Plasmon-Assisted Polarity Switching of a Photoelectric Conversion

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Plasmon-Assisted Polarity Switching of a Photoelectric Conversion Device by UV and Visible Light Irradiation Keisuke Nakamura, Tomoya Oshikiri, Kosei Ueno, Takayoshi Katase, Hiromichi Ohta, and Hiroaki Misawa J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01198 • Publication Date (Web): 10 Apr 2018 Downloaded from http://pubs.acs.org on April 10, 2018

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The Journal of Physical Chemistry

Plasmon-Assisted

Polarity

Switching

of

a

Photoelectric Conversion Device by UV and Visible Light Irradiation Keisuke Nakamura†, Tomoya Oshikiri†, Kosei Ueno†, Takayoshi Katase†, Hiromichi Ohta†, Hiroaki Misawa*,†,§

†

Research Institute for Electronic Science, Hokkaido University N21, W10, CRIS Bldg.,

Kita-ku, Sapporo 001-0021, Japan §

Department of Applied Chemistry & Institute of Molecular Science, National Chiao

Tung University, 1001 Ta Hsueh R., Hsinchu 30010, Taiwan

1

ACS Paragon Plus Environment

The Journal of Physical Chemistry 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

ABSTRACT

The plasmon-induced charge separation between metallic nanoparticles and a semiconductor following an electron transfer process has been extensively studied as one of the mechanisms in plasmonic light energy conversion devices. In this study, we propose that the switching of photocurrent polarity can be realized by changing the rectification properties of plasmonic photoelectric conversion devices and utilizing the difference in carrier mobility between electrons and holes. We fabricated plasmonic photoelectric conversion devices using gold nanoparticles (Au-NPs), nickel oxide (NiO), and mobility-limited TiO2 (ML-TiO2) to control the photocurrent polarity according to irradiation wavelengths of visible and UV light. A pulsed laser deposition technique was employed to deposit the ML-TiO2 and NiO layers. The photoelectric properties were measured, and in situ spectroelectrochemical measurements were performed to investigate the relationship between the rectification properties of the plasmonic photoelectric conversion devices and the change in the Fermi level of the Au-NPs under UV light irradiation condition. Additionally, UV and visible light irradiation selectively induced the current of opposite polarity with the small applied voltage. The electron transfer phenomena from ML-TiO2 to Au-NPs and from Au-NPs to ML-TiO2 give us important information to understand plasmon-related charge separation. 2


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INTRODUCTION

Metallic nanoparticles exhibiting localized surface plasmon resonances (LSPRs) have received considerable attention as light harvesting optical antennae for light energy conversion devices such as solar cells1-4 and artificial photosynthesis systems5-10 due to their versatile spectral tunability. In particular, the plasmon-induced charge separation between metallic nanoparticles and a semiconductor following an excited electron transfer process has been extensively studied as one of the mechanisms in plasmonic light energy conversion devices.11-13 Previously, we constructed solid-state plasmonic photoelectric conversion devices by inserting gold nanoparticles (Au-NPs) between niobium-doped rutile-type titanium dioxide (TiO2) single crystals as an n-type semiconductor and a nickel oxide (NiO) thin film as a p-type semiconductor.14 Upon irradiation with visible light for exciting the LSPR, the plasmonically enhanced near field promotes inter- and intra-band transitions of Au; the excited electron is transferred to the conduction band of the n-type TiO2 layer, and the hole migrates to the p-type NiO layer. Thus, plasmon-induced photocurrent generation can be obtained via plasmon-induced charge separation. Interestingly, principles based on plasmon-induced excited electron transfer are used not only for light energy conversion but also for biochemical and optical sensors.15-17 Furthermore, switching of electric properties utilizing plasmon-induced charge separation is useful as a replacement for conventional optical and electric instruments, such as wavemeters and optical routers, that hold complex functionalities with a bulky and impractical setup.18 We propose that the switching of photocurrent polarity can be realized by changing the rectification properties of plasmonic photoelectric conversion devices and utilizing the difference in carrier mobility between electrons and holes. It is known that the 3



