Local Conduction in Mo_xW_1–xSe₂: The Role of Stacking Faults, Defects, and Alloying

Abstract

Here, we report on the surface conductivity of WSe2 and MoxW1–xSe2 (0 ≤ x ≤ 1) crystals investigated with conductive atomic force microscopy. We found that stacking faults, defects, and chemical heterogeneities form distinct two-dimensional and one-dimensional conduction paths on the transition metal dichalcogenide surface. In the case of WSe2, in addition to step edges, we find a significant amount of stacking faults (formed during the cleaving process) that strongly influence the surface conductivity. These regions are attributed to the alternation of the 2H and 3R polytypism. The stacking faults form regular 2D patterns by alternation of the underlying stacking order, with a periodicity that varies significantly between different regions and samples. In the case of MoxW1–xSe2, its conductivity has a localized nature, which depends on the underlying chemical composition and the Mo/W ratio. Segregation to W-rich and Mo-rich regions during the growth process leads to nonuniform conduction paths on the surface of the alloy. We found a gradual change of the conductivity moving from one region to the other, reminiscent of lateral band bending. Our results demonstrate the use of C-AFM as a nanoscopic tool to probe the electrical properties of largely inhomogeneous samples and show the complicated nature of the surface conductivity of TMDC alloys.

Conductive Atomic Force Microscopy of Semiconducting Transition Metal Dichalcogenides and Heterostructures

“…As a matter of fact, SBH mapping with nanoscale resolution is required to disentangle the effect of surface (or near surface) defects in the TMD materials from that of metal-induced gap states. To this purpose, the conductive tip of C-AFM has been employed as a nanoscopic metal electrode to record local I-V characteristics on the TMDs surface, from which the SBH was quantitatively evaluated [44,45,69]. Contacting the TMD surface with a sliding metal tip presents the additional advantage of excluding eventual reactions that have been reported at the metal/TMD interface (for some metal species) when the contact is fabricated by evaporation or sputtering [70,71].…”

“…Recently, Bampoulis et al [69] used C-AFM to characterize the conductivity of a Mo x W 1−x Se 2 alloy at the nanoscale. They observed the segregation of Mo-rich and W-rich domains and demonstrated that these different regions exhibit distinct SBHs values, reflecting the different band structures of WSe 2 and MoSe 2 .…”

Heterogeneous Electronic and Photonic Devices Based on Monolayer Ternary Telluride Core/Shell Structures

“…Alloying in 2D transition metal dichalcogenides (TMDs) has allowed bandgap engineering and phase transformation, which enables a new series of electronic and photonic devices. Bandgap engineering has been demonstrated in various TMD ternary alloys, including MoS 2(1−x) Se 2x , [1][2][3][4][5][6] WS 2(1−x) Se 2x , [7,8] Mo 1−x W x S 2 , [9][10][11][12][13] and Mo 1−x W x Se 2 [14,15] by adjusting the composition of the elements. A 2H-1T' phase transformation was observed in Mo 1−x Re x Se 2 .…”

Defect-Seeded Atomic Layer Deposition of Metal Oxides on the Basal Plane of 2D Layered Materials

“…Mechanical exfoliation can consequently induce defect patterns and networks that modify the local electronic structure and chemical reactivity of the basal plane surface. [11][12][13] In principle, the local differences in properties induced by defects can consequently alter the chemical reactivity of the surface. Such heterogeneity can be exploited to produce patterned deposition using defect-selective deposition methods and reaction conditions.…”