Laser-Beam-Patterned Topological Insulating States on Thin Semiconducting ${\mathrm{MoS}}_2$

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 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 $1T^{\prime }$ phase embedded into the $2H$ phase of thin semiconducting molybdenum-disulfide 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 $1T^{\prime }$ phases.

Figure 1

(a) Optical microscopy image of the $1T^{\prime }$ phase rectangular (right) and $H$-letterlike (left) patterns formed onto a thin $2H\text{−}{\mathrm{MoS}}_2$ flake by laser beam irradiation. (Inset) Schematic cross section of a crystal structure of $1T^{\prime }\text{−}{\mathrm{MoS}}_2$ monolayer with distortion. (b) AFM image of a cross section of the laser irradiated part. (c) Schematic cross section of $1T^{\prime }$ phase part created by laser beam irradiation onto few-layer ${\mathrm{MoS}}_2$, corresponding to (b). (d) Raman spectra for nonlaser-irradiated region ($2H$ phase; blue curve) and irradiated region ($1T^{\prime }$ phase on $2H$ phase; red curve). Individual peaks correspond to $E_{2g}\sim 382{\mathrm{nm}}^{-1}$ and $A_{1g}\sim 408{\mathrm{nm}}^{-1}$ for $2H$ phase, $J_1\sim 155$, $J_2\sim 225$, and $J_3\sim 330{\mathrm{nm}}^{-1}$ for $1T^{\prime }$ phase. (e),(f) PL spectra of the laser beam irradiated points plotted for wavelengths (e) 530–710 and (f) 640–700 nm. The numbers on the graph are the irradiation time for each plotted line and are common to both (e) and (f). (g) XPS of the sample after laser irradiation. Red and blue lines are data fits for spectra of the $1T^{\prime }$ and $2H$ phases, respectively.

Figure 2

(a),(b) For the $1T^{\prime }$ rectangular patterns formed by two different laser irradiation times on each point; two-terminal resistance measured between electrode pairs 1,3 and 2,4 as a function of $V_{\mathrm{BG}}$ by flowing a constant current between electrode pairs 1,3 and 2,4 (insets). Contact resistances with $1T^{\prime }$ metallic-layer resistances are subtracted. (c) For the $1T^{\prime }$ $H$-letterlike pattern: nonlocal resistance ($R_{\mathrm{NL}}$) observed for electrode pairs 3-4 as a function of $V_{\mathrm{BG}}$, when a constant current flows between electrode pairs 1-2 (inset). (d) For the $1T^{\prime }$ rectangular pattern formed by reduced laser power, with a short channel. Three different colors correspond to three different measurements. Equivalent circuits are shown in insets of (a) and (b). (b, inset) Channel length dependence of $R$ peak values in high $V_{\mathrm{BG}}$ regions. Error bars are for the results of each of the three samples.

Figure 3

(a) Out-of-plane magnetic-field ($B_{\perp }$) dependence of conductance corresponding to inverse of the three $R$ peaks [blue and red symbols for Fig. 2 (rectangular pattern) and pink for high $V_{\mathrm{BG}}$ of Fig. 2 ($H$-letterlike pattern)] and two off-$R$ peak values [green and orange symbols for Fig. 2]. (b) Temperature dependence of conductance corresponding to the three $R$ peaks in (a) in Arrhenius plot format. Dashed lines are guides to the eyes. (c)–(e) STS spectra for nonlaser-irradiated $2H$ region (c) and irradiated $1T^{\prime }$ region (d),(e) (Supplemental Material, Sec. IX [13]). (d) The two bulk signals (blue and green lines) were measured near the center of the $1T^{\prime }$ rectangular pattern, and the edge signal (red line) was measured near the boundary of the $1T^{\prime }/2H$ phases. (e) The edge signals for different three $V_{\mathrm{BG}}$ values in sample of (d). The $1T^{\prime }$ region was formed by irradiation with the condition for Fig. 2. (Insets) Schematic views of band diagram near or away from Kramers degeneracy point with Fermi level ($E_F$).

Figure 4

(a) Overall band structure of the $2H/1T^{\prime }/2H$ heterostructure. (Bottom inset) A view of the whole heterostructure showing the passivated edges. (Top inset) Atomic detail of one interface. The yellowish area indicates the energy window within the $2H$ phase gap. Inside this range and near the center of the Brillouin zone, the bulk band inversion of the $1T^{\prime }$ phase and the gap opened by the SOC can be seen along with nontrivial and trivial bands. (b) Enlargement of the bands into the relevant energy window. The number of band crossings at the Fermi level (placed at zero) and at any energy in the gap is odd, as expected from the presence of protected interface states. (c) Bulk band structure of a bilayer for three different stacking possibilities (indicated in the insets), showing a gap in all of them.