Three-dimensional (3D) air-gap metal-coated nanocavities with tunable geometries, changeable heights, and improved smoothness are fabricated by combining electron beam lithography (EBL), ultra dilute hydrofluoric acid solution wet etching (UDHFE), and metal magnetron sputtering technologies. With different shapes, heights, and separations of the nanocavities, the strong electromagnetic resonances inside the nanocavities are changed in different extent, resulting in broad gamut and sophisticated plasmonic color generation. The nanocavities-based metasurface is also used to construct a real-time and label-free refractive index sensor with 372 nm/RIU sensitivity, which shows distinct colorimetric change between different mediums. This nanocavities may find extensive potential applications in high-fidelity color printing, high-density information storage, and on-chip colorimetric label-free biomedical sensing.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
The variety of colors in nature constitutes our wonderful world. In principle, most of colors are produced by the scattering or partial absorption of materials [1,2]. In the past few years, plasmonics, another important color generation method, has aroused researchers’ great interests because of its extraordinary high resolution [3–6], broad color gamut [7–9], and high saturation [5,6,10,11]. Based on these advanced merits, plasmonic colors are widely used in color filters [12–14], broadband and perfect absorption [6,10,15–17], color printings [3,4,7,8,10,11,18,19], color holograms [20–22] and can even find applications in highly secure information encryption [23,24] and subwavelength dynamic display [9,23–30]. What’s more, plasmonic resonance is very sensitive to local environment, thus very suitable for biological sensing. Researchers expect the metasurface to work in the optical regime, and show clear wavelength shift to the surrounding environment change at the same time . So, biological analytes can be sensed by simply observing the color change of the metasurface, which will greatly simplify the sensing process.
In the past, plasmonic color was mainly generated by nanoparticles composed by noble metal. However, recent progress in plasmonic colors mentioned above is mainly based on nanostructures fabricated by EBL [5–8,11,13,19,24,31] and focused ion beam (FIB) [10,12,32]. EBL is the most commonly used fabrication method to fabricate nanostructures. There are also other novel fabrication methods which are usually used in combination with EBL, such as nanoimprint lithography (NIL)  and lasing writing . For example, Karthik Kumar et al. reported the first color printing at the optical diffraction limit fabricated by NIL . Xiaolong Zhu et al. reported that laser writing can print all primary colors with high speed, high resolution, and low power consumption . FIB is commonly used to fabricate nanohole arrays. For example, Fei Cheng et al. fabricated metal-dielectric-metal (MDM) nanohole arrays to realize high-resolution and angle-insensitive plasmonic color printing .
Despite the success, recent progress is mainly based on simple-patterned two-dimensional (2D) nanostructures, such as nanoholes [10,28,30], nanogrooves [12,16,33,34], nanoposts [3,5,7,13,31] and nanocubes [23,24]. In these 2D nanostructures, only 2D patterns and their separations can be tuned to change colors , making color-tuning limited. Take nanoposts as an example, only diameters and their separations can be tuned to change plasmonic colors, resulting in the lack of color tunability. What’s more, in order to generate broader gamut colors, nanostructures are optimized to be more spectral-sensitive to geometrical variations . But the structural deviations in nanofabrication make these spectral-sensitive nanostructures hard to tune precisely. So, developing nanostructures which can be tuned by diversified geometric parameters with different spectral sensitivities is an urgent and meaningful task.
3D nanostructures, however, can increase the color tunability significantly by more geometrical parameters. What’s more, other extraordinary optical properties can be achieved by the strong light-matter interactions inside the 3D nanostructures [35–39]. Nanocavities, as a specific 3D nanostructure, has drawn a wide attention because of the joint effects of resonance coupling and plasmonic Fabry–Pérot (FP) nanocavities [8,19,23,40], resulting in narrow-band resonance which means high saturation in color printing. Specifically, air-gap nanocavities can be fully immersed in liquid, making air-gap nanocavities-based metasurfaces ideal candidates for liquid biosensing .
