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Self-Assembled Epitaxial Au-Oxide Vertically Aligned Nanocomposites for Nanoscale Metamaterials Leigang Li, Liuyang Sun, Juan Sebastian Gomez-Diaz, Nicki L. Hogan, Ping Lu, Fauzia Khatkhatay, Wenrui Zhang, Jie Jian, Jijie Huang, Qing Su, Meng Fan, Clement Jacob, Jin Li, Xinghang Zhang, Quanxi Jia, Matthew Sheldon, Andrea Alu, Xiaoqin Li, and Haiyan Wang Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b01575 • Publication Date (Web): 17 May 2016 Downloaded from http://pubs.acs.org on May 21, 2016
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Nano Letters
Self-Assembled Epitaxial Au-Oxide Vertically Aligned Nanocomposites for Nanoscale Metamaterials Leigang Li1, Liuyang Sun2, Juan Sebastian Gomez-Diaz3, Nicki L. Hogan4, Ping Lu5, Fauzia Khatkhatay6, Wenrui Zhang1, Jie Jian6, Jijie Huang1, Qing Su1, Meng Fan6, Clement Jacob6, Jin Li7, Xinghang Zhang7, Quanxi Jia8, Matthew Sheldon1,4, Andrea Alù3, Xiaoqin Li2, and Haiyan Wang1,6* 1
Department of Materials Science and Engineering, Texas A&M University, College Station, Texas 77843, United States 2 Department of Physics and the Center for Complex Quantum Systems, The University of Texas at Austin, Austin, Texas 78712, United States 3 Department of Electrical and Computer Engineering, The University of Texas at Austin, Austin, Texas 78712, United States 4 Department of Chemistry, Texas A&M University, College Station, Texas 77843, United States 5 Sandia National Laboratories, Albuquerque, New Mexico 87185, United States 6 Department of Electrical and Computer Engineering, Texas A&M University, College Station, Texas 77843, United States 7 Department of Mechanical Engineering, Texas A&M University, College Station, Texas 77843, United States 8 Center for Integrated Nanotechnologies (CINT), Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States
*
To whom correspondence should be addressed. E-mail:
[email protected] 1 ACS Paragon Plus Environment
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Metamaterials made of nanoscale inclusions or artificial unit cells exhibit exotic optical properties that do not exist in natural materials. Promising applications, such as super-resolution imaging, cloaking, hyperbolic propagation, and ultrafast
phase
velocities
have
been
demonstrated
based
on
mostly
micrometer-scale metamaterials and few nanoscale metamaterials. To date, most metamaterials are created using costly and tedious fabrication techniques, with limited paths towards reliable large-scale fabrication. In this work, we demonstrate the one-step direct growth of self-assembled epitaxial metal-oxide nanocomposites as a drastically different approach to fabricating large-area nanostructured metamaterials. Using pulsed laser deposition, we fabricated nanocomposite films with vertically aligned Au nanopillars (~ 20 nm in diameter) embedded in various oxide matrices with high epitaxial quality. Strong, broad absorption features in the measured absorbance spectrum are clear signatures of plasmon resonances of Au nanopillars. By tuning their densities on selected substrates, anisotropic optical properties are demonstrated via angular dependent and polarization resolved reflectivity measurements and reproduced by full-wave simulations and effective medium theory. Our model predicts exotic properties, such as zero permittivity responses and topological transitions. Our studies suggest that these self-assembled metal-oxide nanostructures provide an exciting new material platform to control and enhance optical response at nanometer scales. KEYWORDS: Nanoscale metamaterial, plasmonic property, gold nanopillar, 2 ACS Paragon Plus Environment
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BaTiO3, vertically aligned nanocomposite (VAN), self-assembled metamaterial
Artificially constructed electromagnetic metamaterials, e.g., metallic rods,1,
2
double-fishnet,3, 4 metal-dielectric layers,5, 6 and split-ring resonators,7 have evolved into a new material science frontier, owing to their exotic electromagnetic properties that do not often have a counterpart in natural materials. Fascinating optical properties such as negative refractive index, large positive refractive index, giant circular dichroism, and zero scattering, have enabled promising applications including super lenses,8, 9 biological sensing,10, 11 subwavelength imaging,12, 13 cloaking,14, 15 etc. In particular, artificial materials composed of noble metal nanorods (specifically, gold and silver), either free standing or embedded in dielectrics, have received wide interest due to their unique plasmonic properties arising from the interaction between surface plasmon polaritons and electromagnetic waves.1, 2, 11, 16
Significant efforts have been devoted to fabricating these metamaterials, including bottom-up template-assisted electroplating2, techniques
(stacked
electron-beam
11
and top-down nanofabrication
lithography,17,
18
membrane
projection
lithography,7 direct laser writing,19 etc.). For example, silver nanowire (60 nm in diameter) arrays supporting negative refraction, and gold nanowire (20 - 40 nm in diameter) arrays operating as plasmonic nanoantennas have been realized by filling porous alumina template with electrochemical plating.1,
2
Three-dimensional
micrometer-scale fishnet metamaterials with negative refractive index were fabricated 3 ACS Paragon Plus Environment
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by focused ion-beam (FIB) milling of silver and magnesium fluoride multilayers.3 Micrometer-scale Ti/Au split ring resonators were deposited using electron-beam evaporation and membrane projection lithography.7 Despite the enormous progress recently made in fabrication and processing techniques, the tedious patterning and lithography steps, as well as the costly nanoscale FIB-based process, have hindered the practical applications of these nanoscale and micrometer-scale metamaterials. For instance, template-assisted electroplating requires complicated procedures and is inherently limited by the fact that the alumina template is difficult to integrate with other wafer-based devices and processes. In addition, nanofabrication involving electron-beam lithography and FIB milling faces issues of low speed and potential material damage due to the high-energy electron-beam or ion-beam involved.
