Size-Controlled Ex-nihilo Ferromagnetism in Capped CdSe Quantum Dots

The potential of exploiting the spin of the electron (in addition to its charge) in novel new electronic devices and the associated opportunities for new science has led to extensive search for viable magnetic semiconductors with room temperature ferromagnetism (RTFM). Some success for ferromagnetism has been reported in dilute magnetic semiconductors (DMS) and dilute magnetic oxides (DMO) containing a few percent of transition metal ions such as (Ga,Mn)As, (Zn,M)O, and (Ti,M)O2 [10–12] with M1⁄4V, Cr, Mn, Co, Ni, and Cu. Despite the numerous reports on the successful observation of room temperature ferromagnetism (RTFM) in a number of these systems in apparent agreement with computations based on density functional theory, the issue remains unsettled and contentious for a number of reasons including lingering doubt of the possible role of undetected ferromagnetic impurities such as Fe, Co, and Ni. In some systems such as (Zn,Ni)O and (Zn,Cr)Te the observed RTFM has been linked to clustering of Ni and Cr nanocrystals, respectively. In this paper, we report the observation of RTFM in CdSe quantum dots (QD) capped with TOPO (tri-n-octylphosphine oxide). This RTFM is labeled as ex-nihilo since the RTFM is due to the marriage of two diamagnetic materials viz. CdSe and TOPO, possibly resulting from charge transfer from Cd d-band to the oxygen atoms of TOPO. We further show that the RTFM varies inversely with the size of the QD, in agreement with our calculations. CdSe is a II-VI semiconductor with direct band gap Eg1⁄4 1.74 eV at 300 K. Semiconductors nanocrystals (NCs)