Aluminum Nanocubes Have Sharp Corners - ACS Publications

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Aluminum Nanocubes Have Sharp Corners Benjamin D Clark, Christian Jacobson, Minhan Lou, David Renard, Gang Wu, Luca Bursi, Arzeena S. Ali, Dayne F. Swearer, Ah-Lim Tsai, Peter Nordlander, and Naomi J. Halas ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.9b05277 • Publication Date (Web): 09 Aug 2019 Downloaded from pubs.acs.org on August 9, 2019

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Aluminum nanocubes have sharp corners 254x190mm (150 x 150 DPI)

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Aluminum Nanocubes Have Sharp Corners Benjamin D. Clark,†,‡ Christian Jacobson,†,‡ Minhan Lou,§,∥ David Renard,†,‡ Gang Wu,⊥ Luca Bursi,§,‡ Arzeena S. Ali,†,‡ Dayne F. Swearer,†,‡ Ah-Lim Tsai,⊥ Peter Nordlander, §,∥,‡ and Naomi J. Halas†,∥,‡,* †

Department of Chemistry, §Department of Physics & Astronomy, ∥Department of Electrical &

Computer Engineering, ‡Laboratory for Nanophotonics, Rice University, 6100 Main Street, Houston, Texas 77005, United States ⊥Division

of Hematology, Department of Internal Medicine, The University of Texas McGovern

Medical School, 6431 Fannin St, Houston, Texas 77030, United States

Abstract: Of the many plasmonic nanoparticle geometries that have been synthesized, nanocubes have been of particular interest for creating nanocavities, facilitating plasmon coupling, and enhancing phenomena dependent upon local electromagnetic fields. Here we report the straightforward colloidal synthesis of single-crystalline {100} terminated Al nanocubes by decomposing AlH3 with Tebbe’s reagent in tetrahydrofuran. The size and shape of the Al nanocubes is controlled by the reaction time and the ratio of AlH3 to Tebbe’s reagent, which, together with reaction temperature, establish kinetic control over Al nanocube growth. Al nanocubes possess strong localized field enhancements at their sharp corners and resonances highly amenable to coupling with metallic substrates. Their native oxide surface renders them extremely air-stable. Chemically synthesized Al nanocubes provide an earth-abundant alternative to noble metal nanocubes for plasmonics and nanophotonics applications.

Keywords: shape-control, plasmonics, field-enhancement, {100} faceted, nanotechnology


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Colloidally synthesized metallic nanocubes are one of the essential building blocks of plasmonics.1–3 Their cubic shape ensures that they couple strongly to adjacent nanoparticles or substrates, creating nanoscale cavities with large local electromagnetic fields. Their edges and corners enhance the local electromagnetic field at the nanometer length scale to a greater extent than spheroidal nanoparticles.4–7 The optical properties of noble or coinage metal nanocubes arise from their ability to support localized surface plasmon resonances, the collective oscillations of conduction band electrons, when excited. Plasmonic nanocubes can couple to a plasmonic substrate, creating a Fano resonance in their scattering spectrum that is highly sensitive to their local dielectric environment, making them useful for plasmonic sensing.8,9 Au and Ag nanocube syntheses are well established, and the properties and applications of these nanoparticles have been pursued extensively. More recently, aluminum (Al) has begun to emerge as an alternative, highly earth-abundant plasmonic metal that can support plasmon resonances from the ultraviolet (UV) into the infrared (IR), depending on the size and shape of the Al nanostructures.10–15 However, Al nanoparticle nucleation and growth is chemically distinct from that of the noble/coinage metal nanoparticles. Al nanocrystal synthesis is mediated by a catalyst which forms a complex with both reactant and solvent molecules under O2- and H2O-free conditions, directing the earliest stages of nanoparticle nucleation and growth.16 Although a shape-controlled synthesis of monodisperse Al nanocrystals using a polymer ligand was recently demonstrated,17 shape-controlled growth of Al nanocrystals is still in its relative infancy. The native oxide surface of Al nanocrystals is amorphous,18,19 possessing basic reactive sites, in contrast to the acidic sites characteristic of Al2O3 crystalline bulk phases.20 Chemical modifications of the surface oxide have expanded Al nanocrystal properties, enabling new applications. These include the growth of ultrasmall reactive metal islands21,22 and metal oxide layers23 for “antenna-reactor” photocatalysts, polymers for


