Science of Electron-conducting Filaments in Ion-conducting Chalcogenide Glasses

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NON-TECHNICAL DESCRIPTION: This proposal aims to address a fundamental question of charge and mass transport in ion-conducting glasses: how does an electric field create an electron-conducting path in metal-doped glasses (that contain S, Se or Te)? Under a strong external electric field, an electron-conducting path is created or annihilated inside the solid electrolyte depending on the direction of the electric field. The observed reversible creation/annihilation of electron-conducting paths by external fields mimics 0 and 1 states of a binary system and is the basis underlying conductive bridging random access memory (CBRAM), a novel non-volatile memory technology based upon bipolar resistive switching. The goal of this project is to understand the destruction and formation mechanism of the conductive filaments through integrated experiment/theory approaches. Success of this project provides fundamental understanding of the emerging CBRAM technology and promises transformation of electronically insulating glasses to electronically conducting glasses. The impact of this research is broadened further by engaging students typically underrepresented in science and engineering to participate in the research project. Summer internships are offered to undergraduate students through the on-campus Research Experience for Undergraduates program. Outreach to children with autism spectra disorder in Appalachian Ohio is offered through science demonstrations, workshops and summer camp activities.TECHNICAL DETAILS: Fast-ion conductors based upon solid electrolyte glasses have many advantages over their crystalline counterparts. For example, Ag doped chalcogenide glasses exhibit extremely high ionic conductivity. An interesting twist for these materials is that under a strong external electric field, an electron-conducting path is created inside the solid electrolyte. It is widely speculated that formation of metal filaments through field-driven electrochemical reactions is responsible for the enhanced electronic conductivity. However, this view leads to an erroneous prediction that superfast motion of both the metal ions and the solid electrolyte host occurs simultaneously under the external field. A hypothesis of this grant is that the electron-conducting filaments created by the external electric field are not made simply of metal, but complex semiconducting compounds that involve concentrated ions trapped by the ion-trap centers in the solid electrolytes. Prototypical metal-doped chalcogenide glasses (i.e., Ag and Cu doped Ge-Se and Sb-Te) were selected for study. Advanced experimental and theoretical techniques are applied to understand the charge and mass transport in the solid electrolytes under external fields. A novel simulation technique called experimentally constrained molecular relaxation (ECMR) is used to ensure maximal coincidence between theory and experiment. Structure, properties and dynamics of the electron-conducting filaments generated by the electric field are studied through the integrated experiment/theory approach. This project provides atomistic insight into the dynamics of filament formation and structure-property relations of the electron-conducting filaments. Undergraduate and graduate students participating in this project are trained on cutting-edge research facilities such as those in Advanced Photon Source and Center for Nanoscale Materials at Argonne National Lab and the Ohio Supercomputing Center. Outreach to children with autism spectral order is offered to enhance their interest in college education and careers in information technology.