Tuning localized surface plasmon resonances of FeS₂ nanocrystals via shape and surface functional groups for enhanced photoconductivity

The anisotropy of nanocrystals and surface functional groups play an important role in their photo-absorption as well as opto-electronic properties. In this manuscript, we report on the synthesis of cluster-like and cubic FeS₂ nanocrystals via a simple colloidal chemistry method. As-prepared FeS₂ nanocrystals exhibit an enhanced absorption in the light range of 400–1200 nm due to the free carrier induced localized surface plasmon resonances (LSPRs). Compared to nanoclusters, FeS₂ nanocubes show a stronger absorption at longer wavelength with larger scattering effect. The surface of as-synthesized FeS₂ nanocrystals has been further modified via post-synthetic ligand exchange to remove the insulating long organic hydrocarbon molecules. An obvious red shift of corresponding LSPRs frequency of FeS₂ nanocrystals is observed, indicating the decrease of free carrier concentration. High quality FeS₂ thin films with thickness of ~500 nm have been spray-painted from colloidal nanocrystal suspensions. The photoresponse activity has been investigated with a structure of FTO/FeS₂ thin film/FTO both in the dark and under illumination using a solar simulator (AM 1.5 G irradiation, 100 mW cm⁻²). The photocurrent of FeS₂ nanocubes is almost two times higher than that of nanoclusters, which is in accordance with stronger light absorption of FeS₂ nanocubes from UV-Vis-NIR absorption spectra. After ligand exchange, an enhancement of photocurrent has been observed for cluster-like and cubic FeS₂ thin films by 136.8 and 125.7% at 1000 mV, respectively. FeS₂ nanocrystals with tunable LSPRs and enhanced photocurrent are attractive for applications in low-cost thin film photovoltics.