Using Femtosecond Laser Irradiation to Grow the Belly of Gold

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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

Using Femtosecond Laser Irradiation to Grow the Belly of Gold Nanorods Pablo Díaz-Núñez, Guillermo González-Rubio, Alejandro Prada, Jesus Gonzalez Izquierdo, Antonio Rivera, Luis Banares, Andrés Guerrero-Martínez, and Ovidio Peña-Rodríguez J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b06375 • Publication Date (Web): 03 Aug 2018 Downloaded from http://pubs.acs.org on August 4, 2018

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The Journal of Physical Chemistry

Using Femtosecond Laser Irradiation to Grow the Belly of Gold Nanorods Pablo Díaz-Núñez,1 Guillermo González-Rubio,2,3 Alejandro Prada,1,† Jesús G. Izquierdo,4 Antonio Rivera,1 Luis Bañares,2,4 Andrés Guerrero-Martínez2 and Ovidio Peña-Rodríguez1,* 1

Instituto de Fusión Nuclear, Universidad Politécnica de Madrid, José Gutiérrez Abascal 2, E28006 Madrid, Spain

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Departamento de Química Física, Universidad Complutense de Madrid, Avenida Complutense s/n, E-28040 Madrid, Spain

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Bionanoplasmonics Laboratory, CIC biomaGUNE, Paseo de Miramón 182, E-20014 DonostiaSan Sebastián, Spain

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Centro de Láseres Ultrarrápidos, Universidad Complutense de Madrid, Avenida Complutense s/n, E-28040 Madrid, Spain

Abstract

Irradiation of gold nanorods can lead to the formation of several new nanostructures. One of the most interesting among them are φ-shaped rods, a stable intermediate shape in the rod-to-sphere conversion process but not much attention has been paid to their characterization. In this work, we irradiated colloidal rods, looking for the conditions to maximize the generation of φ-shaped

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structures. The shape and dimensions of the irradiated samples were then statistically analyzed. Moreover, we have calculated the energy range within which such transformation into φ-shaped rods takes place, which has been determined to be a very narrow energy window (between 0.38 and 0.41 eV/atom). Finally, we have performed simulations to characterize the optical response of such structures.

1. Introduction Noble metal nanoparticles are of great interest due to their unique optical properties, which arise from the interaction of light with its free conducting electrons. This interaction, known as localized surface plasmon resonance (LSPR), is responsible for the absorption and scattering of light at that resonance frequency (or frequencies), a highly appealing feature in many fields such as sensing, photovoltaics and catalysis, among others.1–3 The spectral properties of the LSPR are mainly controlled by the particle size, shape, and composition, and by the surrounding dielectric medium.3,4 As such, continuous efforts are being made toward the generation of new structures with different properties tailored for specific applications. Irradiation with femtosecond (fs) or nanosecond (ns) pulses produces several useful photothermal effects.5,6 In particular, laser irradiation is a technique extensively used to induce morphological changes via reshaping, fragmentation, and/or assembly of colloidal metal nanoparticles. Furthermore, it has been used as a tool to study the electron dynamics of the overall process. The early investigations into the shape transition of gold nanorods carried out by Link et al.7–10 by comparing fs and ns pulses and by Chang et al.11 for ns pulses showed melting, reshaping into spheres, and fragmentation at high enough laser fluences. In addition, they observed a new structure, φ-shaped rods, i.e. rods with a spherically swollen center as a stable intermediate of


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this transition. Moreover, the complex and fast dynamic processes behind the energy transfer from the laser pulse to the gold nanorods (AuNRs), including electron–electron coupling (500 fs), electron–phonon coupling (10 ps), and phonon–phonon coupling (100 ps),12–15 render such a reshaping process extremely difficult to control, producing in most cases a mixture of shapes, among which appear the φ-shaped rods.16–18

