1. S. Narasimhan,
A Tryst with Density: Walter Kohn and Density Functional Theory,
Resonance,
22, 731 - 746 (2017).
, Read abstract. Walter Kohn transformed theoretical chemistry and solid state physics with his development of density functional theory, for which he was awarded the Nobel Prize. This article tries to explain, in simple terms, why this was an important advance in the field, and to describe precisely what it was that he (together with his collaborators Pierre Hohenberg and Lu Jeu Sham) achieved.
2. N. Mammen, L. Spanu, E. Tyo, B. Yang, A. Halder, S. Seifert, M. J. Pellin, S. Vajda and S. Narasimhan,
Reversing Size-Dependent Trends in the Oxidation of Copper Clusters through Support Effects,
European Journal of Inorganic Chemistry,
(2017).
In Press. , Read abstract. Having the ability to tune the oxidation state of Cu
nanoparticles is essential for their utility as catalysts. The degree of
oxidation that maximizes product yield and selectivity is known to
vary, depending on the particular reaction. Using first principles
calculations and XANES measurements, we show that for sub-
nanometer sizes in the gas phase, smaller Cu clusters are more
resistant to oxidation. However, this trend is reversed upon
deposition on an alumina support. We are able to explain this result
in terms of strong cluster-support interactions, which differ
significantly for the oxidized and elemental clusters. The stable
cluster phases also feature novel oxygen stoichiometries. Our
results suggest that one can tune the degree of oxidation of Cu
catalysts by optimizing not just their
3. P. Zalake, S. Ghosh, S. Narasimhan and K. G. Thomas,
Descriptor-Based Rational Design of Two-Dimensional Self-Assembled Nanoarchitectures Stabilized by Hydrogen Bonds,
Chemistry of Materials,
29, 7170 - 7182 (2017).
, Read abstract. The self-assembly of supramolecular architectures bound together by noncovalent interactions offers, at present, the most promising route to constructing devices at the nanoscale. However, the ability to pick molecular components that will result in a desired supramolecular structure and functionality remains a challenge. We suggest these goals can be met by using easily computed descriptors that correlate molecular form and formula with supramolecular architecture and energetics. Using a combination of theory and experiment, we show the feasibility of such an approach for a set of molecules comprised of three hosts (carboxylic acid derivatives of phenyleneethynylene) and five guests (naphthalene, phenanthrene, benzo-c-phenanthrene, benzo-ghi-perylene, and coronene), self-assembled on highly oriented pyrolitic graphite. Both scanning tunneling microscopy experiments and density functional theory calculations show that the host–guest combinations display rich structural diversity, assembling in hexagonal, linear, or random glass-like patterns. For certain host–guest combinations, the introduction of the guest triggers structural reorganization, including a disorder-to-order transition. By correlating with computed free energies, we formulate host and guest descriptors. These descriptors can be evaluated at essentially zero computational cost, as they depend only on the geometry and number of chemical motifs of specific types in the isolated molecules. However, they can successfully predict the structures and energetics of the host–guest assemblies, including systems not included in the original database used when determining the form of the descriptors. Structures of the same kind are found to cluster in the descriptor space. This suggests the way forward for the descriptor-based rational design of self-assembled nanostructured systems on surfaces.
4. J. Songkhao, Rajdeep Banerjee, Saikat Debnath, S. Narasimhan, N. Wannaprom, P. Vanalabhpatana, N. Seriani, R. Gebauer and P. Thamyongkit,
Structure-property relationship of π-extended boron-dipyrromethene derivatives towards optoelectronic applications,
Dyes and Pigments,
142, 558 - 571 (2017).
pdf
, Read abstract. A series of novel boron-dipyrromethenes bearing beta-benzo-fused rings, or benzo-BODIPYs, and phenyl, thienyl and bithiophenyl meso-substituents was synthesized, and investigated for their photophysical and electrochemical properties. Results from the UV-visible spectrophotometry revealed that the boron-complexation and the presence of the beta-benzo-fused rings signifcantly increase absorptivity of the molecules in the longer wavelength region. Moreover, according to the observed electrochemical and photophyscial properties of these compounds, the extended conjugation system and the presence of thiophene-based meso-substituents in the target benzo-BODIPYs created great impact on the energy levels of the molecular frontier orbitals, resulting in narrowing of the energy gap of the materials. We have also demonstrated the use of density functional theory calculations to evaluate the performance of these compounds as dye sensitizers in solar cells. While all the molecules studied have their frontier orbitals aligned appropriately with respect to the band gap of TiO2, they are found to differ in their charge injection properties when anchored on TiO2, and even more markedly in electron-hole separation and change in dipole moment upon excitation. Upon combining these criteria using a simple model and
the experimental results, the bithiophenyl-substituted benzo-BODIPY was suggested as the optimal candidate.
