Monday, Nov 28, 2011 at 9:00 AM
Magnetism and Catalysis of free and supported Pdn Clusters
S. N. Khanna, J. U. Reveles, V. Medel, A. C. Reber, V. Ong
Department of Physics, Virginia Commonwealth University, Richmond, VA 23284-2000, USA.
Email: [email protected]
Vikas Chauhan and Prasenjit Sen
Harish-Chandra Research Institute, Chhatnag Road Jhunsi, Allahabad 211019, India.
A.M. Köster and P. Calaminici
Departamento de Química, Cinvestav, Ave. Instituto Politéc. Nacion. 2508, México.
Clusters of non-magnetic solids can exhibit magnetic order. This includes composite clusters of
simple and 3d transition metals, or small clusters of 4d transition metal elements. For example, while
bulk Pd is paramagnetic, small Pdn clusters display magnetic character. Palladium clusters are also
important catalysts. For example, small clusters of palladium supported on oxide supports constitute
an important class of heterogeneous catalysts employed in a variety of industrially relevant processes
including methane combustion. Consequently, identifying atomic, electronic, and magnetic character
of clusters is an important undertaking.
I will first present results using first principles density functional theory on magnetism in clusters of
non-magnetic solids starting from our recent discovery of magnetic superatoms. I will then present
studies on free Pdn clusters and in particular Pd13. I will show that a bi-layer ground state structure
that can be regarded as a relaxed bulk fragment is most compatible with the experimental results from
Stern Gerlach measurements. An icosahedral structure, considered to be the ground state in
numerous previous studies, is shown to be around 0.14 eV above the ground state. A detailed
analysis of the molecular orbitals reveals the near degeneracy of the bi-layer or icosahedral structures
is rooted in the stabilization by p- or d- like cluster orbitals. The importance of low-lying spin states in
controlling the electronic and magnetic properties of the cluster will be highlighted.
I will then talk about our work on Pdn clusters supported on various supports and in particular on
TiO2. Since the Pdn clusters are important oxidation catalysts, I will present our work on the effect of
oxygen absorption on the atomic and electronic structure. In particular, I will show how the absorption
of oxygen can lead to structural rearrangements in cluster and strong relaxations of the support. The
talk will highlight the microscopic origin of the strong metal-support interaction and the existence of
“spill over” O states.
1
Monday, Nov 28, 2011 at 9:45 AM
Ab initio theory of complex magnetism in Tb clusters
Biplab Sanyal
Department of Physics and Astronomy, Uppsala University, Sweden
Email: [email protected]
Magnetism of rare-earth systems is known to be quite complex in bulk and less known in low
dimensions. Moreover, the presence of f-electrons adds an extra complexity due to the indeterminacy
of their degree of localization and hence, its effects on electronic structure and other properties. Here,
we present a detailed study of structural, electronic and magnetic properties of Tb n (n=2,13) clusters
by ab initio density functional theory within the framework of local density approximation plus Hubbard
U approach. The ground state geometries of these clusters, which are unknown in experiments, have
been obtained by careful energy minimizations of several starting geometries and spin multiplicities.
We show that a non-monotonous behavior of magnetic moment (spin+orbital) as a function of cluster
size appears due to complex (ferro/ferri)magnetic structures. The evolution of magnetic moments in
theory agrees quite well with experimental observations. Finally, we present some interesting results
on the magnetic anisotropy energies of these clusters.
2
Monday, Nov 28, 2011 at 11:00 AM
Electronic structure and magnetism of some novel carbon materials: GNR
channels, and GNRS@nano-tunes.
D G Kanhere,
Central university of Rajasthan, Kishangarh , India
Email: [email protected]
In the first part, we present the results of detailed density functional calculations on narrow nano
ribbons encapsulated inside nano tubes GNRS@SWNT. We have investigated pure carbon GNRS,
partially hydrogenated ones and fully hydrogenated (Graphane) ribbons encapsulated in three tubes
of different diameters, both metallic and semiconducting type. The work is particularly relevant
because of very recent report of experimental synthesis of GNRS inside nano tubes. It turns out that
such novel materials are stable. The geometry of GNRS is dependent on the width and in the case of
hydrogenated grapheme, sensitive to the amount of hydrogenation. It is possible to obtain one
dimensional chains, or rings inside the tubes. All system studied are periodic. We will present
geometries, density of states and charge densities.
