Refractive-Index-Enhanced Raman Spectroscopy and Absorptiometry

Jul 15, 2013 - Waveguide-based spectroscopic techniques utilizing evanescent wave as a probe are ideal tools for surface and interface analysis. Howev...
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Refractive-Index-Enhanced Raman Spectroscopy and Absorptiometry of Ultrathin Film Overlaid on an Optical Waveguide De-Bo Hu and Zhi-Mei Qi* State Key Laboratory of Transducer Technology, Institute of Electronics, Chinese Academy of Sciences, Beijing 100190, P. R. China W Web-Enhanced Feature * S Supporting Information *

ABSTRACT: Waveguide-based spectroscopic techniques utilizing evanescent wave as a probe are ideal tools for surface and interface analysis. However, practical applications of these techniques are largely limited by the weak effective signals resulting from the restricted strength of interaction between the evanescent wave and the ultrathin sample immobilized at the waveguide surface; this is especially the case in light of the evanescent-wave excited waveguide Raman spectroscopy. Hence, we present a non-surface-enhanced Raman spectroscopy (SERS) method in this work to enhance the Raman spectroscopy of ultrathin film overlaid on the waveguide surface by intensifying the evanescent wave−sample interaction via manipulating the refractive index (RI) of the waveguide superstrate. Polarization-controllable and highly reproducible Raman enhancement can be achieved for the 10-nm-thick copper phthalocyanine (CuPc) ultrathin film deposited on the surface of a composite optical waveguide (COWG), with the enhancement factors being five and eight for transverse electric (TE) and transverse magnetic (TM) polarization, respectively. Additionally, enhanced absorption of the CuPc ultrathin film was also observed, with the absorbance increments being 0.155 and 0.165 for TE and TM polarization, respectively. Polarization controllability and good reproducibility are two great advantages of our method, and the implementation of this method should promote the practical applications of waveguide-based Raman and absorbance spectroscopic techniques effectively.

1. INTRODUCTION The integrated optical waveguide in conjunction with all kinds of spectroscopic techniques provides a promising analytical tool for surface and interface scientists due to its high sensitivity to the kinetic properties of the surface species as well as its capability to characterize the static properties of thin films and sub-/monolayers overlaid on the waveguide surface.1−4 Rapid evolvement of the waveguide absorbance spectroscopy technique in the last two decades has made routine characterization of thin films and sub-/monolayers at the waveguide surface possible.5−8 However, as another important spectroscopic technique for surface characterization, evanescent-wave excited waveguide Raman spectroscopy has lagged behind for many years, primarily ascribed to the small Raman signals that can be acquired due to the restricted strength of interaction between the excitation evanescent field extending out of the waveguide surface and the ultrathin sample that is under investigation. The long acquisition time for a distinct Raman spectrum has become a main obstacle that handicaps the practical applications of evanescent-wave excited waveguide Raman spectroscopy.9−13 Recently, plasmon waveguide based Raman spectroscopy14,15 making use of the resonantly enhanced evanescent field and microstructured optical fiber based Raman spectroscopy16 taking advantage of the long light−sample interaction length have shortened the acquisition time within the limit of 60 s, which, however, is still quite long. Surface-enhanced Raman spectroscopy (SERS) based on © 2013 American Chemical Society

plasmonic resonance can provide remarkable Raman enhancement,17−19 so it has been utilized to enhance the evanescentwave excited waveguide Raman spectroscopy.20,21 Although the evanescent-wave excited waveguide Raman spectroscopy can be enhanced significantly by SERS, the requirement of specially designed SERS substrates compatible with both the waveguide and the analyte makes the investigation procedure rather burdensome. What is more, the poor reproducibility of the SERS substrates makes them unlikely to be used repeatedly. To address the problem suffered by the evanescent-wave excited waveguide Raman spectroscopy (i.e., small effective signals), in this paper, a non-SERS enhancement method is present. By applying a liquid superstrate of high refractive index (RI) to the surface of a composite optical waveguide (COWG), the evanescent field is boosted up in the vicinity of the waveguide surface, resulting in an intensified evanescent wave− sample interaction, thus the Raman scattering of the ultrathin film deposited onto the COWG is enhanced. As an adjoint effect, absorption of the guided light by the ultrathin film is also enhanced by the intensified evanescent field, which opens up the opportunity for simultaneous measurement of Raman spectroscopy and absorbance of the ultrathin film immobilized at the waveguide surface. With the molecular fingerprints Received: May 29, 2013 Revised: July 5, 2013 Published: July 15, 2013 16175

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offered by the enhanced Raman spectroscopy and the kinetic properties offered by the enhanced absorptiometry, our enhancement method is believed to be of great value for the in situ analysis of chemical and biochemical analytes such as sub-/monolayers and biomembranes at the waveguide surface.

