Kinetic arrest of front transformation to gain access to the bulk glass transition in ultrathin films of vapour-deposited glasses

Ràfols‐Ribé, Joan; Vila-Costa, Ana; Rodríguez-Tinoco, Cristian; Lopeandía, A. F.; Rodríguez‐Viejo, J.; González-Silveira, Marta

doi:10.1039/c8cp06264a

Cited by 21 publications

(14 citation statements)

References 34 publications

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“…From that moment, many other techniques have been used to identify and investigate the dynamics of the liquid front. In all cases, results were consistent, showing as solid observations the non-dependence of front velocity with film thickness, for films thinner than the cross-over length, and a strong dependence of this velocity with temperature [30,99,106,117,173,175,[179][180][181]]. An empirical relation between velocity and temperature was suggested, where the relaxation time of the equilibrated super-cooled liquid is invoked to introduce the temperature dependence of the propagation front [120]:…”

Section: Front Meltingsupporting

confidence: 61%

“…Most of the work on ultrastable glasses has involved molecular glasses. Since the first measurements in 2007 [1] with 1,3-bis-(1-naphthyl)-5-(2-naphthyl)benzene (TNB) (T g = 347 K) and indomethacin (IMC) (T g = 315 K), more than 45 different organic molecules, ranging from small molecules such as toluene [26][27][28] and ethylbenzene [28] to pharmaceuticals such as IMC and TNB and more recently to organic semiconductors like TPD [29,30], NPD [31,32] and TPBi [2], have shown their ability to form highly stable glasses upon growth by physical vapor deposition at the right processing conditions. Table 1 shows a comprehensive list of molecules and some of the outstanding properties of the vapor-deposited thin film ultrastable glasses obtained from them.…”

Section: Organic Glassesmentioning

confidence: 99%

“…This limitation can be overcome by arresting the mobile regions that would initiate the liquid front. By capping the stable film with a layer of an organic molecule with higher T g , one can inhibit the formation of the front [30,175] at the interfaces forcing the stable glass to transform through a bulk process. In a recent study from Vila-Costa et al [176], capped ultrastable thin films were isothermally annealed at temperatures above the glass transition temperature and their transformation kinetics extracted from the calorimetric trace of partially transformed glasses.…”

Section: Bulk Meltingmentioning

confidence: 99%

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Ultrastable glasses: new perspectives for an old problem

et al. 2022

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Ultrastable glasses (mostly prepared from the vapor phase under optimized deposition conditions) represent a unique class of materials with low enthalpies and high kinetic stabilities. These highly stable and dense glasses show unique physicochemical properties, such as high thermal stability, improved mechanical properties or anomalous transitions into the supercooled liquid, offering unprecedented opportunities to understand many aspects of the glassy state. Their improved properties with respect to liquid-cooled glasses also open new prospects to their use in applications where liquid-cooled glasses failed or where not considered as usable materials. In this review article we summarize the state of the art of vapor-deposited (and other) ultrastable glasses with a focus on the mechanism of equilibration, the transformation to the liquid state and the low temperature properties. The review contains information on organic, metallic, polymeric and chalcogenide glasses and an updated list with relevant properties of all materials known today to form a stable glass.

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Section: Front Meltingsupporting

confidence: 61%

Section: Organic Glassesmentioning

confidence: 99%

Section: Bulk Meltingmentioning

confidence: 99%

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Ultrastable glasses: new perspectives for an old problem

et al. 2022

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“…We explain this behavior by faster crystal nucleation and growth at the free surface, aided by the higher mobility of surface molecules with respect to bulk molecules . Additionally, covering the bare surface of the emission layers may suppress the propagation of mobility fronts as has been observed for similar structures …”

Section: Methodsmentioning

confidence: 52%

Long-Term Stability of Emitter Orientation in Organic Light-Emitting Diodes at Temperatures in the Range of the Active Layer Glass Transition

2022

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Controlling the alignment of emitter molecules in the active layer of organic light-emitting diodes has become a main approach to maximize the device efficiency when emitter molecules with luminescence quantum yields approaching 100% are used. In order to guarantee stable device performance, the initial molecular orientation should not change over time. In this work, we study this property for a time frame of 1.5 years and storage temperatures up to 80 °C which may be reached in displays exposed to direct sun light. For the studied material systems, this temperature is close to the glass transition at which drastic morphological changes occur and a randomization of the molecule arrangement is expected. We compare two different phosphorescent emitter molecules and, additionally, investigate the impact of the substrate temperature during evaporation. Concluding this long-term study, we prove experimentally that the emitter orientation remains unchanged under those device-critical storage conditions. On the contrary, the fatal potential of heat-induced reorientation is revealed by post-annealing experiments that show a strong change of the emitter orientation at about 20 K above the glass transition temperature.

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“…In vapor-deposited toluene, it has been shown that decreasing the film thickness from 70 to 5 nm can increase the thermodynamic stability but decrease the apparent kinetic stability (5,6). In contrast, thin films covered with a top layer of another material do not show a significant evidence of reduced kinetic stability (21), indicating the nontrivial role of mobility gradients in thermal and kinetic stability.…”

mentioning

confidence: 99%

Glasses denser than the supercooled liquid

Jin

Zhang

Wolf

et al. 2021

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

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When aged below the glass transition temperature, Tg, the density of a glass cannot exceed that of the metastable supercooled liquid (SCL) state, unless crystals are nucleated. The only exception is when another polyamorphic SCL state exists, with a density higher than that of the ordinary SCL. Experimentally, such polyamorphic states and their corresponding liquid–liquid phase transitions have only been observed in network-forming systems or those with polymorphic crystalline states. In otherwise simple liquids, such phase transitions have not been observed, either in aged or vapor-deposited stable glasses, even near the Kauzmann temperature. Here, we report that the density of thin vapor-deposited films of N,N′-bis(3-methylphenyl)-N,N′-diphenylbenzidine (TPD) can exceed their corresponding SCL density by as much as 3.5% and can even exceed the crystal density under certain deposition conditions. We identify a previously unidentified high-density supercooled liquid (HD-SCL) phase with a liquid–liquid phase transition temperature (TLL) ∼35 K below the nominal glass transition temperature of the ordinary SCL. The HD-SCL state is observed in glasses deposited in the thickness range of 25 to 55 nm, where thin films of the ordinary SCL have exceptionally enhanced surface mobility with large mobility gradients. The enhanced mobility enables vapor-deposited thin films to overcome kinetic barriers for relaxation and access the HD-SCL state. The HD-SCL state is only thermodynamically favored in thin films and transforms rapidly to the ordinary SCL when the vapor deposition is continued to form films with thicknesses more than 60 nm.

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Kinetic arrest of front transformation to gain access to the bulk glass transition in ultrathin films of vapour-deposited glasses

Abstract: Capping as an strategy to improve thermal stability of ultrastable glasses.

Cited by 21 publications

References 34 publications

Ultrastable glasses: new perspectives for an old problem

Ultrastable glasses: new perspectives for an old problem

Long-Term Stability of Emitter Orientation in Organic Light-Emitting Diodes at Temperatures in the Range of the Active Layer Glass Transition

Glasses denser than the supercooled liquid

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