Low-Loss Near-Infrared Hyperbolic Metamaterials with Epitaxial ITO

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Article Cite This: ACS Photonics XXXX, XXX, XXX−XXX

Low-Loss Near-Infrared Hyperbolic Metamaterials with Epitaxial ITOIn2O3 Multilayers Peijun Guo,† Benjamin T. Diroll,† Wei Huang,‡,§ Li Zeng,‡,∥ Binghao Wang,‡,§ Michael J. Bedzyk,‡,∥,⊥ Antonio Facchetti,§,# Tobin J. Marks,‡,§,∥ Robert P. H. Chang,*,‡,⊥ and Richard D. Schaller*,†,§ †

Center for Nanoscale Materials, Argonne National Laboratory, 9700 South Cass Avenue, Lemont, Illinois 60439, United States Materials Research Center, §Department of Chemistry, ∥Applied Physics Program, and ⊥Department of Materials Science and Engineering, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States # Flexterra Inc., 8025 Lamon Avenue, Skokie, Illinois 60077, United States ‡

S Supporting Information *

ABSTRACT: Artificial metamaterials with hyperbolic dispersions exhibit unusual optical properties not found in Nature. Such hyperbolic metamaterials (HMMs) permit the access to and control of electromagnetic waves with large wave vectors. An important criterion for multilayer-based HMMs is whether the thickness of each individual layer can be far below the operating wavelength while still maintaining the material and interfacial quality. Herein, we report heteroepitaxial growth of HMMs composed of multilayers of ultrathin indium tin oxide (ITO) and indium oxide (In2O3) films. The disparate metallic and dielectric properties of the individual building blocks, in conjunction with the good carrier mobility and film morphology enable a low-loss infrared HMM platform on which we demonstrate ultrafast optical switching and the enhancement of the radiative decay rate of PbS quantum dots in the telecommunication wavelength regime. KEYWORDS: indium tin oxide (ITO), indium oxide (In2O3), epitaxial growth, hyperbolic metamaterials, transient absorption, lead sulfide (PbS) quantum dots

H

plasmonic materials12−14 throughout the visible to the infrared range have been grown as epitaxial thin films with superior electrical, magnetic, and thermal properties.15,16 Such schemes can be adopted for the fabrication of HMMs, provided that the metallic and dielectric components, realized through controlled stoichiometric doping, can be epitaxially grown on top of each other. Epitaxially grown, nitride-based multilayers have been recently demonstrated as low-loss HMMs operating in the visible range.17 Further extending the operation frequency of epitaxial multilayer-based HMMs into the near-infrared (NIR) and mid-infrared (MIR) range can provide a new paradigm for high-sensitivity biomolecular sensing,18 on-chip optical communications,19 and thermal engineering,20,21 with tunability achievable through electrical gating22,23 as well as the possibility of interfacing with phase change materials.24 Here we report the growth and characterization of alternating layers of ITO and In2O3 on lattice-matched yttriastabilized zirconia (YSZ) substrates. The identical crystal structures and excellent lattice match between ITO and In2O3 enable epitaxial growth with sharp interfaces and individual layer thicknesses down to 10 nm. Electrical measurements show that the ITO and In2O3 films obtained under identical growth conditions exhibit comparably high

yperbolic metamaterials (HMMs) are optically anisotropic media with permittivity exhibiting opposite signs along different directions.1 With the large photonic density of states arising from a hyperbolic dispersion,2 HMMs have found numerous applications ranging from negative refraction3 and super-resolution imaging,4 to emission-rate enhancement5 and signal routing.6,7 While hyperbolic dispersion can be achieved with either metallic nanowire arrays8,9 or metal-dielectric multilayers,10 the latter structure presents appealing advantages, such as facile, scalable fabrication via physical or chemical deposition methods and the ease of integration with active materials such as photon or electron emitters.11 Loss is an important governing characteristic of an HMM, which is usually comprised of (1) intrinsic Ohmic loss of the metallic components and (2) additional scattering loss owing to the deviation from an idealized structure, such as from interfacial roughness in a multilayer structure. The second contribution to loss dictates that an HMM with a large feature size should offer low loss, but a small feature size (e.g., the thickness of individual layers in a multilayer structure) is essential for access to the high-k modes. As a result, it is essential to identify combinations of low-loss metallic and dielectric components that can form high-quality multilayers with ultrathin individual layer thickness. Compared to noble metals such as gold and silver, degeneratively doped nitrides and oxides as alternative © XXXX American Chemical Society

