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General Outline:

Our research focuses on fundamental materials science of the novel polymer-composite materials based on nanotubes (NT). During the project period, we performed modeling of the electronic and optical properties of the hybrid complexes of NTs with the single-stranded DNA (ss-DNA) which is helpful for understanding  mechanisms for the light absorption and the photo-voltaic effect in NT-based materials. Modeling of NT-based materials also branched out to cover their specifics for particular device applications. We investigated novel thermal conductance properties and heat dissipation in NT transistors as well as perspectives for NT-NEMS. Theory of collective optical excitations of cylindrical-shell clusters has been developed.

Experimental study of interactions of NTs and various polymers was performed by means of Atomic Force and Scanning Electron Microscopy and electrical transport characterization. The goal of experimental work was an optimization of deposition techniques to achieve better sample morphology.

SRF-08 ACS-PRF Supplement has supported collaboration with Prof. Huang (Chemistry Department, Lafayette College) on the chemistry of polymer coating for the NT photo-diode fabrication. She, and her student at Lafayette, studied a set of polymer materials to find a combination of polymers which gives a well defined hetero-interface.

SUMR-08 and SUMR-09 ACS-PRF Supplements supported work of Mr. Garrett Wadsworth, undergraduate student from Physics Department at Lehigh, and Ms. Adrienne LaFleur, undergraduate student from BioMolecular Program at Lehigh. Wadsworth learned sample fabrication and electrical characterization techniques. LaFleur received training in Molecular Dynamics modeling of NT-polymer complexes. However, the most important outcome of the training is that we were able to attract the students from the underrepresented group, deeply involve them in a scientific research and direct for continuing education beyond the BS level.

This research resulted in 7 papers in 2007-09 (4 in proceedings) and 21 presentations, including 4 invited talks.

Detailed description:

We proposed that the NT-polymer interface may play the major role in the charge carrier extraction and light conversion in solar cells made of NT-based materials. We conducted both theoretical modeling of the ssDNA-NT hybrids and made first steps toward direct experimental studies of NT-polymer samples. 

In theory, we developed a semi-empirical quantum-mechanical approach for the NT wrapped with the ssDNA molecule [Rotkin, Annual Reviews of Physical Chemistry, 2010]. Within second quantization formalism, using the tight-binding Hamiltonian with (a) the selfconsistent Coulomb operator for the electron-electron interactions on the NT and (b) the electrostatic energy operator for the interactions with the charged backbone of the polymer (ss-DNA, for example), we wrote a code for calculating the electronic structure (band structure) of the NT-polymer complexes. The input parameters of the modeling include the geometry of the complex obtained through Molecular Dynamic modeling and from the experimental data.

Our major finding is a symmetry breaking induced in the NT by a helical wrap of the ss-DNA, which was never reported in the past. Based on our theoretical conclusion we proposed non-destructive method to characterize the samples and to determine the wrap by analyzing the optical response in the cross-polarization [Snyder and Rotkin, Small, 2008]. This new theory generated a few collaborations (at LANL and in Japan) to find an experimental confirmation of the effect.

A variety of wrappings and a selection of NTs of different diameters have been studied. A general effect observed in our modeling is an appearance of substantial changes of the absorption line in perpendicular polarization of the photon with respect to NT axis and the (weak) shift of the absorption/emission in the parallel polarization. The latter effect has been detected experimentally, but not yet the former one. The appearance of predicted peak/shoulder in the cross-polarized spectroscopy would be a clear identification of the helical wrap of the DNA.

We developed a theory of the optical modes for the finite-size shell-clusters [Maxson, Rotkin, Nano Letters, 2008]. Using non-retarded theory of coupled quantum oscillators we obtained response function of the shell and analyzed it using the symmetry of the shell lattice.

We developed the near-field thermal coupling theory for nanoscale carbon-based materials and their electronic devices [Rotkin, Proc. of SPIE, 2009].

In experimentation, we worked on obtaining a protocol for fabrication of the NT photodiode with polymer coating that has a distinct and abrupt hetero-interface. NTs dispersed in dichloromethane as well as ss-DNA wrapped NTs from water solution were used together with the poly(ethyleneimine), poly(methylmethacrylate), poly(acryalic acid), and poly(acrylamide) coatings. Sample fabrication was followed by AFM and electrical transport characterization. Next we plan to perform photo-electric measurements for the samples produced using optimized protocols.