Conformal Coating of a Phase Change Material on Ordered Plasmonic Nanorod Arrays for Broadband All-Optical Switching Peijun Guo,† Matthew S. Weimer,‡,§ Jonathan D. Emery,∥ Benjamin T. Diroll,† Xinqi Chen,⊥,# Adam S. Hock,‡,¶ Robert P. H. Chang,∥ Alex B. F. Martinson,§ and Richard D. Schaller*,†,□ †
Center for Nanoscale Materials, §Materials Science Division, and ¶Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Lemont, Illinois 60439, United States ‡ Department of Chemistry, Illinois Institute of Technology, 3101 South Dearborn Street, Chicago, Illinois 60616, United States ∥ Department of Materials Science and Engineering, ⊥Department of Mechanical Engineering, #Northwestern University’s Atomic and Nanoscale Characterization Experimental Center (NUANCE), and □Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States S Supporting Information *
ABSTRACT: Actively tunable optical transmission through artificial metamaterials holds great promise for next-generation nanophotonic devices and metasurfaces. Plasmonic nanostructures and phase change materials have been extensively studied to this end due to their respective strong interactions with light and tunable dielectric constants under external stimuli. Seamlessly integrating plasmonic components with phase change materials, as demonstrated in the present work, can facilitate phase change by plasmonically enabled light confinement and meanwhile make use of the high sensitivity of plasmon resonances to the variation of dielectric constant associated with the phase change. The hybrid platform here is composed of plasmonic indium−tin-oxide nanorod arrays (ITO-NRAs) conformally coated with an ultrathin layer of a prototypical phase change material, vanadium dioxide (VO2), which enables all-optical modulation of the infrared as well as the visible spectral ranges. The interplay between the intrinsic plasmonic nonlinearity of ITO-NRAs and the phase transition induced permittivity change of VO2 gives rise to spectral and temporal responses that cannot be achieved with individual material components alone. KEYWORDS: indium−tin oxide (ITO), vanadium dioxide (VO2), phase change, atomic layer deposition, plasmonics, ultrafast spectroscopy emission control,14 energy-efficient windows,15 and hydrogen storage.16 Downscaling the dimensions of PCM-based devices has been actively pursued recently in order to achieve low power consumption as well as enhanced electrical and/or optical responses.17,18 One strategy is through the integration of PCMs with plasmonic nanoantennas,19 since the latter can focus light down to subwavelength scales to efficiently drive the phase transition of PCMs. Meanwhile, strong optical modulation around the plasmonic resonances can be obtained, as phase
M
aterials that undergo structural or electronic phase transitions have been a topic of intensive research owing to their excellent ability to respond to applied temperature,1 light,2 electric field,3,4 or electrochemical gating.5 Canonical phase change materials (PCMs) include chalcogenide compounds (such as Ge2Sb2Te5) that can be switched between crystalline and amorphous states,6 gallium, which can transform between solid and liquid phases at around room temperature,7 and correlated electron materials such as vanadium dioxide (VO2), which undergo a reversible electronic phase transition between insulating and metallic phases,8 to name a few. Powered by PCMs a variety of applications have been demonstrated, with notable examples being rewriteable data storage,9 reconfigurable metasurfaces,10,11 electro-optic modulators,12 electromechanical sensors and resonators,13 light © 2016 American Chemical Society
Received: October 18, 2016 Accepted: December 8, 2016 Published: December 19, 2016 693
DOI: 10.1021/acsnano.6b07042 ACS Nano 2017, 11, 693−701
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Figure 1. Growth and structural characterization of VO2-coated ITO-NRA. (a) Schematic illustration of the ALD process of VO2. (b) SEM image of sample I-u. (c) SEM image of sample I-c. Regions with fallen nanorods are included on purpose to provide additional angles of view. Scale bars in (b) and (c) are 1 μm; both images were captured at a 30° tilting angle with respect to the nanorod long axis. (d and e) Raman spectra of sample I-c as a function of increasing and decreasing temperature. Spectra in (d) and (e) are plotted under arbitrary units and are vertically offset for clarity. (f) Real and imaginary parts of the refractive index of VO2 (16 nm thick) deposited on a witness Si/SiO2 wafer extracted from fitting the ellipsometric data.
