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

Abstract

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 ligandexchange 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 × 10 2 % and 1.07 × 10 11 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 −2 A/W and 2.2 s, respectively.

Scalable Synthesis of a Sub-10 nm Chalcopyrite (CuFeS2) Nanocrystal by the Microwave-Assisted Synthesis Technique and Its Application in a Heavy-Metal-Free Broad-Band Photodetector

“…Therefore, the bilayer heterojunction photodetector enhanced the overall conductivity of the photodetector in comparison to only CuFeS 2 in the photodetector. A similar phenomenon has been observed in PbS nanocrystal- and CdS nanocrystal-based photodetectors. , …”

“…From these sequential observations, a growth process is proposed that is based on the “orientated attachment” model. 39 , 40 As soon as CuFeS 2 nanocrystals are formed at temperature ∼160 °C, the capping-free nanocrystals aggregate to form nanoplates spontaneously under the high-temperature condition. In this aggregated state, the nanoparticles come together and form a weak bridge that allows the solid–solid diffusion that leads to the formation of the nanoplate structure.…”

“…A similar phenomenon has been observed in PbS nanocrystal- and CdS nanocrystal-based photodetectors. 25 , 39 …”

“…In other words, detectivity is a measure of the lowest possible value of radiant power that can generate a signal. The following mathematical relations show the interrelationship among R λ , D *, and EQE and where I ph is the photocurrent at a particular applied voltage, P is the intensity of the incident light per unit area, h is Planck’s constant, c is the speed of light, 39 e is the electronic charge, J d is the dark current per unit effective area of the photodetector, and λ is the wavelength of light. In the expression of detectivity ( eq 3 ), we have considered that the noise current is dominated by dark current only, which is a good approximation for low-intensity photodetection.…”

“…The morphology of these powder nanoparticles observed by a high-resolution SEM (HRSEM) study shows a nanoplate structure, but after dispersion in DMF and sonication, the HRTEM image shows spherical nanoparticles with an average size of 5 nm. From these sequential observations, a growth process is proposed that is based on the “orientated attachment” model. , As soon as CuFeS 2 nanocrystals are formed at temperature ∼160 °C, the capping-free nanocrystals aggregate to form nanoplates spontaneously under the high-temperature condition. In this aggregated state, the nanoparticles come together and form a weak bridge that allows the solid–solid diffusion that leads to the formation of the nanoplate structure.…”