On the Stability and Electronic Structure of Transition-Metal Dichalcogenide Monolayer Alloys Mo1−xXxS2−ySey with X = W, Nb

Agnieszka Kuc, Thomas Heine
2015 Electronics  
Layered transition-metal dichalcogenides have extraordinary electronic properties, which can be easily modified by various means. Here, we have investigated how the stability and electronic structure of MoS 2 monolayers is influenced by alloying, i.e., by substitution of the transition metal Mo by W and Nb and of the chalcogen S by Se. While W and Se incorporate into the MoS 2 matrix homogeneously, forming solid solutions, the incorporation of Nb is energetically unstable and results in phase
more » ... paration. However, all three alloying atoms change the electronic band structure significantly. For example, a very small concentration of Nb atoms introduces localized metallic states, while Mo 1´x W x S 2 and MoS 2´y Se y alloys exhibit spin-splitting of the valence band of strength that is in between that of the pure materials. Moreover, small, but evident spin-splitting is introduced in the conduction band due to the symmetry breaking. Therefore, transition-metal dichalcogenide alloys are interesting candidates for optoelectronic and spintronic applications. Electronics 2016, 5, 1 2 of 13 change the electronic properties of monolayers, it causes band gap closure and spin-orbit splitting in bi-and multi-layer TMCs. The monolayer band structure can, however, be modulated by tensile strain or hydrostatic pressure, leading to semiconductor-metal transition [8] [9] [10] [11] 14, 15] . Results on TMC nanotubes suggest that doping small quantities of Group 13 or Group 15 atoms results in a metallic character [16] ; however, Nb doping of MoS 2 tubes was found to be unfavorable energetically [17] [18] [19] . The incorporation of a dilute impurity concentration into a host material results in significant changes to the band structure, which was shown in the cases of III-V semiconductors, e.g., the As impurity in GaN leads to a significant band gap reduction, and such a GaNAs alloy can be used for green emission [20, 21] . This discovery opens up a new direction in the field of light-emitting diodes. Similarly, the addition of dilute-N impurity into the GaAs material results in low threshold laser devices [22, 23] . Several experimental and theoretical studies have also been published up to date on the doping and alloy formation in TMC materials [24] [25] [26] [27] [28] [29] [30] . Many of these investigations are focused on the changes of Raman modes, optical or electronic properties in the mixed systems. For example, Raman modes, especially A 1 g and E 2 g , are increasing and decreasing, respectively, with W concentration in MoS 2 monolayers [26] [27] [28] . There is, however, a limited literature available on the spin-orbit splitting in the electronic structure of TMC alloys. Among various TMCs, molybdenum disulfide (MoS 2 ) is the most widely-studied material. TMCs of the 2H TX 2 type are composed of two-dimensional X´T´X sheets stacked on top of one another and held together via weak interlayer interactions, allowing easy exfoliation. Each sheet is trilayered with T atoms sandwiched between two chalcogen layers, as shown in Figure 1 . Therefore, doping/alloying can occur both in the T and X sites. In this paper, we have studied the complete transition of substituted MoS 2 as (i) Mo 1´x W x S 2 , (ii) Mo 1´x Nb x S 2 and (iii) MoS 2´x Se x and discuss the stability and electronic properties of these phases. Such ternary alloys are a straightforward way of tuning the electronic properties of TMC materials and might be of great interest in the fields of electronic transport, optoelectronics, as well as spintronics. We have discussed in detail the electronic band structures and projected densities of the states of MoS 2 -based alloys, showing that band gaps and the spin-splitting in the valence and conduction bands can be effectively tuned, with values in between those of the pure parental materials. Moreover, a small amount of NbS 2 mixed into the MoS 2 matrix makes it metallic. We show that semiconducting WS 2 and MoSe 2 easily form solid solutions with MoS 2 , while metallic NbS 2 results in a phase separation.
doi:10.3390/electronics5010001 fatcat:sc5wi4ydznbzpexkgym4srrbxm