Yu-Seong Seo

Bio: Yu-Seong Seo is an academic researcher from Sungkyunkwan University. The author has contributed to research in topics: Thin film & Phonon. The author has an hindex of 10, co-authored 30 publications receiving 256 citations. Previous affiliations of Yu-Seong Seo include Pusan National University.

Synthesis of a Copper 1,3,5-Triamino-2,4,6-benzenetriol Metal-Organic Framework.

Abstract: We report the synthesis and characterization of a two-dimensional (2D) MX2Y2-type (M = metal, X, Y = N, S, O, and X ≠ Y) copper 1,3,5-triamino-2,4,6-benzenetriol metal-organic framework (Cu3(TABTO)2-MOF). The role of oxygen in the synthesis of this MOF was investigated. Copper metal is formed along with the MOF when the synthesis is done in argon as suggested by XRD. When the reaction was exposed to air with vigorous stirring, copper metal was not observed by XRD. However, if there is no stirring, then copper metal is formed, and we learned that this is because oxygen was not allowed to enter the solvent due to the formation of a MOF film at the air/water interface. For the sample synthesized in argon (Cu3(TABTO)2-Ar), an insulating Cu3(TABTO)2-Ar pellet (σ < 10-10 S cm-1) became a metallic conductor with an electrical conductivity of 0.78 S cm-1 at 300 K after exposure to iodine vapor. This work provides further insights into the role of oxygen in the synthesis of redox-active ligand-based MOFs, expands the family of 2D redox-active ligand-based electrically conductive MOFs, and offers more opportunities in sensing, photocatalytic, electronic, and energy-related applications.

Epitaxial growth and metallicity of rutile MoO2 thin film

Abstract: Molybdenum oxides have various crystal structures and physical properties due to the multiple valence states of the 4d molybdenum. Among them, MoO2 has a distorted rutile structure with chemical stability and metallic behavior. In this study we grew epitaxial (100) MoO2 thin films on (0001) Al2O3 substrates. Through careful control of the Ar-partial pressure and growth temperature, we determined the optimal growth condition. From our structural assessments, MoO2 epitaxial thin films with high crystallinity can only be achieved in very narrow growth conditions such as 500 °C and 7 mTorr. The thin film prepared under optimal condition showed good metallic behavior, which was confirmed by electronic transport and optical reflectance measurements. A detailed electronic structure was also investigated by spectroscopic ellipsometry.

Pressure dependence of the phonon spectrum in BaTiO 3 polytypes studied by ab initio calculations

Abstract: We report the first principles investigations on the phonons of three polytypes of BaTiO${}_{3}$ (BTO): paraelectric (PE) cubic $Pm\overline{3}m$ and two ferroelectric (FE) phases, tetragonal $P$4$mm$ and rhombohedral $R$3$m$. The phonon frequencies were calculated using various exchange-correlation functionals, including density functional theory, Hartree-Fock approximation, and their hybrids were reviewed. The pressure-induced interplays between the modes from individual phases were explored by calculating the phonon modes as a function of pressure, $P$ from $\ensuremath{-}$15 to 230 GPa. The pressure-sensitive modes of the FE phases showed softening and converged to the modes of the PE phase at pressures below $\ensuremath{\sim}$10 GPa. These results on the FE phases can be interpreted as phonon precursors for a change in symmetry from low- to high-symmetry and partly as a theoretical explanation for the pressure-induced mode-coupling behaviors reported by Sood et al. [Phys. Rev. B 51, 8892 (1995)]. As pressure is applied further beyond $\ensuremath{\sim}$50 GPa to the cubic PE phase, the lowest ${F}_{1u}$ mode softens again and diverges into two separate modes of tetragonal FE $P$4$mm$ at above $\ensuremath{\sim}$150 GPa. These phonon-branching behaviors at high pressures provide a clear reconfirmation of the re-entrant ferroelectricity predicted in earlier papers [Phys. Rev. Lett. 95, 196804 (2005); Phys. Rev. B 74, 180101 (2006); Phys. Rev. B 85, 054108 (2012)]. The high-pressure-re-entrant FE polarization was not found in the rhombohedral structure. Instead, the centrosymmetric $R\overline{3}m$ phase was favored at above $\ensuremath{\sim}$30 GPa. The phonon modes calculated for the phonon-propagation vectors in the high-symmetry directions show that the $Pm\overline{3}m$ phase exhibits polar instability at the $\ensuremath{\Gamma}$ point and nonpolar instability at the $X$, $M$, and $R$ points under high pressure.

