Bending Gold Nanorods with Light

Transcription

1 Supporting Information - Bending Gold Nanorods with Light Anastasia Babynina 1, Michael Fedoruk 1, Paul Kühler 1, Alexander Meledin 3, Markus Döblinger 4, Theobald Lohmüller 1,2 * 1 Photonics and Optoelectronics Group, Department of Physics and Center for NanoScience (CeNS), LMU München, Amalienstraße 54, Munich, 80799, Germany 2 Nanosystems Initiative Munich (NIM), Schellingstraße 4, Munich, Germany 3 EMAT, University of Antwerp, Groenenborgerlaan 171, 2020, Antwerp, Belgium 4 Department of Chemistry, LMU München, Butenandtstr (E), Munich, Germany Corresponding Author: T.L. (t.lohmueller@lmu.de) 1

2 Materials and Methods. Materials. Gold nanorods with an extinction maximum at 1064 nm were purchased from Nanopartz. Glass cover slips (24x24 mm) with a thickness of 0.17 mm were purchased from Paul Marienfeld GmbH & Co. KG, Germany. Figure S1. Extinction spectrum of the gold nanorods in water. Preparation of glass substrates. Glass cover slips were intensively cleaned in isopropanol (10 min), acetone (10 min), and ultrapure water (10 min) in an ultrasonic bath and blow-dried with nitrogen. Temperature Simulations. Steady state temperature simulations of gold nanorods under laser illumination in water were performed in COMSOL Multiphysics Software 4.0 ( heat transfer module). A gold nanorod was modelled as a cylinder with two spheres at each end. The particle was assumed to be 124 nm long and 21 nm thick. The surrounding medium was considered to be a water sphere with a radius of 5µm. Thermally insulated boundary conditions (293.15K) were imposed on the surface of the water sphere. Absorption cross sections of the gold rods, the nanorod volume, the laser power, and the FWHM of the focused laser beam (~ 651 nm) were used to calculate the heat transfer. 2

3 Figure S2. Simulations of the nanorod temperature in water for a laser power density of 0.45 MW/cm 2. The nanorod is heated to 1150 C, which is above the bulk melting temperature of gold 1. Dark-field spectroscopy and particle manipulation. A clean glass substrate was placed in an upright dark-field microscope (Zeiss Axio Scope A1), with condenser illumination (NA = ) and a water immersion objective (magnification: 100, NA = 1.0, Zeiss, Germany). A droplet of ultrapure water (100µl) and an aliquot of gold nanorods in solution (10µl) were mixed on the top of the glass substrate. A continuous wave (cw) 1064 nm laser (Cobolt Rumba 05-10, Cobolt, Sweden) was used to print lines of nanorods with different laser powers. The laser was focused 2.8 µm above the surface of a glass substrate. The diameter of the laser beam was measured to be 651 nm at FWHM. This FWHM was also used for all simulations and to calculate the laser power density for all laser powers. Each laser power was measured with a powermeter directly after the water-immersion objective. Rayleigh scattering spectra of bent nanorod particles were collected with a spectrograph (Acton 2300i and Princeton Instruments Spec 1.0). 3

4 FDTD simulations of absorption and scattering cross sections. Simulations of absorption and scattering cross sections of bent nanorods have been performed using Lumerical FDTD solutions (Lumerical Solutions, Canada). The bent nanorods with different bending angles and a total length of 124 nm and a thickness of 21 nm have been placed in water (refractive index: 1,33). The dielectric function of gold was taken from Johnson and Christy 2. The bent gold rods were represented as two cylinders connected by a sphere and two spheres at the end. Convergence tests were performed in order to determine the smallest mesh size. Linear polarized light with a polarization 0 (perpendicular to longer axes) and 90 (parallel to longer axes) degrees was injected. Figure S3. FDTD simulations of the scattering cross-sections for different bending angles. For each angle, the incident light was polarized parallel to long axis of the structure. 4

