Nanoseed Assisted PVT Growth of Ultrathin 2D Pentacene MolecularCrystal Directly onto SiO2 SubstrateS. Atika Arabi,†,‡,# Ji Dong,†,# Misbah Mirza,† Peng Yu,† Liang Wang,† Jun He,§ and Chao Jiang*,†

†CAS Key Laboratory of Standardization and Measurement for Nanotechnology, & CAS Center for Excellence in Nanoscience and§CAS Key laboratory of Nanosystem and Hierarchical Fabrication, National Centre for Nanoscience and Technology, No. 11Zhongguancun Beiyitiao, Beijing 100190, China‡University of Chinese Academy of Science, No. 19A Yuquan Road, Beijing 100049, China

*S Supporting Information

ABSTRACT: High order of molecular packing and perfect semiconductor/dielectric interface are two key factors to achievehigh performance for organic field-effect transistors (OFET). Moreover, the thin crystal offers an improved efficiency of carrierinjection for OFETs. To this aim, formation of thin and large single crystal directly on dielectrics is the basis to obtain the idealcrystal OFETs. Herein, we report the controlled growth of ultrathin 2D Pentacene (Pn) crystal via nanoseed assisted physicalvapor transport (PVT) method grown directly on SiO2. The size, thickness, and density of Pn crystals are systematically studied.Potentially effective parameters such as initially lowered Pn coverage and decreased supersaturation with the aid of carrier gasflow were optimized to grow large, ultrathin 2D Pn crystalline flakes efficient for the fabrication of crystal OFETs. The typical sizeand thickness of as-grown Pn crystalline flakes can be controlled to be large and thin enough. Device of ultrathin crystal withbottom gate and top contact configuration showed mobility as high as 5.6 cm2 V−1 s−1, indicating that the proposed novelarchitecture of organic molecular crystals may pave the way toward the application of large-sized single crystals of Pn in organicelectronics.

1. INTRODUCTION

Over past few years, single crystal field effect transistors(SCFET) have become a subject of interest for understandingthe mechanism of charge transport in organic semiconductors.Single crystals exhibit incomparable advantages over thin filmssuch as high purity, long-range-order molecular packing, and nograin boundaries with minimized structural defects, establishingthe structure property relationship and ensuring higher deviceperformance.1,2 To date, it is generally acknowledged that theSCFETs performance is highly sensitive to semiconductor/dielectric interface. Many groups have reported the growth oforganic single crystal and their device fabrication techniques toattain high mobility. By introducing a new technique of air flownavigation, Zhengran fabricated a crystal of TIPS Pentacene.3

Onojima et al. used a simple method of electrostatic spraydeposition for organic crystal growth.4 Horowitz,5 Kloc,6 andLaudise et al.7 are among the pioneers who developed a PVTtechnique for the growth of several organic semiconductors by

improving the design of high quality organic semiconductorcrystal devices. With emphasis on these PVT-based techniques,Alborghetti et al. fabricated nanogap single-crystal organictransistors8 while anisotropic measurements of fused-ringthienoacene9 and Pn10 have also been reported. The highperformance single-crystal organic semiconductors are en-dowed with clarification of their intrinsic electronic propertiesand significance for potential applications such as displays,11,12

large area electronics,11 and sensors.13

In most previously reported SCFET devices, grown organicsingle crystals were usually thick and transferred onto othersubstrates for the fabrication of SCFET. While in the solutionmethod, the solvent and impurities can inevitably deterioratethe quality of crystal and the interfaces. The contaminated

Received: December 7, 2015Revised: March 19, 2016Published: March 28, 2016

Article

pubs.acs.org/crystal

© 2016 American Chemical Society 2624 DOI: 10.1021/acs.cgd.5b01726Cryst. Growth Des. 2016, 16, 2624−2630

semiconductor/dielectric interface would result in poor deviceperformance and make it difficult to investigate the intrinsictransport mechanism. Therefore, to achieve high deviceperformance, the best way to keep the entire interface is togrow a thin organic single crystal in situ on a substrate with thePVT method. However, the reduced thickness and interfacialdefects still exist as major challenges.14,15