Received: December 5, 2017

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DOI: 10.1021/acsphotonics.7b01485 ACS Photonics XXXX, XXX, XXX−XXX

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ACS Photonics electron mobilities of ∼50 cm2·V−1·s−1 but drastically different free carrier concentrations. HMMs fabricated from ultrathin layers of ITO and In2O3 exhibit both type-I and type-II hyperbolic dispersion in the NIR to MIR range,25 with a minimal interfacial scattering loss. Optical pumping of free electrons in ITO enables all-optical switching in the NIR range with a large relative change of transmission in excess of 60%. Time-integrated and time-resolved photoluminescence (PL) measurements show that the fabricated HMMs can improve both the brightness and spontaneous emission rate of PbS quantum dots in the two important telecommunication wavelengths of 1.35 and 1.5 μm. Table 1 lists the configurations of various samples studied in this work, together with their acronyms (which indicate the film

along the [111] direction were determined to be 39.9 nm for the In2O3-225 sample and 101.0 nm for the ITO-145 sample. Note that the slight shift in the substrate peak likely arises from the commercial batch-to-batch variation. The excellent lattice match between ITO and In2O3 enables the epitaxial growth of ITO-In2O3 multilayers, as evident from the XRD pattern of the HMM-10 sample (Figure 1a) with a domain size of 50.8 nm. The targeted individual layer thickness in the HMM-10 sample is 10 nm, which is consistent with an estimated 22.6 nm of the In2O3−ITO bilayer period identified from the superlattice fringes (black dash lines in Figure 1a). We also carried out ϕ scans to examine the in-plane epitaxy. By tilting the sample by χ = 19.0° and fixing the incident angle θ and diffraction angle 2θ at the off-specular [112] Bragg condition, we collected the diffraction pattern by rotating the ϕ angle from 0° to 360° along the normal direction of the sample surface. As demonstrated in Figure 1b, the 3-fold symmetry along the [111] growth direction is observed for all films, indicating highquality epitaxy. We determined the thickness of ITO and In2O3 films by cross-sectional transmission electron microscopy (TEM). The sputtering rates of ITO and In2O3, 2.15 nm·min−1 and 3.98 nm· min−1, respectively, were deduced from the measured film thickness and sputtering time. High-resolution TEM images (Figure 1c) and selected-area electron diffraction patterns of the ITO-145 and In2O3-225 samples (Figure 1d) indicate smooth, epitaxial ITO and In2O3 films. Cross-sectional TEM results for a bilayer of In2O3−ITO (Figure S1), and for HMM10 and HMM-20 samples (Figures S2 and S3), demonstrate the formation of sharp and smooth interfaces between ITO and In2O3. Atomic force microscopic (AFM) images of the ITO145, In2O3-225, and HMM-10 samples shown in Figure 1e further demonstrate the excellent film smoothness. Multilayers grown on YSZ (111) substrates exhibit improved film smoothness compared to those grown on YSZ (001) and (011) substrates (see Figure S4), possibly owing to a more stabilized In2O3 (111) surface.27 Table 2 summarizes the electrical properties of the ITO-145 and In2O3-225 samples obtained from Hall measurements. The

Table 1. Composition and Thickness of the Samples Used in This Study Grown on YSZ (111) Substrates Sample name

Composition

Film Thickness (nm)

