Among the families of transition metal dichalcogenides (TMDs), Pd-based TMDs have been one of the less explored materials. In this study, we investigate the electronic properties of PdX 2 (X = S, Se, or Te) bulk and thin films. The analysis of structural stability shows that the bulk and thin film (1 to 5 layers) structures of PdS 2 exhibit pyrite, while PdTe 2 exhibits 1T. Furthermore, PdSe 2 exhibits pyrite in bulk and thin films down to the bilayer. Most surprisingly, PdSe 2 monolayer transits to 1T phase. For the electronic properties of the stable bulk configurations, pyrite PdS 2 and PdSe 2 , and 1T PdTe 2 , demonstrate semi-metallic features. For monolayer, on the other hand, the stable pyrite PdS 2 and 1T PdSe 2 monolayers are insulating with band gaps of 1.399 eV and 0.778 eV, respectively, while 1T PdTe 2 monolayer remains to be semi-metallic. The band structures of all the materials demonstrate a decreasing or closing of indirect band gap with increasing thickness. Moreover, the stable monolayer band structures of PdS 2 and PdSe 2 exhibit flat bands and diverging density of states near the Fermi level, indicating the presence of van Hove singularity. Our results show the sensitivity and tunability of the electronic properties of PdX 2 for various potential applications.

Ultrathin Janus two-dimensional (2D) materials are attracting intense interest currently. Substitutional doping of 2D transition metal dichalcogenides (TMDs) is of importance for tuning and possible enhancement of their electronic, physical and chemical properties toward industrial applications. Using systematic first-principles computations, we propose a class of Janus 2D materials based on the monolayers MX 2 (M = V, Nb, Ta, Tc, or Re; X = S, Se, or Te) with halogen (F, Cl, Br, or I) or pnictogen (N, P, As, Sb, or Bi) substitution. Nontrivial phases are obtained on pnictogen substitution of group VB (V, Nb, or Ta), whereas for group VIIB (Tc or Re), the nontrivial phases are obtained for halogen substitution. Orbital analysis shows that the nontrivial phase is driven by the splitting of M-d yz and M-d xz orbitals. Our study demonstrates that the Janus 2D materials have the tunability and suitability for synthesis under various conditions.

Recent studies have demonstrated the feasibility of synthesizing two-dimensional (2D) Janus materials which possess intrinsic structure asymmetry. Hence, we performed a systematic first-principles study of 2D Janus transition metal dichalcogenide...

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Topological Dirac materials are attracting a lot of attention because they offer exotic physical phenomena. An exhaustive search coupled with first-principles calculations was implemented to investigate 10 Zintl compounds with a chemical formula of CaM2X2 (M = Zn or Cd, X = N, P, As, Sb, or Bi) under three crystal structures: CaAl2Si2-, ThCr2Si2-, and BaCu2S2-type crystal phases. All of the materials were found to energetically prefer the CaAl2Si2-type structure based on total ground state energy calculations. Symmetry-based indicators are used to evaluate their topological properties. Interestingly, we found that CaM2Bi2 (M = Zn or Cd) are topological crystalline insulators. Further calculations under the hybrid functional approach and analysis using k · p model reveal that they exhibit topological Dirac semimetal (TDSM) states, where the four-fold degenerate Dirac points are located along the high symmetry line in-between Г to A points. These findings are verified through Green's function surface state calculations under HSE06. Finally, phonon spectra calculations revealed that CaCd2Bi2 is thermodynamically stable. The Zintl phase of AM2X2 compounds have not been identified in any topological material databases, thus can be a new playground in the search for new topological materials.