2. EXPERIMENTAL DETAILS 2.1. Fabrication of COWGs and Deposition of CuPc Ultrathin Films. COWG has been widely used as a chemical and biochemical sensing element of high sensitivity due to its intensified evanescent field compared with that of the ordinary step-index or graded-index planar optical waveguide.22,23 The COWG utilized in this work was manufactured by the same method reported before.23 It is a combination of a graded-index waveguide obtained by potassium ion exchange (PIE) in a soda-lime glass substrate (76 mm × 26 mm, n0 = 1.52) and a strip of RF sputtered amorphous tantalum pentoxide (Ta2O5) thin film (5 mm × 26 mm, n2 = 2.15) in the middle of the PIE glass waveguide. The RI profile of the PIE waveguide follows Gaussian distribution and thus can be expressed by the equation ⎛ x2 ⎞ n1(x) = n0 + Δn exp⎜ − 2 ⎟ ⎝ d ⎠

Figure 1. Experimental setup for refractive-index-enhanced Raman spectroscopy and absorptiometry of ultrathin film overlaid on the optical waveguide.

the waveguide holder permits the laterally scattered Raman light to be collected by the Raman probe, which was positioned with its optical axis coincident with the normal of the COWG surface and its focus in the middle of the 5 mm long bright streak (caused by surface scattering of the waveguide mode) at the surface of the 5 mm width CuPc strip. The collected Raman scattering light was transported to the Raman spectrometer (Avantes, AvaRaman-532TEC) by an optical fiber, and the Raman spectra of the ultrathin CuPc film could be obtained. 2.3. Experimental Process. The integration time of the Raman spectrometer was set to 20 s for a single scan, and every recorded Raman spectrum was an average of four successive scans. The Raman spectrum was first recorded with the silicone chamber occupied by air. Then deionized water and NaCl solutions with sequentially increasing weight concentrations (i.e., increasing RI, measured by Abbe refractometer) were pumped into the silicone chamber one after the other, and the Raman spectrum corresponding to each superstrate RI was recorded in succession. Finally, the NaCl solution in the chamber with the highest RI was replaced by deionized water, which, in turn, was replaced by air later; at the same time, Raman spectra for water and air superstrate were acquired, respectively, to verify the reversibility of the experimental results. The intensity of the output guided light from COWG was recorded synchronously with the Raman spectra measurement process by the PD. The experiments utilizing TE and TM polarized waveguide modes were carried out with the same procedure mentioned above. Four independent experiments were carried out successively for the TM polarized excitation configuration to verify the reproducibility of the experimental results.

(1)

where Δn = 0.008 is the maximal RI change at the waveguide surface; d = 2 μm is the thickness of the PIE waveguide; and x is the vertical distance from the waveguide surface. The Ta2O5 film was tapered at its two ends, each with a 1 mm long slope formed by utilizing a metallic mask during the RF sputtering, to allow the guided light to be coupled in and out of it adiabatically. Two COWGs which differ only in the Ta2O5 film thickness were fabricated for TE (tTE = 25 nm) and TM (tTM = 40 nm) polarized evanescent-wave excitation experiments, respectively. Waveguide parameters of the two COWGs were all evaluated at the wavelength of 532 nm by a spectroscopic ellipsometer (J.A. Woollam Co., M-2000). We took copper phthalocyanine (CuPc) ultrathin film which plays an important role in the present research of organic photovoltaics24 as a sample to verify our enhancement method in this work. The CuPc ultrathin film was deposited onto the surface of COWG via vacuum sublimation at a pressure of 1 × 10−3 Pa. A plastic mask was used to make sure that only the 5 mm width Ta2O5 film would be covered by the CuPc ultrathin film. The ultrathin CuPc films deposited onto the two COWGs for TE and TM polarized evanescent-wave excitation experiments are of the same thickness, which was evaluated to be 10 nm also by spectroscopic ellipsometry. 2.2. Experimental Setup. The experimental setup is illustrated in Figure 1. To perform the Raman spectroscopy and absorptiometry measurements, the COWG covered with CuPc ultrathin film was sandwiched between a silicone chamber and a specially designed waveguide holder. The linearly polarized laser beam (polarization direction oriented at 45° with respect to that of both TE and TM, laser power: 20 mW) with a wavelength of 532 nm was coupled in and out of the waveguide via an input coupling prism and an output coupling prism assembled to the waveguide holder. A polarizer was inserted between the laser and the input coupling prism to switch the polarization state of the waveguide mode in the COWG between TE and TM. The laser beam coupled out from the waveguide was fed into a photodetector (PD) to record the light intensity. The circular window at the back of

