Colloidal nanocrystaline semiconductor materials are at the forefront of innovations in optoelectronics, including photovoltaics, photodetectors, light-emitting diodes and field-effect transistors. These materials feature bright and uniquely narrow, tuneable emission characteristics that are of particular interest for application in highly efficient light-emitting devices. The materials are also solution processable so that large savings in the fabrication cost of devices may be possible. Among the many semiconductor materials studied, lead sulphide nanocrystals have received attention because of the strong confinement of charge carriers, high dielectric constants, tuneable emission across the near infrared telecommunications window, and potential for harnessing multiple excition generation where a single photon is able to generate more than one set of charge carriers (electron hole pair) with implications for the yields of photovoltaic devices.
A major challenge to the application of nanocrystals in optoelectronic devices is the poor conductivity of the as synthesised colloidal materials. Owing to the insulating surfactant molecules that stabilise nanocrystals, transport of charge carriers (electrons/holes) through the materials is disrupted. A number of approaches have been developed to negate these problems that typically involve “exchanging” the surfactant for alternative molecules that may also passivize the nanocrystal surfaces while facilitating better charge transport through the material. While these ligand exchange methods may enhance charge transport, such treatments can also be detrimental to the optical properties of the materials.
In this work, new organic ligands based upon simple aromatic and “dendritic” (branching) molecules were synthesised with different anchoring functionalities. Namely, π-conjugated benzoic acid derived ligands and non-conjugated aryl acetic analogues. The effects of these ligands upon the chemical and photophyscial properties of lead sulfide nanocrystals were investigated. Lead sulfide nanocrystal stabilised with these two types of ligand were prepared via two routes: Direct synthesis and through ligand exchange. In direct synthesis, both types of ligands offered excellent control over the size and shape of lead sulphide nanocrystal formed. For ligand exchange experiments, lead sulphide nanocrystals capped with oleic acid were first prepared by conventional synthetic protocols. The effects of titrating the ligands into solution of these nanocrystals were investigated through measurements of absorption and photoluminescence. While a sharp decline in the intensity of excitionic features in the nanocrystal absorption was found from the addition of the aryl acetic acid derived ligands. This was attributed to a surface charging effect that quenched the excitonic absorption. The studies using the benzoic acid derived ligand revealed an unusal enhancement of the nanocrystal absorption and photoluminescence. This was attributed to an relaxation of the nanocrystal wavefunction into the conjugated ligand.
To explore the effect of the ligand-anchoring group upon nanocrystal properties, the same simple aromatic and dendritic molecules were synthesised featuring thiol and phosphoric acid functionalities. The effects of titrating the thiol into pre-synthesised, oleic acid capped, lead sulphide nanocrystal solutions was again investigated and showed significant quenching of small lead sulfide nanocrystal photoluminescence and large red shifting of the nanocrystal absorption. The photoluminescence quenching was explained by hole transfer from the photoexcited nanocrystal, while the redshift of the absorption again suggested a relaxation of the nanocrystal wavefunction into the ligand shell. The aryl phosphoric acid ligands were found to rapidly degrade the nanocrystal solutions upon addition, likely due to etching of the acid sensitive lead sulphide surface by the strongly acidic group. This prompted an investigation into the effects of cadmium ion exchange treatments to generate lead sulfide/cadmium sulphide core/shell nanocrystals. The effects of the ligands upon the lead sulphide nanocrystals featuring a cadmium sulphide shell of varying thickness were measured and compared with the results of addition to “naked” lead sulphide nanocrystals. By varying the “thickness” of the cadmium shell it was possible to significantly reduce both the photoluminescence quenching effect of the thiol ligands and the etching effect of the phosphoric acid ligands.
The electronic properties of the lead sulphide nanocrystals were explored in field effect transistors structures. While it was not possible to form films of suitable quality for device operation from the directly synthesised materials, an existing technique for generating nanocrystal film was employed. Using a 1,2-ethanedithiol treatment to immobilise the nanocrystal film allowed for processing of multiple nanocrystal layers to generate contiguous films suitable for the devices. The devices showed ambipolar characteristics with high electron charge carrier mobilites up to 1.8 cm2·V–1·s–1 using of a bottom gate top contact architecture. Devices were fabricated with asymmetric source/drain contacts, and notably emission of light was detected during the operation of these devices, suggesting the potential for new light emitting transistors based upon lead sulphide nanocrysals.