Titanium Self-Intercalation Induced Formation of Orthogonal (1 × 1) Edge/Surface Reconstruction in 1T-TiSe2: Atomic Scale Dynamics and Mechanistic Study.

Edges and surfaces play indispensable roles in affecting the chemical-physical properties of materials, particularly in two-dimensional transition metal dichalcogenides (TMDCs) with reduced dimensionality. Herein, we report a novel edge/surface structure in multilayer 1T-TiSe2, i.e., the orthogonal (1 × 1) reconstruction, induced by the self-intercalation of Ti atoms into interlayer octahedral sites of the host TiSe2 at elevated temperature. Formation dynamics of the reconstructed edge/surface are captured at the atomic level by in situ scanning transmission electron microscopy (STEM) and further validated by density functional theory (DFT), which enables the proposal of the nucleation mechanism and two growth routes (zigzag and armchair). Via STEM-electron energy loss spectroscopy (STEM-EELS), a chemical shift of 0.6 eV in Ti L3,2 is observed in the reconstructed edge/surface, which is attributed to the change of the coordination number and lattice distortion. The present work provides insights to tailor the atomic/electronic structures and properties of 2D TMDC materials.

Mechanism Regulating Self-Intercalation in Layered Materials.

Recent experimental breakthrough demonstrated a powerful synthesis approach for intercalating the van der Waals gap of layered materials to achieve property modulation, thereby opening an avenue for exploring new physics and devising novel applications, but the mechanism governing intercalant assembly patterns and properties remains unclear. Based on extensive structural search and energetics analysis by ab initio calculations, we reveal a Sabatier-like principle that dictates spatial arrangement of self-intercalated atoms in transition metal dichalcogenides. We further construct a robust descriptor quantifying that strong intercalant-host interactions favor a monodispersing phase of intercalated atoms that may exhibit ferromagnetism, while weak interactions lead to a trimer phase with attenuated or quenched magnetism, which further evolves into tetramer and hexagonal phases at increasing intercalant density. These findings elucidate the mechanism underpinning experimental observations and paves the way for rational design and precise control of self-intercalation in layered materials.

Self-Intercalated 1T-FeSe2 as an Effective Kagome Lattice.

In kagome lattice, with the emergence of Dirac cones and flat band in electronic structure, it provides a versatile ground for exploring intriguing interplay among frustrated geometry, topology and correlation. However, such engaging interest is strongly limited by available kagome materials in nature. Here we report on a synthetic strategy of constructing kagome systems via self-intercalation of Fe atoms into the van der Waals gap of FeSe2 via molecular beam epitaxy. Using low-temperature scanning tunneling microscopy, we unveil a kagome-like morphology upon intercalating a 2 × 2 ordered Fe atoms, resulting in a stoichiometry of Fe5Se8. Both the bias-dependent STM imaging and theoretical modeling calculations suggest that the kagome pattern mainly originates from slight but important reconstruction of topmost Se atoms, incurred by the nonequivalent subsurface Fe sites due to the intercalation. Our study demonstrates an alternative approach of constructing artificial kagome structures, which envisions to be tuned for exploring correlated quantum states.

Giant g-factor in Self-Intercalated 2D TaS2.

Central to the application of spintronic devices is the ability to manipulate spins by electric and magnetic fields, which relies on a large Landé g-factor. The self-intercalation of layered transitional metal dichalcogenides with native metal atoms can serve as a new strategy to enhance the g-factor by inducing ferromagnetic instability in the system via interlayer charge transfer. Here, scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) are performed to extract the g-factor and characterize the electronic structure of the self-intercalated phase of 2H-TaS2 . In Ta7 S12 , a sharp density of states (DOS) peak due to the Ta intercalant appears at the Fermi level, which satisfies the Stoner criteria for spontaneous ferromagnetism, leading to spin split states. The DOS peak shows sensitivity to magnetic field up to 1.85 mV T-1 , equivalent to an effective g-factor of ≈77. This work establishes self-intercalation as an approach for tuning the g-factor.

Van der Waals Superstructure and Twisting in Self-Intercalated Magnet with Near Room-Temperature Perpendicular Ferromagnetism.

