A novel cuprous ethylenetetrathiolate coordination polymer: Structure characterization, thermoelectric property optimization and a bulk thermogenerator demonstration

Synthetic Metals 193 (2014) 1–7

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A novel cuprous ethylenetetrathiolate coordination polymer: Structure characterization, thermoelectric property optimization and a bulk thermogenerator demonstration Peng Sheng a,b , Yimeng Sun a,b , Fei Jiao a,b , Chongan Di a , Wei Xu a,∗ , Daoben Zhu a,∗∗ a Beijing National Laboratory for Molecular Sciences, Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China b University of Chinese Academy of Sciences, Beijing 100049, China

a r t i c l e

i n f o

Article history: Received 6 January 2014 Received in revised form 9 March 2014 Accepted 14 March 2014 Keywords: Thermoelectric Thermogenerator Bulk organic thermoelectric material Cu(I)-ethylenetetrathiolate

a b s t r a c t A kind of bulk organic thermoelectric material namely Cu(I)-ethylenetetrathiolate was synthesized, and its thermoelectric properties were optimized using chemical oxidation and reduction. Both X-ray absorption fine structure and X-ray photoelectron spectroscopy techniques were used to characterize the complex. The material showed p-type property and the best thermoelectric properties were achieved with a power factor of 118.2 ␮W m−1 K−2 and a ZT of 0.060 at 400 K. A thermogenerator based on this material and n-type poly[Kx (Ni-ethylenetetrathiolate)] had an open voltage of 1.51 V and a short current of 2.71 mA under a temperature difference of 60 K. The maximum power output exceeded 1 mW, which is the best performance of the thermoelectric modules based on organic thermoelectric materials. The thermogenerator could power a liquid crystal display calculator, which shed light on the practical use of organic thermoelectric materials. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Energy shortage is one of the most urgent issues globally nowadays. This stimulated numerous efforts of search for alternative energy sources for fossil fuel and efficient energy conversion technologies. Among them thermoelectric (TE) system as a promising energy conversion way which can use waste heat to generate electrical power received rising interests. TE materials can realize mutual conversion of heat and electricity. Due to its attractive virtues such as no working fluids, no moving parts, quiet operation, lightweight, robustness and rapid response time, thermoelectrics is thought to be an effective energy-harvesting way and it has already been used in DNA synthesizers, car seat cooler/heaters, laser diode coolers and space power generators [1–4]. The performance of thermoelectric materials is appraised by a dimensionless figure-of-merit ZT expressed by ZT = S2 T/, where S denotes Seebeck coefficient,  denotes electrical conductivity, T denotes absolute temperature and  denotes thermal conductivity. The

∗ Corresponding author. Tel.: +86 10 62423103; fax: +86 10 62569349. ∗∗ Corresponding author. E-mail address: [email protected] (W. Xu). http://dx.doi.org/10.1016/j.synthmet.2014.03.024 0379-6779/© 2014 Elsevier B.V. All rights reserved.

term S2  is called power factor (PF) which is pivotal to achieve high performance and a large PF means a high voltage and a high current can be generated. During the past half century, most investigations have been carried out on inorganic TE materials such as bismuth telluride based alloys, filled skutterudites, clathrate compounds and half-Heusler compounds [5–7], but inorganic materials have many limitations such as containing toxic elements, rare natural resources and high-cost processing. Organic TE (OTE) materials did not receive much attention of scientists until recently. Compared to inorganic materials, OTE materials usually have low thermal conductivity and they are plentiful in natural resources, solution-processable, lightweight and flexible. In recent years, the thermoelectric properties of conducting polymers including poly(3-hexylthiophene), poly(thienothiophene), polyaniline, poly(ethylenedioxy-thiophene) (PEDOT), polycarbazole, and poly(metalpoly(methoxy-phenyl-enevinylenes) ethylenetetrathiolate) and polymer-based composites were investigated [8,9]. To date, the highest ZT of OTE materials reached 0.45 for PEDOT: PSS (poly(styrenesulphonate)) [10]. Although the TE performance of OTE materials have been improved a lot, most of current reported OTE materials are in the geometry of thin-film. Currently, thin-film OTE materials are difficult to scale up, fabrication of thermoelectric devices using thin-film OTE materials is

