Nanoscale chemical engineering of Ti3C2Tx MXene multilayers through in situ electron beam irradiation in a TEM
Defect engineering in low-dimensional materials is a powerful approach to tailor their structure and chemistry at the atomic scale, with high relevance for tuning physical properties. Here, we apply this strategy to MXenes, one of the largest families of two-dimensional (2D) materials, and demonstrate the use of a highly focused scanning transmission electron microscope (STEM) beam for the structural and chemical nanoengineering of titanium carbide (Ti3C2Tx) multilayers. By adjusting the beam energy, electron dose, and scanning parameters, Ti3C2Tx multilayers were irradiated at 80 and 300 keV and characterized in situ using annular dark-field imaging and electron energy-loss spectroscopy (EELS). The irradiation induces gradual structural and chemical modifications, largely driven by the selective sputtering of atomic species and MXene layers. We identify irradiation conditions that selectively remove Cl or F surface terminations while largely preserving the Ti3C2 framework, whereas other conditions promote preferential carbon removal, in agreement with theoretical predictions. Most notably, electron irradiation drives the migration and accumulation of initially dilute nitrogen impurities within the irradiated regions, leading to the formation of nitrogen-rich titanium carbonitride (Ti3CyNzTx) nanodomains embedded in the Ti3C2Tx multilayers. These laterally patterned carbide/carbonitride architectures constitute in-plane MXene heterostructures with nanometer-scale spatial control. These results demonstrate that electron irradiation can be used to locally tailor MXene composition and create nanoscale architectures beyond those accessible through conventional chemical synthesis.