3. RESULTS AND DISCUSSION 3.1. RI-Enhanced Raman Spectroscopy and Absorptiometry. The evolution of the Raman spectra of CuPc ultrathin film with respect to the changing superstrate RI is shown in Figure 2(a) and Figure 2(b) for TE and TM experimental setups, respectively; the variation of the output guided light intensity with respect to time (i.e., the time-dependent superstrate RI) is shown in Figure 2(c) and Figure 2(d) for TE and TM experimental setups, respectively. As shown clearly in Figure 2(a) and Figure 2(b), the intensity of the B1g fundamental mode of CuPc at 1534 cm−1 increased abruptly when the superstrate medium was changed 16176

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Figure 2. (a) For TE and (b) for TM, evolution of the Raman intensity of CuPc ultrathin film with the superstrate RI; the inset shows the calculated normal vibrations of the CuPc B1g fundamental mode at 1534 cm−1 (an animation in AVI format is available in the HTML). (c) For TE and (d) for TM, variation of the output guided light intensity with time and superstrate RI.

absorbance increment (compared with the absorbance in air superstrate) were calculated via the equation

from air to water, corresponding to the superstrate RI change from 1.000 to 1.333. Along with the increasing weight concentration of the NaCl solution pumped into the chamber, the RI of the superstrate increased, and so did the Raman intensity of CuPc ultrathin film, which culminated when the weight concentration of the NaCl solution reached 20%, corresponding to the superstrate RI of 1.369. The Raman intensity decreased to the value corresponding to water superstrate when deionized water was pumped into the chamber for the second time to displace the NaCl solution and further decreased to the value corresponding to air superstrate when deionized water was displaced by air finally. Contrary to the upward trend of Raman intensity, Figure 2(c) and Figure 2(d) show that the intensity of the output guided light exhibited a drop-off when the superstrate medium was changed from air to water and then stepped down with the increasing RI of the superstrate, finally ascending to the value corresponding to water and air superstrate in succession. Evidently, both the Raman intensity and the output guided light intensity exhibited good reversibility. The superstrate RI dependences of both the Raman intensity and the CuPc absorption of the guided light are decrypted graphically in Figure 3(a) and Figure 3(b) for TE and TM experimental setups, respectively. The values of the Raman intensity were readily read out from Figure 2(a) and Figure 2(b) for the vibrational mode at 1534 cm−1, and the values of

⎛ I − Idark ⎞ ΔA = log10⎜ air ⎟ ⎝ INaCl − Idark ⎠

(2)

where ΔA is the absorbance increment; INaCl is the output guided light intensity at different weight concentrations of NaCl solution; Iair is the output guided light intensity when the superstrate was air; Idark is the dark voltage of the PD. It is evident that both the Raman intensity and the absorbance increment increase almost linearly with the increasing superstrate RI. Considering the Raman intensity is linearly proportional to the evanescent optical power (i.e., the square of the evanescent field amplitude), it is reasonable to infer that the evanescent optical power is also linearly dependent on the superstrate RI ranging from 1.333 to 1.369. This inference was confirmed by a theoretical simulation (see Supporting Information for detail) of the evanescent optical power at the COWG surface. It is shown in Figure 3(c) for TE and Figure 3(d) for TM that as the RI of the superstrate increases the normalized evanescent optical power at the surface of COWG follows in-phase, which culminates when the superstrate RI reaches 1.369. The largest enhancement factors of the evanescent optical power are about 17 and 14 (compared with the air superstrate case) for the TE and TM experimental setup, respectively, which predict Raman enhancement factors of the same values. However, the experimentally observed 16177

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Figure 3. (a) For TE and (b) for TM, variations of absorbance increment and Raman intensity of the CuPc ultrathin film with the increasing superstrate RI ranging from 1.333 to 1.369. (c) For TE and (d) for TM, variation of normalized evanescent optical power with the increasing superstrate RI. The insets indicate that the normalized evanescent optical power at the COWG surface increases almost linearly with the increasing superstrate RI ranging from 1.333 to 1.369.

