Walking the Walk: A Giant Step toward Sustainable Plasmonics Christopher J. DeSantis,† Michael J. McClain,‡ and Naomi J. Halas*,†,‡ †
Department of Electrical and Computer Engineering and ‡Department of Chemistry, Rice University, Houston, Texas 77005, United States
ABSTRACT: The use of earth-abundant materials is at the frontier of nanoplasmonics research, where their availability and low cost can enable practical mainstream applications and commercial viability. Aluminum is of specific interest in this regard, due to its ability to support plasmon resonances throughout the ultraviolet (UV), visible, and infrared regions of the spectrum. However, the lack of accurate dielectric data has critically limited the agreement between theoretical predictions and experimental measurements of the optical properties of Al nanostructures compared, for example, to the agreement enjoyed by the noble/coinage metals. As reported in this issue of ACS Nano, efforts by Cheng et al. to determine the dielectric function of pristine Al show that Al has substantially lower loss than was indicated by previously reported dielectric data for Al, including a 2-fold lower loss for the UV region compared to that in previous studies. These results provide data that are essential for accurate agreement between theory and experiment for Al plasmonic nanostructures, placing this earth-abundant metal on sound footing as a new and highly promising material for sustainable plasmonics by design.
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drug-delivery agents where the absorption properties of the nanoparticle can be used to release drug cargo selectively.8 Elsewhere, Au or Ag nanocrystals in various formulations can aid in disease diagnosis.8 For biomedical applications such as tumor targeting and sensing, the quantities of Au or Ag required are extremely modest. In many other potential applications, such as optical nanoantenna light harvesters for solar cells, light-scattering pixels for displays, or photocatalysts for industrial-scale processes, noble metal plasmonics will likely not be commercially viable, particularly in cases when a large amount of raw material is required. One only needs to be reminded of the high frequency at which noble-metal-based catalytic converters are stolen to recognize the immediate need to find sustainable alternatives for noble metals in many proposed applications that could be enabled or enhanced by plasmonic nanostructures.9 Al is a proposed alternative material to plasmonic noble/ coinage metals because it is 2−3 orders of magnitude cheaper
n the mid-1850s, Michael Faraday identified why gold colloid could have bright, colorful properties. More than 100 years would pass before the physical origin of the bright, vivid colors of nanoparticles, known as the localized surface plasmon, could be controlled well enough at the nanoscale to be useful in applications.1 Today, plasmonic properties are well-studied in a number of metals ranging from the coinage metals (Au, Ag, Cu) to materials possessing ultraviolet (UV) plasmonic features such as Rh and Ga.2−4 Part of the reason for the exponential growth of this field is due to our increasing abilities to “nanoengineer” absorption and scattering properties of nanomaterials through manipulation of their geometry. Additionally, secondary features such as Fano resonances can emerge due to the interaction between plasmon modes, enabling precise sensing measurements.5−7 In principle, these features could have extremely valuable commercial applications in many areas such as photovoltaics, flat-panel displays, and sensors. The wide variability of plasmonic features is already leading to a multitude of applications. Au nanoparticles, for example, are currently in development to be used as tumor-targeting © 2016 American Chemical Society
Published: November 8, 2016 9772
DOI: 10.1021/acsnano.6b07223 ACS Nano 2016, 10, 9772−9775
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photocatalysts, illuminated Al nanocrystals can efficiently dissociate hydrogen at ambient temperature and pressure (Figure 1A).21 Al nanocrystals can be combined with reactive metals in “antenna−reactor” photocatalyst complexes, greatly expanding the types of light-mediated chemical reactions whose efficiencies and selectivities can be improved by this approach (Figure 1B).22 Al nanoparticles embedded into an aluminum oxide membrane form the critical light-harvesting nanocomponent of an effective and inexpensive solar distillation apparatus (Figure 1C).23 Finally, Al nanoparticles have been developed as materials for CMOS-compatible, nontoxic display monitors with vibrant colors (Figure 1D).24,25 Collectively, these examples clearly show the promise of Al plasmonics for large-area, mainstream plasmon-enabled applications. Alongside these demonstrations of the use of Al nanomaterials, theoreticians have developed approaches to understand and to predict Al plasmonic phenomena.26 However, many of these predictions are based upon measurements of the dielectric function of Al films, where the purity and quality of the Al film growth is not comparable to highly crystalline and high-purity Al. We recently showed that the plasmon resonance frequency of an Al nanodisk is strongly dependent upon the purity of the bulk Al composition of the nanostructure; a Bruggeman model was invoked to describe how the presence of ultrasmall oxide impurities in the Al were responsible for shifting the plasmon resonance.10 Impurities in Al also needed to be accounted for in a more recent study of the narrowing of the dipolar plasmon resonance linewidth when an Al nanocrystalline sphere coupled to an Al film substrate.27 The need for accurate dielectric data for Al is most clearly seen, however, in studies of Al nanocrystals. Here, the dark-field spectra obtained experimentally show substantially narrower peaks than are predicted by the same type of theoretical model that shows highly accurate, quantitative agreement in the case of Au nanoparticles and nanostructures.20 For Al plasmonics to achieve the same predictive power in theoretical calculations that can be achieved for noble/coinage metals, in particular, Au, better dielectric data for ultrapure Al are essential and sorely needed. The work by Cheng et al. in this issue of ACS Nano provides a valuable new set of data for the description of the dielectric function of Al for pure, crystalline systems.28 In their work, Al is deposited onto a Si surface using a molecular beam epitaxy
