1 Supplementary information Synthesis of transparent p-type LaOCuS nanoparticles via soft chemistry Charlotte Doussier-Brochard, a Benoit Chavillon, a Laurent Cario,* a and Stéphane Jobic* a Synthesis. Used precursors for the preparation of LaOCuS nanoparticles were cupric or cuprous oxide (CuO, Alfa Aesar 99.99%, Cu 2 O, Alfa Aesar 99.99%,), hydrated lanthanide chloride (LaCl 3 .7H 2 O, sigma Aldrich 99.9%) dried in an oven at 100°C and elemental sulfur flakes (Sigma Aldrich 99.99+%). Starting materials were weighted in stœchiometric amounts (La/Cu/S = 1:1:1). Namely, 0.715 mg blend was added to 21 ml of ethylene diamine (en) at room temperature. After stirring for 10 min, the mixture was sealed in a Teflon-lined autoclave (filling rate of 70%), and heated at 200°C for 36h. Then, autoclave was slowly cooled to room temperature (i.e. in two hours) and the product was filtered under hood, washed with distilled water and ethanol and dried at ambient temperature. Be careful: owing to the toxicity of en and its decomposition products, solvothermal experiments must be carried out in a ventilated area in case of leaks of the sealed container.