The control of supersaturation during reaction time and insitu growth of crystal may solve the problems addressed above.In our previous work, we reported the in situ growth of organiccrystal using the seed growth method which promotes thegrowth of crystal by reducing the Gibbs free energy of thesystem with nanoseeds.16 This work explores the basis ofmaterials for the experimental research in obtaining large andultrathin 2D crystal and seeks the device applications for Pncrystals. The main aim of this study is to get a betterunderstanding of the underlying mechanism of the formation ofultrathin 2D Pn crystal via PVT process.Here, the growth of large and ultrathin 2D Pn crystals with

specific facets is demonstrated via nanoseed assisted growthmethod stimulated by supersaturation. The obtained Pn crystalshave been carefully characterized and a detailed study on thegrowth of the crystallization mechanism is also provided. Pnsingle crystal of size as large as 50 μm with thickness ranging

from several hundreds of nanometers to about 50 nm wassuccessfully synthesized. To the best of our knowledge, this isthe first report to come up with ultrathin Pn crystals by PVTmethod. Furthermore, growth mechanism of different crystalshapes manipulated by partly tailoring the deposition wasproposed. Significantly, the competing relations of surfaceenergy between the (110), (010), and (11 ̅0) surfaces leadperfect hexagonal to rhombic shape. Moreover, by investigatingfield effect behavior, the maximum mobility reaches up to 5.6cm2 V−1 s−1 with average mobility 2.0 (±1.1) cm2 V−1 s−1. Thehuge variation may be somewhat attributed to the anisotropy ofcharge transport. Thus, our method provides an opportunity toreveal the growth mechanism of Pn crystals and realize theirpractical applications in future organic electronic technologies.

2. EXPERIMENTAL SECTION2.1. Formation of Nanoseeds. Heavily doped Si wafers with

thermally grown SiO2 of 300 nm were chosen as substrates. Thesubstrates were cleaned consecutively with a solution of ethanol,acetone, ammonia, and deionized water with a 1:6 volume ratio and inthe end with deionized water by using ultrasonic cleaner. Finallysubstrates were blown with high purity dry nitrogen. Pn (Sigma-Aldrich used without further purification) monolayer (0.7 ML) wasdeposited on substrates by thermal evaporator (AUTO 306, BOCEdward Co.) at ambient substrate temperature under a vacuum

Figure 1. (a) Scheme of experiments during PVT growth. (b,c) AFM image (30 × 30 μm2) of ultrathin pentacene crystal with its height profile andoptical image, respectively. (d) SEM image of large-sized crystal obtained after PVT. (e) XRD of the corresponding crystal. (f,g) TEM image andrelative SAED pattern, respectively.

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pressure of 7 × 10−5 Pa. After deposition of thin film, substrates weretransferred to the glovebox having argon environment to protect themfrom being oxidized in the air. Substrates with MLs were annealed at120 °C to get seed crystal using a heating plate.2.2. PVT Growth. A conventional horizontal furnace setup has

been used. For crystal enlargement, the annealed samples were shiftedto the furnace for subsequent PVT growth. Pn powder (97%, fromAldrich Chemical Co.) was used as a source material while SiO2

substrates having nanocrystals (from annealing 0.7 ML) were placeddownstream in the furnace. The following procedure was used to growthe Pn crystal as a reference throughout this report: the source washeated at 285 °C, while substrate temperature maintained to 180 °C ata reactor total pressure of 1 atm and 300 sccm flow. The growth timewas fixed to 30 min.This procedure was modified in four series to determine the growth

conditions for nucleating the Pn crystal. Series A consists of substrateswith four Pn coverages of 0.3, 0.5, 0.7, and 1 ML. In the second series(series B) the growth temperature of the Pn crystal was modified toTsub = 210, 180, 150, and 120 °C while otherwise following thereference procedure. Series C was modified by using four different gasflows as 100, 200, 300, and 400 sccm while maintaining otherconditions the same as the reference. In the fourth series (series D) thegrowth time was varied to 10, 30, and 60 min while otherwisefollowing the reference procedure.2.3. Device Fabrication and Characterization. Copper grid was

used as shadow mask for the device fabrication and a configurationwith top contact and bottom gate was adopted. Copper/goldelectrodes with 100 and 40 nm thickness were deposited respectivelydirectly on to the as grown crystal without disturbing its interface. Thechannel length is 10 μm while the width is 20 μm. The electricalproperty of the devices was carried out via a Keithley 4200 instrumentin air at room temperature. The morphology of the monolayer wasmonitored by AFM,16 while Pn structure after PVT was characterizedby field emission scanning electron microscopy (FESEM) S4800(Hitachi).

