Iron Intermediate Band Governs Relaxation Kinetics of Bornite Plasmonic Semiconductor Nanocrystals

Intermediate band (IB) plasmonic semiconductor nanocrystals of Cu x FeS 4 ( x = 3, 5, 7) are investigated by femtosecond transient absorption (fsTA) to correlate the observed LSPR damping behavior with changes in the observed electron–phonon and phonon–phonon relaxation dynamics when excited at the localized surface plasmon resonance (LSPR) and optical band gap ( E g,opt ). Changing the Cu:Fe ratio results in a shift of the Fe 3d IB consistent with an isosbestic shift seen for all fsTA data. While different relaxation pathways can be accessed, the Fe intermediate band is critical for both pump regimes, resulting in an increased hole–phonon ( ho-ph ) coupling constant as a function of increased carrier density and carrier effective mass ( m *). Additionally, evidence is provided for a correlation of LSPR m * and damping versus ho-ph and ph-ph relaxation, respectively, by comparing fsTA data with two-temperature modeling which may be used to guide future development of high photothermal or photoacoustic conversion efficient plasmonic semiconductor nanomaterials.