In the second part we present density functional results for partially dehydrogenated graphane
leading to diagonal and edge channels of bare carbon atoms. Such channels correspond to A-GNRS
and ZGNRS. We have investigated the magnetic response of one and two interacting Fe atoms
(magnetic impurities) placed in both these channels. The response ie the induced magnetic moments
and the nature of the density of states turns out to be very different. A strong ferromagnetic coupling is
observed in edge channels, whereas in the diagonal channel there is hardly any magnetic interaction
between the two Fe atoms. The magnetic coupling is mediated by edge carbon atoms.
Work done in collaboration with Prachi Chandrachde (Pune), S Haldar (Pune) and Biplab
Sanyal(Uppsala).
3
Monday, Nov 28, 2011 at 11:45 AM
Structure prediction of clusters, molecules and solids by the minima
hopping global geometry optimization method
Stefan Goedecker
Department of Physics and Astronomy, Basel University, Basel, Switzerland
E-mail: [email protected]
The minima hopping global geometry optimization method will be introduced together with the
fundamental principles on which it is based. Various applications of the method will be presented and
in particular applications where the potential energy landscape is explored on the density functional
level. It will be shown that many theoretically proposed structures for endohedral silicon cages and
boron fullerenes are only metastable structures and that the energy landscape for these systems is
quite different from the landscape of structures which can by synthesized in experiment. Finally the
structural richness of some simple crystalline materials will be presented.
4
Monday, Nov 28, 2011 at 2:00 PM
Theoretical study of binding and desorption of hydrogen on doped
Magnesium hydride and light metal decorated metal-organic framework 5
Sourav Pal
National Chemical Laboratory, Pune 411 008
Email: [email protected]
For implementation of a hydrogen economy, cost-effective, safe, and efficient storage of hydrogen
is a necessary prerequisite. Storing hydrogen in solid-state materials offers the advantage of
operating in a low pressure and room temperature and also improves the energy density of hydrogen
in comparison to that provided by high pressure tanks and cryogenic vessels. We will discuss
theoretical calculations on the effect of doping on two classes of materials, Magnesium hydrides and
metal-organic-frameworks (MOF).
Magnesium hydride (MgH2) is lightweight and has a low manufacture cost and a high capacity of
7.6 wt.% H2. These features make it an attractive possibility for hydrogen storage. However, MgH 2
has a high thermodynamic stability which is responsible for the high dehydrogenation temperature
requirement of 573 K. Its slow hydrogen sorption kinetics still further limits the application of MgH 2 for
on-board hydrogen storage. Typically, it is realized that hydrogen adsorption energies should be ~2050 Kj/ mole. In the light of the destabilization of MgH2 caused by light metals and the importance of its
metastable phases, study of the electronic structure, thermodynamics and hydrogen desorption
kinetics of Al- and Si-doped -, -, and -MgH2 will be reported using density functional calculations
and the plane wave pseudopotential method.
MOFs,with their extremely high surface areas, fast H 2 desorption kinetics, and reversible H2
uptake and release are one of the most promising class of hydrogen storage materials. However, in
this case, the binding is via van der Waals interactions between the physisorbed H2 molecules and
the host MOFs. We discuss the results of decoration of the organic linker of MOF-5 with light metal
atoms and ions to strengthen the binding.
5
Monday, Nov 28, 2011 at 2:45 PM
Nanoscale Curvature and Confinement Effects on Molecular Interaction
and Properties
Swapan K. Ghosh
Theoretical Chemistry Section, Bhabha Atomic Research Centre, Mumbai 400085, India
Email: [email protected]
Behaviour of molecules and fluids under confinement has attracted considerable attention in
recent years. The interaction and properties of molecules near an interface have been shown [1] to be
significantly influenced by the nanoscale curvature of the concerned surface. Of special importance
has been the interaction of hydrogen [2] and water with carbon nanostructures, due to implications of
relevance to hydrogen energy. The effect of the diffuse electron density profiles on the size
dependence of optical properties of bimetallic nanospheres has also been of interest [3]. The effect of
surface curvature and confinement is, however, not limited to the domain of electronic structure based
description alone and is in fact of considerable interest in the area of soft condensed matter
investigated using tools relevant to the mesoscale. Density functional theory has been known to
provide a practical tool within both the length scales, and has been used to investigate the structure of
fluids at interfaces, nucleation phenomena and size-dependent properties of fluid droplets [4]. Another
aspect of interest in the context of confinement effect is the single-file flow of fluids in nanopores [5]. A
simple unified one-dimensional theoretical approach has also been proposed for the investigation of
dynamics in condensed phase and has been applied to several dynamical processes of importance
[6]. A brief overview of current investigations in these areas will form the subject matter of this talk.
References
[1]. K.R.S. Chandrakumar, K. Srinivasu and S. K. Ghosh, J. Phys. Chem. C112, 15670 (2008).