In this study, arrayed 3D air-gap metal-coated dielectric nanocavities with tunable geometries, changeable heights and improved smoothness were precisely fabricated via a combination of EBL, wet etching and metal magnetron sputtering technologies As an example, 3D air-gap nanopin-cavities are shown in Fig. 1(a). The nanocavities can be tuned by diversified geometrical parameters with different spectral sensitivities. Thus, broad gamut and sophisticated plasmonic colors are generated with precise tunability, as shown in Fig. 1(b). Using the nanocavities-based metasurface, a refractive index sensor with the spectral sensitivity of 372 nm/RIU was developed. This type of 3D nanocavities has a promising prospect in high-fidelity color printing, high-performance color filter, high-density information storage, and high-sensitivity colorimetric sensing.
2. Fabrication methods and results
2.1 Fabrication methods
In this study, we fabricated 3D air-gap metal-coated nanocavities with a series of processes including EBL, UDHFE, and metal magnetron sputtering. Figures 2(a)-2(f) show the detailed fabrication processes.
Step 1: Silicon dioxide membranes with the thickness of 20 nm to 90 nm were grown on bare silicon wafers (four-inch, N-type doped, one-side polished, <1 0 0> crystal orientation, 2–4 Ω cm resistivity) by thermal oxidization, as shown in Fig. 2(a). Compared with other silicon dioxide membrane deposition techniques like low pressure chemical vapor deposition (LPCVD) and plasma enhanced chemical vapor deposition (PECVD), silicon dioxide membranes grown by thermal oxidation are more compact which ensures the controllability in the following wet etching process.
Step 2: A 60 nm-thick silicon nitride membrane was deposited on the surface of silicon dioxide layer by LPCVD which acts as a mask in the following wet etching process, as shown in Fig. 2(b).
Step 3: 80 nm-thick negative electron beam photoresist XR-1541 (HSQ) was spun onto the silicon nitride layer with hexamethyl disilazane (HMDS) for better adhesion. EBL was performed using a NanoBeam nB4 EBL system with an accelerating voltage of 80 kV and a beam current of 1 nA. Subsequently, the film was developed in a 0.26 N TMAH standard aqueous base developer, as shown in Fig. 2(c).
Step 4: 2D nanopatterns were transferred to the silicon nitride layer by reactive ion etching (RIE), as shown in Fig. 2(d).
Step 5: Taking 2D silicon nitride nanopatterns as masks, nanocavities were formed by UDHFE with the volume fraction of 1:60, as shown in Fig. 2(e). The etching time depends on the thickness of the silicon dioxide membrane and the critical sizes of the 2D nanopatterns, in other words, the heights and the morphologies of the nanocavities.
Step 6: 25nm-thick aluminum was deposited on the surface of the 3D air-gap nanocavities and the substrate to form metal-coated nanocavities with plasmonic colors by angle-tilt magnetron sputtering technology, as shown in Fig. 2(f).
After fabrication, we characterized a single 3D air-gap aluminum-coated nanocavity by scanning electron microscope (SEM), transmission electron microscope (TEM), and energy dispersive spectrometer (EDS), as shown in Figs. 3(a)-3(c). From the SEM image and the TEM image, the outline of the nanocavity is clearly shown. And from the EDS image shown in Fig. 3(c), aluminum can enter the nanocavity, which is beneficial for the strong resonance generation.
2.2 Diversified 3D air-gap nanocavities
The key technology is the high controllability during the wet etching process by means of ultra dilute hydrofluoric acid with the volume fraction of 1:60. Our experimental results show that silicon dioxide membranes grown by thermal oxidization were etched at the average speed of 32.07 Å/min in ultra dilute hydrofluoric acid solution with the volume fraction of 1:60 under the environment of 25 degrees Celsius. Such a slow etching speed made the nanocavities’ morphology highly controllable. The standard deviation of the etching speed is 0.96 Å/min which shows ultra high accuracy during the wet etching process. Detailed experimental data and calculation methods are shown in the Appendix A. Furthermore, the silicon nitride membrane was shown to be etched very slowly in such dilute hydrofluoric acid which could act as nanomasks during the silicon dioxide etching.
