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Prediction of intrinsic two-dimensional ferroelectrics in In 2 Se 3 and other III 2 -VI 3 van der Waals materials

View Article: PubMed Central - PubMed

ABSTRACT

Interest in two-dimensional (2D) van der Waals materials has grown rapidly across multiple scientific and engineering disciplines in recent years. However, ferroelectricity, the presence of a spontaneous electric polarization, which is important in many practical applications, has rarely been reported in such materials so far. Here we employ first-principles calculations to discover a branch of the 2D materials family, based on In2Se3 and other III2-VI3 van der Waals materials, that exhibits room-temperature ferroelectricity with reversible spontaneous electric polarization in both out-of-plane and in-plane orientations. The device potential of these 2D ferroelectric materials is further demonstrated using the examples of van der Waals heterostructures of In2Se3/graphene, exhibiting a tunable Schottky barrier, and In2Se3/WSe2, showing a significant band gap reduction in the combined system. These findings promise to substantially broaden the tunability of van der Waals heterostructures for a wide range of applications.

No MeSH data available.


Effects of external electric fields.The calculated activation barrier (black circles) and energy difference (grey squares) between the initial and final states (the insets) in the electric polarization reversal process of 1 quintuple layer In2Se3 via the concerted motion as illustrated in Fig. 2b, plotted as a function of the external electric field applied in the out-of-plane direction (a) and in-plane [110] direction (b), respectively. The directions of the applied external electric fields are indicated by the blue arrows in the insets.
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f3: Effects of external electric fields.The calculated activation barrier (black circles) and energy difference (grey squares) between the initial and final states (the insets) in the electric polarization reversal process of 1 quintuple layer In2Se3 via the concerted motion as illustrated in Fig. 2b, plotted as a function of the external electric field applied in the out-of-plane direction (a) and in-plane [110] direction (b), respectively. The directions of the applied external electric fields are indicated by the blue arrows in the insets.

Mentions: Next, we turn to the key prediction that each of the degenerate ground-state structures of the In2Se3 QL is an intrinsic 2D ferroelectric material with both out-of-plane and in-plane electric polarization. As shown in Fig. 1f,g, the Se atoms in both the top and the bottom surface layers reside on the hollow sites of the respective second-layer In atoms, while each atom in the central Se layer is tetrahedrally coordinated by the two neighbouring In layers, with one Se–In bond connecting to one side vertically and three Se–In bonds to the other side. As a result, the interlayer spacing between the central Se layer and the two In layers is dramatically different, effectively breaking the centrosymmetry and providing the very underlying basis for the emergence of the spontaneous out-of-plane electric polarization. The calculated magnitudes of the electric dipoles for one QL In2Se3 in the degenerate ground states are both around 0.11 eÅ per unit cell (calculated by HSE06, 0.094 eÅ per unit cell by generalized gradient approximation-Perdew-Burke-Ernzerhof (GGA-PBE)). In addition, each ground-state structure hosts two equivalent states with opposite electric polarizations, which only differ by the energetically degenerate positions of the central Se-layer atoms. Specifically, in the FE-ZB′ structure illustrated in Fig. 2a, the atoms in the central Se layer are at the B sites vertically aligned with the lower In layer (left in Fig. 2a), and the resultant electric dipole points downwards; by moving the central Se layer to the neighbouring C sites aligned with the upper In layer (right in Fig. 2a), the resultant electric dipole points upwards. In addition, the α-phase FE-ZB′ and FE-WZ′ structures also have in-plane electric polarization due to the in-plane centrosymmetry breaking. The in-plane electric polarization is along the [110] direction defined by the lattice vectors as illustrated in the insets of Fig. 3b. The magnitude of the in-plane electric polarization is calculated to be 2.36 and 7.13 eÅ per unit cell for the FE-ZB′ and FE-WZ′ phases, respectively, using the Berry phase approach. This difference can be attributed to the ions that make the two structures deviate from the non-polar reference structure possessing different numbers of charge, as illustrated in Supplementary Fig. 10a.


