Since the power conversion efficiency (PCE) of hybrid halide perovskite solar cells (PSCs) could ... more Since the power conversion efficiency (PCE) of hybrid halide perovskite solar cells (PSCs) could exceed 24%, extensive research has been focused on improving their long-term stability for commercialization in the near future. In a previous study, we reported that the addition of a certain amount of ionized iodide (triiodide: I 3-) ions during the perovskite film formation significantly improved the efficiency of PSCs by reducing deep-level defects in the perovskite layer. Understanding the relationship between the concentration of these defects and the longterm chemical aging of PSCs is important not only for obtaining fundamental insight into the perovskite materials, but also for studying the long-term chemical stability of PSCs. Herein we aim to identify the origin of the natural decay in PCE during long-term chemical aging in the dark of PSCs based on formamidinium lead triiodide, by comparing the performance of control and low-defect (LD) devices. After aging for 200 days, the change in PCE of the LD devices (1.3%) was found to be half that of the control devices (2.6%). We investigated this difference
Electronic structures of perovskite oxides of SrMn 1−x Mo x O 3 in the Mo-rich regime (x 0.5) hav... more Electronic structures of perovskite oxides of SrMn 1−x Mo x O 3 in the Mo-rich regime (x 0.5) have been investigated by employing soft x-ray absorption spectroscopy (XAS). Mn 2p XAS spectra of SrMn 1−x Mo x O 3 indicate that the valence states of Mn ions are divalent and that they remain nearly unchanged for x 0.5. On the other hand, the Mo 3p XAS spectra of SrMn 1−x Mo x O 3 show some changes, suggesting that the valence states of Mo ions decrease with increasing x for x 0.5. These conclusions are supported by the unoccupied Mn 3d and Mo 4d states, observed experimentally in the O 1s XAS spectra of SrMn 1−x Mo x O 3 .
Ideal undisturbed functionalized epitaxial graphene on SiC was examined by angle resolved photoem... more Ideal undisturbed functionalized epitaxial graphene on SiC was examined by angle resolved photoemission spectroscopy, scanning photoemission microscopy and atomic force microscope. The surface was prepared by non-covalently attaching tetra(4-carboxyphenyl)-porphine(TCPP) on the surface. This material performs well as a chemically very sensitive and thermally very stable template. The adsorbed TCPP layer does not disturb the intrinsic physical properties of the graphene. A patterned Al 2 O 3 layer can be easily produced by preparing a pre-patterned TCPP on the graphene surface.
Post-patterning of an electronic homojunction in atomically thin monoclinic MoTe 2 To cite this a... more Post-patterning of an electronic homojunction in atomically thin monoclinic MoTe 2 To cite this article: Sera Kim et al 2017 2D Mater. 4 024004 View the article online for updates and enhancements. Related content Reducing the Schottky barrier between few-layer MoTe2 and gold Dianyu Qi, Qixing Wang, Cheng Han et al.-Absorption dichroism of monolayer 1T-MoTe2 in visible range Gang Hee Han, Dong Hoon Keum, Jiong Zhao et al.
Doping two-dimensional (2D) semiconductors beyond their degenerate levels provides the opportunit... more Doping two-dimensional (2D) semiconductors beyond their degenerate levels provides the opportunity to investigate extreme carrier density-driven superconductivity and phase transition in 2D systems. Chemical functionalization and the ionic gating have achieved the high doping density, but their effective ranges have been limited to ∼1 nm, which restricts the use of highly doped 2D semiconductors. Here, we report on electron diffusion from the 2D electride [Ca2N](+)·e(-) to MoTe2 over a distance of 100 nm from the contact interface, generating an electron doping density higher than 1.6 × 10(14) cm(-2) and a lattice symmetry change of MoTe2 as a consequence of the extreme doping. The long-range lattice symmetry change, suggesting a length scale surpassing the depletion width of conventional metal-semiconductor junctions, was a consequence of the low work function (2.6 eV) with highly mobile anionic electron layers of [Ca2N](+)·e(-). The combination of 2D electrides and layered materials yields a novel material design in terms of doping and lattice engineering.
