Article pubs.acs.org/JPCC
Measurement of Plasmon-Mediated Two-Photon Luminescence Action Cross Sections of Single Gold Bipyramids, Dumbbells, and Hemispherically Capped Cylindrical Nanorods Arif M. Siddiquee,† Adam B. Taylor,† Salmaan Syed,† Guh-Hwan Lim,‡ Byungkwon Lim,‡ and James W. M. Chon*,† †
Centre for Micro-Photonics, Dept. of Physics, Faculty of Science, Engineering and Technology, Swinburne University of Technology, PO Box 218, Hawthorn 3122, Victoria, Australia ‡ School of Advanced Materials Science & Engineering, Sungkyunkwan University (SKKU), Suwon 440-746, South Korea S Supporting Information *
ABSTRACT: Recent advances in two-photon excited photoluminescence of plasmonic gold nanorods have greatly expanded their application. Shape-controlling of nanorods can enhance a particular photophysical process to tailor needs, but its application to twophoton luminescence is yet to be fully developed despite its importance in biolabeling. Here, we report direct, independent measurements of two-photon action cross sections (TPACS) of single gold bipyramids, hemispherically capped cylindrical nanorods, and dumbbells. The effect of radius of curvature at the tip on TPACS are measured and compared per same aspect ratio and volume. Bipyramids have shown 33% more TPACS per volume for the aspect ratio range 3.8−4.2, with 0.28 GM/nm3 compared to 0.21 and 0.20 GM/nm3 for nanorods and dumbbells, respectively, at 27% measurement error. This value is not as high as the field strength at the tip, which can be 3−4 times higher for sharp bipyramids. Such moderate increase is attributed to the small surface area of the tip that the field is subjected to which competes with the increase in field. We have used the z-scan technique to measure nonlinear absorption on a single crystalline gold nanosheet as a bulk gold and an analytical field enhancement theory for prolate spheroids to successfully confirm the trend of TPACS/volume with respect to the aspect ratio. It was found that the prolate spheroids do provide a good approximation of TPACS values for these rods. Current plasmonics and nanorod applications with the sharp geometric features used for greater field enhancement and subsequent two-photon luminescence will need to consider the tip surface area that limits the overall efficiency of the process.
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demonstrated the multiphoton excitation of the metal film luminescence. Later, Mohamed et al.3 observed one-photon excited luminescence from ensemble of gold nanorods. The measured quantum yield of ∼10−5 was 2−3 orders of magnitude higher than roughened film and was also attributed to the LRE. Dulkeith et al.6 observed SPR-like luminescence spectrum of spherical gold nanoparticles and proposed hole coupling to surface plasmon to explain the observation. More recently, Yorulmaz et al.22 and Fang et al.23 have confirmed the SPR-like luminescence spectrum from single hemispherically capped cylindrical gold nanorods, with both works showing quantum yield of ∼10−6. Rao et al.35 also measured bipyramidal gold nanorods quantum yield of ∼10−5. For two-photon luminescence (TPL), Beversluis et al.,5 Bouhelier et al.,8 and Wang et al.9 used a near-infrared pulsed laser to excite the luminescence and found this to increase greatly around surface plasmon resonance of the gold nanorods. Two-photon action cross section (TPACS or σTPA) of
INTRODUCTION
Recent progress in gold nanorod and nanoparticle two-photon luminescence has greatly advanced application of gold nanoparticles in optics and plasmonics.1−35 Because of their nonblinking, nonbleaching luminescence, it can be used in biolabeling,9,12,27,31 optical antenna mode sensing,10,11,13,16,33,34 high-density optical storage, and cryptography.17,24 When the excitation laser energy is coupled to the longitudinal surface plasmon resonance (LSPR) of the nanorods, their two-photon action cross section can be increased to exceed that of fluorescent dye molecules or semiconductor quantum dots.9,12 Luminescence from smooth bulk gold film was originally observed to be extremely weak, with quantum yield of the process measured at ∼10−10. Mooradian1 was the first to observe and attributed this to interband excitation of d-band electron to sp-band above the Fermi level and then the radiative recombination with a d-band hole. Boyd et al.2 found the strength of the quantum yield increases to ∼10−7 for roughened surfaces and explained such increase in terms of near-field enhancement due to plasmon and lightening rod effect (LRE) that concentrates the field around the tip of half hemispheroidal bosses that were assumed to construct the surface. They also © XXXX American Chemical Society
Received: August 23, 2015 Revised: November 20, 2015
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DOI: 10.1021/acs.jpcc.5b08214 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
