Transformation of Gold Nanorods in Liquid Media Induced by nIR

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Transformation of Gold Nanorods in Liquid Media Induced by nIR, Visible and UV Laser Irradiation Yasser Attia Attia, M.Teresa Flores-Arias, Daniel Nieto, Carlos VazquezVazquez, Germán F de la Fuente, and M. Arturo Lopez-Quintela J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 21 May 2015 Downloaded from http://pubs.acs.org on May 21, 2015

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The Journal of Physical Chemistry

Transformation of Gold Nanorods in Liquid Media Induced by nIR, Visible and UV Laser Irradiation Yasser A. Attia1, 2, M. Teresa Flores-Arias*3, Daniel Nieto3, Carlos Vázquez-Vázquez*4, Germán F. De La Fuente 5 and M. Arturo López-Quintela4 1 - National Institute of Laser Enhanced Sciences, Cairo University, Egypt. 2 – Department of Chemistry, Faculty of Sciences, Taif University, Saudi Arabia 3 - Microoptics and GRIN Optics group (Unidad Mixta Asociada al CSIC), Department of Applied Physics, Faculty of Physics, University of Santiago de Compostela, Spain 4- Department of Physical Chemistry, Faculty of Chemistry, University of Santiago de Compostela, Spain. 5 - ICMA (CSIC-University of Zaragoza) Zaragoza (Spain)

*Corresponding authors: [email protected], [email protected]

ACS Paragon Plus Environment


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ABSTRACT Gold nanorods (GNRs) were initially prepared with 3 different aspect ratios (2.5, 2.9 and 3.8) via a seed-mediated chemical route, using cetyltrimethylammonium bromide (CTAB) as organic surfactant. These rods were then irradiated under continuous-wave (λ=1064 nm) and pulsed (λ=1064, 532, 355 nm) emission modes using Nd:YVO4 and Nd:YAG lasers, respectively. The photostability and behavior of the Au nanorods under these laser irradiation conditions were studied using UV-Vis-nIR spectroscopy and electron microscopy. Photofragmentation and melting mechanisms were provoked as a function of laser irradiation parameters (emission mode and wavelength) and caused the GNRs to undergo considerable morphological changes. Important differences were observed between the wavelengths used for irradiation, indicating significantly different nanorod breakdown mechanisms.

Keywords: Gold nanorods, nanosecond laser, pulsed laser, photostability


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INTRODUCTION Laser ablation in liquids has been studied intensely during the past decade, and a convenient review has been recently published.1 In addition, many scientific groups have centered on the study of the interaction between metal nanoparticles and laser irradiation.2-5 More particularly, Koda et al. reported the irradiation effect of nanosecond laser pulses (532 nm) on gold nanospheres, which leads to fragmentation of nanodots.6,7 These results were explained on the basis of the slow heat transfer of the laser energy absorbed by the solvent surrounding the gold nanospheres. This slow heat transfer then leads to the melting and vaporization of the nanoparticles as estimated from the amount of absorbed laser energy. More information about the structural dynamics of metal nanoparticles after laser excitation can be obtained when nanoparticles having shapes different from spheres are used. Link et al. studied the effect of high power with short pulse duration lasers –femtosecond (fs) and nanosecond (ns)– on the structure of gold nanorods for comparison's sake, to discern between large differences in laser pulse width.4 They found that nanorods are easily converted to nanospheres by laser irradiation. For femtosecond pulses, pulse energies in the order of 10 mJ are sufficient to completely convert the rods into spheres. However, the exact value of the nanosecond pulse energy needed for melting depends on the experimental conditions, primarily on the laser spot size irradiating the sample. Most of these studies are concentrated on the effect of femtosecond laser irradiation on gold nanoparticles, where essentially no melting takes place, according to Amoruso et al.8 In addition, Gamaly has proposed two types of mechanisms which may break down structures within the femtosecond (fs) laser pulse regime, as well as the fact that no thermal effect is observed below the ca. 3-4 ps timescale.9 The present work concentrates on the behavior of gold nanorods with different aspect ratios (2.5, 2.9 and 3.8), prepared using a seed-mediated method,10 upon irradiation by different nanosecond lasers. The latter emits at wavelengths of 1064, 532 and 355nm and under pulsed and continuous wave (cw) regimes. The photostability of these particles was thus studied in order to improve our understanding of the effects produced by nanosecond laser irradiation, by comparing the results obtained between the two laser emission modes (pulsed and cw).