Figure 4. Reproducibility of the Raman intensity (a) and the absorbance increment (b) in four independent TM experiments.

Raman enhancement factors are about five for TE and eight for TM, which are much less than the theoretically predicated ones. This deviation can be attributed to the approximate waveguide parameters used in the theoretical simulation, the omission of

the CuPc film in the theoretical simulation, and the optical imperfection of the waveguide used in the experiments. The absorbance increments corresponding to superstrate RI = 1.369 are about 0.155 for TE and 0.165 for TM, which are 17 and 18 16178

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Figure 5. (a) RI enhanced waveguide Raman spectra of 10 nm thick CuPc ultrathin film excited by TE and TM polarized evanescent wave, both with superstrate RI = 1.369 and integration time IT = 20 s. (b) Total internal reflection Raman spectra of 100 nm thick CuPc film excited by TE and TM polarized evanescent wave, both with integration time IT = 20 s.

film was noted. As illustrated in Figure 5(a), the vibrational mode at 1489 cm−1 in the TM excited spectrum disappeared in the TE excited one. To follow up on the origin of this difference, both an intensive literature search and additional experiments have been carried out. In the related literature available, the assignment of the symmetry label associated with the vibrational mode at 1489 cm−1 is still in controversy, but a very definite conclusion emerging from the literature is that the vibrational mode is highly polarization dependent;26−28 this polarization dependence was further testified by a total internal reflection (TIR) Raman experiment, of which the experimental setup is illustrated in Figure 5(b). The same laser in the COWG experiment was used with its polarization direction oriented at 45° with respect to that of both TE and TM. A prism (np = 1.818) with the CuPc film deposited at its hypotenuse was used to excite Raman signals, and the Raman probe was positioned with its optical axis coincident with the normal of the prism hypotenuse and its focus on the CuPc film to acquire the strongest Raman signals within the integration time (IT = 20 s). Also, a polarizer was inserted between the laser and the prism to switch the polarization state of the evanescent wave at the hypotenuse of the prism. The experimental results are illustrated in Figure 5(b). Evidently, the TE polarized evanescent field could not effectively excite the vibrational mode of CuPc at 1489 cm−1, while the TM polarized evanescent field could. Considering the almost equal magnitudes of the Raman intensity at 1534 cm−1 for the TM excited COWG Raman spectroscopy in Figure 5(a) and the TE excited TIR Raman spectroscopy in Figure 5(b), it is reasonable to conclude that the vibrational mode of CuPc at 1489 cm−1 can only be effectively excited by TM polarized evanescent wave; therefore, its disappearance in the TE polarized COWG Raman spectroscopy did not result from the low optical power of the TE polarized evanescent wave. It is worthy of note that with the laser beam incident at the critical angle both TE and TM polarized evanescent fields at the hypotenuse of the prism reach their maximal strength,29 which, however, is still far from enough to excite equivalent Raman signals to those demonstrated in Figure 5(a) out of the 10 nm thick CuPc ultrathin film. To obtain the Raman intensities demonstrated in Figure 5(b), the thickness of the CuPc film deposited at the prism hypotenuse was thickened to about 100 nm.

