https://libjncir.jncasr.ac.in/xmlui/handle/10572/21 Sat, 04 Apr 2026 05:27:33 GMT 2026-04-04T05:27:33Z http://https://libjncir.jncasr.ac.in:80/xmlui/bitstream/id/e1e95ae7-0f49-4c4c-9f43-6e3a339434d6/shobhana.jpg https://libjncir.jncasr.ac.in/xmlui/handle/10572/21 https://libjncir.jncasr.ac.in/xmlui/handle/10572/2340 Tuning spin transport properties and molecular magnetoresistance through contact geometry Ulman, Kanchan; Narasimhan, Shobhana; Delin, Anna Molecular spintronics seeks to unite the advantages of using organic molecules as nanoelectronic components, with the benefits of using spin as an additional degree of freedom. For technological applications, an important quantity is the molecular magnetoresistance. In this work, we show that this parameter is very sensitive to the contact geometry. To demonstrate this, we perform ab initio calculations, combining the non-equilibrium Green's function method with density functional theory, on a dithienylethene molecule placed between spin-polarized nickel leads of varying geometries. We find that, in general, the magnetoresistance is significantly higher when the contact is made to sharp tips than to flat surfaces. Interestingly, this holds true for both resonant and tunneling conduction regimes, i.e., when the molecule is in its "closed" and "open" conformations, respectively. We find that changing the lead geometry can increase the magnetoresistance by up to a factor of similar to 5. We also introduce a simple model that, despite requiring minimal computational time, can recapture our ab initio results for the behavior of magnetoresistance as a function of bias voltage. This model requires as its input only the density of states on the anchoring atoms, at zero bias voltage. We also find that the non-resonant conductance in the open conformation of the molecule is significantly impacted by the lead geometry. As a result, the ratio of the current in the closed and open conformations can also be tuned by varying the geometry of the leads, and increased by similar to 400%. (C) 2014 AIP Publishing LLC. Restricted Access Wed, 01 Jan 2014 00:00:00 GMT https://libjncir.jncasr.ac.in/xmlui/handle/10572/2340 2014-01-01T00:00:00Z https://libjncir.jncasr.ac.in/xmlui/handle/10572/2339 Point defects in twisted bilayer graphene: A density functional theory study Ulman, Kanchan; Narasimhan, Shobhana We have used ab initio density functional theory, incorporating van der Waals corrections, to study twisted bilayer graphene (TBLG) where Stone-Wales defects or monovacancies are introduced in one of the layers. We compare these results to those for defects in single-layer graphene or Bernal stacked graphene. The energetics of defect formation is not very sensitive to the stacking of the layers or the specific site at which the defect is created, suggesting a weak interlayer coupling. However, signatures of the interlayer coupling are manifested clearly in the electronic band structure. For the "gamma gamma" Stone-Wales defect in TBLG, we observe two Dirac cones that are shifted in both momentum space and energy. This up/down shift in energy results from the combined effect of a charge transfer between the two graphene layers and a chemical interaction between the layers, which mimics the effects of a transverse electric field. Charge density plots show that states near the Dirac points have significant admixture between the two layers. For Stone-Wales defects at other sites in TBLG, this basic structure is modified by the creation of minigaps at energy crossings. For a monovacancy, the Dirac cone of the pristine layer is shifted up in energy by similar to 0.25 eV due to a combination of the requirement of the equilibration of Fermi energy between the two layers with different numbers of electrons, charge transfer, and chemical interactions. Both kinds of defects increase the density of states at the Fermi level. The monovacancy also results in spin polarization, with magnetic moments on the defect of 1.2-1.8 mu(B). Restricted Access Wed, 01 Jan 2014 00:00:00 GMT https://libjncir.jncasr.ac.in/xmlui/handle/10572/2339 2014-01-01T00:00:00Z https://libjncir.jncasr.ac.in/xmlui/handle/10572/2338 Physical origins of weak H-2 binding on carbon nanostructures: Insight from ab initio studies of chemically functionalized graphene nanoribbons Ulman, Kanchan; Bhaumik, Debarati; Wood, Brandon C.; Narasimhan, Shobhana We have performed ab initio density functional theory calculations, incorporating London dispersion corrections, to study the absorption of molecular hydrogen on zigzag graphene nanoribbons whose edges have been functionalized by OH, NH2, COOH, NO2, or H2PO3. We find that hydrogen molecules always preferentially bind at or near the functionalized edge, and display induced dipole moments. Binding is generally enhanced by the presence of polar functional groups. The largest gains are observed for groups with oxygen lone pairs that can facilitate local charge reorganization, with the biggest single enhancement in adsorption energy found for "strong functionalization" by H2PO3 (115 meV/H-2 versus 52 meV/H-2 on bare graphene). We show that for binding on the "outer edge" near the functional group, the presence of the group can introduce appreciable contributions from Debye interactions and higher-order multipole electrostatic terms, in addition to the dominant London dispersion interactions. For those functional groups that contain the OH moiety, the adsorption energy is linearly proportional to the number of lone pairs on oxygen atoms. Mixed functionalization with two different functional groups on a graphene edge can also have a synergistic effect, particularly when electron-donating and electron-withdrawing groups are combined. For binding on the "inner edge" somewhat farther from the functional group, most of the binding again arises from London interactions; however, there is also significant charge redistribution in the pi manifold, which directly reflects the electron donating or withdrawing capacity of the functional group. Our results offer insight into the specific origins of weak binding of gas molecules on graphene, and suggest that edge functionalization could perhaps be used in combination with other strategies to increase the uptake of hydrogen in graphene. They also have relevance for the storage of hydrogen in porous carbon materials, such as activated carbons. (C) 2014 AIP Publishing LLC. Restricted Access Wed, 01 Jan 2014 00:00:00 GMT https://libjncir.jncasr.ac.in/xmlui/handle/10572/2338 2014-01-01T00:00:00Z https://libjncir.jncasr.ac.in/xmlui/handle/10572/2337 India: shed the bad science image Narasimhan, Shobhana Restricted Access Wed, 01 Jan 2014 00:00:00 GMT https://libjncir.jncasr.ac.in/xmlui/handle/10572/2337 2014-01-01T00:00:00Z