Article pubs.acs.org/JPCC
Enhanced Surface Plasmon Effect of Ag/TiO2 Nanodiodes on Internal Photoemission Hyosun Lee,†,‡ Young Keun Lee,†,‡ Euyheon Hwang,§ and Jeong Young Park*,†,‡ †
Center for Nanomaterials and Chemical Reactions, Institute for Basic Science, Daejeon 305-701, Republic of Korea Graduate School of EEWS, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Republic of Korea § SKKU Advanced Institute of Nanotechnology, Sungkyunkwan University, Suwon 440-746, Republic of Korea ‡
S Supporting Information *
ABSTRACT: Over the last several decades, innovative light-harvesting devices have evolved to achieve high-efficiency solar energy transfer. Understanding the mechanism of plasmon resonance is very desirable to overcome the conventional efficiency limits of photovoltaics. The influence of localized surface plasmon resonance on hot electron flow at a metal− semiconductor interface was observed with a Schottky diode composed of a thin silver layer on TiO2; subsequent X-ray photoelectron spectroscopy characterized how oxygen in the Ag/ TiO2 nanodiode influenced the Schottky barrier height. Photoexcited electrons generate photocurrent when they have enough energy to travel over the Schottky barrier formed at the metal−semiconductor interface. We observed that the photocurrent could be enhanced by optically excited surface plasmons. When the surface plasmons are excited on the corrugated Ag metal surface, they decay into energetic hot electron−hole pairs, contributing to the total photocurrent. The abnormal resonance peaks observed in the incident photons to current conversion efficiency can be attributed to surface plasmon effects. We observed that photocurrent enhancement due to surface plasmons was closely related to the corrugation (or roughness) of the metal surface. While the photocurrent measured on Ag/TiO2 exhibits surface plasmon peaks, the photocurrent on Au/TiO2 does not show any peaks even at the Au surface plasmon energy frequency presumably because of the smoothness of the gold film. We modified the thickness and morphology of a continuous Ag layer using electron beam evaporation deposition and heating under gas conditions and found that morphological changes and the thickness of the Ag film are key factors in controlling the internal photoemission efficiency.
1. INTRODUCTION Schottky nanodiodes, composed of a thin metal layer on a semiconductor, convert photon and chemical energy into electrical energy. High kinetic energy electrons (1−3 eV) can be generated in the metal thin film during light irradiation or exothermic chemical reactions because the low electronic heat capacity of metals permits nonadiabatic energy transfer.1−8 These excited electrons are thermally out of equilibrium with the surrounding metal atoms and are called “hot electrons”.9−13 Hot electrons emitted in the near-surface region travel ballistically toward the metal−semiconductor interface within the length of the electron mean free path, are transported into the semiconductor when the energy of the electrons is high enough to overcome the Schottky barrier, and generate a steady-state current.5,14 The thickness of the metal should not exceed the mean free path of the electrons, which is on the order of 10 nm in metals.5,15,16 The performance of Schottky diodes as photoactive devices, such as photodetectors or solar cells, has been studied theoretically and experimentally.17−20 D.W. Peters suggested the concept of an infrared detector utilizing internal photoemission via a device composed of a Au layer on an n-type silicon substrate. Numerous theoretical models have also been presented in an attempt to elucidate the energy conversion process.17 It is possible to detect infrared radiation below the band gap energy of the semiconductor © 2014 American Chemical Society
because low-energy electrons can overcome the Schottky barrier formed at the interface between the Au and Si layers. McFarland and Tang designed a solar cell structure in which hot electrons can be transported into the photocurrent after absorbing light in the visible range, in contrast to an IR detector sensing only long wavelength photons.14 A molecular layer of dye on a Au/TiO2 Schottky diode acted as a photoreceptor of light in the visible range. However, this requires threedimensional TiO2 structures, such as nanopores, to obtain sufficient light absorption. Recent studies have investigated the use of modified surface plasmon resonances of metal nanostructures to enhance the performance of photovoltaic devices in the visible and nearinfrared regions. Metal nanoparticles that present strong optical extinction caused by plasmonic effects are alternative materials for the light absorption layer.21,22 Strong optical extinction allows a photovoltaic device to be constructed in twodimensional arrays, which are beneficial when developing any solid-state device. Metal nanostructures have received much attention and have been explored in various applications, such as sensors,23,24 photovoltaic cells,25 and photocatalysts.26−28 Au Received: October 5, 2013 Revised: February 23, 2014 Published: February 25, 2014 5650
dx.doi.org/10.1021/jp409894b | J. Phys. Chem. C 2014, 118, 5650−5656
The Journal of Physical Chemistry C
Article
Figure 1. (a) Schematic of a Ag/TiO2 nanodiode. The rectifying contact is formed at the interface between the Ag and TiO2 layer. A layer of Ti (50 nm) makes the ohmic junction with the TiO2, and a Au layer (100 nm) makes the junction with the Ag. (b) Energy band diagram for the metal− semiconductor Schottky diode. Excited electrons that are energetic enough to overcome the Schottky barrier can be detected as a steady-state current. (c) Current−voltage (I−V) curves in semilog scale of a Schottky nanodiode, consisting of a 10 nm Ag layer on a TiO2 substrate, both with and without illumination. This diode generates a short-circuit photocurrent of 775 nA under light irradiation using a tungsten-halogen lamp (9 mW/ cm2) with a normal incident angle when the active area is 4 mm2. (d) Fitting I−V curves for a Ag/TiO2 nanodiode (10 nm thick Ag layer) to the thermionic emission equation. This diode shows a Schottky barrier height of 0.91 eV, ideality factor of 1.35, and series resistance of 720 ohm.
