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Phase transitions in two-dimensional (2D) materials promise reversible modulation of material physical and chemical properties in a wide range of applications. 2D van der Waals layered In 2 Se 3 with bistable out-of-plane ferroelectric (FE) α phase and antiferroelectric (AFE) β′ phase is particularly attractive for its electronic applications. However, reversible phase transition in 2D In 2 Se 3 remains challenging. Here, we introduce two factors, dimension (thickness) and strain, which can effectively modulate the phases of 2D In 2 Se 3 . We achieve reversible AFE and out-of-plane FE phase transition in 2D In 2 Se 3 by delicate strain control inside a transmission electron microscope. In addition, the polarizations in 2D FE In 2 Se 3 can also be manipulated in situ at the nanometer-sized contacts, rendering remarkable memristive behavior. Our in situ transmission electron microscopy (TEM) work paves a previously unidentified way for manipulating the correlated FE phases and highlights the great potentials of 2D ferroelectrics for nanoelectromechanical and memory device applications.

To suppress surface dangling bonds, monolayer oxides derived from non-layered bulks usually undergo a pronounced structural reconstruction. It remains challenging to resolve these structural reconstructions and the induced distinct modulation of intrinsic properties. In this study, the structural reconstruction-modulated electronic, polaronic, and exitonic properties of a non-layered oxide at the monolayer limit are unraveled. Based on first-principles calculations and tight-binding simulations for a stable titanium dioxide (TiO 2 ) monolayer, we show that its distinct surface Kagome sublattices host a topologically nontrivial flat band at the valence band edge. The strong electron−hole interaction in this monolayer oxide gives rise to a large exciton binding energy of around 2.49 eV. Interestingly, the monolayer TiO 2 also exhibits strong electron−lattice coupling, which favors the formation of small electron polarons and thus greatly reduces its band gap energy into the visible light range. This work could be useful to understand the structural reconstructioninduced modulation of exotic physical and chemical properties for a broad range of non-layered oxides at the monolayer limit.

SrTiO3 (STO), a room-temperature paraelectric material in bulk form, has been a rich playground for emergent phenomena for decades. As an emerging material, great attention has been paid to freestanding 2D STO thin films. Recently, the room-temperature ferroelectricity has been unveiled in strained STO thin films; however, it remains an open question whether the strain-free freestanding 2D STO thin film is room-temperature ferroelectric or not. Here, we report the electric field-induced out-of-plane ferroelectric polarization in large-scale, freestanding, and strain-free 2D STO membranes at room temperature. High-resolution piezoresponse force microscopy measurements show that polarization in freestanding strain-free STO membranes can be switched by electric field and persist for an hour. The first-principles calculations suggest that the intrinsic defects such as oxygen vacancies could be linked to the observed spontaneous out-of-plane polarization in 2D STO membranes, which could be further enhanced by external electric field due to the induced symmetry breaking. Our work reports the unprecedented room-temperature ferroelectric polarization in strain-free freestanding 2D STO membranes, unlocking the great potential of freestanding 2D STO for the applications in novel electronic and logic-in-memory devices.

Ferrimagnets with magnetic compensation temperature ( Tcomp) around room temperature are desirable due to their potential applications in low-energy consuming and high-frequency spintronic devices. In this study, the Tcomp of ferrimagnetic Mn2.21Ru0.86Ga (MRG) is tuned to near room temperature by strain. Moreover, we observed unconventional magnetoresistance behaviors for MRG-based Hall bar devices near Tcomp. First-principles calculations suggest two kinds of Mn moments, which lead to two anomalous Hall channels with opposite signs and consequently correspond to the peak structure and triple loops of the anomalous Hall effect loops. The unconventional temperature dependence of longitudinal resistivity is caused by the combined effects of two types of Mn moments and the anisotropic magnetoresistance of the MRG film. Interestingly, the spontaneous Hall angle of the MRG film is calculated to be ∼2.2%, which is one order of magnitude larger than those of other 3 d ferromagnets. Therefore, our study demonstrates MRG to be a ferrimagnet with the Tcomp near room temperature, which enables its potential applications in spintronic devices.