Sudipta Dutta

Intrinsic half-metallicity in modified graphene nanoribbons.

We perform first-principles calculations based on density functional theory to study quasi-one-dimensional edge-passivated (with hydrogen) zigzag graphene nanoribbons of various widths with chemical dopants, boron and nitrogen, keeping the whole system isoelectronic. The gradual increase in doping concentration takes the system finally to zigzag boron nitride nanoribbons (ZBNNRs). Our study reveals that for all doping concentrations the systems stabilize in antiferromagnetic ground states. Doping concentrations and dopant positions regulate the electronic structure of the nanoribbons, exhibiting both semiconducting and half-metallic behaviors as a response to the external electric field. Interestingly, our results show that ZBNNRs with a terminating polyacene unit exhibit half-metallicity irrespective of the ribbon width as well as applied electric field, opening a huge possibility in spintronics device applications.

Novel properties of graphene nanoribbons: a review

Low-dimensional materials are of great interest to both theorists and experimentalists, owing to their novel electronic properties which arise mainly because of a host of quantum confinement effects. Recent experimental findings of graphene have provided a new platform to explore the interesting electronic properties in strictly two dimensions. In this feature article, we review the novel properties of an interesting class of quasi one dimensional materials, known as graphene nanoribbons, which can be obtained by finite termination of graphene sheet with smooth edges. Recent experimental sophistications provide various physical and chemical ways to materialize these systems. Two different edge geometries, namely zigzag and armchair, arising from the finite termination of graphene, control the electronic properties of graphene nanoribbons. Here we attempt to give an overview of their interesting electronic, magnetic, optical, conduction properties and explore possible ways of enhancing their device applicability by a number of ways including external perturbations, doping and chemical modifications.

Half-metallicity in undoped and boron doped graphene nanoribbons in the presence of semilocal exchange-correlation interactions

We perform density functional calculations on one-dimensional zigzag edge graphene nanoribbons (ZGNRs) of different widths, with and without edge doping including semilocal exchange correlations. Our study reveals that, although the ground state of edge-passivated (with hydrogen) ZGNRs prefers to be anti-ferromagnetic, the doping of both of the edges with boron atoms stabilizes the system in a ferromagnetic ground state. Both the local and semilocal exchange correlations result in half-metallicity in edge-passivated ZGNRs at a finite cross-ribbon electric field. However, the ZGNR with boron edges shows half-metallic behavior irrespective of the ribbon width even in the absence of electric field and this property sustains for any field strength, opening a huge possibility of applications in spintronics.

Self-limiting layer-by-layer oxidation of atomically thin WSe2.

Growth of a uniform oxide film with a tunable thickness on two-dimensional transition metal dichalcogenides is of great importance for electronic and optoelectronic applications. Here we demonstrate homogeneous surface oxidation of atomically thin WSe2 with a self-limiting thickness from single- to trilayers. Exposure to ozone (O3) below 100 °C leads to the lateral growth of tungsten oxide selectively along selenium zigzag-edge orientations on WSe2. With further O3 exposure, the oxide regions coalesce and oxidation terminates leaving a uniform thickness oxide film on top of unoxidized WSe2. At higher temperatures, oxidation evolves in the layer-by-layer regime up to trilayers. The oxide films formed on WSe2 are nearly atomically flat. Using photoluminescence and Raman spectroscopy, we find that the underlying single-layer WSe2 is decoupled from the top oxide but hole-doped. Our findings offer a new strategy for creating atomically thin heterostructures of semiconductors and insulating oxides with potential for applications in electronic devices.

Electron-electron interactions on the edge states of graphene: a many-body configuration interaction study

We have studied zigzag and armchair graphene nanoribbons (GNRs), described by the Hubbard Hamiltonian using quantum many-body configuration interaction methods. Due to finite termination, we find that the bipartite nature of the graphene lattice gets destroyed at the edges, making the ground state of the zigzag GNRs a high spin state, whereas the ground state of the armchair GNRs remains a singlet. Our calculations of charge and spin densities suggest that, although the electron density prefers to accumulate on the edges (instead of spin polarization), the up and down spins prefer to mix throughout the GNR lattice. While the many-body charge gap results in insulating behavior for both kinds of GNRs, the conduction upon application of electric field is still possible through the edge channels because of their high electron density. Analysis of optical states suggest differences in quantum efficiency of luminescence for zigzag and armchair GNRs, which can be probed by simple experiments.