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inconvenient and moreover it is difficult to build up a high temperature difference in these devices which limits the power output and thus practical application of the devices. On the contrary, thermoelectric devices based on bulk TE materials are easy to fabricate and they can obtain high temperature difference. What is more, bulk TE materials can be produced in large quantities, so, compared to thin-film TE materials, bulk materials are much easier and faster to introduce to market. Thus it is of great significance to investigate bulk OTE materials. However, researches on bulk OTE materials are rare. Previously, our group synthesized a kind of bulk OTE material namely n-type poly[Kx (Ni-ethylenetetrathiolate)] (poly[Kx (Ni-ETT)]) with ZT = 0.1 at 300 K and ZT = 0.2 at 440 K, which are the highest values of bulk OTE materials to date [11]. The material was easy to synthesize and to shape to construct devices. This inspired us that metal ethylenetetrathiolate might be a new kind of promising OTE material worth further investigation. Lately, we optimized the TE properties of poly[Cux Cu-ETT] and the best TE properties were achieved with a PF of 26.0 ␮W m−1 K−2 and a ZT of 0.019 at 360 K [12]. In the present work, we used cuprous salts instead to prepare bulk cuprous ethylenetetrathiolate (Cu(I)ETT), and the TE properties of the material were optimized by controlling the oxidation level or reduction level using a simple chemical method. The TE properties of the materials were largely enhanced compared to those of poly[Cux (Cu-ETT)]. The materials showed p-type property and the highest PF of 118.2 ␮W m−1 K−2 and ZT of 0.060 were achieved at 400 K, which are the highest values for bulk OTE materials except poly[Kx (Ni-ETT)]. In addition, we constructed a thermogenerator using the Cu(I)-ETT complex as p-type materials and poly[Kx (Ni-ETT)] as n-type materials. An open voltage of 1.51 V and a short current of 2.71 mA could be obtained under a temperature difference of 60 K. The maximum power output exceeded 1 mW, which is the best performance of a TE module based on OTE materials. Encouragingly, the thermogenerator could drive a calculator with liquid crystal display independently, which forwards the practical use of OTE materials. 2. Experimental 2.1. Synthesis 2.1.1. Synthesis of Cu(I)-ETT complex Under an inert atmosphere, 1,3,4,6-tetrathiapentalene-2,5dione (TPD, 1 g, 4.8 mmol) was treated with MeONa (2.07 g, 38.4 mmol) in refluxing methanol for 12 h. Then, CuI (19.2 mmol, 3.66 g) was added, refluxing for another 12 h during which black solids formed. The solution was filtered and the precipitate was washed thoroughly with water, methanol and ether separately. The product was dried in vacuum for 12 h. 2.1.2. Oxidation and reduction of Cu(I)-ETT complexes Oxidation. Under an inert atmosphere, to the pristine material suspended in methanol was added 0.5 or 1 equiv. iodine in methanol solution. The mixture was stirred for 12 h at room temperature. The solids were collected and washed thoroughly with water, methanol and ether, separately followed by drying in vacuum for 12 h. The 0.5 and 1 equiv. iodine oxidized materials are denoted by “0.5 eq I ox” and “1 eq I ox”, respectively. Reduction. Under an inert atmosphere, to the pristine material suspended in THF was added 0.5 or 1.0 equiv. LiBHEt3 (1 M in the THF solution). The mixture was stirred for 12 h at room temperature. The solids were collected and washed thoroughly with THF, water, methanol and ether separately followed by drying in vacuum for 12 h. The 0.5 and 1 equiv. LiBHEt3 reduced materials are denoted by “0.5 eq LiBHEt3 red” and “1 eq LiBHEt3 red”, respectively.

