New Polymorphs of 2D Indium Selenide with Enhanced Electronic Properties

Abstract

The 2D semiconductor indium selenide (InSe) has attracted significant interest due its unique electronic band structure, high electron mobility, and wide tunability of its band gap energy achieved by varying the layer thickness. All these features make 2D InSe a potential candidate for advanced electronic and optoelectronic applications. Here, the discovery of new polymorphs of InSe with enhanced electronic properties is reported. Using a global structure search that combines artificial swarm intelligence with first-principles energetic calculations, polymorphs that consist of a centrosymmetric monolayer belonging to the point group D _3d are identified, distinct from well-known polymorphs based on the D _3h monolayers that lack inversion symmetry. The new polymorphs are thermodynamically and kinetically stable, and exhibit a wider optical spectral response and larger electron mobilities compared to the known polymorphs. Opportunities to synthesize these newly discovered polymorphs and viable routes to identify them by X-ray diffraction, Raman spectroscopy, and second harmonic generation experiments are discussed.

Keywords: 2D materials; electronics and optoelectronics; indium selenide; materials by design.

Figure 1

a) Evolution of the energy of the predicted structures as a function search generation. The zoomed‐in low‐energy region is dominated by three types of polymorphs (shown in green, red, and blue dots). b) The monolayer structures of three types of polymorphs (C _2h, D _3h, and D _3d) shown in the corresponding colored boxes. c) Side views of the three energetically favorable bulk phases, named as δ(D _3d), ω(D _3d), and ϕ(D _3d), respectively. The rectangular frames with arrows are a visual aid to the particular stacking pattern of the layers.

Figure 2

a) Energy barrier and atomic structures during the transformation from the D _3d to the D _3h monolayer. The transition‐state (TS) structure is indicated. b) Fluctuations of the total potential energy of D _3d monolayer during the molecular dynamics simulation at 300 and 500 K, respectively. c) Phonon spectrum of the D _3d monolayer. d) Phonon spectra of δ(D _3d), ω(D _3d), and ϕ(D _3d). The shear and breathing mode branches are shown in red and blue, respectively.

Figure 3

a) Calculated band structures of the D _3d and D _3h monolayers. The band structures have been projected onto atomic orbitals with the blue color representing Se‐p_z and red representing Se‐p_{x / y}. b) Evolution of the band gap with increasing number of layers of the predicted three D _3d monolayer based polymorphs and two known D _3h monolayer based polymorphs. c) Interlayer differential charge densities of bilayer δ(D _3d), ω(D _3d), ϕ(D _3d), β(D _3h), and γ(D _3h), respectively. The isosurface value is set to 1 × 10⁻⁴ electrons per ų. The charge accumulation and depletion are shown in yellow and blue, respectively.

Figure 4

Evolution of electron mobilities along the a) armchair and b) zigzag directions with increasing number of layers for three D _3d monolayer based polymorphs and two known D _3h monolayer based polymorphs.

Figure 5

a) XRD patterns of δ(D _3d), ω(D _3d), and ϕ(D _3d) predicted by theory. The reference PDF data of bulk β(D _3h) and γ(D _3h) are also shown for comparison. b) Raman spectra of ϕ(D _3d) and γ(D _3h) phases from DFT simulations (left panel). The zoomed‐in part of Raman spectra between 150 and 200 cm⁻¹, the inset shows the vibrational modes of the constituent D _3d/D _3h monolayer (right panel).