Growth orientation control of metal nanostructures using linearly polarized light irradiation

Thin Solid Films 621 (2017) 137–144

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Growth orientation control of metal nanostructures using linearly polarized light irradiation Masashi Watanabe a, Chuanjun Liu b, Kenshi Hayashi a,⁎ a b

Graduate School of Information Science and Electrical Engineering, Kyushu University, Fukuoka 8190395, Japan Division of Odor Sensor, Research and Development Center for Taste and Odor Sensing, Kyushu University, Fukuoka 8190395, Japan

a r t i c l e

i n f o

Article history: Received 5 February 2016 Received in revised form 14 November 2016 Accepted 24 November 2016 Available online 26 November 2016 Keywords: Metal nanoparticles Seed-mediated growth Surface Polarized light Nanostructure control

a b s t r a c t Controlled orientation of metal nanostructures on a solid substrate was realized by irradiating a pre-deposited nanoseed layer with linearly polarized light in a growth solution containing metal cations. The resulted nanostructures showed the different transmittance spectra for two orthogonal polarized lights, which indicated an anisotropic growth induced by polarized light. The investigation on the growth conditions demonstrated that the wavelength of the irradiated light and the existence of cetyl cetyltrimethylammonium bromide used as surfactant could affect the anisotropic degree of the oriented nanostructures. It was suggested that the polarized lights enhanced the anisotropic local electric field of Au seed nanoparticles, which resulted in the oriented growth of metal nanostructures during the reduction process in the solution. The approach reported in this work can be used in the device fabrication based on oriented metal nanostructures, such as photocatalysts or optical sensors. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Metal nanostructures show the unique optical/electrical property known as localized surface plasmon resonance (LSPR), where the incident light to the structures can be captured and the energy is concentrated around them. This light-harvesting nature of metal nanostructures has been investigated for various applications such as photocatalysts, including artificial photosynthesis application [1,2], sensitization of solar cells [3], bio/chemical sensing [4–6], and so on. Using anisotropic metal nanostructures like nanorods is one of the strategies to achieve larger field enhancement or to modulate the plasmon band [7,8]. Many synthetic methods have been investigated to obtain metal nanorods, such as the processes using hard or soft templates [9–11] and the photo-assisted processes [12,13]. These methods are generally based on the synthesis in suspensions, which can produce the nanorods suspended in water or organic solvent inexpensively. However, the additional processes of fixation to substrates [14] and simultaneous orientation control are required when they are used to fabricate solid devices. Some groups performed the growth of gold nanorods directly on substrate surfaces, by fixing seeds on surfaces at first followed by the growth process [15–17]. All of them employed cetyltrimethylammonium bromide (CTAB) as a soft template to achieve the anisotropic growth, but no orientation control was performed. The lithographic techniques like electron beam lithography [18] can be also employed to obtain the oriented nanostructures onto substrates. Although those lithographic ⁎ Corresponding author. E-mail address: [email protected] (K. Hayashi).

http://dx.doi.org/10.1016/j.tsf.2016.11.039 0040-6090/© 2016 Elsevier B.V. All rights reserved.

techniques are useful as direct ways to obtain the precise nanostructures on substrates, there are problems of expensiveness and time-consuming because of the intrinsic problem of scanning beam irradiation. In this study, we examined a direct way to obtain the anisotropic metal nanostructures with aligned orientation on solid substrates, by applying the seed-mediated growth processes on the seeds fixed on the substrates, with the help of the irradiation of linearly polarized light. Conventional growth processes of metal nanorods in suspension [10–13] and on substrates [15–17] utilized the anisotropic growth rate depends on crystal faces of seed particles and realized the anisotropic growth. However, the necessity of simultaneous orientation control of grown nanostructures on substrates still remains as next challenge because of the difficulty of fixing nanoparticles on solid substrates with specific direction. Our approach uses anisotropic electron distribution on the surface of seeds induced by polarized light irradiation, not crystal anisotropy, to realize such simultaneous oriented growth on a substrate. Such direction control of the polarization of metal nanostructures has been already harnessed to trap particle and control their position in previous research [19], known as a plasmonic tweezer, while the application for anisotropic growth has not performed yet. Although the photo-induced reduction of metal ions has been utilized to realize anisotropic metal growth [12,13], synthesis of metal nanoparticles [20], and controlling micron-scale metal structures [21], those methods only concerns the reaction rate and excludes the polarization directions from growth mechanisms. The concept of the anisotropic growth assisted with polarized light is shown in Fig. 1. Anisotropic distribution of electric near-field or hot electrons, on the surfaces of gold seeds induced by the irradiation of

