Controlling magnetic and magneto-optical properties of transparent metal oxide semiconductors has a significant potential for spintronics and photonics. Although ferromagnetism has been reported for several nanostructured transparent metal oxides in the absence of magnetic dopants, its origin and the nature of the exchange interactions remain controversial. Here, we report a variable-temperature−variable-field magnetic circular dichroism study of ZnO and SnO 2 nanocrystals prepared under oxidizing and reducing conditions. We observe the band splitting in ZnO and SnO 2 nanocrystals induced by localized doublet (S = 1/2) and triplet (S = 1) ground states, respectively. Photoluminescence measurements suggest that these states are associated with oxygen vacancies, either as isolated paramagnetic sites in ZnO or as local vacancy-based complexes in SnO 2 nanocrystals. The results of this work demonstrate the ability to tune carrier polarization in metal oxide nanocrystals by in situ control of the native defect formation and attest to the anomalous Zeeman splitting of the band states, which may play an important role in generating ferromagnetism in this class of materials.
Plasmonic semiconductor nanocrystals (NCs) have emerged as an attractive alternative to noble metal nanoparticles. They typically exhibit localized surface plasmon resonance (LSPR) in the infrared range, which can be tuned by controlling the type and concentration of charge carriers. Compositionally and electronically complex semiconductor NCs have attracted significant attention due to their reported plasmonic properties, such as bipolar behavior (off-stoichiometric spinel-type gallium iron oxide or GFO), wide tunability (copper iron chalcogenide or Cu x Fe y S2–z Se z ), and extremely broad LSPR (oxygen-deficient WO3–x ). However, the origin of these unusual properties remains unclear. Here, we comparatively investigate the plasmonic properties of these semiconductor NC materials using magnetic circular dichroism (MCD) spectroscopy, owing to the highly specific response of LSPR to the magnetic field and light polarization. In all systems, we find indisputable evidence that the LSPR absorption bands have been mischaracterized. MCD signals that coincide with the absorption bands previously assigned to LSPR of GFO and Cu x Fe y S2–z Se z NCs show rapid saturation with the magnetic field and independence on the sign of charge carriers, indicating that the corresponding absorption bands are not due to LSPR. This behavior is consistent with intra-ionic and inter-band transitions for GFO and Cu x Fe y S2–z Se z NCs, respectively. The MCD signal corresponding to the absorption spectrum of WO3–x NCs shows Brillouin function dependence on the magnetic field at high photon energies (attributed to d–d transitions of W5+) and linear dependence in the near-infrared range (attributed to LSPR), indicating that the broad absorption spectrum of WO3–x NCs is only partly associated with LSPR. The results of this work help resolve notable discrepancies in the literature, underline the challenges in assigning absorption bands of complex semiconductor NCs to LSPR, and demonstrate that MCD spectroscopy is an invaluable tool for characterization of these materials.
We investigated the role of the synthesis method and post-synthesis processing on the plasmonic properties of antimony-doped SnO2 nanocrystals. The nanocrystal samples having variable doping concentrations were prepared by coprecipitation and solvothermal methods, and subsequently thermally annealed for different durations. We found that solvothermally-synthesized Sb-doped SnO2 nanocrystals exhibit strong localized surface plasmon resonance in the near-infrared region, which is distinctly absent in the nanocrystals synthesized by the coprecipitation method. Upon thermal annealing, the plasmon absorption emerges in the nanocrystals prepared by the coprecipitation method, and increases in intensity in solvothermally-synthesized nanocrystals. Using X-ray photoelectron spectroscopy, we correlated the plasmon intensity to the oxidation of Sb3+ to Sb5+. These results demonstrate that synthesis methodology and post-synthesis treatment can dramatically influence the plasmonic properties of aliovalently-doped semiconductor nanocrystals via dopant oxidation state, and can be effectively used to design semiconductor nanocrystals with targeted plasmonic properties.
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