1. Introduction

Thin metal films with rough surfaces exhibit optical enhancement properties due to strong electric field that results from light induced surface plasmons (SPs) [1]. Noble metal particles of subwavelength sizes can sustain resonances of collective electron oscillations known as localized surface plasmons (LSP) [2]. Noble metal structures with SPs and LSP resonance modes have a wide range of applications for their unique properties, such as imaging with subwavelength resolution [3, 4], chemical and biological sensors [5, 6, 7], and novel photonic devices [8, 9]. It is therefore a highly interesting research scope to tune the surface plasmon resonance (SPR) peaks to desired positions since the wavelength and intensity of SPR are highly sensitive to the nanostructure’s material, size, shape, period as well as surrounding environment.

A number of research groups have studied the development of novel structures, such as core/shell nanoparticles, nanorods, nanospins and nanostars which exhibit a number of interesting optical properties [10-13]. Moreover, a theoretical hybridization model for the plasmon response of complex nanostructures has been developed for some structures, such as a multi-layer nanoshell [14], and nanoparticles near metallic surfaces [15]. The interplay between LSP and surface plasmon resonance (SPR) has also been extensively studied [16, 17, 18]. However, most theoretical models and experiment are focused on different structures with a single noble metal. For SPR tuning by hybrid materials, the studies mainly focus on bimetallic colloidal nanoparticles and bimetallic films for biosensor applications. Bimetallic colloidal nanoparticles (Au-Ag, Au-Pt, and Ag-Pt) and nanostructures of gold and silver [19, 20] have been investigated to obtain desirable SPR peaks. The SPR of noble metal thin films has been widely used in biosensing, i.e. silver is preferred for its narrow resonance curve which results in high signal to noise ratio (SNR); gold has the advantage of having good chemical resistance. Silver and gold bimetallic thin film has therefore been frequently adopted to improve the resolution and adhesion for biosensing applications [21, 22]. The thickness of each double metal layer is also an important factor in tuning the SPR peak. Meanwhile, the role of substrate metal (gold or silver) in enhanced SPR imaging with gold nanoparticles has been theoretically studied [23]. It shows the interplay between LSP and SPR for two different noble metals.

To the best of our knowledge, there is yet any extensive research on the surface plasmon behavior of hybrid nanodots localized on quartz substrate. In this paper, we focus on gold and silver bimetallic nanostructures and study SPR peaks of the film and dot arrays. This study offers a novel way for hybrid materials to tune the SPR peaks of noble metals. The spectrum shift is attributed to the electromagnetic interaction between gold and silver atoms. A simplified spring model is adopted to qualitatively explain the phenomena observed.

2. Experiment

2.1. Metal film preparation

Quartz was chosen as the substrate for the sample preparation. Firstly, metal deposition was carried out using an electron beam evaporator. Chromium (Cr) and Titanium (Ti) thin films (~2 nm) were deposited to improve the adhesion between the gold-quartz and gold-silver thin films, respectively. For single layer gold film, 2 nm of Cr film and 25 nm of gold film were deposited. For bimetallic gold and silver film, an additional 2 nm of titanium and 25 nm of silver films were subsequently deposited. These thin layers of Cr and Ti do not affect the SPR observed [7].

2.2. Nanostructure fabrication

Laser interference lithography (LIL) is applied in the nanostructure fabrication. LIL is a maskless lithographic technique, which makes use of the interference pattern of two incident laser beams and records the pattern on a photoresist material. LIL has proven its ability to generate uniform two dimensional patterns over a large area [24]. The interference pattern consists of a standing wave whose intensity varies with a period equal to p=λ/(2sinθ), where λ is the wavelength of the laser beams, and θ the half angle at which two beams intersect. In this experiment, the exposure was carried out using Llyod’s mirror setup [25] with a 325 nm helium-cadmium (He-Cd) continuous wave laser as the light source, as shown in Fig. 1.

A bi-layer resist lift-off process was adopted in the fabrication of the metallic nanodot array, which is an improvement to the single-layer lift-off. An undercut profile was created by the use of two layers of resists, so as to prevent the metal film deposition on the sidewalls of the resist. The top layer is commercial MA-N1407 photoresist from MicroChem, whereas the bottom layer is polydimethylglutarimide (PMGI) resist. PMGI has several unique properties, such as being insoluble in commercially available photoresist solvent, which makes it suitable for the bi-layer lift-off process. PMGI resist also has a highly controllable dissolution/undercut rate. With careful design of the prebake parameters and well defined exposure and development processes, a precise control of the undercut profile can be achieved.

 

Fig. 1. Schematic diagram of laser interference lithography for bimetallic nanostructure fabrication [26].

