Encapsulation of small magnetic clusters in fullerene cages: A density functional theory investigation within van der Waals corrections

Abstract

The encapsulation of magnetic transition-metal (TM) clusters inside carbon cages (fullerenes, nanotubes) has been of great interest due to the wide range of applications, which spread from medical sensors in magnetic resonance imaging to photonic crystals. Several theoretical studies have been reported; however, our atomistic understanding of the physical properties of encapsulated magnetic TM 3d clusters is far from satisfactory. In this work, we will report general trends, derived from density functional theory within the generalized gradient approximation proposed by Perdew, Burke, and Ernzerhof (PBE), for the encapsulation properties of the TM m @C n (TM = Fe, Co, Ni; m = 2−6, n = 60,70,80,90) systems. Furthermore, to understand the role of the van der Waals corrections to the physical properties, we employed the empirical Grimme's correction (PBE + D2). We found that both PBE and PBE + D2 functionals yield almost the same geometric parameters, magnetic and electronic properties, however, PBE + D2 strongly enhances the encapsulation energy. We found that the center of mass of the TM m clusters is displaced towards the inside C n surfaces, except for large TM m clusters (m = 5 and 6). For few cases, e.g., Co 4 and Fe 4 , the encapsulation changes the putative lowest-energy structure compared to the isolated TM m clusters. We identified few physical parameters that play an important role in the sign and magnitude of the encapsulation energy, namely, cluster size, fullerene equatorial diameter, shape, curvature of the inside C n surface, number of TM atoms that bind directly to the inside C n surface, and the van der Waals correction. The total magnetic moment of encapsulated TM m clusters decreases compared with the isolated TM m clusters, which is expected due to the hybridization of the d-p states, and strongly depends on the size and shape of the fullerene cages.

Stair Magnetism: Distinct Magnetic States of Co<sub>5</sub>C<sub>5</sub> Carbide Isomers

“…Nevertheless, as we can see now, I lost evidences about quantum magnetism. It should be mentioned that Ni-atom clusters (n = 2 -6) demonstrated the similar stair magnetic trend in a gas phase and in fullerene cages (m = 60, 70, 80, 90), however, authors did not emphasize this interesting fact [2].…”

“…JMP equator, demonstrated "atypical" high magnetic activity similar to C-3 and C-4 compounds with reduced three and four Co-C bonds, respectively. It should be pointed out that the rough "Dragon's Teeth" appeared in calculations of spin magnetic moments of covered metal clusters via both VASP [2,7] and OpenMX [8,9] software, as well as LDA + U [10], GGA-PW91 [7], or GGA-PBE [2,7-9] functionals. Vienna ab initio Simulation Package (VASP) using the projector augmented wave (PAW) method is one of the most accurate packages, which is also considered as a state of the art in DFT codes [22][23][24][25].…”

“…Several papers were recently published on the putative endohedral metallofullerenes Me n @C m obtained by an encapsulation of transition-metal clusters (Fe, Co, Ni, Tc, n = 2 -7) into carbon fullerenes (m = 60, 70, 80, 82, 90) in the frame of quantum mechanical simulations employing Density Functional Theory (DFT) [2,[7][8][9]. The "encapsulation" means the merging of available or previously calculated atomic coordinates for the TM cluster and the fullerene with the following structural optimization and prediction of physical characteristics.…”

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ABSTRACTRuffle magnetic landscape of transition metals (TM) is a trouble of material science that makes it hard to predict properties of nanomagnets [1,2]. It is not quite clear, whether jumps in magnetic activity is the intrinsic property of TM complexes or a controversy of computational experiments.

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Hybrid density functional study of small Rhn(n=2−15)clusters

“…[6][7][8] Among the 3d, 4d, and 5d TM clusters, rhodium clusters (Rh n ) have attracted great interest due to the wide range of applications in catalysis. 6,9 Furthermore, experimental studies reported unexpected large magnetic moments for Rh n (n = 9-34), 10 which is in contrast with the nonmagnetic crystalline phase of bulk Rh.…”