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Mechanical Chameleon through Dynamic Real-Time Plasmonic Tuning Guoping Wang, Xuechen Chen, Sheng Liu, Ching-Ping Wong, and Sheng Chu ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.5b07472 • Publication Date (Web): 13 Jan 2016 Downloaded from http://pubs.acs.org on January 20, 2016
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Mechanical Chameleon through Dynamic Real-Time Plasmonic Tuning Guoping Wang1, 2,+,*, Xuechen Chen1, +, Sheng Liu2, Chingping Wong3, Sheng Chu1,* 1. State key laboratory for optoelectronics materials and technology & School of physics and engineering, Sun Yat-Sen University; Guangzhou 510275, PR China; 2. School of Power and Mechanical Engineering, Wuhan University, Wuhan 430072, PR China; 3. School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0245,USA. +, These authors contributed equally to this work; *,Corresponding authors: Sheng Chu:
[email protected] Guoping Wang:
[email protected] KEYWORDS: plasmonic modulation, active camouflage, full-visible range, nanodot arrays, biomimetic
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ABSTRACT The development of camouflage methods, often through a general resemblance to the background, has recently become a subject of intense research. However, an artificial, active camouflage that provides fast response to colour change in the full-visible range for rapid background matching remains a daunting challenge. To this end, we report a method, based on the combination of bimetallic nanodot arrays and electrochemical bias, to allow for plasmonic modulation. Importantly, our approach permits real-time light manipulation readily matchable to the colour setting in a given environment. We utilize this capability to fabricate a biomimetic mechanical chameleon and an active matrix display with dynamic colour rendering covering almost the entire visible region. Optical invisibility represents one of the greatest challenges in military and biomimetic research.1-3 Despite tremendous efforts, the camouflage methods under study today are mostly derived from pattern painting dated back to 1940s. A more sophisticated approach involves the redirection of the incident light to develop an "invisibility cloak".4,5 It was only recently that the metamaterials have proved effective in bending electromagnetic waves in the microwave region,6,7 but extending to the entire visible spectrum for practical use has been challenging, especially in surroundings that support ballistic light propagation.8,9 On the other hand, active camouflage that rapidly adapts the surroundings of an object such as a chameleon10,11could be more feasible to achieve effective invisibility. The key to this biomimetic technology is realizing electrically-driven actuation of broad reflection bands, which may be partially enabled by some of the existing "e-paper" or "e-chem" display approaches,12,13 including electrophoretic,14 cholesteric liquid crystalline,15 and electrowetting, etc.16 They can produce multiple colors, and some of these technologies even have progressed to the status of colorful video display capability.
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However, the main drawback of these technologies in camouflage is that they often need colour filters or multiple colour-mixing layers,17 thereby resulting in substantially low reflectivity and more complex devices. Despite the report of single layered devices such as photonic crystals, their multi-colouration capability is still in their infancy.18 Nevertheless, their thermal expansion and contraction19,20 may severely influence the colouration in an environment with a fluctuating temperature due to the inherent dependence of reflection wavelength on the interparticle separation. It is well known that plasmonic metal-nanostructures can be utilized to drastically
tune
optical
reflection
and
absorption
in
the
range
of
ultraviolet-to-near-infrared region by changing their geometries or compositions.21 They also have been demonstrated to produce more saturated colours than standard technologies, albeit the difficulty and
rarity in attaining electrically actuatable
colours.22-24 Therefore, in this letter, dynamic plasmonic-nanostructures based on highly ordered Au/Ag nanodomes are integrated into our design and successfully demonstrate a biomimetic mechanical "chameleon" as well as a 64 x 32 "plasmonic cell"-matrix display, effective in a full visible region of 430 nm to 650 nm, which is operated
by
altering
the
Au/Ag
core
shell
structures
through
electrodepositing/stripping process. RESULTS AND DISCUSSION To ensure outstanding performance of the plasmonic cell, we intentionally designed an Au-core/Ag-shell structure (Schematic Fig. 1 a), rather than the reciprocal one, which will be explained in detail in the simulation section. The device fabrication process is described as following: firstly, an anodized aluminum oxide (AAO) film with highly ordered pores (Fig. S1), the template for etching nano-hole
