The fabrications of sphere-like Au nanoparticles (NPs) on sapphire, GaN, and SiO2 substrates through the irradiation of a few pulses of 266-nm laser onto Au thin films deposited on the substrates are demonstrated. The top-view diameter, contact angle on substrate, surface population density, and surface coverage percentage of the NPs can be controlled by the Au thin film thickness, laser energy density, substrate choice, and the gas or liquid, in which the Au thin film is immersed during laser irradiation. Optical transmission measurements show clear in-plane and out-of-plane localized surface plasmon resonance (LSPR) features, including the air resonance feature dictated by the gas or liquid immersing the NPs during transmission measurement, the in-plane substrate resonance feature controlled by the substrate material and the contact angle, and the out-of-plane resonance feature, which is strongly influenced also by the substrate material and the contact angle. Numerical simulations based on the finite-element method using the experimental parameters show highly consistent LSPR spectral positions and their variation trends. From the simulation results, one can also observe the relative importance between NP absorption and scattering in contributing to the extinction. This simple laser-irradiation method for fabricating sphere-like Au NPs of no aggregation and of strong adhesion to the substrate is useful for developing polarization-sensitive LSPR bio-sensing.
© 2009 OSA
Metal nanoparticles (NPs) can find important applications in recently developed nanotechnologies, particularly in plasmonics for bio-sensing [1–5]. Localized surface plasmon (LSP) resonance (LSPR) features on metal NPs, which are prepared on a substrate, have been used for bio-sensing based on the resonance wavelength dependence on the refractive index of surrounding liquid. Such a measurement can lead to refractive index measurement sensitivity as high as 880 nm spectral shift per refractive index unit . Among the methods of fabricating metal NPs, chemical synthesis is the most widely used approach [7–9]. Normally, in fabricating an LSPR device, chemically synthesized metal NPs, such as Au and Ag, are spin-coated on a transparent substrate. However, the bonding between the metal NPs and the used substrate in such a device is usually weak leading to unstable detection. Also, the NP aggregation can be a major problem in fabricating such a device. Besides spin-coating chemically synthesized metal NPs on a substrate, several methods have been developed for fabricating metal nanostructures on a substrate, such as the structures of nanoring, nanodisk, and nanoprism [6,10–15]. Among them, colloidal lithography [6,10] and nanosphere lithography  have been used for directly fabricating metal NPs on a substrate. Plasmonic nanolithography for forming metal nanodot arrays was also reported . Because the size and shape of a metal NP determine the LSPR wavelengths, shaping a metal NP is an important issue. The shape of an NP can be manipulated through laser-induced LSPR excitation during the chemical synthesis process [13–15]. Also, the geometry of a metal NP can be modified by direct laser irradiation when the NP is in either liquid or gas environment [16–18]. Although various methods for preparing metal NPs on a substrate have been implemented, they either correspond to complicated processes or lead to random orientations of NPs on substrate. NPs on a substrate with a certain degree of alignment can result in polarization-sensitive LSPR excitation for more versatile sensing application. If a simple technique for directly fabricating metal NPs of controllable parameters on a substrate with strong adhesion and a certain degree of alignment, the LSPR sensing technology can be significantly improved.
In this paper, we report a simple method for fabricating substrate adhesive Au NPs of no aggregation based on a process of laser ablation and annealing. The size, shape, surface density, and coverage of Au NPs on a substrate can be controlled by laser energy density, substrate material and surface condition, deposited Au film thickness, and the liquid covering the Au film during laser irradiation. With optical transmission measurements, the in-plane and out-of-plane LSPR features can be clearly identified due to the similar shapes and vertical alignments of NPs on substrate. The spectral features of substrate-affected LSPR modes rely on the size, Au/substrate contact area, substrate material, and surrounding gas or liquid. Numerical simulations based on the finite-element method show highly consistent results in the LSPR-induced extinction spectral features. The used laser came from the fourth-harmonic generation (266 nm) of a Q-switched Nd:YAG laser of 5 ns in pulse width and of adjustable pulse repetition rate. In Au NP fabrication, one or a few laser pulses were applied onto an Au thin film, which was deposited on a double-side-polished substrate with electron-beam evaporation. The used substrates are transparent in the visible range for transmission measurements with a white light source.
