8 research outputs found

    Probing from Both Sides: Reshaping the Graphene Landscape via Face-to-Face Dual-Probe Microscopy

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    In two-dimensional samples, all atoms are at the surface and thereby exposed for probing and manipulation by physical or chemical means from both sides. Here, we show that we can access the same point on both surfaces of a few-layer graphene membrane simultaneously, using a dual-probe scanning tunneling microscopy (STM) setup. At the closest point, the two probes are separated only by the thickness of the graphene membrane. This allows us for the first time to directly measure the deformations induced by one STM probe on a free-standing membrane with an independent second probe. We reveal different regimes of stability of few-layer graphene and show how the STM probes can be used as tools to shape the membrane in a controlled manner. Our work opens new avenues for the study of mechanical and electronic properties of two-dimensional materials

    Atomic Structure of Intrinsic and Electron-Irradiation-Induced Defects in MoTe<sub>2</sub>

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    Studying the atomic structure of intrinsic defects in two-dimensional transition-metal dichalcogenides is difficult since they damage quickly under the intense electron irradiation in transmission electron microscopy (TEM). However, this can also lead to insights into the creation of defects and their atom-scale dynamics. We first show that MoTe<sub>2</sub> monolayers without protection indeed quickly degrade during scanning TEM (STEM) imaging, and discuss the observed atomic-level dynamics, including a transformation from the 1H phase into 1T′, 3-fold rotationally symmetric defects, and the migration of line defects between two 1H grains with a 60° misorientation. We then analyze the atomic structure of MoTe<sub>2</sub> encapsulated between two graphene sheets to mitigate damage, finding the as-prepared material to contain an unexpectedly large concentration of defects. These include similar point defects (or quantum dots, QDs) as those created in the nonencapsulated material and two different types of line defects (or quantum wires, QWs) that can be transformed from one to the other under electron irradiation. Our density functional theory simulations indicate that the QDs and QWs embedded in MoTe<sub>2</sub> introduce new midgap states into the semiconducting material and may thus be used to control its electronic and optical properties. Finally, the edge of the encapsulated material appears amorphous, possibly due to the pressure caused by the encapsulation

    Atomic Structure of Intrinsic and Electron-Irradiation-Induced Defects in MoTe<sub>2</sub>

    No full text
    Studying the atomic structure of intrinsic defects in two-dimensional transition-metal dichalcogenides is difficult since they damage quickly under the intense electron irradiation in transmission electron microscopy (TEM). However, this can also lead to insights into the creation of defects and their atom-scale dynamics. We first show that MoTe<sub>2</sub> monolayers without protection indeed quickly degrade during scanning TEM (STEM) imaging, and discuss the observed atomic-level dynamics, including a transformation from the 1H phase into 1T′, 3-fold rotationally symmetric defects, and the migration of line defects between two 1H grains with a 60° misorientation. We then analyze the atomic structure of MoTe<sub>2</sub> encapsulated between two graphene sheets to mitigate damage, finding the as-prepared material to contain an unexpectedly large concentration of defects. These include similar point defects (or quantum dots, QDs) as those created in the nonencapsulated material and two different types of line defects (or quantum wires, QWs) that can be transformed from one to the other under electron irradiation. Our density functional theory simulations indicate that the QDs and QWs embedded in MoTe<sub>2</sub> introduce new midgap states into the semiconducting material and may thus be used to control its electronic and optical properties. Finally, the edge of the encapsulated material appears amorphous, possibly due to the pressure caused by the encapsulation

    Atomic-Scale <i>in Situ</i> Observations of Crystallization and Restructuring Processes in Two-Dimensional MoS<sub>2</sub> Films

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    We employ atomically resolved and element-specific scanning transmission electron microscopy (STEM) to visualize <i>in situ</i> and at the atomic scale the crystallization and restructuring processes of two-dimensional (2D) molybdenum disulfide (MoS<sub>2</sub>) films. To this end, we deposit a model heterostructure of thin amorphous MoS<sub>2</sub> films onto freestanding graphene membranes used as high-resolution STEM supports. Notably, during STEM imaging the energy input from the scanning electron beam leads to beam-induced crystallization and restructuring of the amorphous MoS<sub>2</sub> into crystalline MoS<sub>2</sub> domains, thereby emulating widely used elevated temperature MoS<sub>2</sub> synthesis and processing conditions. We thereby directly observe nucleation, growth, crystallization, and restructuring events in the evolving MoS<sub>2</sub> films <i>in situ</i> and at the atomic scale. Our observations suggest that during MoS<sub>2</sub> processing, various MoS<sub>2</sub> polymorphs co-evolve in parallel and that these can dynamically transform into each other. We further highlight transitions from in-plane to out-of-plane crystallization of MoS<sub>2</sub> layers, give indication of Mo and S diffusion species, and suggest that, in our system and depending on conditions, MoS<sub>2</sub> crystallization can be influenced by a weak MoS<sub>2</sub>/graphene support epitaxy. Our atomic-scale <i>in situ</i> approach thereby visualizes multiple fundamental processes that underlie the varied MoS<sub>2</sub> morphologies observed in previous <i>ex situ</i> growth and processing work. Our work introduces a general approach to <i>in situ</i> visualize at the atomic scale the growth and restructuring mechanisms of 2D transition-metal dichalcogenides and other 2D materials

    Atomic-Scale <i>in Situ</i> Observations of Crystallization and Restructuring Processes in Two-Dimensional MoS<sub>2</sub> Films

