[Research
Home][Nanocrystals Home]
Isolated and Collective Magnetic
Phenomena in Metallic Nanocrystals and their Superlattices
Funding: NSF/DMR #0203069 (07/01/02 - 06/30/05)
Project summary:
In this project we were studying fundamental issues
involving the synthesis, assembly and magnetic behavior
of metallic cobalt nanocrystals (NCs). We developed
a comprehensive method for the reproducible chemical
synthesis of mono-disperse, metallic, passivated cobalt
nanocrystals with good size/shape control [K1]. We
then investigated the preparation of controlled self-assembled
superlattices from this single component nanocrystals
system by a systematic variation of their size, shape
and inter-particle interactions. We discovered that
the self-assembly of these arrays can be tuned to
selectively achieve square packing, hexagonal close
packing, linear chains, spatially segregated arrays
as a function of particle size and lyotropic liquid-crystal-like
arrays with orientation order. This richness in self-assembly
is obtained when any one of a set of competing forces
(steric, van der Waals, depletion, or magnetostatic)
are chosen to dominate and determine the resulting
organization [K2, K3]. In addition to transmission
electron microscopy [K4], arrays of NCs were also
characterized by small-angle x-ray scattering (SAXS)
and a free energy model developed to describe the
NCs self-assembly process was shown to be in good
agreement with the SAXS experiment [K5]. The nature
of the dipolar, magnetostatic coupling between spherical,
ferromagnetic NCs was elucidated by high-resolution
electron holography measurements [K6]. A variety of
SQUID-based magnetic measurements have shed light
on the surface spin-glass transitions in individual
nanocrystals [K7], their stability [K8], the effects
of surfactant friction on Brownian magnetic relaxation
in these NC ferrofluids [K9] and the nature of inter-particle
interactions in two-dimensional arrays. We have also
initiated coherent, resonant, x-ray scattering measurements
at the Advanced Light Source to determine details
of the magnetic correlation in such nanocrystal arrays.
Recently, we have discovered a general approach to
synthesize binary-element nanoparticles with controlled
morphologies and magnetic properties [K10]. This "discovery"
forms a substantial part of the NSF/DMR renewal application
and will serve as a springboard for a number of experiments
that are being proposed. The work being carried out
in this project has also been recognized to be at
the forefront of research in this field by a number
of invited/plenary talks at symposia of leading professional
societies [Annual Magnetism and Magnetic Materials
Conference(11/01 & 11/04), American Vacuum Society
(11/02), Materials Research Society (12/03), International
Conference on Scientific and Clinical Applications
of Magnetic Carriers (6/04), APS Nanomagnetism Workshop
(8/04) etc.]. In addition to the above, this grant
will also lead to one doctoral thesis in Materials
Science (Ms. Yuping Bao, Doctoral dissertation entitled
"Synthesis, self-assembly and magnetic behavior
of metallic nanocrystals", expected Autumn, 2005).
Other personnel, partially supported by this grant
include graduate students M. Beerman (Ph.D. Candidate
in MSE), Ms. M. Gonzales (Ph.D. Candidate in MSE),
Dr. A. Pakhomov and undergraduate students J. Silvia
(received the Barry Goldwater fellowship in 2003)
Chris Paetz (Sophomore, MSE) and Michael McCrae (Senior,
Chemistry).
Recent Publications:
M. Beerman, A.B.Pakhomov, Y. Bao
and Kannan M. Krishnan, "Influence of oxidation
on the susceptibility of ?-Co nanoparticles",
APL (submitted)
Yuping Bao, Michael Beerman and
Kannan M. Krishnan,
"Controlled self-assembly of colloidal cobalt
nanocrystals mediated by magnetic interactions",
Jour. Mag. Mag. Mat. 272-276, E1367-E1368 (2004)
Gao YH, Bao YP, Beerman M, Yasuhara
A, Shindo D, Krishnan KM,
"Superstructures of self-assembled cobalt nanocrystals",
Appl. Phys. Lett 84 (17), 3361-3363 (2004)
Recent Oral Presentations:
M. Beerman, "Structure of Cobalt
Nanosphere Superlattice Films by Small Angle X-ray
Scattering," Oral presentation, Materials Research
Society Spring Meeting 2004
Kannan M. Krishnan, University of
North Carolina, Chapel Hill, Dept. of Physics, "Synthesis,
self-assembly and magnetism of shape-controlled cobalt
nanocrystals and their superlattices", December,
2003
Kannan M. Krishnan, MRS Fall Meeting,
"Synthesis, Self-assembly and Magnetism of Cobalt
Nanospheres and Nanodisks" , Invited talk, Boston,
December, 2003
Kannan M. Krishnan, National Institute
of Materials Science, Tsukuba, Japan, "Metallic
nanocrystals and their superlattices: Controlled synthesis,
magnetism and technological applications", June
29, 2003
Kannan M. Krishnan, University of
Nebraska, Center for materials research and analysis
and Department of Physics, "Magnetism and self-assembly
of shape-controlled cobalt nanocrystals and their
superlattices", May 23, 2003
Kannan M. Krishnan, University of
Southampton, England , Dept. of Physics, "Magnetism
and self-assembly of shape-controlled cobalt nanocrystals
and their superlattices", March 17, 2003
Kannan M. Krishanan, "Magnetism
and self-assembly of shape-controlled cobalt nanocrystals
and their superlattices", Argonne National Laboratory,
Materials Science Colloquium, February 17, 2003
Kannan M. Krishnan, Japanese JISTEC
Workshop, Invited talk on "Magnetism and nanostructures:
synthesis and characterization at relevant length
scales", November 26, 2002
Surfactant Mediated
Shape Control, Magnetism and Self-assembly of Cobalt
Nanocrystals (in PDF format), invited talk given
by Prof. Krishnan at the annual meeting of the American
Vacuum Society, Denver, CO Nov. 2002
Kannan M. Krishnan, University of
Sheffield, Department of Physics, "Nanomagnetism",
March, 2002
Kannan M. Krishnan, Carnegie Mellon
University, Department of Materials Science Colloquium,
"Magnetic small particles and nanostructures",
February, 2002
Recent Poster Presentations:
Low
Temperature Magnetization Behavior of Epsilon-Cobalt
Nanosphere Systems (in PowerPoint format), Michael
Beerman, ICM 2003, Rome
Controlled
Self-assembly of Colloidal Cobalt Nanocrystals
(in PowerPoint format), Yuping Bao, ICM 2003, Rome
[Research
Home][Nanocrystals Home]
