Full-visible multifunctional aluminium metasurfaces by in situ anisotropic thermoplasmonic laser printing

Yinan Zhanga, Liu Shiab, Dejiao Hua, Shiren Chena, Shouyi Xiec, Yudong Lua, Yaoyu Caoa, Zhuqing Zhub, Long Jina, Bai-Ou Guana, Sven Roggec and Xiangping Li*a
aGuangdong Provincial Key Laboratory of Optical Fiber Sensing and Communications, Institute of Photonics Technology, Jinan University, Guangzhou, 510632, China. E-mail: xiangpingli@jnu.edu.cn
bSchool of Physical Science and Technology, Nanjing Normal University, Nanjing, 210023, China
cSchool of Physics, The University of New South Wales, Sydney 2052, Australia

Received 4th January 2019 , Accepted 29th January 2019

First published on 29th January 2019


Multifunctional metasurfaces fusing multiple sets of plasmonic antennas within a single layer open a new class of functional optical elements with superior integratability. Even though laser printing of metallic nanostructures has provided a new paradigm for post-customizing metasurfaces in a cost-effective means, the intuitively isotropic thermoplasmonic shape transition adversely tampers with the optical responses of complex plasmonic nanostructures to multiplexed attributes of light, failing advanced multi-functionalities. Herein, we demonstrate full-visible multifunctional metasurfaces by in situ anisotropic laser printing of Al cross nanostructures using single femtosecond pulses. The shape transition and corresponding plasmonic resonances of the two orthogonal arms can be independently and exquisitely modulated in the full-visible range with ultralow cross-talk by controlled irradiations. This is achieved by exploiting polarization-controlled ultrafast thermoplasmonic heating of Al (one-order-of magnitude faster than Ag and Au) and subsequent curvature-driven surface atom migration. Thereby, Janus prints featuring structural color/phase-modulated holographic images can be accomplished with polarization-controlled switches. This newly-developed laser post-printing method can be easily generalized to multiple sets of plasmonic antennas at single pixel levels and thus opens a new strategy to customize complex plasmonic nanostructures for multi-functional flat optics with considerable miniaturization and up-scalability including waveplates, holographic encryption and multiplexed optical storage.



Conceptual insights

Laser printing plasmonic nanostructures have emerged as a low-cost and large-scale lithography solution for high resolution structural color and flat optical device printing, holding great promise for integrating with real-life applications. However, limited control over the plasmonic shape transition and resonance tuning has significantly hindered the development of advanced metasurfaces with multifunctionalities. We demonstrate for the first time a new level of plasmonic laser printing, i.e., in situ exquisite and anisotropic thermoplasmonic laser printing for full-visible multifunctional metasurfaces. This is achieved by exploiting ultrafast single femtosecond laser pulses to post-process low-cost and COMS-compatible aluminum (Al) plasmonic nanostructures. The shape transition can be reliably modulated by controlled irradiations for full-visible plasmonic resonance tuning. Thanks to the one-order-of magnitude larger electron–phonon coupling coefficient than their counterparts silver and gold, exquisite and anisotropic shape transition of Al nanostructures can be achieved with negligible crosstalk. Thus, multiplexing and post-customizing optical responses (e.g. color appearances, phase shifts) of single complex nanostructures becomes possible, which unlocks the full potential of laser processing metasurfaces with multi-functionalities for daily life integration in a scalable and cost-effective means.

Metasurfaces,1–3 composed of arrays of subwavelength meta-atoms, offering unprecedented light field management capability have emerged as a new paradigm for planar optical elements such as ultrathin lenses,4 waveplates,5,6 metaholograms,7–12 and color prints beyond diffraction limits.13,14 Recently, multifunctional metasurfaces by combining multiple sets of plasmonic antennas exhibiting distinct optical responses to multiplexed attributes of light have upsurged for versatile functional devices.15–20 In this context, metallic antennas21 with plasmonic resonances sensitive to the structure geometry, wavelength, and polarization have been suggested as indispensable ingredients for meta-atoms. High-precision and complicated lithography procedures such as electron-beam lithography and focused ion beam lithography are a necessity to pattern pre-designed plasmonic nanostructures, which are expensive and prevent their scalable applications from post-processing customization. Alternatively, intense laser exposure can locally generate enormous heat for photothermal melting or even ablation of plasmonic nanostructures,22–31 which opens a new route to customize plasmonic resonances for functional metasurfaces. It should be mentioned that other methods, such as the thermo-optical effect, microfluidic channels, and electrical charge-carrier injection can also be used to tune the resonances of metasurface elements.32–36

