The University of Massachusetts Amherst
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Graduate Seminar: (G.R.A.S.S.) – Ashish Kumar (Maroudas/Dimitrakopoulos) and Vivek Vattipalli (Fan)


Tuesday, November 8, 2016 - 11:30am


LGRT 201


Ashish Kumar and Vivek Vattipalli will be presenting at the upcoming Chemical Engineering student seminar on Tuesday, November 8th, 2016 at 11:30 a.m. in LGRT 201. See below presentation abstracts.

Ashish Kumar – Dimitrios Maroudas an Christos Dimitrakopoulos Lab

Externally Driven Nanopatterning of
Surfaces and Low-Dimensional Materials

The formation and precise manipulation of nanoscale features by controlling macroscopic forces is essential to advancing nanotechnology. Toward this end, we have carried out a comprehensive theoretical study on the current-driven dynamics of single-layer epitaxial islands on the surfaces of electrically conducting face-centered cubic crystalline substrates and the patterns that emerge from such driven dynamics. We report the formation of surface nanopatterns such as nanowires with precisely controlled widths, starting from single-layer rounded conducting islands under the controlled action of an externally applied electric field that drives island edge electromigration. Numerical simulations based on an experimentally validated model and supported by linear stability theory show that large-size islands undergo a current-induced fingering instability, leading to nanowire formation after finger growth. Depending on the substrate surface crystallographic orientation, necking instabilities after fingering lead to the formation of multiple parallel nanowires per island. In all cases, the axis of the formed nanowires is aligned with the direction of the externally applied electric field. The nanowires have constant widths, on the order of 10 nm, which can be tuned by controlling the externally applied electric field strength. We also report formation of surface nanopatterns consisting of single-layer epitaxial islands as a result of an electromigration-induced morphological instability on the straight edge of a single-layer nanowire grown epitaxially on a crystalline substrate. Direct dynamical simulations show that the current-induced nanowire edge instability causes the breakup of the nanowire and leads to formation of uniformly distributed islands, arranged in linear or V-shaped arrays, which are uniformly sized with nanoscale dimensions.  The simulation results are supported by linear stability theory and demonstrate that the geometrical features of the island patterns and the island sizes can be controlled precisely by controlling the electric field direction with respect to the nanowire axis and the electric field strength. Furthermore, we demonstrate the formation of very complex nanopatterns due to current-induced morphological instabilities undergone by large epitaxial islands.  The resulting nanopatterns are symmetric with respect to the direction of applied electric field, can be tuned by controlling the island size and the duration of application of the electric field, and their formation kinetics exhibits a universal scaling relation. Our findings have important implications for developing lithography-free physical approaches to nanopatterning of electronic materials and low-dimensional materials toward enabling future nanofabrication technologies.

Vivek Vattipalli – Wei Fan Lab

Long Walks in Hierarchical Porous Materials due to Combined Surface and Configurational Diffusion

Hierarchical porous materials are an emerging class of porous materials with both microporous (< 2 nm) and mesoporous (2 – 50 nm) structures, and are a subject of active investigation for a number of prospective applications such as catalysis, gas separations, sensors and drug delivery. While these applications are based on the high adsorption capacities and fast molecular transport properties of such materials, the nature of molecular transport in them has been elusive for decades due to the complexity involved – the presence of multiple sizes of pores leads to the simultaneous occurrence of different diffusion phenomena such as configurational diffusion, surface diffusion and Knudsen diffusion. The evidence on the existence of structural “surface barriers” which hamper molecular transport also adds to the complexity in understanding the process. Rational development of these materials for desired applications requires a good fundamental understanding of molecular transport in such materials.

In this study, mesoporous MCM-41 silica samples and hierarchical zeolites of varying microporosity and mesoporosity were synthesized and the diffusion of different probe molecules in them was studied using the Zero Length Column (ZLC) technique. Our results demonstrate that the dominant molecular transport mechanism in MCM-41 is surface diffusion while that for the hierarchical zeolite is configurational diffusion, mainly because of the presence of microporous structures in the hierarchical zeolite. In the case of cyclohexane diffusion in the hierarchical zeolite, our results indicate that the effective diffusion length in these materials might be much longer than what was previously thought and a mechanism is proposed to explain the same. We extend this hypothesis to a system comprising zeolite nanoparticles due to their similarities with hierarchical zeolites; and find further evidence supporting the proposed mechanism as well as a method to accomplish a more than 5-fold improvement in the observed diffusivities.

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