2.2. Characterization The content of Cu and Na was measured on an inductively coupled plasma optical emission spectrometer (Optima 5300DV, Perkin Elmer). Before measurement, the complex was dissolved in concentrated nitric acid and the obtained solution was diluted for analysis of Cu2+ and Na+ . The content of C and H was analyzed by a Flash EA 1112 (Thermo Fisher Scientific). Thermogravimetric analysis (TGA) was performed on a Shimadzu DTG 60 instrument at a heating rate of 10 ◦ C min−1 under an Ar atmosphere. X-ray Photoelectron Spectroscopy (XPS) measurements were performed using an ESCALab220i-XL (VG Scientific). X-ray Absorption Near Edge Structure (XANES) was acquired at the BL14W1 beamline of the Shanghai Synchrotron Radiation Facility (SSRF). The materials were compressed into cuboid pellets for measurement of Seebeck coefficient and electrical conductivity and into discs for measurement of thermal conductivity. The Seebeck coefficient, electrical conductivity, and thermal conductivity were measured by a SB-100 Seebeck Measurement System (MMR Tech.), a KEITHLEY 2002 Multimeter (Keithley Instrument Inc.), and a TCi Thermal Conductivity Analyzer (C-THERM Tech.), respectively. 2.3. Details of device fabrication A 5 mm thick polydimethylsiloxane (PDMS) membrane (50 mm × 50 mm in size) containing 440 equally separated cavities (1 mm × 2 mm in the cross section) penetrating through the membrane was employed as holder for the TE legs to avoid short contact between adjacent legs. Poly[Kx (Ni-ETT)] synthesized according to the previous report was used as n-type materials and the Cu(I) complex as p-type materials, and both of the materials were compressed into bar-shaped samples with a size of 1 mm × 2 mm × 5 mm. Both ends of the each TE leg for electrical contact were covered with 50 nm evaporated Au to reduce the contact resistance. Then the p- and n-legs were connected in series by Al foils with silver paste (EPO-TEK H20E). The device fabrication was finished by sandwiching these leg arrays between two AlN wafers (50 mm × 50 mm) and sealing the edges with epoxy resin. T was created by a hot plane (for Thot ) and a cooling fan (for Tcold ), and was measured by platinum resistance thermometers. The electrical properties were measured by Keithley 4200 SCS. 3. Results and discussion 3.1. Characterization of Cu(I)-ETT complex Upon treatment of TPD with sodium methoxide, the corresponding ethylenetetrathiolate C2 S4 4− is generated in situ and then chelates copper ions to form organometallic polymers. TGA analysis (Fig. S2) shows that the pristine, oxidized and reduced complexes were stable up to 440–480 K. The Cu(I)-ETT complex does not show any diffraction peaks in XRD analysis and it is highly insoluble in organic solvents which makes characterization of the complex difficult. XANES can provide information about the oxidation state and coordination geometry of the absorbing atom. In this work, we used XANES to gain some information on the structure. Fig. 1 depicts the normalized Cu K-edge absorption spectra of Cu(I)-ETT and a reference compound (Me4 N)4 Cu4 (mnt)4 (mnt = maleonitriledithiolate). The Cu atom in (Me4 N)4 Cu4 (mnt)4 is coordinated by three S atoms in a trigonal fashion and the spectrum show three features: ∼8984 eV, ∼8990 eV and ∼9006 eV. The absence of any pre-edge feature attributed to 1s → 3d transition in both Cu(I)-ETT and (Me4 N)4 Cu4 (mnt)4 indicates that Cu atoms in the two compounds are in +1 oxidation state. This corresponds with the Cu 2p XPS spectrum of the Cu(I)-ETT complex

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3.2. Optimization of the TE properties of Cu(I)-ETT

Fig. 1. Normalized Cu K-edge XNAES spectra of Cu(I)-ETT and (Me4 N)4 Cu4 (mnt)4 .

in Fig. S1a which shows no shake-up peaks indicating that the Cu atoms are in +1 oxidation state [13]. The spectrum of Cu(I)-ETT is similar to that of (Me4 N)4 Cu4 (mnt)4 in general except that the features at ∼8984 and ∼8990 eV attributed to 1s → 4p transition [14] are less intense or partially resolved compared to those in (Me4 N)4 Cu4 (mnt)4 . One possible reason could be that the coordination environment of the Cu atoms in Cu(I)-ETT might not be equal. The spectrum seems a combination of those of tetragonally and trigonally coordinated Cu atoms [15]. Hence, in Cu(I)-ETT complex, tetragonally and trigonally coordinated Cu atoms possibly coexist and moreover, most of the Cu atoms are trigonally coordinated according to the spectrum. Another explanation is also reasonable. That is, the Cu(I)-ETT complex probably has an extended conjugated structure, this will increase the ␲-overlap between Cu 4p and ETT ␲ orbitals thus reducing the 4p character in the final state, so the above-mentioned two features are less pronounced compared to those in (Me4 N)4 Cu4 (mnt)4 [16]. Regrettably, XANES can only give information about the coordination information of center atom, and moreover, due to the insolubility of the material making characterization difficult and lack of XRD diffraction peaks, it is difficult to depict a detailed structure of the material.

