We demonstrate fabrication of Au nanorod aggregates microstructures by means of a femtosecond near-infrared laser. The laser light was tightly focused into colloidal Au nanorods dispersed in photopolymerizable metyl-methacrylate (MMA) compound to induce two-photon polymerization (TPP). TPP of MMA glued the nanorods together to form solid microstrucures of aggregates. The laser light excited a local surface plasmon, resulting in confinement of TPP in the vicinity of nanorods. Concurrenly occurring optical accumulation of nanorods created a unique mechanism for the formation of nanorod aggregates into desired microstructures. This technique would be a clue for a novel micro/nanofabrication method for plasmonic materials and devices.
© 2011 OSA
Nanoparticles of noble metal exhibit strong resonant scattering of light through plasmonic oscillation of free electrons. The optical feature of metal nanoparticles, so called local surface plasmon resonances (LSPRs), shows potential as a fundamental platform for novel optical/photonic nanotechnologies. As the free electrons are confined in the nanoscale volume of metal particles, the oscillation of these electrons creates discrete resonant modes, which are strictly defined by the geometry, i.e. the size, the shape, and the orientation of the particles, as well as various ambient parameters causing refractive index change such as temperature, pH, etc . The excitation of LSPRs results in the creation of a strong local electromagnetic field surrounding the nanoparticle, reflecting the spatial distribution of the oscillating electrons. Interaction of adjacent multiple nanoparticles creates conjugation modes of LSPRs. LSPR coupling induces extremely strong electromagnetic field at small gaps between particles, and also inter-particle propagation of electromagnetic waves, allowing us to manipulate and utilize photons in the nanoscale. Recently, chemical synthesis methods provide us a variety of noble metal nanoparticles of well-controlled size and shape, and make it possible to understand these unique properties of LSPRs systematically. Many novel applications based on the plasmonic features in assembled metal nanoparticles have been proposed including surface-enhanced Raman scattering , chemical and bio-sensors [1,2], left-handed materials , optical waveguides  and optical nano-imaging devices .
Currently, for the achievement of such plasmonic technologies, one of the urgent demands is to develop novel micro/nano fabrication methods to provide arbitrary complex micro/nano structures in which metal nanoparticles are assembled in desired orientation and arrangement. So far, lithographic techniques are widely used to fabricate precisely organized metal nanostructures on substrates. Laser ejection of molten metal nanodroplets is a unique method, in which laser light plays roles of creation and deposition of metal nanoparticles simultaneously in order to fabricate 2D and 3D structures of metal nanoparticle arrangements . Alternatively, a variety of mass-produced crystalline metal nanoparticles with uniform shape and size are available, and self-organization is an attractive method to produce well-ordered assemblies of the mass-produced nanoparticles . However, the structure of the array is limited to a few types according to the intrinsic characteristics of self-organization. The assembly of nanoparticles in three-dimensional (3D) arbitrary patterns is therefore a very challenging task. The use of optical manipulation technique has been proposed for the precise assembly of individual colloidal nanoparticles on the surface of substrates [8–10]. Recently, DNA origami templates were used for producing well-ordered metal nanoparticles in large dimension [11,12]. The sequences of the template DNA allow the design of particle arrangement patterns. In this paper, we investigated the use of femtosecond laser micro/nano fabrication technique to assemble Au nanorods into micro/nano structures. Two-photon polymerization (TPP) lithography has been established as a powerful technique of fabricating three-dimensional (3D) micro/nano structures. A tightly focused spot of a femtosecond laser light locally excites the chemical TPP reaction in photopolymer, and the trajectory of the laser spot creates arbitrary 3D polymerized structures with high spatial resolution [13,14]. Our approach in this research is to disperse Au nanorods densely into photopolymerizable resin and perform TPP lithography onto the colloidal Au nanorods. There are already some reports of the application of TPP lithography for fabricating metal nanoparticles/polymer composite microstructures [15,16]. The aim of our research is, rather than creating polymer structures with metal nanoparticles dispersed in, to build an assembly of Au nanoparticles into desired geometries. To this end, we aim to apply photopolymerization as a glue to stick individually dispersed Au nanorods one another into micro/nano structures. We prepared photopolymerizable Au nanorods/monomer compound. We investigated the effect of the existence of Au nanorods on the progress of TPP of monomers and demonstrated fabrication of Au nanorods aggregates. We found a novel mechanism of microstructure formation based on a co-operation between LSPR-induced TPP and laser-induced attraction of Au nanorods.