mobility of holes is smaller than that of electrons. In particular, the hole mobility is much smaller than the electron mobility in anatase TiO 2 crystal.19, 20 The rectification properties can be modulated by trapping the carrier in the semiconductor.21 Additionally, it is reported that the modification of the rectification properties from rectifying to ohmic easily enhances the ohmic current with a very small applied voltage.22 Therefore, the reverse photocurrent polarity can be obtained with a small applied voltage by changing the Fermi level of the Au-NPs while accumulating holes in TiO2 upon irradiation with UV light. We propose active control of the current polarity using two input parameters, namely, the irradiation wavelength and the applied voltage. It is expected that such multiparameter controllable switching can be applied for multichannel wavelength sensors and logic gates. To achieve this purpose, in the present study, we fabricated TiO2 thin films by changing the deposition conditions to find the condition in which the TiO2 deposited has a mobility 1-order smaller than the reported TiO2.23 We employed the mobility limited TiO2 (ML-TiO2) as an n-type semiconductor of the plasmonic photoelectric conversion devices to control the photocurrent polarity. A pulsed laser deposition (PLD) technique was employed to deposit the ML-TiO2 layer. PLD can precisely control the crystallinity, carrier density, and electronic properties of the semiconductor because it transfers the target composition to the deposition film with high reproducibility and because a wide range of gas pressures can be applied. Au-NPs were loaded on the ML-TiO 2 layer by a sputtering and annealing method, and then the NiO layer was deposited as a p-type semiconductor by PLD. The photoelectric properties were measured, and in situ spectroelectrochemical measurements were performed to investigate the relationship between the rectification properties of the plasmonic photoelectric conversion devices 4


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and the change in the Fermi level of the Au-NPs under UV light irradiation condition. We discuss the mechanism for controlling the photocurrent polarity in plasmonic photoelectric conversion devices from the viewpoint of the wavelength-selective rectification properties.

EXPERIMENTAL SECTION Fabrication of plasmonic photoelectric conversion devices Conductive TiO2, ML-TiO2, and NiO were deposited by PLD (PAC-LMBE, PASCAL Co.) using a KrF (=248 nm) excimer laser. A (100) LaAlO3 substrate crystal (10  10  0.5 mm3, Shinkosya, Co. Ltd.) was used as the substrate. The LaAlO3 substrate was washed with acetone, methanol, and deionized water in an ultrasonic bath for 3 min and dried under air flow. TiO2 doped with 6 at% Nb with a thickness of 50 nm was grown on the LaAlO3 substrate as conductive TiO2 at an oxygen pressure (PO2) of 1.310-3 Pa and a temperature of 500°C, where a laser fluence was set at 1.25 J cm-2. The conductive TiO2 layer was partially masked to prevent the deposition of other layers. TiO2 doped with 0.01 at% Nb with a thickness of 10 nm or 110 nm was deposited on the grown conductive TiO2 layer as ML-TiO2 at a PO2 of 10 Pa and a temperature of 500°C with a laser power of 1.25 J cm-2. A 3 nm thin gold film was deposited on the surface of the MLTiO2 layer by helicon sputtering (MPS-4000, ULVAC) with a deposition rate of 1 Å/s and 5



was annealed at 800°C for 1 h under a nitrogen atmosphere to fabricate the Au-NPs. NiO doped with 0.01 at% Li with a thickness of 100 nm was deposited on the ML-TiO2 layer loaded with Au-NPs at an PO2 of 1.010-2 Pa and a temperature of 200°C with a laser power of 1.35 J cm-2. Excluding the PO2 and Nb-doping concentration, which were varied, the TiO2 deposition conditions were same as those used for ML-TiO2 deposition.

Structural analysis of plasmonic photoelectric conversion devices The formation, crystallization and crystal orientation of the TiO2 and NiO films were examined using X-ray diffraction (CuK, D8 Discover, Bruker) on an instrument equipped with a two-dimensional detector. The reciprocal space mapping of ML-TiO2 was obtained by high-resolution X-ray diffraction (CuK1, ATX-G, Rigaku Co.). The surface morphologies of the ML-TiO2 and Au-NPs were observed using field-emission scanning electron microscopy (FE-SEM, JSM-6700FT, JEOL), whose maximum resolution at an electron accelerating voltage of 15 kV was 1 nm.

