Real-Time Synchronous CCD Camera Observation and Reflectance

Mar 20, 2014 - Key Laboratory for Renewable Energy, Chinese Academy of Sciences; Beijing Key Laboratory for New Energy Materials and Devices; Institut...
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Real-Time Synchronous CCD Camera Observation and Reflectance Measurement of Evaporation-Induced Polystyrene Colloidal SelfAssembly Dongfeng Lin, Jinze Wang, Lei Yang, Yanhong Luo, Dongmei Li, and Qingbo Meng* Key Laboratory for Renewable Energy, Chinese Academy of Sciences; Beijing Key Laboratory for New Energy Materials and Devices; Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China S Supporting Information *

ABSTRACT: A new monitoring technique, which combines real-time in-situ CCD camera observation and reflectance spectra measurement, has been developed to study the growing and drying processes of evaporation-induced selfassembly (EISA). Evolutions of the reflectance spectrum and CCD camera images both reveal that the entire process of polystyrene (PS) EISA contains three stages: crack-initiation stage (T1), crack-propagation stage (T2), and crackremained stage (T3). A new phenomenon, the red-shift of stop-band, is observed when the crack begins to propagate in the monitored window of CCD camera. Deformation of colloidal spheres, which mainly results in the increase of volume fraction of spheres, is applied to explain the phenomenon. Moreover, the modified scalar wave approximation (SWA) is utilized to analyze the reflectance spectra, and the fitting results are in good agreement with the evolution of CCD camera images. This new monitoring technique and the analysis method provide a good way to get insight into the growing and drying processes of PS colloidal self-assembly, especially the crack propagation.

1. INTRODUCTION Three-dimensional photonic crystals (PCs), which were first presented in 1987 by Yablonovitch1 and John,2 can confine and control the propagation of light as their photonic stop-bands in the periodic dielectric structures. Owing to this unique property, there are many applications of photonic crystals in the optical communication, photonic computing, switching, sensing, lasing, and solar cells.3−6 Therefore, over the past few decades, many fabricating methods have been developed, such as electrostatic repulsion assembly, gravitational sedimentation, and spin or spray coating.7−18 Among these methods, evaporation-induced self-assembly (EISA) of the colloidal photonic crystals (CPCs) attracts much attention because of its simplicity and low cost.17−25 However, a crack in colloidal self-assembly (CSA) has been observed in many works,26−30 and the crack has a strong influence on optical characteristics and the quality of CPCs. To obtain perfect CPCs and use CPCs into practical applications, it is necessary to find out how crack initiates and propagates in CSA. In the past few decades, many groups have investigated this topic in the following two ways:26−36 (1) Visual image observation of crack during EISA of CPCs. For example, Teh et al. observed that colloidal crystals exhibit crack of different morphologies within the different growth zones as a result of the drying process. Alternating bands of arrayed spheres and empty regions formed on the substrate after EISA of CPCs. A “stick−slip” motion of the meniscus growth front was presented to explain different growth patterns.26 Wang et al. utilized a long working distance optical microscope to real-time © 2014 American Chemical Society

observe crack spacing and gave the relationship between crack spacing and particle radius based on solid theory force analysis.27 (2) Reflectance spectra measurement of CPCs. For example, Koh et al. utilized reflectance spectroscopy to monitor structure changes during CSA at the meniscus of a sessile drop.33 Jiang and Bertone studied the optical stop-band of PCs and found that the stop-bandwidth and peak attenuation depend on the number of layers and on the dielectric contrast between the spheres and the interstitial regions.17,34 Our group also employed an optical microspectroscopy to study the different growing stages of EISA in real time35 and developed modified SWA to give information about crack by analyzing the reflectance spectra.36 However, the visual image observation method cannot present the three-dimensional quantitative information on the CPCs during drying process, such as thickness, volume fraction of water, and especially the real-time optical properties of CPCs. Meanwhile, the reflectance spectra measurement method cannot present the morphologies of crack directly. Although the SEM images of CPCs can be obtained after CSA, it is difficult to guarantee that the SEM images are from the same region with the monitored region. It is also complicated to get real-time SEM images of CPCs during CSA. Thus, it is necessary to seek a more convinced monitoring method to obtain more comprehensive information about CSA. Received: December 13, 2013 Revised: March 18, 2014 Published: March 20, 2014 3949

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Figure 1. Schematic diagram of EISA in the monitored region (a) and schematic diagram of the experimental setup for CCD camera observation and reflectance spectra measurement (b).

