Fast Surface Diffusion of Amorphous <i>o</i>-Terphenyl and Its Competition with Viscous Flow in Surface Evolution

Zhang, Wei; Brian, Caleb W.; Yu, Lian

doi:10.1021/jp5127464

Cited by 83 publications

(122 citation statements)

References 44 publications

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“…−20 m 2 /s [32]. This simple estimation implies that the average dynamics in 8 nm films, are several orders of magnitude faster than the bulk dynamics if measured at T g .…”

mentioning

confidence: 83%

Long-range correlated dynamics in ultra-thin molecular glass films

Zhang

Glor

et al. 2016

The Journal of Chemical Physics

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Physical vapor deposition (PVD) is widely used in manufacturing ultra-thin layers of amorphous organic solids. Here, we demonstrate that these films exhibit a sharp transition from glassy solid to liquid-like behavior with thickness below 30 nm. This liquid-like behavior persists even at temperatures well below the glass transition temperature, Tg. The enhanced dynamics in these films can produce large scale morphological features during PVD and lead to a dewetting instability in films held at temperatures as low as Tg-35 K. We measure the effective viscosity of organic glass films by monitoring the dewetting kinetics. These measurements combined with cooling rate-dependent Tg measurements show that the apparent activation barrier for rearrangement decreases sharply in films thinner than 30 nm. These observations suggest long-range facilitation of dynamics induced by the free surface, with dramatic effects on the properties of nano-scale amorphous materials.Nanometer-sized thin films of small organic molecules are widely used in applications ranging from organic photovoltaics[1] and light emitting diodes [2,3], to protective coatings [4] and high resolution nano-imprint lithography [5]. It is advantageous to use amorphous films because, compared to crystals, they do not have grain boundaries to hinder charge transport, generate cracks and defects, or disrupt the writing processes. Physical vapor deposition (PVD), the common method used to manufacture these films, is usually performed at substrate temperatures below T g to produce films in the glassy state. However, if the properties at nanoscale deviate significantly from bulk properties, the resulting films can have reduced kinetic and thermal stability. Recent experiments suggest that diffusion at the free surface of organic glasses can be several orders of magnitude faster [6,7], with weaker temperature dependence compared to bulk diffusion. Enhanced, weakly temperature-dependent dynamics on the surface of polymeric glasses [8,9] have been shown to significantly affect the properties of ultra-thin polymer films [9][10][11][12][13][14][15][16][17]. In polymeric systems, the molecular weight of the polymer [14], and the temperature range of the measurement [8,9,14] seem to also affect the observed properties, resulting in ambiguity in the relationship between enhanced dynamics at the free surface and properties of ultra-thin glass films. As such, these results can not be extrapolated to molecular and atomic glass systems.

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“…−20 m 2 /s [32]. This simple estimation implies that the average dynamics in 8 nm films, are several orders of magnitude faster than the bulk dynamics if measured at T g .…”

mentioning

confidence: 83%

Long-range correlated dynamics in ultra-thin molecular glass films

Zhang

Glor

et al. 2016

The Journal of Chemical Physics

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“…1 taken from the paper of Zhang et al 2 Deeper into the glassy state, the shifted D S (T) and t b (T) are within half a decade apart and can be considered the same if the uncertainty of t b (T) determined from the broad frequency dispersion of the JG b-relaxation is taken into consideration. Thus eqn (8) and (9) for T r T g in OG of OTP is verified.…”

Section: -430 If D S (T)/d V (T) Is the Same As The Ratio T A (T)/tmentioning

confidence: 99%

“…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%

“…The size of the enhancement of D S over D V depends on the glass-former, as shown by nearly the two orders of magnitude greater enhancement in the case of OTP than that in IMC. In all these previous studies [1][2][3][4] at temperatures below T g the glasses were formed ordinarily by rate cooling or liquid quenching, and are referred to as ordinary glass (OG) to distinguish from a new class of ultrastable glasses (SG) produced by physical vapor deposition. Several theoretical models were proposed to address this huge enhancement of surface mobility of small molecule OGs [5][6][7] and metallic glass.…”

Section: Introductionmentioning

confidence: 99%

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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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“…9,13,14 Such behaviour has also been detected in small-molecule glasses. [15][16][17][18][19][20] An efficient and versatile method to probe the aforementioned attributes is the capillary-driven relaxation of an imposed topography on the film surface. In this context, a recent experimental approach addressed the viscocapillary leveling of polystyrene stepped films.…”

Section: Introductionmentioning

confidence: 99%

Molecular dynamics simulation of the capillary leveling of viscoelastic polymer films

Tanis

Meyer

Salez

et al. 2017

The Journal of Chemical Physics

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Abstract. Surface tension-driven flow techniques have recently emerged as an efficient means of shedding light into the rheology of thin polymer films. Motivated by experimental and theoretical approaches in films bearing a varying surface topography, we present results on the capillary relaxation of a square pattern at the free surface of a viscoelastic polymer film, using molecular dynamics simulations of a coarse-grained polymer model. Height profiles are monitored as a function of time after heating the system above its glass-transition temperature and their time dependence is fitted to the theory of capillary leveling. Results show that the viscosity is not constant, but time dependent. In addition to providing a complementary insight about the local inner mechanisms, our simulations of the capillary-leveling process therefore probe the viscoelasticity of the polymer and not only its viscosity, in contrast to previous experimental approaches.

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Fast Surface Diffusion of Amorphous o-Terphenyl and Its Competition with Viscous Flow in Surface Evolution

Cited by 83 publications

References 44 publications

Long-range correlated dynamics in ultra-thin molecular glass films

Long-range correlated dynamics in ultra-thin molecular glass films

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

Molecular dynamics simulation of the capillary leveling of viscoelastic polymer films

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