Figure 1: Schematic view of the transitions observed for a AuNR, as a function of the absorbed energy. The rod shape is conserved for lower energies whereas for higher energies the AuNR transforms into a sphere. The energy range between E0 and E1corresponds to the energy window where the formation of φ-shaped rods occurs. In a previous work, we demonstrated how control of the reshaping process can be attained by carefully adjusting the energy absorbed by the AuNR and the heat dissipation rate into the medium, leading to virtually monodispersed colloidal solutions (in terms of the aspect ratio) with an exceptionally narrow LSPR.19 Here, we will focus on the evolution of AuNRs when the absorbed energy surpasses the conditions for optimal reshaping so that the final colloid contains


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a mixture of different shapes. Particularly, in this work we have focused on the formation of φshaped rods, a structure of interest because of its stability as an intermediate shape between a rod and a sphere. A statistical study was carried out to characterize the generated φ-shaped structures. Also, their optical response was investigated in the far and near fields. The theoretical results are compared to experimental optical absorption spectroscopy data. In addition, we have calculated the energy range for the efficient formation of φ-shaped rods, i.e. the energy required for a rod to evolve into a stable φ-like shape while avoiding its full transition into a sphere (Figure 1). 2. Procedure 2.1 Synthesis of gold nanorods AuNRs were prepared using the seeded growth method with some modifications, as previously described by Scarabelli et al.20 The seeds were prepared by the standard CTAB/NaBH4 procedure: 25 µL of a 0.05 M HAuCl4 solution was added to 4.7 mL of a 0.1 M CTAB solution; 300 µL of a freshly prepared 0.01 M NaBH4 solution was then injected under vigorous stirring. The excess borohydride was consumed by ageing the seed solution for 30 min at room temperature prior to use. In a typical synthesis of a 50 mL AuNR solution, 45 mg of 5-bromosalicylic acid (BrSal) was added to 50 mL of 0.05 M CTAB and the mixture was mildly stirred for 15 min until complete dissolution. Then, 480 µL of 0.01 M AgNO3, 200 µL of 0.1 M ascorbic acid, and 500 µL of a 0.05 M HAuCl4 solution were added to the mixture. After 2 h at 25 °C (or once Au(III) was completely reduced to Au(I) by BrSal, as monitored by the decrease in the absorbance of the Au(III)-CTAB complex at 390 nm), 50 µL of a 0.1 M ascorbic acid solution was added under vigorous stirring, followed by 80 µL of the seed solution. The mixture was allowed to stand at


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room temperature for at least 2 h. Under such experimental conditions, the resulting AuNRs displayed LSPR maxima ranging from 820 to 890 nm. Fine tailoring of the LSPR to match the 800 nm femtosecond Ti:Sapphire laser was achieved via overgrowth of the synthesized AuNRs. The optimum amount of ascorbic acid needed to achieve controlled tuning of the LSPR was found experimentally by overgrowing small aliquots of the prepared AuNRs with increasing volumes of the ascorbic acid solution: from 0.3 to 0.6 µL per mL for the synthesis of rods with LSPR at 800 nm. The mixture was left undisturbed at room temperature for at least 4 h. Typically, 50 mL of the mixture was centrifuged (8000 rpm, 30 min) and the precipitate was redispersed in the same volume of a 0.002 M CTAB solution. Centrifugation was repeated twice and the particles were finally redispersed in 5 mL of a 0.001 M CTAB solution. 2.2 Irradiation experiments AuNRs synthesized by the seeded growth method and redispersed at the critical micelle concentration (cmc) of CTAB (1mM) with a longitudinal LSPR band centered at 800 nm were irradiated with 50 fs, 800 nm laser pulses generated with an amplified Ti:Sapphire (Spectra Physics) laser system at a repetition rate of 1 kHz and with a beam diameter of about 1 cm. Samples were irradiated in quartz cuvettes with an optical path of 1 cm and a fixed volume of 2.5 mL under constant stirring at 300 rpm with a magnetic bar. Optical absorption spectra were measured in situ during irradiation. The samples were illuminated with a halogen lamp directed by a silica optical fiber of 200µm diameter and converted to a nearly-parallel beam using a silica lens. The transmitted beam was collected and focused with a similar silica lens into a silica optical fiber of 1 mm diameter. The light was guided to a compact spectrometer, USB2000+ (Ocean Optics Inc.), configured with a multichannel array detector to simultaneously measure the whole spectrum in the range 400–1000 nm, with a spectral resolution better than 1 nm.