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greatly enhanced stability in aqueous solution,24 and metal-organic frameworks for precision etching of Al nanocrystal size.25 Here we report the straightforward synthesis of {100}-terminated Al nanocubes using readily available reagents, performed in an inert environment using standard Schlenk line techniques. The reaction proceeds through the decomposition of an AlH3 precursor catalyzed by Tebbe’s reagent (C13H18AlClTi) in the solvent tetrahydrofuran (THF). We examine how reactant and catalyst concentration, along with reaction temperature, control the size and shape of the nanocubes synthesized by this approach. The nanocubes formed are highly crystalline and produced with a high yield, with morphologies that include concave-cubic nanoparticles. We also examine how the plasmonic properties of Al nanocubes are affected by the presence of an adjacent metallic film substrate, examining the coupling between an Al nanocube plasmons and that of underlying noble/coinage metallic films of various types.


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Fig. 1. Chemical synthesis of single-crystalline Al nanocubes using Tebbe’s reagent. (A) Reaction of excess dimethylethylamine alane (DMEAA) with Tebbe’s reagent produces Al nanocubes in THF. (B) High-resolution TEM image of a ~55 nm Al nanocube. (C) High-resolution TEM image of a corner of this nanocube, showing the atomic lattice of Al viewed along the direction of the nanocrystal. (D) Selected area electron diffraction pattern of this cube. (E) Dark-field TEM image from one of the diffracted electron beams. (F) Tilted SEM image of an aggregate of ~150 nm Al nanocubes. (G) Powder X-ray diffraction pattern from dried Al nanocubes. Results and Discussion The reaction of Tebbe’s reagent with an excess of AlH3 in THF and a reaction temperature of 65-70 C produces single-crystalline Al nanocubes within two hours (Fig. 1A, B). High-


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resolution transmission electron microscopy (TEM) of an individual Al nanocube indicates that the corners of this cube have a radius of curvature of ~5 nm (Fig. 1C). Electron diffraction along the [100] zone axis confirms the single-crystalline face center cubic structure of the Al nanocubes (Fig. 1D) and allows for the acquisition of a dark-field TEM image (Fig. 1E). The Al nanocubes characteristically adsorb onto substrates on one of their flat sides, which are all terminated by {100} facets, allowing them to assemble like blocks, as shown in the tilted scanning electron microscopy (SEM) image (Fig. 1F). Their cubic shape and {100} facets were evident on the ensemble level from the anomalously large diffraction peak from the {200} planes in the powder X-ray diffraction spectrum of Al nanocubes dried onto a background-free, amorphous silicon substrate (Fig. 1G).

Fig. 2. Kinetic control of Al nanocube size by reaction time. (A) Extinction spectra from Al nanocubes removed from a 200:1 AlH3:Tebbe ratio reaction at different times. (B) Graph of the peak wavelength of the plasmonic modes of the Al nanocubes as a function of reaction time. (C) Associated TEM images of the Al nanocubes with sizes ranging from ~50 to ~150 nm.


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Al nanocubes with average sizes of 52.9  6.9, 92.8  7.4, 130.6  9.9, and 158.4  14.9 nm were synthesized from a one-pot reaction by removing aliquots at 30 minute intervals and quenching them in a 50 mM solution of dibutyl phosphate in cyclohexane (Fig. 2). The reaction was performed with a 200:1 molar ratio of AlH3 to Tebbe’s reagent, and the size resolution was obtained from more than 100 particle measurements for each size. Control over the size of the Al nanocubes allows their dipolar resonance to be tuned from the UV across the visible and into the near-IR (Fig. 2A). As the size of the Al nanocubes increases, quadrupolar and higher order plasmon modes appear in the visible spectrum (Fig. 2B). Representative TEM images of the aliquots indicate that their cubic morphology is already established early in the reaction, followed by uniform growth on the {100} facets (Fig. 2C). The formation of a cubic morphology so early in this reaction contrasts with the previously reported synthesis of cuboctahedral Al nanocrystals, which uses a polymer ligand that passivates the {100} facets of Al.17 In the present case, Al nanocubes with sharp corners were observed throughout the reaction: in the previously synthesis with titanium(IV) isopropoxide as the catalyst, Al nanocrystals grew from isotropic truncated octahedral particles into cuboctahedral nanocrystals.17 These observations indicate that the kinetically controlled growth of {100} terminated Al nanocrystals using Tebbe’s reagent is fundamentally different than reactions catalyzed with titanium(IV) isoproxide, whether or not a {100} facet-specific polymer ligand is used.16


