Non-Technical Abstract Two-dimensional (2D) materials that can be thinned down to atomically thin sheets represent the ultimate in miniaturization while displaying unique physical and chemical properties. Varying how the atoms are arranged will allow further control of the physical and chemical properties, but has been difficult to achieve in a systematic way. With the recent discovery of a 2D form of silicon dioxide (SiO2), new possibilities for single sheet materials have arisen. The flexibility of the linkages between the SiO2 building blocks provides a pathway to realize atomic arrangements hypothesized for 2D materials, but thus far not realized in a controlled way. The ability to substitute a wide range of elements for Si in SiO2 networks further offers the opportunity to control chemical properties and induce new structures. This project integrates theory and experiment to develop rules for controlling how the SiO2 building blocks organize to form structures. The theory identifies combinations of dopants and growth conditions most likely to produce unique structures, while the experiments are directed towards realizing the materials and characterizing their local structural and chemical properties. With the support of the Solid State and Materials Chemistry program in the Division of Materials Research, the goals are to understand how the properties of the building blocks along with applied external stresses dictate the atomic arrangement of 2D materials, and thus their functional properties. Applications enabled by control of two-dimensional silicon dioxide structures include highly efficient, atomically thin membranes for separating molecules based on their size, controlled-thickness insulating layers for assembling electronic and optical devices by stacking different atomic sheets, and model catalysts. The results will be integrated into course material starting at the high school level where the graphical nature of images of the atomic structures provide an engaging introduction to the differences between crystals and glasses and the interrelationships between processing, structure, and properties.
The recent discovery of 2D SiO2 bilayers created new possibilities for single sheet van der Waals materials. The open structure and high stability provides opportunities to design the ultimate membrane for molecular separations, while the flexibility of the network links can lead to a rich structural chemistry that may allow structures postulated for other tri-coordinated networks to be realized. First principles theory will be used to identify combinations of strain, dopant, dopant concentration, and coverage that provide a large energetic push away from the hexagonal six-membered ring structures favored for pure, unstrained 2D SiO2. The target structures will be synthesized using molecular beam epitaxy and characterized using scanning probe microscopy (SPM). Initial experiments will inform the level of theory needed to accurately model the system (e.g., the treatment of bilayer-substrate interactions), thereby providing a feedback mechanism to improve the predictive capabilities of the modeling. Scanning probe methods will be employed that combine tunneling and force measurements to yield the positions of surface Si, O, and dopant atoms and force and tunneling spectra above every atom. This extensive information together with theory will reveal how strain due to lattice mismatch and doping is relaxed, which in turn influences the chemical interactions of the bilayer with its surroundings. By combining theory with state-of-the-art SPM this project will yield a fundamental understanding of the roles played by network flexibility, strain, surface and interfacial tensions, and dopant properties in determining the diversity of 2D structures that can be achieved.
