Phase transition and in situ construction of lateral heterostructure of 2D superconducting α/β Mo₂C with sharp interface by electron beam irradiation

Abstract

Lateral heterostructures of 2D materials have several interesting properties and potential applications, and they are usually fabricated by chemical vapor deposition. However, it still remains a great challenge to fabricate 2D lateral heterostructures with well-controlled patterns and sharp interfaces. Herein, we found that the 2D α-MoC crystal, a recently emerging 2D superconductor, experiences a phase transition from the α phase to β phase on electron beam irradiation in a transmission electron microscope because of the migration of carbon atoms among the molybdenum octahedrons. Combined with first-principles calculations, the carbon atom migration paths and the corresponding energy barriers were discussed. Utilizing this unique phase transition property of 2D α-MoC crystal, we demonstrated the precise in situ construction of the lateral heterostructure of 2D superconducting α/β MoC with a well-controlled pattern and sharp interface using advanced aberration-corrected scanning transmission electron microscopy.

Identifying surface structural changes in a newly-developed Ga-based alloy with melting temperature below 10 °C

“…Furthermore, TEM, as one powerful characterization method, not only provides a platform for spatially imaging at the atomic scale, chemical analysis and diffraction, but also makes it possible to create defects and induce phase transitions of materials by electron beam irradiation [51][52][53] . Therefore, to explore the effect of electron beam on the structure of surface oxide layer, prolonged parallel electron beam irradiation was performed at a chosen region with a flux of 1.0910 6 e nm -2 s -1 .…”

Preliminary study of the surface reactivity of 2D α‐Mo2C crystallites

“…Assuming that all of the observed photoelectron signal arises from a surface region equal to 3 λ at a take‐off angle of 0° and 0.8 λ at a take‐off angle of 75°, it can be seen that in both cases that 2D‐α‐Mo 2 C layers of greater than 4 nm thickness would effectively mask the Cu signal. However, it must be kept in mind that the samples display regions of non‐covered Cu as well as 2D‐α‐Mo 2 C layers of different thicknesses …”

“…A copper foil is melted over a Mo substrate and Mo atoms that diffuse to the surface can react with a methane flow to form the 2D carbide. Atomic force microscopy (AFM) and optical microscopy measurements reported by Liu et al, Liu et al, and Wang et al show the formation of regular shaped crystallites, such as triangles, rectangles, hexagons, octagons, nonagons, and dodecagons, with lateral dimensions of ∼10 µm and uniform thicknesses of 3–20 nm. Furthermore, their advanced TEM data show that the thin layers are single crystals of orthorhombic α‐Mo 2 C and that they are free of defects .…”

“…In one approach, Anasori et al pioneered the study of a class of 2D carbides called MXenes, such as 2D‐Mo 2 C formed by etching Ga from the layered material Mo 2 Ga 2 C . In a different approach, Xu et al, Liu et al, Liu et al, and Wang et al used chemical vapour deposition (CVD) to grow 2D‐α‐Mo 2 C with lateral dimensions over 100 µm on copper foil . Other groups have elaborated on the latter method to prepare 2D‐Mo 2 C/graphene hybrid materials .…”

Interface Characterization and Control of 2D Materials and Heterostructures

“…In addition to chemically driven transformations, a STEM electron beam can locally convert MoS 2 into the 1T phase . A similar concept using STEM has been explored to convert α‐phase 2D Mo 2 C (a 2D superconductor) locally to β‐phase Mo 2 C with different superconducting properties . Recently, direct selective CVD growth of 1T' and 2H phase MoTe 2 has been achieved by tuning growth temperature, resulting in an atomically well‐defined 1T'‐2H MoTe 2 polymorphic lateral heterostructure interface (Figure e) .…”