2. EXPERIMENTAL DETAILS Fabrication of Diodes. The metal−semiconductor nanodiodes were fabricated as follows: First, we deposited a thin TiO2 layer (250 nm thick) on a p-type Si(100) wafer covered by 500 nm SiO2 using cosputtering from a titanium target in a mixture of O2 and Ar. A patterned aluminum shadow mask (4 × 6 mm2) was used for this step. After that, the wafer was annealed in air at 653 K for 2 h, while the sheet resistance was monitored, to modify the Fermi level of the TiO2 film. The electrode, including a 50 nm Ti layer and a 150 nm Au layer, was then deposited through a second aluminum shadow mask (5 × 5 mm2), which allows for the use of electron beam evaporation. The titanium layer is for ohmic contact with the TiO2 thin film and the Au layer is for the nanodiode’s two ohmic electrodes. Finally, four thicknesses (10, 20, 30, and 50 nm) of Ag thin film were deposited through a third patterned mask (2 × 1 mm2). This produced Ag/TiO2 nanodiodes with a 4 mm2 active area. Measurements. For electrical characterization of the nanodiodes, current−voltage (I−V) curves and the shortcircuit photocurrent were measured using a Keithley Instrumentation 2400 sourcemeter under illumination from a 9 mW/cm2 tungsten-halogen lamp. To measure the I−V curves of the diodes, the voltage was swept from −1 to 1 V both with and without illumination. We also carried out a solar simulation (PEC-L12) and IPCE measurements (PEC-S20). We used a multipurpose X-ray photoelectron spectroscope (Sigma Probe, energy resolution of 0.47 eV fwhm) to analyze the interface between the thin Ag metal and the TiO2 layer while etching the surface using an Ar ion gun (at 2 kV and 1 mA) 10 times every 10 s. Four Ag thin films of different thicknesses were deposited on a quartz window; a clean quartz
and Ag, in particular, are attractive plasmonic metals because of their distinct dielectric properties in the visible and nearinfrared spectral regions.29 Although numerous studies on localized surface plasmon resonance of isolated metal nanoparticles have been reported,20,22,28,30 there is still little research focusing on the performance of connected metal nanostructures on these devices. Recently, Lee et al.31 and Knight et al.20 reported plasmon-induced hot electron flow using Au/TiO2 and Au/Si nanodiodes, respectively. For standard photoemission, the results on enhanced photoyield due to plasmon effects were reported.32−35 Feibelman et al. showed that the enhanced photoyield is due to nonlocal effects leading to electric field enhancements at characteristic plasma frequencies. For confined metallic systems, a strong photoemission is found at photon energies close to the bulk plasmon energy.32 An enhanced photoresponse in Schottky diodes using rough Ag films has been reported recently, implying the intrinsic relation between chemiluminescence and surface plasmon.36 In this study, we investigated a nanodiode composed of a Ag thin film on a TiO2 layer (Figure 1a), verified the formation of a Schottky barrier, and investigated the effect of the silver oxide layer at the metal−semiconductor interface via X-ray photoelectron spectroscopy (XPS) depth profile analysis. We confirmed the optical superiority of Ag by comparing the incident photons to current conversion efficiencies (IPCE) of Ag and Au. Furthermore, we found that the photocurrent and internal photoemission efficiencies of the nanodiodes depend on the thickness and morphology of the Ag layer, which also affect the generation of hot electron flow and surface plasmon effects. 5651
dx.doi.org/10.1021/jp409894b | J. Phys. Chem. C 2014, 118, 5650−5656
The Journal of Physical Chemistry C
Article
window was prepared as the reference. A Nova 230 SEM instrument captured the field emission scanning electron microscope (SEM) images.
