Generic packing motifs in vapor-deposited glasses of organic semiconductors.

We study the structure of vapor-deposited glasses of five common organic semiconductors as a function of substrate temperature during deposition, using synchrotron X-ray scattering. For deposition at a substrate temperature of ∼0.8T (where T is the glass transition temperature), we find a generic tendency towards "face-on" packing in glasses of anisotropic molecules.

Vapor-Deposited Glass Structure Determined by Deposition Rate-Substrate Temperature Superposition Principle.

We show that deposition rate substantially affects the anisotropic structure of thin glassy films produced by physical vapor deposition. Itraconazole, a glass-forming liquid crystal, was deposited at rates spanning 3 orders of magnitude over a 25 K range of substrate temperatures, and structure was characterized by ellipsometry and X-ray scattering. Both the molecular orientation and the spacing of the smectic layers obey deposition rate-substrate temperature superposition, such that lowering the deposition rate is equivalent to raising the substrate temperature.

Origin of Anisotropic Molecular Packing in Vapor-Deposited Alq3 Glasses.

Anisotropic molecular packing is a key feature that makes glasses prepared by physical vapor deposition (PVD) unique materials, warranting a mechanistic understanding of how a PVD glass attains its structure. To this end, we use X-ray scattering and ellipsometry to characterize the structure of PVD glasses of tris(8-hydroxyquinoline) aluminum (Alq3), a molecule used in organic electronics, and compare our results to simulations of its supercooled liquid. X-ray scattering reveals a tendency for molecular layering in Alq3 glasses that depends upon the substrate temperature during deposition and the deposition rate.

Organic Glasses with Tunable Liquid-Crystalline Order.

Liquid crystals (LCs) are known to undergo rapid ordering transitions with virtually no hysteresis. We report a remarkable counterexample, itraconazole, where the nematic to smectic transition is avoided at a cooling rate exceeding 20  K/s. The smectic order trapped in a glass is the order reached by the equilibrium liquid before the kinetic arrest of the end-over-end molecular rotation.

Surface transport mechanisms in molecular glasses probed by the exposure of nano-particles.

For a glass-forming liquid, the mechanism by which its surface contour evolves can change from bulk viscous flow at high temperatures to surface diffusion at low temperatures. We show that this mechanistic change can be conveniently detected by the exposure of nano-particles native in the material. Despite its high chemical purity, the often-studied molecular glass indomethacin contains low-concentration particles approximately 100 nm in size and 0.

Influence of Hydrogen Bonding on the Kinetic Stability of Vapor-Deposited Glasses of Triazine Derivatives.

It has recently been established that physical vapor deposition (PVD) can produce organic glasses with enhanced kinetic stability, high density, and anisotropic packing, with the substrate temperature during deposition (T) as the key control parameter. The influence of hydrogen bonding on the formation of PVD glasses has not been fully explored. Herein, we use a high-throughput preparation method to vapor-deposit three triazine derivatives over a wide range of T, from 0.

Nematic-like stable glasses without equilibrium liquid crystal phases.

We report the thermal and structural properties of glasses of posaconazole, a rod-like molecule, prepared using physical vapor deposition (PVD). PVD glasses of posaconazole can show substantial molecular orientation depending upon the choice of substrate temperature, T, during deposition. Ellipsometry and IR measurements indicate that glasses prepared at T very near the glass transition temperature (T) are highly ordered.

Vapor deposition of a smectic liquid crystal: highly anisotropic, homogeneous glasses with tunable molecular orientation.

Physical vapor deposition (PVD) has been used to prepare glasses of itraconazole, a smectic A liquid crystal. Glasses were deposited onto subtrates at a range of temperatures (Tsubstrate) near the glass transition temperature (Tg), with Tsubstrate/Tg ranging from 0.70 to 1.

Growth actuated bending and twisting of single crystals.

Crystals of a variety of substances including elements, minerals, simple salts, organic molecular crystals, and high polymers forgo long-range translational order by twisting and bending as they grow. These deviations have been observed in crystals ranging in size from nanometers to centimeters. How and why so many materials choose dramatic non-crystallographic distortions is analyzed, with an emphasis on crystal chemistries that give rise to stresses operating either on surfaces of crystallites or within the bulk.