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Quantification of efficient plasmonic hotelectron injection in gold nanoparticle – TiO films 2
Daniel C Ratchford, Adam D. Dunkelberger, I. Vurgaftman, Jeffrey Owrutsky, and Pehr E Pehrsson Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b02366 • Publication Date (Web): 29 Aug 2017 Downloaded from http://pubs.acs.org on August 29, 2017
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Quantification of efficient plasmonic hot-electron injection in gold nanoparticle – TiO2 films Daniel C. Ratchford,*1 Adam D. Dunkelberger,1 Igor Vurgaftman,2 Jeffrey C. Owrutsky,1 and Pehr E. Pehrsson1 1
Chemistry Division and 2Optical Sciences Division, US Naval Research Laboratory,
Washington, D.C. 20375 Abstract: Excitation of localized surface plasmons in metal nanostructures generates hotelectrons 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. In order 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 4 ACS Paragon Plus Environment
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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 of Au NP, which appear as white circular objects. We fabricated each sample with 5 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.
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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.
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 nm and 550 nm with a FWHM of about 220 nm and 140 nm, respectively. These spectral dips arise 6 ACS Paragon Plus Environment
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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 Supporting Information). A similar spectral feature 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 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.
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Figure 2. a) Transmission, b) reflection, and c) absorption from the TiO2-only (black), TiO2-Au stack (red), and Al2O3-Au stack (blue) samples.
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 depicted in Figure 3a, which shows a schematic of the energy band structure of an n-type semiconductormetal interface, representative of the TiO2-Au interface. At the interface, a rectifying junction forms with an Schottky barrier of height . Previous reports have put 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, . 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 8 ACS Paragon Plus Environment
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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 Δ = − in units of optical density (OD) where
and 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 appears with an instrument-limited rise time and then decays. This response is from 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 (>1 ns depending on the trap density, energy, and distance to the NP.38 Therefore, the faster decay of the mid-IR TiO2-Au signal compared to the TiO2-only signal likely originates from the injected electrons falling into the TiO2 traps, although further study is required to determine on what time scale the electrons transfer back to the Au NPs.
Figure 3. a) Cartoon band diagram of metal – semiconductor (n-type) interface. b) The IR transient decay from the TiO2-only film when pumped above band gap (pump = 325 nm) and probed at 5 is shown in black. Transient absorption decay from the TiO2-Au stack, Al2O3-Au 10 ACS Paragon Plus Environment
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stack, and TiO2-only film when pumped at 600 nm in red, green, and blue, respectively. The TiO2-only above bandgap pump decay is scaled by 0.54 for easy comparison of the TiO2-Au stack decay. c) The maximum amplitude of the TiO2-Au transient absorption decay, ∆ , as a function of pump wavelength. The static absorption spectrum of the TiO2-Au stack is shown in red.
In Figure 3c, we plot the pump-wavelength dependence of the maximum transient amplitude, ∆ , from the TiO2-Au stack to show that it qualitatively matches the spectral shape of the Au plasmon resonance, verifying that the transient signals originate from the injection of hot-electrons generated in the decay of localized surface plasmons. Here, the incident pump fluence was kept constant (at ~250 µJ/cm2) as the pump wavelength was varied. The steady-state absorption spectrum from the TiO2-Au stack (red curve) is shown for comparison. There is an additional peak in the transient response centered ≈450 nm, and while there does appear to be a small corresponding spectral feature in the absorption spectrum, it does not exhibit the same scaling as ∆ near the plasmon resonance. The exact origin of this feature is unclear, but we speculate that it might result from interband transitions in the Au NPs or excitation of electrons from sub-bandgap states to the TiO2 conduction band. Since this spectral feature does not have a direct relationship to the main focus of the paper, namely, carrier injection resulting from plasmon decay, we discuss these possible origins in the Supporting Information. We quantify the electron injection efficiency by comparing the TiO2-Au stack transient infrared absorption amplitude to that of a reference TiO2 sample with a known electron density. Since the magnitude of the mid-IR absorption is proportional to the electron density,35 the ratio of the signal amplitudes is equivalent to the ratio of the electron densities. Here, we use abovebandgap pumping of the TiO2-only film sample as a reference, assuming that each absorbed 11 ACS Paragon Plus Environment
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photon excites one electron into the TiO2 conduction band. In Figure 4a, the maximum transient amplitude, ∆ , is plotted for the 325 nm pumping of the TiO2-only film (black circles) as a function of absorbed photon density. We calculate the absorbed photon density as = !"#! $∙& , '∙ !"#!
where ( $ is the total absorption at the pump frequency (taken from Figure
2c), ) is the pump fluence, ( is the pump photon energy, and * is the total sample film thickness. The plot shows that ∆ is proportional to the absorbed photon density, and a fit to the data yields a proportionality constant of +,-. = 215 2 4 5* ∙ 6 ⁄78 9 , with the intercept fixed at 0 5*. The value obtained from fitting both the slope and intercept yielded a slope that is only about 2% different, comparable to the experimental uncertainty.
