Structural engineering of bilayer PtSe2 thin films: a first-principles study

Layer-number and strain effects on the structural and electronic properties of PtSe2material

Bandgap engineering of low-dimensional materials forms a robust basis for advancements in optoelectronic technologies. Platinum diselenide (PtSe2) material exhibits a transition from semi-metal to semiconductor (SM-SC) when going from bulk to monolayer. In this work, density functional theory (DFT) with various van der Waals (vdW) corrections has been tested to study the effect of the layer-number on the structural and electronic properties of the PtSe2material. The considered vdW corrections gave different results regarding the number of layers at which the SM-SC transition occurs. This variation is due to the different interlayer distances found for each correction, revealing the sensitivity of the bandgap to this distance in addition to the layer number. In fact, the bandgap increases with the increasing of the interlayer distance, due to the energy shift of conduction and valence bands dominated by Se-pzorbitals. According to the comparison with the available experimental data, the vdW corrections vdW-DF and rVV10 gave the most accurate results. Moreover, the control of the interlayer distance via vertical compressive strain led to the bandgap tuning of semiconductor PtSe2BL. Indeed, a semi-metal character of PtSe2BL can be obtained under 17% vertical strain. Our work shows a deep understanding of the correlation between the structural and electronic properties, and thus a possibility to tune the bandgap by strain means.

Probing Carrier Dynamics in Large-Scale MBE-Grown PtSe2 Films by Terahertz Spectroscopy

Atomically thin platinum diselenide (PtSe2) films are promising for applications in the fields of electronics, spintronics, and photodetectors owing to their tunable electronic structure and high carrier mobility. Using terahertz (THz) spectroscopy techniques, we investigated the layer-dependent semiconducting-to-metallic phase transition and associated intrinsic carrier dynamics in large-scale PtSe2 films grown by molecular beam epitaxy. The uniformity of large-scale PtSe2 films was characterized by spatially and frequency-resolved THz-based sheet conductivity mapping. Furthermore, we use an optical-pump-THz-probe technique to study the transport dynamics of photoexcited carriers and explore light-induced intergrain carrier transport in PtSe2 films. We demonstrate large-scale THz-based mapping of the electrical properties of transition metal dichalcogenide films and show that the two noncontact THz-based approaches provide insight in the spatial and temporal properties of PtSe2 films.

First-principles study on optoelectronic properties of Cs2PbX4-PtSe2 van der Waals heterostructures

In order to achieve low-cost, high efficiency and stable photoelectric devices, two-dimensional (2D) inorganic halide perovskite photosensitive layers need to cooperate with other functional layers. Here, we investigate the structure, stability and optical properties of perovskite and transition metal dichalcogenide (TMD) heterostructures using first-principles calculations. Firstly, Cs2PbX4-PtSe2 (X = Cl, Br, I) heterostructures are stable because of negative interface binding energy. With the halogen varying from Cl to I, the interface binding energies of Cs2PbX4-PtSe2 heterostructures decrease rapidly. 2D Cs2PbCl4-PtSe2, Cs2PbBr4-PtSe2 and Cs2PbI4-PtSe2 heterostructures have an indirect bandgap with the value of 1.28, 1.02, and 1.29 eV, respectively, which approach the optimal bandgap (1.34 eV) for solar cells. In the contact state, the electrons transfer from the PtSe2 monolayer to Cs2PbX4 monolayer and only the Cs2PbBr4-PtSe2 heterostructure maintains the type-II band alignment. The Cs2PbBr4-PtSe2 heterostructure has the strongest charge transfer among the three Cs2PbX4-PtSe2 heterostructures because it has the lowest tunnel barrier height (ΔT) and the highest potential difference value (ΔEP). Furthermore, the light absorption coefficient of Cs2PbX4-MSe2 heterostructures is at least two times higher than that of monolayer 2D inorganic halide perovskites. With the halogen varying from Cl to I, the light absorption coefficients of the Cs2PbX4-PtSe2 heterostructures increase rapidly in the visible region. Above all, the Cs2PbX4-MSe2 heterostructures have broad application prospects in photodetectors, solar cells and other fields.

Atomic Structure of Dislocations and Grain Boundaries in Two-Dimensional PtSe2

Each 2D material has a distinct structure for its grain boundary and dislocation cores, which is dictated by both the crystal lattice geometry and the elements that participate in bonding. For the class of noble metal dichalcogenides, this has yet to be thoroughly investigated at the atomic scale. Here, we examine the atomic structure of the dislocations and grain boundaries (GBs) in two-dimensional PtSe₂, using atomic-resolution annular dark field scanning transmission electron microscopy, combined with density functional theory and empirical force field calculations. The PtSe₂ we study adopts the 1T phase in large-area polycrystalline films with numerous planar tilt GB distinct dislocations, including 5|7_+Se and 4|4|8_+Se polygons, in tilt-angle monolayer GBs, with features sharply distinguished from those in 2H-phase TMDs. On the basis of dislocation cores, the GB structures are investigated in terms of pathways of dislocation chain arrangement, dislocation core distributions in different misorientation angles, and 2D strain fields induced. Based on the Frank-Bilby equation, the deduced Burgers vector magnitude is close to the lattice constant of 1T-PtSe₂, building the quantitative relationship of dislocation spacings and small GB angles. The 30° GBs are most frequently formed as a stitched interface between the armchair and zigzag lattices, constructed by a string of 5|7_+Se dislocations asymmetrically with a small deviation angle. Another special angle GB, mirror twin 60° GB, is also mapped linearly by metal-condensed asymmetric or Se-rich symmetric dislocations. This report gives atomic-level insights into the GBs and dislocations in 1T-phase noble metal TMD PtSe₂, which is a promising material to underpin extending properties of 2D materials by local structure engineering.