Material-Versatile Ultrabroadband Light Absorber with Self

KEYWORDS: broadband absorber, material-versatile, funnel structure, ...... FDTD simulation: The FDTD simulations were conducted using a commercial FDT...
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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 29884−29892

Material-Versatile Ultrabroadband Light Absorber with SelfAggregated Multiscale Funnel Structures Yunha Ryu,† Changwook Kim,† Junmo Ahn,‡ Augustine M. Urbas,§ Wounjhang Park,∥ and Kyoungsik Kim*,† †

School of Mechanical Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, Republic of Korea The 4th (Energetics and Defense Materials) R&D Institute-3, Agency for Defense Development, Yuseong P.O. Box 35, Daejeon 305-600, Republic of Korea § Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson AFB, Dayton, Ohio 45433, United States ∥ Department of Electrical, Computer & Energy Engineering, University of Colorado, Boulder, Colorado 80309, United States

ACS Appl. Mater. Interfaces 2018.10:29884-29892. Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 09/08/18. For personal use only.

‡

S Supporting Information *

ABSTRACT: Broadband light absorbers are essential components for a variety of applications, including energy harvesting and optoelectronic devices. Thus, the development of a versatile absorbing structure that is applicable in various operating environments is required. In this study, a materialversatile ultrabroadband absorber consisting of metal-coated self-aggregated Al2O3 nanowire bundles with multiscale funnel structures is fabricated. A high absorptance of ∼0.9 over the AM 1.5G spectrum (300−2500 nm) is realized for absorbers with a range of metal coatings, including Al, W, and titanium nitride (TiN). We demonstrate that the plasmonic nanofocusing and index-matching effects of the funnel structure result in strong ultrabroadband absorption for the various metal coatings, even though the coating materials have different optical properties. As an example of applicability in an operating environment, in the evaluation of the thermal-oxidation resistance, the Al-coated solar absorber exhibits superior performance to those coated with refractory materials such as W and TiN because of the protective alumina layer formed on the Al surface. KEYWORDS: broadband absorber, material-versatile, funnel structure, nanowire bundles, thermal-oxidation resistance approach is micro-/nanoscale surface texturing.15,16 The resulting structures show broadband high absorption induced by enhanced light trapping or the graded index effect. However, surface texturing is sometimes inapplicable or detrimental to optical devices wherein the absorbing layer is very thin or is an active layer.17 Moreover, surface texturing of highly durable (either mechanically or chemically) materials such as Ni is challenging.18 Metal-coated nonplanar structures (MCNPSs) have recently gained popularity owing to their ultrabroadband absorption characteristics and efficient fabrication. First, the basis of MCNPSs, that is, nonplanar structures, can be easily fabricated using a facile process such as nanoimprinting, colloidal lithography, or laser-assisted fabrication on various kinds of supporting substrates. Additionally, the thickness of the metal coating layer required for MCNPSs is extremely small (few tens of nanometers), and a single layer is sufficient to achieve ultrabroadband absorption, allowing cost-effective fabrication

1. INTRODUCTION Broadband light absorbers are widely used for a variety of applications, such as solar energy harvesting,1,2 photodetection,3,4 thermal emitters,5,6 and imaging devices.7 Thus, broadband light-absorbing structures such as plasmonic absorbers, metal-dielectric multilayer stacks, and micro-/ nanoscale surface-textured structures have been extensively researched for decades. Plasmonic absorberstypically based on noble metals such as Au and Agshow strong absorption while the absorption band is controllable owing to their resonant characteristics,8,9 but the high cost of rare noble metals limits the practical use of the plasmonic absorbers. Additionally, these absorbers are vulnerable to heat and thus not suitable for high-temperature applications because Au and Ag start to aggregate at temperatures considerably lower than their melting points.10,11 Metal-dielectric multilayer stacks provide a good absorbing characteristic without requiring complicated fabrication methods such as focused ion beam (FIB) milling or e-beam lithography;12,13 however, there can be mechanical stress between the different layers, limiting the durability.14 Additionally, the repeated deposition process results in low-throughput and high-cost fabrication. Another © 2018 American Chemical Society

Received: June 1, 2018 Accepted: August 14, 2018 Published: August 14, 2018 29884

