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Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 5081−5089

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Enhancement of Hot Electron Flow in Plasmonic Nanodiodes by Incorporating PbS Quantum Dots Changhwan Lee,†,‡ Hyekyoung Choi,§,∥ Ievgen I. Nedrygailov,‡ Young Keun Lee,†,‡ Sohee Jeong,*,§,∥ 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 305-701, Republic of Korea § Nanomechanical Systems Research Division, Korea Institute of Machinery and Materials, Daejeon 305-343, Republic of Korea ∥ Department of Nanomechatronics, Korea University of Science and Technology (UST), Daejeon 305-350, Republic of Korea ‡

S Supporting Information *

ABSTRACT: The enhancement of hot electron generation using plasmonic nanostructures is a promising strategy for developing photovoltaic devices. Here, we show that hot electron flow generated in plasmonic Au/TiO2 nanodiodes by incident light can be amplified when PbS quantum dots are deposited onto the surface of the nanodiodes. The effect is attributed to efficient extraction of hot electrons via a threedimensional Schottky barrier, thus giving new pathways for hot electron transfer. We also demonstrate a correlation between the photocurrent and Schottky barrier height when using PbS quantum dots with varying size and ligand treatments that allow us to control the electric properties (e.g., band gap and Fermi level, respectively) of the PbS quantum dots. This simple method introduces a new technique for further improving the power conversion efficiency of thin-film photovoltaic devices. KEYWORDS: hot electrons, plasmonic nanodiode, PbS quantum dots, three-dimensional Schottky barrier, momentum of hot electrons

1. INTRODUCTION The use of metal nanostructures has been explored to increase the solar energy conversion in photovoltaic and photocatalytic devices.1−4 Recently, direct conversion of hot electron flow generated in plasmonic nanostructures into electricity has been proposed as a promising method for achieving high performance.5−9 These energetic electron flows can be enhanced by localized surface plasmon resonance (LSPR), which is the collective electron charge oscillation at the same frequency as that of the incident light that produces a highly concentrated electromagnetic field.10,11 Following excitation, a surface plasmon can decay nonradiatively and transfer energy to electrons in the metal nanostructure, eventually generating hot electrons. These highly excited electrons can be collected via ballistic transport through a Schottky diode formed by contacting a metal and a semiconductor or a tunneling diode composed of metal−insulator−metal contacts, which results in the generation of a steady-state current.9,12−17 In these structures, the potential barriers operate as an energy filter that allows the separation of ground-state and excited electrons.7,18 Hot electrons lose energy via thermalization while moving to the metal−semiconductor interface; only hot electrons with high enough energy can be injected into the conduction band of the semiconductor. Another important point that should be considered is that hot electrons are generated by light absorption and that light can penetrate the © 2018 American Chemical Society

metallic bulk. Once a hot electron is generated, the thickness of the metal should be less than the mean free path of the electron to maximize the probability that the hot electron reaches the interface before thermalization. Therefore, to address the characteristic length scale of hot electron transport, we must consider both factors: the penetration depth of photons and the mean free path of hot electrons.19−21 For efficient hot electron collection, the momentum of the hot electrons must be considered in addition to the incident photon energy and the metal thickness.22−25 In general, the momentum of hot electrons generated in a plasmonic nanostructure is highly directional along the electric field formed perpendicular to the incident light (Figure 1a). However, conventional Schottky nanodiodes based on a planar metal−semiconductor contact have a limitation: the metal− semiconductor interface is parallel to the electric field when a planar Schottky nanodiode is exposed to light with a normal incidence angle. Most of the hot electrons generated in the metal propagate parallel to the Schottky barrier and cannot be captured efficiently, thus resulting in a low probability for electron transport into the semiconductor. To overcome this restriction, three-dimensional (3D) Schottky barrier structures Received: November 4, 2017 Accepted: January 8, 2018 Published: January 8, 2018 5081

