Highly Conductive Cu2-xS Nanoparticle Films through Room-Temperature Processing and an Order of Magnitude Enhancement of Conductivity via Electrophoretic Deposition

Abstract

A facile room-temperature method for assembling colloidal copper sulfide (Cu2-xS) nanoparticles into highly electrically conducting films is presented. Ammonium sulfide is utilized for connecting the nanoparticles via ligand removal, which transforms the as-deposited insulating films into highly conducting films. Electronic properties of the treated films are characterized with a combination of Hall effect measurements, field-effect transistor measurements, temperature-dependent conductivity measurements, and capacitance voltage measurements, revealing their highly doped p-type semiconducting nature. The spin-cast nanopartide films have carrier concentration of 1019 cm(-3), Hall mobilities of similar to 3 to 4 cm(2) V-1 s(-1), and electrical conductivities of similar to 5 to 6 S. cm(-1). Our films have hole mobilities that are 1-4 orders of magnitude higher than hole mobilities previously reported for heat-treated nanopartide films of HgTe, InSb, PbS, PbTe, and PbSe. We show that electrophoretic deposition (EPD) as a method for nanopartide film assembly leads to an order of magnitude enhancement in film conductivity (similar to 75 S.cm(-1)) over conventional spin-casting, creating copper sulfide nanoparticle films with conductivities comparable to bulk films formed through physical deposition methods. The X-ray diffraction patterns of the Cu2-xS films, with and without ligand removal, match the Djurleite phase (Cu1.94S) of copper sulfide and show that the nanopartides maintain finite size after the ammonium sulfide processing. The high conductivities reported are attributed to better interparticle coupling through the ammonium sulfide treatment. This approach presents a scalable room-temperature route for fabricating highly conducting nanopartide assemblies for large-area electronic and optoelectronic applications.