Two-dimensional (2D) materials hold great promise for future nanoelectronics as conventional semiconductor technologies face serious limitations in performance and power dissipation for future technology nodes. The atomic thinness of 2D materials enables highly scaled field-effect transistors (FETs) with reduced short-channel effects while maintaining high carrier mobility, essential for high-performance, low-voltage device operations. The richness of their electronic band structure opens up the possibility of using these materials in novel electronic and optoelectronic devices. These applications are strongly dependent on the electrical properties of 2D materials-based FETs. Thus, accurate characterization of important properties such as conductivity, carrier density, mobility, contact resistance, interface trap density, etc is vital for progress in the field. However, electrical characterization methods for 2D devices, particularly FET-related measurement techniques, must be revisited since conventional characterization methods for bulk semiconductor materials often fail in the limit of ultrathin 2D materials. In this paper, we review the common electrical characterization techniques for 2D FETs and the related issues arising from adapting the techniques for use on 2D materials.
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2D Materials™ is a multidisciplinary, electronic-only journal devoted to publishing fundamental and applied research of the highest quality and impact covering all aspects of graphene and related two-dimensional materials.
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Sekhar Babu Mitta et al 2021 2D Mater. 8 012002
Morten Niklas Gjerding et al 2021 2D Mater. 8 044002
The Computational 2D Materials Database (C2DB) is a highly curated open database organising a wealth of computed properties for more than 4000 atomically thin two-dimensional (2D) materials. Here we report on new materials and properties that were added to the database since its first release in 2018. The set of new materials comprise several hundred monolayers exfoliated from experimentally known layered bulk materials, (homo)bilayers in various stacking configurations, native point defects in semiconducting monolayers, and chalcogen/halogen Janus monolayers. The new properties include exfoliation energies, Bader charges, spontaneous polarisations, Born charges, infrared polarisabilities, piezoelectric tensors, band topology invariants, exchange couplings, Raman spectra and second harmonic generation spectra. We also describe refinements of the employed material classification schemes, upgrades of the computational methodologies used for property evaluations, as well as significant enhancements of the data documentation and provenance. Finally, we explore the performance of Gaussian process-based regression for efficient prediction of mechanical and electronic materials properties. The combination of open access, detailed documentation, and extremely rich materials property data sets make the C2DB a unique resource that will advance the science of atomically thin materials.
Claudia Backes et al 2020 2D Mater. 7 022001
We present an overview of the main techniques for production and processing of graphene and related materials (GRMs), as well as the key characterization procedures. We adopt a 'hands-on' approach, providing practical details and procedures as derived from literature as well as from the authors' experience, in order to enable the reader to reproduce the results.
Section I is devoted to 'bottom up' approaches, whereby individual constituents are pieced together into more complex structures. We consider graphene nanoribbons (GNRs) produced either by solution processing or by on-surface synthesis in ultra high vacuum (UHV), as well carbon nanomembranes (CNM). Production of a variety of GNRs with tailored band gaps and edge shapes is now possible. CNMs can be tuned in terms of porosity, crystallinity and electronic behaviour.
