Single Particle Studies on Two-Photon Photoluminescence of Gold

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Single Particle Studies on Two-Photon Photoluminescence of Gold Nanorod-Nanosphere Heterodimers Monalisa Garai, Taishi Zhang, Nengyue Gao, Hai Zhu, and Qing-Hua Xu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b02941 • Publication Date (Web): 09 May 2016 Downloaded from http://pubs.acs.org on May 15, 2016

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The Journal of Physical Chemistry

Single Particle Studies on Two-Photon Photoluminescence of Gold Nanorod-Nanosphere Heterodimers Monalisa Garai1, Taishi Zhang2, Nengyue Gao1, Hai Zhu1, and Qing-Hua Xu1,2,* 1

Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543

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NUS Graduate School for Integrative Sciences & Engineering (NGS), Singapore 117456 *Corresponding Author E-mail: [email protected]

ABSTRACT

Localized surface plasmon resonance (SPR) property of metal nanoparticles has found lots of potential applications. Coupling between different SPR modes of adjacent metal nanoparticles can result in significantly enhanced optical responses such as surface enhanced Raman scattering (SERS) and two-photon photoluminescence (2PPL). In this work, gold (Au) nanorod (NR)nanosphere (NS) heterodimers with side-linked and end-linked spatial arrangements have been systematically studied on the single particle level to investigate the plasmon coupling effects on their 2PPL properties. Compared to a single Au NR, both end-linked and side-linked Au NR-NS

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heterodimers displayed red-shifted scattering spectra with similar cos2θ excitation polarization dependence, which can be understood in terms of plasmon hybridization theory. However, 2PPL intensities of end-linked and side-linked Au NR-NS heterodimers displayed strikingly different excitation polarization dependence. 2PPL signals of the end-linked Au NR-NS heterodimer displayed cos4θ excitation polarization dependence, similar to that of Au NR monomer. 2PPL intensity of the end-linked heterodimer is largest with polarization along the long axis of Au NR, which is ~300 times that of the Au NR monomer component and even ~5 times that of a longer Au NR with a similar overall size to the heterodimer. In a striking contrast, 2PPL intensity of the side-linked heterodimer is largest for excitation along the transverse direction of Au NR, which is ~25 times that under excitation along the longitudinal direction. Despite these differences, an interesting feature is that the largest 2PPL signals occur along the coupling directions for both end-linked and side-linked heterodimers. Plasmon coupling enhanced 2PPL is generally believed arising from enhanced two-photon excitation efficiency through two effects: improved resonance effects due to plasmon coupling induced red-shifted SPR mode and plasmon coupling induced giant local electric field amplification. Stronger 2PPL signals have been observed for coupled nanostructures with excitation along the coupling direction but less favorable overlap between the SPR scattering spectra and excitation wavelength, which unambiguously supports that plasmon coupling induced local electric field amplification is the dominant effect responsible for the big difference in 2PPL intensities of different nanostructures. This conclusion has been further confirmed by the excellent agreement between the numerically calculated integrated |E/E0|4 and the experimentally obtained 2PPL signal intensities for various nanostructures. These studies offer some fundamental understanding of plasmon coupling effects on optical responses and excitation mechanisms of 2PPL of metal nanostructures, which provide insight on designing


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nanostructures with tailored optical properties for various potential photonic and optoelectronic applications.

INTRODUCTION Noble metal nanoparticles displayed unique optical property known as localized surface plasmon resonance (SPR), which has been utilized for many important applications.1-3 Plasmon coupling arises when metal nanoparticles come to close proximity, resulting in dramatically enhanced local electric field in the gap region and consequently altering optical responses of metal nanoparticles or nearby chromophores.4-8 This phenomenon opens up many interesting applications, such as plasmon assisted polymerization,9 nanometric optical tweezers,10 and signal amplification in various plasmon-enhanced spectroscopic techniques including surface enhanced Raman scattering,11-14 metal enhanced fluorescence,15,

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high-order harmonic generation,17,

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two-photon photoluminescence (2PPL).19-22 2PPL of metal nanoparticles has attracted a lot of interest due to the unique advantages of twophoton excitation in biomedical applications.23-25 These advantages include deep tissue penetration, three-dimensionally confined excitation, and less photo-damage.26, 27 However, these applications are limited by the relatively weak 2PPL signals of metal nanoparticles. Anisotropic nanostructures or nanoparticles with sharp tips generally display much stronger 2PPL signals due to lightening rod effects.28,