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Extinction spectra were recorded using an integration time of 1 s. The irradiation time was limited to 10 min for all the experiments. The irradiation fluence was increased from 5.1 J/m2 to 33.1 J/m2using a variable attenuator wheel. Irradiated samples were characterized by transmission electron microscopy (TEM). Images were obtained with a JEOL JEM-2100 transmission electron microscope operating at an acceleration voltage of 200 kV. All samples were centrifuged prior to blotting on carbon-coated 400 square mesh copper grids. The ImageJ software was employed to analyze the TEM images. The length, external diameter, and internal diameter were used to calculate the volume fraction of the φ-shaped rods, defined as the quotient between the volume of the swollen region and the total volume of the rod, which was then used to calculate the statistical distribution of the AuNRs. We measured about 700–1000 nanoparticles per sample. 2.3 Molecular dynamics simulations To understand the modifications induced by fs-laser pulses on the AuNRs, we carried out classical molecular dynamics (MD) simulations with the LAMMPS code.21 The chosen interatomic potential was EAM,22 which is able to reproduce the behavior of gold at melting temperatures and their recrystallization. We simulated fcc AuNRs (lattice parameter of 0.4078 nm) with aspect ratios from 3.0 to 4.0 containing ∼6.5 × 105 atoms. We elevated the temperature from 0 K (initial box) to 800 K during 10 ps, kept at 800 K for 10 ps, reduced it down to 300 K by velocity rescaling in 10 ps and kept at 300 K for 10 ps to relax the material. MD simulations cannot provide details of the energy absorption by the LSPR and the energy transfer to the lattice (mainly through electron–phonon processes). However, since lattice heating (controlled by electron–phonon interactions) occurs very fast, the initial conditions can be set by imposing an average kinetic energy of the atoms up to a value of 0.5 eV/atom in 7 ps.19 We also


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ignored the details at the AuNR surface, i.e. the CTAB coverage and surrounding water. In addition, by including water, its possible phase transition (bubbling) and complete heat transfer protocol to a cold sink would require a large computational cost, thus rendering unfeasible a simulation effort as the one described in this paper. The approach to obtain practical results involved imposing a temperature decay based on experimental data.23 In this case, the time decay (τ) for a AuNR with CTAB coverage near the cmc was 350 ps.23 Energy dissipation was included through a Langevin frictional term,24 which, in addition to considering fast cooling, leads to shock wave dumping that otherwise would result in undesirable artifacts. This computational model has been applied and validated with experimental results in our previous work.19 2.4 FDTD calculations The optical response of a single φ-shaped rod was studied by finite-differences in the timedomain (FDTD) using the computer code MEEP.25 A detailed description of the FDTD method has been provided elsewhere.26,27 For the simulation, the φ-shaped rod geometry was constructed as a hemispherically capped cylinder with an initial length (L) of 57.5 nm and internal diameter (Din) of 16.5 nm. The swollen region of the structure was incorporated as a sphere of variable external diameter (Dout) concentric with the cylinder. The volume of the sphere was modified (but the total volume of the nanoparticle was kept constant) to evaluate how the swollen region affects the optical response in the far and near fields. The value of the external diameter was increased from 16.5 to 26.5 nm, which corresponds to a length decrease from 57.5 to about 36 nm (the internal diameter remained constant). The computational cell had a size of 4L×4Dout × 4Dout nm3 with a spatial resolution of 0.5 nm and was surrounded by perfectly matched layers with a thickness of 2Dout to absorb the scattered waves. The structure was placed at the center of the simulation box. A broadband Gaussian source (400–1000 nm) was placed at the top of the


