Tuning the Work Function of Ti₃C₂T_x MXene by Molecular Doping without Changing its Surface Functional Groups

Owing to their impressive electronic/optoelectronic properties, MXenes have attracted significant attention among the 2D materials research community. Their work function (WF) tunability, in particular, has permitted efficient interfacial band alignment engineering in several device applications. However, like most of their properties, the WF of MXenes highly depends on their surface terminations, making it hard to individually modify the WF without compromising other fundamental properties, which hinders the exploitation of MXenes to their full potential. Herein, we introduce a surface-termination-independent method to tune the WF of Ti3C2Tx MXene through molecular doping. The achieved stepwise 500-meV increase in WF, in ∼120-meV increments, is induced by subsurface electron depletion from Ti3C2Tx, with no effect on its other key properties. Utilizing electron paramagnetic resonance and ultrafast laser spectroscopy, we reveal that tuning the WF of Ti3C2Tx is entirely surface-termination-independent. Such discrete control over the WF renders MXene-based devices with unprecedented operational degrees of freedom.