Co:CdS Diluted Magnetic Semiconductor Nanoparticles: Radiation Synthesis, Dopant−Defect Complex Formation, and Unexpected Magnetism

Abstract

Incorporating a dopant into a nanoparticle is a nontrivial proposition in view of the size dependent surface versus bulk energy considerations and the intrinsic proximity of the surface to the interior, which facilitates migration to the surface. If realized and controlled, however, it can open up new avenues to novel nanomaterials. Some previous studies have shown the dopability of nanosystems but only with specific surface functionalization. Here, we demonstrate the successful dopant incorporation via a new route of pulsed high energy electron induced synthesis. We choose a system Co:CdS (dilutely cobalt doped cadmium sulfide) in view of the well-known application-worthy properties of CdS and the potential possibility of its conversion to a diluted magnetic semiconductor of interest to spintronics. By using various techniques, we show that matrix incorporation and uniform distribution of cobalt are realized in CdS nanocrystals without the need for additional chemical or physical manipulation. Optical and photoluminescence properties also support dopant incorporation. Interestingly, although magnetism is realized, it is weak, and it decreases at higher cobalt concentration. First principle density functional calculations are performed to understand this counterintuitive behavior. These calculations suggest that the introduction of parent cation or anion vacancies lead to magnetic moment reduction, albeit marginally. However, with some Co impurity fraction in the octahedral interstitial site inside the wurtzite cage, the magnetic moment drops down drastically. This study reveals that defect states may have an interesting role in dopant stabilization in nanosystems, with interesting system dependent consequences for the properties.

Encapsulation of cobalt nanoparticles in cross-linked-polymer cages

“…In the literature, there are small discrepancies in the reference peaks for various oxidation states of cobalt. We have adopted the values from [10][11][12]. The signatures at 285 and 288.4 eV ascribed to the carbon atoms in aliphatic chain (C-C) and the carboxylate (-COO À ), respectively.…”

Recent Advances in Magnetic Ion-Doped Semiconductor Quantum Dots

“…However, it was observed around the same time that, even non-magnetic QDs like CeO 2 show ferromagnetic behaviour with similar moments attributed to oxide vacancies at the surface of QD 49 making it difficult to separate the contribution arising out of the magnetic dopant. More recent papers have not shown substantial improvement in magnetic moment [50][51][52] despite improved synthesis methods, possibly due to clustering of magnetic dopants and/or due to inherent nature of sp-d exchange interaction. Further, the absence of quantitative magnetic moment per ion has also hindered the comparison of absolute magnetic moment with bulk materials.…”

A DFT study of the effects of Sc doping on electronic and optical properties of CdS nanoparticles

“…Transition metal (TM) (Mn, Fe, Co, etc) doped CdS has drawn considerable attention as it offers a great opportunity to integrate electrical, optical and magnetic properties into a single material, which makes it an ideal candidate for non volatile memory, magneto-optical and future spintronics devices [19,20]. The first principle DFT calculations have proven that the system Co-doped CdS (dilutely cobalt-doped cadmium sulphide), in view of the well known application worthy properties of CdS and the potential possibility of its conversion to a diluted magnetic semiconductor, is of interest to spintronics [21]. Similarly, copper impurity changes the type of semiconductor from n to p type [22].…”