It is generally believed that photothermal reshaping of plasmonic nanostructures starts after absorbing an intense pulsed illumination exceeding certain thresholds to melt the material and the subsequent morphological change is governed by surface tension contraction to minimize interfacial energy.24,27 Thus, plasmonic nanostructures incline to isotropically reshape into spherical morphologies, with limited flexibility for controlled shape transition and full-visible resonance tunability, leaving molding optical responses of complex nanostructures to multiplexed attributes of light unattainable and thus failing multifunctionalities. With a relatively lower bulk melting temperature (∼933 K) and larger electron–lattice coupling coefficients than their counterparts silver and gold, aluminum (Al) plasmonic nanostructures can potentially allow anisotropic shape transition by regulating the nanoscale thermoplasmonic effect. In addition, as a low-cost, earth-abundant and CMOS-compatible element alternative to prevailing noble metals, Al nanostructures support full-visible range plasmonic resonances with excellent durability self-terminating by thin surface oxides.37–45

Here, we demonstrate full-visible multifunctional metasurfaces by in situ anisotropic laser printing of complex Al nanostructures using single femtosecond (fs) pulses. In particular, we reveal that the shape transition of the two orthogonal arms of Al cross nanostructures can be independently and exquisitely modulated by polarization-controlled ultrafast thermoplasmonic heating and subsequent curvature-driven surface atom migration. Thereby, the plasmonic resonances in the orthogonal directions can be respectively tuned across the entire visible range with ultralow crosstalk, enabling in situ customization of Janus displays featuring structural color/phase-modulated holographic images with a polarization-controlled switch. Our methods provide freedom for in situ customization and post-processing capability without the need for further lithography procedures.

The concept of in situ anisotropic thermoplasmonic laser printing Al nanostructures for multifunctional metasurfaces is illustrated in Fig. 1. The as-prepared metasurface consists of arrays of Al cross structures on top of a 30 nm thick SiO2 spacer layer on the 100 nm thick Al substrate (Fig. 1a). The width and the length of each arm of the cross structures are designed to be 30 and 150 nm, respectively. The large aspect ratio of such Al cross structures on the metal–insulator–metal (MIM) configuration yields a distinct contrast in thermoplasmonic absorption as well as high curvatures at the tips. In addition, the MIM configuration can serve as an efficient phase shifting device and generate 2π-phase coverage. Single fs pulsed beams with controlled polarization and fluence density46 are tightly focused onto the sample to reshape each constituent arm of the Al cross structures by anisotropic surface diffusion (Fig. 1a). As a consequence, independent and exquisite shape transition with a decreased aspect ratio of the two orthogonal arms can be reliably manipulated, enabling full-visible range optical resonance modulation. Fig. 1b shows the scanning electron microscope (SEM) images of the initial Al cross arrays, arrays with horizontal arms reshaped firstly and then vertical arms reshaped succedently. Thereby, in situ multifunctional metasurfaces can be reproducibly printed by independently modulating the plasmonic resonances in the two arms. As a proof-of-principle demonstration, multifunctional metasurfaces by molding resonance color appearances/phase modulations in the orthogonal arms at single pixel levels are accomplished. Fig. 1c illustrates a representative crypto-display metasurface by encoding color images and holographic images in the orthogonal arms, respectively. Under normal incoherent white light illumination, a color image can be observed in the reflection channel while under coherent laser beam illumination, a holographic image can be retrieved at the diffraction channel. This demonstrated that anisotropic laser processing of Al nanostructures can be easily generalized to achieve other advanced functional meta-devices, such as waveplates, gratings, lenses, and vortex generators operating in the full-visible range.


image file: c9nh00003h-f1.tif
Fig. 1 Concept of full-visible multifunctional metasurfaces by anisotropic laser printing. (a) Schematic illustration of anisotropic thermoplasmonic surface diffusion of Al cross structures in an MIM configuration by polarization-controlled single fs pulses. The red arrows illustrate the curvature-driven surface atom migration. The width, height and length of the Al cross structures are w = 30 nm, h = 30 nm and l = 150 nm, respectively. The thickness of the SiO2 layer is t = 30 nm. The cross structure is arranged in a square lattice array with a periodicity p = 200 nm. (b) Top-view SEM images of (i) initial Al cross arrays, arrays with (ii) horizontal arms reshaped firstly and (iii) then vertical arms reshaped succedently at the laser fluence of 94.6 J m−2. Scale bar: 150 nm. (c) Schematic illustration of a representative dual-function metasurface. Images multiplexed in reflection and diffraction channels can be switched by incoherent white light and coherent illuminations with orthogonal linear polarization.