DOI: 10.1021/acsami.8b09116 ACS Appl. Mater. Interfaces 2018, 10, 29884−29892

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) Schematic of the fabrication process for the metal-coated Al2O3 nanowire bundles. (b,c) Top view, (d,e) tilted view, and (f) crosssectional SEM images of the fabricated structure. (g) Optical photographs of bare Al2O3 nanowire bundles and the bundles after the deposition of an Al, a W, or a TiN coating with a thickness of 80 nm. The scale bars represent 1 μm for (b,d, f) and 500 nm for (c,e).

absorption is induced by the combined effect of plasmonic nanofocusing and index matching of the funnel structure, even though the coating materials have different complex dielectric functions. In addition, we test the thermal durability of the absorbers because broadband light absorbers are often used for solar-thermal energy conversion. In particular, we focus on the change of the absorption properties due to the thermal oxidation depending on the coating material. W and TiN are known to have high thermal stability; however, the absorbers with W and TiN coatings are severely oxidized after annealing at 400 °C. In contrast, an Al-coated absorber shows good thermal-oxidation resistance because of the stable Al2O3 layer at the outer surface. The proposed funnel structure with metalcoated Al2O3 nanowire bundle arrays can be a universal lightabsorbing structure owing to its material-versatility and thus has great potential for applications in various optical devices.

compared with the aforementioned strategies. However, thus far, the candidate metals for MCNPSs with broadband absorption have been noble metals, yielding the disadvantages of plasmonic absorbers.19,20 Therefore, methods for fabricating nonnoble metal-based MCNPSs have been proposed. Chirumamilla et al. reported a titanium nitride (TiN)-coated Si nanopillar having average absorptivities of 0.94 in the wavelength range of 300−2300 nm.21 Ao et al. presented a cone-shaped polymer substrate that had a Fe coating and exhibited absorptivity of ∼95% in the range of 300−2100 nm.22 Although previously reported non-noble metal-coated MCNPSs provide high broadband absorption, the optical mechanism underlying these structures has not been extensively investigated. Furthermore, the materials employed for the metallic coating layer of MCNPSs are limited to a small group of metals. This can be a concern for absorbers operating in a harsh environment, such as a high-temperature or corrosive atmosphere. Therefore, a nonplanar structure that exhibits high absorptivity with a variety of metal coatings would be beneficial. In this study, we fabricate highly efficient broadband absorbers from metal-coated Al2O3 nanowire bundle arrays having a multiscale funnel structure. In our previous work, we have employed the plasmonic nanofocusing of Au-coated Al2O3 nanowire bundles which exhibit ultrabroadband light absorption and efficient light-to-heat conversion for solar vapor generation.23 Here, to extend the metallic coating materials from noble to non-noble metals, we use a wide range of metallic coating materials, such as Al, W, and TiN, and explore their absorption properties. The structures exhibit high absorptance of ∼0.9 in the AM 1.5G solar spectrum (300− 2500 nm). We demonstrate that the broadband high

2. RESULTS AND DISCUSSION 2.1. Fabrication Process and Morphology. Metalcoated Al2O3 nanowire bundle arrays were fabricated via the controlled pore widening of anodic aluminum oxide (AAO). Figure 1a shows a schematic of the fabrication process. First, AAO with a thickness of 6 μm was prepared via anodic oxidation of a pure Al plate. The hexagonal hole array of the AAO layer had a 100 nm lattice constant when the Al plate was anodized at 40 V. Pore widening via wet etching in phosphoric acid thinned the walls of the AAO pores, yielding nanowires with a high aspect ratio. Because of the lateral capillary force between the nanowires, they self-aggregated to form bundles during the subsequent rinsing and drying processes. 29885

DOI: 10.1021/acsami.8b09116 ACS Appl. Mater. Interfaces 2018, 10, 29884−29892

Research Article

ACS Applied Materials & Interfaces

Figure 2. Total reflectance in the UV−vis−NIR region (300−2500 nm) for (a) Al-, (b) W-, and (c) TiN-coated nanowire bundles. The thickness of the metal layer ranged from 20 to 80 nm.