3. RESULTS AND DISCUSSION

Schematic diagram based on experimental setup is given inFigure 1a to depict the important stages of the moleculartransport process during PVT growth. The morphology of insitu grown large Pn crystal was characterized by AFM as shownin Figure 1b. The height of the Pn crystal indicates 48.5 nmthicknesses while Figure 1c shows the optical image of thecorresponding large crystal. The SEM image of the ultrathin Pncrystal fabricated using the reference conditions (seeExperimental section) is shown in Figure 1d. TEMobservations have shed light on the crystal upper surfacewhile selected area diffraction (SAED) patterns of Pn crystal inFigure 1f,g unveil the single crystalline quality and growthdirection. The upper surface of the crystal is indexed to the(001) plane. X-ray diffraction studies verify the high quality ofPn crystals. Figure 1e is the XRD pattern of in situ grown largePn crystals; the strong diffraction peak at 6.24° corresponds to(001) crystal plane. The calculated full width at half maxima ofthe (001) peak is 0.18°.The morphology of the final crystal structures depends upon

the nucleation, orientation, and reaction conditions. Althoughthere are reports on the PVT growth of Pn crystals, but nosystematic studies about the effect of carrier gas, growth timemeasurements on the morphology have been carried out. Thefollowing study shows that the density and thickness of the Pncrystal can be systematically controlled. The determiningfactors for the formation of large crystals are found to bemonolayer pentacene coverage and substrate temperature,while for ultrathin crystal they are gas flow and growth time.

Their roles in the formation of Pn crystals are discussed in thefollowing section.

3.1. Coverage Effect. Nanoseed density provides controlover the nucleation of vapor-grown organic single crystals ontothe SiO2 surface Figure 2 displays the role of nanoseed density

on the growth (series A). AFM images of ∼1, 0.7, 0.5, and 0.3ML are shown in Figure 2 (a1, b1, c1, d1), respectively, for allcases the morphology of Pn is disc-like comparable with thereported work.17 Nanoseeds obtained after annealing ofrespective coverages are presented in Figure 2 (a2, b2, c2,d2). It is clear that density of nanoseeds increases with theincrease in coverage or vice versa. However, the phenomena ofthis molecular redistribution vanished up to a certain thickness,i.e., when coverage exceeds 5 ML. SEM images of large crystalafter subsequent PVT growth are shown in Figure 2 (a3, b3, c3,d3).On the basis of the experimental results, as density of

nanoseeds increases (up to 0.3/μm2), after subsequent PVTgrowth, density of large crystals also increases. This shows thatnanoseeds promote the growth of crystals, as they providespecific sites for the incoming molecules to accommodate. Theselectivity achieved in our system is more likely due to thedifference in thermodynamics on substrates having nanoseedsor without nanoseeds. This view is supported by the followingexplanation. For Pn, the critical island size was assumed to be 3< i < 4 at substrate temperature between 25 and 70 °C.18 In thecase of bare SiO2 substrate, without nanoseeds, when Pnmolecule arrives, it is not easy for it to diffuse with anothermolecule to become a stable nucleus, as its island size is belowthe critical island size. Most of the rare Pn molecules lyingbeyond the mean diffusion length will be desorbed from thesubstrate before they collide with each other to form stable

Figure 2. (a1, b1, c1, d1) AFM images (5 × 5 μm2) of pentacene 1,0.7, 0.5, 0.3 ML, respectively. (a2, b2, c2, d2) SEM images afterannealing of Pn ML with calculated nanoseed density. (a3, b3, c3, d3)SEM images of single crystal after PVT growth.

