Polarization Effect of Hot Electrons in Tandem-Structured Plasmonic

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Polarization Effect of Hot Electrons in Tandem-Structured Plasmonic Nanodiode Changhwan Lee, Young Keun Lee, Yujin Park, and Jeong Young Park ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b00717 • Publication Date (Web): 30 Aug 2018 Downloaded from http://pubs.acs.org on August 30, 2018

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Polarization Effect of Hot Electrons in TandemStructured Plasmonic Nanodiode Changhwan Lee†,‡, Young Keun Lee†,‡, Yujin Park†,‡ and Jeong Young Park*,†,‡

†

Graduate School of EEWS, Korea Advanced Institute of Science and Technology (KAIST) Daejeon 305-701, Republic of Korea. ‡ Center for Nanomaterials and Chemical Reactions, Institute for Basic Science, Daejeon 305701, Republic of Korea.

ABSTACT: Energy conversion from light to electricity mediated by hot electrons in a plasmonic metal nanostructure caused by the decay of surface plasmons has been proposed as a promising way to obtain novel photovoltaics and photocatalytic devices. In Schottky barriers composed of metal nanostructures supported on a semiconductor surface, hot electrons produced in the metal with sufficient photon energy can be extracted into the conduction band of the semiconductor by overcoming the Schottky barrier. An important parameter for the efficient extraction of hot electrons is the polarization of the incident light, which can be tuned by the angle between the electric field of the incident light and the plane of the Schottky barrier. Here, we investigate polarization-dependent hot electrons detected on planar (two-dimensional) and three-dimensional (3D) tandem plasmonic Au/TiO2 nanodiodes. We confirm that the maximum photocurrent was obtained with the planar structure in transverse mode and with the 3D tandem structure in longitudinal mode. These results indicate that hot electrons can be extracted most efficiently when the direction of the electric field of the incident light coincides with the plane of the Schottky interface. This study sheds light on the fundamental mechanism for the polarization effect on hot electrons, with applications in the advanced design of hot-electron-based photonic devices with high energy conversion efficiency. KEYWORDS: hot electrons, surface plasmon, photocurrent, 3D-Schottky interface, polarization 1

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Plasmonic nanomaterials provide a new scheme for a wide range of applications in photovoltaics and photocatalytic devices.1-3 In plasmonic metal nanostructures where light is incident, a unique optical phenomenon can occur, called localized surface plasmon resonance (LSPR), that is caused by the oscillation of free electrons in the metal at the same frequency as the incident photons. Following LSPR excitation, the surface plasmons decay radiatively by scattering4 or non-radiatively by exciting the hot electrons.5,6 When non-radiative decay takes place in a metal nanostructure, the accumulated energy is transferred to electrons in the conduction band of the metal via Landau damping, generating energetic electron–hole pairs.5, 7-9

These excited hot electrons have a very short lifetime that is primarily limited by

relaxation through electron–electron scattering.10-12 To utilize these hot electrons, devices with Schottky or tunneling junctions formed by contact between a plasmonic metal and a semiconductor have been suggested.13-17 Hot electrons produced in a metal by the absorption of photon energy move ballistically toward the Schottky interface within the length of the hot electron mean free path.18,19 Interfacial hot electrons with sufficiently high energy to overcome the barrier are transferred to the conduction band of the semiconductor, resulting in efficient hot electron/hole separation in the metal.20 Although the overall efficiency of photovoltaics based on the production of hot electrons from plasmon decay in Schottky structures is still quite low, much research has been dedicated to constantly improving the performance. Thus, charge transport of light-induced hot carriers on nanowires,21 graphene,22 and quantum dots23 has been investigated. Conversion efficiency based on hot electrons in a Schottky nanodiode was determined by the number of hot electrons excited from the metal and the probability of extracting the hot electrons into the semiconductor. The size, shape, and material of the plasmonic nanostructures are important factors for LSPR excitation and hot electron 2


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production.24 Furthermore, these parameters affect the wavelength at which the LSPR occurs as well as the efficiency of the hot carrier separation process. The hot electron extraction process is influenced by the height of the Schottky barrier and the direction of the movement of the hot electrons. The lower the height of the Schottky barrier, the more advantageous it is for detecting hot electrons. In general, hot electrons travel with momentum determined by LSPR excitation following the direction of the electric field of the incident light.25,26 For efficient detection of hot electrons from a metal to a semiconductor, the momentum of the hot electrons must be directed toward the Schottky interface.27 In planar Schottky structures illuminated at a normal incidence angle, however, the momentum of the hot electrons is parallel to the Schottky interface. This indicates that the hot electrons also primarily move parallel to the interface, resulting in a low probability for capturing hot electrons. Advanced approaches for the detection of hot electrons have been suggested by embedding plasmonic nanostructures for a three-dimensional (3D) Schottky interface to collect hot electrons from all directions and using a metallic tip to focus the surface plasmons with polarized light, inducing the excitation of hot electrons with momentum directed at the Schottky interface.2830

Based on these strategies, a 3D tandem-structured TiO2/plasmonic Au/TiO2 nanodiode was fabricated for this work by coating sol-gel TiO2 onto a planar plasmonic Au/TiO2 nanodiode. Enhancement of the photocurrent measured in the 3D tandem structure indicates that the performance was improved by enabling efficient extraction of hot electrons from all directions compared with the planar structure. To further understand the correlation between the detection probability and the arrangement direction between the Schottky interface and the momentum of the hot electrons, the photocurrent behavior was traced while changing the polarization and the tilting angle of the irradiated light. To clarify the observed 3


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polarization dependence, finite difference time domain (FDTD) simulations were performed; these simulations showed that hot electrons can be efficiently detected on the side of the Schottky interface where the electric field is the strongest.

