Magnetism of Zn-Doped SnO\u3csub\u3e2\u3c/sub\u3e: Role of Surfaces

Surface effects on the magnetization of Zn-doped SnO2 are investigated using first principles method. Magnetic behavior of Zn-doped bulk and highest and lowest energy surfaces—(001) and (110), respectively, are investigated in presence and absence of other intrinsic defects. The Zn-doped (110) and (001) surfaces of SnO2 show appreciable increase in the magnetic moment (MM) compared to Zn-doped bulk SnO2. Formation energies of Zn defects on both the surfaces are found to be lower than those in bulk SnO2. Zn doping favors the formation of oxygen vacancies. The density of states analysis on the Zn-doped (110) surface reveals that the spin polarization of the host band occurs primarily from p-orbitals of bridging oxygen atoms and the Zn atom itself contributes minimally. The present work provides a key understanding on the role played by the surfaces in inducing the magnetism of doped nanoparticles and thin films

Enhanced Li Capacity in Functionalized Graphene: A First Principle Study with van der Waals Correction

We have investigated the adsorption of Li on graphene oxide using density functional theory. We show a novel and simple approach to achieve a positive lithiation potential on epoxy and hydroxyl functionalized graphene, compared to the negative lithiation potential that has been found on prestine graphene. We included the van der Waals correction into the calculation so as to get a better picture of weak interactions. A positive lithiation potential suggests a favorable adsorption of Li on graphene oxide sheets that can lead to an increase in the specific capacity, which in turn can be used as an anode material in Li-batteries. We find a high specific capacity of ~860 mAhg-1 by functionalizing the graphene sheet. This capacity is higher than the previously reported capacities that were achieved on graphene with high concentration of Stone-Wales (75%) and divacancy (16%) defects. Creating such high density of defects can make the entire system energetically unstable, whereas graphene oxide is a naturally occurring substance

Magnetically Ordered Transition-Metal-Intercalated WSe\u3csub\u3e2\u3c/sub\u3e

Introducing magnetic behavior in nonmagnetic transition metal dichalcogenides is essential to broaden their applications in spintronic and nanomagnetic devices. In this article, we investigate the electronic and magnetic properties of transition-metal-intercalated tungsten diselenide (WSe2) using density functional theory. We find that intercalation compounds with composition of T1/4WSe2 (T is an ironseries transition-metal atom) exhibit substantial magnetic moments and pronounced ferromagnetic order for late transition metals. The densities of states of the T atoms and the magnetic moments on the W sites indicate that the moments of the intercalated atoms become more localized with increasing atomic number. A large perpendicular magnetocrystalline anisotropy of about 9 meV per supercell has been found for Fe1/4WSe2. Furthermore, using mean field theory, we estimated high Curie temperatures of 660, 475, and 379 K for Cr, Mn, and Fe, respectively. The predicted magnetic properties suggest that WSe2 may have applications in spin electronics and nanomagnetic devices

Interplay between Bonding and Magnetism in the Adsorption of NO on Rh Clusters

We have studied the adsorption of NO on small Rh clusters, containing one to five atoms, using density functional theory in both spin-polarized and non-spin-polarized forms. We find that NO bonds more strongly to Rh clusters than it does to Rh(100) or Rh(111); however, it also quenches the magnetism of the clusters. This (local) effect results in reducing the magnitude of the adsorption energy, and also washes out the clear size-dependent trend observed in the non-magnetic case. Our results illustrate the competition present between the tendencies to bond and to magnetize, in small clusters.Comment: Submitted to J. of Chem. Phy

Spin and exchange coupling for Ti embedded in a surface dipolar network

We have studied the spin and exchange coupling of Ti atoms on a Cu$_2$N/Cu(100) surface using density functional theory. We find that individual Ti have a spin of 1.0 (i.e., 2 Bohr Magneton) on the Cu$_2$N/Cu(100) surface instead of spin-1/2 as found by Scanning Tunneling Microscope. We suggest an explanation for this difference, a two-stage Kondo effect, which can be verified by experiments. By calculating the exchange coupling for Ti dimers on the Cu$_2$N/Cu(100) surface, we find that the exchange coupling across a `void' of 3.6\AA\ is antiferromagnetic, whereas indirect (superexchange) coupling through a N atom is ferromagnetic. We confirm the existence of superexchange interactions by varying the Ti-N angle in a model trimer calculation. For a square lattice of Ti on Cu$_2$N/Cu(100), we find a novel spin striped phase

Defect Driven Magnetism in Doped SnO\u3csub\u3e2\u3c/sub\u3e Nanoparticles: Surface Effects

Magnetism and energetics of intrinsic and extrinsic defects and defect clusters in bulk and surfaces of SnO2 is investigated using first-principles to understand the role of surfaces in inducing magnetism in Zn doped nanoparticles. We find that Sn vacancies induce the largest magnetic moment in bulk and on surfaces. However, they have very large formation energies in bulk as well as on surfaces. Oxygen vacancies on the other hand are much easier to create than VSn, but neutral and VO+2 vacancies do not induce any magnetism in bulk as well as on surfaces. VO+1 induce small magnetism in bulk and on (001) surfaces. Isolated ZnSn defects are found to be much easier to create than isolated Sn vacancies and induce magnetism in bulk as well on surfaces. Due to charge compensation, ZnSn+VO defect cluster is found to have the lowest for-mation energy amongst all the defects; it has a large magnetic moment on (001), a small magnetic moment on (110) surface and it is non-magnetic in bulk. Thus, we find that ZnSn and ZnSn+VO defects on the surfaces of SnO2 play an important role in inducing the magnetism in Zn-doped SnO2 nanoparticles

Tuning Electronic and Optical Properties of a New Class of Covalent Organic Frameworks

Covalent organic frameworks (COFs) are the new emerging functional materials for constructing novel electronic and optoelectronic devices. However, designing COFs with tunable electronic and optical properties is still a critical challenge of paramount significance. In this work, we demonstrate a novel and simple approach to tuning the electronic and optical properties of a new class of three-dimensional covalent organic frameworks – (X4Y)(O2B–C6H4–BO2)3. Boronic acid (O2B–C6H4–BO2) being the linker group and X4Y being the node, we show that the band gap of these COFs can be tuned desirably via node-alteration in (X4Y)(O2B–C6H4–BO2)3, viz., by changing the elemental combinations in (X4Y). Using density functional theory, these COFs with X = C and Si and Y = C, Si, Ge, Sn, and Pb are predicted to have a high thermodynamic stability, suggesting that these structures can be experimentally accessible under suitable conditions. Lattice parameter, bulk modulus, formation enthalpy, chemical bonding, band gap and optical properties are shown to systematically vary as a function of X and Y. All COFs in the series are found to be semiconductors with band gaps ranging from 2.7 to 3.8 eV. The bulk moduli of the current COFs are found to be larger than that of MOF-5, indicating a robust framework stability of the predicted COFs. Chemical bonding analysis indicates a predominantly covalent bonding along with a modest ionic bonding, which can be tuned from X4C to X4Pb. The optical response of current COFs can be systematically tuned from the UV to the visible spectrum. These findings will pave the way for the utilization of COFs in various applications such as in photovoltaics, photocatalysts, hybrid solar cells, electroluminescence cells and light-emitting, optoelectronic and nanoelectronic devices