Without loss of generality, let us first consider physically anisotropic Al nanorods in an MIM configuration. The Al nanorods with a width of 30 nm and a length of 150 nm are arranged periodically with a periodicity of 200 nm (Fig. 2a). The MIM configuration forms a Fabry–Perot (FP) resonator and significantly enhances the absorption of the incident pulse energy within the nanorods close to 80% centered around the resonance wavelength under a linear polarization along the x direction (Fig. 2b). In contrast, a broadband reflectance over 80% and a drastically reduced field strength under irradiance by y-polarized illumination are displayed compared with that of the x-polarization incidence (Fig. S1, ESI). The polarization-controlled distinct contrast in absorption is key to the subsequent anisotropic thermoplasmonic effects at the nanoscale.


image file: c9nh00003h-f2.tif
Fig. 2 Thermoplasmonic surface diffusion of Al nanorods. (a) Illustration of laser-induced shape transition of Al nanorod arrays in an MIM configuration. (b) Polarization dependent thermoplasmonic absorption (x- and y-polarization are parallel and orthogonal to the rod longitudinal axis, respectively). (c) Temperature distribution of an Al nanorod at 100 ps after the fs pulse with an energy density of 56 J m−2. (d) The calculated temperature sweep with respect to time through the TTM and the aspect ratio evolution as a consequence of thermoplasmonic surface diffusion calculated using eqn (2). (e) The comparison between experimental results of reshaping of Al nanorods at different laser fluence (9.5, 14.2, 23.7, 42.6, 94.6 J m−2) and the calculations using the TTM and eqn (2). The laser fluence corresponding to the bulk melting point is indicated by the vertical dashed line. (f) The SEM images of shape transition at various laser fluences corresponding to data points in (e). Scale bar: 50 nm.

The field is mainly located at the sharp corners of the nanorods and over 80% of absorbed pulse energy is dominantly dissipated in the top Al nanorods, which implies that Joule heating will be generated. The transiently absorbed heat power is proportional to the electric field distribution and can be expressed as47

 
image file: c9nh00003h-t1.tif(1)
where ω is the angular frequency, ε′′ is the imaginary part of the dielectric function, ε0 is the permittivity of vacuum and E(ω,t) is the time-varying electric field. The transient thermoplasmonic heating can increase the temperature of Al nanorods through the electron–lattice relaxation.48 The ultrafast heating behavior and temperature evolution can be described in a two-temperature mode (TTM)48–50 (Fig. S2, ESI). Depending on the given experimental laser fluence of the incident pulse, the peak temperatures of Al nanorods can vary from 400 K to the melting temperature of 933 K. Fig. 2c showcases one example of the temperature distribution at 100 ps after being irradiated by the fs pulse with a fluence of 56 J m−2.

The thermodynamic shape transition of a plasmonic nanostructure induced by a fs pulsed laser can be described in a simplified model of curvature-driven surface diffusion,51,52 which is based on the idea of minimizing the surface energy by mass migration at the surface. The outward diffusion speed of surface atoms in the normal direction can be expressed as (Discussion, ESI)

 
image file: c9nh00003h-t2.tif(2)
where Ω, γs and Ds represent the atomic volume, the free energy and the interface diffusivity, respectively, k is Boltzman's constant, T is the temperature, and image file: c9nh00003h-t3.tif is the mean curvature of the surface. The equation clearly reveals that the speed of surface atom migration is proportional to the surface curvature. The atom migration at each point of the entire surface can be calculated for each time increment at a given temperature. The temperature evolution during the ultrafast thermodynamic process can be obtained through the TTM as the input for eqn (2) (Fig. S2, ESI).

