Elucidating Buried Interfacial Structures Of Complex Materials Using Advanced Spectroscopic And Microscopic Techniques

Abstract

Modern electronic devices are engineered to be compact, many device functions are often encapsulated into multiple layers within one thin device. Such technological advance trend creates multiple buried interfaces between different layers in single electronic device. Optimizing overall device function and lifetime often relies on understanding each layer independently, however approaches used to understand such buried interfaces nondestructively are still lacking, making it intrinsically difficult to perform failure analysis on many devices to fully understand the device failure mechanisms. This thesis combines an interface sensitive technique, sum frequency generation (SFG) vibrational spectroscopy, with various other analytical techniques to elucidate interfacial structure-function correlations related to complex electronic devices and systems. By studying the surface structure and behavior of organic semiconductor (Poly 3-hexyl thiophene, P3HT) thin films prepared using different solvents, it was found that the P3HT molecules at the polymer/air interface would lie down as more acetone was added into the casting solvent, demonstrating the feasibility of varying semiconductor thin film surface structure by altering the solvent composition for thin film preparation. SFG research also elucidated how the properties of the underneath substrate and semiconductor polymer sidechain could affect the orientation of polythiophene at the polymer/air and polymer/substrate interface of a thin polythiophene film. This research provided systematic understanding of the effects of sample preparation solvent, substrate hydrophobicity, and polymer side chain composition on the surface and buried interfacial structures of semiconductor polymer thin films. SFG has also been applied to study interfacial structures of semiconductor polymer thin films in photovoltaics: the interfacial orientations of polythiophene molecules at the perovskite/polythiophene hole transport layer interface were successfully correlated to the overall perovskite device power conversion efficiency. The experimental results indicated that subtle of the tilt angle of the polythiophene backbone at the perovskite/polythiophene interface could lead to over 100% difference on device efficiency. This example demonstrated that the understanding of the interfacial structure of a semiconductor thin film could improve the property of photovoltaic device. SFG has also been used to probe the molecular structures of buried interfaces of polymer-based adhesives to understand the structure-function correlations of such polymer adhesives for microelectronics packaging. In this study, pristine polyethylene and grafted polyethylene were examined. It was found that ordered C=O grafted groups and standing up methylene group at buried polymer adhesive interface could lead to better adhesion strength for polyethylenes. This thesis also developed a microscopic-SFG platform which can collect SFG spectrum and obtain optical/fluorescence image simultaneously from the same sample. With this analytical platform, molecular interactions between biological molecules and 2D material MoS2 were revealed. This research developed a generally applicable approach to design the sequence of a peptide with a preferred orientation on MoS2. The microscope-SFG platform was also used to study the bacterial killing mechanism of surface-immobilized antimicrobial peptides via covalent attachment. SFG was used to monitor the structural change of these peptides upon interacting with bacteria, and the microscope was used to collect the live/dead bacterial information on the surface in situ with fluorescence image, demonstrating the feasibility to probe structure and function of interfacial biological molecules simultaneously. Overall, this thesis developed important approaches using SFG to study buried interfaces related to electronics. Such approaches are general and can be applied to study complex interfaces to understand their structure-function correlations, providing important knowledge for constructing interfaces with improved properties.