arXiv subjects

Hanako Okuno

Publications and source records attributed to Hanako Okuno.

16 recordsLinked to original sources

arXiv2026

High temperature ferromagnetism in epitaxial monolayers of Co-doped Fe5GeTe2

Magnetic van der Waals materials have mainly been investigated in their bulk form or as few-layers flakes. Due to the challenges in producing atomically thin films, only a few have been isolated as monolayers, which typically exhibit long-range magnetic order below 150 K. In this work, we use molecular beam epitaxy to synthesize Co-doped Fe5GeTe2, achieving precise control over both thickness and composition. We demonstrate ferromagnetism well above room temperature in multilayer samples and present clear evidence of ferromagnetic ordering in monolayers up to $\sim$200 K. The changes in Curie temperature and magnetic anisotropy with composition exhibit similar trends in both monolayers and thicker films, indicating that the magnetic properties are primarily governed by intralayer magnetic interactions. Through element-specific X-ray magnetic circular dichroism and density functional theory, we identify the substitution site of Co dopants and reveal the mechanism behind the Curie temperature enhancement induced by Co doping. Our findings suggest that, despite their weak magnetic moment, Co dopants strengthen the magnetic moments on neighboring Fe atoms and enhance the intralayer ferromagnetic exchange interactions.

cond-mat.mtrl-sci↗

arXiv2026

A new method to probe conducting filaments in MoS$_2$-based memristors

Two-dimensional (2D) transition metal dichalcogenides (TMDs), such as molybdenum disulfide (MoS$_2$), are emerging as promising materials for next-generation electronic devices. They have proved to be serious candidates for integration with memristors in non-volatile memory and radio frequency (RF) applications. However, the physical mechanisms behind their resistive switching, particularly the formation and resorption of conducting filaments, remain unclear. In this study, we present a novel mechanical exfoliation technique that selectively removes the top metallic electrode from MoS$_2$-based memristors by exploiting the weak van der Waals interaction between MoS$_2$ and the top electrode. This method enables direct and multi-scale characterization of the MoS$_2$ surface in different states (initial, ON and OFF) using Kelvin Probe Force Microscopy (KPFM) and Raman spectroscopy mapping. To complete this study, cross-sectional Transmission Electron Microscopy (TEM) was also performed in different conductive states. Our results reveal that the conducting filament is formed by metallic atom migration from the top electrode into the MoS$_2$ layer. Additionally, we demonstrate that the choice of metallic electrodes (gold vs. nickel) significantly impacts the switching behavior due to differences in adsorption and diffusion energies. This work not only clarifies the filament formation mechanism and introduces a reproducible approach for in-operando characterization but also represents a real progress in the understanding and optimization of 2D material-based memristors.

cond-mat.mtrl-sci↗

arXiv2025

Advanced structural characterization of single-walled carbon nanotubes with 4D-STEM

Single wall carbon nanotubes (SWCNT) exhibit remarkable optical and electrical properties making them one of the most promising materials for next generation electronic and optoelectronic devices. Their electronic properties strongly depend on their chirality, i.e., their structural configuration, as well as on the presence and nature of atomic defects. Currently, the lack of versatile and efficient structural characterization techniques limits SWCNT applications. Here, we report how four-dimensional scanning transmission electron microscopy (4D-STEM) can address critical challenges in SWCNT structural analysis. Using modern fast pixelated electron detectors, we were able to acquire rapidly a large number of low noise electron diffraction patterns of SWCNTs. The resulting 4D-STEM data allow to precisely determine the local chirality of multiple nanotubes at once, with limited electron dose (down to 1750 e-/{\AA}^2) and nanometric spatial resolution (down to 3.1 nm). We also show how this approach enables to track the chirality along a single nanotube, while giving access to the strain distribution. Then, we report how 4D-STEM data enable to reconstruct high-resolution images with electron ptychography. With this second approach, structural information can be obtained with atomic scale spatial resolution allowing atomic defect imaging. Finally, we investigate how multi-slice electron ptychography could provide even further insight on nanotube defect structures thanks to its close to 3D imaging capabilities at atomic resolution.

cond-mat.mtrl-sci↗

arXiv2025

Growth of Highly Conductive PtSe2 Films Controlled by Raman Metrics for High-Frequency Photodetectors and Optoelectronic Mixers at 1.55 {\mu}m