2. Preparation of gold nanorods/MMA compound
A schematic of the procedure for preparation of photopolymerizable Au nanorods/MMA compound is illustrated in Fig. 1 . First, Au nanorods were prepared by the seed-mediated method [17,18]. 7.5 mL of 0.1 M hexadecyltrimethylammonium bromide (CTAB) solution was mixed with 0.25 mL of 0.01 M HAuCl4 aqueous solution. 0.60 mL of ice-cold 0.01 M NaBH4 aqueous solution was quickly added, resulting in the formation of colloidal Au seed clusters with a light brown colored solution. The Au seed solution was vigorously stirred for 2 min and then left for 2 hours in water bath at 30 þC. The growth solution of Au nanorods consists of 40 mL of 0.1 M CTAB aqueous solution, 2 mL of 0.01 M HAuCl4 aqueous solution, 0.30 mL of 0.01 M AgNO3 aqueous solution, 0.80 mL of 0.5 M H2SO4, and 0.32 mL of 0.1 M L-ascorbic acid aqueous solution. After the color of the growth solution changed from orange to colorless with mild mixing, 0.096 mL of the Au seed solution was added to the growth solution and left for 2 hours in a water bath at 30 þC. The Au seed clusters were grown into a rod shape for a solution with brownish red color. The obtained Au nanorods have a uniform rod shape with an average length of 66 ± 2 nm and a diameter of 20 ± 1 nm, confirmed by scanning electron microscope (SEM) observation shown in Fig. 2(a) . The obtained Au nanorods were stable in water as hydrophilic micelles wrapped by CTAB. In order to transfer the Au nanorods from aqueous solution into MMA, we performed surface modification of the CTAB-wrapped Au nanorods by thiol-terminated poly(ethylene glycol) (mPEG-SH) . 40 mL of as-prepared Au nanorods dispersion was centrifuged at 8000 rpm for 8 min and the precipitation was re-dispersed in 3.0 mL of ultra-pure water. 3.0 mL of 0.002 M mPEG-SH aqueous solution was added to the dispersion and stirred for 2 hours. The mixture was centrifuged at 8000 rpm for 8 min. The precipitation was dispersed in ethanol and centrifuged again, in order to remove CTAB, water, as well as excess mPEG-SH. After the centrifugation, ethanol was evaporated completely from the precipitation, and the remnant mPEG-SH wrapped Au nanorods were dispersed in 0.01 mL of MMA monomer by mild sonication. Finally, 0.134 mL of photo-polymerizable resin which contained crosslinker (DPE-6A, Kyoeisha Chemical Co., Ltd.), photo-initiator (Benzil), and photo-sensitizer (2-benzyl-2-(dimethylamino)-4’-morpholinobutyrophenon, Aldrich), was mixed. The final concentrations of MMA, crosslinker, photo-initiator, and photo-sensitizer were 64.8 wt%, 31.3 wt%, 0.88 wt%, and 0.88 wt%, respectively. This recipe of MMA, crosslinker, initiator, and photo-sensitizer has been used in our previous research of TPP lithography by using a Ti:sapphire laser at the wavelength of 780 nm . A rough estimation of the density of Au nanorods in the obtained compound is about 15 - 20 nanorods per 1 µm3, corresponding to 1.9 – 2.1 wt% of Au in the compound.
Figure 2(b) shows a series of visible (Vis) – near-infrared (NIR) spectra of mPEG-SH wrapped Au nanorods dispersed in water, ethanol, and MMA obtained during the compound preparation process. In all the spectra, prominent peaks are seen at around 530 nm and 800 nm, representing excitation of transverse mode and longitudinal mode of LSPRs. It is well known that the longitudinal mode appearing at around 800 nm is particularly sensitive to the change of size, shape, the refractive index of surrounding media, as well as the aggregation of nanorods, all of which cause spectral shifts and spectral shape changes [20,21]. In Fig. 2(b), the longitudinal mode did not show a significant variation except for a small peak shift caused by the refractive index difference between the media. This indicates that the nanorods kept their original shape through the surface modification and the following dispersion process into MMA, and the processes did not cause an aggregation of Au nanorods. The spectra also show that the resonant wavelength of the longitudinal LSPR well matches the excitation wavelength for TPP of MMA, indicating the possibility of an efficient initiation of TPP by the electromagnetic evanescent field excited by LSPR.