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(a)

M + (metal cation) - e- e e e-

Linearly Polarized Light

AuNP seed Electric field

e- e- e- e-

ereducing agent

M+

(b) glass substrate seed layer

(c)

grown particle

polarizer

Fig. 1. Schematic illustrations of (a) the estimated mechanism of anisotropic growth by the irradiation of polarized light to (b) metallic seeds fixed on substrate. (c) is the concept of the actual growth experiments to obtain the orientation-aligned metal nanostructures using polarized light, where the direction of electric field, indicated with black arrows, decides the orientation of grown metal. Two orthogonal directions were defined as vertical and horizontal for identification.

polarized light are expected to be a driving force for anisotropic and oriented growth of those seeds, assuming that the growth due to the reduction of metal ions caused by the hot electrons is more active than other region (Fig. 1a,c). This scheme of polarized-light-assisted growth is an approach to realize the oriented growth control of metal nanostructures on the solid substrates directly. 2. Experimental Firstly soda-lime glasses were cut into 9 mm wide and 26 mm high and were cleaned with continuous sonication in deionized water, acetone, ethanol for 10 min for each cleaning solvent. Subsequently, a seed layer was prepared by deposition of gold onto those glass substrates with 5 nm of nominal thickness using sputtering with a quick coater (SC-701HMCII, SANYU ELECTRON), followed by annealing of them in 580 degrees Celcius for 10 h in an electric furnace (SMF-1, AS ONE), which resulted in the formation of stably-fixed [22,23] and moderately separated gold seeds as shown in Fig. 2 a. Then we examined the influence of the irradiation with polarized light to seeds during the growth, in the growth solution that contains metal ions (0.86 mM of

HAuCl4 or 2.4 mM of AgNO3), reducing agent (1.8 mM trisodium citrate), and surfactant (240 μM of CTAB). Although this condition can cause the spontaneous particle formation in room-temperature without light irradiation, the rate is much slower than the growing reaction at the seed surfaces with light irradiation. The concentrations of the growth solutions were chosen to observe significant changes in anisotropic growths. However, in the gold-containing solution, the concentration of gold ions had to be limited because high concentration caused the detachment of gold seeds from the substrates. The incident light was irradiated perpendicularly to the seed-fixed substrates with the power of 10 mW/cm2 for 1 h. Six band-pass filters (FB450-40, FB500-40, FB550-40, FB600-40, FB650-40 and FB700-40, whose center wavelengths were 450, 500, 550, 600, 650, 700 nm respectively and their FWHMs were 40 nm, Thorlabs Inc.) were used for wavelength selection of the light irradiations. All growth processes were performed in room temperature. We used a xenon light source (LAX-103, ASAHI SPECTRA) as the light source, and analysed the resulted nanostructures by using a field-emission scanning electron microscope (FE-SEM, SU8000, Hitachi High-Technologies) at the operating voltage of 1 kV, and a UV–vis spectrometer (UV-1800, SHIMADZU).

