Subscriber access provided by United Arab Emirates University | Libraries Deanship
Article
Directing energy into a sub-wavelength nonresonant metasurface across the visible spectrum. Timothy U. Connell, Gus O. Bonin, Christopher D. Easton, Enrico Della Gaspera, Anthony Sidney Richard Chesman, Timothy J. Davis, and Daniel Gomez ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01704 • Publication Date (Web): 07 Jan 2019 Downloaded from http://pubs.acs.org on January 12, 2019
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Applied Energy Materials
Directing Energy into a Sub-Wavelength Nonresonant Metasurface Across the Visible Spectrum. Timothy U. Connell,*,† Gus O. Bonin,† Christopher D. Easton,‡ Enrico Della Gaspera,† Anthony S. R. Chesman,‡,∆ Timothy J. Davis,‖ Daniel E. Gómez*,†,∆
† School of Science, RMIT University, Melbourne, VIC 3000, Australia ‡ CSIRO Manufacturing, Bayview Ave, Clayton, VIC 3168, Australia ‖ School of Physics, The University of Melbourne, Parkville, VIC 3010, Australia ∆ Melbourne Centre for Nanofabrication, Australian National Fabrication Facility, Clayton, VIC 3168, Australia
KEYWORDS
Hot charge carriers, metasurfaces, absorbers, photocatalysis, plasmonics, palladium, critical coupling
1 ACS Paragon Plus Environment
ACS Applied Energy Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 2 of 35
ABSTRACT
Group 10 metals (i.e. Ni, Pd, Pt) catalyze a wide range of chemical transformations but the weak interaction of their nanoparticles with light hinders their development for photocatalytic applications. Conversely, coinage metals nanoparticles (particularly Ag and Au) exhibit intense localized surface plasmon resonances in the visible spectrum, but are relatively unreactive, limiting the scope and efficiency of their photochemical processes. Here we demonstrate the design, fabrication and characterization of a new structure containing a single layer of Pd nanoparticles that absorbs up to >98% of visible light. Furthermore, the wavelength of absorption is controlled throughout the visible range of the electromagnetic spectrum by modulating the thickness of a supporting metal oxide film. We show that the absorbed energy is concentrated in the nanoparticle layer, crucial for energy conversion applications including photocatalysis and photothermal processes.
2 ACS Paragon Plus Environment
Page 3 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Applied Energy Materials
The efficient generation of energetic charge carriers through light-matter interactions is intrinsic to the design of opto-electronic devices,1-2 heterogeneous photocatalytic materials and processes.3-6 Achieving the concerted absorption and conversion of light energy into chemical energy requires an ideal photocatalyst to interact strongly with both light and chemical reagents. The produced charge carriers must meet the energy requirements of a desired chemical transformation.7 Charge carriers must also localize in close proximity to the site of reagent interaction, often at the interface between catalyst and reagent/solvent. Photocatalytic reactions that operate at ambient temperature and pressure are heralded as sustainable alternatives to current industrial-scale chemical processes with high energy demands, which are often satisfied using non-renewable fuels.8 Nanostructured noble metals such as silver (Ag) and gold (Au) interact strongly with light due to the excitation of localized surface plasmon resonances (LSPRs),9-11 and numerous reports now exist on the capacity of these nanoparticles to induce direct photochemical transformations.12-13 Despite the ever increasing number of experimental demonstrations, the range of chemical transformations using noble metals remains limited.14 The electronic structure of group 10 elements (i.e. nickel (Ni), palladium (Pd) and platinum (Pt)) 3 ACS Paragon Plus Environment
ACS Applied Energy Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 4 of 35
promote the adsorption and reaction of a variety of chemical reagents, and catalysts made with these metals are extensively applied in industrial-scale processes.15-20 While select examples of group 10 nanoparticles employed in photocatalytic systems are reported,21-24 their poor interaction with light, due to the high dissipation rate of their LSPRs,25-26 has hindered wider application. It is possible to alleviate this shortcoming through the design of hybrid bimetallic structures that blend catalytic and optical functionality.27 A variety of nanostructures have been proposed and demonstrated, including noble metal nanoparticles capped with Pt shells,14,
27-28
palladium-gold (Pd-Au) alloy nanoparticles supported on metal-oxides,29 Pt-30 and Pdtipped
Au
nanorods31
and
aluminum-palladium
(Al-Pd)
‘antenna–reactor’
nanostructures.32-33 An alternative approach is to drastically alter the optical properties of catalytic metals through their incorporation into electromagnetic metamaterials,34 specifically those capable of complete absorption of light either at a single wavelength or across a broadband. Maximization of light absorption can be realized by designing structures where reflection and transmission of light are strongly suppressed.35-36 The most common architecture to achieve this goal consists of placing a layer of metal nanostructures at 4 ACS Paragon Plus Environment