Fig. 2 shows the temperature dependence of the TE properties of pristine Cu(I)-ETT. Overall the electrical conductivity, Seebeck coefficient and thermal conductivity all increase as the temperature increases. The positive sign of Seebeck coefficient means p-type charge carriers in the complex. The calculated power factor and ZT also rise with elevated temperature. At 300 K, the electrical conductivity, Seebeck coefficient and thermal conductivity are 88.6 S cm−1 , 54.8 ␮V K−1 and 0.52 W m−1 K−1 , respectively, which acquires a power factor of 26.6 ␮W m−1 K−2 and a ZT of 0.015. At 400 K, the electrical conductivity, Seebeck coefficient and thermal conductivity reach 151.8 S cm−1 , 66.0 ␮V K−1 and 0.76 W m−1 K−1 , respectively. The power factor and ZT are raised to 66.1 ␮W m−1 K−2 and 0.035, respectively. It is well known that metal dithiolenes compounds can get multiple oxidation states, and the conducting properties vary with the oxidation state [17]. This inspired us to investigate the effect of oxidation state on the thermoelectric properties of Cu(I)-ETT and possibly optimization of the thermoelectric properties could be realized by tuning the oxidation state using simple chemical reduction and oxidation. Hence we used iodine as the oxidation reagent and LiHBEt3 as the reduction reagent to try to optimize the TE properties. The TE properties of the optimized complexes are summarized in Fig. 3. Of all the complexes, the electrical conductivity, Seebeck coefficient and thermal conductivity increase with elevated temperature. The calculated power factor and ZT are also raised when the temperature increases. It is apparent that oxidation reduces the electrical conductivity while enhancing the Seebeck coefficient; however, reduction has an adverse effect, i.e. it improves the electrical conductivity and lowers the Seebeck coefficient. The effect of oxidation and reduction on the thermal conductivity is a little complicated. Generally speaking, oxidation reduces the power factor and ZT, however, reduction improves the two parameters. When reduced the pristine material with 1 equiv. LiHBEt3 , the power factor and ZT reach the highest value. At 300 K, the corresponding

Fig. 2. Temperature dependence of (a) electrical conductivity, Seebeck coefficient, (b) thermal conductivity, (c) power factor and (d) ZT of pristine Cu(I)-ETT.

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Fig. 3. Temperature dependence of (a) electrical conductivities, (b) Seebeck coefficients, (c) thermal conductivities, (d) power factors, and (e) ZT values of pristine, oxidized and reduced Cu(I)-ETT complexes.

power factor and ZT are 46.7 ␮W m−1 K−2 and 0.024, respectively, with the corresponding electrical conductivity, Seebeck coefficient and thermal conductivity being 186.5 S cm−1 , 50.0 ␮V K−1 and 0.58 W m−1 K−1 , respectively. At 400 K, a power factor of 118.2 ␮W m−1 K−2 and a ZT of 0.060 are achieved, with the corresponding electrical conductivity, Seebeck coefficient and thermal conductivity being 314.4 S cm−1 , 61.3 ␮V K−1 and 0.79 W m−1 K−1 , respectively. To the best of our knowledge, pristine and reduced Cu(I)-ETT showed TE performance of bulk OTE materials among the most excellent ones. As shown in Table S1, the copper content in the complexes decreases with the increase of the oxidation level but increases as the reduction level increases. Fig. S1 demonstrates the Cu 2p XPS spectra of the complexes. Fig. S1 shows that all of the complexes exhibit a sharp Cu 2p3/2 peak and a sharp Cu 2p1/2 peak and the Cu 2p3/2 binding energies are in the range from 932.6 to 932.8 eV. Fig. 4 depicts the S 2p XPS spectra of the complexes. The S 2p spectra have been fitted with doublets with an intensity ratio of 2:1 for S 2p3/2 to S 2p1/2 and a separation of 1.0 eV. Three kinds of S atoms exist in the complexes with the S 2p3/2 binding energies in the range of