5. G. Mettela, N. Mammen, J. Joardar, S. Narasimhan and G.U. Kulkarni,
Non-FCC rich Au crystallites exhibiting unusual catalytic activity,,
Nano Research,
(2017).
pdf
, Read abstract. Bipyramidal Au microcrystallites have
been synthesized by thermalizing a
Au-organic complex in the presence of
Ag(I) ions, the latter acting as a shape-
directing agent. With a highly corrugate
d morphology leading to strain-induced
non-face-centered cubic (non-FCC) Au ph
ases, the non-FCC portion can be tuned
by varying the Ag/Au ratio, as verified by diffraction measurements. For a Ag/Au
ratio of 0.34, the non-FCC Au portion was as high as 85%. X-ray microdiffraction
and electron diffraction measurements reveal that the non-FCC contribution
comes primarily from bipyramids, while other microcrystallites, namely,
tetrahexahedrons and hexagrams, host no
n-FCC phases only at the edges and, to
an even lesser extent, at the corners. Wh
en used as a catalyst for p-nitrophenol
reduction, the non-FCC microcrystallites exhibit a significantly enhanced activity
compared to FCC Au, which shows only negligible activity. These results are in
accordance with trends in the values of
two descriptors of reactivity calculated
from first principles: The effective coor
dination number is found to decrease
and the d-band center is found to increase in energy going from the FCC to the
non-FCC phases of Au. Our findings cont
radict the general notion that Au is
catalytically active only in nanodimensions
and is otherwise inert; in this system,
its activity arises from the non-FCC phases.
6. Sananda Biswas and Shobhana Narasimhan,
Bromine as a Preferred Etchant for Si Surfaces in the Supersaturation Regime: Insights from Calculations of Atomic Scale Reaction Pathways,
J. Phys. Chem. C,
120, 15230 - 15234 (2016).
, Read abstract. Etching of semiconductors by halogens is of vital importance in device manufacture. A greater understanding of the relevant processes at the atomistic level can help determine optimal conditions for etching to be carried out. Supersaturation etching is a seemingly counter-intuitive process where the coverage of the etchant molecules on the surface to be etched is > 1. Here we use density functional theory computations of reaction pathways and barriers to suggest that supersaturation etching of Si(001) by Br2 should be more effective than conventional etching by Br2, as well as both conventional and supersaturation etching by Cl2. Analysis of our results shows that this is due in part to the larger size of bromine atoms, and partly due to Br-Si bonds being weaker than Cl-Si bonds. We also show that for both conventional and supersaturation etching, the barrier for the rate limiting step of desorption of SiX2 units is lower when the halogen X is Br rather than Cl. This contributes to the overall reaction barrier for supersaturation etching being lower for Br2 than Cl2.
7. Nisha Mammen, S. de Gironcoli and S. Narasimhan,
Substrate Doping: A Strategy for Increasing the Reactivity of Gold Nanocatalysts by Tuning sp Bands,
J. Chem. Phys.,
143, 144307 (2015).
pdf
, Read abstract. We suggest that the reactivity of Au nanocatalysts can be greatly increased by doping the oxide substrate on which they are placed with an electron donor. To demonstrate this, we perform density functional theory calculations on a model system consisting of a 20-atom gold cluster placed on a MgO substrate doped with Al atoms. We show that not only does such substrate doping switch the morphology of the nanoparticles from the three-dimensional tetrahedral form to the two-dimensional planar form, but it also significantly lowers the barrier for oxygen dissociation by an amount proportional to the dopant concentration. At a doping level of 2.78%, the dissociation barrier is reduced by more than half, which corresponds to a speeding up of the oxygen dissociation rate by five orders of magnitude at room temperature. This arises from a lowering in energy of the s and p states of Au. The d states are also lowered in energy, however, this by itself would have tended to reduce reactivity. We propose that a suitable measure of the reactivity of Au nanoparticles is the difference in energy of sp and d states.
8. Rajiv Kumar Chouhan, Kanchan Ulman and S. Narasimhan,
Graphene Oxide as an Optimal Candidate Material for Methane Storage,
J. Chem. Phys.,
143, 044704 (2015).
, Read abstract. Methane, the primary constituent of natural gas, binds too weakly to nanostructured carbons to meet the targets set for on-board vehicular storage to be viable. We show, using density functional theory calculations, that replacing graphene by graphene oxide increases the adsorption energy of methane by 50%. This enhancement is sufficient to achieve the optimal binding strength. In order to gain insight into the sources of this increased binding, that could also be used to formulate design principles for novel storage materials, we consider a sequence of model systems, that progressively take us from graphene to graphene oxide. A careful analysis of the various contributions to the weak binding between the methane molecule and the graphene oxide shows that the enhancement has important contributions from London dispersion interactions as well as electrostatic interactions such as Debye interactions, aided by geometric curvature induced primarily by the presence of epoxy groups.
9. S. Narasimhan,
Teaching Density Functional Theory Through Experiential Learning,
Proceedings of the XXVI Conference on Computational Physics, CCP2014.,
(2015).
In Press.