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EXPERIMENTAL METHODS Materials: Hexadecyltrimethylammonium bromide (CTAB, 98%), Sodium borohydride (NaBH4, 95%), Hydrogen tetrachloroaurate trihydrate (HAuCl4·3H2O, 99,999%), Silver nitrate (AgNO3, 99%) Polyvinyl pyrrolidone (PVP-K30 with average molecular weight = 30,000 to 40,000), and L-ascorbic acid (99%) were purchased from Aldrich.

Method of preparation: Seed solution by using CTAB: 2.5 ml of 0.2 M CTAB solution was mixed with 2.5 ml of 5x10-4 M HAuCl4. To the stirred solution, 30 µl of ice-cold 0.1M NaBH4 was added, which results in the formation of a solution with brownish yellow color. Vigorous stirring of the seed solution was continued for 2 minutes. Growth solution: Different amounts (50, 100, and 150µl) of 4x10-3 M AgNO3 solution were added to 2.5 ml of 0.2 M CTAB. To this solution, 2.5 ml of 10-3M HAuCl4 was added and after gentle mixing of the solution, 35 µl of 0.0788 M ascorbic acid was added. Ascorbic acid as a reducing agent changes the growth solution from dark yellow to colorless within 10 minutes. The final step was the addition of the seed solution to the growth solution. The color of the resultant solution gradually changed within 10-20 minutes. Instrumentation: The absorption spectra of the colored solutions were measured with a Perkin Elmer Lambda 25 spectrophotometer before and after irradiation with the different lasers. The aspect ratio of the gold nanorods was obtained by means of the linear relationship between the absorption maximum of the longitudinal plasmon resonance λmax and the aspect ratio R and the dielectric constant of the surrounding medium εm (using 1.77 for water):11

λmax = (53.71R - 42.29)εm + 495.14

(1)

λmax = 95.07R + 420.29

(2)

Gold nanorods were imaged with a PHILIPS CM-12 and a JEOL JEM-1011 transmission electron microscopes working at 80kV. A drop of the gold nanorods


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solution was spread on a 400-mesh copper grid coated with a Formvar film, and the extra droplet was instantly wiped off by filter paper. The effect of laser irradiation on gold nanorods was tested by exposing 3ml of gold nanorods with different aspect ratios to three different ns laser types in a quartz cuvette: Figure 1 shows the configurations used for performing the experiments. 1. Pulsed Nd:YAG laser, operating in Q-switch regime: A lamp-pumped Nd:YAG laser, model Quanta-Ray GCR-130 (Spectra Physics) with a fixed repetition rate of 50 Hz was used. The Au NR suspension was irradiated under a cylinder-type geometry with a 1 cm diameter beam cross section (see illustration in Figure 1). The output configuration of the laser does not allow the movement of the laser beam. The corresponding collimated beam diameters were, respectively, ca. 10 mm in the fundamental mode and 8 mm in the second and third harmonic modes. Pulse width, power and irradiance for each wavelength of the laser are given in the Table below. 1064 nm

Wavelength

532 nm

355 nm

(fundamental) (second harmonic) (third harmonic)

Pulse width (τ) / ns

8-9

Power / W

12.5 -2

Irradiance / W·cm

6-7

5-6

2.9 7

≈ 3.5x10

0.55 7

≈ 1.65x10

≈ 3.56x106

2- Pulsed Nd:YVO4 laser, operating in Q-switch regime: A diode laser end-pumped, Q-switched Nd:YVO4 solid state laser, model RSM Powerline 20E/LP (Rofin) operating at its fundamental wavelength of 1064nm was used. The laser beam parameters are controlled by commercial software and allow defining the diode pump current intensity (power level output), the pulse repetition rate and the beam scanning speed, among other parameters. Beam steering is controlled by a galvanometer system placed at the exit of the laser beam (see illustration in Figure 1), which is fitted with a 100 mm-focal length flat-field lens ensuring a uniform 30 µm diameter beam spot within a working area of 80x80mm2. 3- Nd:YAG laser (continuum SLI-10): A model SLI-10 (Continuum) Nd:YAG laser operating at its second harmonic emission wavelength of 532nm (repetition rate of 10 Hz) and working in its continuous wave



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mode (cw). The beam diameter is 6 mm. This laser is used without any additional focusing lens. The irradiance value is ≈2.04x104 W·cm-2), measured with a MolectronEPM 2000:Tar5 Laser Power Meter.

