TECHNICAL EXPLANATION This proposal aims to synthesize novel one-dimensional nanomaterials based on technologically and fundamentally important rare earth sulfides, such as ferromagnetic semiconducting europium sulfide (EuS) and intermediate valence metal samarium sulfide (SmS), and study their unique physical properties. These chemically synthesized single crystalline nanomaterials would enable both potential applications in spintronics and fundamental physical studies in nanoscale structures with highly correlated electrons. Taking a multidisciplinary approach, the PI and his co-workers will develop organometallic precursors to synthesize uniform or mixed rare earth sulfide nanowires by chemical vapor deposition, characterize the nanoscale structures and physical properties of the resulting nanomaterials using a variety of techniques in order to understand the synthesis-structure-property relationship. They will further investigate the relationship between ferromagnetism and semiconductivity of EuS nanowires and examine the size evolution of the fundamental properties. The PI also proposes to build an integrated educational program in materials chemistry by developing a graduate level course in nanomaterials chemistry, by encouraging and incorporating minority and underrepresented undergraduate students to participate in scientific research, and by collaboratively developing a Web based course on nanoscience and nanotechnology for high school teachers so that teachers can have the toolbox needed to incorporate elements of cutting edge science and technology into their curriculum.
NON-TECHNICAL EXPLANATION Nanometer scale wires (nanowires) that are made of technologically useful and fundamentally important rare earth sulfide materials will be created, and their unique physical properties will be investigated for a variety of potential applications, such as the next generation nanoscale spin electronic ("spintronic") and magento-optical information storage devices. Despite the interesting combination of magnetic and semiconducting properties that rare earth sulfide materials possess, materials issues remain to be addressed. This interdisciplinary research proposal will lay the scientific foundation to enable the creation and understanding of these high quality nanoscale materials. Furthermore, an integrated educational program in materials chemistry will be built by developing a new graduate level course in nanomaterials chemistry, by encouraging and incorporating minority and underrepresented undergraduate students to participate in cutting edge scientific research, and by collaboratively developing the first Web based course on nanoscience and nanotechnology for high school teachers so that teachers can have the toolbox needed to incorporate elements of cutting edge science and technology into their curriculum. All of these will stimulate and engage the interest of students in science and technology and help to prepare the new generation of scientific and technological workforce.
We have discovered a nanowire (NW) growth mechanism driven by axial screw dislocations, (which are a type of crystal defects) that is fundamentally different from the traditional mechanisms using metal catalysts. The self-perpetuating steps of a screw dislocation spiral provide the fast crystal growth front under low supersaturation (see attached Fig. B) to enable the anisotropic crystal growth of 1D NWs. The fast growing NWs driven by dislocations, when combined with the slower VLS-driven epitaxial overgrowth of NW branches, result in unprecedented "Christmas tree" nanostructures of lead sulfide (PbS) (see attached Fig. A). These trees with rotating branches are the clearest demonstration of "Eshelby twist" — the rotation of a crystal lattice around a screw dislocation as the consequence of its associated stress. We have further elucidated the growth and direct formation of single-crystal inorganic nanotubes due to screw dislocations with large magnitude. By rationally controlling the supersaturation using a continuous flow cell reactor, nanotubes (NTs) and NWs of zinc oxide (ZnO) can be controllably grown from solution without catalysts. With these NTs, we conclusively showed that an axial screw dislocation provides 1D anisotropic crystal growth, and its associated strain energy is large enough to favor the creation of a new internal surface along the dislocation core, leading to spontaneous formation/growth of a hollow tube. Dislocation-driven growth is a general mechanism that should be applicable to many nanomaterials grown in solution or vapor phase but has been greatly under appreciated. Furthermore, we are using dislocation mechanism and elasticity theory to explain the spontaneous formations of plate-like structures and helical nanostructures and other intriguing crystal growth phenomena. They will provide the general and unifying concepts for many nanomaterial morphologies that commonly observed but are often unconvincingly explained. Our research illustrate the general theoretical framework in which one can rationally design the catalyst-free synthesis of various 1D nanomaterials, especially from solutions. NWs and other 1D nanomaterials possess interesting properties that have already found many applications in nanoelectronics, nanophotonics, sensing, solar energy conversion, thermoelectrics, and energy storage. The fundamental understanding of 1D nanomaterial growth such as that developed in this project is critically important to developing rational and controllable synthesis that advances these applications. Our discoveries and continuing fundamental study will create a new dimension in the rational design and synthesis of nanomaterials. It will enable applications of novel complex hierarchical nanostructures in solar energy harvesting and 3D electronics. We have identified the general guidelines to promote dislocation-driven nanomaterials growth, namely by controlling the low supersaturations, which will open up the exploitation of large scale/low cost solution growth for rational catalyst-free synthesis of 1D nanomaterials for a variety of large-scale applications, such as those for renewable energy. We have also successfully prepared pure and doped ferromagnetic semiconducting EuS nanocrystals using single source precursors. Magnetometry (SQUID) and X-ray magnetic circular dichroism (XMCD) measurements confirm their magnetic properties. These novel nanomaterials and fundamental understandings can help the development of spintronic devices that promise to enable faster computing with less energy cost.