    No full text
    We employ atomically resolved and element-specific scanning transmission electron microscopy (STEM) to visualize <i>in situ</i> and at the atomic scale the crystallization and restructuring processes of two-dimensional (2D) molybdenum disulfide (MoS<sub>2</sub>) films. To this end, we deposit a model heterostructure of thin amorphous MoS<sub>2</sub> films onto freestanding graphene membranes used as high-resolution STEM supports. Notably, during STEM imaging the energy input from the scanning electron beam leads to beam-induced crystallization and restructuring of the amorphous MoS<sub>2</sub> into crystalline MoS<sub>2</sub> domains, thereby emulating widely used elevated temperature MoS<sub>2</sub> synthesis and processing conditions. We thereby directly observe nucleation, growth, crystallization, and restructuring events in the evolving MoS<sub>2</sub> films <i>in situ</i> and at the atomic scale. Our observations suggest that during MoS<sub>2</sub> processing, various MoS<sub>2</sub> polymorphs co-evolve in parallel and that these can dynamically transform into each other. We further highlight transitions from in-plane to out-of-plane crystallization of MoS<sub>2</sub> layers, give indication of Mo and S diffusion species, and suggest that, in our system and depending on conditions, MoS<sub>2</sub> crystallization can be influenced by a weak MoS<sub>2</sub>/graphene support epitaxy. Our atomic-scale <i>in situ</i> approach thereby visualizes multiple fundamental processes that underlie the varied MoS<sub>2</sub> morphologies observed in previous <i>ex situ</i> growth and processing work. Our work introduces a general approach to <i>in situ</i> visualize at the atomic scale the growth and restructuring mechanisms of 2D transition-metal dichalcogenides and other 2D materials

    Atomic-Scale <i>in Situ</i> Observations of Crystallization and Restructuring Processes in Two-Dimensional MoS<sub>2</sub> Films

    No full text
    We employ atomically resolved and element-specific scanning transmission electron microscopy (STEM) to visualize <i>in situ</i> and at the atomic scale the crystallization and restructuring processes of two-dimensional (2D) molybdenum disulfide (MoS<sub>2</sub>) films. To this end, we deposit a model heterostructure of thin amorphous MoS<sub>2</sub> films onto freestanding graphene membranes used as high-resolution STEM supports. Notably, during STEM imaging the energy input from the scanning electron beam leads to beam-induced crystallization and restructuring of the amorphous MoS<sub>2</sub> into crystalline MoS<sub>2</sub> domains, thereby emulating widely used elevated temperature MoS<sub>2</sub> synthesis and processing conditions. We thereby directly observe nucleation, growth, crystallization, and restructuring events in the evolving MoS<sub>2</sub> films <i>in situ</i> and at the atomic scale. Our observations suggest that during MoS<sub>2</sub> processing, various MoS<sub>2</sub> polymorphs co-evolve in parallel and that these can dynamically transform into each other. We further highlight transitions from in-plane to out-of-plane crystallization of MoS<sub>2</sub> layers, give indication of Mo and S diffusion species, and suggest that, in our system and depending on conditions, MoS<sub>2</sub> crystallization can be influenced by a weak MoS<sub>2</sub>/graphene support epitaxy. Our atomic-scale <i>in situ</i> approach thereby visualizes multiple fundamental processes that underlie the varied MoS<sub>2</sub> morphologies observed in previous <i>ex situ</i> growth and processing work. Our work introduces a general approach to <i>in situ</i> visualize at the atomic scale the growth and restructuring mechanisms of 2D transition-metal dichalcogenides and other 2D materials

    Chemical Oxidation of Graphite: Evolution of the Structure and Properties

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    Graphene oxide is a complex material whose synthesis is still incompletely understood. To study the time evolution of structural and chemical properties of oxidized graphite, samples at different temporal stages of oxidation were selected and characterized through a number of techniques: X-ray photoelectron spectroscopy for the content and bonding of oxygen, X-ray diffraction for the level of intercalation, Raman spectroscopy for the detection of structural changes, electrical resistivity measurements for probing charge localization on the macroscopic scale, and scanning transmission electron microscopy for the atomic structure of the graphene oxide flakes. We found a nonlinear behavior of oxygen uptake with time where two concentration plateaus were identified: Uptake reached 20 at % in the first 15 min, and after 1 h a second uptake started, reaching a highest oxygen concentration of >30 at % after 2 h of oxidation. At the same time, the interlayer distance expanded to more than twice the value of graphite and the electrical resistivity increased by seven orders of magnitude. After 4 days of chemical processing, the expanded structure of graphite oxide became unstable and spontaneously exfoliated; more than 2 weeks resulted in a significant decrease in the oxygen content accompanied by reaggregation of the GO sheets. These correlated measurements allow us to offer a comprehensive view into the complex oxidation process

    High-Performance Hybrid Electronic Devices from Layered PtSe<sub>2</sub> Films Grown at Low Temperature

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    Layered two-dimensional (2D) materials display great potential for a range of applications, particularly in electronics. We report the large-scale synthesis of thin films of platinum diselenide (PtSe<sub>2</sub>), a thus far scarcely investigated transition metal dichalcogenide. Importantly, the synthesis by thermally assisted conversion is performed at 400 °C, representing a breakthrough for the direct integration of this material with silicon (Si) technology. Besides the thorough characterization of this 2D material, we demonstrate its promise for applications in high-performance gas sensing with extremely short response and recovery times observed due to the 2D nature of the films. Furthermore, we realized vertically stacked heterostructures of PtSe<sub>2</sub> on Si which act as both photodiodes and photovoltaic cells. Thus, this study establishes PtSe<sub>2</sub> as a potential candidate for next-generation sensors and (opto-)­electronic devices, using fabrication protocols compatible with established Si technologies
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