times the value measured with the conventional transmission method, respectively (A = 0.009, see Figure 5S in Supporting Information). With the simultaneous RI enhanced Raman spectroscopy and absorptiometry of the investigated sample, our enhancement method has paved the way for in situ qualitative (i.e., molecular recognition) and quantitative analysis of chemical and biochemical analytes such as sub-/monolayers and biomembranes at the waveguide surface. Compared with the experimental results of the TM polarized excitation configuration illustrated in Figure 2(b) and Figure 2(d), those of the TE polarized configuration (Figure 2(a) and Figure 2(c)) did not behave so well: the Raman enhancement factor is less, while variations of both the Raman intensity and the output guided light intensity are not as regular. The probable reason is that the TE polarized waveguide mode suffers much more transmission loss than the TM mode in the Ta2O5 film, which is manifested by the lower TE polarized output light intensity as demonstrated in Figure 2(c). Large transmission loss results in less TE polarized optical power that can be utilized to excite the CuPc Raman signal or can be absorbed by the CuPc ultrathin film, thus the deterioration of the experimental results. 3.2. Reproducibility of Experimental Results. Figure 4 is a demonstration of the reproducibility of the experimental results in four successive and independent TM experiments. Statistical calculations showed that the Raman intensity exhibits a maximum of 3.92% variance from the average value over the four repetitions, and the corresponding value for the absorbance increment is 3.65%. So, both the Raman intensity and the absorbance increment exhibited good reproducibility. Good reproducibility, which is quite a remarkable character of our enhancement method (Raman enhancement produced by SERS is usually described as “inordinate”25), is an essential requirement for any transducers and sensors in practical applications, thus our enhancement method is believed to be of great potential in promoting the practical applications of Raman and absorbance spectroscopic techniques based on an optical waveguide. 3.3. Polarization Dependence of CuPc Raman Spectra. Aside from the disparity in the magnitude of the Raman intensity, another difference between the TE and TM polarized evanescent-wave-excited Raman spectra of the CuPc ultrathin 16179

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ization of Orientation and Charge-Transfer Kinetics by Waveguide Spectroelectrochemistry. J. Phys. Chem. Lett. 2012, 3, 1154−1158. (3) Umemura, T.; Hotta, H.; Abe, T.; Takahashi, Y.; Takiguchi, H.; Uehara, M.; Odake, T.; Tsunoda, K. Slab Optical Waveguide HighAcidity Sensor Based On an Absorbance Change of Protoporphyrin IX. Anal. Chem. 2006, 78, 7511−7516. (4) Wiederkehr, R. S.; Hoops, G. C.; Aslan, M. M.; Byard, C. L.; Mendes, S. B. Investigations On the Q and CT Bands of Cytochrome c Submonolayer Adsorbed On an Alumina Surface Using Broadband Spectroscopy with Single-Mode Integrated Optical Waveguides. J. Phys. Chem. C 2009, 113, 8306−8312. (5) Mendes, S. B.; Li, L.; Burke, J. J.; Lee, J. E.; Dunphy, D. R.; Saavedra, S. S. Broad-Band Attenuated Total Reflection Spectroscopy of a Hydrated Protein Film On a Single Mode Planar Waveguide. Langmuir 1996, 12, 3374−3376. (6) Puyol, M.; Del Valle, M.; Garcés, I.; Villuendas, F.; Domínguez, C.; Alonso, J. Integrated Waveguide Absorbance Optode for Chemical Sensing. Anal. Chem. 1999, 71, 5037−5044. (7) Bradshaw, J. T.; Mendes, S. B.; Saavedra, S. S. Planar Integrated Optical Waveguide Spectroscopy. Anal. Chem. 2005, 77, 28−36. (8) Araci, I. E.; Mendes, S. B.; Yurt, N.; Honkanen, S.; Peyghambarian, N. Highly Sensitive Spectroscopic Detection of Hemeprotein Submonolayer Films by Channel Integrated Optical Waveguide. Opt. Express 2007, 15, 5595−5603. (9) Mitsuhata, T.; Fujii, S.; Itoh, K.; Itoh, K.; Murabayashi, M. Tapered Velocity Coupling Method for Optical Waveguide Raman Scattering Spectroscopy: Application to Iron(III) Phosphate Thin Films. J. Phys. Chem. 1992, 96, 8813−8817. (10) Walker, D. S.; Hellinga, H. W.; Saavedra, S. S.; Reichert, W. M. Integrated Optical Waveguide Attenuated Total Reflection Spectrometry and Resonance Raman Spectroscopy of Adsorbed Cytochrome c. J. Phys. Chem. 1993, 97, 10217−10222. (11) Ellahi, S.; Hester, R. E. Enhanced Waveguide Raman Spectroscopy with Thin Films. Plenary Lecture. Analyst 1994, 119, 491−495. (12) Kanger, J. S.; Otto, C.; Slotboom, M.; Greve, J. Waveguide Raman Spectroscopy of Thin Polymer Layers and Monolayers of Biomolecules Using High Refractive Index Waveguides. J. Phys. Chem. 1996, 100, 3288−3292. (13) Kanger, J. S.; Otto, C. Orientation Effects in Waveguide Resonance Raman Spectroscopy of Monolayers. Appl. Spectrosc. 2003, 57, 1487−1493. (14) Meyer, M. W.; McKee, K. J.; Nguyen, V. H. T.; Smith, E. A. Scanning Angle Plasmon Waveguide Resonance Raman Spectroscopy for the Analysis of Thin Polystyrene Films. J. Phys. Chem. C 2012, 116, 24987−24992. (15) McKee, K. J.; Meyer, M. W.; Smith, E. A. Plasmon Waveguide Resonance Raman Spectroscopy. Anal. Chem. 2012, 84, 9049−9055. (16) Calkins, J. A.; Peacock, A. C.; Sazio, P. J. A.; Allara, D. L.; Badding, J. V. Spontaneous Waveguide Raman Spectroscopy of SelfAssembled Monolayers in Silica Micropores. Langmuir 2011, 27, 630− 636. (17) Paxton, W. F.; Kleinman, S. L.; Basuray, A. N.; Stoddart, J. F.; Van Duyne, R. P. Surface-Enhanced Raman Spectroelectrochemistry of TTF-Modified Self-Assembled Monolayers. J. Phys. Chem. Lett. 2011, 2, 1145−1149. (18) Sonntag, M. D.; Klingsporn, J. M.; Garibay, L. K.; Roberts, J. M.; Dieringer, J. A.; Seideman, T.; Scheidt, K. A.; Jensen, L.; Schatz, G. C.; Van Duyne, R. P. Single-Molecule Tip-Enhanced Raman Spectroscopy. J. Phys. Chem. C 2012, 116, 478−483. (19) Li, J. F.; Huang, Y. F.; Ding, Y.; Yang, Z. L.; Li, S. B.; Zhou, X. S.; Fan, F. R.; Zhang, W.; Zhou, Z. Y.; Wu, D. Y.; et al. Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy. Nature 2010, 464, 392− 395. (20) Su, L.; Lee, T. H.; Elliott, S. R. Evanescent-Wave Excitation of Surface-Enhanced Raman Scattering Substrates by an Optical-Fiber Taper. Opt. Lett. 2009, 34, 2685−2687. (21) Dong, B.; Zhang, W.; Li, Z.; Sun, M. Remote Excitation Surface Plasmon and Consequent Enhancement of Surface-Enhanced Raman

