Observation of Unconventional Ferroelectricity in Non-Moir'\e Graphene on Hexagonal Boron Nitride Boundaries and Interfaces

TL;DR

This work demonstrates unconventional ferroelectricity in non-moiré graphene on hBN boundaries by deliberately introducing hBN edges and line defects near the graphene. Using dual-gate measurements, the authors observe pronounced hysteresis tied to the displacement fields $D_{ ext{TG}}$ and $D_{ ext{BG}}$, and extract a population of localized charges via $n_L = n_{ ext{tot}} - n_H$, with $n_{ ext{tot}} = C_{ ext{TG}}V_{ ext{TG}} + C_{ ext{BG}}V_{ ext{BG}} - n_i$ and $n_L$ saturating around $2$–$3\times10^{12}\,\text{cm}^{-2}$. The results reveal a threshold in $D_{ ext{TG}}$ (≈0.6 V/nm) for TG-driven hysteresis and immediate BG-driven responses, highlighting distinct charging dynamics of localized states and suggesting a time-scale dependent mechanism. The findings indicate that carefully engineered hBN defects can induce and control unconventional charge polarization in vdW heterostructures, offering a pathway to defect-engineered ferroelectric functionalities beyond moiré-based systems.

Abstract

Interfacial interactions in two parallel-stacked hexagonal boron-nitride (hBN) layers facilitate sliding ferroelectricity, enabling novel device functionalities. Additionally, when Bernal or twisted bilayer graphene is aligned with an hBN layer, unconventional ferroelectric behavior was observed, though its precise origin remains unclear. Here, we propose an alternative approach to engineering such an unconventional ferroelectricity in graphene-hBN van der Waals (vdW) heterostructures by creating specific types of hBN boundaries and interfaces. We found that the unconventional ferroelectricity can occur--without the alignments at graphene-hBN or hBN-hBN interfaces--when there are hBN edges or interfaces with line defects. By systematically analyzing the gate dependence of mobile and localized charges, we identified key characteristics of localized states that underlie the observed unconventional ferroelectricity, informing future studies. These findings highlight the complexity of the interfacial interactions in graphene/hBN systems, and demonstrate the potential for defect engineering in vdW heterostructures.

Figures (18)

• Figure 1: Resistance hysteresis in three hBN-encapsulated monolayer graphene devices.a, d, g Left: Schematics comparing hysteretic devices (D1-D3) with non-hysteretic reference devices (R1-R3). Right: Optical images highlighting different types of hBN defects: a bilayer hBN edges (D1), a trilayer hBN crack (D2), and a trilayer hBN edges (D3). Red lines in the optical images indicate the hBN edges or fracture lines, which are represented by the red-colored lines in the schematic diagrams on the left. b, e, h Resistance $R_{xx}$ as a function of top gate voltage $V_{\text{TG}}$ swept in the forward (red) and backward (blue) directions for devices D1-D3 (solid lines) and R1-R3 (broken lines) at a fixed back gate voltage $V_{\text{BG}}=0$ V. c, f, i$R_{xx}$ in $V_{\text{BG}}$ swept in the forward (red) and backward (blue) directions for devices D1-D3 (solid lines) and R1-R3 (broken lines) at $V_{\text{TG}}=0$ V. Measurements were performed at temperatures of 1.4 K (D1, R1), 43 K (D2, R2), and 5.0 K (D3, R3).
• Figure 2: Dual-gate resistance hysteresis.a, b Dual-gate maps of $R_{xx}$ by sweeping $V_{\text{TG}}$ in the forward (a) and backward (b) directions as the fast axis, while gradually ramping up $V_{\text{BG}}$. c The resistance difference between (b) and (a), $\Delta R_{xx}(V_{\text{TG}},V_{\text{BG}})$. d, e Dual-gate maps of $R_{xx}$ by sweeping $V_{\text{BG}}$ in the forward (d) and backward (e) directions as the fast axis, with $V_{\text{TG}}$ slowly increasing. f The resistance difference between (e) and (d), $\Delta R_{xx}(V_{\text{TG}},V_{\text{BG}})$. Solid (dashed) arrows represent the fast (slow) sweep direction.
• Figure 3: Density of localized charges $\bm n_{\text{L}}$ and its dependence on $\bm D_{\text{TG}}$ and $\bm D_{\text{BG}}$.a Hall density $n_{\text{H}}$ versus top gate displacement field $D_{\text{TG}}$ from negative to positive (red), and from positive to negative (blue) at $V_{\text{BG}}=0$ V. The dashed black line represents $n_{\text{tot}} = C_{\text{TG}}V_{\text{TG}}$. b$n_{\text{L}}\equiv n_{\text{tot}}-n_{\text{H}}$ as a function of $D_{\text{TG}}$. c$n_{\text{L}}$ versus $n_{\text{H}}$ in a sweeping loop of $D_{\text{TG}}$. The sweeping trajectory is indicated in a and b. d$n_{\text{H}}(D_{\text{BG}})$ at $V_{\text{TG}}=0$ V from negative to positive (red), and from positive to negative (blue). The dashed black line represents $n_{\text{tot}}=C_{\text{BG}}V_{\text{BG}}$. e$n_{\text{L}}(D_{\text{BG}})$. f$n_{\text{L}}$ versus $n_{\text{H}}$ in a sweeping loop of $D_{\text{BG}}$. The sweeping trajectory is indicated in d and e. All Hall measurements are done at $B=0.5\,\text{T}$.
• Figure 4: Dependence of the localized charge density $\bm n_{\text{L}}$ on sweep range and rate.a$n_{\text{L}}$ versus $D_{\text{TG}}$ for forward (negative to positive) and backward (positive to negative) sweeps at different sweep ranges (from bottom to top: $\pm$8.1 V, $\pm$6 V, $\pm$4 V, and $\pm$2 V). The sweep rate is $6.8\,\text{mV/s}$. b$n_{\text{L}}$ versus $D_{\text{BG}}$ for forward and backward sweeps at different sweep ranges (from bottom to top: $\pm$90 V, $\pm$40 V, $\pm$10 V, and $\pm$5 V). The sweep rate is $14\,\text{mV/s}$. c$n_{\text{L}}$ versus $D_{\text{TG}}$ for forward and backward sweeps at different sweep rates. d$n_{\text{L}}$ versus $D_{\text{BG}}$ for forward and backward sweeps at different sweep rates. Curves in a--c are offset by $5\times10^{12} \, \text{cm}^{-2}$ from bottom to top, and curves in d by $7\times10^{12} \, \text{cm}^{-2}$, for clarity.
• Figure 5: top and back gate sweep dependent hysteresis.a Localized charge density $n_{\text{L}}$ versus $D_{\text{TG}}$ in forward and backward sweeps at three different values of $D_{\text{BG}}=-0.88$ V/nm (red), 0.00 V/nm (black), and 0.88 V/nm (blue). b, c$n_{\text{L}}(D_{\text{BG}})$ in forward and backward sweeps at five different values of $D_{\text{TG}}=-0.80$ V/nm (blue, b), $-0.34$ V/nm (pink, c), 0.00 V/nm (black, b), 0.34 V/nm (gray, c), and 0.80 V/nm (red, b).