Synthesis of Anisotropic Concave Gold Nanocuboids with Distinctive

May 24, 2013 - Engineering Gold Nanoparticles in Compass Shape with Broadly Tunable Plasmon Resonances and High-Performance SERS. Youju Huang , Liwei ...
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Synthesis of Anisotropic Concave Gold Nanocuboids with Distinctive Plasmonic Properties Youju Huang,†,∥ Lin Wu,‡,∥ Xiaodong Chen,§ Ping Bai,*,‡ and Dong-Hwan Kim*,† †

School of Chemical and Biomedical Engineering and §School of Materials Science and Engineering, Nanyang Technological University, 637457 Singapore ‡ Institute of High Performance Computing, A*STAR (Agency for Science, Technology and Research), 1 Fusionopolis Way, no. 16-16 Connexis North, Singapore 138632 S Supporting Information *

ABSTRACT: Gold nanoparticles have attracted considerable attention owing to their appealing plasmonic properties that have found applications in sensing, imaging, and energy harvesting. In the present article, we report the synthesis of anisotropic concave Au nanocuboids using a seeded growth method controlled by a seed concentration. Unlike conventional nonconcave counterparts which typically present two fundamental plasmonic modes (transverse and longitudinal modes), our experimental measurements and theoretical analysis show that the anisotropic concave Au nanocuboid has three plasmonic resonances. Theoretical calculations based on a finite element method confirm that the third resonance is a transverse “edge” mode, which is enhanced by the sharpened edges of the concave surfaces. This third resonance is found to be separated from the conventional broad transverse mode band. Because of the separation of the resonance mode, the quality-factor of the original transverse mode shows nearly a 3-fold enhancement. KEYWORDS: anisotropic, concave, gold nanoparticles, plasmonic, high-index facets

1. INTRODUCTION

Anisotropic gold nanoparticles (AuNPs) exhibit high chemical and plasmonic sensitivity as well as multiple plasmon resonance bands along with their individual axes, which enables tuning the plasmon resonance from visible to near-IR spectral regions.24−29 Inspired by such appealing optical and chemical properties of anisotropic AuNPs, we have, for the first time, synthesized concave Au rectangular nanocuboids by a seeded growth method. The plasmonic properties of synthesized concave Au nanoparticles were systematically studied by Ultraviolet−visible (UV−vis) absorption spectra and singleparticle dark-field spectroscopy. Contrary to two resonance peaks (transverse and longitudinal modes) typically appearing in anisotropic AuNPs,30−32 three resonance peaks were observed in the measured extinction spectra of the concave Au nanocuboids, which has not been reported. To understand the nature of the newly observed third plasmonic resonance, we theoretically studied light interactions with the Au nanocuboids using a finite element method which solves the full set of threedimensional Maxwell’s Equations.

The unique optical and catalytic properties of metallic nanoparticles arising from their geometric alteration, that is, sizes1−5 and shapes,6−11 have aroused considerable scientific interest in the development of various applications, such as biosensors,12−14 optical labeling,15−17 and imaging.18,19 Among the intriguingly structured nanostructures, concave nanoparticles have attracted great attention because of their extraordinarily high chemical activity originating from highindex facets. Recently, Mirkin and co-workers20 have reported concave gold nanocubes that exhibit a high chemical activity and 80 nm red-shifted plasmon resonance contrary to similarly sized cubes with flat surfaces. Despite such distinct optical and chemical properties of concave nanoparticles, there are only few reports on such concave metal nanoparticles probably because of challenges in synthesis, which arises from their high surface energy.21,22 Compared to flat surface counterparts with lowindex facets, a concave surface constructed with high-index facets possesses a high surface energy, which provides metal atoms with a favorable surface for deposition in a seeded growth process.21,22 This typically results in the disappearance of a concave surface as metal atoms preferably grow onto highindex facets. Thus, lowering surface energy can be a way to tune the growth rate of individual facets and the resultant morphology of synthesized nanoparticles.10,23 © XXXX American Chemical Society

Received: March 8, 2013 Revised: May 17, 2013

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Figure 1. Representative SEM images of AuNRs seeds (a), synthesized AuNPs (b) and (c) at the AuNR seed concentrations of 5 × C0, and C0 respectively. The scale bars in panels a, b, and c represent a 100 nm. TEM image of single nanocuboid tilted 0° (d) and 20° (e) to illustrate the concave faces. Scale bars in panels d and e represent 20 nm. (f) HRTEM image of the edge with high-index facet, projected from the [001] direction (Corresponding to the region indicated with a blue box in panel d). (Inset image in panel f is a FFT pattern of the nanocuboid shown in this panel).