Beamline 8A (BL 8A) is an undulator-based soft X-ray beamline at Pohang Accelerator Laboratory. T... more Beamline 8A (BL 8A) is an undulator-based soft X-ray beamline at Pohang Accelerator Laboratory. This beamline is aimed at high-resolution ambientpressure X-ray photoelectron spectroscopy (AP-XPS), soft X-ray absorption spectroscopy (soft-XAS) and scanning photoemission microscopy (SPEM) experiments. BL 8A has two branches, 8A1 SPEM and 8A2 AP-XPS, that share a plane undulator, the first mirror (M1) and the monochromator. The photon beam is switched between the two branches by changing the refocusing mirrors after the monochromator. The acceptance angle of M1 is kept glancing at 1.2 , and Pt is coated onto the mirrors to achieve high reflectance, which ensures a wide photon energy range (100-2000 eV) with high resolution at a photon flux of $ 10 13 photons s À1. In this article, the main properties and performance of the beamline are reported, together with selected experiments performed on the new beamline and experimental system.
Artificial van der Waals heterostructures with two-dimensional (2D) atomic crystals are promising... more Artificial van der Waals heterostructures with two-dimensional (2D) atomic crystals are promising as an active channel or as a buffer contact layer for next-generation devices. However, genuine 2D heterostructure devices remain limited because of impurity-involved transfer process and metastable and inhomogeneous heterostructure formation. We used laser-induced phase patterning, a polymorph engineering, to fabricate an ohmic heterophase homojunction between semiconducting hexagonal (2H) and metallic monoclinic (1T') molybdenum ditelluride (MoTe 2) that is stable up to 300°C and increases the carrier mobility of the MoTe 2 transistor by a factor of about 50, while retaining a high on/off current ratio of 10 6. In situ scanning transmission electron microscopy results combined with theoretical calculations reveal that the Te vacancy triggers the local phase transition in MoTe 2 , achieving a true 2D device with an ohmic contact.
Abstract This study examined the catalytic activities of three distinct N–Pd@TiO 2 nanoparticles,... more Abstract This study examined the catalytic activities of three distinct N–Pd@TiO 2 nanoparticles, post-annealed (at 700, 800, and 900 °C) after fabrication on silicon substrates. Systematically, HRPES and SPEM indicated that nitrogen was predominantly doped around PdO nanostructure. Particularly, we prove that photocatalytic activity in effective nitrogen doped surface area is far higher than in undoped area of Pd@TiO 2 nanoparticles by measuring for oxidation reaction of benzenethiol, thioacetic acid, and thiobenzoic acid. Besides to the band gap narrowing, nitrogen doping leads to generate Ti 3 + species or oxygen vacancies site of the surface, which leads to enhance the activity of surface photocatalysis.
Journal of Inorganic and Organometallic Polymers and Materials, Aug 7, 2013
A sputter-cleaned amorphous In-Ga-Zn-O thin-film surface was exposed to O 2 and air, and the spec... more A sputter-cleaned amorphous In-Ga-Zn-O thin-film surface was exposed to O 2 and air, and the spectral changes at the O 1s, In 3d, Ga 3d, Zn 3d, and the valence band were investigated by soft-X-ray photoelectron spectroscopy. Both exposures reduced the density of the oxygen-vacancy-representing deep subgap state, which was located above the maximum of the valence band. The exposures also reduced the densities of the metallic states, which were observed near the Fermi energy and at In 3d. A higher oxidation state than that of the unexposed metal oxide was negligibly observed at the metal 3d orbitals. These results imply that O 2 and air exposures effectively fill oxygen vacancies and decrease the number of free electrons. Keywords In-Ga-Zn-O Á IGZO Á Threshold voltage shift Á Transparent semiconducting oxides Á Oxygen-exposure effect Á Air-exposure effect S. J. Kang and J. Baik contributed equally.
The electronic structure of superoxide KO 2 was investigated by employing soft x-ray absorption s... more The electronic structure of superoxide KO 2 was investigated by employing soft x-ray absorption spectroscopy ͑XAS͒ and core-level photoemission spectroscopy ͑PES͒. The clean and single component was observed in both the O 1s and K 2p core-level PES spectra of KO 2 , which confirms the good quality of the KO 2 sample employed in this work. The measured O 1s XAS spectrum was compared with the calculated density of states ͑DOSs͒, obtained from the first-principles electronic-structure calculations that include both the spin-orbit ͑SO͒ effect and the on-site Coulomb interaction ͑U͒ between O 2p electrons. In the O 1s XAS, both the unoccupied O 2p antibonding g and u states are identified. It is found that the unoccupied states, observed in the O 1s XAS, agree reasonably well with the calculated DOSs, which suggests the importance of the SO and U effects for KO 2 having the high-symmetry tetragonal structure.
