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Keywords: Crystal Structure, Semiconductors,
Structure-Property Relationships, Magnetism, Spintronics,
Second Harmonic Generation, Non-Linear Optics,
Molten Flux Synthesis, Solvothermal Synthesis,
Hydrothermal Synthesis, Crystal Growth, Phosphides,
Antimonides, Sulfides, Selenides
Overview
Our laboratory focuses on the synthesis,
structure and physicochemical characterization
of new inorganic solid-state materials. In particular
we are focusing on phosphides, antimonides, sulfides
and selenides. We are investigating several synthetic
strategies to further develop these classes of
solid-state materials. The underlying theme in
our research is the quest for novel materials
with unique technologically useful properties.
From an academic perspective we wish to develop
the chemistry of these systems. In studying structure-property
and composition-property relationships among these
new materials, we should be better able to predict
and design new materials with desired properties.
We have identified several areas, discussed below,
in which we take an exploratory approach to new
materials followed by a developed understanding
of the systems and an ultimate predictability
in the chemistry.
New Diamond-Like Semiconductors
with Novel Magnetic and Optical Properties
New semiconductors with unique
qualities and combinations of properties are constantly
needed. Our laboratory is pursuing the synthesis
and study of new, diamond-like semiconductors
(DLS) possessing novel magnetic and optical properties.
Diamond-like semiconductors (DLS) are normal valence
compounds based on the structure of diamond. For
example, InP is an ordered variant of the diamond
structure in which half the carbon sites are occupied
by In and the other half by P in an orderly fashion.
Further ordered substitutions on the cation and
anion sites lead to ternary and quaternary DLS.
Reports of quaternary DLS are scarce and their
properties are virtually unexplored. The motivation
for further research in DLS is the unique optical
and magnetic properties expected.
Derivation
of diamond-like
semiconductors by cross-subtitution.
Evolution from InP to CdGeP2 to CdIn2GeP4.
All diamond-like semiconductors
possess a noncentrosymmetric crystal structure,
which is the first criterion for second harmonic
generation (frequency doubling of light). In the
past decade, the ternary diamond-like, chalcopyrite
semiconductors have come into prominence because
of their potential for nonlinear optical, photovoltaic
and luminescent applications. One emerging area
of interest is diluted, magnetic semiconductors
(DMS) because of the manner in which the magnetic
behavior can modify and complement the semiconductor
properties.
Noncentrosymmetric
Crystal structure
of InP viewed down the [101] direction
showing all tetrahedra pointing in the same
direction.
Compounds with
tetrahedral structures represent only a small
group of inorganic compounds but they assume a
unique position since they are one of the rare
groups of inorganic compounds for which all possible
chemical compositions can be calculated and for
which a set of possible structures can be postulated.
There are several rules, including valence electron
rules and Pauling's 1st and 2nd
rule, which must be obeyed in order for a compound
to possess a diamond-like structure. While maintaining
the diamond-like structure, we are altering the
compositions of these compounds and expecting
to find the enhancement or realization of useful
properties.
We are synthesizing new II-III2-IV-V4
and I-III-IV2-V4 pnictides.
These materials may exist as discrete compounds,
or a whole range of solid solutions may be accessible,
which can be expressed as II-IV-V2:III-V.
The compositional flexibility of these systems
allows for the tuning of optical properties, where
the formation of a series of solid solutions leads
to new materials with a wide range of band gaps.
For example, if the nonlinear optical response
of the compound is large and the material can
be phase-matched, the compositional flexibility
can be exploited to tune the bandgap to the desired
region of the electromagnetic spectrum. We are
measuring the non-linear optical properties of
these materials in collaboration with the research
group of Dr. Shiv Halasyamani from the University
of Houston. We will also prepare diluted magnetic
semiconductors based on these new materials.
Two synthetic methods: (1) traditional solid-state,
high temperature reactions and (2) salt or metal
flux synthesis are used to pursue these new materials.
We are also involved in the crystal growth and
characterization of the resulting new materials.
Conclusions will be drawn concerning structure/composition-property
relationships.
Diluted Magnetic Semiconductors
As we move further towards
the miniaturization of electronic and memory devices
we look for multifunctional materials. One such
area emerging from this rationale is the field
of spintronics, where researchers wish to exploit
not only the charge carriers of a material but
also the spin of those charge carriers. A material
with room-temperature ferromagnetism and an existing
technology base for use in applications would
be an ideal candidate for spin-based devices.
The goal of our project is to predict and synthesize
new diluted magnetic semiconductors with technologically
useful properties. Diluted magnetic semiconductors
(DMS) are, by definition, semiconductors in which
one or more cations of a semiconductor are partially
substituted by a magnetic ion. A sizable amount
of work has been done in the area of binary semiconductors,
namely the II-VI and III-V based systems, for
example CdTe:Mn and GaAs:Mn. In the II-VI systems,
the diluted magnetic semiconductors are usually
antiferromagnetic or spin glass. In the case of
the III-V based DMS materials, ferromagnetic behavior
is observed; however, the magnetic transition
temperatures (Tc) are far below room temperature
limiting their practical application in spintronic
devices, for example 110 K for GaAs:Mn. Furthermore,
only a small concentration of Mn can be incorporated
in these materials.