Epitaxial metal-oxide self-assembled nanocomposites are proposed as an ideal platform to realize nanoscale metamaterials, as illustrated in the schematic diagram in Figure 1. The growth of such epitaxial metal-oxide vertically aligned nanocomposite (VAN) structures, however, presents critical challenges. For example, potential inter-diffusion between metal and oxide phases during both high-temperature target processing and film deposition, the vastly different surface energies and dissimilar growth kinetics of metal and oxide phases, are all previously considered as insurmountable challenges.
In this work, we demonstrate a new self-assembly approach to fabricating nanoscale 4 ACS Paragon Plus Environment
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metamaterials consisting of epitaxial metallic nanopillars in oxide matrices using a one-step pulsed laser deposition (PLD) method. To overcome the above mentioned challenges in creating metal-oxide epitaxial composites, we select thermally stable face-centered cubic gold (Au, a = 4.080 Å) and tetragonal barium titanium BaTiO3 (BTO, a = 3.992 Å, c = 4.036 Å). Detailed microstructural characterizations demonstrate high-quality vertically aligned epitaxial Au nanopillars embedded in the BTO matrix. Optical measurements supported by theoretical simulations reveal anisotropic optical properties of these VAN films. The success of growing different metal-oxide VAN structures, outperforming conventional fabrication methods, suggests great potential of our new one-step self-assembly growth method for producing new nanoscale metamaterials with a wide variety of applications, including graded metasurfaces to manipulate light at will,20 hyperlenses,21, 22 and hyperbolic structures,20, 22 offering strong light-matter interactions at nanoscale.
To identify the phases and growth orientations of the fabricated nanocomposite thin films, X-ray diffraction (XRD) patterns were collected for the Au-BTO thin films grown on SrTiO3 (STO) (001) and c-cut α-Al2O3 substrates as shown in Figure 2, respectively. The STO (001) substrate was chosen because of its excellent lattice matching with both Au and BTO, while the c-cut α-Al2O3 was selected to facilitate the optical measurements. There are several notable features in the XRD patterns. First, in the XRD θ-2θ scan (in log scale, Figure 2a) of the STO substrate sample, the distinct (00l) peaks of BTO indicate the highly textured growth of BTO matrix on the STO 5 ACS Paragon Plus Environment
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substrate along [001] direction. The broadened BTO (002) peak indicates its overlapping with Au (002) peak because of the quite similar out-of-plane lattice parameters of Au (a = 4.080 Å) and BTO (c = 4.036 Å). Besides the primary Au (002) orientation, there are additional diffraction peaks from Au (111) and (220) indicating the existence of other minor orientations. Different from the case on STO substrate, the Au nanopillars grow primarily along [111] direction while BTO shows no obvious preferred growth orientation on c-cut α-Al2O3 substrate (Figure 2b). The four-fold symmetry in φ-scans of Au (202), BTO (101), and STO (101) indicates cube-on-cube growth of Au/BTO on STO without any in-plane rotation (Figure 2c). The four-fold symmetric (202) φ-scan of Au also indicates its growth along [001] direction. Both the 2θ- and φ-scans of the Au-BTO films indicate no apparent intermixing of the metallic Au and BTO. θ-2θ scan of Au-ZnO (Figure 2d) indicate that Au-oxide VAN structure can also be obtained in other systems on different substrates besides the Au-BTO system. The versatile selection of oxide matrices demonstrates that this one-step self-assembled growth method could be applicable to a large variety of epitaxial metal-oxide VAN structures.
As illustrated in the schematic in Figure 3e, the Au-BTO VAN structure with two phases grows as self-assembled nanopillars embedded in an oxide matrix on a STO substrate.
Both
plan-view
and
cross-sectional
transmission
electron
microscopy/scanning transmission electron microscopy (TEM/STEM) analysis were conducted to resolve the 3D nature of the nanocomposites. A high-resolution 6 ACS Paragon Plus Environment
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plan-view STEM (Figure 3b) image of Au-BTO film deposited on a STO substrate clearly demonstrates that Au nanopillars are uniformly distributed in the BTO matrix. The rectangular nanopillars (~ 66.4%) arise from the Au (001) oriented growth while non-rectangular nanopillars (~ 33.6%) are attributed to Au pillars with either (111) or (220) orientations. Size distribution statistics of the rectangular-shaped nanopillars shows an average width of 15.5 ± 0.12 nm and length of 36.6 ± 0.45 nm (Figure S1). High-resolution plan-view STEM image of a typical rectangular shaped Au nanopillar (Figure 3c) embedded in the BTO matrix shows a lattice matching relationship of (002)Au║(002)BTO║(002)STO, and (202)Au║(101)BTO║(101)STO, which also confirms the cube-on-cube growth of Au nanopillars. Both the distinct lattices together with the sharp Au-BTO phase interface demonstrate the highly textured growth of Au nanopillars
and
BTO
matrix
without
obvious
intermixing.
