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Plasmoelectric potential mapping of single nanoparticle Feng Zhao, Wei-Min Yang, Tien-Mo Shih, Shi-Liang Feng, Yuejiao Zhang, Jian-Feng Li, Jiawei Yan, and Zhi-Lin Yang ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b00763 • Publication Date (Web): 02 Aug 2018 Downloaded from http://pubs.acs.org on August 3, 2018

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Plasmoelectric potential mapping of single nanoparticle Feng Zhao1, Weimin Yang1, Tien-Mo Shih1, 3, Shiliang Feng1, Yuejiao Zhang2, Jianfeng Li2, Jiawei Yan2*, Zhilin Yang1* 1

Deparment of Physics, Collaborative Innovation Center for Optoelectronic Semiconductors and Efficient Devices, Xiamen University, Xiamen, Fujian, 361005, P. R. China 2 State Key Laboratory for Physical Chemistry of Solid Surfaces, and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, Fujian, 361005, P. R. China 3 Tianming Physics Research Institute, Changtai, Fujian, 363900, P. R. China

ABSTRACT The plasmoelectric potential effect, based on all-metal nanostructures, is a newly discovered plasmon-induced photoelectric converting mechanism. In this work, we have experimentally demonstrated that gold and silver nanoparticles exhibit opposite plasmoelectric potentials under the same excitation source, depending on their distinct surface plasmon resonance (SPR) frequencies. Three-dimensional mapping of plasmoelectric potentials on gold and silver nanoparticles depicts height-related surface potential information as well as the charge density evolution process down to the single nanoparticle level with Kelvin Probe Force Microscopy (KPFM). Upon employing a statistical method, the quantitative plasmoelectric potential is obtained. Under the power intensity of 5.4 mW/cm2, the maximum value of the plasmoelectric potential reaches -79 mV on the gold nanoparticle and 54 mV on silver nanoparticles. Finally, our work may provide guidance to the understanding of plasmoelectric potential on the single nanoparticle level.

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KEYWORDS: plasmoelectric potential, SPR, three-dimensional mapping, KPFM, charge transfer

INTRODUCTION Noble metal nanostructures have attracted researchers’ interests in a wide range of areas for their unique optical properties that originate from surface plasmon resonance (SPR).1-4 One of most important applications of such plasmonic materials was photovoltaic devices where the field confinement and enhancement of the nanoparticles were employed to improve the power conversion efficiency.5-10 Although there were plenty of work using metal nanoparticle for light trapping and manipulation in photovoltaic research, few had realized that these nanoparticles themselves also possess the photo-electric converting effect until H. Atwater’s pioneering work on plasmoelectric potential was published in Science in 2014.11 Plasmoelectric potential effect arises from photo-induced charge transfer processes based on all-metal nanostructures. Metal nanoparticles or metal nanohole arrays can gain a static electric potential when the frequency of the excitation slightly deviates from that of SPR of the nanostructure. The absolute value as well as the sign of this potential will be affected by the deviation between the excitation frequency and the SPR frequency. Promisingly, the discovery of the plasmoelectric potential defies the photon-energy limitation of the photoelectric conversion process, and has increasingly attracted researcher’s focal interest.12-16 Via the introduction of a well-defined thermodynamic model, A. Polman and co-workers have discovered that the charge transfer in plasmoelectric potential is dictated by the principle of minimal free energy under steady state excitations.17 M. Moskovits et al. have reported that, owing to this plasmoelectric effect, gold nanoparticles can change their charge states when the UV-Visible spectrum is measured.18 H. Misawa and co-workers have found that the existence of plasmoeletric potential adequately explains the enhancement of the external quantum efficiency at wavelengths shorter than the plasmon peak in

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inverted thin-film perovskite solar cells.19 Meanwhile, Jin and co-workers have revealed a long-range plasmon field and plasmoelectric effect on catalysis by using pinhole-free Au@SiO2 coreshell nanoparticles as photo catalysts.20 There are also some applications of plasmoelectric potential on plasmonically induced potential devices.21-24 Although there has been a growing number of work concerning the plasmoelectric potential, the real space mapping of the plasmoelectric potential on individual nanoparticle level, which can directly reflect the charge transfer property of noble metal nanoparticle with specific surface plasmon resonance frequency, has not been reported yet to the best of our knowledge. In this study, the plasmoelectric potential on gold and silver nanoparticles are investigated by KPFM with a deliberately selected excitation wavelength that is located exactly between SPR frequencies of gold and silver nanoparticles.25,26 Surface potentials under different power densities are recorded and a three-dimensional mapping method is developed to offer an intuitive insight into the height-related information of the plasmoelectric potential as well as the charge transfer process under different power densities. Experimental results are associated with the thermodynamic model by analyzing the surface potential shift during illumination.11,17 To gain a quantitative recognition of the plasmoelectric potential under different power densities, a statistical method is employed to analyze the data obtained from KPFM measurements.

