HAADF‐STEM observation of twinned structure formed in gold nanorods by near‐infrared pulsed laser irradiation

Abstract

Gold nanorods have been drawing much interests widely in various science and engineering fields because of their characteristic optical properties [1]. Due to their anisotropic rod shape, gold nanorods generally absorb visible and near‐infrared light by localized surface plasmon resonances. Absorbed light more or less heats up the nanorods through electron‐phonon coupling. This heating due to photo‐thermal conversion sometimes causes deformation of gold nanorods under light illumination. Actually, it has been reported that gold nanorods suspended in an aqueous solution changes their shape into sphere, singular Φ‐shape or elongated rod when irradiated with pulsed laser light [2]. Recently our group set up a pulsed laser light illumination system attached to a high voltage electron microscope (HVEM) and performed in‐situ observation of deformation process under pulsed laser light illumination [3]. In the present study, we carried out HAADF‐STEM characterization of atomic structural change in a gold nanorod due to pulsed laser illumination. Gold nanorods used in the present study were produced in CTAB micelle solution by a photochemical method and synthesized to be about 50 nm in length and 10 nm in diameter (products of Dai Nihon Toryo Co. Ltd in Japan). The Au nanorods show two optical absorption peaks around 520 and 980 nm in wavelength. Quantifoil TM carbon films were used for sample supporting mesh. Laser illumination to the samples was performed in a JEM‐1300NEF HVEM equipped with an optical guide path of laser pulses into its specimen chamber [3]. The wavelength of laser pulses was 1064 nm, and the pulse duration was 6 to 8 ns. The averaged intensity was 7.3×10 3 J/m 2 pulse. HAADF‐STEM atomic‐resolution observation was carried out with a JEM‐ARM200CF operated at an acceleration voltage of 120 kV. In order to suppress the influence due to sample drift during STEM operation, the observation was performed on a drift compensating operation, where rapidly scanned plural images of an interesting area were overlaid with autocorrelation. Figure A shows an atomic resolution image of an original gold nanorod before irradiation with pulsed laser light. Here the incident electrons were illuminated along the [110] zone axis. We are convinced that the drift compensation operation is quite powerful to obtain an atomic structure HAADF image of the whole of a nanorod without any serious distortion. It is clearly shown that the virgin nanorod is a single crystal oriented to [001] along its longer axis. One may confirm faceting tendencies of surfaces; {100} for top and bottom ends and {111} in the tip sides. The main side surfaces are quite smooth and flat {110}. Figure B gives a HAADF image of the same nanorod but after experiencing one shot illumination of a laser pulse. The outer shape has been significantly deformed to be nearly spherical. The particle interior also has been complexly changed in atom configuration, and has been divided into tiny blocks in different crystal orientations. The surface is surrounded with mixture of {111} and {100} facets, and {110} surface has disappeared. A rectangle region in Fig. B is further magnified in Fig. C. One may clearly recognize in the close‐up view that the particle interior consists of blocks with twinned orientation relationships. The five orientations rotating on a common [110] axis are classified with different colors in Fig. C. The blocks are separated by single layer twin boundaries with {111} mirror symmetry and double layered twin or stacking faults. One can find multiple twin junctions in squared areas in Fig. C. As the rotating angle between two twined orientations is 70.53 degree, five‐fold decagonal junction of twins results in a solid‐angle deficiency of 7.35 degree [4]. The angle deficiency due to five‐fold junction in the left squared region is mostly accommodated with insertion of double layered stacking faults in the right‐hand side blocks. At junctions of four blocks recognized in the right square, on the other hand, blue and yellow ones are not in twined relationship any longer, and are separated by a wide angle grain boundary. One may notice that atom columns close to the junction in the yellow block are significantly displaced from their regular positions.