of VO2 (see Methods for full synthetic details). The thickness of the VO2 layer was controlled by the number of cycles (VO2 was grown at 0.31 ± 0.02 Å/cy) and subsequently deduced by fitting the data from spectroscopic ellipsometric measurements on VO2 films grown on witness Si/SiO2 substrates (placed near the ITO-NRA samples during the ALD process). For all ITONRA samples, a thin ITO film (∼10 nm) was first epitaxially sputtered on the entire substrate; then part of the ITO film was patterned with a periodic array of gold dots that yielded nanorod growth. This sample configuration ensures the conformal coating of VO2 on both regions of the ITO-NRA and bare ITO film, which can be compared to elucidate the plasmonic effect of the ITO-NRA on the phase change of VO2. Scanning electron micrographs of an ITO-NRA before and after 16.0 nm VO2 deposition are presented in Figure 1b and c, respectively, which reveal a smooth and conformal VO2 layer on the crystalline-flat (100) surface of the ITO nanorods that suggests a successful VO2 ALD process. The ITO-NRA has a pitch size of 1 μm and height of 2 μm and is denoted as sample I; note that the film region without nanorod is denoted as sample I-film (see Table 1). We also assign uncoated and coated samples as sample I-u and sample I-c (see Table 1), and similarly for other samples as discussed later. The phase evolution of VO2 on sample I-c was studied by temperaturecontrolled confocal Raman spectroscopy as shown in Figure 1d (heating from room temperature to 90 °C) and 1e (cooling from 90 °C to room temperature). The Raman spectrum near room temperature with a characteristic peak at ∼690 cm−1 and the featureless Raman profile at high temperature are assigned to the monoclinic, insulating phase and the rutile, metallic phase,25,26 respectively. The reversible phase change occurring at around 50−55 °C, identified from the Raman spectra series, is reasonably consistent with the phase change temperature of 68 °C.20 Interestingly, in the present case the phase transition temperature of VO2 on ITO-NRA is 15−20 °C lower than the
transitions of PCMs are accompanied by abrupt changes of dielectric constants. Among various investigations of PCMs, the reversible insulator-to-metal transition of VO2 is particularly interesting, as it occurs at relatively low temperature around 68 °C.20 Coupling plasmonic components with VO2 to form a designed hybrid system with spatially varying optical characteristics may enable enhanced optical responses.21 In this work, we demonstrate the conformal coating of VO2 on indium−tinoxide nanorod arrays (ITO-NRAs) using atomic layer deposition (ALD). ALD is a vapor phase deposition technique, where precursors are introduced into a deposition zone in a sequential fashion and undergo self-limiting chemistry at the solid/gas interface.22 Inherent in the benefits of the self-limiting nature of surface chemistry is the ability to uniformly coat various oxides and sulfides onto intricate three-dimensional (3D) nanostructures with digital thickness control at the angstrom scale.23 In this work, we demonstrate the conformal coating of an ultrathin, pinhole-free VO2 film on 3D ITO-NRAs via ALD. Using both static and time-resolved spectroscopic techniques, we investigate the phase change of VO2 and its effect on the ultrafast optical dynamics of the hybrid system. We show that the phase change of VO2 enables modulation of the visible to infrared spectral range even with a deepsubwavelength film thickness of a couple tens of nanometers. Our work highlights the advantage of conformal coating of phase change materials, as it permits full access to the hot spots offered by the plasmonic component.
