Plasmonic Biomimetic Nanocomposite with Spontaneous

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Plasmonic Biomimetic Nanocomposite with Spontaneous Subwavelength Structuring as Broadband Absorbers Mingzhu Li, Urcan Guler, Yanan Li, Anthony Rea, Ekembu Kevin Tanyi, Yoonseob Kim, Mikhail A. Noginov, Yanlin Song, Alexandra Boltasseva, Vladimir M. Shalaev, and Nicholas A. Kotov ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b00583 • Publication Date (Web): 14 May 2018 Downloaded from http://pubs.acs.org on May 14, 2018

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ACS Energy Letters

Plasmonic Biomimetic Nanocomposite with Spontaneous Subwavelength Structuring as Broadband Absorbers Mingzhu Li,† Urcan Guler,§ Yanan Li, † Anthony Rea, ‡ Ekembu K. Tanyi,┴ Yoonseob Kim,‡ Mikhail A. Noginov,┴ Yanlin Song, † Alexandra Boltasseva,§ Vladimir M. Shalaev,§ and Nicholas A. Kotov‡*. †Key Laboratory of Green Printing, Institute of Chemistry, Chinese Academy of Sciences, Beijing Engineering Research Center of Nanomaterials for Green Printing Technology, Beijing National Laboratory for Molecular Sciences, Beijing, 100190, P. R. China. ‡Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48109-2136, United States. §School of Electrical and Computer Engineering and Birck Nanotechnology Center, Purdue University, West Lafayette, Indiana, 47907, United States. ┴ Center for Materials Research, Norfolk State University, 700 Park Ave., Norfolk, VA 23504, United States. Corresponding Author: [email protected]

ABSTRACT: Broadband plasmonic absorbers are essential components for photovoltaic, photothermal, and light-emitting devices. They are often made by lithographic processes that impart out-of-plane surface features with subwavelength dimensions to metallic films. However, lithographic subwavelength patterning of inexpensive plasmonic ceramics, such as TiN is challenging due to high-temperature processing and chemical robustness of these materials. In this work, we show that layer-by-layer assembly (LBL) of TiN plasmonic nanoparticles with polyelectrolytes results in spontaneous formation of out-of-plane topography with subwavelength dimensions. Columnar morphology of these corrugated coatings and their plasmonic functionality results in broadband absorption capabilities exemplified by 90% of the light from UV to IR parts of the spectrum. The method is applicable to large flexible and conformal surfaces with complex geometry. It is also fast, scalable, and environmentally friendly. LBL processing of TiN nanoparticles demonstrates the possibility of replacement of lithographic patterning with stochastic self-assembly processes in manufacturing of photonic metasurfaces.

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TABLE OF CONTENTS FIGURE

Broadband absorbers are essential as components for photovoltaic, photothermal, and some light-emitting devices.1–4

An ideal black body absorbs 100% of light at all angles for all

wavelength, which is difficult to realize in common materials because of scattering and reflection off the surface leading to incomplete absorption. Low-density “forest” from vertically aligned carbon nanotubes has been reported to be a nearly ideal black body coating with corresponding integrated total reflectance of 0.045%.5

Integration of these coatings in photonic devices and

their subsequent use are possible but their fragility makes it practically cumbersome.6 Coatings from metal nanoparticles (NPs) capable of strong interaction with light via plasmonic resonances broadened by the strong coupling between nanoscale subwavelength structures have also been studied for the same purpose.7–11 However, such patterned substrates, also known as metasurfaces,12–14 require top-to-bottom lithographic techniques capable of producing subwavelength surface features on the order of and below 100 nm scale, which limit


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their technological impact especially for emerging broadband absorber technologies related to harvesting of solar and thermal energy.15

The latter require large and often curved surface areas

to capture the maximum amount of light under different daylight conditions. However, existing methods of high-resolution lithography of plasmonic absorbers are mostly limited to planar wafers and cannot be fabricated on surfaces with complex multiscale topographies. Furthermore, lithographic patterns of noble plasmonic metals are convenient as models but their limited availability make them less attractive as practical large-scale absorbers. Less expensive plasmonic materials, such as nanoscale copper, are unstable in the environment due to oxidation. Here we utilized layer-by-layer assembly (LBL) known for its ability to engineer unusual combination of properties in biomimetic composites, to produce titanium nitride (TiN) nanocomposites from negatively charged NP and positively charged polyurethane (PU).

TiN

shows plasmonic absorption in the spectral window from around 500 nm to ~1100 nm.16

TiN

NPs demonstrated, in fact, 50% higher absorption efficiency than other materials when the peak positions were comparable.17

Robustness and thermal stability of plasmonic ceramics

represented by TiN makes them a desirable material for broadband absorbers in almost all the aspects broadband absorption except metasurface processing.18 With some surprise, we noticed that LBL assembly of TiN composites causes spontaneous formation of subwavelength structures due to self-assembly of TiN during film deposition.