[2]. (a) K. R. S. Chandrakumar and S. K. Ghosh, Nano Lett. 8, 13 (2008).
(b) K.Srinivasu, K.R.S. Chandrakumar and S.K. Ghosh, Phys. Chem. Chem. Phys. 10, 5832 (2008)
[3]. M.K. Nayak and S.K. Ghosh, J. Chem. Phys. 130, 204702 (2009)
[4]. (a)
S. Ghosh and S.K. Ghosh, J. Chem. Phys. 134, 024502 (2011)
(b) S. Ghosh and S.K. Ghosh, J. Chem. Phys. 135, 124710 (2011)
[5]. (a) S. Roy Majumder, N. Choudhury and S. K. Ghosh, J. Chem. Phys. 127, 054706 ( 2007)
(b) S. Roy Majumder, T. Bandyopadhyay and S. K. Ghosh, J. Chem. Phys. 125, 201103 (2006)
[6]. (a) A. Patra, A. Samanta and S.K. Ghosh, Phys. Rev. E 83, 026104 (2011)
(b) A. Samanta, and S.K. Ghosh, J. Phys. Chem. A 112, 752 (2008).
(c) K. Dhole, B. Modak, A. Samanta and S.K. Ghosh, Phys. Rev. E 82, 16110 (2010)
6
Monday, Nov 28, 2011 at 4:00 PM
Electronic Structure of CdSe Nanorods and doped Chalcogenide Clusters
Anjali Kshirsagar
Director, Centre for Modeling and Simulation
Director, Interdisciplinary School of Scientific Computing
University of Pune, Pune 411 007
Email: [email protected]
At nanoscale, crystalline particles, tubes and wires exhibit a variety of optical, electrical and
magnetic properties that depend explicitly on the their specific size, shape and aspect ratio. It has
been reported that if the nanocrystals have a highly symmetrical crystal structure (e.g., zinc blend or
rock-salt), the synthesis frequently yields no unique direction of growth, producing spherical
nanostructures but, if the nanocrsystals have lower symmetry structures, more anisotropic shapes are
possible. Hence, cadmium chalcogenide nanocrystals, which often form hexagonal wurtzite structure
that has a unique polar axis, can be synthesized in shapes ranging from spherical to rod-like and even
more complex shapes. In this case, the surfactant particles also play a decisive role in deciding the
properties.
The wurtzite polymorph of Cadmium Selenide (CdSe) is an important II-VI semiconducting
compound that is widely used in optoelectronics. CdSe nanostructures, due to their tunable sizedependent properties make an ideal model system for the study of various nanoscale phenomena. In
our work we study the effects of change in the diameter of the nanorods as compared to their lengths,
on their electronic structure. It has been previously reported by Katz at al. [1] that the electronic level
structure of CdSe nanorods primarily depends on their diameter than their length. The level structure
for the rods with same diameter but different lengths remain remarkably similar. We intend to discuss
electronic structure of CdSe nanorods with the help of Density Functional Theory (DFT) implemented
through Vienna Ab-initio Simulations Package (VASP). For this, we use cluster derived CdSe
nanorods of different lengths and diameters where small CdSe clusters are chosen as the basic
building blocks and are grown along z-axis. These nanorods are then “passivated” with the help of the
pseudo-atoms having appropriate nuclear and valence charge, as per the scheme developed by
Huang et al. [2]. Electronic level structure as generated, would help us to understand the dependence
of band-gaps and excited state transitions on the aspect ratios of the nanorods.
Another interesting issue in the study of II-VI semiconducting nanostructures is doping with
transition metal impurities. Doping leads to magnetic structures which makes these materials useful
for spintronic applications. We will discuss few Zn/Cd(TM)S/Se/Te nanostructures to understand the
magnetic behavior. There are some exceptionally stable structures which can form building blocks for
nanostructured assemblies for use in nanodevices.
[1]. David Katz at al., PRL (2002) 89(8), 086801
[2]. X. Huang et al., Phys. Rev. B (2004) 69, 153302
7
Monday, Nov 28, 2011 at 4:45 PM
Atomic clusters: A possible building motif for inorganic nanomaterials
Debesh Ranjan Roy
Department of Applied Physics, S. V. National Institute of Technology, Surat 395007, India
E-mail: [email protected] ; [email protected]
Small clusters are the sub-nanoscale particles that have been found to exhibit novel electronic,
magnetic, chemical and optical properties. There is currently, enormous interest, in developing
materials with clusters as the building blocks. Since the properties of clusters can be tuned by
changing size, composition and charged state, it is envisioned that novel materials with tunable
properties could be synthesized using such building motifs [1]. A detail investigation on the electronic,
magnetic and optical properties of small transition metal clusters (i.e., Fe, Ni, Pd etc) and their oxides
is performed [2-4]. Possible noncollinear magnetism in iron clusters and their oxides is searched for
[2]. Magnetic structure of the experimentally observed cobalt oxides is predicted [3]. A minute
investigation on the ground state structure of Pd13 is also explored [4].