Plan-view
energy-dispersive X-ray spectroscopy (EDS) elemental mapping of Au, Ba, and Ti confirms the clear phase separation of Au and BTO without obvious inter-diffusion (Figure 3a).
Important for tunable optical properties, the density of self-assembled Au nanopillars in the VAN films can be easily tailored by controlling the content of Au in the composite target. Figure S2a,b clearly shows the plan-view TEM images of Au-BTO grown on STO substrate with a lower (~ 3.1 × 1010 cm-2) density and a higher (~ 4.9 × 1010 cm-2) density of Au nanopillars. The flexible density tunability of this one-step self-assembly method makes it clearly superior to the previously reported phase 7 ACS Paragon Plus Environment
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decomposition method of fabricating metal pillars with limited density tunability.23 Beyond the control of density, we are further optimizing the growth parameters to achieve ordered distribution of Au nanopillars in the matrix.
High-resolution cross-sectional STEM imaging coupled with EDS mapping has been performed to investigate the 3D composition distribution of the nanopillars. Figure 3g clearly illustrates uniformly distributed Au nanopillars in the BTO matrix on the STO substrate. The corresponding selected area electron diffraction (SAED) pattern with sharp lattice fringes shown in Figure 3h also proves the highly epitaxial growth of the Au-BTO VAN structure on the STO substrate (image taken along [100] zone axis). The cross-sectional EDS mapping in Figure 3d further confirms the chemical compositions of the VAN structure and clear phase separation between Au and BTO without inter-diffusion. To further analyze the lattice coupling between Au nanopillars and the BTO matrix along the vertical interfaces, a typical high-resolution cross-sectional STEM image of an Au nanopillar growing along [001] direction is shown in Figure 3f. The enlarged image of the phase interface (Figure 3i) clearly shows the 1:1 lattice matching relationship between Au nanopillar and BTO matrix along the [001] growth direction (Figure 3j). The 3D nature of Au-BTO VAN structures grown on the c-cut α-Al2O3 is also clearly demonstrated by the plan-view and cross-sectional TEM images (Figure S3a,b).
The formation mechanism of self-assembled epitaxial metal-oxide VAN structures can 8 ACS Paragon Plus Environment
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be described by three steps: adatom diffusion, initial two-phase nucleation, and further growth of the two phases. Owing to the vastly different surface energies (1.50 - 1.63 J/m2 for Au, ~ 1.24 J/m2 for BTO, ~ 1.26 J/m2 for STO)24, 25 and wettability, Au adatoms diffuse and nucleate as islands to minimize its high surface energy via the Volmer–Weber (VW) growth mode (3D) while BTO adatoms nucleate in Stranski–Krastanov (2D + 3D) or Frank-van der Merve (2D) growth mode forming the overall matrix via layer-by-layer growth. After the initial nucleation, the Au and BTO adatoms further diffuse and grow in the form of Au nanopillars embedded in BTO matrix. Thus a unique Au nanopillar-oxide matrix nanocomposite structure has been formed through the film thickness. The growth mode is quite different from the oxide-oxide VAN nanocomposite systems26, 27, 28, 29 demonstrated by two oxide phases with very similar surface energies, which leads to different growth morphologies compared with this metal-oxide nanocomposite case.
Compared to the previously reported cermet bulk materials (i.e., ceramic (cer) and metallic (met) composite materials) where metal and ceramic are mixed in polycrystalline bulk form,30,
31
the metal-oxide VANs in our work demonstrate
self-assembled epitaxial vertically aligned nanopillars embedded in an oxide matrix with high ordering and flexibility in density control. This one-step direct growth of Au-oxide metamaterials exhibits advantages over the previously reported gold nanorods aligned in polymer composites32 and bulk gold nanorod dispersed in anisotropic fluids which requires more fabrication steps, albeit with switchable 9 ACS Paragon Plus Environment
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polarization-sensitive plasmon resonances.33,
34
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Furthermore, the simple one-step
self-assembly method for creating 3D Au-BTO VAN structure in this work also outperforms the template-assisted electrochemical deposition method11, 35 in several important aspects. First, the epitaxial metal-oxide VAN structure is achieved by simple one-step pulsed laser deposition process which prevails over the tedious electrochemical deposition with complicated procedures.35 Second, no template is required for this method compared to the electrochemical deposition assisted by anodic aluminum oxide template. Using this self-assembly method, the selection of oxide matrices could be more flexible as demonstrated in Figure 2d and Figure S5a (Au-ZnO nanocomposite), as well as Figure S5b,c (Ag-BTO nanocomposite). Lastly, functional coupling between metallic pillars and oxide matrices in the epitaxial metal-oxide VANs could be stronger than that in the structures processed by electrochemical deposition. The interface coupling between metallic pillars and oxide matrices could be utilized to achieve versatile functionalities by combining different metals and oxides.
