Optical and Dynamical Properties of Chemically Synthesized Gold

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Optical and Dynamical Properties of Chemically Synthesized Gold Nanoplates Todd A. Major, Mary Sajini Devadas, Shun Shang Lo, and Gregory V. Hartland* Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States S Supporting Information *

ABSTRACT: Single crystal, micrometer-sized nanoplates were formed by reducing Au(III) in the presence of surfactants using a modified polyol protocol. The shapes of the plates range from triangular to hexagonal. The nanoplates have {111} surfaces with an average edge length of 5 ± 2 μm and an average width of 107 ± 30 nm. Scanning electron microscopy (SEM) images reveal that the plates grow through a re-entrant groove created by twinning. The optical properties of the plates were studied by scattered light and transient absorption experiments. The scattered light measurements show that propagating surface plasmon polariton (SPP) modes of the Au nanoplates can be excited when a laser beam is focused at the edge of the nanoplate. We also demonstrate that the direction of propagation of the SPP modes can be controlled through the polarization of the laser beam. The transient absorption traces for single suspended nanoplates show oscillations, which are assigned to thickness vibrations of the plates. The quality factors for the oscillations are smaller than those recently measured for suspended gold nanowires, indicating possible contributions to the vibrational damping from surface bound molecules or from the crystal structure.

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by a wet chemical method16−18 and grow as single crystals. Thus, they offer the possibility of longer SPP propagation lengths compared to polycrystalline metal films created by thermal evaporation. We show that SPP modes in the nanoplates can be excited by focusing a laser at the edge, analogous to what happens in metal nanowires.8,9 This scheme avoids using grating couplers or a prism for excitation, which may be useful for integrating these materials into devices. Importantly, we demonstrate that the direction of propagation can be simply controlled by varying the polarization of the excitation laser. Transient absorption traces were also recorded for several single nanoplates. The traces show a fast decay due to electron−phonon coupling and oscillations which are assigned to thickness vibrations of the nanoplates. In these experiments the plates were suspended over a trench, so that the lifetimes reflect intrinsic damping of the vibrations, rather than damping through interactions with the surroundings.19

INTRODUCTION The optics of metal nanostructures is an active area of research in physical chemistry. For particles, the optical spectra are dominated by the localized surface plasmon resonance (LSPR), which is a collective oscillation of the conduction electrons.1 The frequency of the LSPR depends on the size and shape of the particles, and wet chemical synthesis can create materials with spectral features ranging from the near-UV to the near-IR.2 For extended metal nanostructures, such as nanowires or thin films, propagating surface plasmon polariton (SPP) modes can also occur.3,4 Normally these modes do not couple to light due to the momentum mismatch between photons and electron motions. For metal films coupling can be achieved either using a grating structure or by exciting through a prism.5−7 In metal nanowires, SPP modes can also be excited by focusing a laser at the end of the wire, where the break in symmetry relaxes the momentum matching constraints.8,9 In these experiments the SPP modes can travel down the nanowire and re-emit photons at the distal end. The intensity of the emitted photons provides information about the propagation length for the SPPs,9 which depends on the dielectric constant of the material as well as the crystal structure and surface roughness.10−14 Long propagation lengths are desirable for applications such as coupling metal films to semiconductor nanostructures to create lasers with ultrasmall mode volumes.15 In this contribution we examine the optical and dynamic properties of Au nanoplates. The nanoplates were synthesized © 2013 American Chemical Society

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EXPERIMENTAL METHODS Au Nanoplate Synthesis and Characterization. The synthesis is a modified procedure from previous reports.16−18 For the formation of gold nanoplates 4 mL of ethylene glycol was heated at 160 °C for 5 min in an oil bath, and then 4 mL of Received: November 20, 2012 Revised: December 21, 2012 Published: January 3, 2013 1447

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were spatially overlapped with a dichroic beamsplitter and focused onto the sample with an Olympus UPlan FLN 100×, 1.30 NA oil immersion objective. The reflected probe was detected with a Hamamatsu C5331-11 avalanche photodiode, with a short pass filter to eliminate the pump. For images the sample was raster scanned over the laser spot using a piezoelectric stage (Physik Instrumente, P-527.3Cl), and the transient absorption signal was recorded with the lock-in amplifier with a time constant of 100 ms. Transient absorption traces were recorded for nanoplates suspended over trenches on a glass coverslip, which were fabricated by photolithography and reactive ion etching. The trench dimensions were measured to be several hundred nanometers deep and several micrometers wide. Atomic force microscopy (AFM) images of the nanoplates examined in the time-resolved measurements were recorded with a Veeco Bioscope II AFM operating in tapping mode for height imaging. Alignment marks were created on the coverslips used in these experiments in order to locate the same nanoplate in the optical and AFM measurements.