2. Au Nanoparticle Fabrication and Transmission Measurement
The four parts of Fig. 1 show the results of laser irradiation with two pulses onto a 10-nm Au thin film on a sapphire substrate (sample A). The laser pulse energy density was around 30 mJ/cm2. Figs. 1(a) and 1(b) show the plan-view scanning electron microscopy (SEM) images of the surfaces before and after laser irradiation, respectively. One can see the cracks of the Au thin film on sapphire substrate in Fig. 1(a). After laser irradiation, sphere-like Au NPs of various sizes can be observed in Fig. 1(b). Fig. 1(c) shows a tilted SEM image of the Au NPs. One can observe the cut-facet of the sphere-like NPs. To understand the NP size distribution, we developed a computer program to evaluate the sizes and population density of those NPs based on the plan-view SEM image. The NP diameter distribution is shown in Fig. 1(d). Here, the NP diameter shows a nearly normal distribution ranging from 30 through 170 nm with the average diameter around 85 nm. The contact angle, θc, between an Au NP and the substrate is defined as that between the contact facet and the tangential of the Au body at one of the contact edges, as demonstrated in the insert of Fig. 1(d). During the laser irradiation process, the gold film is melted due to the absorption of UV laser. The contact angle of an Au NP in solid phase is close to that of a liquid Au droplet at a temperature around the melting point. Therefore, this contact angle can be ideally evaluated by using the Young-Dupré formulas as 
Here, γLV, γSV, and γSL are the surface tensions at the liquid/vapor (Au/air), solid/vapor (substrate/air), and solid/liquid (substrate/Au) interfaces, respectively. Also, ΔWSLV is the adhesion energy per unit area between the metal and the substrate. Note that a larger contact angle corresponds to an Au NP shape closer to a perfect sphere. The size of the contact angle or contact area depends on the material type of the substrate and the metal melting temperature. Based on the reaction-free theory, the contact angle of an Au sphere-like NP on sapphire is between 135 and 140 degrees . Although it is difficult to accurately read the contact angle from the SEM image in Fig. 1(c), the estimated angle is close to the theoretical value.
Figs. 2(a) and 2(b) show the similar results to those in Figs. 1(b) and 1(c), respectively, by using a GaN template as the substrate (sample B). The GaN template of 2 μm in thickness was grown on sapphire substrate with metalorganic chemical vapor deposition along the c axis at 1000 °C. Its surface roughness is below 0.5 nm. Under laser irradiation of five pulses at around 20 mJ/cm2 in energy density onto a 7.5-nm thick Au film, sphere-like Au NPs are formed on GaN as shown in the plan-view and tilted SEM images of Figs. 2(a) and 2(b), respectively. The top-view diameter is distributed between 40 and 120 nm with the average value around 75 nm. From Fig. 2(b), the contact angle is estimated to be 125 degrees. Au NPs were also fabricated on a SiO2 template (sample C), which was obtained by depositing a 30-nm SiO2 layer on a GaN template with plasma-enhanced chemical vapor deposition. A tilted SEM image of such NPs is shown in Fig. 2(c). Here, one can see a few relatively larger NPs, which have a contact angle close to 180 degrees. The details of laser irradiation conditions and resultant NP parameters are listed in Table 1. To demonstrate the process besides thermal effect in laser irradiation, we prepared a few samples with furnace heating under the conditions of different temperatures ranging from 800 through 1000 °C and different ambient N2 flow rates. In Fig. 2(d), we show a plan-view SEM image of Au nanostructures fabricated with a furnace-based process at 800 °C for 30 min from a 7.5-nm Au film on a GaN template. Here, one can observe disk-like nanostructures of various sizes and random shapes. The random nanostructures originate from thermally activated spinodal dewetting of the Au thin film . As temperature increases, the nanostructure becomes more separated to form individual NPs. However, the disk-like geometry is unchanged. With high-power pulsed laser irradiation, the rapidly melted Au film begins to contract with a velocity of several tens m/s, starting from those locations of cracks, where the radii of curvature are small and the forces due to surface tension are strong . During contraction, a melted Au NP experiences an upward force due to inertia. This upward-pushing mechanism can help in forming sphere-like NPs. If the upward force is strong enough to overcome the adhesion energy, an NP may detach from the substrate. On the other hand, in the case of furnace heating, with a relatively slower heating process and hence a slower dewetting process, the upward-pushing mechanism is weaker. Therefore, the formed NPs show the disk-like shapes and irregular planar geometry.