Section II covers 'top down' techniques. These rely on breaking down of a layered precursor, in the graphene case usually natural crystals like graphite or artificially synthesized materials, such as highly oriented pyrolythic graphite, monolayers or few layers (FL) flakes. The main focus of this section is on various exfoliation techniques in a liquid media, either intercalation or liquid phase exfoliation (LPE). The choice of precursor, exfoliation method, medium as well as the control of parameters such as time or temperature are crucial. A definite choice of parameters and conditions yields a particular material with specific properties that makes it more suitable for a targeted application. We cover protocols for the graphitic precursors to graphene oxide (GO). This is an important material for a range of applications in biomedicine, energy storage, nanocomposites, etc. Hummers' and modified Hummers' methods are used to make GO that subsequently can be reduced to obtain reduced graphene oxide (RGO) with a variety of strategies. GO flakes are also employed to prepare three-dimensional (3d) low density structures, such as sponges, foams, hydro- or aerogels. The assembly of flakes into 3d structures can provide improved mechanical properties. Aerogels with a highly open structure, with interconnected hierarchical pores, can enhance the accessibility to the whole surface area, as relevant for a number of applications, such as energy storage. The main recipes to yield graphite intercalation compounds (GICs) are also discussed. GICs are suitable precursors for covalent functionalization of graphene, but can also be used for the synthesis of uncharged graphene in solution. Degradation of the molecules intercalated in GICs can be triggered by high temperature treatment or microwave irradiation, creating a gas pressure surge in graphite and exfoliation. Electrochemical exfoliation by applying a voltage in an electrolyte to a graphite electrode can be tuned by varying precursors, electrolytes and potential. Graphite electrodes can be either negatively or positively intercalated to obtain GICs that are subsequently exfoliated. We also discuss the materials that can be amenable to exfoliation, by employing a theoretical data-mining approach.
The exfoliation of LMs usually results in a heterogeneous dispersion of flakes with different lateral size and thickness. This is a critical bottleneck for applications, and hinders the full exploitation of GRMs produced by solution processing. The establishment of procedures to control the morphological properties of exfoliated GRMs, which also need to be industrially scalable, is one of the key needs. Section III deals with the processing of flakes. (Ultra)centrifugation techniques have thus far been the most investigated to sort GRMs following ultrasonication, shear mixing, ball milling, microfluidization, and wet-jet milling. It allows sorting by size and thickness. Inks formulated from GRM dispersions can be printed using a number of processes, from inkjet to screen printing. Each technique has specific rheological requirements, as well as geometrical constraints. The solvent choice is critical, not only for the GRM stability, but also in terms of optimizing printing on different substrates, such as glass, Si, plastic, paper, etc, all with different surface energies. Chemical modifications of such substrates is also a key step.
Sections IV–VII are devoted to the growth of GRMs on various substrates and their processing after growth to place them on the surface of choice for specific applications. The substrate for graphene growth is a key determinant of the nature and quality of the resultant film. The lattice mismatch between graphene and substrate influences the resulting crystallinity. Growth on insulators, such as SiO2, typically results in films with small crystallites, whereas growth on the close-packed surfaces of metals yields highly crystalline films. Section IV outlines the growth of graphene on SiC substrates. This satisfies the requirements for electronic applications, with well-defined graphene-substrate interface, low trapped impurities and no need for transfer. It also allows graphene structures and devices to be measured directly on the growth substrate. The flatness of the substrate results in graphene with minimal strain and ripples on large areas, allowing spectroscopies and surface science to be performed. We also discuss the surface engineering by intercalation of the resulting graphene, its integration with Si-wafers and the production of nanostructures with the desired shape, with no need for patterning.
Section V deals with chemical vapour deposition (CVD) onto various transition metals and on insulators. Growth on Ni results in graphitized polycrystalline films. While the thickness of these films can be optimized by controlling the deposition parameters, such as the type of hydrocarbon precursor and temperature, it is difficult to attain single layer graphene (SLG) across large areas, owing to the simultaneous nucleation/growth and solution/precipitation mechanisms. The differing characteristics of polycrystalline Ni films facilitate the growth of graphitic layers at different rates, resulting in regions with differing numbers of graphitic layers. High-quality films can be grown on Cu. Cu is available in a variety of shapes and forms, such as foils, bulks, foams, thin films on other materials and powders, making it attractive for industrial production of large area graphene films. The push to use CVD graphene in applications has also triggered a research line for the direct growth on insulators. The quality of the resulting films is lower than possible to date on metals, but enough, in terms of transmittance and resistivity, for many applications as described in section V.