0.019 M cetyltrimethylammonium bromide (CTAB) and 0.27 M poly(vinylpyrrolidone) (PVP, mol wt 40 000 g/mol) was added while stirring. After the temperature equilibrates, 0.031 M HAuCl4 in ethylene glycol was added. This solution was continuously and vigorously stirred for 30 min. The solution became colorless, quickly turned brown, and finally a metallic gold color appeared, indicating the formation of Au plates. The solution was brought to room temperature, and the precipitated plates were washed with acetone, ethanol, and then water. If the plates agglomerate, they can be redispersed in water or ethanol by ultrasonication. All glassware was cleaned with aqua regia and rinsed with deionized water before use. Although the majority of the sample was made up of anisotropic hexagonal plates, some spheres were also produced. These can be separated from the plates by centrifuging the solution, removing the turbid brown supernatant solution, and washing the plates with deionized water. The phases of the samples were identified by X-ray diffraction (XRD) patterns recorded on a Bruker D8 Advance using θ/2θ configuration with Cu radiation. The morphology and microstructures of the samples were determined by field emission scanning electron microscopy (FESEM, Magellan 400, 5 or 10 kV voltage), transmission electron microscopy (TEM, Titan 80-300), and selected area electron diffraction (SAED) (Titan 80-300), using an accelerating voltage of 300 kV. Optical Studies. Samples for optical studies were prepared by drop-casting the Au nanoplate solution onto a flamed glass coverslip. The Au nanoplates were allowed to dry on the surface for roughly 5 min, and then excess solvent and Au was removed by spinning at 1000 rpm for 3 min. Scattered light images were recorded with a color CMOS camera (Thorlabs DCC1645C) with either a lamp or a linearly polarized HeNe laser (633 nm, Thorlabs HRP020-1) as the light source. In these experiments the light was focused onto the sample with an Olympus UPlanApo 100×, 1.35 NA oil immersion objective, and the scattered light was collected with the same objective. Au nanoplates were spatially overlapped with the HeNe laser beam using a piezoelectric stage. In order to best see emission from the SPP modes, the focus of the HeNe was slightly expanded to sharpen the contrast of the re-emitted SPP modes around the nanoplate. The pixel/μm ratio of the camera was calibrated using an Edmund Optics precision ruling glass slide to determine the size of the nanoplates in the optical measurements. The intensity of the re-emitted SPP modes was quantified by averaging the brightness of the camera pixels over a line along the edge of the nanoplate. Transient absorption experiments were performed with a Coherent Chameleon Ultra-II Ti:sapphire laser tuned to 720 nm (repetition rate of 80 MHz). The output of the oscillator was split with a 90/10 beamsplitter. The weaker portion was used as the pump and was chopped by an acousto-optical modulator (IntraAction) at 500 kHz, triggered by the internal function generator of a lock-in amplifier (Stanford Research Systems SR844). The stronger portion of the laser output pumped an optical parametric oscillator (Coherent MiraOPO), which provided 530 nm probe pulses. A Thorlabs DDS220 linear translation stage was used to control the time delay between the pump and probe beams. The intensity of the pump and probe beams were controlled by half-wave platepolarizer combinations, and quarter-wave plates were used to create circularly polarized beams. Typical powers were 500 and 100 μW for the pump and probe, respectively, corresponding to pulse energies of 12 and 1.2 pJ. The pump and probe beams

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RESULTS AND DISCUSSION Figure 1A shows a scanning electron microscopy (SEM) image of gold nanoplates synthesized by the optimized protocol

Figure 1. (A) FESEM image of Au nanoplates. The scale bar represents 30 μm. (B) Lateral view of a hexagonal nanoplate. The scale bar represents 5 μm. (C, D) Plots of the size distribution histograms for the edge length and width, respectively, of the nanoplates. The dimensions were measured from the SEM images using Image J. (E) Bright-field TEM image of a truncated triangular nanoplate. (F) Powder XRD pattern of the Au nanoplates revealing a strong diffraction peak from the {111} lattice planes.