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array on SiO2, was carefully transferred onto a SiO2 (50 nm)/ITO glass. The contact between these films is robust (Fig. S2) for further controllable process. Reactive ion etching (RIE) was then performed precisely in order to remove all 50 nm SiO2 film underneath the AAO holes, while still leaving the 50 nm ITO almost intact. Thus, an ordered array of nano-holes on SiO2 was achieved (Fig. 1 b), with a conductive ITO bottom (inset of Fig. 1 b). This conductive bottom is important for current feed through in electrodeposition process. Then, the sample was subjected to Au evaporation, creating an ordered array of Au "nanodomes", as shown in the scanning electron microscope (SEM) image of Fig. 1 c. The colour of the sample evolves from transparent to red in Fig. S3, which is typically due to plasmonic absorption/reflection bands of Au nanostructures.25
Finally, the sample was packaged into device and
filled with gel electrolyte containing Ag+ ion (Fig. 1 d). The major merit of the current structure lies in the integration of plasmonic colour tunability and electrodeposition induced structural transformability. Usually, plasmonic nanostructures can induce dramatic change of reflection just by slightly tuning the physical configurations such as geometries or compositions, but nanostructures obtained by normal synthesis only stay stationary, making dynamic plasmonic tunability nearly impossible. Electrodeposition, on the other hand, can dynamically alter material thickness and has been applied in single-colour electrochromic cell,26 but their potential in controlling plasmonic nanostructures has not been fully explored. Hence, the approach of electrodepositing and stripping Ag shells on plasmonic Au nanodomes can solve the above dilemma and produce a reversible plasmonic cell, covering the entire visible spectrum. Fig. 1 is the microscope image of the Au/Ag nanodomes sample, which contains red, green, and blue colours, suggesting fairly wide tunability of the nanodome. For the Au
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nanodome-electrode,
the
cyclic
voltammogram
(CV)
based
on
standard
three-electrode system (Fig. S4) presents an obvious reversible redox reaction of Ag and also indicates the viability of rendering a continuous colour change.
For device
working in "mechanical chameleon", two electrodes (1.5 V is applied between them) are used because of device size constraint, but still offer good stability and repeatability. The transmission versus oxidation/reduction time is recorded (Fig. 1 f) and clearly shows a redox behavior, which is in consistent with the CV measurement. The device displays a broad colour change from red to blue at the initial point and ending point of the CV curve (indicated by black arrows), respectively, as seen from inset of Fig. 1 f.
Figure 1 | Structure and function of the plasmonic cell devices. a, A schematic diagram of the plasmonic cell. The double layered hemi-ellipsoids represents nanodomes with different Ag shell thickness. b, SEM image of the SiO2 nano-hole array formed after etching and removal of AAO. Scale bar: 100 nm. Inset:
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A cross-sectional SEM image of one SiO2 nano-hole. Scale bar: 50 nm. c,Top-view SEM image of the Au nanodome- array. Scale bar: 100 nm. d, The formation of the working plasmonic cell including electrodes, gel electrolyte, and sealing. e, Microscopic image of the device's colour in RGB colour. Scale bar: 50 µm. f, Transmission of 600 nm light as a function of electrodeposition voltage. The scan rate: 0.2 V/s. Inset: photo of the plasmonic cell device at the starting and ending point.
Fig. 2 a depicts the measured reflection spectra of the device with the increase of Ag deposition time. The trend is monolithic blue-shift with longer Ag deposition time and later well modeled in the finite difference time domain (FDTD) simulation part. Fig. 2 b presents the extracted reflection peak positions versus depositing/stripping time, while the superimposed images are corresponding to these representative points selected. Fig. 2 c shows the chromatic diagram of the device with points extracted from Fig. 2 a. Those results suggest that the reversible quasi-full visible colour plasmonic cell is achieved, and it is capable of jumping between any two colour states within seconds. In addition, the concept of plasmonic cell device is also able to be fabricated on flexible substrates, as shown in Fig. S5 with a narrower colour window, implying potential applications in wearable active-camouflage and functional soft machines, etc. Furthermore, the device possesses good stability, which is proved in endurance characterization in Fig. S6. The device did not lose colouration capability after 200 cycles of operation. It is noted that the "Ag oxidation" (forming silver oxides) might affect the device' optical properties after significant long time (e.g. three months). Nevertheless, the current research has successfully demonstrated the feasibility of making a versatile plasmonic colour-changing device.