EXPERIMENTAL SECTION Fabrication of the plasmonic Au/TiO2 nanodiode. To measure the hot electron flow enhanced by surface plasmons in a nanostructure, we fabricated a plasmonic Schottky diode that was composed of vertically oriented Au and TiO2 on an insulating silicon oxide wafer covered by a 500 nm layer of SiO2. More details on the fabrication of the nanodiode are described elsewhere.22 The first step is the deposition of a 150 nm thick film of Ti patterned by a metal mask using electron beam evaporation on the 500 nm SiO2 wafer. After that, the wafer is annealed at 470 °C for 2 hours in air to form TiO2 with a sheet resistance suitable for the device. The resulting TiO2 film is transparent (see Supporting Information, Figure S1). Next, a 50 nm thick film of Ti and a 150 nm thick film of Au patterned by a second metal mask were deposited sequentially using electron beam evaporation to form the two ohmic electrodes of the nanodiode. Finally, a thin gold film (20 ± 2 nm thick) was deposited by a third metal mask using electron beam evaporation. A Schottky interface is obtained between the Au and TiO2 layer that can collect the hot electrons produced in the Au. For a plasmonic Schottky diode, the thin film Au/TiO2 diode was annealed at 240 °C for 1 hour at ambient conditions. Owing to the difference in surface energy between the Au and TiO2 films, the Au agglomerated and was finally modified into a connected nanoisland structure that exhibits surface plasmons. Sol-gel synthesis of TiO2. A sol-gel method was used to synthesize the TiO2 to 4


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be deposited on the Au nanostructure. First, 0.5 ml of titanium isopropoxide (TTIP), the precursor, was added to 9.5 ml of ethanol, the solvent. This was then mixed with 5 µl of nitric acid, which helps the TiO2 sols to form a gel and prevents precipitation of the TiO2 particles. Measurement

of

electrical,

optical

properties

and

polarization

dependence. The current–voltage and short-circuit photocurrent measurements were carried out using a sourcemeter (Keithley Instrumentation, 2400) with and without illumination from a halogen–tungsten lamp (9 mW/cm2). To detect the hot electron flow dependent on wavelength, the incident photon-to-current conversion efficiency (IPCE) was measured. To confirm the polarization dependence of hot electron generation, a linear polarizer (Glan-Thompson Polarizer, wavelength range: 350 ~ 2200 nm) was utilized in this experiment. The linear polarizer was placed directly below the light source to separate the linear electromagnetic field from the incident light. The polarization dependence was controlled by tilting the linear polarizer to the diode from 0° to 60°. To determine the optical properties of the films, films were deposited on a quartz window and were measured using a UV-Vis spectrometer (Shimadzu, UV-2600). Optical Simulation. The optical properties of the 3D tandem plasmonic Au/TiO2 structure were simulated using a FDTD simulation supplied by Lumerical Inc. The 3D models were designed by inputting the various parameters (e.g., length, shape, and dielectric constant of the nanostructure with a 1 nm mesh size). On the x- and y-axis, the periodic boundary condition was adjusted, while the perfectly matched layer (PML) boundary condition was applied on the z-axis. A plane wave with 2.0 eV of photon energy was induced along the z-axis and tilted from 0° to 60°.

RESULTS AND DISCUSSION 5


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Figure 1a illustrates the scheme of the tandem-structured Schottky nanodiode consisting of sol-gel TiO2 as the top layer and a plasmonic Au nanostructure and thermally oxidized TiO2 as the bottom layer. The planar plasmonic Au/TiO2 nanostructure, known as a connected gold island nanostructure, was formed by annealing the thin film Au/TiO2 in air.31 After annealing, the plasmonic Au/TiO2 nanostructure was covered by the sol-gel TiO2 using the spin coating method.32 To confirm the shape of the fabricated stacked structure, top and cross-sectional scanning electron microscopy (SEM) images were obtained (Figure 1b,c and Figure S2 in the Supporting Information). The plasmonic Au nanostructures were sandwiched between the bare TiO2 and the sol-gel TiO2, resulting in the formation of 3D interfaces. The thicknesses of the plasmonic Au and TiO2 top layers are about 40 and 90 nm, respectively. At the interface formed by the contact between the Au and the TiO2, a Schottky barrier was established by the difference in the electrical characteristics of the two materials, as shown in Figure 1d. This potential barrier serves as an energy filter to efficiently separate the hot electron–hole pairs in the Au. Highly energetic electrons photoexcited with enough energy to overcome the Schottky barrier are extracted by the TiO2 through ballistic transport. Afterwards, these hot electrons travel along the edge of the conduction band of the TiO2 and are finally detected as current. The electrical properties of the nanodiodes were explored by measuring the current– voltage (I–V) curves of the planar and 3D tandem plasmonic Au/TiO2 nanodiodes (Figure 2a). Rectification of the nanodiodes was well maintained after deposition of the sol-gel TiO2 onto the plasmonic Au/TiO2 nanodiode, although the shape of the I–V curves changed slightly in the forward bias regime. For a detailed and accurate understanding, the series resistances and the Schottky barrier heights of the planar and 3D tandem plasmonic Au/TiO2 structures were obtained by fitting the data from the current–voltage curves to the thermionic emission 6