Fig. 3 shows the transmission spectra of the samples of Au NPs on sapphire (sample A) and SiO2 (sample C) with the s (Sapphire-s and SiO2-s) and p (Sapphire-p and SiO2-p) incident polarization conditions when the incident angle is 60 degrees with respect to the normal of the substrate surface. With such an incident angle, in the case of s-polarized excitation, only the in-plane LSPR features can be excited. On the other hand, in the case of p-polarized excitation, both the in-plane and out-of-plane LSPR features are excited. As shown in Fig. 3, in the transmission spectrum of s-polarized excitation of the sapphire-based sample, two clear dip features can be identified, with one around 515 nm and the other around 565 nm. The feature around 565 nm is due to in-plane electron oscillation, which is affected by the contact facet of the Au NPs on the substrate, and is named as the in-plane substrate resonance (IPSR) feature. That around 515 nm corresponds to the LSPR of an Au NP completely surrounded by air and is named as the air resonance (AR) feature. This feature is red-shifted when the NPs are immersed in certain liquid during transmission measurement. In the transmission spectrum of the “Sapphire-p” case, a broad dip with the minimum around 545 nm can be observed. This broad dip is supposed to consist of three features, including the aforementioned AR and IPSR, and an out-of-plane resonance (OPR) feature around 545 nm. Similar in-plane and out-of-plane LSPR features have been observed in a periodic metal nanodisk structure . This OPR LSP feature is also affected by the contact facet of an Au NP and the substrate material. In Fig. 3, the transmission behaviors of NPs on SiO2 are quite different from those on sapphire. Because the contact angle of the NPs on SiO2 is close to 180 degrees, the IPSR and OPR features are unclear. Only the AR feature is observed in either s-or p-polarized excitation. The significant contact between NPs and substrate is crucial for the generations of the IPSR and OPR features. The oscillating transmission behaviors in the SiO2 sample are due to the Fabry-Perot effect between the air/SiO2, SiO2/GaN, and GaN/sapphire interfaces. To guide eyes, low-pass-filtering fitting curves are drawn on the oscillating transmission curves.
Fig. 4 shows the transmission spectra of the Au-nanostructure samples on the GaN template (sample B) with the curves of GaN-s and GaN-p for s- and p-polarized excitations, respectively. Here, the data of both samples fabricated by laser treatment and furnace-based thermal annealing are demonstrated. In all the curves here, Fabry-Perot oscillations can be clearly seen. Again, low-pass-filtering fitting curves are drawn to guide eyes. Here, in the sample of laser treatment, the AR feature merges into the dominating IPSR feature and cannot be clearly identified. The IPSR and OPR features are located around 655 and 533 nm, respectively. The relatively smaller contact angle and larger refractive index (~2.5) of GaN substrate leads to a larger spectral separation between the IPSR and OPR features, when compared with the case of sapphire substrate. The transmission behaviors of the sample fabricated with furnace-based thermal annealing are quite different from those fabricated with laser treatment. In this thermal annealing case, broad-range depression features can be observed under both excitation polarization conditions. Such a feature may include the contributions of various LSP modes induced by a variety of metal geometry. In this situation, it becomes difficult to clearly identify the AR, IPSR or OPR mode.