Transmittance (%)

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100 90

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Gold seed

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Growth with Au3+

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Au

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Wavelength (nm)

Fig. 2. (a) SEM image of gold seeds before the growth processes and transmittance spectra of (b) seed-fixed substrate and (c) resulted Au nanostructures and (d) Ag nanostructures after the growth processes with irradiation of non-polarized light (wavelength: 600 nm). The red lines and the blue lines indicate the transmittance of vertically and horizontally polarized light, respectively, although these lines are almost overlapped in these spectra. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

3. Results and discussion The typical transmittance spectra of the substrates before and after growth processes with the irradiation of non-polarized were shown in Fig. 2b–d. Fig. 2b shows the transmittance spectra of seed-fixed substrate before the growth, while Fig. 2c,d are the spectra of nanostructures after the growth with Au cations and Ag cations, respectively. These spectra were measured by also using two orthogonal polarized light with same direction as those of growth processes, namely vertical and horizontal, to analyse the structural orientation. Although the transmittances of grown seeds were significantly decreased after the growth with the irradiation of non-polarized light, no anisotropic transmission properties were observed. A notable point was the significantly decreased transmittance of the sample grown with silver ions at longer wavelength from 700 to 1000 nm (Fig. 2d), that was not consistent with the plasmonic absorption peak of individual silver nanoparticles. We presumed plasmonic coupling, i.e., the new absorption band was derived from the inter-particle electric interactions between adjacent grown particles, that could be especially strong if the gaps between particles got close to a few nanometers [23]. The transmittance spectra of the substrates after growth processes with the irradiation of polarized were shown in Fig. 3. The polarization directions of growth-assisting light were defined as vertical (Fig. 3a,b) and horizontal (Fig. 3c,d). In the spectra of grown particles, new absorption bands were appeared around 700 nm that indicate the changes caused by growth processes. Furthermore, the absorptions of polarized light whose polarization directions were same as those of the light irradiated at growth processes were larger than those of the other polarization directions, implying that growth as elongating along the polarization directions occurred.

The seeds grown with Ag cation and assist of polarized light showed especially significant anisotropy (Fig. 3b,d). We can presume that the resulted structures are anisotropic and oriented to aligned direction, if this spectral dissociation become larger. In order to evaluate and compare the anisotropy of grown particles for each growth condition from the obtained transmittance spectra, we calculated the deference between absorbances for two orthogonal polarized lights as: ΔA ¼ AV ðλ0 Þ−AH ðλ0 Þ ¼ log10

T H ðλ0 Þ T V ðλ0 Þ

ð1Þ

where AV(λ) and AH(λ) are the absorbances for vertically and horizontally polarized light with the wavelength of λ, TV(λ) and TH(λ) are the transmittances as with the absorbances, and λ0 is the wavelength that gives a maximum deference between AV(λ) and AH(λ), within the range from 600 nm to 1000 nm. ΔA expresses the degree of difference between two spectral curves for each polarization direction, calculated in accordance with the Lambert-Beer law. Fig. 4 shows ΔA of the Au/Ag grown products for the polarized and non-polarized light irradiation of various wavelengths from 450 nm to 700 nm. The peaks of ΔA were observed around 600 nm for the products grown with Ag cation and polarized light irradiation, while ΔA were much smaller for ones grown with nonpolarized light. The Au particles grown with polarized light also showed larger ΔA than those grown with non-polarized light, although the differences were not so significant compared to the case of Ag growth. It can be considered that the anisotropic polarization of free electrons in gold seeds induced by the irradiation of polarized light makes a dominant contribution to the anisotropic growth because the growth experiments with Ag

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Growth with Ag+

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Vertical polarization

Growth with Au3+ 100

(c)

90

80

80

70

70

Au

60

(d) Ag

60 50

50 40 400

600

800

Wavelength (nm)

40 400 1000

600

Wavelength (nm)

Fig. 3. Transmittance spectra of (a,c) grown Au nanostructures and (b,d) Ag nanostructures after the growth processes with irradiation of polarized light (wavelength: 600 nm). The result of the growth with (a,b) vertically polarized light irradiation and (c,d) horizontally polarized light are shown. The growth conditions are shown in insetted schemes. The red lines and the blue lines indicate the transmittance of vertically and horizontally polarized light, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

cations showed a significant tendency, where the peak of ΔA appears around 600 nm that can be absorbed due to the LSPR nature of seeds than the light around 450 nm or 700 nm. LSPR-derived light absorption excites more hot electrons having reducing ability of metal cations. The similar phenomena were observed in a report of other group [24]. Although the atomic diffusion, not the growth, was occurred in this report, the anisotropic distribution of the occurrence of photochemical reaction was observed depending on the polarization direction of the free electrons in metal particles induced by the irradiation of polarized light. The gap between the peak wavelength of ΔA at longer than 600 nm and

