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Controlled fabrication of nanopores in 2D materials offer the means to create robust membranes needed for ion transport and nanofiltration. Techniques for creating nanopores have relied upon either plasma etching or direct irradiation; however, aberration‐corrected scanning transmission electron microscopy (STEM) offers the advantage of combining a sub‐Å sized electron beam for atomic manipulation along with atomic resolution imaging. Here, a method for automated nanopore fabrication is utilized with real‐time atomic visualization to enhance the mechanistic understanding of beam‐induced transformations. Additionally, an electron beam simulation technique, Electron‐Beam Simulator (E‐BeamSim) is developed to observe the atomic movements and interactions resulting from electron beam irradiation. Using the MXene Ti3C2Tx, the influence of temperature on nanopore fabrication is explored by tracking atomic transformations and find that at room temperature the electron beam irradiation induces random displacement and results in titanium pileups at the nanopore edge, which is confirmed by E‐BeamSim. At elevated temperatures, after removal of the surface functional groups and with the increased mobility of atoms results in atomic transformations that lead to the selective removal of atoms layer by layer. This work can lead to the development of defect engineering techniques within functionalized MXene layers and other 2D materials. [ABSTRACT FROM AUTHOR]

Tuning the structural and electronic properties of atomically thin two-dimensional (2D) materials via defect and vacancy engineering is the key to enabling their potential use in various applications, including electronics, energy, and sensing devices. Vacancies are, for instance, becoming highly promising for the enhanced interaction of gases and biomolecules with 2D materials in energy and sensing applications. However, the deterministic generation of desirable vacancies with tunable concentrations remains a challenge in 2D materials due to the limitations in the current growth methods, such as the complex reaction chemistries and gas flow dynamics. Therefore, engineering defects and vacancies in 2D materials have been mainly limited to destructive top-down processes such as heating, ion bombardments, and laser postprocessing. Here, we introduce a single-step bottom-up synthesis approach for the growth of monolayer MoSe2 crystals with tunable vacancy concentrations. This method utilizes the spatiotemporal properties and adjustable power densities of the lasers to control the vaporization dynamics of the stoichiometric MoSe2 powders. Such a mechanism in the vaporization allows us to grow tunable stoichiometry monolayer MoSe2–x crystals on the substrates. The localized and time-controlled (250 ms to 2 s) vaporization of the MoSe2 powder by a CO2 laser enables the formation of monolayer crystals with controlled vacancy concentrations ranging from ∼1 to 20%. The effects of laser power, laser irradiation time, and background pressure on the tuning range and subsequent properties of the crystals are investigated and quantified using Raman and photoluminescence spectroscopy, scanning transmission electron microscopy (STEM), and time-correlated single-photon counting (TCSPC). This bottom-up synthesis is a promising approach that allows the deterministic vacancy tuning for future electronics and, in particular, gas and biosensing applications without the need for further postprocessing and potential structural disruption of the crystals. [ABSTRACT FROM AUTHOR]

Abstract: Group 14 nanocrystals (NCs) have gained significant attention in the last decade largely owing to their unique and tunable optoelectronic properties, allowing for diverse applications across a range of fields. Herein we report a gram‐scale method to prepare Si and Ge NCs from mesoporous metal oxides using a solid‐state metallothermic reduction method. Mesoporous SiO2 and GeO2 were prepared using a templated sol‐gel method. The influence of pore size of the metal oxide precursor and the nature of reducing metal on the formation of Si and Ge was investigated. The NCs were functionalized with dodecyl groups via microwave‐assisted hydrosilylation and hydrogermylation reactions, respectively. Si and Ge NCs were found to have tunable visible or near‐IR luminescence with high emission quantum yields. [ABSTRACT FROM AUTHOR]

Sodium-ion batteries are attractive because of their potentially lower cost than lithium-ion systems. However, the larger ionic radius of Na+ means that candidate electrode materials often undergo more substantial volumetric changes during reaction as compared to Li-ion batteries, and these changes must be understood and controlled for the development of electrode materials with long cycle life. Here, nanoscale-to-macroscale transformation pathways are investigated in real time in Cu2S (a sulfide electrode material) during electrochemical reaction with Na and Li. In situ and ex situ X-ray diffraction reveal that the overall phase transformations in Cu2S electrodes are similar within both Na and Li cells. However, in situ transmission electron microscopy (TEM) shows that the nanoscale reaction pathways differ significantly, which likely contributes to observed differences in electrochemical behavior. Despite these dissimilarities, Na/Cu2S electrochemical cells are shown to exhibit excellent cycle life for the first time (negligible capacity decay over 400 cycles), which is similar to the Li case. Thus, although the more substantial volume changes during the sodiation of Cu2S induce a new reaction pathway, they do not cause accelerated capacity decay, as is commonly argued for Na-ion materials. These results suggest that other large-volume-change electrode materials may also be engineered for long cycle life in next-generation Na-ion batteries. [ABSTRACT FROM AUTHOR]

Multiple polytypes of MoTe 2 with distinct structures and intriguing electronic properties can be accessed by various physical and chemical approaches. Here, we report electrochemical lithium (Li) intercalation into 1T'-MoTe 2 nanoflakes, leading to the discovery of two previously unreported lithiated phases. Distinguished by their structural differences from the pristine 1T' phase, these distinct phases were characterized using in situ polarization Raman spectroscopy and in situ single-crystal X-ray diffraction. The lithiated phases exhibit increasing resistivity with decreasing temperature, and their carrier densities are two to 4 orders of magnitude smaller than the metallic 1T' phase, as probed through in situ Hall measurements. The discovery of these gapped phases in initially metallic 1T'-MoTe 2 underscores electrochemical intercalation as a potent tool for tuning the phase stability and electron density in two-dimensional (2D) materials.

