Anup Pramanik

Theoretical prediction of a new two-dimensional carbon allotrope and NDR behaviour of its one-dimensional derivatives

By using state of the art theoretical methods we have predicted a new two-dimensional (2-D) carbon allotrope. This new planar carbon framework is made of hexagons, octagons and pentagons and hence named as HOP graphene (HOPG). The possibility of existence of HOPG is evident from its dynamical stability as confirmed by phonon-mode analysis and also from an energetic point of view since it is energetically more favorable than recently synthesized graphdiyne. The band structure shows the metallic behaviour of this new form of carbon allotrope. We also explored the electronic structure and transport properties of a 1-D derivative (nanoribbon) of HOPG. Most of the nanoribbons exhibit multiple negative differential resistance (NDR) behaviour with high peak to valley ratio.

Electronic structure and transport properties of sulfur-passivated graphene nanoribbons

Electronic structure of newly synthesized sulfur-terminated graphene nanoribbons (S-GNRs) has been presented from the calculations based on ab initio density functional theory and non-equilibrium Greens function (NEGF) method. The calculations reveal that zigzag-edged S-GNRs (Z-S-GNRs) are thermodynamically more stable than armchair edged S-GNRs (A-S-GNRs). It has been observed that the band gap of S-GNRs depends both on ribbon width and edge symmetry. The calculated band gap, in case of A-S-GNRs, is also supported by the presence of threshold bias in the I-V characteristics obtained from NEGF formalism. It is shown that all A-S-GNRs having width up to 1.50 nm are semiconducting but the Z-S-GNRs of similar widths are metallic. For A-S-GNRs, the width dependent band-gap hierarchy follows three different trends which seem to be different from that of H-passivated GNRs. The band-gaps for A-S-GNRs arise from both quantum confinement as well as crucial effect of edge, where the passivating S atoms play an imp...

Doped defective graphene nanoribbons: a new class of materials with novel spin filtering properties

We present the results of our spin polarized density functional study of the electronic and transport properties of defective graphene nanoribbons doped with boron or nitrogen atoms. We have analysed the formation energy, electronic band structure, magnetic charge density and quantum conductance of the doped defective graphene nanoribbon systems. We have demonstrated the half metallic behaviour of the doped defective graphene nanoribbons. The primary cause of the half metallic behaviour of this particular system is the charge transfer from carbon to dopant atoms. We have also shown that the band gap of the doped defective graphene nanoribbons decreases with the intensity of a transverse electrical field and reaches the state of a spin gapless semiconductor. The current–voltage characteristics of the doped defective graphene nanoribbons show the polarization of the spin current and have high spin filtering efficiencies.

Isoelectronically doped CdSe/Te nanoalloys as alternative solar cell materials: insight from computational analysis

Nanocrystals play an important role in various fields of technology, especially in catalysis, optoelectronics, bio-diagnostics, magnetic sensing, etc. These materials are more demanding as their optoelectronic properties can be fine-tuned by external doping and also by varying the dopant composition. Herein, we take the advantages of SCC-DFTB methods and investigate the effects of substitution of Se by Te atoms on the structure and electronic properties of the CdSe1−xTex nanoalloys of different compositions. It has been observed that, incorporation of bigger Te atoms induces structural strain which has a great impact on the overall electronic structure of the CdSe/Te nanoalloys. The charge distributions in the frontier molecular orbitals clearly indicate spatial separation of the electrons and holes in some nanoalloys which promise their potential applications in optoelectronic devices.

Computational studies on the mechanism and selectivity of Al 8 O 12 nanocluster for different elimination reactions

Quantum mechanical investigations are performed to show the mechanism and selectivity of Al8O12 nanocluster during different competing elimination reactions of acyclic and cyclic compounds. The studies reveal that, the nanocluster can selectively eliminate hydrogen halide in presence of both hydroxyl and halide as leaving groups. Furthermore, for competing dehydrohalogenations, the chemoselectivity trend is just opposite to the leaving character of the halides, i.e., the fluoride is eliminated first. The regioselectivity of the nanocluster is also remarkable during elimination. Most importantly, the dehydrohalogenations proceed through a so called ’unfavorable’ syn-elimination pathway thereby producing stereospecified products which are difficult to be produced via normal base catalytic condition.

