Communication: Slow relaxation, spatial mobility gradients, and vitrification in confined films

Mirigian, Stephen; Schweizer, Kenneth S.

doi:10.1063/1.4900507

Cited by 86 publications

(136 citation statements)

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“…Models based on a constant length scale of interfacial effects [37,38] may not fully capture the strong, almost sigmoidal transition in apparent activation barriers observed here. Instead, models based on long range elastic response [26], or which use growing cooperative length scales [27,28] may be able to predict such strong correlated dynamics.…”

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confidence: 99%

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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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confidence: 99%

“…As such, it is imperative to study the extent and the length scales of the effect of enhanced surface mobility on the dynamics of ultra-thin films. Furthermore, these length scales may also be directly compared with fundamental length scales of glass transitions as proposed by various theories [25][26][27][28].…”

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confidence: 99%

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

Zhang

Glor

et al. 2016

The Journal of Chemical Physics

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“…Elastically cooperative activated barrier hopping model developed by Mirigian and Schweizer [41,42] predicts surface diffusion as D s = d 2 /(6 τ α ), where d is the molecular size and τ α is the average dynamics of the near-surface layer at different penetration depths, either half or one molecular size [42]. Thus in this model D s is only associated with the near-surface relaxation dynamics as opposed to bulk τ α that strongly vary between stable, aged, and liquid-quenched glasses.…”

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confidence: 99%

Invariant Fast Diffusion on the Surfaces of Ultrastable and Aged Molecular Glasses

Zhang

Fakhraai

2017

Phys. Rev. Lett.

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“…Previous experiments and simulations using confinement found, depending on the conditions, mostly an increase, but sometimes a decrease of the viscosity with the system size [45][46][47][48][49][50][51][52][53][54][55][56] . However previous finite systems simulations [22][23][24][25][26][27][28][29][30][31][32] found to our knowledge always an increase of the viscosity together with a decrease of the cooperativity when the system size decreases.…”

Section: Introductionmentioning

confidence: 99%

“…In this work we study finite size effects using molecular dynamics simulations of a simple molecular liquid. We use finite size simulations [22][23][24][25][26][27][28][29][30][31][32] instead of confinement [45][46][47][48][49][50][51][52][53][54][55][56] because finite size simulations have the advantage over confinement to cut off the cooperativity without introducing any confining wall nor modifying the symmetry or the dimensionality of the system, as long as periodic boundary conditions are used. The simplicity of the molecule permits us to access large time scales with aging times larger than the micro-second.…”

Section: Introductionmentioning

confidence: 99%

Enhanced diffusion in finite-size simulations of a fragile diatomic glass former

et al. 2016

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Using molecular dynamics simulations we investigate the finite size dependence of the dynamical properties of a diatomic supercooled liquid. The simplicity of the molecule permits us to access the microsecond time scale. We find that the relaxation time decreases simultaneously with the strength of cooperative motions when the size of the system decreases. While the decrease of the cooperative motions is in agreement with previous studies, the decrease of the relaxation time opposes what has been reported to date in monatomic glass-formers and in silica. This result suggests the presence of different competing physical mechanisms in the relaxation process. For very small box sizes the relaxation times behavior reverses itself and increases strongly when the box size decreases, thus leading to a non-monotonic behavior. This result is in qualitative agreement with defect and facilitation theories.

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Communication: Slow relaxation, spatial mobility gradients, and vitrification in confined films

Cited by 86 publications

References 50 publications

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

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

Invariant Fast Diffusion on the Surfaces of Ultrastable and Aged Molecular Glasses

Enhanced diffusion in finite-size simulations of a fragile diatomic glass former

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