Relating Ultrastable Glass Formation to Enhanced Surface Diffusion

Letter pubs.acs.org/JPCL

Relating Ultrastable Glass Formation to Enhanced Surface Diffusion via the Johari−Goldstein β‑Relaxation in Molecular Glasses K. L. Ngai,*,†,‡ Li-Min Wang,*,‡ and Hai-Bin Yu*,§ †

CNR-IPCF, Universita di Pisa, Largo B. Pontecorvo 3, I-56127 Pisa, Italy State Key Lab of Metastable Materials Science and Technology, and College of Materials Science and Engineering, Yanshan University, Qinhuangdao, Hebei 066004, China § Wuhan National High Magnetic Field Center, Huazhong University of Science and Technology, WuHan, Hubei 430074, China ‡

ABSTRACT: Glasses are materials essential for modern technology; they are usually prepared by cooling liquids. Recently, novel ultrastable glasses (SGs) with extraordinary thermodynamic and kinetic stability have been created by vapor deposition at appropriate substrate temperatures. However, the underlying mechanism for the formation of SGs is still not established. For most of the molecular SGs created so far, we demonstrate that the formation of SGs is closely related to the Johari−Goldstein β-relaxation from the fact that the lowest substrate temperatures possible for the formation of SGs match the secondary glasstransition temperatures, where the β-relaxation time reaches 103 s. Theoretically the βrelaxation time via the primitive relaxation time of the coupling model has proven capable of accounting for the enhancement of molecular mobility at the surface. Thus our findings provide evidence to support that the immense enhancement of molecular diffusion at the surface is critical for the formation of SGs. The result has implications in the design and fabrication of SGs.

G

standing the underlying formation mechanism of SGs needs no further emphasis. We establish such a link quantitatively with the help of the Johari−Goldstein (JG) β-relaxation30−32 from experiments and a theoretical model for explanation. First, we demonstrate the existence of a strong correlation between the JG β-relaxation and the formation of SGs in almost all of the molecular SGs created to date. Then, we show that JG β-relaxation time τβ is quantitatively the indicator of enhanced surface diffusions. Putting the two together, our findings provide the long sought evidence that the formation of SGs critically depends on the enhanced diffusion at the surface The upper panels of Figure 1 show the primary (α) and secondary (i.e., JG β-) relaxation times as a function of temperature T for four different simple molecule glasses. Presented in the panels in the middle are the relative changes of specific heat Cp of their vapor-deposited SGs compared with OGs as a function of the substrate temperature Tsub. These molecules include toluene (TOL),33 ethylbenzene (EBZ),33 ethyl cyclohexane (ECH),34 and methyl-m-toluate (MMT),35,36 and their chemical structures are shown in the bottom panels. The parameter, 100 × (1 − Cp,SG/Cp,OG), is used as a measure of the thermodynamics stability of SGs. A larger value of this parameter corresponds to higher stability of the SGs.33 As usual, we define the glass temperature Tg when the structural α-relaxation time τα reaches 103 s (the horizontal dashed lines in the upper panels). In a similar way, we define

lasses are disordered materials that lack the long-range order of crystals but behave mechanically like solids, and they are usually prepared by cooling liquids to bypass crystallizations.1−3 Recently, some novel glasses, termed the “ultrastable glasses”, have been prepared by vapor deposition routes at appropriate substrate temperature.4−7 These materials assume extraordinary low-energy states with high density and exhibit remarkable thermodynamic and kinetic stability, which would otherwise be obtained only if ordinary glasses (OGs) were annealed for thousands to millions of years.4,8−10 Ultrastable glasses (SGs) are thus of special interest for understanding many fundamental issues regarding the nature of glass.8−17 Moreover, the enhanced stability of SGs is potentially important for a wide range of applications, such as protective coatings,18−21 organic electronics and optoelectronics,22−25 and amorphous pharmaceuticals.4,26 Despite their importance, the mechanism of formation of SGs by vapor deposition is still not clearly established.4,10 Since the beginning and continued until the present time, it was suggested that the key to the formation of these SGs by vapor deposition is the enhanced molecular mobility at the surface27−29 that enables molecules to explore the lower basins in the energy landscape to reach near-equilibrium packing arrangements before they are buried by further depositions. This suggestion was motivated by the observations of large enhancement of surface diffusion coefficient of some molecule glasses from which SGs had been prepared, such as indomethacin (IMC)27 and ortho-terphenyl (OTP).28 Notwithstanding, no direct link between SG formation and enhanced surface diffusion has been established so far. Considering the broad potential applications of SGs, the urgency of under© XXXX American Chemical Society