Figure 3. (a) SEM image of a fabricated sample (scale bar: 1 μm). (b) Models of V-shaped grooves with different groove angles. FDTD-simulated reflectance spectra of (c) Al-, (d) W-, and (e) TiN-coated nanowire bundles. The solid and dashed lines correspond to the simulated reflectance spectra with x- and y-polarized illumination, respectively. (f) E-field profile of Al-coated nanowire bundles with a 6° groove angle under 500, 900, 1,500, and 2500 nm illumination. The aspect ratio of a pixel is 8:1 for the x-direction to the y-direction.

diameter. The distance between adjacent bundles was approximately 1−4 μm. At a higher magnification, single nanowires are clearly resolved (Figure 1c,e). Transmission electron microscopy (TEM) images in Figure S1 (Supporting Information) show the more detailed structures of Al-, W-, TiN-coated Al2O3 nanowires. Between the nanowires were

Finally, a thin metal layer of Al, W, or TiN was deposited on the Al2O3 nanowire bundles via sputtering. Scanning electron microscopy (SEM) images of the fabricated structures are shown in Figure 1b−f. The aggregated nanowires formed funnel-shaped bundle structures with a broad size distribution, although the bundles were mostly a few micrometers in 29886

DOI: 10.1021/acsami.8b09116 ACS Appl. Mater. Interfaces 2018, 10, 29884−29892

Research Article

ACS Applied Materials & Interfaces

sectional SEM image in Figure 1f. To represent six nanowires linked to the hexagonal lattice of the underlying Al substrate, we modeled a unit bundle comprising six combined nanowires. The unit bundles were merged into a larger nanowire bundle. In this larger bundle, the angle between the nanowires and the Al substrate was gradually changed to form a bundle of nanowires with a V-shaped groove between adjacent bundles. The sidewalls of each nanowire bundle were coated with a thin metal layer. Although the metal-coated nanowire has inhomogeneous coating for different regions, we assume a homogeneous coating thickness to simplify the modeling in FDTD simulation. Each metal-coated Al2O3 nanowire was modeled as a core−shell triangular pillar with a length of 6 μm. The side lengths of the Al2O3 core and metallic shell were 26 and 40 nm, respectively. As shown in the top-view SEM image of Figure 3a, the orientation angle of the nanowires was different on each part of the substrate, resulting in various angles of the V-shaped groove between bundles. We modeled V-shaped grooves with angles of 6°, 12°, and 30° according to the SEM image of an actual array (Figure 3b). Figure 3c−e shows the FDTD-simulated reflectance spectra of the Al-, W-, and TiN-coated nanowire bundles, respectively, under x- and y-polarized light. The simulated spectrum of the Al-coated sample (Figure 3c) under x-polarized illumination exhibits reflectance below 20% over the whole spectral range (300−2500 nm) at a groove angle of 6°. When the groove angle was increased, the reflectance also increased, especially in the NIR region. Under y-polarization, the reflectance in the NIR region was significantly larger than that under x-polarization for the models with groove angles of 6° and 12°. For the structure with a 30° groove angle, the simulated reflectance was large over the whole spectral range. These optical characteristics are consistent with plasmonic nanofocusing in metallic grooves.25,26 The V-shaped grooves coated with W (Figure 3d) and TiN (Figure 3e) showed similar simulated reflectance spectra. At a larger groove angle, these samples exhibited larger reflectance, similar to the Al-coated samples. However, the Wand TiN-coated-samples with groove angles of 6° and 12° did not show large differences in their reflectance spectra under xand y-polarization. These samples exhibited stronger polarization dependence when their groove angle was 30°. The simulation results reveal that the angle between the nanowire bundles affects the absorption behavior of the structure and that the absorption spectrum can be modulated by changing the groove angle. The reflectance spectra of the V-shaped groove structures exhibit polarization dependence. The experimental samples had a broad distribution of groove angles as well as a nondirectional arrangement of the funnels because of the randomly collapsed nanowires in the arrays. Thus, the fabricated structures can show strong broadband absorption under an unpolarized illumination source. We obtained FDTD-simulated electric-field (E-field) profiles (|E|/|E0|) of the metal-coated V-shaped grooves at four different wavelengths of incident light (500, 900, 1,500, and 2500 nm). Figure 3f shows the E-field profile of the structures with a 6° groove angle. The E-field was enhanced and formed standing-wave patterns along the sidewall of the V-shaped groove. Under illumination of λ = 500 nm, the E-field was enhanced at the top of the V-shaped groove. As the illumination wavelength increased to λ = 2500 nm, the Efield enhancement extended from the opening to the bottom of the V-shaped groove, and the distance between the nodes of

