as core materials for future spintronics given their potential for high Curie temperature (T C ) and efficient field-tunable magnetic properties. [1][2][3] Despite half-century-long research efforts, the following three primary issues remain unsolved: i) the uncertain origin of ferromagnetism, called phantom ferromagnetism, owing to a lack of structural analysis of nanodefects; [4,5] ii) solubility limit to only a few percent without forming aggregation; [6] and iii) activation of short-range antiferromagnetic transitions in the high-dopingconcentration regime, [7] which limits the improvement of the magnetic moment and T C .The recent emergence of magnetic order in 2D van der Waals layered materials, which is enabled by strong magnetic anisotropy, [8] has stimulated interest in 2D-DMSs owing to their exotic spindependent physical properties, including long spin-relaxation time, light-controlled magnetism, [9] and spin-valley locking, inherent to their atomically thin nature. [10][11][12] In particular, transition metal dichalcogenide (TMD) semiconductors with magnetic dopants synthesized via chemical vapor deposition (CVD) offer room-temperature T C and gate-tunable magnetism. [13][14][15] Although vanadium dopants in WSe 2 and WS 2 semiconductors have been successfully distributed randomly without aggregation to a relatively high doping concentration of approximately 10%, their saturation magnetization is still limited to approximately 10 −5 emu cm −2 , thus making further analysis and applications difficult. [14,15] While magnetism has been proposed for inducing various defects such as vacancies, [16] anti-sites, [17] and grain boundaries [18] in III-V, oxides, and nitride DMSs, the underlying mechanism of magnetism is little known mainly due to the lack of structural analysis. On the contrary, because of facile monolayer growth, a variety of defects, including transition metal and chalcogen vacancies in 2D-TMDs, can be precisely analyzed using state-of-the-art scanning transmission electron microscopy (STEM) with atomic elemental mapping. [19] This affords the possibility of elucidating the origins of magnetism from defects and further enhancing magnetic order by tailoring intrinsic defects and impurities in 2D-TMD semiconductors. Here, we present a comprehensive atomic analysis of Se-vacancy defects Magnetic order has been proposed to arise from a variety of defects, including vacancies, antisites, and grain boundaries, which are relevant in numerous electronics and spintronics applications. Nevertheless, its magnetism remains controversial due to the lack of structural analysis. The escalation of ferromagnetism in vanadium-doped WSe 2 monolayer is herein demonstrated by tailoring complex configurations of Se vacancies (Se Vac ) via post heat-treatment. Structural analysis of atomic defects is systematically performed using transmission electron microscopy (TEM), enabled by the monolayer nature. Temperature-dependent magnetoresistance hysteresis ensures enhanced magnetic order after high-temperature heat-tre...

Atomic dopants and defects play a crucial role in creating new functionalities in 2D transition metal dichalcogenides (2D TMDs). Therefore, atomic‐scale identification and their quantification warrant precise engineering that widens their application to many fields, ranging from development of optoelectronic devices to magnetic semiconductors. Scanning transmission electron microscopy with a sub‐Å probe has provided a facile way to observe local dopants and defects in 2D TMDs. However, manual data analytics of experimental images is a time‐consuming task, and often requires subjective decisions to interpret observed signals. Therefore, an approach is required to automate the detection and classification of dopants and defects. In this study, based on a deep learning algorithm, fully convolutional neural network that shows a superior ability of image segmentation, an efficient and automated method for reliable quantification of dopants and defects in TMDs is proposed with single‐atom precision. The approach demonstrates that atomic dopants and defects are precisely mapped with a detection limit of ≈1 × 1012 cm−2, and with a measurement accuracy of ≈98% for most atomic sites. Furthermore, this methodology is applicable to large volume of image data to extract atomic site‐specific information, thus providing insights into the formation mechanisms of various defects under stimuli.

