Structural Semiconductor-to-Semimetal Phase Transition in Two-Dimensional Materials Induced by Electrostatic Gating for Novel Electronic Applications

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Our work shows that dynamic control of conductivity via atomic structure change in two-dimensional (2D) materials can be achieved by electrostatic gating. Our findings identify a new physical mechanism, not existing in bulk materials, to dynamically control structural phase transitions in 2D materials, enabling potential applications in phase-change electronic devices and unique refrigeration effects (Electrostaticaloric effect).

By developing new density functional-based methods, we discover that electrostatic gating device configurations have the potential to drive structural semiconductor-to-semimetal phase transitions in some monolayer transition metal dichalcogenides (TMDs). We show that the semiconductor-to-semimetal phase transition in monolayer MoTe2 can be driven by a gate voltage of several volts with appropriate choice of dielectric.


Phase control of monolayer MoTe2 through gating at constant stress and constant area.

We find that the transition gate voltage can be reduced arbitrarily by alloying, for example, for MoxW1-xTe2 monolayers.


Reducing transition gate voltages with the alloy MoxW1-xTe2.

Our discoveries have exciting potential applications in ultrathin flexible 2D electronic devices. If the kinetics of the transformation are suitable, nonvolatile phase-change memory or neuromorphic computing may be an application. One might expect 2D materials to have energy consumption advantages over bulk materials due to their ultrathin thickness. If the kinetics is sufficiently fast, another potential application may be subthreshold swing reduction in field-effect transistors to overcome the scaling limit of conventional transistors. In addition, the change in the transmittance of light due to the phase transition of 2D materials may be employed in infrared optical switching devices, such as infrared optical shutters and modulators for cameras, window coating and infrared antennas with tunable resonance.

Using these results, we have predicted the theoretical existence of new alternate cooling mechanism, the “electrostaticaloric effect”. We have predicted a temperature change of 10–15 K may be possible in devices that utilize monolayer MoTe2 as the active phase change material. This mechanism may prove useful for future electrical devices which require cooling at the component level, and for which small monolayer devices are necessary.


A Carnot cycle of the electrostaticaloric cooling effect compared with the traditional PV Carnot cycle

Current Research: Currently we are studying the impact of this effect on the thermodynamics of electrons to try and design a system that can achieve refrigeration at very small timescales (femtoseconds).

Contact: Akash Ramdas, akashr_at_stanford.edu

Publication:

Rehn, D.A., Li, Y., and Reed, E.J. Refrigeration in 2D: Electrostaticaloric effect in monolayer materials. Physical Review Materials 2, 114004 (2018). doi:10.1103/PhysRevMaterials.2.114004

Li, Y., Duerloo, K.-A. N., Wauson, K., Reed, E. J., Structural semiconductor-to-semimetal phase transition in two-dimensional materials induced by electrostatic gating. Nature Communications, doi:10.1038/ncomms10671 (2016).

News coverage:

Imagine a "cool" data-storage technology that's just a few atoms thick. Stanford Engineering News, May 4, 2016