For simplicity, the Al nanorods are approximated as ellipsoidal shapes. The transient shape transition and large alternation of curvature from the tip to the waist under ultrafast pulsed irradiation can be observed (Fig. S3, ESI). The surface diffusion is driven by the curvature with pronounced changes starting from the sharp tips. Fig. 2d depicts one example of temperature rise and decay with respect to time and the corresponding aspect ratio evolution extracted from the reshaped profiles under a laser fluence of 94.6 J m−2. Remarkably, the temperature increases to the peak value within 2 ps, one-order-of magnitude faster than those of Au and Ag nanorods (∼20 ps) under the same laser fluence due to one-order-of magnitude larger electron–lattice coupling efficiency (5.69 × 1017 J (m3 K s)−1) (Fig. S4, ESI). The competition between the ultrafast thermoplasmonic heating of Al nanorods (∼2 ps) and heat homogenization time across the nanorod (hundreds of ps) serves as a key to the anisotropic surface diffusion. It is also worth noting that the nanorod experiences sharper aspect ratio change at the rapid temperature increasing stage owing to higher curvatures at the initial stage. The successive evolution of the aspect ratio slows down even when the nanorod experiences higher temperatures, which can be attributed to the enlarged energy barrier for the surface atom to diffuse at lower curvatures (Fig. S4, ESI).

The experimental results obtained at variant incident laser fluences (9.5, 14.2, 23.7, 42.6, 94.6 J m−2) corroborate with the simulation with reasonable congruence. The Al nanorods with an initial aspect ratio of ∼4.7 (Fig. S5, ESI) decrease to a final value of ∼2.6 when increasing the irradiation fluence to 94.6 J m−2 (Fig. 2e and f). The energy consumption is only ∼30% of that by Au nanorods with the same initial aspect ratio and reshaping to the same final aspect ratio (Fig. S4, ESI). It is noteworthy that the temperature sweep is below the bulk melting point and the fast temperature decay under ambient conditions on the order of nanoseconds protects Al nanorods from complete reshaping into spheres. Apparently, the final profile with a reshaped aspect ratio depends on the total absorption of the incident laser energy, which provides a new route to anisotropic and precise shape control for molding optical responses at will. A pronounced blue shift of the reflectance valley from 720 to 450 nm can be revealed while the aspect ratio decreases from ∼4.7 to ∼2.6, indicating a full-visible tunability and subtractive plasmonic color appearances (Fig. S6, ESI). The concomitant phase shifts of the reshaped Al nanorods with respect to the wavelength which play the key role in light field manipulation can be calculated using the finite-difference time-domain (FDTD) simulation and experimentally verified through laser fabricated nano-gratings (Fig. S7, ESI). The diffraction efficiency varies between 2 and 4%, suffering from optical losses from intrinsic absorption and the coarse quality of evaporated Al thin films.38,53,54

Taking the thermal diffusivity of Al (6.9 × 10−5 m2 s−1) into consideration, it is estimated that the heat homogenization across hundreds of nanometer sized nanostructure takes about hundreds of picoseconds. Considering the ultrafast lattice temperature rise and rapid shape transition, significantly faster than the heat homogenization time, it is possible to independently mold optical responses in each constituent arm of Al cross structures through polarization-controlled thermoplasmonic heating. Fig. 3a showcases SEM images of cross nanostructures irradiated by horizontally-polarized fs pulses as well as the corresponding measured reflectance and color evolutions at horizontal and vertical polarization illumination. Interestingly, the ultrafast temperature sweep introduced by horizontal polarization thermoplasmonic heating produces distinct curvature changes of the horizontal arms from tip to waist with a remarkable color appearance change from cyan to yellow, while the vertical arms are intact with negligible color alternations. The spectral evolution indicates that the horizontal arm by thermoplasmonic surface diffusion exhibits drastic shifts of the reflection valley over 200 nm to cover full visible frequencies upon increasing the laser fluence. Whereas the orthogonal arm displays trivial shifts less than 30 nm. The International Commission on Illumination (CIE) 1931 chromaticity diagram based on the measured reflectance under vertical (stars) and perpendicular (circles) polarized white light illumination in Fig. 3b further confirms that the thermoplasmonic heated Al arm can be reliably modulated in the full-visible range with minor influence on the orthogonal arm. The crosstalk-free operation power window can be scrutinized through the power dependent spectral shifts and reflectance modifications (Fig. S8, ESI). The measured variation of the arm lengths verified the occurrence of anisotropic surface diffusion in vertical arms from ∼150 to ∼80 nm, and the intact horizontal arms (Fig. 3c).