In search for suitable building motifs, a detail study is performed on the group III and V combined
polygonal units [5]. Also, magic behavior of group II and IV combined bimetallic clusters is explored in
detail [6].
References
[1]
[2]
[3]
[4]
W. Castleman and S. N. Khanna J Phys. Chem. C 113 (2009) 2664.
D. R. Roy, R. Robles and S. N. Khanna, J. Chem. Phys. 132 (2010) 194305.
C. N. van Dijk, D. R. Roy et al., submitted (2011).
A. Koster, P. Calaminici, E. Orgaz, D. R. Roy, J. U. Reveles, S. N. Khanna, J. Am. Chem.
Soc. 2011, 133, 12192.
[5] D. R. Roy, submitted (2011).
[6] D. R. Roy, submitted (2011).
8
Tuesday, Nov 29, 2011 at 9:00 AM
Planar Boron Clusters: hydrogen Analogues
Lai-Shang Wang
Department of Chemistry, Brown University, Providence, RI 02912
http://www.chem.brown.edu/research/LSWang/
Email: [email protected]
Experimental and computational studies in our group and collaborators over the pastt decade
have shown that boron clusters possess planar or quasi-planar structures [1 -4], in contrast to the
structures of bulk boron, which is dominated by three-dimensional cage-like building blocks. All planar
or quasi-planar boron clusters are observed to consist of a monocyclic circumference with one or
more interior atoms. The propensity for planarity has been found to be due to both ơ- and \pi-electron
delocalizations throughout the molecular plane giving rise to the concepts of ơ- and \pi double
aromaticity. The \pi bonding in the planar boron clusters has been found to follow the Hückel ruls for
aromaticity (4n + 2) and all the planar boron clusters can be viewed as analogues of hydrocarbons
(Fig. 1) [5]. Among the planar boron clusters, some display slight out-of-plane distortions, resulting in
quasiplanar structures. The main cause for the quasiplanarity is mechanical in nature, i.e., the
circumference of the cluster is too small to fit the interior atoms. Recently, we have investigated
planarization of two quasiplanar clusters, B7 and B12 , using isoelectronic substitution by A1[6]. The A1
atom is slightly larger in size and it can expand the circumference of a cluster to induce polarization.
Other effects of doping boron clusters will also be discussed.
References:
[1]. “Hepta- and Octa-coordinated Boron in Molecular Wheels of 8- and 9-Atom Boron Clusters: Observation and
Confirmation” (H.J.Zhai, A.N.Alexandrova, K.A.Birch, A>I.Boldyrev, and L.S.Wang), Angew.Chem. Int. Ed. 42, 6004-6008
(2003).
[2]. “Hydrocarbon Analogs of Boron Clusters: Planarity, Aromaticity, and Antiaromaticity” (H.J.Zhai, B.Kiran, J.Li, and
L.S.Wang), Nature Materials 2, 202-206 (2010).
[3]. A Photoelectron Spectroscopic and Theretical Study of B16- and B162-7- : An All-Boron Naphthalene” (a.P. Sergeeva,
D.YuZubarev, H.J.Zhai, A.I.Boldyrev, and L.S.wang), J.Am. Chem. Soc. 130, 7244-7246 (2008).
[4]. “A Concentric Planar Doubly /pi Aromatic B19- Cluster” (w.Huang, A.P.Sergeeva, H.J.Zhai, B.B.Averliev, L.S.Wang, and
A.I.Boldyrev), Nature Chem. 2, 202 -206 (2010).
[5]. “All-Boron Analogues of Aromatic Hydrocarbons: B17- and B18-” (A.P.Sergeeva, B.B. Averkiev, H.J.Zhai, A.I.Boldyrev, and
L.S.Wang, J.Chem.Phys., in press (2011).
[6]. “Planarization of B7- and B12- Clusters by Isoelectronic Substitution: AIB6- and AIB11-” (C.Romanescu, A.P.Sergeeva, W.L.Li,
A.I.Boldyrev, and L.S.Wang). J. Am. Chem. Soc., in press (2011).