0.06 0.05

ΔA

0.04

Ag

0.03 0.02

Au Ag Au

0.01 0 -0.01 450

500

550

600

650

700

Wavelength (nm) Fig. 4. ΔA of the grown products for the polarized light (solid lines) and non-polarized light (dotted lines) irradiation of various wavelengths, from 450 nm to 700 nm. The blue line and orange line indicate the anisotropy indices of the products of the growth process with Ag+ and Au3+, respectively. The bars represent the standard errors. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

that of the light absorption of seeds around 530 nm may imply that the longer wavelength is favorable for oriented anisotropic growth since it causes slower growth [13]. The heating effect due to a non-radiative relaxation of excited localized plasmon can be a cause of low anisotropies at shorter-wavelength irradiations [25]. The low ΔA at 700 nm can be explained by the existence of the trade-off: the longer the wavelength of incident light becomes, the less the gold seeds absorb the light. The reason of high ΔA at 450 nm is unclear, but the relation of the light absorption due to the interband transition of free electrons in the seeds can be considered. The SEM images of the nanostructures grown with Au and Ag cations under the irradiation of vertically polarized light with various wavelengths are shown in Figs. 5 and 6, respectively. The shape transformation of nanoparticles compared with seeds before growth shown in Fig. 2a can be observed in all of these images. In Fig. 5, the particles fused together with adjacent ones were appeared, particularly in seeds grown with polarized light of shorter wavelength. Possible phenomena to cause these fused structures at short wavelength are a direct photodegradation of Au complexes and a plasmonic heating. Fig. 5 f shows a little growth compared to Fig. 5a–c due to the small light absorbance of seeds at the wavelength of 700 nm. As expected from the result in Fig. 4, the structures grown with Au cations showed not so significant anisotropy and orientation to the vertical polarization direction. The particles grown with Ag cations showed more significant growth compared to those grown with Au cations, in Fig. 6. Especially, Fig. 6c,d, the samples grown with 550 and 600 nm irradiation, showed notably anisotropic structures which were the chain-like ones oriented to vertical direction, that was the polarization direction of the irradiation at growth processes. These results are corresponding to ΔA shown in Fig. 4, where the nanoparticles grown with Ag cations under the irradiation with wavelengths of 550 and 600 nm indicates high anisotropies. Box plots of the maximum Feret's diameters and the logarithms of the ratios of vertical (DV) and horizontal (DH) Feret's

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(b)

200 nm

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200 nm

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200 nm

Fig. 5. SEM images of nanoparticles after growth processes with Au cations, and under irradiation of vertically polarized light of the various wavelengths (a–f: 450, 500, 550, 600, 650, 700 nm, respectively).

diameter of seeds and particles grown with vertically polarized light obtained from more than 300 particles picked up from SEM images for each condition are shown in Fig. 7a,b. The former represents the distribution of particle sizes, and the latter represents the degree of anisotropies of particles along vertical direction. The wavelengths shown in Fig. 7 indicate those of the polarized light irradiation. From Fig. 7a, particles grown with polarized light of shorter wavelengths showed larger particle sizes except for Ag growth at 450 nm irradiation. This tendency would reflect the direct photodegradation or heating of seeds caused by irradiation of light with shorter wavelengths as mentioned above. Exceptionally small

(a)

maximum Feret's diameter of particles grown with Ag cation at 450 nm irradiation could be explained by the highly isotropic growth causing large spherical particles rather than chain-like structures. In Fig. 7b, the highest anisotropy of particles grown at 600 nm irradiation of vertically polarized light in the case of Ag growth was shown, which was the same tendency as the result shown in Fig. 4. The positive bias of log10(DV/DH) appeared in Fig. 7b, including seeds without growth, whose reason was discussed below. The SEM images of the typical anisotropically-grown particles and the distribution of the orientation angles of them are shown in Figs. 8 and 9, which were grown with Ag cations,

(b)

200 nm

(d)

(c)

200 nm

(e)

200 nm

200 nm

(f)

200 nm

200 nm

Fig. 6. SEM images of nanoparticles after growth processes with Ag cations, and under irradiation of vertically polarized light of the various wavelengths (a–f: 450, 500, 550, 600, 650, 700 nm, respectively).