10. P. Campiglio, R. Breitwieser, V. Repain, S. Guitteny, C. Chacon, A. Bellec, J. Lagoute, Y. Girard, S. Rousset, A. Sassella, M. Imam and S. Narasimhan,
Change of cobalt magnetic anisotropy and spin polarization with alkanethiolates self-assembled monolayers,
New Journal of Physics,
17, 063022 (2015).
pdf
, Read abstract. Wedemonstrate that the deposition of a self-assembled monolayer of alkanethiolates on a 1 nmthick
cobalt ultrathin film grown on Au(111) induces a spin reorientation transition from in-plane to outof-
plane magnetization. Using ab initio calculations, we show that a methanethiolate layer changes
slightly both the magnetocrystalline and shape anisotropy, both effects almost cancelling each other
out for a 1 nmCo film. Finally, the change in hysteresis cycles upon alkanethiolate adsorption could be
assigned to a molecular-induced roughening of the Co layer, as shown by STM. In addition, we
calculate how a methanethiolate layer modifies the spin density of states of the Co layer and we show
that the spin polarization at the Fermi level through the organic layer is reversed as compared to the
uncovered Co. These results give new theoretical and experimental insights for the use of thiol-based
self-assembled monolayers in spintronic devices.
11. J. Das, S. Biswas, A. K. Kundu, S. Narasimhan and K. S. R. Menon,
Structure of Cr monolayer on Ag(001): A buried two-dimensional c2x 2 antiferromagnet,
Physical Review B,
91, 125435 (2015).
pdf
, Read abstract. The growth, morphology, and magnetic structure of ultrathin Cr films grown on a Ag(001) substrate are studied using low-energy electron diffraction (LEED), angle-resolved photoemission spectroscopy (ARPES), and ab initio density functional theory (DFT) calculations. The presence and temperature dependence of c(2×2) half-order spots in the LEED pattern, for low electron energies, along with the presence of characteristic Cr 3d bands in the ARPES spectra, confirm the existence of antiferromagnetic ordering for the Cr monolayer case. Our experiments are also consistent with the presence of a p(1×1) Ag overlayer on top of the Cr layer, suggesting the existence of a Ag/Cr/Ag(001) sandwich structure at the surface. Our DFT calculations confirm that this is the most favored geometric and magnetic structure of the system. The Cr layer is found to retain a “two-dimensional” character with enhanced Cr 3d magnetic moments, despite being buried below a Ag monolayer, due to the absence of significant hybridization between Cr 3d and Ag 4d electronic states. The coverage dependence of the magnetic ordering indicates a maximum ordering above the expected monolayer coverage, possibly due to intermixing between Ag and Cr atoms in the overlayer.
12. Kanchan Ulman and Shobhana Narasimhan,
Point Defects in Twisted Bilayer Graphene: A Density Functional Theory Study,
Physical Review B,
89, 245429 (2014).
pdf
, Read abstract. We have used ab initio density functional theory, incorporating van der Waals corrections, to study twisted
bilayer graphene (TBLG) where Stone-Wales defects or monovacancies are introduced in one of the layers. We compare these results to those for defects in single-layer graphene or Bernal stacked graphene. The energetics of defect formation is not very sensitive to the stacking of the layers or the specific site at which the defect is created, suggesting a weak interlayer coupling. However, signatures of the interlayer coupling are manifested clearly in the electronic band structure. For the “γγ” Stone-Wales defect in TBLG, we observe two Dirac cones that are shifted in both momentum space and energy. This up/down shift in energy results from the combined effect of a charge transfer between the two graphene layers and a chemical interaction between the layers, which mimics the effects of a transverse electric field. Charge density plots show that states near the Dirac points have significant admixture between the two layers. For Stone-Wales defects at other sites in TBLG, this basic structure is modified by the creation of minigaps at energy crossings. For a monovacancy, the Dirac cone of the pristine layer is shifted up in energy by ∼ 0.25 eV due to a combination of the requirement of the equilibration of Fermi energy between the two layers with different numbers of electrons, charge transfer, and chemical interactions. Both kinds of defects increase the density of states at the Fermi level. The monovacancy also results in spin polarization, with magnetic moments on the defect of 1.2–1.8 μB.
13. Kanchan Ulman, Debarati Bhaumik, Brandon C. Wood and Shobhana Narasimhan,
Physical Origins of Weak H2 Binding on Carbon Nanostructures: Insight from Ab Initio Studies of Chemically Functionalized Graphene Nanoribbons,
J. Chem. Phys.,
140, 174708 (2014).