for laser irradiation. By using a numbered grid, optical and electron microscope images were taken from the same location. Optical Setup. A home-built confocal laser scanning microscopy (CLSM) system was used to perform laser scattering and two-photon luminescence microscopy of single gold nanorods (refer to Supporting Information Section (1), Figure S1, for details of experimental setup). A Ti:sapphire femtosecond laser (Broadband Tsunami, Spectra-Physics) was used as the laser source. A spectrometer (USB4000, Ocean Optics) was used to monitor the laser spectrum. In order to measure laser pulse width, frequency-resolved optical grating (FROG, Grenouille, Swamp Optics) was used. A quarter-wave plate was used for generating circularly polarized light. The laser beam was first expanded, spatially filtered, and focused onto the sample through a high numerical aperture (NA = 1.4, Olympus oil immersion lens) objective. Sample was mounted on a computer controlled 3D high-resolution scanning stage (Physik Instrumente). TPL Image Acquisition of Nanorods. Raster scan on the sample was performed to acquire simultaneously both scattering and TPL signal from the nanoparticles. The same objective was used to collect the emitted signal (scattering or TPL) from the illuminated volume of the sample. The scattered signal is detected by photomultiplier tube (Oriel PMT), and the TPL signal is detected by another PMT (Hamamatsu, H7422P-40). At a particular bias of the PMT, current generation was calibrated to count the number of photons detected. From the raster scanned spot, taking a cross section yields the radially dependent intensity of the focal spot. For 740 nm the full width at half-maximum (fwhm) of the spot is found to be 396 nm, close to the theoretically predicted value of 360 nm (calculated using vectorial Debye theory). Estimation of Collection Efficiency. Collection efficiency at the wavelength range of the luminescence (400−650 nm) was estimated by measuring transmission/reflection efficiency of all the optical elements in the collection path (objective lens, long-pass dichroic beam splitter, and PMT cathode). In addition, percentage of the collection cone was considered for TPL from the focal volume, which amounts to 30.5% for numerical aperture 1.4 objective lens. Collection efficiency at 532 nm, mid-wavelength of TPL spectrum, was 2.4%, from the focal spot of the objective lens to the PMT cathode. The wavelength response of our collection efficiency is shown in Supporting Information Section (1), Figure S2. Estimation of Energy Absorption per Rod. In terms of the nanorod absorption, we employ the method described in Taylor et al.46 First scattering cross sections σmeas scat of nanorods are measured, and then using Gan’s theory with shape and damping corrections, 47,48 the relative strengths of the absorption and scattering of the gold nanorod cross sections, theory σtheory abs /σscat , are calculated. From this ratio, we determined the meas absorption cross section of each nanorod as σmeas abs = σscat × theory theory σabs /σscat . Average peak absorption cross-sectional measurement for the nanorods was found to be ∼0.82 × 10−11 cm2. With the measured absorption cross section, absorbed energy is estimated. As an example, a typical average input power at the back aperture was measured to be 200 μW (at 740 nm, 82 MHz repetition rate, pulse width ∼150 fs). Normalizing this peak power by the area of the Airy disk measured above, noting that 86% of the laser power falls within this disk, gives the laser energy per pulse at the focal spot I = 0.37 mJ/cm2. This corresponds to an average per pulse absorption of 3.5 fJ per rod by the nanorods of volume 36 000 nm3, or 4.7 J/g. Similarly,
nanorods was measured by referencing relative intensity to molecules of known TPACS value, i.e., Rhodamine 6G. Using this technique, Wang et al.9 measured for aspect ratio 3 rods (∼50 nm length), ∼2 × 103 GM, which was comparable to that of quantum dots. TPL strength of various aspect ratio gold nanorods has been also reported by Wang.27 Zijlstra et al.17 measured TPACS by direct calibration of the microscopy and estimated it to be 3 × 104 GM for single gold nanorods of aspect ratio 3.7. Gao et al.30 used this value as a reference to examine a series of increasingly sharp edged nanoparticles, from nanospheres to nanostars, correlating the TPL signal at single excitation wavelength with the shapes of the nanoparticles. Increased TPL strength with increasingly sharp particle shapes was observed. Two-photon absorption is proportional to |E|4; hence, the geometries that maximize the electric near fields at the metal surface will maximize the rate of two-photon absorption.2,36 Structures with sharp features will concentrate charges during oscillation and lead to enhanced near fields around these features.10,30,37,38 By deliberately fabricating or synthesizing structures that concentrate