described in the Experimental Methods section (the morphology and dimensions of the product strongly depend on temperature and concentration of reactants). The as-prepared solution contains a mixture of triangular, hexagonal, and truncated triangular plates, along with some three-dimensional nanoparticles. From the SEM image it can be seen that the 1448

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the transient absorption signal for the Au nanoplates is not greatly affected by the polarization of the pump and probe beams. Figure 3 shows scattered light images of a hexagonal (top) and a truncated triangular Au nanoplate (bottom). Panels A and E show scattered light images of the nanoplates recorded with a lamp, and panels B and F shows images where a HeNe laser is focused at the edge of the Au nanoplates for different laser polarizations. The HeNe excites propagating SPP modes, which travel radially from the laser spot, and re-emit light at the distant edges of the nanoplate. Focusing the laser spot in the middle of the nanoplate does not excite the SPP modes, and no re-emission is seen in this case.9 Panels C, D, G, and H show the relative intensity of the re-emitted light from the SPP modes at different edges of the nanoplates as a function of laser polarization. For the hexagonal plate, changes in laser polarization of 30° shift the emission maximum from one edge to another (α → β → γ). The triangular nanoplate shows the same trend, with a change in the polarization angle of 60° shifting the emission between the different edges (α → β). These angles are consistent with the geometry of the objects. This effect occurs because the SPPs are preferentially launched along the direction of the laser polarization.10 The Supporting Information shows a video of the re-emitted light as a function of polarization. It is important to note that, in contrast to what is observed for nanowires,9 the intensity of the re-emitted light never reaches zero as the angle is changed. The observation that laser polarization can control the direction of propagation of the SPP modes in metal nanostructures is one of the main results of this paper. To the best of our knowledge, this has not been previously seen experimentally. The fact that different laser polarizations excite SPP modes with different propagation directions is the reason for the polarization insensitivity in the transient absorption images: there is always some absorption into the SPP modes for the plates irrespective of the polarization of the pump laser. This simple method to control the direction of propagation of the SPP modes described here may enable the development of nanoscale lasers with controllable directional output. Note that micrometer sized silver nanoplates can also be produced by wet chemical methods.25,26 These materials should also show propagating SPP modes, but with longer propagation lengths compared to the gold nanoplates in the present study, because of the reduced optical damping of silver compared to gold.27 Figure 4 shows transient absorption traces for two suspended Au nanoplates. These samples were prepared by drop-casting the nanoplates in aqueous solution over a glass coverslip that contained several micrometer wide trenches created by photolithography.19 The transient absorption traces show a fast decay due to electron−phonon coupling, followed by oscillations that arise from acoustic modes that are coherently excited by the ultrafast pump laser pulse.28−30 The insets in Figure 4 show scattered light and atomic force microscopy (AFM) images of the suspended nanoplates. Studying suspended nanoplates removes the effect of the substrate from the experiments. This makes it much easier to model the vibrational response: simple analytical continuum mechanics results can be used rather than finite element modeling.31−33 For suspended nanostructures the lifetimes of the vibrational modes also provide information about intrinsic damping processes in the metal.19 The AFM images show that the two nanoplates in Figure 4 have similar widths of 160 ± 10 nm, which is larger than the average thickness measured by SEM.

majority of the sample consists of hexagonal and truncated triangular nanoplates, rather than complete triangular structures. Figure 1B gives a lateral view of a hexagonal nanoplate, obtained by tilting the sample in the SEM, where a re-entrant groove created by a twin plane is clearly visible.20,21 The twin plane gives rise to concave- and convex-type edges on the plate. Atoms are more stable on the concave edges; thus, these edges grow faster, resulting in the transformation of the hexagonal plates into triangular plates.20,21 Histograms of the edge length (measured as the longest edge of the nanoplate) and width are presented in Figure 1C,D. The average edge length is 5 ± 2 μm, and the average width is 107 ± 30 nm (errors equal standard deviations). Figure 1E shows a TEM image of a truncated triangular plate. The plates are sufficiently thin that underlying plates can be seen in the image. X-ray diffraction (XRD) was also used to characterize the sample, and the powder XRD pattern is presented in Figure 1F. A prominent peak for the {111} diffraction plane was recorded with a very weak {200} peak of ∼0.6%. From this we conclude that the surface of the Au nanoplates comprise of {111} planes.16,17 This is confirmed by selected area diffraction pattern (SAED) images, which are presented in the Supporting Information. The SAED pattern shows a hexagonal symmetry diffraction pattern, demonstrating that the gold nanoplate is a single crystal with a preferential growth direction along the gold {111} planes.22 Figure 2A shows a scattered light image of several stacked Au nanoplates. The nanoplates strongly reflect orange light, which