2. RESULTS AND DISCUSSION Synthesis of Anisotropic Concave Au Nanocuboids. Although the creation of concave structures is not thermodynamically favorable, particularly for the case of seeded growth,33 the growth rate of atoms on seeds can be manipulated by kinetic control. AuNRs (60 ± 9 × 20 ± 3 nm) were synthesized and utilized as seeds for further growth (Figure 1a). As the concentration of seed plays an essential role in the control of nanoparticle growth, we explored various seed concentrations ranging from 5 × C0 to 0.2 × C0 (C0 represents concentration of AuNR seed stock solution) to obtain concave nanoparticles, and the resultant products are designated as AuNP-5C0, AuNP2C0, AuNP-C0, AuNP-0.5C0, andAuNP-0.2C0 respectively. In previous literature,31,34 including ours,35 high seed concentration of 5 × C0 has been frequently used for AuNP synthesis. Such high seed concentration typically leads to isotropic overgrowth of Au seeds, resulting in the formation of a rod-like structure with flat surfaces as shown in Figure 1b. When the seed concentration is reduced to 2 × C0, grown AuNPs begin to exhibit facets with irregular trapezoidal and triangular structures on the surface thereof (Supporting Information, SFigure 1a). The concave nanocubiods can be obtained in moderate seed concentration between 0.5 × C0 (Supporting Information, S-Figure 1b) and to 1 × C0 (Figure 1c). The AuNPs synthesized at a seed concentration of 1 × C0 (AuNPC0) was used as a representative concave nanocuboid throughout the following experiment. The concave nanocuboids appear to be darker in the middle region than the edges, indicating the presence of the concave feature of the particles, which was further confirmed by the Transmission Electron Microscopy (TEM) image of a single concave nanocuboid in a tilted position of 20° (Figure 1e). To better understand the structure of the concave Au nanocuboid, the facets on its surface are indexed. The high-index facets of an Au

nanocuboid can be theoretically obtained by the interfacial angles.20,36 The angles between the facets of the concave nanocuboid and the [001] plane of an ideal nanocuboid are calculated to be 16.9°, 16.8°, 17.4°, 17.2°, 18.0°, 15.7°, 18.1°, 16.7° and 14.4° (Figure 1d). The average value of those measured angles is 16.8° which is in good agreement with a theoretical [720] plane of 16.0°,20,36 indicating that the synthesized concave nanocuboid indeed possesses high-index facets. At seed concentration below 0.2 × C0, grown AuNPs lose their concave features (Supporting Information, S-Figure 1c). We note that this concave Au nanocuboid clearly differs from a, so-called, dog bone structure37 in structure, optical properties, and synthesis method, as described in the Supporting Information, S-Figure 2. The size of Au nanocuboids can be facilely tuned by varying the volume ratio of the AuNR-seeds and the growth solution; 70 × 40, 110 × 50, and 140 × 68 nm Au nanocuboids can be obtained (Supporting Information, S-Figure 3). Optical Properties of the Anisotropic Concave Au Nanocuboids. Absorbance of the AuNPs synthesized at different seed concentrations were measured using UV−vis spectroscopy in aqueous solutions. The AuNPs synthesized at high seed concentration of 5 × C0, forming a nonconcave, rodlike structure (Figure 1b), show two resonance peaks at 524 and 721 nm (black curve in Figure 2a), each of which is corresponding to a transverse and longitudinal scattering mode, respectively.31,32,38,39 The AuNP-C0, forming a concave Au nanocubiod, uncommonly exhibits three discrete plasmonic peaks at 530, 615, and 830 nm (indicated by black inverted triangles in Figure 2a). Three plasmonic peaks were reported using UV−vis spectroscopy measured from a suspension of an ensemble of nanoparticles, which inevitably includes contribution from all nanoparticles present in suspension.37 In our present work, to exclude the possibility that the three plasmonic B