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equation33

where

is the area,

elementary charge, the ideality factor, and

is the Richardson constant, is the Boltzmann constant,

is the temperature,

is the

is the Schottky barrier height,

is

is the series resistance of the nanodiode (Figure 2a). The Schottky

barrier height and series resistance of the plasmonic Au/TiO2 nanodiodes were determined to be

eV and

, respectively. After fabrication of the 3D tandem

plasmonic Au/TiO2 nanodiode, the Schottky barrier and series resistance changed to eV and

, respectively. We found that the deposition of sol-gel TiO2

onto the plasmonic Au/TiO2 structure hardly changed the Schottky barrier height, but created a significant reduction in the series resistance, suggesting that the electrical properties of the sol-gel TiO2 are reasonably close to that of the thermally annealed TiO2. The hot electron flow was observed by measuring the short-circuit photocurrent of several types of Au/TiO2 Schottky nanodiodes when illuminated by a 9 mW/cm2 tungsten– halogen lamp with a normal incidence angle. (Figure 2b). The spectral profile of the light source is shown in Figure S3 in the Supporting Information. The initial thin film Au/TiO2 diode has a short-circuit photocurrent of 28 nA. After modification of the plasmonic Au structure, the short-circuit photocurrent dramatically increased to approximately 80 nA. In previous work, we demonstrated that the connected island nanostructure of Au induced LSPR excitation, which enhances hot electron excitation.31 Interestingly, the deposition of sol-gel TiO2 onto the plasmonic Au/TiO2 nanostructure leads to a significant increase in the photocurrent of about 170 nA. These results imply that the efficiency for collecting hot 7


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electrons is greatly enhanced by increasing the Schottky interface in the 3D tandem plasmonic Au/TiO2 structure. To trace the response of the photocurrent depending on the incident angle and the polarized direction of the light, a linear polarizer was set up on top of the diode, as illustrated in Figure 3a,b. The direction of the electric field of the irradiated light was controlled by rotating the linear polarizer from 0° to 360°, indicated as α. The angle between the incident light and the diode (β) was also tilted from 0° to 60°. The behavior of the short-circuit photocurrent depending on the polarization of the incident light (α) was measured on three types of Au/TiO2 nanodiodes while changing the tilting angle of the light irradiation (β), as shown in the scheme in Figure 3a. Under illumination with a normal incidence angle (β = 0°), the direction of the polarization of the light had little effect on the detection of hot electrons (Figure 3c). Interestingly, as the angle of the irradiated light increased, the photocurrent had a different behavior, dependent on the nanostructure. Figure 3d shows the photocurrents measured on the three types of Au/TiO2 nanodiodes as a function of the angle of the linear polarizer (α) at β = 60°; the results were clearly classified by two kinds of polarization dependence. The planar thin Au film/TiO2 and plasmonic Au/TiO2 structures exhibit maximum photocurrents at 90° and 270° of α. On the other hand, the 3D tandem plasmonic Au/TiO2 structure shows maximum photocurrents when α is at 0° and 180°, which is obviously in contrast with the planar thin Au film/TiO2 and the plasmonic Au/TiO2. The reversed trend of the polarization dependence of the photocurrent observed on the planar and 3D plasmonic Au/TiO2 nanodiodes is more clearly shown in Figure S4 in the Supporting Information. The deviation between the minimum and maximum values of the photocurrent measured in the different polarization modes on the planar and 3D tandem structures are 53 and 34 nA, respectively. The effect of polarization is large in the planar plasmonic Au/TiO2 8