2. EXPERIMENTAL SECTION

mm cuvette. The cuvettes were first immerged in chromosulfuric acid for more than 3 h and rinsed copiously with tap water and then ultrapure water, finally dried in a stream of nitrogen. After suspensions were placed into a cuvette, a thermal lance which ejects 100 °C hot air stream was placed 6 cm away from the top of cuvette to elevate the evaporation of solvent, the primary driving force for the convective transfer of particles.38 The walls of the cuvettes serve as substrate for PS spheres assembling into CPC (Figure 1a). The incident light goes through the substrate and the crystal/suspension successively, which is opposite to the normal condition. Then, we chose a monitored region in the front wall of cuvette, which is 3 mm below the initial meniscus to obtain real-time CCD camera images and to measure real-time reflectance spectrum simultaneously. After 4 h, the meniscus entered the monitored region, and the image of meniscus appeared in the monitored window of CCD camera. At the same time, Bragg diffraction signal reflected from the growing CPCs in the monitored region became detectable. Soon after that, we started to record the real-time CCD camera images and reflectance spectrum of the monitored region every 3 min. In order to normalize the reflectance, we also measure the reflectance spectrum by replacing the cuvette with a plane mirror. The reflectance spectrum reflected by the plane mirror is regarded as the incident spectrum because the plane mirror can reflect nearly all light. The normalized reflectance spectrum is obtained after dividing reflectance spectrum by incident spectrum.

2.1. Experimental Setup. The new monitoring technique, which combines real-time in-situ CCD camera observation and reflectance spectra measurement, was carried out to study the growing and drying processes of EISA. As shown in Figure 1b, the white light, coming from a tungsten bromine lamp (200−2500 nm), was reflected by the first beamsplitter, passed through a magnification objective ×20 and then illuminated the monitored region. The size of the monitored region can be changed by controlling the diaphragms and was about 240 μm in our experiments. Reflected by the sample, the light was collected by the objective and went through the first and the second beamsplitter successively. After passing through the second beamsplitter, two beams of light were divided: one into the CCD camera (INFINITY 2-3C, Canada) and another one into the monochromator (Omni-λ 500, Zolix, Inc., China). The CCD camera was used to capture the real-time images of CPCs, and the monochromator, together with a Si diode (200−1100 nm), a lock-in amplifier (SR380, Stanford Research Systems), and chopper, was used to obtain the real-time reflectance spectra of CPCs. Through combing these two monitoring setups and adjusting instruments, we finally got the proper condition and setup for monitoring the EISA of CPCs. 2.2. Materials and Investigated Procedure. In our experiment, 1 vol % aqueous suspensions of polystyrene (PS) latex (Thermo Scientific 240 nm ±3%, 290 nm ±3%) dispersed in ultrapure water (Milli-Q Synthesis System, Millipore S.A., Mol-sheim, France) were used to self-assemble CPCs. Just as the in-situ observation experiment used by our group,35 PS suspensions were contained in a 10 × 10 × 44

3. RESULTS AND DISCUSSION 3.1. Different Growing and Drying Stages of PS EISA Monitored by CCD Camera and Monochromator. The information on entire PS EISA process, including real-time CCD image and reflectance spectra, was detected under this new monitoring technique. For the convenience of description and discussion, Figures 2, 3, 5, and 6 as well as movie SI1 and Figures S1 and S2 in the Supporting Information are the results of 240 nm PS sphere while the results of 290 nm PS sphere are presented in Figures S3−S5 of the Supporting Information. As shown in movie SI1, meniscus enters and sweeps through the monitored window of CCD camera while the stop-band of Bragg diffraction in reflectance spectra becomes distinct. We define as zero time (0 min) for the entire process the instant at which we observe the appearance of meniscus in CCD camera and detect the weakest Bragg diffraction peak generated by CPCs in reflectance spectra. Then the peak attenuation increases, and peak position of reflectance shifts to shorter wavelength. There is a slight crack observed from the CCD camera. At 150 min, the obvious vertical crack propagates, and the peak position of reflectance shifts back to longer wavelength (red-shift). As time goes by, more crack propagates and the