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performed a theoretical study of the optical response of an ideal single concave nanocuboid whose dimensions are extracted from representative concave nanocuboids. The ideal concave nanocuboid is modeled with six high-index {720} facets having an angle of 16° with respect to the {001} facets. Using a finite element method to solve the three-dimensional Maxwell’s equations, we have calculated the extinction (including absorption and scattering) cross sections of a single nanocuboid (details of the calculation are shown in the Methods section). The peaks in the extinction cross section spectrum correspond to various plasmonic modes. Because of the symmetry of the cuboidal geometry, the fundamental plasmonic modes, that is, transverse or longitudinal modes, can be separately excited when the incident light wave is propagating in the -z direction with an electric field consisting of only a y component (or x component) as shown in Figure 3. Our simulation shows that the anisotropic concave nanocuboid has six fundamental modes in total. A detailed plot of the distribution of “+” and “−” charges clearly shows the nature of each plasmonic mode. For the peak at 556 nm shown in Figure 3a, the “+” and “−” charges are separated along the transverse direction and the charges are all accumulated along the edges; therefore we have designated it a transverse edge mode (T1). Similarly, for the peak at 778 nm, the charges are mainly transversely separated and accumulated at corners; therefore, designated a transverse corner mode (T3). Lastly for the peak at 706 nm, the charges are accumulated at both edges and corners; accordingly we have designated it a transverse hybrid mode (T2). The other three peaks appearing with an electricfield along the long axis are designated a longitudinal edge mode (L1), a longitudinal hybrid mode (L2), and a

Figure 2. UV−vis spectra (a) of synthesized AuNPs at the AuNR seed concentrations of 5 × C0 and C0 in aqueous solution. Three discrete plasmonic peaks on AuNP-C0 are denoted by black inverted triangles. LSPR spectrum (b) of a single Au nanocuboids and its corresponding SEM image on an indium−tin-oxide substrate.

peaks come from inhomogeneity of the synthesized nanoparticles, the LSPR of a single particle was examined using a pattern-matching technique that combines SEM with singleparticle dark-field spectroscopy35,40−42 (Supporting Information, S-Figure 4). Figure 2b shows the measured LSPR spectrum of the single concave Au nanocubiod (corresponding SEM image shown in the inset). Three discrete resonance peaks are observed at 563, 666, and 910 nm. We note that the resonance peaks of a single nanoparticle may appear at wavelengths slightly different from the absorption peaks of an ensemble counterpart measured by UV−vis. This discordance is attributed to the effect of surrounding media with different refractive indices30,43 (i.e., water for UV measurement and oil for dark-field imaging) and the shape-dependent plasmonic variations among individual nanoparticles.42 To understand the nature of the three plasmonic peaks observed from the synthesized concave nanocuboid, we

Figure 3. (a) Simulated spectra of absorption, scattering and extinction cross sections for single concave nanocuboid and under plane wave excitation, where the incident light wave is propagating in the −z direction with the electric field having only either the y component or the x component. Therefore the plasmonic resonant peaks shown are transverse modes (namely T1, T2, and T3) and longitudinal modes (namely L1, L2, and L3). The simulated charge density distributions at various plasmonic resonant wavelengths correspond to T1, transverse edge mode; T2, transverse hybrid mode; T3, transverse corner mode; L1, longitudinal edge mode; L2, longitudinal hybrid mode; and L3, longitudinal corner mode. For comparison, similar studies on a nonconcave nanocuboid with flat surfaces are shown in (b). C

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possess sharpened edges. More importantly, transforming a single broad peak (nonconcave) into two sharp peaks (concave) as shown in Figure 3 results in an improved quality factor (Q-factor) of T2, which is defined as the ratio of the resonant frequency to its line-width. As T2 red-shifts from 650 to 700 nm, its line-width reduces from 120 to 50 nm. Consequently, the Q-factor of T2 is greatly enhanced from 5.4 (≈ 650/120) to 14 (≈ 700/50). To further confirm the transverse and longitudinal modes of the concave nanocuboid, plasmonic spectra under a polarized light source was examined. When the polarization of incident light is parallel to the long axis of the concave Au nanocuboids, the peak at 910 nm becomes most dominant (Figure 5a),