Tuning Charge and Spin Excitations in Zigzag Edge Nanographene Ribbons

Graphene and its quasi-one-dimensional counterpart, graphene nanoribbons, present an ideal platform for tweaking their unique electronic, magnetic and mechanical properties by various means for potential next-generation device applications. However, such tweaking requires knowledge of the electron-electron interactions that play a crucial role in these confined geometries. Here, we have investigated the magnetic and conducting properties of zigzag edge graphene nanoribbons (ZGNRs) using the many-body configuration interaction (CI) method on the basis of the Hubbard Hamiltonian. For the half-filled case, the many-body ground state shows a ferromagnetic spin-spin correlation along the zigzag edge, which supports the picture obtained from one-electron theory. However, hole doping reduces the spin and charge excitation gap, making the ground state conducting and magnetic. We also provide a two-state model that explains the low-lying charge and spin excitation spectrum of ZGNRs. An experimental setup to confirm the hole-mediated conducting and magnetic states is discussed.

End-on nitrogen insertion of a diazo compound into a germanium (II) hydrogen bond and a comparable reaction with diethyl azodicarboxylate

A happy ending: The germanium(II) hydride [LGeH], where L = [HC{(CMe)(2,6-iPr(2)C(6)H(3)N)}(2)], reacts with a diazoalkane to form the hydrazone derivative (see picture). The reaction proceeds through the unprecedented end-on nitrogen insertion of the diazo compound.

Soluble Molecular Dimers of CaO and SrO Stabilized by a Lewis Acid

The reaction between a β-diketiminate aluminum methyl hydroxide and [M{N(SiMe 3 ) 2 } 2 (thf) 2 ] (M=Ca, Sr) results in alkaline-earth-metal oxides such as [{LAl(Me)(μ-O)Ca(thf)}2] (see core structure; L is deprotonated β-diketiminate CH{C(CH 2 )}(CMe)(2,6-iPr 2 C 6 H 3 N) 2 ). These soluble complexes are dimeric in the solid state and contain unprecedented M 2 O 2 cores.

Magnetization due to localized states on graphene grain boundary

Magnetism in graphene has been found to originate from various defects, e.g., vacancy, edge formation, add-atoms etc. Here, we discuss about an alternate route of achieving magnetism in graphene via grain boundary. During chemical vapor deposition of graphene, several graphene nucleation centers grow independently and face themselves with unusual bonding environment, giving rise to the formation of grain boundaries. We investigate the origin of magnetism in such grain boundaries within first-principles calculations, by letting two nucleation centers interact with each other at their interface. We observe formation of unprecedented point defect, consisting of fused three-membered and larger carbon rings, which induces net magnetization to graphene quantum dots. In case of periodic lattices, the appearance of array of point defects leads to the formation of magnetic grain boundaries. The net magnetization on these defects arises due to the deviation from bipartite characteristics of pristine graphene. We observe magnetic grain boundary induced dispersion less flat bands near Fermi energy, showing higher localization of electrons. These flat bands can be accessed via small doping, leading to enhanced magnetism. Moreover, the grain boundaries can induce asymmetric spin conduction behavior along the cross boundary direction. These properties can be exploited for sensor and spin-filtering applications.

Effect of electric field on one-dimensional insulators: a density matrix renormalization group study

We perform density matrix renormalization group (DMRG) calculations extensively on one-dimensional Mott and Peierls chains with explicit inclusion of the static bias to study the insulator-metal transition in those systems. We find that the electric field induces a number of insulator-metal transitions for finite-size systems and, at the thermodynamic limit, the insulating system breaks down into a completely conducting state at a critical value of bias that depends strongly on the insulating parameters. Our results indicate that the breakdown, in both the Peierls and Mott insulators, at the thermodynamic limit, does not follow the Landau-Zener mechanism. Calculations on various size systems indicate that an increase in the system size decreases the threshold bias as well as the charge gap at that bias, making the insulator-metal transition sharper in both cases.