Measurement of photovoltaic properties

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The cathode was prepared by sputtering Au (MPS-4000, ULVAC) onto the NiO thin film to a thickness of 4 nm. An In-Ga alloy (4:1 in weight ratio) paste was applied to the exposed area of the conductive TiO2 layer to make an ohmic contact. Then, a Cu foil (Nilaco) was adhered to the In-Ga alloy with Ag paste (D-550, Fujikura Kasei). When the light was irradiated from the NiO side, the terminals of the anode and cathode are attached onto the Cu foil and the gold thin film, respectively (Figure S1a). In contrast, when the light was irradiated from the ML-TiO2 side, the cathode terminal was attached onto the Cu foil pasted on the gold thin film (Figure S1b). The voltage of the NiO film was applied against the TiO2 layer. The current-voltage (I-V) characteristics and current-time (I-t) characteristics of the devices were measured at room temperature using a semiconductor parameter analyzer (B1500A, Keysight Technologies). The device performance was measured under AM1.5 at 100 mW/cm2 using a solar simulator (WXS-156S-L2, AM1.5GMM, WACOM ELECTRIC). To obtain a visible and UV light response, a bandpass filter (ASAHI SPECTRA) with a full-width at half-maximum (FWHM) of less than 15 nm was used. To acquire the incident photon-to-current efficiency (IPCE) action spectrum, a bandpass filter with an FWHM of less than 15 nm was used. The active area of the devices was adjusted to 0.7 × 0.7 cm2. 7



The resistivity, the carrier density and the mobility of TiO2 were obtained by the DC four-probe method using an In–Ga alloy electrode in the van der Pauw configuration. The carrier density of the 0.01 at% Li-doped NiO was estimated as 6 x 1015 cm-3 from the value reported for 10 at% Li-doped NiO24 because the conductivity of the fabricated NiO layer was too low.

Measurement of optical properties The extinction spectra of the semiconductor film and Au-NPs were measured by a UV-visible spectrophotometer (UV-3100PC, SHIMADZU). The plasmon resonance band of the Au-NPs in the plasmonic photoelectric conversion devices under UV and visible light irradiation was measured using the following setup.25 The ML-TiO2 and LSPR were excited by irradiation of the ML-TiO2 side of the device with monochromic light of 350 nm from a mercury lamp with bandpass filters with a bandwidth less than 15 nm (FWHM). The incident light from the halogen lamp (TH4-100, Olympus Co.) was irradiated onto the NiO side of the device via an inverted microscope (IX71, Olympus Co.). The transmitted light was spectrally dispersed by the spectrometer (SpectraPro-300i, Acton Research Co.) and analyzed by a cold CCD optical detector (Princeton Instrument LN/CCD-1340/400-EB). 8


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RESULTS AND DISCUSSION Design of polarity-switchable plasmonic photoelectric conversion devices The plasmonic photoelectric conversion devices were composed of a conductive TiO2 layer, an ML-TiO2 layer as an n-type semiconductor, gold nanoparticles (Au-NPs) as plasmonic metals, and a NiO layer as a p-type semiconductor as shown in Figure 1a. When the LSPR is excited, the plasmon-induced charge separation, which generates the photocurrent, will increase. However, the device can be switched from rectifying to ohmic behavior by irradiation with UV light. Therefore, it is expected that switching from positive to negative current is achieved under a small applied voltage by successive irradiation with visible and UV light (Figure 1b). In this manuscript, the opposite direction of the ohmic current under a forward voltage on the p-n junction is defined as “positive,” as shown in Figure 1b. The detailed mechanism will be discussed later in the manuscript.

Fabrication and characterization of the plasmonic photoelectric conversion devices First, we investigated the PO2 dependence of the deposition conditions of the TiO2 to control the carrier density and mobility, as shown in Table S1. The electron mobility decreased as the PO2 increased. Additionally, the carrier density dramatically 9



decreased when the PO2 was 10 Pa. We employed the PO2 of 10 Pa and precisely controlled the carrier density by doping with an arbitrary concentration of niobium. The out-of-plane X-ray Bragg diffraction pattern and the reciprocal space mapping of the 0.01 at% NbTiO2 thin film indicate that the single-phase (001) anatase TiO2 grows epitaxially on the LaAlO3 substrate as shown in Figure 2a. The carrier density and mobility were 4×1016 cm-3 and 5.1 cm2 V-1 s-1, respectively. We define the fabricated Nb-TiO2 as ML-TiO2 because the mobility was 1-order smaller than the reported value of Nb-doped anatase TiO2 (20-40 cm2 V-1 s-1).23 The hole diffusion in the fabricated TiO2 was significantly restricted because the hole mobility is generally smaller than the electron mobility, as described above. To determine the origin of the small mobility, the morphology of the thin film was observed by SEM, as shown in Figure 2b. The crystal grain and the boundary were clearly observed. The large boundary is believed to result from the high PO2 during deposition.26 These results suggest that the grain boundary restricts the carrier diffusion, even though each crystal has the same orientation. Au-NPs were loaded on ML-TiO2 by thin film deposition and subsequent annealing.27 An SEM image of the Au-NPs is shown in Figure 2c, and the average diameter of the Au-NPs was calculated as 10.4±8.0 nm.