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Fig. 3. Formation of the active catalyst for Al nanocube growth. (A) AlH3 reacts with Tebbe’s reagent to form Ti3+Cp22H2AlH-THF, which can undergo C-H bond activation of the Cp ring to form an Al-C bond (bold red) (B) EPR spectroscopy confirms the formation of the intermediate complex Ti3+Cp22H2AlH-THF, as the spectrum can be accurately simulated by considering the spin density of the unpaired electron delocalized among the nuclei indicated by purple spheres (The protons of the cyclopentadienyl rings in this complex were included in the simulation of the EPR spectrum but were omitted for clarity). (C) TEM of a 200:1 AlH3:Tebbe ratio after ~3.5 hours of reveals the kinetic growth of Al nanocubes using Tebbe’s reagent. (D) AlH3 reacts with TiCp*2Cl2 to form the radical complex Ti3+Cp*22H2AlH-THF. (E) EPR spectroscopy corroborates the formation of Ti3+Cp*22H2AlH-THF, which has electron spin density delocalized onto the nuclei indicated by light blue spheres. (F) TEM image of thermodynamic {111} passivated Al nanocrystals resulting from the reaction of TiCp*2H2AlH-THF with excess AlH3. To further probe the growth chemistry of Al nanocubes, we performed UV-visible, proton nuclear magnetic resonance (1H NMR) and electron paramagnetic resonance (EPR) spectroscopies following the reaction of AlH3 with Tebbe’s reagent at room temperature. As shown in the photographic images and UV-visible spectra, when a small excess of AlH3 is added to Tebbe’s reagent at room temperature, the solution turns from bright orange to brown and then a pale lavender color, indicating the reaction of Tebbe’s reagent with AlH3 to form a new molecular species (Fig. S1). The 1H NMR spectrum of Tebbe’s reagent with an equivalent of AlH3 (Fig. S2)


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confirms that these molecules react, based on the disappearance of NMR signals from Tebbe’s reagent (CH2, singlet 8.3 ppm, C5H5 singlet 5.6 ppm, and Al(CH3)2 singlet -0.25 ppm).26 Meanwhile, new spectral signatures corresponding to CH4 (singlet 0.16 ppm), H2 (singlet 4.55 ppm) and AlCl(CH3)2 (singlets -0.42 and -0.5 ppm) were observed. Addition of a second equivalent of AlH3 turned the solution to a purple color, consistent with analysis of the reaction by UV-visible spectroscopy (Fig. S1). EPR spectroscopy provides conclusive evidence for the formation of a purple bimetallic Ti-Al complex that is stable at room temperature. The reaction between Tebbe’s reagent and an excess of AlH3 produces Ti3+Cp22H2AlH-THF (Fig. 3A). The molecular structure of Ti3+Cp22H2AlH-THF was constructed based on the exceptional match between the measured and simulated EPR spectra acquired from the purple solution (Fig. 3B). With a 50:1 ratio of AlH3 to Tebbe’s reagent or greater and at elevated reaction temperatures (~6570C), Ti3+Cp22H2AlH-THF catalyzes reductive elimination of H2 from AlH3, leading to the formation of low-valent aluminum hydride intermediates that grow into Al nanocubes. The presence of concave nanocubes with high index facets at the end of the reaction between this catalyst and excess AlH3 (~3.5 hours with 200:1 AlH3:Tebbe, Fig. 3C) indicates that the growth of Al nanocubes is kinetically controlled. Concave nanocubes are kinetic products that have been observed in cases where the deposition of adatoms onto a nanocrystal seed occurs faster than the surface diffusion of the adatoms to thermodynamically favorable positions on the nanocrystal. 27 To investigate which ligands from Tebbe’s reagent are responsible for the growth of Al nanocubes, analogues

including

titanocene(IV)

dimethyl,

titanocene(IV)

dichloride,

titanium(IV)

cyclopentadienyl trichloride, zirconocene(IV) dihydride, vanadocene(IV) dichloride, vanadocene and bispentamethylcyclopentadienyl titanium(IV) dichloride (TiCp*2Cl2) were screened (representative TEM images of the resulting Al nanocrystals are shown in Fig. S3 and Fig. 3F for