Besides expected in-plane

self-organization, the resulting composite coatings with distinct out-of-plane mesostructures unlike previous examples of multilayers for which spontaneous flattening were observed.19 The high roughness of the three-dimensional nanocomposite revealed characteristic dimensions in nano- and mesoscale.

Combined with plasmonic characteristics of TiN NPs, such topography

of the multilayers enabled their broadband absorption functionality.

Light absorbance


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approached ca. 90% in the wavelength range from ultraviolet (UV) to infrared (IR) parts of the spectrum making it a comparable to lithographic metasurfaces,15,20 while providing a new approach to their scalable production and simplicity of realization of broadband absorbers for surfaces of complex geometries.21 We utilized gas-phase synthesized TiN powders with an average NP size of 50 nm and broad size distribution with some of the particles as large as 300 nm. The NPs are predominantly cubic in shape with some presence of polyhedral with an average NP size of 50 nm and broad size distribution (Figure S1a). Their plasmon resonance peak is broad and featureless (Figure S1b), which is both expected from polydispersed NPs and desirable for broadband absorbers. The oxide layer on the NPs provides convenient chemical “anchor” for surface functionalization (Figure 1a) enabling adsorption of hydroxypropyl cellulose (HPC) to make the TiN NPs stabile in aqueous dispersion (Figure 1b). In accord with the classical LBL process,19 a single deposition cycle included sequential adsorptions of all the components that was repeated N times to obtain a film with nominal structure as (PU/TiN/PSS)N. The HPC shell prevented the NPs merged together resulting in a golden color of the composite.31 The light absorption by the composite coatings can be controlled by the number of LBL cycles, N (Figure 2a) and the concentration of the TiN dispersion (Figure 2b,c). The increase of the NP loading on the slides resulted in rapid increase of light absorption. Considering how fast the accumulation of light absorbing species on the substrate takes place, one can speculate that LBL process here differs from the classical multilayer deposition pattern typically assuming adsorption of self-limited monolayers of NPs or polymers.22–25

Concomitantly, neither the size of the NPs nor the

chemical structure of the polymeric chains used here are conducive with the exponential LBL deposition involving diffusion of the components in and out of the composite film.26


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The TiN nanocomposite film has structure that is different from the composite film usually obtained by LBL method22–25 or other methods used for deposition of TiN NPs on complex surfaces, for instance using covalent conjugation.27

Depending on the conditions, the NPs may

cluster or deposit uniformly over the entire surface.28

We found that the formation of the

mesoscale clusters takes place for TiN NPs in the initial stages of the deposition (Figure 3). At the first 1, 2, and 3 cycles, the TiN composite cluster formed islands dispersing all over the substrate. As the N > 3, the islands are transformed into peaks (Figure 3b) indicating that the lateral expansion is accompanied by the out-of-plane growth of the composite films. The diameters of the porous columnar assemblies are from 200 nm to 500 nm with some of the features as large as 1 µm.

The diameters of the nanopores in the mesoscale columns are from

50 nm to 300 nm (Figure 3d). The spontaneous out-of-plane structuring of the composite films can be associated with a specific combination of attractive and repulsive forces between TiN NPs.

Their electrokinetic

potential is small, it does not cause electrostatic repulsion between the NP nor strong attraction to the positively charged PU layer.

Simultaneously the van der Waals attraction between the NPs

due to size in polarizability is high, which makes the NP to cluster with each other. Optical properties of (PU/TiN/PSS)N were investigated for N = 5, 7, 9 and 11 that were 111 nm, 308 nm, 798 nm and 1042 nm thick, respectively (Figure 4a). The light extinction passing through (PU/TiN/PSS)11 was at least 90% in the spectral window of interest (Figure 2b).

The

combination of mesoscale and nanoscale structuring reduced reflectance that helps approach black body (Figure 4b). The total normal reflectance of (PU/TiN/PSS)1 and (PU/TiN/PSS)2 is reduced to 9%. For (PU/TiN/PSS)21, the absorbance is up to 90% from 300 nm to 2500 nm (Figure 4c).

Particularly, in the range from 350 nm to 700 nm, the total reflectance is lower


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than 5% and the absorbance is over 95% even at normal incidence.

With increase of the

incidence angle, the reflectance increases and the absorbance decreases (Figure 5). The absorbance A is larger and the reflectance R is smaller in p polarization than in s polarization. The 3D topography of the composite film is the key factor to the strong broad absorption.29,30 The polymer components of the (PU/TiN/ PSS)N, PU and PSS, serve as not only as “cement” to stick the nanoparticles into 3D structure and keep the film robust, but also as insulators to prevent the nanoparticle merging together and be well dispersed. Additionally, the polymer components can reduce the refractive index, which is crucial for anti-reflectance.

