In this talk, I will cover two topics on the control of materials’ properties by introducing defects. First, the modification of the electrical property in Bi2Te3 by 60 degree twin boundary will be presented. Interfaces, such as grain boundaries in a solid material, are excellent regions to explore novel properties that emerge as the result of local symmetry-breaking. For instance, at the interface of a layered-chalcogenide material, the potential reconfiguration of the atoms at the boundaries can lead to a significant modification of the electronic properties because of their complex atomic bonding structure. Here, we report the experimental observation of an electron source at 60 degree twin boundaries in Bi2Te3, a representative layered-chalcogenide material. First-principles calculations reveal that the modification of the interatomic distance at the 60 degree twin boundary to accommodate structural misfits can alter the electronic structure of Bi2Te3. The change in the electronic structure generates occupied states within the original bandgap in a favorable condition to create carriers and enlarges the density-of-states near the conduction band minimum.
 
Second, the control of crystal structure by nitrogen doping in SnO2 will be presented. Controlling crystalline phases in polymorphic materials is critical not only for the fundamental understanding of the physics of phase formation, but also for the technological application of forbidden, but potentially useful physical properties of the nominally unstable phases. Here, using tin oxide (SnO2) as a model system, we demonstrate a new way to enhance the mechanical hardness of an oxide by stabilizing a high-pressure dense phase through nitrogen integration in the oxide. Pristine SnO2 has a tetragonal structure at the ambient pressure, and undergoes phase transitions to orthorhombic and cubic phases with increasing pressure. Leveraging the enhanced reactivity of nitrogen in plasma, we are able to synthesize tin oxynitride (SnON) thin films with a cubic phase same as the high-pressure phase of SnO2. Such nitrogen-stabilized cubic SnON films exhibit a mechanical hardness of ~23±4 GPa significantly higher than even the nitride counterpart (Sn3N4) as the result of the shortened atomic distance of the denser, high-pressure cubic phase.

References:
[1] K.C. Kim et al., Nature Communications, 7, 12449 (2016) doi: 10.1038/ncomms12449
[2] H.J. Gwon et al., Chemistry of Materials, (2016) doi:10.1021/acs.chemmater.6b02888