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cell and the EM wave propagated along the Z direction. Each structure was calculated twice for longitudinal (Ex) and transversal (Ey) polarizations. The fields were allowed to evolve and the simulation was terminated once |E|2/|Emax|2decayed down to 10-8 at the bottom of the cell. The optical response in the far field was analyzed by means of the optical extinction efficiency factor (Qext) and the differential scattering cross section (DSCS). For the near field, the response was evaluated by means of the near-field enhancement, |E|/|E0|. 3. Results and Discussion 3.1 Experimental The experimental results are summarized in Figures 2 and 3. Figure 2 shows the TEM images for each irradiated sample, as well as the corresponding absorption spectra, while Figure 3 shows the statistical analysis of the previous images. There is clear evidence showing that the shape of the AuNRs is not preserved even at 5.1 J/m2, as we have reported previously.19 Also, with increasing laser fluence, the fraction of spheres, φ-shaped rods, and other irregular shapes increases, being the shape polydispersity the main reason for the decrease of the absorption intensity. For a complete transformation of the original sample, laser fluence needs to be at least larger than 7.6 J/m2, as it can be deduced from the optical spectrum at that fluence. For 5.1 J/m2 and 10 minutes of irradiation there are still some unmodified AuNRs of the original fraction that explain the presence of the small band at ~870 whereas at 10.2 J/m2 the complete original band, i.e. the original sample, centered at 800 nm has disappeared. This is explained due to the polidispersity of the original sample and because of the random orientation of the AuNRs in colloidal solution with respect to the polarization of the incident laser beam.19


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Figure 2: (a) Optical absorption spectra before and after irradiation at different fluences. The red vertical bar represents the spectral width of the laser pulses. Transmission electron microscopy images of AuNRs (b) before and after irradiation with a laser fluence of (c) 5.1 J/m2, (d) 10.2 J/m2 and (e) 33.1 J/m2. A statistical analysis of TEM images like those depicted in Figure 2 is shown in Figure 3 as a function of the laser fluence. The fraction of φ-shaped rods increases from about 15% at 5.1 J/m2 to 23% at 12.7 J/m2. For 33.1 J/m2 the φ-shaped rods decrease to about 18% because most of the final products are spheres and spheroids. A more specific analysis of the geometry of the φshaped rods was performed, as shown in the insets of Figure 3. For this purpose, the φ-shaped


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rods were classified by means of the volume fraction, as previously defined. The volume fraction reached as high as 50% and was centered at about 25% irrespective of the laser fluence. The typical distribution obtained at 5.1 J/m2 is no longer observed at higher fluences.

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Fluence ( J/m2 ) Figure 3: Analysis of the TEM images. The main figure represents the fraction of φ-shaped rods generated by laser irradiation. The insets present the characteristics of the φ-shaped rods as a function of their volume fraction. 3.2 Conditions for the reshaping of a AuNR into a φ-shaped structure As previously demonstrated, the gentle reshaping of AuNRs requires low-fluence and multi-shot conditions in order to produce nearly perfect AuNR colloids.19 In this work, the irradiation conditions are far above that energy threshold, and so the transformation from a AuNR into a φshaped structure (and other irregular structures) is accomplished in a single pulse as formerly determined in other works.9–11 In order to find the energy interval within which the generation of φ-shaped structures is most efficient, we simulated the evolution of AuNRs with initial aspect


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ratios ranging from 3.0 to 4.0 excited with energies from 0.35 to 0.5 eV/atom by means of MD simulations.

Figure 4: Molecular dynamics simulations of AuNRs in CTAB (τ = 350 ps). (a) Increase of the nanoparticle diameter as a function of the absorbed energy. The vertical red dashed lines indicate the energy conditions for the formation of φ-shaped nanoparticles with a single pulse. The red continuous line is just a guide to the eye. (b) Shape evolution for a AuNR with an initial aspect ratio of 3.0 as a function of the absorbed energy. The results from the MD simulations are summarized in Figure 4. Due to the shape modifications occurring at higher energies, the evolution of the AuNR was characterized by means of the increment in the external radius instead of the variation in the aspect ratio. For example, Figure 4b presents the evolution of a AuNR with an initial aspect ratio of 3.0 excited