By simply changing the silicon dioxide membrane thickness and the etching time, 3D air-gap nanopin-cavities with the air-gap height (H) of 30 nm, 50 nm, 70 nm, and 90 nm were fabricated as illustrated in Figs. 4(a)-4(d). Note that the SEM images shown here and below are filmed before metal sputtering, which shows the morphology of the nanocavities more clearly. Since the nanopin-cap height (h) is designed to be 40 nm, light is guaranteed to irradiate into the nanocavities through such a thin silicon nitride cap, which is also transparent under the electron irradiation, as shown in Figs. 4(a)-4(d). What’s more, the diameters of the nanopillars are nearly the same which indicates precise fabrication ability by this method. The diameters of the nanopin-caps were chosen to be 190 nm. Since the air gap plays an important role in the optical properties of the nanocavites, the supporting nanopillars under the nanopin-caps are expected to be narrow to enlarge the air gap. But too narrow the supporting nanopillars are fragile and will easily be destroyed by external factors, such as fluid capillary force originated from wet etching and nitrogen gas pressure produced by drying. In our experiments, the diameters of the nanopillars under the nanopin-caps were etched to be around 1/3 (~60nm) of the nanopin-cap diameters, which is a good compromise between optical property and structure stability. The bright edges of the nanopillars were caused by charge accumulation after solution treatment and did no harm to the nanostructures.
More experiments show that the controllability of this fabrication method is universal. As shown in Appendix B and Appendix C, 3D air-gap nanocavities with 20 nm air-gap height and other sized nanocap were also successfully fabricated. In fact, by this fabrication method, the height of the nanocavities can be lower than 10 nm. Such low nanocavities can generate strong resonance coupling. Thus, it may be very helpful in many applications, including singe-photon emitters, nonlinear optics, and tracked or directed molecular reactions . Besides nanopin-cavities, other shape nanocavities with different number of arms were also fabricated. Figures 5(a)-5(d) show 50 nm-height nanocavities with two, four, six, and eight arms’ nanocaps. So many nanocavity morphology provides abundant color pixel candidates, resulting in broad gamut and sophisticated plasmonic colors generation.
2.3 Characterization of fabrication accuracy
Benefitting from the slow and controllable wet etching speed by ultra dilute hydrofluoric acid, the wet etching process was uniform, resulting in subtle nanocavities with highly controllable height and ultra flat surface. First, because of the self-stop SiO2-Si interface during wet etching process and the corrosion resistant merits of Si3N4-made nanomasks in ultra dilute hydrofluoric acid, the total height of the nanocavity (H + h) mainly depends on the high-accuracy thermal oxidization process and LPCVD process during the growing of SiO2 membrane and Si3N4 membrane. Figures 6(a) and 6(b) show the Atomic Force Microscope (AFM) image of the nanocavities and their corresponding altitude curve. The mean total height (H + h) of the nanocavity was measured to be 92.4 nm, which only has 2.7% deviation from the design height 90 nm.
Another advantage of ultra dilute hydrofluoric acid-etched nanocavities is their ultra flat surface, including both bottom surface and top surface. As for the bottom surface, the ultra dilute hydrofluoric-etched nanocavities’ bottom surface is smoother than the ones fabricated by reactive ion etching (RIE) method . By means of AFM, the root mean square (RMS) Rq of the bottom surface fabricated by UDHFE was measured to be 0.531 nm which indicates ultra flat surface. More information are shown in the Appendix D. On the contrary, the bottom surface fabricated by RIE is quite rough which can be easily distinguished by SEM images . More importantly, the nanocavities’ top surface roughness is nearly as smooth as the substrate, which shows great advantages compared to other fabrication methods such as buffered hydrofluoric acid etching. Figures 7(a)-7(d) show the differences between wet etching process by ultra dilute hydrofluoric acid (volume fraction HF(49%):H2O = 1:60) and buffered hydrofluoric acid (BHF, volume fraction HF(49%):NH4F(60%) = 1:6). We can clearly see that the BHF-etched nanocavities’ surfaces are quite unsmooth. This may be caused by the etching instability during rapid etching process. At the same time, such rapid etching rate is hard to control. Conversely, the ultra dilute hydrofluoric-etched nanocavities’ surface is quite smooth. Detailed roughness analyses are shown in the Appendix E. Such smooth surface and precise nanocavity fabrication are especially beneficial to avoiding the broadening of the resonance and reduction in spectral intensity because of its low optical loss . Both the accurate nanocavity height and ultra flat surface ensure that the nanocavities’ optical properties can accurately correspond to the optical designs by electromagnetism theories and finite element analyses, which will be discussed next.
