Stick-and-ball representation of GaS crystal. (a) Side view of bilayer GaS, showing a unit cell c=15.492 Å made of two GaS layers. (b) Top view of the GaS crystal. The GaS crystal lattice is composed of Ga–Ga and Ga–S covalent bonds that extend in two dimensions, forming a stratified crystal made of planes that are held together by van der Waals attractions. The Ga atoms are green and the S atoms are blue. (c,d) Functionalization of the substrate with FDTES changes the contact angle between a Ga drop and the substrate by 12.9°. The SiO2 substrate becomes more hydrophobic and thus resists wetting by liquid Ga. (e,h) Schematics of the synthesis process for patterning 2D GaS via printing the skin oxide of liquid Ga. (e) Lithography process for establishing the negative pattern of the photoresist. (f) Covering the exposed area of the substrate with vaporized FDTES. (g) Placing Ga liquid metal and removing it with soft PDMS that leaves a cracked layer of Ga oxide. (h) Two concurrent steps of chemical vapour treatment: first, GaCl3 layer is formed via exposure to HCl vapour and, second, sulfurization by exposure to S vapour forming GaS.