nanogaps a few nanometers in size. When we cut through the middle of a funnel, as shown in Figure 1f, the cross section of the nanowire bundle had a triangular shape with a base that was 3.5 μm and had a height of 6 μm. The nanowires in the middle part of each triangular bundle were vertical, whereas the collapsed nanowires on the bundle edges were oriented at an angle of 75° with respect to the Al substrate. The photographs in Figure 1g reveal that the self-aggregated nanowire array before the deposition of a metal layer was white because of the strong diffuse scattering.24 The structure showed dark colors after the deposition of Al, W, and TiN. 2.2. Optical Properties. To characterize the optical properties of the metal-coated nanowire bundle arrays, we measured their reflectance spectra in the ultraviolet−visible− near-infrared (UV−vis−NIR) region. Al, W, and TiN were used as coating materials to study the absorption properties of various kinds of metals. The nominal thicknesses of the metal coating on the nanowires were controlled to 20, 40, 60, and 80 nm. We use the term nominal thickness which is a thickness of the metal layer by assuming the deposition on a flat substrate. If we deposit the metal coating on randomly collapsed nanowire bundles, the coating thickness is inhomogeneous and different from the case of the flat substrate. For simplicity, the nominal thickness is calibrated with the same sputtering condition as a flat substrate. The SEM images of the Al-, W-, and TiN-coated nanowire bundles with the different nominal thicknesses are shown in Figure S2 (Supporting Information). Figure 2 shows the measured total reflectance (sum of the specular and diffuse reflectance) of the arrays in the wavelength range of 300−2500 nm. No light was transmitted through the samples because of the thick Al substrate; thus, the absorption can be calculated as follows: absorption (%) = 100 − reflectance (%). For the Al-coated samples (Figure 2a), a 20 nm thick coating yielded the highest reflectance, with Fabry− Pérot fringes in the NIR region. When the thickness of the metal coating was increased to 40 nm, the reflectance decreased, along with the amplitude of the Fabry−Pérot fringes. The array with a 60 nm thick Al coating showed the best performance among the samples, with reflectances of 60 nm was needed to achieve low reflectance. The average reflectance of the array with a 60 nm thick W coating in the range of 300−2500 nm was 11.5%. The TiN-coated samples (Figure 2c) showed higher reflectance than the Al- and W-coated arrays. Additionally, the reflectance spectra varied considerably with respect to the thickness of the TiN coating. The average reflectance of the array with a 20 nm thick TiN coating from 300 to 2500 nm was 56.2%, and it decreased to 19.2% when the TiN thickness was 80 nm. These results indicate that the self-collapsed nanowire bundle arrays with a range of metal coating materials effectively absorb ultrabroadband light, with the metal coating influencing the optical behavior. 2.3. Theoretical Analysis of Absorption Mechanism. To theoretically analyze the absorption mechanism of the metal-coated nanowire bundle arrays, we performed threedimensional finite-difference time-domain (FDTD) simulations. The funnel structure of the nanowire bundles was simplified as a V-shaped groove, according to the cross29887

