Thickness Dependence of Glass Transitions Measured by AC-Chip

Aug 22, 2013 - When most prior studies on thin polymer films have shown that glass transition temperature (Tg) decreases under nanoconfinement, the ...
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Thickness Dependence of Glass Transitions Measured by AC-Chip Calorimetry in Films with Controlled Interface Jiao Chen,† Jie Xu,† Xiaoliang Wang,† Dongshan Zhou,*,† Pingchuan Sun,‡ and Gi Xue*,† †

Department of Polymer Science and Engineering, School of Chemistry and Chemical Engineering, State Key Laboratory of Co-ordination Chemistry, Nanjing National Laboratory of Microstructures, Nanjing University, Nanjing 210093, P. R. China ‡ Key Laboratory of Functional Polymer Materials, Ministry of Education, College of Chemistry, Nankai University, Tianjin 300071, P. R. China S Supporting Information *

ABSTRACT: When most prior studies on thin polymer films have shown that glass transition temperature (Tg) decreases under nanoconfinement, the differential alternating current chip (ac-chip) calorimetric method shows little dependence of Tg on thickness for supported film. To reveal this contradiction, we have manipulated a controlled interface by spin-coating polystyrene (PS) with immiscible surfactants such as tetraoctylammonium bromide or citric acid. Since the immiscible surfactants did not show plasticizing effect for PS, there was no observable reduction of Tg from the bulk value ether in powdered blends or in thick films. However, the ultrathin film with thickness h ∼ 25 nm, consisting of 95 wt % PS and 5 wt % surfactants, showed a reduction of Tg by 6−7 °C, as compared to thick film with the same composition. We propose that the surfactant molecules assembled on the interface between thin film and substrate due to phase separation. The molecular mobility of molecules at the interface was dramatically increased, which was detected by 1NMR with dipolar filter sequence. It appeared that the deviation range was not so large as that measured by other methods. But considering that we were measuring Tg at a high frequence (10 Hz), this amount of deviation was quite significant for ac-chip calorimetry. As a result, ac-chip calorimetry measured Tg data unambiguously demonstrate that thickness dependence of Tg is a real property of confined thin film.

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INTRODUCTION A general description of the glass transition remains a key problem even as glassy materials find widespread applications in industry and in nanotechnology. Many studies have been carried out on the dependence of glass transition temperature with decreasing film thickness h.1−10 However, the results reported have shown disagreement about the behavior of ultrathin polymer films relative to their behavior in the bulk and raised many fundamental questions. When most prior studies on supported thin film have shown that Tg decreases under nanoconfinement, several measurements claimed no thickness dependence of Tg, among which calorimetric measurements attract more attention in these studies.2,11−14 The classical technique for measuring Tg in bulk materials is differential scanning calorimetry (DSC).2,11−14 Calorimetry takes a special place among other methods due to its simplicity and universality. Moreover, the energetic characteristics (heat capacity Cp and its integral over temperature T−enthalpy H), measured via calorimetry, have a clear physical meaning even though sometimes interpretation may be difficult.2,11−14 The significant part of theoretical considerations and glass transition models are constructed on the basis of ΔH(T) functions detected using calorimetry. However, the measurements of heat capacity data of ultrathin polymer films (h < 30 nm) are difficult because of the limited sensitivity of commercial DSC © 2013 American Chemical Society

devices. Differential alternating current (ac) chip calorimetry, originally developed by Schick et al.,12,14 which is a combination of chip calorimetry and ac calorimetry, can measure thermal properties of small amount of samples at thermodynamic equilibrium. The ac-chip calorimeter has very high sensitivity (∼50 pJ K−1) and wide modulation frequency range (10−1−103 Hz). It can be used to detect the glass transition of polymer thin films with thickness below 10 nm. We have studied the glass transition of poly(2,6-dimethyl-1,5phenylene oxide) (PPO) thin films by an ac-chip calorimeter, and no thickness dependence of Tg was found for the supported film with thickness ranging from about 6 nm (approximately half of radius of gyration, Rg) to 330 nm (∼29 Rg).14 Comparing with other measurements, the results studied by calorimetric measurements were different. It has become increasingly evident that it is necessary to provide a distinction between no h dependence in Tg measured by ac-chip calorimeter and those which are intrinsic properties of the sample geometry and dimensions. Forrest et al. suggested that the reductions in Tg are strongly related to the presence of free surface.15 This relation was Received: June 18, 2013 Revised: August 6, 2013 Published: August 22, 2013 7006