Page 5 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Applied Energy Materials
a sub-wavelength distance from a mirror with the aid of either thin dielectric films (also known as mirror-dielectric-metal architectures)34,
37-44
or polymeric spacers.45-47 In
most experimental demonstrations of this concept, the nanostructure layer supports a unique optical resonance that can interact with the incident electric field. Coupling to the incident magnetic field is achieved via the excitation of anti-parallel currents supported in the ground plane (e.g. mirror) and resonant layer.48-49 These localized magnetic and electric dipole resonances provide a mechanism for optical impedance matching to free space, which traps light energy effectively and provides sufficient time for complete non-radiative energy dissipation (and therefore near-perfect absorption) within the metals.34, 36, 48-49 Nanostructured Pd layers in previous examples of nearperfect absorption have supported either a single localized plasmon resonance,44 or multiple resonances (achieved with lithographically-defined structures) resulting in broadband absorption.50 Here we demonstrate record levels of visible light absorption (up to 98%), using a three-layer metal-dielectric-metal architecture comprising of a metasurface absorber layer of Pd nanoparticles supported on a dielectric thin film deposited on a mirror (Figure 1A), prepared without any lithographic procedures. Importantly, and unlike 5 ACS Paragon Plus Environment
ACS Applied Energy Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 6 of 35
previous reports, the absorber layer possesses no optical resonance across the studied region of the electromagnetic spectrum. The resulting near-perfect absorption is caused by optical impedance matching and achieved through rational design enabled by spectroscopic ellipsometry and a theoretical model.51-52 The wavelength at which maximum absorption occurs is controllable by modulating the dielectric layer thickness. The absorption of electromagnetic energy showed minimal dependence on the angle of incident light. Finally, we demonstrate that most of the incident electromagnetic energy is harvested by the Pd layer, where it is dissipated in the form of energetic (hot) electron-hole pairs. The complete separation of optical and hotcarrier functions demonstrated in these materials enables unprecedented control of optically-driven hot-carrier phenomena with chemically reactive metals.
Structure and design principles of the absorber – In an idealized model, the nearperfect absorber design consists of three layers where a single layer of Pd nanoparticles (of thickness d, vide infra) is separated from an optical mirror by a dielectric spacer of tantalum pentoxide (Ta2O5, Figure 1A). The thickness of the nanostructured layer is much smaller than the wavelength of the incident electromagnetic field (electric field Eo). This ultra-thin layer is characterized by an 6 ACS Paragon Plus Environment
Page 7 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Applied Energy Materials
amplitude reflection coefficient (r) and an effective complex refractive index (n = neff +𝑖keff) that differs significantly from that the bulk material (Figure 1B). The maximum amount of light this Pd layer could absorb in a symmetric dielectric environment (i.e. identical super/substrate) is limited to ≤50%.52 By adjusting the optical properties of the substrate (eg. the mirror-dielectric support) to strongly suppress transmission, it is possible to increase this value up to 100%, therefore achieving complete absorption. In particular, it is possible to achieve perfect absorption by adjusting the thickness (f) of the dielectric layer to satisfy the critical coupling condition: 𝑟 = ― 𝑟𝑠/(1 ± 2𝑟𝑠), where
rs represents the amplitude reflection coefficient of the mirror-dielectric support and the upper (+) and lower (-) sign applies to s- and p-polarization respectively (for a detailed derivation of this condition see Supporting Information). The amplitude reflection coefficient of the substrate (rs) is also a function of its refractive index, which enables the design and optimization of perfect absorption in these multi-layer structures. Knowing the refractive indices of the nanostructured layer and the mirrordielectric support, it is straightforward to estimate that to achieve near-perfect absorption at a desired wavelength (λc = 480 nm where λc is the wavelength of maximum/near-perfect absorption), the thickness of the corresponding Ta2O5 layer 7 ACS Paragon Plus Environment
ACS Applied Energy Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 8 of 35
must be approximately 40 nm to result in near-critical coupling, i.e. 𝑟 ≈ ― 𝑟𝑠/(1 ± 2𝑟𝑠) (Figure 1C). We experimentally realized this concept using sequential physical vapor (electronbeam) deposition of a mirror layer (consisting of 100–150 nm of Ag, aluminum (Al) or Au ), a thin Ta2O5 film of a thickness determined by the model (Figure 1A,C), and an ultra-thin layer (4 nm nominal thickness) of Pd (experimental details in Supporting Information). The prepared structures revealed distinct layers by cross-sectional scanning electron microscopy (SEM) with the mirror and dielectric layer clearly visible (Figure 2A). Top-down SEM revealed sub-percolation of the ultra-thin Pd film resulting in a layer of polydisperse nanoparticles