3. RESULTS AND DISCUSSION 3.1. Characterization of the Ag/TiO2 Nanodiode. The higher electrical and thermal conductivities of silver result in an enhanced rate of hot electron generation, compared to gold, during light irradiation on the Schottky nanodiode. However, the work function of silver is affected by both the subsurface oxygen and surface oxide layer, which should be considered when measuring the Schottky barrier height in a Ag/TiO2 nanodiode.37,38 A Schottky junction and an ohmic junction were formed on the TiO2 layer with silver and titanium, respectively, as shown in Figure 1a.39 When the active area of the diode is subjected to light irradiation, hot electrons excited from the Ag thin layer can move ballistically into the metal layer and surmount the Schottky barrier formed at the Ag/TiO2 interface if the thickness of the Ag is less than the mean free path of the electrons (Figure 1b). In this study, we increased the thickness of the Ag layer from 10 to 50 nm to confirm the mean free path of the hot electrons and to modify the morphology of the silver surface. Figure 1c shows a graph of the current−voltage (I−V) curves in semilogarithmic scale measured on the Ag/TiO2 nanodiode composed of a 10 nm Ag layer on TiO2, with and without illumination. We confirmed that a rectifying contact formed at the interface of the Ag and TiO2, indicating that a Schottky barrier exists. This rectifying characteristic was preserved regardless of the thickness of the Ag (from 10 to 50 nm). To estimate the Schottky barrier height of the Ag/TiO2 nanodiode, we fit I−V curves of this device to the thermionic emission equation (Figure 1d), exhibiting a barrier height of 0.91 eV, ideality factor of 1.35, and a series resistance of 720 ohm for the diode with a 10 nm silver layer deposited on TiO2. In terms of the Schottky−Mott rule that is valid for abrupt metal-semiconductor contacts, the lower Schottky barrier height is predicted given the value of the work function of Ag (4.26 eV40,41) and the electron affinity of TiO2 (3.9 eV42). The higher Schottky barrier height that is measured experimentally is associated with the presence of interface states and dipoles between the Ag and TiO2, which was welldemonstrated theoretically (via simulations) and experimentally by W. X. Li et al.37 and F. Hossein-Babaei et al.,38,39 respectively. We performed depth profiling using XPS to establish the existence and role of oxygen, which is found in both the absorbed oxygen and surface oxide on the Ag layer, and to quantify the components in the 10 nm thick Ag layer of the Ag/ TiO2 nanodiodes (Figure 2a). The XPS spectra of O 1s, Ti 2p, and Ag 3d, which indicate changes in the quantities of these elements, are shown in Figure S1 of Supporting Information where we increased the etch time incrementally by 10 s. The active area of the nanodiodes was etched using an Ar ion gun (at 2 kV and 1 mA) 7 times from 0 to 60 s. To clearly visualize the change in the quantity of the components at the surface as a function of etch time, we integrated the area of the peaks for Ag 3d, O 1s, and Ti 2p. Figure 2b shows the ratio of the amount of each component on the entire etched surface of the Ag/TiO2 nanodiodes. As we etched the surface of a thin Ag layer, less Ag and more Ti were detected because the TiO2 layer was exposed on the surface. Therefore, we can determine the interface near the cross section where the ratios of Ag and Ti are
Figure 2. XPS depth profile characterization. (a) The spectra measured on a 10 nm Ag thin layer on a TiO2 film were recorded during etching using an Ar ion gun (at 2 kV and 1 mA) 6 times at intervals of 10 s. (b) Composition depth profile of the Ag film on a TiO2 film: Ag 3d (yellow), O 1s (red), and Ti 2p (blue).
approximately identical (i.e., at the point between 20 and 30 s of etch time where the yellow and blue lines meet). We observed high oxygen content at the top of the surface (i.e., focusing on the red line, which indicates the ratio of oxygen). Thus, we attributed the observed substantial Schottky barrier height to the oxygen in the Ag layer. We annealed a diode at 363 K for 30 min under atmospheric conditions to facilitate oxidation of the Ag layer and then conducted the same analysis again (Figure S2 of Supporting Information). We note that the Schottky barrier decreased as the thickness of the Ag layer increased from 10 to 50 nm (Table S1 of Supporting Information). The thickness and corrugation of the Ag layer was confirmed via atomic force microscopy imaging, as shown in Figure S3 of Supporting Information. 3.2. Photocurrent Measurements. Figure 3a shows a graph of the short-circuit photocurrent measured on Ag/TiO2 nanodiodes having several different Ag layer thicknesses. The short-circuit photocurrent of the nanodiode with the thinner deposited Ag layer is higher than that of the thicker Ag layer. For example, the hot electron flow generated from the Ag/TiO2 diode with a 10 nm Ag layer is 2.1 and 2.6 times larger than that generated from the Ag/TiO2 diodes with 20 and 30 nm Ag layers, respectively. As the thickness of the Ag increased to 50 nm, photocurrent rarely flowed. This decrease in current with increasing Ag film thickness is due to enhanced scattering of the hot electrons in the metal within the elastic mean free path, followed by absorption of the photons in the Ag film. The probability of hot electron transfer in the metal layer decays exponentially because of inelastic scattering of the electrons;43 moreover, the probability of hot electron generation is proportional to the intensity of the absorbed light, which obeys Beer−Lambert’s law (∼e−αz).44,45 Because the change in photocurrent, which is dependent on the thickness of Ag, is governed by integration of these two exponential terms, the photocurrent we measured did not fit linearly in the graph using a semilogarithmic scale (inset of Figure 3a). In addition, the roughness of the thinner Ag film induces optical properties different than those of the flat film, which changes the mean free path as a result of surface scattering, high-density grain boundaries, and porosity. However, it is difficult to interpret the difference in photocurrent between the 10 nm Ag layer, which has a connected island structure, and the 20 nm Ag layer, which is a flat film, because of the complex relationship between thickness and photocurrent. These results imply that the decrease in photocurrent with increasing Ag film thickness, especially when the film is thinner than 50 nm, was due to not only the mean free path of the hot electrons but also the surface plasmon effect. This effect can be elucidated more clearly by 5652
dx.doi.org/10.1021/jp409894b | J. Phys. Chem. C 2014, 118, 5650−5656
The Journal of Physical Chemistry C
Article
Figure 3. (a) Plot of a short-circuit photocurrent detected on Ag/TiO2 nanodiodes under illumination from a tungsten-halogen lamp emitting 9 mW/cm2. The photocurrent decreases with increasing Ag film thickness due to the mean free path of the hot electrons, which is close to 50 nm. The inset shows the photocurrent attenuation using a semilogarithmic scale. (b) Solar simulation performance of the Ag/TiO2 nanodiodes as a function of the thickness of the Ag layer under 1.5 AM illumination.
