processes.26,37 Two of the most extensive single particle spectroscopy studies were conducted by Novo et al. and Juve et al. and were nicely reviewed by Vallée and Del Fatti.28,49,50 These studies disentangled bulk damping and the size dependent contributions of electron-surface scattering and radiation damping,28,49,50 summarized in the following relationship for the plasmon line width Γ: A ·v Γ = γb + 2ℏπκ radV + surf F leff

RESULTS AND DISCUSSION In our experiments, gold nanorods were immobilized to the bottom side of a microfluidic flow cell that allowed us to change the liquid environment around the nanoparticles. The flow cell was imaged in a home-built dark field microscope under white light illumination (Figure 1a, see also Supporting Information S1). This setup allowed us to record a real color image of the nanoparticles in the flow cell (Figure 1b). A software automatically identified single nanorods as bright spots and directed a scanning stage so that the scattered light from each one could be spectrally resolved consecutively with a spectrometer. A representative scattering spectrum of a single nanorod is shown in Figure 1c and fits well to a Lorentzian function in order to extract the resonance position Eres = ℏωres, scattering intensity Ires, and line width Γ measured as the full width at half-maximum (fwhm). Adsorption of DDT on gold nanorods was used to change the chemical interface of the gold nanorods and to study the contribution of CID to the plasmon line width. DDT is known to efficiently adsorb on gold surfaces within minutes by formation of strong covalent gold−sulfur bonds.40−42 At longer

(2)

In their experiments, the radiation damping is proportional to the particle volume V with a proportionality constant Krad of 5.5 × 10−7 fs−1 nm−3. The electron-surface scattering Γsurf is replaced with a term introduced by Kreibig and depends on the average distance of electrons to the surface leff, the Fermi velocity vF, and a proportionality constant Asurf.3 They did not consider CID though as all nanoparticle studied had the same ligands. CID is probably the most poorly understood damping mechanism. Kreibig and co-workers proposed many years ago that CID is related to the ability of a plasmon to decay by coupling to interfacial electronic states based on the comparison of the plasmon line widths measured for an ensemble of silver nanoparticles in the gas phase and then embedded in a SiO2 matrix.3,32,33 Recently, Wu et al. suggested an interfacial charge transfer mechanism similar to CID in order to explain the high efficiency for photo-initiated electron 2887


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Figure 2. Functionalization with DDT weakens and broadens the plasmon resonance of gold nanorods due to CID. Experimental single particle spectrum of a gold nanorod before (gray dots, t0 = 0 min) and after (yellow dots, tend = 60 min) functionalization with DDT (a). An example from the 17 by 55 nm nanorod sample is shown here. Changes in the scattering intensity ΔI (b), the resonance energy ΔEres (c), and the line width ΔΓ (d) following functionalization with DDT can be quantified via Lorentzian fits and normalizing the intensity (c and d) and resonance energy (b and d) to the corresponding values before thiol addition.

Figure 1. Dark field contrast was used to obtain single particle scattering spectra (a). White light illumination (yellow) excites gold nanorods supported on a glass slide through a dark field condenser. Only light scattered by gold nanorods is collected by the objective (red arrows). Nanoparticles appear as bright spots in the real color image shown in (b). Scale bar is 50 μm. The light from a single nanoparticle (indicated by the small circle in (b)) is spectrally dispersed by a spectrometer (c). The resulting spectrum (dots) is proportional to the scattering efficiency Csca of the plasmon resonance. The line shape is well described both by a classical dipole antenna model (red line; Supporting Information S2) and a Lorenzian fit (black line). The latter is used to extract the resonance position ωres (light blue), the relative intensity, and the full width at half-maximum Γ (fwhm, dark blue). The inset shows a cartoon of the charges and field lines associated with a localized surface plasmon of a gold nanorod. An example from the 17 by 55 nm nanorod sample is shown here.