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TiCp*2Cl2). Because Tebbe’s reagent, titanocene(IV) dichloride and titanocene(IV) dimethyl all produced Al nanocubes, one can rule out contributions from the AlCl(CH3)2 byproduct in producing {100} facetted Al nanocrystals. Indeed, reactions catalyzed with titanium(IV) isoproxide with an equivalent of AlCl(CH3)2 added produced truncated octahedral Al nanocrystals with a mixture of {111} and {100} facets (Fig. S4). Only TiCp*2Cl2, titanium(IV) cyclopentadienyl trichloride and vanadocene failed to produce nanocubes, suggesting that the tetrahedral molecular geometry of the transition metal catalyst and both cyclopentadienyl rings are essential for the synthesis of Al nanocubes. TiCp*2Cl2 reacts quickly with AlH3 to form a bimetallic Ti-Al radical complex, Ti3+Cp*22H2AlH-THF (Fig. 3D), which was verified using EPR spectroscopy (Fig. 3E). However, in stark contrast to Tebbe’s reagent, which yields {100} terminated Al nanocubes, Ti3+Cp*22H2AlH-THF produced {111} faceted Al nanocrystals (Fig. 3F). Analysis of these two catalysts by EPR spectroscopy indicates that in both cases, Ti3+ catalyzes the reductive elimination of H2 from AlH3, reducing Al in a manner analogous to our previously described mechanism of titanium(IV) isopropoxide-catalyzed Al nanocrystal growth.16 This comparison between the active Ti3+Cp22H2AlH-THF and Ti3+Cp*22H2AlH-THF catalysts suggests that C-H bond activation of the cyclopentadienyl ring results in the formation of an Al-C bond, which is a crucial mechanistic step in the growth of {100} facetted Al nanocubes. Preventing C-H bond activation of the cyclopentadienyl ring, as in the case with TiCp*2Cl2, precludes the formation of Al nanocubes. Presumably, occurrence of this Al-C bond mediates the formation of a molecular species that binds selectively to {100} facets, although the exact structure of this molecule is presently unknown. Additional spectroscopic evidence is required to further understand the growth mechanism of Al nanocubes from AlH3 using Tebbe’s reagent. Density functional theory (DFT) calculations of the equilibrium geometry of Ti3+Cp22H2AlH-THF and


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time-dependent DFT of its optical properties support the hypothesis of Al-C bond formation between a carbon atom in the cyclopentadienyl ring and Al (Fig. S5). In the equilibrium geometry of Ti3+Cp22H2AlH-THF the H atom bonded to the C atom nearest to the Al atom is pushed above the plane of the cyclopentadienyl ring, which indicates an Al-C interaction. This Al-C interaction appears to slightly buckle the square Ti2H2Al fragment in the bimetallic complex, pushing the hydrides together and predisposing the molecule to the reductive elimination of H 2. Formation of Al-C bonds were previously observed through X-ray crystallography of an analogous bimetallic Ti-Al

complex

stabilized

by

a

tris(trimethyl

silane)

methyl

(C(TMS)3)

ligand,

TiCp22H2AlHC(TMS)3.28 When heated to 100 C in benzene, TiCp22H2AlHC(TMS)3 loses a molecule of H2 and forms the dimer (TiCp(C5H4)2H2AlC(TMS)3)2, which contains two Al-C bonds created by C-H bond activation of the cyclopentadienyl rings.28

Fig. 4. (A) TEM images of reactions after three hours with different ratios of AlH3 to Tebbe’s reagent. 10:1 Inset: possibly an Al nanocube seed. (B) SEM of Al nanorods from an Al nanocube reaction with a 200:1 ratio of AlH3 to Tebbe’s reagent. Inset: 45 tilted SEM image of an individual Al nanorod with a pentagonal cross-section. (C) 45 tilted SEM of trigonal right bipyramidal Al


