Developing novel structural modelling methods for optical glasses

Organic chemical bonds, such as C-O, C-H and O-H, absorb light strongly at specific mid-infrared (mid-IR) frequencies. This gives every molecule a distinctive 'chemical fingerprint' that can be detected. The development of sensors that can measure these interactions would have applications in a diverse range of fields, from monitoring pollution in manufacturing environments and detecting drugs and explosives, to ensuring food and drink production lines are not contaminated and monitoring cancer margins during surgery. However, established silica-based fibre technologies only transmit light to the near-IR, and to exploit this "fingerprinting" method for chemical identification new mid-IR transmissive glasses are required. Research into mid-IR light technologies tends to focus on device development, utilising a small set of glass compositions that are known to exhibit adequate behaviour. However, non-optimal material properties can result in unnecessary problems, from loss of light intensity through a fibre to non-linear optical (NLO) effects that change the light characteristics. These need to be addressed when constructing a working device, but could be avoided if the starting material were specifically designed to exhibit the functional properties needed. The aim of this project will be to address the fundamental gap in knowledge that links glass composition to structure and functional properties. Current compositional development is, perforce, trial-and-error and requires a significant investment in time and money. A better understanding of composition-structure-property relationships in optical glasses will provide a road map to allow new glasses to be predicted, providing a short cut to determining optimised compositions for optical applications. Once established, the research protocols can be applied to improve glass performance for other applications such as energy, biomedical devices, architectural glasses and nuclear waste forms.
The aim of this project will be achieved by studying glass compositions in the tellurite (TeO2) and chalcogenide (Sb2Se3) glass families. These glasses have been chosen because they transmit light into the mid-IR and exhibit strong NLO effects that can interact with light in a number of potentially useful ways. The research will be comprised of two stages. The first stage will be to measure the functional properties of the glasses. It is well-established that the functional properties of a glass, such as softening and melting temperatures, densities, refractive indices and light transmittance windows, depend upon atomic structure. The second stage of the study will be a quantitative analysis of glass structures through the direct and computational analysis of data obtained using a range of techniques. These will include Neutron and X-ray scattering, X-ray Absorption Spectroscopy, Raman Scattering and Nuclear Magnetic Resonance. Preliminary results show that small changes in the composition of tellurite glasses alter the local environment of tellurium, changing the number of nearest neighbours. In comparison, variations in the composition of chalcogenide glasses can lead to changes in the types of nearest neighbours around antimony. The number and type of nearest neighbours can have a large impact on the glass properties, affecting how we make, shape and use the glass. However, our current understanding of these changes is qualitative, rather than quantitative, particularly in the complex multicomponent glasses required for applications.
A determination of structure and functional properties for carefully chosen compositional series will allow robust relationships to be developed. These will be used to predict new glass compositions that exhibit specific properties, allowing the precise functional property requirements of a specific application to be fulfilled. The application of these new materials will result in a step change in the development of new devices that operate in the mid-IR.

Due to the focus on fundamental understanding, the short term impacts for this project are predominantly academic. However, the success of the project will have important implications for the photonics industry. The potential ability for EPSR to predict the structure of new glasses will provide a fast screening method to identify new glass compositions with useful properties. This will result in substantial time and cost savings for device development by allowing the rapid identification of functional materials. In particular the structural insights into chalcogenide and tellurite glasses will accelerate the development of devices designed to exploit the mid-IR. One example of a device which currently utilises tellurite glass is a non-invasive glucose sensor for diabetes patients, developed at the University of Leeds. This device utilises a sodium-zinc-tellurite glass layer to achieve sensing, but this is a standard composition rather than one which has been optimised for the requirements of the device. A custom-designed glass composition could offer increased sensitivity, and hence reliability, in measurements. Improving glass formulations to meet the requirements of specific applications would impact on the development of a host of devices including optical switches and planar waveguides, where precise control of the optical properties of the glasses are required, optical fibres, where matched thermal properties are vital to ensure ease of processing, and a range of chemical sensors such as the glucose monitor.
In addition to their use in mid-IR optics, chalcogenides, particularly Ge-Sb-Te based compositions such as Ge2Sb2Te4, are part of a family of "phase change" materials that are widely used in optical DVDs and non-volatile memories due to their ability to switch rapidly between amorphous and crystalline phases. The insights into Ge-Sb-Se glass structure, and specifically the interaction of Ge with Se in chalcogen deficient glasses, will give new insights into the relationship between glassy and crystalline phases. A clearer understanding of how the switching mechanism occurs could result in the development of new, faster switching phase change materials and better control over the process. Ultimately this could lead to faster data-writing speeds and larger data storage capacity to benefit the electronics industry.
Finally, the structural analysis protocols developed in this project are highly relevant to heavy metal tellurite based glasses used in conductive pastes for photovoltaic applications, (Johnson Matthey) as well as for glasses used for biomedical materials and nuclear waste storage (e.g. Sellafield Ltd). These industries require glasses with carefully controlled functional properties, achieved by the development of multicomponent glasses. Of great interest for biomedical and nuclear fields is the potential for computational analysis to shed light on the mechanisms such as mixed modifier effects. These can alter glass dissolution rates, allowing more precise control of ion release into the body from bioactive glasses, or increase the durability of glasses used to contain nuclear waste forms. The research methodology used in this project would be directly applicable to a wide range of glass compositions and its application would lead to a step change in finding new glass compositions for these industries. One example for potential impact arises from the investigation of lone pair ions in glass. A better understanding of the structural role of lone pair cations, such as Te and Sb, will give insight into the behaviour of another lone-pair ion, lead. A greater understanding of the structural role played by lead could potentially allow the development of new lead-free materials for piezoelectrics. Use of lead is regulated and to replace it with a less toxic element in devices would remove environmental concerns and minimise regulatory problems for companies seeking to bring new devices to market.