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nuclei. As a result, crystals with very low density can beobserved. However, when identical Pn molecules arrived at thesubstrate having nanoseeds, it is easy for them to diffuse to theneighboring nanoseeds because the distance between thesespecies is limited by the mean diffusion length. This proposesthat there should be formation of a critical nucleus whichpromotes nucleation. SEM images in Figure S1 show thecomparison of samples without nanoseeds and with nanoseeds.Furthermore, thermodynamic analysis implies that the numberof molecules required for a stable island increases withincreasing substrate temperature19 which suggests that, forour case, the value of critical island size may exceeds 4. So, inthis case nanoseed assisted growth helps incoming moleculesmake a stable island. From the images in Figure 2, it can bepredicted that the density of large crystals depends on thedensity of nanoseeds.When monolayer coverage is beyond 0.7 ML, the nanoseed

density is so high that incoming molecules just take part inincreasing the density and thickness because they have lessspace to enlarge the crystal size. Our experimental resultsuggests that the low density of nanoseeds is indeed helpful forachieving the large crystals.3.2. Substrate Temperature. Temperature at the

substrate has a key effect on the crystal growth. For betterunderstanding of the formation of Pn crystal, we systematicallystudied different growth temperature (series B) as illustrated bySEM images in Figure 3. It is observed that growth is different

at different substrate temperatures. In our case, substratetemperature of 180 °C shows the best results. These results canbe explained in terms of supersaturation and nucleationprobability of grown adatoms (here Pn molecules) given bySears20

σ πα

∝ −⎡⎣⎢

⎛⎝⎜

⎞⎠⎟⎤⎦⎥P

k Texp

lnN

2

2 2(1)

where σ is surface energy, T is substrate temperature, and α issupersaturation. Assume that vapor pressure of Pn coming fromthe source is steady over the entire tube with backgroundpressure of 1 atm. Vapor pressure can be calculated from theintegrated form of the Clausius−Clapeyron equation.

= −⎜ ⎟⎛⎝⎜⎜

⎞⎠⎟⎟ ⎛

⎝⎞⎠

pp

ABT

lno (2)

where p and po is equilibrium and ambient vapor pressure,respectively, of Pn, while A and B are constants, and it isrecognized that B = ΔHsub/R, where “ΔHsub” defines enthalpyof sublimation while “R” is the gas constant. By taking values ofA and B21 we can calculate the vapor pressure at source (285°C) and different substrate temperatures. Graph (a) in FigureS2 shows linear behavior of vapor pressure with respect totemperature. It demonstrates that the vapor pressure of Pn alsodecreases as temperature decreases from 210 to 120 °C. Thecalculated vapor pressure at 120 °C is exp (−12) Pa, whichsuggests that most of the Pn molecules will not desorb and helpin increasing the density of crystals.At 210 and 180 °C, because of high substrate temperature,

desorption of Pn molecules occurs along with the absorptionand diffusion processes. At these temperatures supersaturationis low, which results in lower nucleation probability. Due tothis, the density of crystals is less as shown by the SEM imagesin Figure 3, while at 150 and 120 °C, because of lower vaporpressure value ∼Psub = exp (−12) Pa desorption nearly ceasesand incoming molecules from the source will be immobile. Theincreased supersaturation results in higher nucleation proba-bility, and growth occurs only because of adsorption anddiffusion. This leads to high density crystals. Hence, there aretwo important observations: (i) at higher substrate temper-atures low density crystal is obtained because of lowersupersaturation; (ii) at lower substrate temperatures super-saturation is higher which results in high density crystals.Calculated values of nucleation probability with varying Pnvapor concentrations at different substrate temperatures isillustrated in Figure S2(b). The observed trend of growth is inconsistent with the calculated values.

3.3. Gas Flow Rate Effect. Table 1 shows a series of SEMimages to demonstrate the detailed growth process of Pn crystalwith various gas flow rates from 100 to 400 sccm while keepingother parameters as reference (series C). Corresponding AFMimages shown in Table 1 illustrated the thickness of Pn crystalsranging from 700 nm down to 48 nm. At 100 sccm, weobtained thick and large-sized crystals. In horizontal PVT, onlya small amount of vapor reverses flow either descending orascending in close proximity of the crystal and the heat source,respectively.22 This results in less condensation of vapor on thesubstrate through the boundary layer of gas. Thus, at lower gasflow rate the crystals are large but with very low density. Lowerflow rates made the Pn vapors have sufficient residence time tointeract with the nanoseeds, which results in thick crystals. Theforced diffusion of vapor using carrier gas has more impact thanthe molecular diffusion driven by concentration gradient asproposed by Nam et al.23 Under this advection state, theregenerally exists a maximum Pn vapor concentration andsupersaturation downstream from the source. As the flow rateof N2 carrier gas increases, the supersaturation shifts furtherdownstream, as flow gives direction to the vapor, which meansby adjusting gas flow we can change supersaturation at a point.At 300 sccm flow we got the best results. The typical thicknessof as-grown Pn crystalline flakes can be narrowed in a rangefrom 200 nm to around 50 nm. The upper surface of the crystalis indexed to the (001) plane which has the lowest surfaceenergy.24 This results in offering the molecules to diffuse to theside surfaces ((11 ̅0), (010), (110)), where the surface energies