3. Optical performance and applications of 3D air-gap nanocavities
3.1 Numerical simulations
In order to understand the light-matter interactions inside the nanocavities, numerical simulations were adopted. The structural model of a single 3D air-gap nanopin-cavity is depicted in Fig. 8(a). The height of the nanocap (h) is designed to be 40 nm, which is low enough to ensure that light irradiates into the underlying nanocavity, and also high enough to ensure the structural stability. The height of the nanocavity (H) is designed to be 50 nm, which is much smaller than the wavelength of visible light. Such low nanocavities ensure efficient plasmon coupling. Aluminum is coated both on the cap of the nanocavity, and on the bottom surface of the nanocavity, as shown in Fig. 3(c). The simulated spectrum is shown in Fig. 8(b). There are two optical resonant wavelengths, which correspond to different spatial electric-field distribution shown in Fig. 8(c). The spatial electronic-field intensity distribution shows that incident light is effectively confined into the nanocavity, leading to large field enhancement and thus resulting in narrow band resonance. We can also see that not only the electric intensity, but also the resonant spatial locations are totally different between the two resonant wavelengths, which indicates different resonant modes. It is further confirmed theoretically by electric charge distribution simulation as shown in Fig. 8(d). In fact, the air-gap nanocavities in our design can also be classified to a kind of modified MDM nanoresonators. In this design, surface plasmons are generated along the surface of multilayered metal and dielectric. What’s more, light is tightly confined in the nanocavities. Thus, coupling resonances with enhanced intensities of plasmon modes are generated which can convert incident white light into vivid colors efficiently.
3.2 Broad gamut color generation
By means of the strong electromagnetic resonance inside the nanocavities and their flexible tunability, broad gamut colors are generated by changing 3D air-gap nanocavities’ multiple geometrical parameters, including nanopin-cavities’ diameters (D), their separations (S) and their heights (H). A group of 3D nanopin-cavities with diameters (D) ranging from 160 nm to 370 nm, separations (S) ranging from 90 nm to 240 nm, heights (H) ranging from 30 nm to 90 nm were fabricated to realize broad gamut colors. By means of S-variation and D-variation color-tuning strategies, red, green and blue PKU letters were fabricated respectively, as shown in Fig. 9(a). Bright PKU logo was also generated with the diameter of 200 μm, as shown in Fig. 9(b). The resolution of the logo is 63,500 dots per inch (d.p.i.) which is comparable with the results of other researchers and at the optical diffraction limit. Furthermore, the nanocavity height influences the light-matter interactions inside the nanocavity directly, making itself another ideal geometrical parameter to tune the colors. Figure 9(c) shows a 120 × 120 μm2 colorful checkerboard with various nanocavity heights and diameters. The reflective colors are very sensitive to H and D. So, tunable H and D can lead to broad gamut colors, which was further confirmed by measured reflection spectra shown in Fig. 9(d). We have also mapped the colors in CIE 1931 to quantify the color printing performance in Appendix F. Note that all of the optical micrographs shown in Figs. 9(a)-9(c) are bright-field optical micrographs measured with an optical microscope (Olympus) with halogen illumination through a 20 × objective (numerical aperture 0.40). But the nanocavities can also display vivid colors in dark-field, as shown in Appendix G. The spectra shown in Fig. 9(d) were measured by a fiber-coupled spectrometer (Ideaoptics). It is also worth mentioning that the bright boundaries inside the two color blocks with D = 160 nm, H = 70 nm and D = 160 nm, H = 90 nm in Fig. 9(c) are caused by the writefield splicing during the EBL process. This can be solved by adopting larger writefield in the EBL equipment.
3.3 Sophisticated color generation
Theoretically, colors can be continuously tuned by changing the geometrical parameters of the nanostructures. But the structural deviations in nanofabrication make nanostructures hard to tune precisely. So, on the contrary to the pursuit of spectral-sensitive broad gamut colors, a spectral-insensitive geometrical parameter is also needed to realize precise color tuning. In our design, the shapes of the nanocaps are introduced to tune the colors precisely. Different shaped 3D air-gap nanocavities with 2, 4, 6, and 8 arms’ nanocaps were fabricated. Figures 10(a)-10(d), 10(e)-10(h), 10(i)-10(l) show their reflection spectra, optical micrographs, and SEM images respectively. Though there are huge changes in the shapes of the nanocavities, the changes of colors are unobvious. This phenomenon may be caused by two reasons. First, it is the strong resonances inside the nanocavities that mainly affect the 3D nanocavities’ color generation. The nanocaps, as a kind of 2D patterns, don’ have too much impacts in color generation. Second, as analyzed in Fig. 8(d), the electric dipoles are concentrated on the edges of the nanostructures. But although the 2D patterns are changed by choosing different arms, the distance between the edge and the center of the nanocavities is fixed. Changing the number of arms is equivalent to only changing the shapes of the edges. This spectral-insensitive arm-tuning strategy ensures the precise color tuning, reducing the influence of fabrication deviations tremendously. The high fabrication accuracy and the spectral-insensitive arm-tuning strategy jointly ensure the precise color generation. So, similar colors can be generated as design, which shows advanced sophisticated color generation ability.
