Photoemission Insight on Narrow Band Gap PbS Quantum Dots Relevant for Infrared Imaging
Abstract
In recent years, the interest for narrow band gap colloidal quantum dots (CQDs) has shifted from materials optimized for solar cells (with EG≈1.2 eV) to materials with properties at longer wavelengths. This shift necessitates a systematic investigation of their electronic properties to optimize photodiode design effectively. In this study, we utilized X-ray photoemission to systematically determine how the band gap (with absorption maximum wavelengths of 1000, 1200, 1400, and 1550 nm) and surface chemistry (short halides and thiols) influence the band alignment and the core levels in PbS nanocrystals. Our research provides evidence for a shift from quasiintrinsic behavior for the narrowest band gap to an exclusively n-type nature for the particles with largest size, potentially indicating the emergence of degenerate doping. Core level analysis also revealed that the effect of ligands extends beyond dipole behavior, inducing charge transfer leading to the formation of metallic islands, which might be detrimental to photodetection. Furthermore, by employing scanning photoemission microscopy, we gained direct access to the photodiode's builtin potential-a parameter typically obtained through indirect modeling. We then discussed how particle size influences this property. This study lays the foundation for a more rational design of PbS CQD-based photodiodes operating in the shortwave infrared spectrum.