Photoinduced generation of mobile charge carriers is the fundamental process underlying many applications, such as solar energy harvesting, solar fuel production, and efficient photodetectors. Monolayer transition-metal dichalcogenides (TMDCs) are an attractive model system for studying photoinduced carrier generation mechanisms in low-dimensional materials because they possess strong direct band gap absorption, large exciton binding energies, and are only a few atoms thick. While a number of studies have observed charge generation in neat TMDCs for photoexcitation at, above, or even below the optical band gap, the role of nonlinear processes (resulting from high photon fluences), defect states, excess charges, and layer interactions remains unclear. In this study, we introduce steady-state microwave conductivity (SSMC) spectroscopy for measuring charge generation action spectra in a model WS2 mono- to few-layer TMDC system at fluences that coincide with the terrestrial solar flux. Despite utilizing photon fluences well below those used in previous pump–probe measurements, the SSMC technique is sensitive enough to easily resolve the photoconductivity spectrum arising in mono- to few-layer WS2. By correlating SSMC with other spectroscopy and microscopy experiments, we find that photoconductivity is observed predominantly for excitation wavelengths resonant with the excitonic transition of the multilayer portions of the sample, the density of which can be controlled by the synthesis conditions. These results highlight the potential of layer engineering as a route toward achieving high yields of photoinduced charge carriers in neat TMDCs, with implications for a broad range of optoelectronic applications.

The imperfect interfaces between 2D transition metal dichalcogenides (TMDs) are suitable for boosting the hydrogen evolution reaction (HER) during water electrolysis. Here, the improved catalytic activity at the spatial heterojunction between 1T' Re x Mo 1−x S 2 and 2H MoS 2 is reported. Atomic-scale electron microscopy confirms that the heterojunction is constructed by an in-situ two-step growth process through chemical vapor deposition. Electrochemical microcell measurements demonstrate that the 1T' Re x Mo 1−x S 2 -2H MoS 2 lateral heterojunction exhibits the best HER catalytic performance among all TMD catalysts with an overpotential of ≈84 mV at 10 mA cm −2 current density and 58 mV dec −1 Tafel slope. Kelvin probe force microscopy shows ≈40 meV as the work function difference between 2H MoS 2 and 1T' Re x Mo 1−x S 2 , facilitating the electron transfer from 2H MoS 2 to 1T' Re x Mo 1−x S 2 at the heterojunction. Firstprinciples calculations reveal that Mo-rich heterojunctions with high structural stability are formed, and the HER performance is improved with the combination of increased density of states near the Fermi level and optimal ΔG H* as low as 0.07 eV. Those synergetic effects with many electrons and active sites with optimal ΔG H* improve HER performance at the heterojunction. These results provide new insights into understanding the role of the heterojunction for HER.

Transition metal dichalcogenides (TMDs) are being actively studied in nextgeneration semiconductor applications owing to their excellent optoelectronic properties. Therefore, numerous defect-related studies have been conducted to improve TMD quality. In the study of defects, Raman spectroscopy is widely used to obtain information regarding the defects on a surface. A single sulfur-vacancy-induced Raman peak was recently reported. However, the origin of this vibrational mode has not yet been identified. Therefore, quantum mechanical calculations were performed on the sulfur-vacancy-containing supercell structure to elucidate the origin. By calculating the band structure and phonon dispersion, the phonon momentum was obtained, considering the possible scattering of electrons. After comparing the phonon momentum and phonon dispersion, it was identified that the phonon vibrational origin of a single sulfurvacancy-induced Raman peak is A′ 1 (k).

