Highly Conductive Cu_2–<i>x</i>S Nanoparticle Films through Room-Temperature Processing and an Order of Magnitude Enhancement of Conductivity via Electrophoretic Deposition

A facile room-temperature method for assembling colloidal copper sulfide (Cu_2–<i>x</i>S) 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 nanoparticle films have carrier concentration of ∼10¹⁹ cm^–3, Hall mobilities of ∼3 to 4 cm² V^–1 s^–1, and electrical conductivities of ∼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 nanoparticle films of HgTe, InSb, PbS, PbTe, and PbSe. We show that electrophoretic deposition (EPD) as a method for nanoparticle film assembly leads to an order of magnitude enhancement in film conductivity (∼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 Cu_2–<i>x</i>S films, with and without ligand removal, match the Djurleite phase (Cu_1.94S) of copper sulfide and show that the nanoparticles 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 nanoparticle assemblies for large-area electronic and optoelectronic applications

Direct Measurements of Surface Scattering in Si Nanosheets Using a Microscale Phonon Spectrometer: Implications for Casimir-Limit Predicted by Ziman Theory

Thermal transport in nanostructures is strongly affected by phonon-surface interactions, which are expected to depend on the phonon’s wavelength and the surface roughness. Here we fabricate silicon nanosheets, measure their surface roughness (∼1 nm) using atomic force microscopy (AFM), and assess the phonon scattering rate in the sheets with a novel technique: a microscale phonon spectrometer. The spectrometer employs superconducting tunnel junctions (STJs) to produce and detect controllable nonthermal distributions of phonons from ∼90 to ∼870 GHz. This technique offers spectral resolution nearly 10 times better than a thermal conductance measurement. We compare measured phonon transmission rates to rates predicted by a Monte Carlo model of phonon trajectories, assuming that these trajectories are dominated by phonon-surface interactions and using the Ziman theory to predict phonon-surface scattering rates based on surface topology. Whereas theory predicts a diffuse surface scattering probability of less than 40%, our measurements are consistent with a 100% probability. Our nanosheets therefore exhibit the so-called “Casimir limit” at a much lower frequency than expected if the phonon scattering rates follow the Ziman theory for a 1 nm surface roughness. Such a result holds implications for thermal management in nanoscale electronics and the design of nanostructured thermoelectrics