ITO-145 ITO-80 In2O3-225 HMM-10 HMM-20

ITO ITO In2O3 ITO + In2O3 ITO + In2O3

145 80 225 8 × (10 + 10) = 160 4 × (20 + 20) = 160

thickness). The epitaxial ITO, In2O3, and HMM films were grown on single-crystalline YSZ (111) substrates using magnetron sputtering. For the HMM samples, we grew ITO as the first layer, and In2O3 as the last layer to prevent the exposure of a metallic surface that can quench the PL of PbS (as discussed later). The film epitaxy was studied by highresolution X-ray diffraction (XRD). YSZ has a face-centered cubic structure (a = 0.512 nm), while ITO and In2O3 have the bixbyite structure with a space group of Ia3̅.26 Due to the substitution of ∼10% indium by tin, the lattice constant of ITO (1.0146 nm) is ∼0.3% larger than that of In2O3 (1.0116 nm), which is confirmed by our θ-2θ XRD results shown Figure 1a. After subtracting the instrumental broadening and applying the Scherrer equation to the XRD peak widths, the domain sizes

Figure 1. Structural characterization and surface morphology of the ITO, In2O3, and ITO-In2O3 multilayer samples. (a) XRD results of the ITO-145, In2O3-245, and HMM-10 samples obtained from θ−2θ scans. The scattering vector q is q = 4π sin(2θ/2)/λ. Black-dashed lines indicate the superlattice-fringes for the HMM-10 sample. (b) ϕ scans obtained for the ITO-145, In2O3-245, and HMM-10 samples at the (112) diffraction conditions. Curves are vertically offset for clarity of presentation. (c) Cross-sectional TEM images of the ITO-145 and In2O3-225 samples. (d) Selected-area diffraction pattern of the ITO-145 sample (along the [112]̅ direction) and the In2O3-225 sample (along the [110̅ ] direction). (e) AFM images showing the surface morphologies of the ITO-145, In2O3-245, and HMM-10 samples, with respective root-mean-square roughnesses of 0.48 nm, 0.97 nm, and 0.74 nm. Scalebars are 1 μm. B

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ACS Photonics Table 2. Electrical and Optical Properties of the Sputtered ITO and In2O3 Films ITO In2O3

Sputtering rate (nm·min−1)

Free electron conc (cm−3)

Mobility (cm2·V−1·s−1)

Plasma frequency (eV)

Damping factor (eV)

2.15 3.98

7.72 ± 0.14 × 10 5.89 ± 0.08 × 1019

50.1 52.7

2.19 0.35

0.10 0.10

20

Figure 2. Static optical properties of the ITO and In2O3 film and the ITO-In2O3 HMM. (a) ε′(ω) and ε″(ω) of ITO and In2O3 extracted from ellipsometric measurements. (b) The calculated permittivity tensor components for the ITO-In2O3 HMMs with 1:1 volume ratio (for both HMM10 and HMM-20 samples). (c) Iso-frequency plots of the real and imaginary parts of k⊥ as functions of k∥ at four representative wavelengths (1 μm is in the nonhyperbolic regime; 1.35 and 1.5 μm are in the type-I regime; 2 μm is in the type-II regime). (d) Color plot of the imaginary part of reflection (with p-polarization) as a function of photon energy and in-plane wavevector, calculated for HMM-10 using actual material permittivity shown in (a); the substrate (superstrate) index of 2.1 (1.0) gives rise to slight discontinuity at k∥/k0 = 2.1 (k∥/k0 = 1.0). (e and f) Angular resolved transmission spectra for the ITO-80 sample and HMM-10 sample, respectively, both obtained with p-polarization.