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Figure 1. Three-dimensional Schottky nanodiode. (a) A comparison of hot electron collection in metal−semiconductor nanostructures with a planar and a 3D Schottky barrier. (b) Schematic illustration of a typical plasmonic Au/TiO2 nanodiode covered with PbS quantum dots. (c) Scanning transmission electron microscopy and high-angle annular dark field image of the cross-section of PbS quantum dots deposited on the plasmonic Au/ TiO2 nanostructure. (d) Energy-dispersive X-ray spectroscopy elemental mapping image of the cross-section of PbS quantum dots deposited on the plasmonic Au/TiO2 nanostructure. Specifically, 1.2 g of oleic acid (OA), 0.46 g of lead oxide (PbO) and 10 mL of 1-octadecene (ODE) was degassed in three-neck flask at 120 °C for 2 h. The solution turned transparent. Hereafter, the flasks were removed from the heating mantle and changed under nitrogen atmosphere and allowed to cool down the solution to required temperature 70 °C and 120 °C for PbS quantum dots with 2.6 nm and 3.4 nm, respectively. 210 μL of bis(trimethylsilyl)sulfide (TMS2 sulfide) in 4 mL of ODE was loaded into a 12 mL of syringe and then rapidly injected into the precursor solution. PbS quantum dots were isolated from the growth mixture by centrifugation with a polar solvent such as acetone. The resulting precipitate was redispersed in hexane. The purification was repeated two times to remove surfactant residual reaction debris. Final isolated PbS quantum dots were dispersed in octane for further device fabrication. 2.2. Fabrication of Plasmonic Au/TiO2 Nanodiodes. The metal−semiconductor Au/TiO2 nanodiodes were fabricated as follows. First, a 150 nm thick Ti film was deposited on a 500 nm thick SiO2 layer grown on a p-type Si (100) wafer by wet oxidation. The process was carried out using a 4 × 7 mm2 stainless steel mask by electron beam evaporation under vacuum of 2 × 10−6 Torr, with a deposition rate of 0.5 Å/s. The wafer was then annealed in air at 500 °C for 2 h to oxidize the Ti film to a TiO2 film. Next, two contact pads composed of a 50 nm thick Ti film and a 150 nm thick Au film were deposited using a 5 × 5 mm2 mask by electron beam evaporation under a vacuum of 2

have been proposed via embedding or using a cavity structure that has vertical interfaces (Figure 1a).23,26,27 In this study, we propose a new concept for an advanced 3D Schottky barrier plasmonic nanodiode, in which PbS quantum dots are deposited directly onto the plasmonic Au/TiO2 structure. In this structure, a perpendicular Schottky barrier can be formed by contacting the sides of the plasmonic Au nanostructures and the PbS quantum dots in addition to the planar Schottky barrier between the plasmonic Au nanostructures and the TiO2. We show that the combined action of the plasmonic Au nanostructure and the PbS quantum dots results in a 5-fold increase in the photocurrent. Furthermore, using a variety of sizes of PbS quantum dots with different ligands, we investigate the effect of the electrical characteristics of the PbS quantum dots on the Schottky barrier height formed at the interface between the plasmonic Au and the PbS quantum dots.

2. EXPERIMENTAL SECTION 2.1. Synthesis of PbS Quantum Dots. We prepared PbS quantum dots with 2.6 nm and 3.5 nm in diameter following previously reported method with slight modifications.28 All manipulations were performed using the standard Schlenk line techniques. 5082