image file: c9nh00003h-f3.tif
Fig. 3 Anisotropic thermoplasmonic shape transition of Al cross nanostructures. (a) The SEM images (i) and the corresponding measured reflectance and optical images at parallel polarization (ii) and perpendicular polarization (iii) of reshaped Al cross nanostructures by a horizontal polarization fs pulse with different laser fluences (9.5, 14.2, 23.7, 42.6, 94.6 J m−2). Scale bar: 100 nm. (b) International Commission on Illumination (CIE) 1931 chromaticity diagram based on the measured reflectance under vertical (stars) and perpendicular (circles) polarized white light illumination. (c) The length evolution of horizontal (black circles) and vertical (red squares) arms of Al cross structures as a function of laser fluence. (d) The simulated temperature evolution of the two arms with respect to time after the thermoplasmonic heated arm reaches the transient temperature peak. The temperature monitors are placed 15 nm from the tips of each arm. The corresponding temperature distributions at various times: 0.01, 0.02, 0.04, 0.08 and 0.16 ns.

To get insights into the anisotropic surface diffusion, we simulated the temperature evolution of both arms with respect to time immediately after one arm was thermoplasmonically heated by a polarized fs pulse. Fig. 3d shows one example of the cross nanostructure with the transient peak temperature approaching 927 K (corresponding to the largest laser fluence of 94.6 J m−2 in the experiment) in the thermoplasmonically heated arm. The subsequent homogenization of their temperature across the entire cross nanostructure through heat diffusion takes place within the first 100 ps. Finally, the temperature of the cross-nanostructure decays to the ambient temperature in the following 1 ns. Therefore, the second arm is exposed to a shorter temperature sweep with a significantly reduced strength, which slows the shape transition to an undetectable level. Depending on the incident pulse density, the two arms experience distinct transient peak temperatures which can be calculated through the TTM and the thermodynamic simulation (Fig. S9, ESI). Therefore, it is possible for anisotropic shape transition to occur with negligible crosstalk.

The fs laser induced anisotropic shape transition enables customized post-processing of each constituent arm of Al cross structures for polarization-controlled multifunctional metasurfaces through independently molding the optical responses in terms of color appearance/phase modulation. In Fig. 4a, we demonstrate a crypto-display metasurface by encoding a color image and a holographic image in horizontal and vertical arms of Al cross structures. The shape transition can be reproducibly controlled by standardizing the laser recipe through establishing a relationship between the final aspect ratio of the constituent arm of Al cross structures and the laser fluence. The SEM images of the laser processed cross structures are shown in Fig. 4b. The multiplexed images in the incoherent white light reflection mode and coherent laser diffraction mode can be switched and distinctly reconstructed by the polarization (Fig. 4c and Fig. S10, ESI). To unlock the full potential of laser printing multifunctional metasurfaces, we printed dolphin and kangaroo multiplexed holograms (Fig. 4d), and a collection of encrypted color images in Fig. 4e–g in the two orthogonal arms. The dual holograms can operate in a bandwidth of ∼200 nm across the visible spectrum range (Fig. S11, ESI). The laser printed Jinan University logo in Fig. 4e can display two color states when switching between the orthogonal polarizations of the white light illumination. Furthermore, two different color images, such as two QR codes (Fig. 4f) can be encrypted in the same layer with minor cross talks. And multiplexed color images (flower/bee) sharing partial pixels in the orthogonal directions are demonstrated in Fig. 4g. Remarkably, the printed optical images are relatively stable in air because the self-terminating native oxide layer (∼3 nm) of Al can effectively passivate the nanostructures and thereby prevent the resonance shift due to the oxidization effect.37–39


image file: c9nh00003h-f4.tif
Fig. 4 Demonstration of multifunctional metasurfaces by anisotropic laser printing. (a) Illustration of a crypto-display metasurface by encoding a color image and a holographic phase pattern in Al cross nanostructure arrays through polarization-controlled thermoplasmonic surface diffusion. (b) The SEM images of the laser processing Al cross nanostructures with a butterfly printed and a zoomed-in view of one section (scale bars: 50 μm and 100 nm, respectively). (c) The color image in reflection mode by horizontally-polarized incoherent white light illumination and the holographic image in diffraction mode by vertically-polarized coherent illumination at the wavelength of 632 nm. (d) Multiplexed holographic images retrieved in the horizontal and vertically polarized laser beam, respectively. (e–g) Encrypted color images, including the Jinan University logo with different color appearance, two different QR-codes in the two polarization channels, and a flower and bee image with shared wing/leaves (scale bar: 20 μm).