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Recent studies showed that 2PPL signals can be significantly

enhanced in the aggregated metal nanoparticles,22, 30 which has found lots of potential biological applications as metal nanoparticles generally tend to form aggregates in the biological environment.31-34 Up to 840-fold enhancement in 2PPL signal has been observed in the aggregated metal nanoparticles in solution.31 As the ensemble measurements in solution are averaged results of many nanostructures with large variations in size and shape, single particle


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studies on optical responses from well-defined nanoparticles allow us to explore the fundamental mechanisms behind aggregation induced 2PPL enhancement.35-38 A previous study by Schuck et al. showed a ~103 fold enhancement in 2PPL intensity for Au bow-tie dimers separated by 20 nm gap distance.39 Our recent work demonstrated a 105 fold differences between 2PPL intensities of Au nanosphere monomer and trimer clusters.22 2PPL of coupled nanoparticles has been found strongly dependent on the coupling strength and decrease exponentially with increasing interparticle gap distance.22, 30, 39 Different contributions are believed to be responsible for the observed enhanced 2PPL signals in aggregated metal nanoparticles.30, 39 First, the amplified local electric field in the gap region or sharp edges resulted in enhanced two-photon excitation efficiency. Second, the excitation process of the observed 2PPL was believed to involve two sequential one-photon absorption process.40 Red-shifted SPR band of the aggregated nanostructures allows better spectral overlap with the excitation wavelength, which facilitate the excitation processes. So far these studies are mainly focused on symmetric homodimers which consist of two identical nanoparticles, such as Au nanosphere-nanosphere,22 Au nanorod-nanorod30 or Au nanotriangle-nanotriangle homodimers.39 Heterodimers of metal nanoparticles enable symmetry breaking and exhibit richer plasmon coupling behavior and interesting optical properties. Plasmon coupling in Au nanosphere-nanosphere of different sizes,41 Au nanosphere-Ag nanosphere,42,

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Au nanosphere-nanoshell,44 Au nanodisk-nanodisk of different sizes,45,

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and

Au nanorod-nanosphere47 have been studied. So far most of these studies were focused on their linear optical properties such as scattering of the hetero-nanostructures and only a few studies were focused on nonlinear optical properties such as second harmonic generation. Studies on nonlinear optical properties are important as they are more sensitive to the local electric field


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induced by plasmon coupling. As second harmonic generations are generally complicated by the interactions of the metal nanostructures with the substrate, studies on 2PPL property of heteronanostructure provide a sensitive probe for plasmon coupling effects on linear and nonlinear optical responses of metal nanostructures to explore their potential photonic and bio-medical applications. Herein, we report a systematic study of single particle properties of two model heterodimer structures composed of an Au nanorod (NR) and an Au nanosphere (NS) with different spatial arrangements: end-linked heterodimer and side-linked heterodimer. An easy wet chemical approach was utilized to prepare these heterodimers with ~1 nm interparticle gap distance. The attachment of Au NS occurred at the end or side of an anisotropic Au NR. The consequent plasmon coupling resulted in a modulation in plasmon resonance bands. The polarization dependent scattering and 2PPL spectra were simultaneously measured to understand how 2PPL properties are affected by different plasmon coupling modes of these heterodimers. Numerical simulations using the finite-difference time-domain (FDTD) methods were conducted to understand the excitation mechanisms of the 2PPL properties of these nanostructures heterodimers and the plasmon coupling effects on their optical properties.

EXPERIMENTAL SECTION Chemicals and materials. Gold(III) chloride trihydrate (HAuCl4·3H2O, 99.9%), sodium borohydride (NaBH4, 98%), silver nitrate (AgNO3, 99%), sodium iodide (NaI, 99.99%), sodium oleate (NaOL, ≥99%), trisodium citrate dihydrate (>99%)

and L-Cysteine (>97%) were

purchased from Sigma Aldrich. L-(+)-ascorbic acid (AA, 99%) and cetyltrimethyl-ammonium bromide (CTAB, 99%) were purchased from Alfa Aesar. Hydrochloric acid (HCl, 37% in water)