10


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The plasmonic photoelectric conversion devices were fabricated by the deposition of materials on LaAlO3 in the following order: conductive TiO2 (6 at% Nbdoped, 50 nm), ML-TiO2 (0.01 at% Nb-doped, 10 or 110 nm), Au-NPs, and NiO (0.01 at% Li-doped, 100 nm). The (012) and (110) orientations of the NiO crystal structure were confirmed by the XRD patterns (Figure S2). Figure 2d shows the extinction spectra of the ML-TiO2 film deposited on LaAlO3, the NiO film deposited on glass, and the Au-NPs in the fabricated plasmonic device. The absorption edges of TiO2 and NiO were estimated as ca. 380 nm and 350 nm and almost corresponded to the bandgaps of 3.2 eV and 3.7 eV, respectively. Additionally, the peak of the LSPR was observed at 670 nm. The relatively longer wavelength of the resonance is due to the large refractive index of the TiO2 and NiO surrounding the Au-NPs.

Photoelectric properties of plasmonic photoelectric conversion devices The rectification and charge separation properties under excitation of the MLTiO2 and the LSPR of Au-NPs were investigated using the current-voltage (I-V) characteristics of the plasmonic device under irradiation of the ML-TiO2 side with 350 nm and 650 nm wavelength light, respectively, as shown in Figure 3. When the thickness of the ML-TiO2 layer was less than 10 nm, the rectification properties were observed 11



under irradiation with both 350 and 650 nm light (Figure 3a). Additionally, the shortcircuit current was obtained, indicating that charge separation was achieved via excitation of the ML-TiO2 and the LSPR of the thinner device. However, when the device with the thicker ML-TiO2 layer was irradiated, the situation was different (Figure 3b). Upon irradiation with 350 nm light, the rectification properties disappeared, and ohmic characteristics were observed, whereas the rectification properties remained under irradiation with 650 nm light. The resistance of the thicker device under 350 nm irradiation was estimated as 68 , which is similar to the series resistance of the thicker device in the dark (74 . This result indicates that a thicker ML-TiO2 layer is required to change the rectification properties. The small diffusion voltage in Figure 3b is due to charge recombination in the thicker film. Furthermore, the enhancement of the reverse saturation current due to the ohmic properties by photoirradiation induces a further decrease in the voltage. To investigate the wavelength dependence of the rectification properties, the I-V characteristics were investigated under irradiation with monochromatic light with wavelengths of 330, 350, 370, and 400 nm onto the NiO and ML-TiO 2 sides of the thicker device, as shown in Figure 4. Although the absolute values of the current and voltage are different in Figure 4a and Figure 4b because the electrode structures are different, as 12


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shown in Figure S1, a qualitative comparison and discussion can be conducted. Irradiation of both sides with 350 and 370 nm light induced the disappearance of the rectification properties. However, the 330 nm light induced the disappearance of the rectification properties only when the light was irradiated onto the ML-TiO2 side. As shown in Figure 2d, the 350 and 370 nm light passes through NiO and reaches the ML-TiO2 side even when the NiO side is irradiated. Additionally, all light in the range of 330-370 nm was absorbed by the ML-TiO2 layer when the ML-TiO2 side was irradiated. These results suggest that the disappearance of the rectification properties can be induced by ML-TiO2 excitation rather than NiO excitation. Based on the thickness dependence and material dependence results, we conclude that the bulk region of ML-TiO2 plays an important role in the rectification control. In fact, the ideal thickness of the depletion region formed by ML-TiO2 and NiO was estimated as approximately 50 nm on the ML-TiO2 side, which is thicker than 10 nm and thinner than 110 nm. We will discuss the mechanism of the rectification change and charge separation induced by the LSPR and ML-TiO2 in the next section.