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Maximum Feret’s Diameter (nm)

(a)

seed

growth with Ag+

growth with Au3+

450 500 550 600 650 700 450 500 550 600 650 700

Wavelength (nm)

Log10(DV /DH)

(b)

seed

growth with Ag+

growth with Au3+

450 500 550 600 650 700 450 500 550 600 650 700

Wavelength (nm) Fig. 7. Box plots of (a) maximum Feret's diameters and (b) logarithms of the ratios of vertical (DV) and horizontal (DH) Feret's diameter of seeds and grown particles with the irradiation of vertically polarized light. × represents the average value and ○ represents outlier value.

(a)

under the irradiation of vertically (Figs. 8b, 9a) and horizontally (Figs. 8c, 9b) polarized light of 600 nm wavelength. A SEM image of the particles grown with non-polarized light (Fig. 8a) was also shown for comparison. It can be seen that the particles oriented along the vertical axis (in Fig. 8b) and horizontal axis (in Fig. 8c), that are the directions of polarization, are observed more than the particles oriented along the other axes. In both of Fig. 8b,c the grown particles with chain-like shapes were observed, which indicated that the reduction of metal cations took place at the gaps of gold seeds. At those gaps, the near fields were induced especially strongly, that therefore resulted in the oriented growth in a manner of bridging between seeds like Fig. 1c. The angle distributions in Fig. 9 were calculated to evaluate the particles' orientation quantitatively. Although the results can be affected by imaging noises of SEM and excessive fusion of grown particles, which makes such evaluation difficult, we can discriminate their differences from the calculated distributions. The anisotropic noises of SEM images because of the position of electron detectors in SEM might make the images anisotropic, which could result in the difference of products' anisotropies depending on the directions of samples placed in SEM, as shown in Figs. 8 and 9. As another reason to make the anisotropy of horizontally grown particles lower than the vertical ones, we consider the effect of the gravity-directed convections in growth solutions induced by local heating due to the light irradiation. The shape of cell containing the solution was anisotropic that also could cause the anisotropic flow of solution, resulting in the morphological differences in the products. The effect of the amount of CTAB was also investigated. Fig. 10a,b shows ΔA of the grown products, applied various concentrations of CTAB in the growth solutions. Existence of the high concentration of CTAB led to highly anisotropic growth along the polarization direction, which is especially clear on the growth with Au cations (Fig. 10a). One possible way of the contribution of CTAB to the anisotropic growth is cationic CTAB-micelle formation and its binding with gold ions, considering in accordance with Ref. [10] where the electric-field-directed growth of gold nanorods have been proposed. Those gold ions bound to CTAB micelles can be reduced when they get close to gold seeds, which depends on the electric double layer interaction between micelles and gold seeds coated with CTAB. The electric double layer of a gold seed can be affected by light irradiation, which can become anisotropic by irradiating with the polarized light. The anisotropic growth without the addition of CTAB was also observed. Those growths can be directly caused by the anisotropic distribution of energetic electrons owing to the plasmonic resonance induced by polarized light. Those energetic electrons were considered to be consumed in the reduction of metal cations near them like the illustration in Fig. 1a, that resulted in

(b)

200 nm

(c)

200 nm

200 nm

Fig. 8. SEM images of nanoparticles after growth processes with Ag cations, under irradiation of (a) non-polarized and polarized light with two orthogonal polarization directions of (b) vertical and (c) horizontal. The wavelength of the irradiation was 600 nm.

