Structural, Optical and Transport Properties of Copper Chalcogenide Nanocrystal Superlattices

This cumulative thesis is based on three publications. It investigates the self-assembly of nanocrystal (NC) superlattices, charge transport in NC assembly, and application of these superlattices in optoelectronic and vapor sensing. The materials of choice are copper chalcogenide NCs such as binary copper sulfide Cu1.1S NCs, binary copper selenide Cu2Se NCs and ternary Cu2-xSeyS1-y NCs and the organic semiconductors metal (Cu or Co) centered -4,4′,4″,4″,4‴-tetraaminophthalocyanine (Cu/CoTAPc). Macroscopic superlattices of NCs are prepared by Langmuir-type self-assembly at the air/liquid interface followed by simultaneous ligand exchange with an organic semiconductor. To enhance interparticle coupling, we cross-link the nanocrystals with the organic π-system Cu-4,4′,4″,4″,4‴-tetraaminophthalocyanine and observe a significant increase in electrical conductivity. Ultraviolet-visible-near-infrared (UV-vis-NIR) and Raman spectroscopy are used to track the chemical changes on the nanocrystals’ surface before and after ligand exchange and develop a detailed picture of the various components which dominate the surface chemistry of this material. Grazing-incidence small-angle X-ray scattering (GISAXS) serve to study the importance of electronic conjugation in the organic π-system vs interparticle spacing for efficient charge transport. Transport measurements reveal that Cu4APc provides efficient electronic coupling for neighboring Cu1.1S NCs. The electrical properties of monolayers of this hybrid ensemble are consistent with a two-dimensional semiconductor and exhibit two abrupt changes at discrete temperatures (120 and 210 K), which may be interpreted as phase changes. This material provides the opportunity to apply the hybrid ensemble as a chemiresistor in organic vapor sensing. The vapor sensing experiments exhibits a strong selectivity between polar and nonpolar analytes, which we discuss in light of the role of the organic π-system and its metal center. Next, we choose ternary alloyed Cu-based chalcogenide NCs Cu2SeyS1–y and checked the effect of ligand exchange with the organic π-system Cobalt β-tetraaminophthalocyanine (CoTAPc) along with its binary counterpart Cu2Se NCs. We analysed changes in the structural, optical as well as electric properties of thin films of these hybrid materials. Strong ligand interaction with the surface of the NCs is revealed by UV/vis absorption and Raman spectroscopy. GISAXS studies show a significant contraction in the interparticle distance upon ligand exchange. For copper-deficient Cu2-xSe, this contraction has a negligible effect on electric transport, while for copper-deficient Cu2-xSeyS1-y, the conductivity increases by eight orders of magnitude and 8 results in metal-like temperature-dependent transport. We discuss these differences in the light of varying contributions of electronic vs. ionic transport in the two materials and highlight their effect on the stability of the transport properties under ambient conditions. With photocurrent measurements, we demonstrate high optical responsivities of 200-400 A/W for CoTAPc-capped Cu2SeyS1–y and emphasize the beneficial role of the organic π-system in this respect, which acts as an electronic linker and an optical sensitizer at the same time. Finally, we report on the in-situ monitoring of the formation of conductive superlattices of Cu1.1S nanodiscs via cross-linking with semiconducting Co-4,4′,4″,4″,4‴-tetraaminophthalocyanine (CoTAPc) molecules at the liquid/air interface by real-time grazing incidence small angle X-ray scattering (GISAXS). We determine the structure, symmetry and lattice parameters of the superlattices, formed during solvent evaporation and ligand exchange on the self-assembled nanodiscs. Cu1.1S nanodiscs self-assemble into two-dimensional hexagonal superlattice with a minor in-plane contraction (~ 0.2 nm) in the lattice parameter. A continuous contraction of the superlattice has been observed during ligand exchange, preserving the initial hexagonal symmetry. We estimate a resultant decrement of about 5% in the in-plane lattice parameters. The contraction is attributed to the continuous replacement of the native oleylamine surface ligands with rigid CoTAPc. The successful cross-linking of the nanodiscs is manifested in terms of the high electrical conductivity observed in the superlattices. This finding provides a convenient platform to understand the correlation between the structure and transport of the coupled superstructures of organic and inorganic nanocrystals of anisotropic shape