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Photothermal-Assisted Optical Stretching of Gold Nanoparticles Shuangshuang Wang and Tao Ding* Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education of China, School of Physics and Technology, Wuhan University, Wuhan 430072, China
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S Supporting Information *
ABSTRACT: The synergy of photothermal energy and optical forces generated by tightly focused laser beams can be used to transform the shape of gold nanoparticles. Here, the combination of these two effects is demonstrated to be an effective way of elongating gold nanoparticles (Au NPs), massively tuning their plasmonic properties. The photothermal effect of the laser increases the temperature of Au NPs above the melting point, and optical forces deform the molten Au NPs. As a result, the shape of Au NPs transforms from nanospheres into nanorods or dimers, depending on the power and time of irradiation as well as the surface energy of the substrate. This process is reversible by using high laser power to transform nanorods back to nanospheres due to capillary dewetting. Such light-induced transformations of nanostructures not only provide a facile way to tune plasmon resonances but also shed light on how the synergistic effect of photothermal energy and optical forces works on plasmonic nanoparticles. KEYWORDS: plasmons, optical forces, films, rods, dimers required resonances set by nanophotonic devices.22 The main issue is how to tailor the direction of optical forces and partially liquefy Au NPs so that anisotropic elongation can be achieved in parallel to the substrate. The combination of optical forces with photothermal effect has been demonstrated to be a neat technique to squeeze Au NPs,22 bend Au NRs,23 and reversibly transport Au microplates.24 However, the tuning range of surface plasmon resonances by these methods is relatively small (100 nm), no elongation is observed but slight rounding (Figure S4). This immobilization is probably because of much larger viscous resistance that counteracts the gradient and radial forces. Irradiation of Multiparticles within the Gaussian Beam. As the magnitude of optical forces differs greatly within the Gaussian beam spot (Figure 3c,d), different particles situated at different locations of the Gaussian beam may experience different orientations and magnitudes of deformation. To verify this, we perform laser irradiation on several Au NPs with separation less than the beam size, and we found that the particles located at different positions of the Gaussian beam are elongated differently and their orientation is also quite different (Figure 4). For particles near the center of the
(2)
where α is the polarizability of the Au NPs (see SI for details), z is wave impedance in a vacuum, and ∇I is the laser intensity gradient. This dipole approximation for the optical force calculations basically agrees with Maxwell stress tensor analysis (SI, Figure S2). The magnitude of the gradient force is proportional to the field gradient and the real part of the polarizability of the Au NPs (Re{α}), and the direction is related to the sign of Re{α}. We define negative value of Fgrad as forces pointing to the beam center and positive as pointing outward. Because of the high losses of Au NPs (due to the interband transition), the polarizability of Au NP remains positive across the resonance (Figure 3a). Therefore, the gradient force generated is always pointing to the beam center in air. As the laser applied here has a wavelength of 446 nm (3 mW), the gradient force (Fgrad) generated by the laser beam is in the range 50−120 fN with trapping potential up to ∼6 kT (Figure 3b). As the intensity and its gradient vary within the
Figure 4. SEM images of multiple particles within the same Gaussian beam (a, c) before and (b, d) after the irradiation. Laser power 3 mW, time 15 s. The white dashed lines represent the Gaussian beam spots, and the red arrows indicate the elongation directions. Figure 3. Simulations of optical forces experienced by the Au NPs. (a) Polarizability of an 80 nm Au NP on a Si substrate. Solid and dashed lines represent the real and imaginary part of the polarizability, respectively. (b) Gradient force and trapping potential along the x direction of the Gaussian beam at the focal plane. (c) Radiation pressure and (d) gradient force mapping of the Gaussian beam at the focal plane with the wavelength of 446 nm (3 mW). The signs of “⊗” and arrows indicate the direction of the forces.
Gaussian beam (Figure 4a), the high-intensity laser melts and merges the Au NPs, which are then elongated radially (Figure 4b). A careful comparison before and after the irradiation finds the particle elongated both toward and away from the beam asymmetrically (Figure 4d), suggesting both an attractive gradient force and a pushing radial force are acting on the particle. For particles near the rim of the Gaussian beam (300 nm away from the beam center, Figure 4d), negligible C
DOI: 10.1021/acsnano.8b06087 ACS Nano XXXX, XXX, XXX−XXX
Article
ACS Nano
Based on this principle, we can selectively shrink the elongated Au NRs back to spheres by applying high-power irradiation, making this deformation process reversible. For a proof-of-concept demonstration, we use a 2 mW laser for lowpower irradiation, which stretches the Au NS into an NR, and then a 4 mW laser to fully melt and shrink the Au NR into an NS again. This process can be repeated several times with partial reversibility (Figure 6) mainly because each time the particle is not positioned at the exact same position within the Gaussian beam, the particle may experience a different magnitude of optical force.
elongation is observed, as the optical forces at these locations are weak compared to the viscous resistance (Figure 3c,d). Influence of Laser Power. It can be concluded that the laser power applied is a critical factor that influences both the magnitude of optical forces and the temperature that affects the viscous resistance of the molten gold (Figure 2e). To verify this, we tune down the laser power to 2 mW; the deformation shows a much slower speed (Figure 5). Figure 5a−f clearly
Figure 6. Reversible transformation between a Au NS and a Au NR when irradiated with a low- (2 mW, 40 s) and a high-power (4 mW, 2 s) laser. Scale bars are 100 nm in the SEM images.