5 Figure S4. Polarization dependent scattering spectra. a, Polarization-dependent Rayleigh scattering spectra of a single V-shaped gold particle in water (bending angle: 87 ). The two peaks with a maximum at 700 nm and 850 nm correspond to the symmetric (black curve) and the antisymmetric (blue curve) plasmon resonances of the bent rod. The scattering spectra for 45 polarization is indicated by the red curve. The antisymmetric plasmon resonance of the bent nanorod is strongly blue shifted compared to the calculated value from FDTD. The difference originates from the shape of the particle. In this particular example, the bent rod was still heated by the laser after it was already printed to the glass substrate. Therefore, the rods start to melt at the tips, while there is no further change of the bending angle. Due to the molten tips, the effective length of plasmon propagation is reduced, which in turn leads to a further blue shift of the antisymmetric plasmon mode. b, Intensity dependence of both plasmon resonances as a function of the polarization angle. TEM measurements. TEM and HAADF-STEM measurements were carried out using a probe corrected FEI Titan Themis at 300 kv (Department of Chemistry, LMU Munich). HAADF-STEM was performed with a semi-convergence angle of 16.6mrad and a semi-collection angle of 45mrad. Gold nanorods were printed on silicon nitride membranes (Si3N4) grid with a thickness of 50 nm (Plano GmbH, Germany). 5

6 Figure S5. HAADF-STEM images of a straight gold nanorod and a spherical particle after a complete rod-to-sphere melting. a-b, STEM images of straight gold nanorod. No defects can be detected. c-d, STEM image of a particle molten to a sphere. The molten particle is crystalline with a number of defects visible. Images b and d were filtered with a high-pass filter to reduce the thickness contrast. SEM measurements. Electron microscopy was performed with a Zeiss Ultra 55 SEM. The images were collected with a SE2 detector and an electron accelerating voltage of 5-10 kv. A 3 nm goldpalladium layer was sputtered (20s sputtering at 15mA) on the sample s surface prior to the measurement to make the sample conductive. Figure S6. Examples for different particle shapes after melting. a, Gold nanorods printed with a laser power of 0.45MW/cm 2. b, Example of a bent nanorod. c, Examples of molten nanorods that were heated while attached to the surface of the glass substrate. Spheres, bone shaped particles, and ϕ-shaped morphologies were observed. 6

7 Hydrodynamics Simulations. Simulations of the hydrodynamic pressure acting on the gold nanorods under an external stationary laminar flow have been performed by solving the Navier- Stokes equations using COMSOL Multiphysics software 5.2 ( Fluid Flow module): ρ(u )u = [ pi + μ( u + ( u) T ] + F and ρ (u) = 0, where u denotes the velocity [m/s], ρ corresponds to the density of water (1000 kg/cm 3 ), μ denotes the dynamic viscosity of water (0.001 Pa s), p corresponds to the pressure [Pa], and I denotes the unity matrix. For the simulation, a solid nanorod with a length of 124 nm and a width of 21 nm was placed in the center of a water box (4µm x 4µm x 4µm). The rod experiences an external stationary laminar flow perpendicular to its long axis. A laminar flow speed of 0.02 m/s was estimated from the Stokes Drag equation for a spherical particle: F d = 6πμRv F d is assumed to be equal to a scattering force of 20 pn. μ denotes the dynamic viscosity of water (0.001 Pa s), R denotes the particle radius (60nm). Here, v denotes the velocity of particle movement. Optical force calculations. Scattering forces on gold nanorods with different bending angles and laser powers were calculated in Wolfram Mathematica 9.0 according to Agayan et. al. 3 For the calculation of straight nanorods, the FWHM of the Gaussian laser beam was assumed be 651 nm. 7

8 References: 1. Buffat, P.; Borel, J.-P. Phys. Rev. A 1976, 13, Johnson, P.B.; Christy, R.W. Phys. Rev. B, 1972, 6, Agayan, R.R.; Gittes, F.; Kopelman, R.; Schmidt, C.F. Appl Optics, 2002, 41,