Electronic structures of perovskite oxides of SrMn1-xMoxO3 (0 ≤x≤ 0.75) have been investigated by... more Electronic structures of perovskite oxides of SrMn1-xMoxO3 (0 ≤x≤ 0.75) have been investigated by employing soft x-ray absorption spectroscopy (XAS) and photoemission spectroscopy (PES). It is found that the valence states of Mn ions change systematically from Mn4+ for x = 0, to nearly Mn3+ for x = 0.3, and Mn2+ for x = 0.5, and then remain as Mn2+ for x≥0.5. Both the Mo 3d core-level PES and Mo 3p XAS spectra show that the valence states of Mo ions remain unchanged as being hexavalent (Mo6+) for x≤0.5 but that they decrease for x>0.5 with increasing x. This study shows that Mo ions are mixed-valent for x>0.5 in SrMn1-xMoxO3, either with the Mo4+–Mo6+ configuration or with the Mo4+–Mo5+–Mo6+ configuration.
Journal of the Korean Physical Society, Jul 1, 2016
We firstly report an experimental visualization of a cycloaddition reaction on RT frozen asymmetr... more We firstly report an experimental visualization of a cycloaddition reaction on RT frozen asymmetric Si dimers. The frozen Si dimers with a local c(4 × 2) order were prepared by pinning flip-flopping Si dimers by using molecules. This RT pristine c(4 × 2) structure was used to determine what Si atom of an asymmetric Si dimer bonds to a molecule at the initial stage of the RT cycloaddition reaction, which has been a long-standing puzzling issue. This made it possible to compare directly experimental cycloaddition reactions with theoretical ones. As a prototype for the experiment, a 1,3-butadiene molecule adsorbed between Si dimer rows was used. The 1,3-butadiene molecule was found to prefer a symmetric Si pair on the frozen Si dimers, i.e., two electrophilic lower atoms of asymmetric Si dimers. This result is consistent with the theoretical prediction that a 1,3-diene molecule prefers a symmetric Si pair on the Si(001)c(4 × 2) surface. This experimental approach can also be applied to other studies for the adsorption of a molecule on a Si(001) surface at room temperature.
Since the power conversion efficiency (PCE) of hybrid halide perovskite solar cells (PSCs) could ... more Since the power conversion efficiency (PCE) of hybrid halide perovskite solar cells (PSCs) could exceed 24%, extensive research has been focused on improving their long-term stability for commercialization in the near future. In a previous study, we reported that the addition of a certain amount of ionized iodide (triiodide: I 3-) ions during the perovskite film formation significantly improved the efficiency of PSCs by reducing deep-level defects in the perovskite layer. Understanding the relationship between the concentration of these defects and the longterm chemical aging of PSCs is important not only for obtaining fundamental insight into the perovskite materials, but also for studying the long-term chemical stability of PSCs. Herein we aim to identify the origin of the natural decay in PCE during long-term chemical aging in the dark of PSCs based on formamidinium lead triiodide, by comparing the performance of control and low-defect (LD) devices. After aging for 200 days, the change in PCE of the LD devices (1.3%) was found to be half that of the control devices (2.6%). We investigated this difference
Electronic structures of perovskite oxides of SrMn 1−x Mo x O 3 in the Mo-rich regime (x 0.5) hav... more Electronic structures of perovskite oxides of SrMn 1−x Mo x O 3 in the Mo-rich regime (x 0.5) have been investigated by employing soft x-ray absorption spectroscopy (XAS). Mn 2p XAS spectra of SrMn 1−x Mo x O 3 indicate that the valence states of Mn ions are divalent and that they remain nearly unchanged for x 0.5. On the other hand, the Mo 3p XAS spectra of SrMn 1−x Mo x O 3 show some changes, suggesting that the valence states of Mo ions decrease with increasing x for x 0.5. These conclusions are supported by the unoccupied Mn 3d and Mo 4d states, observed experimentally in the O 1s XAS spectra of SrMn 1−x Mo x O 3 .
Ideal undisturbed functionalized epitaxial graphene on SiC was examined by angle resolved photoem... more Ideal undisturbed functionalized epitaxial graphene on SiC was examined by angle resolved photoemission spectroscopy, scanning photoemission microscopy and atomic force microscope. The surface was prepared by non-covalently attaching tetra(4-carboxyphenyl)-porphine(TCPP) on the surface. This material performs well as a chemically very sensitive and thermally very stable template. The adsorbed TCPP layer does not disturb the intrinsic physical properties of the graphene. A patterned Al 2 O 3 layer can be easily produced by preparing a pre-patterned TCPP on the graphene surface.