Influence of the Surface Tension. It is clear that the surface adhesion of the molten Au NPs to the substrate is additionally a critical factor that influences the mobility of Au NPs and their aspect ratios. As the surface energy of liquid gold and silicon are 1100 and 1240 mJ/m2,26 the interfacial tension between liquid Au and Si is high (1790 mJ/m2).31 In order to decrease the adhesion of the Au NP, a thin layer of fluoride copolymer poly(polyvinyledenedifluoride trifluoroethylene) (P(VDF-TrFE)) film is spin-coated on the Si substrate, which lowers the surface tension of the substrate and the resistivity of viscous flow. Indeed, the deformed Au NPs on these florinated polymer films present a much larger aspect ratio (Figure 7a−c) in contrast to the untreated Si substrates (Figure 5). Such a large stretching ratio is mainly attributed to the low adhesion from the substrate. Optical analysis shows that the plasmon resonance red-shifts to a wavelength of 740 nm. However, further irradiation results in breaking of the formed rods into dimers (Figure 7d), which rapidly shrink into separated spheres (Figure 7e), which is mainly attributed to the Rayleigh instability of fluid-like Au NPs on the substrate. As a result, the plasmon resonance rapidly blue-shifts back to 590 nm (Figure 7f), and the turning point of the fitted curve (white dashed line in Figure 7f) indicates breaking of the Au NR into a Au dimer. As the irradiation progresses, the Au dimer may be stretched again, red-shifting the plasmon peak from 590 nm to 640 nm (Figure 7f). The whole mechanism is depicted in Figure 7g, where the Au NP first experiences optical stretching, after which it breaks into a dimer. Such a transformation from an NR to a dimer experiences a quantum regime of plasmonically coupled resonances with gaps in the sub-0.5 nm range.32,33 However, this transient state is fast and difficult to capture.
Figure 5. Irradiation time dependence of a Au NP on a Si substrate. (a−f) SEM images of a Au NP after different irradiation times: (a) 0 s, (b) 5 s, (c) 10 s, (d) 30 s, (e) 40 s, (f) 60 s, and (g) the corresponding scattering spectra. Note: The particles are not the same particle but representative at different irradiation times. Scale bars are 100 nm. (h) Change of extracted plasmon resonance and aspect ratio of Au NPs with irradiation time.
show that the Au NP evolves from a sphere to a rod with increasing aspect ratio (from 1 to 1.8) as the irradiation time increases (Figure 5h). This trend is consistent and reproducible over many particles with no polarization (linear) dependence at this wavelength (SI Figure S6), although it shows some twisting effect on the circularly polarized beam (Figure S5). Clearly, it takes much longer time (∼10 times; see Figure 5g and SI Figure S6) to reach a similar plasmon peak position as compared to 3 mW (5 s; see Figure 1b). This is mainly because of the lowered temperature of Au NPs (Figure 2a), which significantly increases the viscous resistance, as well as the decrease of optical forces. Note that the longitudinal modes (Figure 5g) are more red-shifted compared to standard Au NRs with the same aspect ratio, which is due to a flattening effect by radiation pressure (SI Figure S7). However, if the laser power is further increased above 3 mW, laser ablation can be observed, which transforms Au NPs into tiny particles scattered around the main particle (SI Figure S8).30 In this case, the thermal capillary dewetting (∼nN; for detailed calculation see the SI) dominates, while optical forces are not strong enough to sustain an anisotropic shape of the Au NPs.
CONCLUSIONS In summary, irradiation of NPs with a tightly focused laser beam of 446 nm generates both thermal and optical forces on the Au NPs. The thermal effect turns Au NPs into a semifluidD
DOI: 10.1021/acsnano.8b06087 ACS Nano XXXX, XXX, XXX−XXX
Article
ACS Nano
METHODS Sample Preparation and Characterizations. Gold nanoparticles (80 nm, BBI Solutions) are deposited on plasma-cleaned Si or a glass substrate via drop-casting, followed by rinsing with DI water and blow-drying using nitrogen. For the preparation of the P(VDF-TrFE)/Si substrate, an ethyl methyl ketone solution of P(VDF-TrFE, 70/30, Piezotech) (3 mg/mL) was prepared and spincoated on a Si substrate at the speed of 5000 rpm for 1 min. The film thickness is determined to be
