A glass act

Materials World magazine
3 Oct 2015

The uniqueness of a variety of glass discovered by the University of Wisconsin-Madison and the University of Chicago, USA, has been questioned by John Parker, University of Sheffield, UK. Khai Trung Le reports.

The paper

Detailed in the paper Tunable molecular orientation and elevated thermal stability of vapor-deposited organic semiconductors, published in PNAS, glass samples were created via vapour deposition, vaporising large organic molecules in a high vacuum before being deposited in thin layers onto a substrate at a tightly controlled temperature. Detecting unexpected peaks that indicated molecular order in the structure that was expected to be amorphous, Professors Mark Ediger, Wisconsin, and Juan de Pablo, Chicago, identified that the interaction of liquid surface layers and the atmosphere cause the surface molecules to compact and arrange in a structure distinct from the body of the liquid. Coupled with the vapour deposition process, which consists of arranging layers on top of one another, this observation allows for the creation of structured glass, with the degree of molecular order controlled by varying the temperature of the deposition process. This results in highly anisotropic glasses, formed with high density and thermal stability, and possessing properties similar to stable glass prepared from model glass formers. 

De Pablo was keen to stress that further work will need to be conducted, noting, ‘Glasses are one of the least understood classes of materials. They have the structure of a liquid – disorder – but they’re solids. That’s a concept that has mystified people for many decades. So, the fact that we can now control the orientation of these disordered materials is something that could have profound theoretical and technological implications. We don’t know what they are yet – this is a new field of research and a class of materials that didn’t exist before. So we’re just at the beginning.’

Ediger said, ‘The most obvious application is in organic light emitting diodes (OLEDs). It has been shown that horizontal emitter orientation in OLEDs can increase efficiency by a factor of at least 1.3.’ However, Ediger also recognised that most commercial OLEDs use proprietary materials and deposition conditions, ‘I could not say that I know for sure that our process could improve the performance of any particular commercial device. More generally, we expect deposition conditions to change charge mobility substantially, and this could be important for OLEDs and organic PVs.’

Unlike other structured glass, the existence of which ‘established that it was possible to prepare films in which molecular orientation is not isotropic’, Ediger stated that the focus of the paper was to ‘lay out the conditions required to prepare a particular orientation and propose a mechanism for the process.’

The opinion

John Parker, Emeritus Professor and Director of the Centre for Glass Research, University of Sheffield, UK, recognised the paper as ‘yet another excellent example of the rich art of materials scientists in discovering novel materials. Equilibrium thermodynamics provides the backbone to materials processing and structural ordering, although this report highlights kinetics, annealing near the glass transition temperature to tailor the arrangement of organic molecules in a thin film. Indeed, for millennia, technologists in disparate materials disciplines have appreciated the importance of thermal history and processing in achieving a particular goal. This has been no different in glass technology.

‘Unfortunately for present day glass experts, the careful and, at the time, groundbreaking achievements of X-ray crystallographers in the early 20th Century resulted in an oversimplified textbook description of vitreous silica based on a random network. Even at the time, the concept of random was disputed. The present authors perpetuate this model – for example, with the development of the modified random network model for silicates, and the creation of thermodynamic models for glasses based on mixtures of structures mimicking the crystalline counterparts expected from phase diagrams.

‘Their materials have a molecular structure while many commercial glasses are based instead on 3D connected networks. But chalcogenide, phosphate, borate and organic polymeric glasses all have structures where an accepted concept is a degree of ordering influenced by thermal history within an overall disordered structure.’

In spite of this, Parker was enthused by the researcher’s efforts, stating, ‘Transformation range behaviour and the use of fictive temperature as a parameter indicating the frozen–in state of a particular glassy composition have, for many decades, been a rich area of study in glass. That the current authors are extending this tradition is an important positive.’