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.
The electronic and magnetic properties of transition metal dichalcogenides are known to be extremely sensitive to their structure. In this paper we study the effect of structure on the electronic and magnetic properties of mono-and bilayer VSe 2 films grown using molecular beam epitaxy. VSe 2 has recently attracted much attention due to reports of emergent ferromagnetism in the two-dimensional (2D) limit. To understand this compound, high-quality 1T and distorted 1T films were grown at temperatures of 200°C and 450°C, respectively, and studied using 4 K scanning tunneling microscopy and spectroscopy. The measured density of states and the charge density wave (CDW) patterns were compared to band structure and phonon dispersion calculations. Films in the 1T phase reveal different CDW patterns in the first layer compared to the second. Interestingly, we find the second layer of the 1T film shows a CDW pattern with 4a × 4a periodicity which is the 2D version of the bulk CDW observed in this compound. Our phonon dispersion calculations confirm the presence of a soft phonon at the correct wave vector that leads to this CDW. In contrast, the first layer of distorted 1T phase films shows a strong stripe feature with varying periodicities, while the second layer displays no observable CDW pattern. Finally, we find that the monolayer 1T VSe 2 film is weakly ferromagnetic, with ∼3.5 μ B per unit similar to previous reports.
Probing the effects of thin-film
thickness on transition metal
dichalcogenides offer novel insights into their electronic properties
and tunability, which leads to a new avenue of research and applications.
A comprehensive first-principles study on thickness-dependent structural
stabilities and electronic properties of ZrX2 (X = S, Se,
or Te) thin films from 1 layer (L) to 6L and bulk was performed. The
calculated formation energies show that ZrX2 adopts the
1T phase as the most stable structure. Furthermore, 1T-ZrS2 and ZrSe2 thin films and bulk are indirect semiconductors
and their band gaps decrease as the number of layers is increased
up to 6L, while 1T-ZrTe2 thin films and bulk are semimetallic.
Interestingly, we demonstrate that the surface band structure of bulk
and monolayer ZrTe2 under generalized gradient approximation
+ U and HSE06 methods is in excellent agreement with
the angle-resolved photoelectron spectroscopy measurement. Finally,
we discover the existence of van Hove singularities in strained 2L
and unstrained 3L 1T-ZrS2 thin films, implying the existence
of superconductivity in these thin films. These results showcase the
tunable electronic properties of ZrX2 thin films because
of thickness dependence and strain.
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.
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.
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