3. Numerical Simulation
To confirm the LSPR features described above, we performed numerical simulation of plane wave incidence upon the structure of an Au NP on the substrate with the finite-element method (COMSOL) to evaluate the absorption (metal dissipation) and scattering cross-section spectra of the NP/substrate system. Then, the extinction cross-section spectra can be obtained by taking the summation of absorption and scattering cross-section spectra. To simulate the scattering of a single NP on a substrate under an incident plane wave with any incident angle, we use a spherical computation domain with a shell layer to serve as the perfectly matched layer (PML), as shown in the insert of Fig. 5(a). The inner and outer radii of the PML shell are 400 and 550 nm, respectively, corresponding to 150 nm in PML thickness. The problem geometry contains two media surrounding the Au NP with individual PMLs. In computation, the maximum mesh size in each region is set at one tenth the shortest wavelength of the concerned spectral range (400–800 nm) in that region. To investigate the scattering effect of the NP, we use the scattered-field formula for simulation, in which the total electric field E⃑ is divided into an unperturbed part E⃑0 and a scattered part E⃑sc. The unperturbed field E⃑0 is defined as the total field of the same structure without the Au NP under the incidence of a plane wave. It can be easily obtained from an explicit analytic expression. The scattering due to the NP is then described by E⃑sc. Note that in the situation with NP, E⃑0 is given only in the non-PML regions. Also, note that the tangential component of the scattered field Esc is continuous at the interfaces between the PML regions and the non-PML regions. Once the near field of E⃑sc is obtained, we can transfer it into its far-field counterpart through the near-field to far-field transformation. The far field of E⃑sc is then used to compute the scattering power. The total field inside the NP is used to evaluate the metal dissipation. The dielectric characteristics of Au can be found in literature . For simplicity, the refractive indices of GaN, sapphire, and SiO2 are set at 2.5, 1.76, and 1.54, respectively, through the whole concerned spectral range.
It is noted that the resonance peaks of absorption and scattering are not necessarily at the same spectral positions . The six curves in Fig. 5(a) labeled by A-s, S-s, and E-s (A-p, S-p, and E-p) correspond to the absorption, scattering, and extinction cross-section spectra of NPs on sapphire (for comparing with the data of sample A) when the incident wave is s- (p-) polarized with the incident angle at 60 degrees. The top-view NP diameter and the contact angle are assumed to be 85 nm and 135 degrees, respectively. In the E-s curve, one can see a peak around 570 nm and a shoulder on the high-energy side. The peak around 570 nm corresponds to the IPSR feature of the experimental data shown in Fig. 3. The shoulder originates from the AR feature around 515 nm. On the other hand, the broad hump of curve E-p consists of the contributions of IPSR, AR, and the OPR feature around 550 nm. The general agreements of the LSPR features between the experimental data and numerical simulations assure the correctness of data interpretation. In particular, as shown in Figs. 5(b)–5(d) for the distributions of electrical field magnitudes at 515 and 570 nm of the s-polarized excitation case, and at 550 nm of the p-polarized excitation case, respectively, the physical meanings of the AR, IPSR, and OPR features become clearer. The slight differences in spectral peak between the simulation and experimental results can be due to the imprecise assignments of NP parameters for simulation. It is noted that the distribution in Fig. 5(d) contains the contributions of the AR, IPSR, and OPR features. As shown in Fig. 5(c) for the IPSR feature, the LSP field is mainly distributed near the two edges of the NP/substrate contact facet. This feature describes the in-plane electron oscillation on the Au NP near the substrate such that its characteristics are strongly influenced by the substrate material (refractive index) and the contact angle. On the other hand, as shown in Fig. 5(b), the AR feature describes a more uniform energy distribution on the two sides of the NP in the in-plane dimension. This feature is supposed to be strongly dependent on the refractive index of the surrounding medium above the substrate. Then, in Fig. 5(d), one can clearly see more energy distributions at the top and bottom of the NP corresponding to the contribution of the OPR feature. In Fig. 5(a), one can also observe the relative importance of the absorption and scattering contributions to the three LSPR features. The AR feature is mainly caused by the absorption of Au NP. The scattering contribution to the IPSR feature becomes relatively more important even though it is still dominated by absorption. As to the OPR feature, the absorption and scattering contributions become comparable.