Figure 1: Illustration diagram showing the difference between the setups used for the two different lasers. In a) the laser is able to irradiate a much larger volume of the suspension because the galvanometer system is placed at its output permitting to irradiate the whole volume of the sample, and thus interacting more readily with the GNR than in b), where the geometry of irradiation is a cylinder with a 1 cm diameter beam cross section

RESULTS AND DISCUSSION I: Effect of laser irradiation in continuous wave mode (cw). It has been reported by Link et al. that laser irradiation of gold nanorods causes melting of the rod shape particles into spheres.4 These rods were stabilized by a mixture of two surfactants (Cetyltrimethylammonium bromide (CTAB) and Tetraoctylammonium bromide (TOAB)); however, in our case the particles were prepared using CTAB alone. Laser emission from a Continuum SLI-10 and a Nd:YVO4 laser was guided into a quartz cell containing the sample solution of gold nanoparticles. The output energy from the Continuum SLI-10 Nd:YAG laser yielded irradiance values around 2.04 x 104 Wcm-2. On the other hand, Nd:YVO4, working with an average power of 12 W, yields irradiance values of ca. 1.70 x 106 Wcm-2. Both of these irradiance values are below and above, respectively, the threshold value of 4 x 104 Wcm-2 established for plasma formation in metals.12,13


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The cw laser output is at a continuous energy level as a function of time, and it works much like a lamp or an IR heater (since it emits in the near IR region of the electromagnetic spectrum). This type of photons interact basically with low energy electrons which, in turn, undergo nonradiative transitions and transfer energy to lattice vibrations, thus increasing the temperature of the material irradiated. As it can be seen in Figure 2, the absorption spectra suggest that gold nanorods prepared by the seed-mediated method and stabilized by CTAB are very stable towards cw laser irradiation. Only a slight decrease in its absorbance occurred upon exposure for 30 minutes to the Continuum SLI-10 Nd:YAG laser working in cw regime (Figure 2a).

2.1

Absorbance

1.8 1.5 1.2

Irradiation time t = 0 min t = 1 min t = 2 min t = 10 min t = 30 min

t

0.9 0.6 0.3

Aspect ratio = 2.9

a

0.0 1.6 Irradiation time t = 0 min

1.4

Absorbance

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1.2 1.0

t = 30 min 0.8 0.6 0.4 0.2 0.0 400

Aspect ratio = 2.5

b 500

600

700

Wavelength / nm


800

900


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 2: Absorption spectra showing the effect of CW-mode laser on CTAB-stabilized gold nanorods for exposures up to 30 minutes: a) aspect ratio = 2.9, Nd:YAG laser; b) aspect ratio = 2.5, Nd:YVO4 laser.

Similar behavior is observed for the Nd:YVO4 laser, working at 1064 nm in continuous regime. Figure 2b shows almost no changes in the UV-Vis absorption spectra for nanorods with aspect ratio of 2.5, even when the solution has been observed to heat considerably (above 60ºC) due to laser absorption in water. Although the irradiance level is above that considered as the threshold for plasma formation, when calculated from the laser emission point of view, only a small volume of particles, or even none, may be illuminated at the focal length by the beam spot. This is because in this case there is no galvanometer system coupled to the output of the laser, and the irradiation geometry is a cylinder with a 1 cm diameter beam cross section. At this emission wavelength, the interaction between the laser and the species in suspension, as well as with the liquid, is expected to be essentially of a photothermal nature. In addition, the focalized spot diameter is approximately 30 µm, so that the interaction time with the gold nanorods is quite limited because the species in suspension are not present for a long enough time in this reduced area.14 Interaction of the beam is mainly taking place with the liquid, and thus it is expected to raise its temperature significantly above room temperature, since the liquid absorbs an appreciable percentage of the laser energy.15-16

II: Effect of laser irradiation in nanosecond pulsed mode. A- Effect of Pulsed Nd:YVO4 laser, working in Q-switch regime. For this study all the parameters of this laser system, such as diode laser pump intensity (I), pulse repetition frequency (Fpulse), beam scanning rate (ʋ), beam line overlap (%), raster orientation, etc., were evaluated to observe their influence on gold nanorods. As shown in Figure 3a, a slight blue shift and absorbance decrease occurred when we used with a velocity of the galvanometer system ʋ = 100 mm/s, Fpulse = 50 kHz, a pulse overlap of 90 %, raster orientation with 90º angle (up to down fill in) (changed to 0º angle in the second line), and the intensity (I) was changed from 25 to 32 A (corresponding to a laser output power variation of ≈ 4 to 10.5 W). Also the same effect