pdf
, Read abstract. We have performed ab initio density functional theory calculations, incorporating London dispersion
corrections, to study the absorption of molecular hydrogen on zigzag graphene nanoribbons whose edges have been functionalized by OH, NH2, COOH, NO2, or H2PO3. We find that hydrogen molecules always preferentially bind at or near the functionalized edge, and display induced dipole moments. Binding is generally enhanced by the presence of polar functional groups. The largest gains are observed for groups with oxygen lone pairs that can facilitate local charge reorganization, with the biggest single enhancement in adsorption energy found for “strong functionalization” by H2PO3 (115 meV/H2 versus 52 meV/H2 on bare graphene). We show that for binding on the “outer edge” near the functional group, the presence of the group can introduce appreciable contributions from Debye interactions and higher-order multipole electrostatic terms, in addition to the dominant London dispersion interactions. For those functional groups that contain the OH moiety, the adsorption energy is linearly proportional to the number of lone pairs on oxygen atoms. Mixed functionalization with two different functional groups on a graphene edge can also have a synergistic effect, particularly when electron-donating and electron-withdrawing groups are combined. For binding on the “inner edge” somewhat farther from the functional group, most of the binding again arises from London interactions; however, there is also significant charge redistribution in the π manifold, which directly reflects the electron donating or withdrawing capacity of the functional group. Our results offer insight into the specific origins of weak binding of gas molecules on graphene, and suggest that edge functionalization could perhaps be used in combination with other strategies to increase the uptake of hydrogen in graphene. They also have relevance for the storage of hydrogen in porous carbon materials, such as activated carbons
14. Debosruti Dutta, Brandon C. Wood, Shreyas Y. Bhide, K. Ganapathy Ayappa and Shobhana Narasimhan,
Enhanced Gas Adsorption on Graphitic Substrates via Defects and Local Curvature: A Density Functional Theory Study,
J. Phys. Chem. C,
118, 7741 - 7750 (2014).
pdf
, Read abstract. Using van derWaals-corrected density functional theory calculations, we explore the possibility of engineering the local structure and morphology of high surface-area graphene-derived materials to improve the uptake of methane and carbon dioxide for gas storage and sensing. We test the sensitivity of the gas adsorption energy to the introduction of native point defects, curvature and the application of strain. The binding energy at topological point defect sites is inversely correlated with the number of missing carbon atoms, causing Stone-Wales defects to show the largest enhancement with respect to pristine graphene (~ 20%). Improvements of similar magnitude are observed at concavely curved surfaces in buckled graphene sheets under compressive strain, whereas tensile strain tends to weaken gas binding. Trends for CO2 and CH4 are similar, though CO2 binding is generally stronger by ~ 4–5 kJ mol-1. However, the differential between the adsorption of CO2 and CH4 is much higher on folded graphene sheets and at concave curvatures; this could possibly be leveraged for CH4/CO2 flow separation and gas-selective sensors.
15. Kanchan Ulman, Shobhana Narasimhan and Anna Delin,
Tuning Spin Transport Properties and Molecular Magnetoresistance Through Contact Geometry,
J. Chem. Phys.,
140, 044716 (2014).
pdf
, Read abstract. Molecular spintronics seeks to unite the advantages of using organic molecules as nanoelectronic
components, with the benefits of using spin as an additional degree of freedom. For technological
applications, an important quantity is the molecular magnetoresistance. In this work, we show that
this parameter is very sensitive to the contact geometry. To demonstrate this, we perform ab initio
calculations, combining the non-equilibrium Green’s function method with density functional theory,
on a dithienylethene molecule placed between spin-polarized nickel leads of varying geometries. We
find that, in general, the magnetoresistance is significantly higher when the contact is made to sharp
tips than to flat surfaces. Interestingly, this holds true for both resonant and tunneling conduction
regimes, i.e., when the molecule is in its “closed” and “open” conformations, respectively. We find
that changing the lead geometry can increase the magnetoresistance by up to a factor of ∼5. We
also introduce a simple model that, despite requiring minimal computational time, can recapture our
ab initio results for the behavior of magnetoresistance as a function of bias voltage. This model re-
quires as its input only the density of states on the anchoring atoms, at zero bias voltage. We also find
that the non-resonant conductance in the open conformation of the molecule is significantly impacted
by the lead geometry. As a result, the ratio of the current in the closed and open conformations can
also be tuned by varying the geometry of the leads, and increased by ∼400%
16. Madhura Marathe, Alejandra Diaz Ortiz and Shobhana Narasimhan,
Ab initio and cluster expansion study of surface alloys of Fe and Au on Ru(0001) and Mo(110): Importance of magnetism,
Physical Review B,
88, 245442 (2013).
pdf
17. Sananda Biswas, Sadanand Deshpande, Derren Dunn and Shobhana Narasimhan,
Tuning Patterning Conditions by Co-adsorption of Gases: Br2 and H2 on Si(001),
J. Chem. Phys.,
139, 184713 (2013).
pdf
, Read abstract. We have studied the co-adsorption of Br2 and H2 on Si(001), and obtained co-adsorption energies and the surface phase diagram as a function of the chemical potential and pressure of the two gases. To do this, we have used density functional theory calculations in combination with ab initio atomistic thermodynamics. Over large ranges of bromine and hydrogen chemical potentials, the favored configuration is found to be either one with only Br atoms adsorbed on the surface, at full coverage, in a (3 × 2) pattern, or a fully H-covered surface in a (2 × 1) structure. However, we also find regions of the phase diagram where there are configurations with either only Br atoms, or Br and H atoms, arranged in a two-atom-wide checkerboard pattern with a (4 × 2) surface unit cell. Most interestingly, we find that by co-adsorbing with H2, we bring this pattern into a region of the phase diagram corresponding to pressures that are significantly higher than those where it is observed with Br2 alone. We also find small regions of the phase diagram with several other interesting patterns.
