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Abstract
In this work, we present high-performance surface plasmonic sensors using gold nanostructures and Bragg photonic structures. The gold film on the Bragg structure provides Tamm plasmon states (TPs). The Fano coupling between higher order TPs and Bloch-wave surface plasmon polariton (BW-SPP) on the gold nanoslit array results in a new hybrid Tamm-plasmon mode. Using finite-difference time-domain calculations, we demonstrate that the hybrid mode has the advantages of high surface sensitivity of BW-SPP mode and high resonant quality of Tamm state. The calculated plasmonic field distribution shows that the hybrid mode has a similar evanescent distribution with BW-SPP mode on gold surface and TPs field in the Bragg structure. The experimental results verify that the hybrid mode has one hundred times higher wavelength sensitivity than the Tamm state. The figure of merit of the hybrid mode is five times better than the BW-SPP mode in conventional nanoslit arrays. The real-time sensorgram further confirms that the hybrid mode has a much higher sensitivity and better signal to noise ratios in the biomolecular interaction measurement.
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1. Introduction
The surface plasmon resonance (SPR) has the advantages of real-time, label-free and sensitive detections [1–4]. The common SPR biosensors use a planer gold thin film coated on a glass substrate. However, the SPR has a large absorption loss in the metal which results in a low resonant quality. The resonant quality factor (Q) plays an important role in the detection limit of a SPR sensor. One simple way to increase the Q value is to reduce the evanescent field near the metal or increase the field in the transparent substrate. However, the overlapping be-tween the evanescent field and surface biomolecules is also decreased. It results in a low detection sensitivity to surface biomolecules. There is a tradeoff between the quality factor and surface sensitivity. Different from conventional prism-based SPR, the SPR can be directly excited by gold nanostructures. Plasmonic biosensors based on gold nanoholes, nanoslits, nanorods, and other complex nanostructures have been widely studied [5–7]. Most gold nanostructures have stronger optical evanescent field, but the resonant quality is poor. It leads to a lower figure of merit (FoM). To get a narrow linewidth of the surface plasmon polariton (SPP) resonance, the mode coupling such as waveguide or cavity mode coupled with SPR mode have been proposed [8,9]. They were improved by generating Fano coupling modes in the plasmonic nanostructures [10–13]. In this work, we present using distributed Bragg reflector (DBR) to enhance the sensing performance of gold-nanostructures. Metallic film on the DBR structures has been extensively studied recently. There is an optical Tamm plasmon state (TPs) which is directly excited at the continuous gold film and DBR interface [14]. Unlike conventional SPP mode, TPs presents a high-Q resonance and allows for directly coupling from free space and provide an extremely high reflective rate in spectrum. It can be applied for lasers, switches, filters, thermal emitters, and sensor [15–20]. However, most of its optical field is located at the metal/DBR surface. For this reason, TPs has a low surface sensitivity and are not effectively applied in biosensing applications. The wavelength sensitivity of TPs sensor is only about 40-60 RIU/nm [21] which is much lower than conventional SPR mode. To improve the sensitivity, hybrid plasmonic modes between TP and SPP [22–24] under strong coupling have been developed for the application for biosensing [25,26]. In this study, the gold film is replaced by a gold nanoslit array. The SPR on the periodic nanostructures is the Bloch-Wave surface plasmon polariton (BW-SPP). We found the Fano coupling between higher order TPs [27–29] and the BW-SPP mode produced a sharp hybrid mode in reflective system. The resonant quality, surface sensitivity and near-field distribution of the hybrid mode were studied by the finite-difference time-domain (FDTD) calculations. The resonance linewidth was reduced from 20 nm (BW-SPP mode) to 4 nm (hybrid mode). The hybrid mode had an evanescent field distribution similar to the BW-SPP field on the gold surface and a Tamm state-like field in the Bragg substrate. The Bragg structure helped reducing the SPR loss in the metal. It greatly increased the resonant quality without decreasing the surface sensitivity. The theoretical FoM was increased by thirty times as compared to gold nanoslit arrays. The performance of the hybrid mode was also experimentally verified by wavelength/thickness sensitivity tests and biomolecular interaction measurement. Those results confirm that the high performance of the new hybrid mode.
