A grand challenge in materials chemistry is to control the structure and properties of materials at nanoscale. One of the focal points in the effort to meet this challenge is the ability to introduce well-defined nanoscale voids (pores) in the structure of materials. While the last two decades brought milestones in the design of materials with accessible (open) nanopores, advances in the development of materials with closed (isolated) nanoscale pores were limited, even though the latter materials are of significant interest in electronics industry as low-dielectric-constant on-chip insulating media in integrated circuits. The current project is focused on exploring a predictive way to synthesize closed-pore materials, which is based on the use of surfactant micelles as templates to generate well-defined pore arrays. The surfactant templating has revolutionized the synthesis of nanoporous materials, but because of the need to remove the surfactant template from the pores, it seemed to be inherently suitable for open-pore rather than closed-pore architectures. Recently, the micelle-templating approach to generate closed-pore materials was developed which was based on identifying conditions at which the template is removed before the pores close. The current research effort is focused on extending the scope of this powerful approach on new material compositions and on achieving more beneficial structural properties of the closed-pore materials. Thus, the project will provide a knowledge base for the development of closed-pore materials with designed structures and properties and is expected to benefit a broad range of scientists and engineers that study and apply porous materials. The project involves postdoctoral fellows, graduate students and undergraduate students, with a special focus on minority undergraduate students.
TECHNICAL DETAILS: This work will explore a new and transformational approach to the synthesis of well-defined materials with closed (isolated) nanopores, which is based on recently discovered thermally-induced pore closure phenomenon in micelle-templated mesoporous materials. This process promises to be a predictive pathway for the synthesis of closed-pore nanoporous silicas and is expected to be applicable for organosilicas. Ways to obtain silicas and organosilicas with closed spherical pores of diameter 5 nm or lower, which are most appropriate in applications as low-k materials, are being explored. Materials that exhibit the pore closure at as low temperatures as possible (preferably around 300°C) are sought. Ways to achieve the thermally-induced pore closure with minimized extent of shrinkage will be delineated. Periodic mesoporous organosilicas, which are inherently more suitable than silicas for low-k applications due to their lower bulk dielectric constant, are being explored as closed-pore materials candidates. Ways to increase the pore volume of closed-pore materials, thus reducing the dielectric constant, are being developed. Synthetic pathways to materials with closed cylindrical pores are being explored. The study focuses on samples in powder form due to the lack of readily accessible facilities to characterize thin films, but conclusions of the study are expected to extend to materials in the thin-film form. The project will generate the knowledge base for the synthesis of closed-pore nanoporous materials with tailor-made framework composition, pore size, pore shape and pore volume. This effort contributes to the current quest to overcome limitations in the processing speed of electronic devices by exploring a new avenue to the low dielectric constant materials. The project involves the training of scientists and students in the synthesis and characterization of cutting-edge nanoscale materials.
The project was intended to explore the synthesis of materials with arrays of nano-scale pores of diameter of several to several tens on nanometers that are not accessible from the surrounding (in other words: closed). Such materials are of interest in the electronics industry because of their low dielectric constant. Our aim was to generate closed nano-pores using surfactant micelles as templates. Surfactants, some of which are components of household detergents, are known from their ability to form aggregates of molecules, which are known as micelles. The micelles can be very uniform in size and tend to have well-defined shapes, such as spherical or cylindrical. Such well-defined nanoscale objects are used in our strategy to define the nano-pore size and shape. The use of a template to generate a closed pore seems to be self-contradictory, because the closed pore is expected to remain filled with the template that was used to generate it. However, if the pore is initially open (accessible) and the template is removed from the pore before the pore closes (for instance at a lower temperature) while the pore can be closed subsequently (for instance at a higher temperature), the strategy can be successful. We primarily focused on porous silicas as the subject of our work. The silica is hydroxylated silicon dioxide, a "relative" of sand and quartz. Silicas are network solids, which are composed of three-dimensionally cross-linked networks of atoms, and because of that, they can be readily shaped at nano-scale. Silicas shrink on heating at high temperatures, primarily due to the condensation involving the release of water. This creates a mechanism for the pore closing process. Our work demonstrated that silicas with closed mesopores can be readily synthesized. In some cases, they can be obtained at temperatures as mild as 400-450 °C (752-842 °F), if the pore shape is spherical. We also succeeded in generating materials with closed cylindrical nano-pores, which are nano-scale honeycombs with closed cells. This development was surprising, because it was widely believed that when micelles are used as templates, the cylindrical nano-pores have open ends. However, we demonstrated that this is often not the case, at least at early stages of the synthesis processes involving common non-ionic surfactants. Our work indicates that the micelle templating process involves building blocks in which micelles (typically spherical or cylindrical) are surrounded from all sides by a silica envelope, which has minute perforations (micropores). The closed nan-pores can be obtained if the silica envelope is not disrupted in the process and the micropores present in it are shrunk or eliminated after the micelle template has been removed. This process is envisioned to be operative for different shapes and sizes of templating micelles, creating vast opportunities in the synthesis of materials with closed nano-pores. This understanding can be used in the development of low-dielectric-constant materials and in other cases where closed-pore nano-scale materials can be employed. Moreover, the insight from our work can be used to avoid the formation of closed pores in cases where well-accessible (open) nano-scale pores are desired, for instance in heterogeneous catalysis and separations. The project involved numerous participants, primarily graduate students and undergraduate students, and contributed to advancement of their professional and educational goals, making them better trained for careers in industrial and academic research.