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2.3. Experimental procedure

Quartz substrate was prebaked on a hotplate at 180 °C for 3 min to enhance the adhesion between PMGI resist and the substrate. PMGI (SF6) resist was then spin-coated onto the substrate as shown in Fig. 2(a). A spin speed of 3500 rpm was used for 50 sec to obtain a resist thickness of 300 nm. This was followed by another prebake at 180 °C for 3 min. The function of this prebake step is to dry the PMGI film and determine the undercut rate. Negative photoresist MA-N1407 was spin-coated on the top of PMGI layer, followed by a prebake at 100 °C for 1 min 30 sec. The spin-coating was done at 3500 rpm for 30 sec. The resulting photoresist has a thickness of around 700 nm, as shown in Fig. 2(b).

The pattern was formed by LIL. The exposure time used was 2 min 15 sec, and the angle θ was set to be 3°, as shown in Fig. 2(c). Arrays of resist pillar were achieved by rotating the sample by 90° and subjecting it to a second exposure. Development of the photoresist was done by immersing the sample in the developer for 25 sec. The developer dissolved the negative photoresist that was not exposed by the laser, and had no effect on the PMGI resist underneath, as shown in Fig. 2(d). Undercut profile was achieved by 1.9% TMAH (tetramethylammonium hydroxide) developer. TMAH dissolves PMGI isotropically. This resulted in an undercut in the PMGI layer under the photoresist, as shown in Fig. 2(e).

Metal deposition was carried out using an electron beam evaporator, as shown in Fig. 2(f). Around 2 nm of Cr and Ti was deposited to increase the adhesion between the gold and quartz and the gold and silver thin films, respectively. For the single-layer gold dots array, 2 nm of chromium film and 25 nm of gold film were deposited. For double-layer gold and silver dots array, another 2 nm of titanium film and 25 nm of silver film were subsequently deposited. Finally, an commercially available PG remover was used to lift off the bi-layer resist stack, leaving only the metal film deposited directly onto the substrate. The lift-off time taken in this experiment was 3 min 30 sec for the single layer gold dot array, and 5 min 30 sec for the double layer Ag/Au structure, as shown in Fig. 2(g).

 

Fig. 2. Experimental procedure: (a) PMGI coating; (b) photoresist coating; (c) exposure; (d) photoresist development; (e) PMGI undercut; (f) metal film sputtering and (g) lift-off

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3. Results and discussion

3.1 Bimetallic nanodot array characterization

The bimetallic nanodot array was characterized and observed by SEM (Hitachi) and AFM (Veeco) images shown in Fig. 3. As seen in Fig. 3(a) where the sample was observed at a 50° tilt, uniform circular array of oblate spheroids can be observed on the substrate. Measured from the top view image, the diameter of each spheroid is around 900 nm, and individual dots are approximately 700 nm apart from each other. The structure exhibits excellent large area uniformity over an area of 5 mm by 5 mm. The bimetallic structure was confirmed by additional AFM characterization as shown in Fig. 3(b). The line scan profile of the nanodot array shows that the height of the dots is 50 nm while the designed value set by the electron beam evaporator is around 54 nm for deposition. This is possibly due to a small quantity of Ag metal (~5 nm) etched away by the PG remover during the lift-off process. The remaining Ag layer is 20 nm thick. This could also be verified from the rough surface of the nanodots as observed from the line scan profile. Subsequently, this small amount of reduction at the surface metal (silver) does not significantly influence the surface plasmon behavior of silver to the gold dots as most of the silver deposition remained. The bottom plane of the dot features suggests that the photoresist has been lifted off thoroughly. Therefore the resulting fabricated structure is a bimetallic dot array with a height of 25 nm gold and 20 nm silver.

 

Fig. 3. Images of bimetallic (Ag/Au) dots array on a quartz substrate: (a) SEM image at a 500 tilt angle (scale bar value: 1 µm) and (b) Line scan profile and 2D AFM image (coloured arrows correspond to the positions in the scan profile).

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3.2 Transmission spectra analyses

The transmission spectra were measured by UV-Vis scanning spectrophotometer (Shimadzu Corporation). Light from the source was irradiated normally onto the samples. The transmission spectra for single Au and double Ag/Au layers are shown in Fig. 4. The graphs have been normalized for peak position comparison. A normal resonance peak of noble metal materials exists on the metal thin films, even without regular nanostructures, due to the granular structures on the thin films [27, 28]. It can be seen that this spectral peak position is at 524 nm in the Au thin film while there are two peaks at the positions of 320 nm and 496 nm in the bimetallic Ag/Au thin film. The two peaks were assigned to silver and gold, respectively since silver has its surface plasmon resonance at a shorter wavelength [29]. Therefore, the Au surface plasmon resonance peak was observed to be blue shifted from 524 nm to 496 nm, after coating the silver film over it.

As previously verified, the dot structure consists of two metallic layers: 25 nm of gold and 20 nm of silver, each with its own individual optical resonances. Meanwhile, when the two metallic layers are sufficiently close together, an additional hybridized mode is formed due to their interactions [14].

 

Fig. 4. Normalized UV-Vis transmission spectra of Au and Ag/Au bimetallic thin films. The plasmon resonance peak of Au in the bimetallic film at 496 nm shows a blue shift from the single film, which peaks at 524 nm.