longitudinal corner mode (L3). Taken together, the fundamental components of all the six peaks for the concave nanocuboid could be identified. On the basis of these fundamental modes, each peak in our experimental measurement can be matched to a single fundamental mode or their combinations. The measured peaks at 563, 666, and 910 nm from a single nanocuboid (Figure 2b) can be attributed to the T1 transverse edge mode, the T2 transverse hybrid mode (possibly with shielded L1 mode), and the L3 longitudinal corner mode, respectively. In between the 666 and 910 nm peaks, there probably exist the L2+T3 modes which are shielded. For the purpose of comparison, physical pictures of the plasmonic modes of the anisotropic nonconcave nanocuboid with six flat surfaces are also examined as shown in Figure 3b. For consistency, the fundamental modes are similarly designated to be T1−T3 and L1−L3. By transforming the six surfaces from nonconcave to concave, we discovered two distinctive features. First, all the plasmonic modes are redshifted, which was also experimentally observed in the isotropic nanocubes.20 Second, the transverse edge mode (T1) becomes distinguishable in a concave nanocuboid (Figure 3a), while it is almost completely shielded by the transverse hybrid mode for a nonconcave nanocuboid, making these two modes nondiscernible (Figure 3b). This second feature is clearly shown in the field-enhancement plots (Figure 4), calculated as the

Figure 5. Representative experiential (a) and simulated (b) LSPR spectra of a single Au nanocuboid under different polarization angles. The polarization angle was defined by the angle between the long axis of concave Au nanocuboid and the polarization of incident light.

indicating that the peak at 910 nm is a longitudinal mode. In contrast, when the polarized light is orthogonal to the long axis of concave Au nanocuboids, the intensity of the peak at 910 nm is significantly reduced, and the peak at 666 nm becomes prominent (Figure 5a), implying that the peak at 666 nm is a transverse mode. This has been confirmed by our simulations as shown in Figure 5b, where we freely tune the polarization to control the magnitude of peaks at 910 and 666 nm. In addition, we found that the transverse mode in both experimental and theoretical analysis appeared to be less sensitive to the polarization angle. This is because our light source for both the experimental and the theoretical setup is the incident light at 30° (rather than a normal incidence along the −z direction shown in Figure 3a). So for any polarizations, we will have electric field components along the transverse directions (either z or y), that is, the transverse modes are always excited.

Figure 4. Simulated electric-field enhancements for concave nanocuboids at (a) T1 and (c) T2 plasmonic modes and nonconcave nanocuboids at (b) T1 and (d) T2 plasmonic modes, where only the upper half of each nanocuboid is shown. For T1 transverse modes, significant field enhancement (hot spot) is observed in the middle of the edges of the concave nanocuboids (a), while at the corners of the nonconcave nanocuboids (b).

3. CONCLUSIONS In conclusion, we have shown that anisotropic concave Au nanocuboids could be synthesized using a seeded growth method with a precise control of seed concentration and demonstrated that the concave Au nanocuboids exhibited distinctive optical properties compared with nonconcave equivalents. UV−vis absorbance and single particle LSPR spectra of the concave Au nanocuboids showed that they revealed three extinction peaks, which were identified to be a transverse edge mode, transverse hybrid mode, and longitudinal corner mode. Theoretical studies showed that the extra third resonance mode, that is, transverse edge mode, was caused by the sharpened edges arising from a concave structure. This, in turn, transferred the single transverse mode of a conventional nonconcave nanocuboid into two well-separated, sharp transverse modes of a concave nanocuboid. The resulting sharp resonance peak has nearly a 3-fold higher Q-factor.

ratio of the electric-fields at plasmonic resonance with respect to the incident electric-field. Hot spots, the strongest electricfield enhancements, appear at edges of a concave nanocuboid, while they are located at corners of a nonconcave nanocuboid. This is an indication that, for a nonconcave nanocuboid, the T1 mode is no longer an edge mode, but a weak hybrid mode (Figure 4b and 4d). In other words, concave nanocuboids can exhibit one additional plasmonic mode, that is, a transverse edge mode, as compared to a nonconcave nanocuboid. This additional mode is observed in our experiments as shown in Figure 2b, that is, the peak at 563 nm, and we attribute this additional edge mode to the fact that the concave nanocuboids D

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4. METHODS

matching technique for single particle observation. This material is available free of charge via the Internet at http:// pubs.acs.org.

