Mechanical control of the electronic properties of 2D ferroelectrics

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Nontechnical description Ferroelectric materials are characterized by a separation and arrangement of negative vs positive charges that can be instantaneously flipped by applying of an electric field. Those oppositely aligned, or polarized, states can be stored and read as the 1s and 0s of binary data, with the states remaining stable even when a power source has been cut. This provides a basis for use of these materials in nonvolatile memory applications as well as in a number of other advanced electronic devices. Typically, this property is found in the bulk crystals, ceramics and thin films. Recently, theoretical modeling predicted that two-dimensional (2D) layered materials consisting of the atomic planes bonded by weak forces could also exhibit polarized states. However, electrical flipping of the electrical charges in these materials is extremely difficult as application of the electrical bias gives rise to a high current that simply burns the sample. This project explores mechanical stress as a novel method to modulate the electronic charges and the associated changes in electrical resistance. A tiny conducting probe is used to apply local pressure to the sample and to detect the resulting polarization as well as the change in the sample conductance at the nanoscale level. The project enhances the research, education, and outreach missions of the University of Nebraska by bringing students of all races, genders and backgrounds into science, technology and engineering.Technical description Realizing advanced electronic devices based on two-dimensional (2D) ferroelectrics depends on the ability to deterministically control their polarization state. High electrical conductance of these materials precludes using conventional electric methods for testing and manipulating the polarization response. The overarching objective of this research is to investigate the tunability of the electronic properties of 2D ferroelectrics via deterministic voltage-free control of polarization. The proposed research focuses on three closely related components: (1) experimentally investigating the flexoelectric behavior of 2D ferroelectrics; (2) investigating the polarization reversal in 2D materials by mechanical means; (3) evaluating the role of strain and strain gradient in modulation of their polarization-coupled electronic properties. Scanning probe microscopy techniques will be used to explore an interplay between the local transport properties and flexoelectrically-enabled nanoscale domain engineering. Utilizing the flexoelectric effect represents a new paradigm for controlling the polarization-coupled electronic behavior, which is applicable to any 2D ferroelectric material irrespective of its electronic properties. The most important outcome of this research is to develop conceptually novel electronic devices where dynamic mechanical stimulation allows modulation of a variety of the functional properties coupled to polarization, such as electromechanical response, two-dimensional conductivity and photovoltaic effect.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.