Formation and Third-Order Optical Nonlinearities of Fractal Ge

Apr 10, 2013 - Laboratory for Microstructures, Shanghai University, Shanghai 200444, People,s Republic of China. §. Department of Physics and Materia...
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Formation and Third-Order Optical Nonlinearities of Fractal Ge Nanocrystals Embedded in Au Matrix Wenfeng Wang,† Zhiwen Chen,*,†,§ Linggui Hou,† Pengfei Hu,‡ Chan-Hung Shek,§ C. M. Lawrence Wu,§ and Joseph K. L. Lai§ †

Shanghai Applied Radiation Institute, School of Environmental and Chemical Engineering, Shanghai University, Shanghai 200444, People’s Republic of China ‡ Laboratory for Microstructures, Shanghai University, Shanghai 200444, People’s Republic of China § Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon Tong, Hong Kong ABSTRACT: Nonequilibrium growth processes of lowdimensional materials have attracted considerable attention, which controlled micro/nanostructures, properties, and various performances of materials. Semiconductor Ge in contact with some metals, e.g., Au, Pd, and Al, etc., is a class of distinctive materials with noninteger dimensions that differ from integer dimensional materials such as nanoparticles, nanowires, nanorods, nanotubes, nanoribbons, and thin films. Here, Au/Ge bilayer films with interesting fractal Ge nanocrystals were successfully prepared by high-vacuum thermal evaporation techniques. It was found that Ge nanocrystals embedded in Au matrix showed fascinating fractal morphologies with average size of fractal clusters at 550 nm and fractal dimension at 1.756 when the films were annealed at 150 °C for 30 min. Third-order optical nonlinearities of the annealed Au/Ge bilayer films were investigated in detail by Z-scan technique using a femtosecond laser. Experimental results indicated that the nonlinear absorption coefficient and refractive index of the fractal Ge nanocrystals embedded in Au matrix were in the ranges 4.2 to 4.7 × 10−7 cm/W and 5.2 to 5.6 × 10−12 cm2/W, respectively, when the input irradiance (Ip) ranged from 0.58 to 1.65 GW/cm2. This nonlinear optical material may be tailor-made for a large number of applications such as highspeed microelectronics and infrared optical micro/nanodevices.

1. INTRODUCTION

for micro/nanodevices and offer exciting opportunities for both fundamental research and technological applications. The crystallization behavior of amorphous semiconductors such as germanium (Ge) and silicon (Si) in contact with some metals, e.g., gold (Au), palladium (Pd), and aluminum (Al), etc., is an intricate relaxation process of nonequilibrium growth, which is an important area of nonlinear science.12,13 The crystallization behavior of amorphous semiconductors in metal/ semiconductor composite films has the following features: (i) crystallization temperature of amorphous semiconductors in metal/semiconductor composite films was much lower than that of an isolated amorphous semiconductor film; (ii) fractal semiconductor micro/nanoclusters can be formed by crystallization of amorphous semiconductor under some conditions; (iii) some metastable compounds can be formed in fractal edges via metals reacted with amorphous semiconductors. In fact, influence of fractal micro/nanoclusters and metastable compounds on optical and electrical properties of metal/ semiconductor composite films is an important research subject

The key scientific issues of semiconductor micro/nanodevices and optoelectronics components in application and development have driven scientists to explore indepth the design, preparation, micro/nanostructure and performance of semiconductor materials.1−3 The formation and evolution of nonequilibrium growth processes in low-dimensional materials have grown into a subject of fruitful inquiry for a long time.4−6 These investigations have been performed in various disciplines such as physics, chemistry, materials science, nanoscience, and nanotechnology, and have been applied to low-dimensional semiconductor materials characterized by different length scales from macroscopic to nanoscale regimes.7−9 These consequences have provided us insight into fundamental issues pertaining to the interplay and balance governing the bulk and surface contributions to the energetics of materials, their morphological stability, and the relaxation dynamics in nonequilibrium complex systems.10,11 At present, considerable effort is focusing on the preparation of metal/semiconductor composite films with nonequilibrium growth processes and the exploration of their novel properties.10−13 It is expected that these semiconductor materials may constitute important building blocks © 2013 American Chemical Society

Received: December 20, 2012 Revised: March 30, 2013 Published: April 10, 2013 8903

