Out-of-plane Piezoelectricity and Ferroelectricity in Layered $α$-In2Se3 Nano-flakes

Abstract: Piezoelectric and ferroelectric properties in the two dimensional (2D) limit are highly desired for nanoelectronic, electromechanical, and optoelectronic applications. Here we report the first experimental evidence of out-of-plane piezoelectricity and ferroelectricity in van der Waals layered ${\alpha}$-In2Se3 nano-flakes. The non-centrosymmetric R3m symmetry of the ${\alpha}$-In2Se3 samples is confirmed by scanning transmission electron microscopy, second-harmonic generation, and Raman spectroscopy measurements. Domains with opposite polarizations are visualized by piezo-response force microscopy. Single-point poling experiments suggest that the polarization is potentially switchable for ${\alpha}$-In2Se3 nano-flakes with thicknesses down to ~ 10 nm. The piezotronic effect is demonstrated in two-terminal devices, where the Schottky barrier can be modulated by the strain-induced piezopotential. Our work on polar ${\alpha}$-In2Se3, one of the model 2D piezoelectrics and ferroelectrics with simple crystal structures, shows its great potential in electronic and photonic applications.

• The paper demonstrates switchable out-of-plane piezoelectricity and ferroelectricity in α-In2Se3 nanoflakes using STEM, PFM, and SHG techniques.
• Advanced imaging confirmed the non-centrosymmetric R3m structure and highlighted strong nonlinear optical properties compared to benchmark materials.
• The results suggest promising applications in flexible nanoelectromechanical systems, including strain sensors and non-volatile memory devices.

Overview of Out-of-plane Piezoelectricity and Ferroelectricity in Layered α-In<sub\>2</sub>Se<sub\>3</sub> Nanoflakes

This paper presents a comprehensive study of the piezoelectric and ferroelectric properties of van der Waals layered α-In<sub\>2</sub>Se<sub\>3</sub> nanoflakes. In recent years, two-dimensional (2D) materials have garnered significant interest due to their multifunctional applications in the fields of nanoelectronics, optoelectronics, and electromechanics. However, out-of-plane piezoelectricity and ferroelectricity, which are critical for efficient device architecture, particularly in sensor and memory applications, have been less explored in 2D materials. The study addresses this gap by reporting empirical evidence of switchable out-of-plane piezoelectricity and ferroelectricity in α-In<sub\>2</sub>Se<sub\>3</sub> with rhombohedral $R3m$ symmetry.

Experimental Methods and Observations

Techniques such as scanning transmission electron microscopy (STEM) and piezo-response force microscopy (PFM) were utilized to establish the structural and electric properties of α-In<sub\>2</sub>Se<sub\>3</sub> nanoflakes. The non-centrosymmetric $R3m$ structure was validated through these high-resolution imaging techniques, providing clear visualization of domains with opposite polarization. This implies potential switchable polarization functionality in flakes as thin as approximately 10 nm.

Second-harmonic generation (SHG) microscopy demonstrated non-zero signals emanating from the bulk crystal rather than mere surface effects, a feature confirming the non-centrosymmetric nature underlying effective ferroelectric behavior. Furthermore, the experimentation with PFM showed defined domains with a 180° phase difference, aligning with the presence of switchable polarization perpendicularly oriented to the flake’s surface.

Strong Numerical Results and Implications

Quantitative insights were achieved through SHG, revealing that the SHG signal in thick α-In<sub\>2</sub>Se<sub\>3</sub> exhibits considerably higher intensity than that from benchmark materials like GaAs, which thereby underscores the strong non-linear optical properties of α-In<sub\>2</sub>Se<sub\>3</sub>.

In the domain of nano-electromechanical systems (NEMS), the study evidences practical implications by demonstrating a flexible device architecture utilizing α-In<sub\>2</sub>Se<sub\>3</sub>. By applying mechanical strain, a piezotronic effect was observed, wherein the source-drain current was modulated, signifying the utility of these nanoflakes in electromechanical energy transduction. The Schottky barrier modulation due to piezopotential variation paves the way for future technological applications such as strain sensors and components in flexible electronics.

Speculations on Future Development

The paper suggests that by addressing extrinsic leakage through optimal doping strategies or alternative fabrication techniques like molecular-beam epitaxy, the bulk carrier density could be minimized, enabling ferroelectric behavior even for monolayer materials. Such advancements are expected to enhance the role of α-In<sub\>2</sub>Se<sub\>3</sub> in non-volatile memory and sensor technologies. This research contributes significantly towards the expansion of 2D van der Waals heterostructures with potential breakthroughs in integrated photonics and electronics.

Ultimately, this work aligns with the broader trend of advancing functional and structural design of 2D materials, lending insights into fabricating new generations of electronic devices characterized by high performance and efficiency. The results prompt further exploration in the manipulation of polarization dynamics and the development of heterostructure-based multifunctional devices.