This Faculty Early Development (CAREER) Program grant provides funding for the development of an integrated research and education program to investigate the microstructural and mechanical properties of nanoporous thin films. The integrated, multiscale experimental approach including nanoindentation, laser-induced surface acoustic waves (LiSAW), laser-induced thin film spallation (LiTFS), and ultrasonic holography based digital image correlation (UHDIC) will be used to determine the microstructure-mechanical property relationship of nanoporous thin films. In particular, the study will focus on the effect of porosity, pore structure, and crystallinity etc on the bulk, surface and interface properties of nanoporous thin films, and their deformation, delamination, damage and densification mechanisms under various loading conditions. Due to their ordered pore structure, crystalline framework and dominating role in nanoporous materials market, nanoporous zeolite thin films will be the initial focus of this research while the fundamental knowledge and the established techniques will be transferable to many other ordered and non-ordered nanoporous materials.
If successful, the results of this research will lead to: 1) improved science-based understanding of the process-microstructure-mechanical property relationship of nanoporous thin films; 2) a set of innovative experimental tools that can be used to study a wide variety of nanoporous materials; 3) promotion of processing and application of nanoporous thin films in a wide range of fields such as microelectronics, energy materials, and films and coatings; 4) the involvement of more underrepresented undergraduate students, women and minorities in science and engineering; 5) more outreach opportunities to local high school students and teachers, especially to those economically or educationally disadvantaged students; 6) a larger and more diverse science and technology workforce in the under developed regions at the Inland Empire of Southern California served by UC Riverside.
Nanoporous materials (materials with pore size below 100 nm) are of significant interest to the scientific community. Due to the size scale of the pores they contain and the wide range of applications they can satisfy, thin films containing nanometer sized pores have potential applications as anti-reflection optical coatings, low-dielectric-constant (low-k) interlayer materials for interconnects in semiconductors, nanoscopic chemical reactors for catalysis, sensors, reactors and many others. Despite of these promising potential applications, the study of nanoporous thin films is still in its scientific infancy as most of the work so far are proof-of-concept and has not reached a level where engineering criteria can be applied. While increased levels of porosity are beneficial for achieving the desired functionality, high levels of porosity deteriorate the mechanical integrity of the films. To achieve both the functional property and the mechanical integrity of nanoporous thin films requires exact control and understanding of both microstructural (pore size/shape, uniformity, crystallinity, etc) and mechanical properties as well as their inter-relationships. One critical step in achieving this goal is to develop highly sensitive and reliable metrologies that can accurately quantify these properties. The metrology for characterizing microstructural properties is relatively more mature, e.g., nitrogen adsorption/desorption method is routinely used for porosity measurement although more advanced microscopic techniques are still desired to obtain more direct information about the pore structure (e.g., size and shape). However, reliable techniques for investigating the mechanical properties of nanoporous thin films are significantly lacking. Although some preliminary results have been obtained using the traditional techniques established for dense films of similar dimensions, their limitation on nanoporous thin films have not been sufficiently addressed. In this project, we developed an integrated, multiscale characterization approach including nanoindentation, laser-induced surface acoustic wave (LiSAW) and laser-induced thin film spallation techniques to systematically investigate the microstructure-mechanical property relationship of nanoporous thin films. Due to their unique ordered crystalline pore structure and their dominating role in the nanoporous materials market, nanoporous zeolite thin film was chosen as the model system in the context of two important technical applications: (1) as low-k thin films for microelectronics, and (2) as wear resistant coatings for metals in aircraft components. The developed techniques were also further validated by other porous thin films such as porous V2O5 films for battery electrode applications. We demonstrated that due to the high level of porosity, traditional thin film characterization techniques such as nanoindentation is meeting the limitations for very thin and soft porous films due to either substrate effect or in-situ densification. Although some correction schemes are available to achieve more accurate nanoindentation analysis, it is at the price of intensive computation time. Complement to nanoindentation, LiSAW technique is much more advantageous for characterizing thin and soft porous thin films as the large film/substrate acoustic mismatch enhances the sensitivity and resolution of the technique. In addition to mechanical property characterization, we have also demonstrated the capability of laser induced thin film spallation for characterizing the interfacial strength of the porous film/substrate interfaces. Current ongoing work is focusing on the study of the wave absorption/attenuation by nanoporous thin films for potential energy dissipation applications. This will be reported in future research.