measuring the short-circuit photocurrent as a function of the energy of the light via IPCE measurements, as discussed later. As shown in Figure 3b, solar simulation results under 1.5 AM show a trend similar to the short-circuit photocurrent measurement and other parameters for solar cell performance of this diode (see Figure S4 of Supporting Information). Because silver nanostructures have fascinating optical properties that can scatter light and enhance near-fields an order of magnitude more than gold, utilizing the localized surface plasmon resonance of Ag is effective for conversion of solar energy.46−48 We investigated both Ag- and Au-based photodiodes. By measuring the IPCE, we compared the functionality of both photodiodes. The short-circuit photocurrent produced from a monochromatic photon flux was used to estimate the IPCE on the nanodiodes. The photocurrent of a diode with a 10 nm thick nanostructured Ag film was over 70 times higher than that with a 10 nm thick Au layer (Figure 4). In addition, we obtained three peaks at 2.3, 2.7, and 3.1 eV in the IPCE data shown in Figure 4 for the 10 nm Ag case, whereas the IPCE of the diode with a 10 nm thick Au film increased monotonically without showing any peaks as the photon energy increased. These results imply that the 10 nm Ag thin film has localized surface plasmon resonance, which
improves the conversion efficiency from hot electrons to a steady-state current, contrary to the 10 nm Au case, which confirms that Ag is a better candidate than Au for utilizing the plasmonic effect. The surface plasmon-related peaks of the IPCE for the Ag photodiode can be understood as the photocurrent produced by hot electrons created by decay of plasmons (see the following subsection for more details). A corrugated metallic silver surface helps the surface plasmons decay into energetic hot electrons, which are injected over the Schottky barrier at the Ag/TiO2 interface and directly contribute to the photocurrent. Because the gold surface is relatively smooth, enough to not induce plasmon decay, the resonance peaks of the photocurrent arising from plasmon effects do not appear in the Au/TiO2 nanodiode. Note that photocurrent enhancement assisted by surface plasmons has been reported for corrugated Au films.31 To further clarify the nature of the plasmon-enhanced hot electron flow, we prepared a Ag/TiO2 nanodiode without any treatment and an additional nanodiode heated under 300 sccm Ar at 328 K for 10 min to more clearly elucidate how morphological changes affect surface plasmon resonance. We noticed that a flat Ag film covered the entire TiO2 layer when the Ag thickness was greater than 20 nm and that the morphology of the Ag surface was modified from a flat surface to connected island nanostructures after annealing the diode, as shown in Figures 5b and 5c, respectively. Through IPCE measurements on these nanodiodes after heating, we saw the photocurrent increase and even observed surface plasmon resonance peaks that exactly correspond to those of 10 nm Ag, even though it showed a smaller IPCE value (Figure 5a). However, the IPCE of the diode with a flat, 20 nm thick Ag layer increased with photon energy, showing very weak resonance plasmon peaks at the same energies as those with a rough Ag surface. Therefore, we conclude that a modified, 20 nm thick metal thin film with a connected island structure is expected to behave as a source of surface plasmon resonance (Figure 5c), while a flat, 20 nm thick Ag thin film should show a much weaker effect from surface plasmons (Figure 5b). When we review the previous data, the photocurrent of the diode with a 10 nm thick nanostructured Ag film was over 2 times higher than that with a 20 nm thick Ag layer, a much higher enhancement than when the thickness of the Ag decreased from 30 to 20 nm (Figure 3a). In addition to this, we obtained
Figure 4. IPCE of the Ag/TiO2 nanodiode in comparison with the Au/TiO2 nanodiode as a function of photon energy. The metal layers are equivalent to 10 nm thick in both cases. SEM images of the metal surface of the diodes with 10 nm Ag (upper image) and 10 nm Au (lower image) are shown in the insets. 5653
dx.doi.org/10.1021/jp409894b | J. Phys. Chem. C 2014, 118, 5650−5656
The Journal of Physical Chemistry C
Article
Figure 5. (a) IPCEs of the Ag/TiO2 nanodiodes as a function of photon energy. The Ag thin film was modified in both thickness and morphology. Scanning electron microscope images of the Ag surface in diodes with (b) a 20 nm thick Ag layer without treatment and (c) a 20 nm thick Ag layer after annealing under 300 sccm Ar at 328 K for 10 min. (b) A flat thin film was formed with 20 nm Ag deposited on a titanium oxide layer. (c) A connected island nanostructure is shown on the modified 20 nm Ag surface after heating.
