Sequential transmission electron microscopy observation of the shape change of gold nanorods under pulsed laser light irradiation

Microscopy, 2019, 174–180 doi: 10.1093/jmicro/dfy136 Advance Access Publication Date: 12 December 2018 Article Sequential transmission electron microscopy observation of the shape change of gold nanorods under pulsed laser light irradiation Kohei Aso1,*, Koji Shigematsu1, Tomokazu Yamamoto1, and Syo Matsumura1,2 1 Department of Applied Quantum Physics and Nuclear Engineering, Kyushu University, Motooka 744, Nishi-ku, Fukuoka, Japan and 2The Ultramicroscopy Research Center, Kyushu University, Motooka 744, Nishi-ku, Fukuoka, Japan * To whom correspondence should be addressed. E-mail: Received 7 May 2018; Editorial Decision 3 November 2018; Accepted 28 November 2018 Abstract In situ sequential high-resolution observations were performed on gold nanorods under near-infra-red pulsed laser irradiation using a high-voltage electron microscope attached to a pulsed laser illumination system. The original nanorods were single crystals; the longer axes were oriented along [001]. Under laser light irradiation with λ = 1064 nm with an average intensity per pulse of 980 or 490 J/m2, the shape of the nanorods changed from rod to barrel surrounded by the {111} and {001} facets, while the original single-crystalline structure was maintained. The side surfaces with <110> direction were reconstructed into zig–zag fine structures consisting of narrow {111} facets. The temporal evolution of the volume and surface area during irradiation was evaluated based on the images, assuming that the particles have a rotational symmetry along their longer axes. The surface area was stepwise decreased during the shape change using pulse shots of 980 J/m2 while the volume was maintained. On the other hand, several repeated shots were required to induce the shape change when the averaged intensity was reduced to 490 J/m2 per pulse. In addition to the surface area, the volume was reduced under the latter condition during the shape change due to the evaporation of atoms. The quantitative analysis of the temporal changes indicates the heterogeneity of the atomic excitation or heating of gold nanorods induced by pulsed laser illumination. Key words: nanorods, laser light irradiation, high-voltage electron microscopy, in situ observation, high-resolution electron microscopy Introduction Gold nanorods have attracted great interest in scientific and engineering fields due to their characteristic optical properties [1–3]. The anisotropic rod shape of gold nanorods results in two light absorption peaks associated with the longitudinal and transverse modes of localised surface © The Author(s) 2018. Published by Oxford University Press on behalf of The Japanese Society of Microscopy. All rights reserved. For permissions, please e-mail: 174 Microscopy, 2019, Vol. 68, No. 2 Experimental The gold nanorods used in this study were produced in a hexadecyltrimethylammonium bromide (CTAB) micellar solution with a photochemical method (products of the Dai Nihon Toryo Co. Ltd., Japan) [15]. The nanorods were synthesised to have an ~50 nm long axis and 10 nm diameter. The optical absorption spectrum of the nanorod solution shows peak maxima at wavelengths of 520 and 980 nm. The CTAB micelles were removed by centrifuging the aqueous solution for 10 min at 2 × 104 g (g is the magnitude of gravitational acceleration). One drop of the solution was placed onto a Quantifoil™ carbon film of sample-supporting mesh, which had been rendered hydrophilic by exposure to Ar ions in a plasma cleaner. In situ high-resolution observations were made in an HVEM equipped with a laser irradiation system [12]. Laser light pulses generated by an yttrium aluminium garnet (YAG) laser (Quantel YAG 981 C) were introduced to the HVEM (JEM-1300NEF) via optical equipment consisting of mirrors and lenses. The wavelength of the laser pulses was 1064 nm and the pulse duration was 6–8 ns, repeating at 10 Hz. Based on measurements using a PIN photodiode in the specimen chamber, the averaged intensity per pulse in this experiment was 980 or 490 J/m2 over a specimen illumination area with 500 μm diameter. It should be noted that the laser intensities mentioned above are averaged over the illuminated area with a Gaussianlike intensity variation and do not exactly correspond to the values of local observation areas of orders of several tens of nanometres in the HVEM. Hereafter, we call conditions with higher and lower intensities H and L, respectively. The HVEM with an acceleration voltage of 1250 kV was used to acquire high-resolution TEM (HRTEM) images. Results and discussion Figure 1 shows the results of pulse-by-pulse sequential observations of a nanorod under condition H. The nanorod in Fig. 1a has a length of 40 nm, width of 8.5 nm and a aspect ratio of 4.7 before laser irradiation. The rod was confirmed to be a single crystal with its long axis along [001] because there were no defect contrasts such as twins or stacking faults. Both of the side edges are quite smooth, Fig. 1. HRTEM images of a nanorod under pulsed laser irradiation at an average intensity per pulse of 980 J/m2. The nanorod before irradiation (a), after irradiation with one pulse shot (b) and after irradiation with two pulse shots (c). plasmon resonance in their optical absorbance spectra [4]. The light absorption heats up the nanorods through electron–phonon coupling, transforming their shapes [5–8]. Transmission electron microscopy (TEM) revealed that the gold nanorods change from rod shape to various shapes, such as spheres, singular Φ-shapes and elongated rods, when irradiated with pulsed laser light [9–11]. To understand such an interaction between the materials and light on the nanoscale, we connected a pulsed laser light illumination system to a high-voltage electron microscope (HVEM) [12]. Using this laser-HVEM, we observed the in situ morphological and structural change of gold nanorods in response to near-infra-red pulsed laser irradiation [12]. First, the laser pulse effectively rendered the nanorods more spherical; however, this effect diminished after additional shots because the energy transfer from a laser pulse to the nanorods decreases as the longitudinal SP band is blue-shifted by the reduced aspect ratio of the rod [4,13]. Recently, we also obtained atomicresolution high-angle annular dark-field scanning TEM images of individual gold nanorods before and after pulsed laser irradiation using a TEM instrument with a spherical aberration corrector. These images revealed that a nanorod changes its structure after pulsed laser irradiation to a multiple twinned particle with large displacements of gold atoms near multiple twin junctions [14]. In the present study, we made in situ sequential highresolution observations of the gradual shape change of gold nanorods under laser irradiation with moderate intensity, where the rods maintain their original singlecrystalline structure. The results are expected to promote the understanding of the interaction between the n (...truncated)