The emergence of van der Waals (vdW) magnets has created unprecedented opportunities to manipulate magnetism for advanced spintronics based upon all-vdW heterostructures. Among various vdW magnets, Cr1+δTe2 possesses high temperature ferromagnetism along with possible topological spin textures. As this system can support self-intercalation in the vdW gap, it is crucial to precisely pinpoint the exact intercalation to understand the intrinsic magnetism of the system. Here, we developed an iterative method to determine the self-intercalated structures and show evidence of vdW "superstructures" in individual Cr1+δTe2 nanoplates exhibiting magnetic behaviors distinct from bulk chromium tellurides. Among 26,332 possible configurations, we unambiguously identified the Cr-intercalated structure as 3-fold symmetry broken Cr1.5Te2 segmented by vdW gaps. Moreover, a twisted Cr-intercalated layered structure is observed. The spontaneous formation of twisted vdW "superstructures" not only provides insight into the diverse magnetic properties of intercalated vdW magnets but may also add complementary building blocks to vdW-based spintronics.

Phase and Composition Engineering of Self-Intercalated 2D Metallic Tantalum Sulfide for Second-Harmonic Generation.

Self-intercalation in two-dimensional (2D) materials is significant, as it offers a versatile approach to modify material properties, enabling the creation of interesting functional materials, which is essential in advancing applications across various fields. Here, we define ic-2D materials as covalently bonded compounds that result from the self-intercalation of a metal into layered 2D compounds. However, precisely growing ic-2D materials with controllable phases and self-intercalation concentrations to fully exploit the applications in the ic-2D family remains a great challenge. Herein, we demonstrated the controlled synthesis of self-intercalated H-phase and T-phase Ta1+xS2 via a temperature-driven chemical vapor deposition (CVD) approach with a viable intercalation concentration spanning from 10% to 58%. Atomic-resolution scanning transmission electron microscopy-annular dark field imaging demonstrated that the self-intercalated Ta atoms occupy the octahedral vacancies located at the van der Waals gap. The nonperiodic Ta atoms break the centrosymmetry structure and Fermi surface properties of intrinsic TaS2. Therefore, ic-2D T-phase Ta1+xS2 consistently exhibit a spontaneous nonlinear optical (NLO) effect regardless of the sample thickness and self-intercalation concentrations. Our results propose an approach to activate the NLO response of centrosymmetric 2D materials, achieving the modulation of a wide range of optoelectronic properties via nonperiodic self-intercalation in the ic-2D family.

Room-Temperature Magnetic Skyrmions and Large Topological Hall Effect in Chromium Telluride Engineered by Self-Intercalation.

Room-temperature magnetic skyrmion materials exhibiting robust topological Hall effect (THE) are crucial for novel nano-spintronic devices. However, such skyrmion-hosting materials are rare in nature. In this study, a self-intercalated transition metal dichalcogenide Cr1+ x Te2 with a layered crystal structure that hosts room-temperature skyrmions and exhibits large THE is reported. By tuning the self-intercalate concentration, a monotonic control of Curie temperature from 169 to 333 K and a magnetic anisotropy transition from out-of-plane to the in-plane configuration are achieved. Based on the intercalation engineering, room-temperature skyrmions are successfully created in Cr1.53 Te2 with a Curie temperature of 295 K and a relatively weak perpendicular magnetic anisotropy. Remarkably, a skyrmion-induced topological Hall resistivity as large as ≈106 nΩ cm is observed at 290 K. Moreover, a sign reversal of THE is also found at low temperatures, which can be ascribed to other topological spin textures having an opposite topological charge to that of the skyrmions. Therefore, chromium telluride can be a new paradigm of the skyrmion material family with promising prospects for future device applications.

Composition-Controllable Syntheses and Property Modulations from 2D Ferromagnetic Fe5 Se8 to Metallic Fe3 Se4 Nanosheets.

Exploring new-type 2D magnetic materials with high magnetic transition temperature and robust air stability has attracted wide attention for developing innovative spintronic devices. Recently, intercalation of native metal atoms into the van der Waals gaps of 2D layered transition metal dichalcogenides (TMDs) has been developed to form 2D non-layered magnetic TMDs, while only succeeded in limited systems (e.g., Cr2 S3 , Cr5 Te8 ). Herein, composition-controllable syntheses of 2D non-layered iron selenide nanosheets (25% Fe-intercalated triclinic Fe5 Se8 and 50% Fe-intercalated monoclinic Fe3 Se4 ) are firstly reported, via a robust chemical vapor deposition strategy. Specifically, the 2D Fe5 Se8 exhibits intrinsic room-temperature ferromagnetic property, which is explained by the change of electron spin states from layered 1T'-FeSe2 to non-layered Fe-intercalated Fe5 Se8 based on density functional theory calculations. In contrast, the ultrathin Fe3 Se4 presents novel metallic features comparable with that of metallic TMDs. This work hereby sheds light on the composition-controllable synthesis and fundamental property exploration of 2D self-intercalation induced novel TMDs compounds, by propelling their application explorations in nanoelectronics and spintronics-related fields.