than Au or Ag and one of the most abundant elements in the Earth’s crust.10 Already, Al is widely used in substrates for catalysis and lightweight materials for construction. Al is also complementary metal−oxide−semiconductor (CMOS) compatible for integrated device applications. Unlike other airsensitive metals, Al naturally forms a 2−4 nm protective oxide surface layer that inhibits further oxidation. At the nanoscale, Al is predicted to have extinction maxima tunable throughout the UV, visible, and infrared (IR) regions of the spectrum.10 Achieving nanoscale Al features is difficult due to the airsensitive nature of Al, but work with lithography and selective deposition has yielded tunable Al nanostructures that show the predicted plasmonic features.11−14
Al is a proposed alternative material to plasmonic noble/coinage metals because it is 2−3 orders of magnitude cheaper than Au or Ag and one of the most abundant elements in the Earth’s crust. For a great many large-scale commercial applications, however, bottom-up nanoparticle synthesis is one of the best approaches.15 In the case of noble metal and semiconductor nanomaterials, synthetic rules have been derived to control size, shape, and composition through the manipulation of kinetics and use of surface-capping agents.16 A supersaturation model, for example, has been developed to explain kinetic control of Au nanocrystal growth.17,18 However, for materials that are airsensitive, this growth approach remains challenging. Even so, there is a pressing need to push for scalable protocols for nanomaterials of earth-abundant materials. Recently, an approach was developed to prepare Al nanocrystals through the controlled decomposition of an amine alane precursor.19,20 Although this synthesis requires careful attention to synthetic preparation to overcome precursor oxidation, the results will enable commercially viable and widely scalable materials for a wide range of important purposes. Al nanomaterials are already showing unique optical properties that lead to high-value applications in photocatalysis, solar distillation, and pixel displays (Figure 1). As plasmonic
Figure 1. Illustrations depicting the versatility and commercial viability of Al nanoparticles. (A) Photodissociation of H2 using plasmonic Al nanoparticles. Reprinted from ref 21. Copyright 2016 American Chemical Society. (B) Photocatalytic hydrogenation of acetylene using Al nanoparticles that enhance hot electron generation of Pd islands. Reprinted with permission from ref 22. Copyright 2016 Proceedings of the National Academy of Sciences. Photodistillation of seawater using Al nanoparticles embedded in an Al2O3 membrane. Reprinted with permission from ref 23. Copyright 2016 Nature Publishing Group. (D) Chromatic display composed of Al nanorod arrays. Reprinted with permission from ref 24. Copyright 2014 Proceedings of the National Academy of Sciences. 9773
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system, which can precisely control the temperature and rate of Al deposition. Electron diffraction images confirmed that this approach can provide pure, crystalline Al. They find a lower loss through damping throughout the IR, visible, and UV regions compared to currently available literature values, which exhibit the same loss as disordered Al prepared by thermal deposition. This difference was particularly striking in the UV region, where pristine Al outperforms theory by a factor of 2. For these reasons, the authors recognized that the imaginary part of the dielectric function for Al for the high-purity case achievable in nanocrystals and UHV-grown films should be substantially less than what was previously determined.
The work by Cheng et al. in this issue of ACS Nano provides a valuable new set of data for the description of the dielectric function of Al for pure, crystalline systems. This important work provides invaluable input for theory and enables the accurate predictive design of Al plasmonic nanoparticles and nanostructures. Their findings of improved signals through pristine surfaces show an important direction for research in the field. This finding, corroborated by other recent research on pristine Al surfaces, clearly indicates the need to avoid defects and oxygen impurities in plasmonic Al materials.29 Although this work in and of itself is extremely important, there is still more work to be done: extending these measurements further into the UV range will be essential for describing the UV plasmonic properties of Al nanoparticles and nanostructures in a fully comprehensive manner. The work by Cheng et al. is a major step forward in the field of Al plasmonics, advancing this earth-abundant material toward equal footing with that of the noble/coinage metals, so that predictive design of plasmonic nanoparticles and nanostructures is more readily achievable. This work will hopefully stimulate a greater number of plasmonics researchers to focus on more sustainable materials. Rather than “talking the talk” of sustainability, this work will enable more researchers to “walk the walk” and to investigate applications that will enable a greater number of practical, manufacturable, commercializable, and sustainable applications of plasmonics with truly transformational impact.
AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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