RESULTS AND DISCUSSION ITO-NRAs were grown via the vapor−liquid−solid process from gold seeds fabricated by electron-beam lithography, as reported previously.24 VO2 was conformally coated on the surface of ITO-NRAs using the ALD process. A schematic for the ALD surface chemistry is illustrated in Figure 1a. A 350 °C postannealing step was performed to crystalize the rutile phase 694
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absorbance spectra of sample I-u (sample I-film-u) at 25 °C and sample I-c (sample I-film-c) at 25 and 85 °C. The VO2 coating leads to a red-shift of the transverse localized surface plasmon resonance (trans-LSPR27) of the ITO-NRA from 1600 to 1850 nm due to an increased dielectric constant of the surrounding medium of ITO-NRA. Switching of VO2 from the insulating to the metallic phase is accompanied by a blue-shift and broadening of the plasmonic resonance, consistent with earlier observations made on hybrids of a planar VO2 film and gold nanoantennas.17 The blue-shift of the plasmonic absorbance peak is attributed to a reduction of the real part of the refractive index of VO2 when switched from the insulating to the metallic phase (evident from Figure 1f), whereas the broadening stems from a damping of the plasmonic resonance of ITO-NRA by the oscillations of mobile electrons in the metallic VO2. The dynamic optical properties of sample I-c in the NIR were investigated by transient absorption (TA) experiments at normal incidence, in which the sample was pumped at 1580 nm and probed for the range from 1600 to 1850 nm (an available spectral window for the probe based on the spectral bandwidth of our femtosecond optical parametric amplifier); both pump and probe are on the blue side of the static plasmonic resonance (∼1850 nm) of sample I-c (Figure 2a). The transient spectral maps of ΔOD (change of optical density) under the pump fluence of 4.78 mJ·cm−2 are presented in Figure 2c and Supporting Information Figure S3a for delay times up to 3 ps and 3 ns, respectively. A conduction band non-parabolicityinduced, subpicosecond red-shift27 of the trans-LSPR of the ITO-NRA is manifested as transient bleach with a peak ΔOD amplitude of −0.1; it is followed by an induced absorption (ΔOD ∼0.1) lasting for longer than 3 ns, which exceeds the maximal delay time available from the mechanical delay stage. By examination of the static absorbance spectra for sample I-c at 85 and 25 °C (Figure 2a), we can conclude that the nanosecond response in Figure S3a arises from the insulator-tometal phase change of VO2. The subpicosecond negative-to-
Table 1. Details on Various Samples Studied in This Work, Including Nanorod Height, Pitch Size, VO2 Thickness, and Spectral Range Probed in the Transient Absorption Measurements sample namea
nanorod height
VO2 thickness
probed range
sample I
2 μm
1 μm
16 nm
N/A
N/A
16 nm
1.65 μm
600 nm, 700 nm, 800 nm, 1 μm 600 nm
16 nm
NIR, visible NIR, visible MIR
sample I-film samples II, III, IV, V sample VI
25 nm
N/A
2.1 μm
pitch size
a
Samples before and after the VO2 coating are denoted as sample I-u and sample I-c, and so on.
