Tailoring Topological Phases in Two-dimensional Transition Metal Dichalcogenides

Feng-Chuan Chuang

Recently, topological materials and two-dimensional materials have attracted a significant amount of research interest in condensed matter physics. In particular, topological insulators (TIs) have been intensively studied for the past decade due to their interesting electrical, optical, and mechanical properties. In addition, two-dimensional topological insulators (2D TIs), also known as quantum spin Hall (QSH) insulators, exhibit unique symmetry-protected helical metallic edge states with an insulating interior, making these materials well-suited for optoelectronics, spintronics, quantum computing, and other applications due to the robustness of their edge states against backscattering. A distinct characteristic of 2D TIs is the band inversion between the valence band maximum (VBM) and conduction band minimum (CBM) at the Fermi level. Since the electronic properties of materials can be engineered by adding or removing an electron or a hole, the location of the Fermi level can be tuned in such a way that it coincides with the band inversion.

The computational materials research group in the Department of Physics, National Sun Yat-sen University led by Dr. Feng-Chuan Chuang has aggressively studied the fascinating research topic on 2D topological materials. One of the driving factors is that most of the previously found 2D TIs exhibited small band gaps which are not suitable for room-temperature applications, thus, the campaign to search for large band gap 2D TIs began. Another factor is that most of these free-standing 2D materials are highly sensitive to the supporting substrate resulting in the vanishment of topological phases. These factors inspired Chuang's group to find promising material alternatives with larger band gaps and suitable substrates.

One of the most successful material designs of large-bandgap 2D TIs by Chuang's group is the case of planar bismuthene grown on SiC(0001) in 2015 [1]. Bismuthene adapted a similar honeycomb-like structure with a stronger spin-orbit interaction. However, no experimental group had successfully synthesized monolayer bismuthene due to its instability. Chuang's group was the first to predict the realization of 2D TIs via substrate modulation. The prediction had then been successfully synthesized and verified in 2017 [2]. The key to this brilliant design is the effect of substrate via chemical bonding. The planar bismuthene is a trivial insulator, but due to the bonding with the substrate, two extra electrons are provided to the system resulting in the shift of the Fermi level to a higher energy level where a band inversion is present due to strong spin-orbit coupling (SOC). This finding is considered a crucial breakthrough in 2D topological materials design.

Further, transition metal dichalcogenides (TMDs), with a chemical formula of MX₂, have been stimulating a lot of research interest in the field of 2D materials because of their fascinating tunable properties that arise upon dimensional transition from bulk to the 2D regime. This raises the question whether the quantum spin Hall effect exists in these materials. Chuang's group again had designed and demonstrated the tailoring of 2D TIs via functionalization and substitutional doping of monolayer TMDs. Figure 1 illustrates the structural engineering of 2D TMDs through halogen/pnictogen substitution, resulting in Janus 2D TMDs [3], and hydrogenation [4]. These methods significantly alter and manipulate their electrical, topological, and magnetic properties. The proposed Janus 2D TMDs, named after a Roman god with 2 faces, leads to symmetry breaking which results in novel characteristics and phenomena.

Fig. 1: (a) Schematic diagram of the fabrication process of monolayer Janus MXY (X,Y = Halogen or Pnictogen) from the parent MX₂ material. Structures of the 1T and 2H of (b, f) parent MX₂ material, (c, g) Janus MXY material after halogen/pnictogen substitution. (d, h) One-sided hydrogenation, and (e, i) two-sided hydrogenation

Chuang's group probed the topological properties of all the possible combinations of Janus 2D TMDs via halogen and pnictogen substitution, including one-sided hydrogen adsorption of MX₂ (M = V, Nb, Ta, Tc, or Re; X = S, Se, or Te) films in both 1T and 2H structures [3] (see Fig. 1b-i). Using a high throughput approach, a total of 294 compounds were examined. Referring to Fig. 2a, e, TaS₂ has an odd number of electrons, hence, the Fermi level crosses the highest half-occupied band. By substituting one chalcogen with one halogen, one extra electron is introduced, which could also be done by adsorbing one hydrogen on TaS₂. Both induced an upward shift of the Fermi level resulting in a trivial phase as shown in Fig. 2b, c, f, g. Contrary to halogen substitution and one-sided hydrogenation, pnictogen substitution results in the loss of one electron, causing the downshift of the Fermi level. In the case of TaSBi film (without SOC), as seen from Fig. 2d, the VBM and CBM levels now touch at Γ. When strong SOC is included, a 108 meV gap opens at the Γ point as shown in Fig. 2h.

Fig. 2: Band structures of the stable structural phases without SOC (a-d) and with SOC (e-h). (a, e) Monolayer pristine TaS2 film in the 2H structure. (b, f) One adsorbed hydrogen on TaS2 in the 2H structure. (c, g) One CI atom substituting an S atom in the 2H structure. (d, h) One Bi atom substituting an S atom in the 1T structure. Reprinted from Reference 3

Moreover, the high throughput method was again used by Chuang's group to study the electronic and magnetic properties of pristine and hydrogenated 1T, 1T', and 2H TMD monolayers [4]. Group IV, VI, and X pristine TMD monolayers were found to mostly adopt 1T and 2H as their stable structures, except for WTe2 which exhibits 1T', and has been identified as a topological insulator. Upon hydrogenation, a structural phase transition was also observed. Surprisingly, nineteen 2D magnetic materials were found through hydrogenation (see Fig. 1d, e, h, i). After the pre-screening of the materials using standard PBE-based calculations, further analysis of band topologies under hybrid functional calculations revealed that four of these identified magnetic monolayer structures exhibit quantum anomalous Hall effect (see Fig. 3). Here, PdS2 with one hydrogen passivation elevates the Fermi level where a band inversion is observed. Furthermore, this results in a strong ferromagnetic state with a non-trivial topological Chern number.

Fig. 3: Calculated band structure under HSE06 of 1T PdS₂-1h. (a, b) NM pristine 1T PdS₂ without and with SOC (c, d) NM 1T PdS₂-1h without and with SOC, and (e, f) FM 1T PdS₂-1h without and with SOC. Reprinted from Reference 4

Fig. 4: (a, b) Illustrations of the topologically protected edge states. (c) Illustration of possible synthesis of Janus 2D materials via lithography or etching method. Reprinted from Reference 3