3.4 High-sensitivity refractive index sensing
Plasmonic nanostructures, especially nanocavities , are very suitable for high-sensitivity refractive index sensing. The nanocavities-based metasurface can be used as a label-free sensor to test the refractive index in liquid environment. In our experiment, the reflection spectra of the device were measured in glycerin solution with the volume fraction of 10%, 20%, 30%, and 40%. As shown in Fig. 11(a), there are clear spectral shifts by changing the surrounding refractive index of the device. When the surrounding refractive index changes from 1.333 (water) to 1.389 (40% glycerin solution), the measured main resonant dip red-shifts from 538.64 nm to 559.95 nm. The linear fit to the experimental data is shown in Fig. 11(b), where the slope of the fit line was 372. As the sensitivity of refractive index sensing is commonly defined as spectral shifts (nm)/ refractive index changes, the sensitivity of our design is fitted to be 372 nm/RIU. Besides, there is a minor resonant dip around 430 nm showing a relative low sensitivity. This indicates the two resonant dips are generated by different resonant modes . Furthermore, since our device works in the optical regime, spectral shifts bring color-changing effects. So, this device is very suitable for colorimetric refractive index sensing, and is a promising candidate for naked-eye colorimetric label-free biomedical sensing. Here, as a demo, the color change of a PKU logo in different media is shown in Figs. 11(c) and 11(d). Obvious color change from yellow (in air) to purple (in alcohol) was realized. As shown in Appendix H, More color-changeable PKU logos with different geometrical parameters were also tested, which indicates the advanced colorimetric sensing ability of the device. It is also worth noting that although gold-based nanostructures are very suitable for biochemical sensing because of the narrow resonant band, convenient surface modification method, and outstanding biocompatibility of gold, aluminum-based nanostructures may also play an important role in the future biochemical sensing systems. Some researchers have already presented effective functionalization methods on aluminum surface for biosensing applications .
To judge the performance of our plasmonic refractive index sensing device, we have summarized several representative works in Table 1. For the convenience of comparison, only the resonant wavelength with the highest sensitivity at the optical regime is listed. The summary of papers shows that the sensitivity of our work is comparable to other studies, but there are still works which have higher sensitivities than ours. It is because in order to balance with the color printing application, we made some sacrifices in sensitivity. 3D nanocavities can be filled by liquid or gas analytes and generate strong light-matter interactions at the same time, which are very suitable for sensing applications. The sensitivity of 3D nanocavities will improve significantly by optimizing the nanostructures and materials for sensing applications. Besides plasmonic refractive index sensors, there are still refractive index sensors based on the diffraction effect of dielectrics, which provides another plan for sensing applications .
In conclusion, 3D air-gap nanocavities have been designed, fabricated, and applied in color printing and refractive index sensing. By numerical simulations, we have analyzed the resonant behaviors of the 3D nanocavities. By EBL-UDHFE nanofabrication method presented in this paper, nanocavities with tunable geometries, changeable heights and improved smoothness were precisely fabricated. Strong surface plasmon coupling resonances with enhanced intensities of plasmon modes were realized, resulting in multi-resonant and narrow-band spectra. So much tunable geometrical parameters of the nanocavities with different spectral sensitivities to geometrical variations provide efficient choices for wide gamut and sophisticated color generation in the practical color printing application. By nanocavities-based sensors, obvious color changes were observed in different media and high-sensitivity refractive index sensing was successfully realized. We believe that our proposed plasmonic metasurfaces may find extensive potential applications in high-fidelity color printing, high-density information storage, on-chip colorimetric label-free biomedical sensing in the future.
