Profile Prediction and Fabrication of Wet-Etched Gold Nanostructures for Localized Surface Plasmon Resonance
© to the authors 2009
Received: 7 October 2009
Accepted: 29 October 2009
Published: 13 November 2009
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© to the authors 2009
Received: 7 October 2009
Accepted: 29 October 2009
Published: 13 November 2009
Dispersed nanosphere lithography can be employed to fabricate gold nanostructures for localized surface plasmon resonance, in which the gold film evaporated on the nanospheres is anisotropically dry etched to obtain gold nanostructures. This paper reports that by wet etching of the gold film, various kinds of gold nanostructures can be fabricated in a cost-effective way. The shape of the nanostructures is predicted by profile simulation, and the localized surface plasmon resonance spectrum is observed to be shifting its extinction peak with the etching time.
(See supplementary material 1)
Nanoparticles have revolutionized conventional sensing technologies by magnifying signals and introducing unprecedented functionalities such as cloaking, image distortion correction, surface-enhanced Raman spectroscopy (SERS), electrical conduction in nanocircuits, cancer therapy with photothermal effects , etc., and these nanoparticles can be metallic [2–9] or bimetallic nanoparticles  and nanowires [11, 12]. Localized surface plasmon resonance (LSPR) [2–4, 7–9], generated by the interaction between the incident light and conduction electrons in noble metal nanoparticles to detect the refractive index variation around the nanoparticles, is one of the typical applications. As an alternative of surface plasmon resonance (SPR), which is an optical phenomenon on noble metal film for detecting analytes in real-time based on ambient refractive index variations, LSPR employs noble metal nanoparticles to enhance the electromagnetic field, simplifies the measurement setup, and has demonstrated similarity and superiority on the detections of biomarkers, DNA, and low molecular proteins.
Dispersed nanosphere lithography (NSL) is one of the best candidates to fabricate identical gold nanoparticles on a large area of glass substrate for LSPR in a mass productive way [7–9], because in LSPR, each metal nanoparticle serves as a separate emission element and thus periodicity is not required as long as they are evenly distributed. In dispersed NSL, metal film is evaporated in one to several times at different directions onto the nanospheres dispersed on the substrate, and the gold is subsequently dry etched. Due to the shade effect of the nanospheres, metal nanostructures are left on the substrate after the removal the nanospheres. Dispersed NSL can be used to acquire numerous shapes of metal nanostructures for tuning the peak wavelength and sensitivity of LSPR signal; however, its dry etching process increases the cost of LSPR chip, and glass or gold might contaminate the chamber of the expensive etching equipment such as argon milling or inductively coupled plasma (ICP) machine. To circumvent this problem, this paper demonstrates the approach to predict and fabricate gold nanostructures for LSPR by wet etching.
During the gold wet etching with the potassium iodide (KI) solution, glass will not be etched and keep intact, this is another advantage over dry etching, because dry etching tends to leave some over-etched trenches on the glass substrate  which will introduce some scattering loss. We consider two kinds of nanospheres: wet etching endurable and unendurable nanospheres. For example, polystyrene nanospheres will become waxy in the gold etchant, cover the gold and stop its etching, thus they have to be removed prior to wet etching. On the other hand, silica nanospheres keep intact during the wet etching, but it is difficult to be removed after etching. This paper simulated the obtainable gold nanostructure profiles after wet etching for both kinds of nanosphere masks, thus the fabrication process is designable and controllable by simulations.
In our preliminary experiments, we demonstrate that for a glass substrate with a gold film obliquely evaporated on the silica nanospheres, different wet etching time varies the shape of the gold nanostructures and shifts the LSPR spectra accordingly. Because the 3D gold nanostructures after wet etching are mainly under the silica nanospheres and cannot be observed by scanning electron microscope (SEM) or atomic force microscope (AFM), the profiles of the nanostructures at different etching intervals are also simulated, and we find that the trend of this wavelength shift is predictable by profile simulation. We have carried out the wet etching experiments with polystyrene nanospheres without removing them. These nanospheres became waxy (Fig. S1 in supplementary material) during the etching and inhabited the gold etching. However, we have successfully removed the polystyrene nanospheres by heating at 350 °C for 90 min (Fig. S2 in supplementary material). The experiments of wet etching after removing the nanospheres are under investigation and will be reported in the future.