SPR wavelength is tuned by the metal particle size and shape), the dielectric properties of the metal, and the surrounding dielectric medium.47,50−52 It has also been reported that a silver nanoparticle supported on an alumina substrate allows for resonant plasmon absorptions at 2.5, 3.1, and 3.7 eV due to the breaking of symmetry and a nonhomogeneous local field from the substrate.53 Thus, the three resonance peaks in the IPCE are closely related to the surface plasmons in the Ag/TiO2 nanodiode and the observed red-shifted peaks, in comparison with the surface plasmon excitations on the alumina substrate, can be understood by an increase in the dielectric constant from alumina to titanium.54 Enhancement of the photoemission efficiency near surface plasmon frequencies is attributed to hot electrons produced via plasmon decay.55 It is well-known that excited plasmons can decay into electron−hole pairs through electron−electron interactions in the absence of any disorder and/or thermal broadening (i.e., Landau damping).56−58 In general, the Landau damping region is located at large wave vectors, and plasmons within this region can produce hot electrons by Landau damping. The plasmons located outside the Landau damping region (i.e., at small wave vectors) may be damped by other dissipation channels (e.g., disorder scattering, thermal broadening, etc.), which do not produce hot electrons. Thus, to produce hot electrons that contribute to photoemission efficiency, the excited plasmons on the silver surface must have a large wave vector and be located in the Landau damping region. The roughness of the surface is also crucial for the excitation of plasmons with a large wave vector where Landau damping is the dominant dissipation channel. For the corrugated metal surface, the roughness of the surface operates as the effective grating coupler. Consequently, surface plasmons with a wave vector roughly proportional to 1/a, where a is the average size of the silver islands, are excited by coupling to the incident light. Thus, plasmons excited in the rough surface of a metal can have a wave vector high enough for the plasmons to be inside the Landau damping region and decay into electron− hole pairs. On a smooth metal surface, the surface plasmons are excited with a short wave vector (because the average size of the silver island may be very large) and cannot decay directly into electron−hole pairs because they are located outside the Landau damping region. Thus, plasmons on a smooth surface do not produce hot electrons by decay or contribute to the
three peaks in the IPCE data at 2.3, 2.7, and 3.1 eV for the 10 nm Ag case (Figure 4), whereas the diode with 20 nm Ag showed a smooth line with much weaker peaks (Figure 5a). Our results further confirmed that the reason for the increased photocurrent was due to not only the thinner metal layer, which reduced the attenuation of the hot electrons, but also the nanostructured (or corrugated) metal film, which enhanced the surface plasmon behavior at a specific wavelength of light. 3.3. Photocurrent Enhancement Mechanisms by Surface Plasmons. In Figure 5, we show that the nanostructured metal surface functions as a source to amplify the hot electron flow at the resonance of surface plasmons controlled by the size and shape of the nanostructures, whereas a flat thin film has current generated only from hot electrons. According to Fowler’s law,49 the internal photoemission efficiency (η) should comply with the following relationship in the absence of any resonant effects η = c(hν − φ)n /hν
(2)
where c is the Fowler emission coefficient, φ the Schottky barrier energy, and hν the photon energy. Nonzero quantum efficiency starts to appear when the photon energy is equal to the Schottky barrier energy, and the quantum efficiency rises with exponent n = 2 for most metals. When the photon energy increases above φ within the range of the TiO2 band gap (i.e., 0.9 eV < hν < 3.2 eV), as shown in Figure 5, the measured IPCE (i.e., yellow line) concedes with the dotted line (i.e., Fowler’s law) below 3.2 eV, which confirms that the measured photocurrent of the flat metal film arises from the hot electrons. At high photon energies (>3.2 eV), a steep increase is seen in the IPCE data and a significant contribution is due to electron− hole pair generation from the TiO2 combining with the hot electron flow. However, the photoemission efficiency for samples with a rough surface as a result of heating behaves differently from that for samples with a smooth surface. As shown in Figure 5, the three resonance peaks below 3.2 eV appear for the samples with a rough surface and obviously deviate from Fowler’s law. These resonance peaks may be attributed to localized surface plasmon resonance rather than typical internal emission or electron−hole pairs from the semiconductor. Surface plasmon resonance (SPR) of silver nanostructures has been explored by controlling the shape (either via chemical synthesis or nanofabrication because the 5654
