Department of Materials Science and Engineeringclos

Jo, Seungki

Department of Materials Science and EngineeringChalcogens and metal chalcogenides have been regarded as the most promising classes of materials for optoelectronic, thermoelectric, and electronic applications because of their outstanding electrical properties and diverse functionalities. Moreover, the fabrication of thin films with a suitable composition and microstructure in these materials has been of great importance to extend their applicability into thin film-based devices. Among various fabrication methods of thin films, a solution-phase deposition of chalcogenides thin films provides a convenient route to fabricate high-quality thin-film semiconductors in a cost-effective and scalable manner. The recent development of the alkahest solution chemistry allows for a facile dissolution of diverse inorganic semiconductors and thus enables to synthesize the ink solutions. Although the basic chemistry of the alkahest synthesis has been proposed, the available classes of materials and the functional applications are still limited. Herein, I report the synthesis of the soluble chalcogens and metal chalcogenides including Te-, Sb2Te3-, SnSe-, and Ag2S-based anions, which have been used as the inks for the fabrication of thin films or as the additive for the interface control for thermoelectric and electronic applications.

For the thermoelectric applications, firstly, a synthesis of interface-engineered BiSbTe nanostructured thermoelectric materials was developed by exploiting a synthesized soluble Ten2- polyanions. The fluidic Ten2- polyanions were homogeneously impregnated into BiSbTe micro-particles and formed the Te layers between BiSbTe grains via sintering process. These Te layers create a contact potential at the BiSbTe-Te interfaces to realize the energy dependent carrier scattering and phonon scattering, leading to simultaneous improvement in electrical and thermal properties. Thus, the resulting ZT value increased to 1.3 at 75 ??C. Secondly, a synthetic route for molecular telluroantimonate precursor was developed by the reduction of Sb2Te3 powder in ethylenediamine and ethandithiol with superhydride. The fabricated Sb2Te3 thin film on flexible substrate exhibits a power factor of ???8.5 ??W cm???1 K???2 at 423 K and maintains the electrical property during 1000 bending cycles. In addition, FePt nanoparticles were homogeneously embedded in the Sb2Te3 thin film, reducing the thermal conductivity. Lastly, I successfully synthesized Sn2Se6 precursor for the fabrication of SnSe thin films with thermoelectric power factors at the single crystal level. Purification step in the synthetic process produced a SnSe-based chalcogenidometallate precursor, which decomposes to form the SnSe2 phase. The SnSe thin films showed the strong textures in the b???c plane, originate from the transition of two dimensional SnSe2 to SnSe. The optimum SnSe thin films exhibited a thermoelectric power factor of 4.27 ??W cm???1 K???2.

For the electronic applications, I synthesized a soluble Ag2S-based precursor, used to fabricate all-inorganic Ag2S thin film-based resistive random-access memory (RRAM) with an excellent durability against external mechanical, chemical, and thermal stresses. This Ag2S-based RRAM showed bipolar switching characteristic with a Ion/Ioff ratio of ~102 as well as a stable retention and endurance properties. Owing to the intrinsic ductility of Ag2S crystal, the fabricated device exhibited excellent flexibility and stretchability, withstanding 1000 bending and stretching cycles at a bending radius of ~1.9 mm and stretching strain of 30%, respectively. Furthermore, the sustainable operation of Ag2S-based RRAM was presented even after being chemically and thermally stimulated.clos

Synthesis of soluble chalcogens and metal chalcogenides for thermoelectric and electronic applications

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