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was purchased from Fluka. All aqueous solutions were prepared in deionized (DI) water (resistance = 18.1 MΩ.cm). Preparation of Au NRs. Au NRs were prepared by using a previously reported seed-mediated method.48 Briefly, 5 mL of 0.5 mM HAuCl4 solution was mixed with 5 mL of 0.2 M CTAB solution in a 25 mL conical flask. A freshly prepared 0.6 mL of 0.01 M NaBH4 solution was diluted to 1 mL with water and quickly injected into the Au(III)-CTAB solution under vigorous stirring for 2 min. The resultant seed solution was aged for 30 min at room temperature before use. The growth solution was prepared by dissolving 0.9 g CTAB and 0.15 g NaOL in 50 mL warm DI water (50 °C) in a conical flask. The solution was allowed to cool down to 30 °C and 3.6 mL of 4 mM AgNO3 solution was added. The mixture was kept undisturbed at 30 °C for 15 min before adding 50 mL of 1 mM HAuCl4 and stirring at 700 rpm for 90 min. The pH of the mixture was adjusted by adding 0.3 mL of 12.1 M HCl and stirred at 400 rpm for another 15 min. 0.25 mL of 0.064 M AA was then added into the mixture under stirring for 30 s. 0.08 mL seed solution was added into the mixture, swirled for 30 s, and then left undisturbed for 12 h at 30 °C to obtain Au NRs. Preparation of Au NSs. Au NSs were prepared by using a previously reported seeded growth method.49 18 nm seed solution was prepared by heating 100 mL of 2.5×10-4 M HAuCl4 solution at 120 °C temperature for 30 min. 10 ml 1% citrate solution was then added into the solution followed by boiling for another 20 min. The growth solution was prepared by sequentially mixing 24 mL DI water, 270 µL of 25 mM HAuCl4, 270 µL of 0.1 M CTAB into a 50 mL conical flask. 150 µL of 0.1 M AA was then added into the mixture and stirred for 2 min before 2 mL of 18 nm seed solution was quickly added into the growth solution. The mixture was swirled for 30 s and left undisturbed at 30 °C temperature for at least 6 h.


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Self-assembly between Au NRs and Au NSs. The Au NRs and Au NSs solutions were washed to remove the extra surfactants and then mixed with the concentration of 0.02 nM for Au NRs and 0.03 nM for Au NSs, respectively. The pH of the mixture was adjusted to 2.3 by adding proper amount of 1 M HCl. Formation of aggregated nanoparticles was induced by adding cysteine into the mixture.50, 51 The aggregate solution was diluted 60 times and drop-casted onto a pre-cleaned and marked ITO glass substrate. The substrate was dried in an oven at 50 °C and then rinsed with DI water to remove extra cysteine molecules, and then dried under N2 stream before subsequent single particle experiments. Instrumentation. Extinction spectra were measured by using a SHIMADZU UV-2250 spectrophotometer. Scanning electron microscope (SEM) images were taken on a JEOL JSM6701F Field Emission at 5 kV. Transmission electron microscopy (TEM) images were taken on a Philips CM10 TEM microscope at accelerating voltage of 100 kV. Single particle dark field scattering measurements. A home-built dark-field scattering system was used for measuring the scattering spectra of various nanostructures.22 An inverted dark field microscope (Nikon Eclipse Ti) was equipped with a 100 W quartz-tungsten-halogen lamp, a Nikon dark field condenser (NA=0.80-0.95), an oil immersion objective (100×, NA=0.51.25, NA of 0.5 was used in our experiment). The spectra were measured by using a monochromator (Acton Spectra Pro 2150i) coupled with a CCD camera (Andor DR-328G-C01SIL). The position of the sample was controlled by a three dimensional piezoelectric translational stage (PI E-710) to move the particle so as to be imaged into the slit region. The background signals during the measurement were recorded at the spot without any particle and subtracted to obtain the final scattering spectra. The polarization dependent scattering spectra were measured by placing a sheet polarizer in the beam path before the detector.


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Single particle two-photon photoluminescence (2PPL) measurements. 2PPL spectra and intensities of these nanostructures were measured by using a home-built two-photon optical microscope. A mode-locked Ti:sapphire oscillator

