Theoretical Sciences Unit
Jawaharlal Nehru Centre for Advanced Scientific Research

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Shobhana Narasimhan

Research Interests

 Research Interests

 

Simulated STM images of Reconstructed Pt(111)

Simulated STM Images of surface reconstructions on Pt(111)  (Pushpa & Narasimhan, 2003) 

 

Computational Nanoscience

 

In our group, we use theoretical techniques to explore novel physics and chemistry at the nanoscale. Our main interest is in examining how properties (structural, mechanical, electronic, magnetic and chemical) change upon lowering dimensionality and/or reducing size. In some cases, we then use this understanding for designing novel materials with desired properties. While we are mainly driven by intellectual curiosity, some of the problems we work on have technological relevance for pressing problems such as clean energy and device miniaturization.

To address such issues in a precise way, the main tool used in our group is quantum mechanical density functional theory. We perform first-principles calculations, i.e., with no input other than atomic numbers and masses, we are able to compute the structure and properties of a wide range of systems and materials.

     

    Some examples of problems of recent and current interest:

     

The 'Rational Design' of Catalysts:

Much of modern-day life is unthinkable without the use of catalysts, and catalysis is a multi-billion dollar industry. Surprisingly, most commercially used catalysts have been developed by a process of trial and error. Nowadays, however, there is an effort to gain a greater theoretical understanding of how catalysts operate, and then use this knowledge to design better and/or cheaper catalysts.

One of the areas we are looking at is the effort to design a better gold catalyst. Bulk gold is famously inert, yet this is not true of nanosized gold. When used as a catalyst, gold nanoparticles are usually supported on an oxide substrate. Theoretical calculations performed in our group (in collaboration with Stefano de Gironcoli from SISSA, Trieste) showed that it is possible to switch the morphology of these nanoparticles from three-dimensional shapes to two-dimensional ones by doping the oxide substrate with an electron donor. This concept has subsequently been proven by experiments in the group of H-J Freund in Berlin. We have now also shown that these two-dimensional gold clusters on doped substrates are better catalysts.

We are also looking at other reactions such as NO reduction, and processes such as sintering of nanoparticles.

N. Mammen, S. Narasimhan & de Gironcoli, J. Am. Chem. Soc., 133, 2801 (2011).

 

Plots showing the charge transferred from the substrate to three-dimensional (left) and planar (right) twenty-atom gold clusters placed on a MgO substrate doped with Al atoms.

     

Mixing and magnetism on surfaces:

From ancient times, it has been known that mixing two or more metals to form an alloy can result in materials with superior properties. However, many combinations of metals don't form alloys in three-dimensional bulk systems. Is this still true in two-dimensional surface systems? We have shown that many combinations of bulk-immiscible metals can form strain-stabilized surface alloys. By assembling and analyzing a large database of ab initio results, we have improved the understanding of the factors that govern mixing in two dimensions.

We suggested to our experimental collaborators in Sylvie Rousset's group at the Universite de Paris-Diderot that Fe-Au/Ru(0001) would be a good candidate system in which to observe surface alloying - and voila - their STM and LEED experiments showed that FeAu2/Ru(0001) indeed forms a beautiful long-range-ordered surface alloy! Our theoretical calculations led to the surprising finding that the main driving force for alloying in this system is NOT the reduction of surface stress (as was generally believed), but magnetism.

S. Mehendale, Y. Girard, V. Repain, C. Chacon, J. Lagoute, S. Rousset,M. Marathe and S. Narasimhan, Phys. Rev. Lett. 105, 056101 (2010). (Editor's Suggestion).

 


 

     

(a)Experimentally obtained and (b) theoretically simulated STM images of FeAu2/Ru(0001) surface alloy. Insets show LEED image and atomistic model of surface alloy.

 

Gas storage:

Many cities in India have mandated that public transport vehicles should run on natural gas rather than petrol, in a bid to reduce pollution; India also has more natural gas resources than petroleum. However, as occasional newspaper reports make clear, the current way of storing compressed natural gas in cylinders is unsafe. Further, the cylinders used are heavy, not very adaptable to vehicular space constraints, and the pressurization can be expensive. Adsorptive storage of natural gas, if achievable economically, presents an attractive alternative. Activated carbons are cheap and work as a good 'sponge' to soak up methane (the main constituent of natural gas); however it has not been very clear why this is so. By performing ab initio density functional calculations on a variety of defective or chemically functionalized graphene systems we have been looking at strategies to increase the adsorptive capacity of carbon. We are now also starting to look at hydrogen storage. This work is done in collaboration with BPCL, and the group of Ganapathy Ayappa at the Indian Institute of Science.

B.C. Wood, S.Y. Bhide, D. Dutta, V. Kandagal, A. Pathak, S. Punnathanam, K.G. Ayappa, and S. Narasimhan, J. Chem. Phys. 137, 054702 (2012).

 

 

Spintronics:

Spintronics is an exciting technology of the future, in which one exploits the intrinsic spin of electrons, in addition to their charge (as is done in conventional electronics). We have been looking at spin polarized transport through molecules placed between magnetic leads, in order to understand how to control properties such as magnetoresistance.

Spin polarized charge density plot depicting a conduction pathway through a 'closed' DTE molecule placed between Ni leads. (K. Ulman)

 

Nanomagnetism:

We are interested in many aspects of magnetism at the nanoscale, such as the properties of magnetic islands and overlayers on substrates, magnetic surface alloys, and the search for materials that display exotic spin structures such as spin spirals.


   Last modified date: 11-10-2012