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diode, but it should be noted that the photocurrent of the 3D tandem diode is always greater than that of the planar diode. The reversed trend of the photocurrent among these structures may originate from the difference in the direction between the Schottky interface and the electric field that causes the LSPR excitation for the generation of hot electrons. At β = 0°, the electric field is in parallel to the Schottky interface. Therefore, although the magnitude of the photocurrent was different depending on the three types of nanodiode, the behavior of the photocurrent depending on the polarization was the same. However, when the incident angle of light was tilted, the direction between the electric field and the Schottky interface also changed as the polarization rotated. Figure 3b shows the arrangement of the electric field and the Schottky interface depending on the rotating angle of the polarization at β = 60° in the planar plasmonic Au/TiO2 structure. At α = 0° and 180°, the electric field was horizontal to the Schottky interface. Thus, a direction mismatch occurs between the momentum of the hot electrons and the Schottky interface, which lowers the probability of extraction for hot electrons. On the other hand, at α = 90° and 270°, the electric field is partially perpendicular to the Schottky interface. Therefore, a part of the momentum of the hot electrons is toward the interface, so that the hot electrons can be efficiently injected into the acceptor layer. In the 3D tandem plasmonic structure, at α = 90° and 270°, the momentum of the hot electrons is arranged such that it partially matches the top and bottom of the Schottky interface. In this case, as the incident light angle increased, the electric field was aligned with the short side of the plasmonic Au nanostructure, corresponding to the thickness, which is similar to transverse mode. However, at α = 0° and 180°, the momentum of the hot electrons was always toward the side interface of the Au/TiO2, regardless of β. Therefore, the electric field was oriented with the long side of the nanostructure, leading to a longitudinal mode. Generally, in LSPR excitation, the longitudinal 9


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mode is stronger than the transverse mode34,35 because the magnitude of the momentum in the longitudinal mode is larger than that in transverse mode. Therefore, in the 3D tandem plasmonic structure at β = 60°, the maximum value of the photocurrent was obtained at α = 0° and 180°. To investigate the enhancement of photocurrent in the 3D tandem plasmonic Au/TiO2 nanodiode, the response of the photocurrent was observed as function of photon energy (i.e., IPCE). Figure S5 in the Supporting Information shows plots of the IPCE measured on the planar and 3D tandem plasmonic devices. For comparison, the IPCE of the planar and tandem thin film devices were also measured. In the thin-film Au/TiO2 nanodiodes, IPCE increased as the photon energy increased because of internal photoemission of the hot electrons overcoming the Schottky barrier at the Au/TiO2 interface. The dramatic rise in IPCE at higher photon energies (i.e.,

) is caused by the generation of electron–hole pairs by band-

to-band excitation in the TiO2. For the plasmonic Au/TiO2 nanodiode, the increased IPCE peak was observed at 2.1 eV because of LSPR excitation. A tiny increase in the IPCE was also observed at photon energies of 2.6 ~ 3.0 eV, which is attributed to interband excitation in the plasmonic Au structure. After the deposition of sol-gel TiO2 onto the plasmonic Au/TiO2, the IPCE peak corresponding to LSPR excitation of the Au was significantly enhanced by the increased area of the Schottky interface. Figure 4a shows the polarization dependence of the IPCE measured on the planar and 3D tandem plasmonic Au/TiO2 nanodiodes at β = 0°. In this case, the IPCE was almost the same in both polarization modes. On the other hand, at β = 60°, the difference in the IPCE was dependent on the structure of the nanodiode and the polarization mode (Figure 4b,c). In the plasmonic Au/TiO2 device, the IPCE is stronger in transverse mode than in longitudinal 10


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mode. Whereas, in the 3D tandem plasmonic Au/TiO2 structure, the longitudinal mode has a higher IPCE than that in the transverse mode. For a more detailed analysis, the behavior of the IPCE peak following LSPR excitation of the Au was observed with different polarization modes depending on the angle of the light irradiation. Figure 4d,e shows the changes in the IPCE plots measured on the 3D tandem plasmonic device in longitudinal and transverse modes, respectively, while changing the tilting angle (β) from 0° to 60° by 10° increments. At β = 0°, the broad peak of LSPR excitation in the IPCE was observed at 2.0 eV in both polarization modes. As the value of β increased in longitudinal mode, the position and width of the IPCE peaks were almost preserved. In contrast, the IPCE peak measured in transverse mode shifted to a higher photon energy as the tilting angle of the light (β) increased. The IPCE behavior dependent on the modes can be elucidated by the oscillation direction. Excitation of surface plasmon oscillation is always induced along the width of the plasmonic Au nanostructure in both modes. In longitudinal mode, even if the value of β increases, the direction of the surface plasmon oscillation along the width of the plasmonic Au nanostructure is maintained. On the other hand, the oscillation direction in transverse mode changes from a long axis corresponding to the width to a relatively short axis corresponding to the thickness of the plasmonic Au nanostructure. As a result, the position of the LSPR excitation was shifted to a relatively short wavelength as the angle of the irradiated light increased. To more accurately confirm the change in the two types of LSPR excitation peaks in both polarization modes depending on the incident angle of the light, the absorbance was measured on the plasmonic Au nanostructures on quartz. As the angle of the irradiated light increased, the absorbance measured in the longitudinal and transverse modes showed the opposite trend. As shown in Figure S6a in the Supporting Information, in longitudinal mode, the intensity of the absorbance rises overall without any peak shifting as the angle of the 11