Considering the advantages of these two monitoring methods, a new monitoring technique which combines these two methods is developed in this work. In this new monitoring technique, real-time in-situ morphology of crack is obtained by CCD camera observation while real-time in-situ three-dimensional quantitative information is obtained by reflectance spectra measurement. This new technique has mainly three advantages: (1) More comprehensive information is obtained, including the visual morphology of crack and three-dimensional quantitative optical information. (2) Because the observation and measurement are investigated in situ simultaneously, the two result aspects are guaranteed being from the same region of CPCs. (3) The above two advantages make results more convinced and help deeper understanding of crack propagation during EISA. Under this monitoring technique, a new phenomenon during PS colloidal self-assembly, the red-shift of stop-band when crack propagates, was observed, and the model of colloidal spheres’ deformation37 and modified scalar wave approximation (SWA)36 is applied to explain this phenomenon.

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As one part of the monitoring results, the real-time CCD camera images of CPCs (Figure 2) show that the entire process of PS EISA in the monitored region can be divided into three stages. From 0 to 150 min, meniscus appears and sweeps through the monitored window of CCD camera (Figure 2a). Soon the meniscus drops out of the monitored window, and the image remains with no obvious change (Figure 2b). During this stage, only a slight crack initiates in the monitored window of CCD camera, so it is defined as crack-initiation stage (T1). From 150 to 312 min, an obvious crack propagates in the monitored region (Figure 2c−e). It is defined as crackpropagation stage (T2). From 312 min, the crack does not change obviously (Figure 2f). It is defined as crack-remained stage (T3). Another part of monitoring results, the real-time reflectance spectra shows the optical characteristics of CPCs during EISA. The reflectance of CPC is normalized through dividing reflected light intensity by incident light intensity. Previous works have shown that monodispersed colloidal spherical particles are more inclined to self-assemble into a face-centered cubic (FCC) structure which is the most energetically stable for the CPCs, with the (111) planes parallel to the substrate interface during vertical deposition.39−41 The diffracted wavelength (λ) is given by the Bragg equation23 mλ = 2d111 neff 2 − sin 2 θ

(1)

where m is the diffraction order, d111 is the interplanar spacing of (111) planes, which equals (2/3)1/2D (D is the diameter of colloidal particles), neff is the effective refractive index of the crystal, and θ is the incident angle. Here, θ is zero as the light is normal incidence. The effective refractive index of the crystal neff is given by a volume-weighted average

Figure 2. Real-time CCD camera images of CPC fabricated from 240 nm PS spheres at different times: T1 (a, b), T2 (c−e), and T3 (f): (a) 0 min, the beginning of first stage, when the meniscus appears in the observed window; (b) 144 min, the nearly end of first stage, when crack still does not propagate; (c) 168 min, the beginning of second stage, when crack begins to propagate; (d) 219 min and (e) 246 min, when crack propagates; (f) 414 min, third stage, crack remains without obvious change.

neff =

φsphnsph 2 + φwatern water 2 + (1 − φsph − φwater)nair 2 (2)

where φsph and φwater are the volume fractions occupied by PS spheres and water between the spheres, respectively. It is different from the previous definition in that φsph is taken as 0.74. Here, φsph can be smaller or bigger than 0.74, which is the volume fraction of spheres for a close-packed structure. By setting m = 1 in eq 1, we can get the peak position of the stopband λ. The peak position and stop-band attenuation of reflectance spectra are plotted against the time in Figure 3. According to the optical characteristics obtained from reflectance spectra, the entire process of PS EISA in the monitored region can be also separated into three stages. This division of the entire process agrees with the division based on CCD camera images. And it should be noted that these three stages are different from the three stages described in previous work.35 In the first stage, the peak position decreases rapidly and then remains at a relatively low level. The stop-band attenuation goes up rapidly, then goes down a little, and finally remains at a relatively high level. The changes of these two parameters derived from reflectance spectra indicate that PS spheres first assemble rapidly into an ordered structure during this stage, consequently leading to the increase of the stop-band attenuation. Later CPCs begin drying and φwater decreases. Thus, the effective refractive index of the crystal neff decreases, leading to the decrease of peak position λ. The decrease of peak position also results from the shrinkage of electric double layer on the surface of spheres.33 At the later period of T1, CPCs are nearly dried and the number of layers