readily sputtered ITO film exhibits a mobility of 50.1 cm2·V−1· s−1, which is nearly identical to the highest reported mobility of single-crystalline ITO grown by pulsed-laser deposition28 and higher than other transparent conducting oxides at comparable doping levels.29 Despite the identical sputtering conditions, the free electron concentration of ITO obtained from Hall measurements (7.72 × 1020 cm−3) is ∼13 times larger than the value of In2O3 (5.89 × 1019 cm−3). This suggests that free electrons in ITO films are primarily produced by tin substitutions via 1 · x 3 0 In 2InIn + 2SnO2 → 2SnIn + 2 O2 (g) + In2O3 + 2e′. comparison, electrons formed by oxygen vacancies via 1 OO x → 2 O2 (g) + VO·· + 2e′, which are the dominating carriers in the In2O3 films, only play a minor role in ITO. The highly metallic and dielectric characters of ITO and In2O3 grown under the same condition are crucial for the fabrication of the HMMs. Figure 2a presents the relative permittivity of the individual ITO and In2O3 films, both modeled with the Drude formula, ε(ω) = ε′(ω) + ε″(ω) = ε∞ − ωp2/(ω2 + iγω). Here ε∞ was taken to be 3.95 for ITO and 4 for In2O3;31 ωp (the plasma frequency) and γ (the damping factor) were subsequently extracted by fitting the ellipsometric data (Figure S5) and are listed in Table 2. Using the extracted ωp and γ of the individual components, we calculated the ellipsometric response of the HMM-10 sample, which is in good agreement with the measured result (Figure S5). This implies that the ITO-In2O3 interfaces are of good optical and electronic quality, resulting in minimal additional interfacial scattering of free electrons. Figure 2a shows that the highly conductive ITO has an ε-near-zero (ENZ) wavelength of 1.13 μm, to the red of which it behaves as a metal. In contrast, In2O3 exhibits a nearly constant positive

ε′(ω) throughout 0.8 to 2.5 μm, hence behaving as a dielectric with negligible optical loss. With the extracted permittivity for the individual films, we calculated the effective permittivity tensor of the HMMs treated as uniaxial optical media. Here, the components along the inplane and out-of-plane directions can be respectively written as ε∥ = f ITOεITO + f In2O3εIn2O3 and (ε⊥)−1 = f ITO/εITO + f In2O3/εIn2O3, where f denotes the filling ratio (in this work we fixed f ITO = f In2O3 = 0.5). Figure 2b shows that the calculated ε⊥′ is negative from 1.13 to 1.58 μm (and positive elsewhere), whereas ε∥′ switches from positive to negative at 1.61 μm. Hence, by definition,25 the ITO-In2O3 multilayer acts as a type-I hyperbolic metamaterial (ε⊥ < 0 and ε∥ > 0) in 1.13 μm ∼ 1.58 μm, and a type-II hyperbolic metamaterial (ε⊥ > 0 and ε∥ < 0) to the red of 1.61 μm, which is depicted in Figure 2b. The ENZ in ε∥′ and epsilon-near-pole (ENP) in ε⊥′ are both located within the range of 1.58−1.61 μm. The dispersion equation for p-polarized waves inside the HMMs can be expressed as k∥2/ε⊥ + k⊥2/ε∥ = k02, where k∥ and k⊥ are wave vectors in the in-plane and out-of-plane directions, and k0 is the wave vector in free space. Using this equation, we calculated the figure-of-merit of the HMM,10 which is defined as Re(k⊥)/ Im(k⊥) and characterizes the propagation loss. In the Type-I regime this ratio is in the range of 1.5−20 (Figure 2b), which is among the highest values achieved for HMMs.32 The complex iso-frequency contours for p-polarized waves at several characteristic wavelengths (1 μm, 1.35 μm, 1.5 μm, and 2 μm) are shown in Figure 2c, plotted for both the real and imaginary parts of k⊥. Hyperbolic dispersions are observed at 1.35 and 1.5 μm in the Type-I regime and at 2 μm in the TypeII regime. In these calculations we treated k∥ as a real positive number because it is preserved in the HMM upon the C