To further understand the effects of the refractive-index contrast between the substrate and the surrounding medium, and those of the imperfect sphere shape of the NP on the LSP spectral features, we performed the simulations under the conditions of an Au NP completely surrounded by air and by a medium of sapphire refractive index to give the spectral curves in Figs. 6(a) and 6(b), respectively. The NP diameter and the contact angle are still assumed to be 85 nm and 135 degrees, respectively. The label notations in Figs. 6(a) and (b) are the same as those in Fig. 5 for both s- and p-polarized excitations. Also, in either Fig. 6(a) or 6(b), the simulation results under the condition of perfect Au sphere (85 nm in diameter), labeled by A-sphere, S-sphere, and E-sphere for absorption, scattering, and extinction, respectively, are demonstrated. Here, one can see that when the NP is completely surrounded by air, the extinction is dominated by the contribution of absorption, whose resonance feature is slightly blue shifted from that of the scattering contribution. Also, the spectral position of the unique LSPR feature (AR) around 515 nm is not significantly affected by the NP orientation, i.e., excitation polarization. A small blue shift of the LSPR feature in p-polarized excitation by ~5 nm from that in s-polarized excitation is observed. Meanwhile, with perfect sphere shape, the extinction resonance feature is located at almost the same spectral position as those of the sand p-polarized LSPR of an imperfect sphere. However, as shown in Fig. 6(b), when an NP is completely surrounded by a medium of 1.76 in refractive index, scattering feature around 650 nm dominates the contribution to extinction. The spectral peak positions of E-s, E-sphere, and E-p curves are located at 650, 640, and 635 nm, respectively. In other words, the imperfect sphere geometry and the excitation orientation are important factors in determining the LSPR features when the surrounding refractive index is larger than unity. From Fig. 6, one can realize the importance of the NP contact angle and substrate material in controlling the LSPR spectral position. It is noted that a second LSPR feature exists around 560 nm leading to the kinks on the high-energy side of the extinction curves. This feature is due to the quadrupolar LSPR in the NP. Its distribution of electrical field magnitude is shown in the insert of Fig. 6(b) .
Fig. 7 shows the simulation results of the Au NP sample on GaN by assuming that the NP diameter is 75 nm and the contact angle is 125 degrees (for comparing with the data of sample B). In the extinction spectrum of s-polarized excitation, the features of IPSR around 640 nm and AR around 515 nm can be clearly identified. Then, in the extinction spectrum of p-polarized excitation, the dominating feature of OPR around 545 nm can be observed besides the IPSR and AR (as the shoulder) features. In this sample with GaN as the substrate, although the major contribution to the AR extinction is absorption, the contributions of absorption and scattering to the IPSR and OPR extinctions are comparable. The simulation results in Fig. 7 are highly consistent with the experimental data in Fig. 4.
It is noted that the LSPR wavelength is related to the size of Au NP. A larger NP size leads to a longer LSPR wavelength. Therefore, the measured transmission depression actually covers the components of various NP sizes. Although it is difficult to precisely evaluate the widths of those LSPR features in either experimental or simulation results, we can still observe the generally broader LSPR features in the experimental data when compared with the simulation results. The relatively broader LSPR features in the experimental observations are due to the distributions of NP size. Regarding the spatial distribution of NP, some of NPs are closely located that may induce LSP coupling between neighboring NPs. Such LSP coupling also leads to the broadening of an LSPR feature. In our simulation, such coupling behavior is not included since single NP is assumed and the PML boundary condition is used. The relatively broader LSPR features in experiment may also include the effect of spectral broadening due to LSP coupling.