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Figure 2. Enhancement of hot electron flow in PbS quantum dots deposited on the plasmonic Au/TiO2 nanostructure. (a) Short-circuit photocurrents measured on the thin film and on the plasmonic Au/TiO2 nanodiodes covered with 1−5 layers of 2.6 nm PbS quantum dots. (b) IPCE measured on the thin-film Au/TiO2, plasmonic Au/TiO2, and PbS quantum dots deposited on the plasmonic Au/TiO2 nanodiode. (c) A comparison of the transport of hot electrons generated in the Au/TiO2 and in PbS quantum dots deposited on the Au/TiO2 nanostructure under illumination by light incident normal to the surface. (d) Energy band diagram of the PbS quantum dots deposited on the plasmonic Au/TiO2 nanostructure. × 10−6 Torr with a deposition rate of 1.0 Å/s. One of the pads was deposited onto the TiO2 surface and used as an ohmic contact. The other pad was deposited onto the SiO2 wafer to make a terminal for connecting to the Schottky contact. To form the Schottky contact, a 10 nm thin Au film was deposited in the space between the TiO2 and Ti/Au pad by means of a 2 × 10 mm2 mask using electron beam evaporation under vacuum of 2 × 10−6 Torr, with a deposition rate of 0.2 Å/s. Finally, to transform the 10 nm Au film into a plasmonic Au nanostructure, the wafer was annealed in air at 200 °C for 1 h in a furnace. 2.3. Deposition of PbS Quantum Dots on the Nanodiode Surface. PbS quantum dots (0.2 mg/mL) dispersed in octane were deposited onto the surface of plasmonic Au/TiO2 nanodiodes using a layer-by-layer spin-coating method (2500 rpm) that was followed by treatment with 1,2-ethanedithiol (EDT) in acetonitrile (10 mM) and tetrabutylammonium iodide (TBAI) in MeOH (10 mM) to exchange the native ligand (oleic acid). Each layer was then rinsed with acetonitrile and octane while spinning at 2500 rpm. This coating/ ligand exchange/rinse cycle was repeated to obtain the desired thickness of PbS quantum dot film. After deposition, the samples were annealed at 65 °C for 5 min. 2.4. Characterization. The short-circuit photocurrent and current−voltage curves were measured on the fabricated plasmonic Au/TiO2 nanodiodes in air using a source meter (Keithley Instrumentation 2400). A tungsten halogen lamp emitting light with an intensity of 9 mW/cm2 was used as the light source, with the incident light perpendicular to the nanodiode. The incident photon-tocurrent conversion efficiency (IPCE) was measured from 400 to 1100 nm using a tunable xenon arc lamp source (Newport, TLS-300XU) in air. An ultraviolet−visible−near-infrared spectrometer (Shimadzu,

UV3600) was used to characterize the optical properties of the PbS quantum dots dissolved in tetrachloroethylene and deposited on the plasmonic Au nanostructures on a 1/16 in. thick quartz window. Another clean quartz window with the same properties and size was used as a reference. The ultraviolet photoelectron spectroscopy measurements were performed using a hemispherical electron energy analyzer (SES-100, VG-Scienta) with a charge-coupled device camera. The ultraviolet photoelectron spectroscopy measurement used a He I (ℏω = 21.22 eV) gas discharge lamp as an excitation source with a sample bias of −10 V for the secondary electron cutoff region.

3. RESULTS AND DISCUSSION 3.1. PbS Quantum Dots on a Thin-Film Gold versus Plasmonic Gold. A scheme for the plasmonic Au/TiO2 nanodiodes covered with several layers of PbS quantum dots is shown in Figure 1b. The plasmonic Au/TiO2 nanodiodes can be fabricated by annealing thin-film Au/TiO2 nanodiodes in air.9 This process transforms the thin-film Au into a random array of electrically connected nanoparticles, also known as connected gold island nanostructures (Figure S1) that show strong LSPR excitation. After annealing, PbS quantum dots were deposited directly onto the surface of the plasmonic Au nanostructures. For efficient electronic transport in the PbS quantum dots, the ligands are changed from oleic acid with long alkyl chains to the short EDT ligand using a layer-by-layer spin-coating method.29 To confirm the morphology of the fabricated nanostructures, cross-sectional scanning transmission electron microscopy and high-angle annular dark field images 5083

