Ferroelectric 2D ice under graphene confinement

Abstract

We here report on the direct observation of ferroelectric properties of water ice in its 2D phase. Upon nanoelectromechanical confinement between two graphene layers, water forms a 2D ice phase at room temperature that exhibits a strong and permanent dipole which depends on the previously applied field, representing clear evidence for ferroelectric ordering. Characterization of this permanent polarization with respect to varying water partial pressure and temperature reveals the importance of forming a monolayer of 2D ice for ferroelectric ordering which agrees with ab-initio and molecular dynamics simulations conducted. The observed robust ferroelectric properties of 2D ice enable novel nanoelectromechanical devices that exhibit memristive properties. A unique bipolar mechanical switching behavior is observed where previous charging history controls the transition voltage between low-resistance and high-resistance state. This advance enables the realization of rugged, non-volatile, mechanical memory exhibiting switching ratios of 10⁶, 4 bit storage capabilities and no degradation after 10,000 switching cycles.

Fig. 1. Nanoelectromechanical actuation scheme.

(a) concept of nanoelectromechanical confinement of water between graphene, (b) schematic of employed device structure, (Inset) finite-element simulation of deformation and strain distribution upon contact (c) tapping mode AFM image of suspended top layer over drum structure, (d) map of Raman 2D band position, (e) strain distribution extracted from Raman scaling analysis corresponding to the cross-section in (d).

Fig. 2. Observation of ferroelectric polarization.

a Spatial charge distribution across drum from Raman scaling analysis of drum in Fig. 1(d), (b) time-resolved current measurement with fits to switching ($\tau =1\mu \mathrm{s}$) and polarization ($\tau =1\mathrm{ms}$) processes, (inset) high speed measurement of switching transition, (c) current-voltage characteristics obtained by PUVD technique at different polarization conditions (inset) schematic of PUVD scheme, (d) time evolution of polarization with charging, (e) high-frequency measurement of dielectric constant with fit to Debye relaxation model, (f) plot of extracted relaxation time constant $\tau$ during switching.

Fig. 3. Origin of ferroelectric effect.

(a) DFT simulation results of confined water relaxed structure displaying ice XI configuration, (b) electrostatic potential distribution (relative to origin) along the z-axis of pristine graphene and graphene/ice structure with overlaid 3D image of relaxed structure (c) current-voltage characteristics of the device under different amounts of water coverage, (d) tunneling distance between electrodes and polarization as a function of water coverage, (e) bond distance histogram of structures relaxed at different temperatures through molecular dynamics simulation, (f, g) Temperature dependence of (f) high-frequency relaxation time constant and (g) PUVD polarization.

Fig. 4. Nanoelectromechanical memristor.

a current-voltage characteristics of a mechanical switch demonstrating a novel bipolar switching behavior, b change of electrical performance (on/off ratio) and defectiveness (Raman I_D/I_G ratio) during repeated cycling (c) current after single write cycle at 25 V and various delay and read cycles at 0 V and −5 V, respectively, (inset) representation of pulse sequence, (d) remnant polarization as a function of previously applied highest voltage (V_prime), (inset) representation of polarization hysteresis, (e) evolution of device resistance depending on previously applied priming voltage, (f) change of reset transition voltage as a function of previously applied priming voltage.