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Count

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Orientation Angle (degree)

Orientation Angle (degree)

Fig. 9. The distributions of the orientation angles of the particles grown with Ag cations under the irradiation of (a) vertically and (b) horizontally polarized light of 600 nm wavelength, counted from the SEM images of those samples (ones of images are shown in Fig. 8b,c).

the oriented growth. Furthermore, the growth without CTAB and with Au cation could cause the detachment of seeds from substrates and resulted in numbers of cavities on glass surface as shown in Fig. 10c, which was not observed in the case of Ag growth (Fig. 10d). This seed detachment could be caused by the excessive rate of growth reactions with Au cations, which could be prevented by adding CTAB to growth solutions. More investigations are required to clarify the detailed mechanisms of the anisotropic growth induced by polarized light irradiation and how the additives contribute to the anisotropy.

4. Conclusion In summary, the anisotropic growth of meatal nanostructures with controlled orientation on glass substrates was performed by irradiating the seeds with linearly polarized light, in the growth solutions containing metal cations, reducing agent and CTAB. The existence of CTAB worked as the assisting materials for the anisotropic growth and the polarized light induced the orientation of the nanostructures. Although the conditions of seed formation and growth process are not optimized

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Concentration of CTAB (µM)

Concentration of CTAB (µM)

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200 nm

Fig. 10. ΔA of the grown products for various CTAB concentrations from 0 to 240 μM, with the growth solution containing (a) Au3+ and (b) Ag+, which were grown with the polarized light irradiation of 600 nm wavelengths. The bars represent the standard errors. (c) and (d) are the SEM images of the typical morphologies of grown structures without CTAB, with Au and Ag cations respectively.

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enough to practical applications yet, this growth process can be a useful tool in device fabrication in the future, because of its ability of on-site building in nanoscale, which enables light-addressed and polarizationdirected arrangement of nanomaterials as building blocks. To make the growth more anisotropic and highly oriented, the density and the size of gold seeds might be important factor for optimization, because the anisotropic growth has taken place between adjacent seed particles and has formed chain-like structures along the polarization direction. Acknowledgement This work was partly supported by Grant-in-Aid for JSPS Fellows (JP15J04707). References [1] D. Astruc, F. Lu, J.R. Aranzaes, Nanoparticles as recyclable catalysts: the frontier between homogeneous and heterogeneous catalysis, Angew. Chem. Int. Ed. Eng. 44 (2005) 7852–7872. [2] S. Mubeen, J. Lee, N. Singh, S. Krämer, G.D. Stucky, M. Moskovits, An autonomous photosynthetic device in which all charge carriers derive from surface plasmons, Nat. Nanotechnol. 8 (2013) 247–251. [3] S. Mubeen, G. Hernandez-Sosa, D. Moses, J. Lee, M. Moskovits, Plasmonic photosensitization of a wide band gap semiconductor: converting plasmons to charge carriers, Nano Lett. 11 (2011) 5548–5552. [4] J.N. Anker, W.P. Hall, O. Lyandres, N.C. Shah, J. Zhao, R.P. Van Duyne, Biosensing with plasmonic nanosensors, Nat. Mater. 7 (2008) 442–453. [5] B. Chen, C. Liu, M. Ota, K. Hayashi, Terpene detection based on localized surface plasma resonance of Thiolate-modified Au nanoparticles, IEEE Sensors J. 13 (2013) 1307–1314. [6] K. Kneipp, Y. Wang, H. Kneipp, L.T. Perelman, I. Itzkan, R.R. Dasari, M.S. Feld, Single molecule detection using surface-enhanced Raman scattering (SERS), Phys. Rev. Lett. 78 (1997) 1667–1670. [7] J. Aizpurua, G.W. Bryant, L.J. Richter, F.J. García de Abajo, B.K. Kelley, T. Mallouk, Optical properties of coupled metallic nanorods for field-enhanced spectroscopy, Phys. Rev. B 71 (2005) 235420. [8] K.-S. Lee, M.A. El-Sayed, Gold and silver nanoparticles in sensing and imaging: sensitivity of plasmon response to size, shape, and metal composition, J. Phys. Chem. B 110 (2006) 19220–19225.

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