and strengthen the plasmonically generated near field, such as coupled nanoantennas,10,13,16,25,38 roughened nanoparticles,30,32 or nanoparticle oligomers,28,29,31 generated TPL strength was significantly increased. With the myriad of nanoparticle geometries available via modern synthesis and fabrication routes39−44 accurate knowledge of how these geometries enhance the TPL strength is essential, to allow the best geometry to be chosen for a given application. However, most of TPACS measurements in the literature utilize method referencing to known TPACS of molecules, i.e., Rhodamine 6G,9,27 which can be highly dependent on the environment the molecules are subjected to causing quantum yield and TPACS to vary greatly. In this work, we measure TPACS of single gold nanorods of varying tip geometry (bipyramids, hemispherically capped cylinders, and dumbbells, which have increasing radii of curvature at the tip) to understand how the geometry of nanorods enhances the strength of their TPL generation. In particular, we measure TPACS directly and independently using calibration method by Xu and Webb45 rather than referencing values to the literature.9,27,30 The TPACS were also measured at LSPR conditions of nanorods to account for the true plasmon effect on TPACS, unlike previous report on measuring TPACS at one particular wavelength.30 We also use the z-scan technique to measure nonlinear absorption on a single crystalline gold nanosheet as a bulk gold, and relate it to the measured TPACS values using an analytical field enhancement theory of prolate spheroids to successfully confirm the trend of TPACS/volume of these rods with respect to the aspect ratio. Exact dimensions of the rods were measured by correlating with electron microscopy. Accurate knowledge of the TPACS will be extremely important for biolabeling and cancer therapy applications of gold nanorods.9,12,27
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EXPERIMENTAL METHODS Sample Preparation. Three different types of nanorods bipyramids (BPs, AR 4.3 ± 0.4), hemispherically capped cylindrical nanorods (NRs, AR 3.2 ± 0.6), and dumbbells (DBs, AR 3.6 ± 0.6)were purchased from Nanoseedz. The nanorod solution was mixed with poly(vinyl alcohol) (PVA) and was centrifuged on a TEM grid, before the TEM imaging. Once the imaging is completed, the grid is transferred to an optical setup B
DOI: 10.1021/acs.jpcc.5b08214 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Figure 1. (a−c) High-resolution TEM images of a bipyramid (208 × 43 nm, AR 4.8), nanorod (84 × 26 nm, AR 3.4), and dumbbell (73 × 19 nm, AR 3.8), respectively. Scale bar length is 50 nm. (d−f) Exact geometries from the TEM images are then used in COMSOL Multiphysics to model the 3D electromagnetic response of the nanoparticles under electromagnetic excitation. For each particle, the peak LSPR wavelength is determined (λBP = 810 nm, λNR = 850 nm, and λDB = 880 nm), and the electric field norm produced under excitation at this wavelength is plotted in log10 scale. The tip E-field norm was measured to be 77, 34, and 9 (arbitrary values) for the bipyramid, nanorod, and dumbbell, respectively. The mean dimensions of the population those particles are shown as histograms of lengths (g), widths (h), and tip radius (i).
average BPs and DBs showed peak absorption of 15.1 fJ per rod, corresponding to 12.1 J/g, and 3.4 fJ per DB, corresponding to 5.3 J/g. They are far below the ∼50 J/g threshold observed by Taylor et al.46 for single pulse thermal reshaping of nanorods of similar aspect ratios. During the laser exposure, negligible change to the two-photon luminescence strength indicates that cumulative heating was also negligible.
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RESULTS AND DISCUSSION Transmission electron microscopy (TEM) images of a typical single BP, NR, and DB are shown in Figures 1a,b,c, respectively, with geometries close to mean dimensions of each particle type. Dimensional histograms of the nanorods that were used in measuring TPACS are shown in Figures 1g−i, and ensemble extinction spectra of the rods in solution are shown in Figure 1j. With particles shown on the same scale, the increase in tip radius of curvature is evident as the particle geometry transitions from a BP, NR, to a DB. For each nanorod, electromagnetic simulations (finite element method, COMSOL Multiphysics; details of numerical simulation meshing and method are shown in Supporting Information Section (2), Figure S3) are conducted at its peak LSPR wavelength, and the resulting electric field is plotted in Figures 1d−f. As the tip sharpness increases, the strength of the electric field around and at the tip is also seen to be increased, albeit the confinement area is reduced. Optical images of a constellation of nanorods, imaged with 770 nm wavelength, 150 fs pulse width, and maximum 200 μW laser power measured at the back of the aperture (