Appendix A Wet etching by ultra dilute hydrofluoric acid
In order to measure the silicon dioxide wet etching speed by hydrofluoric acid with the volume fraction of 1:60, 5 samples with the silicon dioxide membrane thickness of around 980 Å were fabricated by thermal oxidization. The wet etching time ranges from 5 min to 20 min. The membrane thickness was measured again after wet etching, as shown in Table 2. All the membrane thickness is measured by Thickness Measurement Instrument (ST2000, K-MAC).
The average etching speed μ is defined by:Eq. (1), the average etching speed is 32.07 Å/min.
The standard deviation of the etching speed σ is defined by:
According to Eq. (2), the standard deviation of the etching speed is about 0.96 Å/min.
Appendix B 20nm height 3D air-gap nanocavities
20 nm height 3D air-gap nanocavities were successfully fabricated with the sizes of nanopin caps and nanopillars nearly the same as the 30 nm-90 nm height ones which proves the universality of this fabrication method, as shown in Fig. 12.
Appendix C 3D air-gap nanocavities with larger sizes
Besides the nanocavities with the nanopin cap diameter of 190 nm, and the nanopillar diameter of 60 nm. fabricated in the main body of this paper, nanocavities with other sizes were also successfully fabricated. For example, Fig. 13 shows different height nanocavities with the nanopin cap diameter of 250 nm, and the nanopillar diameter of 120 nm.
Appendix D Substrate surface roughness analyses by AFM
By means of AFM, we can measure the roughness of ultra dilute hydrofluoric wet-etched surface precisely and analyze it quantitatively. The AFM image is shown in Fig. 14, which exhibits little unsmooth. By using AFM data processing software NanoScope Analysis, the RMS Rq is calculated to be 0.531 nm and the mean roughness Ra is calculated to be 0.424 nm, which are both small enough to be ignorable. In fact, the AFM measurement noise has a great influence on such a flat surface and the actual roughness of ultra dilute hydrofluoric wet-etched surface is less.
Appendix E Nanocavities’ surface roughness analyses by AFM
In order to analyze the nanocavities’ surface roughness quantitatively, nanopin-cavities were chosen as examples to be measured by AFM and post processed by NanoScope Analysis software. Nanopin-cavities fabricated by ultra dilute hydrofluoric acid were much smoother than the BHF-fabricated ones as shown in Fig. 15. The ultra dilute hydrofluoric-etched nanopin-cavities’ RMS Rq is calculated to be 1.00 nm and their the mean roughness Ra is calculated to be 0.79 nm. However, the BHF-etched nanopin-cavities’ RMS Rq is calculated to be 3.26 nm and their the mean roughness Ra is calculated to be 2.64 nm, which are both much rougher than the ultra dilute hydrofluoric-etched ones. All the analyses above prove the excellent nanocavities’ fabrication capability by UDHFE.
Appendix F CIE 1931 chromaticity coordinate quantification
In order to further quantify the color printing performance of the 3D nanocavities, we have mapped the checkerboard colors of Fig. 9(c) to CIE 1931 chromaticity coordinate. As shown in Fig. 16, nanocavities with different heights and diameters show distinct coordinate differences. So, height and diameter can be used as spectral-sensitive geometrical parameters to generate vivid plasmonic colors.
Appendix G Dark-field PKU logo
Plasmonic colors generated by 3D air-gap nanocavities can be measured in both bright-field and dark-field. And different colors are shown by different measurement methods. Taking the measurement of PKU logo (D=310 nm, S=90 nm, H=50 nm) as an example, bright-field micrography is shown in Fig. 9(b), and dark-field micrography is shown in Fig. 17. The dark-filed vivid colors manifest that 3D air-gap nanocavities can also be used in dark-field color generation, exhibiting universal applicability.
Appendix H Color changes of PKU logos in air and alcohol
Nanocavities with various height (H) and nanocap diameter (D) have already been proved to generate broad gamut plasmonic colors as shown in Fig. 9(c). What’s more, they are also very sensitive to the surrounding refractive index. Figure 18 shows the color-changing merit of PKU logos with different heights and diameters in air and alcohol. Thus, broad gamut label-free colorimetric sensing device can be realized by our design.
National Basic Research Program of China (973 Program, 2015CB352100); National Natural Science Foundation of China (61474006); National Key Laboratory Foundation.
The author thanks Dr. Yifei Mao for TEM sample preparation, and Prof. Rui Zhu for helpful discussions and fabrication advices.
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