9
Tuesday, Nov 29, 2011 at 9:45 AM
Sub-nanometer and Nanometer Size Clusters: Bridging the Size -and Pressure
Gap between Model and Practical Catalysts
Stefan Vajda
Materials Science Division and Center for Nanoscale Materials, Argonne National Laboratory,
Argonne, IL 60439, USA
&
Department of Chemical and Environmental Engineering, Yale University,
New Haven, CT 06520, USA
Email: [email protected]
The elucidation of the size/composition/shape/structure and function correlation, the effect of
support along with the determination of the nature of the catalytic particles under reaction conditions
are instrumental for addressing fundamental aspects of catalysis and for the design of new catalysts,.
1
Highly uniform particles on technologically relevant supports are prerequisites for such studies .
The experimental studies are based on 1) chemically uniform support fabrication, 2) size-selected
cluster deposition, 3) ex situ and in situ microscopies and 4) in situ synchrotron X-ray characterization
of clusters under working conditions, combined with mass spectroscopy analysis of the reaction
products. The experimental work is complemented with DFT calculations performed by collaborating
groups.
The main part of the lecture will outline, by using examples of selective bond activation, the
applicability of this approach in identifying optimal cluster sizes using model, sub-nanometer and
2-10
nanometer size-selected clusters.
As time allows, perspectives of using sub-nanometer size
clusters as building blocks for nanometer size aggregates will be discussed.
1. "The Impact of Nanoscience on Heterogeneous Catalysis", A.T. Bell, Science 299, 1688 (2003)
2. “Combined Temperature Programmed Reaction and in-Situ X-ray Scattering Studies of Size Selected Silver Clusters under Realistic Reaction
Conditions in the Epoxidation of Propene”, S. Vajda et al., J. Chem. Phys., 131, 121104 (2009)
3. “Selective Propene Epoxidation on Immobilized Au6-10 Clusters: The Effect of Hydrogen and Water on Selectivity and Activity”, S. Lee et al,
Angew. Chemie. Int. Ed. 48, 1467 (2009)
4. “Subnanometre Platinum Clusters as Highly Active and Selective Catalysts for the Oxidative Dehydrogenation of Propane”, S. Vajda et al.,
Nat. Mater. 8, 213 (2009)
5. “Combined TPRx, in situ GISAXS and GIXAS Studies of Model Semiconductor-Supported Platinum Catalysts in the Hydrogenation of Ethene”,
S. Wyrzgol et al, Phys. Chem. Chem. Phys. 12, 5585 (2010)
6. “Increased Silver Activity for Direct Propylene Epoxidation via Subnanometer Size Effects”, Y. Lei et al., Science 328, 224 (2010)
7. “Size-Dependent Selectivity and Activity of Silver Nanoclusters in the Partial Oxidation of Propylene to Propylene Oxide and Acrolein: A Joint
Experimental and Theoretical Study”, L. Molina et al., Catal. Today 160, 116 (2011)
8. “Cleavage of the C-O-C bond on Size-Selected Subnanometer Cobalt Catalysts and on ALD-Cobalt Coated Nanoporous Membranes”, W.
Deng et al, Appl. Catal. A: General 393, 29-35 (2011)
9. “Simultaneous Measurement of X-ray Small Angle Scattering, Absorption, and Reactivity: A Continuous Flow Catalysis Reactor”, S. Lee et al,
Nucl. Instr. and Meth. A, 649. 200-203 (2011)
10. “Size and Support-Dependent Reactivity of Subnanometer Cobalt Catalysts with CO and H 2: A combined GIXAS, GISAXS and TPRx Study",
S. Lee et al, Phys. Chem. Chem. Phys., invited article, submitted
10
Tuesday, Nov 29, 2011 at 11:00 AM
Theoretical Studies of Structural, Electronic, and Chemical Reactivity
Properties of Pure Pt and Mo, and Mixed Pt/Mo Nanocatalysts
Julius Jellinek
1,2
and Aslihan Sumer
2
1
2
Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, IL 60439, USA
Institute for Atom-Efficient Chemical Transformations, Argonne National Laboratory, Argonne, IL 60439, USA
E-mail: [email protected]
Results of an extensive density functional theory study on structural, electronic, and chemical
reactivity properties of pure Pt and Mo, and mixed Pt/Mo catalytic clusters will be presented and
discussed. The discussion will include analyses of isomeric forms, bonding energetics, and electronic
features of the pure clusters as a function of their size, and how these are affected by admixing Mo to
Pt clusters and vice versa. The issue of the energetically preferred homotopic conformations (particular
placement of different types of atoms among the cites of a given isomeric form) and the role of the
stoichiometric composition will be addressed as well. The chemical reactivity of the nanocatalysts will
be analyzed for the case of CO adsorption. The energetic and electronic aspects of this reaction will be
characterized as a function of cluster size, structure, and composition. The issues of site and coverage
dependence will also be addressed. The implications of our findings for the well-known problem of COpoisoning of Pt catalysts will be pointed out. Finally, we will remark on the role of the synthesis pathway
in defining the structural and, consequently, chemical reactivity characteristics of mixed Pt/Mo
nanocatalysts.
