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<jats:title>Abstract</jats:title> <jats:p>Heterostructures based on two-dimensional transition metal dichalcogenides (TMDC) are highly intriguing materials because of the layers' pronounced excitonic properties and their nontrivial contributions to the heterostructure (HS). These heterostructures exhibit unique properties that are not observed in either of the constituent components in isolation. Interlayer excitons, which are electron-hole pairs separated across the heterostructures, play a central role in determining these heterostructure properties and are of interest both fundamentally and for device applications. In recent years, a major focus has been on understanding and designing heterostructures composed of two or more TMDC materials. Less attention has been paid to heterostructures composed of one TMDC layer and a layer of perovskite material. A central challenge in the understanding of HS properties is that basic measurements such as optical spectroscopic analysis can be misinterpreted due to the complexity of the charge transfer dynamics. Addressing these aspects, this review presents an overview of the most common and insightful optical spectroscopic techniques used to study TMDC/TMDC and TMDC/halide perovskite HSs. Emphasis is placed on the interpretation of these measurements in terms of charge transfer and the formation of interlayer excitons. Recent advances have started to uncover highly interesting phenomena, and with improved understanding these heterostructures offer great potential for device applications such as photodetectors and miniaturized optics.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Grain boundaries play a major role for electron transport in graphene sheets grown by chemical vapor deposition. Here we investigate the electronic structure and transport properties of idealized graphene grain boundaries (GBs) in bi-crystals using first principles density functional theory (DFT) and non-equilibrium Greens functions (NEGF).&amp;#xD;We generated \TotalStructs different grain boundaries using an automated workflow where their geometry is relaxed with DFT. We find that the GBs generally show a quasi-1D bandstructure along the GB.&amp;#xD;We group the GBs in four classes based on their conductive properties: transparent, opaque, insulating, and spin-polarizing and show how this is related to angular mismatch, quantum mechanical interference, and out-of-plane buckling. Especially, we find that spin-polarization in the GB correlates with out-of-plane buckling. We further investigate the characteristics of these classes in simulated scanning tunnelling spectroscopy and diffusive transport along the GB which demonstrate how current can be guided along the GB.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Van der Waals heterostructures utilizing semiconducting transition metal dichalcogenide (TMDC) monolayers have surfaced as compelling candidates due to their intriguing optical characteristics, which can be effectively controlled by the manipulation of the stacking twist angle. This study investigates the intricate correlation between twist angle, band offset, and interlayer exciton emission within twisted WSe2/MoSe2 heterostructures. Our findings suggest a crucial influence of monolayer stacking order on the band offset and the dipole orientation in twisted heterostructures that leads to either blueshift or redshift in emission energy. Herein, we fabricate heterobilayers with twist angles varying from 1° to 56° and observe an anomalous redshift energy of 100 meV in the interlayer exciton emission. Additionally, photoluminescence excitation spectroscopy measurements highlight the systematic twist angle dependence of intralayer exciton resonances, indicating significant angle dependent effects on individual monolayer bandgaps and on the interlayer coupling strength. Our fundamental study of exciton resonances provides comprehensive insights into the nuanced interplay between twist angle, dipole orientation, and dielectric asymmetry, providing a deeper understanding of the factors governing the optical properties of layered TMDC heterostructures.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Studying and controlling the properties of individual exfoliated materials is one of the first steps towards the fabrication of complex van der Waals systems. In this work, we present a systematic study of optical properties and micro-Raman spectroscopy on exfoliated flakes of the high-temperature superconductor Bi<jats:sub>2</jats:sub>Sr<jats:sub>2</jats:sub>CaCu<jats:sub>2</jats:sub>O<jats:sub>8+δ</jats:sub> (BSCCO-2212). We demonstrate that these are quick and non-invasive techniques for studying air sensitive materials. The apparent color of BSCCO-2212 exfoliated flakes on SiO<jats:sub>2</jats:sub>/Si has allowed a rough and fast identification of the number of layers. By analyzing the optical contrast of different flakes we determined the complex refractive index in the visible range and found the optimal combination of illumination wavelength and substrate properties for the identification of flakes with different thickness. In addition, we report the hardening of the most characteristic Raman modes as flake thickness decreases, possibly caused by strain in the BiO and CuO<jats:sub>2</jats:sub> planes. Moreover, the evolution of the Raman modes establishes a second approach to determine the thickness of BSCCO-2212 thin flakes. As BSCCO-2212 is a challenging material due to its sensitivity to ambient conditions, the present work provides a guide for the fabrication and characterization of complex van der Waals systems paving the way for studying heterostructures based on unconventional superconductors in the 2D limit.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Chalcogen vacancies in the semiconducting monolayer transition-metal dichalcogenides (TMDs) have frequently been invoked to explain a wide range of phenomena, including both unintentional p-type and n-type conductivity, as well as sub-band gap defect levels measured via tunneling or optical spectroscopy. These conflicting interpretations of the deep versus shallow nature of the chalcogen vacancies are due in part to shortcomings in prior first-principles calculations of defects in the semiconducting two-dimensional TMDs that have been used to explain experimental observations. Here we report results of hybrid density functional calculations for the chalcogen vacancy in a series of monolayer TMDs, correctly referencing the thermodynamic charge transition levels to the fundamental band gap (as opposed to the optical band gap). We find that the chalcogen vacancies are deep acceptors and cannot lead to n-type or p-type conductivity. Both the (0/−1) and (−1/−2) transition levels occur in the gap, leading to paramagnetic charge states <jats:inline-formula> <jats:tex-math><?CDATA $S = 1/2$?></jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>S</mml:mi> <mml:mo>=</mml:mo> <mml:mn>1</mml:mn> <mml:mrow> <mml:mo>/</mml:mo> </mml:mrow> <mml:mn>2</mml:mn> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="tdmad2108ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> and <jats:italic>S</jats:italic> = 1, respectively, in a collinear-spin representation. We discuss trends in terms of the band alignments between the TMDs, which can serve as a guide to future experimental studies of vacancy behavior.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Graphene and related materials (GRMs) promise ample application potential throughout numerous industries. A dedicated graphene market gradually forms around emerging suppliers aspiring to satisfy future demands. Its growth critically depends on the interplay of supply stream maturation and initial utilizations to drive the demand. The present issue of Graphene Roadmap Briefs provides quantitative insights into the current state and future development of the emerging graphene market. We aggregate the underlying expectations and projections from commercial market reports and critically discuss the results. Established science and technology metrics complement our analyses and provide deeper insights into the global market landscape and key actors. In particular, we resolve composites, batteries, and electronics as major application areas likely to drive the overall development of the graphene market towards mass production.</jats:p> <jats:p> <jats:bold>About: Graphene Roadmap Briefs</jats:bold> </jats:p> <jats:p>Graphene Roadmap Briefs highlight key innovation areas impacted by graphene and related materials (GRMs) as well as overarching aspects of GRM innovation status and prospects. The series bases on the evolving technology and innovation roadmap process initiated by the European Graphene Flagship. It covers crucial innovation trends beyond fundamental scientific discovery and applied research on GRM utilization opportunities.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Self-intercalation is an efficient strategy for tailoring the property of layer structured materials like transition metal dichalcogenides (TMDCs), while the associated kinetics and mechanism remain scarcely explored. In this study, we investigate the atomic-scale dynamics and mechanism of vanadium (V) self-intercalation in multi-layer 1T-VSe<jats:sub>2</jats:sub> using <jats:italic>in situ</jats:italic> high resolution scanning transmission electron microscopy. The results reveal that the self-intercalation of V induces structural transformation of pristine VSe<jats:sub>2</jats:sub> into three V-enrich intercalated compounds, i.e. V<jats:sub>5</jats:sub>Se<jats:sub>8</jats:sub>, V<jats:sub>3</jats:sub>Se<jats:sub>4</jats:sub> and VSe. The self-intercalated V follows an ordered arrangement of <jats:inline-formula> <jats:tex-math><?CDATA $2 \times 2$?></jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>2</mml:mn> <mml:mo>×</mml:mo> <mml:mn>2</mml:mn> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="tdmad2193ieqn1.gif" xlink:type="simple" /> </jats:inline-formula>, <jats:inline-formula> <jats:tex-math><?CDATA $2 \times 1$?></jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>2</mml:mn> <mml:mo>×</mml:mo> <mml:mn>1</mml:mn> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="tdmad2193ieqn2.gif" xlink:type="simple" /> </jats:inline-formula>, and <jats:inline-formula> <jats:tex-math><?CDATA $1 \times 1$?></jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>1</mml:mn> <mml:mo>×</mml:mo> <mml:mn>1</mml:mn> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="tdmad2193ieqn3.gif" xlink:type="simple" /> </jats:inline-formula> within the interlayer octahedral sites, corresponding to an intercalation concentration of 25%, 50% and 100% in V<jats:sub>5</jats:sub>Se<jats:sub>8</jats:sub>, V<jats:sub>3</jats:sub>Se<jats:sub>4</jats:sub> and VSe, respectively. The V intercalants induced lattice distortions to the host 1T-VSe<jats:sub>2</jats:sub> such as the dimerization of neighboring lattice V is observed experimentally, which are further supported by density functional theory (DFT) calculations. Finally, a superstructure model generalizing the possible structures of self-intercalated compounds in layered TMDCs is proposed and then validated by the DFT determined formation energy landscape. This study provides comprehensive insights on the kinetics and mechanism of the self-intercalation in layered TMDC materials, contributing to the precise control for the structure and stoichiometry of self-intercalated TMDC compounds.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Two-dimensional (2D) ferroelectrics can maintain electrical polarization up to room temperature and are, therefore, promising for next-generation nonvolatile memories. Although natural 2D ferroelectrics are few, moiré superlattices provide us with a generalized method to construct ferroelectrics from non-ferroelectric parent materials. We report a realization of ferroelectric hysteresis in an AB-BA stacked twisted double bilayer graphene (TDBG) system. The ferroelectric polarization is prominent at zero external displacement field and reduces upon increasing displacement fields. TDBG in the AB-BA configuration is an intriguing system, which facilitates ferroelectricity even without the assistance of any boron nitride layers; however, in the AB-AB stacking case, the development of polarization necessitates the presence of a second superlattice induced by the adjacent boron nitride layer. Therefore, twisted multilayer graphene offers us a fascinating field to explore 2D ferroelectricity.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Atomically thin heterostructures formed by twisted transition metal dichalcogenides can be used to create periodic moiré patterns. The emerging moiré potential can trap interlayer excitons into arrays of strongly interacting bosons, which form a unique platform to study strongly correlated many-body states. In order to create and manipulate these exotic phases of matter, a microscopic understanding of exciton–exciton interactions and their manifestation in these systems becomes indispensable. Recent density-dependent photoluminescence (PL) measurements have revealed novel spectral features indicating the formation of trapped multi-exciton states providing important information about the interaction strength. In this work, we develop a microscopic theory to model the PL spectrum of trapped multi-exciton complexes focusing on the emission from moiré trapped single- and biexcitons. Based on an excitonic Hamiltonian we determine the properties of trapped biexcitons as function of twist angle and use these insights to predict the luminescence spectrum of moiré excitons for different densities. We demonstrate how side peaks resulting from transitions to excited states and a life time analysis can be utilized as indicators for moiré trapped biexcitons and provide crucial information about the excitonic interaction strength.</jats:p>