Assisted by our enhancement method, the polarization dependence of the Raman spectroscopy of CuPc ultrathin film has manifested itself. Since polarized Raman spectroscopy can reveal unique characteristics of the investigated sample, such as molecular symmetry, molecular orientation, or intermolecular interactions,30 our enhancement method can be utilized in the polarization-controlled evanescent-wave excited waveguide Raman spectroscopy to extract much more information about the sample rather than merely the molecular vibrational frequencies.

4. CONCLUSIONS In summary, a nonplasmonic method to enhance the evanescent-wave excited Raman spectroscopy as well as the absorptiometry of ultrathin films deposited onto the optical waveguide surface was demonstrated in this paper. This double enhancement was realized by intensifying the evanescent field acting on the investigated ultrathin film via increasing the refractive index of the waveguide superstrate. The implementation of this method is very convenient, and the polarization state of the excitation evanescent field can be easily manipulated. Moreover, the performance of this method is highly reversible and reproducible. The polarization controllability and the good reproducibility are two great advantages of our method. The implementation of our method would greatly promote the practical applications of both Raman and absorbance spectroscopic techniques based on optical waveguide.

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ASSOCIATED CONTENT

S Supporting Information *

Details of the theoretical simulation and the transmission absorbance spectrum of the 10 nm thick CuPc ultrathin film. These materials are available free of charge via the Internet at http://pubs.acs.org. W Web-Enhanced Feature *

An animation of calculated normal vibrations of the CuPc B1g fundamental mode at 1534 cm−1 in AVI format is available in the HTML version of the manuscript.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86 10 5888 7196. Fax: +86 10 5888 7196. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the Major National Scientific Instrument and Equipment Development Project of China (2011YQ0301240802) and the National Natural Science Foundation of China (No. 61078039).

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REFERENCES

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