Letter pubs.acs.org/NanoLett
Quantification of Efficient Plasmonic Hot-Electron Injection in Gold Nanoparticle−TiO2 Films Daniel C. Ratchford,*,† Adam D. Dunkelberger,† Igor Vurgaftman,‡ Jeffrey C. Owrutsky,† and Pehr E. Pehrsson† †
Chemistry Division and ‡Optical Sciences Division, U.S. Naval Research Laboratory, Washington, D.C. 20375, United States S Supporting Information *
ABSTRACT: Excitation of localized surface plasmons in metal nanostructures generates hot electrons that can be transferred to an adjacent semiconductor, greatly enhancing the potential light-harvesting capabilities of photovoltaic and photocatalytic devices. Typically, the external quantum efficiency of these hot-electron devices is too low for practical applications (40%), near the upper limit based on the excited electron distribution, and would not limit the ability to achieve external quantum efficiencies greater than a percent. To characterize the electron injection efficiency from Au NPs into a semiconductor, Au NPs were sandwiched between TiO2 thin films fabricated by atomic layer deposition (ALD). The Au NPs were deposited using electron beam evaporation to create subpercolation thin films of Au. In addition to the TiO2−Au stacks, we fabricated two control samples: (1) similar to the TiO2−Au stacks but with ALD Al2O3 in place of the TiO2 and (2) an ALD TiO2-only film with no Au NPs (see the Supporting Information for full fabrication details). The sample morphology for the TiO2−Au and Al2O3−Au stacks is depicted in the scanning electron microscopy (SEM) images of Figure 1a,b. For electron microscopy, we used samples fabricated on silicon substrates to improve image quality, in contrast to the samples for optical measurements, which were fabricated on sapphire. The samples were fabricated on sapphire substrates because sapphire is transparent to the visible and mid-IR wavelengths used in the transient absorption measurements presented below. Figure 1a shows the side-view image of a completed Al2O3−Au stack and illustrates the layers
Figure 1. (a) Side-view image of Al2O3−Au NP stack on a silicon substrate. (b) Top-down SEM image of Au NPs after annealing the sample at 500 C. (c) Histogram of the approximate NP diameter for each sample with log-normal fits to the TiO2−Au (Al2O3−Au) sample shown as a black (red) dashed line. Both fits yield a mean of 9.8 nm with a standard deviation of 0.35 nm.
of Au NP, which appear as white circular objects. We fabricated each sample with five layers of Au NPs, which produced strong optical absorption in both the Al2O3−Au and TiO2−Au stacks. Top-down images of the Au NPs on a TiO2 film (Figure 1b) show that the NPs have a range of shapes and sizes. To verify that the NP size distributions were similar for the TiO2−Au and Al2O3−Au stacks, we quantified the NP size distribution using top-down SEM images of the NPs. Individual NPs were identified, and the surface area of each NP was calculated using the computer software ImageJ. We approximated the NP diameter as the diameter of a circle with the same surface area. As shown in Figure 1c, the histograms of the approximate NP diameters for the TiO2−Au and Al2O3−Au stacks are nearly identical. We found that a log-normal distribution fit each data set well, yielding a mean NP diameter of 9.8 nm and a standard deviation of 0.35 nm for both samples. The absolute absorption of the fabricated samples must be measured to determine the charge injection efficiency. The absorption was determined by measuring the total reflection and transmission from the samples with an integrating sphere. Figure 2a,b shows the total fraction of transmitted light, T, and reflected light, R, respectively, for the TiO2-only film (black lines), TiO2−Au stack (red lines), and Al2O3−Au stack (blue lines). In the Supporting Information, we show that the spectra for the TiO2−Au and Al2O3−Au samples can be accurately calculated using an effective medium approximation. In Figure 2a, the TiO2-film transmission decreases starting at about 400 nm, consistent with the onset of above-bandgap absorption. The TiO2−Au and Al2O3−Au transmission spectra are dominated by broad spectral dips centered at about 650 and 550 nm with a full width half-maximum value of about 220 and 140 nm, respectively. These spectral dips arise from the local surface-plasmon resonances of the embedded Au NPs. The difference in the band centers is due to the different refractive indexes of TiO2 (n ≈ 2.5) and Al2O3 (n ≈ 1.7). In Figure 2b, the TiO2−Au reflection spectrum shows a spectral peak at about 660 nm, which our calculations show is the result of an increase in the effective refractive index of the TiO2−Au layer (see the Supporting Information). A similar spectral feature 6048