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with energies ranging from 0.38 to 0.42 eV/atom. From these images, it is evident that the AuNR evolves by swelling first its internal region, while reducing its length to conserve the total volume, due to the formation of point and line defects that evolve into planar defects, where melting begins, accompanied by surface diffusion of atoms in a process that clearly reproduces the experimental transition described by Link and collaborators,10 In addition, by analyzing the evolution of the external radius for all the studied aspect ratios, we can conclude that there exists a narrow energy window, represented by the vertical red dashed lines in Figure 4a, where the dominant phenomenon is the formation of φ-shaped rods. This window lies in the interval between 0.38 and 0.41 eV/atom and seems to be independent of the initial aspect ratio. At lower energies, the AuNR retains its original shape, while at higher energies, the AuNR transforms directly into a sphere or spheroid. 3.2 Optical behavior of φ-shaped rods The properties of a single φ-shaped rod in the far and near fields were studied for volume fractions from 0 (which corresponds to an unmodified AuNR) to ∼0.5 (which corresponds to a φ-shaped rod with half its volume in the swollen region). Figure 5 presents the extinction efficiency factor for unpolarized light (Figure 5a), along with the differential scattering cross section for the longitudinal and transversal polarization in the scattering XZ and YZ planes (Figures 5b and 5c). In Figure 5a, the growth of the volume fraction leads to a simultaneous blue-shift and intensity reduction of the longitudinal band of the AuNR, whereas the opposite is true for the transversal band. The reason behind this behavior is the appearance of the swollen region, which results in a reduction of the aspect ratio and simultaneous growth of the spherical center. When the volume fraction is around 50%, the longitudinal and transversal bands overlap, generating a broadband


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mode in the range of 500–600 nm. This is an extreme case, where the geometry of the φ-shaped rod resembles more that of a sphere with two extreme bulbs than that of a swollen AuNR. This evolution of the extinction efficiency is in good agreement with the experimental optical density data. The DSCS for longitudinal polarization in the XZ plane and for the transversal polarization on the YZ plane is depicted in Figures 5b and 5c, respectively. The evolution of the DSCS follows the same trend as the extinction efficiency. For longitudinal polarization, the higher the volume fraction, the lower the DSCS in both scattering planes. On the contrary, for transversal polarization, the DSCS increases with the volume fraction. This evolution can be explained again by the presence of the swollen sphere-like structure. The transversal polarization directly interacts with this swollen structure, and so the DSCS intensity increases with its volume. The opposite occurs for longitudinal polarization due to the reduction of the aspect ratio of the AuNR and, hence, of its length. 12

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Figure 5: (a) Extinction efficiency of a φ-shaped rod as a function of its volume fraction and its differential scattering cross section for (b) Ex polarization in the XZ plane and (c) Ey polarization in the YZ plane. The electromagnetic wave propagates along the z axis. Finally, we also examined the variations in the near field as a function of the volume fraction. Figure 6a shows the near field enhancement spectra for longitudinal polarization (the geometrical point of the maximum electric field is located at the tips of the structure, indicated by point p in the inset scheme of Figure 6a). The maximum of the field enhancement spectra blue-shifts and its intensity moderately increases, although only for small volume fractions as it decreases again at higher fractions. To illustrate this effect, electric field enhancement maps in the XY plane (the electromagnetic wave propagates parallel to the Z direction) for an unmodified AuNR and φshaped rods with volume fractions of 0.12 and 0.52 are presented in Figures 6b, 6c, and 6d, respectively. It appears from the maps that the presence of the swollen structure stops the electric field enhancement from totally surrounding the nanoparticle, contrary to what can be observed for the unmodified AuNR.