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nonlinear absorption coefficient and refractive index of the fractal Ge nanocrystals embedded in Au matrix were in the ranges 4.2 to 4.7 × 10−7 cm/W and 5.2 to 5.6 × 10−12 cm2/W, respectively, when the input irradiance (Ip) ranged from 0.58 to 1.65 GW/cm2. Our findings may enable this novel functional material with fractal Ge nanocrystals to be tailor-made for a large number of applications such as the high-speed microelectronics and infrared optical micro/nanodevices and provide new opportunities for future study of optical nonlinearities in semiconductor architectures, with the goal of optimizing photoelectronic functional material properties for specific applications.

since its widespread technological applications in microelectronic devices and optoelectronic components. In our previous works, fractal micro/nanoclusters including Mn3O4 and SnO2 have been successfully prepared by chemical liquid homogeneous precipitation14 and pulsed laser deposition method,15 respectively. Ge and Si micro/nanoclusters are of fundamental importance to the development of smart and functional materials, devices, and systems.16−18 Notwithstanding the fact that Ge has been applied to many areas, its use is not as extensive as that of Si, and nebulous domains in our understanding of its precise technical functions still remain. It is known that Ge has an excitonic Bohr radius of 24.3 nm, which is much larger than that of silicon (4.9 nm).19 Thus, Ge should exhibit more pronounced quantum size effects, which will be more prominent in Ge nanocrystals even for larger size of the crystallites. These electronic conditions lead to an expectation that it is much easier to change the electronic structure around the band gap of Ge, resulting in strong modification of its optical properties.20 Over the years, Ge and Si micro/ nanoclusters have been extensively investigated because they would open a new possibility for indirect gap semiconductors as new materials for optoelectronic applications.21,22 The development of semiconductor materials with novel linear and nonlinear optical properties is the key to realizing the full potential of all types of optical computing and signal processing.23−25 To date, several preparation methods have been implemented to produce semiconductor nanocrystals with narrow and reproducible size distribution.26−28 Altering the average nanocrystal size allows a broad modification of their energy structure, particularly in the energy band gap. Moreover, three-dimensional quantum confinement results in discrete energy structures and atomic-like behavior for nanocrystal optical transitions. Strong and fast optical nonlinearities, and strong photo- and electroluminescence, have been observed in materials composed of semiconductor nanocrystals.29,30 However, the understanding of nonlinear optical response in these materials is still not well established. In low-dimensional semiconductors, the optical transitions across originally an indirect energy gap can be allowed in the first order of the perturbation theory due to confinement-induced mixing of states. However, calculations performed, e.g., for Si nanocrystals, show that the probability of these transitions remains low unless the nanocrystal size is smaller than 2−3 nm. Thus, it is not clear whether we can expect a large confinement-induced increase in absorption in the case of the relatively large fractal Ge nanocrystals. In this article, we report an experimental investigation of third-order optical nonlinearities of fractal Ge nanocrystals embedded in Au matrix using femtosecond laser pulses at 800 nm wavelength. New strategies of fractal assessment for Au/Ge bilayer films after annealing are of fundamental importance in the development of micro/nanodevices. Au/Ge bilayer films with interesting Ge nanocrystals, which have noninteger dimensions and are called fractals, were successfully prepared by high-vacuum thermal evaporation techniques. It was found that Ge nanocrystals embedded in Au matrix showed fascinating fractal morphologies with average size of fractal clusters at 550 nm and fractal dimension at 1.756 when the films were annealed at 150 °C for 30 min. Third-order optical nonlinearities of the annealed Au/Ge bilayer films were investigated in detail by the Z-scan technique using a femtosecond laser. Experimental results indicated that the