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occurred when the parameters changed to ʋ = 50 mm/s, Fpulse = 5 kHz, a pulse overlap of 20 %, orientation with 0º angle and I varying from 20 to 32 A (≈ 1.5 to 10.5W). The irradiance of pulsed Q-switched mode Nd:YVO4 is about 1.13 x 108 Wcm-2 (the laser permits to vary the repetition frequency, and optimum results were obtained for values ranging between 5 and 10 kHz). In this case, slight modifications of the nanogold structures are observed as a function of intensity (W or pulse energy) as can be seen in Figures 3a and 3b. The difference between the cw and pulsed modes may be assigned to the levels of irradiance, which lie around 106 and 108 Wcm-2, respectively. In the latter case, some effect on the surface plasmon fundamental (transverse) band of Au nanorods, within the visible region of the spectrum, is observed. This is in contrast to the null influence that cw irradiation has over this visible transverse mode resonance band, as shown in the previous discussion.



1.6

Absorbance

1.4

Rods I = 25 A I = 28 A I = 30 A I = 32 A

Aspect ratio = 2.5

1.2 1.0 0.8 0.6 υ = 100 mm / s

0.4 0.2 0.0 1.6

Fpulse = 50 kHz Pulse overlap = 90 %

a

Aspect ratio = 2.5

Rods I = 20 A I = 22 A I = 25 A I = 28 A I = 30 A I = 32 A

1.4

Absorbance

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1.2 1.0 0.8 0.6 υ = 50 mm / s

0.4 0.2

Fpulse = 5 kHz Pulse overlap = 20 %

b

0.0 400

500

600

700

800

900

Wavelength / nm

Figure 3: Absorption spectra showing the effect of pulsed Nd:YVO4 laser irradiation on gold nanorods (aspect ratio = 2.5): (a) after changing the intensity from 25 to 32 A (from ≈ 4 to 10.5 W), ʋ = 100 mm/s, Fpulse = 50 KHz, pulse overlap 90 %, and scan line orientation of 0º; (b) after changing the intensity from 20 to 32 A (from ≈ 1.5 to 10.5 W), ʋ = 50 mm/s, Fpulse = 5 kHz, pulse overlap 20 %, and scan line orientation of 0º.

B- Effect of pulsed Nd:YAG laser, working in Q-switch regime. Experiments carried out with a pulsed Nd:YAG nanosecond regime emission laser yielded significantly different results for Au nanorods of different aspect ratios (2.5, 2.9, and 3.8), as follows from the discussion below. This Nd:YAG pulsed laser can emit at its fundamental wavelength (λ= 1064 nm) and at its second and third harmonics (λ= 532 nm and 355 nm, respectively).


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The first experiment was carried out at its fundamental wavelength (1064 nm). Fragmentation takes place under these irradiation conditions (Figure 4) and it is observed by the continuous decrease in the absorbance of the longitudinal gold NRs band as a function of irradiation time (Figure 5). As reported by Link et al.,4 laser pulses are claimed to cause excitation of the hot lattice (stage 1, fast process) in the experiments with nanosecond pulses. In view of the latter, it is thus expected that absorption of additional photons by the hot lattice, occurring in the nanosecond experiment at a pulse repetition rate of 50 Hz, is what leads to a substantial increase in the lattice internal energy and, consequently, to the fragmentation of gold NRs (stage 2, slow process). hν ( ns)

Stage 1: ( Au) NR  fast  →( Au) NR  vib

Stage 2: ( Au) NR 

vib∗∗

vib∗∗

(3)

Fragmentation

slow   → n ( Au) NP 

SNS

(4)

The longitudinal absorption band of gold NRs changes its maximum to shorter wavelength (from 784nm to 696nm for gold NRs of aspect ratio 3.8, Figures 4 and 5a), suggesting that the laser exposure changes the distribution into smaller rods and smaller nanospheres (SNS). In addition, the formation of smaller nanospheres is demonstrated by the absorption increase of the transversal mode (Figure 5b). These absorption changes in both longitudinal and transversal modes have been fitted to one single exponential component (Figures 5a and 5b). The obtained relaxation times for both longitudinal and transversal modes are very similar − τL = (115 ± 7) s and τT = (102 ± 6) s −, giving an average relaxation time for the slow process (fragmentation stage) of = (108 ± 6) s. This proposed mechanism has been confirmed by transmission electron microscopy (TEM) measuring the irradiated sample at different exposure times (Figure 6). TEM micrographs clearly show the gold nanorods length reduction and the formation of gold nanospheres after the irradiation. Instead of a mechanism based on photofragmentation, Chang et al. proposed a photothermal mechanism when they irradiated Au NRs with a 1064nm pulsed Nd:YAG laser.17 However, in their case, the Au NRs showed a larger aspect ratio with the maximum of the longitudinal absorption band at ≈1000nm, close to the wavelength of the laser. In our case, the photofragmentation is more plausible because the maximum of the longitudinal absorption band is far from the wavelength of the laser and it is observed an increase of



