  Controlled fabrication of nanopores in atomically thin two-dimensional
material offers the means to create robust membranes needed for ion transport,
nanofiltration, and DNA sensing. Techniques for creating nanopores have relied
upon either plasma etching or direct irradiation using electrons or ions;
however, aberration-corrected scanning transmission electron microscopy (STEM)
offers the advantage of combining a highly energetic, sub-angstrom sized
electron beam for atomic manipulation along with atomic resolution imaging.
Here, we utilize a method for automated nanopore fabrication with real-time
atomic visualization to enhance our mechanistic understanding of beam-induced
transformations. Additionally, an electron beam simulation technique,
Electron-Beam Simulator (E-BeamSim) was developed to observe the atomic
movements and interactions resulting from electron beam irradiation. Using the
2D MXene Ti3C2Tx, we explore the influence of temperature on nanopore
fabrication by tracking atomic transformation pathways and find that at room
temperature, electron beam irradiation induces random displacement of atoms and
results in a pileup of titanium atoms at the nanopore edge. This pileup was
confirmed and demonstrated in E-BeamSim simulations around the small, milled
area in the MXene monolayer. At elevated temperatures, the surface functional
groups on MXene are effectively removed, and the mobility of atoms increases,
which results in atomic transformations that lead to the selective removal of
atoms layer by layer. Through controllable manufacture using e-beam milling
fabrication, the production and then characterization of the fabricated defects
can be better understood for future work. This work can lead to the development
of defect engineering techniques within functionalized MXene layers.