Acknowledgments
This material is based upon work supported as part of the Institute for Atom-efficient Chemical
Transformations (IACT), an Energy Frontier Research Center funded by the U.S. Department of
Energy, Office of Science, Office of Basic Energy Sciences, and by the Office of Basic Energy
Sciences, Division of Chemical Sciences, Geosciences and Biosciences, U.S. Department of Energy
under Contract No.DE-AC02-06CH11357.
11
Tuesday, Nov 29, 2011 at 11:45 AM
Controlling the Morphology of Supported Clusters by Substrate Doping
Shobhana Narasimhan
JNCASR, Bangalore, India
Email: [email protected]
For many applications, such as catalysis, small metal nanoparticles and clusters are placed on an
oxide substrate. These clusters can assume various shapes, which may be three-dimensional or twodimensional. In the literature, various approaches have been suggested to control the morphology.
Recently, we proposed a new strategy, that of doping the substrate. I will present results of
calculations performed using density functional theory, that show that the morphology of gold clusters
on a MgO substrate can be switched from a tetrahedral to a planar form, when the substrate is doped
with a small concentration of Al atoms. Recently, experimental proof of this concept has
beenobtained, by other groups. This work was performed in collaboration with Nisha Mammen and
Stefano de Gironcoli.
12
Tuesday, Nov 29, 2011 at 2:00 PM
Tailoring the catalytic properties of supported transition metal clusters
G.P. Das and S. Barman
Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur, Kolkata-700032,
INDIA
Email: [email protected]
Size selected transition metal clusters deposited on suitable substrates are being extensively
used for oxidative catalysis. The reactivity which is controlled by electronic structure can be tailored by
varying the cluster size. We shall discuss how first principles density functional investigation of
adsorbed Tungsten Clusters (W n, n=1-6) on anchored graphite (0001) surface. When CO and O 2
molecules are allowed to react simultaneously with the supported W 2 dimer, CO gets oxidized via the
detachment of O-O bond. For cluster size n4, the supported W n clusters adopt 3D geometry. We
shall show how W n clusters on the anchored graphite (0001) surface may serve as an ideal system for
converting CO to CO2.
13
Tuesday, Nov 29, 2011 at 2:45 PM
Growth of Nanoclusters on Support
Chiranjib Majumder
Chemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 094
Email: [email protected]
The subject of cluster physics has remained a fascinating field of research during past three
decades to underscore “how the behavior of substances evolve as it built up atom by atom.” In this
respect there is already a great deal of knowledge regarding the behavior of free clusters in the gas
phase. The interactions of clusters with a support is, however, crucial to understanding the behavior of
real catalysts since this will modify both the morphology and the electronic structure. Cluster
properties that may be influenced via substrate manipulations include: adsorption (adhesive)
energies, cluster geometries and di-mensionalities, cluster diffusion barriers and coalescence
propensity, electronic spectra, charge distributions, and chemical reactivities.
This presentation will focus on two aspects of deposited clusters; (i) the growth pattern of noble
metal clusters on metal oxide support and (ii) the higher catalytic efficiency of the deposited clusters
with respect to the free clusters. These studies have been carried out using the density functional
theory formalisms using the plane wave – pseudo potential approach. The primary objective of these
studies is to show the role of a substrate in changing the geometrical and electronic properties of
isolated clusters towards enhancing their catalytic efficiency.
References:
1.
U. Heiz and U. Landman, Nanocatalysis (Springer, New York, 2006).
14
Wedneesday, Nov 30, 2011 at 9:00 AM
Electronic Density of States at Fermi Level in the Extreme Two-Dimensional Limit
B. N. Dev
Department of Materials Science
Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India
Email: [email protected]
Surface contribution to electronic density of states and related properties like electronic heat
capacity and spin susceptibility would depend on the ratio of the number of surface atoms to the
number of volume atoms [1]. For an ultrathin metal film, i.e., for a two-dimensional system, the
electronic density of states (DOS) at the Fermi level would depend on the thickness of the film. We
have explored this aspect in thin Ag films on Si(111) substrates, where the film thickness varies from
a few atomic layers down to one atomic layer, by scanning tunneling spectroscopy experiments. The
observed thickness dependence is explained by analytical theory, simple free electron model
calculation and calculations using the density functional theory.
