The Long and Winding Road: Predicting Material Properties Through Theory and Computation
Written by Ashley White
Galli’s Thursday evening Materials Theory Award talk was centered around three scientific examples, or “short stories,” as she called them. The stories had a common thread of the relationship between structure and function, and how we can understand, predict, and eventually control this relationship to design an optimal material. This approach was discussed in the context of optimal materials to absorb light in photo-electrochemical cells, optimal nanostructured materials for solar cells and electronic devices, and defects in semiconductors for quantum information devices.
In her first story on photo-electrochemical cells, Galli emphasized the importance of understanding, first and foremost, the interface between the electrode and water, as well as the band offsets, which control how charge travels in the system. Galli discussed two examples—silicon surfaces and tungsten oxide. In computing the absolute positions of the bands of liquid water and the band offsets between liquid water and the solids, Galli found that: (1) the solid-liquid interaction is not negligible; (2) multiple combined effects are present; and (3) surface functionalization can be tailored to optimize the photoabsorbent properties. In particular, she emphasized that understanding the electronic structure of solvated surfaces at finite temperature is critical for optimizing the materials system. Overall, she cautioned that studying the intrinsic properties of a material is insufficient, and in some cases misleads one away from predicting the optimal material.
Galli’s second story focused on electronic transport properties in nanoparticles, which may be enhanced through the use of inorganic ligands. In this example, she emphasized the importance of closely and iteratively incorporating experiment with simulations. To develop a structural motif for their models, Galli’s group had to work closely with experimentalists to refine and validate their calculations. Only after the model was validated by experiment could they use it as an input for further molecular dynamics simulations to successfully predict the electronic properties of their materials.
Galli’s third story was about manipulating defects in semiconductor materials for quantum information science. In this case, Galli explained, it is important to understand not only the electronic structure but also the spin decoherence, since it is related to interconnections between defects. Through a combination of approaches, including coupling the electronic structure calculation to the spin Hamiltonian, Galli was able to get excellent agreement between theory and experimental results for di-vacancy in silicon carbide—an approach which can now be extended to other systems as well.
An overarching challenge and goal for the future, Galli said, is to solve the “inverse problem” of being able to design new materials with pre-determined properties based on theoretical and computational information. In closing, Galli dedicated her talk and her award to Alessandro De Vita, a professor of physics and materials science at King’s College London and a collaborator of Galli’s, who passed away unexpectedly in October.
The Materials Theory Award, endowed by Toh-Ming Lu and Gwo Ching Wang, recognizes exceptional advances made by materials theory to the fundamental understanding of the structure and behavior of materials. Galli received the award “for the development of advanced first-principles simulation methods and their application to the understanding, prediction and design of complex nanostructured materials.”