Given the

out-of-plane topography, nanoscale pores, and interparticle gaps light entering the composite is efficiently scattered and absorbed by it.32,33 Strong coupling between the NPs10,20,34,35 stimulate strong dissipation of light energy. The performance of nanocomposite film for solar energy absorption was tested in Solar Simulator recording the surface temperature with thermographic camera (Figure 6b,c). Temperature during light irradiation of (PU/TiN/PSS)11 increased by 68.6 º (from 18º to 86.6 º) with homogenous spatial distribution, which is 3x higher than that of the control sample whose temperature increased by 22.9 º (from 18 º to 40.9 º). One could notice that the topography of TiN composite films with columhar agglomerates is similar to that observed in scales of Cyphochilus beetle.36

Its disorder and its dimensions are

enhance scattering over a large range of wavelengths and angles of incidence.32 In summary, spontaneous subwavelength structuring of (PU/TiN/PSS)N films allows remarkable enhancement of the overall absorption efficiency. In contrast to other broadband absorbers, e.g. metal nanopatterned metasurfaces fabricated using lithography, the LBL-based absorber can be


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ACS Energy Letters

fabricated over macroscopic areas in the roll-to-roll format and on arbitrary surface using simple solution based deposition. The technical realization of precision photonic heating with microscale resolution is expected to be the foundation of their practicality. Considering universality of NP self-assembly, one may also look for other bioinspired applications of composites with columnar out-of-plane topography in other technological areas.

CONFLICT OF INTERESTS: The authors declare no competing financial interests.

ACKNOWLEDGMENTS:

This material is based upon work partially supported by the

Center for Solar and Thermal Energy Conversion, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, and Office of Basic Energy Sciences (DE-SC0000957). The authors would like to than National Science Foundation for NSF (Grant No1538180). We are also greatly grateful to China Scholarship Council. This work was also supported by the NSFC (Grant Nos. 21522308, and 51573192). We also acknowledge Dr. Haiwei Yin and Shanghai Ideaoptics Corp. Ltd. for the help in reflection spectra measurement. NSU: Air Force Office of Scientific Research (AFOSR) (FA9550-14-1-022); National Science Foundation (NSF) (DMR 1205457 and DGE 0966188); Army Research Office (ARO) (W911NF-14-1-0639). Purdue co-authors acknowledge support from NSF (NSF-DMR-1506775 grant) and AFOSR MURI grant (FA9550-14-1-0389).

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FIGURES

Figure 1. TEM images of (a) pristine TiN NPs and (b) after stabilization by hydroxypropyl cellulose (HPC).


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Figure 2. (a) The absorbance spectra of the TiN NP dispersion. The photographs of TiN composite layers made after the specified number of LBL cycles. “0” is the glass slides without LBL films, “c” is the control sample carrying NP-free LBL film (PU/PSS)1, 1~11 are the series sample of (PU/TiN/PSS)N, N = 1, 2, 3, 5, 7, 9, 11, respectively. (b,c) The extinction spectra of the TiN composite absorbers made from TiN-HPC dispersions stabilized by HPC for different N indicated in the graphs. The concentration of TiN-HPC dispersion was 0.5 mg/ml.


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Figure 3. SEM images of TiN composite absorber. (a-e) top views of (PU/TiN/PSS)N, N = 1, 3, 5, 7, 9, respectively. (f) SEM image of (PU/TiN/PSS)9 at 30o angle view of surface. Scale bars in a,b,c,d,e are 1µm; scale bar in f is 500 nm.


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Figure 4. (a,b) Absorbance and total reflectance of TiN composite absorbers with (PU/TiN/PSS)N multilayer sequence and N = 1, 2, 3, 5, 7, 9, 11, 21 LBL processes. (c) The absorbance (blue line), total reflectance (the red line) and transmission (the black line) spectra of (PU/TiN/PSS)21.


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Figure 5. Absorbance spectra of the (PU/TiN/PSS)9, taken at multiple angles of incidence (as indicated in the figure) in s polarization (a) and p polarization (b). Reflectance spectra of the (PU/TiN/PSS)9, taken at multiple angles of incidence (as indicated in the figure) in s polarization (c) and p polarization (d).


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Figure 6. (a) Temperature plotted with respect to time for TiN composite absorbers. (b, c) Typical thermal images of (PU/TiN/PSS)11 and PU/PSS after 2 min exposed under Solar Simulator illumination. At the edge of the films, heat dissipated slight along the circular edges of the coatings.


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Plasmonic Biomimetic Nanocomposite with Spontaneous

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