[1]. V. E. Kenner and R. E. Allen, Phys. Rev. B 11 (1975) 2858
15
Wednesday, Nov 30, 2011 at 9:45 AM
First-principles calculations of electronic properties of Carbon and Boron
Nitride based two-dimensional Nanomaterials
Sugata Mukherjee
S.N. Bose National Centre for Basic Sciences, Block-JD, Sector III
Salt Lake, Kolkata 700098, India
[email protected]
The electronic properties of Graphite, Graphene and hexagonal Boron Nitride was studied in detail
using first-principles method based on density functional theory (DFT) [1]. We further study the effect
of doping of Graphene either by Boron or by Nitrogen or co-doping by both Boron and Nitrogen using
ab-initio pseudopotential calculation. Our extensive banstructure and density of states (DOS)
calculations indicate that upon doping by Nitrogen (electron doping), the Dirac point in the
Graphene bandstructure shifts below the Fermi level and an energy gap appears at the high
symmetric K-point. In case of doping by Boron (hole doping) the Dirac point shifts above the Fermi
level and a gap appears. Upon co-doping of Graphene by Boron and Nitrogen, the energy gap
between valence and conduction bands appears at Fermi level and the system behaves as narrow
gap semiconductor. The energy gap depends sensitively on the thickness of the layers and on the
percentage of doping. These results will be discussed in the light of recent experimental
measurements[2,3].
[1]. T.P. Kaloni and S. Mukherjee, Mod Phys Lett B 25, 1855 (2011).
[2]. X. Wang et al Science 324, 768 (2009).
[3]. L. Cie et al Nature Materials 9, 430 (2010)
16
Wednesday, Nov 30, 2011 at 11:00 AM
In-Silico perspectives in Nano-bio-technology: The Convergence of Chemistry,
Biology and Physics
Avinash C. Pandey
Nanotechnology Application Centre, University of Allahabad, Allahabad-211002, India.
Email: [email protected]
The primary drift in current scientific trend is to blob the restrictions not only between the
subareas of a discipline but also between different disciplines. For an example, now-a-days, we all
have accepted the term, “Nano-bio-technology” which gives an idea that chemical, biological, physical
and computational sciences can work together and advance our understanding of these fields in ways
that were hard to imagine even a few decades ago. In this talk, I shall cover the different aspects of
nano-bio-technology that were taken up in our laboratory. For instance, we have made noteworthy
progression in the multifunctional magnetic naoparticle based biomedical applications and have
developed some novel MRI contrast agents. We have demonstrated that when nanomagnets are
deployed in Magnetic Resonance Imaging (MRI), a high contrast of cancerous tumor images in mice
is revealed. Upon concentrating the nanomagnets with the use of external magnetic field, the MRI
showed extraordinary details of the tumor. The rare earth based novel luminescent nanomagnets
were synthesized for the first time. These novel luminescent nanomagnets were functionalized with
folic acid as a ligand for several over expressed folate receptors on cancer cells and methotrexate
(MTX) as a dihydrofolate reductase inhibitor. MTX is a chemotherapeutic drug that can target many
cancer cells whose surfaces are over expressed by folate receptors. Drug release experiments
demonstrated that MTX was cleaved from the nanoparticles under low pH conditions mimicking the
intracellular conditions in the Lysosome. In-vitro studies of the first ever room temperature
3+
synthesized Gd2S3:Eu nanoparticles modified with cytosine show better accumulation of blood
platelets as compared to unmodified one posing them a potential candidate for platelet isolation from
the plasma for different applications and a way for easy separation and simultaneous visualization of
blood platelets from the mice-blood platelet rich plasma is proposed. The low-temperature synthesis
of quantum size gadolinium monosulfide nanoparticles has been achieved and their pathogen capture
efficiency has been demonstrated. The origin and root cause of such strong magnetic behavior of the
host Gd2S3 nanoparticles were studied with the help of electronic structure calculations of Gd 2S3
cluster and fragments/nanostructures of -Gd2S3 of various sizes. Nanoparticles of Gd2S3 were
simulated by considering the bulk fragments of -Gd2S3 which are spherical for two different radii,
resulting in Gd8S16 and Gd12S20 (sub nanometer) clusters. Spin polarized geometry optimization
calculations were carried out on the Gd2S3, Gd8S16, and Gd12S20 clusters without any symmetry
constraints in the framework of generalized gradient approximation to density functional theory (GGADFT) using the DMol3 software package. For Gd 2S3 and Gd8S16 clusters the AFM and FM states are
energetically degenerate with AFM being lower in energy ( = 0.01 – 0.02 eV), while in case of
Gd12S20 cluster, the FM state is more favorable ( = 0.11 eV). We have demonstrated the use of one
of the nucleobases, „cytosine‟ as a new capping agent in controlling the size of the CdSe
nanoparticles. Electronic structure calculations, based on the density function theory, were performed
on Cd16Se16 sub nanometer clusters representing a CdSe QD. The cytosine capped CdSe QD is
predicted to be stable, capping does not appear to change the electronic structure near the Fermi
energy of as-prepared CdSe QD.