Mechanism of rectification change and charge separation

13



As described below, we formulated a hypothesis regarding the rectification change and charge separation phenomena. Figure 5 depicts the time course of the current when the ML-TiO2 and LSPR in devices were excited and the proposed mechanism of each event. First, we considered the plasmon-induced charge separation. When the visible light irradiation wavelength corresponds to the LSPR wavelength, electron-hole pairs are generated in the Au-NPs near the depletion region of the ML-TiO2/NiO p-n junction. The generated electrons and holes are rapidly separated by the internal electric field of the depletion region. Next, the electrons and holes are transferred to the conduction band of the ML-TiO2 and valence band of the NiO, respectively. As a result, a positive photocurrent is observed (Figure 5a-i, Figure 5b-i). In this situation, the drift current caused by electric fields is dominant, and the diffusion current caused by a carrier concentration gradient is negligible. This mechanism has already been proposed for conventional plasmonic photoelectric conversion devices.14, 28 Second, we move our focus to the excitation of the ML-TiO2 layer. When the thickness of the ML-TiO2 is low enough, the depletion region spread almost all the way through the ML-TiO2. Therefore, the photogenerated electrons and holes are separated by the internal electric field, and the drift current is dominantly obtained, as in the case of 14


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plasmon-induced charge separation (Figure 5a-ii). In that case, the rectification properties are maintained because the carrier concentration in the device does not dramatically change via compensated electron and hole transfer. In contrast, when UV light is irradiated onto the 110 nm-thick ML-TiO2 layer, the bulk region outside of the depletion region is excited. A spike current was observed after irradiation, and decay of the current was subsequently observed. It is considered that the fast spike current is derived from the charge current due to the limited mobility of holes. The slow decay of the current can be explained by two competing events. The small number of photogenerated holes reaches the depletion region after irradiation and contributes to the positive photocurrent (Figure 5b-ii). The compensated number of electrons are transferred to the anode because the mobility of electrons is sufficiently high. However, the majority of the holes accumulate near the anode because the diffusion length is significantly limited. Simultaneously, the electrons could reach the region close to the Au-NPs due to the diffusion caused by the carrier concentration gradient because the diffusion length is relatively long (Figure 5b-iii). The excited electrons in TiO 2 can be transferred into the attached gold, as previously reported.29-32 Both temporary events (Figure 5b-ii: hole transportation to the cathode and electron transportation to the anode, and Figure 5b-iii: hole aggregation in ML-TiO2 and electron injection to the Au-NPs) start 15



immediately after initiating the photoirradiation. Similarly, the electrons can reach the NiO through the Au-NPs (Figure 5b-iv). As a result, the Fermi levels of the Au and NiO increase, and the diffusion potential formed with ML-TiO2 decreases. Finally, the contact among ML-TiO2, Au-NPs, and NiO changes to ohmic behavior, and the photovoltaic response disappears (Figure 5b-v). According to the proposed hypothesis, a special situation in which only the electron reaches the NiO layer via the Au-NPs is required to change the rectification properties. The thick ML-TiO2 layer with Au-NPs satisfied this condition. Additionally, the photoelectric conversion device composed of the ML-TiO2 and NiO without Au-NPs maintained the rectification properties, accompanying only small changes even under irradiation with 350 nm light, as shown in Figure S3. This result supports the Au-NPs playing an important role in changing the rectification properties through electron transfer. If the hypothesis that the excited electrons in ML-TiO2 are transferred through the Au-NPs is correct, the electron density of Au should increase when irradiated with UV light. Mulvaney et al. reported a measurement technique for the electron density change in plasmonic nanoparticles using the spectral shift of the LSPR band.33, 34 The relationship between the electron density in plasmonic particles and the LSPR peak wavelength is described by eq 1. 16


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𝛥𝜆 = −

𝛥𝑁 𝜆 2𝑁

𝜀 +

1−𝐿 𝜀 ・・・(1) 𝐿

In this equation, N is the electron density in an uncharged Au-NP, ∞ is the high frequency contribution to the metal dielectric function, m is the dielectric constant of the medium,

p is the Au bulk plasma wavelength, and L is the particle shape factor. We measured the extinction spectrum of the Au-NPs under irradiation and estimated the electron density change in the Au-NPs. Figure 6 shows the extinction spectra of the Au-NPs in the plasmonic photoelectric conversion devices under irradiation. The LSPR band blueshifted to blue when the 350 nm light was irradiated onto the plasmonic photoelectric conversion device with 110 nm-thick ML-TiO2, indicating electron injection from the ML-TiO2 to the Au-NPs (Figure 6a). The shift value  is 7 nm, which corresponds to a 12.2% increase in the electron density of the Au-NPs according to eq. 1. Additionally, the LSPR peak position did return to the original position after removing the irradiation, indicating that the change in the electron density in the AuNPs is reversible. When 350 nm light was irradiated onto the plasmonic photoelectric conversion device with 10 nm-thick ML-TiO2, the LSPR band peak did not change as shown in Figure 6b.