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irradiated light increased from 0° to 60°. On the other hand, in transverse mode, the absorbance peak of the transverse excitation increased, while the longitudinal excitation peak decreased and shifted to a higher photon energy (See Supporting Information, Figure S6b). To quantify the internal transport efficiency at different α and β, we calculated the internal quantum efficiency (IQE), which is the value obtained by removing any absorption enhancement from plasmonic excitation. As shown in Figure S7a in the Supporting Information, similar to the IPCE results, an increased peak at 2.0 eV was observed because of the LSPR excitation occurring as the thin film Au was transformed into the plasmonic Au nanostructure. After deposition of the sol-gel TiO2, the IQE was greatly enhanced. In addition, Figure S7b,c in the Supporting Information shows a comparison of the IQE plots in the longitudinal and transverse modes on the planar and 3D tandem plasmonic Au/TiO2 diodes at β = 60°. The IQE of the planar diode was higher in transverse mode than in longitudinal mode, and the IQE of the 3D diode showed the opposite trend. Furthermore, deviation of the IQE depending on the polarization mode is larger in the planar diode than in the 3D diode. This means that the polarization effect on hot electron detection is greater in a planar diode; these results are consistent with the photocurrent measurements shown in Figure 3d. However, note that the value of the IQE is higher in the 3D tandem plasmonic Au/TiO2 than in the planar plasmonic Au/TiO2 diode. To normalize the peak shift in the IPCE from any resonance effects, the IQE plots were obtained at β = 0° and 60° on the 3D tandem diode in transverse and longitudinal modes, as shown in Figure S8 in the Supporting Information. As β increased from 0° to 60°, a significant peak shift of IQE was only observed in transverse mode, while the peak shift in the longitudinal mode can be ignored. For a more detailed analysis, optical simulations were carried out using FDTD to 12


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estimate the electric field distribution and strength in the two types of plasmonic gold nanostructures depending on the polarization of the light. Since a randomly ordered plasmonic Au structure was used in the study, modeling was performed based on a crosssectional SEM image for the FDTD simulations (See Supporting Information, Figure S8a,b). A nanostructure with a width of 35 nm and height of 100 nm surrounded by TiO2 was used as the unit cell and the calculations were performed on the XZ and YZ planes. All the simulations were periodic in the x- and y-directions, with the PML in the z-direction. The incident radiation was a plane wave at 620 nm, which corresponds to LSPR excitation of the plasmonic Au nanostructure. Figure 5a–d shows the electric field distribution in the XY and YZ planes of the 3D tandem plasmonic Au/TiO2 structures depending on the polarization mode. A strong electric field formed at the lateral Schottky interface in longitudinal mode (Figure 5a,b), whereas the electric field was broad and weak on the horizontal Schottky interface in transverse mode (Figure 5c,d). Note that the strength of the electric field implies a significant increase in the excitation of hot electrons via the decay of surface plasmons. Furthermore, in the 3D tandem structure, hot electrons can be detected in all directions because the gold nanostructures are completely surrounded by the TiO2. Therefore, hot electrons enhanced in the longitudinal mode can be efficiently extracted through the lateral Schottky barrier, resulting in a larger photocurrent than when in transverse mode. FDTD simulations of the planar plasmonic Au/TiO2 nanostructures were also performed at the same conditions as those used for the planar structure. Figure S9c–f in the Supporting Information shows the simulated electric field distribution in the planar plasmonic Au/TiO2 structure under illumination at 620 nm while varying the polarization of the light 13


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while tilted at 60°. The intensity of the electric field in longitudinal mode (See Supporting Information, Figure S9c,d) is stronger than when in transverse mode (See Supporting Information, Figure S9e,f). However, in longitudinal mode, the electric field formed along the side of the gold nanostructure is comparable to the interface between the Au and TiO2. On the other hand, in transverse mode, the electric field was located at the interface between the Au and TiO2, resulting in a larger enhancement of the photocurrent than when in longitudinal mode.

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CONCLUSION In conclusion, a 3D tandem plasmonic nanodiode (TiO2/plasmonic Au nanostructure/TiO2) was fabricated by modifying thin film Au to connected nanoislands to create a plasmonic effect at the interface formed by the contact between the Au and TiO2. Compared with planar plasmonic Au/TiO2, hot electrons enhanced by LSPR excitation in 3D tandem plasmonic Au/TiO2 can be extracted through the 3D Schottky interface, resulting in the amplification of the short-circuit photocurrent. Furthermore, inverse dependence on polarization was observed with tilted incident light, indicating that the extraction efficiency of hot electrons is determined by the arrangement of the electric field and the Schottky barrier. Planar plasmonic Au/TiO2 nanodiodes exhibit a much higher photocurrent in transverse mode than in longitudinal mode. On the other hand, in the 3D tandem plasmonic Au/TiO2 nanodiodes, the photocurrent in longitudinal mode is much larger than in transverse mode. From the FDTD simulation results, the detection of hot electrons produced in the 3D tandem plasmonic Au/TiO2 structure through the side Schottky interface is dominant because of the formation of strong electric fields in longitudinal mode. This study can provide an understanding of the effect of surface plasmons on hot electron generation and the mechanism for efficient collection of hot electrons in all directions of the momentum.

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ASSOCIATED CONTENT Supporting Information. Additional information and figures. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (J.Y.P). Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the Institute for Basic Science (IBS) [IBS-R004]. AUTHOR CONTRIBUTIOS Y.K.L. and J.Y.P. conceived the experiments. C.L. and Y.K.L prepared the samples and performed the measurements. C.L. carried out the FDFD simulations. C.L. and Y.P. analyzed the data. J.Y.P supervised the project and all authors contributed to the preparation of this manuscript and the supporting information and participated in discussions about this work.