Understanding the conductance switching of permethyloligosilanes: A theoretical approach

On the basis of ab initio density functional theory coupled with non-equilibrium Greens function technique, we have presented a molecular level understanding on the stereoelectronic switching of conducting properties of oligosilane molecules. Su et al. [Nat. Chem. 7, 215-220 (2015)] demonstrated that these types of oligosilane molecules exhibit three stereoconformers which show two distinct conducting profiles. On the basis of break-junction technique, the authors show that manipulating a specific dihedral angle and thereby controlling the length of the molecular contact, it is possible to switch the conducting states minutely. However, their discussions scarce the proper energy level alignment upon which the molecule-lead tunneling amplitude depends. On the basis of electronic structure and non-equilibrium electron transport calculations, we interpret such switching behavior and thus quantify the switching parameter demonstrating how the metal-molecule contact geometry along with the electronic energy level alignment is responsible for such kind of junction process. We also provide the variation of switching parameter and the type of majority carrier with the conjugation length of the oligosilanes.

Pure carbon-based Schottky diode, an implication of stretched carbon nanowire

Density functional theory calculations are performed on various forms of linear carbon chains. It has been predicted that stretched carbon nanowire may be stabilized through interchain interaction thereby forming a quasi-bound state of carbon, naming parallel carbon nanowire (PCNW). The electronic structure analysis on PCNW indicates that this ladder type of structure is achieved through lateral overlap between unhybridised p orbitals of sp C atoms. Furthermore, electronic transport calculations using nonequilibrium Greens function reveal that this material can be properly utilized as Schottky barrier diode with appreciable voltage rectifying capability when connected to asymmetrical metallic contacts, which may have potential application as field effect transistor.

Essential Role of Ancillary Ligand in Color Tuning and Quantum Efficiency of Ir(III) Complexes with N-Heterocyclic or Mesoionic Carbene Ligand: A Comparative Quantum Chemical Study

Tuning photoluminescence properties is of prime importance for designing efficient light emitting diode (LED) materials. Here, we perform a computational study on the effect of normal N-heterocyclic carbene (NHC) and abnormal mesoionic carbene (MIC) ligands on the photoluminescence properties of some Ir(III) complexes, which are very promising LED materials. We find MIC as the privileged ligand in designing triplet emitters. The strong σ-donating and moderate π-accepting properties of MIC render a lower access to the nonemissive triplet metal-centered state (3MC), resulting in lowering the nonradiative rate constant ( knr) and correspondingly achieving higher quantum efficiency. We also demonstrate that the judicial choice of ancillary ligand can improve the efficiency of these materials even further. This quantum chemical investigation focuses on the importance of MIC as cyclometalating ligand and the substantial effects of ancillary ligands in controlling the color tuning and quantum efficiency for optoelectronic applications.

Computational studies on the hydride transfer barrier for the catalytic hydrogenation of CO2 by different Ni(II) complexes

AbstractHydride transfer is the most crucial step for the catalytic hydrogenation of CO2 in homogeneous condition. Here, we perform state-of-the-art calculations to show the effect of geometry and spin states of Ni-hydride complexes containing different types of multidentate phosphine ligands on their hydride transfer barrier. For doing this, we first choose Ni-bis(diphosphine) complexes of the type NiP4, which have been synthesized recently and then by extrapolating the idea we propose a new type of NiP2N2 complex showing much lower hydride transfer barrier. We also compute the hydricities of the Ni-hydride complexes in aqueous medium and try to correlate these thermodynamic quantities with the kinetic barrier of hydride transfer. Graphical AbstractNiP2N2 complex can efficiently hydrogenage CO2 with a quite low hydride transfer barrier.