A key feature of the VAN thin films is their anisotropic and largely tunable optical properties. This optical response strongly depends on the plasmon resonances of individual Au nanopillars, and on the density with which they are embedded in the matrix. We characterized the fundamental optical properties of these VAN films by performing absorbance measurements at normal incidence, as well as angle dependent and polarization resolved reflectivity measurements. A summary of the optical data is 10 ACS Paragon Plus Environment
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provided in Figures 4 and 5. The absorbance spectrum (Figure 4) shows strong broad absorption features between 500 – 700 nm that are attributed to the resonant surface plasmon polaritons of Au nanopillars (Figure 4a), which is reproduced well from simulation (Figure 4b) using approximated geometry and according to real sample (Figure 4c,d). There are two pronounced features that correspond to pillars embedded in the BTO matrix, near 650 nm (Figure 4f), or Au pillars that transect the BTO-air interface, providing the greatest absorbance experimentally observed near 530 nm (Figure 4e). Comparison with control BTO films (see supplemental Figure S6) confirms that the strong resonance is due to the Au inclusions, indicating the high optical quality of the composite VAN thin film structure.
We used angle dependent and polarization resolved reflectivity to measure optical anisotropy of the VAN films. Full-wave simulations of these data reproduced well the experimentally measured spectra, demonstrating that the optical properties of VAN films can be captured by a simple model of a uniform pillar array embedded in the oxide matrix, as described below and in the Experimental Section. The subwavelength nature of the pillars allows us to model the VAN films as a homogenous media with an anisotropic permittivity, providing additional physical insights into their optical response. We present representative reflectivity measurements performed on an Au-BTO VAN grown on STO in Figure 5b with the measurement geometry illustrated in Figure 5a. First, the measurements were compared with full-wave simulations of a uniform nanopillar array embedded in the oxide matrix as illustrated in Figure 5c. An 11 ACS Paragon Plus Environment
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average single unit-cell representing the fabricated composite material is simulated using periodic-boundary conditions, thus yielding the optical response of an ideally infinite periodic sample. The cell is composed of an average nanopillar radius of
r = 9 nm and an average squared unit-cell size of
L = 45 nm . These geometrical
parameters are chosen based on the analysis of SEM images (see Figure S4). The simulation results, obtained using the full-wave commercial software CST Design Studio, are shown in Figure 5d, demonstrating excellent agreement with our measurements.
In order to fully characterize the optical properties of the VANs, one needs to obtain the anisotropic permittivity of the films. To this end, we exploit the subwavelength dimensions of the individual nanopillar to describe the composite material as a homogeneous medium with a uniaxial permittivity tensor, as illustrated in Figure 5e. This permittivity can be expressed using an effective medium theory (EMT) that includes spatial dispersion (nonlocality) effects36 as
ε xx = ε yy =
pε Au E Au + (1 − p ) ε BaTiO3 E0 pE Au + (1 − p ) E0
ε zz = pε Au + (1 − p ) ε BaTiO + δ zz 3
where, E Au =
2 E0ε BaTiO3
(ε
Au
+ ε BaTiO3
)
k z2c 2
ω2
,
,
(1)
(2)
, δ zz ≈ 0.04 − 0.006i is an empirical parameter that
measures the importance of nonlocality in the response of the composite material for excitation parallel to the rods, p is the filling factor, ε BaTiO3 is the bulk BaTiO3 permittivity, and ε Au represents the gold permittivity. The mean free path of 12 ACS Paragon Plus Environment
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electrons in Au depends on the crystalline quality and the finite-size of the material.36 Consequently, ε Au varies with the nanopillar radius (see Experimental Section). The wavenumber k z is obtained by solving the dispersion relation for the three modes supported by the VAN (see Experimental Section for details). The effective optical permittivity, plotted in Figure 5f for the sample described above, is essentially a linear combination of those from the Au nanopillar and the oxide matrix, with corrections introduced by additional electromagnetic modes due to coupling between the nanopillars. This effective homogeneous model not only closely reproduces the measured reflectivity spectra (see Experimental Section), but also provides an easy explanation for the key features of the measured/simulated reflectivity spectra. In the case of s-polarized light (see Figure 5b-d), whose electric field is parallel to the surface, the reflectivity increases for tilted incident angles, as expected from classical electromagnetic theory.37 A peak at around 550 nm observed in the imaginary part of the in-plane permittivity ( ε xx = ε yy ) is associated with an increase of absorption losses at this frequency. This reflectivity reduction is also found in the case of p-polarized incident light. The electric field of p-polarized incident light possess a component aligned along the nanopillars, thus allowing a strong interaction with the VAN and leading to a resonance at around 1000 nm. The fact that this resonance does not appear in the case of s-polarized light confirms the extreme anisotropy of the fabricated structures. Specifically, this peak is associated with the extremely dispersive response of the perpendicular component of the effective permittivity ( ε zz ) in this frequency range, which reaches epsilon near zero values at around 1000 nm. 13 ACS Paragon Plus Environment
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This unusual electromagnetic response, can be exploited in applications, for instance, to fabricate hyperlenses,21,
22
or to shape the radiation pattern of nearby emitters
located nearby.38 In addition, ε zz changes sign at slightly higher wavelengths and therefore our composite structure behaves as a hyperbolic metamaterial36, 39 for higher wavelengths, indicating an optical topological transition.40,
41
This regime also
provides very unique optical properties, allowing exciting phenomena such as negative refraction, focusing, and a dramatic enhancement of the spontaneous emission rate of dipoles located nearby. Most importantly, the analytical homogeneous model derived here allows one to design and fabricate specific nanocomposites with designed optical response taking advantage of the rich electromagnetic features that these nanoscale metal-oxide VANs can offer.