RESULTS AND DISCUSSION The shape and the size of gold and silver nanoparticles dispersed on fluorine-doped tin oxide (FTO) /glass substrate were characterized by Scanning Electron Microscopy (SEM). The FTO/glass was treated as a transparent conducting substrate, and the diameter was 150 nm averagely for both gold and silver nanoparticles (Fig. 1 (a, b)) to ensure a similar interface between the nanoparticles and the conducting substrate. We can also infer from the SEM graph that nanoparticles were well distributed on the

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substrate. The SPR frequencies of gold and silver nanoparticles were determined by the transmission spectra shown in Fig. 1 (c). They differ appreciably for gold and silver nanoparticles with similar sizes due to unique optical properties for different materials. Peaks of SPR were centered at 560 nm and 460 nm for gold and silver nanoparticles respectively, with the central wavelength of the incident light source at 514 nm pointed out by the dashed green line in Fig. 1 (c). As shown, the excitation wavelength was exactly located between SPR frequencies of gold and silver nanoparticles. Surface potential variations of gold and silver nanoparticles under different light conditions were investigated by KPFM, which was regarded as a powerful method in the research concerning the electrical property of the sample surface. The topography and surface potential information could be recorded simultaneously through KPFM measurements, and the working principle of our system was shown in Fig. S1 in the supplement information. The topography was recorded under the tapping mode while the surface potential was recorded under the amplitude modulation kelvin force microscopy (AM-KFM). Topographic graphs and corresponding surface potential graphs of silver nanoparticles under power density of 0, 2.6, and 5.4 mW/cm2 are shown in Fig. 2. We could deduce from Fig. 2 (a) that, due to the roughness of the substrate, it is difficult for us to discern some nanoparticles from the substrate itself. For the surface potential graph (Fig. 2 (d)), however, the separation between the surrounding substrate and nanoparticles becomes conspicuous in comparison with the topography graph. Shapes of silver nanoparticles remain the same under different power densities during the entire measurement (Fig. 2 (a-c)). The surface potential of silver nanoparticles, however, experienced an evident increase with the increasing power intensity in Fig. 2 (d-f), indicating that the surface potential change did not originate from the deformation of nanoparticles but from the photo-induced charge transfer of nanoparticles. Structural and electronic properties of silver nanoparticles could be separately reflected by these two-dimensional topography and surface potential graphs respectively. To comprehensively understand both properties within a single graph, we have established a three-dimensional mapping method by rebuilding the ACS Paragon Plus Environment

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topographic graph along with the surface potential graph. The three-dimensional mapping of silver nanoparticles is demonstrated in Fig. 2 (g-i) in which the structural information can be directly acquired from the three-dimension topography, while the electrical information was depicted by the color bar. The surface potential information at different positions of silver nanoparticles and the evolution process of the surface potential under different illumination conditions are clearly demonstrated by using this three-dimensional mapping method. For gold nanoparticle dispersed on FTO/glass substrate, the topography and surface potential measurements have been carried out on an area of the same size as that of silver nanoparticles. The size of the gold nanoparticle was similar to that of silver nanoparticles in the topography graph in Fig. 3 (a), which was consistent with the SEM measurement on gold and silver nanoparticles. The topography of the gold nanoparticle remains unchanged except an obvious displacement during the measurement as shown in Fig. 3 (a-c). This displacement originates from the drift of the sample and does not influence the surface potential of the gold nanoparticle. The surface potential of the gold nanoparticle under different power densities exhibits a quite different evolution trend in comparison with that of the silver nanoparticle. In the dark state (0 mW/cm2), the surface potential of the gold nanoparticle differs insignificantly with the surrounding substrate (Fig. 3 (d)), indicating that the gold nanoparticle and the substrate bear similar values of the work function. Under the excitation, the surface potential of gold nanoparticle gradually decreases while that of the surrounding substrate gradually increases. Therefore, a clear boundary between the nanoparticle and the substrate emerges in the surface-potential graph (Fig. 3 (e, f)). As the power density increases, the surface potential decreases for the gold nanoparticle. This downshift appears in contrast with the upshift for silver nanoparticles mentioned above. Such contrast indicates that a completely opposite electron-transfer behavior for these two types of nanoparticles relies on the deviation of the surface plasmon resonance frequency from the excitation frequency. The surface-potential shift for the gold nanoparticle can be easily recognized despite the displacement of the nanoparticle on the substrate (Fig. 3 (g-i)). ACS Paragon Plus Environment

