Large-Area Broadband Near-Perfect Absorption from a Thin

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Large-area broadband near-perfect absorption from a thin chalcogenide film coupled to gold nanoparticles Tun Cao, Kuan Liu, Li Lu, Hsiang-Chen Chui, and Robert E. Simpson ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b21452 • Publication Date (Web): 11 Jan 2019 Downloaded from http://pubs.acs.org on January 13, 2019

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ACS Applied Materials & Interfaces

Large-area Broadband Near-perfect Absorption from a Thin Chalcogenide Film Coupled to Gold Nanoparticles

Tun Cao, *,† Kuan Liu , † Li Lu, ‡ Hsiang-Chen Chui, § and Robert E. Simpson ‡ †School

of Optoelectronic Engineering and Instrumentation Science, Dalian University of Technology, Dalian 116024, China

‡

Singapore University of Technology and Design, 8 Somapah Road, 487372, Singapore

§Center

for Micro/Nano Science and Technology, National Cheng-Kung University, Tainan 70101, Taiwan

Abstract Perfect absorbers that can efficiently absorb electromagnetic wave over a broad spectral range are crucial for energy harvesting, light detection, and optical camouflage. Recently, perfect absorbers based on the metasurface have attracted intensive attention. However, highperformance metasurface absorbers in the visible spectra require strict fabrication tolerances, and this is a formidable challenge. Moreover, fabricating sub-wavelength meta-atoms requires a top-down approach thus limiting their scalability and spectral applicability. Here, we introduce a plasmonic nearly perfect absorber that exhibits a measured polarisation-insensitive absorptance of ~92% across the spectral region from 400 nm to 1000 nm. The absorber is realised via a one-step self-assembly deposition of 50 nm gold (Au) nanoparticle (NP) clusters onto a 35 nm thick Ge2Sb2Te5 (GST225) chalcogenide film. An excellent agreement between the measured and theoretically simulated absorptance was found. The coalescence of the lossy GST225 dielectric layer and high density of localized surface plasmon resonance modes induced by the randomly distributed Au NPs plays a vital role in obtaining the nearly perfect absorptance. The exceptionally high absorptance together with the large-area high-throughput

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self-assembly fabrication demonstrates their potential for industrial scale manufacturability, and consequential widespread applications in thermophotovoltaics, photodetection, and sensing. Keywords: perfect absorption, chalcogenide, surface plasmon resonance, phase change material, metamaterials 1. Introduction A wide range of exciting applications, such as thermophotovoltaics, 1-4 controlled-emissivity surfaces,5-7 infrared imaging,8,9 biosensing,10 and camouflage11 need to absorb electromagnetic (EM) waves efficiently. A perfect EM-absorber should eliminate the reflection and transmission over a broad spectral range. However, natural materials tend to have a strong reflectance, exhibit a low absorptance, or have weakly defined spectral features in the visible spectra.12 To perfectly absorb the light, the materials must be designed to support a high density of optical modes,13,14 not reflect light,15,16 and efficiently couple the light into the supported optical modes.17 Metasurfaces have been designed and developed to satisfy these requirements. Indeed, nearly ideal absorbers that can absorb light across a broad spectral bandwidth from near-infrared to microwave frequencies have been demonstrated.18-27 The majority of the metasurface absorbers employ a metallic ground layer, which not only eliminates the transmittance but also interacts with the upper metallic resonator to provide strong electric and magnetic resonances. Such a design can efficiently localise the EM-field within a dielectric interlayer sandwiched between the top and bottom metallic resonators.28,29 However, some drawbacks hinder the commercial potential of these absorbers. First, each meta-atom needs to be sub-wavelength in size, requiring top-down lithography techniques such as electron-beam lithography (EBL) or focused ion beam (FIB) milling,24 which have limited spatial resolution, are difficult to scale-up to large area patterning, and since they are serial writing techniques, they have a low fabrication throughput. Second, although the metasurface absorbers do not always require periodicity,30 a well-defined array of resonators with a subwavelength pitch is still necessary for many kinds of metasurface absorbers,31-33 which limits the absorption

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bandwidth and efficiency. Third, the metasurface absorbers typically require back reflectors, which substantially increase the complexity.

Alternatively, particle-based plasmonic nanostructures have been fabricated by electrochemical deposition on porous templates,34-37 self-assembling,38-40 dealloying,41-43 a combination of these techniques,44,45 and sputtering.[46-50] As these absorbers do not require a high degree of periodicity, they were considered suitable for large area surfaces; yet, experimental works showed low absorptance over a limited spectral bandwidth. This places severe barriers to the materials that can be used in the broadband perfect EM-absorber.51 Moreover, in these absorbers, the dielectric materials are passive (i.e., SiO2 and polymer). Therefore they cannot convert the absorbing light energy into the electron-hole pair. This limits their applications for photovoltaic cells and photodetectors. These issues motivated the current study, where we explore active EM perfect absorbers over genuinely macroscopic areas using a straightforward, scalable, and low-cost fabrication technique. The family of Ge-Sb-Te phase change material (PCM) chalcogenide semiconductor has been heavily studied for photonics applications that range from multi-level storage and displays to integrated nanophotonic systems.52 Thanks to its high-speed phase change, excellent thermal stability, and high cyclability, these materials are of significant technological importance for fast and tunable photonic devices.53-55 Indeed thermal imaging systems, thermal emitters,56,57 tunable negative index metamaterials,58 rewritable metasurfaces lenses,59 beam steering,60,61 and active circular dichroism62,63 have been suggested. Chalcogenide semiconductor thin film plasmonic absorbers are another research area that is providing efficient approaches not only the tunable perfect absorptance in the mid-infrared region29 but also the ideal broadband absorptance in the visible region.28,64 However, fabricating periodic resonators arrays typically requires either EBL or FIB patterning, which is expensive and time-consuming.