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becomes more pronounced as the thickness of the PbS quantum dot coating increases. To understand the processes underlying the observed enhancement of photocurrent, let us consider the changes in morphology of the surface of the Au/TiO2 nanodiodes after depositing the PbS quantum dots. Figure 2c shows a comparison of hot electron mechanisms in the plasmonic Au/TiO2 nanostructures both with and without PbS quantum dots. Planar plasmonic Au/TiO2 nanostructures without PbS quantum dots only allow one-dimensional collection of hot electrons through the bottom interface of the metal−semiconductor contact, resulting in low photocurrent generation efficiency. On the other hand, deposition of PbS quantum dots onto the plasmonic Au/TiO2 nanostructure forms a new pathway for hot electron transfer. In this case, hot electrons can transfer to the TiO2 through the 3D interface. The energy band diagram for PbS quantum dots deposited on the plasmonic Au/ TiO2 nanostructure is determined by the electrical properties of each material (Figure 2d). The PbS quantum dots show p-type characteristics, which were observed in the ultraviolet photoelectron spectroscopy and absorbance spectra (Figure S4). We know that a Schottky barrier was formed at the interface between the Au and the PbS quantum dots, and that a p−n junction was formed at the contact between the PbS quantum dots and the TiO2. On the basis of the energy band diagram shown in Figure 2d, the increases in IPCE at 1.7, 2.0, and 2.6− 3.0 eV can be explained. As described above, LSPR was induced in the direction of the electric field formed in the plane perpendicular to the incident light. Therefore, hot electrons enhanced by LSPR excitation essentially have momentum toward the Schottky barrier at the Au/PbS quantum dot interface. Thus, these charge carriers can be transported to the conduction band of the TiO2, leading to an increase in IPCE at 2.0 eV. The increase in IPCE at 2.6−3.0 eV, in turn, can be ascribed to interband electron transitions in the Au nanostructures. As reported elsewhere, the interband absorption of light in gold nanoparticles is caused by the transition of an electron from the filled d-band to one of the empty energy levels in the conduction band above the Fermi level. Because the d-band in gold is located approximately 2.4 eV below the Fermi level, the number of photoexcited hot electrons created by the interband transition exceeds the number created via LSPR excitation at photon energies higher than 2.4 eV.32,33 However, the contribution of these electrons to the net photocurrent measured from the plasmonic Au/TiO2 nanodiodes is minor,31,34 which is clearly demonstrated in Figure 2b. Interestingly, deposition of PbS quantum dots onto the plasmonic Au/TiO2 nanostructure results in an increase in IPCE in the interband transition range. This implies that hot electrons with a lower energy (i.e., caused by interband transition) can transport to the TiO2 through the PbS quantum dot layer. Because the thickness of the PbS quantum dots is very thin (i.e., 15 nm), band bending at the interface between the plasmonic Au and the PbS quantum dots is very narrow and sharp and is similar to potential barriers formed in tunnel junctions. Such a barrier can be easily overcome by hot electrons excited from the d-band of the Au nanostructures via tunneling or hopping processes. Thus, the IPCE value increases when the photon energy is 2.6−3.0 eV (Figure 2d). Finally, the new peak at 1.7 eV can be attributed to photocurrent generation via light absorption by the quantum dots themselves. The position of this peak agrees well with the band gap of the quantum dots, which was measured from the

and energy-dispersive X-ray spectroscopy elemental mapping images were taken of the plasmonic Au/TiO2 nanodiodes with three layers of PbS quantum dots (Figure 1c,d). The PbS quantum dots were primarily deposited on the exposed TiO2 surface, filling the voids in the Au nanostructures. The thickness of the PbS quantum dot film was 10 ± 2 nm. Deposition of five layers of PbS quantum dots creates a continuous film that covers both the Au and TiO2 surfaces (Figure S3). Figure 2a shows a plot of the short-circuit photocurrent measured on the plasmonic Au/TiO2 nanodiodes with and without 2.6 nm PbS quantum dots under illumination with light (9 mW/cm2 intensity). For comparison, the photocurrent measured on the thin-film Au/TiO2 nanodiodes is also shown. Prior to deposition of the PbS quantum dots, the short-circuit photocurrent generated by the plasmonic Au/TiO2 nanodiodes is significantly higher than that measured on the nanodiodes with thin-film Au. As reported elsewhere,9,12 this shows an effect caused by the presence of LSPR excitation of the Au. Interestingly, after depositing the first layer of PbS quantum dots, the short-circuit photocurrent increases notably by about 60%. Further deposition of quantum dots leads to an even more noticeable amplification of the photocurrent. Finally, after depositing five layers of quantum dots, the photocurrent is approximately 3 times larger than that of the plasmonic nanodiodes without quantum dots. It is worth noting that the deposition of PbS quantum dots on the thin-film Au/TiO2 nanodiodes does not lead to any significant change in the photocurrent. The newly formed interface between the thinfilm Au and the PbS quantum dots is parallel to the highly directional momentum of the hot electrons, and thus this interface does not make a large contribution to capturing hot electrons. To find a correlation between photon energy and the photocurrent enhancement caused by the deposition of PbS quantum dots on the surface of plasmonic Au/TiO2 nanodiodes, the IPCE was obtained using eq 1, where h is the Planck constant, c is the speed of light, and λ is the wavelength. IPCE =

hc ⎡ photocurrent density (μ A/cm ⎢ λ⎣ light power (μ W/cm 2)