the intensity of the transversal band while it keeps the position, indicating the increasing number of Au nanospheres.2 These changes can be also observed for gold nanorods with smaller aspect ratios (a.r. = 2.5 and 2.9) but the effect is less pronounced (See Figure S1).

3.3 Irradiation time t = 0s t = 15s t = 30s t = 45s t = 60s

3.0 2.7 2.4

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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2.1 1.8 1.5

t = 680s

1.2 0.9 0.6 0.3

aspect ratio = 3.8

0.0 400

500

600

700

800

900

1000

1100

Wavelength / nm

Figure 4: UV-Vis absorption spectra showing the effect of pulsed Nd:YAG laser irradiation (λ=1064 nm) at different intervals between 0 and 680 s, of a suspension containing gold nanorods with aspect ratio 3.8. Note: the arrows are just guides to the eye.


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Amax(t) / Amax(t=0)

1.00

Aspect ratio = 3.8

0.90

Longitudinal band

Model: Exponential Decay y0 + A1e^(-x/t1) Chi^2/DoF 4.59411E-4 R^2 0.98952

0.80 0.70

Parameter Value Error --------------------------------------------y0 0.308890 0.01017 A1 0.636300 0.01557 t1 115.33858 6.80504 ---------------------------------------------

0.60 0.50 0.40 0.30

a

0.20 1.30

Amax(t) / Amax(t=0)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


b

1.25 Model: Exponential Growth y = y0 + A1e^(x/t1)

1.20 1.15 1.10 1.05 Transverse band

1.00 0

100

200

300

Chi^2/DoF R^2 ---------------------------------------0.00008 0.98967 ---------------------------------------Parameter Value Error ---------------------------------------y0 1.277850 0.00384 A1 -0.26532 0.00651 t1 -102.00772 5.77147 ----------------------------------------

400

500

600

700

time / s

Figure 5: Relative change of the absorption spectra maxima for the transverse (A) and longitudinal (B) bands of rods with aspect ratio 3.8, under the effect of pulsed Nd: YAG laser irradiation at λ=1064 nm.



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Figure 6: Transmission electron micrographs of the gold nanorods (aspect ratio 3.8) subjected to different laser irradiation times (0, 250 and 720s) using a pulsed Nd: YAG laser working at λ=1064 nm. All scale bars are 50 nm. An illustration of the possible fragmentation that can occur at intermediate stages of the laser irradiation is also given.

A drastically different behavior is observed for the second harmonic laser irradiation (green, 532 nm) (Figures 7 and S2), as compared to similar conditions in the nIR (1064 nm). Although the pulse width was slightly shorter (7 ns versus 9 ns) it should not cause a significant effect on the irradiation results in terms of nanorod modification. The irradiance values, calculated based on the laser emission are of the same magnitude, thus other type of mechanism (melting) could be considered responsible for such a change in the laser-nanorod interaction. The main differences of the second harmonic (532nm) with respect to the fundamental line (1064nm) is that the longitudinal absorption band of gold NRs decreases its intensity but keeps its maximum wavelength unchanged during the full irradiation period (Figures 7 and 8a). Because the longitudinal mode is directly related to the aspect ratio of the GNRs, the fact that the maximum wavelength remains unchanged is a clear indication that the aspect ratio is