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The transmission spectra of the Au nanodot array and bimetallic Ag/Au nanodot array are compared in Fig. 5. There is a plasmon band for gold nanodot array with the peak wavelength at 510 nm. Previous investigation showed that arrays of particles with similar circular section (spheroids) parallel to the substrate deposited on ITO or glass display a single plasmonic resonance [17]. This band at 510 nm is attributed to the plasmon band for Au nanodot array. The shoulders around the band may be due to the surface roughness [18]. In the transmission spectrum of the bimetallic nanodot array, two plasmon bands are observed, which correspond to the Au and Ag components, respectively. As shown in Fig. 5, there is a relatively narrow band with peak wavelength at 324 nm and a broader band with peak wavelength at 450 nm. Since the Au plasmon resonance wavelength is longer than that of Ag with the same shape and size [29], it is reasonable to assign the narrow band to the Ag component and the broader band to the Au component. Comparing the Au plasmon bands in these two structures, there is a significant blue shift in Au peak wavelengths of the bimetallic nanodot array. This shift is attributed to the addition of the Ag thin film, since the only difference between these two structures is the presence of the Ag layer.

 

Fig. 5. Normalized UV-Vis transmission spectra for metallic nanodot arrays fabricated by LIL technique. The plasmon resonance peaks are blue shifted. The larger shift of the Au peak (as compared to the film structure) is due to the independent increase in repulsive force within each atom pairs formed by the isolation of the periodic dot structures.

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The observed blue shifts in the transmission spectra for Ag/Au thin film and nanodot arrays can be explained qualitatively by a simple dipole-dipole interaction model, which is a well known form of molecular system [30, 31]. The electric field of the irradiated light causes the positive and negative charges within the atom to be displaced from their equilibrium positions. For a single metal, an atom with electrons bound to the nucleus can be represented by a small mass bound to a larger mass by a spring. Each of the charges in the atom is under the influence of the electric field and driven back and forth. For metals, electrons move freely in the lattice to form an “electron sea”. Due to the great difference between atoms and electrons, the electrons are expected to contribute dominantly to the induced dipole moment.

The simplified spring model of gold and silver atoms is shown in Fig. 6(a). A larger circle represents a gold atom and a relatively smaller circle represents a silver atom. The gold spring model could be adopted for a single gold nanodot, and the frequency of spring could be considered as the resonance frequency of the gold nanodot. When the gold and silver atoms are placed in adjacent to each other, the additional polarization forces act on both atoms as sketched in Fig. 6(b). The top and bottom circles represent silver and gold atoms, respectively. When the driving field is parallel to the particle surface, the repulsive forces within each atom are enhanced due to the attractive forces between the positive and negative charges in different atoms, giving rise to a correspondingly higher resonance frequency. Thus the resonance wavelength of the gold dots array is blue shifted in the bimetallic structure due to the interaction with the silver dots array.

 

Fig. 6. Sketches to illustrate the electromagnetic interaction between closely spaced atoms: (a) a gold atom (left) or a silver atom (right) and (b) a pair of closely placed atoms with the polarization of the exciting field parallel to the particle surface.

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In addition, it is noted that there is a more apparent blue shift for Au SPR peak from 510 nm to 450 nm in the nanodot arrays (Fig. 5) while the peak wavelength blue shifted from 524 nm to 496 nm in the thin film (Fig. 4). With reference to the simplified spring model, there is an electrical attraction between adjacent atoms. Compared with the thin film structure, the nanodot array has gaps which weaken the attractive forces between isolated nanodots. Consequently, this leads to a larger unrestrained restoring force and a higher resonance frequency. This translates physically to a lower resonant wavelength in its UV-vis transmission spectrum and therefore, a more apparent blue shift occurs.

4. Conclusions

Bimetallic nanostructures have been fabricated using LIL and bi-layer resist lift-off. LIL is a maskless lithographic technique and has the ability to generate uniform two dimensional patterns over a large area. The bi-layer resist lift-off process ensures good quality structure fabrication. The combination of these two techniques shows potential in low-cost large-area plasmonic nanostructure fabrication. The transmission spectra of thin films and nanodot arrays fabricated with single metal and bimetallic layers were analyzed by UV-Vis spectroscopy and compared. The results show that the Au SPR peak position is blue shifted when additional Ag thin films or nanodot structures are patterned over it. The spectrum shift is attributed to the interaction between gold and silver atoms. A simplified spring model was adopted to qualitatively explain the phenomena observed. The paper also suggests that the extent of plasmon band tuning can be explained using the same model. These results offer a new approach to design and fabricate hybrid materials or structures for the tuning of SPR peaks in noble metals. Tuning of SPR peaks can be practically applied to many potential areas, such as the tuning of solar cell absorption spectrum for enhanced absorption performance. Future works also include the study of other hybrid models beyond gold and silver. The possibilities of this include silver/nickel combination which can be used to achieve magnetic tuning of surface plasmon resonances in magnetic nanodevices.