damping simultaneously changes the resonance frequency, the oscillation amplitude, and line width. Plasmon resonance spectra can be completely understood within this classical antenna theory, which allows us to establish a consistent picture of the amount of damping, the scattering efficiency (intensity), and the resonance position. Usually, plasmon spectra are modeled with a Lorentzian function. However, the line shape predicted by the classical model of a dipole antenna describes the measured spectra equally well (Figure 1c). Most importantly, the classical dipole antenna theory predicts a connection between additional damping and loss of scattering intensity ΔI as well as resonance energy shifts (Figure 3a). However, the resonance energy shift is relatively small, if damping is smaller than the resonance energy,56 as is the case here. The radiative emission of a dipole antenna at the resonance is inversely proportional to the amount of damping. Our experimental results, where the damping was increased due to the adsorption of a thiol layer on the nanorods, followed exactly this prediction. Figure 3b shows the loss in scattering intensity as damping increased over the time course of thiol adsorption. This dipole antenna theory is precisely confirmed by experimental results, giving us confidence to correctly interpret line width changes with the damped harmonic oscillator model. The adsorption of an alkanethiol layer on the nanoparticle surface has two additional effects which could in principle influence the nanorod spectra: (1) The layer increases the refractive index around the nanorod, and (2) each thiol bond (partially) removes an electron from the gold conduction band. Electromagnetic calculations show that both effects only have a negligible effect on the resonance position (Supporting Information S5) and are not able to explain the observed large drop in scattering intensity and the simultaneous line width broadening (Supporting Information S7 and S8). In order to investigate the influence of particle size on the amount of CID, four different sizes of gold nanorods with similar plasmon resonance frequencies were synthesized.

time scales, the adsorbed DDT molecules reorganize into denser monolayers aided by the attractive van der Waals interactions between the carbon chains of the DDT molecules.51 The adsorption of thiols on the gold surface significantly changed the plasmon resonance (Figure 2a):39 The plasmon resonance energy shifted toward lower energies (Figure 2c), the scattering intensity decreased (Figure 2b), and the line width broadened (Figure 2d). There are three potential reasons for the plasmon resonance energy shift: (a) increase of the local refractive index by the backbone of the thiols; (b) static decrease of electron density in the metal induced by the electron withdrawing effect of the sulfur atom; and (c) increase in damping of the plasmon due to CID.39 All three effects potentially contribute and it is not possible to clearly assign which one is the dominant factor in our experiments (please see Supporting Information S5 for more details). However, the loss of scattering intensity and the resonance broadening can be attributed to CID as will be shown in more detail below. First, we introduce an analysis to establish a quantitative link between plasmon broadening and intensity loss. For an externally driven, damped harmonic oscillator, increasing 2888


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Figure 3. Damping reduces the scattering intensity of gold nanorods, because a fraction of the plasmon energy is lost nonradiatively. The classical model of a dipole antenna predicts an exact relationship between damping and relative intensity. (a) Left: Radiative emission of a classical dipole antenna for increasing damping. Right: Relative intensity at the resonance maximum as a function of increased damping expressed as the change in line width ΔΓ. (b) Relative scattering intensity of single gold nanorods (circles) plotted as a function of increased line width ΔΓ during the functionalization with DDT that caused increased plasmon damping as more thiol molecules attached to the surfaces of the nanorods. The result of a 17 by 55 nm gold nanorod sample is shown. Dotted line shows the intensity loss predicted by the model of a classical dipole antenna. Both intensity and damping represent average values obtained from single nanorod spectra recorded at each time step during thiol adsorption (Supporting Information S1).

Figure 4. Four gold nanorod samples having different sizes but similar aspect ratios were synthesized to investigate the size dependence of CID. (a) Size distributions for the short and long axes of the 4 gold nanorod samples as obtained by TEM. Error bars indicate the standard deviation of the length and width for 419, 1751, 484, and 378 analyzed particles corresponding to the 14 × 41 nm, 18 × 55 nm, 22 × 66 nm, and 27 × 78 nm nanorod samples, respectively. Further details are given in the Supporting Information S3. (b) Representative single particle scattering spectra from each of the synthesized gold nanorod samples illustrating that the plasmon resonance is almost the same because of the similar aspect ratios.