established phase transition temperature of 68 °C. This is likely due to strain in the VO2 layer imposed by the ITO surface, when correlated with the strain-sensitive Raman peak at 690 cm−1 (Figure 1d and e) that is higher in energy than the 620 cm−1 Raman peak for VO2 deposited on the Si/SiO2 substrate, as shown in Supporting Information Figure S1; the latter more closely matches earlier literature reports. Temperature-controlled ellipsometric results on the Si/SiO2/ VO2 stack (shown in Supporting Information Figure S2) permit the determination of the complex refractive indices of the insulating and metallic phases of VO2, which are plotted in Figure 1f. A decrease of the real component of the refractive index throughout the range from 250 to 1000 nm (the measurement limits of our ellipsometer) is observed upon VO2 phase change from the insulating to the metallic phase. The metallic phase exhibits a slightly increased imaginary part of the refractive index from 800 to 1000 nm.1 Static, temperature-controlled near-infrared (NIR) absorbance measurements were performed on sample I, which was mounted in a cryostat. Figure 2a (2b) presents the NIR
Figure 2. Dynamic optical response of sample I in the NIR spectral range. (a) FTIR spectra of sample I-u at 25 °C and sample I-c at 25 and 85 °C. (b) Same as (a) but taken on sample I-film. (c) NIR transient spectral map of sample I-c plotted for delay times up to 3 ps under a pump wavelength of 1580 nm and fluence of 4.78 mJ·cm−2. (d) Fluence dependent ΔOD for sample I-c at 1700 nm averaged over a delay time from 2 to 3 ns. (e and f) ΔOD kinetics at 1750 nm for sample I-c plotted for delay times up to 3 ps and 3 ns, respectively. Legend right of (f) applies to both (e) and (f). 695
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Figure 3. Dynamic optical response of sample II to sample V in the MIR spectral range. (a) Absorbance spectra for samples II-u to V-u. (b) Absorbance spectra for samples II-c to V-c. (c) MIR transient spectral map for sample III-c plotted for a delay time up to 3 ps under a pump wavelength of 1580 nm and fluence of 6.45 mJ·cm−2. (d) Transient spectra averaged over a delay time from 2 to 3 ns for samples II-c to V-c. Pump fluence was fixed at 6.45 mJ·cm−2. (e) Fluence-dependent transient spectra for sample III-c averaged over a delay time from 2 to 3 ns. Legend in (e) also applies to (f). (f) Fluence-dependent ΔOD kinetics at 3680 nm for sample III-c plotted for delay times up to 4 ps. All static and TA experiments were performed at an incident angle of 30° under p-polarization.
positive transition of ΔOD in Figure 2c suggests that the ultrafast electronic phase change of VO2 is primarily ascribed to the direct optical pumping28 rather than due to heat transfer from ITO (whose lattice temperature increases on the subpicosecond time scale following the pump excitation through an electron−phonon coupling process), as in the latter case the time scale is dictated by heat transport over a distance of tens to hundreds of nanometers and hence is expected to lie in the nanosecond regime,29 especially given that the thermal conductivity of ITO is lower than that of noble metals. The optical response due to phase change of VO2 is clearly demonstrated in Figure 2d, which presents the fluencedependent ΔOD amplitude at 1700 nm averaged over the delay time window from 2 to 3 ns (extracted from the corresponding fluence-dependent transient spectra shown in Supporting Information Figure S3b). The sharp rise of ΔOD amplitude when fluence is increased from 3.02 mJ·cm−2 to 7.58 mJ·cm−2 is associated with the threshold behavior of the VO2 phase change. The kinetic traces at 1750 nm plotted for delay times up to 3 ps are shown in Figure 2e. Notably, while the ultrafast plasmonic response of ITO yields negative ΔOD signals during the first 100 to 200 fs under all fluences, the positive change of ΔOD induced by VO2 phase transition with a subpicosecond time scale30 competes with ITO’s plasmonic response and leads to a swing of ΔOD from 0 to −0.1 and back to 0 within ∼220 fs at high fluences. The kinetic traces at 1750 nm for delay times up to 3 ns are presented in Figure 2f. The gradual increase of ΔOD under high pump fluences (7.58 mJ· cm−2 and above) is attributed to heat transfer from ITO to VO2, because the pump-induced lattice temperature rise of ITO is expected to be initially higher than that of VO2 due to the metallic nature of ITO (and therefore a stronger pump absorption) near room temperature. Transient spectra taken on sample I-film-c plotted in Supporting Information Figure S3c
demonstrate that although the pump wavelength employed (1580 nm) did not match the plasmonic resonance of sample Ic at 1850 nm (shown in Figure 2a), the ITO-NRA enables both stronger VO2 phase-change-induced ΔOD (0.2 for sample I-c versus 0.15 for sample I film-c) and lower threshold fluence (
