Investigating The Effect Of Chemical Structure On The Physical Properties Of Vapor-Deposited Organic And Inorganic Glasses

The existence of nanometer-sized amorphous material (glasses) is ubiquitous and plays a crucial role in the rapid growing technology community. Physical vapor deposition is commonly used to produce nano-scaled glasses, with the assumption that thin film properties will resemble those of the bulk. However, intermolecular forces, air-material interface and deposition conditions can all affect the behavior of thin glassy films. This thesis presents experimental findings and discussions on 50 nm to 3 um thick vapor-deposited glasses, which aids the understanding of fundamental properties and future design for various applications. Chapter 2 and 3 introduce a chemoselective approach to synthesize the tailored molecular glassformers. With these molecules, physical properties of vapor-deposited glasses are studied via spectroscopic ellipsometry. Using cooling rate-dependent Tg measurement, relaxation dynamics near Tg is probed to evaluate the activation barrier for rearrangement. A simple linear relationship does not hold between molecular weight and various physical properties. Surface-mediated equilibration (SME) allows vapor-deposited glasses to overcome kinetic barriers and achieve low-energy, high density and kinetically stable states. Many properties of these stable glasses resemble those of liquid-quenched glasses aged over long periods of time. Chapter 4 systematically studies the correlation between chemical structure and enhanced stability in vapor-deposited organic glasses. The frequently observed optical birefringence in SME glasses often implies molecular alignment, analogous to those observed in liquid crystals. Chapter 5 demonstrates that birefringence can be observed in an SME glass system with isotropically oriented molecules. This result implies the accessibility to low-energy liquid states using physical vapor deposition. As such, the robust nature of stable glass formation is established and helps elucidate the mechanism of SME processes. The relaxation dynamics in supported polymer and organic molecular films transition from liquid-like to glassy behavior for film thickness greater than 30nm. Chapter 6 uses cooling rate-dependent Tg measurements to demonstrate that an inorganic network glass, amorphous selenium, exhibits the same transition, but at a much larger thickness. This observation suggests a much longer-range correlated dynamics in amorphous selenium than seen before. Chapter 7 extends the SME stable glass formation to selenium. Preliminary findings here suggest the accessibility to the stable state of amorphous selenium via vapor deposition. These results can help elucidate the origin of enhanced dynamics in thin glasses, and also broaden the existing literatures on correlated dynamics in amorphous thin films