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REFERENCES (1) Atwater, H. A.; Polman, A., Plasmonics for Improved Photovoltaic Devices. Nat. Mater. 2010, 9, 205-213. (2) Clavero, C., Plasmon-Induced Hot-Electron Generation at Nanoparticle/Metal-Oxide Interfaces for Photovoltaic and Photocatalytic Devices. Nat. Photonics 2014, 8, 95-103. (3) Christopher, P.; Xin, H. L.; Linic, S., Visible-Light-Enhanced Catalytic Oxidation Reactions on Plasmonic Silver Nanostructures. Nat. Chem. 2011, 3, 467-472. (4) Sonnichsen, C.; Franzl, T.; Wilk, T.; von Plessen, G.; Feldmann, J.; Wilson, O.; Mulvaney, P., Drastic Reduction of Plasmon Damping in Gold Nanorods. Phys. Rev. Lett. 2002, 88, 077402. (5) Endriz, J. G.; Spicer, W. E., Surface Plasmon-One Electron Decay and Its Observation in Photoemission. Phys. Rev. Lett. 1970, 24, 64-68. (6) Lehmann, J.; Merschdorf, M.; Pfeiffer, W.; Thon, A.; Voll, S.; Gerber, G., Surface Plasmon Dynamics in Silver Nanoparticles Studied by Femtosecond Time-Resolved Photoemission. Phys. Rev. Lett. 2000, 85, 2921-2924. (7) Fuchs, R.; Kliewer, K. L., Surface Plasmon in a Semi-Infinite Free-Electron Gas. Phys. Rev. B 1971, 3, 2270-2278. (8) Li, X. G.; Xiao, D.; Zhang, Z. Y., Landau Damping of Quantum Plasmons in Metal Nanostructures. New J. Phys. 2013, 15, 023011. (9) Lee, Y. K.; Lee, H.; Lee, C.; Hwang, E.; Park, J. Y., Hot-electron-based solar energy conversion with metal-semiconductor nanodiodes. J. Phys.: Condens. Matter 2016, 28, 254006. (10) Inagaki, T.; Kagami, K.; Arakawa, E. T., Photoacoustic Observation of Nonradiative 17


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Decay of Surface-Plasmons in Silver. Phys. Rev. B 1981, 24, 3644-3646. (11) Inouye, H.; Tanaka, K.; Tanahashi, I.; Hirao, K., Ultrafast Dynamics of Nonequilibrium Electrons in a Gold Nanoparticle System. Phys. Rev. B 1998, 57, 11334-11340. (12) Furube, A.; Du, L.; Hara, K.; Katoh, R.; Tachiya, M., Ultrafast Plasmon-Induced Electron Transfer from Gold Nanodots into TiO2 Nanoparticles. J. Am. Chem. Soc. 2007, 129, 14852-14853. (13) Knight, M. W.; Sobhani, H.; Nordlander, P.; Halas, N. J., Photodetection with Active Optical Antennas. Science 2011, 332, 702-704. (14) Tian, Y.; Tatsuma, T., Mechanisms and Applications of Plasmon-Induced Charge Separation at TiO2 Films Loaded with Gold Nanoparticles. J. Am. Chem. Soc. 2005, 127, 7632-7637. (15) Somorjai, G. A.; Frei, H.; Park, J. Y., Advancing the Frontiers in Nanocatalysis, Biointerfaces, and Renewable Energy Conversion by Innovations of Surface Techniques. J. Am. Chem. Soc. 2009, 131, 16589-16605. (16) Park, J. Y.; Baker, L. R.; Somorjai, G. A., Role of Hot Electrons and Metal-Oxide Interfaces in Surface Chemistry and Catalytic Reactions. Chem. Rev. 2015, 115, 2781-2817. (17) Lee, C.; Nedrygailov, I. I.; Lee, Y. K.; Ahn, C.; Lee, H.; Jeon, S.; Park, J. Y., Amplification of hot electron flow by the surface plasmon effect on metal-insulator-metal nanodiodes. Nanotechnology 2015, 26. 445201. (18) Brown, A. M.; Sundararaman, R.; Narang, P.; Goddard, W. A.; Atwater, H. A., Nonradiative Plasmon Decay and Hot Carrier Dynamics: Effects of Phonons, Surfaces, and Geometry. Acs Nano 2016, 10, 957-966. (19) Lee, H.; Lee, Y. K.; Hwang, E.; Park, J. Y., Enhanced Surface Plasmon Effect of Ag/TiO2 Nanodiodes on Internal Photoemission. J. Phys. Chem. C 2014, 118, 5650-5656. 18