2. EXPERIMENTAL SECTION Specimens were prepared by evaporation on a freshly cleaved NaCl (100) single crystal substrate at a pressure of 2.67 × 10−3 Pa at room temperature.31 We deposited Ge first and then Au without breaking the vacuum (about 2.67 × 10−3 Pa) by evaporating high-purity germanium (99.9%) and gold (99.99%) from two resistive-heated tungsten boats, viz., the bottom layer was amorphous Ge (a-Ge) and the top one was polycrystalline Au (p-Au). According to the evaporation equation t = m/4πr2ρ, where t is the thickness of the films, m is the mass of the Au or Ge, ρ is the density of the Au or Ge, and r is the distance from the specimen to the evaporation source, r = 10 cm in present experiments. The thickness ratio of the p-Au and a-Ge films was designed to be 25 and 20 nm, respectively. All as-deposited specimens were annealed in vacuum of about 2.67 × 10−3 Pa at 150 °C for 30 min. After annealing, the specimens were floated on distilled water and then placed on copper meshes to be observed with a Philips CM20 transmission electron microscope at an acceleration voltage of 200 kV. By such annealing, self-similar fractal structures may be formed in these bilayer films. Since the annealing temperatures can effectively control the morphology of the fractal patterns, the density of the different fractal clusters formed at a given annealing temperature is also approximately uniform at different sites of the sample. The average value of the evaluated dimension, obtained from different regions, can be approximately considered as the whole sample’s fractal dimension (D). The fractal dimension for these samples was calculated by measuring the fractal dimensions of these self-similar clusters using conventional box-counting method.32,33 The linear optical absorption spectrum of the Au/Ge bilayer films annealed at 150 °C for 30 min was measured by using a Hitachi U-3040 spectrophotometer at room temperature. The sensitive and reliable Z-scan techniques were used to determine the nonlinear absorption coefficient (β) and nonlinear refractive index (n2) values of the sample. In Z-scan experiments, the laser pulses were delivered by a mode-locked Ti:sapphire laser (Coherent MIRA 900-F) operating at a repetition rate of 76 MHz. The full width at half-maximum (fwhm) pulse duration was 120 fs. All measurements were performed at a wavelength of 800 nm. 3. RESULTS AND DISCUSSION Figure 1a shows a typical bright-field image of transmission electron microscopy (TEM) and the corresponding selected area electron diffraction (SAED) patterns (the inset at the upper right-hand corner) of the morphology of the as-prepared Au/Ge bilayer films. As seen in the TEM bright-field image, the as-prepared bilayer films were homogeneous in morphology 8904

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along the Au−Ge interfaces, making it possible for Ge nanoclusters can be formed in Au matrix. Figure 2a shows the plot of ln(N) versus ln(1/L) of the fractal Ge nanocrystal regions in Figure 1b, where L is the box

Figure 1. (a) TEM bright-field image and SAED patterns (the inset at the upper right-hand corner) of the as-prepared Au/Ge bilyer films. (b) TEM bright-field image and SAED patterns (the inset at the lower left-hand corner) of the Au/Ge bilayer films annealed at 150 °C for 30 min.

Figure 2. (a) Plot of ln(N) versus ln(1/L) of fractal Ge nanocluster in Figure 1b, where L is the box size and N is the number of boxes occupied by fractal Ge nanocrystals. (b) Linear absorption spectrum of the Au/Ge bilayer films annealed at 150 °C for 30 min.

size and N is the number of boxes occupied by the fractal Ge nanocrystals. It can be seen that the plot shows good linear relationship, which means that the morphologies of fractal Ge nanocrystals have scale invariance within these ranges. So these nanocrystals can be regarded as fractals. In order to obtain the fractal dimension (D), we fit a linear relationship for the function ln(N) versus ln(1/L). The results testify that the fractal dimension (D) is about 1.756 as shown in Figure 2a. This fractal structure may lead to improvement in the design of micro/nanodevices for microelectronic industry applications. Figure 2b shows a typical linear absorption spectrum for the annealed Au/Ge bilayer films with fractal Ge nanocrystals. Interestingly, it is found that the linear absorption spectrum shows two resonant absorption ramps at around 1.1 and 3.3 eV, respectively, which may correspond to the Γ′25 → Γ′2 (ΔE ∼ 0.8 eV) transitions and transitions Γ′25 → Γ′15 (ΔE ∼ 2.7−3.6 eV) in bulk Ge.34 The above experimental results indicated that the linear absorption is not negligible in the visible and nearinfrared ranges when the fractal Ge nanocrystals formed in the annealed Au/Ge bilayer films. It is common knowledge that the real part of the refractive index could be altered by cumulative sample heating in Z-scan measurements with high repetition rate lasers. The nonlinear absorption arises from either direct multiphoton absorption or saturation of single photon absorption. The Z-scan with no aperture is expected to be symmetric with respect to the focus (Z = 0) where it has the minimum transmittance for two or multiphoton absorption or maximum transmittance for saturation of absorption. Figure 3a,b shows, respectively, the normalized open-aperture (OA) and closed-aperture (CA) Z-