Figure 3. SEM images of obtained single crystal at different substratetemperatures (series A).

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are higher; thus, the molecules diffuse much more easily intothe lattice planes on these surfaces. The faster the growth ratesof the side surfaces, the larger the area for the top surface. Thiscompetitive behavior among different planes leads to theextension of crystal size along the (010) plane. At the sametime accelerated speed of gas flow decreases the deposition rate,which helps to obtain ultrathin crystals down to 48 nm. OurPVT approach, therefore, resulted in very thin Pn crystalsendowing immense application in organic electronic.Exceeding 300 sccm, flakes of Pn appeared which stand on

their edge onto the substrate. Because of high flow speed, mostof the volatilized molecules travel through the nanoseedswithout helping them with maturation.These results suggest that to obtain ultrathin crystal, it is

necessary to control supersaturation which can be adjusted bythe aid of gas flow rate.3.4. Time Effect. To further understand the formation of

Pn crystals, we also studied the time effect (series D). From theimages in Figure 4a, it is clear that size and thickness increase astime extends. Different Pn shapes have also been observed. Theprominent shape of the crystal obtained is pentagonal andrhombic, while the lifetime for the hexagonal crystal is verysmall. The result indicates that the shape of Pn crystals withdistinct facets can be partially tailored by the deposition time.In essence, the formation of different crystal shapes of Pncrystals at different deposition times is governed by minimizingthe surface energy of the crystal system. An overall growthmechanism has been proposed. Normally, during thenucleation and growth, the vapor atoms prefer to fuse withthe high energy crystallographic planes in the crystal latticesystem. The high-energy planes preferentially disappear aftergrowth, and then the final crystals are terminated by low energyplanes.25 We have calculated the angles among the side faces forlarge-sized crystals. The corresponding plane indexes of sidefaces and the calculated values of angles which are consistentwith the reported work26 are presented in Figure 4a.Based on the above analysis for Pn shapes, the evolution

process of their edge planes could be described as follows. Forhexagonal shape, large-sized crystal are confined by (110),(010), and (11 ̅0) surface energy planes. Due to the lowersurface energy, the growth rate of the (001) plane could be veryslow. Then, as time goes on, since the growth rate of the (010)plane is high, so are those of (110) and (11 ̅0) planes. In our

experiments, up to 60 min of growth, only (110) planes will beleft behind to become the final edge planes of Pn crystal and tomaintain the whole crystal with a minimum surface energy.Above 60 min growth time, because of high supersaturation,growth of the crystal along the (001) direction is high andslower along other crystal planes. This is the limiting stepwhere the enhancement in size is diminished while thicknessincreases. Over a growth period of 120 min it is observed thatthe thickness of the crystal increases from a few tenths ofnanometers up to several micrometers. This infers that crystalsize and thickness are influenced by evolution of crystallineplanes. Growth mechanism of Pn crystal with increasingsupersaturation and surface energy is schematically illustrated inFigure 4b.

Table 1. SEM and AFM Images of Obtained Pn Crystals with Different Gas Flow (Series B)

Figure 4. (a) Crystal shape with corresponding angles betweendifferent planes. (b) Schematic illustration of shape evolution withrespect to supersaturation and surface energy.