dx.doi.org/10.1021/jp409894b | J. Phys. Chem. C 2014, 118, 5650−5656
The Journal of Physical Chemistry C
Article
(3) Gadzuk, J. On the Detection of Chemically-Induced Hot Electrons in Surface Processes: From X-ray Edges to Schottky Barriers. J. Phys. Chem. B 2002, 106 (33), 8265−8270. (4) Park, J. Y.; Renzas, J.; Hsu, B. B.; Somorjai, G. A. Interfacial and Chemical Properties of Pt/TiO2, Pd/TiO2, and Pt/GaN Catalytic Nanodiodes Influencing Hot Electron Flow. J. Phys. Chem. C 2007, 111 (42), 15331−15336. (5) Nienhaus, H.; Bergh, H.; Gergen, B.; Majumdar, A.; Weinberg, W.; McFarland, E. Electron-Hole Pair Creation at Ag and Cu Surfaces by Adsorption of Atomic Hydrogen and Deuterium. Phys. Rev. Lett. 1999, 82 (2), 446−449. (6) Hasselbrink, E. Non-Adiabaticity in Surface Chemical Reactions. Surf. Sci. 2009, 603 (10), 1564−1570. (7) Karpov, E.; Nedrygailov, I. Solid-State Electric Generator Based on Chemically Induced Internal Electron Emission in MetalSemiconductor Heterojunction Nanostructures. Appl. Phys. Lett. 2009, 94 (21), 214101−214101-3. (8) Stella, K.; Kovacs, D. A.; Diesing, D.; Brezna, W.; Smoliner, J. Charge Transport Through Thin Amorphous Titanium and Tantalum Oxide Layers. J. Electrochem. Soc. 2011, 158 (5), P65−P74. (9) Park, J. Y.; Somorjai, G. A. Energy Conversion from Catalytic Reaction to Hot Electron Current with Metal-Semiconductor Schottky Nanodiodes. J. Vac. Sci. Technol., B: Microelectron. Nanometer Struct.− Process., Meas., Phenom. 2006, 24 (4), 1967−1971. (10) 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 (46), 16589−16605. (11) Park, J. Y.; Lee, H.; Renzas, J. R.; Zhang, Y.; Somorjai, G. A. Probing Hot Electron Flow Generated on Pt Nanoparticles with Au/ TiO2 Schottky Diodes during Catalytic CO Oxidation. Nano Lett. 2008, 8 (8), 2388−2392. (12) Karpov, E. G.; Hashemian, M. A.; Dasari, S. K. ChemistryDriven Signal Transduction in a Mesoporous Pt/TiO2 System. J. Phys. Chem. C 2013, 117 (30), 15632−15638. (13) Karpov, E. G.; Nedrygailov, I. Nonadiabatic Chemical-toElectrical Energy Conversion in Heterojunction Nanostructures. Phys. Rev. B 2010, 81 (20), 205443. (14) McFarland, E. W.; Tang, J. A Photovoltaic Device Structure Based on Internal Electron Emission. Nature 2003, 421 (6923), 616− 618. (15) Prietsch, M. Ballistic-Electron Emission Microscopy (BEEM): Studies of Metal/Semiconductor Interfaces with Nanometer Resolution. Phys. Rep. 1995, 253 (4), 163−233. (16) Frese, K. W.; Chen, C. Theoretical Models of Hot Carrier Effects at Metal-Semiconductor Electrodes. J. Electrochem. Soc. 1992, 139 (11), 3234−3243. (17) Peters, D. An Infrared Detector Utilizing Internal Photoemission. Proc. IEEE 1967, 55 (5), 704−705. (18) Scales, C.; Breukelaar, I.; Berini, P. Surface-Plasmon Schottky Contact Detector Based on a Symmetric Metal Stripe in Silicon. Opt. Lett. 2010, 35 (4), 529−531. (19) Scales, C.; Berini, P. Thin-Film Schottky Barrier Photodetector Models. IEEE J. Quantum Electron. 2010, 46 (5), 633−643. (20) Knight, M. W.; Sobhani, H.; Nordlander, P.; Halas, N. J. Photodetection with Active Optical Antennas. Science 2011, 332 (6030), 702−704. (21) Bohren, C. F.; Huffman, D. R. Absorption and Scattering of Light by Small Particles. Wiley-Vch: New York, 2008. (22) Takahashi, Y.; Tatsuma, T. Solid State Photovoltaic Cells Based on Localized Surface Plasmon-Induced Charge Separation. Appl. Phys. Lett. 2011, 99 (18), 182110−182110-3. (23) Mayer, K. M.; Hafner, J. H. Localized Surface Plasmon Resonance Sensors. Chem. Rev. 2011, 111 (6), 3828. (24) Anker, J. N.; Hall, W. P.; Lyandres, O.; Shah, N. C.; Zhao, J.; Van Duyne, R. P. Biosensing with Plasmonic Nanosensors. Nat. Mater. 2008, 7 (6), 442−453. (25) Atwater, H. A.; Polman, A. Plasmonics for Improved Photovoltaic Devices. Nat. Mater. 2010, 9 (3), 205−213.
photocurrent. We note that the available phase space to produce electron−hole pairs is allowed only at large wave vectors (e.g., in the case of a rough surface) and severely restricted at short wave vectors (e.g., in the case of a smooth surface). If the energy of the optically excited surface plasmons exceeds the Schottky barrier, the decay of the plasmons should contribute to the emission of photoelectrons. Thus, the roughness-aided optical coupling to surface plasmons and the consequent decay to electron−hole pairs enhance the photoemission efficiency.
