Surface Self-Diffusion of Organic Glasses

Brian, Caleb W.; Yu, Lian

doi:10.1021/jp404944s

Cited by 113 publications

(143 citation statements)

References 48 publications

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“…In fact, the existence of highly mobile regions in the vicinity of free surfaces with diffusivity values as high as 10 6 times the bulk diffusivity has been demonstrated in organic glasses at temperatures near the glass transition temperature (Tg). 5 Highly stable glasses are located much lower in the potential energy landscape than any glass obtained by cooling the liquid in human time scales. 2 Their exceptional stability impacts many of the striking properties of these glasses, among them the differentiated mechanism by which these glasses transform into the liquid.…”

Section: ■ Introductionmentioning

confidence: 99%

Evaluation of Growth Front Velocity in Ultrastable Glasses of Indomethacin over a Wide Temperature Interval

et al. 2014

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Ultrastable thin film glasses transform into supercooled liquid via propagating fronts starting from the surface and/or interfaces. In this paper, we analyze the consequences of this mechanism in the interpretation of specific heat curves of ultrastable glasses of indomethacin for samples with varying thickness from 20 nm up to several microns. We demonstrate that ultrastable films above 20 nm have identical fictive temperatures and that the apparent change of onset temperature in the specific heat curves originates from the mechanism of transformation and the normalization procedure. An ad hoc surface normalization of the heat capacity yields curves which collapse into a single one irrespective of their thickness. Furthermore, we fit the surface-normalized specific heat curves with a heterogeneous transformation model to evaluate the velocity of the growth front over a much wider temperature interval than previously reported. Our data expands previous values up to Tg + 75 K, covering 12 orders of magnitude in relaxation times. The results are consistent with preceding experimental and theoretical studies. Interestingly, the mobility of the supercooled liquid in the region behind the transformation front remains constant throughout the thickness of the layers.

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Section: ■ Introductionmentioning

confidence: 99%

Evaluation of Growth Front Velocity in Ultrastable Glasses of Indomethacin over a Wide Temperature Interval

et al. 2014

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“…The properties of stable glasses are explained by the high mobility of the free surface during the vapor deposition process (20,25). Because of lowered constraints to motion (26), molecules near the free surface can adopt near-equilibrium packing arrangements during deposition even at temperatures where the bulk structural relaxation time is thousands of years (21,27). Subsequent deposition traps this efficient packing into the bulk solid.…”

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confidence: 99%

“…At lower values of T substrate /T g , there is a thermodynamic driving force to form the equilibrium SCL at that temperature resulting in higher stability, higher density materials. The mobility of the surface enables molecules to find these tight packing arrangements (20,25,27), even though this would require a prohibitively long time for a bulk material. T onset /T g and Δρ are maximized when high surface mobility is paired with a large thermodynamic driving force.…”

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confidence: 99%

Tunable molecular orientation and elevated thermal stability of vapor-deposited organic semiconductors

Dalal

Walters

Lyubimov

et al. 2015

Proc. Natl. Acad. Sci. U.S.A.

202

351

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Physical vapor deposition is commonly used to prepare organic glasses that serve as the active layers in light-emitting diodes, photovoltaics, and other devices. Recent work has shown that orienting the molecules in such organic semiconductors can significantly enhance device performance. We apply a high-throughput characterization scheme to investigate the effect of the substrate temperature (T substrate ) on glasses of three organic molecules used as semiconductors. The optical and material properties are evaluated with spectroscopic ellipsometry. We find that molecular orientation in these glasses is continuously tunable and controlled by T substrate /T g , where T g is the glass transition temperature. All three molecules can produce highly anisotropic glasses; the dependence of molecular orientation upon substrate temperature is remarkably similar and nearly independent of molecular length. All three compounds form "stable glasses" with high density and thermal stability, and have properties similar to stable glasses prepared from model glass formers. Simulations reproduce the experimental trends and explain molecular orientation in the deposited glasses in terms of the surface properties of the equilibrium liquid. By showing that organic semiconductors form stable glasses, these results provide an avenue for systematic performance optimization of active layers in organic electronics.glasses | organic semiconductors | molecular orientation | physical vapor deposition | high-throughput characterization

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“…Surface diffusion coefficients D S (T) of several molecular glassformers including indomethacin (IMC), nifedipine (NIF), and ortho-terphenyl (OTP), [1][2][3] and a canonical metallic glass, Pd 40 Cu 30 Ni 10 P 20 (Pd 40 ) 4 were measured at temperatures slightly above and mostly below the bulk glass transition temperature T g using the method of surface-grating decay. The surface diffusion coefficients D S (T) are found to be many orders of magnitude larger than the bulk diffusion coefficients D V (T) at the same temperature.…”

Section: Introductionmentioning

confidence: 99%

Why is surface diffusion the same in ultrastable, ordinary, aged, and ultrathin molecular glasses?

Ngai

Paluch

Rodríguez-Tinoco

2017

Phys. Chem. Chem. Phys.

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Recently Fakhraai and coworkers measured surface diffusion in ultrastable glass produced by vapor deposition, ordinary glass with and without physical aging, and ultrathin films of the same molecular glassformer, N,N 0 -bis(3-methylphenyl)-N,N 0 -diphenylbenzidine (TPD). Diffusion on the surfaces of all these glasses is greatly enhanced compared with the bulk diffusion similar to that previously found by others, but remarkably the surface diffusion coefficients D S measured are practically the same. The observed independence of D S from changes of structural a-relaxation due to densification or finite-size effect has an impact on the current understanding of the physical origin of enhanced surface diffusion. We have demonstrated before and also here that the primitive relaxation time t 0 of the coupling model, or its analogue t b , the Johari-Goldstein b-relaxation, can explain quantitatively the enhancement found in ordinary glasses. In this paper, we assemble together considerable experimental evidence to show that the changes in t b and t 0 of ultrastable glasses, aged ordinary glasses, and ultrathin-films are all insignificant when compared with ordinary glasses. Thus, in the context of the explanation of the enhanced surface diffusion given by the coupling model, these collective experimental facts on t b and t 0 further explain approximately the same D S in the different glasses of TPD as found by Fakhraai and coworkers.

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Surface Self-Diffusion of Organic Glasses

Cited by 113 publications

References 48 publications

Evaluation of Growth Front Velocity in Ultrastable Glasses of Indomethacin over a Wide Temperature Interval

Evaluation of Growth Front Velocity in Ultrastable Glasses of Indomethacin over a Wide Temperature Interval

Tunable molecular orientation and elevated thermal stability of vapor-deposited organic semiconductors

Why is surface diffusion the same in ultrastable, ordinary, aged, and ultrathin molecular glasses?

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