1 Optical Signatures of Impurity-Impurity Interactions in Copper Containing II-VI Alloy Semiconductors Biswajit Bhattacharyya, 1 Kushagra Gahlot, 2 Ranjani Viswanatha 2,3 and Anshu Pandey 1* 1 Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India. 2 New Chemistry Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bangalore-560064, India. 3 International Centre for Materials Science, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bangalore-560064, India. Correspondence to: anshup@iisc.ac.in Supplementary Information Materials and Methods: Materials: Selenium (Se, 99.99%), Sodium borohydride (NaBH 4 , 96%), Zinc acetate (Zn(CH 3 COO) 2 , 99%), Copper acetate (Cu(CH 3 COO) 2 , 98%), Cadmium acetate (Cd(CH 3 COO) 2 , tri-octylphosphine (TOP, technical grade, 90%), 1-octadecene (ODE, technical grade, 90%),