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ACS Photonics incidence from free space.33 The hyperbolic modes outside the light line can be investigated by calculating the poles in the reflectivity.25,34 Figure 2d displays the energy- and momentumresolved imaginary component of reflectivity under p-polarized wave, calculated using material permittivities shown in Figure 2a. We observe clear hyperbolic modes supported by the HMM-10 sample (results for additional samples are shown in Figure S6), which can be conceivably accessed by tip-based measurements.23 Angle-resolved transmission spectra of the ITO-80 sample with p-polarization are shown in Figure 2e. Note that the ITO80, HMM-10, and HMM-20 samples have the same volume of ITO (80 nm thick). A transmission dip coincident with the ENZ wavelength of ITO (Figure 2a) is observed at oblique incidence angles. The transmission dip with a nearly fixed spectral location and an increasing amplitude as a function of incidence angle corresponds to an absorption resonance as verified by finite-element simulations (Figure S7). Additional transmission spectra shown in Figure S8 to S9 demonstrate a lack of absorption resonance for all the samples under spolarization. Excited by free space illumination, this resonance cannot arise from surface plasmon polaritons, but instead is attributed to the so-called Ferrell-Berreman mode (FB mode),34−36 a bulk polariton mode associated with volume electron oscillations along the perpendicular direction, which are excited by an electric field along the out-of-plane direction. Note that the FB mode is different from the so-called epsilonnear-zero (ENZ) mode, which lies out of the light line and only can be excited with a grating (see Figure S6 for a more detailed discussion).37−39 A similar absorption resonance is observed for the HMM-10 sample at its ε⊥′-near-zero wavelength but not the ε∥′-near-zero wavelength. The absorption resonance associated with the FB mode can be further evaluated by the imaginary component of k⊥, which is expressed as k⊥ = [ε∥(k02 − k∥2/ε⊥)]1/2 that characterizes the propagation loss. As shown in Figure S10, the calculated Im(k⊥) spectrally match the measured transmission dips with p-polarization. At ENZ of ε⊥, the term k∥2/ε⊥ diverges, so the term (k02 − k∥2/ε⊥) can become extremely negative and the imaginary part of k⊥ reaches a maximum, giving rise to enhanced absorption. With the understanding of the pronounced optical features, we proceeded with transient extinction experiments by pumping at 1.45 μm and probing at 1.0−1.3 μm (spanning the FB mode), both with p-polarization. The off-resonance, intraband pumping nearly instantaneously heats up the free electrons in ITO by hundreds to thousands of degrees (depending on the pump fluence), resulting in an increased effective mass and with it a reduced ωp.40,41 As shown in Figure 3a, the induced transparency and extinction on the respective blue and red side of the ENZ wavelength of the HMM-10 sample signify a transient spectral redshift of the ENZ wavelength (magenta line in Figure 3a), consistent with the decrease of ωp. The fluence dependent spectra shown in Figure 3b demonstrate a differential change of transmission up to 65% at the ENZ wavelength of ε⊥′ achieved with a fluence of 9.33 mJ·cm−2. The fluence can be reduced by on-resonance pumping at the ENZ wavelength where the sample absorbs more strongly. The kinetic traces shown in Figure 3c demonstrate subpicosecond kinetics correlated with the fast electron cooling, a result of the small electron heat capacity compared to that of the lattice. Complete spectral maps and transient response for the HMM-20 and ITO-80 samples are shown in Figure S12 to S14. Consistent with the static results,

Figure 3. Transient optical response of the HMM-10 sample. (a) ΔT/ T transient spectral map acquired with 1.45-μm p-polarized pump, and broadband p-polarized probe. The magenta line traces out the epsilon (ε⊥)-near-zero wavelength as a function of delay time. (b) Fluence dependent ΔT/T spectra at the delay time of maximal ΔT/T intensity. (c) Fluence dependent ΔT/T kinetics at the wavelength of maximal ΔT/T intensity. Both (b) and (c) were acquired with p-polarized pump and p-polarized probe at 37° incidence angle. (d) Estimated relative changes of plasma frequency for ITO as a function of fluence.