Two-dimensional PtSe2 exhibits outstanding intrinsic properties such as high carrier mobility, tunable bandgap, broadband absorption and air stability, making it ideal for (opto)electronic applications. In particular, thick PtSe2 is semimetallic and well suited for ultrafast optoelectronics in the infrared domain. However, achieving PtSe2 films of high crystalline quality with controlled properties on low-cost and insulating substrates remains challenging. Here, highly crystalline semimetallic PtSe2 films are grown by molecular beam epitaxy on sapphire substrates. It is shown how an optimized post-growth annealing remarkably improves the out-of-plane crystallinity and leads to record sheet conductances, up to 1.6 mS. In-depth structural analyses reveal the strong influence of the domain arrangement within the films on their electrical properties. Films that are mainly composed of vertically single crystalline domains exhibit high sheet conductance (1.1 - 1.6 mS), whereas films that contain superimposed twisted domains present low sheet conductance (0.5 - 0.6 mS). Moreover, it is demonstrated that the A1g Raman peak width, in addition to the commonly used Eg peak width, are both effective metrics for evaluating the quality of PtSe2: films with narrower Eg and A1g peaks exhibit higher in-plane and out-of-plane crystalline quality, respectively, as well as higher sheet conductance. Finally, coplanar waveguides integrating a semimetallic PtSe2 channel are fabricated on a 2-inch sapphire substrate to demonstrate optoelectronic devices operating at the 1.55 {\mu}m telecom wavelength. This includes photodetectors with a record 60 GHz bandwidth and the first PtSe2-based optoelectronic mixer with a bandwidth above 30 GHz.

cond-mat.mtrl-sci↗

arXiv2024

Development and optimization of large-scale integration of 2D material in memristors

Two-dimensional (2D) materials like transition metal dichalcogenides (TMD) have proved to be serious candidates to replace silicon in several technologies with enhanced performances. In this respect, the two remaining challenges are the wafer scale growth of TMDs and their integration into operational devices using clean room compatible processes. In this work, two different CMOS-compatible protocols are developed for the fabrication of MoS$_2$-based memristors, and the resulting performances are compared. The quality of MoS$_2$ at each stage of the process is characterized by Raman spectroscopy and x-ray photoemission spectroscopy. In the first protocol, the structure of MoS$_2$ is preserved during transfer and patterning processes. However, a polymer layer with a minimum thickness of 3 nm remains at the surface of MoS$_2$ limiting the electrical switching performances. In the second protocol, the contamination layer is completely removed resulting in improved electrical switching performances and reproducibility. Based on physico-chemical and electrical results, the switching mechanism is discussed in terms of conduction through grain boundaries.

physics.app-ph↗

arXiv2023

Mapping domain junctions using 4D-STEM: toward controlled properties of epitaxially grown transition metal dichalcogenide monolayers

Epitaxial growth has become a promising route to achieve highly crystalline continuous two-dimensional layers. However, high-quality layer production with expected electrical properties is still challenging due to the defects induced by the coalescence between imperfectly aligned domains. In order to control their intrinsic properties at the device scale, the synthesized materials should be described as a patchwork of coalesced domains. Here, we report multi-scale and multistructural analysis on highly oriented epitaxial WS$_2$ and WSe$_2$ monolayers using scanning transmission electron microscopy (STEM) techniques. Characteristic domain junctions are first identified and classified based on the detailed atomic structure analysis using aberration corrected STEM imaging. Mapping orientation, polar direction and phase at the micrometer scale using four-dimensional STEM enabled to access the density and the distribution of the specific domain junctions. Our results validate a readily applicable process for the study of highly oriented epitaxial transition metal dichalcogenides, providing an overview of synthesized materials from large scale down to atomic scale with multiple structural information.

cond-mat.mtrl-sci↗

arXiv2023

Epitaxial van der Waals heterostructures of Cr2Te3 on 2D materials

Achieving large-scale growth of two-dimensional (2D) ferromagnetic materials with high Curie temperature (TC) and perpendicular magnetic anisotropy (PMA) is highly desirable for the development of ultra-compact magnetic sensors and magnetic memories. In this context, van der Waals (vdW) Cr2Te3 appears as a promising candidate. Bulk Cr2Te3 exhibits strong PMA and a TC of 180 K. Moreover, both PMA and TC might be adjusted in ultrathin films by engineering composition, strain, or applying an electric field. In this work, we demonstrate the molecular beam epitaxy (MBE) growth of vdW heterostructures of five-monolayer quasi-freestanding Cr2Te3 on three classes of 2D materials: graphene (semimetal), WSe2 (semiconductor) and Bi2Te3 (topological insulator). By combining structural and chemical analysis down to the atomic level with ab initio calculations, we confirm the single crystalline character of Cr2Te3 films on the 2D materials with sharp vdW interfaces. They all exhibit PMA and TC close to the bulk Cr2Te3 value of 180 K. Ab initio calculations confirm this PMA and show how its strength depends on strain. Finally, Hall measurements reveal a strong anomalous Hall effect, which changes sign at a given temperature. We theoretically explain this effect by a sign change of the Berry phase close to the Fermi level. This transition temperature depends on the 2D material in proximity, notably as a consequence of charge transfer. MBE-grown Cr2Te3/2D material bilayers constitute model systems for the further development of spintronic devices combining PMA, large spin-orbit coupling and sharp vdW interface.