Charge transfer plays a key role in the structural transformation of amyloid-β proteins (Aβs), as it fibrillizes from small monomers to intermediate oligomers and to ordered fibrils. While the protein fibrillization states have been identified using cryo-electron microscopy, X-ray diffraction, Raman, infrared, terahertz spectroscopies, etc., there is little known about the electronic states during the fibrilization of Aβ protein. Here, we probe the charge transfer of Aβ42 proteins at different aggregation stages adsorbed on monolayer graphene (Gr) and molybdenum disulfide (MoS2) using Raman spectroscopy. Monomers, oligomers, and fibrils prepared in buffer solutions were deposited and dried separately on Gr and MoS2 where well-established characteristic Raman modes (G, 2D for Gr and E2g, A1g for MoS2) were monitored. The shifts in Raman parameters showed that the small Aβ monomers withdraw electrons, whereas fibrils donate electrons to Gr and MoS2. Oligomers undergo transient charge states near the neutrality point. This is explained in terms of modulated carrier concentration in Gr and MoS2. This finding provides insight into the electronic properties of Aβs that could be essential to identifying the onset of toxic fibril forms and developing a straightforward, label-free diagnosis using Gr and MoS2.

High pressure or strain is an effective strategy for generating phase transformations in van der Waals (vdW) layered materials without introducing defects, but this approach remains difficult to perform consistently. We present a scalable and facile method for achieving phase transformation in vdW materials, wherein solid vdW materials are subject to internal thermal stress within a molten metal mantle as it undergoes cooling. This internal thermal stress is principally the product of differential thermal expansion between mantle and core and can be tuned by the mantle material and temperature conditions. We validated this approach by achieving phase transformation of red phosphorus to black phosphorus, and metallic 1T′- to semiconducting 2H-MoTe2 crystals. We further demonstrate quantum electronic phase transformation of suppressed charge density wave in TiSe2 by means of electron–phonon coupling using the same system.

Excitonic-insulating phases can be realized in semimetals or narrow bandgap semiconductors where the bandgap is smaller than the excitonbinding energy (E ex ). [2,3] Since the theoretical suggestion of excitonic insulator in 1960s, [1][2][3] recent experimental studies have successfully demonstrated evidence of the excitonic-insulating phases in real crystal systems. [7][8][9][10][11][12][13][14][15][16] Among them, Ta 2 NiSe 5 has been investigated intensively because of its relatively higher excitonic transition temperature (T c ) at ≈326 K. [13,14] Ta 2 NiSe 5 consists of three layers with alternating chemically bonded Ta and Ni atoms sandwiched by two Se layers that are further stacked by van der Waals interaction along the (020) direction. [13][14][15] An orthorhombic structure, space group of Cmcm, with a direct bandgap at Γ point in the Brillouin zone is energetically stable at high temperature above T c . [11][12][13][14][15] The exciton-binding energy (E ex ) is larger than the bandgap of orthorhombic phase. The excitonic-insulating phase transition occurs with structural transformation into monoclinic (space group C2/c) when T is lower than T c . [15] The excitonicinsulating transition in Ta 2 NiSe 5 is mainly driven by strong Coulomb interaction between electron and hole and, moreover, does not involve charge density wave (CDW), [11][12][13][14][15] which is well contrasted to emergence of CDW in 1T-TiSe 2 excitonic Excitonic insulators exhibit intriguing quantum phases that further attract numerous interests in engineering the electrical and optical properties of Ta 2 NiSe 5 . However, tuning the electronic properties such as spin-orbit coupling strength and orbital repulsion via pressure in Ta 2 NiSe 5 are always accompanied with electron-hole pair breaking, which is a bottleneck for further applications. Here, the robust excitonic-insulating states invariant with electron-doping concentrations in Ta 2 NiSe 5 are demonstrated. The electron doping is conducted by substituting Cu into Ni site (Ta 2 Ni 1-x Cu x Se 5 ). The majority carrier of pristine sample is a hole-type and is converted to electrontype with a doping concentration over x = 0.01, whose carrier density can be controlled by varying the Cu concentration. The excitonic transition temperature (T c ) does not significantly alter with electron-doping concentrations, which is stark contrast with the declining T c as the hole-type dopant of Fe or Co increases. The optical conductivity data also demonstrate the invariant excitonic-insulating states in Cu-doped Ta 2 NiSe 5 . The findings of invariant excitonic-insulating states in n-type Cu-substituted Ta 2 NiSe 5 can be utilized for further electronic device applications by using excitons.