no transient change of transmission is observed under an spolarized probe (Figure S12d), besides the coherent artifact produced by the YSZ substrate (see Figure S15). Figure 3d presents the fluence dependence of the ωp change for the three samples (ITO-80, HMM-10 and HMM-20). Under a fixed fluence, the maximal change of ΔT/T, and with it the relative change of ωp, increases in the order of ITO-80 → HMM-20 → HMM-10. This phenomenon is attributed to hot electron transfer from ITO to In2O3 in the HMM samples, which reduces the free electron density in ITO and results in a more strongly reduced ωp of ITO, as compared to the case of a pure ITO film. Note that we observed slow sample degradation (especially for the HMM-10 sample) at the highest fluence of 9.33 mJ·cm−2. The large photonic density of states supported by HMMs can be exploited for enhancing the radiative decay rates of quantum emitters.5,42 Here, the ITO-In2O3 multilayers exhibit hyperbolic dispersion in the technologically important NIR telecommunication range. Compared to dye molecules which typically emit to the blue of 1 μm,43 semiconductor quantum dots are more promising candidates for NIR light sources because of their size-tunable bandgaps. There is a rich library of materials that emit in the NIR, including the lead chalcogenides (PbS, PbSe, PbTe) and HgTe.44−47 However, the radiative decay rates of many NIR emitters are intrinsically low, which arises owing to the proportionality of the decay rates to the square of the emission frequency. Here we demonstrate the coupling of the visibly transparent ITO-In2O3 HMMs with the archetypal PbS colloidal quantum dots. We synthesized two PbS samples of different sizes,44 with emission targeted at the widely used O band (1.26 to 1.36 μm) and S band (1.46 to 1.53 μm) of the telecommunication window. As shown in Figure 4a, time-integrated PL spectra of the two samples in octane are centered at 1.3 μm (denoted as PbS-1.3) and 1.5 μm (denoted as PbS-1.5), respectively. Time-resolved PL dynamics with nearly single-exponential decays (Figure 4b) reveal the excited state lifetimes of 433 ns for PbS-1.3 and 910 ns for PbS-1.5. We D

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Figure 4. Time-integrated and time-resolved photoluminescence of PbS quantum dots in various environments. (a) Time-integrated emission spectra and (b) time-resolved emission dynamics of PbS-1.3 and PbS-1.5 dispersed in octane. (c) Time-integrated emission spectra and (d) timeresolved emission dynamics (at 1.35 μm) of thin layers of PbS-1.3 on HMM-10, HMM-20, ITO-80, and bare YSZ substrate. (e) Time-integrated emission spectra and (f) time-resolved emission dynamics (at 1.5 μm) of thin layers of PbS-1.5 on HMM-10, HMM-20, ITO-80, and bare YSZ substrate. The photoluminescence signal from PbS-1.5 on ITO-80 sample was too weak to detect for the time-resolved measurement. Black-dashed lines in (b), (d), and (f) show exponential fits to the photoluminescence dynamics.

PbS-1.5 placed on the ITO-80 sample, in which case substantially weaker photoluminescence is observed and no sufficient counts can be collected for the determination of decay dynamics. Our preliminary study suggests that further enhancement of the radiative decay rates may be achievable by adjusting the individual and total layer thickness of the HMMs, as well as nanostructuring the HMM to allow for out-coupling of the high-k modes.42,50,51 In summary, we report on a low-loss NIR HMM comprised of alternating layers of ITO and In2O3 epitaxially grown on lattice matched substrates. Hall measurements confirm the high contrast of free electron concentration in ITO and In2O3 with good mobility. Optical transmission measurements demonstrate a strong absorption resonance arising from the FB mode of the HMMs, which is dynamically tunable by optical pumping. The HMMs yield enhanced radiative decay rates for PbS quantum dots in the telecommunication wavelength range. Further reduction in the individual layer thickness in oxide-based HMMs can be realized by atomic layer deposition of such superlattices with digital control of thickness.52 The oxide-based HMMs demonstrated in this work can be easily extended into the MIR range by reducing the carrier concentration of ITO via high-temperature annealing in oxygen-rich environments, offering the ability to tailor radiative lifetimes and spectral profiles of MIR emitters.

