The ability to develop in a single material, ferromagnetic and semiconducting functionalities which are desirable for spintronics as well as thermoelectric devices is a great challenge in solid state chemistry. This CAREER grant supported by the Solid State and Materials Chemistry (SSMC) program seeks to exploit the marked structural features of synthetic and naturally occurring ternary and quaternary heavy main-group metal chalcogenides with general composition Pb(n+2)Sb2(m+1)Q(n+3m+5 and MtPb(n-t+2)Sb2(m+1)Q(n+3m+5) (M = Mn, Fe, Co; Q = S, Se, Te) to: (i) investigate the stability of the framework structure upon changing the nature of transition metal elements and/or the nature of chalcogen atom; (ii) investigate how the magnetism and the charge transport properties of this family of compounds change with modification of the composition and structure; and (iii) understand the correlation between the charge carrier's concentration and mobility and the coupling between magnetic centers in the structure. This will be done by combining two approaches: (a) the nature and stoichiometry of elements within a given structure type will be systematically varied and their effects on the magnetism and electronic transport of the resulting materials will be assessed, (b) for a given system, phases with structures closely related to each other (pseudo-homologous series) will be used to investigate the effects of structural evolution (changes in structure type within the homologous series) on the magnetism and charge transport properties of the materials.
NON-TECHNICAL SUMMARY:
This research program aims to develop ferromagnetic and semiconducting functionalities in a single material. The availability of such materials could impact the architecture of modern computers where an entire computer is built on a single semiconducting chip using a device capable of electronically writing and reading information to and from a magnetic material. Additionally, the materials can also be used to build a device capable of cooling the whole computing system. The multi-component educational program proposed in this CAREER grant will provide research and teaching experiences to K-12 teachers, high school students, undergraduate students, graduate students and postdoctoral associates. Graduate students and postdoctoral researchers will receive outstanding training on state-of-the-art techniques in high temperature solid-state synthesis, X-ray diffraction and physical properties measurements. They will also be involved in data analysis as well as preparation of manuscripts for publication. Although, the University of New Orleans (UNO) is located in an "urban city" with about 58% of the population from African American origin, the number of high school seniors from this group entering higher education is still low. The proposed Community Services and Learning Experiences for Chemistry Freshmen program will provide hands-on experience and training to current UNO Chemistry freshmen while simultaneously stimulating interest among high school students (sophomores, juniors and seniors) in chemistry.
Intellectual Merit: In this work, we exploit the concept of structural homology to develop new ferromagnetic semiconducting structures encompassing ordered arrays of the magnetic atoms embedded within the semiconducting network of non-magnetic atoms and to investigate their crystal structure as well as their magnetic and semiconducting properties. This strategy resulted in the discovery of complex structures with tremendous compositional flexibilities such as MPn2Se4 (M = Mn, Fe; Pn = Sb, Bi); (M2-xPbx)Pb2Sb4Se10 (M = Mn, Fe) ; FePb4Sb6Se14; and MnPb16Sb14S38. The MPn2Se4 structure (Figure 1) for example represents a fascinating class of ternary compounds with tremendous diversity of magnetic and electronic properties. We have synthesized several compositions of the MPn2Se4 phases and investigated their crystal structure, magnetic properties and electronic transport properties. We find that, despite the structural similarity of these phases, their magnetic behavior is defined by the nature of magnetic atoms within the [MnSe4n+2] chains of edge-sharing octahedra. For instance, the dominant type of magnetic ordering in MPn2Se4 can be tuned from ferromagnetic to antiferromagnetic behaviors by altering the transition metal atoms within the [MSe6] chain from Fe to Mn. Likewise, the dominant charge carrier type in MPn2Se4 can be manipulated by altering the composition of the [Pn2Se] network separating adjacent [MSe6] chains in the structure. For example, substituting Sb for Bi according to the Pn sites in MPn2Se4 induces drastic change in the conduction type from p-type for MSb2Se4 to n-type for MBi2Se4. This ability to independently control the magnetic ordering and carrier type within the same crystal lattice of MPn2Se4 compounds through chemical manipulation of the composition of various structural subunits enables us to create both n-type and p-type ferromagnetic semiconductors with Curic temperature (Tc) above 300 K and to examine the mechanism of carrier-mediated ferromagnetism in spintronic materials. Broader Impact: Over the past five years, this project has provided a broad range of training in solid-state materials research to several students. The graduate student, Honore Djieutedjeu (PhD Fall 2013, MSE, UM) working on this project has co-authored nine publications (five as first author) and has produced enough data for at least five additional manuscripts that are currently in working progress. Dr. Djieutedjeu continued for 1 year, as a postdoctoral associate on this project and is currently a visiting lecturer at the University of Indiana Southeast. A new graduate student Mr. Juan Lopez is continuing working on the MBi2Se4 analogs systems and related solid solutions. The postdoctoral researcher supported for 18 months through this Career award (Dr. Sanjaya Ranmohotti) has co-authored four papers (two as first author). Dr. Ranmohotti is currently an Assistant Professor at Governors State University. In addition, this Career Award has directly impacted the training of 23 undergraduate students, 04 high school students and 01 high school teacher. In this respect, the most notable achievements is by Juan Callejas who was supported by this Career grant as a full time undergraduate student for two years (2010 and 2011). Juan Callejas won several awards within the department of chemistry at the University of New Orleans (UNO) and received outstanding oral presentation prizes (Rice University, 2010 and University of Memphis, 2011) at regional undergraduate research symposia. Juan is now a Chemistry graduate student at Penn State (since fall 2012). One of the high school students, Arianna Rivera won the Louisiana State University (LSU) LA-STEM Research Scholars scholarship, which will support her for four years at LSU. Another high student, Omkar Shende, was ranked first place "Engineering" at the Science and Engineering Fair of Metro Detroit (March 2014).
