We present a comparative study of DNA nucleobases [guanine (G), adenine (A), thymine (T), and cytosine (C)] adsorbed on hexagonal boron nitride (h-BN) sheet and graphene, using local, semilocal, and van der Waals (vdW) energy-corrected density-functional theory (DFT) calculations. Intriguingly, despite the very different electronic properties of BN sheet and graphene, we find rather similar binding energies for the various nucleobase molecules when adsorbed on the two types of sheets. The calculated binding energies of the four nucleobases using the local, semilocal, and DFT+vdW schemes are in the range of 0.54 ∼ 0.75 eV, 0.06 ∼ 0.15 eV, and 0.93 ∼ 1.18 eV, respectively. In particular, the DFT+vdW scheme predicts not only a binding energy predominantly determined by vdW interactions between the base molecules and their substrates decreasing in the order of G>A>T>C, but also a very weak hybridization between the molecular levels of the nucleobases and the π-states of the BN sheet or graphene. This physisorption of G, A, T, and C on the BN sheet (graphene) induces a small interfacial dipole, giving rise to an energy shift in the work function by 0.11 (0.22), 0.09 (0.15), −0.05 (0.01), and 0.06 (0.13) eV, respectively.

A recent density-functional calculation for fcc C 60 H n (n = odd) [K. W. Lee and C. E. Lee, Phys. Rev. Lett. 106, 166402 (2011)] proposed the existence of Stoner ferromagnetism based on an itinerant band model. However, our density-functional calculation shows that the antiferromagnetic (AFM) configuration is slightly more stable than the ferromagnetic (FM) one. This preference for antiferromagnetism over ferromagnetism is analogous to the case of a dimer (C 60 H) 2 , where each C 60 H is spin polarized by an intramolecular exchange and the two magnetic moments are antiferromagnetically coupled with each other. The results demonstrate that the underlying mechanism of the magnetic order in fcc C 60 H n is associated with the AFM superexchange between the magnetic moments created by H dopants. The observation of ferromagnetism in fullerenes has attracted intensive attention due to its interest concerning carbon magnetism and potential technological application in the emerging field of spintronics.1-5 However, there has long been controversy about whether the origin of the observed ferromagnetism is associated with extrinsic iron impurities [6][7][8] or intrinsic defects.9-14 The latter intrinsic defects involve carbon vacancies in polymerized fullerenes 9-11 or doped fullerenes 12-14 C 60 R n (R: nonmagnetic elements such as H and O atoms) where doping creates fullerene radical adducts with unpaired spins localized on fullerene. Experimental studies for photo-oxidated fullerenes 3,4 and hydrofullerite 5 C 60 H 24 observed a signal of ferromagnetism at room temperature. However, a density-functional theory (DFT) calculation 13 for C 60 O did not support the existence of ferromagnetism, whereas a DFT calculation 14 for fcc C 60 H n predicted a strong itinerant ferromagnetism with odd-numbered H dopants. From their DFT calculation within the local-density approximation, Lee and Lee (LL) 14 found that H dopants on an fcc C 60 crystal create quasilocalized π electrons leading to a narrow half-filled band, and concluded that a direct overlap of the π electrons between adjacent C 60 H n molecules gives rise to Stoner (itinerant) ferromagnetism with an exchange splitting of ∼0.2 eV. However, this exchange splitting is not due to the Stoner-type FM exchange but to an intramolecular exchange (i.e., Stoner parameter I ), as discussed below. As a matter of fact, the ferromagnetic (FM) and antiferromagnetic (AFM) order is governed by the exchange interaction energy between neighboring magnetic moments (i.e., exchange coupling constant J in the Heisenberg model).Recently, it was reported 15 that the microscopic mechanism of defect-induced magnetism in dilute magnetic semiconductors could be similar to that in carbon-based materials such as C 60 polymers, TDAE-C 60 , graphene ribbons, and irradiated graphite. There are two different mechanisms that describe the exchange interaction in diluted magnetic semiconductors: The double-exchange mechanism favors FM coupling, whereas the superexchange mechanism usually leads to AFM co...

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