(Avesta TiF-100M) was used as the

excitation source, which generates 80 fs pulses with central wavelength at 785 nm and repetition rate of 84.5 MHz. The laser beam was passed through a 785/10 nm band pass filter (Semrock LD 01-785/10-25), 5× spatially expanded, and then reflected by a 50/50 beam-splitter into the objective lens (NA=0.5). The polarization of the excitation beam was adjusted by using a combination of waveplate and polarizer. The laser beam was focused onto the single nanoparticle. The emission signals were collected by the same objective lens and filtered by two 785 nm notch filters (Semrock NF03-785E-25) to reduce the laser scattering and a long pass filter with cutoff wavelength at 450 nm (Chroma) to remove the second harmonic generation signal. The spectra of the emission signals were measured by using a monochromator (Acton, Spectra Pro 2300i) coupled CCD (Princeton Instruments, Pixis 100B) using an optical fiber. The 2PPL intensities were measured by using a raster scanning imaging method. In this method, the 2PPL intensities were detected by a photon counting photomultiplier (PMT) (PicoQuant, PMA 182-N-M) and the signals were processed by using PicoHarp 300. The integrated pixel intensities of single particles were corrected by subtracting the integrated background intensity without any particle to obtain the net 2PPL intensity of each single particle. Scattering spectra was measured before and after each 2PPL measurement to ensure no photothermal transformation happened during the measurements. Finite-difference time-domain (FDTD) simulation. FDTD calculation was performed by using the FDTD Solution 8.6 by Lumerical Solutions Inc. The light from a total-field scatteredfield source with a spectrum ranging from 400 to 900 nm was introduced into a 2 µm box


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surrounded by a perfectly matched layer boundary condition. The nanostructures were assumed at the center of the simulation region. Au NR and NS were modeled as a hemi-sphere capped cylinder at both ends and a sphere, respectively. The length and diameter of the Au NR were set as 100 nm and 40 nm respectively while the diameter of the Au NS was set as 40 nm. The dielectric constant of gold was taken from Johnson and Christy’s measurements with polynomial fitting.52 The gap between Au NR and NS was fixed at 1 nm. The nanostructures and the surrounding space were divided into 1 nm meshes and the interparticle gap region was divided into 0.2 nm meshes. The light source propagated towards the coupled nanostructures through a slab of ITO coated coverslip which was placed 2 nm underneath the nanostructure. The refractive index of ITO substrate and the surrounding were set as 1.9 and 1.0, respectively.

RESULTS AND DISCUSSIONS Preparation of Au NR-NS heterodimers. Au NRs and NSs were prepared by using previously reported seed-mediated growth methods.48, 49 The obtained nanoparticles are uniform in size and shape based on their TEM images (Figure 1a-b). Nanoparticle aggregates of different sizes, including Au NR-NS heterodimers, were formed upon addition of cysteine into the mixture solution of Au NRs and NSs at pH of ~2.3. The zwitterionic structure of cysteine molecules facilitates formation of Au NR-NS heterodimers (Figure 1c).50,

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Formation of

nanoparticle assembly was confirmed by the change in their extinction spectra after incubating the mixture solution for 30 min at 45 °C (Figure 1d). The diluted solution of aggregated nanoparticles was drop-casted onto a conductive indium tin oxide (ITO)-coated glass substrate for subsequent single particle experiments.


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Figure 1. (a, b) TEM images of Au NRs (a) and Au NSs (b); (c) Cysteine assisted coupling between an Au NR and Au NS; (d) Extinction spectra of Au NR-NS mixture solution before and after addition of cysteine. A pattern matching method53 was performed between the SEM and dark field scattering images of the same sample to locate Au NR-NS heterodimers on the ITO substrate and correlate their sizes and morphologies with the corresponding optical responses (Figure S2).The SEM image showed a random distribution of Au NR-NS heterodimers on the substrate (~9% endlinked heterodimers, ~5% side-linked heterodimers) together with some bigger clusters and monomers. Heterodimers with end-linked and side-linked geometries were chosen for our studies. Statistical analysis based on 15 end-linked heterodimers and 15 side-linked heterodimers showed that the average length and diameter of Au NRs are 100±8 nm and 43±2 nm and the diameter of Au NSs is 40±5 nm in those coupled structures. The approximate distance between Au NR and NS is ~1 nm. More SEM images are shown in Figure S3.


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Figure 2. Normalized dark field scattering spectra of end-linked Au NR-NS heterodimer, side linked Au NR-NS heterodimer, and Au NR monomer under unpolarized white light excitation. Dark field scattering spectra. Single particle dark field scattering spectra of Au NR-NS heterodimers were measured by using a slit imaging technique54 under a home-built microscope (detail experimental setup shown in Figure S4a). Unpolarized scattering spectra of an end-linked Au NR-NS heterodimer, a side-linked Au NR-NS heterodimer and an Au NR monomer (Figure 2) displayed a dominant SPR peak at 682±4, 651±5 and 645±2 nm, respectively. Compared to the Au NR monomer, an obvious red-shift of 37±3 nm was observed for end-linked Au NR-NS heterodimer while only 6±3 nm red-shift was observed for the side-linked heterodimer. A slight fluctuation in the peak position and intensity was observed due to the size distribution in the coupled Au NR-NS nanostructures. The red-shifted scattering spectra of the heterodimers can be explained in terms of plasmon coupling between the dipolar mode of Au NR and Au NS using the plasmon hybridization theory as discussed in detail later.