before annealing. The crystallites close to Bragg orientations were recognizable by their dark contrast. The average grain size of the polycrystalline Au (p-Au) in the as-prepared bilayer films was about 42 nm. The SAED patterns of the as-prepared bilayer film confirmed that the films consisted of amorphous Ge (aGe) and p-Au, e.g., two diffuse rings of a-Ge and p-Au (111) and (200) rings. Experimental results indicated that part of the a-Ge has begun crystallization in the as-prepared bilayer films, e.g., Ge (112) ring. Figure 1b shows TEM bright-field image and SAED patterns (the inset at the lower left-hand corner) of the Au/Ge bilayer films annealed at 150 °C for 30 min. After annealing at 150 °C, the diffuse ring of a-Ge was replaced by crystalline Ge (c-Ge) as shown in the inset of Figure 1b. It can be seen that the films display unique fractal Ge pattern. The maximum size of fractal Ge nanocluster was measured by the intercept method and found to be about 550 nm. Referring to Figure 1b, Au grains exhibited black contrast in the TEM image, which were enchased by the white Ge nanoclusters (indicated by the blue arrows). The average Au grain size estimated from this TEM image ranged from 8 to 15 nm in diameter. It was found that the white Ge nanoclusters consisted of Ge nanocrystals with diameters ranging from 20 to 50 nm. The Au grains were clearly enchased by the Ge nanocrystals. During annealing, Ge nanocrystals nucleated at Au−Ge interfaces due to the breaking of Ge−Ge bonds. The surrounding Ge atoms diffused along the interfaces and through Au layer to form the nucleus (nanocrystal seed); meanwhile, the Au atoms aggregated in an opposite direction. The Au extrusion aggregated into the black spots, and the continuous Au layer provided sufficient Au atoms transported 8905

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Figure 4. Measured nonlinear absorption coefficient (β) and nonlinear refractive index (n2) values versus the input irradiance (Ip) of the Au/ Ge bilayer films annealed at 150 °C for 30 min.

T= Figure 3. Normalized open-aperture (OA, part a) and closed-aperture (CA, part b) Z-scan data of the Au/Ge bilayer films annealed at 150 °C for 30 min. The solid squares are the experimental data. The solid curves are the theoretical fits.

1 ⎡ 1 − exp(‐αL) ⎤ 2 1 + βI 0 ⎣ ⎦ /(1 + ξ ) α ⎤ ‐2 ⎡ C1z C2z ×⎢ + − 1⎥ ⎦ ⎣ (1 + ξ 2)2 (1 + ξ 2)

(1)

where scan data of the Au/Ge bilayer films annealed at 150 °C for 30 min obtained with linearly polarized 800 nm and 120 fs pulses. It should be noted that all the input irradiances are the peak irradiances at the focus within the samples. For each z position, the traces of the intensity collected by the OA and CA detectors were acquired. An exponential fit of these traces allowed us to produce the curves at different input irradiances. The curves obtained in this way, representative of single-pulse and cumulative effects, were used to estimate the values of nonlinear absorption coefficient (β) and nonlinear refractive index (n2).35 We assumed that the total absorption coefficient can be written as α = α0 + βI, where I was the irradiance of the laser beam within the sample. Using the OA Z-scan theory,36 we numerically calculated the normalized power transmittance as a function of the z position of the sample using β as a free parameter. The β value can be extracted from the best fit curve. The solid line in Figure 3a shows the best fit to experimental Zscan data at an irradiance of 0.98 GW/cm2, which yields β = 4.5 × 10−7 cm/W. We also conducted OA Z-scan measurements at input irradiances ranging from 0.58 to 1.65 GW/cm2. It was found that the measured β value was independent of the laser irradiance (see Figure 4), which implied that the observed nonlinear absorption was a third-order process. Similarly we could express the total refractive index of the sample as n = n0 + n2I, where n0 is the linear index of refraction. It should be pointed out that, in Z-scan measurements with high-repetitionrate laser pulses, the thermal-lens effect is important due to the strong linear absorption in the sample at 800 nm wavelength. Therefore, the CA Z-scans are sensitive to nonlinear refraction of either electronic or thermal origin, and to nonlinear absorption. The normalized transmitted power for CA Zscans is given by22