4.1. Preparation of AuNR Seeds. AuNR seed preparation includes two subsequent experimental steps,32,44 that is, the formation of spherical Au seeds (∼3 nm in diameter) followed by growth of AuNR seeds (∼61 nm in length and ∼18 nm in width). Briefly, the spherical Au seeds were prepared by the addition of a freshly prepared, ice-cold, aqueous NaBH4 solution (0.01 M, 0.6 mL) into an aqueous mixture composed of HAuCl4 (0.01M, 0.25 mL) and CTAB (0.1 M, 9.75 mL). The resultant solution was mixed by rapid inversion for 2 min and then kept at room temperature for at least 2 h before use. To fabricate AuNR seeds, a growth solution was prepared by mixing an aqueous solution of HAuCl4 (0.01 M, 4 mL), AgNO3 (0.01 M, 0.8 mL), and CTAB (0.1 M, 80 mL) together. Then, a freshly prepared, aqueous ascorbic acid solution (0.1 M, 0.64 mL) was added to the above mixture, followed by the addition of an aqueous HCl solution (1.0 M, 1.6 mL). The resultant solution was mixed by rapid inversion, followed by the addition of the spherical Au seed solution (0.02 mL). The above mixture was subjected to gentle inversion for 10 s and then left undisturbed for at least 6 h, resulting in the formation of AuNR seeds. 4.2. Preparation of Anisotropic Concave Au Nanocuboids. To prepare the Au nanocuboids, 7.5 mL of a growth solution containing a mixture of 2.5 × 10−4 M HAuCl4 and 0.01 M CTAB solutions were added to five 20 mL conical flasks. Then, 41 μL of 0.1 M freshly prepared ascorbic acid was added into each flask followed by gentle stirring for 2 min. Finally, 0.2 mL of the AuNR seed solutions at different concentrations (0.2C0, 0.5C0, C0, 2 C0, and 5 C0, where C0 represents concentration of AuNR seed stock solution) were added into each flask, and the mixtures were kept at 30 °C in a water bath for at least 6 h. The concentrated AuNR seed solutions (i.e., 5C0 and 2C0) were prepared by redispersing centrifuged precipitation of AuNR seeds into 0.1 M CTAB solution. 4.3. Modeling Method. In our modeling, we consider a single nanocuboid (either in nonconcave or concave shape) and solve the scattering problem for such subwavelength conductive nanoparticles in an oscillating electromagnetic field. This is done by solving the full set of 3D Maxwell’s equations for the electric and magnetic fields ∇ × E⃗ = ikH⃗ , ∇ × H⃗ = ikεE⃗ , where k is the vacuum wavenumber, and the material’s dielectric permittivity ε(ω) is taken from the Palik handbook.45 In our simulations, we assume that (i) both nanocuboids have the same size (50 nm × 50 nm × 110 nm); (ii) the nonconcave nanocuboid is drawn with six flat {001} facets, while the concave nanocuboid is drawn with six high-index {720} facets having an angle of 16° with respect to the {001} facets; (iii) single nanocuboids are embedded in an oil environment (refractive index 1.516); and (iv) plane wave excitation. Besides the electric and magnetic fields, the model calculates the spectrum of cross sections for scattering Csca and absorption Cabs, then the extinction Cext = Csca + Cabs, as well as the charge density (or electric field) distributions at resonant wavelengths. More specifically, the scattering cross section Csca is calculated as the power scattered from the nanocuboid (i.e., the surface integration of the scattered normal flux, ∮ S (E × H) n dS, over an auxiliary closed surface enclosing the nanocuboid) divided by the constant incident flux (i.e., (1/2) ncε0|E0|2 with c as the speed of light, ε0 as the vacuum permittivity, n as the refractive index of the dielectric environment, and |E0| as the magnitude of the incident electric field). The absorption cross section Cabs is calculated as the absorbed power within the nanocuboid (i.e., the volume integration of the resistive heating in the nanocuboid, ∫ v J·E dV, with J as the current density) divided by the incident flux. All these calculations are performed based on the scattered-field formulation in the COMSOL multiphysics RF module, and a perfectly matched layer (PML) absorbing boundary is applied to absorb the incident radiation without producing reflections.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (D.-H.K.), [email protected]. edu.sg (P.B.). Author Contributions ∥

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge financial support from the Science and Engineering Research Council (102 152 0014) and the Ministry of Education (MOE2012-T2-1-058) of Singapore.

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S Supporting Information *

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