18. Kanchan Ulman, Rajesh Sathiyanarayanan, R.K. Pandey, K.V.R.M. Murali and Shobhana Narasimhan,
Dielectric properties of Si3−ξGeξN4 and Si3−ξCξN4: A density functional study,
Journal of Applied Physics,
113, 234102 - 234103 (2013).
, Read abstract. Using first principles calculations, we have studied the dielectric properties of crystalline α- and β-phase silicon germanium nitrides and silicon carbon nitrides,
A3−ξBξN4
(A = Si, B = Ge or C,
ξ = 0,1,2,3
). In silicon germanium nitrides, both the high-frequency and static dielectric constants increase monotonically with increasing germanium concentration, providing a straightforward way to tune the dielectric constant of these materials. In the case of silicon carbon nitrides, the high-frequency dielectric constant increases monotonically with increasing carbon concentration, but a more complex trend is observed for the static dielectric constant, which can be understood in terms of competition between changes in the unit-cell volume and the average oscillator strength. The computed static dielectric constants of C
3
N
4
, Si
3
N
4
, and Ge
3
N
4
are 7.13, 7.69, and 9.74, respectively.
19. Kanchan Ulman, Mighfar Imam, Shobhana Narasimhan, Anders Odell and Anna Delin,
Theoretical Study of Spin Conduction in the Ni/DTE/Ni Nanohybrid,
Nano Hybrids,
4, 1 - 20 (2013).
20. B. Wood, S. Bhide, D. Dutta, V. Kandagal, A. Pathak, S. Punnathanam, K. G. Ayappa and S. Narasimhan,
Methane and carbon dioxide adsorption on edge-functionalized graphene: A comparative DFT study,
J. Chem. Phys.,
137, 054702 - 054703 (2012).
, Read abstract. With a view towards optimizing gas storage and separation in crystalline and disordered nanoporous
carbon-based materials, we use ab initio density functional theory calculations to explore the effect
of chemical functionalization on gas binding to exposed edges within model carbon nanostructures.
We test the geometry, energetics, and charge distribution of in-plane and out-of-plane binding of
CO2 and CH4 to model zigzag graphene nanoribbons edge-functionalized with COOH, OH, NH2,
H2PO3, NO2, and CH3. Although different choices for the exchange-correlation functional lead to a
spread of values for the binding energy, trends across the functional groups are largely preserved for
each choice, as are the final orientations of the adsorbed gas molecules. We find binding of CO2 to
exceed that of CH4 by roughly a factor of two. However, the two gases follow very similar trends with
changes in the attached functional group, despite different molecular symmetries. Our results indicate
that the presence of NH2, H2PO3, NO2 , and COOH functional groups can significantly enhance
gas binding, making the edges potentially viable binding sites in materials with high concentrations
of edge carbons. To first order, in-plane binding strength correlates with the larger permanent and
induced dipole moments on these groups. Implications for tailoring carbon structures for increased
gas uptake and improved CO2/CH4 selectivity are discussed
21. M. Miao, J. Kurzman, N. Mammen, S. Narasimhan and R. Seshadri,
Trends in the electronic structure of extended gold compounds: Implications for the use of gold in heterogeneous catalysis,
Inorganic Chemistry,
51, 7569 - 7578 (2012).
, Read abstract. First-principles electronic structure calculations are presented on a
variety of Au compounds and speciesencompassing a wide range of formal
oxidation states, coordination geometries, and chemical environmentsin order
to understand the potentially systematic behavior in the nature and energetics of d
states that are implicated in catalytic activity. In particular, we monitor the
position of the d-band center, which has been suggested to signal catalytic activity
for reactions such as CO oxidation. We find a surprising absence of any kind of
correlation between the formal oxidation state of Au and the position of the dband
center. Instead, we find that the center of the d band displays a nearly linear
dependence on the degree of its filling, and this is a general relationship for Au
irrespective of the chemistry or geometry of the particular Au compound. Across
the compounds examined we find that even small calculated changes in the d-band filling result in a relatively large effect on the
position of the d-band center. The results presented here have some important implications for the question of the catalytic
activity of Au and indicate that the formal oxidation state is not a determining factor
22. M. Marathe, J. Lagoute, V. Repain, S. Rousset and S. Narasimhan,
Spin-polarized Surface States on Fe-deposited Au(111) Surface: A Theoretical Study,
Surface Science,
606, 950 - 955 (2012).