Keeping the plasmon resonance frequency constant avoids complications associated with the frequency dependent bulk damping γb(ω) (Supporting Information S4). For rod shaped nanoparticles, the plasmon resonance depends mainly on the aspect ratio (AR = length/diameter) of the gold nanorod.43 Figure 4a shows that the four synthesized gold nanorod batches contained nanorods clearly distinguishable by size (diameter and length) but with comparable aspect ratios. Consequently, these four nanorod samples had similar plasmon resonance energies (Figure 4b and Supporting Information S3). To quantitatively compare the amount of CID as a function of nanorod size it was necessary to establish that a comparable, if not complete, coverage of adsorbed alkanethiols was achieved. The gold nanorods were first extensively washed with ethanol to create a surface as pristine as possible. After that procedure, an ethanoic solution of DDT was flushed through the flow cell and the thiols started to adsorb on the gold nanorod surface. During this process, we recorded the scattering spectra of about 40 single gold nanorods repeatedly for a period of around 60 min and extracted the additional line width ΔΓ with respect to the starting value (Figure 5a). The plasmon line width broadening was determined for each single gold nanorod and then averaged to obtain the data points in Figure 5a (bars indicate the error of the mean). We found a time evolution typical for a Langmuir adsorption isotherm, i.e., θ = 1−exp(−kL·c·t) where the coverage θ depends on the time t, the thiol concentration c, and the Langmuir adsorption constant kL. As the thiol concentration was constant, we simplified the adsorption constant to k = kL·c and fitted the line width broadening due to thiol adsorption with ΔΓ(t) = ΔΓCID[1−exp(−kt)]. This Langmuir adsorption model yielded the equilibrium value of CID, ΔΓCID, for the four gold nanorod sizes at complete thiol adsorption. These ΔΓCID values compare well with the line width broadening measured for individual nanorods after 60 min (Figure 5b, compare black dots to

circles). These results clearly demonstrate that the plasmon line width broadening by the layer of DDT increased for smaller gold nanorod sizes. We investigated more carefully how CID scales with the size of the nanoparticles to test the predictions by Persson’s theory. Within his model, CID should depend on the Fermi velocity vF, a constant A that describes the probability that an electron scattered at the particle surface transfers its energy to attached molecules, and the effective path length of electrons leff, which gives the average distance an electron needs to travel before reaching a surface: ΔΓCID = ACID · vF/leff. The particle size dependency enters through the effective path length leff, which can be calculated from the particle volume V and surface area S according to leff = 4V/S.44 We calculated the effective electron path length by approximating the gold nanorods as cylinders and using the average dimensions determined by TEM (Supporting Information S3 and S6). If we plot ΔΓCID as a function of the inverse effective electron path length, we indeed find the predicted linear scaling (Figure 6). A linear regression to the data points in Figure 6a allowed us to determine the probability of an electron to scatter at the nanorod-thiol interface as ACID = 0.34 ± 0.02 (using the value of vF = 1.4 nm/fs from Hartland).1 This value implies that each electron, which reaches the surface, transferred its energy to the attached DDT with an efficiency of 34%. This probability is the same for all nanorod sizes. However, CID becomes stronger in smaller nanorods because more electrons are able to reach the interface. However, it is important to keep in mind that ACID as 2889


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Figure 5. Time dependence of DDT adsorption observed via plasmon line width broadening (a). Each data point indicates the mean plasmon broadening for >26 single gold nanorods for each of the 4 samples. Insets capture schematically the functionalization process: naked gold nanorod (t0 = 0), partially functionalized gold nanorod (0 < t < ∞), and dense thiol monolayer coverage on a gold nanorod (t = ∞). The adsorption process can be described by a Langmuir adsorption process (see equation and fits shown as black lines) to obtain the CID caused plasmon broadening ΔΓCID due to a dense monolayer of DDT. Plasmon broadening due to CID increases with smaller gold nanorod sizes (b). Circles indicate the ΔΓCID values for single gold nanorods at the end of the experiment (tend = 60 min) with different colors distinguishing the 4 nanorod samples. The ΔΓCID value of each gold nanorod is sorted by the four nanorod sizes and arbitrarily distributed along the x-axis for better visualization. The equilibrium ΔΓCID obtained by fitting to a Langmuir adsorption process is represented by a black cross for each sample with error bars calculated from the confidence bounds of the fit.