We are synthesizing new, ternary diluted magnetic
semiconductors with the chalcopyrite structure,
which we believe will possess interesting and
technologically useful properties. In the course
of our investigations we wish to study the effect
of the magnetic-ion concentration and the choice
of magnetic dopant on the magnetic, structural,
thermal, electronic and optical properties of
these new materials. To complement this time-consuming
synthetic avenue we are incorporating solid-state
electronic structure methods. Theoretical calculations
will help us gain fundamental insight to these
systems, as well as guide us in selecting which
systems will be the most promising. We are synthesizing
these new materials via simple high-temperature
solid-state reactions. We are characterizing these
materials and comparing our findings to the calculated
properties. This will help to fine-tune our calculations,
which will then be used to look into many more
new DMS systems.
Crystal structure
of Chalcopyrite
This research stands at the
cross-roads between chemistry, physics and engineering
and exposes the graduate and undergraduate students
in my laboratory to characterization methods such
as powder X-ray diffraction, magnetic susceptibility,
scanning electron microscopy and solid state electronic
structure methods. Dr. Jeffry Madura from the
Department of Chemistry and Biochemistry here
at Duquesne is working together with us on a computational
approach to finding diamond-like semiconductor
materials with enhanced physical properties. Dr.
Monica Sorescu from the Physics Department at
Duquesne University has extensive experience in
magnetic measurements and is working closely with
us on the magnetic property measurements of these
materials. The results of this project will provide
some insight towards where we should look in the
future for new diluted magnetic semiconductor
materials.
Development of a New Class
of Solid-State Compounds
Our laboratory is also working
on the synthesis and characterization of oxothiophosphate
materials. Oxothiophosphates are compounds that
contain oxidized phosphorus bound to both oxygen
and sulfur. There is a practical paucity of oxothiophosphates
in the literature, especially considering the
overwhelming number of (oxo)phosphate and thiophosphate
relatives. Explorations of oxothiophosphates are
warranted because of the interesting structural
chemistry and physicochemical properties expected.
Since few oxothiophosphates have been synthesized,
we are relying heavily upon the established oxo-
and thiophosphate chemistry to aid us in developing
our synthetic methodologies. Therefore, we are
pursuing four synthetic strategies for the discovery
of new oxothiophosphates: (1) high temperature
solid-state, (2) molten flux, (3) solution, and
(4) solvothermal syntheses. In many cases, each
technique is expected to yield unique materials
not obtainable via the other methods. Solvothermal
synthesis using structure-directing organic amines
is expected to yield the first inorganic/organic
hybrid materials based on oxothiophosphate ligands.
 
The cyclic
oxothiophosphate ligands
[P4O8S4]4-
(left) and [P3O6S3]3-
(right).
O atoms are blue, sulfur atoms are yellow and
P atoms are purple.
The new oxothiophosphates
will be studied both structurally and physicochemically.
The structures of these new compounds will be
compared and contrasted and correlations between
their structures and the ratio of O:S in their
anions will be made. In the case where the ratio
of O:S can change while maintaining the same structure
we can tune in the properties of the resulting
materials, for example band-gap energies. Together
the new compounds will be studied as a class and
generalizations about structure-property and composition-property
relationships will be proposed. In addition, similarities
to and differences from the all oxygen and all
sulfur chemistry will be examined.
Laboratory Equipment
To see some of the state-of-the-art
laboratory equipment available in the Aitken Laboratory
click here.
Recent Publications
Jayasekera, B.; Aitken, J. A.;
Heeg, M. J.; Brock, S. L "Towards an Arsenic
Analog of Hittorf's Phosphorus: Mixed Pnictogen
Chains in Cu2P1.8As1.2I2"
Inorg. Chem. 2002, 42, 658-660.
Aitken, J. A.; Evain, M.; Iordanidis,
L.; Kanatzidis, M. G. "NaCeP2Se6,
Cu0.4Ce1.2P2Se6,
Ce1.33P2Se6 and
the Incommensurately Modulated, AgCeP2Se6:
New Selenophosphates Featuring the Ethane-Like
[P2Se6]4- Anion"
Inorg. Chem. 2002, 41, 180-191.
Aitken, J. A.; Larson, P.; Mahanti,
S. D.; Kanatzidis, M. G. "Li2PbGeS4
and Li2EuGeS4: Polar Chalcopyrites
with a Severe Tetragonal Compression" Chem.
Mater. 2001, 13, 4714-4721.
Aitken, J. A.; Kanatzidis, M.
G. "New Information on the Na-Ti-Se Ternary
System" Z. Naturforsch 2001, 56b,
49-56.
Aitken, J. A.; Kanatzidis, M.
G. "alpha-Na6Pb3(PS4)4,
A Noncentrosymmetric Thiophosphate with the Novel
Saucer-Shaped [Pb3(PS4)4]6-
Cluster, and Its Metastable, 3-Dimensionally Polymerized
Allotrope beta-Na6Pb3(PS4)4"
Inorg. Chem. 2001, 40, 2938-2939.
Aitken, J. A.; Canlas,
C.; Weliky, D. P.; Kanatzidis, M. G. "[P2S10]4-:
a Novel Polythiophosphate Anion Containing a Tetrasulfide
Fragment" Inorg. Chem. 2001, 40, 6496-6498.
Please click here to visit my personal web page.
Office Phone: 412.396.1670
Email: aitkenj@duq.edu
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