Atomic Structure of Intrinsic and Electron-Irradiation-Induced Defects in MoTe₂

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 MoTe₂ 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 MoTe₂ 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 MoTe₂ 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 MoTe₂

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 MoTe₂ 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 MoTe₂ 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 MoTe₂ 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 MoS₂ Films

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 (MoS₂) films. To this end, we deposit a model heterostructure of thin amorphous MoS₂ 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 MoS₂ into crystalline MoS₂ domains, thereby emulating widely used elevated temperature MoS₂ synthesis and processing conditions. We thereby directly observe nucleation, growth, crystallization, and restructuring events in the evolving MoS₂ films <i>in situ</i> and at the atomic scale. Our observations suggest that during MoS₂ processing, various MoS₂ 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 MoS₂ layers, give indication of Mo and S diffusion species, and suggest that, in our system and depending on conditions, MoS₂ crystallization can be influenced by a weak MoS₂/graphene support epitaxy. Our atomic-scale <i>in situ</i> approach thereby visualizes multiple fundamental processes that underlie the varied MoS₂ 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 MoS₂ Films

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 (MoS₂) films. To this end, we deposit a model heterostructure of thin amorphous MoS₂ 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 MoS₂ into crystalline MoS₂ domains, thereby emulating widely used elevated temperature MoS₂ synthesis and processing conditions. We thereby directly observe nucleation, growth, crystallization, and restructuring events in the evolving MoS₂ films <i>in situ</i> and at the atomic scale. Our observations suggest that during MoS₂ processing, various MoS₂ 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 MoS₂ layers, give indication of Mo and S diffusion species, and suggest that, in our system and depending on conditions, MoS₂ crystallization can be influenced by a weak MoS₂/graphene support epitaxy. Our atomic-scale <i>in situ</i> approach thereby visualizes multiple fundamental processes that underlie the varied MoS₂ 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 MoS₂ Films

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 (MoS₂) films. To this end, we deposit a model heterostructure of thin amorphous MoS₂ 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 MoS₂ into crystalline MoS₂ domains, thereby emulating widely used elevated temperature MoS₂ synthesis and processing conditions. We thereby directly observe nucleation, growth, crystallization, and restructuring events in the evolving MoS₂ films <i>in situ</i> and at the atomic scale. Our observations suggest that during MoS₂ processing, various MoS₂ 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 MoS₂ layers, give indication of Mo and S diffusion species, and suggest that, in our system and depending on conditions, MoS₂ crystallization can be influenced by a weak MoS₂/graphene support epitaxy. Our atomic-scale <i>in situ</i> approach thereby visualizes multiple fundamental processes that underlie the varied MoS₂ 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