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Figure 6: (a) Electric field enhancement (at point p, located at a distance of ∼0.5 nm from the tip) spectra for longitudinal polarization of a φ-shaped rod as a function of its volume fraction (defined as the ratio between the volume of the swollen region and the total volume of the nanoparticle), and electric field enhancement maps around nanoparticles with a volume fraction of (b) 0, (c) 0.12 and (d) 0.52. The behavior of the transversal polarization is basically the opposite, as shown in Figure 7. The electric field enhancement increases and red-shifts with the increasing volume fraction, generating a broader band (Figure 7a). The electric field enhancement maps for the same volume fractions of Figures 6b to 6d are presented in Figures 7b to 7d. In this case, the presence of the swollen region generates a lateral increase of the electric field compared to the case of the unmodified AuNR. From all these results, it can be concluded that the optical properties of φshaped rods can be tailored by controlling the volume fraction of the swollen structure.


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Figure 7: (a) Electric field enhancement (at point p, located at a distance of ∼0.5 nm from the side) spectra for transversal polarization of a φ-shaped rod as a function of its volume fraction (defined as the ratio between the volume of the swollen region and the total volume of the nanoparticle), and electric field enhancement maps around nanoparticles with a volume fraction of (b) 0, (c) 0.12 and (d) 0.52. 4. Conclusions In this work, we have irradiated AuNRs with femtosecond laser pulses at a fluence optimized to induce the transformation of the rods into φ-shaped structures, and found that the swollen middle region can reach up to 50% of the total volume of the nanoparticle. Moreover, by means of molecular dynamics calculations, we found that the energy absorbed by a AuNR for it to evolve into a φ-shaped structure must be within the narrow window of 0.38–0.41 eV/atom. At lower energies, the structure retains its rod shape and, at larger energies, it directly reshapes into


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a sphere. Finally, the optical response of the φ-shaped structures was calculated by means of FDTD simulations and characterized as a function of the volume fraction, which we previously defined as the quotient between the volume of the swollen region and the total volume of the AuNR. We found that control of the volume fraction, which can be attained by controlling the energy absorbed by the AuNRs, can be applied to control the plasmonic behavior of φ-shaped rods. The main appeal of these φ-shaped nanoparticles resides in this control of their plasmonic response, resulting from the great tunability of these hybrid structures between a rod and a sphere. At low volume fractions, the structure acts more like a rod with a high longitudinal plasmon response; however, as the volume fraction increases, the transversal response gains significance, for example generating higher near fields than the transversal response of an unmodified AuNR while still retaining a longitudinal rod-like response. ASSOCIATED CONTENT The following files are available free of charge. Optical spectra and TEM images of AuNRs before and after irradiation at different fluences. Geometrical analysis of φ-shaped AuNRs. Details of FDTD simulations. (PDF) AUTHOR INFORMATION Corresponding Author * Correspondence should be addressed to Ovidio Peña-Rodríguez, e-mail: [email protected] Present Addresses † Alejandro Prada is now at Centro para el Desarrollo de la Nanociencia y la Nanotecnologı́a (CEDENNA), Santiago, Chile and Centro de Nanotecnología, Facultad de Ciencias, Universidad Mayor, Santiago, Chile.


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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources This work was partially funded by DIMMAT project, provided by Madrid regional government under program S2013/MIT-2775. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors acknowledge the computer resources and technical assistance provided by CESVIMA (Centro de Supercomputación y Visualización de Madrid, Universidad Politécnica de Madrid) and the facilities provided by the Center for Ultrafast Lasers and the National Center of Microscopy (Universidad Complutense de Madrid). P.D.-N. thanks the predoctoral grant provided by Madrid regional government via the DIMMAT project, funded under program S2013/MIT-2775. A.P. acknowledges the postdoctoral funding provided by Financiamiento Basal para Centros Cientı́ficos y Tecnológicos de Excelencia (Chile) through the Center for Development of Nanoscience and Nanotechnology (CEDENNA, Contract FB0807). ABBREVIATIONS LSPR, localized surface plasmon resonance; AuNRs, gold nanorods; fs, femtosecond; ns, nanosecond; BrSal, 5-bromosalicylic acid; CTAB, cetyltrimethylammonium bromide; cmc, critical micelle concentration; TEM, transmission electron microscopy; MD, molecular