It has been proposed to use magnesium oxide (MgO) to separate carbon dioxide directly from the atmosphere at the gigaton level. We show experimental results on MgO single crystals reacting with the atmosphere for longer (decades) and shorter (days to months) periods with the goal of gauging reaction rates. Here, we find a substantial slowdown of an initially fast reaction as a result of mineral armoring by reaction products (surface passivation). In short-term experiments, we observe fast hydroxylation, carbonation, and formation of amorphous hydrated magnesium carbonate at early stages, leading to the formation of crystalline hydrated Mg carbonates. The preferential location of Mg carbonates along the atomic steps on the crystal surface of MgO indicates the importance of the reactive site density for carbonation kinetics. The analysis of 27-year-old single-crystal MgO samples demonstrates that the thickness of the reacted layer is limited to ∼1.5 μm on average, which is thinner than expected and indicates surface passivation. Thus, if MgO is to be employed for direct air capture of CO 2 , surface passivation must be circumvented.

Iron trifluoride (FeF 3 ), a conversion-type cathode for sodium-ion batteries (SIBs), is based on cheap and abundant Fe and provides high theoretical capacity. However, the applications of FeF 3 -based SIBs have been hindered by their low-capacity utilization and poor cycling stability. Herein, we report greatly enhanced performance of FeF 3 in multiple types of ionic liquid (IL) electrolytes at both room temperature (RT) and elevated temperatures. The Pyr 1,4 FSI electrolyte demonstrated the best cycling stability with an unprecedented decay rate of only ∼0.023% per cycle after the initial stabilization and an average coulombic efficiency of ∼99.5% for over 1000 cycles at RT. The Pyr 1,3 FSI electrolyte demonstrated the best cycling stability with a capacity decay rate of only ∼0.25% per cycle at 60 °C. Cells using Pyr 1,3 FSI and EMIMFSI electrolytes also showed promising cycling stability with capacity decay rates of ∼0.039% and ∼0.030% per cycle over 1000 cycles, respectively. A protective and ionically conductive cathode electrolyte interphase (CEI) layer is formed during cycling in ILs, diminishing side reactions that commonly lead to gassing and excessive CEI growth in organic electrolytes, especially at elevated temperatures. Furthermore, the increased ionic conductivity and decreased viscosity of ILs at elevated temperatures help attain higher accessible capacity. The application of ILs sheds light on designing a protective CEI for its use in stable SIBs.

MoS 2 has important applications in (electro)catalysis and as a semiconductor for electronic devices. Other chemical species are commonly added to MoS 2 to increase catalytic activity or to alter electronic properties through substitutional or adsorption-based doping. While groundbreaking work has been devoted to determining the atomic-scale structure of MoS 2 and other layered transition-metal dichalcogenides (TMDCs), there is a lack of understanding of the dynamic processes that govern the evolution of these materials during synthesis. Here, in situ transmission electron microscopy (TEM) heating, in combination with larger length scale ex situ experiments, is used to investigate the effects of added Ni on the growth of MoS 2 during the thermolysis of the solid-state (NH 4 ) 2 MoS 4 precursor. Low concentrations of Ni are observed to cause significant differences in the MoS 2 crystallization and growth process, leading to an increase in MoS 2 crystal size. This is likely a result of the altered mobility of interfaces between crystals during growth. These findings demonstrate the important role of additional elements in controlling the evolution of TMDCs during synthesis, which should be considered when designing these materials for a variety of applications.

Nanoscale transition-metal dichalcogenide (TMDC) materials, such as MoS 2 , exhibit promising behavior in next-generation electronics and energy-storage devices. TMDCs have a highly anisotropic crystal structure, with edge sites and basal planes exhibiting different structural, chemical, and electronic properties. In virtually all applications, two-dimensional or bulk TMDCs must be interfaced with other materials (such as electrical contacts in a transistor). The presence of edge sites vs basal planes (i.e., the crystallographic orientation of the TMDC) could influence the chemical and electronic properties of these solid-state interfaces, but such effects are not well understood. Here, we use in situ X-ray photoelectron spectroscopy (XPS) to investigate how the crystallography and structure of MoS 2 influence chemical transformations at solid-state interfaces with various other materials. MoS 2 materials with controllably aligned crystal structures (horizontal vs vertical orientation of basal planes) were fabricated, and in situ XPS was carried out by sputter-depositing three different materials (Li, Ge, and Ag) onto MoS 2 within an XPS instrument while periodically collecting photoelectron spectra; these deposited materials are of interest due to their application in electronic devices or energy storage. The results showed that Li reacts readily with both crystallographic orientations of MoS 2 to form metallic Mo and Li 2 S, while Ag showed very little chemical or electronic interaction with either type of MoS 2 . In contrast, Ge showed significant chemical interactions with MoS 2 basal planes, but only minor chemical changes were observed when Ge contacted MoS 2 edge sites. These findings have implications for electronic transport and band alignment at these interfaces, which is of significant interest for a variety of applications.