In
summary,
self-assembled
highly
epitaxial
Au-oxide
vertically
aligned
nanocomposite (VAN) thin films have been fabricated by a simple one-step pulsed laser deposition method on different substrates. Microstructural characterizations have shown that Au nanopillars with flexibly tunable densities are uniformly distributed in the oxide matrices with high ordering and excellent epitaxial quality. The strong broad absorption features in the absorbance spectrum are attributed to resonant surface plasmon polaritons supported by the Au nanopillars. Angular dependent and polarization resolved reflectivity measurements supported by extensive simulation and modeling have demonstrated anisotropic optical properties. We emphasize that, compared to other fabrication techniques, our self-assembled one-step epitaxial 14 ACS Paragon Plus Environment
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growth of metal-oxide nanocomposites permits control over the density, size, and alignment of nanopillars instead of random distribution of particles in the oxide matrix. These controls open unprecedented possibilities for large scale and reliable development of nanoscale photonic materials enhancing light-matter interactions at the nanoscale for novel applications.
Experimental Section. Sample fabrication. Composite two-phase Au-BaTiO3 (with the volume ratio of Au ~ 21% and 35%, respectively) and Au-ZnO (with the volume ratio of Au ~ 41%) targets were prepared by a conventional solid state sintering process under flowing Ar/H2 atmosphere. Self-assembled epitaxial Au-BaTiO3 and Au-ZnO vertically aligned nanocomposite thin films were deposited on single crystal SrTiO3 (001) and c-cut α-Al2O3 substrates, respectively, by a pulsed laser deposition method with a KrF excimer laser (Lambda Physik, λ = 248 nm). The laser beam was focused onto the target surface at an incident angle of 45o obtaining an energy density of about 4.0 J/cm2. Before depositions, the chamber was pumped to a base pressure of 9.7 × 10-7 mbarr or lower. A substrate temperature ranging from 400 to 800 oC was maintained during depositions. After depositions, the samples were cooled down to room temperature under high vacuum. XRD, SEM, TEM, and STEM HAADF imaging. The microstructures of the films were investigated by high resolution X-ray diffraction (XRD, PANalytical Empyrean), ultra-high resolution field emission scanning electron microscopy (SEM, JEOL JSM-7500F), and transmission electron microscopy (TEM, FEI Tecnai G2 F20 ST 15 ACS Paragon Plus Environment
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Materials). FEI TitanTM G2 80-200 microscope with a Cs probe corrector and TEAM 0.5, a modified FEI Titan microscope with a special high-brightness Schottky field emission electron source and an improved hexapole-type illumination aberration corrector, were employed to record the scanning transmission electron microscopy (STEM) images in high-angle annular dark-field (HAADF) mode. The samples used for TEM and STEM analysis were prepared by a standard manual grinding and thinning procedure followed by final ion polishing in a precision ion polishing system (PIPS 691, Gatan). EDS chemical mapping. A FEI TitanTM G2 80-200 STEM with a Cs probe corrector and ChemiSTEMTM technology (X-FEGTM and SuperXTM EDS with four windowless silicon drift detectors), operated at 200 kV was used in this study for energy-dispersive X-ray spectroscopy (EDS) chemical mapping.42 EDS spectral images were acquired with an electron probe of size < 1.2 Å, convergence angle of 18.1 mrad, and current of ~100 pA. Spectral images were acquired as a series of frames, where the same region was scanned multiple times. The frames were spatially drift-corrected to build up spectral image data using a reference HAADF image. The instantaneous dwell time on each pixel was 20 µsec, and a typical frame was 400 × 400 pixels. Spectral image collection typically took about 2000 sec, yielding a total per-pixel dwell time of about 12 msec.43, 44 Optical measurements. Absorbance was determined by measuring reflection and transmission spectra with an optical microscope (Witec Alpha 300). Broadband, non-polarized LED illumination at normal incidence was focused with an NA = 0.4 16 ACS Paragon Plus Environment
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objective, and reflection from the sample was normalized by comparison to an Al mirror. Transmission measurements were normalized to the free space transmission collected by an NA = 0.8 objective positioned below the sample stage. Angular resolved reflection spectra were measured using a spectroscopic ellipsometer (Woollam VASE® ellipsometer M2000). Collimated white light with approximately 3 mm diameter and either p- or s- linear polarization was used to illuminate the sample. The intensity of the reflected beam was recorded and normalized to a reference level, which was obtained by sending the white light source directly into the detector. The plane of incidence is parallel to the vertically aligned nanopillars. Numerical simulations. Full-wave simulations accounting for the contribution to absorption provided by individual nanopillars (Figure 4) were performed using the finite-difference time-domain method. Optical constants for gold were taken from Rakic.45 Constants for BTO and STO were taken from Wemple,46 and Dodge,47 respectively, and adjusted to match experimental data. The simulation geometry was modeled from a cross-sectional TEM of an 80 nm thick Au VAN layer in BTO deposited on an STO substrate. Perfect matching layer boundary conditions were used on the top and bottom edges of the simulation window, while the four edges normal to the surface employed periodic boundary conditions to approximate an infinite film. A plane wave source injected light (λ = 400-800 nm) with polarization along the x-axis. A mesh size of 2.5 nm was used to resolve the BTO layer with gold nanopillars. The generally broader spectral features and enhanced absorbance observed experimentally likely indicates a greater distribution of Au pillar widths and orientations compared 17 ACS Paragon Plus Environment
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with the simulated geometry.