2. Theory and FDTD calculations
There are three kinds of plasmonic nanostructures in this study as illustrated in Fig. 1(a). They are gold film on DBR structure, periodic gold nanostructure on glass, and periodic gold nanostructure on DBR structure, respectively. For a thin gold layer on DBR, the TPs occur in the bandgaps of the DBR structure. The eigenmode of TPs can be determined by the phase condition [30]. The resonant frequency of TPs near the center of the Bragg reflector stop band is approximated by:
Fig. 1. (a) The layouts of three different kinds of nanoplasmonic sensors: (left) gold film on DBR structure; (middle) periodic gold nanostructure on glass; (right) periodic gold nanostructures on DBR structure. (b) The calculated reflection spectra for DBR structure (Bragg state) and gold film on DBR (Tamm plasmon state). The gold film thickness is 50 nm. A sharp Tamm state (TP1) in the first-order Bragg band and broad Tamm states (TP2, TP3) in high-order Bragg bands. (c) The calculated spectra for gold nanoslit array on a glass substrate (BW-SPP mode) and the same gold nanoslit array on a DBR structure (hybrid mode). The hybrid mode is formed due to the Fano coupling between the broad Tamm state (TP3) and BW-SPP mode. The gold film thickness is 50 nm, the period of the gold nanoslit is 580 nm and the slit width is 100 nm. (d) The calculated optical distribution of first-order Tamm plasmon state. Most optical energy locates in the DBR structure which leads to a low surface sensitivity.
As seen in Fig. 1(c), the gold nanoslit array on the DBR structure can generate a new hybrid mode with very narrow linewidth. We performed the FDTD simulations by varying the period of the gold nanoslit array for the optimal condition of the coupling effect. To compare the sensing performance, gold nanoslit array on glass is also simulated with the same nanoslit structure: 100-nm slit width and 50-nm thickness of gold film. The BW-SPP wavelength as a function of gold nanoslit period is shown in Fig. 2(a). According to Eq. 2, the resonant wavelength can be simply expressed by λ∼nmediumP. The line 1 (L1) is the surface BW-SPP mode which is the sensing mode in the surrounding medium (water). The line 2 (L2) is the substrate (glass) BW-SPP mode. This mode is insensitive to surface refractive index change. Both lines follow well with the prediction of Eq. 2 (${\textrm{n}_{medium}} = 1.33\; \textrm{and}\; 1.46)$. Figure 2(b) shows the spectra for the gold nanoslit arrays on DBR. Compared to Fig. 2(a), there are dark bands related to TPs in the sidebands of the DBR. The L1’ is associated with hybrid mode. It occurs when the resonant profiles of TPs and the surface BW-SPP mode are overlapped. The L2’ is associated with hybrid mode which is the coupling mode between the substrate BW-SPP and TPs. The L2’ and L1’ come from the couplings of the TP mode and the BW-SPP mode. The TP mode happens at the bandgap of the DBR. If the resonant wavelength of the BW-SPP modes are not in the bandgap region, there will have no enhanced resonance (the hybrid mode). For these regions, we thus see splitting of the lines in Fig. 2(b). It is noted that there is a new dark line (L3’) in the spectral graph. The mode is the grating coupling to the waveguide mode. The line appears at about 600 nm in the spectra below L1’, as compared to the photonic bandgap diagram of the gold film on the DBR structure (Figure S1(b) in Supplement 1) which is the first order of Tamm plasmon mode. The waveguide mode is also insensitive to surface refractive index change. Figure 2(c) shows the resonant wavelength and Q value for different period of gold nanoslit array on glass. The Q value is calculated by (dip wavelength)/(resonant linewidth). The linewidth of BW-SPP mode is large which leads to a small Q value. Figure 2(d) shows the resonant wavelength and Q value for different period of gold nanoslit array on DBR. The Tamm state is coupled to BW-SPP. It has a small full width at half maximum (FWHM) and high Q value. In Fig. 2(d), the maximum Q value occurs at 580-nm period. The resonant wavelength is around 790 nm, which is close to the TPs in the second sideband of the Bragg reflector.