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Figure 3. Polarization dependent scattering spectra (a-c) and intensity plot (d-f) of an end-linked Au NR-NS heterodimer, side-linked Au NR-NS heterodimer, and Au NR monomer. Insets are their corresponding SEM images. The red arrows on SEM images indicate the polarization angle of 0o. The scattering spectra of two types of Au NR-NS heterodimers and Au NR monomer showed strong polarization dependence (Figure 3a-c). Similar to the Au NR monomer, both end-linked and side-linked heterodimers displayed strongest scattering signals at the polarization angle of 0o (along the longitudinal direction of the Au NR) and the weakest at the polarization angle of 90° (along the transverse direction of the Au NR). The polarization dependent scattering intensity plots displayed a cos2θ dependence.


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Figure 4. Plasmon hybridization models for Au NR-NS end-linked heterodimer (a) and sidelinked heterodimer (b). The optically active modes of end-linked and side-linked heterodimers are summarized in (c). Plasmon hybridization model. A plasmon hybridization model can be utilized to understand the extinction spectra of the heterodimers (Figure 4). The energy of dipolar surface plasmon resonance (SPR) modes is at 540 nm (2.29 eV) for Au NSs, 646 nm (1.92 eV, longitudinal SPR mode) and 529 nm (2.34 eV, transverse SPR mode) for Au NR, respectively, based on the FDTD calculation. The dipolar mode of an Au NS was hybridized with both longitudinal or transverse SPR modes of Au NR, resulting in different bonding and antibonding modes in the Au NR-NS heterodimers. According to this model, plasmon hybridization in the end-linked heterodimer evolves into a total of four hybridized modes (two bonding and two antibonding modes) (Figure 4a). The SPR mode of Au NS interacts with the longitudinal SPR mode of Au NR to generate a lower energy bonding mode (symmetric, optically active) and a higher energy antibonding mode (antisymmetric, partially active as the dipole moment is not completely canceled). Similarly, the interactions between the SPR mode of Au NS and the transverse SPR mode of Au NR result in a lower energy bonding mode (antisymmetric, very weak due to nearly canceled dipole moments) and a higher energy antibonding mode (symmetric, optically active). The end-linked heterodimer


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is expected to display two distinct SPR bands with different polarizations: the bonding longitudinal hybridized mode at 683 nm (1.81 eV) polarized along 0° and the antibonding transverse hybridized mode at 532 nm (2.33 eV) polarized along 90°. The two partially active modes did not show up in the simulation results due to their weak extinction intensities. Similarly, the side-linked Au NR-NS dimer has two bonding and two antibonding modes upon plasmon hybridization (Figure 4b). All four modes will be optically active as their dipole moments do not cancel out. The scattering spectra of side-linked dimer showed a distinct band at 651 nm (1.9 eV) under 0° excitation while a weak band at 595 nm (2.08 eV) under 90° polarizations (Figures 3b and 5b). The 651 nm peak can be assigned as the bonding mode due to antisymmetric hybridization between Au NS and the longitudinal mode of Au NS, while the peak at 595 nm can be assigned as the bonding mode due to symmetric hybridization between the Au NS and the transverse mode of Au NR. FDTD simulation of heterodimers. FDTD simulations were performed for quantitative understanding of different plasmon coupling modes in these heterodimers. The gap distance between the Au NR and the Au NS was set as 1 nm. The Au NR was modeled as a hemisphere capped cylinder at both ends with diameter of 40 nm and length of 100 nm. The Au NS was modeled as a sphere with a diameter of 40 nm. As only the scattering signals were detected by the dark field method, only the scattering spectra were calculated for direct comparison. Figure 5a-c shows the calculated scattering spectra of end-linked and side-linked Au NR-NS heterodimers, and Au NR monomer under both 0° and 90° polarizations. The simulation results are in excellent consistence with the experimental data. The extinction intensity is highest at 0° polarization while lowest for 90° polarization for all three different situations. Under 0° polarization, a single SPR band was observed at 683 nm for end linked heterodimer, 651 nm for


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side-linked heterodimer, and 646 nm for Au NR monomer, respectively. Under 90° polarization, a single weak SPR band was observed at 529 nm for end-linked dimer, at 532 nm for Au NR monomer respectively. The scattering spectrum at 90° polarization for side-linked heterodimer is slightly complicated: a SPR band at 595 nm with a relatively stronger intensity and a weak band at 528 nm. The former can be ascribed to the bonding mode of the hybridization between dipolar mode of Au NS and the transverse SPR mode of Au NR, while the latter can be ascribed to the corresponding antibonding mode with partial cancellation of their dipole moments.