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Figure 1. Cooling curves for calorimetric ac-chip measurement for PS films: (a) neat PS; (b) PS thick and thin films consisted of 95 wt % of PS and 5 wt % of TOAB. and distilled. Toluene, tetrahydrofuran (THF), and chloroform were commercially available and distilled before use. The monodisperse polystyrene (Mn = 205 kg/mol, d = 1.06) was purchased from Polymer Source Inc., Canada. Tetraoctylammonium bromide (TOAB), citric acid, and pyrene were used as received. Polymer thin films with different thicknesses were prepared by spincoating polymer from filtered toluene solutions. Each solution was spin-coated directly onto the central position of a cleaned sensor for ac-chip calorimetry measurement and onto a cleaned silicon wafer for the thickness measurement by ellipsometry. The mixed powder for DSC measurement was prepared by dissolving polystyrene and diluents in their good solvent (benzene for PS with TOAB and pyrene and THF for PS with citric acid) overnight to get a homogeneous solution then stirring at room temperature under the fume hood to let most of the solvent evaporate. Subsequently, they were dried in vacuum at room temperature for 12 h, 80 °C for 12 h, and 120 °C for 2 h to ensure that there was no residual solvent. The samples PS and TOAB adsorbed on SiO2 (Aldrich, 7 nm in diameter) particles were prepared by mixing PS, TOAB, and SiO2 particles in purified chloroform and stirring at room temperature for 12 h. The suspending liquid was centrifuged and then washed by chloroform for three times to remove residual PS and TOAB in free state. At last samples were dried as mentioned above. Calorimetric Measurements. Standard differential scanning calorimeter (DSC) measurement was run on Metter-Toledo DSC1 STAR system with a FRS5 sensor under a dry nitrogen atmosphere (N2 flow of 50 mL/min). The instrument was calibrated with indium and zinc standards. Temperature scans were made as follows: All samples were first heated from 50 to 150 °C at 10 °C/min, held for 2 min at 150 °C, then cooled from 150 to 50 °C at 10 °C/min, and held for 2 min at 50 °C. The above procedure was repeated twice. The midpoint of the slope change of the heat capacity plot was taken as the glass transition temperature (Tg). Differential alternating current (ac) chip calorimeter: The full details of the method are described in ref 14. In this work, thin films were supported by the sensor XI392 (Xensor integrations, NL), which had a large smooth heated area (100 μm × 100 μm). The experiments were undertaken at the frequency of 10 Hz and voltage of 1.5 V. The underlying heating/cooling rate was 1 °C/min under a protecting nitrogen atmosphere, and each sample was scanned for three times. The amplitude of the complex differential voltage as a function of measuring temperature was obtained. The dynamic glass transition temperature was determined as the half-step temperature of the amplitude. NMR Measurements. 1H solid-state NMR spectra were recorded on a Varian Infinityplus-400 wide-bore (89 mm) NMR spectrometer at a proton frequency of 399.7 MHz using a 2.5 mm T3 doubleresonance CPMAS probe, and this probe can provide stable sample spinning up to 30 kHz using a zirconia PENCIL rotor. The magic angle spinning (MAS) frequency used here was 25 kHz. All the NMR data were processed with Varian Spinsight software, and all

strengthened considerably by the observation of a quantitative mapping between supported films of thickness h and freestanding films of thickness 2h.15 Despite the consensus among many measurements, a key fundamental question has persisted regarding the contribution of sample preparation to the measured Tg value. Spin-coating is an effective method to produce uniform polymer thin films on a planar substrate. It is most often employed in the microelectronics industry for the production of photoresists and has been widely used in scientific research.16 However, spin-coated polymer films are affected by chain configurations far from equilibrium and potentially large stresses.17,18 These interfacial stresses between film and substrate are crucial to thin film studies. These effects can not only provide possible alternative explanations to measured Tg values in thin films but may also be very difficult to remove by conventional annealing.18,19 Very recently, Dalnoki-Veress et al.20 examined the direct effect of manipulating the number of free surfaces on the measured Tg of thin PS films studied by ellipsometry. They recorded a reduction of Tg for a freestanding thin film and then transferred it to a SiO2/Si substrate and found no deviation of Tg between thin film and the bulk polymer. Their experiments unambiguously show the importance of free interfaces on the Tg reductions for the film study. Shin et al.21 reported an enhancement in mobility of polymer confined within nanoscopic cylindrical pores (2D confinement) having radius smaller than the radius of gyration (Rg) of the chains in the bulk. Napolitano et al.22 demonstrated the experiment in capped films and suggested that below 50 nm both the glass transition temperature and the thermal expansion coefficients decreased. Such a mixed behavior implies an enhancement of the molecular mobility, without the presence of any free surface, but dead layers. It seems that the correlation of free surface and reduction in Tg is not universal. To investigate the effect of interface stresses on ac-chip measurements for Tg of thin films, we prepared PS thin films with addition of surfactant molecules, such as tetraoctylammonium bromide (TOAB), which was immiscible with PS and adsorbed on substrate resulting in a mobile interface. Our experiments show a dependence of Tg on film thickness using ac-chip measurement. Moreover, we prepared freestanding PS and PPO films for thermal analysis to confirm our results.