Figure 2. (A) Scattered light image of overlapping trianglar, truncated triangular, and hexagonal Au nanoplates. The scale bar represents 10 μm. (B) Transient absorption image of a truncated triangular nanplate when the pump and probe pulses are temporally overlapped. The transient absorption signal is the voltage measured by the lock-in amplifier. The edges of the nanoplate show increased signal compared to the middle region, which is analogous to behavior seen in nanowires. The scale bar represents 2 μm.

is expected given that they are much thicker than the optical penetration depth of Au.23 Like the SEM images, a variety of different shapes can be seen in the scattered light images. A transient absorption image of a single truncated triangular plate is presented in Figure 2B. The transient absorption signal is stronger at the edges of the nanoplate, analogous to the transient absorption images of metal nanowires, which are more intense at the ends.24 For metal nanowires the increased transient absorption signal at the end is due to excitation of propagating SPPs. The transient absorption image in Figure 2B shows that a similar effect occurs for the nanoplates. The signal is strongly polarized for metal nanowires, with a maximum when the near-IR pump laser is polarized parallel to the nanowire axis (this geometry launches propagating SPP modes that travel down the wire).4,24 In contrast to metal nanowires, 1449

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Figure 3. (A) Scattered light image of a hexagonal Au nanoplate under white light illumination. Scale bar is 2 μm. Panel B shows the Au nanoplate from panel A illuminated with HeNe on the top right edge at polarization angles of 150° (left), 180° (middle), and at 210° (right). (C) Plot of the brightness of the re-emitted SPP mode as a function of polarization angle. The lines represent a sine wave fit. The colors corresponds to the colors of the Greek letters in (A). (D) Polar plot of the data in (C) for the α (red) and γ (blue) sides of the nanoplate. The polar plot shows that the maxima in brightness for the α and γ sides are 60° apart. (E−H) Analogous images and plots for a truncated triangular nanoplate.

This is possibly because the thinner plates are not wide enough to span the trenches. The plates in Figure 4 also appear to be bowed, presumably due to capillary forces that act on the nanoplates as the solvent dries during sample preparation.34 SEM images that show distortion of nanoplates around objects on the surface are presented in the Supporting Information. The bowing makes it difficult to estimate the width of the nanoplates from the AFM images, which leads to the relatively large error quoted above. On the nanosecond time scale of our experiments we expect to observe longitudinal vibrational modes corresponding to changes in the width of the nanoplates, which we take as the zaxis. The governing equation for the displacement u(z) of these modes is35 d2u(z) ω 2ρ + u(z) = 0 2 Eijk dz

(1)

where ω is the vibrational frequency, ρ is the density, and Eijk is the value of Young’s modulus along the direction of wave motion.36 For a nanoplate with a thickness w and free surfaces (du/dz = 0 at z = 0 and z = w), the vibrational frequencies are

Figure 4. Transient absorption traces for two suspended Au nanoplates. The insets show scattered light and AFM images of the nanoplates, and an expanded view of a portion of the transient absorption trace that shows the oscillations from the vibrational modes. The blue line is a fit to the data using a damping cosine function. For the nanoplate in the top panel the period is 150 ± 2 ps and the damping time is 1260 ± 80 ps. For nanoplate in the bottom panel the period and damping times are 145 ± 2 ps and 950 ± 70 ps, respectively. Both nanoplates have a width of w = 160 ± 10 nm, as measured by AFM.

