Exploring the Importance of Surface Diffusion in Stability of Vapor-Deposited Organic Glasses

Samanta, Subarna; Huang, Georgia; Gao, Gui; Zhang, Yue; Zhang, Aixi; Wolf, Sarah; Woods, Connor; Jin, Yi; Walsh, Patrick J.; Fakhraai, Zahra

doi:10.1021/acs.jpcb.9b01012

Cited by 23 publications

(27 citation statements)

References 86 publications

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“…It is understood that SG formation is due to enhanced surface mobility (5,8,12,(14)(15)(16)(17). When T dep < Tg , at slow deposition rates (∼0.2 nm/s for most organic glasses), the surface region has sufficient mobility (18) to reach more energetically favored states before being buried into dynamically arrested states.…”

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confidence: 99%

Polyamorphism of vapor-deposited amorphous selenium in response to light

Zhang

Jin

Liu

et al. 2020

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

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Enhanced surface mobility is critical in producing stable glasses during physical vapor deposition. In amorphous selenium (a-Se) both the structure and dynamics of the surface can be altered when exposed to above-bandgap light. Here we investigate the effect of light on the properties of vapor-deposited a-Se glasses at a range of substrate temperatures and deposition rates. We demonstrate that deposition both under white light illumination and in the dark results in thermally and kinetically stable glasses. Compared to glasses deposited in the dark, stable a-Se glasses formed under white light have reduced thermal stability, as measured by lower density change, but show significantly improved kinetic stability, measured as higher onset temperature for transformation. While light induces enhanced mobility that penetrates deep into the surface, resulting in lower density during vapor deposition, it also acts to form more networked structures at the surface, which results in a state that is kinetically more stable with larger optical birefringence. We demonstrate that the structure formed during deposition with light is a state that is not accessible through liquid quenching, aging, or vapor deposition in the dark, indicating the formation of a unique amorphous solid state.

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confidence: 99%

Polyamorphism of vapor-deposited amorphous selenium in response to light

Zhang

Jin

Liu

et al. 2020

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

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“…Developing methods for the deposition of organic thin film has been an important research topic in the material sciences fields for a long time [ 4 , 5 , 6 , 7 , 8 ]. One of many urgent issues regarding organic thin film is to achieve high quality—in terms of surface smoothness—that is largely hindered by the uncontrolled nucleation of target molecules or the formation of irregular aggregates [ 7 , 9 , 10 , 11 , 12 ]. Therefore, inhibiting abrupt nucleation and suppressing aggregates during film formation are two reasonable pathways for the formation of highly smooth films.…”

Section: Introductionmentioning

confidence: 99%

“…The most conventional and standard method for the preparation of organic film is to deposit organic vapors under a high-vacuum environment (referred to as the ‘high-vacuum method’) [ 7 , 10 , 16 , 17 , 18 , 19 ]. A low-vacuum method, represented by physical vapor deposition (PVD) is an alternative [ 20 , 21 , 22 , 23 ].…”

Section: Introductionmentioning

confidence: 99%

Ultrasmooth Organic Films Via Efficient Aggregation Suppression by a Low-Vacuum Physical Vapor Deposition

Yoon

Lee

et al. 2021

Materials

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Organic thin films with smooth surfaces are mandated for high-performance organic electronic devices. Abrupt nucleation and aggregation during film formation are two main factors that forbid smooth surfaces. Here, we report a simple fast cooling (FC) adapted physical vapor deposition (FCPVD) method to produce ultrasmooth organic thin films through effectively suppressing the aggregation of adsorbed molecules. We have found that thermal energy control is essential for the spread of molecules on a substrate by diffusion and it prohibits the unwanted nucleation of adsorbed molecules. FCPVD is employed for cooling the horizontal tube-type organic vapor deposition setup to effectively remove thermal energy applied to adsorbed molecules on a substrate. The organic thin films prepared using the FCPVD method have remarkably ultrasmooth surfaces with less than 0.4 nm root mean square (RMS) roughness on various substrates, even in a low vacuum, which is highly comparable to the ones prepared using conventional high-vacuum deposition methods. Our results provide a deeper understanding of the role of thermal energy employed to substrates during organic film growth using the PVD process and pave the way for cost-effective and high-performance organic devices.

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“…As such, the degree of enhanced surface mobility and mobility gradients are critical factors in the formation of stable glasses (3,11,17,18). While the effect of film thickness on the surface mobility and gradients of liquid-quenched (LQ) glasses has been studied in the past (19,20), there are limited data on the role of film thickness in the stability of vapor-deposited glasses.…”

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confidence: 99%

“…During PVD, stable glasses are formed because of the enhanced mobility at the free surface (3,11,36,46) and layers directly underneath (13,17). Surface mobility gradients allow molecules in the surface region to adopt more efficient packing structures, before falling out of equilibrium at locations deeper into the film.…”

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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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Exploring the Importance of Surface Diffusion in Stability of Vapor-Deposited Organic Glasses

Cited by 23 publications

References 86 publications

Polyamorphism of vapor-deposited amorphous selenium in response to light

Polyamorphism of vapor-deposited amorphous selenium in response to light

Ultrasmooth Organic Films Via Efficient Aggregation Suppression by a Low-Vacuum Physical Vapor Deposition

Glasses denser than the supercooled liquid

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