DOI: 10.1021/acsami.8b09116 ACS Appl. Mater. Interfaces 2018, 10, 29884−29892

Research Article

ACS Applied Materials & Interfaces the standing wave increased. The Al-coated structure showed greater E-field enhancement than the structures with the Wand TiN-coated V-shaped grooves. The difference in the optical behaviorssuch as the reflectance and polarization dependenceamong the structures coated with different metals is related to their complex dielectric functions, that is, permittivity ϵ(ω) = ϵr(ω) + iϵi(ω). The real part ϵr(ω) indicates the polarization response, and the imaginary part ϵi(ω) represents the optical losses. The ϵ(ω) values of Al, W, and TiN are shown in Figure S3 (Supporting Information). Al has a negative ϵr(ω) in the vis−NIR region, which means that surface plasmons can be launched on the Al nanostructure. TiN also has a negative ϵr(ω); however, its absolute value |ϵr(ω)| is significantly smaller than that of Al, leading to a weaker plasmonic response. In contrast, TiN is a lossy material with a large ϵi(ω), which is advantageous for increasing the absorption. W has a positive ϵr(ω) from 240 to 920 nm and a negative ϵr(ω) for λ > 920 nm, with a relatively small |ϵr(ω)|. The Al-coated structure shows stronger polarization dependence in its reflectance spectra than the W- and TiN-coated structures because of its stronger plasmon resonance.27 We also simulated the E-field profiles of V-shaped grooves with angles of 12° and 30°, as shown in Figure S4 (Supporting Information). For the structures with groove angles of 6° and 12°, incident light propagated toward the bottom of the Vshaped groove with a parallel wavefront. The structure with a groove angle of 30° induced deformation of the wavefront, which may have resulted from the interference of reflected light due to the metallic sidewalls. This behavior was common among the Al-, W-, and TiN-coated structures. To observe the light propagation over time, we included a movie monitor in the FDTD simulations. The E-field profiles captured at specific times are shown in Figure S5 (Supporting Information). The incident light propagated inside the V-shaped groove of the structure without back reflection, showing the index-matching effect of the tapered structure.28 At the sidewall, light propagated along the metal-coated nanowires and dissipated through absorption by the metal. As a result, a very small fraction of the incident light was reflected from the substrate. Without a metal coating, the incident light propagated within the structure without loss and was then reflected back to the air side. Considering the aforementioned FDTD simulation results, the strong broadband absorption of the metal-coated arrays was induced by the combined effects of plasmonic nanofocusing and index matching. In addition to the nanofocusing and index-matching effects, light concentration at the nanogaps between nanowires,29 the surface roughness of the metal layer,30 and multiple scattering in the funnel31 can increase the absorption efficiency of the arrays. Furthermore, we demonstrated that our structure has material-versatility of the coating layer for materials other than Al, W, and TiN. The simulated reflectance spectra of Vshaped grooves coated with Au, Cu, Fe, Ni, and Ti are presented in Figure 4. These structures show broadband low reflectance, indicating that our self-assembled multiscale funnel structure can be used as a universal platform for producing broadband solar absorbers with a wide selection of coating materials. The coating materials could be carefully chosen depending on the applications and operating environment. 2.4. Thermal-Oxidation Resistance. As a test of adaptability to a tough environment, we studied the resistance to thermal oxidation of the fabricated broadband absorbers because broadband absorbers are often used for solar-thermal

Figure 4. FDTD-simulated reflection spectra of metal-coated nanowire bundles with various kinds of coating materials. The groove angle of the V-shaped structure was fixed at 6°.

conversion devices with a high working temperature. First, we measured the optical spectra of thermally annealed samples. Thermal annealing was performed at 200, 300, 400, and 500 °C in air for 2 h using a muffle furnace. After annealing, the samples were naturally cooled to room temperature. Figure 5

Figure 5. Reflectance spectra of (a) Al-, (c) W-, and (e) TiN-coated absorbers before and after thermal annealing in air at 200−500 °C for 2 h. The thickness of the metal coating was 80 nm. Optical photographs of annealed absorbers with (b) Al, (d) W, and (f) TiN coatings of different thicknesses (20−80 nm).

shows the total reflectance spectra of Al-, W-, and TiN-coated absorbers with a coating thickness of 80 nm from 300 to 2500 nm. As the annealing temperature increased, the reflectance of the absorbers tended to increase, that is, their absorption decreased. This was because the metal layer, which absorbs light, was converted into a metal oxide layer through thermal 29888