Post-patterning of an electronic homojunction in atomically thin monoclinic MoTe 2 To cite this a... more Post-patterning of an electronic homojunction in atomically thin monoclinic MoTe 2 To cite this article: Sera Kim et al 2017 2D Mater. 4 024004 View the article online for updates and enhancements. Related content Reducing the Schottky barrier between few-layer MoTe2 and gold Dianyu Qi, Qixing Wang, Cheng Han et al.-Absorption dichroism of monolayer 1T-MoTe2 in visible range Gang Hee Han, Dong Hoon Keum, Jiong Zhao et al.
Doping two-dimensional (2D) semiconductors beyond their degenerate levels provides the opportunit... more Doping two-dimensional (2D) semiconductors beyond their degenerate levels provides the opportunity to investigate extreme carrier density-driven superconductivity and phase transition in 2D systems. Chemical functionalization and the ionic gating have achieved the high doping density, but their effective ranges have been limited to ∼1 nm, which restricts the use of highly doped 2D semiconductors. Here, we report on electron diffusion from the 2D electride [Ca2N](+)·e(-) to MoTe2 over a distance of 100 nm from the contact interface, generating an electron doping density higher than 1.6 × 10(14) cm(-2) and a lattice symmetry change of MoTe2 as a consequence of the extreme doping. The long-range lattice symmetry change, suggesting a length scale surpassing the depletion width of conventional metal-semiconductor junctions, was a consequence of the low work function (2.6 eV) with highly mobile anionic electron layers of [Ca2N](+)·e(-). The combination of 2D electrides and layered materials yields a novel material design in terms of doping and lattice engineering.
Beamline 8A (BL 8A) is an undulator-based soft X-ray beamline at Pohang Accelerator Laboratory. T... more Beamline 8A (BL 8A) is an undulator-based soft X-ray beamline at Pohang Accelerator Laboratory. This beamline is aimed at high-resolution ambientpressure X-ray photoelectron spectroscopy (AP-XPS), soft X-ray absorption spectroscopy (soft-XAS) and scanning photoemission microscopy (SPEM) experiments. BL 8A has two branches, 8A1 SPEM and 8A2 AP-XPS, that share a plane undulator, the first mirror (M1) and the monochromator. The photon beam is switched between the two branches by changing the refocusing mirrors after the monochromator. The acceptance angle of M1 is kept glancing at 1.2 , and Pt is coated onto the mirrors to achieve high reflectance, which ensures a wide photon energy range (100-2000 eV) with high resolution at a photon flux of $ 10 13 photons s À1. In this article, the main properties and performance of the beamline are reported, together with selected experiments performed on the new beamline and experimental system.
Artificial van der Waals heterostructures with two-dimensional (2D) atomic crystals are promising... more Artificial van der Waals heterostructures with two-dimensional (2D) atomic crystals are promising as an active channel or as a buffer contact layer for next-generation devices. However, genuine 2D heterostructure devices remain limited because of impurity-involved transfer process and metastable and inhomogeneous heterostructure formation. We used laser-induced phase patterning, a polymorph engineering, to fabricate an ohmic heterophase homojunction between semiconducting hexagonal (2H) and metallic monoclinic (1T') molybdenum ditelluride (MoTe 2) that is stable up to 300°C and increases the carrier mobility of the MoTe 2 transistor by a factor of about 50, while retaining a high on/off current ratio of 10 6. In situ scanning transmission electron microscopy results combined with theoretical calculations reveal that the Te vacancy triggers the local phase transition in MoTe 2 , achieving a true 2D device with an ohmic contact.
Abstract This study examined the catalytic activities of three distinct N–Pd@TiO 2 nanoparticles,... more Abstract This study examined the catalytic activities of three distinct N–Pd@TiO 2 nanoparticles, post-annealed (at 700, 800, and 900 °C) after fabrication on silicon substrates. Systematically, HRPES and SPEM indicated that nitrogen was predominantly doped around PdO nanostructure. Particularly, we prove that photocatalytic activity in effective nitrogen doped surface area is far higher than in undoped area of Pd@TiO 2 nanoparticles by measuring for oxidation reaction of benzenethiol, thioacetic acid, and thiobenzoic acid. Besides to the band gap narrowing, nitrogen doping leads to generate Ti 3 + species or oxygen vacancies site of the surface, which leads to enhance the activity of surface photocatalysis.