cond-mat.mtrl-sci↗

arXiv2022

The influence of illumination conditions in the measurement of built-in electric field at p-n junctions by 4D-STEM

Momentum resolved 4D-STEM, also called center of mass (CoM) analysis, has been used to measure the long range built-in electric field of a silicon p-n junction. The effect of different STEM modes and the trade-off between spatial resolution and electric field sensitivity are studied. Two acquisition modes are compared: nanobeam and low magnification (LM) modes. A thermal noise free Medipix3 direct electron detector with high speed acquisition has been used to study the influence of low electron beam current and millisecond dwell times on the measured electric field and standard deviation. It is shown that LM conditions can underestimate the electric field values due to a bigger probe size used but provide an improvement of almost one order of magnitude on the signal-to-noise ratio, leading to a detection limit of 0.011MV/cm. It is observed that the CoM results do not vary with acquisition time or electron dose as low as 24 $e^-/A^2$, showing that the electron beam does not influence the built-in electric field and that this method can be robust for studying beam sensitive materials, where a low dose is needed.

physics.app-ph↗

arXiv2022

Assessment of active dopants and p-n junction abruptness using in-situ biased 4D-STEM

A key issue in the development of high-performance semiconductor devices is the ability to properly measure active dopants at the nanometer scale. 4D scanning transmission electron microscopy and off-axis electron holography have opened up the possibility of studying the electrostatic properties of a p-n junction with nm-scale spatial resolution. The complete description of a p-n junction must take into account the precise evolution of the concentration of dopants around the junction, since the sharpness of the dopant transition directly influences the built-in potential and the maximum electric field. Here, a contacted silicon p-n junction is studied through in-situ biased 4D-STEM. Measurements of electric field, built-in voltage, depletion region width and charge density in the space charge region are combined with analytical equations as well as finite-element simulations in order to evaluate the quality of the junction interface. The nominally-symmetric, highly doped ($N_A = N_D = 9\space x \space10^{18} cm^{-3}$) junction presents an electric field and built-in voltage much lower than expected for an abrupt junction. These experimental results are consistent with electron holography data. All measured junction parameters are compatible with the presence of an intermediate region with a graded profile of the dopants at the p-n interface. This hypothesis is also consistent with the evolution of the electric field with bias. These results demonstrate that in-situ biased 4D-STEM enables a better understanding of the electrical properties of semiconductor p-n junctions with nm-scale resolution.

physics.app-ph↗

arXiv2021

High carrier mobility in single-crystal PtSe2 grown by molecular beam epitaxy on ZnO(0001)

PtSe2 is attracting considerable attention as a high mobility two-dimensional material with envisionned applications in microelectronics, photodetection and spintronics. The growth of high quality PtSe2 on insulating substrates with wafer-scale uniformity is a prerequisite for electronic transport investigations and practical use in devices. Here, we report the growth of highly oriented few-layers PtSe2 on ZnO(0001) by molecular beam epitaxy. The crystalline structure of the films is characterized with electron and X-ray diffraction, atomic force microscopy and transmission electron microscopy. The comparison with PtSe2 layers grown on graphene, sapphire, mica, SiO2 and Pt(111) shows that among insulating substrates, ZnO(0001) yields films of superior structural quality. Hall measurements performed on epitaxial ZnO/PtSe2 with 5 monolayers of PtSe2 show a clear semiconducting behaviour and a high mobility in excess of 200 cm2V 1s-1 at room temperature and up to 447 cm2V-1s-1 at low temperature.

cond-mat.mtrl-sci↗

arXiv2021

Large-scale epitaxy of two-dimensional van der Waals room-temperature ferromagnet Fe5GeTe2

In recent years, two-dimensional van der Waals materials have emerged as an important platform for the observation of long-range ferromagnetic order in atomically thin layers. Although heterostructures of such materials can be conceived to harness and couple a wide range of magneto-optical and magneto-electrical properties, technologically relevant applications require Curie temperatures at or above room-temperature and the ability to grow films over large areas. Here we demonstrate the large-area growth of single-crystal ultrathin films of stoichiometric Fe5GeTe2 on an insulating substrate using molecular beam epitaxy. Magnetic measurements show the persistence of soft ferromagnetism up to room temperature, with a Curie temperature of 293 K, and a weak out-of-plane magnetocrystalline anisotropy. Surface, chemical, and structural characterizations confirm the layer-by-layer growth, 5:1:2 Fe:Ge:Te stoichiometric elementary composition, and single crystalline character of the films.

cond-mat.mtrl-sci↗

arXiv2021

Current-driven domain wall dynamics in ferrimagnetic Ni-doped Mn4N films : very large domain wall velocities and reversal of motion direction across the magnetic compensation point