Fig. 2. (a) The reflection spectra of conventional gold nanoslit array in water as a function of the period. There are two dark lines: line L1 is the BW-SPP mode on the gold surface which is the surface sensing mode; line L2 is the BW-SPP mode in the substrate (glass) which is insensitive to surface refractive index. (b) The reflection spectra of gold nanoslit array on DBR as a function of the period. Line L1’ is the hybrid mode with surface BW-SPP, line L2’ is the hybrid mode with substrate BW-SPP, line L3’ is the grating-coupling waveguide mode. (c) The calculated dip wavelengths and Q values of surface BW-SPP mode for different periods of gold nanoslit arrays in water. (d) The calculated dip wavelengths and Q values of hybrid mode for different periods of gold nanoslit arrays on DBR under water environment. There is a maximum Q value at 580-nm period. The dip wavelength is overlapped with the Tamm state (TP2) of the DBR structure
The optical field distribution is a key factor for surface sensitivity. The calculated optical field distribution for BW-SPP mode of 580-nm-period nanoslit array on glass is shown in Fig. 3(a). The optical field distribution for the hybrid mode at the gold/DBR interface is shown in Fig. 3(b). The evanescent distributions on the gold surface are almost the same for both cases. Nevertheless, there is large difference under the gold layer. The DBR layer helps localizing the optical field in the dielectric multilayer and leads to a smaller SPP loss. Both BW-SPP mode and the hybrid Fano mode have similar distribution on the gold surface as seen in Fig. 3(c). Therefore, their wavelength sensitivities would also be similar. To verify this point, we have calculated the spectra of gold nanoslit array in different refractive index medium from 1.33 to 1.37. Figures 3(d) and 3(e) show the spectral results. At the nanoslit period of 580 nm, the dip of surface BW-SPP mode is near 780 nm and the hybrid mode is located at 790 nm. Clear redshifts of the resonant dips are observed when the refractive index increases. Both BW-SPP mode and hybrid mode have similar bulk refractive index sensitivity about 580 nm/RIU and 570 nm/RIU, respectively. To compare the performance of a biosensor, the FoM is widely used. The FoM is determined by the wavelength sensitivity (Sλ) and FWHM of the resonant spectrum, FoM = Sλ/FWHM. Figure 3(f) shows the calculated FoM of the hybrid mode, BW-SPP mode, and first-order Tamm plasmon mode. The FoM of the hybrid mode is about 32 times higher than general BW-SPP mode and 210 times than TPs in reflective system. Although the Tamm state has a highest Q value but it is less useful for a biosensor because of the ultralow wavelength sensitivity and its distribution of electrical field is mostly inside the DBR layer.
Fig. 3. (a) The plasmonic field distribution (Ez) of conventional gold nanoslit array. (b) The plasmonic field distribution (Ez) of the hybrid mode for gold nanoslit array on the DBR. (c) The plasmonic field distribution along the z axis. The evanescent fields on metal surface are similar for both nanostructures. The hybrid mode has a Tamm-state like distribution in the DBR layer. (d) The calculated reflection spectra of conventional gold nanoslit array under different refractive index medium. (e) The calculated reflection spectra of the hybrid structures under different refractive index medium. (f) (upper) The shifts of dip wavelengths as a function of refractive index for three different modes: hybrid mode, BW-SPP mode and Tamm plasmon mode. (lower) The calculated FoM values for three different modes: hybrid mode, BW-SPP mode and Tamm mode.
There are other optimal parameters related to the gold nanoslits/DBR structure, such as thickness of gold film, width of gold nanoslits, numbers of DBR layer and dielectric constants of DBR materials. Figure 4(a) shows resonant profiles for different thickness of gold film. The period is 580 nm and the slit width is 100 nm. The sharpest resonance occurs at 50-nm gold thickness. Figure 4(b) shows the influence of the nanoslit width. Although, the width which is smaller than 100 nm has better FWHM, but the reflective rate is shallow. The width near 100 nm has chance to reach the reflective rate to 0. When the width is wider than 100 nm, the resonance profile is broadened. It is almost disappeared when the width is larger than 200 nm. Figure 4(c) shows the calculated FoM value as a function of slit thickness and slit width. Based on the simulation results, the optimal condition of gold nanoslit on DBR structure is about 50 nm gold layer with a slit width of 100 nm. The number of periods of DBR also affects the resonant profiles as shown in Fig. 4(d). The periodic nanoslits result in a grating coupling effect which couples incident photons to the DBR waveguide. Increasing the number of layers will increase the effective thickness of multilayer waveguide. Some incident photons near the hybrid mode will be guided to the waveguide. It reduces the resonant quality of the hybrid mode. There is an optimal condition occurred at 6 layers of DBR structures. The sharpest dip also owns the minimum FWHM of 3 nm as shown in Fig. 4(e). In Fig. 4(f), the material of lower refractive index is fixed at 1.46 (SiO2) and the index of the higher index material is increased. At the beginning, the refractive index near 1.53 is close to refractive index of SiO2. The resonant profile is close to BW-SPP mode on the glass substrate. When the substrate refractive index increases, a red shift is found. It indicates that some optical field is redistributed into the high index material. The hybrid mode has a higher equivalent refractive index and thus the resonant wavelength is redshifted. The increase of the refractive index also results in the increase of the resonant quality. Figure 4(g) shows the calculated Q factor as a function of the refractive index of dielectric materials. There is a maximum Q-factor when the refractive index is about 2.6, which is close to the refractive index of TiO2. This is the reason why we choose TiO2 as a high refractive material in DBR composition. With these calculated conditions from FDTD simulations, this work provides a optimal coupling condition between plasmonic nanostructures and photonic structures that hybrid mode sensor could performs a high sensitivity, high FoM, amd high reflective rate in reflection measurement system.
