A single-ion conductor was synthesized and used to electrostatically gate suspended MoTe₂ field-effect transistors (FETs).  MoTe₂ is a two-dimensional, layered material with in-plane bonds and no dangling bonds out of plane.  MoTe₂ is a particularly interesting 2D crystal because it can undergo a semiconducting (2H) to metallic (1T) phase transition when strained by only ~3%.  That is, it behaves like a semiconducting material under no strain, and a good electrical conductor under a small amount of strain. The goal of this project was to use a single-ion conductor over a suspended MoTe₂ channel to induce the 2H to 1T phase transition by field effect. The practical goal was to induce this transition to create a low-voltage switch.

Intellectual merit:  This project made a fundamental contribution by reporting on the basic physics of using a single ion conductor as an electric double layer gate.  In the first paper, (Xu et al., ACS Appl. Mater. Interfaces 2019, 11, 35879), we experimentally demonstrated that a custom synthesized, single-ion conductor can gate a supported MoTe₂ FET to a similar extent as a dual-ion conductor, and used modeling to quantify the electrostatic imbalance that resulted. In the second paper (Woeppel, et al, ACS Appl. Mater. Interfaces 2020, 12, 36, 40850), we quantify the sheet density as 7 x 10¹³ ions/cm^-2, which is the same order of magnitude as a dual ion conductor.  We also showed the importance of having a large gate-to-channel size ratio when gating with a single ion conductor. An important (and as yet unpublished) outcome of this project is demonstrating the semiconducting (2H) to (1T) transition for a partially suspended MoTe₂ FET using a combination of Raman spectroscopy and electrical measurements. This project also contributed fundamental insights into a polymerizable single-ion conductor (Arora, et al., ACS Materials Letters 2020, 2, 4, 331) and how this material can be used to lock a p-n junction (Liang, Materials 2020, 13, 1089).

Broader impacts: This work advanced fundamental understanding in the area of electrolyte-gated electronics.  Ion-gating is a technique used by both the inorganic and organic semiconductor communities, and so insights gained here will be largely translatable to other communities.  Moreover, this study showed a completely new approach to tune the electrolyte and to impart a new functionality (i.e., inducing the phase change).  Although only part of the channel was transformed to a new phase (because it was only partially suspended to start with), this work demonstrated proof-of-concept for what could lead to a low-voltage switch; notably, the partial transition occurred at 2V which is the smallest voltage for a field-induced transition of MoTe2 that could be identified in the literature. This project contributed significantly to the PhD thesis of 2 students, the MS thesis of 1 student, and one undergraduate researcher (Woeppel) published a first-authored paper based on his work on this project.  The PI participated in several K-12 outreach activities with demonstrations on various phases of materials and active learning about polymers. The students supported on this grant participated in outreach to high school students through the Pittsburgh Quantum Institute (PQI).    

Last Modified: 10/17/2021
Modified by: Susan Fullerton