Tunable Multipolar Surface Plasmons in 2D Ti₃C₂T_x MXene Flakes

2D Ti3C2Tx MXenes were recently shown to exhibit intense surface plasmon (SP) excitations; however, their spatial variation over individual Ti3C2Tx flakes remains undiscovered. Here, we use scanning transmission electron microscopy (STEM) combined with ultra-high-resolution electron energy loss spectroscopy (EELS) to investigate the spatial and energy distribution of SPs (both optically active and forbidden modes) in mono- and multilayered Ti3C2Tx flakes. With STEM-EELS mapping, the inherent interband transition in addition to a variety of transversal and longitudinal SP modes (ranging from visible down to 0.1 eV in MIR) are directly visualized and correlated with the shape, size, and thickness of Ti3C2Tx flakes. The independent polarizability of Ti3C2Tx monolayers is unambiguously demonstrated and attributed to their unusual weak interlayer coupling. This characteristic allows for engineering a class of nanoscale systems, where each monolayer in the multilayered structure of Ti3C2Tx has its own set of SPs with distinctive multipolar characters. Moreover, the tunability of the SP energies is highlighted by conducting in situ heating STEM to monitor the change of the surface functionalization of Ti3C2Tx through annealing at temperatures up to 900 °C. At temperatures above 500 °C, the observed fluorine (F) desorption multiplies the metal-like free electron density of Ti3C2Tx flakes, resulting in a monotonic blue-shift in the SP energy of all modes. These results underline the great potential for the development of Ti3C2Tx-based applications, spanning the visible–MIR spectrum, relying on the excitation and detection of single SPs.