Figure 4. a) ∆ for the TiO2-only film with 325 nm pump (black dots) and TiO2-Au stacks with 615 nm pump (red dots) as a function of absorbed photon density. Linear fits to each data set yield slopes of 215 and 82 mOD*nm3/photon for the TiO2-Au and Al2O3-Au stacks, respectively. The ratio of the slopes is used to estimate electron injection efficiency. b) Black circles are the electron injection efficiencies for the TiO2-Au stack for different pump 12 ACS Paragon Plus Environment
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wavelengths, as estimated from the mid-IR transient data. Red and blue lines are calculated electron injection efficiencies, using Eq. (3), for an interfacial barrier height of 0.9 eV and 1.2 eV, respectively.
From the discussion above, it follows that the black line in Figure 4a estimates the expected ∆ as a function of absorbed photon density for an electron injection efficiency of one. For visible pumping of the TiO2-Au stack, the injection efficiency is less than one, which results in a smaller ∆ for a particular absorbed photon density compared to the UV pumping of the TiO2-only film. This is illustrated by the red data in Figure 4a which shows a shallower slope for ∆ vs. absorbed photon density for the TiO2-Au stack for a pump wavelength of 615 nm. A linear fit to the data yields a proportionality constant of +,-. =
?@ABCDE" ?@ABC
, which results in an electron injection efficiency of ≈38%
at a pump wavelength of 615 nm. We measured the electron injection efficiency for several different pump wavelengths, with the results plotted in Figure 4b (black circles). The error bars on the injection efficiency account for the possible decay of signal during the measurement response time of the pumpprobe setup. This error is approximated as the ratio of the instrument response time (≈180 fs) to the decay time of the carriers in the TiO2 (≈2 ps) or ≈10%. The electron injection efficiency decreases roughly linearly with wavelength. This is consistent with the intuition that as the pump photon energy decreases, a lower percentage of the electrons with energies above the barrier are generated and injected into the semiconductor. The measured efficiency ranges from ≈ 45% at 550 nm down to ≈ 25% at 750 nm.
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There are a few other reports in the literature that specifically measure the electron injection efficiency from Au NPs to TiO2 using mid-IR transient absorption.1, 39, 40 The injection efficiencies we measured above compare favorably to the work of Furube et al., who measured an electron injection efficiency of ~40% for a different sample geometry consisting of Au NPs interfaced with TiO2 NPs.1 However, in contrast to the monotonic behavior seen in Figure 4b, a subsequent study from the same group reported a non-monotonic injection efficiency as a function of pump wavelength.39 In particular, they noted a bimodal distribution for the injection efficiency, with a large dip in efficiency around the plasmonic resonance. They assigned the two peaks to different effects: one of the peaks was attributed to the injection of Au d-band electrons into the TiO2, and the other peak to the electron injection from plasmon decay. The reason for the dip on the plasmon resonance remained unresolved. In what follows, we describe a simple model for estimating the hot-electron injection efficiency, which agrees well with the experimental injection efficiencies shown in Figure 4b. For the calculations presented here, we follow a method similar to that of Leenheer et al. and consider the specific case of carrier injection from small metal NPs fully embedded within a semiconductor.26 The probability of electron tunneling into the TiO2 is negligible (see Supporting Information), and we only consider direct electron injection. A photon incident on a metal NP can generate an electron-hole pair by either direct excitation or decay of an excited localized surface plasmon. We begin by estimating the excited electron energy distribution to determine the probability that an excited electron injects into the surrounding semiconductor because the electron must have sufficient energy to cross the interfacial barrier. As depicted in Figure 3a, the energy separation of the electron and hole is equal to the energy of the incident photon, , with the electron having an energy, G , above the Fermi level that ranges from 0 eV 14 ACS Paragon Plus Environment
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to . While we do not consider the various plasmon decay processes, we can approximate the probability that an electron is excited to a particular energy, G = GH + G , as given by the product of the electron density of states, G$, at the initial and final electron energy, normalized by the same product integrated over all possible initial and final electron energy combinations:27 JK G$ ≈
KP Q
MK
P
G$ G − $
G N $ G N − $OG N
1$
We assume that the excited electrons are well described using the free electron model with parabolic dispersion and G$ ∝ √G. This analysis is applicable to intraband transitions. For gold, this is a good approximation for photon energies less than about 2 eV, i.e., below the onset of interband transitions from the d-bands. Once an electron is excited to a particular energy, the second factor to consider is the transport efficiency, i.e., the probability that the electron reaches the interface. Here, we assume that the electrons have an isotropic momentum distribution. The probability that an excited electron reaches the interface depends on where it is generated in the NP and its mean free path, which depends on the electron energy.41 For a spherical NP, the probability that a hot-electron produced within the NP with initial energy G makes it to the NP surface is given by 6
UV
JT G $ = . M
W
V .
W M