17



These results support the hypothesis that excitation of the thicker ML-TiO2 layer induces electron injection from the ML-TiO2 to Au-NPs, resulting in the disappearance of the rectification properties.

Switching the polarity of plasmonic photoelectric conversion devices We tried to switch the current polarity of the plasmonic photoelectric conversion devices utilizing their unique rectification properties. When the LSPR was excited, the current polarity was positive in the wide voltage region shown in Figure 3 because the photovoltaic voltage generated by the plasmon-induced charge separation overcame the externally applied voltage. In contrast, the current polarity of the device after excitation of the bulk region of the ML-TiO2 corresponds to the applied voltage. Therefore, it is expected that the current polarity can be separately controlled by a combination of the irradiation wavelength and the applied voltage. Figure 7a and Figure 7b show the time course of the photoresponse of the current and the IPCE action spectrum of the plasmonic photoelectric conversion device with a very small applied voltage (3 x 10-4 V), respectively. The result indicates that UV and visible light irradiation can selectively induce the current of opposite polarity with the small applied voltage. The detailed voltage dependence of the photoresponse shown in 18


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Figure S4 suggests that the current polarity can be arbitrarily controlled by the irradiation wavelength and the applied voltage. Figure 7b clearly shows the wavelength dependence of the polarity. When the irradiation wavelength was longer than 370 nm, positive current was observed. However, when the irradiation wavelength was shorter than 370 nm, negative current was observed. It is important that the shape of the absolute value of the IPCE spectrum shows good agreement with the extinction spectra of the ML-TiO2 and the LSPR in both wavelength regions. The relatively higher IPCEs from 400 to 500 nm are due to the interband transition of the Au.27 Furthermore, plasmon-induced photocurrent showed interesting phenomena. The maximum IPCE in the visible region reached 0.8%. This value is 7-fold larger than that of the plasmonic photoelectric conversion device composed of single-crystal rutile TiO2 with a thickness of 0.5 mm, Au-NPs, and multicrystalline NiO. The large enhancement of the IPCE may be due to the contribution of the drastically decreased thickness of the TiO2 layer, even though the carrier mobilities were limited.

Conclusion We successfully fabricated plasmonic photoelectric conversion devices composed of ML-TiO2, Au-NPs, and NiO. We also demonstrated the wavelength19



dependent control of the rectification properties of these plasmonic photoelectric conversion devices by controlling carrier mobility in the semiconductor. Furthermore, polarity switching has been achieved by the combination of photoirradiation and applied voltage. The polarity-switching device structure is simple and works with small voltages. Therefore, it is easy to apply to actual devices from the perspective of fabrication and working energy reduction. Furthermore, we used photoelectric and optical measurements to investigate the mechanism of the rectification change, which involved electron transfer from ML-TiO2 to Au-NPs and NiO. The Au-NPs play 2 roles in controlling the current polarity. When UV light is irradiated to excite ML-TiO2, the Au-NPs work as the electron path and then generate negative current. In contrast, when visible light is irradiated, the Au-NPs exhibit LSPR and generate positive current via plasmon-induced charge separation. The electron transfer phenomena from ML-TiO2 to Au-NPs and from Au-NPs to ML-TiO2 give us important information to understand plasmon-related charge separation. Quantitative discussion of the dynamics and transportation of hot carriers is required to more precisely control the photoelectric properties of plasmonic devices in the near future.

ACKNOWLEDGMENTS 20


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K.N., T.O., K.U. and H.M. gratefully acknowledge financial support from JSPS KAKENHI (Grant Nos. JP17H01041, JP17J00763, JP17H05245, JP17H05459 and JP15K04589), the Nanotechnology Platform (Hokkaido University), and the Dynamic Alliance for Open Innovation Bridging Human, Environment and Materials (Five-Star Alliance) of MEXT. H.O. was supported by Grants-in-Aid for Scientific Research A (17H01314).