Received: May 14, 2017 Accepted: June 6, 2017 Published: June 6, 2017 2739

DOI: 10.1021/acs.jpclett.7b01192 J. Phys. Chem. Lett. 2017, 8, 2739−2744

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The Journal of Physical Chemistry Letters

Figure 1. Relaxation dynamics and stability of vapor-deposited simple molecule glasses. (a) Toluene (TOL), (b) ethylbenzene (EBZ), (c) ethyl cyclohexane (ECH), and (d) methyl-m-toluate (MMT). For each column, the upper panels show the relaxation time (τ) of α and β relaxations as a function of temperature, except in panel b, where the β relaxations of EBZ are inferred from toluene and isopropyl benzene (IPB). The middle panels show the relative specific heat Cp change of deposited stable glass (SG) compared with ordinary glass (OG). The chemical structures of the molecules are shown in the bottom panels. The horizontal dashed lines indicate when τα or τβ reaches 103 s, where Tg and Tg,β are defined as vertical lines.

Figure 2. Relaxation dynamics and stability of vapor-deposited molecule glasses with relatively more complex structures. Relaxation dynamics (upper panels) and the onset temperatures (middle panels) of (a) Celecoxib and (b) ortho-terphenyl. (c) Mean-square displacements of cis-decalin (upper panel), the onset temperature Tonset (middle panel, left axis), and the relative Cp changes (middle panel, right axis). The chemical structures of the related molecules are shown in the bottom panel. 2740

DOI: 10.1021/acs.jpclett.7b01192 J. Phys. Chem. Lett. 2017, 8, 2739−2744

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The Journal of Physical Chemistry Letters the secondary glass-transition temperature Tg,β for the βrelaxation (i.e., the temperature at which τβ = 103 s). The values of Tg and Tg,β are indicated by the vertical dashed lines in each column of Figure 1. In general, Tg can be routinely measured by various techniques of calorimetry such as differential scanning calorimetry (DSC). Although not readily observed by DSC, the secondary glass transition was detected by adiabatic calorimetry,37,38 and positronium annihilation lifetime spectroscopy and the values of Tg,β were determined for several glasses.39−41 This suggests that Tg,β could also be an intrinsic property of the glasses. The importance of Tg,β, especially its relationships with other dynamic properties of glasses, have been discussed in refs 39 and 40. Interestingly, Figure 1a−d reveals that around the temperature Tg,β, the stability of SGs undergoes a sharp change of the dependence on the substrate temperature Tsub. For TOL, as shown in Figure 1a, 100 × (1 − Cp,SG/Cp,OG) shows a pronounced decrease around Tsub = 73 K, close to Tg,β ≈ 70 K.42 For EBZ, because of its prone to crystallization, there is no direct measurement of the β-relaxation to date. Nevertheless, one can use the data of TOL42 and isopropyl benzene (IPB)37 to estimate its τβ(T), as shown in the upper panel of Figure 1b. The estimated Tg,β of EBZ in this way is between 70 and 74 K, and the stability of vapor-deposited EBZ starts to decrease rapidly on lowering Tsub to below Tg,β, as can be seen in the middle panel of Figure 1b. The correlation between Tg,β and the lower bound of Tsub for formation of SGs holds also in the other two simple molecule glasses as well: ECH34,43 and MMT35 shown in Figure 1c,d, respectively. To further verify this correlation, we examine molecular glasses with relatively more complex chemical structures shown at the bottom of Figure 2. These are celecoxib, which is a pharmaceutical,26 OTP,44 and cis-decalin.45 Figure 2a shows the temperature-dependent relaxation times of celecoxib (upper panel) and the onset temperature Tonset of its SG (where heat capacity measured by calorimetry exhibits a jump26) as a function of Tsub (middle panel). We note that like the suppressed Cp, which characterizes the thermodynamics stability of SGs, the higher value of Tonset of SGs is another measure of the enhanced kinetic stability of SGs, and these two parameters usually follow the same trends.33 As indicated by a vertical dashed line in Figure 2a, Tg,β ≈ 221 K for celecoxib. On the contrary, for celecoxib SGs deposited at this temperature of the substrate, the value Tonset is almost equal to that of OG (as indicated by the dashed horizontal line in Figure 2a). At lower Tsub than Tg,β, the Tonset becomes even smaller than that of the OG. These results suggest that Tsub ≈ Tg,β is the lowest substrate temperature allowed in the formation of celecoxib SGs, and it would be impossible to obtain celecoxib SGs having higher stability than OGs by vapor deposition if Tsub < Tg,β. Figure 2b validates the similar observation in OTP44 as well. Figure 2c shows the results of cis-decalin. Because of its small dipole moments, the β-relaxation of cis-decalin has not been probed directly by dielectric spectroscopy. Nevertheless, recently it has been shown39,40 that for many molecular glasses the mean-squared displacements measured by quasielastic neutron scattering experiments would change its slope with temperature when crossing Tg,β. Therefore, one can use the temperature-dependent of cis-decalin to estimate its Tg,β. The upper panel of Figure 2c reproduces the of cisdecalin,46 and one can see that the slope of indeed shows an appreciable change at T = ∼102 K in the glassy state. From this, we take Tg,β ≈ 102 K for cis-decalin. The middle panel of