DOI: 10.1021/acs.nanolett.7b02366 Nano Lett. 2017, 17, 6047−6055
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Nano Letters
depicted in Figure 3a, which shows a schematic of the energy band structure of an n-type semiconductor−metal interface, representative of the TiO2−Au interface. At the interface, a rectifying junction forms with an Schottky barrier of height ϕb. Previous reports have put ϕb in the 0.9−1.2 eV range for the TiO2−Au interface.3,21,30−32 The decay of a surface plasmon in the metal can lead to the generation of an electron−hole pair in the metal, and the energy of the electron, Ee, above the Fermi energy level, Ef, can range from 0 eV up to the surface plasmon energy, ℏω. An excited electron with sufficient energy and momentum may reach and cross the barrier, injecting into the semiconductor. The rectifying junction acts to separate the excited electron and hole, preventing energy relaxation within the metal. The electron injection efficiency is the ratio of the number of electrons injected into the semiconductor to the total number of excited electrons in the metal. We used mid-IR transient absorption spectroscopy, which reports on the presence of photogenerated free carriers, to establish the injection of electrons into the semiconductor in the TiO2−Au stacks and to quantify the injection efficiency based on the magnitude of the infrared absorption.33−36 Figure 3b shows the mid-IR transient response from the films described above with the transient absorption, plotted as
Figure 2. (a) Transmission, (b) reflection, and (c) absorption from the TiO2-only (black), TiO2−Au stack (red), and Al2O3−Au stack (blue) samples.
( ) in units of optical density (OD), where T
ΔA = −log10
with a smaller overall magnitude is shown in the Al2O3−Au stack reflection spectrum at ∼570 nm. We calculated the total absorption, shown in Figure 2c, as A = 1 − T − R. Notice that the TiO2-only film (black line) exhibits absorption at wavelengths below ∼500 nm. The band gap of crystalline anatase TiO2 is roughly ∼3.2 eV (∼390 nm); therefore, the redder onset of absorption at ∼500 nm likely suggests the presence of trap states in the energy gap. Indeed, we acquired photoluminescence spectra from this TiO2-only film (see the Supporting Information) by exciting the sample at 443 nm and detected weak emission at wavelengths >450 nm, confirming the existence of the trap states. Due to its relatively weak, flat reflection spectrum, the Al2O3−Au stack absorption (blue line in Figure 2c) is largely determined by its transmission spectrum. The Al2O3−Au stack absorption exhibits a broad plasmon resonance centered at ≈550 nm. In contrast, the TiO2−Au stack has an asymmetric absorption resonance (red line in Figure 2c) due to the increase in reflection near the center of the Au plasmon that peaks at ∼615 nm with a broad shoulder centered at ∼715 nm. Below, we quantify the plasmonic hot-electron injection efficiency from the Au NPs into the TiO2 film after excitation of the Au plasmon band. The electron injection process is
T T0
and T0 are the transmission through the sample with and without the pump pulse, respectively. When the TiO2-only film is excited with 325 nm pulses (above the bandgap, black circles), an increased mid-IR extinction at 5 μm appears with an instrument-limited rise time and then decays. This response is from the free carrier absorption of the electrons in the TiO2 conduction band.34 When the TiO2-only film is excited with 600 nm pulses (below bandgap, blue circles), there is no measurable response (85%), these results suggest that it is the electron energy distribution that mainly limits the calculated injection efficiency. Given the high injection efficiencies measured above, it is intriguing to ask if electron injection modifies the electron dynamics within the Au NPs as well. For instance, Harutyunyan et al. observed a strong, ultrafast (40%, and we corroborated these results by a separate experiment in which we observed modified carrier dynamics in the Au NPs, consistent with the previously measured charge injection efficiencies. The agreement of these two transient absorption measurements implies that the mid-IR signals we observed are indeed the result of efficient charge injection from the Au NPs, and not for instance, due to the excitation of sub-bandgap TiO2 transitions. The external quantum efficiencies of plasmonic hot-electron injection devices reported in literature are frequently