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nanocrystals produced alongside Al nanocubes. (D) Statistics of Al nanocrystal shapes from three separate Al nanocube reactions (472 particles measured). The ratio of AlH3 to Tebbe’s reagent is critical to controlling the Al nanocube morphology. For optimal synthesis of Al nanocubes, a diluted stock of Tebbe’s reagent should be prepared in anhydrous toluene, since Tebbe’s reagent slowly decomposes in THF.29 However, reactions in pure toluene do not produce Al nanocubes with sharp corners so THF is the optimal reaction solvent (a representative TEM image of the products of a synthesis reaction performed in pure toluene is shown in Fig. S6). When there is too much Tebbe’s reagent in the reaction, irregular cubic Al nanocrystals with lumpy surfaces are produced (Fig. 4A, 10:1 ratio of AlH3:Tebbe). As the ratio of AlH3 to Tebbe’s reagent increases, Al nanocubes with concave high index facets are produced alongside the cubic Al nanoclusters with bumpy surfaces (Fig. 4A, 50:1 ratio). Between a 100:1 and 200:1 ratio of AlH3 to Tebbe’s reagent, regular Al nanocubes terminated by {100} facets are the major product of the reaction (Fig. 2 and Fig. 4A). Further increasing the ratio of AlH3 to Tebbe’s reagent results in the truncation of the Al nanocubes, exposing {111} facets at the corners of the cubes (Fig. 4A, 500:1 ratio). The formation of truncated Al nanocubes with low amounts of Tebbe’s reagent is attributed to the reduced concentration of the molecular species responsible for producing {100} faceted Al nanocubes. X-ray photoelectron spectroscopy of the sample with a 10:1 ratio of AlH3 to Tebbe’s regent indicated that less than 1% of the sample consisted of Ti, which was in the 4+ oxidation state (Fig. S7). With a 100:1 ratio of AlH3 to Tebbe’s reagent, no Ti was detected by X-ray photoelectron spectroscopy (Fig. S7). Consistent with previous investigations, these results indicate that Ti is not doped into the Al clusters, but most likely resides on their surfaces during Al nanocrystal growth.16 However, in this case, the ligands of Tebbe’s reagent facilitate the synthesis of {100} faceted Al nanocrystals through kinetic control as opposed to thermodynamic growth that was observed with titanium(IV) isopropoxide, which


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resulted in truncated octahedral Al nanocrystals consisting of {111} and {100} facets. Multiply twinned Al nanorods with a pentagonal cross-section and consistent aspect ratios of ~3 were observed concomitant with the growth of single-crystalline Al nanocubes with a 200:1 AlH3:Tebbe ratio reaction (Fig. 4B). The common aspect ratio of the Al nanorods is attributed to passivation of the {100} facets along the edges of the nanorods while growth preferentially occurs at the {111} facetted endcaps of the nanorods. These pentagonal Al nanorods grow in the direction consistent with previous work describing the synthesis of single-crystalline Al nanorods and pentagonal Au nanorods grown from decahedral seeds.15,30 Meanwhile, the singly-twinned Al nanocrystals adopted a trigonal right bipyramidal morphology terminated by {100} facets (Fig. 4C) as previously observed with Ag and palladium nanocrystals.31,32 With a fresh solution of Tebbe’s reagent, the percent yield of Al nanocubes is 66  15%, while 20  9% of the particles are twinned trigonal right bipyramidal nanocrystals and 5  4% are multiply twinned nanorods that have pentagonal cross-sections (Fig. 4D). Irregularly shaped Al nanocrystals constituted 9  3% of the reactions, primarily at the expense of the Al nanocube yield. Over 90% of the Al nanocrystals synthesized using Tebbe’s reagent possess {100} terminated facets that manifest into cubic, right bipyramidal and pentagonal nanorods morphologies depending on the crystallinity of the Al seeds. The synthesis of Al nanocrystals with high shape homogeneity remains an ongoing research challenge, since high-precision control of the seed crystallinity remains elusive. Seeded growth of Al nanocrystals appears to be an obvious route to samples of a single morphology, considering the success of this method in the synthesis of noble metal nanorods.30 However, seeded growth of Al nanocrystals using noble metal nanoparticles has not been experimentally realized due to the distinctive chemistry involved in Ti3+ catalyzed decomposition of AlH3 into metallic Al, which is fundamentally different from noble metal nanoparticle synthesis. For this reason, post-


