Explanation of the Opposing Shifts in the Absorption Edge and the Optical Resonance in CuFeS2 Nanoparticles br

Size-dependent change of the electronic band structure is one of the keyfeatures of nanoparticles in the quantum confinement region. CuFeS2nanoparticles have astrong absorption feature in the visible region that has, controversially, been described asneither an excitonic transition nor a free carrier plasmon oscillation. Instead, theabsorption feature in CuFeS2nanoparticles has been attributed to quasi-static opticalresonances from inter-band transitions between the valence band (VB), intermediate band(IB), and conduction band (CB). As such, we hypothesized that the feature should besubject to quantum confinement effects through modification of the electronic bands. Inthis paper, we show experimentally that the optical resonance absorption peak red-shiftsand the optical band gap blue-shifts as the particle size decreases. Through densityfunctional theory (DFT) and the tight binding (TB) modeling, we elucidate the size dependence of the band structure, especiallyfocusing on the change in the IB. Using a Lorentzian oscillator optical model to simulate the absorption spectrum with inputs fromthe DFT-calculated band structure and band shifts from the TB model, wefind that the size-dependent shifts of the opticalresonance peak position in CuFeS2are due to a tri-band quantum confinement effect that results in both the VB to IB and IB to CBgap expansion that accompanies a decrease in particle size. We alsofind that the transitions between the IB and CB play only aminor role in the optical spectrum. Moreover, the linear optical Lorentzian model predicts that the optical resonance peak is tunableacross the visible range by partiallyfilling the IB, lowering the CB, or expanding the IB.