)⎤ ⎥ ⎦

2

(1)

Figure 2b shows a plot of the IPCE as a function of photon energy measured on the plasmonic Au/TiO2 nanodiode with 2.6 nm PbS quantum dots. For comparison, the IPCE measured on the thin-film Au/TiO2 and plasmonic Au/TiO2 nanodiodes without PbS quantum dots are also shown. On the thin-film Au/TiO2 nanodiode, the IPCE increases monotonically as the photon energy increases. The photocurrent in this case originates from internal photoemission of hot electrons (i.e., generated in the Au thin film by incident light) over the Schottky barrier formed at the metal−semiconductor interface. The sharp rise in the IPCE at higher photon energies (i.e., hν ≥ 3.0 eV) is caused by band-to-band excitation of electron−hole pairs when light is absorbed in the TiO2. The plasmonic Au/ TiO2 nanodiodes, in turn, show a well-defined IPCE peak at a photon energy of 2.0 eV due to LSPR excitation.9,12,30 In addition to the plasmonic peak, a slight increase in IPCE can be observed for photon energies of 2.6−3.0 eV. The increase in photocurrent in this energy range is attributed to electronic excitation by interband absorption of light in the Au nanostructures.2,21,31 Deposition of PbS quantum dots onto the surface of the plasmonic Au/TiO2 nanodiodes leads to a notable increase in the IPCE at 1.7, 2.0, and 2.7 eV. The effect 5084

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Figure 3. Size effect of PbS quantum dots on photocurrent generation. (a) Absorbance spectra measured on 2.6 and 3.5 nm PbS quantum dots. Inset shows the band gaps of the 2.6 and 3.5 nm PbS quantum dots. (b) A comparison of the photocurrents measured on the plasmonic Au/TiO2 nanodiodes covered with 2.6 and 3.5 nm PbS quantum dots. (c) IPCE measured on thin-film and plasmonic Au/TiO2 nanodiodes covered with 3.5 nm PbS quantum dots. The increase in IPCE is classified into four steps depending on the process of hot electron transfer. (d) Schematic representation of Schottky barrier height reduction caused by increasing the size of the PbS quantum dots from 2.6 to 3.5 nm.

peak shifts from 1.7 to 1.4 eV when the quantum dot size increased. Such behavior is clear evidence of band gap reduction in the PbS quantum dots with increasing size. Figure 3b shows a comparison of the short-circuit photocurrents measured on the plasmonic Au/TiO2 nanodiodes with films composed of 2.6 and 3.5 nm PbS quantum dots. The nanodiodes covered with 3.5 nm PbS quantum dots generate larger photocurrents. Thus, after depositing five layers of quantum dots, the nanodiodes covered with 3.5 nm quantum dots have a photocurrent that is approximately 1.6 times larger than that measured on the nanodiodes covered with 2.6 nm quantum dots. From the IPCE results, the enhancement mechanism can be analyzed through the transfer process for electrons generated by each photon. As shown in Figure 3c, which is measured on plasmonic Au/TiO2 coated with 3.5 nm PbS quantum dots, the increase in IPCE can be separated into four parts: (1) Enhancement of the IPCE from the thin-film Au to plasmonic Au at 1.8 eV is from the detection of hot electrons amplified by LSPR excitation through the planar Schottky interface of the Au/TiO2. (2) Enhancement of the IPCE at 1.8 eV after the deposition of PbS quantum dots onto the plasmonic Au nanostructure is caused by two types of electron transfer processes. The first process is the additional extraction of hot electrons generated in the Au by LSPR excitation via the side Schottky barrier formed at the Au/PbS quantum dot interface. Before depositing the PbS quantum dots, hot electrons were detected only through the planar Schottky interface of the Au/ TiO2, resulting in a low energy conversion efficiency. The second process is from electrons excited in the PbS quantum dots by photon energy higher than the band gap of the

