Strained noble metal di chalcogenides PtX2 (X = S, Se) mono-layer: Ab initio study of electronic and lattice dynamic properties
Introduction
After the successful synthesis of graphene [1], [2], [3], a new wave of research has also been taking place in other two-dimensional (2D) materials such as boron nitride, silicene, germanene and layered transition metal di-chalcogenides (TMDCs) [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15]. TMDCs, especially their two-dimensional (2D) counterparts, are a class of materials with fascinating and technologically useful properties [16], [17]. Depending on the stoichiometry, the three-dimensional TMDCs with a formula MX2, where M represents transition metals and X is chalcogen, can be either metals or semiconductors with indirect band gaps. Whereas the 2D monolayers are direct-band-gap semiconductors with sizable band gaps around 1–2 eV [18]. This band-gap phenomenon together with the versatile chemistry of MX2 enables a variety of fields of applications including field-effect transistors, energy storage, and catalysis [16], [17], [18], [19]. Several TMDCs (e.g., MoS2, MoSe2, and WS2) have been extensively investigated, both experimentally and theoretically [20], [21], [22], [23], [24], [25], [26], [27] however there are not many studies of platinum and palladium dichalcogenides.
It has been demonstrated that noble metals, like Pt and Pd can also form layered structures with S atoms [28], [29], [30], [31]. Inspired by the extensive studies of 2D TMDCs, it is expected that PdS2 and PtS2 can also be exfoliated into a mono-layer and find some important applications in electronics. First principles calculations suggest semiconducting behavior for monolayer PdSe2 with indirect gap value of 1.43 eV. [32]. Monolayer of PtSe2 has been grown experimentally [33] by direct selenization on a Pt substrate and the semiconducting behavior with band gap of 1.20 eV was observed in it. There are very few studies on mono-layer chalcogenides of Pt and Pd. Miro et al. [34] theoretically studied the electronic properties of PdS2 and PtS2 mono-layer and found these mono-layers as semiconducting with band gap 1.17 eV and 1.78 eV respectively. However, they reported that the most stable configuration of these mono-layers is 1 T, which is different from that of the layers in PtS2 bulk. Very recently we carried out ab initio calculations to study the strain dependent electronic properties of PdS2 and PdSe2 mono-layers in 1 T phase [35]. To extend our work, here we report results of strain induced tuning electronic properties of mono-layers of sulfides and selenides of Pt. A trend in band gap variation with strain is discussed. Phonon spectrum of PtX2 (X = S, Se) and its strained structure have also been studied to check the stability of system.
Section snippets
Computational methodology
We used the plane wave pseudo potential method as implemented in the QUANTUM-ESPRESSO code [36] to perform all the calculations. The exchange correlation potential was approximated by generalized gradient approximation using Perdew-Burke-Ernzerhof exchange-correlation functional [37]. We used norm conserving ultra-soft pseudopotentials for Pt, S and Se each. The atomic positions and cell parameters were fully relaxed until an energy convergence of 10−9 eV reached. We used wave function and
Results and discussion
We studied platinum disulphides and diselenides mono-layers in 1 T polytypic form as shown in the Fig. 1. The unit cell consists of one Pt and two chalcogenides whose coordinates and lattice constants are mentioned in Table 1. The band structure and density of states (DOS) of PtX2 (X = S, Se) mono-layers have been studied. As shown in Fig. 2, where the Fermi level is set at zero, PtS2 mono-layer is a semiconductor with indirect band gap of ca. 1.94 eV. The band structure shown in the Fig. 2 may
Conclusion
We have performed a study of the effect of uniform bi-axial tensile strains on the electronic properties of PtX2 mono-layer. Our calculation shows that strain can play an important role in tuning the band gap. Some trends of band gap engineering due to the strain have been summarized. For PtS2, the band gap decreases from 1.94 to 1.20 eV (1.42 eV) on applying bi-axial tensile (compression) strain. A similar trend of linear decrease in band gap from 1.37 eV is observed in PtSe2. The phonon spectra
Acknowledgements
Support from Deanship of Scientific Research at King Khalid University by research project number G.R.P-365-38 is gratefully acknowledged. All calculations were performed using high performance computing facility at IUAC, New Delhi.
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