Controlling the Mechanism of Excitonic Splitting in In₂O₃ Nanocrystals by Carrier Delocalization

Abstract

Degenerately doped metal oxide nanocrystals have emerged as infrared plasmonic materials with promising applications in optoelectronics, surface-enhanced infrared spectroscopies, and sensing. They also have potential for technological applications in electronics and photonics owing to the possibility of coupling between plasmon and exciton in the absence of a heterojunction. Here, we demonstrate the control of excitonic splitting in In 2 O 3 nanocrystals upon excitation with circularly polarized light in an external magnetic field by simultaneous control of the electronic structure of donor defects and the nanocrystal host lattice. Using variable-temperature− variable-field magnetic circular dichroism spectroscopy, we show that the nanocrystal band splitting has two distinct contributions in plasmonic In 2 O 3 nanocrystals. Temperature-independent splitting arises from the cyclotron magnetoplasmonic modes, which impart angular momentum to the conduction band excited states near the Fermi level, and increases with the intensity of the corresponding plasmon resonance. Temperature-dependent splitting is associated with the localized electron spins trapped in defect states. The ratio of the two components can be controlled by the formation of oxygen vacancies or introduction of aliovalent dopants. Using these experimental results in conjunction with the density functional theory modeling, relative contribution of the two mechanisms is discussed in the context of the perturbation theory taking into account energy separation between the nanocrystal excited states and the localized defect states. The results of this work demonstrate the ability to control carrier polarization in nonmagnetic metal oxide nanocrystals using both individual and collective electronic properties and allow for their application as an emerging class of multifunctional materials with strongly interacting degrees of freedom.

Plasmon-induced carrier polarization in semiconductor nanocrystals

“…[24][25] It has been demonstrated that when a larger concentration of oxygen vacancies is favored by the reaction equilibrium, In2O3, TiO2, MoO3 and WO3 NCs could have sufficient free electrons to generate LSPR. 11,20,[26][27] It is worth mentioning that, some metal oxides, such as VO2 and ReO3, are intrinsically metallic and could possess the LSPR in the NIR and even visible light region. [28][29] Aliovalent doping is a commonly used protocol to tune the electric properties of a semiconductor material in industry.…”

“…38 Similar behavior of LSPR was also observed in oxygen-deficient TiO2 NCs. 76 The other discrepancy is related to the quantum confinement associated with small sized NCs. As mentioned above, Schimpf et al observed a blue shift of LSPR in ZnO NCs with decreasing sizes in the quantum confinement regime.…”

“…As we have shown in the Chapter 3, the spin-splitting (anomalous Zeeman splitting) of excitonic states in plasmonic metal oxide NCs can arise not only from coupling of the conduction band states with unpaired d electrons on aliovalent transition metal dopant ions, but also with localized electrons on oxygen vacancy sites. 26 In this study, dopant-induced spin-splitting would require the molybdenum or tungsten to exist in NCs in a reduced form relative to their precursors (Mo 6+ or W 6+ ). The appreciable amount of reduced dopants is found only in IMO NCs.…”

Magnetoplasmon Resonances in Semiconductor Nanocrystals: Potential for a New Information Technology Platform

“…have shown that the percentage of {001} and {101} facets in anatase TiO 2 NCs can be engineered through the choice of the titanium precursor and co‐surfactant, resulting in the oxygen deficiency and the formation of LSPR. Importantly, well‐defined LSPR absorption band has been found to occur even in In 2 O 3 NCs synthesized under reducing conditions, illustrating the tendency of oxygen vacancy formation in this class of materials. In metal chalcogenide NCs, cation vacancies are typically responsible for inducing free valence band holes that determine the LSPR properties (Figure d) .…”

“…The novel concept of magnetoplasmon‐induced excitonic Zeeman splitting in semiconductor NCs allows for many interesting opportunities to control carrier polarization through colloidal synthesis conditions. An illustrative example is the ability to control oxygen vacancy formation in non‐stoichiometric plasmonic metal oxide NCs via redox environment or NC morphology . We have recently demonstrated the variation in plasmon‐induced excitonic splitting in colloidal TiO 2 NCs by controlling NC faceting .…”