Figure 3. Peak position (red circles) and stop-band attenuation (blue squares) as functions of elapsed time in the experiment of 240 nm PS spheres.

peak position of reflectance goes on shifting to longer wavelength. After 312 min, the crack change barely, and the peak position of reflectance shifts back again to shorter wavelength smoothly. 3951

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nearly dried, the attenuation is nearly unchanged. This explanation of the new phenomenon will be further verified by theory modified SWA in the following section. It should be noted that the deformation of PS sphere is not isotropic while Figure 4 is a simplified schematic diagram. 3.2. Analysis of Reflectance Spectra by the Modified Scalar Wave Approximation. To further understand the mechanism of PS EISA and explain the new phenomenon, a theoretical model is applied to analyze the reflectance spectra. A widely used theoretical model to calculate the reflectance of CPCs is scalar wave approximation (SWA).41,43,44 The reflectance is determined by the expression35

of CPCs remains stable as water front is far below the monitored region. T1 is consistent with the three growth stages of EISA described by our group.35 In the second stage, peak position of reflectance starts to shift back to long wavelength (red-shift). This is a new phenomenon which has not been mentioned before. The red-shift of peak position can be explained by the deformation of PS spheres. It will be discussed in detail in the following paragraph. In the third stage, peak position λ decreases again together with tiny decrease of attenuation. At this stage, deformation of PS spheres stops. It can be verified from the crack-remained in CCD image. It is believed that the tiny decrease of attenuation and peak position is due to quality decline of CPCs. The new phenomenon, stop-band red-shifts when the crack propagates, can be explained by the deformation of PS spheres. The earliest theory of deformation for spheres in contact, proposed by Hertz,42 relates the change in the center to center distance between two touching spheres to their elastic modulus and the external force squeezing them together. It is believed that the interstice of CPCs is gradually out of water in T1, and the force between the PS spheres accumulates for a period of time, leading to the deformation of PS spheres in T2. The CPCs film binds to the substrate and resists deformation in the transverse direction, giving rise to transverse tensile stress. Then crack in the film are formed to release the transverse tensile stress.24 As the model of deformation shown in Figure 4,

R=

(1 + β0)(1 + η)kg − (1 − β0)[kc + η(kc − G)]

2

(1 + β0)(1 + η)kg + (1 − β0)[kc + η(kc − G)] (3)

where β0 =

(kc − k 0)eikcT + η(kc − k 0 − G)ei(kc − G)T (kc + k 0)e−ikcT + η(kc + k 0 − G)e−i(kc − G)T

(4)

Here, kg, k0, and kc are the wave vectors in glass slab, air, and photonic crystal, respectively. T is the thickness of the photonic crystals, G is the magnitude of the reciprocal lattice vector, and η is the dielectric contrast between the two materials which comprise the composite structures. In our previous work,36 we have extended SWA theories to describe the reflectance spectra of CPCs with defects. Specifically, we take the short-range disorder (such as grain boundaries, point defects, and unstructured colloids) into consideration and introduce a component of imaginary part of dielectric constant to rewrite the dielectric constant as ε0 = εr + αi. The imaginary part of dielectric constant α is defined as short-range disorder parameter. Meanwhile, G is rewritten as G0(1 + βG) to describe the influence of the long-range disorder (such as distortion of lattice constant related to size polydispersion of spheres and nonhomogeneity of the CPCs). G0 is the reciprocal lattice of perfect crystal, βG is a random variable related to lattice distortion, and βG is defined as long-range disorder parameter. Finally, an expression of modified averaged reflectance R considered the defects is obtained as +∞

R̅(ω) =

∫−∞

p(βG)R(G(βG), ω) dβG

(5)

where the distribution density function of βG is p(βG) = e−βG

2

/2σ 2

/(σ 2π )

(6)

