Microwave-Polyol Synthesis of Sub-10-nm PbS Nanocrystals for Metal Oxide/Nanocrystal Heterojunction Photodetectors

A rapid microwave-polyol technique is developed to grow bare-surface sub-10-nm lead sulfide (PbS) colloidal nanocrystals (NCs) of uniform size. Growth involves three distinct dynamical stages. PbS NCs homogeneously nucleate and undergo a fast growth process that spontaneously aggregates them into a continuous microstructure of PbS. Next, the interaction of low-power microwave irradiation with the NCs’ dipole moments along different crystallographic axes leads to the development of strain at the NCs’ grain boundaries, which ultimately causes dissociation of the aggregated microstructures into individual uniform-sized PbS NCs. The most distinctive feature of this growth technique is the bare nature and uniform size of the NCs produced. This gives the freedom to fabricate a highly packed NC thin film, which is essential for enhanced charge-transport properties. It simultaneously also precludes the need of undergoing a detrimental solid-state ligand-exchange process, which is indispensably required for casting thin films from conventional counterpart long-aliphatic-ligand-capped NCs. To realize the photosensitivity and charge-transport nature of these microwave-synthesized NCs, we fabricated a lateral heterojunction photoconductor and a phototransistor that work on the principal of electron transfer to the metal oxide layer. The photoconductor exhibits prominent multispectral photosensitivity in the visible region, demonstrating high external quantum efficiency and detectivity of 7.42 × 102% and 1.07 × 1011 jones, respectively. Further, it also shows superior photocurrent generation with respect to the conventional hot-injection-method-synthesized PbS NC-based photoconductor, indicating more efficient inter-NC charge transport in its NC thin film. The unencapsulated photoconductor exhibits a long air-storage lifetime, which reflects air stability of the device and PbS NCs. In addition to the photoconductor, the metal oxide/PbS heterojunction phototransistor also shows superior photosensitivity with a responsivity and a response time of 3 × 10–2A/W and 2.2 s, respectively.