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EXPERIMENTAL SECTION

Materials. Benzene (Aldrich, spectrophotometric grade) was shaken with concentrated sulfuric acid and then with water, dilute NaOH, and again water. And then it was dried with anhydrous Na2SO4 7007

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Figure 2. (a) DSC heating traces for neat PS and PS with addition of 5 and 0.5 wt % TOAB. (b) Heating curves in calorimetric ac-chip measurement for PS thin films with addition of 5 wt % TOAB; the thickness of film is indicated on each curve.

Figure 3. (a) Calorimetric ac-chip measurement cooling curves for PS thick and thin films via addition of citric acid; the ratio is 95:5 in weight. (b) DSC heating traces of neat PS and PS powder with addition of 5 wt % citric acid. experiments were carried out at room temperature. The 1H chemical shifts were referenced to external TMS. More details of NMR experiments could be found elsewhere.23

measurement. The DSC result is shown in Figure 2a. Tg for PS samples blended with 5 or 0.5 wt % TOAB is 103 °C, which is nearly the same as the neat PS. In the curve for PS sample which contains 5 wt % TOAB, we can see a melting peak at about 95 °C for TOAB. It seems that in powdered states PS and TOAB are immiscible, so TOAB does not show plasticizing effect on glass transition. Figure 2b shows the heating curves in calorimetric ac-chip measurement for PS films with addition of 5 wt % TOAB. There is a clear melting point near 99 °C for thick film (h = 150 nm), and its Tg is at 128 °C. However, we do not see melting transition of TOAB in the heating curve (Figure 2b) for thin film (h = 25 nm), but its Tg is reduced by 7 °C from the bulk. Since each TOAB molecule contains polar portion and nonpolar portion, it is reasonable to understand that TOAB molecules adsorbed onto SiO2/Si substrate at the interface, so that there is no crystalline TOAB formed in ultrathin film. This result seems in controversy with previous calorimetric ac-chip measurements for thin films.14 One might argue that a small amount of dissolved TOAB could reduce Tg. However, we did not find a obvious reduction for calorimetric Tg in either bulk blend or thick films (h > 150 nm) composed with PS and TOAB. Changing the sensor for ac-chip calorimetry measurement gives the same thickness dependence of Tg for thin films. To prove that the result obtained from PS-TOAB system is a general phenomenon, we measured Tg for PS film blended with another surfactant citric acid. Figure 3 shows that Tg for 25 nm film was reduced by 7 °C as compared with that for 250 nm film, indicating clearly a thickness dependence of Tg for PS with citric acid. The DSC result shows no Tg reduction for the mixed powder of PS and citric acid, so PS and citric acid are also immiscible. We should address again that Tg changes here was

