(a) XRD of printed GaS film (after multiple printing steps) with XRD of the printed oxide skin and SiO2/Si substrate for comparison and GaS planes indicated (traces have been normalized and offset for clarity). The GaS peak corresponding to the 002 plane (11.6°) reveals a layer spacing of 7.62 Å that is in good agreement with previous studies11,13. (b) Raman spectrum of the 2D GaS with GaS vibrational peak shifts indicated. (c,d) XPS of the 2D GaS for the regions of interest (c) Ga 3d and (d) S 2p. A small peak from elemental Ga is also observed in the Ga 3d region that has been associated with the slow decomposition of GaS in moist air11. XPS analysis of the printed 2D GaOx and GaCl3 are shown in Supplementary Fig. 4.