Amitesh Maiti

Bead–bead interaction parameters in dissipative particle dynamics: Relation to bead-size, solubility parameter, and surface tension

Dissipative particle dynamics (DPD) is a mesoscale modeling method for simulating equilibrium and dynamical properties of polymers in solution. The basic idea has been around for several decades in the form of bead-spring models. A few years ago, Groot and Warren established an important link between DPD and the Flory-Huggins chi-parameter theory for polymer solutions. We revisit the Groot-Warren theory and investigate the DPD interaction parameters as a function of bead size. In particular, we show a consistent scheme of computing the interfacial tension in a segregated binary mixture. Results for three systems chosen for illustration are in excellent agreement with experimental results. This opens the door for determining DPD interactions using interfacial tension as a fitting parameter.

Electronic transport through carbon nanotubes: Effects of structural deformation and tube chirality

Atomistic simulations using a combination of classical force field and density-functional theory (DFT) show that carbon atoms remain essentially sp(2) coordinated in either bent tubes or tubes pushed by an atomically sharp atomic-force microscope (AFM) tip. Subsequent Greens-function-based transport calculations reveal that for armchair tubes there is no significant drop in conductance, while for zigzag tubes the conductance can drop by several orders of magnitude in AFM-pushed tubes. The effect can be attributed to simple stretching of the tube under tip deformation, which opens up an energy gap at the Fermi surface.

Nanotube–polymer composites: insights from Flory–Huggins theory and mesoscale simulations

Carbon nanotube (CNT)-polymer composites, with potential applications in structural materials, optoelectronics, sensors, biocatalysis, and thermal and electromagnetic shielding are an important emerging area of nanotechnology. However, progress has been slow due to difficulties in dispersing CNTs into the polymer matrix. We attack the problem from a Flory-Huggins theory point of view, and present novel simulations of the dispersion process at the mesoscale. The solubility parameter of the CNTs is mapped out as a function of tube diameter, and compared with that of well-known polymers. Parallel alignment of CNTs with the application of shear, and dispersion by attaching organic functional groups are also investigated.

Structural and electronic properties of PbTiO3, PbZrO3, and PbZr0.5Ti0.5O3: First-principles density-functional studies

Perovskites of the PbZr1−xTixO3 type are among the most important ferroelectric materials and highly active catalysts. The structural and electronic properties of PbTiO3, PbZrO3, and PbZr0.5Ti0.5O3 were examined using first-principles density-functional (DF) calculations with the local-density-approximation (LDA) or the generalized-gradient approximation (GGA, Perdew–Wang and Perdew–Burke–Ernzerhoff functionals). A series of crystal structures were considered for each compound. In several cases, the structural parameters predicted by the GGA functionals were clearly in better agreement with experimental results than the LDA-predicted values, but in qualitative terms the LDA and GGA approaches always predicted similar trends for crystal geometries and differences in thermochemical stability. DF calculations at the LDA level could underestimate the ferroelectric character of PbTiO3 and PbZr1−xTixO3. In the perovskites, the most stable structures belong to tetragonal (PbTiO3), orthorhombic (PbZrO3), and mono...

Reaction of SO2 with pure and metal-doped MgO: Basic principles for the cleavage of S-O bonds

Synchrotron-based high-resolution photoemission, x-ray absorption near-edge spectroscopy, and first-principles density-functional calculations are used to examine the interaction of SO2 with pure and modified surfaces of magnesium oxide. On a MgO(100) single crystal, SO2 reacts with O centers to form SO3 and SO4 species. The bonding interactions with the Mg cations are weak and do not lead to cleavage of S–O bonds. An identical result is found after adsorbing SO2 on pure stoichiometric powders of MgO and other oxides (TiO2, Cr2O3, Fe2O3, NiO, CuO, ZnO, V2O5, CeO2, BaO). In these systems, the occupied cations bands are too stable for effective bonding interactions with the LUMO of SO2. To activate an oxide for S–O bond cleavage, one has to create occupied metal states above the valence band of the oxide. DF calculations predict that in the presence of these “extra” electronic states the adsorption energy of SO2 should increase, and there should be a significant oxide→SO2(LUMO) charge transfer that facilitates the cleavage of the S–O bonds. In this article, we explore three different approaches (formation of O vacancies, promotion with alkali metals, and doping with transition metals) that lead to the activation of SO2 and S–O bond breaking on MgO and oxides in general. Basic principles for a rational design of catalysts with a high efficiency for the destruction of SO2 are presented.Synchrotron-based high-resolution photoemission, x-ray absorption near-edge spectroscopy, and first-principles density-functional calculations are used to examine the interaction of SO2 with pure and modified surfaces of magnesium oxide. On a MgO(100) single crystal, SO2 reacts with O centers to form SO3 and SO4 species. The bonding interactions with the Mg cations are weak and do not lead to cleavage of S–O bonds. An identical result is found after adsorbing SO2 on pure stoichiometric powders of MgO and other oxides (TiO2, Cr2O3, Fe2O3, NiO, CuO, ZnO, V2O5, CeO2, BaO). In these systems, the occupied cations bands are too stable for effective bonding interactions with the LUMO of SO2. To activate an oxide for S–O bond cleavage, one has to create occupied metal states above the valence band of the oxide. DF calculations predict that in the presence of these “extra” electronic states the adsorption energy of SO2 should increase, and there should be a significant oxide→SO2(LUMO) charge transfer that facilita...

