Glasses are out of equilibrium materials. They are typically produced by quenching a supercooled liquid to a temperature where it can no longer maintain its equilibrium properties and is kinetically arrested. The pathway by which a glass is produced can define the specific state of the glass and its physical properties. In this project, we produced highly stable glasses through physical vapor deposition (PVD). In this process, the enhanced mobility of the liquid at its surface is used to trap glasses at low energy states that are not accessible through liquid-quenching. As such, uniquely stable, high-density glasses can be produced with interesting physical properties.

While the enhanced mobility at the surface allows for surface-mediated equilibration, the structure of the liquid at the surface can also affect the structure of the resulting glass. For example, in computer simulations we have demonstrated how an organic molecule with a fluorinated tail can produce networked structures due to the trapping of the tails in fluorine-rich domains. This provides a unique opportunity to study structure-property relationships in vapor-deposited glasses and engineer glasses with desired properties.

Intellectual Merit. In this project, we combined chemical design, high-throughput experiments, and coarse-grained computer simulations to understand structure-property relationships in stable vapor-deposited glasses. Specifically, we investigated:

• The role of inter-molecular interactions in glass properties by introducing fluorinated tails of various lengths in the structure and thermodynamic stability of molecules. We demonstrated that increasing interactions could lead to a slow-down of enhanced mobility, resulting in networked structures that are thermally less stable.
• The role of substrate temperature and deposition rate in the orientation of molecules, templated by the free surface, as well as layering structures that make stable glasses optically birefringent.
• The effect of intra-molecular degrees of freedom in the structural entropy of the liquid and its ability to equilibrate rapidly at the free surface. We demonstrated that increasing rotational degrees of freedom leads to increased stability and reduced birefringence.
• The effect of film thickness and substrate interactions on the stability of PVD glasses. In particular, we demonstrated that exceptionally dense glasses can be produced in the thickness range of 30-50 nm, with packings that correspond to a uniquely high-density liquid state that has not been previously observed.

To achieve these goals we developed new synthesis that enables modular design of homologous series of molecules, designed and custom built an ultra-high vacuum chamber with high-throughput temperature-gradient stage and in-situ characterization capabilities, and developed new modeling tools to predict properties of vapor-deposited glasses.

Broader Impacts. Understanding structure-property relationships in molecular glasses is helpful in designing new materials for various applications. For example, it has been known that thin molecular glass films used for photoresist or thin film electronics are prone to dewetting if their thickness is below 30 nm. Our studies highlight that in the proper range of substrate deposition and rates, these films can be made into stable layers that can resist dewetting. A fundamental understanding of such properties can have impact in various fields such as coatings, organic electronics, and lithography.  Our studies also elucidate fundamental properties of glasses by producing super-cooled liquid states at low temperatures, were it was previously impossible to access.

We have started a new database for molecular glass materials, which includes the measured properties of glass molecules synthesized for this project. We aim to broaden this database by adding data obtained in other research groups. All of this data is made available online, which facilitates big-data analysis of the structure-property relationships obtained in this project.

Graduate and undergraduate education was an integrated part of this study. The starting material for the synthesis of all of our molecules, tri halide benzene, is synthesized in an undergraduate organic lab, by undergraduate students. This material is then collected, purified, and used for synthesizing organic molecules. Both graduate and undergraduate students were involved in all aspects of the project, including synthesis, method development, simulations, and characterization.

Last Modified: 08/30/2020
Modified by: Zahra Fakhraai