Colloidal lead sulphide nanostructures for optoelectronics applications
Keywords: nanostructures
quantum dots
transfer printing
colloidal
FETs
nanowires

Colloidal semiconductors are nanosized semiconducting materials that are grown in solution and dispersed in solvents. Since the first synthesis reports of colloidal nanocrystal quantum dots in the early 1990s, considerable progress has been made, enabling the production of monodisperse colloids that are tunable in size and shape over a wide range of semiconductors and alloys. In recent years, the deployment of these nanocrystals , has been considered in numerous optoelectronic devices such as solar cells, photodetectors, and light emitting diodes, covering areas as diverse as renewable energies, communications, medical imaging, displays, and lighting.
Lead sulphide (PbS) colloidal nanocrystals stand out, following ground-breaking discoveries in energy harvesting and near infra-red (NIR) photodetection. In order to achieve successful integration in the next generation of commercially available devices, several improvements must be made at the material, device, and manufacturing level. This thesis attempts to address these challenges by focusing on the application of PbS colloidal nanowires and quantum dots in NIR photodetection. Field effect transistors have been fabricated to study the transport properties of PbS nanowires and it is shown that p-type mobilities can reach values as high as 160 cm2V-1s-1 thanks to the high degree of crystallinity. The devices also showed interesting photo-response behaviour in the infrared with a responsivity as high as 1.9x104 A/W, together with response times of the order of 30 ms.
Hybrid devices consisting of PbS nanowire FETs capped with PbS QDs were also made for the first time thanks to an innovative fabrication technique involving the dry printing of PbS QDs. It was shown that such hybrid structures exhibited substantial reduction in gate voltage hysteresis, which was attributed to charge transfer occurring at the QD/nanowire interface, helping to passivate localized trap states in the nanowire. PbS QDs were also used as standalone active materials in IR photodetectors. The impact on device performance of ligand exchange, a strategy used to increase the conductivity of QD films, was investigated in a study of two commonly used ligands, namely 1,2-Ethanedithiol (EDT) and tetrabutylammonium (TBAI). EDT-exchanged films showed better conductivity in the dark but a lower photosensivity than the TBAI-treated films.
The study on ligands served as a basis for the design of two photodetection devices, starting with the fabrication of nanogap Schottky photodiodes consisting of ligand exchanged PbS QDs that were sandwiched between gold (Au) and aluminium (Al) electrodes, with spacings of only a few tens of nanometres. The electrodes were made via adhesion lithography, a novel fabrication technique that holds great promise for probing charge transport properties as well as for large scale manufacturing of devices. The devices fabricated in this work exhibited a responsivity of R = 38.7A/W in reverse bias combined with a detectivity of 3.5×1012 and a bandwidth of 180 Hz. These results highlight the compatibility of colloidal PbS QDs with adhesion lithography fabrication processes for the development of IR detectors for the next generation of flexible and transparent electronics. The second type of devices consisted of patterned photodetectors made from printed micro arrays of PbS QDs. It was shown that transfer printing was a suitable method for the patterning of such devices and offered a viable alternative to commonly used non-patternable deposition techniques such as spin-coating. Because of the versatility in the process, it was also shown that this technique could be used with transparent and plastic substrates, leading to the realisation of transparent flexible photodetectors made of PbS QDs.