Mass spectrometry imaging of untreated wet cell membranes in solution using single-layer graphene.

We report a means by which atomic and molecular secondary ions, including cholesterol and fatty acids, can be sputtered through single-layer graphene to enable secondary ion mass spectrometry (SIMS) imaging of untreated wet cell membranes in solution at subcellular spatial resolution. We can observe the intrinsic molecular distribution of lipids, such as cholesterol, phosphoethanolamine and various fatty acids, in untreated wet cell membranes without any labeling. We show that graphene-covered cells prepared on a wet substrate with a cell culture medium reservoir are alive and that their cellular membranes do not disintegrate during SIMS imaging in an ultra-high-vacuum environment. Ab initio molecular dynamics calculations and ion dose-dependence studies suggest that sputtering through single-layer graphene occurs through a transient hole generated in the graphene layer. Cholesterol imaging shows that methyl-ß-cyclodextrin preferentially extracts cholesterol molecules from the cholesterol-enriched regions in cell membranes.

Epitaxial Synthesis of Molybdenum Carbide and Formation of a Mo2C/MoS2 Hybrid Structure via Chemical Conversion of Molybdenum Disulfide.

The epitaxial synthesis of molybdenum carbide (Mo2C, a 2D MXene material) via chemical conversion of molybdenum disulfide (MoS2) with thermal annealing under CH4 and H2 is reported. The experimental results show that adjusting the thermal annealing period provides a fully converted metallic Mo2C from MoS2 and an atomically sharp metallic/semiconducting hybrid structure via partial conversion of the semiconducting 2D material. Mo2C/MoS2 hybrid junctions display a low contact resistance (1.2 kΩ·µm) and low Schottky barrier height (26 meV), indicating the material's potential utility as a critical hybrid structural building block in future device applications. Density functional theory calculations are used to model the mechanisms by which Mo2C grows and forms a Mo2C/MoS2 hybrid structure. The results show that Mo2C conversion is initiated at the MoS2 edge and undergoes sequential hydrodesulfurization and carbide conversion steps, and an atomically sharp interface with MoS2 forms through epitaxial growth of Mo2C. This work provides the area-controllable synthesis of a manufacturable MXene from a transition metal dichalcogenide material and the formation of a metal/semiconductor junction structure. The present results will be of critical importance for future 2D heterojunction structures and functional device applications.