3. Characteristics of two-photon polymerization under the existence of gold nanorods
Before the demonstration of TPP lithography using the Au nanorods/MMA compound, we first confirmed the efficiency of TPP of MMA and the stability of the Au nanorods under laser irradiation. It is known that irradiation of intense laser light can cause permanent distortion of Au nanorods by photothermal effect [22–24]. In order to perform TPP lithography, it is required that TPP can be excited with a light intensity much lower than the threshold of this photothermal distortion. For this purpose, we performed a following preliminary experiment as shown in Fig. 3(a) . We dispersed CTAB wrapped Au nanorods on a glass substrate and poured MMA (mixed with initiator and photo-sensitizer) over the Au nanorods. A Ti:sapphire laser (Tsunami, Spectra-Physics, Newport Corp.) emitting at 780 nm, with a pulse width and a repetition rate of 100 fs and 82 MHz, respectively, was focused by an oil-immersion objective lens with a numerical aperture (NA) of 1.4, and irradiated onto the substrate. The time of the laser exposure was 60 ms. After laser irradiation, the glass substrate was immersed in ethanol so that non-polymerized MMA was removed.
Figure 3(i) shows SEM image of the Au nanorods exposed by the laser light. It can be seen that the nanorods were covered by polymerized MMA surrounding their surfaces. The PMMA layer sometimes showed slight larger thickness in the direction perpendicular to the nanorods, but in most cases the thickness of PMMA layer was almost uniform. Figure 3(b) shows the relation of the thickness of the PMMA layer and the average laser intensity. For each case of different laser intensity, we measured the thickness of the formed PMMA layer in longitudinal and perpendicular directions of 10 different particles from high magnification SEM images, and plotted the average of these values. When the average laser intensity was below 1.3 GW/cm2, the PMMA layer was not observed on the Au nanorods. As the laser intensity increased from 1.3 GW/cm2, PMMA layer started to appear on the surfaces of Au nanorods as shown in Fig. 3(i). For reference, the threshold laser intensity for ordinary TPP of our compound without Au nanorods was measured to be 5.3 GW/cm2 as shown in Fig. 3(ii). Namely, the PMMA layer on Au nanorods was formed with the laser intensity about 4 times lower than the threshold for TPP of MMA compound without Au nanorods. This result indicates the contribution of LSPRs to the excitation of TPP. As it is mentioned previously, the wavelength of the laser light (780 nm) corresponds to both the excitation of TPP and the excitation of the longitudinal LSPR in Au nanorods. It is known that LSPRs create a strong local electromagnetic field on the metal structure, and that the intensity of the local field often reaches several orders of magnitude higher than the initial excitation light field. Even with the laser intensity lower than the threshold for TPP, the enhanced electromagnetic field can exceed the threshold intensity locally, and excite TPP only at the surface of Au nanorods. Similar experimental results of LSPR induced TPP were also reported by several groups [25–27]. In Fig. 3(b), the thickness of the PMMA layer was increased with a nonlinear manner, until the laser intensity reaches to the threshold for the TPP of original MMA compound. It was observed that, in the case of conventional TPP with radical polymerization, the size of the polymerized voxel shows nonlinear relation on the laser intensity [28,29]. The intensity dependence of the thickness of the PMMA layer in Fig. 3(b) looks similar to the case of the conventional TPP. If we assume that the intensity of LSPR-induced local electromagnetic field is in proportion to the intensity of the incident light, Fig. 3(b) may also be a clue that the polymerization was directly excited by LSPR-induced optical field through two-photon absorption. Another important issue confirmed by this experiment is that we can excite TPP without causing any damage to Au nanorods when using proper laser intensity and exposure time. As shown in Fig. 3(i), Au nanorods maintained their original shape without any deformation under laser irradiation. On the contrary, when the laser intensity was higher than 5.3 GW/cm2, which is the threshold intensity of TPP for MMA compound without nanorods, MMA started to photo-polymerize not only at the surface of Au nanorods but the whole irradiated volume. At almost the same time, Au nanorods induced bubbling of MMA caused by a sudden temperature increase around the irradiated nanorods as shown in Fig. 3(iii). As a result, polymerized structures were often distorted. Consequently, in the Au nanorods/MMA compound with femtosecond Ti:sapphire laser irradiation, stable TPP of MMA can be achieved only at the surface of Au nanorods, while avoiding such disruptive thermal effects.