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kept constant all along the irradiation process, suggesting that the gold nanorods are getting thinner in all the directions. This drastic decrease of the longitudinal band can be associated to melting of GNRs. In addition, the transverse absorption band is shifted to shorter wavelengths and increases its intensity, being a clear indication of the formation of thinner gold nanorods and an increase in the amount of nanoparticles present in the solution (Figure 7 and 8b). An illustration of the suggested melting mechanism is shown in Figure 7. Coincidently, the changes observed are very similar to those reported for irradiation with fs lasers (Link et al.4), where different interaction mechanisms have been well established from those taking place within the ns regime.8-9,18 In addition, this laser output wavelength matches with the transverse resonance plasmon band of Au, thus it is not surprising to observe considerable effects on the corresponding visible band in the UV-Vis spectrum recorded just after 532 nm irradiation. Similar effects have been reported for ablation of metals, when comparing the efficiency between 1064 and 532 nm wavelengths and have been assigned to the difference in absorption.19 This is, in part, because of the presence of the Localized Surface Plasmon Resonance (LSPR) in the coinage metals used (Cu, Ag). Figure 8 shows the absorption change at the transverse and longitudinal bands. These absorption changes in both longitudinal and transversal modes have been also fitted to single exponentials (see Table 1, Supp. Info). It is observed that both longitudinal and transversal modes have similar relaxation times for the two gold NR with larger aspect ratios. However, these two values are quite different for the shorter gold nanorods. This is possibly related to the less anisotropic shape that these gold NRs have. Figure 9 shows the TEM micrographs with the effect of second harmonic Nd:YAG laser irradiation on GNRs.


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Absorbance


1.3 1.2 aspect ratio = 2.9 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 Irradiation time t = 0s t = 7s t = 17s t = 32s 0.3 t = 67s t = 127s 0.2 t = 187s t = 277s t = 412s t= 532s 0.1 t = 652s 0.0 400 500 600

700

800

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900

Wavelength / nm

Figure 7: UV-Vis absorption spectra showing the effect of pulsed Nd: YAG laser (Second harmonic with λ=532 nm) on gold nanorods with aspect ratio 2.9. An illustration of the suggested melting mechanism is also shown.


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Amax(t) / Amax(t=0)

1.0 0.9

Longitudinal band

a.r. = 2.5 a.r. = 2.9 a.r. = 3.8

0.8 0.7 0.6 0.5 0.4 0.3 a

0.2 1.6

Amax(t) / Amax(t=0)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


b

1.5 1.4 1.3 1.2 1.1 Transverse band 1.0 0

100

200

300

400

500

600

700

800

time / s

Figure 8: Relative change of the absorption spectra maxima for the transverse (A) and longitudinal (B) bands of rods with different aspect ratios (2.5, 2.9 and 3.8) under the effect of pulsed Nd: YAG laser irradiation at the second harmonic (λ= 532 nm).



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Figure 9: TEM images showing the effect of pulsed Nd:YAG laser irradiation (second harmonic, λ=532 nm) of gold nanorods (left), which transform into nanospheres (right) via a melting mechanism (scale bar 200 nm). UV irradiation at the third harmonic (λ= 355 nm) causes a similar melting behavior to that observed for the (532 nm) visible laser irradiation (see Figure S3). Exposure to continued illumination at both wavelengths induces gradual changes until approx. 400 s of exposure; however, these changes have different kinetics at the two wavelengths. In particular, the absorption changes in both longitudinal and transversal modes have been fitted to single exponentials and the obtained relaxation times are shown in the Supp. Info - Table 1 and Figure S4. It is observed that both longitudinal and transversal modes have similar relaxation times for each gold NR. It can be also observed that relaxation times are similar for the smaller aspect ratios (2.5 and 2.9) but, as far as the aspect ratio becomes larger, the relaxation times increase considerably. In addition, it is observed that, for the same aspect ratio, these values are much larger than those obtained with the laser working at larger wavelengths (e.g. for the gold nanorods with aspect ratio 3.8: = (555 ± 52) s for λ = 355nm but = (156 ± 7) s for λ = 532nm). These different behaviors must be related to the difference in laser-nanorod interaction at these wavelengths, which may be related to several phenomena. The LSPR absorption coincides with the visible laser emission at 532 nm,20 but should not be appreciably affected by UV irradiation at 355 nm. At this shorter wavelength, it is expected that the photon-electron interaction regime may take into account higher energy electrons and associated nonlinear phenomena that become much more important at this UV emission.9 According to the literature (reference 8 and work cited therein) nonlinear effects which cause multiphoton absorption on irradiated surfaces, are gradually more important as the wavelength is changed in the sequence nIR-Vis-UV (refer to graph Fig 80 in reference 13, p 129), but are considered important only at the ultrashort pulse regime (pico- and femtosecond lasers),17,21 which is not our case. Core shell electrons may be conveniently excited and different laser-matter interaction mechanisms may thus be induced within the ns regime at 355 nm.9, 13, 18, 22 It is known, for example, that the refractive index of a laser-irradiated medium depends, among other parameters, on the laser irradiance. When the irradiance is above a threshold, and the medium exhibits partial absorption of the laser light, self-focusing and plasma formation are reported to occur readily at the ns timescale.18,