Collective Magnetic Behavior in Vanadium Telluride Induced by Self-Intercalation.

Self-intercalation of native magnetic atoms within the van der Waals (vdW) gap of layered two-dimensional (2D) materials provides a degree of freedom to manipulate magnetism in low-dimensional systems. Among various vdW magnets, the vanadium telluride is an interesting system to explore the interlayer order-disorder transition of magnetic impurities due to its flexibility in taking nonstoichiometric compositions. In this work, we combine high-resolution scanning transmission electron microscopy (STEM) analysis with density functional theory (DFT) calculations and magnetometry measurements, to unveil the local atomic structure and magnetic behavior of V-rich V1+xTe2 nanoplates with embedded V3Te4 nanoclusters grown by chemical vapor deposition (CVD). The segregation of V intercalations locally stabilizes the self-intercalated V3Te4 magnetic phase, which possesses a distorted 1T'-like monoclinic structure. This phase transition is controlled by the electron doping from the intercalant V ions. The magnetic hysteresis loops show that the nanoplates exhibit superparamagnetism, while the temperature-dependent magnetization curves evidence a collective superspin-glass magnetic behavior of the nanoclusters at low temperature. Using four-dimensional (4D) STEM diffraction imaging, we reveal the formation of collective diffuse magnetic domain structures within the sample under the high magnetic fields inside the electron microscope. Our results shed light on the studies of dilute magnetism at the 2D limit and on strategies for the manipulation of magnetism for spintronic applications.

Active Sulfur-Host Material VS4 with Surface Defect Engineering: Intercalation-Conversion Hybrid Cathode Boosting Electrochemical Performance of Li-S Batteries.

Transition-metal sulfides as late-model electrocatalysts usually remain inactive in lithium-sulfur (Li-S) batteries in spite of their advantages to accelerate the rapid conversion of lithium polysulfides (LiPSs). Herein, a series of cobalt-doped vanadium tetrasulfide/reduced graphene oxide (x%Co-VS4/rGO) composites with an ultrathin layered structure as an active sulfur-host material are prepared by a one-pot hydrothermal method. The well-designed two-dimensional ultrathin 3%Co-VS4/rGO with heteroatom architecture defects (defect of Co-doping and defect of S-vacancies) can significantly improve the adsorption ability on LiPSs, the electrocatalytic activity in the Li2S potentiostatic deposition, and the active sulfur reduction/oxidation conversion reactions and greatly boost the electrochemical performances of Li-S batteries. On the one hand, the ultrathin 3%Co-VS4/rGO possesses good conductivity inheriting from rGO which contributes to the capacity of internal redox reactions on lithiation from VS4. On the other hand, the hybrid architectures provide strong adsorption and excellent electrocatalytic ability on LiPSs, which benefit from the surface defects caused by heteroatom doping. The S@3%Co-VS4/rGO cathode displays a high specific capacity of 1332.6 mA h g-1 at 0.2 C and a low-capacity decay of only 0.05% per cycle over 1000 cycles at 3 C with a primary capacity of 633.1 mA h g-1. Furthermore, when the sulfur loading (single-side coating) reaches 4.48 mg cm-2, it still can deliver 756.2 mA h g-1 after the 100th cycle at 0.2 C with 89.5% capacity retention. In addition, the in situ X-ray diffraction test reveals that the sulfur conversion mechanism is the processes of α-S8 → Li2S → ß-S8 (first cycle) and then ß-S8 ↔ Li2S during the subsequent cycles. The designing strategy with heteroatom doping and self-intercalation capacity adopted in this work would provide novel inspiration for fabricating advanced sulfur-host materials to achieve excellent electrochemical capability in Li-S batteries.

On-Site Synthesis and Characterizations of Atomically-Thin Nickel Tellurides with Versatile Stoichiometric Phases through Self-Intercalation.

