Transforming Colloidal Nanocrystals into a Novel Class of Electronic Materials
To employ colloidal nanocrystals (NCs) in electronic and optoelectronic devices (e.g., in solar cells, light-emitting diodes, and radiation detectors), it is necessary to establish efficient electronic communication between individual nanocrystals. The conduction in nanocrystal solids involves charge transfer between individual nanocrystals [Prospects of NCs for Electronics & Optoelectronics (Review), n-Type Conducting CdSe NC Solids, Electron-Conducting Quantum Dot Solids (Review)].
Surface ligands with long hydrocarbon chains, used for nanocrystal synthesis, form highly insulating interparticle barriers and result in a very weak exchange coupling between the nanocrystals. Charge transport can be improved by replacing bulky ligands with smaller capping molecules such as pyridine, n-butylamine, etc. so that the electronic coupling between adjacent nanocrystals increases [All-Inorganic NC Solar Cells, Ultrasensitive Quantum Dot Photodetectors].
Also, chemical treatments of nanocrystal solids with dilute solutions of hydrazine, methylamine, or 1,2-ethanedithiol can significantly improve carrier mobility, up to about 1 cm2V-1s-1 [Hydroxide Treatment of n-Type CdSe NC Films, Photoconductivity of Treated CdSe NC Films, Structural, Optical, Electronic Properties of PbSe NC Films, Field-Effect Transistors from PbSe NC Solids]. Unfortunately, such small linking molecules are volatile and susceptible to oxidation, imparting instabilities in the electronic properties. Our group has proposed entirely new chemical approach for designing electronic materials from colloidal nanocrystals.
Colloidal Nanocrystals with Molecular Metal Chalcogenide Surface Ligands
Spin-coated arrays of CdSe nanocrystals capped with In2Se44- molecular metal chalcogenide complex (MCC) ligands exhibited electron mobility higher than 15 cm2V-1s-1, which was at least one order of magnitude higher than the carrier mobility in best solution processed organic and nanocrystal-based devices [High Mobility of MCC-Capped CdSe]. Most probably, no other approach could provide a dense film of a direct gap semiconductor with comparable carrier mobility via a simple low temperature solution-based route. Moreover, our high mobility nanocrystal solids preserved the properties of quantum-confined semiconductors (Figure 6). High carrier mobility, combined with size-tunable electronic structure, makes MCC-capped nanocrystals very attractive for printable electronics, photovoltaics, and photodetector applications.
Our group has recently employed lead- and bismuth chalcogenide nanocrystals capped with Sb2Te74- MCCs as soluble building blocks for nanostructured thermoelectric materials [Nanostructured Thermoelectric Materials with Antimony Telluride Zintl Ions]. Properly designed MCC bridges between nanocrystals can provide high electron mobility and very low thermal conductivity supressed by the interfacial phonon scattering, which is the combination required for high thermoelectric figures of merit (ZTs). Thermoelectric properties of our nano-composite materials (ZT=0.7 and 0.6 at 300K and 523K, respectively) compared favorably to all previously reported solution-processed thermoelectric materials, with the potential for further improvements. This chemical methodology has been licensed by Evident Technologies, Inc. (NY). This company launched a development effort toward its practical implementation for designing advanced thermoelectric materials.
Charge Transport in Metal-Semiconductor Nano-Heterostructure Arrays
Another direction of research in our group involves synthesis of heterostructured nanocrystals and studies of charge transport through the arrays of these multifunctional building blocks. Figure 7 shows an example of nano-heterostructures combining a magnetic FePt core and semiconducting PbS shell [Magnet-in-Semiconductor FePt-PbS & FePt-PbSe Nanostructures]. We explored such nanostructures as the building blocks for unprecedented materials combining magnetic and semiconducting functionalities, where the inter-component interactions were additionally enforced by tight spatial confinement. The arrays of “magnet-in-the-semiconductor” nanostructures showed semiconductor-type transport properties with magnetoresistance typical for magnetic tunnel junctions, thus combining the advantages of both functional components.
Another example of multifunctional materials has been assembled from colloidal nano-heterostructures combining plasmonic Au core and semiconducting PbS shell [Au-PbS Core-Shell NCs]. In Au-PbS core-shells we observed enhancement of the absorption cross section due to the synergistic coupling between plasmon and exciton in the core and the shell, respectively. Field-effect devices with channels assembled from arrays of Au-PbS core-shell nanostructures demonstrated strong p-type doping that we attributed to the formation of an intra-particle charge transfer complex.