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Figure 2 | Demonstration of the electrically driven colour changing from the device. a, Reflection spectra of the device after different electrodeposition time. b, The dependence of reflection peak wavelength with different electrodepositing (blue square) and electro-stripping time (orange dot). Insets are photo of the devices corresponding to selected points. c, Chromatic diagram of the plasmonic cell. To confirm the successful Ag shell electrodeposition process, the evolution of nanodomes was characterized by scanning electron microscopy (SEM), transmission electron microscope (TEM) and energy dispersive spectroscopy (EDS) mapping. The SEM images in Fig. 3 a clearly present that the size of the nanodomes inside the
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well-arranged holes and these holes were growing larger with longer Ag deposition times (0 s, 1.5 s, 3.5 s and 5 s.), which is in good accordance with the TEM images of the representative single nanodomes for an individual colour state, as shown in Fig. 3 b (lower magnification TEM in Fig. S7), with the measured heights for each nanodome to be 10 nm, 17 nm, 30 nm, and 40 nm，respectively. EDS mapping coupled with the TEM was then used to characterize the locations of Au and Ag elements, which is shown in Figs. 3 c and 3 d, with the corresponding EDS spectra given in Figs. S8 a-d, as can be seen from these images it’s undoubtedly confirming the presence of Au cores and Ag shells in the nanodomes. The above results clearly indicate a controllable and desirable Ag shell on Au core was achieved by electrodeposition, which strongly suggests that the mechanism of plasmonic cell is due to the formation of Ag/Au hybrid nanostructure, rather than ion insertion or carrier-concentration mediated effect.27
Figure 3 | Micro-structural characterizations of the Au/Ag core-shell nanodomes.
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a, 45 ̊ tilted-view SEM images of the nanodomes array for the sample with electrodeposition time of 0 s, 1.5 s, 3.5 s and 5 s. Scale bar: 100 nm. b, TEM image of a single nanodome from samples in a. Scale bar: 20 nm. c, EDS mapping of Au element associated with samples in b. d, EDS mapping of Ag element. The evolution of reflection dependence on Ag shells was further studied by the FDTD method. The model structure is shown schematically in Fig. 4 a. The Au/Ag nanostructures were set as hemi-ellipsoids to reasonably approximate the structure shape observed from experiments. The radii of Au (rAu1) and Ag (rAg1) in XY plane are 25 nm and varying in the range of 0-6 nm, respectively. Heights of Au (rAu2) and Ag (rAg2) are 10 nm and ranging from 0 to 30 nm on the Z axis, respectively, while the ratio between rAg2 and rAg1 is approximately constant of about 5 during the evolution process, according to TEM results. It is noted that the actual Au nanodomes are not exactly uniform as seen from Fig. S7.
However, as can be realized from our optical
characterization, the relative low shape uniformity did not lead to significant broadening of the spectra, which is probably because of the relatively large sizes of the nanostructures. Fig. 4 b shows the two-dimensional reflection pattern for continuously variable thickness of Ag shell, which indicates that the peak wavelength can be continuously tuned from 670 nm to 450 nm, by varying rAg2 from 0 to 30 nm, which well agrees with the experimental range of 650 nm to 430 nm. Strong resonant electric field intensities can be found at the bottom edge of Au/Ag nanodomes in Fig. 4 c: with the resonance peak λ = 670 nm when rAg2 = 0 nm (Fig. 4 c left) and λ = 450 nm when rAg2 = 30 nm (Fig. 4 c right) , and they are responsible for giving rise to the colours of the sample. The plasmonic resonance can be established when the frequency of incident light matches the natural frequency of surface plasmon oscillation. From these analyses, it theoretically proves that the electrodeposited shell
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is responsible for the reflection shift. Even though, whether the shift comes mainly from the Ag material (material factor) or simply the nanodome shape change (geometric factor) still need to be further distinguished. The magnitude of plasmonic shifting due to pure shape change in hemi-ellipsoid is analyzed as follows. For ellipsoid particles, the absorption cross section (which is proportional to the intensity of the absorption) can be expressed as:28
δ
/ = 3
( ) (λ )
(λ ) + [
] + (λ )
where , c , and V denote the vacuum light angular frequency, the speed of the light, the surrounding material’s dielectric constant, and the volume of the particle, respectively. The depolarizing factor Pj includes PX, PY and PZ, corresponding to three axes of the ellipsoid. In the case, axis length RZ>RX=RY，the depolarizing factors are defined as:
= .
1 − ! 1 1+! " ln & ' − 1( , ! 2! 1−!