Figure 6. CID scales inversely with the effective path length of electrons, and hence CID depends on the average distance of electrons to the surface. Extrapolation of our data to a 5 × 15 nm gold nanorod predicts that CID becomes the most important damping channel in small gold nanorods and nearly half of the plasmon energy is transferred to the adsorbate. (a) Average ΔΓCID of single gold nanorods covered by a dense layer of DDT (crosses, from Figure 5a) as a function of the effective electron path length leff. Inset: Equation predicted by Persson’s model and used for linear regression (black line). (b) Size dependent contributions to the experimental plasmon line width Γ of gold nanorods in ethanol (black crosses and black line) and functionalized with DDT (red crosses and red line) and extrapolation toward a 5 × 15 nm gold nanorod. Shaded areas visualize the role of the four competing damping channels: radiation damping, bulk damping, electronsurface scattering, and CID. Details regarding this analysis can be found in the Supporting Information S9.

determined here only considered the contribution to CID by the adsorbed monolayer of DDT on the gold nanorods. Gold nanorods were already in contact with molecules before thiol adsorption (ethanol). Thus, it is very probable that the plasmon line width was already influenced by CID for the gold/ethanol interface and that the value of ACID = 0.34 ± 0.02 is only a lower bound to the total CID. The combined surface scattering and CID parameter ADDT = ASurf + ACID = 0.46 for the DDT coated gold nanorods in this study (see Supporting Information S9 for further details) is larger than the value obtained for cetyltrimethylammonium bromide (CTAB) coated gold nanorods by Novo et al. and smaller than the value measured for SiO2 encapsulated gold nanorods reported by Juve et al.28,49 The A parameter for CTAB coated gold nanorods was obtained by single particle scattering spectroscopy similar to this work. ACTAB = 0.30 ± 0.02 contains both surface scattering and CID contributions. A larger A parameter for thiols can be rationalized considering that thiol molecules covalently bind to gold, causing significant orbital mixing between the sulfur and gold atoms, while the CTAB molecules are predominantly physiosorbed.52 The study by Juve et al. employed single particle extinction spectroscopy and determined a value of ASiO2 = 1.4 ± 0.2 for SiO2 encapsulated gold nanorods.49 In order to quantitatively compare this result to our study, it is important to adjust the definition of the effective mean free path. They used Leff,1 = D/ √AR, while we calculated it according to Leff,2 = D · AR/(AR +

0.5), consistent with other previous work (Supporting Information S6).3,32,28 As we kept the aspect ratio AR ≈ 3 the same for the different sizes of gold nanorods (Figure 3a), we can convert our ADDT parameter using the ratio between the two effective mean free path definitions (Leff,2/Leff,1 = 1.48), yielding ADDT = 0.46 · 1.48 = 0.68. The SiO2 interface therefore causes damping that is about twice as large as observed here for a thiol coated gold nanoparticle surface. Calculations considering the molecular orbitals of the environment and the metal are needed to further understand the mechanism underlying plasmon damping in the presence of different chemical environments. It is certainly clear from previous work and our study here that the plasmon “feels” more than just a change in refractive index when the local environment is changed. For applications of plasmonic nanoparticles in light harvesting and energy conversion, it is important to understand how to maximize the fraction of light energy that is transferred to attached molecules. Using the parameters determined for CID in this work together with the established relationships of radiation, surface, and bulk damping, we are able to compare the various damping channels as a function of nanorod size. 2890