Spin-transfer torque (STT) and spin-orbit torque (SOT) are spintronic phenomena allowing magnetization manipulation using electrical currents. Beyond their fundamental interest, they allow developing new classes of magnetic memories and logic devices, in particular based on domain wall (DW) motion. In this work, we report the study of STT driven DW motion in ferrimagnetic manganese nickel nitride (Mn4-xNixN) films, in which a fine adjustment of the Ni content allows setting the magnetic compensation at room temperature. The reduced magnetization, combined with the large spin polarization of conduction electrons, strongly enhances the STT so that domain wall velocities approaching 3000 m/s can be obtained for Ni compositions close to the compensation point. In addition, a reversal of the domain wall motion direction is observed when the magnetic compensation composition is crossed. This striking feature, related to the change of direction of the spin polarization with respect to that of the net magnetization, is clarified by ab initio band structure calculations.

cond-mat.mtrl-sci↗

arXiv2020

Growth, charge and thermal transport of flowered graphene

We report on the structural and transport properties of the smallest dislocation loop in graphene, known as a flower defect. First, by means of advanced experimental imaging techniques, we deduce how flower defects are formed during recrystallization of chemical vapor deposited graphene. We propose that the flower defects arise from a bulge type mechanism in which the flower domains are the grains left over by dynamic recrystallisation. Next, in order to evaluate the use of such defects as possible building blocks for all-graphene electronics, we combine multiscale modeling tools to investigate the structure and the electron and phonon transport properties of large monolayer graphene samples with a random distribution of flower defects. For large enough flower densities, we find that electron transport is strongly suppressed while, surprisingly, hole transport remains almost unaffected. These results suggest possible applications of flowered graphene for electron energy filtering. For the same defect densities, phonon transport is reduced by orders of magnitude as elastic scattering by defects becomes dominant. Heat transport by flexural phonons, key in graphene, is largely suppressed even for very low concentrations.

cond-mat.mtrl-sci↗

arXiv2020

Bound hole states associated to individual vanadium atoms incorporated into monolayer WSe$_2$

Doping a two-dimensional semiconductor with magnetic atoms is a possible route to induce magnetism in the material. We report on the atomic structure and electronic properties of monolayer WSe$_2$ intentionally doped with vanadium atoms by means of scanning transmission electron microscopy and scanning tunneling microscopy and spectroscopy. Most of the V atoms incorporate at W sites. These V$_W$ dopants are negatively charged, which induces a localized bound state located 140 meV above the valence band maximum. The overlap of the electronic potential of two charged V$_W$ dopants generates additional in-gap states. Eventually, the negative charge may suppress the magnetic moment on the V$_W$ dopants.

cond-mat.mtrl-sci↗

arXiv2019

New approach for the molecular beam epitaxy growth of scalable single-crystalline WSe$_2$ monolayers

The search for high-quality transition metal dichalcogenides mono- and multi-layers grown on large areas is still a very active field of investigation nowadays. Here, we use molecular beam epitaxy to grow 15$\times$15 mm large WSe$_2$ on mica in the van der Waals regime. By screening one-step growth conditions, we find that very high temperature ($>$900$^{\circ}$C) and very low deposition rate ($<$0.015 nm/min) are necessary to obtain high quality WSe$_2$ films. The domain size can be larger than 1 $μ$m and the in-plane rotational misorientation less than $\pm$0.5$^{\circ}$. The WSe$_2$ monolayer is also robust against air exposure, can be easily transferred over 1 cm$^2$ on SiN/SiO$_2$ and exhibits strong photoluminescence signal. Moreover, by combining grazing incidence x-ray diffraction and transmission electron microscopy, we could detect the presence of few misoriented grains. A two-dimensional model based on atomic coincidences between the WSe$_2$ and mica crystals allows us to explain the formation of these misoriented grains and gives suggestion to remove them and further improve the crystalline quality of WSe$_2$.

cond-mat.mtrl-sci↗

arXiv2019

The valley Nernst effect in WSe$_2$

The Hall effect can be extended by inducing a temperature gradient in lieu of electric field that is known as the Nernst (-Ettingshausen) effect. The recently discovered spin Nernst effect in heavy metals continues to enrich the picture of Nernst effect-related phenomena. However, the collection would not be complete without mentioning the valley degree of freedom benchmarked by the observation of the valley Hall effect. Here we show the experimental evidence of its missing counterpart, the valley Nernst effect. Using millimeter-sized WSe$_{2}$ mono-multi-layers and the ferromagnetic resonance-spin pumping technique, we are able to apply a temperature gradient by off-centering the sample in the radio frequency cavity and address a single valley through spin-valley coupling. The combination of a temperature gradient and the valley polarization leads to the valley Nernst effect in WSe$_{2}$ that we detect electrically at room temperature. The valley Nernst coefficient is in very good agreement with the predicted value.

cond-mat.mtrl-sci↗