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(20) Zheng, B. Y.; Zhao, H. Q.; Manjavacas, A.; McClain, M.; Nordlander, P.; Halas, N. J., Distinguishing between Plasmon-Induced and Photoexcited Carriers in a Device Geometry. Nat. Commun. 2015, 6, 7797. (21) Lee, H.; Lee, Y. K.; Van, T. N.; Park, J. Y., Nanoscale Schottky Behavior of Au Islands on TiO2 Probed with Conductive Atomic Force Microscopy. Appl. Phys. Lett. 2013, 103, 173103. (22) Lee, Y. K.; Choi, H.; Lee, H.; Lee, C.; Choi, J. S.; Choi, C. G.; Hwang, E.; Park, J. Y., Hot Carrier Multiplication on Graphene/TiO2 Schottky Nanodiodes. Sci. Rep. 2016, 6, 27549. (23) Lee, C.; Choi, H.; Nedrygailov, I. I.; Lee, Y. K.; Jeong, S.; Park, J. Y., Enhancement of Hot Electron Flow in Plasmonic Nanodiodes by Incorporating PbS Quantum Dots. Acs Appl. Mater. Inter. 2018, 10, 5081-5089. (24) Linic, S.; Christopher, P.; Ingram, D. B., Plasmonic-Metal Nanostructures for Efficient Conversion of Solar to Chemical Energy. Nat. Mater. 2011, 10, 911-921. (25) Babicheva, V. E.; Zhukovsky, S. V.; Ikhsanov, R. S.; Protsenko, I. E.; Smetanin, I. V.; Uskov, A., Hot Electron Photoemission from Plasmonic Nanostructures: The Role of Surface Photoemission and Transition Absorption. Acs Photonics 2015, 2, 1039-1048. (26) Govorov, A. O.; Zhang, H.; Gun'ko, Y. K., Theory of Photoinjection of Hot Plasmonic Carriers from Metal Nanostructures into Semiconductors and Surface Molecules. J. Phys. Chem. C 2013, 117, 16616-16631. (27) Schuck, P. J., Nanoimaging: Hot electrons Go through the Barrier. Nat. Nanotechnol. 2013, 8, 799-800. (28) Knight, M. W.; Wang, Y. M.; 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. 19


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(29) Giugni, A.; Torre, B.; Toma, A.; Francardi, M.; Malerba, M.; Alabastri, A.; Zaccaria, R. P.; Stockman, M. I.; Di Fabrizio, E., Hot-Electron Nanoscopy using Adiabatic Compression of Surface Plasmons. Nat. Nanotechnol. 2013, 8, 845-852. (30) Goddeti, K. C.; Lee, C.; Lee, Y. K.; Park, J. Y., Three-dimensional Hot Electron Photovoltaic Device with Vertically Aligned TiO2 Nanotubes. Sci. Rep. 2018. (31) Lee, Y. K.; Jung, C. H.; Park, J.; Seo, H.; Somorjai, G. A.; Park, J. Y., Surface PlasmonDriven Hot Electron Flow Probed with Metal-Semiconductor Nanodiodes. Nano Lett. 2011, 11, 4251-4255. (32) Lee, Y. K.; Lee, H.; Park, J. Y., Tandem-Structured, Hot Electron based Photovoltaic Cell with Double Schottky Barriers. Sci. Rep. 2014, 4, 4580. (33) Sze, S. M., Ng, K. K., Physics of Semiconductor Devices; John Wiley & Sons: New York. 2006. (34) Huang, X. H.; Neretina, S.; El-Sayed, M. A., Gold Nanorods: From Synthesis and Properties to Biological and Biomedical Applications. Adv. Mater. 2009, 21, 4880-4910. (35) Chalabi, H.; Schoen, D.; Brongersma, M. L., Hot-Electron Photodetection with a Plasmonic Nanostripe Antenna. Nano Lett. 2014, 14, 1374-1380.

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(a) Sol-gel TiO2 Plasmonic Au nanostructure Thermally oxidized TiO2

(d)

(b)

Hot electron

e-

Ballistically

ESB 100 nm

EC EF

hv

(c) Au nanostructures Sol-gel TiO2 EV

Thermally oxidized TiO2 SiO2 wafer

Au

200 nm

TiO2

Schottky interface

Figure 1. (a) Schemes of the 3D tandem plasmonic Au/TiO2 nanodiodes. The stacking layer consists of sol-gel TiO2 (top layer), a plasmonic Au nanostructure (middle layer), and thermally oxidized TiO2 (bottom layer). (b) SEM image of the randomly connected gold island nanostructures. (c) Cross-sectional SEM image of the 3D tandem plasmonic Au/TiO2 structure. (d) Mechanism for hot electron transfer. Energetic electrons generated in the gold when illuminated can travel ballistically through the Schottky barrier and be detected as a photocurrent.

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(b)

(a) 3D tandem plasmonic Au/TiO2 Planar plasmonic Au/TiO2 Thermionic emission fitting

2

200

Photocurrent (nA)

4

Log (current (mA))

6

Current (mA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1 0 -1 -2 -3 -4 -5 0.0

0.5

Voltage (V)

1.0

150 100

-1.0

-0.5

0.0

0.5

1.0

on

50 0

0

on

on off

Planar thin film Au/TiO2

Voltage (V)

off

off

off

3D-tandem Planar plasmonic plasmonic Au/TiO2 Au/TiO2

Figure 2. (a) Current–voltage curves measured on the 3D tandem and planar plasmonic Au/TiO2 nanodiodes. (Inset) Current–voltage curves in log scale. The solid red line is from the measured current–voltage data fitted to the thermionic emission equation. (b) Shortcircuit photocurrents measured on the planar thin film, plasmonic Au/TiO2, and 3D tandem plasmonic Au/TiO2 nanodiodes.

