Browsing by Author "Isobe, Masahiko"

Filter results by typing the first few letters
Now showing 1 - 4 of 4
  • Thumbnail Image
    ItemOpen Access
    Direct visualization of stacking-selective self-intercalation in epitaxial Nb1+xSe2 films
    (2024) Wang, Hongguang; Zhang, Jiawei; Shen, Chen; Yang, Chao; Küster, Kathrin; Deuschle, Julia; Starke, Ulrich; Zhang, Hongbin; Isobe, Masahiko; Huang, Dennis; van Aken, Peter A.; Takagi, Hidenori
    Two-dimensional (2D) van der Waals (vdW) materials offer rich tuning opportunities generated by different stacking configurations or by introducing intercalants into the vdW gaps. Current knowledge of the interplay between stacking polytypes and intercalation often relies on macroscopically averaged probes, which fail to pinpoint the exact atomic position and chemical state of the intercalants in real space. Here, by using atomic-resolution electron energy-loss spectroscopy in a scanning transmission electron microscope, we visualize a stacking-selective self-intercalation phenomenon in thin films of the transition-metal dichalcogenide (TMDC) Nb1+xSe2. We observe robust contrasts between 180°-stacked layers with large amounts of Nb intercalants inside their vdW gaps and 0°-stacked layers with little detectable intercalants inside their vdW gaps, coexisting on the atomic scale. First-principles calculations suggest that the films lie at the boundary of a phase transition from 0° to 180° stacking when the intercalant concentration x exceeds ~0.25, which we could attain in our films due to specific kinetic pathways. Our results offer not only renewed mechanistic insights into stacking and intercalation, but also open up prospects for engineering the functionality of TMDCs via stacking-selective self-intercalation.
  • Thumbnail Image
    ItemOpen Access
    Floating zone growth of large tetragonal Ruddlesden-Popper bilayer nickelate YySr3-yNi2-xAlxO7-δ single crystals
    (2025) Yilmaz, Hasan; Sosa-Lizama, Pablo; Knauft, Manuel; Küster, Kathrin; Starke, Ulrich; Isobe, Masahiko; Clemens, Oliver; Aken, Peter A. van ; Suyolcu, Y. E.; Puphal, Pascal
    The discovery of superconductivity under high pressure in Ruddlesden-Popper (RP) type phase bilayer La3Ni22.5+O7 and trilayer La4Ni32.66+O10 has initiated the frontier of nickelate-based superconductors. In this context, RP-type phases within the Sr-Ni-O system offer promising alternatives as they offer unconventional high oxidation states and Sr-T-O comprises the usual RP series. Here, the intrinsic stability of the undoped Sr-Ni-O framework is investigated using density functional theory (DFT). While Sr3Ni2O7 (SNO) is stable synthesis so far requires Al co-substition in Sr3Ni2-xAlxO7-δ (SNAO). Y-doping resulting in YySr3-yNi2-xAlxO7-δ (YSNAO) effectively mitigates the challenge posed by an insulating ground state. This modification yields a substantial reduction in resistivity, with the crystals exhibiting semiconducting behavior. To explore phase formation within the narrow compositional window of the Y-Sr-Ni-Al-O system, single crystals were grown using the optical floating zone (OFZ) technique under an oxygen partial pressure of approximately 10 bar. The optimized growth conditions for YSNAO enabled the production of large (6 × 5 x 3 mm3), high-quality crystals suitable for neutron scattering experiments. In the absence of Al, crystal growth yielded the n = 1 RP phase Sr1.66Y0.33NiO4-δ, for which single crystals were obtained. The structural, chemical, electrical, and magnetic properties of both the as-grown and topochemically reduced YSNAO compounds were comprehensively characterized through diffraction, spectroscopy, transport, and magnetization measurements.
  • Thumbnail Image
    ItemOpen Access
    Realization of a classical Ruddlesden Popper type bilayer nickelate in Sr3Ni2-xAlxO7-δ with unusual Ni4+
    (2024) Yilmaz, Hasan; Küster, Kathrin; Starke, Ulrich; Clemens, Oliver; Isobe, Masahiko; Puphal, Pascal
    The discovery of 80 K superconductivity in bilayer La3Ni2O7 at pressures greater than 14 GPa presents a unique opportunity to study a novel class of high-temperature superconductors. Therefore, other bilayer nickelates following the classical (T4+) Ruddlesden-Popper (RP) series of Sr3Ni2O7 would present an interesting new candidate. In this work, we study the stabilization of RP n = 2 phase in Sr3Ni2-xAlxO7-δ, via floating zone growth of crystals. With powder and single-crystal XRD, we study the stability range of the RP-type phase. Our Thermogravimetric Analysis (TGA), X-ray photoelectron spectroscopy (XPS) and gas extraction studies reveal a remarkably high oxidation state of Ni4+ stabilized by chemical strain from Al. The obtained black crystals are insulating in transport and show a magnetic transition around 12 K.
  • Thumbnail Image
    ItemOpen Access
    Synthesis and optical properties of single‐crystalline phosphors Gd3In2Ga3O12:RE3+ (RE = Nd3+ and Ho3+) grown via the optical float zone method
    (2025) Yilmaz, Hasan; Kinik, Gülsüm; Isobe, Masahiko; Puphal, Pascal; Suta, Markus; Clemens, Oliver
    The continuous development of innovative optical materials with lanthanoid ions as activators has emerged as a modern sector of materials chemistry. The experience with the fabrication of single crystals with the optical float zone has motivated one to investigate the luminescence of Nd3+ and Ho3+ ions in the garnets (Gd3−xREx)In2Ga3O12 (RE = Nd and Ho, x = 0; 0.15–0.30). Upon usage of an Ar/O2 (80:20 ratio) atmosphere and application of an auxiliary pressure (6 bar) to suppress In2O3 evaporation, single‐crystalline domain sizes in the order of ≈6 × 6 × 1 mm3 are obtained. Structural analysis confirms the formation of a cubic garnet phase with space group Ia3¯d, with the substituents incorporated in accordance with Vegard's law. Backscattered electron imaging and energy‐dispersive X‐ray spectroscopy are conducted, demonstrating a homogeneous elemental distribution within the crystals. Photoluminescence studies are carried out, revealing the characteristic narrow‐line 4fn → 4fn transitions of Nd3+ and Ho3+, with decay times in the submillisecond range, suggesting non‐negligible cross‐relaxation effects are present. Despite this, the large nearest‐neighbor Gd-Gd distance (3.88 Å) in Gd3In2Ga3O12 and the low phonon cutoff energy (≈700 cm-1) are found to limit cross‐relaxation pathways, preserving significant photoluminescence brightness. These results highlight the potential of Gd3In2Ga3O12:RE3+ single crystals as promising candidates for advanced optical applications.