4. CONCLUSION We have investigated plasmon-induced internal photoemission in Ag/TiO2 nanodiodes and its dependence on the thickness and morphology of the metal film using XPS depth profile and IPCE measurements and by fitting the I−V curve to the thermionic emission equation. The results presented here demonstrate that the silver oxide layer at the interface plays an important role in forming and controlling the Schottky barrier. By measuring the IPCEs of Au/TiO2 and Ag/TiO2 nanodiodes, we confirmed that Ag is a much more effective than Au for enhancing the plasmonic effect on hot electrons. The internal photoemission efficiency of Ag/TiO2 nanodiodes is influenced simultaneously by the thickness and morphology of the metal layer, which are related to the attenuation length of hot electrons and the plasmonic effect, respectively. These two factors affecting internal photoemission are considered separately through the IPCE; the surface plasmon resonance wavelengths of the Ag thin film, which involve one quadrupole and two dipole modes, are confirmed. These results propose that plasmon resonance is an effective way to improve photocurrent and internal photoemission.
■
ASSOCIATED CONTENT
S Supporting Information *
X-ray photoelectron spectroscopy depth profile characterization (Figures S1 and S2) of the Ag/TiO2 nanodiodes, an atomic force microscopy image and line profile (Figure S3), and solar simulator performance (Figure S4). This material is available free of charge via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work was supported by Institute for Basic Science (IBS) [CA1401-04], Republic of Korea. E.H. acknowledges support from Basic Science Research Program through the National Research Foundation of Korea Grant funded by the Ministry of Science, ICT & Future Planning (2009-0083540).
■
REFERENCES
(1) Nienhaus, H. Electronic Excitations by Chemical Reactions on Metal Surfaces. Surf. Sci. Rep. 2002, 45 (1), 1−78. (2) Park, J. Y.; Somorjai, G. A. The Catalytic Nanodiode: Detecting Continous Electron Flow at Oxide−Metal Interfaces Generated by a Gas-Phase Exothermic Reaction. ChemPhysChem 2006, 7 (7), 1409− 1413. 5655
dx.doi.org/10.1021/jp409894b | J. Phys. Chem. C 2014, 118, 5650−5656
The Journal of Physical Chemistry C
Article
Silver Nanodots and Nanorods. J. Phys. Chem. B 1999, 103 (40), 8410−8426. (49) Fowler, R. H. The Analysis of Photoelectric Sensitivity Curves for Clean Metals at Various Temperatures. Phys. Rev. 1931, 38 (1), 45. (50) Haynes, C. L.; Van Duyne, R. P. Nanosphere Lithography: A Versatile Nanofabrication Tool for Studies of Size-Dependent Nanoparticle Optics. J. Phys. Chem. B 2001, 105 (24), 5599−5611. (51) Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C. The Optical Properties of Metal Nanoparticles: The Influence of Size, Shape, and Dielectric Environment. J. Phys. Chem. B 2003, 107 (3), 668−677. (52) Wang, F.; Shen, Y. R. General Properties of Local Plasmons in Metal Nanostructures. Phys. Rev. Lett. 2006, 97 (20), 206806. (53) Lazzari, R.; Roux, S.; Simonsen, I.; Jupille, J.; Bedeaux, D.; Vlieger, J. Multipolar Plasmon Resonances in Supported Silver Particles: The case of Ag/α-Al2O3(0001). Phys. Rev. B 2002, 65 (23), 235424. (54) Xu, G.; Tazawa, M.; Jin, P.; Nakao, S. Surface Plasmon Resonance of Sputtered Ag Films: Substrate and Mass Thickness Dependence. Appl. Phys. A: Mater. Sci. Process. 2005, 80 (7), 1535− 1540. (55) Hofmann, J.; Steinmann, W. Plasma Resonance in the Photoemission of Silver. Phys. Status Solidi B 1968, 30 (1), K53−K56. (56) Endriz, J.; Spicer, W. Surface-Plasmon-One-Electron Decay and Its Observation in Photoemission. Phys. Rev. Lett. 1970, 24, 64−68. (57) Fetter, A. L.; Walecka, J. D. Quantum Theory of Many-Particle Systems; Courier Dover Publications: Mineola, NY, 2003. (58) Raether, H. Surface Plasmons on Smooth Surfaces. In Surface Plasmons on Smooth and Rough Surfaces and on Gratings; Springer: Berlin, 1988.
