Quantum Phase Transition from Superparamagnetic to Quantum Superparamagnetic State in Ultrasmall Cd_1–<i>x</i>Cr(II)_<i>x</i>Se Quantum Dots?

Despite a long history of success in formation of transition-metal-doped quantum dots (QDs), the origin of magnetism in diluted magnetic semiconductors (DMSs) is yet a controversial issue. Cr(II)-doped II–VI DMSs are half-metallic, resulting in high-temperature ferromagnetism. The magnetic properties reflect a strong p–d exchange interaction between the spin-up Cr(II) t_2g level and the Se 4p. In this study, ultrasmall (∼3.1 nm) Cr(II)-doped CdSe DMSQDs are shown to exhibit room-temperature ferromagnetism, as expected from theoretical arguments. Surprisingly, a low-temperature phase transition is observed at 20 K that is believed to reflect the onset of long-range ordering of the single-domain DMSQD

Evidence of a ZnCr₂Se₄ Spinel Inclusion at the Core of a Cr-Doped ZnSe Quantum Dot

Herein we report doping of ZnSe by Cr ions leads to formation of small ZnCr₂Se₄ spinel inclusions within the cubic sphalerite lattice of a 2.8 nm CrZnSe quantum dot (QD). The Cr ion incorporates as a pair of Cr­(III) ions occupying edge-sharing tetragonal distorted octahedral sites generated by formation of three Zn ion vacancies in the sphalerite lattice in order to charge compensate the QD. The site is analogous to the formation of a subunit of the ZnCr₂Se₄ spinel phase known to form as inclusions during peritectoid crystal growth in the ternary CrZnSe solid-state compound. The oxidation state and site symmetry of the Cr ion is confirmed by X-ray absorption near edge spectroscopy (XANES), crystal field absorption spectroscopy, and electron paramagnetic resonance (EPR). Incorporation as the Cr­(III) oxidation state is consistent with the thermodynamic preference for Cr to occupy an octahedral site within a II–VI semiconductor lattice with a half-filled t_2g d-level. The measured crystal field splitting energy for the CrZnSe QD is 2.08 eV (2.07 eV form XANES), consistent with a spinel inclusion. Further evidence of a spinel inclusion is provided by analysis of the magnetic data, where antiferromagnetic (AFM) exchange, a Curie–Weiss (C–W) temperature of θ = −125 K, and a nearest-neighbor exchange coupling constant of <i>J</i>_NN = −12.5 K are observed. The formation of stable spinel inclusions in a QD has not been previously reported