Detailed discussion of modified scalar wave approximation can be found in ref 36. It was also presented that the modified SWA achieved excellent fitting of theory to experiment results.36 Moreover, it was found in previous researches that short-range disorder parameter α mainly contributes to the diminishing of reflectance peak45,46 and long-range disorder parameter β mainly contributes to the widening of the gap.47−51 Taking the short-range disorder and long-range disorder into account, we use the modified SWA to fit the reflectance spectra of CPCs and analyze the propagation of crack quantitatively. First, we use the Fabry−Perot Fringes17 in the long wavelength (600−700 nm) to determinate the thickness and number of layers of the CPCs (Figure 5, number of layers is 16). The diameter of colloidal sphere is marked as 240 nm ± 3% on the label (Thermo Scientific) and is measured to be 232.4 nm by

Figure 4. Model images of deformation of colloidal spheres which results in a crack: (a) and (c) are the images before deformation of colloidal spheres; (b) and (d) are the images after the deformation of colloidal spheres.

PS spheres get closer, squeeze with each other, and then the remaining space gathers into a crack. With PS spheres getting closer and squeezing with each other, the interstice between spheres decreases and φsph increases. Therefore, the effective refractive index neff increases, resulting in the increase of peak position λ. At this time, the water front is further below the monitored region, so the little decreases of small φwater cannot change the final increase of neff. As the number of layers of CPCs in the monitored region does not change and CPCs are 3952

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Figure 5. Reflectance of CPCs fabricated from 240 nm PS spheres given by experiment (black round circle) and theory simulation (blue dashed line for perfect SWA, and red solid line for modified SWA). T1: (a) 33 min, β = 0.007, α = 0.01, φwater = 0.288, φsph = 0.64, D = 237 nm; (b) 150 min, β = 0.0015, α = 0.0595, φwater = 0.03975, φsph = 0.735, D = 234.82 nm; T2: (c) 156 min, β = 0.005, α = 0.058, φwater = 0.0369, φsph = 0.754, D = 234.3 nm; (d) 189 min, β = 0.0052, α = 0.062, φwater = 0.03525, φsph = 0.765, D = 234 nm; T3: (e)336 min, β = 0.005, α = 0.075, φwater = 0.0152, φsph = 0.81, D = 233 nm; (f) 408 min, β = 0.005, α = 0.077, φwater = 0.0076, φsph = 0.81, D = 233 nm. (N = 16 all above).

results between theory reflectance and experimental data are obtained after fine adjustment. For comparison, reflectance spectra accumulated from perfect SWA and modified SWA at different time are obtained (Figure S1 in Supporting Information). In Figure 5, some fitting results of typical moments are shown with the corresponding φsph, φwater, α, and β. The fitting results of reflectance spectra considered defects (red solid line) show a good agreement with the experimental data (black round circle). The volume fractions of spheres φsph and water φwater are plotted against the elapsed time to support the model presented above (Figure 6a). With the time going on, water evaporates continuously and φwater decreases. The speed of water evaporation slows down with the drying proceeding. The change of φwater is in coincidence with previous experimental result.35 The volume fraction of spheres φsph increases from

the zeta potential analyzer (Brookhaven). As the deformation of spheres is anisotropic, the interplanar and in-planar spacings are different. Considering the anisotropic deformation and error range, the diameter in the fitting process (D) only relates to the interplanar spacing and is set as 237 nm. Second, we adjust the volume fraction of water φwater referring the water evaporation model in ref 35 and then adjust volume fraction of spheres φsph to change the effective refractive index neff to fit the peak position of reflectance. Third, we adjust disorder parameters α and β to fit stop-band attenuation and stopband bandwidth. Finally, considering the complicated dynamic process of EISA, many factors influence the optical properties of the growing film, including the thickness polydispersity, medium in the interstices of the crystal, the degree of order in the CPC, and so on. We focus on the agreement in peak position and Fabry−Perot fringe, and a series of good fitting 3953