4. Fabrication of gold nanorods assembly microstructures
The demonstration of TPP lithography using the Au nanorods/MMA compound was performed by the same optical setup used in the previous experiment. The Au nanorods/MMA compound was cast on a glass substrate, and the laser light was focused on the boundary of the glass surface and the compound. After laser irradiation, the glass substrate was immersed in ethanol to remove non-polymerized MMA and nanorods, followed by drying of the substrate. The laser intensity was 1.5 GW/cm2, which is in the window of LSPR-induced TPP and lower than the threshold intensity for TPP without nanorods as shown in Fig. 3(b). At first, we set the exposure time at 60 ms, same as in the previous experiment. In this case, we did not find any solid structures, although we sometimes found a few PMMA-wrapped Au nanorods (similar to the case in Fig. 3(a)) remaining on the substrate. This is reasonable because the nanorods were individually isolated in the compound and photopolymerization of MMA did not occur, except for the surface of nanorods at this laser intensity. However, when we increased the exposure time, we started to observe a solid spot forming at the position of the laser spot, as shown in Fig. 4(a) . In the case of the exposure time at 10 s, the aggregation grew in a diameter of about 1 µm (Fig. 4(b)). In high magnification SEM images (Fig. 4(c)) it is observed that Au nanorods were individually wrapped with PMMA layers. The shape of the spot is not smooth like the profile of the laser spot, but rather follows a profile of the distribution of Au nanorods. This means that the nanorods were originally mono-dispersed in the compound, and the aggregation of nanorods was formed during the laser irradiation. It should be noted that the density of Au nanorods in the solid spot is much higher than that of the original Au nanorod/MMA compound. The original concentration of Au nanorods in the compound is about 20 nanorods per 1 µm3. On the other hand, from SEM observation, about 5,000 ~10,000 nanorods were aggregated in a volume of 1 µm3, indicating 2 ~3 orders of magnitude higher concentration than that in the pristine compound. This result indicates that the formation of a TPP-induced Au nanorods/PMMA composite structure caused by the combination of laser-induced attraction of Au nanorods with concurrent LSPR-induced TPP of MMA.
By employing this unique mechanism, we demonstrated the fabrication of arbitrary microstructures of Au nanorods aggregates. Figure 5(a) shows a series of SEM images of the letters “GOLD” structures as an example. We performed plasma-enhanced chemical vapor deposition of osmium onto the sample to obtain clear contrast in SEM observation. The letters were fabricated by a single-stroke scan of the laser spot by a step of 50 nm with the exposure time of 3 s at each position. The line width of the structure was 1.2 µm and the total size of each letter was about 10 µm. High magnification images as shown in Figs. 5(b, c) show that the solid line consists of close-packed Au nanorods wrapped by thin PMMA. We evaluated the spatial resolution at different exposure time with fixed laser intensity. Figure 6 shows a plot of the line widths and a series of SEM images of the fabricated line structures at different exposure time. Similar to the typical TPP lithography, as reducing the exposure time we observed an improvement of the spatial resolution. However, from a certain level, further reduction of the exposure time started to cause inhomogeneous line width with forming discrete aggregation. The modulation of the line width may be caused by the depletion and consequent temporal change of the local density of nanorods surrounding the laser spot. In the current experiment, we achieved the smallest line width of about 1.0 µm.
Although the results shown in Fig. 5 and Fig. 6 were obtained with the laser intensity at 1.6 GW/cm2, the available laser intensity for the fabrication is not restricted to this value. The available intensity range was almost the same as the window for LSPR-induced TPP shown in Fig. 3(b). However, higher intensity resulted in a boost of the accumulation speed, which sometimes caused instability in the fabrication line width. Not only to obtain higher spatial resolution but also to avoid the above-mentioned instability, we needed to control the exposure time. For example, with the laser intensity at 1.9 GW/cm2 and 3.2 GW/cm2, stable fabrication was achieved with the exposure time of 2 s and 0.5 s, respectively.
On the other hand, the use of femtosecond laser was necessary to achieve the fabrication. We performed the same experiment with using CW laser light at the same wavelength. The accumulation of nanorods into the laser spot was observed, however, TPP of MMA was not observed on the surface of Au nanorods in the case of CW light.
