AbstractAtomically thin transition metal dichalcogenides (TMDs) are currently receiving significant attention due to their promising opto-electronic properties. Tuning optical and electrical properties of mono and few-layer TMDs, such as tungsten diselenide (WSe₂), by controlling the defects, is an intriguing opportunity to synthesize next generation two dimensional material opto-electronic devices. Here, we report the effects of focused helium ion beam irradiation on the structural, optical and electrical properties of few-layer WSe₂, via high resolution scanning transmission electron microscopy, Raman spectroscopy and electrical transport measurements. By controlling the ion irradiation dose, we selectively introduce precise defects in few-layer WSe₂ thereby locally tuning the resistivity and transport properties of the material. Hole transport in the few layer WSe₂ is degraded more severely relative to electron transport after helium ion irradiation. Furthermore, by selectively exposing material with the ion beam, we demonstrate a simple yet highly tunable method to create lateral homo-junctions in few layer WSe₂ flakes, which constitutes an important advance towards two dimensional opto-electronic devices.