[CCoE Notice] PhD Defense Presentation

[Khator, Suresh]

      NANOSTRUCTURED SURFACES AND EMERGENT PHYSICAL BEHAVIOR
                                                       By: Parnia Mohammadi
                              Date: January 30th 2012, Time: 10 am, Room: W232
Committee members: Dr Pradeep Sharma, Dr Lewis Wheeler, Dr Stanko Brankovic, Dr Yashashree Kulkarni, Dr Ashutosh Agrawal, Dr Liping Liu

                                                          Abstract
Due to larger surface to volume ratio, surfaces play a significant role at the nanoscale. Surface atoms have different coordination numbers, charge distribution and subsequently different physical, mechanical and chemical properties. These differences are interpreted phenomenologically by postulating the existence of surface energy and acknowledging that the various bulk properties such as elastic modulus, melting temperature, electromagnetic properties are different for surfaces.
In this dissertation we consider two types of surfaces: those bounding a three-dimensional entity, and independent two-dimensional deformable surfaces that can be used to represent (for example) graphene sheets, thin films, lipid bilayers among others.
In this dissertation:
(i)            We develop a theoretical framework, complemented by atomistic calculations, that elucidates the effect of roughness on surface energy, stress and surface elasticity. We find that the residual surface stress is hardly affected by roughness while the superficial elastic properties are dramatically altered and, importantly, may also result in a change in its sign----this has significant ramifications in the interpretation of sensing based on frequency measurement changes. In particular, we also comment on the effect of roughness on the (generally ignored) term that represents the curvature dependence of surface energy (crystalline Tolman's length).
(ii)           In the context of independent deformable surfaces, our focus is on electromechanical coupling---in particular the rapidly emerging topic of flexoelectricity. Recent developments in flexoelectricity, especially in nanostructures, have lead to several interesting notions such as piezoelectric materials without using piezoelectric materials, enhanced energy harvesting at the nanoscale among others. In the biological context also, membrane flexoelectricity has been hypothesized to play an important role e.g. biological mechano-transduction, hearing mechanisms. In this dissertation we consider a heterogenous flexoelectric membrane, and derive the homogenized flexoelectric, dielectric and elastic response. In particular for purely fluid (lipid type) membranes, we obtain exact results-one of very few in homogenization theory. A simple application is illustrated for graphene sheets. Using quantum mechanical calculations, we also show that graphene can be designed to be pyroelectric thus providing an avenue to create the thinnest possible thermo-electro-mechanical material.


_______________________________________________________
Suresh K. Khator, Ph.D., P.E.                         Phone: 713-743-4205
Associate Dean, College of Engineering    Fax: 713-743-4214
University of Houston                                     Email: skhator at uh.edu<mailto:skhator at uh.edu>
E421 Engineering Bldg 2                                www.egr.uh.edu/ie<http://www.egr.uh.edu/ie>
Houston, TX 77204-4008

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