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Plasmoelectric potential was defined as the difference of the surface potential between dark and illuminated state, that is Vpp = SPilluminated - SPdark, where Vpp is the value of the plasmoelectric potential while SPilluminated and SPdark correspond to the surface potential in illuminated state and dark state respectively. Through regarding the band structure in dark state as a reference and considering the amount of surface potential shift under different illumination conditions, the influence of the difference in band structures of gold and silver nanoparticles relative to that of FTO/glass can be eliminated. For silver nanoparticles, the upshift of the surface potential indicates that electrons exit from the nanoparticles. By contrast, for the gold nanoparticle, the downshift of the surface potential indicates that the electrons enter the nanoparticle.27 This directional charge transfer behavior originates from the plasmoelectric potential effect. According to the Drude model of dielectric function, the SPR frequency of the noble nanoparticle could be modulated reversibly by changing the charge density.28 Atwater group has experimentally and theoretically demonstrated that the charge density could also be modulated by the impingement of off-resonance excitation as an inverse process, which is called the plasmoelectric potential effect.11 The charge density can affect the free energy of the nanoparticle and will remain stable at a value which satisfies the thermodynamic law of Minimum Free Energy ∂‫ܰ(ܨ‬, ܶ)/ ∂ܰ = 0 under the steady state illumination, where F and N denote the free energy and the number of electrons of the nanoparticle respectively. As a result, excitation at the red side of the SPR frequency results in a lower charge density, while that at the blue side of the SPR frequency results in a higher charge density. In our work, the excitation frequency is located at the red side of SPR frequency of silver nanoparticle and at the blue side of SPR frequency of gold nanoparticle. The charge-transfer direction for nanoparticles with specific SPR frequency in our experimental results is consistent with that predicted by the thermodynamic model, which reveals that the plasmoelectric potential effect plays a key role in this process.11,17 At the same time, we also notice that the surface potential increased for both the substrate surrounding the gold and silver nanoparticle. For the purpose of understanding the impact of the illumination on the substrate, the control experiment is carried out on bare FTO/glass ACS Paragon Plus Environment

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substrate and the surface potential of the substrate remains almost the same under different densities as shown in Fig. S2, indicating that the illumination exerts little influence on the surface potential of the substrate. Therefore, it is reasonable that the increase of the surface potential of the substrate originates from the complex electric field induced by charge transfer as well as the thermal effect of the nanoparticle when the sample is illuminated by light.29,30 Through the three-dimensional mapping method, the charge density evolution as well as the charge transfer process were demonstrated on individual nanoparticle level, which gives us a better understanding of the fundamental properties of the plasmoelectric potential effect. To analyze the value of plasmoelectric potential for gold and silver nanoparticles under different conditions, we employ a statistic method to show the surface-potential difference between illuminated and dark states. For comparison, an area with the size of 350 nm by 350 nm enclosing gold and silver nanoparticle is taken and marked by red boxes in Fig. S3. Histograms in Fig. 4 show the surface-potential distribution under three different power densities with a distribution-fitting curve. In Fig. 4 (a), for silver nanoparticles in dark state (0 mW/cm2), the short peak (low surface potential) and the tall peak (high surface potential) in the red curve denote counts contributed by the nanoparticle and the substrate. Upon the illumination, the two-hump solid curve in red gradually upshifts to the two-hump long-dashed curve in blue (2.6 mW/cm2) and the two-hump short-dashed curve in green (5.4 mW/cm2). The plasmoelectric potential value is estimated to be 54 mV under power density of 5.4mW/cm2. In Fig. 4 (b), for the gold nanoparticle in dark state (0 mW/cm2), since surface potentials of the nanoparticle and the substrate share similar values, only one peak appears in the red solid curve. Under the impingement of the light, the single-hump surface-potential distribution splits into a two-hump distribution, as well as the substrate-associated hump and the nanoparticle-associated hump shift upward and downward away from each other. At the power density of 5.4 mW/cm2, the plasmoelectric potential value up to -79 mV is obtained. Therefore, the quantitative recognition of the plasmoelectric potential effect on gold and silver nanoparticles under the same illumination condition can be achieved by introducing the statistical method. ACS Paragon Plus Environment