In our previous paper , for the convenience of calculating the profile of the nanostructure, four intertransformable coordinate systems are introduced: the original coordinate system xoyozo, where the gold is evaporated at the angles of θ and φ (Fig. 1a); the coordinate system xφyφzφ (where yφ = yo) with φ = 0, θ ≠ 0 (Fig. 1a); the coordinate system x θ y θ z θ (where z θ = z φ ) with φ = 0, θ = 0 (Fig. 1d); and the coordinate system xφcyφczφc for non-conformal gold deposition (Fig. 1b), where the non-conformal angle θc is the angle between yθ and yφc. θc can be positive or negative depending on the materials and evaporation conditions. When θc is negative, the non-conformal part forms an undercut instead of the extension in Fig. 1b.
The areas fulfill x θ 2 + z θ 2 <r2 have no gold evaporated on the glass substrate, while other areas of the substrate have a gold deposition thickness of t cos θ. For multiple gold evaporation, the gross thickness is summated along the direction of each evaporation.
Since the calculated P′(xθ′,yθ′,zθ′) or P′(xφc′,yφc′,zφc′) can be converted back to the xo–yo–zo coordinate system, Eqs. 9 and 12 form the profile of the gold nanostructure after wet etching.
The software is programmed with Fortran90. The output data from the 5 layers are plotted together in Mathcad to obtain the 3D profile of the gold nanostructure after gold deposition and after gold etching. They also can be plotted by other software such as Mathematica, Matlab, etc.
By comparing in pairs with Fig. 4c and 4D, or 4g and 4h, or 4k and 4l, it is found that the gold nanostructure obtained by keeping the nanosphere on and wet etching 30 nm of gold looks similar to the one obtained by removing the nanosphere and wet etching 15 nm of gold, because the former is single-side etching and the latter is double-side etching; but obviously the gold on the substrate is thicker for the latter one, as the gold on the substrate only experiences single-side etching. Since the gold is thick on the substrate due to three times of gold evaporation, gold remains on the substrate after 30 nm of wet etching.
Figure 5d is with 60 nm of anisotropic dry etching. The leftover gold nanostructure on the nanosphere after 60 nm of dry etching is much larger than that of wet etching, because in dry etching the size of the nanostructure only reduces 60 nm from the top to bottom, while in wet etching, the 60 nm of gold is reduced in all directions; but the gold on the substrate is etched at the same depth for wet and dry etching, because in wet etching, one side of the gold on the substrate is protected by the substrate. In Fig. 5b and 5c, clusters of gold nanoparticles are left on the substrate, similar to those in Fig. 4. But in dry etching in Fig. 5d, clusters on the substrate are not generated. The clusters are interesting nanostructures in plasmonics, since it is reported that a dimer of nanoparticles emits much higher electrical field than a single nanoparticle , the narrow gaps between the clustered nanoparticles are expected to strongly enhance the plasmonic signal.
The simulations in Figs. 4 and 5 indicate that the size and shape of the gold nanostructure after wet etching are sensitive to the fabrication conditions such as the size of the nanosphere, gold evaporation angle and thickness, wet etching thickness, and whether the nanosphere is removed or not before etching. Because of these too many influential factors, the profile simulation provides a useful tool to control the fabrication of the gold nanostructure by wet etching. We regard this simulation as important, also because the nanostructures after wet etching are hidden under unremovable silica nanospheres and are hard to be measured by SEM or AFM. Even the substrate can be tilt or cut to observe its side view in SEM, the 3D nanostructures cannot be fully inspected. On the other hand, the electrical charges in SEM are high when silica nanospheres and glass substrate are observed, the substrate has to be evaporated with a few nanometers of gold, and this layer of gold evaporation will deform the actual shape of the nanostructures. So in the following wet etching experiments, we simulated the wet-etched gold nanostructures to get their 3D shapes.
In the experiments, silica nanospheres in the diameter of 175 nm were purchased from Microspheres-Nanospheres (a subsidiary of Corpuscular Inc.), and the silica nanospheres in the diameter of 500 nm were purchased from Duke Scientific Ltd. The chemicals poly(diallyldimethyl ammonium chloride) (PDDA), potassium iodide (KI), and iodide were from Sigma–Aldrich.
In order to disperse the silica nanospheres, a 4” Pyrex 7740 glass wafer was first implanted with silicon ions in Varian EHP-200 ion implanter at an energy of 5 keV and a dose of 1E14/cm 2 prior to being diced into 2 cm × 2 cm chips as substrates. After cleaning, the glass substrate was dip coated with a 1:5 diluted PDDA solution for 30 s; and after rinsing and drying, it was drop coated with 1 mL diluted nanosphere solution. The positive charges of PDDA canceled out the negative charges of the nanospheres as well as the charges of the implanted silicon ions on the glass substrate, thus the nanospheres were dispersed on the surface.