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Millipore was used in all experiments. All chemicals were used as received without further purification. Chemical Synthesis of Gold Nanorods. 18 × 55 nm and 22 × 66 nm gold nanorods were synthesized using the method by Ye et al.45 The batch of 18 × 55 nm gold nanorods was produced by adjusting the amounts of the following ingredients: 1.1 g of 5-bromosalicylic acid, 12.0 mL of 4 mM silver nitrate solution, and 800 μL seed solution. The batch of 22 × 66 nm gold nanorods was produced using the procedure given in the Supporting Information by Ye et al.45 14 × 41 nm gold nanorods were synthesized using the method by Nikoobakht.46 The amount of silver nitrate was adjusted to 375 μL of a 4 mM silver nitrate solution. 27 × 78 nm gold nanorods were synthesized using the method by Ye et al.47 for gold nanorods with larger dimensions with the following adjustments: 360 μL of a 4 mM silver nitrate solution and 25 μL of seed solution. Determination of Gold Nanorod Dimensions. Dimensions of synthesized gold nanorods were analyzed by transmission electron microscopy (TEM) using a FEI Tecnai G2 12 BioTwin and FEI Tecnai G2 Spirit Twin. The average dimensions determined from the TEM images are given in Table S1. Preparation of Flow Cells. Parafilm was used as a spacer between two 24 mm by 60 mm coverslips #1 (Menzel-Gläser). Three channels were cut into the parafilm and six holes were cut into one of the coverslips by using a commercially available CO2-lasercutter system (Speedy 100 from trotec). The parafilm and coverslips were overlaid so that the holes in one of the coverslips matched with the ends of the channels in the parafilm. The flow cell was placed on a heating plate at 120 °C for a few seconds so that the parafilm melted and the flow cell became airtight. The flow cell was then mounted into a home-built microscope sample holder. Preparation of Nanoparticles for Single Particle Spectroscopy. After incorporation of the flow cell in the home-built dark field microscopy setup, a diluted solution of gold nanorods was flowed through the sample cell. Some of the nanorods were immobilized on the glass substrate by adding a 0.1 M KNO3 solution. Subsequently, roughly 2 mL of 200 proof ethanol was flowed through the cell to remove remaining surfactants from the surface of the immobilized gold nanorods. Thiol Binding Experiments. Single particle spectra were obtained at the start of the experiment for reference. Then a 3 mM ethanolic dodecanethiol solution in 200 proof ethanol was flowed through the cell while continuously taking single particle spectra of the same gold nanorods. The same procedure was performed for each gold nanorod size. Home-Built Dark Field Microscopy Setup. An inverted microscope from Zeiss (Axio Observer Z1) was equipped with a PI542 XY-Piezo stage (Physik Instrumente, 200 μm × 200 μm travel range) for xy-positioning of the sample and a PI721 Z-Piezo (Physik Instrumente, 100 μm travel range) for focusing a Plan-Apochromat 40x/1.3 objective from Zeiss. An Imspector V10E transmissive imaging spectrograph with an Andor Luca R EM-CCD camera and a Canon EOS 5D Mark II camera were attached to the microscope for obtaining single particle spectra and real color images of the sample, respectively. A MatLab-based software was used for automated acquisition and analysis of many single particle spectra. A detailed description is given by Rosman et al.48

The average experimental plasmon line width decomposed into those four damping channels is shown in Figure 6b. As bulk damping is size independent and surface scattering and CID are negligible for large sizes, radiation damping is the dominating contribution. On the other hand, in smaller gold nanorods radiation damping becomes negligible and the fractions of surface scattering and CID compared to the total damping increase. Extrapolation to small nanorods for the parameters determined here (ACID = 0.34 and Asurf = 0.12, see Supporting Information S9 for details) predicts that CID becomes the dominating damping channel. For example, in a 5 × 15 nm gold nanrod nearly half of the plasmon energy is transferred to the adsorbate. Our analysis has assumed that surface scattering and CID are pure T1 relaxation processes that result in the loss of plasmon energy. However, based on the line width broadening alone it is also possible that electron scattering at the interface leads to pure dephasing without energy loss. While the phenomenological model applied here cannot distinguish between loss of energy and loss of phase coherence, recent studies by Wu et al. have linked plasmon damping to energy loss through interfacial charge transfer.23 CID has furthermore been observed for gold nanostructures covered with graphene,25 a system that has also been shown to produce a photocurrent via charge injection from the metal to the graphene.53−55 More work is needed to confirm that interface damping corresponds to the loss of plasmon energy and not just its phase.

CONCLUSION In this work we showed experimentally that the classical picture of a dipole antenna and a damped harmonic oscillator correctly describes the interplay between plasmon intensity and line width. Based on this analogue, we used the measured plasmon line width of single gold nanorods to quantify the amount of energy that is transferred from the plasmon to a model adsorbate, DDT. We established that this energy transfer to adsorbates, referred to as CID, is inversely proportional to the average distance of electrons to the surface and, as a result, increases rapidly with decreasing size of the gold nanorods. We determined the proportionality factor ACID = 0.34 ± 0.02, which allowed us to extrapolate our results to even smaller nanorods. We predict that in very small DDT covered gold nanorods CID overcomes the other three competing damping channels: radiation, bulk, and surface damping. These results have only been possible by measuring the homogeneous plasmon line width during the entire thiol adsorption process for the same single nanorods. Finally, because plasmon assisted light harvesting requires the transfer of the plasmon energy to attached material, our work encourages size reduction of plasmonic nanoparticles to an extent that yields the best tradeoff between plasmon energy transfer and absorption cross section.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b08010. Single particle spectroscopy of gold nanoparticles, radiative emission of a dipole antenna, characterization of gold nanorods, energy dependent bulk damping in gold nanorods, plasmon resonance shift due to thiol coating, electron effective path length in gold nanorods, effect of refractive index change and electron density reduction on the plasmon line width and scattering