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CONCLUSION Upon selecting the incident frequency properly, we have obtained specific plasmoelectric potential on gold and silver nanoparticles with particular SPR frequencies through light-irradiated KPFM measurements. Under the impingement of light at 514 nm, the surface potential of silver and gold nanoparticles exhibit different evolution trends, indicating a completely opposite charge transfer direction between the nanoparticles and the substrate. This shift of the surface potential is associated with the charge transfer induced by the plasmoelectric potential effect. Through a three-dimensional mapping method, we demonstrate the real space mapping of the plasmoelectric potential on gold and silver nanoparticles. This demonstration reveals the height-related surface potential information and the charge density evolution process. Quantitative analyses of the plasmoelectric potential have also been conducted by introducing a statistic method. METHOD Gold and silver nanoparticles were synthesized through wet chemical route.31,32 The surfactant employed in the synthesis process was both sodium citrate for gold and silver nanoparticles. Nanoparticles were washed in ultrapure water twice to remove most of the surface ligands and then re-dispersed in enthonal. The FTO/glass substrates were purchased from NSG and were cleaned by ultrasonic in the mixture of acetone, methanol and isopropyl alcohol for 3h, then the FTO/glass was dried with an N2 gun and baked for 2h at 125oC in a drying oven. The nanoparticles were directly dropped on FTO/glass with a pipette gun and the sample was placed in the vacuum desiccator to remove the solvent. SEM graph was taken by s-4800, Hitachi and the transmission spectra of the sample were obtained using an Avantes fiber spectrometer system. The topography graph and the surface potential graph were recorded with Atomic Force Microscopy (Agilent 5500, Keysight Inc) equipped with a Mode III lock-in amplifier (LIA) and the KPFM module. The AFM tip was a commercial tip ACS Paragon Plus Environment

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(PPP-EFM, Nanosensor) with a Pt/Ir coating layer of 30nm and the resonant frequency of the tip was 75 kHz. Some modifications were made on the sample stage of the AFM instrument to allow the impingement from the downside of the sample. The excitation light source was a light emitting diode (LED) with a central wavelength of 514 nm and a full width at half maximum of 30nm. ASSOCIATED CONTENT Supporting information Further details on the measurement setup, control experiment and the area employed in quantitative analysis. NOTES The authors declare no competing financial interest. ACKNOWLEDGMENTS The authors acknowledge support from National Science Foundation of China (No. 11474239, No. 21673192), MOST (2016YFA0200601, 2017YFA0204902). The authors acknowledge Prof. Martin Moskovits for the helpful discussion in the revision process. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. *E-mail: [email protected].

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Fig. 1 SEM images of (a) gold nanoparticles and (b) silver nanoparticles on FTO substrate and (c) the corresponding transmission spectra. The dashed green line in fig. 1 (c) indicated the excitation frequency while the inset arrows in fig. 1 (c) indicated the SPR frequency of gold and silver nanoparticle respectively.

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Fig. 2 AFM topography (a-c), surface potential (d-f) and the corresponding three-dimensional surface potential mapping (g-i) of the silver nanoparticles under power 2

densities of 0 mW/cm2 (a, d, g), 2.6

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mW/cm (b, e, h) and 5.4 mW/cm (c, f, i).

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Fig. 3 AFM topography (a-c), surface potential (d-f) and the corresponding three-dimensional surface potential mapping (g-i) of the gold nanoparticle under power densities of 0 mW/cm2 (a, d, g), 2.6 mW/cm2 (b, e, h) and 5.4 mW/cm2 (c, f, i).

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Fig. 4 The surface potential distribution and the fitting curve of the selected silver (a) and gold nanoparticles (b).

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For Table of Contents Use Only Plasmoelectric potential mapping of single nanoparticle Feng Zhao1, Weimin Yang1, Tien-Mo Shih1, 3, Shiliang Feng1, Yuejiao Zhang2, Jianfeng Li2, Jiawei Yan2#, Zhilin Yang1* We have experimentally demonstrated that gold and silver nanoparticle exhibit opposite surface potentials with a carefully designed excitation, depending on their distinct SPR frequencies. The dramatically surface potential shift originates from the directional charge transfer, as shown in this graphic.

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