C1 =

2LI0n2 n02ω02

, C2 =

LPincKT πn02ω02

Here, ζ = z/z0, z0 = πω20/λ, I0 is the peak irradiance at the focal plane, Pinc is the beam average power on the sample, L is the sample thickness, ω0 is the Gaussian beam radius, and KT is a constant related to the thermo-optic coefficient and thermal conductivity of the films. It should be emphasized that the n2 in eq 1 is the electronic contribution to nonlinear refractive index, excluding the thermal contribution. C1 and C2 represent electronic and thermal contribution to nonlinear phase shift, respectively. The differing z dependence of the two terms with C1 and C2 inside the bracket of the eq 1 comes about because the electronic contribution to the refractive nonlinearity is proportional to the peak irradiance, whereas the thermal contribution is proportional to the laser average power. Using the eq 1, the n2 and KT values were extracted from the best fit curves to experimental CA Z-scan data at five different input irradiances. The obtained value was n2 = 5.3 × 10−12 cm2/W, which was independent of the input irradiance up to 1.65 GW/ cm2 (see Figure 4), implying that the observed nonlinear refraction is of Kerr nonlinearity. This indicates that the fractal Ge nanocrystals give rise to the observed nonlinearities in the annealed Au/Ge bilayer films. This nonlinear optical film with fractal Ge nanocrystals may be tailor-made for a large number of applications such as high-speed microelectronics and infrared optical micro/nanodevices.

4. CONCLUSIONS In summary, the Au/Ge bilayer films with interesting Ge nanocrystals, which have noninteger dimension and are called fractals, were successfully prepared by high-vacuum thermal evaporation techniques. It was found that Ge nanocrystals 8906

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(9) Stoyanov, S.; Kaschiev, D. Current Topics in Materials Science; Kaldis, E., Ed.; North-Holland: Amsterdam, 1981; p 7. (10) Chen, Z. W.; Shek, C. H.; Lai, J. K. L. Insights into Microstructural Evolution and Polycrystalline Compounds Formation from Pd-Ge Thin Films. Phys. B 2005, 358, 56−62. (11) Bales, G. S.; Chrzan, D. C. Dynamics of Irreversible Island Growth During Submonolayer Epitaxy. Phys. Rev. B 1994, 50, 6057− 6067. (12) Chen, Z. W.; Lai, J. K. L.; Shek, C. H. Microstructural Changes and Fractal Ge Nanocrystallites in Polycrystalline Au/Amorphous Ge Thin Bilayer Films upon Annealing. J. Phys. D: Appl. Phys. 2006, 39, 4544−4548. (13) Chen, Z. W.; Lai, J. K. L.; Shek, C. H.; Chen, H. D. Nanocrystals Formation and Fractal Microstructural Assessment in Au/Ge Bilayer Films upon Annealing. Appl. Surf. Sci. 2005, 250, 3−8. (14) Chen, Z. W.; Jiao, Z.; Pan, D. Y.; Li, Z.; Wu, M. H.; Shek, C. H.; Wu, C. M. L.; Lai, J. K. L. Recent Advances in Manganese Oxide Nanocrystals: Fabrication, Characterization, and Microstructure. Chem. Rev. 2012, 112, 3833−3855. (15) Chen, Z. W.; Pan, D. Y.; Zhao, B.; Ding, G. J.; Jiao, Z.; Wu, M. H.; Shek, C. H.; Wu, C. M. L.; Lai, J. K. L. Insight on Fractal Assessment Strategies for Tin Dioxide Thin Films. ACS Nano 2010, 14, 1202−1208. (16) Dai, Z. R.; Gole, J. L.; Stout, J. D.; Wang, Z. L. Tin Oxide Nanowires, Nanoribbons, and Nanotubes. J. Phys. Chem. B 2002, 106, 1274−1279. (17) Chen, Z. W.; Lai, J. K. L.; Shek, C. H. Insights into Microstructural Evolution from Nanocrystalline SnO2 Thin Films Prepared by Pulsed Laser Deposition. Phys. Rev. B 2004, 70, 165314− 1−165314−7. (18) Chen, Z. W.; Lai, J. K. L.; Shek, C. H. Quantum Dot Formation and Dynamic Scaling Behavior of SnO2 Nanocrystals Induced by Pulsed Delivery. Appl. Phys. Lett. 2006, 88, 033115−1−033115−3. (19) Wu, Y.; Yang, P. Germanium Nanowire Growth via Simple Vapor Transport. Chem. Mater. 2000, 12, 605−607. (20) Dowd, A.; Elliman, R. G.; Samoc, M.; Luther-Davies, B. Nonlinear Optical Response of Ge Nanocrystals in a Silica Matrix. Appl. Phys. Lett. 1999, 74, 239−241. (21) Takeoka, S.; Fujii, M.; Hayashi, S.; Yamamoto, K. SizeDependent Near-Infrared Photoluminescence from Ge Nanocrystals Embedded in SiO2 Matrices. Phys. Rev. B 1998, 58, 7921−7925. (22) Li, H. P.; Kam, C. H.; Lam, Y. L.; Jie, Y. X.; Ji, W.; Wee, A. T. S.; Huan, C. H. A. Nonlinear Optical Response of Ge Nanocrystals in Silica Matrix with Excitation of Femtosecond Pulses. Appl. Phys. B: Lasers Opt. 2001, 72, 611−615. (23) Hon, N. K.; Soref, R.; Jalali, B. The Third-Order Nonlinear Optical Coefficients of Si, Ge, and Si1‑xGex in the Midwave and Longwave Infrared. J. Appl. Phys. 2011, 110, 011301−1−011301−8. (24) Wan, Q.; Lin, C. L.; Zhang, N. L.; Liu, W. L.; Yang, G.; Wang, T. H. Linear and Third-Order Nonlinear Optical Absorption of Amorphous Ge Nanoclusters Embedded in Al2O3 Matrix Synthesized by Electron-Beam Coevaporation. Appl. Phys. Lett. 2003, 82, 3162− 3164. (25) Debrus, S.; Lafait, J.; May, M.; Pincon, N.; Prot, D.; Sella, C.; Venturini, J. Z-Scan Determination of the Third-Order Nonlinearity of Gold: Silica Nanocomposites. J. Appl. Phys. 2000, 88, 4469−4475. (26) Choi, W. K.; Chim, W. K.; Heng, C. L.; Teo, L. W.; Ho, V.; Ng, V.; Antoniadis, D. A.; Fitzgerald, E. A. Observation of Memory Effect in Germanium Nanocrystals Embedded in an Amorphous Silicon Oxide Matrix of a Metal-Insulator-Semiconductor Structure. Appl. Phys. Lett. 2002, 80, 2014−2016. (27) Yamamoto, M.; Koshikawa, T.; Tasue, T.; Harima, H.; Kajiyama, K. Formation of Size Controlled Ge Nanocrystals in SiO2 Matrix by Ion Implantation and Annealing. Thin Solid Films 2000, 369, 100−103. (28) Xu, J.; He, Z. H.; Chen, K.; Huang, X.; Feng, D. Fabrication of Luminescent Ge Nanocrystals Started from Unlayered Hydrogenated Amorphous SiGe Films or Hydrogenated Amorphous Si/Hydro-

