Control and Design of Two Dimensional Silica Structures

二维（2D）材料可以被减薄到原子级薄片，代表了微型化的终极，同时显示出独特的物理和化学性质。 改变原子的排列方式将允许进一步控制物理和化学性质，但很难以系统的方式实现。 随着最近发现的2D形式的二氧化硅（SiO2），出现了单片材料的新可能性。 SiO2结构单元之间的连接的灵活性提供了一种实现2D材料假设的原子排列的途径，但迄今为止还没有以受控的方式实现。 在SiO2网络中用各种元素取代Si的能力进一步提供了控制化学性质和诱导新结构的机会。 该项目将理论和实验相结合，以制定控制SiO2构建块如何组织形成结构的规则。 该理论确定了最有可能产生独特结构的掺杂剂和生长条件的组合，而实验则旨在实现材料并表征其局部结构和化学性质。 在材料研究部门的固态和材料化学计划的支持下，目标是了解构建块的特性沿着施加的外部应力如何决定2D材料的原子排列，从而决定其功能特性。 通过控制二维二氧化硅结构实现的应用包括用于基于分子大小分离分子的高效原子级薄膜、用于通过堆叠不同原子片组装电子和光学器件的受控厚度绝缘层以及模型催化剂。 结果将被整合到课程材料开始在高中水平的原子结构的图像的图形性质提供了一个迷人的介绍晶体和玻璃之间的差异和加工，结构和性能之间的相互关系。最近发现的2D SiO2双层为单层货车德瓦尔斯材料创造了新的可能性。开放结构和高稳定性提供了设计用于分子分离的最终膜的机会，而网络连接的灵活性可以导致丰富的结构化学，这可以允许实现为其他三配位网络假设的结构。第一原理理论将被用来确定应变，掺杂剂，掺杂剂浓度和覆盖范围的组合，提供了一个大的精力充沛的推动远离六方六元环结构有利于纯，无应变的2D二氧化硅。目标结构将使用分子束外延合成，并使用扫描探针显微镜（SPM）进行表征。 初始实验将告知精确建模系统所需的理论水平（例如，双层-衬底相互作用的处理），从而提供反馈机制以提高建模的预测能力。将采用扫描探针方法，结合联合收割机隧道和力的测量，以产生表面硅，O和掺杂剂原子的位置和力和隧道光谱以上的每个原子。 这些广泛的信息与理论将揭示由于晶格失配和掺杂导致的应变是如何放松的，这反过来又影响了双层与周围环境的化学相互作用。 通过将理论与最先进的SPM相结合，该项目将产生对网络灵活性，应变，表面和界面张力以及掺杂剂特性在确定可以实现的2D结构多样性中所起作用的基本理解。

Non-Technical AbstractTwo-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. Technical AbstractThe 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.