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synthetic sorting of the various nanocrystalline reaction products based on their shape may be more practically feasible but also remains an open challenge, due in part to the reactivity of Al nanocrystals in aqueous solutions.24,32 Although the stability of Al nanocrystals and nanocubes in aqueous solution is limited, Al nanocubes have shown outstanding stability in air. We have imaged and analyzed Al nanocubes that have been stored in ambient laboratory air for as long as 9 months and have not observed changes in composition or in morphology that would indicate additional oxidation relative to their 2-4 nm thick native oxide shell layer (Fig. S8).


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Fig. 5. Size- and shape-dependent optical properties of Al nanocubes. Experimental measurements are shown with solid lines while dashed lines show the theoretical scattering spectra computed using the finite difference time domain (FDTD) method for Al nanocubes on quartz substrates with dimensions corresponding to the SEM images. (A) Effect of Al nanocube size on the dark-field scattering spectrum of cubes with edge lengths of 140 (dark blue), 110 (red) and 60 nm (black) determined from the associated SEM images. (B) Effect of the corner geometry demonstrated by cubes with concave higher index faces (orange), regular {100} sides (purple) and truncated corners with {111} facets (green) as revealed by correlated SEM images. (C) P (blue) and S (pink) polarized scattering spectra of a 110 nm Al nanocube on a quartz substrate. (D) Corresponding charge distribution plots for the dipole and quadrupole modes excited by P (blue) and S (pink) polarized light.

The optical scattering of individual cubes of different sizes was characterized with single nanoparticle dark-field scattering, using a custom-built instrument capable of measuring from 1.8 to 5.5 eV.12 The scattering measurements (Fig. 5A) show size-tunable dipolar and higher-order modes from the UV across the visible spectrum, separated by a pronounced Fano resonance, as previously observed with Ag nanocubes. However, an Al nanocube that is the same size as a Ag nanocube has a quadrupole resonance at a shorter wavelength. Due to phase retardation, for a 100 nm Al nanocube or nanosphere positioned on a dielectric substrate, the quadrupole mode is a relatively bright mode with a width that is narrower than the dipole mode. Destructive quadrupoledipole mode interference in the far-field can result in backward scattering suppression which partially contributes to the observed scattering dip between dipole and quadrupole in Al nanocubes.33 The dipole-quadrupole coupling introduced by the substrate further enhances the scattering of the hybridized quadrupole and weakens the hybridized dipole.34 This Fano resonance is clearest when exciting with S-polarized light, as the S-polarized dipole and quadrupole modes have the same symmetry in their charge distribution at the substrate, whereas the P-polarized modes do not (Fig. 5D). This substrate-mediated charge distribution symmetry allows the Spolarized modes to couple with each other, forming the Fano resonance (Fig. 5C).35 The scattering


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measurements were complemented by finite difference time domain (FDTD) calculations, which allow for mode identification and further analysis of the system (Fig. 5C, D). Consistent with previous theoretical calculations for single-crystalline Ag and Al nanoparticles, the cube Fano resonance is highly substrate dependent and not observed in the absence of a substrate, or when the particle-substrate interaction is too strong, such as the case of a metal cube on a metal film.9,36 Single particle measurements and FDTD calculations were also used to examine the effect of the corner geometry (Fig. 5B). Similarly sized particles were measured, approximately 110 nm across, with either truncated corners, regular cube corners and flat {100} faces, or pointed corners (with high-index concave faces). A slight blue-shift is observed with increasing truncation of the corners due to the increased separation between the induced surface charges on the corners and the substrate.


