Opto-thermoelectric nanotweezers

Abstract

Optical manipulation of plasmonic nanoparticles provides opportunities for fundamental and technical innovation in nanophotonics. Optical heating arising from the photon-to-phonon conversion is considered as an intrinsic loss in metal nanoparticles, which limits their applications. We show here that this drawback can be turned into an advantage, by developing an extremely low-power optical tweezing technique, termed opto-thermoelectric nanotweezers (OTENT). Through optically heating a thermoplasmonic substrate, alight-directed thermoelectric field can be generated due to spatial separation of dissolved ions within the heating laser spot, which allows us to manipulate metal nanoparticles of a wide range of materials, sizes and shapes with single-particle resolution. In combination with dark-field optical imaging, nanoparticles can be selectively trapped and their spectroscopic response can be resolved in-situ. With its simple optics, versatile low-power operation, applicability to diverse nanoparticles, and tuneable working wavelength, OTENT will become a powerful tool in colloid science and nanotechnology.

Figure 1. Working principle of OTENT

a, Surface charge modification of a metal nanoparticle by CTAC adsorption. b, Formation of CTAC micelles. c, Schematic view of a Cl^- ion. d, Dispersion of a single metal particle and multiple ions in the solution without optical heating. e, Thermophoretic migration of the ions under optical heating. f, Steady ionic distribution under optical heating generates a thermoelectric field E_T for trapping the metal nanoparticle. The repulsive electric field E_r arises from the positive charge of the thermoplasmoic substrate and balances E_T. g, Simulated in-plane temperature gradient and direction of the corresponding trapping force. h, Simulated out-of-plane temperature gradient and direction of the corresponding trapping force. The incident laser beam in (e-h) has a diameter of 2 μm and an optical power of 0.216 mW. The green arrows in (g, h) show the direction of the trapping force.

Figure 2. Single-nanoparticle trapping and manipulation

Schematic illustration and successive optical images showing a, trapping b, dynamic manipulation, and c, release of a single 100 nm AgNS. The grey disks and the red disks mean that the laser is turned on or turned off, respectively. The grey lines show the manipulation trajectory of the trapped AgNS. Measured trapping stiffness of single d, 100 nm AgNSs and e, 100 nm AuNSs as a function of CTAC concentration. κ_x and κ_y are the trapping stiffness along x and y axis, respectively. The error bars show the deviation in multiple measurements with different particles. f, Trapping potential and g, trapping force of a single 100 nm AgNS at CTAC concentration of 20 mM. The laser has a wavelength of 671 nm and an optical power of 0.4 mW in d-g. Scale bars: 10 μm (a, c) and 20μm (b).

Figure 3. In-situ optical spectroscopy of different metal nanoparticles trapped via OTENT

a, Optical setup of OTENT with in-situ dark-field optical imaging and spectroscopy. Dark-field optical images, experimental and simulated scattering spectra, and electric field profiles of single AgNSs b, with diameters of 70, 90 and 100 nm; single AuNSs c, with diameters of 80, 90 and 100 nm; single AuNTs d, with side lengths of 60 and 140 nm; single AuNRs e, with lengths of 50-60 and diameters of 19-25 nm and corresponding absorption peaks at around 650 nm (top panel); with lengths of 63-73 nm and diameters of 19-25 nm and corresponding absorption peaks at around 700 nm (bottom panel). The solid and dashed curves represent experimental and simulated scattering spectra, respectively. The grey rectangles represent the peak distributions recorded in multiple experiments. Scale bars: 2 μm.

Figure 4. Parallel and multiple trapping via OTENT

Parallel trapping of a, six 100 nm AgNSs into a circular pattern, and b, six 140 nm AuNTs into a triangular pattern. c, Interaction forces between two trapped nanoparticles. d, Calculated interaction potential between two AuNSs at different CTAC concentrations. Scattering spectra of a single AuNS (top) and two AuNSs (bottom) in e, 1 mM and f, 20 mM CTAC solution. g, Simulated scattering spectra of a single AuNS (top) and two AuNSs (bottom) in 20 mM CTAC solution. The red and green dashed curve represent the longitudinal and transverse plasmon mode, respectively (f and g). Trapping dynamics of h, a single AuNS and i, multiple AuNSs in 1 mM CTAC solution. The grey rectangles represent the peak distributions recorded in multiple experiments. The particle diameter is 100 nm in d-i. Scale bars: 5μm (a, b) and 2 μm (f).