We model the optical permittivity determined from ellipsometry measurements (Figure 5) based on an idealized composite consisting of identical Au nanopillars arranged in a square lattice. We keep the first order correction to permittivity due to spatial dispersion arising from the embedded Au nanopillars. We also apply Maxwell-Garnett (MG) theory to derive effective permittivity tensor by treating the composite as a uniaxial bulk material where the extraordinary optical axis is along the Au nanopillars. Both homogenized Au nanopillars lattice model and MG model reproduce optical property of this metamaterial qualitatively and quantitatively. The homogeneous uniaxial permittivity tensor is described in Eqs. (1)-(2), whereas Au permittivity is given by
ε Au (ω ) = ε Au
Bulk
+
iω p2τ ( Rb − R )
ω (ωτ + i )(ωτ R + iRb )
,
(3)
where τ = 2.53 ⋅10−14 sec. is free electron’s relaxation time, Rb ≈ 35.7 nm and R are the mean free patch of electrons in bulk and finite-size Au, respectively (with R ≈ 2nm < Rb in our structures), ω p = 13.7 ⋅1015 Hz is the plasma frequency, and
ε Au
Bulk
is the frequency-dependent permittivity of bulk Au. In addition, the
wavenumber k z required by Eq. (2) is obtained by solving the dispersion relation of the three modes supported by the nanopillars, namely
k x2 + k y2 + k z2
ε yy
=
ω2 c2
,
(4)
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2δ zz k z2 =
ω2
ε zz + δ zz ε yy ± c 2
(ε
− δ zz ε yy ) + 4ε yyδ zz 2
zz
k z2 c 2 . ω 2
(5)
ASSOCIATED CONTENT Supporting Information Statistical size distribution, plan-view TEM images showing density tuning, TEM images of Au-BaTiO3 on c-cut sapphire, SEM images of Au-BaTiO3 and Au-ZnO, TEM image of Ag-BaTiO3, experimental absorbance, reflectance, and transmittance spectra of a control BaTiO3 film. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Author *E-mail:
[email protected].
Notes The authors declare no competing financial interest.
ACKNOWLEDGMENTS The high resolution STEM work was supported in part by the U.S. National Science Foundation (DMR-0846504). A portion of the electron microscopy experiments were performed at the National Center for Electron Microscopy, Molecular Foundry, which is supported by the Office of Science, Office of Basic Energy Sciences of the U.S. 19 ACS Paragon Plus Environment
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Department of Energy under Contract No. DE-AC02-5CH11231. Sandia National Laboratory is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the US Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000. The work at Los Alamos was partially supported by the Laboratory Directed Research and Development Program and was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. L.S. and X.L. gratefully acknowledge support from NSF DMR-1306878, and the Welch Foundation (F-1662). X.L. and A.A. acknowledge the support from the U. S. Army Research Office W911NF-11-1-0447. J.S.G.D and A.A. acknowledge the support of the ONR MURI grant No. N00014-10-1-0942 and the Welch Foundation (F-1802). M.S. and N.L.H. acknowledge the support from the Welch Foundation (A-1886). L.L. gratefully thanks the financial support from the China Scholarship Council (CSC).
REFERENCES 1. Yao, J.; Liu, Z.; Liu, Y.; Wang, Y.; Sun, C.; Bartal, G.; Stacy, A. M.; Zhang, X. Science 2008, 321 (5891), 930. 2. Dickson, W.; Wurtz, G. A.; Evans, P.; O’Connor, D.; Atkinson, R.; Pollard, R.; Zayats, A. V. Phys. Rev. B 2007, 76 (11), 115411. 3. Valentine, J.; Zhang, S.; Zentgraf, T.; Ulin-Avila, E.; Genov, D. A.; Bartal, G.; Zhang, X. Nature 2008, 455 (7211), 376-9. 4. García-Meca, C.; Hurtado, J.; Martí, J.; Martínez, A.; Dickson, W.; Zayats, A. V. Phys. Rev. Lett. 2011, 106 (6), 067402. 5. Hoffman, A. J.; Alekseyev, L.; Howard, S. S.; Franz, K. J.; Wasserman, D.; Podolskiy, V. A.; Narimanov, E. E.; Sivco, D. L.; Gmachl, C. Nat. Mater. 2007, 6 (12), 946-50. 6. Verhagen, E.; de Waele, R.; Kuipers, L.; Polman, A. Phys. Rev. Lett. 2010, 105 20 ACS Paragon Plus Environment
Page 20 of 28
Page 21 of 28