absorbance spectrum for the 2.6 nm PbS quantum dots in an aqueous solution (Figure S4c). Furthermore, it is important that the band alignment promotes the transport of hot electrons generated in the PbS quantum dots. The conduction band offset formed at the interface between the Au and the PbS quantum dots provides a driving force for the transport of hot electrons from the PbS quantum dots to the TiO2, leading to more efficient collection of the photoexcited charge carriers. Electrons excited in the PbS quantum dots by photon energy higher than its band gap (1.7 eV) would also contribute slightly to the increase in IPCE. Thus, PbS quantum dots play a dual role in photocurrent amplification: they form a 3D Schottky barrier that leads to more efficient capture of hot electrons created in the Au nanostructures by incident light, and the PbS quantum dots themselves can effectively absorb light and create hot electrons that are then transferred to the TiO2. Altogether, these effects lead to the amplification of photocurrent shown in Figure 2a. 3.2. Size Effect of PbS Quantum Dots: Energy Band Gap Control. The energy band gap in quantum dots can change significantly by changing the size of the dots because of quantum confinement.35,36 This provides an easy way to control the optical properties of quantum dots that can be utilized when manufacturing multijunction photovoltaic devices, light-emitting diodes, etc. To study the effect of PbS quantum dot size on the performance of the plasmonic Au/ TiO2 nanodiodes, we performed a series of measurements using samples containing 2.6 and 3.5 nm PbS quantum dots. Figure 3a shows the optical absorbance spectra measured on the 2.6 and 3.5 nm PbS quantum dots dispersed in a liquid solution. The photon energy corresponding to the absorbance 5085

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ACS Applied Materials & Interfaces quantum dots. In this case, the excited electrons are amplified by increasing the absorption of light in the PbS quantum dots by the electromagnetic field formed around the plasmonic Au nanostructure. As the PbS quantum dots are coated on the plasmonic Au/TiO2 nanodiode, the maximum peak position of the IPCE was shifted to a lower photon energy, which is a clear indication of the contribution of electrons excited in the quantum dots. (3) The increase in IPCE around 2.7 eV is from hot electrons generated by interband excitation in the plasmonic Au nanostructure and electrons excited in the PbS quantum dots. (4) The increase in IPCE at 1.4 eV, which corresponds to the band gap of the 3.5 nm PbS quantum dots, is attributed to electron−hole pair generation in the PbS quantum dots. These electrons flow along the conduction band of the p−n junction consisting of PbS quantum dots/TiO2. The positions of both peaks for LSPR excitation at 1.85 eV and interband transition at 2.8 eV differ slightly from previous results with 2.6 nm PbS quantum dots because the plasmonic nanostructures form randomly. The total IPCE value for the plasmonic nanodiode with 3.5 nm PbS quantum dots is greater than that for the nanodiode with 2.6 nm PbS quantum dots. Because the band gap is smaller, the bottom of the conduction band for the 3.5 nm PbS quantum dots is lower, whereas the valence band top is higher than that for the 2.6 nm PbS quantum dots (Figure 3d). This change in energy band alignment results in a reduction of the Schottky barrier height at the interface between the plasmonic Au and the PbS quantum dots, leading to improved photocurrent collection efficiency. 3.3. Ligand Effect on PbS Quantum Dots: Energy Level Control. Control of the surface of the quantum dots is crucial because of their large surface-to-volume ratios.37,28 In particular, ligands on the surface affect the electronic properties of quantum dots when in thin-film form.38 Thus, the choice of ligand, which changes the surface dipole, can determine both the absolute energy levels and the doping polarity.39 To study the effect of ligands on the performance of plasmonic Au/TiO2 nanodiodes, we modified the surface properties of the PbS quantum dots via ligand exchange using TBAI and EDT ligands. Figure 4a shows the relative position of the energy levels for the 3.5 nm PbS quantum dots capped with EDT and TBAI ligands, with a band gap of 1.4 eV. The position of the energy levels in the PbS quantum dots is determined by combining ultraviolet photoelectron spectroscopy and absorbance measurements (Figure S6). EDT-PbS quantum dots show p-type behavior, and the top of the valence band is 0.4 eV below the Fermi level. The difference between the Fermi level and the valence band edge in the TBAI-PbS quantum dots is greater than that in the EDT-PbS quantum dots, indicating that the Fermi level and valence band shift up of about 0.5 eV and 0.3 eV. Figure 4b shows the photocurrent measured on plasmonic Au/TiO2 nanodiodes with either TBAI-PbS or EDT-PbS quantum dots and on plasmonic Au/TiO2 nanodiodes without quantum dots. The nanodiodes with EDT-PbS quantum dots generate a short-circuit photocurrent about 226% larger than that measured on the nanodiodes without quantum dots. At the same time, the nanodiodes with TBAI-PbS quantum dots only show a 53% increase in photocurrent. A similar trend is demonstrated by IPCE measurements on the Au/TiO2 nanodiodes with TBAI-PbS or EDT-PbS quantum dots (Figure 4c). Here, three peaks at 1.4, 1.82, and 2.85 eV are detected for both the TBAI-PbS and EDT-PbS quantum dots. These results

