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 ∼0.8Tg (where Tg is the glass transition temperature), we find a generic tendency towards "face-on" packing in glasses of anisotropic molecules. At higher substrate temperature however this generic behavior breaks down; glasses of rod-shaped molecules exhibit a more pronounced tendency for end-on packing. Our study provides guidelines to create face-on and end-on packing motifs in organic glasses, which can promote efficient charge transport in OLED and OFET devices respectively.

Stable Glasses of Organic Semiconductor Resist Crystallization.

The instability of glassy solids poses a key limitation to their use in several technological applications. Well-packed organic glasses, prepared by physical vapor deposition (PVD), have drawn attention recently because they can exhibit significantly higher thermal and chemical stability than glasses prepared from more traditional routes. We show here that PVD glasses can also show enhanced resistance to crystallization. By controlling the deposition temperature, resistance toward crystallization can be enhanced by at least a factor of ten in PVD glasses of the model organic semiconductor Alq3 (tris(8-hydroxyquinolinato) aluminum). PVD glasses of Alq3 first transform into a supercooled liquid before crystallizing. By controlling the deposition temperature, we increase the glass → liquid transformation time thereby also increasing the overall time for crystallization. We thus demonstrate a new strategy to stabilize glasses of organic semiconductors against crystallization, which is a common failure mechanism in organic light emitting diode devices.

Over What Length Scale Does an Inorganic Substrate Perturb the Structure of a Glassy Organic Semiconductor?

While the bulk structure of vapor-deposited glasses has been extensively studied, structure at buried interfaces has received little attention, despite being important for organic electronic applications. To learn about glass structure at buried interfaces, we study the structure of vapor-deposited glasses of the organic semiconductor DSA-Ph (1,4-di-[4-(N,N-diphenyl)amino]styrylbenzene) as a function of film thickness; the structure is probed with grazing incidence X-ray scattering. We deposit on silicon and gold substrates and span a film thickness range of 10-600 nm. Our experiments demonstrate that interfacial molecular packing in vapor-deposited glasses of DSA-Ph is more disordered compared to the bulk. At a deposition temperature near room temperature, we estimate ∼8 nm near the substrate can have modified molecular packing. Molecular dynamics simulations of a coarse-grained representation of DSA-Ph reveal a similar length scale. In both the simulations and the experiments, deposition temperature controls glass structure beyond this interfacial layer of a few nanometers.

Direct in situ observation of ZnO nucleation and growth via transmission X-ray microscopy.

The nucleation and growth of a nanostructure controls its size and morphology, and ultimately its functional properties. Hence it is crucial to investigate growth mechanisms under relevant growth conditions at the nanometer length scale. Here we image the nucleation and growth of electrodeposited ZnO nanostructures in situ, using a transmission X-ray microscope and specially designed electrochemical cell. We show that this imaging technique leads to new insights into the nucleation and growth mechanisms in electrodeposited ZnO including direct, in situ observations of instantaneous versus delayed nucleation.

Structure of oxidized bismuth nanoclusters.

Synchrotron X-ray diffraction has determined that beta-Bi(2)O(3) is the dominant oxide phase covering hexagonal bismuth nanoclusters produced in an inert gas aggregation source. Simulated Debye-Scherrer patterns have indicated that the oxide is 20 +/- 5 Angstroms thick on average, at the surface of 320 +/- 40 Angstroms diameter clusters. A Williamson-Hall analysis of the peak broadening was used to measure the non-uniform strain in clusters. The oxidized clusters were in -0.11 +/- 0.06% uniform compressive strain compared with other clusters without oxides detectable by X-ray diffraction which only have a small tensile uniform strain. High-resolution transmission electron microscopy (HRTEM) and multislice image simulations indicated a beta-Bi(2)O(3) thickness of 20-50 Angstroms. The HRTEM micrographs show the relative orientation between the oxide and the cluster core.