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RESULTS AND DISCUSSION Calorimetric ac-chip measured cooling curves for PS thick films with thickness (h) of 250, 70, and 25 nm are listed in Figure 1. Each sample was scanned for three times until Tg came to an constant temperature. Here the Tg measured for pure PS is 124 °C (in this report, all of the Tgs are determined at half-height of heat capacity change). This temperature is about 20 °C higher than that measured by normal DSC because we detected the films by ac-chip at a higher frequency (10 Hz).24 Figure 1a indicates no Tg deviation on thickness h for pure PS. This is in agreement with some previous reports.2,11−14 However, a distinct thickness dependence in Tg is shown in Figure 1b for PS with addition of surfactant (TOAB) detected by calorimetric ac-chip. Usually, polymer shows a lowered Tg when it contains miscible diluents or solvents than pure polymer. Torkelson et al. reported a reduction of Tg for PS with addition of small diluents pyrene, but no thickness dependence for thin films.3,25 Here we investigated the effect of addition of surfactant molecules on the Tg in PS thin films. Figure 1b shows the Tg of PS thick and thin films with addition of TOAB by 5 wt %. The 150, 60, and 25 nm PS films blended with 5 wt % TOAB have Tgs of 124, 122, and 118 °C, respectively. For 25 nm ultrathin film, the calorimetric Tg reduction is about 6 °C as compared with thick films with h ∼150 nm consisting of 95 wt % of PS and 5 wt % of TOAB. The changes in Tg in Figure 1b clearly indicate a thickness dependence of glass transition for this supported thin film. The Tg deviation is obviously not due to plasticizing effect since both thick and thin films were composed of PS and TOAB in weight ratio of 95:5. We prepared the mixed powder of PS and TOAB for standard DSC 7008

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Figure 4. (a) Calorimetric ac-chip measurement cooling curves for PS thin films via addition of pyrene; the ratio is 95:5 in weight. (b) DSC heating traces of neat PS and PS powder with addition of 5 wt % pyrene.

monolayer molecules when confined at nanometer length scales differ greatly from properties in the bulk. Napolitano et al.27 revealed the correlation between shift in Tg and confinement effects. They investigated the impact of adsorption on the deviations from bulk behavior and found that ΔTg is proportional to the polymer interface width, that is, the thickness of the interfacial layer with nonbulk density. This evidence better proves our result. By using fluorophores to label a single layer within a multilayer PS film, Ellison and Torkelson4,28 performed direct measurement of Tg gradients in substrate-supported PS films. Paeng and Ediger et al.25 measured the molecular mobility in freestanding PS films dispersed with fluorescent probes by an optical photobleaching technique. It seems hard to distinguish mobility for TOAB molecules in a blend by the use of fluorescence spectroscopy. We detect the mobility of adsorbed TOAB by the use of a NMR dipolar filter pulse sequence experiment.29−31 A 12-pulse dipolar filter NMR experiment developed by Schmidt-Rohr et al.29,31 is used to select the 1H magnetization with a weak dipole−dipole interaction. In this 1H NMR experiment, as recycle times (Ncycle) of 12-pulse dipolar filter increase, the signals of the “rigid” parts are gradually suppressed, while the mobile component shows an isolated signal, as illustrated by the schematic diagram in Figure 5 (upper). NMR spectra for TOAB, PS, and PS−TOAB coadsorbed on silica surface at increased Ncycle were collected, as shown in Figures 5a, 5b, and 5c, respectively. The proton peak at ca. 1.3 ppm is mainly due to aliphatic chain of TOAB molecules, which is reduced greatly in intensity at Ncycle = 10 (Figure 5a). The NMR signals (7.0 ppm) for aromatic protons of PS are also suppressed at high Ncycle (Figure 5b). However, NMR spectra for the adsorbed PS and TOAB on SiO2 (Figure 5c) show a distinguished peak at 1.3 ppm at high Ncycle, indicating that the adsorbed methylene groups of TOAB on SiO2 are in high mobility. The surface of SiO2 particles is hydrophilic; the peak at ca. 4 ppm is from some amount of H2O unavoidably adsorbed on SiO2 particles.32 The change of NMR integral for the peak at 1.3 ppm versus Ncycle is plotted in Figure 6. One can see from Figure 6 that the integral of proton peak at 1.3 ppm for neat TOAB or TOAB blended with PS is suppressed to near zero after Ncycle = 10. However, almost 60% of the intensity of the proton peaks at 1.3 ppm for TOAB adsorbed on SiO2 remains unchanged (Figure 6), indicating a high mobility for the surfactant molecules adsorbed on surface. Here we should point out that NMR peaks for both aliphatic and aromatic protons in PS are suppressed significantly after