Figure 4. Ligand effect of PbS quantum dots on photocurrent generation. (a) Schematic of energy levels for the 3.5 nm PbS quantum dots fabricated using 1,2-ethanedithiol (EDT) and tetrabutylammonium iodide (TBAI) ligands. The Fermi levels (EF, dashed line), valence band edge (EV, blue lines), and conduction band edge (EC, red lines) are indicated. (b) Short-circuit photocurrent and (c) IPCE measured on the plasmonic Au/TiO2 nanodiodes with EDT-PbS or TBAI-PbS quantum dots.

can be explained by a change in the energy band alignment. The Schottky barrier height is calculated by the difference between the work function of the Au and the electron affinity of the PbS quantum dots. As the ligand is changed from EDT to TBAI, the energy band structure shifts up by 0.3 eV (Figure 4a). This implies that the Schottky barrier height for the interface between the plasmonic Au and the TBAI-PbS quantum dots is higher than that for the plasmonic Au and the EDT-PbS quantum dots, resulting in a reduction in the photocurrent. 3.4. Comparison of the Schottky Barrier Heights. For a more detailed analysis, the Schottky barrier height was obtained by fitting the data from the current−voltage curve to the thermionic emission equation40 (Figure 5a,b). 5086

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ACS Applied Materials & Interfaces ⎛ qφ ⎞⎡ ⎛ q(V − IR s) ⎞⎤ − 1⎟⎥ I = AA**T 2 exp⎜ − b ⎟⎢exp⎜ ⎝ k bT ⎠⎢⎣ ⎝ ηk bT ⎠⎥⎦

can be reduced when PbS quantum dots are deposited onto the surface of the nanodiodes. The Schottky barrier in the plasmonic Au/TiO2 nanodiodes was lowered to a value of about φb = 0.71 eV after depositing five layers of 2.6 nm EDTPbS quantum dots. Furthermore, we also confirmed that as the size of the PbS quantum dots increased from 2.6 to 3.5 nm, the height of the Schottky barrier decreased to about 0.59 eV. Finally, we found that exchanging the ligand from EDT to TBAI increased the Schottky barrier height to about 0.77 eV. The photocurrent of the plasmonic Au/TiO2 nanodiode with 3.5 nm EDT-PbS quantum dots (i.e., which has the lowest barrier height) is the largest, whereas the photocurrent for the plasmonic Au/TiO2 nanodiode without quantum dots (i.e., that has the highest barrier height) is the lowest (Figures 3b and 4b). These results indicate that the height of the Schottky barrier determines the hot electron detection efficiency. 3.5. Angle Dependence on Photocurrent. To investigate the correlation between the momentum of hot electrons and the Schottky interface in generating photocurrent, experiments were carried out measuring the behavior of the photocurrent on the plasmonic Au/TiO2 nanodiode with and without PbS quantum dot coating while varying the incident angle of the light. Figure 5c shows an enhancement of the photocurrent as a function of the incident angle of light from 0 to 60°. Here, 0° refers to the direction perpendicular to the light incidence on the device plane. As the incidence angle of the light on the plasmonic Au/TiO2 nanodiode without PbS quantum dots increased, the mismatch between the momentum of the hot electrons and the interface of the Au/TiO2 was reduced, thereby enhancing the photocurrent. In the plasmonic Au/TiO2 with PbS quantum dot coating, there are two orthogonal Schottky interfaces that are arranged vertically. As the angle of the incident light increased on the plasmonic Au/TiO2 coated with PbS quantum dots, a mismatch occurred between the interface of the Au/PbS quantum dots and the momentum of the hot electrons, whereas the interface of the Au/TiO2 was partially matched with the momentum of the hot electrons. Note that the Schottky barrier height of the plasmonic Au/TiO2 interface is much larger than that of the Au/PbS quantum dots, as shown in Figure 5b. This means that the detection efficiency of the hot electrons is much lower at the interface of the plasmonic Au/TiO2 than that of the Au/PbS quantum dots. As a result, the photocurrent of the plasmonic Au/TiO2 with PbS quantum dot coating was reduced as the angle of incident light increased.