162.0–162.4 eV, 163.6–163.9 eV and 168.1–168.9 eV, respectively. The low intensity peaks in the range of 168.1–168.9 eV probably arise from surface oxidation which are neglected in the subsequent discussion. S atoms of 162.0–162.4 eV and 163.6–163.9 eV are represented by S1 and S2 , respectively. The binding energies of Cu 2p3/2 and S 2p3/2 along with the content ratio of S2 /S1 are summarized in Table 1. It is seen from Table 1 that the binding energies of Cu

Table 1 Binding energies (unit: eV) of Cu 2p3/2 , S 2p3/2 and the ratio of the content of high oxidation state S to that of low oxidation state S, i.e. S2 /S1 of pristine, oxidized and reduced Cu(I)-ETT complexes. Cu

1 eq I ox 0.5 eq I ox Pristine 0.5 eq Li red 1 eq Li red

932.6 932.7 932.8 932.6 932.6

S S1

S2

S2 /S1

162.3 162.4 162.3 162.0 162.1

163.7 163.9 163.9 163.6 163.6

0.242 0.229 0.211 0.199 0.187

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Fig. 4. S 2p XPS spectra of pristine, oxidized and reduced Cu(I)-ETT complexes: (a) pristine, (b) 0.5 eq I ox, (c) 1 eq I ox, (d) 0.5 eq LiHBEt3 red, and (e) 1 eq LiHBEt3 red. Dark cyan and dark yellow colored lines represent S 2p3/2 peaks with their corresponding S 2p1/2 peaks denoted by magenta and navy colored lines, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

do not vary obviously after oxidation and reduction. However, the S2 /S1 alters upon oxidation and reduction, becoming larger with elevated oxidation level and smaller with elevated reduction level. It changes from 0.187 for “1 eq LiHBEt3 red” sample to 0.242 for “1 eq I ox” sample. As is known that the binding energy reflects the oxidation state of a kind of element and a higher oxidation state element has a higher binding energy [18]. Therefore, S2 is in a higher oxidation state and S1 in a lower oxidation state. Iodine oxidizes part of S1 to S2 and LiHBEt3 reduces part of S2 to S1 . This explains why S2 /S1 changes in that way. From the above analysis, it is concluded that the oxidation and reduction of Cu(I)-ETT is a ligand-centered process; the relative content of the high oxidation state and low oxidation state S atoms is a key factor influencing the thermoelectric properties of the complexes; low S2 /S1 ratio is favorable for improvement of the thermoelectric properties. As demonstrated in Fig. 5, the XANES of Cu(I)-ETT is also affected by the oxidation and reduction level. It seems that the effect of reduction on the XANES of Cu(I)-ETT is little, while the oxidation process clearly affects the XANES and hence the coordination geometry of the Cu atom. CuS in the figure is used for reference. Two-thirds of the Cu atoms in CuS are tetrahedrally coordinated to S atoms and the remaining coordinated by S atoms in a trigonal geometry [13]. The feature ∼8986 eV is an indicator of tetrahedrally coordinated geometry [13,15]. With the increase of the oxidation level, the features of the spectrum become more analogous to those

of CuS. As discussed supra, the Cu(I)-ETT complex probably contains both trigonally and tetrahedrally coordinated Cu atoms and most of the Cu atoms are in a trigonal geometry. This means that upon oxidation the relative content of trigonally and tetrahedrally coordinated Cu atoms varies and oxidation increases the amount of the tetrahedral Cu atoms. It is possible that the unchanged oxidation state of Cu atoms is a result of the combination of oxidation of S atoms and the increase of tetrahedrally coordinated Cu atoms upon oxidation. In addition, compared to trigonal coordination, tetrahedral coordination destroys the planarity between Cu and S atoms

Fig. 5. Normalized XNAES spectra of Cu K-edge of pristine, oxidized, reduced Cu(I)ETT complexes along with CuS.