not due to the plasticizing effect since we were comparing the films with different thickness but with the same composition. Figure 3b shows no observable shift of Tg between PS composite and neat PS, indicating there is no plasticizing effect of citric acid for PS. But Figure 3a clearly indicates that thin film (h ∼ 25 nm) composed of 95 wt % PS with 5 wt % of citric acid shows a reduced Tg (by 7 °C) as compared to thick film (h ∼ 250 nm) with the same composition of PS and citric acid. Definitely, this reduction is not due to plasticizing effect. According to the above results, in immiscible systems, Tg does not reduce for bulk belnds but shows a clear thickness dependence for film. It is interesting that a miscible blend of PS and pyrene gave an opposite result. The Tg of PS with 5 wt % pyrene was reduced due to the plasticizing effect but did not show thickness dependence for Tg. Torkelson et al. reported a dramatic reduction of confinement for PS thin film with addition of small diluents pyrene using fluorescence spectroscopy.3 They point out that at a fundamental level the presence of small-molecule diluents in polymers reduces the extent of cooperativity by relaxing constrains on cooperative segmental mobility defining Tg.3,4 To distinguish the effect of miscibility of the added small molecules on Tg measurement, we used calorimetric method to investigate PS−pyrene films. Figures 4a and 4b show cooling curves for PS−pyrene thin films measured by calorimetric ac-chip and DSC heating traces for powdered blend, respectively. For the film blend (Figure 4b), the Tg is reduced by 8 °C as compared with pure PS due to the plasticizing effect. However, little thickness dependence for Tg in thin film is recorded (Figure 4a). Our calorimetric results confirm Torkelson’s report.3,26 Results in Figure 4 also demonstrate the reliability and sensitivity in investigation of plasticizing effect on Tg by the addition of miscible small molecules. Importantly, calorimetric detection for PS thin film with addition of immiscible TOAB, an unexpected reduction was observed for ultrathin film as compared with thick film with the same compositions. The distinguish factor is phase separation in the PS−TOAB blend. A melting endothermic peak for crystalline TOAB appeared in DSC curves for powdered blend and in ac-chip heating curves for thick film (Figure 2). But for ultrathin film of PS−TOAB (h ∼ 25 nm), no melting peak was observed. It is reasonable to propose that TOAB molecules adsorb onto substrate from the vicinity of the film near the interface due to strong attraction between the polar group of surfactant molecules and SiO2/Si. The nonpolar parts of TOAB molecules exhibit high mobility at the interface. The mobility and thermal dynamic properties of 7009

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released the interfacial stress caused by spin-coating process, as illustrated by schematic diagram in Figure 7.

Figure 7. Schematic diagram for TOAB molecules (upper) and a mobile interface in PS/substrate interface (lower).

Napolitano33 reported that the reduction in Tg measured in thin films (polymer/solid interface) upon confinement is proportional to the degree of adsorption and, thus, to the interfacial free volume. Also in our previous report,34 we showed that the reduction in packing density can increase the segmental mobility in the glassy state. Here in our experiment, we suggest that the TOAB adsorb on the substrate and produce a mobile interface. Since the TOAB runs off, the interfacial free volume increases and packing density decreases. It is another possible explanation for the reduction of Tg. For the first time we show the Tg dependence on thickness of supported film, using ac-chip measurement, which is in agree with Simon’s report by the flash DSC method.35 Our experiments unambiguously confirm the importance of free interfaces of the Tg reductions for the film study by Forrest and Dalnoki-Veress.15,20 In order to verify the free surface effect on Tg measurement proposed by Forrest et al.,5,6,15,20 we prepared stacked freestanding PS film (h = 50 nm) and PPO film (h = 70 nm), and measured their Tgs by DSC. We find that their Tgs are reduced by 7 and 15 °C from the bulk, respectively (see Figure S4). This is in agreement with current reports by Boucher and Cangialosi et al.36

Figure 5. Upper: schematic diagram of detection of a mobile component by NMR dipolar filter sequence. Ncycle indicates the multipulse dipolar filter number in 1H NMR experiments. Lower: the dipolar filtered 1H NMR spectra for (a) TOAB in solid state, (b) neat PS, and (c) PS and TOAB coadsorbed on SiO2 (d = 7 nm) particles.

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CONCLUSION In summary, calorimetry is an effective tool to characterize the glass transition under confinement. The inconsistencies and controversies related to data obtained by ac-chip calorimetry and other techniques seem to be caused by the chain configuration at the interface far from equilibrium and potentially large stresses caused by spin-coating. Releasing the stresses by introducing a mobile interface can effectively recover the thickness dependence of Tg for supported polymer film. Comparing with other techniques, there is only a 6−7 °C depression of Tg measured by ac chip. It appears that the deviation range was not so large as that measured by other methods.20 But considering that we were measuring Tg at a high frequence (10 Hz), this amount of deviation was quite significant for ac-chip calorimetry. The experiments presented in this report combined with recent reported results by Dalnoki-Veress20 and Simon29 et al. represent complete quantification of the effective free interface on measured Tg values and rule out a dominant contribution due to sample preparation.

