Bikash Mandal

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.

Energetics and electronic structure of encapsulated graphene nanoribbons in carbon nanotube.

We report results of our total energy electronic structure calculation of encapsulation of graphene nanoribbon (GNR) in the carbon nanotube (CNT). The encapsulation of both coronene and perylene based graphene nanoribbons in zigzag (n,0) carbon nanotubes (where n ranges from 14 to 18 for perylene based nanoribbon and from 16 to 20 for coronene based nanoribbons) is an exothermic process. Our study shows that in certain cases arm-chair GNR (aGNR) encapsulated CNT results in type II band alignment and may be useful in the application in solar cells. We have also studied the potential of this composites for hydrogen storage. We found that the encapsulated GNR composite systems have higher hydrogen adsorption energies than the individual components of either GNR and CNT. The hydrogen molecules oriented perpendicular to GNR are found to be more stable as compared to hydrogen molecules parallel to GNR.

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...

Coronene-based metal–organic framework: a theoretical exploration

With the help of first-principle calculations we have proposed a new 2D metal-organic framework (MOF) consisting of a -NH substituted coronene molecule and transition metals. Our study reveals that formation of such crystals is exothermic in nature, i.e. it is an energetically favourable process. The mode of magnetic coupling between the local magnetic moments and hence the overall magnetic moment of the MOF can be tuned by changing the transition metal. Not only the magnetic properties, but also the electronic structure of the MOF can be regulated from half-metal to spin-semiconductor to semimetallic-semiconductor by altering the metal center from Cr to Mn to Fe/Co. Our study also indicates that the change in the ligand structure and its anchoring group is also very effective in tuning the electronic properties of MOFs. The study of transport properties reveals that the half-metallic crystal possesses the maximum value of the spin-filtering efficiency, i.e. 100%.

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.

Electrochemical studies on Li + /K + ion exchange behaviour in K4Fe(CN)6 cathode material for Li, K-ion battery

AbstractThe electrochemical studies of anhydrous K4Fe(CN)6 is reported. Anhydrous material was produced after dehydrating K4Fe(CN)6.3H2O crystal at 200°C in an open atmosphere. The material, as obtained, was characterized by various spectroscopic techniques, such as UV-Visible, FTIR, powder X-ray diffraction and FESEM-EDX. Electrochemical and Li+/K+ ion exchange behaviour of the synthesized material were studied by cyclic voltametry (CV), chronoamperometry (CA) and galvanostatic charge-discharge method after preparing a laboratory model cell against lithium anode instead of potassium. During anodic scan in the 1st cycle, peak maximum was observed at 3.93 V vs. Li+/Li due to removal of K+ ions from the ferrocyanide matrix, whereas, in the reverse scan (cathodic sweep) as well as in consequent cycles, peak maxima due to Li+ ion insertion and extraction were observed at 2.46 V and 3.23 V vs. Li+/Li, respectively. Cell, assembled using ferrocyanide cathode and lithium anode, shows an open circuit potential of 3.08 V and delivers a maximum capacity of 61 mAh g−1 (theoretical capacity 72 mAh g−1) at a rate of 0.2 C at room temperature. Graphical AbstractHighly dispersed crystalline K4Fe(CN)6 has been synthesized by simple cost effective route. The material was charge-discharged using lithium-salt electrolyte against Li metal. The redox peak current increment, shift of peak position in CV and capacity enhancement during charge-discharge cycles were explained in terms of faster exchange rate of K+ by Li+ ion.

Understanding the Electronic Structure of Graphene Quantum Dot-Fullerene Nanohybrids for Photovoltaic Applications

Abstract By using density-functional tight-binding method we have calculated the electronic structure of graphene quantum dot (GQD)-fullerene hybrid systems and explored the efficacy of their use in designing solar cells. We have shown that the electronic energy levels of the nanohybrids can be tuned either by varying the size of the quantum dots or by proper functionalization of the quantum dot (QD). The GQD-fullerene nanohybrids form type-I or type-II band energy alignment depending upon the size of the GQD. Thus, hybrid systems with smaller sized QDs form type-II band energy alignment while those of larger GQDs form type-I alignment. The type-II band alignment confirms the spatial charge separation for the systems and thus the rate of recombination of charge carriers will be low. The value of ΔG i.e. the difference in energy between the LUMO of the donor (GQD) and LUMO of the acceptor (fullerene) which measures the rate of electron injection from the donor to the acceptor is also large for the nanohybrids with smaller GQDs. So, we suggest that GQD-fullerene nanocomposites with smaller GQD will be a suitable system for photovoltaic devices. We also show that the type-II band energy alignment for the nanohybrids with larger QDs can be achieved through the functionalization of the GQD with electron donating group such as -NH2.

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.

A novel aqueous Li4Fe(CN)6 cathode and hydrophobic ionic liquid electrolyte combined lithium-ion battery

Present study reports an easy and cost-effective method of synthesis of Li4Fe(CN)6 cathode from K4Fe(CN)6.3H2O and LiClO4 in aqueous medium for its use in lithium-ion battery. The synthesized Li4Fe(CN)6 is characterized by UV-Vis, FTIR and cyclic-voltammetry studies. A special laboratory model lithium-ion battery is designed, where aqueous Li4Fe(CN)6 solution acts as a cathode, metallic lithium as anode and 1 molar solution of LiPF6 dissolved in water immiscible ionic liquid, 1-Butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF6) as electrolyte. The cell exhibits an open circuit potential of 3.12 volt and a good charge-discharge cycling behaviour. The cell delivered a maximum discharge capacity of 86 mAhg (theoretical capacity 112 mAhg) at 0.2 C rates with an average discharge potential of 2.1 volts. Although the ionic liquid is a little bit cost intensive, but the easy synthesis methodology with the cheapest raw materials and overall cycling efficiency, makes this technology available as a green economical energy storage device in the current battery industry. Copyright