ω=

nπ w

Eijk ρ

(2)

where n = 1, 2, 3, .... Using a value of E111 = 115 GPa,37 we estimate a period of T = 2w/(E111/ρ)1/2 = 130 ± 10 ps for the fundamental mode of the nanoplates in Figure 4 (w = 160 ± 10 nm). This value is in reasonable agreement with the experimental measurements (periods of 145 ± 2 and 150 ± 2 ps), especially considering the difficulty in accurately 1450

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performed for suspended nanoplates. The traces show modulations due to coherently excited vibrational modes. The measured periods are consistent with a simple calculation for the period of a longitudinal vibration, corresponding to sound waves bouncing across the nanoplate.35 The quality factors for these vibrational modes are significantly smaller than those measured for gold nanowires,19 indicating that additional damping processes occur for the nanoplates. These processes could include effects from the crystal structure of the nanoplates and/or surface bound molecules as well as propagation of acoustic energy out of the excitation region.

determining the width of the nanoplates from the AFM images. This agreement confirms the assignment of the vibrational modes. The lifetimes of the vibrational modes are also of interest in these experiments.29,30 For single nanoparticles the vibrational lifetimes are best discussed as quality factors Q = πτ/T, where τ is the vibrational lifetime and T is the period, as this removes the dimensional scaling, allowing different sized nano-objects to be compared.19,24,29,38−40 In all, we examined seven suspended Au nanoplates and found an average quality factor of ⟨Qplate⟩ = 29.7 ± 8.6 (error equals standard deviation). This value is significantly smaller than our recent measurement for suspended Au nanowires in air of ⟨Qwire⟩= 90 ± 29.19 For suspended nano-objects in air, the damping of the vibrational motions arises from processes that are inherent to the object.19 Contributions to the inherent damping for the present measurements include: damping from surface bound molecules and/or defects, anelastic effects,41 flow of energy into lower frequency acoustic modes of the nanostructure, and propagation of acoustic energy out of the excitation region. All of these factors could explain the differences between the values of ⟨Qplate⟩ and ⟨Qwire⟩. For example, the nanowires in ref 19 were heated in a reducing atmosphere to remove surface bound molecules. In contrast, the nanoplates in the present study were directly studied without heating and, thus, contain at least one layer of CTAB at their surface. This layer could act to dissipate energy.38,42 The vibrational motion for the nanoplates also occurs along a specific direction (the ⟨111⟩ direction), whereas for the nanowires the breathing motion averages over many different crystal directions. It is quite possible that anelastic effects depend on crystal direction and could be different for these two nano-objects. The number and density of acoustic modes are also different for the nanowires and nanoplates, implying that flow of energy out of the excited mode into the background phonon modes will be different. Finally, for the nanowires in ref 19 we concluded that propagation of acoustic energy out of the excitation region was not a significant effect. This conclusion was based on a comparison between our measured Q values for nanowires with values for the intrinsic damping for single Au nanorods.40 This may not be true for the nanoplates. In the present experiments the pump laser launches an acoustic pulse that propagates from one face of the nanoplate to the opposite face. Because of the finite spot size of the pump laser, the acoustic pulse is not a pure plane wave: it will have components in the in-plane (x,y) directions of the nanoplate. These components can act to carry energy away from the excitation region. Experiments are planned to try and differentiate these different mechanisms for damping, for example, by comparing the dynamics of suspended nanoplates with and without treatment to remove surface bound molecules.

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ASSOCIATED CONTENT

S Supporting Information *

Additional SEM and TEM images of the nanoplates, selected area electron diffraction data, and movies showing the polarization dependence of the SPP propagation direction. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work as support by the National Science Foundation through Awards CHE-1110560 and CHE-0946447 and by the University of Notre Dame Strategic Research Initiative. The authors are also grateful to Dr. Libai Huang (Notre Dame Radiation Laboratory) for use of the AFM and to Dr. Allen Oliver for help acquiring and analyzing the SRD data. We also acknowledge the Notre Dame Integrated Imaging Facility for electron microscopy support.

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SUMMARY AND CONCLUSIONS The optical properties of metal nano-objects that are extended in at least one dimension are different than those of metal nanoparticles, in that they can support propagating surface plasmon polariton (SPP) modes.8−10 In this paper we have examined propagating SPP modes of gold nanoplates. These modes can be excited by focusing a laser at the edge of the nanostructure, analogous to how propagating SPP modes are excited in metal nanowires. Significantly, we show that the direction of propagation of the SPP modes can be controlled by laser polarization. Transient absorption experiments were also 1451

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