, Read abstract. We have studied electronic structure of Fe-deposited Au(111) by performing ab initio density functional
theory calculations. We find that the magnetic moment on the deposited Fe layer is enhanced as compared to that in bulk iron. We observe a large number of new states on the Fe-deposited surface — one of which is in the majority spin channel having similar dispersion to that on the clean surface, and others in the minority spin channel. The effective mass of electrons in surface states near the Fermi level increases on Fe deposition. The electronic properties are found to be insensitive to the stacking of near-surface layers. We need to use very thick slabs in our calculations to avoid splitting of surface states due to spurious interactions between the two surfaces of the slab. Using the local density of states profiles for different surface states, we conclude that in scanning tunneling microscope experiments one can detect two of the surface states — one in the majority channel below the Fermi level, and another in the minority channel appearing just above the Fermi energy. We compare our results to those from scanning tunneling spectroscopy experiments.
23. S. Mehendale, M. Marathe, Y. Girard, V. Repain, C. Chacon, J. Lagoute, S. Rousset and S. Narasimhan,
Prediction of Reconstruction in Heteroepitaxial Systems Using the Frenkel-Kontorova Model,
Physical Review B,
84, 195458 (2011).
pdf
24. Anders Odell, Anna Delin, Borje Johansson, Kanchan Ulman, Shobhana Narasimhan, Ivan Rungger and Stefano Sanvito,
Comparison between s- and d-electron mediated transport in a photoswitching dithienylethene molecule using ab initio transport methods,
Physical Review B,
84, 165402 (2011).
, Read abstract. The influence of the electrodes Fermi surface on the transport properties of a photoswitching molecule is investigated with state of the art ab initio transport methods. We report results for the conducting properties of the two forms of dithienylethene attached either to Ag or to non-magnetic Ni leads. The I-V curves of the Ag/dithienylethene/Ag device are found to be very similar to those reported previously for Au. In contrast, when Ni is used as electrode material the zero-bias transmission coefficient is profoundly different as a result of the role played by the Ni d bands in the bonding between the molecule and the electrodes. Intriguingly, despite these differences the overall conducting properties depend little on the electrode material. We thus conclude that electron transport in dithienylethene is, for the cases studied, mainly governed by the intrinsic electronic structure of the molecule.
25. A. Delga, J. Lagoute, V. Repain, C. Chacon, Y. Girard, M. Marathe, S. Narasimhan and S. Rousset,
Electronic Properties of Fe Clusters on a Au(111) Surface,
Physical Review B,
84, 035416 - 035417 (2011).
, Read abstract. The electronic states of self-organized Fe nanoislands on a Au(111) surface have been investigated
using low-temperature scanning tunneling microscopy and spectroscopy. We show that the local
density of state is dominated by Shockley surface states conned in the nanostructures. Comparing
the experimental dispersion diagram with a free-electron model we derive the eective mass
m=0.39 me and the band onset E0=420 meV of these states. Ab initio calculations show the
existence of the Shockley surface states in the Fe layer, in agreement with the experiment, and reveal
that they are fully spin polarized.
26. Mighfar Imam and Shobhana Narasimhan,
Magnetism of surface alloys of the type MxN1 À x/Rh(1 1 1),
Journal of Magnetism and Magnetic Materials,
323, 1873 - 1881 (2011).
, Read abstract. We present a density functional theory study on the magnetic properties of two-dimensional surface
alloys of the type MxN1 À x (M 1⁄4 Fe, Co and Ni; N 1⁄4 Pt, Au, Ag, Cd and Pb) on Rh(1 1 1) for x 1⁄40.0, 0.25,
0.33, 0.5, 0.67, 0.75 and 1.0, in two types of geometric arrangements—striped phases or linear-chain
type, and non-striped phases or mixed checkerboard type. Many pairs among these are bulk-immiscible
but show mixing on the surface. We find that the trend in the magnetic moment of surface alloys of N
with a given M follows the number of valence electrons in N: the higher the number of valence
electrons, the lower the magnetic moment. Overlayer atoms when put on hcp sites show higher
moment compared to fcc sites. In general, for a given composition x, linear-chain type structures show a
reduced magnetic moment compared to checkerboard type structures. We find that Pb, when alloyed
with magnetic elements (Fe, Co and Ni), has a lowering effect on their magnetic moments.
27. Nisha Mammen, Shobhana Narasimhan and Stefano de Gironcoli,
Tuning the Morphology of Gold Clusters by Substrate Doping,
JACS,
133, 2801 - 2802 (2011).
pdf
, Read abstract. The morphology of small metal clusters can have a big impact on their electronic, magnetic, and chemical properties. This has been shown earlier, for example, for Au
20
clusters on MgO(001), where planar and tetrahedral geometries are possible for the gold atoms. While the planar geometry is more desirable for catalytic applications, it is disfavored in the usual situation. While earlier suggestions that have been made for tilting this balance in favor of the planar isomer are of considerable fundamental interest, they do not easily lend themselves to practical applications. Here, we suggest a conceptually simple but practicable way of achieving the same goal: viz., by doping the MgO substrate with Al atoms. We show, by performing density functional theory calculations, that this stabilizes the planar over the tetrahedral arrangement by an energy difference that is linearly proportional to the dopant concentration and is insensitive to the position of the dopant atom. The charge transferred to the Au cluster also depends monotonically on the doping concentration. This work is of interest for possible applications in the field of gold nanocatalysis.
