LAUSR

Due to their shape/size-tunable electronic and optical properties, ease of fabrication, and solution processability, colloidal semiconductor nanocrystals (NCs) have gained substantial attention as promising building blocks for advanced materials and devices. They already play a relevant role in field-effect transistors (FETs), light-emitting diodes (LEDs), photodiodes and photovoltaic cells (PVCs) as well as in biological applications, and important perspectives in future nanoelectronic and nano-optoelectronic devices are expected.[1–7] Among different classes of structures, arrays of few monolayers of NCs connected via organic ligands have recently attracted significant attention due to their numerous novel emerging applications.[3,8–10] The architecture of these devices includes inorganic nanocrystals physically and electronically connected to each other through organic semiconductor bridges. These nanostructures have demonstrated variable sensitivity and selectivity utilizing different linker molecules which make them promising for developing artificial noses and multivariable sensors.[11–14] To this end, a fundamental understanding of charge transport in NCs arrays containing hybrid molecule–nanoparticle junctions is of particular relevance. Charge transport in NC assemblies is often reported to follow the variable range hopping (VRH) model, g(T) ∝ exp[−(T0/T)], where T0 is a characteristic temperature and γ = 1⁄2, 1⁄3, and 1⁄4 for Efros–Shklovskii (ES) or Coulomb Gap (CG), 2D, and 3D transport models, respectively. The similarity of charge transport properties in arrays of NCs versus VRH in doped semiconductors was first noticed by Beverly et al.[18] Since that observation, many experiments on metallic and semiconducting nanocrystal arrays at low temperatures (inside the Coulomb blockade regime) revealed γ = 1⁄2, suggesting that transport in these systems can be satisfactorily described by the ES-VRH model.[19–21] This model is based on direct, singlecharge tunnel events between distributed defect sites. However, when applied to NC arrays, the derived hopping lengths are often equivalent to several NC diameters, which questions its applicability to arrays of NCs. Thus, an alternative picture Macroscopic superlattices of tin-doped indium oxide (ITO) nanocrystals (NCs) are prepared by self-assembly at the air/liquid interface followed by simultaneous ligand exchange with the organic semiconductors M-4,4′,4′′,4′′′tetraaminophthalocyanine (M4APc, M = Cu, Co, Fe, Ni, Zn). Transport measurements, focusing on the effect of the metal center of the ligand, reveal a ligand-dependent increase in electrical conductance by six to nine orders of magnitude, suggesting that M4APc provides efficient electronic coupling for neighboring ITO NCs. The resulting I–V characteristics as well as the temperature dependence (7–300 K) of the zero-voltage conductance indicates that at low temperatures, transport across the arrays occurs via a sequence of inelastic cotunneling events, each involving ≈3 ITO NCs. At higher temperatures, a crossover to 3D Mott-variable range hopping mechanism is observed. Finally, the vapor sensitivity of chemiresistors is investigated made from ITO NCs coupled via Cuand Zn4APc by dosing the sensors with 4-methyl2-pentanone (4M2P), toluene, 1-propanol, and water in the concentration range of 100–5000 ppm at 0% relative humidity. The nanocrystal superlattices respond with an increase in resistance to these analytes with the highest sensitivity to 4M2P.