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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.
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