Ultrabroadband Absorption and High-Performance Photodetection in Europium-Doped 2D Topological Insulator Bi₂Se₃ Nanosheets

In the pursuit of materials with exceptional optoelectronic functionalities in the visible to mid-IR region, the topological insulators are very promising because they have unique Dirac-like surface states and high light-absorption properties. Herein, we report on a low-temperature, facile, solution-processable method for the synthesis of undoped and Eu-doped 2D layered Bi2Se3 nanosheets (NSs) with exceptional optical functionalities. 2D geometry provides highly effective light absorption and efficient charge transportation via topologically protected surface states of Bi2Se3 NSs. Our study reveals that as-grown Bi2Se3 NSs possess major native defects, particularly the Se vacancies that are responsible for ultrabroadband absorption and bulk conduction. Interestingly, Eu doping in Bi2Se3 partially compensates the intrinsic defects, and the Eu-doped Bi2Se3 NSs show reduced band gap and very high absorption coefficient (2.87 × 105 cm–1) over the broad range of 400–2500 nm. We achieve a lower dark current and a higher photocurrent with increasing doping concentration. Interestingly, the undoped Bi2Se3 NSs exhibited negative photoconductivity, which is completely suppressed in the Eu-doped Bi2Se3 NSs due to the reduction in trapping sites. The role of defects in the ultrabroadband absorption and high photoconductivity was elucidated through the rapid thermal annealing studies. To demonstrate its application, we fabricate a planar photodetector using the Eu-doped 2D Bi2Se3 NSs and investigate device characteristics at specific wavelengths 405, 657, and 808 nm. The device with undoped Bi2Se3 NSs shows the highest photoresponsivity at 405 nm, while the Eu-doped NSs show superior performance at 657 and 808 nm. Additionally, the doped photodetectors possess broadband and high responsivity/detectivity with a peak responsivity of 130 A/W and a peak detectivity of 1.482 × 1013 Jones, which are the highest among the reported values. The present report demonstrates an effective strategy to develop ultrabroadband and high-performance optoelectronic applications of doped 2D topological insulators.