Recent experiments on bulk Zintl CaAl2Si2 reveal the presence of nontrivial topological states. However, the large family of two-dimensional (2D) Zintl materials remains unexplored. Using first-principles calculations, we discuss the stability and topological electronic structures of 12 Zintl single-quintuple-layer (1-QL) AM2X2 compounds in the CaAl2Si2-structure (A = Ca, Sr, or Ba; M = Zn or Cd; and X = Sb or Bi). Considering various layer-stackings, we show that the M-X-A-X-M stacking, where the transition metal M is exposed, is energetically most favorable. Phonon dispersion computations support the thermodynamic stability of all the investigated compounds. Nontrivial topological properties are ascertained through the calculation of Z2 invariants and edge states using the hybrid functional. Insulating topological phases driven by a band inversion at the Γ-point involving Bi-(px + py) orbitals are found in CaZn2Bi2, SrZn2Bi2, BaZn2Bi2, CaCd2Bi2, SrCd2Bi2, and BaCd2Bi2 with bandgaps (eV) of 0.571, 0.500, 0.025, 0.774, 0.650, and 0.655, respectively. Interestingly, van Hove singularities are found in CaCd2Bi2 and BaCd2Bi2, implying the possibility of coexisting insulating and superconducting topological phases. We discuss how topological 1-QL Zintl compounds could be synthesized through atomic substitutions resulting in Janus materials (1-QL AM2XY). In particular, the thermodynamically stable Janus BaCd2SbBi film is shown to exhibit both an insulating topological state and the Rashba effect. Our study identifies a new family of materials for developing 2D topological materials platforms and paves the way for the discovery of 2D topological superconductors.

Two-dimensional transition metal dichalcogenides (TMDs) have become well-known due to their versatile and tunable physical properties for potential applications, specifically on low-power and optical devices. Here, we explored the structural stability and electronic properties of bulk and thin-film (from 1 up to 6 layers) structures of hafnium dichalcogenides (HfX 2 , X = S, Se, or Te) using first-principles calculations. Our calculations reveal that the most stable phase is 1T for both thin films and bulk. The bulk and thin-film structures of HfTe 2 are semimetallic, while those of HfS 2 and HfSe 2 are insulating. Both HfS 2 and HfSe 2 thin films exhibit a decreasing band gap with increasing thickness, while HfTe 2 thin films remain semimetallic with increasing number of layers. Moreover, van Hove singularity (vHs), due to the contribution of the p z orbital from S atoms, is observed in 3L-HfS 2 at the valence band maximum, which can be further enhanced by applying an in-plane biaxial strain, suggesting possible superconductivity. Finally, the bulk and monolayer band structures of HfTe 2 , under HSE06 and GGA + U with the effective Hubbard U parameter of 4.6 eV, are in good agreement with the experimental ARPES data. Our results indeed show that HfX 2 have sensitive and tunable electronic properties through film thickness control and strain for future potential applications.

A family of two-dimensional (2D) materials, particularly the complex-structured Zintl phase compounds, has attracted tremendous research attention because of tunable material properties and exceptional applications. Utilizing first-principles computations under the hybrid functional approach, we performed a systematic study on A2MX2 (A = Ca, Sr, or Yb; M = Zn or Cd; X = P, As, Sb, or Bi). Among the three surface terminations considered, the metal element XM-termination (T2) was found to be the most stable structural phase with the lowest total ground state energies. Thermodynamic stability was further confirmed through phonon dispersion and formation energy calculations. Surprisingly, the T2 monolayer Sr2CdBi2 was found to host the topological insulating phase with a sizable bandgap of 556 meV, as well as a Z2 topological invariance of 1. The corresponding topologically protected edge states link the conduction and valence bands. Moreover, the topological phase is driven by the spin–orbit coupling, which causes the inverted bands at Γ point close to the Fermi level concerning the Cd- s + Bi2- s and Bi2- px + py orbitals. Moreover, Rashba and chiral spin-splittings were also observed. The computed Rashba strengths along Γ-M (αR Γ‑M) are 1.970, 0.676, and 0.669 eV·Å, whereas the computed values along Γ-K (αR Γ‑K) are 1.701, 0.731, and 0.646 eV·Å, for Ca2ZnAs2, Sr2CdBi2, and Yb2ZnAs2, respectively. Our study provides fundamental knowledge to further experimental investigations and synthesis, which may lead to electronic applications of A2MX2 compounds in quantum computing or spintronics.