Journal of Inorganic and Organometallic Polymers and Materials, Aug 7, 2013
A sputter-cleaned amorphous In-Ga-Zn-O thin-film surface was exposed to O 2 and air, and the spec... more A sputter-cleaned amorphous In-Ga-Zn-O thin-film surface was exposed to O 2 and air, and the spectral changes at the O 1s, In 3d, Ga 3d, Zn 3d, and the valence band were investigated by soft-X-ray photoelectron spectroscopy. Both exposures reduced the density of the oxygen-vacancy-representing deep subgap state, which was located above the maximum of the valence band. The exposures also reduced the densities of the metallic states, which were observed near the Fermi energy and at In 3d. A higher oxidation state than that of the unexposed metal oxide was negligibly observed at the metal 3d orbitals. These results imply that O 2 and air exposures effectively fill oxygen vacancies and decrease the number of free electrons. Keywords In-Ga-Zn-O Á IGZO Á Threshold voltage shift Á Transparent semiconducting oxides Á Oxygen-exposure effect Á Air-exposure effect S. J. Kang and J. Baik contributed equally.
The electronic structure of superoxide KO 2 was investigated by employing soft x-ray absorption s... more The electronic structure of superoxide KO 2 was investigated by employing soft x-ray absorption spectroscopy ͑XAS͒ and core-level photoemission spectroscopy ͑PES͒. The clean and single component was observed in both the O 1s and K 2p core-level PES spectra of KO 2 , which confirms the good quality of the KO 2 sample employed in this work. The measured O 1s XAS spectrum was compared with the calculated density of states ͑DOSs͒, obtained from the first-principles electronic-structure calculations that include both the spin-orbit ͑SO͒ effect and the on-site Coulomb interaction ͑U͒ between O 2p electrons. In the O 1s XAS, both the unoccupied O 2p antibonding g and u states are identified. It is found that the unoccupied states, observed in the O 1s XAS, agree reasonably well with the calculated DOSs, which suggests the importance of the SO and U effects for KO 2 having the high-symmetry tetragonal structure.
Electronic structures of perovskite oxides of SrMn1-xMoxO3 (0 ≤x≤ 0.75) have been investigated by... more Electronic structures of perovskite oxides of SrMn1-xMoxO3 (0 ≤x≤ 0.75) have been investigated by employing soft x-ray absorption spectroscopy (XAS) and photoemission spectroscopy (PES). It is found that the valence states of Mn ions change systematically from Mn4+ for x = 0, to nearly Mn3+ for x = 0.3, and Mn2+ for x = 0.5, and then remain as Mn2+ for x≥0.5. Both the Mo 3d core-level PES and Mo 3p XAS spectra show that the valence states of Mo ions remain unchanged as being hexavalent (Mo6+) for x≤0.5 but that they decrease for x>0.5 with increasing x. This study shows that Mo ions are mixed-valent for x>0.5 in SrMn1-xMoxO3, either with the Mo4+–Mo6+ configuration or with the Mo4+–Mo5+–Mo6+ configuration.
Journal of the Korean Physical Society, Jul 1, 2016
We firstly report an experimental visualization of a cycloaddition reaction on RT frozen asymmetr... more We firstly report an experimental visualization of a cycloaddition reaction on RT frozen asymmetric Si dimers. The frozen Si dimers with a local c(4 × 2) order were prepared by pinning flip-flopping Si dimers by using molecules. This RT pristine c(4 × 2) structure was used to determine what Si atom of an asymmetric Si dimer bonds to a molecule at the initial stage of the RT cycloaddition reaction, which has been a long-standing puzzling issue. This made it possible to compare directly experimental cycloaddition reactions with theoretical ones. As a prototype for the experiment, a 1,3-butadiene molecule adsorbed between Si dimer rows was used. The 1,3-butadiene molecule was found to prefer a symmetric Si pair on the frozen Si dimers, i.e., two electrophilic lower atoms of asymmetric Si dimers. This result is consistent with the theoretical prediction that a 1,3-diene molecule prefers a symmetric Si pair on the Si(001)c(4 × 2) surface. This experimental approach can also be applied to other studies for the adsorption of a molecule on a Si(001) surface at room temperature.
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