Self-intercalation of native metal atoms in two-dimensional (2D) transition metal dichalcogenides has received rapidly increasing interest, due to the generation of intriguing structures and exotic physical properties, however, only reported in limited materials systems. An emerging type-II Dirac semimetal, NiTe2, has inspired great attention at the 2D thickness region, but has been rarely achieved so far. Herein, we report the direct synthesis of mono- to few-layer Ni-tellurides including 1T-NiTe2 and Ni-rich stoichiometric phases on graphene/SiC(0001) substrates under ultra-high-vacuum conditions. Differing from previous chemical vapor deposition growth accompanied with transmission electron microscopy imaging, this work combines precisely tailored synthesis with on-site atomic-scale scanning tunneling microscopy observations, offering us visual information about the phase modulations of Ni-tellurides from 1T-phase NiTe2 to self-intercalated Ni3Te4 and Ni5Te6. The synthesis of Ni self-intercalated NixTey compounds is explained to be mediated by the high metal chemical potential under Ni-rich conditions, according to density functional theory calculations. More intriguingly, the emergence of superconductivity in bilayer NiTe2 intercalated with 50% Ni is also predicted, arising from the enhanced electron-phonon coupling strength after the self-intercalation. This work provides insight into the direct synthesis and stoichiometric phase modulation of 2D layered materials, enriching the family of self-intercalated materials and propelling their property explorations.

Elimination of Interlayer Potential Barriers of Chromium Sulfide by Self-Intercalation for Enhanced Hydrogen Evolution Reaction.

The van der Waals (vdW) gaps in layered transition-metal dichalcogenides (TMDs) with an interlayer poor charge transport are considered the bottleneck for higher hydrogen evolution reaction (HER) performance of TMDs. Filling the vdW gap of TMDs materials with intercalants is considered a good way to generate new interesting properties. However, postsynthesis intercalation with foreign atoms may bring extra crystalline imperfections and low yields. In this work, to overcome the interlayer potential barriers of TMDs, CrS2-Cr1/3-CrS2 is produced by naturally self-intercalating native Cr1/3 atom plane into the vdW layered CrS2. The CrS2-Cr1/3-CrS2 exhibits strong chemical bonds and high electrical conductivity, which can provide excellent HER electrocatalytic performance. Moreover, based on the first-principles calculations and experimental verification, the intercalated Cr atoms exhibit a Gibbs free energy of the adsorbed hydrogen close to zero and could further improve the electrocatalytic HER performance. Our work provides a new view in self-intercalation for electrocatalysis applications.

Electron Density Optimization and the Anisotropic Thermoelectric Properties of Ti Self-Intercalated Ti1+ xS2 Compounds.

Polycrystalline Ti1+ xS2 (0.111 ≤ x ≤ 0.161) with high density and controllable composition were successfully prepared using solid-state reaction combined with plasma-activated sintering. Ti1+ xS2 showed strong (00 l) preferred orientation with Lotgering factor of 0.32-0.60 perpendicular to the pressing direction (⊥), whereas the preferred orientation was not obvious along the pressing direction (∥). This structural anisotropy resulted in distinct anisotropic thermoelectric transport properties in Ti1+ xS2. At 300 K, while the Seebeck coefficient was weak anisotropic, the power factor and lattice thermal conductivity of Ti1+ xS2 was much larger in the perpendicular direction as compared to that of the parallel direction, with an anisotropic ratio of 1.8-2.7 and 1.3-1.7, respectively. Theoretical calculations of formation energy of defects suggested that the excess Ti was most probably intercalated into the van der Waals gaps in metal-rich Ti1+ xS2, consistent with X-ray diffraction, high-resolution transmission electron microscopy characterization and transport measurements. With increasing x, the carrier concentration and power factor of Ti1+ xS2 dramatically increased because of the donor behavior of Ti interstitials, which was accompanied by a significant decrease in the lattice thermal conductivity owing to the strengthened phonon scattering from structural disorder. Because of its strongest (00 l) preferred orientation and largest carrier mobility among all samples, Ti1.112S2 had the highest power factor of 22 µW cm-1 K-2 at 350 K perpendicular to the pressing direction, close to the value (37.1 µW cm-1 K-2) achieved in single-crystal TiS2. We found out that the maximum power factor and dimensionless figure of merit ZT could be achieved at an optimum carrier concentration of about 5.0 × 1020 cm-3. Finally, Ti1.142S2 acquired the highest ZT value of 0.40 at 725 K perpendicular to the pressing direction because of the beneficial preferred orientation, improved power factor, and reduced lattice thermal conductivity.