Furthermore, a novel procedure for visible detection and separation of thiols and disulfides has
been described. The controlled drug release characteristics and enhanced antibacterial effect of
graphene nanosheets containing drug (gentamicin sulfate) have been also investigated. So, these
nanoparticles could be used for labelling and manipulating biomolecules, in vivo as site specific drug
delivery agents, sensing, cell sorting, bio-separation, magnetic resonance imaging (MRI), as well as
hyperthermia treatment. But before the clinical trials scientists working in the disciplines of chemistry,
biology, physics and computational studies should come together to understand the properties of the
nanoparticles in better way.
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Wednesday, Nov 30, 2011 at 11:45 AM
Clusters in Hydrogen storage: From catalyst to an efficient sorbent
Abhishek Kumar Singh
Materials Research Centre, Indian Institute of Science, Bangalore 560012 India
Email: [email protected]
Hydrogen has emerged as most promising carrier of clean energy due to possibility of its
generation from renewable sources. Hydrogen sorption on graphitic receptors via catalytic spillover
process has been considered as effective way to store hydrogen. The fundamental question that how
would an H atom binds to graphene if it is more favorable for it to form H 2, however, remains open.
Using ab initio calculations we show that it is indeed not possible to get hydrogen sorbed on a pristine
graphene. Presence of a phase of hydrogenated graphene makes the spillover thermodynamically
and kinetically favorable. Furthermore, we demonstrate the high hydrogen storage capacity of
metallacarborane clusters. The transition metal (TM) atoms in the metallacarborane can bind up to 5
H2‟s with the average binding energy of ~0.4 eV/H, which lies within the reversible adsorption range.
Among the first row TM atoms Sc and Ti are found to be maximize the H2 storage (~8 wt%) on the
metallacarborane. Used as a linker in MOF design and being integral part of the MOF cage, TMs do
not suffer from the clustering problem, which has been a biggest hurdle for the success of TM
decorated graphitic materials.
References:
1. K. Singh, M. A. Ribas, and B. I. Yakobson, H-Spillover through the catalyst saturation: An
Ab Initio Thermodynamics Study, ACS Nano 3, 1657 (2009).
2. K. Singh, A. Sadrzadeh, and B. I. Yakobson, Metallacarboranes: Toward Promising
Hydrogen Storage Metal Organic Frameworks, JACS 132, 14126 (2010).
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Wednesday, Nov 30, 2011 at 2:00 PM
Clusters at Finite Temperatures
Kavita Joshi
Center of Excellance in Scientific Computing, Physical Chemistry Division,
National Chemical Laboratory, Pune, 411008 India.
Email: [email protected]
Melting in finite size systems is different compared to the melting in bulk systems. Unlike bulk, in
clusters the transition is not well defined but spread over range of temperatures. There is a dynamic coexistance of solid-like and liquid-like phases. Further, it is generally expected that these systems melt at
much lower temperatures compared to their bulk counter parts because of the higher surface to volume
ratio. However, small gallium and tin clusters are exceptions to this rule of thumb. Recent experiments
measuring heat capacities of Ga clusters in the range of 30-55 brought out many interesting and some
counter intutive observations. These clusters melt at much higher temperatures compared to the bulk
melting temperature of Ga (303K). Moreover, their finite temperature behavior is highly size sensitive.
Some clusters are called magic melters because of the distinguishable peak in their heat capacities
whereas others, termed as non-melters, have no distinguishable peak in the heat capacity curve.
Another interesting observation is the variation in the melting temperature as a function of the size the
cluster. For example, the melting temperature varies by more than 300 K for gallium cluster cations with
30-50 atoms. In this talk we discuss the origin of this behaviour. Our simulations reveal that the higher
than bulk melting temperatures of Ga clusters are artifacts of the change in the nature of bonding. The
finite temperature simulations of Ga30 and Ga31 demonstrate that the magic melters have a geometric
origin. We will also present some recent results on Al and other Ga clusters.
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