Figure 5. (a-c) Comparison of the experimental and calculated scattering spectra of Au NR-NS end-linked heterodimer, side-linked heterodimer and Au NR monomer; (d-f) Comparison of calculated scattering spectra under 0° and 90° polarizations and normalized sum for end-linked heterodimer, side-linked heterodimer and Au NR monomer.


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Figure 6. 2PPL (red) and scattering (black) spectra of end-linked heterodimer (a, d), monomer (b, e) and side-linked heterodimer (c, d) under excitation polarization at 0° (a-c) and 90° (d-f); (g-i) Excitation polarization dependent 2PPL intensities of these nanostructures. The arrows on the SEM images indicate the directions of excitation polarization. The solid lines in g-i for just for eye-guiding.


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Polarization dependent 2PPL of Au NR-NS heterodimers. Single particle 2PPL spectra and intensities of Au NR-NS heterodimers and Au NR monomers were measured under femtosecond laser excitation at 785 nm. 2PPL spectra of various nanostructures under excitation with polarization parallel (0° polarization) and perpendicular (90° polarization) to the longitudinal axis of Au NR were found to display close resemblance to their corresponding scattering spectra (Figure 6a-f). Two different mechanisms have been previously proposed to understand the resemblance between the photoluminescence and scattering spectra: plasmonic emission55, 56 and plasmon modulated emission.27,

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According to the plasmonic emission model, the

photoluminescence of Au nanoparticle originates from non-radiative recombination of d-band holes with sp band electrons, emitting particle plasmons.55,

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According to the plasmon

modulated emission model, the photoluminescence originates from the radiative electron-hole recombination which is strongly modulated by SPR.27, 57 Both models support a good agreement between observed 2PPL spectra and SPR bands. The excitation polarization dependence of 2PPL intensities of Au NR-NS heterodimers and Au NR monomer were measured and direct comparison of their results are shown in Figure 6g-i. 2PPL intensities of these nanostructures show strong dependence on the excitation polarization. For both Au NR-NS end-linked heterodimer and Au NR monomer, the integrated 2PPL intensity was highest under 0° excitation polarization and lowest under 90° excitation polarization. Similar to that of Au NR monomer, 2PPL intensity of end-linked heterodimer can be well fit with cos4θ function with respect to the excitation polarization angle, which suggests that the observed 2PPL of end-linked heterodimer is strongly coupled with its newly formed longitudinal SPR band. In a striking contrast, the excitation polarization dependent 2PPL intensity of the side-linked heterodimer displayed an opposite trend compared to that of end-linked heterodimer: 2PPL


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signal of side-linked heterodimer under 90° excitation is much stronger than that under 0° excitation instead. The 2PPL intensity was lowest under 0° polarization (along the long axis of Au NR), and then gradually increased with the increasing polarization angle, and reached highest under 90° polarization. It is intriguing that the polarization dependence of the 2PPL intensity of side-linked NR-NS dimer is opposite to that of its scattering intensity (Figure 6h). This result indicates that 2PPL intensity of side-linked NR-NS dimer along the coupling direction were significantly enhanced because of the plasmon coupling between the Au NS and the transverse mode of Au NR.

Figure 7. (a) Relative 2PPL intensities of Au NR-NS heterodimers and Au NR monomers (L=100 nm and 140 nm) under excitation at 0° and 90° polarization; (b) Comparison of experimental relative 2PPL intensities (solid lines) and calculated relative integrated |E/E0|4 values at 785 nm (dotted lines) relative to that of 100 nm Au NR under 90° excitation under 0° and 90° polarization for Au NR-NS heterodimers and Au NR monomers. Relative 2PPL intensities of Au NR-NS heterodimers and Au NR monomers. Figure 7a shows relative 2PPL intensities of Au NR-NS heterodimers and Au NR monomers (L=140 nm and 100 nm) under excitation polarization at 0° and 90°. 2PPL of Au NR with the dimension of