* = + =
1 − 2
where ρ = (1 − - / 1 )/ .0
Thus, the change in axis length RZ and RX will lead to the shift of extinction spectra. In fact, a detailed derivation (see Fig. S9 and corresponding analysis) shows that when the ratio of RZ/RX increases (RZ, RX are corresponding to rAg2+rAu2, rAg1+rAu1 in our experiment, respectively. RZ/RX indeed increase with Ag deposition), the resonant peak in XY plane (in the normal incidence case) blueshifted, which coincides with the peak-shift direction of adding Ag-shell onto Au-core,29 indicating that the geometric factor is a positive enhancement role considerably helping achieve a wide tuning
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range of 670 nm to 450 nm. In order to verify the superiority of our designed scheme, another simulation of "Au-shell" on original Au-core is carried out for comparison, which can be seen in the Fig. 4 d. The reflection spectra of 30 nm Ag-shell and 30 nm "Au-shell" on the same original Au-core blueshift to 450 nm and 560 nm, respectively, from the 670 nm peak of the original Au-core. The above result clearly suggests that only by coupling of both geometric factor and material (Ag shell) factor, the capability of tuning-wavelength down to 450 nm is then able to be realized. Geometric factor alone, in other words, can merely blueshifit 110 nm, which is far from meeting the full-colour requirement of plasmonic active-camouflage. Moreover, if the geometric factor related peak-shift contradicts the shift-trend induced by material itself, then the overall colour tunability will be substantially attenuated, which may be reasonably predicted in the Ag-core/Au-shell nanostructure (different with the Au-core/Ag-shell adopted in our design): the deposition of Au-shell on Ag-core will redshift the peak wavelength due to intrinsic plasmonic spectral properties of Au,30 but the geometric factor (RZ/RX) changing during the Au-deposition will lead to the blueshift as mentioned above. The afore-described contradiction between material itself and geometric factor is inevitable in the Ag-core/Au-shell case, creating a real and substantial constraint for accomplishing full visible colour performance. Thus, the design of Au-core/Ag-shell is a logical choice for the artificial active-camouflage application.
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Figure 4 | FDTD numerical study of the optical properties of the Au/Ag nanodomes array. a, Schematic of the simulation and related
parameters. b,
Two-dimensional reflection spectra as a function of Ag shell thickness denoted in a. Blueshift with Ag shell increase is evident. c, Simulated electric field distribution for Ag-shell thickness of 0 nm at 650 nm light (left), and Ag shell thickness of 30 nm at 450 nm (right). d, Comparison of reflection spectra: the orange curve for Ag (30 nm)/Au core-shell nanodome, and the blue curve for the pure Au nanodome with the same size and shape . Having the excellent ability of plasmonic cells, they can be implemented in many profound applications. Here, we focus firstly on the biomimetic active-camouflage embodied in a mechanical chameleon. Fig. 5 a shows the schematic of the mechanical chameleon with working principle. The whole body of the chameleon is covered with fish-scale like colour patches, which correspond to plasmonic cells in our experiment. An ideal mechanical chameleon will be equipped with miniature colour sensors to sense the colour patterns of the environment. The acquired information from camera will be automatically analyzed and delivered to individual colour patches, changing
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the chameleon' body appearance accordingly. The demonstration used in mechanical chameleon’s structure is shown in Fig. S10, in which two simplified colour sensors are used, corresponding to the front part and rear part of the concept-proven mechanical chameleon following respectively with an independent homemade micro-control system (Fig. S 11). Sensors are capable of detecting the red, green and blue, one at a time and outputing the voltage signals into the micro-control system, which can analyze signals and subsequently apply a voltage of 1.5 V onto the corresponding plasmonic cells with suitable time duration, presenting the colours matching the background. The top and bottom of Fig. 5 b show the picture of a red-state mechanical chameleon and the colour patterns of our mechanical chameleon in the natural environment, respectively. Fig.5c shows the screenshots of our movie, displaying the walking chameleon of corresponding time in front of a triple-colour background. The colours presented in time series, 0 s (red), 8 s (red and green) and 29 s (blue), are shown in the left, middle and right panels, respectively. As we can see from Fig. 5 c, when the mechanical chameleon is moving from red background to the green one, the colour of plasmonic cells in the front part will switch to green automatically, merging into the background, but the rear part is still in the red background, with no colour-switching until its sensor moves to green background. The colour-changing effect can also be duplicated in the interface of green/blue (movie S1, Supplementary information). Evidently, the mechanical chameleon possesses every fundamental feature that is needed for realistic active camouflage. As a by-product, we demonstrate the potential capability of displaying of plasmonic cells, which can be used in low-powered electronic papers and display units. Fig 5 d is a 10 ×10 “fast” display screen that the total refresh speed (
