Research
Covalent Organic Frameworks (COFs)
- an important class of nanoporous materials that was first synthesized in 2005 by Yaghi and co-workers. COFs consist of light elements and exist both in two and three-dimensions. COF-108, which has a topology formed by stitching together tetragonal and triangular molecular building units, is one of the lowest density (0.17 gm/cm3) materials. Our research on 2D COFs questioned the then known eclipsed arrangement of layers and resulted in suggesting energetically more preferred offset between layers. This is an important result capable to influence charge transport between COF layers, an important property underlies in the suggested photovoltaic applications of COFs. We also explored the concept of reticular chemistry for designing a set of new COFs and calculated their energies of formation from molecular building units. We used Density Functional Tight-Binding (DFTB) method to study the set containing some reported as well as hypthetical COFs. Our studies of 3D COFs included unraveling their structural stability and electronic properties.
Porous Aromatic Frameworks (PAFs)
- a class of organic framework materials that have a diamond-like topology. They differ from diamond in their long organic linkers in place of diamond's C-C bonds. The presence of longer linkers (ligands that connect on both ends) such as quaterphenyl promise low-weight (lower than COF-108) and high hydrogen adsorption capacity (more than any known COF). Coupling reactions that enable condensation of PAFs lack reversibility and therefore long range order in the final products - this sets a synthetic limitation for its current commercial use. We have studied hydrogen storage capacity and structural, electronic, and mechanical properties for a set of PAFs built from a selection of different organic linkers.
Metal-Organic Frameworks (MOFs)
- a very versatile class of nanoporous materials made of metal clusters and organic linkers. Blessed by the high coordination number of constituent metal atoms, they can form different topologies. A large number of MOFs have been synthesized so far by many research groups . They have been used for gas storage, gas separation, catalysis, filtration, and drug delivery applications. Unit cells of MOFs typically include several hundreds of atoms, which make the application of standard density-functional methods computationally very expensive, sometimes even unfeasible. We focused on MOFs comprising three common connector units: copper paddlewheels (HKUST-1), zinc oxide Zn4O tetrahedron (MOF-5, MOF-177, DUT-6 (MOF-205)), and aluminum oxide AlO4(OH)2 octahedron (MIL-53). We validated self-consistent charge-DFTB (SCC-DFTB) method for MOFs containing Cu, Zn, and Al metal centers. This method was validated against full hybrid density-functional calculations on model clusters, against gradient corrected density-functional calculations on supercells, and against experimental results.
Paddle-wheel based two-dimensional MOFs are well-known for over two decades, however reports on their interlayer orderings found in the literature are ambiguous. With our simulations, we have given some clear insights on their interlayer interactions and the preferred stackings. We also have found out why such MOFs are stuck in a metastable state when grown on templates. Our studies further extended to determine the stability of MOF-MOF heterojunctions of large lattice constant gradients.
Quantum Dots (Nanocrystals)
- 0-dimensional crystals with an energy-gap usually in the semiconductors range. They are typically used for photovoltaic applications. We have studied mainly lead chalcogenide (PbS and PbSe) nanocrystals. Electronic structure of dots up to 3 nm in size (using DFT), influence of surface passivation on energy states (using DFT), feasibility of planar, conducting ligands binding on dot surfaces (using DFT), and charge (electron and hole) transport within the bulk arrangement of a million dots in a simulation box of micrometer side length (using kinetic Monte Carlo) are some of the problems that we have worked on.
Polymer: Fullerene Bulk Heterojunction
- an inexpensive, flexible material used for solar cell applications. We studied morphology of an aggregate of P3HT polymers interfaced with an aggregate of PCBM, a fullerene-like molecule. We employed classical molecular dynamics to study them at room temperature as well as at annealing temperature and devised statistical tools to quantify their morphology. We observed key structural transitions, namely changes in the tacticity of the polymers. This leads us to the suggest that P3HT polymers are generally atactic especially at high temperatures.
III-V Semiconductors (InGaAs)
InGaAs - a material with high electron mobility and high electron-to-hole mobility, considered a candidate material for potential replacement for silicon in semiconductor electronics. We have studied the activation of silicon dopant in InGaAs, specifically we calculated energies of formation and migration of silicon dopants (treated as defects) in InGaAs. We used density functional theory for determining formation energies for neutral as well as charged defects. We paramterized Tersoff potential for the Si-InGaAs system and calculated defect formation and migration energies using molecular mechanics.
Frustrated Lewis Pairs (FLPs)
- a mixture of Lewis acid (LA) and Lewis base (LB), which cannot combine to form a classical adduct and therefore stays reactive. The LA and LB components can be parts of a single molecule or two different molecules. Because of their "unquenched" reactivity, such systems are reactive toward substrates that can undergo heterolysis. We have studied their abilities to split hydrogen molecule and to capture CO2. In each case, using ab-initio based metadynamics simulations, we calculated its free energy landscape containing the specific reaction pathway and determined the steps of the process leading to the particular functionality.
