Energy dissipation and transport in carbon nanotubes and graphene

Ajit Kumar Vallabhaneni, Purdue University

Abstract

The emergence of new carbon-based nanomaterials, like carbon nanotubes and graphene, in the past decade has provided new opportunities in many areas of scientific research. Despite their promise, the devices based on these materials are facing several challenges that need to be addressed to reap complete advantage of their extraordinary properties. In the current work, we studied the intrinsic scattering processes among the energy carriers and how it effects the energy dissipation and transport in these devices which would set the upper limit on their performance. In the first half of this work, the energy dissipation in carbon nanotube resonators is studied using molecular dynamics simulations. We studied various ways to calculate the quality factor (Q) which quantifies the efficiency of a resonator from the temporal response. We have also pointed out the drawbacks of the previously proposed methods which lead to incorrect conclusions on the temperature dependence of Q. A new method based on a band-pass filter is proposed which can be used to calculate the Q of any mode within the linear regime. Then, using the same method, the impact of the CNT size (length and diameter) on Q is studied and comparisons are made with classical theoretical models is made wherever applicable. A non-classical dependence on size is clearly observed for both primary axial and transverse mode vibrations emphasizing the significance of nanoscale phenomena like ballistic transport and size effects. Later the impact of higher-order modes on the Q is considered, where it was observed that Q decreases with increasing order of the mode. Finally, the effect of the presence of the defects and the challenges it poses in the design of NEMS devices is discussed. In the second half of the thesis, the energy transport in laser irradiated graphene and the effect of non-equilibrium between energy carriers on thermal conductivity measurements in experiments are discussed We primarily used a first principles Density Functional theory (DFT) approach to calculate the scattering rates between electrons and phonons. We subsequently developed diffusive multi-temperature model to spatially resolve the temperature distribution of carriers. We showed that Raman spectroscopy under-estimate the thermal conductivity almost by a factor of 2.5. We also extended the work to model the phonon transport with a Boltzmann Transport equation (BTE) which captures the non-equilibrium in the reciprocal space more effectively which adds further weight to the earlier conclusion that Raman spectroscopy technique is not reliable for measuring the thermal conductivity of 2-D materials.

Degree

Ph.D.

Advisors

Murthy, Purdue University.

Subject Area

Mechanical engineering|Nanotechnology

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