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L=140 and D=40 nm, which has the same overall dimension as end-linked Au NR-NS heterodimer, was measured under the same experimental conditions for direct comparison to clarify the possible length effects. The results are based on averaging over 3 measurements. The relative intensity was defined as the 2PPL intensities of various nanostructures over that of L=100 nm Au NR monomer under 90° excitation polarization (along its transverse mode), i.e. the 2PPL intensity of L=100 nm Au NR was set as 1. Under 0° excitation, end-linked heterodimer was found to display the highest 2PPL intensity, ~300 times that of L=100 nm Au NR monomer. In contrast, 2PPL intensity of side-linked NR-NS heterodimer was slightly (~25%) larger than that Au NR (L=100 nm) monomer. The huge difference between 2PPL intensities of end-linked Au NR-NS heterodimers and the Au NR monomer can be attributed to two main factors. First, the bonding hybridization between Au NR and NS caused a red-shifted longitudinal SPR mode (which overlaps better with the excitation wavelength of 785 nm and facilitate the resonance enhancement of two-photon excitation efficiency.40 On the other hand, plasmon coupling can dramatically amplify the local electric field in the gap region (generation of "hot-spot"), which further contributes to enhanced excitation efficiency. The 2PPL intensity of Au NR-NS heterodimer is even larger than (~4.6 times) that of Au NR monomer of the same total length (L=140 nm), although the longitudinal SPR band of Au NR with L=140 nm (band maximum at 768 nm, Figure 8a) has a better spectral overlap with the excitation wavelength compared to end-linked heterodimer. This result suggested that the 2nd effect, amplified local electric field in the gap region due to symmetric plasmon coupling plays a dominant role in the observed huge 2PPL intensities in end-linked Au NR-NS heterodimer. In contrast, the sidelinked heterodimer only showed very subtle change in 2PPL intensity under 0° excitation polarization. In the side-linked heterodimer, the lowest energy mode along 0° direction arises


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from the asymmetric hybridization between the longitudinal SPR mode of NR and NS, which reduce the electron density and counteract the resonance effect of slightly red-shifted longitudinal SPR mode.

Figure 8. Comparison of relative 2PPL intensity and normalized scattering spectra of (a) endlinked Au NR-AS heterodimer and 140 nm long Au NR monomer under 0° polarization; (b) side-linked Au NR-NS heterodimer under 0° and 90° polarization. Larger 2PPL intensities were observed in both cases despite less favorable spectral overlap between scattering spectra and excitation wavelength. The results under 90° excitation are strikingly different from those under 0° excitation. Under 90° excitation, end-linked Au NR-NS dimer, L=140 nm Au NR monomer showed similar 2PPL intensities to that of L=100 nm Au NR monomer. However, the 2PPL intensity of side-linked heterodimer under 90° excitation is 850 times that of L=100 nm Au NR monomer (Figure 7a). In particular, 2PPL intensity of side-linked heterodimer under 90° excitation is even ~25 times stronger that its 2PPL under 0° excitation (Figures 7a and 8b), despite the fact that SPR band associated with the former (band maximum at 595 nm) has less overlap with the excitation wavelength compared to that under 0° excitation (band maximum at 651 nm). The difference


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between 2PPL intensities of the heterodimer under 0° and 90° excitation can only be explained by the plasmon coupling due to symmetric hybridization between the transverse band of Au NR and Au NS along the transverse directions. This again confirms that amplified local electric field in the gap region due to plasmon coupling plays a dominant role in the observed huge 2PPL intensities. Numerical simulations have been performed to understand 2PPL properties of different nanostructures (Figure 7b) by using the FDTD method. To compare the 2PPL experimental data with the numerical simulation results, we used a method similar to that was previously described

by Schuck et al.39 The 2PPL intensity is expected to be proportional to ∬ , where |E/E0| is electric field enhancement at excitation wavelength (785 nm). Here the two-dimensional integral over the plane that bisects the nanoparticle through its center was used as a good approximation for the full three-dimensional calculation.39 The relative 2PPL enhancement factor of Au NR-NS heterodimers and Au NR at different polarization directions were calculated by the above integral of various nanostructures normalized by that of L=100 nm Au NR at 90o

polarization, i.e. to =

∬

∬

( )

. Figure 7b shows the comparison of the relative

2PPL intensities based on our experimental measurements and FDTD calculations. The excellent agreement between the experimental data (relative 2PPL intensities) and simulation results (relative integrated |E/E0|4) confirms that the observed 2PPL in different nanostructures are primarily due to different excitation efficiency. Plasmon coupling between Au NR and Au NS resulted in huge enhancement in electric field of the coupled nanostructure, which are responsible for the huge 2PPL intensities in the corresponding heterodimers under excitation


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polarization along its plasmon coupling directions, i.e. end-linked heterodimer under excitation at 0o polarization and side-linked Au NR-NS heterodimer under excitation at 90o polarization.