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dynamics; τ, time decay; FDTD, finite-differences in the time-domain; L, rod length; Din, rod diameter; Dout, diameter of the swollen region; E, electric field; E0, incident electric field; Ex, longitudinal component of the electric field; Ey, transversal component of the electric field; Qext, optical extinction efficiency; DSCS, differential scattering cross section. REFERENCES (1) Lakowicz, J. R. Plasmonics in Biology and Plasmon-Controlled Fluorescence. Plasmonics 2006, 1 (1), 5–33. (2) Brongersma, M. L.; Halas, N. J.; Nordlander, P. Plasmon-Induced Hot Carrier Science and Technology. Nat. Nanotechnol. 2015, 10 (1), 25–34. (3) Chen, H.; Shao, L.; Li, Q.; Wang, J. Gold Nanorods and Their Plasmonic Properties. Chem. Soc. Rev. 2013, 42 (7), 2679–2724. (4) Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C. The Optical Properties of Metal Nanoparticles: The Influence of Size, Shape, and Dielectric Environment. J. Phys. Chem. B 2003, 107 (3), 668–677. (5) Boulais, E.; Lachaine, R.; Hatef, A.; Meunier, M. Plasmonics for Pulsed-Laser Cell Nanosurgery: Fundamentals and Applications. J. Photochem. Photobiol. C Photochem. Rev. 2013, 17, 26–49. (6) Boulais, É.; Lachaine, R.; Meunier, M. Plasma-Mediated Nanocavitation and Photothermal Effects in Ultrafast Laser Irradiation of Gold Nanorods in Water. J. Phys. Chem. C 2013, 117 (18), 9386–9396.


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(7) Link, S.; Burda, C.; Nikoobakht, B.; El-Sayed, M. A. How Long Does It Take to Melt a Gold Nanorod?: A Femtosecond Pump-Probe Absorption Spectroscopic Study. Chem. Phys. Lett. 1999, 315 (1), 12–18. (8) Link, S.; Burda, C.; Mohamed, M. B.; Nikoobakht, B.; El-Sayed, M. A. Laser Photothermal Melting and Fragmentation of Gold Nanorods: Energy and Laser Pulse-Width Dependence. J. Phys. Chem. A 1999, 103 (9), 1165–1170. (9) Link, S.; Burda, C.; Nikoobakht, B.; El-Sayed, M. A. Laser-Induced Shape Changes of Colloidal Gold Nanorods Using Femtosecond and Nanosecond Laser Pulses. J. Phys. Chem. B 2000, 104 (26), 6152–6163. (10) Link, S.; Wang, Z. L.; El-Sayed, M. A. How Does a Gold Nanorod Melt? J. Phys. Chem. B 2000, 104 (33), 7867–7870. (11) Chang, S. S.; Shih, C. W.; Chen, C. D.; Lai, W. C.; Wang, C. R. C. The Shape Transition of Gold Nanorods. Langmuir 1999, 15 (3), 701–709. (12) Ahmadi, T. S.; Logunov, S. L.; El-Sayed, M. A. Picosecond Dynamics of Colloidal Gold Nanoparticles. J. Phys. Chem. 1996, 100 (20), 8053–8056. (13) Logunov, S. L.; Ahmadi, T. S.; El-Sayed, M. A.; Khoury, J. T.; Whetten, R. L. Electron Dynamics of Passivated Gold Nanocrystals Probed by Subpicosecond Transient Absorption Spectroscopy. J. Phys. Chem. B 1997, 101 (19), 3713–3719. (14) Kurita, H.; Takami, A.; Koda, S. Size Reduction of Gold Particles in Aqueous Solution by Pulsed Laser Irradiation. Appl. Phys. Lett. 1998, 72 (7), 789–791.