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(a)

(b) E

Light

E

k

α = 90°, 270° (Transverse mode) β = 60°

β

E

α

E

k

α = 0°, 180° (Longitudinal mode)

Linear polarizer

β = 60°

(c)

(e)

(d) 240

Photocurrent (nA)

120

Plasmonic Au/TiO2 80

Thin film Au/TiO2

40

3D tandem plasmonic Au/TiO2

200

3D tandem plasmonic Au/TiO2

160

0

240

β = 0°

200

Photocurrent (nA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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β =060°

Planar plasmonic Au/TiO2

160

TiO2 Plasmonic Au/TiO2

120

3D-tandem plasmonic Au/TiO2

80 40

Thin film Au/TiO2

TiO2

0 0

60

120

180

240

300

360

Rotation angle of polarizer, α (degree)

0

60

120

180

240

300

360

Rotation angle of polarizer, α (degree)

Transverse mode

Longitudinal mode

Figure 3. (a) Scheme of the linear polarizer setup on the diodes. α and β represent the rotating angle of the linear polarizer and the tilting angle between the linear polarizer and the diode, respectively. 0° represents that the linear polarizer is normal to the top surface of the diode. (b) Direction of the electric field in transverse (α = 90°, 270°) and longitudinal (α = 0°, 180°) modes at β = 60°. Photocurrent measured on the three diodes as a function of α at (c) β = 0° and (d) β = 60°. (e) Comparison of the pathway that hot electrons can transfer through depending on the polarization direction in the planar and 3D tandem plasmonic Au/TiO2 nanostructures.

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(a)

(b)

2.4

1.8

1.5

Longitudinal mode Transverse mode ο 3D-tandem plasmonic Au/TiO2, β = 0

IPCE (%)

IPCE (%)

1.0 1.2

0.6

0.0

Longitudinal mode Transverse mode ο Planar plasmonic Au/TiO2, β = 60

0.5

0.0 1.6

2.0

2.4

2.8

3.2

1.6

Photon energy (eV)

(c)

2.4

(d)

Longitudinal mode Transverse mode ο 3D-tandem plasmonic Au/TiO2, β = 60

Intensity of IPCE (a.u.)

1.8

2.0

2.8

1.2

0.6

(e) 60° 60°

0.0

0°

0°

1.6

2.0

2.4

2.8

3.2

Photon energy (eV)

2.4

IPCE (%)

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3.2

Photonenergy (eV)

1.6 1.8 2.0 2.2 2.4

1.6 1.8 2.0 2.2 2.4

Photon energy (eV)

Photon energy (eV)

Figure 4. Comparison of the IPCE plots when in longitudinal or transverse modes, measured on (a) the 3D tandem plasmonic Au/TiO2 at β = 0°, (b) the planar plasmonic Au/TiO2 at β = 60°, and (c) the 3D tandem plasmonic Au/TiO2 at β = 60°. The change in the peak position of the LSPR excitation measured in the 3D tandem plasmonic Au/TiO2 nanodiode depending on the tilted incident light angle from 0° to 60° in (d) longitudinal and (e) transverse modes.

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x (nm)

z (nm)

z (nm)

E TiO2

(b) Au E TiO2 y (nm)

3

3

x (nm)

z (nm)

z (nm)

E TiO2

(d) Au E

TiO2 y (nm)

Electric field intensity lEl2 0

Au


(c)


Au


(a)

3

3

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ACS Photonics

Figure 5. Electric field distribution and intensity in the 3D tandem plasmonic Au/TiO2 nanostructure simulated by FDTD under light in longitudinal mode on the (a) XZ and (b) YZ planes, and in transverse mode on the (c) XZ and (d) the YZ planes. The arrows in the bottom right corners indicate the direction of the electric field.

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For Table of Contents Use Only

Polarization Effect of Hot Electrons in TandemStructured Plasmonic Nanodiode Changhwan Lee†,‡, Young Keun Lee†,‡, Yujin Park†,‡ and Jeong Young Park*,†,‡

We investigate polarization-dependent hot electrons detected on planar (two-dimensional) and three-dimensional (3D) tandem plasmonic Au/TiO2 nanodiodes. We confirm that the maximum photocurrent was obtained with the planar structure in transverse mode and with the 3D tandem structure in longitudinal mode.

3D-tandem plasmonic Au/TiO2

Planar plasmonic Au/TiO2

Transverse mode (90°, 270°)

Photocurrent (nA)

Photocurrent (nA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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100 80 60 0

60

120 180 240 300 360

Rotation angle of polarizer (degree)

Longitudinal mode (0°,180°,360°) 200 180 160 0

60

120 180 240 300 360

Rotation angle of polarizer (degree)

Table of Content graphic

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Polarization Effect of Hot Electrons in Tandem-Structured Plasmonic

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