(26) Kowalska, E.; Mahaney, O. O. P.; Abe, R.; Ohtani, B. VisibleLight-Induced Photocatalysis through Surface Plasmon Excitation of Gold on Titania Surfaces. Phys. Chem. Chem. Phys. 2010, 12 (10), 2344−2355. (27) Christopher, P.; Xin, H.; Linic, S. Visible-Light-Enhanced Catalytic Oxidation Reactions on Plasmonic Silver Nanostructures. Nat. Chem. 2011, 3 (6), 467−472. (28) Tian, Y.; Tatsuma, T. Mechanisms and Applications of PlasmonInduced Charge Separation at TiO2 Films Loaded with Gold Nanoparticles. J. Am. Chem. Soc. 2005, 127 (20), 7632−7637. (29) Zorić, I.; Zäch, M.; Kasemo, B.; Langhammer, C. Gold, Platinum, and Aluminum Nanodisk Plasmons: Material Independence, Subradiance, and Damping Mechanisms. ACS Nano 2011, 5 (4), 2535−2546. (30) 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 (48), 14852−14853. (31) Lee, Y. K.; Jung, C. H.; Park, J.; Seo, H.; Somorjai, G. A.; Park, J. Y. Surface Plasmon-Driven Hot Electron Flow Probed with MetalSemiconductor Nanodiodes. Nano Lett. 2011, 11 (10), 4251−4255. (32) Feibelman, P. J. Surface Electromagnetic Fields. Prog. Surf. Sci. 1982, 12 (4), 287−407. (33) Levinson, H. J.; Plummer, E.; Feibelman, P. J. Effects on Photoemission of the Spatially Varying Photon Field at a Metal Surface. Phys. Rev. Lett. 1979, 43 (13), 952. (34) Wang, Y.; Plummer, E.; Kempa, K. Foundations of Plasmonics. Adv. Phys. 2011, 60 (5), 799−898. (35) Aballe, L.; Rogero, C.; Horn, K. Quantum-Size Effects in Ultrathin Mg Films: Electronic Structure and Collective Excitations. Surf. Sci. 2002, 518 (1), 141−154. (36) Becker, F.; Krix, D.; Hagemann, U.; Nienhaus, H. Internal Detection of Surface Plasmon Coupled Chemiluminescence during Chlorination of Potassium Thin Films. J. Chem. Phys. 2013, 138 (3), 034710. (37) Li, W.-X.; Stampfl, C.; Scheffler, M. Subsurface Oxygen and Surface Oxide Formation at Ag (111): A Density-Functional Theory Investigation. Phys. Rev. B 2003, 67 (4), 045408. (38) Hossein-Babaei, F.; Abbaszadeh, S.; Esfahani, M. S. Gas Sensitive Porous Silver-Rutile High-Temperature Schottky Diode on Thermally Oxidized Titanium. IEEE Sens. J. 2009, 9 (3), 237−243. (39) Hossein-Babaei, F.; Rahbarpour, S. Titanium and Silver Contacts on Thermally Oxidized Titanium Chip: Electrical and Gas Sensing Properties. Solid-State Electron. 2011, 56 (1), 185−190. (40) Michaelson, H. B. The Work Function of the Elements and Its Periodicity. J. Appl. Phys. 1977, 48 (11), 4729−4733. (41) Skriver, H. L.; Rosengaard, N. Surface Energy and Work Function of Elemental Metals. Phys. Rev. B 1992, 46 (11), 7157. (42) Rothenberger, G.; Fitzmaurice, D.; Graetzel, M. Spectroscopy of Conduction Band Electrons in Transparent Metal Oxide Semiconductor Films: Optical Determination of the Flatband Potential of Colloidal Titanium Dioxide Films. J. Phys. Chem. 1992, 96 (14), 5983−5986. (43) Glass, S.; Nienhaus, H. Continuous Monitoring of Mg Oxidation by Internal Exoemission. Phys. Rev. Lett. 2004, 93 (16), 168302. (44) Crowell, C.; Spitzer, W.; Howarth, L.; LaBate, E. Attenuation Length Measurements of Hot Electrons in Metal Films. Phys. Rev. 1962, 127 (6), 2006. (45) Fox, A. M. Optical Properties of Solids; Oxford University Press: Oxford, U.K.,2001; Vol. 3. (46) Benjamin, J.; Im, S. H.; Li, Z.-Y.; McLellan, J.; Siekkinen, A.; Xia, Y. Maneuvering the Surface Plasmon Resonance of Silver Nanostructures through Shape-Controlled Synthesis. J. Phys. Chem. B 2006, 110 (32), 15666−15675. (47) Benjamin, J.; Chen, Y.; McLellan, J. M.; Xiong, Y.; Li, Z.-Y.; Ginger, D.; Xia, Y. Synthesis and Optical Properties of Silver Nanobars and Nanorice. Nano Lett. 2007, 7 (4), 1032−1036. (48) Link, S.; El-Sayed, M. A. Spectral Properties and Relaxation Dynamics of Surface Plasmon Electronic Oscillations in Gold and 5656
dx.doi.org/10.1021/jp409894b | J. Phys. Chem. C 2014, 118, 5650−5656