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The model of spheres’ deformation we utilize to explain the red-shift and crack propagation is supported by the CCD camera observations of crack, reflectance spectra measurement, and fitting simulation of the reflectance spectrum. The above analyses are based on the results of CPC formed from 240 nm PS particles. The self-assembly of CPC formed from 290 nm PS particles is also analyzed in Figures S3−S5 and shows similar behaviors. Furthermore, the model of spheres’ deformation may also suit for different thicknesses of the artificial opal film along the growth direction. As observed by Vos et al., the sample thickness increases strongly from top to bottom (along the grow direction) and the lateral domains increases linearly with ́ thickness,52 and Miguez et al. observed the thickness fluctuates53 and found that the amount of defects fluctuates periodically and decreases along the growth direction of the lattice.54 They explained the phenomenon by temporal periodic fluctuation of the lattice growth velocity, a phenomenon that is inherent to the evaporation induced self-assembly process. It can be explained by our model of spheres deformation as follows: as the thickness increase, more spheres share the stress and deformation. It results in the increase of the spacing of crack, that is, the increase of lateral domain size and decrease of defects. Nevertheless, the model also has its limits and needs systematic research in the further work. For example, the elastic constant has been not introduced in the model as the elastic constant is not a single elastic modulus but a tensor in this system. It is difficult to introduce the microscopic elastic constant tensor into the simplified macroscopic SWA simulation to give the macroscopic crack parameter. To reveal the quantitative relationship between elastic constant and crack formation, further experiments and theoretical calculations should be investigated, including the elasticity of spheres, stress of film, and crack spacing. Considering that many complicated factors influence the self-assemble of CPC, we just utilize a macroscopic and simple model to do SWA simulation and obtain agreement with the observations. In future work, we will apply this monitoring technique to research crack formation systematically considering different particles, different radius, and initial volume fraction.

Figure 6. Fitting parameters as functions of the elapsed time in the experiment of 240 nm PS spheres: (a) volume fraction of water (red circles) and volume fraction of spheres (blue squares); (b) long-range disorder parameter β (blue squares) and short-range disorder parameter α (red circles).

0.64 to 0.74 and finally to 0.81. It verifies the explanation presented in section 3.1. In the first stage, the spheres get closer to each other and gradually assemble into closed-packed structure, whose volume fraction is 0.74. There is slight crack initiation during this stage. Then in the second stage, with the water evaporating, spheres get closer and squeeze with each other. Thus, the interstice between spheres decreases and φsph increases. The crack propagates and peak position λ red-shifts. To estimate the quantitative deformation to reach high volume fraction, a simplified isotropic deformation is presented in Figure S2. In the third stage, the repulsive force between the spheres that prevents the spheres from getting closer and the structure is stable, so φsph and crack stay barely changed. Figure 6b displays disorder parameters α and β against the elapsed time. The change of long-range disorder parameter β can also be divided into three different parts. The long-range disorder parameter β decreases first in T1, then increases quickly in T2, finally remains stable in T3. It indicates that the CPCs turn into more ordered structure in T1, crack propagates in T2, and crack does not change obviously in T3. It is well consistent with experiment description in section 3.1. The time when β begins to increase is just the time when crack propagates in the monitored window of CCD camera (see in Figure 3c). Another disorder parameter α increases, indicating the short-range disorder such as point defect initiates and propagates. The changes of α and β support the explanation of red-shift and crack propagation.

4. CONCLUSIONS A new monitoring technique, which contains CCD camera observation and microspectroscopy setup measurement, has been utilized to study the real-time in-situ growing and drying processes of PS CPCs. Together with the evolutions of optical properties, including peak position and stop-band attenuation, CCD camera observation results indicate that the growing and drying processes can be divided into three stages, including crack-initiation stage, crack-propagation stage, and crackremained stage. A new phenomenon, that the peak position of reflectance red-shifts when crack begins to propagate, is observed. The red-shift of peak position is due to the deformation of PS spheres, which leads to the increase of volume fraction of spheres. Furthermore, this explanation is verified by the modified SWA with the change of sphere volume fraction. The fitting reflectance spectra by modified SWA are in good agreement with experimental results. Obtained from the fittings, the changes of disorder parameters are also in accordance with PS EISA process we described. The new monitoring technique in this work appears as a useful approach to studying the growing and drying processes of CPCs, especially the crack propagation. 3954

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ASSOCIATED CONTENT

S Supporting Information *

Video clip (SI1) shows real-time in-situ CCD camera images and reflectance spectrum; file SI2 shows fitting figures of experimental reflectance based on SWA and modified SWA, computational simulation of spheres’ deformation, results and analyses of 290 nm PS sphere. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel +86-10-82649242; Fax +86-10-82649242; e-mail [email protected] (Q.M.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors appreciate the financial support of the National Key Project for Basic Research (No. 2012CB932903) and National Natural Science Foundation of China (No. 51072221).

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