(2)

Figure 5. Schottky barrier height. (a) Current−voltage curves measured on plasmonic Au/TiO2, and 2.6 nm EDT-PbS, 3.5 nm EDT-PbS, and 3.5 nm TBAI-PbS quantum dots deposited on plasmonic Au/TiO2 nanodiodes. The black solid lines are the fit of the measured current−voltage curve to the thermionic emission equation. (b) Comparison of the Schottky barrier heights obtained by fitting the data from the measured current−voltage curves to the thermionic emission equation. (c) Enhancement of photocurrent as a function of the incident angle of light from 0 to 60°.

4. CONCLUSIONS In conclusion, the mechanism for photocurrent generation in plasmonic Au/TiO2 nanodiodes with PbS quantum dots is studied. The combined action of localized surface plasmon resonance in the Au nanostructures and optical absorption of the PbS quantum dots leads to amplification of the photocurrent. The effect is attributed to the more effective capture of photoexcited hot electrons because of the formation of a 3D Schottky barrier. An important feature of this method is its simplicity. The enhancement in photocurrent generation is achieved by an easy-to-implement procedure for depositing PbS quantum dots directly onto the surface of plasmonic nanodiodes. The energy band alignment in the nanodiodes also promotes the transport of hot electrons because of light absorption in the PbS quantum dots themselves. The hot electron collection efficiency of the nanodiodes can be controlled by modifying the size of the quantum dots and by

where A is the area, A** is the Richardson constant, T is the temperature, q is the elementary charge, kb is the Boltzmann constant, φb is the Schottky barrier height, η is the ideality factor, and Rs is the series resistance of the nanodiode (Table S1). Regression analysis data (average R2 = 0.99987) indicate that the experimental results are in good agreement with the calculated fitting results. In the plasmonic Au/TiO2 nanodiodes, the Schottky barrier height was determined to be about φb = 0.95 eV. We found that the height of the Schottky barrier 5087

DOI: 10.1021/acsami.7b16793 ACS Appl. Mater. Interfaces 2018, 10, 5081−5089

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ligand exchange. Furthermore, we confirmed the effect of the direction of hot electron momentum relative to the Schottky interface on the generation of photocurrent by controlling the angle of incident light. The results of this study are of interest for developing a strategy to further improve the power conversion efficiency of hot electrons in photovoltaic devices.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b16793. Supporting characterization data (scanning electron microscopy, transmission electron microscopy, energydispersive X-ray spectroscopy, absorbance spectrum, ultraviolet photoelectron spectroscopy, calculated parameters from the thermionic emission equation) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.J.). *E-mail: [email protected] (J.Y.P.). ORCID

Sohee Jeong: 0000-0002-9863-1374 Jeong Young Park: 0000-0002-8132-3076 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Institute for Basic Science (IBS) [IBS-R004-A2-2017-a00]. This work was also supported by NRF grant funded by MSIP. (2016R1A2B3014182, 2017M3A6A7051087).

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