28. Alison Hatt, Brent Melot and Shobhana Narasimhan,
Harmonic and anharmonic properties of Fe and Ni: Thermal expansion, exchange-correlation
errors, and magnetism,
Physical Review B,
82, 134418 (2010).
, Read abstract. We have investigated the source of errors in ab initio calculations of thermal properties of the magnetic
metals Fe and Ni and their dependence on the form of the exchange and correlation functional. We used
density-functional theory and density-functional perturbation theory together with the quasiharmonic approxi-
mation to compute the coefficient of thermal expansion, bulk modulus and its pressure derivative, phonon
modes, and Grueneisen parameters of bcc Fe and fcc Ni. In nonmagnetic metals the main source of error in
calculated thermal properties can be attributed to evaluation of properties at incorrect lattice constants, which
in turn may be traced to the choice of exchange and correlation functional. However, for magnetic metals the
properties may be evaluated at both incorrect lattice constant and incorrect magnetic moment. This affects
vibrational properties so that it is no longer true that anharmonic errors are significantly less than errors at
harmonic order.
29. S. Mehendale, Y. Girard, V. Repain, C. Chacon, J. Lagoute, S. Rousset, M. Marathe and S. Narasimhan,
Ordered Surface Alloy of Bulk-Immiscible Components Stabilized by Magnetism,
Phys. Rev. Lett,
105, 056101 (2010).
, Read abstract. Using scanning tunneling microscopy and a diffraction experiment, we have discovered a new ordered
surface alloy made out of two bulk-immiscible components, Fe and Au, deposited on a Ru(0001)
substrate. In such a system, substrate-mediated strain interactions are believed to provide the main
driving force for mixing. However, spin-polarized ab initio calculations show that the most stable
structures are always the ones with the highest magnetic moment per Fe atom and not the ones minimizing
the surface stress, in remarkable agreement with the observations. This opens up novel possibilities for
creating materials with unique properties of relevance to device applications.
30. G. Prevot, Y. Girard, V. Repain, S. Rousset, A. Coati, Y. Garreau, J. Paul, N. Mammen and S. Narasimhan,
Elastic Displacements and Step Interactions on Metallic Surfaces: GIXD and ab initio study of Au(332),
Physical Review B,
81, 075415 (2010).
pdf
31. P. Ghosh, R. Pushpa, S. de Gironcoli and S. Narasimhan,
Effective coordination number: a simple indicator of activation energies for NO dissociation on Rh(100) surfaces,
Physical Review B,
80, 233406 - 233506 (2009).
32. M. Marathe, M. Imam and S. Narasimhan,
Mixing and Magnetic Properties of Surface Alloys: The Role of the Substrate,
Applied Surface Science,
256, 449 - 454 (2009).
33. R. Pushpa, P. Ghosh, S. Narasimhan and S. de Gironcoli,
Effective Coordination as a Predictor of Adsorption Energies: a Model Study of NO on Rh(100) and Rh/MgO(100) Surfaces,
Physical Review B,
79, 165406 (2009).
34. M. Marathe, M. Imam and S. Narasimhan,
Elastic and chemical contributions to the stability of magnetic surface alloys on Ru(0001),
Physical Review B,
79, 085413 (2009).
35. M. Imam, M. Marathe and S. Narasimhan,
Nix Pt1−x /Rh(111): A Stable Surface Alloy with Enhanced Magnetic Moments,
Solid State Communications,
149, 559 - 563 (2009).
36. M. Imam, M. Marathe and S. Narasimhan,
Competition between elastic and chemical effects in the intermixing of Ag and Co on Rh(111),
J. Chem. Sci.,
120, 621 - 626 (2008).
37. P. Ghosh, R. Pushpa, S. de Gironcoli and S. Narasimhan,
Interplay between bonding and magnetism in the binding of NO to Rh clusters,
J. Chem. Phys.,
128, 194708 (2008).
38. R. P ushpa, U.V. Waghmare and S. Narasimhan,
Bond Stiffening in Small Nanoclusters and its Consequences,
Physical Review B,
77, 045427 (2008).
39. M Upadhyay Kahaly, S. Narasimhan and U.V. Waghmare,
Size dependence of structural, electronic, elastic and optical properties of selenium nanowires: A first-principles study,
J. Chem. Phys.,
128, 044718 (2008).
40. J. Paul and S. Narasimhan,
Effect of coordination on bond properties: A first principles study,
Bulletin of Materials Science,
31, 569 - 572 (2008).