Figure 2c depicts the behaviors of cis-decalin SGs as a function of Tsub, and it is clear that the stability, in terms of both Cp and Tonset, vanishes dramatically around Tg,β ≈ 102 K, which is consistent with the results of other SGs in Figures 1 and 2. Thus all of the results in Figures 1 and 2 demonstrate the close relation between the formation of SGs and the JG βrelaxation by the fact that the lowest substrate temperature accessible for the formation of SGs is around Tg,β where τβ approaches ∼103 s. If the enhanced molecular mobility at the surface is the key to the formation of SGs by vapor deposition, the correlation found between the β-relaxation and the formation of SGs suggests a possible connection between β-relaxations and surface diffusions. To explore this possibility, we start by reconsidering the enormous increase in the diffusion coefficient of molecular glass-formers at the surface Ds compared with Dv in the bulk observed by the technique of surface-grating decay. The surface diffusion coefficients Ds at Tg of OTP are larger than Dv by about eight orders of magnitude.28 Intuitively, the enhancement can be rationalized by the reduced intermolecular interaction with other molecules, and the free space available for molecular motions is particularly orthogonal to the plane of the surface. However, it is challenging to account for the magnitudes of the enhancement for each material and to predict its trend on varying the material based on some parameters of the bulk dynamics. At present there are three theoretical models47−49 proposed to predict the degree to which diffusion is enhanced at the surface of small-molecule van der Waals glass-formers. The enhancement given by the ratio Ds/Dv is deduced from the ratio τα/τs, where τα and τs are, respectively, the α-relaxation time of the bulk glass-former and its value at the surface. Here we consider only the coupling model (CM).48 The equations of the CM relating the relaxation times τα, τβ, and τ0 of the αrelaxation, the JG β-relaxation, and the primitive relaxation, respectively, are given by τα = [tc−nτ0]1/(1 − n) ≈ [tc−nτβ ]1/(1 − n)

(1)

Here n is the coupling parameter of the bulk material, which appears in the exponent of the Kohlrausch correlation function of the α-relaxation, φa = exp[−(t/τa)1‑n], and tc for molecular glass-formers including IMC and OTP is 1 to 2 ps. The second part of eq 1 comes from the approximate relation, τ0 ≈ τβ, supported by experiments in many glass-formers,32,50 for example, in OTP at Tg shown in Figure 3. Assuming that intermolecular coupling is totally removed and n = 0 at the surface, it follows from eq 1 that τs becomes the same as τ0 of the CM29,32,50 and approximately the same as the JG βrelaxation time τβ. Putting τs = τ0 ≈ τβ into eq 1, the prediction of the enhancement is given by Dv /Ds = τα /τs = τα /τ0 = (τ0/tc)n /(1 − n) ≈ (τβ /tc)n /(1 − n) (2)

The comparison of this prediction with experiments was made before for IMC.48 With the value of n = 0.41 determined by the dielectric experiments, the CM eqs 1 and 2 give τα(Tg)/τs(Tg) ≈ 105.7, which is favorably comparable to the experimental value of Ds(Tg)/Dv(Tg).13 Here we consider the case of OTP as an example of the molecular glass-formers to verify eq 2. In Figure 3 we show the α and β relaxation times, τα and τβ, of OTP51 and the bulk values of τ0 calculated with n = 0.5 from refs 52 and 53. At Tg 2741