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(22), 223901. 7. Burckel, D. B.; Wendt, J. R.; Ten Eyck, G. A.; Ginn, J. C.; Ellis, A. R.; Brener, I.; Sinclair, M. B. Adv. Mater. 2010, 22 (44), 5053-7. 8. Zhang, X.; Liu, Z. Nat. Mater. 2008, 7 (6), 435-41. 9. Casse, B. D. F.; Lu, W. T.; Huang, Y. J.; Gultepe, E.; Menon, L.; Sridhar, S. Appl. Phys. Lett. 2010, 96 (2), 023114. 10. Kawata, S.; Ono, A.; Verma, P. Nat. Photon. 2008, 2 (7), 438-42. 11. Kabashin, A. V.; Evans, P.; Pastkovsky, S.; Hendren, W.; Wurtz, G. A.; Atkinson, R.; Pollard, R.; Podolskiy, V. A.; Zayats, A. V. Nat. Mater. 2009, 8 (11), 867-71. 12. Fang, N.; Lee, H.; Sun, C.; Zhang, X. Science 2005, 308 (5721), 534-7. 13. Barnes, W. L.; Dereux, A.; Ebbesen, T. W. Nature 2003, 424 (6950), 824-30. 14. Zhang, S.; Genov, D. A.; Sun, C.; Zhang, X. Phys. Rev. Lett. 2008, 100 (12), 123002. 15. Valentine, J.; Li, J.; Zentgraf, T.; Bartal, G.; Zhang, X. Nat. Mater. 2009, 8 (7), 568-71. 16. Ranjan, M.; Oates, T. W. H.; Facsko, S.; Mller, W. Opt. Lett. 2010, 35 (15), 2576-8. 17. Liu, N.; Guo, H.; Fu, L.; Kaiser, S.; Schweizer, H.; Giessen, H. Nat. Mater. 2008, 7 (1), 31-7. 18. Liu, N.; Fu, L.; Kaiser, S.; Schweizer, H.; Giessen, H. Adv. Mater. 2008, 20 (20), 3859-65. 19. Gansel, J. K.; Thiel, M.; Rill, M. S.; Decker, M.; Bade, K.; Saile, V.; von Freymann, G.; Linden, S.; Wegener, M. Science 2009, 325 (5947), 1513-5. 20. Mohammadi Estakhri, N.; Argyropoulos, C.; Alù, A. Philos. Trans. Ser. A Math. Phys. Eng. Sci. 2015, 373 (2049). 21. Salandrino, A.; Engheta, N. Phys. Rev. B 2006, 74 (7), 075103. 22. Jacob, Z.; Alekseyev, L. V.; Narimanov, E. Opt. Express 2006, 14 (18), 8247-56. 23. Mohaddes-Ardabili, L.; Zheng, H.; Ogale, S. B.; Hannoyer, B.; Tian, W.; Wang, J.; Lofland, S. E.; Shinde, S. R.; Zhao, T.; Jia, Y.; Salamanca-Riba, L.; Schlom, D. G.; Wuttig, M.; Ramesh, R. Nat. Mater. 2004, 3 (8), 533-8. 24. Vitos, L.; Ruban, A. V.; Skriver, H. L.; Kollár, J. Surf. Sci. 1998, 411 (1–2), 186-202. 25. Meyer, B.; Padilla, J.; Vanderbilt, D. Faraday Discuss. 1999, 114 (0), 395-405. 26. MacManus-Driscoll, J. L.; Zerrer, P.; Wang, H.; Yang, H.; Yoon, J.; Fouchet, A.; Yu, R.; Blamire, M. G.; Jia, Q. Nat. Mater. 2008, 7 (4), 314-20. 27. Chen, A.; Bi, Z.; Tsai, C.-F.; Lee, J.; Su, Q.; Zhang, X.; Jia, Q.; MacManus-Driscoll, J. L.; Wang, H. Adv. Funct. Mater. 2011, 21 (13), 2423-9. 28. Zheng, H.; Wang, J.; Lofland, S. E.; Ma, Z.; Mohaddes-Ardabili, L.; Zhao, T.; Salamanca-Riba, L.; Shinde, S. R.; Ogale, S. B.; Bai, F.; Viehland, D.; Jia, Y.; Schlom, D. G.; Wuttig, M.; Roytburd, A.; Ramesh, R. Science 2004, 303 (5658), 661-3. 29. HarringtonSophie, A.; Zhai, J.; Denev, S.; Gopalan, V.; Wang, H.; Bi, Z.; RedfernSimon, A. T.; Baek, S.-H.; Bark, C. W.; Eom, C.-B.; Jia, Q.; Vickers, M. E.; MacManus-Driscoll, J. L. Nat. Nanotechnol. 2011, 6 (8), 491-5. 30. Verdon, C.; Dunand, D. C. Scripta Mater. 1997, 36 (9), 1075-80. 21 ACS Paragon Plus Environment
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31. Cheng, Z.; Wang, J.-H.; Choi, Y.; Yang, L.; Lin, M. C.; Liu, M. Energy Environ. Sci. 2011, 4 (11), 4380-409. 32. Murphy, C. J.; Orendorff, C. J. Adv. Mater. 2005, 17 (18), 2173-7. 33. Liu, Q.; Cui, Y.; Gardner, D.; Li, X.; He, S.; Smalyukh, I. I. Nano Lett. 2010, 10 (4), 1347-53. 34. Liu, Q.; Campbell, M. G.; Evans, J. S.; Smalyukh, I. I. Adv. Mater. 2014, 26 (42), 7178-84. 35. Liakakos, N.; Blon, T.; Achkar, C.; Vilar, V.; Cormary, B.; Tan, R. P.; Benamara, O.; Chaboussant, G.; Ott, F.; Warot-Fonrose, B.; Snoeck, E.; Chaudret, B.; Soulantica, K.; Respaud, M. Nano Lett. 2014, 14 (6), 3481-6. 36. Pollard, R. J.; Murphy, A.; Hendren, W. R.; Evans, P. R.; Atkinson, R.; Wurtz, G. A.; Zayats, A. V.; Podolskiy, V. A. Phys. Rev. Lett. 2009, 102 (12), 127405. 37. Novotny, L.; Hecht, B. Principles of nano-optics; Cambridge University Press: Cambridge, 2012. 38. Alù, A.; Silveirinha, M. G.; Salandrino, A.; Engheta, N. Phys. Rev. B 2007, 75 (15), 155410. 39. Poddubny, A.; Iorsh, I.; Belov, P.; Kivshar, Y. Nat. Photon. 2013, 7 (12), 948-57. 40. Maas, R.; Parsons, J.; Engheta, N.; Polman, A. Nat. Photon. 2013, 7 (11), 907-12. 41. Krishnamoorthy, H. N. S.; Jacob, Z.; Narimanov, E.; Kretzschmar, I.; Menon, V. M. Science 2012, 336 (6078), 205-9. 42. Harrach, H. v.; Dona, P.; Freitag, B.; Soltau, H.; Niculae, A.; Rohde, M. Microsc. Microanal. 2009, 15 (SupplementS2), 208-9. 43. Lu, P.; Zhou, L.; Kramer, M. J.; Smith, D. J. Sci. Rep. 2014, 4, 3945. 44. Lu, P.; Romero, E.; Lee, S.; MacManus-Driscoll, J. L.; Jia, Q. Microsc. Microanal. 2014, 20 (06), 1782-90. 45. Rakić, A. D.; Djurišić, A. B.; Elazar, J. M.; Majewski, M. L. Appl. Opt. 1998, 37 (22), 5271-83. 46. Wemple, S. H.; Didomenico Jr, M.; Camlibel, I. J. Phys. Chem. Solids 1968, 29 (10), 1797-803. 47. Dodge, M. J. Refractive index, in M. J. Weber (ed.), Handbook of Laser Science and Technology, Volume IV, Optical Material: Part 2, CRC Press, Boca Raton, 1986 (as cited in Handbook of Optics, 3rd edition, Vol. 4. McGraw-Hill 2009).