Fig. 4. (a) The calculated spectra for different thickness of gold film in the nanoslit array on DBR structure. (b) The calculated spectra for different slit width in the nanoslit array on DBR structure. (c) The calculated FoM of the hybrid mode as a function of slit width (upper) and gold film thickness (lower). (d) The calculated spectra for different number of DBR layers in the nanoslit array on DBR structure. (e)The Dip intensity (upper) and FWHM (lower) of the hybrid mode as a function of the number of layers. There is an optimal condition at 6 layers. (f) The calculated spectra for different dielectric material in the high refractive index layer of the DBR structure. (g) The Q value of the hybrid mode as a function of refractive index and the corresponding material in the high refractive index layer of the DBR structure.
3. Measurement setup and results
Based on the simulation results, the fabrication parameters of nanostructures: 50-nm gold film, 100nm-slit width, 580-nm period for gold nanoslit; six pairs of TiO2/SiO2 layers are used for the DBR structure. The optical sensor was fabricated by following steps. A glass substrate was cut as 400 mm2 and cleaned by piranha solution (sulfuric acid and hydrogen peroxide, vt % 3:1). Then, the glass substrate was coated with the alternative dielectric layer through RF-sputtering of SiO2 (80 nm) and TiO2 (40 nm) with 6 pairs. After RF-sputtering, the 50-nm gold thin layer was coated on the DBR structure by an electron-beam evaporator. For the gold nanoslit array on the DBR, periodic nanoslits were directly written using focused ion beam (FIB) (Helios NanoLab 660, Thermo Fisher Scientific). The SEM images were taken right after the FIB fabrication. The sensing chip was combined with the microfluidic channels. The upper PMMA cover with multiple inlets and outlets was designed and fabricated by a CO2 laser engraving machine. The middle layer was the multiple microfluidic channels in a 3M tape with 40 $\mathrm{\;\ \mu m}$ thickness, which was also prepared by the laser engraving machine and the width of channel was 1.5 mm. For measuring system, a collimated white light source is incident from the solution. (The illustrate of measuring system and microfluidic channel is shown in Supplement 1 Figure S3.) The light is polarized to p-polarization for the excitation of BW-SPP mode. The polarized light is focused onto the sample. The 20X long working distance objectives lens (Mitutoyo) was used to focus light onto the surface of the nanoslit array. The spectrum of the reflective light was measured by a commercial spectrometer (BWTEK). For comparisons, both conventional gold nanoslit array on a glass substrate and 50-nm gold film on the DBR substrate were also fabricated. Figure 5(a) shows the SEM images of the fabricated DBR nanostructure and the gold nanoslit array. Figure 5(b) shows the measured and calculated spectra of the gold nanoslit array. Both experimental and calculated spectra are quite consistent. The FWHM of the BW-SPP mode is about 40 nm. Figure 5(c) shows the measured and calculated spectra of the gold nanoslit array on DBR structure. The FWHM is reduced to 8 nm and the spectrum is quite consistent with FDTD simulation result. To test the performance of the sensor, both BW-SPP mode and hybrid mode were measured under different refractive index environment. The glucose solution buffer with different refractive index ranged from 1.33 to 1.35 were injected into the fluidic channel. Figure 5(d) shows the dip wavelengths as a function of refractive index for three kinds of sensing modes: TPs, BW-SPP and the hybrid mode. The slope of the curves shows the wavelength sensitivities for these modes. The sensitivity of the hybrid mode (533 nm/RIU) is close to the BW-SPP mode (564 nm/RIU). The TP mode has a sensitivity only 45.4 nm/RIU. The measured wavelength sensitivity is consistent with the FDTD simulations. Moreover, the experimental FOM can be calculated by measured wavelength sensitivity and FWHM. The FoM of BW-SPP mode is about 12 RIU−1. It is improved to 60 RIU−1 when using the hybrid mode. For the bio-sensing application, the thickness sensitivity is more important than the bulk wavelength sensitivity. To determine the thickness sensitivity, different thickness of transparent layer was coated on the gold surface. The coating layer was formed by Al2O3 dielectric layer using an atomic layer deposition machine (Syskey Technology Co., Ltd.) to simulate the conformal coating of the biomaterials. The dip wavelength has an obvious redshift as shown in Fig. 5(e). Figure 5(f) shows the measured dip wavelengths for Al2O3 coating film with the thickness from 0 to 30 nm under the same bulk medium (nwater = 1.33). Both the experimental and theoretical results are quite consistent. The experimental and calculated thickness sensitivity of the hybrid mode are 0.93 nm/nm and 0.96 nm/nm, respectively. It indicates 0.1 nm thickness of biolayer can be directly measured with a wavelength resolution of 0.1 nm.