then spin-coated thin layers of these particles onto various samples (representative TEM images are shown in Figure S16).48 Spatially uniform PL intensities and kinetics were observed over each sample. Figures 4c−4f present the timeintegrated PL spectra and lifetimes of the two PbS quantum dots on different samples, with Table 3 summarizing the Table 3. Photoluminescence Lifetimes of PbS-1.3 and PbS1.5 Samples in Different Environments

PbS-1.3 PbS-1.5

in octane

on HMM-10

on HMM-20

on ITO-80

on YSZ

433 ns 910 ns

9.16 ns 39.1 ns

9.40 ns 33.4 ns

0.85 ns N/A

12.9 ns 109.6 ns

measured decay rates obtained from single-exponential fits (black-dashed lines). We find that, in general, PbS dots in octane solution exhibit much longer lifetimes compared to those on the YSZ solid substrate control. This can be explained by the difference in the dielectric constants of surrounding media, ε1 (2.0 for octane and 1.0 for air), which impacts the radiative decay rate through the equation τ=

(

3ε1 ε2 + 2ε1

2

)

2γe 2ω2fε11/2 49 . 3me 2c 3

Here ω is the emission frequency,

f is the oscillator strength, e is the elementary charge, me is the free electron mass, c is the speed of light, γ is the exciton degeneracy, and ε2 = 17 is the dielectric constant of PbS. PbS1.3 on the HMM samples exhibit enhanced PL intensities, whereas PbS-1.5 on the HMM samples show comparable or slightly enhanced PL intensities. Because the high-k modes supported by the HMM samples are impedance-mismatched, and hence not expected to contribute to the measured PL intensities in these planar structures, the PL enhancement is mainly contributed by a stronger absorption due to near-field enhancements (Figure S17). The PL lifetime enhancements observed for both HMM samples suggest the coupling of PbS with the HMMs (see Figure S18 for simulated Purcell factors). Note that the ITO film significantly increases the decay rate of PbS-1.3 (by a factor of 15 compared to YSZ), but at a cost of decreased emission intensity attributable to quenching by the metal. Such nonradiative quenching is much more severe for

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METHODS Sample Fabrication. The epitaxial ITO and In 2 O 3 multilayers were grown in a multigun magnetron sputtering system (AJA Orion). Both the ITO target (In2O3/SnO2 90/10 wt %, 99.99% purity) and In2O3 target (99.99% purity) were purchased from the Kurt J. Lesker Company. The ITO films were sputtered with an RF gun at a power of 75 W; the In2O3 films were sputtered with a pulsed DC gun at 75 W, respectively, with the substrate holder maintained at 630 °C. The chamber was first evacuated to a base pressure of ∼2 × 10−7 Torr, and then the sputtering was performed under a 20 sccm flow of Ar gas (of ultrahigh-purity) at a constant pressure of 5 mTorr. Film thickness was controlled by the growth time. E

