Rajeev R. Pandey

Theoretical design of bioinspired macromolecular electrets based on anthranilamide derivatives

Polypeptide helices possess considerable intrinsic dipole moments oriented along their axes. While for proline helices the dipoles originate solely from the ordered orientation of the amide bonds, for 310− and α‐helices the polarization resultant from the formation of hydrogen‐bond network further increases the magnitude of the macromolecular dipoles. The enormous electric‐field gradients, generated by the dipoles of α‐helices (which amount to about 5 D per residue with 0.15 nm residue increments along the helix), play a crucial role in the selectivity and the transport properties of ion channels. The demonstration of dipole‐induced rectification of vectorial charge transfer mediated by α‐helices has opened a range of possibilities for applications of these macromolecules in molecular and biomolecular electronics. These biopolymers, however, possess relatively large bandgaps. As an alternative, we examined a series of synthetic macromolecules, aromatic oligo‐ortho‐amides, which form extended structures with amide bonds in ordered orientation, supported by a hydrogen‐bond network. Unlike their biomolecular counterparts, the extended π‐conjugation of these macromolecules will produce bandgaps significantly smaller than the polypeptide bandgaps. Using ab initio density functional theory calculations, we modeled anthranilamide derivatives that are representative oligo‐ortho‐amide conjugates. Our calculations, indeed, showed intrinsic dipole moments oriented along the polymer axes and increasing with the increase in the length of the oligomers. Each anthranilamide residue contributed about 3 D to the vectorial macromolecular dipole. When we added electron donating (diethylamine) and electron withdrawing (nitro and trifluoromethyl) groups for n‐ and p‐doping, respectively, we observed that: (1) proper positioning of the electron donating and withdrawing groups further polarized the aromatic residues, increasing the intrinsic dipole to about 4.5 D per residue; and (2) extension of the π‐conjugation over some of the doping groups narrowed the band gaps with as much as 1 eV. The investigated bioinspired systems offer alternatives for the development of broad range of organic electronic materials with nonlinear properties.

Synthesis and characterization of peptide nucleic acid–platinum nanoclusters

Peptide nucleic acid (PNA) is an analogue of deoxyribonucleic acid (DNA) and possesses a neutral backbone that affords stronger hybridization, greater stability and higher specificity in base pairing. However, it has not been explored as much as DNA in self-assembling functional nanostructures or nanoelectronic devices. We report here for the first time the metallization of PNA with platinum (Pt) nanoparticles via chemical binding, reduction and deposition. Pt ions from a precursor salt solution are allowed to bind over the PNA fragments followed by a reduction and then growth into metal nanoparticles. PNA–Pt complexes form chains several hundred nanometres in length and by varying the duration of chemical reduction step, the dimension of the Pt nanoparticles can be controlled. The structural features and chemical composition of PNA–Pt nanoparticles have been characterized via scanning electron microscopy, transmission electron microscopy and Fourier transform-infrared spectroscopy. These results are also supported by modelling and analysis of the nature of high-lying molecular orbitals on PNA using density functional theory (DFT) method.

Electronic transport through a CNT-Pseudopeptide-CNT hybrid material

We present electron transmission studies on a pseudopeptide fragment (P) linking two (10,0) semi-infinite carbon nanotubes (CNTs). Calculations are performed using the non-equilibrium Green function formalism (NEGF) implemented within the tight-binding molecular dynamics density functional theory code FIREBALL. Results are compared with the transmission for an ideal (10,0) infinite CNT and a hydrogen passivated CNT-pseudopeptide-CNT (CNT–P–CNT) structure. The transmission spectrum indicates that the pseudopeptide fragment acts as a good bridge for hole transfer and strongly suppresses electron transfer.

Modeling and Design of Beyond the Roadmap Materials and Devices: Nanowires, Nanotubes, and Molecules

As device dimensions shrink to the molecular scale, theory and modeling assume an ever greater role. The analytical ability to experimentally determine the chemistry and geometry for individual molecular devices does not yet exist. Heroic experiments can be required to fabricate devices at this scale. Theory and modeling can relatively quickly explore the effect of the microscopic chemistry and geometry determining the electron and hole transport. The performance metrics of extremely scaled, difficult-to-fabricate designs can be compared. Theory and modeling can identify promising directions and provide physical understanding of experimental results. We apply theory and modeling to understand, analyze, design and optimize CNT, nanowire, and molecular based devices. The theory, modeling and design of chemically and biologically assembled carbon nanotubes (CNTs) is described. A CNT-molecular-resonant tunneling device is analyzed. Interface geometry is shown to have a large effect on the electron and hole transport. The intrinsic performance metrics of a CNT field effect transistor on insulator (COIFET) are determined.