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Large scale optical images of crystals over a broad range oftime are shown in Figure S3. However, it is important to notethat if the heating of the furnace stops, the growth of the crystalwill also be terminated. Thus, besides the majority of crystals inthe final stage, there should some products staying in theintermediate stage probably due to the late nucleation.Furthermore, the growth condition for each nanoseed cannotbe precisely the same.3.5. Device Characterization. In this study, the device

structure is shown schematically in Figure 5a; the interfacebetween single crystal and substrate is preserved during thewhole device fabrication. Figure 5b,c shows the transfer andoutput curves of an organic single crystal field effect transistor.The mobility of the device in the saturation region is derived

from the following equation

=μ

−IC W

LV V

2( )i

DS GS th2

(3)

where IDS is the drain current, μ is the mobility, Ci is thedielectric capacitance, W and L are the channel width andlength, respectively, VGS is the gate voltage, and Vth is thethreshold voltage. Our devices have good device performance,and the highest hole mobility reaches up to 5.6 cm2/(V s).We found that the device performance is influenced by the

crystal thickness because different crystal thickness results indifferent injection barrier (hereafter called injection resistance)from the top contact to the semiconductor/dielectric interface.The effect of injection resistance (Ri) on the deviceperformance can be directly shown from the differentialconductance of the device output curves as displayed in Figure5d. If we neglect Ri, the differential conductance in the channelshould monotonically decrease to 0 when increasing the source-drain voltage (VSD) from VSD = 0 V. This is because, at fixed

gate voltage, the distribution of the gate-induced charges alongthe channel becomes narrower as the VSD increases and at thesaturation regime a pinched off point occurs. This means thechannel resistance would increase with increasing VSD.However, the differential conductance vertical to the substratein the source region has a positive correlation with the appliedvoltage. It should increase with increasing the VSD from 0 in thevicinity of VSD = 0 V. The total differential conductance isdominated by the smaller differential conductance. When twodifferential conductances are nearly the same, there will be aturning point as shown in the Figure 5d. Compared to the thincrystal (95-nm-thick) device, the turning point in the thickcrystal (360-nm-thick) device has a shift to higher VSD as shownin Figure 5d. This indicates that the injection resistance leads tomuch deterioration in the thick crystal device performance. Wecan conclude that ON current increases with reduction incrystal thickness. This suggests that it is meaningful to get anultrathin single crystal.

4. CONCLUSION

Our experiments have demonstrated the controlled growth oflarge and ultrathin 2D Pn crystal via nanoseed assisted growththrough PVT method stimulated by supersaturation. Byoptimizing the deposition parameters, we successfully illustratethe synthesis of high quality, Pn single crystal with tens ofmicrometers in size and around 50 nm in thickness. Wesystematically monitor the effect of coverage and substratetemperature on crystal size. We also demonstrated that use ofhigh gas flow rates decreases the deposition rate while at thesame time lowering the supersaturation, thus helping thegrowth of ultrathin 2D crystal. Moreover, growth mechanism ofthe shape evolution of as-grown Pn crystal has also beenexplained with respect to different surface energies of various

Figure 5. (a) Schematic of crystal device (inset: optical image of device). (b,c) Transfer and output curve of device, respectively. (d) Differentialoutput conductance of different thickness OSCFET.

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planes. The interface between the crystal and the SiO2 dielectrichas been preserved during all processes of device fabrication,which is important for the electrical transport study. Electricalstudy based on crystals showed mobility as high as 5.6 cm2 V−1

s−1 revealed that the in situ growth of ultrathin crystals has asignificant result in transport performance. We believe that thesynthesis technique in this work manifests a new strategy forthe growth of large and ultrathin Pn crystals.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.cgd.5b01726.

SEM images for the comparison of growth on substrateswithout nanoseeds and with nanoseeds, graphs showingPn vapor pressure in furnace and nucleation probabilityof Pn at different substrate temperatures depending onlocal supersaturation, and optical images of Pn crystalafter PVT at different times (PDF)

■ AUTHOR INFORMATIONCorresponding Author*Tel: 86-10-82545563. Fax: 86-10-62656765. E-mail: jiangch@nanoctr.cn.Author Contributions#S. Atika Arabi and Ji Dong contributed equally. Themanuscript was written through contributions of all authors.All authors have given approval to the final version of themanuscript.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work is supported by the National Natural ScienceFoundation of China (Grants 21432005, 11374070,61327009), and the “Strategic Priority Research Program” ofthe Chinese Academy of Sciences (Grant No. XDA 09040201).

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