The silica nanospheres on the substrate were coated with gold film in R-Dec thermal evaporator, and then the sample was etched in the gold etchant formulated with 95% MilliQ water, 4% potassium iodide, and 1% iodide. In one of the experiments, the etchant was diluted 10 times to slow down the gold etching. The etching process was controlled both by LSPR spectra and SEM images at different etching intervals. The LSPR spectra were taken with Ocean Optics USB2000-UV–VIS optical fibre spectrometer at the wavelength range of 400–875 nm, and the dispersed nanospheres and wet-etched samples were inspected by JEOL JSM7400F field emission gun SEM.
After evaporating 50 nm of gold onto the nanospheres of 175 nm in diameter at 70°, the obtained sample is as shown in Fig. 7a. The gold evaporation under this condition forms a 3D nanostructure around the nanosphere, based on the 3D formation condition r sin θ + t cosθ >r, where r is the radius of the nanosphere,θ is the gold deposition angle, and t is the gold evaporated thickness . At the etching durations of 1, 2, 3, 4, 5, 6, 8, 10, 15, 20, 30 min, the sample was taken out, rinsed with DI water and dried for LSPR measurements, with the resultant spectra presented in Fig. 7b. The peak wavelengths of the LSPR spectra after 1–6 min of wet etching are, respectively, 543.4, 531.6, 537.5, 540.1, 541.4, and 588.2 nm, as plotted in the insert of Fig. 7b, and the LSPR peak disappeared after 8 min. Because it is difficult to inspect the gold nanostructures under SEM or AFM, we use some profile simulations to explain the trend of the LSPR shift.
Empirically, we know 3D gold nanostructure fabricated by gold evaporation is non-conformal with an angle θc. By fitting of the simulated profile, the possible gold nanostructures should have a non-conformal angle θc = −10°. Because under this condition, after etching 8.55 × 8 = 68.4 nm of gold, only a little gold remains on the substrate as demonstrated in Fig. 7j, thus no LSPR spectrum is distinguishable. Taking θc = −10°, we further simulated the nanostructure profiles after 0, 1, 2, 3, 4, 5, and 6 min of etching as illustrated in Fig. 7c–7i. Comparing the size and shape variations of these nanostructures by time, the blueshift in the first 1–2 min was due to the quick size reduction [7, 9]; the peak wavelength kept almost unchanged at 3–5 min, because the reduced size of the nanostructure tended to blueshift the spectrum, while the reduced thickness to width ratio redshifted the spectrum [4, 7], two effects canceled out and did not exhibit obvious spectra shift; at 6 min, the LSPR spectrum redshifted a lot, because the thickness to width ratio of the nanostructures was greatly reduced when the gold on the nanosphere was etched away. This continuous peak tuning is an important feature for LSPR biosensing. In this experiment only 60 nm of LSPR wavelength shift was observed, because the original size of the silica nanospheres was only 175 nm. We expect to have larger LSPR wavelength shift range with larger silica nanospheres and thicker gold deposition.
As drawn in the insert of Fig. 7b, the extinction of the LSPR spectrum reduces with nanoparticle size reduction, because absorbance scales with the volume of the nanoparticle and scattering scales with the volume squared .
The fabrication of gold nanostructures through dispersed nanosphere lithography and wet etching was investigated. The profile simulation of wet-etched gold nanostructures under different fabrication conditions was carried out and could be used as a reference for parameter settings in fabrication process. In the preliminary experiments, we selected silica nanospheres as the mask as they endure gold etchant, we observed the LSPR spectrum of the wet-etched nanostructures shifted at different etching duration, correlated with the simulated profile of the nanostructures.
This wet etching method can acquire different kinds of gold nanostructures for LSPR sensing. Compared with dry etching conventionally used in dispersed NSL, wet etching is more cost-effective, it also reduces the optical scattering due to the rough glass surface caused by unspecific dry etching, and can form nanoparticle clusters that might further enhance the electromagnetic field of the nanostructures.
We acknowledge Institute of Materials Research and Engineering (IMRE), A*STAR for its financial support of the project IMRE09/1C0420. We thank Miss Farhana Bibi Mahmud Munshi and Mr. Huei Ming Tan from National University of Singapore (NUS) for carrying out some wet etching experiments during their internship in IMRE, and Ms Sharon Oh in IMRE for beneficial discussions and help of taking some of the SEM images.