In summary, we have demonstrated for the first time a new level of exquisite and anisotropic shape transition of Al complex nanostructures at the single structure level. The polarization-controlled ultrafast thermoplasmonic heating and subsequent surface atom diffusion enable independent reshaping of the two arms of Al cross nanostructures opening a new route to mold the optical responses of complex antennas to multiplexed attributes of light. By leveraging this anisotropic laser printing, full-visible multifunctional optical elements can be accomplished for scalable applications by in situ customization. This can enrich the knowledge and know-how of thermoplasmonic effects and thermodynamics and tremendously extend the boundary of emerging Al plasmonics and metasurfaces.

Methods

Sample preparation

All the initial samples used in this work are fabricated through the standard electron-beam lithography for proof of principle studies. For low-cost and large-scale applications, the well-developed nanoimprint lithography can be employed to fabricate the initial nanostructures. The multilayers were prepared by the following procedures. An optical thick (100 nm) Al thin film was firstly deposited on a polished Si wafer, followed by 30 nm thick SiO2 deposition by electron beam evaporation. Then a positive electron beam resist was spin-coated on top of SiO2 and baked for 90 s at 180 °C. After patterning and development, a layer of 30 nm thick Al thin film was deposited under a pressure of ∼1 × 10−7 Torr. Then the lift-off process was conducted by resolving the resist in acetone after soaking for 24 hours.

Laser printing and characterization

fs laser pulses at the wavelength of 730 nm and with a repetition rate of 80 MHz pass through a pulse picker and a polarizer, then were tightly focused on the as-prepared sample by an object lens with a numerical aperture of 0.65. A piezo stage was used to mount and scan the sample. The pixel size used in this work is tested to be ∼500 nm and therefore the periodicity for printing was kept at 500 nm. For the color image printing, we firstly divided the original color image into multiple images of different color tones. Then, each color tone image was printed through the established relationship between color and laser fluence. The morphologies of the nanostructures were imaged by a scanning electron microscope (FEI, Apreo Hivac) while the color images were obtained by an Olympus microscope (BX53). The reflectance of the laser-printed nanostructures was measured by a home-made micro-spectrometer. The white light source was coupled to a microscope to illuminate the sample. A spectrometer (Andor, SR-500i-D2-1F1) coupled to a CCD camera (Andor, DU920P-OE) was employed to acquire the spectrum.

Numerical simulations

The optical response of the structure was simulated by Lumerical's FDTD solutions. A plane wave source at wavelengths ranging from 300 to 900 nm was employed to illuminate the structure. Periodic boundary conditions were used in the lateral direction to simulate a periodic structure and perfectly matched layers were put in the vertical direction to prevent any interference between the reflected light and the incident light. One reflection monitor was positioned above the light source to obtain the reflection and phase shift. The thermal simulation of the structure was carried out by using Comsol Multiphysics. The model contains a box area where the aluminum cross structure is placed on top of 30 nm thick SiO2 with a ground layer of 100 nm thick Al thin film. The boundaries are set as “outflow” to absorb any heat that flows to it. One arm of the cross is set with an initial temperature to simulate the temperature gained immediately after absorbing the laser pulses, while the other one and the surroundings are set with an initial temperature of 293.15 K. The diffusion was calculated with a temporal solver in a time range of 0 to 10 nanoseconds.

Author contributions

Y. Z. and X. L. conceived the ideas and designed the experiments. Y. Z. and L. S. performed the experiments. Y. Z. conducted the optical simulations. D. H. contributed to the thermal simulations. Y. L. contributed to the code of surface diffusion modelling. S. X. and S. R. prepared the pre-designed samples. All authors contributed to the discussion and paper writing.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research was supported by the National Key R&D Program of China (2018YFB1107200), the National Natural Science Foundation of China (NSFC) (Grant No. 61605065, 61522504 and 21317137), Guangdong Provincial Innovation and Entrepreneurship Project (Grant 2016ZT06D081), and Guangzhou Science and Technology Program (Grant No. 201804010322). The authors would like to thank Professor Yaohua Mai (Jinan University) for the access to the SEM facilities. The authors acknowledge the help from Xu Ouyang for 3D drawings and Zilan Deng for useful discussion. This work was performed in part at the NSW node of the Australian National Fabrication Facility, a company established under the National Collaborative Research Infrastructure Strategy to provide nano and micro-fabrication facilities for Australia's researchers.

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c9nh00003h

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