SUMMARY Single particle dark-field scattering and 2PPL properties of side-linked and end-linked Au NRNS heterodimers were systematically studied under direct comparison with Au NR monomer to investigate the plasmon coupling effects on their 2PPL properties. The scattering spectra of sidelinked and end-linked Au NR-NS heterodimers were found red-shifted by 6±3 nm and 37±3 nm with respect to that of Au NR monomer, respectively, which can be understood in terms of plasmon hybridization theory. Similar to single Au NR monomer, scattering spectra of both heterodimers displayed a cos2θ polarization dependence with a dominant contribution from the component with polarization along the long axis of the Au NRs. 2PPL spectra of both heterodimers and Au NR monomers were found to resemble their corresponding scattering spectra under both 0o and 90o excitation polarizations, suggesting that the observed 2PPL signals were strongly modulated by their corresponding SPR modes. The excitation polarization dependent 2PPL intensity of end-linked heterodimer showed cos4θ dependence, similar to that of Au NR monomer. 2PPL intensities of end-linked dimer under 0o excitation polarization is ~300 times that of L=100 nm Au NR monomer and even ~4.6 times that of L=140 nm Au NR (similar total length and diameter as the heterodimer) under same experimental conditions. However, 2PPL of side-linked heterodimer displayed strikingly different excitation polarization dependence. The 2PPL excitation polarization dependence of side-linked heterodimer is orthogonal to the polarization dependence of its scattering spectra: the largest 2PPL intensity was found along the excitation polarization at 90o (perpendicular to the long axis of Au NR) instead


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of 0o as in its scattering spectra. 2PPL of side-linked heterodimer under 90o excitation is 25 times that 0o excitation, 850 times that of Au NR under 90° excitation. Despite these differences, an interesting feature is that the largest 2PPL occur along the coupling directions intensities for both end-linked and side-linked heterodimers. FDTD numerical simulations give excellent agreement between the experimentally obtained 2PPL signal intensities and calculated integrated |E/E0|4 at the excitation wavelength. These results indicated that the observed different 2PPL intensities in different heterodimers are primarily due to their different excitation efficiency. Although redshifted spectra of hybridized SPR mode due to plasmon coupling partially contributes to enhanced two-photon excitation efficiency due to improved resonance effects, stronger 2PPL signals have been observed for coupled nanostructures with excitation along the coupling direction but less favorable spectral overlap between SPR band and excitation wavelength. Plasmon coupling induced electric field amplification of the coupled nanostructure is the dominant mechanism responsible for the huge 2PPL intensities in the corresponding heterodimers under excitation polarization along its plasmon coupling directions, i.e. end-linked Au NR-NS heterodimer under excitation at 0o polarization and side-linked heterodimer under excitation at 90o polarization. These studies provide insight on fundamental understanding of plasmon coupling effects on optical properties and excitation mechanism of 2PPL of metal nanostructures, which provide insight on designing nanostructures with tailored optical properties for various potential photonic and optoelectronic applications.

ASSOCIATED CONTENT Supporting Information


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The extinction spectra of Au NR and Au NS solution; pattern matching between dark-field scattering image and SEM image; SEM images of Au NR-NS heterodimers and NR monomers; experimental setup for dark field scattering and 2PPL measurements; comparison of single particle dark field scattering spectra of different nanoparticles; polarization dependent 2PPL spectra of end-linked heterodimer; 2PPL imaging of end-linked heterodimer; electric field enhancement (|E/E0|) of Au NR-NS heterodimers, and Au NR monomer under different excitation polarizations; 2PPL and scattering spectra of L=140 nm Au NR.

AUTHOR INFORMATION Corresponding Author Tel: (65)-6516-2847; E-mail: [email protected] ACKNOWLEDGMENT This work is supported by the Ministry of Education, Singapore (Tier 1, R-143-000-607-112), National Research Foundation, Prime Minister’s Office, Singapore under its Competitive Research Program (CRP Award No. NRF-CRP10-2012-04), and Office of Deputy President (Research & Technology), National University of Singapore (R143-000-595-733). REFERENCES: 1.

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ToC Graphic


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Single Particle Studies on Two-Photon Photoluminescence of Gold

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