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Abstract We report the growth of InSe films on semi-insulating GaAs(111)B substrates by molecular beam epitaxy (MBE). Excellent nucleation behavior resulted in the growth of smooth, single-phase InSe films. The dominant polytype was the targeted γ-InSe. Transmission electron microscopy revealed the presence of three bulk polytypes β, γ, and ε-InSe arranged in nanosized domains, which can be interpreted as sequences of stacking faults and rotational twin boundaries of γ-InSe. Additionally, a centrosymmetric Se-In-In-Se layer polymorph with$$P\bar{3}m$$ $P\bar{3}m$symmetry was identified as typically not present in bulk. Sizeable differences in their electronic properties were found, which resulted in sizeable electronic disorder arising from the nanoscale polytype arrangement that dominated the electronic transport properties. While MBE is a viable synthesis route towards stabilization of InSe polytypes not present in the bulk, an improved understanding to form the targeted polymorph is required to ultimately inscribe a layer sequence on demand utilizing bottom-up synthesis approaches. 

Abstract Two-dimensional (2D) materials have garnered significant attention in recent years due to their atomically thin structure and unique electronic and optoelectronic properties. To harness their full potential for applications in next-generation electronics and photonics, precise control over the dielectric environment surrounding the 2D material is critical. The lack of nucleation sites on 2D surfaces to form thin, uniform dielectric layers often leads to interfacial defects that degrade the device performance, posing a major roadblock in the realization of 2D-based devices. Here, we demonstrate a wafer-scale, low-temperature process (<250 °C) using atomic layer deposition (ALD) for the synthesis of uniform, conformal amorphous boron nitride (aBN) thin films. ALD deposition temperatures between 125 and 250 °C result in stoichiometric films with high oxidative stability, yielding a dielectric strength of 8.2 MV/cm. Utilizing a seed-free ALD approach, we form uniform aBN dielectric layers on 2D surfaces and fabricate multiple quantum well structures of aBN/MoS₂and aBN-encapsulated double-gated monolayer (ML) MoS₂field-effect transistors to evaluate the impact of aBN dielectric environment on MoS₂optoelectronic and electronic properties. Our work in scalable aBN dielectric integration paves a way towards realizing the theoretical performance of 2D materials for next-generation electronics. 

Abstract Understanding surface stability becomes critical as 2D materials like SnSe are developed for piezoelectric and optical applications. SnSe thin films deposited by molecular beam epitaxy showed no structural changes after a two-year exposure to atmosphere, as confirmed by X-ray diffraction and Raman spectroscopy. X-ray photoelectron spectroscopy and reflectivity show a stable 3.5 nm surface oxide layer, indicating a self-arresting oxidative process. Resistivity measurements show an electrical response dominated by SnSe post-exposure. This work shows that SnSe films can be used in ambient conditions with minimal risk of long-term degradation, which is critical for the development of piezoelectric or photovoltaic devices. Graphical Abstract 

Abstract The functionality of atomic quantum emitters is intrinsically linked to their host lattice coordination. Structural distortions that spontaneously break the lattice symmetry strongly impact their optical emission properties and spin-photon interface. Here we report on the direct imaging of charge state-dependent symmetry breaking of two prototypical atomic quantum emitters in mono- and bilayer MoS₂by scanning tunneling microscopy (STM) and non-contact atomic force microscopy (nc-AFM). By changing the built-in substrate chemical potential, different charge states of sulfur vacancies (Vac_S) and substitutional rhenium dopants (Re_Mo) can be stabilized.$${\mathrm{Vac}}_{{{{{{{{\rm{S}}}}}}}}}^{-1}$$ ${\mathrm{Vac}}_S^{-1}$as well as$${{\mathrm{Re}}}_{{{{{{{{\rm{Mo}}}}}}}}}^{0}$$ ${\mathrm{Re}}_{\mathrm{Mo}}^0$and$${\mathrm{Re}}_{{\rm{Mo}}}^{-1}$$ ${\mathrm{Re}}_{\mathrm{Mo}}^{-1}$exhibit local lattice distortions and symmetry-broken defect orbitals attributed to a Jahn-Teller effect (JTE) and pseudo-JTE, respectively. By mapping the electronic and geometric structure of single point defects, we disentangle the effects of spatial averaging, charge multistability, configurational dynamics, and external perturbations that often mask the presence of local symmetry breaking. 