41. A. Aravindh, + 66 other authors and S. Narasimhan (corresponding author),
Six C1−x O2 alloys: a possible route to stabilize carbon-based silica-like solids,
Solid State Communications,
144, 273 - 276 (2007).
42. P. Ghosh, S. Narasimhan, S. Jenkins and D.A. King,
Lifting of Ir(100) reconstruction by CO adsorption: an ab initio study,
J. Chem. Phys.,
126, 244701 (2007).
43. J. Bhattacharjee, S. Narasimhan and U.V. Waghmare,
Distiribution of Electron Charge Centres: A Picture of Bonding Based on Geometric Phases,
arXiv,
06, 12468 (2006).
44. R. Pushpa, S. Narasimhan and U.V. Waghmare,
Symmetries, vibrational instabilities and routes to stable structures of clusters of Al, Sn and As,
J. Chem. Phys.,
121, 5211 - 5220 (2004).
45. S. Narasimhan,
Ab Initio Calculations of Metal Surfaces,
Transactions of the Materials Research Society of Japan,
29, 19 - 24 (2004).
46. S. Narasimhan,
Stress, Strain and Charge Transfer in the Ag/Pt(111) System: A Test of Continuum Elasticity Theory,
Physical Review B,
69, 045425 - 045442 (2004).
47. Raghani Pushpa and Shobhana Narasimhan,
Reconstruction of Pt(111) and Domain Patterns on Close Packed Metal Surfaces,
Physical Review B,
67, 205418 (2003).
48. Raghani Pushpa and Shobhana Narasimhan,
Double Stripe Reconstruction of the Pt(111) Surface,
Bulletin of Materials Science,
26, 91 (2003).
49. Shobhana Narasimhan and Stefano de Gironcoli,
Exchange-correlation errors at harmonic and anharmonic orders: the case of bulk Cu,
Bulletin of Materials Science,
26, 75 (2003).
50. Shobhana Narasimhan and Pushpa Raghani,
Honeycombs, Triangles and Bright Stars: Pattern Formation on Metal Surfaces,
Physics at Surfaces and Interfaces, B.N. Dev (ed.), World Scientific, Singapore,
3 - 12 (2003).
51. Shobhana Narasimhan,
Ab Initio Lattice Dynamics of Ag(110),
Surface Science,
496, 331 (2002).
52. Raghani Pushpa and Shobhana Narasimhan,
Stars and Stripes: Nanoscale Misfit Dislocation Patterns on Surfaces,
Pure and Applied Chemistry,
74, 1663 (2002).
53. Shobhana Narasimhan and Stefano de Gironcoli,
Ab initio calculation of the thermal properties of Cu: Performance of the LDA and GGA,
Physical Review B,
65, 64302 (2001).
54. Shobhana Narasimhan,
Reversed Anisotropies and Thermal Contraction of FCC(110) surfaces,
Physical Review B,
64, 125409 (2001).
55. Shobhana Narasimhan,
Ab Initio Calculations on the Anomalous Thermal Behaviour of FCC(110) Surfaces,
Applied Surface Science,
182, 293 (2001).
56. Shobhana Narasimhan,
Surprises in the Physics of Metal Surfaces,
Journal of the Indian Institute of Science,
81, 15 (2001).
57. Shobhana Narasimhan,
Phonon Softening and the Anomalous Thermal Expansion of Ag(111),
Surface Science Letters,
417, (1998).
58. Shobhana Narasimhan and Matthias Scheffler,
A Model for the Thermal Expansion of Ag(111),
Zeitschrift fuer Physikalische Chemie,
202, 253 (1997).
59. Shobhana Narasimhan and J. W. Davenport,
An ab initio study of polytetrahedral packing: the Al-Mg system,
Physical Review B, Rapid Communications,
51, 659 (1995).
60. J. W. Davenport, N. Chetty, R. B. Marr, S. Narasimhan, J. E. Pasciak, R. F. Peierls and M. Weinert,
First Principles Pseudopotential Calculations on Aluminum and Aluminum Alloys,
The Minerals, Metals, and Materials Society,Warrendale,
(1994).
61. Shobhana Narasimhan and David Vanderbilt,
Elastic-Stress Domains and the Herringbone Reconstruction on Au(111),
Physical Review Letters,
69, 1564 (1992).
62. Shobhana Narasimhan and David Vanderbilt,
Anharmonic self energies of phonons in silicon,
Physical Review B, Rapid Communications,
43, 4541 (1991).
63. Shobhana Narasimhan and David Vanderbilt,
Lifetimes and frequency shifts of phonons in Si,
Phonons,
1, 89 (1990).
64. David Vanderbilt, S. H. Taole and Shobhana Narasimhan,
Anharmonic Elastic and Phonon Properties of Si,
Physical Review B,
40, 5657 (1989).
65. Srinivas Krishnagopal, Shobhana Narasimhan and S. H. Patil,
Multipolar Polarizabilities and Rydberg States,
J. Chem. Phys.,
83, 5772 (1985).
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