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Figures and Figure Captions
Figure 1 | Design and schematic illustration of a self-assembled epitaxial metal (e.g., Au)-oxide (e.g., BaTiO3 (BTO)) vertically aligned nanocomposite (VAN) structure. The schematic demonstrates the concept of metal-oxide VAN structures as nanostructured metamaterials with tunable metallic nanopillar density. Such ordered nanostructured metamaterials pave a new avenue for controlled light-matter interactions at the nanoscale.
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Figure 2 | Phase identification of Au-oxide VAN thin films using XRD. a, XRD θ-2θ pattern of Au-BTO thin film grown on SrTiO3 (STO) (001). The distinct Au and BTO peaks indicate the highly textured growth of both Au and BTO phases without secondary phase formation. b,d, XRD θ-2θ patterns of Au-BTO and Au-ZnO thin films grown on c-cut α-Al2O3 suggesting the epitaxial growth of Au nanopillars both along Au (111) in BTO or ZnO matrix, respectively. c, φ-scans of Au (202), BTO (101), and STO (101) with four-fold symmetry demonstrating the Au and BTO in-plane matching relationship with STO substrate.
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Figure 3 | 3D microstructural characteristics of epitaxial Au-BTO VAN thin films. a, Plan-view EDS mapping of Au-BTO thin film grown on the STO substrate. The scale bar is 40 nm. b, A plan-view STEM image of the epitaxial Au-BTO film. c, An atomic-scale plan-view STEM image showing an epitaxial Au nanopillar embedded in the BTO matrix. d, Cross-sectional EDS mapping of Au-BTO thin film. The scale bar is 30 nm. e, A 3D schematic drawing showing the concept of epitaxial metal-oxide VAN structure. f, An atomic-scale cross-sectional STEM image showing an epitaxial Au nanopillar within the BTO matrix. g, A typical cross-sectional STEM image showing epitaxial Au nanopillars embedded in the BTO matrix. h, The corresponding selected area diffraction pattern of Au-BTO thin film grown on the STO substrate. i, A magnified atomic-scale STEM image illustrating the out-of-plane lattice match between Au nanopillar and BTO matrix along the [001] growth direction. j, The atomic model showing the out-of-plane lattice match between Au and BTO. All plan-view images were taken from axis and all cross-sectional TEM/STEM images were from axis. 25 ACS Paragon Plus Environment
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Figure 4 | Experimental and simulation of absorbance spectrum. Experimental absorbance (blue), reflectance (green) and transmittance (red) spectra (a) are compared with full-wave simulations (b) of an Au-BTO film on STO. A cross-sectional TEM image of the sample (c) was approximated in the 3-D simulation geometry depicted in profile in (d). Simulated optical field enhancement maps corresponding to incident illumination at 530 nm (e) and 650 nm (f). The measured absorbance exhibited 0.2% variation across the 25 mm2 sample surface.
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Figure 5 | Optical reflectivity measurement, full-wave simulation and effective medium theory of the Au-BTO VAN metamaterial. a,c, Measurement geometry of Au-BTO VAN metamaterial with actual and uniform (for simulation) Au nanopillar array embedded in BTO matrix, respectively. b,d, Measured and simulated reflectivity spectra of the Au-BTO VAN metamaterial for different angles of incidence for both s-polarized and p-polarized incident lights. e,f, Homogeneous medium illustration and effective optical permittivity of the composite Au-BTO metamaterial.
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TOC
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