Strain-Induced Ferroelectricity and Second-Harmonic Generation in a TaOI₂ Monolayer

Area of Science:

• Materials Science
• Condensed Matter Physics
• Solid State Chemistry

Background:

• Two-dimensional (2D) layered niobium oxide dihalides (NbOX2) exhibit notable ferroelectric and second-harmonic generation (SHG) properties.
• 2D TaOI2 materials, despite structural similarity to NbOX2, lack ferroelectric polarization and SHG due to inversion symmetry.

Purpose of the Study:

• To investigate the potential for inducing ferroelectric polarization and SHG in 2D TaOI2 monolayers.
• To explore the role of strain engineering in overcoming inversion symmetry limitations.
• To understand the interplay between strain, exciton effects, and nonlinear optical properties.

Main Methods:

• First-principles calculations were employed to model 2D TaOI2.
• Tensile strain was applied along the Ta-O direction to induce phase transitions.
• Exciton effects were analyzed for their influence on optical and SHG responses.
• Volcano plots were used to correlate SHG coefficients with ferroelectric polarization and optical gap.

Main Results:

• A nonpolar to ferroelectric phase transition was achieved in 2D TaOI2 under tensile strain.
• Significant ferroelectric polarization and SHG response were induced in the strained TaOI2 monolayer.
• Exciton effects were found to be crucial for linear absorbance and SHG, tunable by strain.
• A competitive relationship between ferroelectric polarization and optical gap in determining SHG coefficients was revealed.

Conclusions:

• Tensile strain is an effective method to engineer ferroelectric and nonlinear optical properties in 2D TaOI2.
• This work demonstrates a viable pathway for developing 2D ferroelectric and nonlinear optical materials from inversion-symmetric precursors.
• The findings have significant implications for the design of next-generation electronic and photonic devices.