Ab-initio studies on the phonons of BaTiO3 polytypes: pressure dependences with a hybrid functional

Abstract: We report the first principles investigations on the phonons of three polytypes of BaTiO3 (BTO): paraelectric (PE) cubic Pm-3m and two ferroelectric (FE) phases, tetragonal P4mm and rhombohedral R3m. The phonon frequencies calculated using various exchange-correlation functionals, including density functional theory, Hartree-Fock approximation, and their hybrids were reviewed. The pressure-induced interplays between the modes form individual phases were explored by calculating the phonon modes as a function of pressure, P from -15 to 230 GPa. The pressure-sensitive modes of the FE phases showed softening and converged to the modes of the PE phase at pressures below ~ 10 GPa. These results on the FE phases can be interpreted as phonon-precursors for a change in symmetry from low- to high-symmetry and partly as a theoretical explanation for the pressure-induced mode-coupling behaviors reported by Sood et al. [Phys. Rev. B 51, 8892 (1995)]. As pressure is applied further beyond ~ 50 GPa to the cubic PE phase, the lowest F1u mode softens again and diverges into two separate modes of tetragonal FE P4mm at above ~ 150 GPa. These phonon-branching behaviors at high pressures provide a clear re-confirmation of the re-entrant ferroelectricity predicted in [Phys. Rev. Lett. 95, 196804 (2005); Phys. Rev. B 74, 180101 (2006); ibid. 85, 054108 (2012)]. The high-pressure-re-entrant FE polarization was not found in the rhombohedral structure. Instead, the centosymmetric R-3m phase was favored at above ~ 30 GPa. The phonon modes calculated for the phonon-propagation vectors in the high-symmetry directions show that the Pm-3m phase exhibits polar instability at the \Gamma point and non-polar instability at the X, M, and R points under high pressure.

Temperature-dependent excitonic superfluid plasma frequency evolution in an excitonic insulator, Ta 2 NiSe 5

Abstract: An interesting van der Waals material, Ta2NiSe5 has been known one of strong excitonic insulator candidates since it has very small or zero bandgap and can have a strong exciton binding energy because of its quasi-one-dimensional crystal structure. Here we investigate a single crystal Ta2NiSe5 using optical spectroscopy. Ta2NiSe5 has quasi-one-dimensional chains along the a-axis. We have obtained anisotropic optical properties of a single crystal Ta2NiSe5 along the a- and c-axes. The measured a- and c-axis optical conductivities exhibit large anisotropic electronic and phononic properties. With regard to the a-axis optical conductivity, a sharp peak near 3050 cm−1 at 9 K, with a well-defined optical gap ($${{\boldsymbol{\Delta }}}_{{\boldsymbol{o}}{\boldsymbol{p}}}^{{\boldsymbol{E}}{\boldsymbol{I}}}\,{\boldsymbol{\simeq }}$$ 1800 cm−1) and a strong temperature-dependence, is observed. With an increase in temperature, this peak broadens and the optical energy gap closes around ∼325 K ($${{\boldsymbol{T}}}_{{\boldsymbol{c}}}^{{\boldsymbol{E}}{\boldsymbol{I}}}$$). The spectral weight redistribution with respect to the frequency and temperature indicates that the normalized optical energy gap $$({{\boldsymbol{\Delta }}}_{{\boldsymbol{o}}{\boldsymbol{p}}}^{{\boldsymbol{E}}{\boldsymbol{I}}}({\boldsymbol{T}})/\,{{\boldsymbol{\Delta }}}_{{\boldsymbol{o}}{\boldsymbol{p}}}^{{\boldsymbol{E}}{\boldsymbol{I}}}{\bf{(0)}})$$ is $${\bf{1}}{\boldsymbol{-}}{({\boldsymbol{T}}/{{\boldsymbol{T}}}_{{\boldsymbol{c}}}^{{\boldsymbol{E}}{\boldsymbol{I}}})}^{{\bf{2}}}$$. The temperature-dependent superfluid plasma frequency of the excitonic condensation in Ta2NiSe5 has been determined from measured optical data. Our study may pave new avenues in the future research on excitonic insulators.

Screening in crystalline liquids protects energetic carriers in hybrid perovskites

Abstract: Hybrid lead halide perovskites exhibit carrier properties that resemble those of pristine nonpolar semiconductors despite static and dynamic disorder, but how carriers are protected from efficient scattering with charged defects and optical phonons is unknown. Here, we reveal the carrier protection mechanism by comparing three single-crystal lead bromide perovskites: CH3NH3PbBr3, CH(NH2)2PbBr3, and CsPbBr3. We observed hot fluorescence emission from energetic carriers with ~102-picosecond lifetimes in CH3NH3PbBr3 or CH(NH2)2PbBr3, but not in CsPbBr3. The hot fluorescence is correlated with liquid-like molecular reorientational motions, suggesting that dynamic screening protects energetic carriers via solvation or large polaron formation on time scales competitive with that of ultrafast cooling. Similar protections likely exist for band-edge carriers. The long-lived energetic carriers may enable hot-carrier solar cells with efficiencies exceeding the Shockley-Queisser limit.

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Local polar fluctuations in lead halide perovskite crystals

Abstract: Hybrid lead-halide perovskites have emerged as an excellent class of photovoltaic materials. Recent reports suggest that the organic molecular cation is responsible for local polar fluctuations that inhibit carrier recombination. We combine low-frequency Raman scattering with first-principles molecular dynamics (MD) to study the fundamental nature of these local polar fluctuations. Our observations of a strong central peak in the cubic phase of both hybrid (CH_{3}NH_{3}PbBr_{3}) and all-inorganic (CsPbBr_{3}) lead-halide perovskites show that anharmonic, local polar fluctuations are intrinsic to the general lead-halide perovskite structure, and not unique to the dipolar organic cation. MD simulations indicate that head-to-head Cs motion coupled to Br face expansion, occurring on a few hundred femtosecond time scale, drives the local polar fluctuations in CsPbBr_{3}.