23-24

A most relevant question here is whether a


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photochemical (bond-breaking) mechanism is contributing at all to the largely photothermal (heating) mechanism characteristic of ns laser-GNR interactions. From the differences observed in the above results, it is reasonable to propose that an additional interaction mechanism is exerted under UV irradiation. Clarifying more precisely its origin, however, still remains an open question, although the presence of subnanometric Ag clusters in nanorod tips may have to be taken into consideration in order to clarify this point satisfactorily.25

Amax(t) / Amax(t=0)

1.0 0.9

a.r. = 2.5 a.r. = 2.9 a.r. = 3.8

Longitudinal band

0.8 0.7 0.6 0.5 0.4 0.3

a

0.2 1.30

Amax(t) / Amax(t=0)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


b

1.25 1.20 1.15 1.10 1.05 Transverse band 1.00 0

100

200

300

400

500

600

700

800

time / s

Figure 10: Relative change of the absorption spectra maxima for the transverse (A) and longitudinal (B) bands of rods with different aspect ratios (2.5, 2.9 and 3.8) under the effect of pulsed Nd: YAG laser irradiation at the third harmonic (λ= 355 nm).



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CONCLUSIONS GNRs with aspect ratios of 2.5, 2.9 and 3.8, prepared using CTAB as surfactant within a seed-mediated chemical route were subjected to nIR, Vis and UV laser irradiation in CW and pulsed mode. It was found that the GNRs are practically not affected by 1064 nm CW laser irradiation. However, they breakdown under 1064, 532 and 355 nm pulsed laser irradiation. The latter occurs under different mechanisms. Photofragmentation takes place under nIR irradiation, while melting takes place under 532 nm and 355 nm irradiation wavelengths. This clearly indicates that a significantly different mechanism is responsible for the GNR morphological changes observed under Vis and UV irradiation, where the laser-GNR interaction appears to change from that at the nIR. Increased absorption due to the presence of the LSPR band of GNRs within the Vis, as well as the potential presence of nonlinear effects within the UV, appear to contribute significantly to the above differences. Further studies proposed for future work are necessary, however, to distinguish between the laser-GNR interaction mechanisms under 532 and 355 nm laser irradiation.

ACKNOWLEDGMENTS / DEDICATION This work was supported by the MCI, Spain (MAT2010-20442; MAT2011-28673-C0201); MINECO, Spain (MAT2012-36754-C02-01); and Xunta de Galicia, Spain (EM019/2012, Grupos Ref. Comp. GRC2013-044, FEDER Funds). This work is dedicated to the memory of Prof. Carlos Gómez-Reino, head of the Microoptics and GRIN Optics Group (University of Santiago de Compostela, Spain).

ASSOCIATED INFORMATION Supporting

Information

Available:

Relaxation

times

associated

with

the

transformation of Au NRs after being irradiated with a pulsed Nd:YAG laser irradiation (λ = 532 and 355 nm). Description of the effect of the second and third harmonic of the pulsed Nd:YAG laser on gold nanorods with different aspect ratios. This material is available free of charge via the Internet at http://pubs.acs.org


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REFERENCES (1) Zeng H.; Du X.; Singh S. C.; Kulinich S. A.; Yang S.; He J.: Cai W. Nanomaterials via Laser Ablation/Irradiation in Liquid: A Review. Adv. Funct. Mater. 2012, 22, 1333– 1353. (2) Link S.; Burda C.; Mohamed M. B.; Nikoobakht B.; El-Sayed M. A. Laser Photothermal Melting and Fragmentation of Gold Nanorods: Energy and Laser PulseWidth Dependence. J. Phys. Chem. A 1999, 103, 1165-1170. (3) 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, 6152-6163. (4) Link S.; Wang Z. L.; El-Sayed M. A. How Does a Gold Nanorod Melt? J. Phys. Chem. B 2000, 104, 7867-7870. (5) Petrova H.; Perez Juste J.; Pastoriza-Santos I.; Hartland G. V.; Liz-Marzan L. M.; Mulvaney P. On The Temperature Stability Of Gold Nanorods: Comparison Between Thermal And Ultrafast Laser-Induced Heating. Phys. Chem. Chem. Phys. 2006, 8, 814821. (6) Kurita H.; Takami A.; Koda S. Size Reduction Of Gold Particles In Aqueous Solution By Pulsed Laser Irradiation. Appl. Phys. Lett. 1998, 72, 789. (7) Takami A.; Kurita H.; Koda S. Laser-Induced Size Reduction of Noble Metal Particles. J. Phys. Chem. B 1999, 103, 1226-1232. (8) Amoruso S., Bruzzese R., Wang X., Nedialkov N. N., Atanasov P. A. Femtosecond Laser Ablation of Nickel in Vacuum. J. Phys. D: Appl. Phys. 2007, 40, 331-340. (9) Gamaly E. G. Ultra-fast Disordering by fs-Lasers: Lattice Superheating prior to the Entropy Catastrophe. Appl. Phys. A 2010, 101, 205-208.