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In this work, we fabricated wafer-scale self-assembled Au nanoparticles (NPs) coupling to a Ge2Sb2Te5 (GST225) film. The structure possesses polarizationindependent, broadband, omnidirectional, and nearly perfect absorptance (~92%) from the visible (VIS) to near-infrared (NIR) spectrum. Compared with the top-down fabrication method, our conformal solution-based assembly process does not require lithographic patterning, which leads to several advantages, such as scalability, high spatial resolution (1- to 2- nm distance between the particles), and high throughput. We compare the absorption of a bare GST225 layer with the Au NPs-coupled to the GST225 film. The measured absorption substantially increases from 52% to 92% when the Au NPs are positioned over the GST225. The near-perfect absorptance is achieved by the coalescence of the lossy GST225 semiconductor layer and a high density of localized surface plasmon resonance (LSPR) modes, which are induced due to a random distribution of Au NPs. Namely, the strongly confined LSPR modes in the metallic NP-coupled layer system enhances the electric (E) field in the gap between the GST225 film and the Au NPs,65,66 allowing the high lossy GST225 semiconductor layer to absorb the incident light efficiently. As the GST225 is an active semiconductor, the light energy absorbed by the GST225 can create electron-hole pairs,67 which

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enables our proposed absorber to be useful for photovoltaic cells or photodetectors. A numerical model based on the finite-difference-time-domain (FDTD) method was built to understand the physics underlying the nearly perfect absorptance. We propose that this large-area, broadband, omnidirectional, and polarization-insensitive light absorber can be used to enhance the performance of steam heating, thermopiles, and absorptive spectral imaging devices. The high-throughput and one-step solution-based deposition process may lead towards emissivity and absorption controllable surfaces with arbitrary size and geometry. 2. Results and discussions To achieve a large-area broadband plasmonic perfect absorber, a wide variety of anisotropicshaped and random-sized metallic NPs need to be patterned or deposited compactly. We avoided using conventional bottom-up lithographic methods in favor of a simple and inexpensive drop cast deposition of NPs. We first deposited a 35 nm thick GST225 film on a silicon (Si) substrate followed by a 5 nm thick Si3N4 barrier layer using Radio Frequency (RF) sputtering. The Si3N4 layer is necessary to avoid interlayer diffusion and reactions.68 Finally, the Au NPs were deposited from a colloidal solution onto the Si3N4 layer, which forms a structure exhibiting the LSPR (see Methods for details). Figure 1(a) illustrates the Au NPcoupled GST225 plasmonic absorber. Figure 1(b) shows a high-resolution scanning electron microscopy (SEM) image of self-assembled Au NPs on the GST225 film. The Au NP radius distribution on the GST225 layer was 25±1 nm. In Figure S1 of the Supporting Information (SI), we provide the measured complex refractive index (n = nGST + i×kGST) of a 35 nm thick GST225 film on the Si substrate in the amorphous state. Variable angle spectroscopic 5



ellipsometry (VASE) was used to measure the n over the entire VIS-NIR spectral range. We fitted the measured data by using a Tauc-Lorentz model. The real part of the GST225 refractive index (nGST, solid line) is very dispersive, while the imaginary part (kGST, dashed line) is large, implying a high absorptance. The optical absorptance (A) spectra of the Au NPs coupled to the amorphous GST225 film was derived from the measured reflectance R using an infrared microscope coupled to a grating spectrometer (Horiba, iHR320). A was calculated from A= 1R-T, where the reflectance spectra were normalized to the Au mirror, and the transmittance (T) spectra were assumed to be zero. The measured and calculated absorptances of the structure under normal incidence are shown in Figure 1(c)-(d). In Figure 1(c), we show the measured wideband nearly perfect absorptance (A >90 %) from 400 nm to 1000 nm. A good agreement is observed between the simulated and measured absorptance spectra. In Figure 1(d), we calculate the absorptance spectra of the absorber using the Finite-Difference Time-Domain (FDTD) method to solve Maxwell’s equations within the Lumerical FDTD Solutions software package. The model used the distribution profiles of the Au NPs that were measured by SEM (Figure 1(b)). More details of the numerical model were given in methods. The simulations validated the measured broadband high absorptance under normal incidence. In the simulation, we neglected the surface roughness and fabrication imperfection, which may cause the small differences between the modeled and measured absorptance. Note, the reflectance spectra measurement was performed in the far-field on a finite acceptance angle within the 0.5 numerical aperture (NA) microscope objective light cone. Whereas, in the model, the reflectance spectra were calculated in the near field of the absorber, which may also cause differences between the experiment and simulation. We were able to neglect scattering due to the small size of the Au NPs, which inefficiently scatter the incident light (

Large-Area Broadband Near-Perfect Absorption from a Thin

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