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the range [Tg,β, Tg]. Under this condition, the glass already formed beneath the surface has become so stable that it exerts no influence on the dynamics of the molecules freshly deposited at the surface. Hence the molecules at the free surface are able to take advantage of their enhanced molecular mobility and, in turn, attain equilibrium and continue the process of formation of the SGs. In summary, we have shown that vitrification of the JG βrelaxation determines the lower limit of the substrate temperature required for the formation of SGs by physical vapor deposition. This is rationalized by the fact that the JG βrelaxation time τβ is quantitatively an indicator of the enhancement of surface diffusion of glass-formers. These two major results of this paper confirm in a quantitative manner that the enhanced surface diffusion is critical for the formation of ultrastable glasses. The correlation between the formation of SGs and the β-relaxation or the primitive relaxation could be useful in the design and fabrication of glassy materials with enhanced thermal stabilities. The secondary glass-transition temperature associated with the vitrification of the β-relaxation might have wider implications.

Figure 3. Relaxation times and surface diffusion coefficient of OTP. Green closed circles and diamonds are the α-relaxation times and the primitive relaxation times (calculated). The sold line is the VFT fit. The red closed squares are the β-relaxation times. The dashed line is the fit by the Arrhenius dependence. The larger blue open circles represent −log Ds(T) + C with C = −19.05. The inset shows the temperature-dependent β-relaxation times, and the lone closed star at the corner represents the adiabatic calorimetry β-relaxation time, τβ = 103 s at 133 K. The line is a fit by the Arrhenius dependence.

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the prediction from eqs 1 and 2 for τα(Tg)/τs(Tg) = τα(Tg)/ τ0(Tg) is ∼107.7. In obtaining this estimate, it is assumed that τ0(Tg) at the surface is the same as in the bulk. Because of more free space available at the surface than in the bulk, τ0(Tg) = τs(Tg) at the surface can be shorter. Hence the value of τα(Tg)/ τs(Tg) at the surface can be larger than 107.7, and it is in order of magnitude agreement with the experimental value of Dv(Tg)/ Ds (Tg) = 108. Although τ0(T) is only determined at temperatures at Tg and above, the approximate equality, τ0 ≈ τβ, is verified by extrapolating the Arrhenius T dependence of τβ(T) to higher temperature, as done in Figure 3. Shown also in the inset of Figure 3 are the dielectric τβ(T), the adiabatic calorimetry relaxation time of τβ = 103 s at 133 K,37 and the fit to the Arrhenius T dependence. The CM explanation of surface diffusion enhancement has another prediction from eq 1, which is that the JG β-relaxation time τβ(T) should have about the same temperature dependence as 1/Ds(T). It is substantiated in Figure 3 as well, where the Ds(T) from ref 28 are plotted as −log Ds(T) + C versus 1000/T, and C= −19.05 is chosen to shift the data to near-log τβ(T) for making the comparison. Thus all of the results presented in Figure 3 support the fact that the ratio τα(T)/ τβ(T) can be considered as the quantitative measure of the enhancement of molecule diffusion at the surface. For the formation of SGs by vapor depositions, one empirical rule is that the stability of SGs would be maximized when Tsub is in the range of approximately 0.8 to 0.91Tg for molecule glasses54 (which is material-specific). For Tsub outside this optimal range, vapor depositions would produce less stable glasses with increasingly smaller difference from the OGs in onset temperature, (Tonset,SG − Tonset,OG), and heat capacity, (1 − Cp,SG/Cp,OG). The higher limit of Tsub for the formation of SGs is usually set by the glass-transition Tg because for Tsub ≥ Tg the α-relaxation of the resulting materials would be comparable to the experimental time scales, and the deposition process is, in fact, like quenching the liquid. Regarding the lower limit of Tsub, our results of almost all molecular SGs demonstrate it is set by the secondary glasstransition temperature Tg,β. It is thus clear that for the formation of SGs with higher stability than OGs, Tsub is within

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (K.L.N.). *E-mail: [email protected] (L.-M.W.). *E-mail: [email protected] (H.-B.Y.). ORCID

Hai-Bin Yu: 0000-0003-0645-0187 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS L.-M.W. acknowledges support given by National Basic Research Program of China (973 Program No. 2015CB856805) and National Natural Science Foundation of China (NSFC) (Grant No. 11474247). H.-B.Y. acknowledges the support from Huazhong University of Sciences and Technology of China.