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Fig. 6. Electromagnetic field enhancements of Al nanocubes. (A) Maximum field enhancement plots under normal incidence excitation of 110 nm concave, normal and truncated Al nanocubes at their Fano resonance (403, 360 and 330 nm respectively) compared to a 180 nm sphere (which has a scattering spectrum comparable to the 110 nm Al nanocubes) at its dipole (515 nm) on quartz substrates. (B) Maximum field enhancement as a function of wavelength for the particles in (A). (C) Maximum field enhancement (solid, left) and calculated scattering cross-section (dashed, right) under normal incidence excitation as a function of wavelength for a 70 nm Al nanocube on Al, Ag, and Au substrates. (D) Calculated scattering cross-section under 60 degree incidence excitation of a 70 nm Al nanocube on a Au substrate with varying air gap size. (E) Experimental dark-field scattering under 60 degree incidence excitation and corresponding theory of a 70 nm Al nanocube on a smooth Au film. Inset: SEM image of the 70 nm Al nanocube on a Au substrate.


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Although there are only small changes in the scattering spectra as the corners are increasingly rounded, the maximum field enhancement is highly sensitive to corner geometry and was calculated to exceed that of an Al sphere of similar resonance energy by a factor of ~5 (Fig 6A). The maximum field enhancements were found to correspond to the dipole and Fano resonances of the cubes, and can be tuned across the visible spectrum (Fig. 6B). Theoretical calculations indicate that placing an Al nanocube onto a metallic substrate dramatically shifts the optical scattering of the edge dipolar mode and the maximum field enhancement into the visible and near-infrared region while greatly suppressing the corner dipolar mode, particularly in the case of Au and Ag substrates (Fig. 6C). These large redshifts in the scattering and field enhancement for Al nanocubes on metal substrates are comparable to those observed with Ag nanocubes,37 but resonances near 850 nm are damped by the Al interband transition.12 Notably, theoretical calculations reveal that this redshift is strongly dependent on and sensitive to the distance between the Al nanocubes and the Au substrate, as well as the specifics of the excitation geometry (Fig. 6D). When a 70 nm Al nanocube was placed on a smooth Au film and excited at a 60 degree incidence, a facet dipole mode was observed at 700 nm with a Fano resonance at 650 nm as result of coupling to the highly localized facet quadrupole, in excellent agreement with theoretical calculations.38 The gap size-dependent scattering of Al nanocubes on metal substrates makes the incorporation of optically active two-dimensional materials into the gap a tantalizing prospect for nanophotonic applications, since Al nanocubes concentrate electromagnetic energy into the gap with a size-dependent and substrate metal-dependent dispersive response.

Conclusions


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ACS Nano

The synthesis of {100} facetted Al nanocrystals using Tebbe’s reagent enables the growth of single-crystalline Al nanocubes that are earth-abundant, cost-effective alternatives to noble/coinage metal nanocubes for plasmonics. Because of their highly tunable plasmonic properties and ease of colloidal synthesis, Al nanocubes can expand the scope of plasmonic and nanophotonic applications that rely on metallic nanocubes. The growth of Al nanocubes from an excess of AlH3 reacting with Tebbe’s reagent is mechanistically distinct from their noble metal counterparts, validating the notion that molecular transition metal catalysts can be designed to control the shape of Al nanocrystals. This research starts a fresh chapter in the chemistry of Tebbe’s reagent by expanding the complex’s scope beyond organic transformations into the realm of shape-controlled Al nanocrystal growth.39 Encapsulation of Al nanocubes within functional protective shells will further allow Al nanocrystals to supersede traditional plasmonic materials in a wide variety of applications. The cubic shape of these Al nanocrystals will enable straightforward experimental incorporation onto various substrates to create devices that are guided by theoretical predications, bringing sustainable materials to the forefront of plasmonics, nanophotonics, and nanotechnology.

Methods Materials. All materials were obtained from Sigma Aldrich and used as received unless otherwise noted. DMEAA and Tebbe’s reagent were obtained as 0.5 M solutions in toluene. Alternatively, a 0.5 M solution of Tebbe’s reagent was prepared according to literature.40 THF was dried and sparged with argon using a solvent system. Anhydrous toluene was prepared and sparged using a solvent system or purchased from Sigma Aldrich. While inside an argon filled glovebox with

Aluminum Nanocubes Have Sharp Corners - ACS Publications

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