Fig. 5. (a) The SEM images of the fabricated DBR structure (upper) and the gold nanoslit array (lower). (b) The measured and calculated reflection spectra of gold nanoslit array on glass. (c) The measured and calculated reflection spectra of the hybrid nanostructure (gold nanoslit array on DBR). (d) The measured dip wavelength wavelengths of TPs, BW-SPP mode and hybrid mode for different refractive index medium. The slopes show the measured wavelength sensitivities. RIU is the refractive index unit. (e) The reflection spectra of the gold nanoslit array on DBR before and after the coating of dielectric layer. (f) The calculated and measured thickness sensitivity for different thickness of aluminium oxide layer on the hybrid nanostructure.
The antibody and antigen interaction experiment were conducted to compare the performance of hybrid mode and BW-SPP mode. Before the measurement, the surface of gold-coated sensor was modified by cysteamine for 2 hours [26]. Then glutaraldehyde in 2-(N-Morpholino) ethane sulfonic acid (MES) buffer was used as a linker for 1 hour. It links the amino group of the cysteamine and the amino group of the protein. In the bio-interaction measurement, we choose the antibody (Anti-Human IgG) and antigen (IgG) interaction as an example. These proteins were prepared in phosphate buffered saline (PBS) buffer. All chemicals and proteins are purchased from Sigma-Aldrich. The sensor surface was cleaned by deionized water (DI water). Figure 6(a) shows the antibody-antigen binding and washing processes. In order to verify the advantages of the sharp resonant profile of the hybrid mode, we analyzed the spectral data using the resonant wavelength tracking method as shown in Fig. 6(b) [33,34]. In this method, the resonant spectrum is divided into two fixed areas: left spectral area (B) and right spectral area (A). The analysis of the raw spectral data is calculated by the equation: $\mathrm{\gamma } = ({\textrm{A} - \textrm{B}} )/({\textrm{A} + \textrm{B}} )$. When the spectrum is red-shift due to binding of the protein, $({\textrm{A} - \textrm{B}} )/({\textrm{A} + \textrm{B}} )> 0$. On the other hand, $\mathrm{\gamma }$ is decreased when unbound protein is washed away. This resonant wavelength tracking approach is analogous to the conventional position tracking method using two segments of photodetectors, which can precisely track the central position of an optical spot. The tracking resolution is closely related to the spot size. A smaller spot has a better sensitivity to the central position. The conventional position tracking is in real space, while the resonant wavelength tracking approach is in the wavelength domain. A sharper resonant profile results in a better detection sensitivity. Figure 6(c) shows the response signals for different steps of bio-interactions. The black line and red line show the responses using gold nanoslit on glass and gold nanoslit on DBR structures, respectively. There is an abrupt change occurred at injection from water to PBS due to the change of bulk refractive index. Different concentrations of IgG from 100 ng/ml to 10 µg/ml are injected into the fluidic channel. As observed from the sensorgram, the performance of the hybrid mode on the gold nanoslit/DBR sensor is much better than the BW-SPP mode on gold nanoslit array. The $\mathrm{\gamma }$ value of the hybrid mode has a higher response and lower noises. Figure 6(d) shows the steady state responses for different concentrations of IgG. The noise level is about $4.6 \times {10^{ - 4}}$ and $2.5 \times {10^{ - 3}}$ for hybrid mode and BW-SPP mode, respectively. The mean signal is increased about 1.5 times and the noise level is reduced about 5 times. It verifies that the hybrid mode-based sensor has a better sensing performance than BW-SPP based sensor due to the high Q value and sharp Fano resonance. Figure 6(e) shows the kinetic measurement during the antigen and antibody interactions, the Anti-IgG/IgG association states are clearly observed. The curves can be well fitted by using the binding equation for the association binding process [35,36]. Anti-IgG has a high affinity for the IgG antigen. The binding constant can be fitted by an effective observed first-order rate constant (kobs),