Prediction of intrinsic two-dimensional ferroelectrics in In 2 Se 3 and other III 2 -VI 3 van der Waals materials
Effects of external electric fields.The calculated activation barrier (black circles) and energy difference (grey squares) between the initial and final states (the insets) in the electric polarization reversal process of 1 quintuple layer In2Se3 via the concerted motion as illustrated in Fig. 2b, plotted as a function of the external electric field applied in the out-of-plane direction (a) and in-plane [110] direction (b), respectively. The directions of the applied external electric fields are indicated by the blue arrows in the insets.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5385629&req=5

f3: Effects of external electric fields.The calculated activation barrier (black circles) and energy difference (grey squares) between the initial and final states (the insets) in the electric polarization reversal process of 1 quintuple layer In2Se3 via the concerted motion as illustrated in Fig. 2b, plotted as a function of the external electric field applied in the out-of-plane direction (a) and in-plane [110] direction (b), respectively. The directions of the applied external electric fields are indicated by the blue arrows in the insets.
Mentions: Next, we turn to the key prediction that each of the degenerate ground-state structures of the In2Se3 QL is an intrinsic 2D ferroelectric material with both out-of-plane and in-plane electric polarization. As shown in Fig. 1f,g, the Se atoms in both the top and the bottom surface layers reside on the hollow sites of the respective second-layer In atoms, while each atom in the central Se layer is tetrahedrally coordinated by the two neighbouring In layers, with one Se–In bond connecting to one side vertically and three Se–In bonds to the other side. As a result, the interlayer spacing between the central Se layer and the two In layers is dramatically different, effectively breaking the centrosymmetry and providing the very underlying basis for the emergence of the spontaneous out-of-plane electric polarization. The calculated magnitudes of the electric dipoles for one QL In2Se3 in the degenerate ground states are both around 0.11 eÅ per unit cell (calculated by HSE06, 0.094 eÅ per unit cell by generalized gradient approximation-Perdew-Burke-Ernzerhof (GGA-PBE)). In addition, each ground-state structure hosts two equivalent states with opposite electric polarizations, which only differ by the energetically degenerate positions of the central Se-layer atoms. Specifically, in the FE-ZB′ structure illustrated in Fig. 2a, the atoms in the central Se layer are at the B sites vertically aligned with the lower In layer (left in Fig. 2a), and the resultant electric dipole points downwards; by moving the central Se layer to the neighbouring C sites aligned with the upper In layer (right in Fig. 2a), the resultant electric dipole points upwards. In addition, the α-phase FE-ZB′ and FE-WZ′ structures also have in-plane electric polarization due to the in-plane centrosymmetry breaking. The in-plane electric polarization is along the [110] direction defined by the lattice vectors as illustrated in the insets of Fig. 3b. The magnitude of the in-plane electric polarization is calculated to be 2.36 and 7.13 eÅ per unit cell for the FE-ZB′ and FE-WZ′ phases, respectively, using the Berry phase approach. This difference can be attributed to the ions that make the two structures deviate from the non-polar reference structure possessing different numbers of charge, as illustrated in Supplementary Fig. 10a.

View Article: PubMed Central - PubMed

ABSTRACT

Interest in two-dimensional (2D) van der Waals materials has grown rapidly across multiple scientific and engineering disciplines in recent years. However, ferroelectricity, the presence of a spontaneous electric polarization, which is important in many practical applications, has rarely been reported in such materials so far. Here we employ first-principles calculations to discover a branch of the 2D materials family, based on In2Se3 and other III2-VI3 van der Waals materials, that exhibits room-temperature ferroelectricity with reversible spontaneous electric polarization in both out-of-plane and in-plane orientations. The device potential of these 2D ferroelectric materials is further demonstrated using the examples of van der Waals heterostructures of In2Se3/graphene, exhibiting a tunable Schottky barrier, and In2Se3/WSe2, showing a significant band gap reduction in the combined system. These findings promise to substantially broaden the tunability of van der Waals heterostructures for a wide range of applications.

No MeSH data available.