Structure, Stability, and Superconductivity of Two-Dimensional Janus NbSH Monolayers: A First-Principle Investigation

Abstract

Two-dimensional Janus materials have unique structural characteristics due to their lack of out-of-plane mirror symmetry, resulting in many excellent physical and chemical properties. Using first-principle calculations, we performed a detailed investigation of the possible stable structures and properties of two-dimensional Janus NbSH. We found that both Janus 1T and 2H structures are semiconductors, unlike their metallic counterparts MoSH. Furthermore, we predicted a new stable NbSH monolayer using a particle swarm optimization method combined with first-principle calculations. Interestingly, the out-of-plane mirror symmetry is preserved in this newly found 2D structure. Furthermore, the newly found NbSH is metallic and exhibits intrinsic superconducting behavior. The superconducting critical temperature is about 6.1 K under normal conditions, which is found to be very sensitive to stress. Even under a small compressive strain of 1.08%, the superconducting critical temperature increases to 9.3 K. In addition, the superconductivity was found to mainly originate from Nb atomic vibrations. Our results show the diversity of structures and properties of the two-dimensional Janus transition metal sulfhydrate materials and provide some guidelines for further investigations.

Superconductivity in monolayer Janus titanium-sulfur hydride (TiSH) at ambient pressure

“…There is a clear from figure 3(b), that the S dominant phonon modes and Ti dominant optical modes are the major contribution, which contribute around 78% of λ. This observation is unique in this class of 2D materials, as similar behavior is observed in MoSH and NbSH, where λ increases to ∼95% and ∼99% of the total value due to the acoustic and the first optical regions [41,43]. Nevertheless, the comparison with the superhydrides, such as LaH 10 , shows contrary behavior where the optical phonon modes contribute mostly to λ [56].…”

“…In our case, the predicted value of T c is 9.24 K with the corresponding value of ω ln = 21.74 meV. This can be compared with those of NbSH (6.10 K, 14.7 meV), MoSH (28.92 K, 15.36 meV) and WSH (12.20 K, 13.90 meV) [41][42][43]. For a detailed comparison between TiSH and other reported 2D monohydrides (NbSH, MoSH and WSH), we extracted spectral function data from the graph reported in MoSH [41] using an online graph digitization tool [60].…”

“…The first set is constituted by the acoustic modes, and it ranges from 0 meV to 17.63 meV with the biggest contribution from the S atom. This observation is quite deviating from rule-of-thumb relationship (ω ∝ 1 / √ m ) between frequency and mass of the system, It is obeyed in similar way for MoSH [41], NbSH [43] or WSH [42]. Where all the three central atoms (Mo, Nb, W) contribute lower vibrational frequencies and the higher frequencies are orderly occupied by S and H atoms, respectively.…”

“…This unique feature observed in the band structure of TiSH is the presence of non-degenerate band starting at ∼0.5 eV with reference to Fermi level, and crossing Fermi line at least at two points, marked by red arrow in figure 2(a). This is a deviation from the usual trend of JTMDCs of this class indicating a novel behavior [41,43]. The corresponding sharp peak in the PDOS close to Fermi level indicates high electron densities of which ∼74% are contributed from Ti_d, as shown in figure 2(b).…”

“…Up till now, there have been only a few 2D Janus hydrides which have been investigated. The most recent simulation analysis reports stable dynamical structures in MoSH, NbSH, and WSH 2D Janus hydrides [41][42][43]. Moreover, the recent successful synthesis of MoSH has removed the barrier in this research direction and help to establish a research flow for ambient pressure, phonon triggered superconductivity [28,35].…”