Expanding our vision of glass: Physical vapor deposition prepares ultrastable and anisotropic materials

Glasses are non-equilibrium materials and a vast array of amorphous structures are possible in principle.  However, glasses are generally regarded as disordered and the idea of “controlling” molecular packing in glasses is reasonably met with skepticism. We have shown that physical vapor deposition (PVD) produces glasses with properties that cannot be achieved by other preparation routes, including high stability and controlled anisotropy.   In addition, the low temperature (< 3 K) heat capacity of ultrastable glasses follows the Debye law typical for crystals, in contrast to the behavior reported for liquid-cooled glasses.  The exotic properties of PVD glasses can be explained by a surface equilibration mechanism: mobility near the free surface allows substantial equilibration during deposition, even well below the conventional glass transition temperature. Our work with organic glasses has now been extended to show aspects of ultrastability in polymeric, metallic, and chalcogenide glasses.
 The major application of organic glasses is the production of commercial OLEDs, where PVD is utilized to prepare glassy active layers of organic semiconductors.  It has recently been shown that ultrastable glasses of organic semiconductors make longer-lasting OLEDs.  In addition, the surface equilibration mechanism can be used to control the orientation of emitter molecules, to produce more efficient OLEDs. OLED production often utilizes co-deposition.  We have recently shown that co-deposition of two organic semiconductors can produce a homogeneous ultrastable glass but, in other systems, component separation occurs in the plane of the sample, on a controllable length scale.