RUI: Time-Resolved Point Kelvin Probe Force Microscopy for Non-Traditional Semiconductors

Mount Holyoke College, South Hadley MA

Investigators

Abstract

Nontechnical description: Non-traditional semiconductors such as polymers and quantum dots offer great promise for flexible and cheap electronics and novel devices. However, the way charges move through these materials can be complex due to their disordered nature or sensitivity to their environment. The better one understands charge motion in these materials and at their interfaces, the better one can devise materials for applications such as solar cells, efficient lighting and electronic devices. Given their disordered nature, full characterization of these materials must be done with nanoscale resolution. Furthermore, it is possible to use an atomic force microscope - a tool that enables imaging of materials on the nanoscale - to visualize charge motion through these materials in real time. This project harnesses an atomic force microscope to develop a technique for accurate probing of charge motion through a variety of non-traditional semiconductors. This technique can provide insights into material behavior by following real-time charge motion at different points in response to changing conditions. The female undergraduate students from Mount Holyoke College are involved in every part of the research. Women are severely underrepresented in physics and engineering - fewer than 25% of the bachelor's degrees are granted to women in physics - and early involvement in research encourages persistence in the major. To promote public engagement with science, the PI organizes and hosts a monthly SciTech Café open to the public, where participants discuss scientific topics with researchers in an informal setting. Technical description: This proposal develops a scanning probe technique that records real-time charge motion in thin films in response to a changing electric field, applying the technique to non-traditional semiconductors. These materials show complex transport behavior, often due to their disorder or sensitivity to the environment. The morphological non-uniformity within many of these materials and the importance of interfaces necessitates characterization tools that provide spatial resolution, while the time dependence on long and short timescales requires time resolution. Time-resolved point Kelvin probe force microscopy records the potential of the thin film at a point as it evolves in time after the application of a gate voltage, as charges are injected or extracted from the film to screen the gate potential. This real-time screening behavior permits characterization of the carrier traps, critical for understanding hysteresis in devices and in predicting and understanding open circuit voltage in solar cells. The spatial resolution enables discrimination among contributions to the observed transport behavior. Studying well characterized organic semiconductors is used to validate the technique before investigating colloidal quantum dot thin films and monolayers of molybdenum disulfide. The project is conducted at Mount Holyoke College, with the female undergraduate student body participating in all aspects of the research. The project also supports SciTech Café, which brings scientists into an informal setting to share their excitement about their work with the general public. The PI founded SciTech Café in 2012 and continues to organize and host the events, which draw 60 - 100 people each month.