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Fig. 6. (a) Photograph of the PDMS membrane with the n- and p-type legs imbedded in it. (b) I–V curve the devices under a temperature difference of 60 K. (c) The TE device was used to drive a calculator independently.

thus hindering the ␲-overlap between Cu d and ETT ␲ orbitals. So we consider that tetrahedral coordination is possibly unfavorable for charge transport which may partly explains why the electrical conductivities and hence the calculated TE properties of the oxidized samples are reduced. The effect of oxidation and reduction on the charge transport of this material is different from that of conventional p-type polymers. Upon oxidation, the electrical conductivities of conventional p-type polymers are usually increased because of the increased hole carriers. The reason is possible that, for Cu(I)-ETT, upon oxidation or reduction, not only does the quantity of carriers change, but also the structure changes as well. Upon oxidation the coordination structure of the polymer was modified with tetrahedral coordination increasing, leading to reduction in Cu-S conjugation and ␲ overlap. Consequently, the band structure of the material is turned from more dispersive to be much flatter, resulting in significant reduction in carrier mobility. In contrast, flatter band structure is preferred for Seebeck coefficient because the profile of density-of-states (DOS) is sharper in this case. Therefore, when the polymer was oxidized to such extent that the material structure was changed, carrier mobility would decrease while Seebeck coefficient would increase although carrier concentration was increased at the same time. Similarly, reduction may have an opposite effect. The combination of these two effects made the effect of oxidation and reduction on the electric conductivity and Seebeck coefficient of the material different from that of conventional p-type polymers. Previously, for demonstrating the possibility of OTE materials for practical energy generating application we have fabricated a thermogenerator with 35 thermocouples [11]. At that time, the poly[Cux (Cu-ETT)] used as p-type TE materials only possessed a ZT value about 0.01 around 400 K. Due to the inferior performance of this p-type material and the lower integral degree, that device could only give a power output of 750 ␮W under a temperature difference of 82 K (Thot = 423 K) with an open voltage of 0.26 V. This performance is still far behind the requirement for a practical application. Here, as the ZT value of pristine Cu(I)-ETT complex improved more than three-fold compared to that of the previous polymer, it seems much easier for us to fabricate a thermogenerator with higher performance. In order to improve the open voltage, more thermocouples should be integrated into one module. Here we fabricated

a new device with 220 pairs of thermocouples. Poly[Kx (Ni-ETT)] synthesized according to the previous report was used as n-type materials and the Cu(I) complex was used as p-type materials. Fig. 6b shows the I–V curve of the device under a temperature difference of 60 K (Thot = 373 K). An open voltage of 1.51 V and a short current of 2.71 mA could be obtained. The internal resistance of the generator was 557 . The maximum power output (Pmax = U2 /4RI , where U is the Seebeck voltage and RI is the internal resistance) exceeded 1 mW, which is the best performance of a TE module based on OTE materials. The relative high output voltage and large power output of our constructed module were sufficient to drive a calculator with liquid crystal display independently (Fig. 6c). 4. Conclusion Bulk OTE material – cuprous ethylenetetrathiolate was synthesized and the TE properties were optimized using simple chemical oxidation and reduction. XANES spectra show that Cu atoms possibly exist in both trigonal and tetrahedral geometries and oxidation increases the amount of tetrahedral Cu atoms. From XPS spectra, it is concluded that the oxidation and reduction is a ligand-centered process and the relative content of the high oxidation state and low oxidation state S atoms varies upon oxidation and reduction. It seems lower S2 /S1 improves the TE properties of the complex. The highest PF and ZT for this material reached 118.2 ␮W m−1 K−2 and 0.060, respectively, at 400 K, which are among the highest values for bulk OTE materials. The thermogenerator composed of pristine p-type Cu-ETT and n-type poly[Kx (Ni-ETT)] exhibited the highest power output for TE modules based on OTE materials and it could power a liquid display calculator. These results gave us confidence that practical use of OTE materials could be realized in the future. Acknowledgments The authors acknowledge the financial support from the National Natural Science Foundation of China (21021091, 21333011), Chinese Ministry of Science and Technology (2013CB632506, 2011CB932304) and Chinese Academy of Sciences.

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