Laser-Beam-Patterned Topological Insulating States on Thin Semiconducting MoS2

Abstract

Identifying the two-dimensional (2D) topological insulating (TI) state in new materials and its control are crucial aspects towards the development of voltage-controlled spintronic devices with low power dissipation. Members of the 2D transition metal dichalcogenides (TMDCs) have been recently predicted and experimentally reported as a new class of 2D TI materials, but in most cases edge conduction seems fragile and limited to the monolayer phase fabricated on specified substrates. Here, we realize the controlled patterning of the 1T'-phase embedded into the 2H-phase of thin semiconducting molybdenum-disulfide (MoS2) by laser beam irradiation. Integer fractions of the quantum of resistance, the dependence on laser-irradiation conditions, magnetic field, and temperature, as well as the bulk gap observation by scanning tunneling spectroscopy and theoretical calculations indicate the presence of the quantum spin Hall phase in our patterned 1T' phases.Two-dimensional (2D) topological insulting (TI) states have been mainly investigated in HgTe/CdTe or InAs/GaSb quantum well systems (1-3). In the 2D TI state the quantum spin Hall (QSH) effect emerges thanks to the simultaneous presence of a bulk energy gap and gapless helical edge states protected by time-reversal symmetry, namely, opposite and counter-propagating spin states forming a Kramers doublet. Interestingly, 2D TI states were first theoretically predicted for graphene (4-6), but experimentally reported in only few related systems (7-9) such as low-coverage Bi2Te3 nanoparticle-decorated graphene (8). Moreover, control of the QSH phase in graphene-based systems remains a challenge.Recently, a family of atom-thin transition metal dichalcogenides (TMDCs) materials has also been predicted to exhibit the QSHE (10-12), having its origin in the natural band inversion of the 1T' phase (one of the phases of TMDC; see Supplementary Material (SM) 1) and the spin-orbit coupling (SOC)induced band-gap opening. Moreover, the TI state has been experimentally verified in the case of WTe 2 (13-15) thanks to the stability and high-quality of WTe 2 monolayers carefully formed on bilayer graphene/atom-thin hBN. Various signatures of the TI state have been demonstrated in this 2 material (13,15), including the latest observation of a half-integer quantum value of resistance (RQ/2 = h/2e 2 = 12.9 k, where h is Planck's constant and e is the charge on the electron) (14).However, the TI phenomenon in WTe2 is rather sensitive to the substrates, synthesis process, and the chemical environment, making its controlled use in practical applications challenging. Moreover, although the (metastable) 1T' phase can be found or induced in other TMDCs (23,25), nobody has demonstrated the existence of the QSHE in these other TMDCs. The conditions under which helical edge states can exist at the 1T'-2H interfaces is a crucial problem which should be mastered for both TI physics and its applications. Here, we pattern a metallic 1T'-phase (SM 1) embedded into the nontopological and semiconducting 2H p...

Quantum Spin Hall Materials

“…Such 1 T ′‐MoTe 2 is reported to be stable up to 300 °C. This technique is also performed on 2 H ‐MoS 2 with similar results …”

“…Previously mentioned laser‐induced phase patterning technique can create hetero‐phase homojunctions between 2 H and 1 T ′ phase. Measurements on laser‐patterned 1 T ′‐MoS 2 show peak resistances near the helical edge transport value, where the edge is defined by the interface between 1 T ′ and 2 H phases . The existence of interface edge states is confirmed by STS spectra and the peak resistance is temperature‐independent below 25 K, which may point to the existence of QSH states.…”

Local engineering of topological phase in monolayer MoS2

“…The weak-plasma treatment developed in this work can be easily generalized to a large variety of 2D TMDCs materials, where the 1T 0 phase is considerably less stable than the 2H phase. Combined with shadow mask or focused ion/e-beam/laser-beam pattern techniques [22,27], it is possible to pattern well-defined strips of 1T 0 phase within 2H phase and built topological circuits consisting of highly dense one-dimensional conduction channels which has recently been achieved by laser beam irradiation in thin MoS 2 film [21]. However laser beam irradiation is destructive to monolayer films [21,22].…”

“…It has been shown that the 2H ? 1T 0 phase transition of monolayer TMDCs can be induced by alkali ion intercalation [18][19][20], laser irradiation [21,22], or electrostatic gating [23]. However, a scalable, controllable and nondestructive method for engineering the topological phase of monolayer MoS 2 is still lacking and the helical edges states are yet to be explored [24,25].…”

Optothermoplasmonic Nanolithography for On‐Demand Patterning of 2D Materials

“…Laser processing of low‐dimensional materials is a remotely controlled, one‐step, maskless, and low‐cost fabrication technique which provides an alternative approach . The direct or indirect light‐matter interactions at the laser spots, e.g., laser‐induced thermal effect, radiation force, laser‐induced phase transition, or photon‐assisted chemical reaction, open new ways to control or modify the composition, structure, and geometry of materials . In the past few years, laser processing of 2D materials using femtosecond laser has also been achieved .…”