Supporting Information. The measurement configurations of the device, electronic properties of TiO2 films, XRD pattern of NiO film, rectification properties of the photoelectric conversion device without Au-NPs, and applied voltage dependence of the I-t characteristics of the plasmonic photoelectric conversion devices.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Phone: +81-11-706-9358. Fax: +81-11-706-9359. The present address of Takayoshi Katase: Laboratory for Materials and Structures, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan 21



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Figure 1. (a) Schematic of the plasmonic photoelectric conversion device. (b) Concept of a current polarity-switchable device controlled by the irradiation wavelength and applied voltage.

25



Figure 2. Out-of-plane X-ray Bragg diffraction pattern (a) and scanning electron microscope image (b) of 0.01 at% Nb-TiO2 thin film on a LaAlO3 substrate. The inset at (a) shows the reciprocal space mapping of the 028 diffraction spot of anatase TiO2. (c) Scanning electron microscope image and (d) extinction spectra of ML-TiO2 on LaAlO3, NiO on glass and Au-NPs in the plasmonic conversion device.

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20

(a) 100 80 60 40 20 0 -20 -40 -60 -80 -100 -0.2

10

0

-10 -0.10

-0.1

0.0

4

(b) Current density (A/cm2)

Current density (A/cm2)

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


-0.05

0.1

Voltage (V)

0.00

0.2

0.05

0.10

0.3

3

100 80 60 40 20 0 -20 -40 -60 -80 -100 -0.2

2 1 0 -1 -2 -3 -4 -4 -5.0x10 -2.5x10

-0.1

0.0

0.1

Voltage (V)

0.2

0.0

-4

2.5x10 5.0x10

0.3

Figure 3. I-V characteristics of devices whose thickness of the ML-TiO2 layer was 10 nm (a) and 110 nm (b). Black: dark current. Red and blue: under irradiation with 650 nm and 350 nm light, respectively, of AM1.5 with a bandpass filter.

27


-4


(b) 100


(a) Current density (A/cm2)

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

Page 28 of 32

dark 330 nm 350 nm 370 nm 400 nm

50

0

-50

-100 -0.2

-0.1

0.0

0.1

0.2

1000

500

0

-500

-1000 -0.2

0.3

dark 330 nm 350 nm 370 nm 400 nm

-0.1

0.0

0.1

0.2

0.3

Voltage (V)

Voltage (V)

Figure 4. Irradiation wavelength dependence of the I-V characteristics of the device with the 110 nm-thick ML-TiO2 layer. The incident light is irradiated from the NiO side (a) and ML-TiO2 side (b).

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3

e650 nm

2

(i)

350 nm

650 nm (ii) 650 nm on off 350 nm on

1

h+

350 nm off

0

(b)

(ii)

ML-TiO2 NiO

Cathode

4

Anode


(i) (a)


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


Au

4

650 nm 650 nm on off

(ii)

(iii)

(iv)

(v)

350 nm on

2

1

0

(i)

(i)

3

(ii) (iii) 0

(iv)

350 nm off (v)

100

200

Time (s)

Figure 5. I-t characteristics (left) and corresponding events (right) of the plasmonic photoelectric conversion devices with the ML-TiO2 layer whose thickness is 10 nm (a), and 110 nm (b).

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(b)

 Normalized extinction

Normalized extinction

Normalized extinction

(a)

 Normalized extinction

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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500

550

600

650

700

750

800

500

Wavelength (nm)

550

600

650

700

750

800

Wavelength (nm)

Figure 6. Extinction spectra and the differences between the plasmonic photoelectric conversion devices with ML-TiO2 layers whose thicknesses are 110 nm (a) and 10 nm (b). Black: dark conditions. Red: under irradiation of 350 nm. The broken lines in the lower figures indicate zero.

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(b)

650 nm 650 nm on off

4 3

1

350 nm on

2

IPCE (%)

(a) Current density (A/cm2)

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


1 0

350 nm off

-1 -2

-6 -8

-3 -4

0

-10

0

200

400

600

300

Time (s)

400

500

600

700

800

900

Wavelength (nm)

Figure 7. I-t characteristics (a) and IPCE action spectrum (b) of the plasmonic photoelectric conversion device with the 110 nm-thick ML-TiO2 layer under a 0.3 mV applied voltage. The blue lines in Figure 7b indicate the measurement errors, which were calculated from the standard deviation of multiple measurements under irradiation with 650 nm light for positive currents and under irradiation with 350 nm light for negative currents.

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TOC Graphic

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Plasmon-Assisted Polarity Switching of a Photoelectric Conversion

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