Glasses are amorphous solids, characterised by the absence of a long-range order, which gives them interesting physical and chemical properties. Thanks to the variety of glassy materials that can be produced and the large flexibility they offer to tune their composition and characteristics, glasses find a wide range of applications, among which optoelectronic devices are of great interest. Ultrastable glasses, exhibiting remarkable thermodynamic and kinetic stability and high density, can be obtained by vapour-deposition, a technique introduced in 2007. Since then, many studies have been carried out with the aim to both improve our understanding of the physics and chemistry of glasses (in particular, the glass transition phenomenology) and take advantage of their enhanced properties.
A review paper published in “La Rivista del Nuovo Cimento” summarizes the state of the art of vapour-deposited ultrastable glasses, discussing their mechanism of formation, the existing different families –organic, metallic, polymeric and chalcogenide glasses— and their characteristics and performance, also in correlation with possible applications. This work, whose first author is Dr Cristian Rodriguez-Tinoco, from the Department of Physics of the Universitat Autónoma de Barcelona (UAB), was coordinated by Prof. Javier Rodriguez-Viejo, leader of the Thermal Properties of Nanoscale Materials Group at the ICN2 and professor at the UAB, and Dr Marta Gonzalez-Silveira, from Prof. Rodriguez-Viejo’s team. Prof. Miguel Angel Ramos, from the Universidad Autónoma de Madrid (UAM), collaborated in the study.
The outstanding properties of glasses obtained by vapour deposition have opened the way to novel applications, for which conventional liquid-cooled glasses are not suitable. In fact, glasses are usually obtained by cooling a liquid, from a temperature above the melting point of the material to a lower one where viscosity is high enough for the material to be considered a solid, bypassing crystallization. Of course, this is possible only for materials having a very slow rate of crystallisation, so that they solidify into glass before giving crystalline structures time to form.
Vapour deposition enables the preparation of glasses whose stability would be achieved by liquid cooling only over an unreasonable period of time: the cooling process would have to last months, years, or even centuries. Most of the work on stable glasses until recent times has revolved around organic glasses, with thin films of these materials being used to improve the efficiency and lifetime of organic optoelectronic devices. However, lately, the other families of vapour-deposited glasses have been gaining ground.
The review is structured in five sections. The first provides a general introduction to the main characteristics of vapour-deposited glasses in comparison with the liquid-cooled ones, an overview of the differences among the various families, and a comprehensive list of most of the glassy materials that can be prepared by means of this technique and their properties. After a description of the formation mechanism of glasses by vapour deposition and the factors that play an important role on the characteristics of the materials obtained this way (in the second section), the transformation mechanism of organic stable glasses into super-cooled liquids is discussed, identifying different behaviours in very thin films compared with thicker structures (section thee). Following, a summary of the peculiar properties of glasses at low temperatures is given in section four. Here the authors point out the disappearance observed in some vapour-deposited ultrastable glasses of a behaviour that low-temperature glasses normally exhibit, which is described by a two-level system (TLS) model. This is particularly relevant since such TLS phenomenon is thought to be the major source of noise and decoherence in superconducting quantum circuits. Finally, section five deals with the remarkable (opto)electronic and thermal transport properties of vapour-deposited stable glasses, in particular the organic ones, which are key to develop devices such as organic field effect transistors, organic light emitters and photovoltaic cells with improved performance. Reference Ultrastable glasses: new perspectives for an old problem
Cristian Rodriguez-Tinoco, Marta Gonzalez-Silveira, Miguel Angel Ramos & Javier