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Electrical and Structural Characterization. Carrier type, carrier concentration, and carrier mobility of ITO and In2O3 films were measured with a Hall system (Ecopia 3000) on samples using the van der Pauw configuration with 0.58 T magnetic field. TEM samples used for thickness measurement were prepared with the standard focused ion beam milling technique in an FEI Helios NanoLab 600 instrument. Crosssectional TEM measurements were performed with a JEOl JEM-2100F field-emission transmission electron microscope. Xray diffraction measurements were performed with a Rigaku Smartlab workstation using Cu Kα1 radiation with a Ge (220) channel-cut crystal. AFM images were acquired with a Veeco Dimension Icon Scanning Probe Microscope in the tapping mode. Optical Characterization. Near-infrared transmission spectra were captured by FTIR (Thermo Nicolet 6700). A ZnSe lens was used to focus the light into a 1 mm diameter spot. Ellipsometric measurements were performed at 70° incidence angle using a Horiba Jobin Yvon system. Timeintegrated photoluminescence measurements were performed with a Horiba Jobin-Yvon Nanolog Spectrofluorimeter with excitation at 500 nm. In time-resolved photoluminescence measurements, the samples were excited using a 705 nm pulsed diode laser; the photoluminescence dynamics was produced using an InGaAs photomultiplier tube with time-to-amplitude conversion photon counting electronics. Transient Absorption Measurement. Transient absorption measurements were performed using a 35 fs amplified titanium:sapphire laser operating at 800 nm at a repetition rate of 2 kHz. Broadband probe pulses were generated by focusing a portion of the amplifier output into a 2 mm thick CaF2 window. Pump pulses at 1.45 μm were generated via a white light seeded optical parametric amplifier and were reduced in repetition rate to 1 kHz. Probe pulses were mechanically time delayed using a translation stage and retroreflector. The pump spot diameter on the sample was ∼531 μm. Synthesis of PbS Quantum Dots. Lead oxide (Aldrich, 99.999%), octadecene (Aldrich, 90%), oleic acid (Aldrich, 90%), and hexamethyldisilathiane (Aldrich, synthesis grade) were used as received. The synthesis of PbS quantum dots followed the procedure of Hines and Scholes.44 Briefly, 90 mg of lead oxide, 1−4 mL oleic acid, and 4−7 mL octadecene (8 mL total liquid) were added to a 25 mL three neck flask and held under vacuum at 125 °C for 1 h, as the solution turned clear. Then the reaction was maintained under a nitrogen atmosphere and 42 μL of hexamethyldisilathiane dissolved in 2 mL of dried and degassed octadecene was rapidly injected. The flask was removed from heating and allowed to cool to room temperature. The quantum dots were isolated by precipitation with isopropanol and subsequently dispersed into hexanes and precipitated with isopropanol two additional times before being dispersed into octane for photoluminescence measurements and spin-coating. Reflection Calculation, and Optical Simulation. The reflection of the multilayer stack was calculated using a customized transfer-matrix code. The Purcell factor was estimated using finite-element simulations (COMSOL Multiphysics), wherein a point dipole with either parallel or perpendicular orientation was placed 15 nm above the HMM. The power outflow was integrated over the surface of a sphere centered at the dipole with a radius of 10 nm.

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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/acsphotonics.7b01485. Additional information about the structural and optical characterizations (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]; [email protected]. ORCID

Peijun Guo: 0000-0001-5732-7061 Benjamin T. Diroll: 0000-0003-3488-0213 Wei Huang: 0000-0002-0973-8015 Li Zeng: 0000-0001-6390-0370 Binghao Wang: 0000-0002-9631-6901 Michael J. Bedzyk: 0000-0002-1026-4558 Tobin J. Marks: 0000-0001-8771-0141 Richard D. Schaller: 0000-0001-9696-8830 Author Contributions

P.G. fabricated the thin films and performed TEM measurements. P.G., B.T.D., and R.D.S. performed optical measurements. B.T.D. synthesized the PbS nanoparticles. W.H., L.Z., and B.W. performed structural characterizations. P.G. wrote the manuscript with inputs from all authors. R.P.H.C. and R.D.S. supervised the project. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was performed at the Center for Nanoscale Materials, a U.S. Department of Energy Office of Science User Facility, and supported by the U.S. Department of Energy, Office of Science, under Contract No. DE-AC02-06CH11357. Work at Northwestern University was funded by the MRSEC program (NSF DMR-1121262) at Northwestern University. This work made use of the J.B.Cohen X-ray Diffraction Facility supported by the MRSEC program of the National Science Foundation (DMR-1720139) at the Materials Research Center of Northwestern University and the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF ECCS1542205.)

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