Electronic changes induced by surface modification of Cu_2-xS nanocrystals

Copper sulfide nanocrystals (Cu_2-xS NCs) consisting of earth-abundant and nontoxic elements have attracted attention for optoelectronic and plasmonic applications due to their tunable light absorption and emission properties. In this work, we present a study of the electronic changes induced in organic-capped Cu_2-xS NCs by surface modification treatments using charge transport and optical spectroscopy measurements. We have investigated surface treatments yielding ligand exchange and also ligand removal as well as changes in electronic defect density. The structural and morphological changes induced by the treatments were monitored by infrared spectroscopy, electron microscopy, and electron paramagnetic resonance. Untreated Cu_2-xS NCs exhibit a strong absorption band arising from a localized surface plasmon resonance (LSPR). We found that using a ligand exchange procedure (ethanedithiol treatment), the electrical conductivity in films of Cu_2-xS NCs can be enhanced by 5 orders of magnitude, while maintaining other electronic properties of the individual NCs like optical absorption and LSPR. The improvements in the electrical conductivity were attributed to the reduction of the inter-NC separation in the films, as revealed by the structural and morphological studies. We also have observed that ligand removal treatments such as thermal annealing and hydrazine treatment yield a LSPR red-shift, while the electrical conductivity increases by up to 5 and 7 orders of magnitude, respectively. We proposed a model for the surface reactions taking place during these treatments. Our work highlights the potential of simple chemical or thermal treatments in tailoring the electronic properties of Cu_2-xS NCs, making thermally treated Cu_2-xS NCs interesting for tunable plasmonic and optoelectronic applications.