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REFERENCES

(1) Angell, C. A. Formation of Glasses from Liquids and Biopolymers. Science 1995, 267, 1924−1935. (2) Debenedetti, P. G.; Stillinger, F. H. Supercooled liquids and the glass transition. Nature 2001, 410, 259−259. (3) Ediger, M. D.; Harrowell, P. Perspective: Supercooled liquids and glasses. J. Chem. Phys. 2012, 137, 080901. (4) Swallen, S. F.; Kearns, K. L.; Mapes, M. K.; Kim, Y. S.; McMahon, R. J.; Ediger, M. D.; Wu, T.; Yu, L.; Satija, S. Organic glasses with exceptional thermodynamic and kinetic stability. Science 2007, 315, 353−6. (5) Guo, Y.; Morozov, A.; Schneider, D.; Chung, J. W.; Zhang, C.; Waldmann, M.; Yao, N.; Fytas, G.; Arnold, C. B.; Priestley, R. D. Ultrastable nanostructured polymer glasses. Nat. Mater. 2012, 11, 337−43. (6) Singh, S.; Ediger, M. D.; de Pablo, J. J. Ultrastable glasses from in silico vapour deposition. Nat. Mater. 2013, 12, 139−44. (7) Ishii, K.; Nakayama, H.; Hirabayashi, S.; Moriyama, R. Anomalously high-density glass of ethylbenzene prepared by vapor deposition at temperatures close to the glass-transition temperature. Chem. Phys. Lett. 2008, 459, 109−112. (8) Yu, H. B.; Tylinski, M.; Guiseppi-Elie, A.; Ediger, M. D.; Richert, R. Suppression of β Relaxation in Vapor-Deposited Ultrastable Glasses. Phys. Rev. Lett. 2015, 115, 185501. 2742