Figure 6. Percentage of remaining integrals of aliphatic proton peak at ca. 1.3 ppm versus the Ncycle values in 1H NMR dipolar filter experiments. We normalize the intensity of the initial spectra as 1. The sample composition is indicated on each curve.

Ncycle = 10, as shown in Figure 5b. We can conclude that the isolated signal at 1.3 ppm (Figure 5c) are mainly due to protons in TOAB after Ncycle = 10. Moreover, it is interesting to see no crystalline peak in DSC heating curve (Figure 2), but Figure 6 shows TOAB molecules are rigid in a low content blend (containing 0.5 wt % of TOAB). Forrest et al. suggested that the reduction in Tg is strongly related to the presence of free surface.15 Dalnoki-Veress et al.20 examined the direct effect of manipulating the number of free surfaces on the measured Tgs of thin PS films. Here, we introduced a mobile layer between PS and substrate and 7010

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

Article

(22) Napolitano, S.; Pilleri, A.; Rolla, P.; Wubbenhorst, M. ACS Nano 2010, 4, 841−848. (23) Wang, X. L.; Tao, F. F.; Sun, P. C.; Zhou, D. S.; Wang, Z. Q.; Gu, Q.; Hu, J. L.; Xue, G. Macromolecules 2007, 40, 4736−4739. (24) Jiang, W.; Du, M.; Gu, Q.; Jiang, J.; Huth, H.; Zhou, D.; Xue, G.; Schick, C. Eur. Phys. J.: Spec. Top. 2010, 189, 187−195. (25) Paeng, K.; Swallen, S. F.; Ediger, M. D. J. Am. Chem. Soc. 2011, 133, 8444−8447. (26) Kim, S.; Mundra, M. K.; Roth, C. B.; Torkelson, J. M. Macromolecules 2010, 43, 5158−5161. (27) Napolitano, S.; Wubbenhorst, M. Nat. Commun. 2011, 2, 260. (28) Kim, S.; Torkelson, J. M. Macromolecules 2011, 44, 4546−4553. (29) Egger, N.; Schmidt-Rohr, K.; Blumich, B.; Domke, W. D.; Stapp, B. J. Appl. Polym. Sci. 1992, 44, 289−295. (30) Guo, M.; Zachmann, H. G. Polymer 1993, 34, 2503−2507. (31) Schmidt-Rohr, K.; Spiess, H. W. Multidimensional Solid-State NMR and Polymers; Academic Press: New York, 1994. (32) Nosaka, A. Y.; Nosaka, Y. Bull. Chem. Soc. Jpn. 2005, 78, 1595− 1607. (33) Napolitano, S.; Rotella, C.; Wubbenhorst, M. ACS Macro Lett. 2012, 1, 1189−1193. (34) Xu, J.; Li, D. W.; Chen, J.; Din, L.; Wang, X. L.; Tao, F. F.; Xue, G. Macromolecules 2011, 44, 7445−7450. (35) Gao, S. Y.; Koh, Y. P.; Simon, S. L. Macromolecules 2013, 46, 562−570. (36) Boucher, V. M.; Cangialosi, D.; Yin, H. J.; Schonhals, A.; Alegria, A.; Colmenero, J. Soft Matter 2012, 8, 5119−5122.

S Supporting Information *

Ac-chip results for PS thin films with different contents of TOAB; ellipsometry measurement; standard DSC results for stacked PS and PPO thin films. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author

*Tel 86-25-83686136; e-mail: [email protected] (G.X.), [email protected] (D.Z.); Fax 86-25-83317761. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge Prof. Christoph Schick at the University of Rostock for his help in building the equipments for differential alternating current (ac) chip calorimetric study in Nanjing University. We appreciate the helpful discussions with Prof. S. L. Simon and Prof. G. B. McKenna at Texas Tech University. We also acknowledge support from National Basic Research Program of China (973 Program, 2012 CB 821503) and National Natural Science Foundation of China (Nos. 51133002, 21274060, 21274059, 21027006, and 21174062).

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