where [AB] is the surface concentration of bound complex of Anti-IgG and IgG, [A] is the concentration of IgG analyte and [B] is the surface concentration of Anti-IgG. The association rate constant ka and dissociation rate constant kd are calculated by fitting equation (4) to the measured kinetic curves for different concentrations of IgG. The fitted kobs is a linear function of [A]. The slope of the fitted line is ka and the intercept is kd. From the binding curves, the kd is calculated $3.3 \times {10^{ - 4}}\; \textrm{se}{\textrm{c}^{ - 1}}$ and ka is $6.6 \times {10^4}\; \; {\textrm{M}^{ - 1}}\textrm{se}{\textrm{c}^{ - 1}}$. The resultant binding affinity, kd/ka, is 5 nM which is close to related research31. Through the calculation of kd/ka, the hybrid sensor is suitable to observer the equilibrium dissociation between antibody and antigen. The conventional Tamm plasmon has a poor surface sensitivity in the bio-reaction measurement. However, when the gold film on the Tamm structure is modified to a nanoslit array, the coupling of higher order Tamm mode and BW-SPP mode can generate a hybrid mode. This hybrid mode greatly improves the sensitivity of the Tamm plasmon structure.Fig. 6. (a) The flow chart of biomolecular binding condition on the surface of the hybrid mode-based sensor. (b) The illustration of the resonant wavelength tracking method. The resonant profile is divided into two areas: left spectral area (B) and right spectral area (A). The analysis of the raw spectral data is calculated by the equation: γ=(A-B)/(A + B). This resonant wavelength tracking approach is analogous to the central position tracking of an optical spot using two segments of photodetectors. (c) The sensorgram of spectral contrast signals (γ) for different surface binding conditions. The responses of hybrid mode-based sensor have clear signal changes and lower noise levels. (d) The binding signals and standard deviations for different surface binding conditions in the equivalent states. The performance of the hybrid mode-based sensor is much better than the conventional BW-SPP based sensor. (e) The measured association kinetic curves and the fitting curves using Eq. (4).
4. Conclusion
The general Tamm state-based sensor is insensitive to surface biomolecules. In this work, we use gold nanoslit array to replace gold thin film on the DBR layer. The hybrid mode has a similar evanescent field with the BW-SPP mode on the gold nanoslit array. Below the gold film, the optical field is redistributed to the DBR layer, similar to the Tamm plasmon state. Such hybrid mode combines the advantages of Tamm state-based sensor and SPP-mode sensor. In the experiments, the hybrid mode-based sensors were fabricated and measured using a simple reflection spectral system. The real-time and label-free measurement of the antibody-antigen interactions verified that the hybrid mode has a much better sensing performance than conventional BW-SPP mode. In this work, we used FIB machine to make the nanoslit array on the DBR structure. The structure can also be massively produced using advanced photolithographic processes. The measurement system is simple and cheap. Such high-performance chip-based sensors will benefit many kinds of label-free biosensing applications.
Funding
Academia Sinica (AS-GC-111-M02, AS-IDR-111-15); Ministry of Science and Technology, Taiwan (108-2221-E-001-019-MY3, 108-2221-E-002-155-MY3).
Acknowledgments
R.-L. Chern and P.-K. Wei are corresponding authors of this work. This work was supported by the Ministry of Science and Technology of Taiwan (Project No. 108-2221-E-001-019-MY3), (Project No. 108-2221-E-002-155-MY3), and Academia Sinica (Project No. AS-GC-111-M02, AS-IDR-111-15). Technical support from the core facilities for nanoscience and nanotechnology, in Taiwan, is acknowledged. K. Flockhart thanks the National Science Foundation for help identifying collaborators for this work.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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