Browsing by Author "Cho, Kyeongiae"
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Item A Fermi-Level-Pinning-Free 1D Electrical Contact at the Intrinsic 2D MoS₂–Metal Junction(Wiley-VCH Verlag, 2019-05-08) Yang, Z.; Kim, C.; Lee, K. Y.; Lee, M.; Appalakondaiah, S.; Ra, C. -H; Watanabe, K.; Taniguchi, T.; Cho, Kyeongiae; Hwang, E.; Hone, J.; Yoo, W. J.; 0000-0003-2698-7774 (Cho, K); Cho, KyeongiaeCurrently 2D crystals are being studied intensively for use in future nanoelectronics, as conventional semiconductor devices face challenges in high power consumption and short channel effects when scaled to the quantum limit. Toward this end, achieving barrier-free contact to 2D semiconductors has emerged as a major roadblock. In conventional contacts to bulk metals, the 2D semiconductor Fermi levels become pinned inside the bandgap, deviating from the ideal Schottky–Mott rule and resulting in significant suppression of carrier transport in the device. Here, MoS₂ polarity control is realized without extrinsic doping by employing a 1D elemental metal contact scheme. The use of high-work-function palladium (Pd) or gold (Au) enables a high-quality p-type dominant contact to intrinsic MoS₂, realizing Fermi level depinning. Field-effect transistors (FETs) with Pd edge contact and Au edge contact show high performance with the highest hole mobility reaching 330 and 432 cm² V⁻¹ s⁻¹ at 300 K, respectively. The ideal Fermi level alignment allows creation of p- and n-type FETs on the same intrinsic MoS₂ flake using Pd and low-work-function molybdenum (Mo) contacts, respectively. This device acts as an efficient inverter, a basic building block for semiconductor integrated circuits, with gain reaching 15 at V_{D} = 5 V. ©2019 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimItem Ab-Initio Design of Novel Cathode Material LiFeP₁₋ₓSiₓO₄ for Rechargeable Li-Ion Batteries(Elsevier Ltd, 2019-04-27) Yi, S.; Moon, J.; Cho, M.; Cho, Kyeongiae; 0000-0003-2698-7774 (Cho, K); Cho, KyeongiaeIn this study, newly designed cathode material LiFeP₁₋ₓSiₓO₄, with silicon mixed in LiFePO₄ is investigated using the density functional theory. Its most optimized structure is the olivine structure of the Pnma space group. Bonding length show the anti-site defect which hinders Li diffusivity is prevented in the LiFeP₁₋ₓSiₓO₄. Lithium migration energy barriers in the (010) path of LiFeP₁₋ₓSiₓO₄ (x = 0, 0.5, and 1) are calculated by using nudged elastic band calculations, and the average values are determined as 0.180, 0.245, and 0.280 eV for LiFePO₄, LiFeP₀․₅Si₀․₅O₄, and LiFeSiO₄, respectively. This signifies that the Li ionic diffusivity is degraded thermodynamically, which is contrary to that indicates by the calculated bonding length, however, the difference is negligibly small. Furthermore, the intercalation voltage increases up to 4.97 V, depending on the Si ratio to P, and is much higher than that of the pristine cathode materials LiFePO₄ (~3.47 V) enabling voltage optimization by Si substitution. The energy density is proportional to the intercalation voltage, hence the energy density is increased, respectively. Finally, the Total density of states show that the electronic conductivity of LiFeP₁₋ₓSiₓO₄ (x = 0–1) is better than that of LiFePO₄.