4. Au Nanoparticle Fabrication under Different Liquid Coverage Conditions
As mentioned earlier, the Au NP size and contact angle can be slightly controlled by the laser pulse energy, Au film thickness, and substrate selection. The Au NP parameters can also be controlled by the covering liquid on the Au thin film during laser irradiation. In Table 1, we list the parameters of Au NPs on sapphire fabricated under the conditions of air (sample A), water (sample D), and methanol (sample E) coverage during laser irradiation. The results of the air coverage condition have been discussed earlier related to Fig. 1. For comparison, the results of the NPs on GaN and SiO2 discussed earlier are also summarized in Table 1. Among the three samples of sapphire substrate fabricated under the conditions of air, water, and methanol coverage of Au thin film during laser irradiation, i.e., samples A, D, and E, respectively, one can see the changes of NP average size, contact angle, NP density, and surface coverage. Although it is difficult to exactly determine the contact angles under the conditions of water and methanol coverage during laser irradiation, they are surely significantly larger than that under the fabrication condition of air coverage. Generally speaking, under the fabrication condition of methanol coverage, the NP size is the largest, the NP density is the smallest, and the surface coverage is the smallest, all followed by the condition of water coverage, among the three samples of sapphire substrate. The differences in contact angle are due to the changes of surface tensions γLV and γSV (see Eq. (1)) under different coverage media. However, it is difficult to find a major cause for the increase of NP size when water and methanol are used to cover the Au thin films during laser irradiation. The small increases in NP size can be due to a complicated interplay mechanism, including the effects of different laser ablation processes, different heating processes, and different laser energy densities. In Fig. 8, we show the transmission spectra of the three samples with s- and p-polarized excitations. In the curves of s-polarized excitation, one can see the slight blue shift of the IPSR feature and the relatively more important contribution of the AR feature when water and methanol are applied during laser irradiation. On the other hand, the broad dip (containing the AR, IPSR, and OPR features) under p-polarized excitation is red shifted when water or methanol is used, indicating that the OPR feature is red shifted. In Figs. 9(a) and 9(b), the simulated absorption, scattering and extinction spectra under the conditions of air, water, and methanol coverage with s- and p-polarized excitations, respectively, are shown. The NP parameters for simulations, including NP diameters and contact angles, are shown in Table 1. Here, one can see the slight blue shift trend (570 to 565 nm) of the IPSR feature and the slight red shift trend (550 to 565 nm) of the OPR feature when air coverage is replaced by water and then by methanol coverage. Such trends are consistent with the experimental observations shown in Fig. 8. It is noted that the relative extinction levels among the three samples shown in Figs. 9(a) and 9(b) are not consistent with the relative dip levels shown in Fig. 8. Such an inconsistency is attributed to the different NP densities among the three samples. As shown in Table 1, the sample of air (methanol) coverage has the highest (lowest) NP density that may reverse the variation trend of the total extinction. However, it is believed that other factors should be included for explaining the variation trend of extinction, including the possible variation in measurement condition and the LSP coupling between the neighboring NPs. Because of the random distribution nature of NPs, some of NPs are close enough to generate LSP coupling. Such coupling behaviors are expected to shift the LSPR spectral position and statistically result in a broadened LSPR spectral feature.
In summary, we have demonstrated the fabrication of sphere-like Au NPs of similar shapes and alignments on sapphire, GaN, and SiO2 substrates by the irradiation of a few UV laser pulses onto Au thin films, which were deposited on the substrates. The top-view diameter, contact angle on substrate, surface density and coverage of the NP could be controlled by Au thin film thickness, laser energy density, substrate choice, and the gas or liquid covering the Au thin film during laser irradiation. White light transmission measurements showed clear in-plane and out-of-plane LSPR features, including the AR feature dictated by the surrounding gas or liquid immersing the NPs during transmission measurement, the IPSR feature controlled by the substrate material and the contact angle, and the OPR feature, which was strongly influenced also by the substrate material and the contact angle. Numerical simulations based on the finite-element method using the experimental parameters showed highly consistent LSPR spectral positions and their variation trends. From the simulation results, one could also see the relative importance between NP absorption and scattering in contributing to the total extinction.
This research was supported by National Science Council, The Republic of China, under the grant of NSC 97-2120-M-002-005, NSC 97-2221-E-002-044, NSC 97-2622-E-002-011-CC1, and NSC 97-2628-E-002-044-MY3, by the Excellent Research Projects of National Taiwan University under 97R0061-04, by US Air Force Scientific Research Office under the contracts AOARD-07-4010 and AOARD-09-4117, and by Epistar Corporation, Taiwan.
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