DOI: 10.1021/acs.jpclett.7b01192 J. Phys. Chem. Lett. 2017, 8, 2739−2744

Letter

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(31) Yu, H. B.; Wang, W. H.; Bai, H. Y.; Samwer, K. The β-relaxation in metallic glasses. Natl. Sci. Rev. 2014, 1, 429−461. (32) Ngai, K. L.; Paluch, M. Classification of secondary relaxation in glass-formers based on dynamic properties. J. Chem. Phys. 2004, 120, 857−73. (33) Ahrenberg, M.; Chua, Y. Z.; Whitaker, K. R.; Huth, H.; Ediger, M. D.; Schick, C. In situ investigation of vapor-deposited glasses of toluene and ethylbenzene via alternating current chip-nanocalorimetry. J. Chem. Phys. 2013, 138, 024501−024501. (34) Chua, Y. Z.; Ahrenberg, M.; Tylinski, M.; Ediger, M. D.; Schick, C. How much time is needed to form a kinetically stable glass? AC calorimetric study of vapor-deposited glasses of ethylcyclohexane. J. Chem. Phys. 2015, 142, 054506. (35) Sepúlveda, A.; Tylinski, M.; Guiseppi-Elie, A.; Richert, R.; Ediger, M. D. Role of Fragility in the Formation of Highly Stable Organic Glasses. Phys. Rev. Lett. 2014, 113, 045901. (36) Tylinski, M.; Sepúlveda, A.; Walters, D. M.; Chua, Y. Z.; Schick, C.; Ediger, M. D. Vapor-deposited glasses of methyl-m-toluate: How uniform is stable glass transformation? J. Chem. Phys. 2015, 143, 244509. (37) Oguni, M.; Sekine, J.-I.; Mine, H. Effects of the presence/ absence of hydroxyl or amine groups in mono/di-substituted benzenes on the α and β structural relaxation properties in liquids and glasses. J. Non-Cryst. Solids 2006, 352, 4665−4671. (38) Fujimori, H.; Oguni, M. Correlation index (Tgα − Tgβ)/Tgα and activation energy ratio as parameters characterizing the structure of liquid and glass. Solid State Commun. 1995, 94, 157−162. (39) Ngai, K. L.; Capaccioli, S.; Prevosto, D.; Wang, L. M. Coupling of Caged Molecule Dynamics to JG beta-Relaxation II: Polymers. J. Phys. Chem. B 2015, 119, 12502−18. (40) Ngai, K. L.; Capaccioli, S.; Prevosto, D.; Wang, L. M. Coupling of Caged Molecule Dynamics to JG beta-Relaxation III: van der Waals Glasses. J. Phys. Chem. B 2015, 119, 12519−25. (41) Capaccioli, S.; Ngai, K. L.; Thayyil, M. S.; Prevosto, D. Coupling of Caged Molecule Dynamics to JG β-Relaxation: I. J. Phys. Chem. B 2015, 119, 8800−8808. (42) Kudlik, A.; Tschirwitz, C.; Benkhof, S.; Blochowicz, T.; Rössler, E. Slow secondary relaxation process in supercooled liquids. Euro. Phys. Lett. 1997, 40, 649. (43) Mandanici, A.; Huang, W.; Cutroni, M.; Richert, R. On the features of the dielectric response of supercooled ethylcyclohexane. Philos. Mag. 2008, 88, 3961−3971. (44) Whitaker, K. R.; Tylinski, M.; Ahrenberg, M.; Schick, C.; Ediger, M. D. Kinetic stability and heat capacity of vapor-deposited glasses of o-terphenyl. J. Chem. Phys. 2015, 143, 084511. (45) Whitaker, K. R.; Scifo, D. J.; Ediger, M. D.; Ahrenberg, M.; Schick, C. Highly Stable Glasses of cis-Decalin and cis/trans-Decalin Mixtures. J. Phys. Chem. B 2013, 117, 12724−12733. (46) Plazanet, M.; Schober, H. Anharmonicity in a fragile glassformer probed by inelastic neutron scattering. Phys. Chem. Chem. Phys. 2008, 10, 5723−9. (47) Stevenson, J. D.; Wolynes, P. G. On the surface of glasses. J. Chem. Phys. 2008, 129, 234514. (48) Capaccioli, S.; Ngai, K. L.; Paluch, M.; Prevosto, D. Mechanism of fast surface self-diffusion of an organic glass. Phys. Rev. E 2012, 86, 051503. (49) Mirigian, S.; Schweizer, K. S. Communication: Slow relaxation, spatial mobility gradients, and vitrification in confined films. J. Chem. Phys. 2014, 141, 161103. (50) Ngai, K. L. Relaxation and Diffusion in Complex Systems; Springer: New York, 2010. (51) Hansen, C.; Stickel, F.; Berger, T.; Richert, R.; Fischer, E. W. Dynamics of glass-forming liquids. III. Comparing the dielectric α- and β-relaxation of 1-propanol and o-terphenyl. J. Chem. Phys. 1997, 107, 1086−1093. (52) Richert, R. On the dielectric susceptibility spectra of supercooled o-terphenyl. J. Chem. Phys. 2005, 123, 154502.