embedded in Au matrix showed fascinating fractal morphologies with average size of fractal nanoclusters at 550 nm and fractal dimension at 1.756 when the films were annealed at 150 °C for 30 min. Third-order optical nonlinearities of the annealed Au/Ge bilayer films were investigated in detail by the Z-scan technique using a femtosecond laser. Experimental results showed that the nonlinear absorption coefficient and nonlinear refractive index of the fractal Ge nanocrystals embedded in Au matrix were in the ranges 4.2 to 4.7 × 10−7 cm/W and 5.2 to 5.6 × 10−12 cm2/W, respectively, when the input irradiance (Ip) ranged from 0.58 to 1.65 GW/cm2. Our findings may enable this novel functional material with fractal Ge nanocrystals to be tailor-made for a large number of applications such as the high-speed microelectronics and infrared optical micro/nanodevices and provide new opportunities for future study of optical nonlinearities in semiconductor architectures, with the goal of optimizing photoelectronic functional material properties for specific applications.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86 21 66137503. Fax: +86 21 66137787. E-mail: [email protected] (Z.C.); [email protected] (C.-H. S). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work described in this article was financially supported by the National Natural Science Foundation of China (Project 11074161), Shanghai Pujiang Program (Project 10PJ1404100), Key Innovation Fund of Shanghai Municipal Education Commission (Project 10ZZ64), Science and Technology Commission of Shanghai Municipality (Project 10JC1405400), and Shanghai Leading Academic Discipline Project (Project S30109). This work was also supported by a General Research Fund (Project CityU 119212) from the Research Grants Council, Hong Kong.

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