DOI: 10.1021/acsami.8b09116 ACS Appl. Mater. Interfaces 2018, 10, 29884−29892

Research Article

ACS Applied Materials & Interfaces

The thermal durability of the absorbers is related to the oxidation behavior of the metal coating. Depending on factors such as the temperature, oxygen pressure, and surface treatment, the metal oxidation rate obeys a logarithmic, parabolic, or linear law. Logarithmic oxidation involves a very high initial oxidation rate that decreases over time and is usually observed for thin layers. For the parabolic case, the oxidation rate continuously decreases over time because of the limited diffusivity of the oxidant in the oxide layer, resulting in a protective oxide film. Parabolic oxidation usually occurs when the temperature is relatively low. In linear oxidation, which occurs at higher temperature than parabolic oxidation, oxygen diffuses to the metal/oxide interface at a constant rate, leading to a nonprotective, growing oxide film.40 It has been reported that the transition temperature between the parabolic and linear oxidation of Al is approximately 450 °C, with a narrow temperature transition zone. W is known to exhibit parabolic oxidation between 400 and 500 °C, but its oxidation rate is higher than that of Al, even below its transition temperature.41−43 In our experiment, severe oxidation of W started between 300 and 400 °C. TiN is generally reported to exhibit parabolic and linear oxidation and is severely oxidized above 400 °C,44,45 although the TiN oxidation rate varies because it depends on the stoichiometry of TiN.46 The different oxidation behaviors of the metals explain the superior resistance to thermal oxidation in air for the Al-coated structure compared with the W- and TiN-coated ones. The photographs shown in Figure 5b,d,f indicate the color changes of the Al-, W-, and TiN-coated absorbers after thermal annealing. The Al-coated samples remained black until the annealing temperature reached 400 °C and turned gray after annealing at 500 °C. The W-coated absorbers were black before annealing and changed to yellowish-white, which is the color of WO3, after annealing at 400 °C. The TiN-coated absorbers changed from dark gray before annealing to golden at 400 °C and then to white at 500 °C. The SEM images in Figure S7−S9 (Supporting Information) show the structural changes of Al-, W-, and TiN-coated absorbers after thermal annealing. The Al-coated nanowires do not show distinct structural changes after annealing. The W-coated nanowires maintain their original morphology under the annealing temperature up to 300 °C. Above 400 °C, the surface of the W coating layer turns rough. The morphological changes of TiN-coated nanowires were not observed after annealing at 200−500 °C. To analyze the change of the material composition caused by thermal oxidation, we obtained X-ray diffraction (XRD) patterns of the absorber samples before and after annealing (Figure 6). An XRD pattern of Al2O3 nanowire bundles on an Al substrate is presented in Figure S10 (Supporting Information) for reference. The XRD patterns of the Alcoated absorbers (Figure 6a) exhibit no distinct changes after annealing. XRD peaks originating from Al were observed at 2θ values of 38.47°, 44.74°, 65.13°, and 78.23° (ICDD card no. 00-004-0787). The peaks at 40.05° and 42.75° are consistent with κ-Al2O3 (ICDD card no. 00-001-1305). The XRD patterns of the W-coated absorbers (Figure 6b) exhibit peaks attributed to W at 2θ values of 40.26°, 58.28°, and 73.20° (ICDD card no. 00-004-0806), and the intensity of these peaks decreased as the annealing temperature increased. When the annealing temperature was >400 °C, the formation of WO3 was confirmed by the appearance of XRD peaks that well

oxidation. The reflectance of the Al-coated absorbers (Figure 5a) increased slightly with the thermal annealing temperature. In contrast, those of the W- (Figure 5c) and TiN- (Figure 5e) coated absorbers increased substantially, especially when the annealing temperature was above 400 °C. (We measured the reflectance data using BaSO4 as a reflectance standard, which is not a perfect mirror in the NIR region; thus, the reflectance of the TiN-coated sample annealed at 500 °C exceeds 100% at some wavelengths.) W- and TiN-coated absorbers annealed at 500 °C showed a drastic decrease in reflectance below 480 and 400 nm, respectively. This is attributed to the formation of the metal oxide. WO3 and TiO2, which are the metal oxides of W and TiN, respectively, are semiconductor materials with band gaps of 2.75 and 3.2 eV, respectively.32,33 We calculated the difference between the reflectance of annealed and unannealed absorbers using Rdiff = Rannealed − Ras, where Rannealed is the reflectance of the absorber after annealing and Ras is the reflectance of the absorber before annealing. For the Al-coated absorbers annealed at 300, 400, and 500 °C, Rdiff values were