(9) Pérez-Castañeda, T.; Rodríguez-Tinoco, C.; Rodríguez-Viejo, J.; Ramos, M. A. Suppression of tunneling two-level systems in ultrastable glasses of indomethacin. Proc. Natl. Acad. Sci. U. S. A. 2014, 111, 11275−80. (10) Ishii, K.; Nakayama, H. Structural relaxation of vapor-deposited molecular glasses and supercooled liquids. Phys. Chem. Chem. Phys. 2014, 16, 12073−12092. (11) Reid, D.; Lyubimov, I.; Ediger, M.; de Pablo, J. Age and Structure of a Model Vapor-Deposited Glass. Nat. Commun. 2016, 7, 13062. (12) Chen, Z.; Richert, R. Dynamics of glass-forming liquids. XV. Dynamical features of molecular liquids that form ultra-stable glasses by vapor deposition. J. Chem. Phys. 2011, 135, 124515. (13) Kearns, K. L.; Swallen, S. F.; Ediger, M. D.; Wu, T.; Sun, Y.; Yu, L. Hiking down the energy landscape: Progress toward the Kauzmann temperature via vapor deposition. J. Phys. Chem. B 2008, 112, 4934− 4942. (14) Berthier, L.; Ediger, M. D. Facets of glassy physics. Phys. Today 2016, 69, 40. (15) Zhang, Y.; Fakhraai, Z. Invariant Fast Diffusion on the Surfaces of Ultrastable and Aged Molecular Glasses. Phys. Rev. Lett. 2017, 118, 066101. (16) Zhang, W.; Douglas, J. F.; Starr, F. W. Dynamical heterogeneity in a vapor-deposited polymer glass. J. Chem. Phys. 2017, 146, 203310. (17) Jack, R. L.; Berthier, L. The melting of stable glasses is governed by nucleation-and-growth dynamics. J. Chem. Phys. 2016, 144, 244506. (18) Ngai, K. L.; Wang, Z.; Gao, X. Q.; Yu, H. B.; Wang, W. H. A connection between the structural α-relaxation and the β-relaxation found in bulk metallic glass-formers. J. Chem. Phys. 2013, 139, 014502. (19) Kearns, K. L.; Still, T.; Fytas, G.; Ediger, M. D. High-Modulus Organic Glasses Prepared by Physical Vapor Deposition. Adv. Mater. 2010, 22, 39. (20) Liu, M.; Cao, C. R.; Lu, Y. M.; Wang, W. H.; Bai, H. Y. Flexible amorphous metal films with high stability. Appl. Phys. Lett. 2017, 110, 031901. (21) Yu, H.-B.; Luo, Y.; Samwer, K. Ultrastable metallic glass. Adv. Mater. 2013, 25, 5904−5908. (22) Dalal, S. S.; Walters, D. M.; Lyubimov, I.; de Pablo, J. J.; Ediger, M. D. Tunable molecular orientation and elevated thermal stability of vapor-deposited organic semiconductors. Proc. Natl. Acad. Sci. U. S. A. 2015, 112, 4227−32. (23) Gómez, J.; Jiang, J.; Gujral, A.; Huang, C.; Yu, L.; Ediger, M. D. Vapor deposition of a smectic liquid crystal:highly anisotropic, homogeneous glasses with tunable molecular orientation. Soft Matter 2016, 12, 2942−2947. (24) Walters, D. M.; Richert, R.; Ediger, M. D. Thermal stability of vapor-deposited stable glasses of an organic semiconductor. J. Chem. Phys. 2015, 142, 134504. (25) Gujral, A.; Gómez, J.; Jiang, J.; Huang, C.; O’Hara, K. A.; Toney, M. F.; Chabinyc, M. L.; Yu, L.; Ediger, M. D. Highly Organized Smectic-like Packing in Vapor-Deposited Glasses of a Liquid Crystal. Chem. Mater. 2017, 29, 849−858. (26) Rodríguez-Tinoco, C.; Gonzalez-Silveira, M.; Ràfols-Ribé, J.; Garcia, G.; Rodríguez-Viejo, J. Highly stable glasses of celecoxib: Influence on thermo-kinetic properties, microstructure and response towards crystal growth. J. Non-Cryst. Solids 2015, 407, 256−261. (27) Zhu, L.; Brian, C. W.; Swallen, S. F.; Straus, P. T.; Ediger, M. D.; Yu, L. Surface Self-Diffusion of an Organic Glass. Phys. Rev. Lett. 2011, 106, 256103. (28) Zhang, W.; Brian, C. W.; Yu, L. Fast Surface Diffusion of Amorphous o-Terphenyl and Its Competition with Viscous Flow in Surface Evolution. J. Phys. Chem. B 2015, 119, 5071−5078. (29) Ngai, K. L. Interpreting the dynamics of nano-confined glassformers and thin polymer films: Importance of starting from a viable theory for the bulk. J. Polym. Sci., Part B: Polym. Phys. 2006, 44, 2980− 2995. (30) Johari, G. P.; Goldstein, M. Viscous Liquids and the Glass Transition. II. Secondary Relaxations in Glasses of Rigid Molecules. J. Chem. Phys. 1970, 53, 2372−2372. 2743

DOI: 10.1021/acs.jpclett.7b01192 J. Phys. Chem. Lett. 2017, 8, 2739−2744

Letter

The Journal of Physical Chemistry Letters (53) León, C.; Ngai, K. L. Rapidity of the Change of the Kohlrausch Exponent of the α-Relaxation of Glass-Forming Liquids at TB or Tβ and Consequences. J. Phys. Chem. B 1999, 103, 4045−4051. (54) Kearns, K. L.; Swallen, S. F.; Ediger, M. D.; Wu, T.; Yu, L. Influence of substrate temperature on the stability of glasses prepared by vapor deposition. J. Chem. Phys. 2007, 127, 154702.

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