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Jennifer Aitken, Ph.D.

Associate Professor
Bayer School of Natural and Environmental Sciences
Chemistry & Biochemistry

Mellon Hall 302A
Phone: 412.396.1670

http://www.scienceresearch.duq.edu/chem/chemdept/aitken.html

Education:

B.S., Rider University
Ph.D., Michigan State University
Post Doctoral Fellow, Wayne State University
Research

 

Solid-State Inorganic Materials Chemistry

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.