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(15) Werner, D.; Furube, A.; Okamoto, T.; Hashimoto, S. Femtosecond Laser-Induced Size Reduction of Aqueous Gold Nanoparticles: In Situ and Pump−Probe Spectroscopy Investigations Revealing Coulomb Explosion. J. Phys. Chem. C 2011, 115 (17), 8503–8512. (16) Horiguchi, Y.; Honda, K.; Kato, Y.; Nakashima, N.; Niidome, Y. Photothermal Reshaping of Gold Nanorods Depends on the Passivating Layers of the Nanorod Surfaces. Langmuir 2008, 24 (20), 12026–12031. (17) Gordel, M.; Olesiak-Banska, J.; Matczyszyn, K.; Nogues, C.; Buckle, M.; Samoc, M. Post-Synthesis Reshaping of Gold Nanorods Using a Femtosecond Laser. Phys. Chem. Chem. Phys. 2013, 16 (1), 71–78. (18) Babynina, A.; Fedoruk, M.; Kühler, P.; Meledin, A.; Döblinger, M.; Lohmüller, T. Bending Gold Nanorods with Light. Nano Lett. 2016, 16 (10), 6485–6490. (19) González-Rubio, G.; Díaz-Núñez, P.; Rivera, A.; Prada, A.; Tardajos, G.; GonzálezIzquierdo, J.; Bañares, L.; Llombart, P.; Macdowell, L. G.; Palafox, M. A.; et al. Femtosecond Laser Reshaping Yields Gold Nanorods with Ultranarrow Surface Plasmon Resonances. Science 2017, 358 (6363), 640–644. (20) Scarabelli, L.; Grzelczak, M.; Liz-Marzán, L. M. Tuning Gold Nanorod Synthesis through Prereduction with Salicylic Acid. Chem. Mater. 2013, 25 (21), 4232–4238. (21) Plimpton, S. Fast Parallel Algorithms for Short-Range Molecular Dynamics. J. Comput. Phys. 1995, 117 (1), 1–19. (22) Foiles, S. M.; Baskes, M. I.; Daw, M. S. Embedded-Atom-Method Functions for the Fcc Metals Cu, Ag, Au, Ni, Pd, Pt, and Their Alloys. Phys. Rev. B 1986, 33 (12), 7983–7991.


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(23) Nguyen, S. C.; Zhang, Q.; Manthiram, K.; Ye, X.; Lomont, J. P.; Harris, C. B.; Weller, H.; Alivisatos, A. P. Study of Heat Transfer Dynamics from Gold Nanorods to the Environment via Time-Resolved Infrared Spectroscopy. ACS Nano 2016, 10 (2), 2144–2151. (24) Schneider, T.; Stoll, E. Molecular-Dynamics Study of a Three-Dimensional OneComponent Model for Distortive Phase Transitions. Phys. Rev. B 1978, 17 (3), 1302–1322. (25) Oskooi, A. F.; Roundy, D.; Ibanescu, M.; Bermel, P.; Joannopoulos, J. D.; Johnson, S. G. Meep: A Flexible Free-Software Package for Electromagnetic Simulations by the FDTD Method. Comput. Phys. Commun. 2010, 181 (3), 687–702. (26) Taflove, A.; Hagness, S. C. Computational Electrodynamics: The Finite-Difference TimeDomain Method, 3rd ed.; Artech House: Boston, 2005. (27) Taflove, A.; Oskooi, A.; Johnson, S. G. Advances in FDTD Computational Electrodynamics: Photonics and Nanotechnology; Artech House, 2013.


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Table of contents image:


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Figure 1: Schematic view of the transitions observed for a AuNR, as a function of the absorbed energy. The rod shape is conserved for lower energies whereas for higher energies the AuNR transforms into a sphere. The energy range between E0 and E1 corresponds to the energy window where the formation of φ-shaped rods occurs. 79x83mm (300 x 300 DPI)


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Figure 2: (a) Optical absorption spectra before and after irradiation at different fluences. The red vertical bar represents the spectral width of the laser pulses. Transmission electron microscopy images of AuNRs (b) before and after irradiation with a laser fluence of (c) 5.1 J/m2, (d) 10.2 J/m2 and (e) 33.1 J/m2. 116x165mm (300 x 300 DPI)



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Using Femtosecond Laser Irradiation to Grow the Belly of Gold

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