Abstract 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 Ti₃C₂T_x, 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 There is tremendous interest in employing collective excitations of the lattice, spin, charge, and orbitals to tune strongly correlated electronic phenomena. We report such an effect in a ruthenate, Ca₃Ru₂O₇, where two phonons with strong electron-phonon coupling modulate the electronic pseudogap as well as mediate charge and spin density wave fluctuations. Combining temperature-dependent Raman spectroscopy with density functional theory reveals two phonons,B₂^PandB₂^M, that are strongly coupled to electrons and whose scattering intensities respectively dominate in the pseudogap versus the metallic phases. TheB₂^Psqueezes the octahedra along the out of planec-axis, while theB₂^Melongates it, thus modulating the Ru 4d orbital splitting and the bandwidth of the in-plane electron hopping; Thus,B₂^Popens the pseudogap, whileB₂^Mcloses it. Moreover, theB₂phonons mediate incoherent charge and spin density wave fluctuations, as evidenced by changes in the background electronic Raman scattering that exhibit unique symmetry signatures. The polar order breaks inversion symmetry, enabling infrared activity of these phonons, paving the way for coherent light-driven control of electronic transport. 

Abstract Recent advancements in the field of two-dimensional (2D) materials have led to the discovery of a wide range of 2D materials with intriguing properties. Atomistic-scale simulation methods have played a key role in these discoveries. In this review, we provide an overview of the recent progress in ReaxFF force field developments and applications in modeling the following layered and nonlayered 2D materials: graphene, transition metal dichalcogenides, MXenes, hexagonal boron nitrides, groups III-, IV- and V-elemental materials, as well as the mixed dimensional van der Waals heterostructures. We further discuss knowledge gaps and challenges associated with synthesis and characterization of 2D materials. We close this review with an outlook addressing the challenges as well as plans regarding ReaxFF development and possible large-scale simulations, which should be helpful to guide experimental studies in a discovery of new materials and devices. 

Abstract Vapor‐based deposition techniques are emerging approaches for the design of carbon‐supported metal powder electrocatalysts with tailored catalyst entities, sizes, and dispersions. Herein, a pulsed CVD (Pt‐pCVD) approach is employed to deposit different Pt entities on mesoporous N‐doped carbon (MPNC) nanospheres to design high‐performance hydrogen evolution reaction (HER) electrocatalysts. The influence of consecutive precursor pulse number (50‐250) and deposition temperature (225–300 °C) are investigated. The Pt‐pCVD process results in highly dispersed ultrasmall Pt clusters (≈1 nm in size) and Pt single atoms, while under certain conditions few larger Pt nanoparticles are formed. The best MPNC‐Pt‐pCVD electrocatalyst prepared in this work (250 pulses, 250 °C) reveals a Pt HER mass activity of 22.2 ± 1.2 A mg⁻¹_Ptat ‐50 mV versus the reversible hydrogen electrode (RHE), thereby outperforming a commercially available Pt/C electrocatalyst by 40% as a result of the increased Pt utilization. Remarkably, after optimization of the Pt electrode loading, an ultrahigh Pt mass activity of 56 ± 2 A mg⁻¹_Ptat ‐50 mV versus RHE is found, which is among the highest Pt mass activities of Pt single atom and cluster‐based electrocatalysts reported so far. 

Abstract Intercalation forms heterostructures, and over 25 elements and compounds are intercalated into graphene, but the mechanism for this process is not well understood. Here, the de‐intercalation of 2D Ag and Ga metals sandwiched between bilayer graphene and SiC are followed using photoemission electron microscopy (PEEM) and atomistic‐scale reactive molecular dynamics simulations. By PEEM, de‐intercalation “windows” (or defects) are observed in both systems, but the processes follow distinctly different dynamics. Reversible de‐ and re‐intercalation of Ag is observed through a circular defect where the intercalation velocity front is 0.5 nm s⁻¹± 0.2 nm s.⁻¹In contrast, the de‐intercalation of Ga is irreversible with faster kinetics that are influenced by the non‐circular shape of the defect. Molecular dynamics simulations support these pronounced differences and complexities between the two Ag and Ga systems. In the de‐intercalating Ga model, Ga atoms first pile up between graphene layers until ultimately moving to the graphene surface. The simulations, supported by density functional theory, indicate that the Ga atoms exhibit larger binding strength to graphene, which agrees with the faster and irreversible diffusion kinetics observed. Thus, both the thermophysical properties of the metal intercalant and its interaction with defective graphene play a key role in intercalation. 

Abstract Reproducible wafer-scale growth of two-dimensional (2D) materials using the Chemical Vapor Deposition (CVD) process with precise control over their properties is challenging due to a lack of understanding of the growth mechanisms spanning over several length scales and sensitivity of the synthesis to subtle changes in growth conditions. A multiscale computational framework coupling Computational Fluid Dynamics (CFD), Phase-Field (PF), and reactive Molecular Dynamics (MD) was developed – called the CPM model – and experimentally verified. Correlation between theoretical predictions and thorough experimental measurements for a Metal-Organic CVD (MOCVD)-grown WSe₂model material revealed the full power of this computational approach. Large-area uniform 2D materials are synthesized via MOCVD, guided by computational analyses. The developed computational framework provides the foundation for guiding the synthesis of wafer-scale 2D materials with precise control over the coverage, morphology, and properties, a critical capability for fabricating electronic, optoelectronic, and quantum computing devices.