METHODS Materials. The following chemicals were purchased from SigmaAldrich: Dodecanethiol, potassium nitrate (KNO3), sodium borohydride (NaBH4), hydrogen tetrachloroaurat(III) (HAuCl4), 5-bromosalicylic acid, L-ascorbic acid, and cetyltrimethylammonium bromide (CTAB). 200 proof ethanol was purchased from Fisher Scientific. Silver nitrate and hydrochloric acid (HCl, 37 wt% in water) was purchased from Carl Roth. Sodium oleat was purchased from TCI America. Ultrapure water produced by a Milli-Q Direct 8 system from 2891


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intensity, and extrapolation of experimentally obtained damping to smaller gold nanorods (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Benjamin Foerster: 0000-0003-2622-2405 Stephan Link: 0000-0003-2570-4285 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was funded by the European Research Council (259640). B.F. acknowledges support from the Excellence Initiative by the Graduate School Materials Science in Mainz (GSC 266) through a DFG-fellowship position. A.H. acknowledges support by the National Science Foundation Graduate Research Fellowship Program under Grant No. (1450681). K.K. acknowledges financial support from Max Planck Graduate Center at the Johannes Gutenberg University of Mainz. S.L. thanks the Robert A. Welch Foundation (C-1664) and AFOSR (MURI FA9550-15-1-0022) for financial support and the Graduate School of Excellence Materials Science for a MAINZ Visiting Professorship. We thank A. Henkel and A. Neiser for implementing and V. Wulf and K. Wandner for maintaining the MatLab-based software for our home-built dark field microscopes. We thank W.-S. Chang for his technical support and scientific feedback. REFERENCES (1) Hartland, G. V. Optical Studies of Dynamics in Noble Metal Nanostructures. Chem. Rev. 2011, 111, 3858−3887. (2) Willets, K. A.; Van Duyne, R. P. Localized Surface Plasmon Resonance Spectroscopy and Sensing. Annu. Rev. Phys. Chem. 2007, 58, 267−97. (3) Kreibig, U. Optical Properties of Metal Clusters; Springer: Berlin, 1995. (4) Atwater, H. A.; Polman, A. Plasmonics for Improved Photovoltaic Devices. Nat. Mater. 2010, 9, 205−213. (5) Kakavelakis, G.; Vangelidis, I.; Heuer-Jungemann, A.; Kanaras, A. G.; Lidorikis, E.; Stratakis, E.; Kymakis, E. Plasmonic Backscattering Effect in High-Efficient Organic Photovoltaic Devices. Adv. Energy Mater. 2016, 6, 1501640. (6) Pustovalov, V. K.; Astafyeva, L. G.; Fritzsche, W. Analysis of Optical Properties of Spherical Metallic Nanoparticles for Effective Absorption of Solar Radiation and their Heating. Sol. Energy 2015, 122, 1334−1341. (7) Linic, S.; Christopher, P.; Ingram, D. B. Plasmonic-Metal Nanostructures for Efficient Conversion of Solar to Chemical Energy. Nat. Mater. 2011, 10, 911−921. (8) Aydin, K.; Ferry, V. E.; Briggs, R. M.; Atwater, H. A. Broadband Polarization-Independent Resonant Light Absorption Using Ultrathin Plasmonic Super Absorbers. Nat. Commun. 2011, 2, 517. (9) Knight, M. W.; Wang, Y.; Urban, A. S.; Sobhani, A.; Zheng, B. Y.; Nordlander, P.; Halas, N. J. Embedding Plasmonic Nanostructure Diodes Enhances Hot Electron Emission. Nano Lett. 2013, 13, 1687− 1692. (10) Knight, M. W.; Sobhani, H.; Nordlander, P.; Halas, N. J. Photodetection with Active Optical Antennas. Science 2011, 332, 702− 705. (11) Wang, F.; Melosh, N. A. Plasmonic Energy Collection through Hot Carrier Extraction. Nano Lett. 2011, 11, 5426−5430. 2892


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Chemical Interface Damping Depends on Electrons Reaching the

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