Studies on the behavior of mixed-metal oxides: Adsorption of CO and NO on MgO(100), NixMg1−xO(100), and CrxMg1−xO(100)

Ultraviolet photoelectron spectroscopy (UPS), thermal desorption mass spectroscopy (TDS), and first-principles density functional (DF) generalized-gradient-corrected calculations were used to study the adsorption of CO and NO on MgO(100), Ni0.06Mg0.94O(100), and Cr0.07Mg0.93O(100) surfaces. UPS spectra and DF calculations show clear differences in the electronic properties of these oxides. After doping MgO with nickel, states with Ni 3d character appear ∼1.5 eV above the occupied {O 2p+Mg 3s} band. A similar phenomenon is found after adding Cr, but now the dopant levels are ∼3 eV above the {O 2p+Mg 3s} band. In CO- and NO-TDS experiments, the reactivity of the oxide surfaces increases in the sequence: MgO(100)<Ni0.06Mg0.94O(100)<Cr0.07Mg0.93O(100). Cr-bonded molecules exhibit adsorption energies as large as 15 (CO) and 20 kcal/mol (NO). For CO and NO on MgO(100), the mixing between the frontier orbitals of the adsorbate and the bands of the surface is poor, and the low adsorption energy is mainly due to w...

Application of Carbon Nanotubes as Electromechanical Sensors — Results from First‐Principles Simulations

A combination of First-Principles Density Functional Theory (DFT) and classical molecular dynamics with interatomic potential is used to examine bonding differences between two types of nanotube deformation: 1. bending, and 2. pushing with atomically sharp AFM tips. Bent tubes maintain an all-hexagonal network up to large bending angles. AFM-probed tubes, in contrast, display a more interesting behavior, which depends on the representation of the AFM tip.

Effect of surface phosphorus on the oxidative dehydrogenation of ethane: A first-principles investigation

Oxidative dehydrogenation (ODH) of small-chain alkanes has the potential to displace thermal cracking as the preferred method of light olefin production. Many heterogeneous catalysts for the ODH reaction have been discussed in the literature, including oxides, vanadates, and phosphates of rare earth and transition metals. Our experiments and the literature indicate that for most of these catalysts, including silica gel and alumina, a phosphorus-enriched surface enhances the ODH yield of ethane to ethylene. To understand the role of P, the ODH reactions were simulated on a silica surface, with and without P, using the density functional theory code DMol3 in a periodic supercell. Optimized structures for all intermediates as well as transition states were obtained for full catalytic cycles. The simulations reveal that activation barriers for the rate-limiting steps are lowered by ∼10 kcal/mol in the presence of P. The decrease results from a transition state in which the P atom remains quasi-5-valent and fo...

Sensing mechanical deformation in carbon nanotubes by electrical response: a computational study

Recent experimental advances have made carbon nanotubes promising material for utilizing as nano-electro-mechanical systems (NEMS). The key feature of CNT-based NEMS is the ability to drastically change electrical conductance due to a mechanical deformation. The deformation effects can be divided into two major groups: bond stretching of sp2 coordinated nanotubes and transition from sp2 to sp3 coordination. The purpose of this work is to review the change in electrical response of nanotubes to different types of mechanical deformation. The modeling consists of a combination of universal force-field molecular dynamics (UFF), density functional theory (DFT) and Greens function theory. We show that conductance of metallic carbon nanotubes can decrease by 2-3 orders of magnitude, when deformed by an AFM tip, but is insensitive to bending. These results can explain the experiment of Ref. [1]. Such a decrease is chirality dependent, being maximum for zigzag nanotubes. In contrast, twisting and radial deformation result in bandgap openning only in armchair nanotubes. In addition, radial deformation of armchair nanotubes leads to dramatic oscillations of conductance.

Electromechanical and Chemical Sensing at the Nanoscale: Molecular Modeling Applications

Atomistic modeling and simulations are becoming increasingly important in the design of new devices at the nanoscale. Of the myriad of potential application areas commonly associated with Nanotechnology, sensors based on carbon nanotubes (CNT) and metal-oxide nanoribbons are one of the closest to commercial reality. In this work, we review some recent molecular modeling investigations on: (1) CNT-based electromechanical sensors, and (2) gas-sensing properties of SnO2 nanoribbons. Simulation methods include: First-Principles Density Functional Theory (DFT), classical molecular mechanics, and Greens-function-based tight-binding transport.