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(10) Nikoobakht, B.; El-Sayed, M. A. Preparation and Growth Mechanism of Gold Nanorods (NRs) Using Seed-Mediated Growth Method. Chem. Mater. 2003, 15, 19571962. (11) Link S.; Mohamed M. B.; El-Sayed M. A. Simulation of the Optical Absorption Spectra of Gold Nanorods as a Function of Their Aspect Ratio and the Effect of the Medium Dielectric Constant. J. Phys. Chem. B, 1999, 103, 3073-3077; Correction: J. Phys. Chem. B, 2005, 109, 10531-10532. (12) Sappey A.; Nogar N. in: Miller J. (Editor) Laser Ablation; Springer-Verlag: Berlin, Germany, 1994, p. 157. (13) Rubahn H.-G. Laser Applications in Surface Science and Technology; Wiley: Chichester, U.K., 1999. (14) Simakin A.V.; Voronov, V. V.; Kirichenko, N. A.; Shafeev, G. A. Nanoparticles Produced by Laser Ablation of Solids in Liquid Environment. Appl. Phys. A 2004, 79, 1127-1132. (15) Curcio, J. A.; Petty, C. C. The Near Infrared Absorption Spectrum of Liquid Water. J. Opt. Soc. Am. 1951, 41, 302-302. (16) Stone, J. Measurements of the Absorption of Light in Low-Loss Liquids. J. Opt. Soc. Am. 1972, 62, 327-333. (17) 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, 701-709. (18) Bäuerle D. Laser Processing and Chemistry, Springer-Verlag: Berlin, Germany, 2000, 3rd Edition. (19) Castelo, A.; Nieto D.; Bao C.; Flores-Arias M. T.; Pérez M. V.; Gómez-Reino C.; López-Gascón C.; de la Fuente G. F. Laser Backwriting Process On Glass Via Ablation Of Metal Targets. Optics Communications 2007, 273, 193-199.


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(20) Nedyalkov N. N.; Imamova S. E.; Atanasov P. A.; Toshkova R. A.; Gardeva E. G.; Yossifova L. S.; Alexandrov M. T.; Obara M. Interaction Of Gold Nanoparticles With Nanosecond Laser Pulses: Nanoparticle Heating. Appl. Surf. Sci. 2011, 257, 5456-5459. (21) Dahotre N. B.; Harimkar S. P. Laser Fabrication and Machining of Materials; Springer: New York, U.S.A., 2008. p 61. (22) Schaaf P. (Editor) Laser Processing of Materials: Fundamentals, Applications and Developments; Springer Series in Materials Science 139; Springer: Heidelberg, Germany, 2010. (23) Miller J. C.; Haglund R. F. Laser Ablation and Desorption, in Experimental Methods in the Physical Sciences, Celotta R. Lucatorto T. (Editors); Academic Press: San Diego, U.S.A., 1998. (24) Duley W. W. UV Lasers: Effects And Applications In Materials Science, Cambridge University Press: Cambridge, U.K., 1996. (25) Attia Y. A.; Buceta D.; Blanco-Varela C.; Mohamed M. B.; Barone G.; LópezQuintela M. A. Structure-Directing and High-Efficiency Photocatalytic Hydrogen Production by Ag Clusters. J. Amer. Chem. Soc. 2014, 136, 1182-1185.



Table of Contents (TOC) Image

1.8 a.r. = 2.5 a.r. = 2.9 a.r. = 3.8

1.6 1.4

Absorbance

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1.2 1.0 0.8 0.6 0.4 0.2 0.0 400

500

600

700

800

900

Wavelength / nm

a.r.=2.5

a.r.=2.9


a.r.=3.8

Transformation of Gold Nanorods in Liquid Media Induced by nIR

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