Since TPP can proceed only in a few tens of nm-thick layer from the surface of Au nanorods with safe laser intensity as mentioned previously, the common mechanism of TPP lithography is not possible to create photopolymerized microstructures. In order to form such aggregations of PMMA-wrapped Au nanorods observed in Fig. 4, the nanorods must be concentrated into the focus spot and be connected one another during and/or after TPP of MMA. In practice, we observed continuous flickering of scattered light around the focus spot through our charge coupled device (CCD) camera equipped on the optical microscope, till a concrete aggregation of PMMA-wrapped Au nanorods was formed. The flickering is due to the scattering of laser light by the nanorods. We always observed a collective flow of the flickering nanorods toward the center of laser spot. One of the possible mechanisms of the creation of such flow may be a contribution of the optical gradient force. Our hypothetical mechanism of the formation of Au nanorods aggregation is illustrated in Fig. 4(d). Optical gradient force is the Lorentz force occurring on small particles in the intensity gradient of an optical field. The force is in proportion to the intensity gradient of light and to the optical polarizability of the particles . So far, a variety of nanoparticles and nanorods were trapped by attractive gradient force of focused laser light [31–35], and the optical trap was utilized for manipulation and deposition of individual particles [7–9]. Especially, in the case of Au nanorods, if the wavelength of the trapping laser is chosen to be near the resonant wavelength of the LSPR, the attractive gradient force could be enhanced by the increase of the polarizability of Au nanorods . It is also reasonable to propose that for our case the Au nanorods feel an attractive force toward the center of the laser focus during laser irradiation. Concurrently, the laser light excites a local electromagnetic field around the Au nanorods through LSPR, which initiates TPP of MMA. Thus, during the attraction of the particles into the focus spot, the PMMA layer on the Au nanorods grows gradually. Since PMMA has a refractive index slightly higher than that of MMA, the PMMA layer could also enhance the attraction force further. However, the optical trap of nanorods is still not so rigid to stop the random Brownian motion of nanorods in the focus spot. If one nanorod happens to move near another nanorod during its random walk, the two nanorods could create a gap mode LSPR with an optical hot spot at the gap between the two rods [26,36,37]. The hot spot accelerates TPP further at the gap and make a joint between the two rods. This effect was demonstrated with using lithographically fabricated metal islands with nanogap in between [38–40]. Such a process could occur with many nanorods continuously. Finally, many nanorods are stacked against one another, resulting in the formation of an aggregation of PMMA-wrapped Au nanorods.
Figure 7 shows the dependence of the structures fabricated with several objective lenses with different NA. We performed the fabrication using 0.4, 0.8, as well as 1.4 NA with the same laser intensity. In the case of 0.8 NA (in Fig. 7(a)), similar structures were fabricated, but the line width became wider and inhomogeneous compared with the case of 1.4 NA (in Fig. 7(c)). In high magnification image, it is seen that the distance (or the packing density) of PMMA-wrapped nanorods in the aggregation became larger (lower). Sometimes air gaps were observed in the formed structures. In the case of 0.4 NA (in Fig. 7(b)), we could not perform stable fabrication any more. On the trace of the laser spot we observed nanorods wrapped with PMMA on the substrate. However, the nanorods were not continuously connected, and the packing density of the nanorods was further decreased from the result of 0.8 NA. The decrease of the packing density of nanorods and concurrently occurring increase and instability of the fabrication line width can be interpreted as follows. The decrease of NA number causes the decrease of the intensity and the gradient of optical field, which gives rise to the decrease of the trapping force. This interpretation also supports our hypothesis that the driving force of the accumulation of nanorods is the optical gradient force.
As shown in Fig. 4(c), nanorods are randomly oriented in the aggregation, which could be due to the Brownian motion of nanorods in the focus spot. It is known that the gap mode of multiple metal particles varies by the relative positions of particles. Then, there should be an optimal configuration of particles to efficiently form a stronger gap mode. Optical gradient forces usually show a tendency to align nanorods in the focus spot [30–33]. From these facts, it can be expected that, by precise tuning of experimental parameters such as the wavelength, polarization, and/or the intensity of the laser light, it may be possible to find a condition where Au nanorods form a regular arrangement in the optical field.
We demonstrated a novel method to fabricate Au nanorod assembly microstructures. The method is based on a co-operation of concurrently occuring two optical processes; laser-induced accumulation of Au nanorods, and sticking of concentrated nanorods by LSPR-induced TPP. In order to generate the two processes simultaneously, femtosecond laser light was tightly focused into colloidal Au nanorods dispersed in MMA-based photopolymerizable compound. By scanning the laser spot several microstructures were created, demonstrating flexible fabrication capability of this method. We showed the available range of the laser intensity, the exposure time, and the NA of focusing objective lens in order to achieve stable fabrication and to avoid photothermal damage onto the Au nanorods through a series of experimental investigations. The driving force of the accumulation of Au nanorods was interpreted as the optical gradient force generated by tightly focusing laser light. Our approach provides a method of using chemically mass-produced Au nanorods as a building block to create a variety of metal micro/nanostructures. Further investigation of the dynamics of Au nanorod aggregates formation may provide higher spatial resolution and controllability of the orientation of Au nanorods in the future, leading toward novel methods for creating plasmonic optical materials and devices.
This research is supported by Iketani Science and Technology Foundation, and by KAKENHI Grant-in-Aid for Young Scientists (No. 21686010, 19810012), MEXT, Japan. This work was also financially supported by the National Natural Science Foundation of China (Grants No. 50973126, 61077028), 973 Project (2010CB934103) and ICP (2008DFA02050, S2010GR0980) of MOST.
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