Transfer technologies are the focus of section VI. CVD synthesis of graphene on metals and bottom up molecular approaches require SLG to be transferred to the final target substrates. To have technological impact, the advances in production of high-quality large-area CVD graphene must be commensurate with those on transfer and placement on the final substrates. This is a prerequisite for most applications, such as touch panels, anticorrosion coatings, transparent electrodes and gas sensors etc. New strategies have improved the transferred graphene quality, making CVD graphene a feasible option for CMOS foundries. Methods based on complete etching of the metal substrate in suitable etchants, typically iron chloride, ammonium persulfate, or hydrogen chloride although reliable, are time- and resource-consuming, with damage to graphene and production of metal and etchant residues. Electrochemical delamination in a low-concentration aqueous solution is an alternative. In this case metallic substrates can be reused. Dry transfer is less detrimental for the SLG quality, enabling a deterministic transfer.
There is a large range of layered materials (LMs) beyond graphite. Only few of them have been already exfoliated and fully characterized. Section VII deals with the growth of some of these materials. Amongst them, h-BN, transition metal tri- and di-chalcogenides are of paramount importance. The growth of h-BN is at present considered essential for the development of graphene in (opto) electronic applications, as h-BN is ideal as capping layer or substrate. The interesting optical and electronic properties of TMDs also require the development of scalable methods for their production. Large scale growth using chemical/physical vapour deposition or thermal assisted conversion has been thus far limited to a small set, such as h-BN or some TMDs. Heterostructures could also be directly grown.
Section VIII discusses advances in GRM functionalization. A broad range of organic molecules can be anchored to the sp2 basal plane by reductive functionalization. Negatively charged graphene can be prepared in liquid phase (e.g. via intercalation chemistry or electrochemically) and can react with electrophiles. This can be achieved both in dispersion or on substrate. The functional groups of GO can be further derivatized. Graphene can also be noncovalently functionalized, in particular with polycyclic aromatic hydrocarbons that assemble on the sp2 carbon network by π–π stacking. In the liquid phase, this can enhance the colloidal stability of SLG/FLG. Approaches to achieve noncovalent on-substrate functionalization are also discussed, which can chemically dope graphene. Research efforts to derivatize CNMs are also summarized, as well as novel routes to selectively address defect sites. In dispersion, edges are the most dominant defects and can be covalently modified. This enhances colloidal stability without modifying the graphene basal plane. Basal plane point defects can also be modified, passivated and healed in ultra-high vacuum. The decoration of graphene with metal nanoparticles (NPs) has also received considerable attention, as it allows to exploit synergistic effects between NPs and graphene. Decoration can be either achieved chemically or in the gas phase. All LMs, can be functionalized and we summarize emerging approaches to covalently and noncovalently functionalize MoS2 both in the liquid and on substrate.
Section IX describes some of the most popular characterization techniques, ranging from optical detection to the measurement of the electronic structure. Microscopies play an important role, although macroscopic techniques are also used for the measurement of the properties of these materials and their devices. Raman spectroscopy is paramount for GRMs, while PL is more adequate for non-graphene LMs (see section IX.2). Liquid based methods result in flakes with different thicknesses and dimensions. The qualification of size and thickness can be achieved using imaging techniques, like scanning probe microscopy (SPM) or transmission electron microscopy (TEM) or spectroscopic techniques. Optical microscopy enables the detection of flakes on suitable surfaces as well as the measurement of optical properties. Characterization of exfoliated materials is essential to improve the GRM metrology for applications and quality control. For grown GRMs, SPM can be used to probe morphological properties, as well as to study growth mechanisms and quality of transfer. More generally, SPM combined with smart measurement protocols in various modes allows one to get obtain information on mechanical properties, surface potential, work functions, electrical properties, or effectiveness of functionalization. Some of the techniques described are suitable for 'in situ' characterization, and can be hosted within the growth chambers. If the diagnosis is made 'ex situ', consideration should be given to the preparation of the samples to avoid contamination. Occasionally cleaning methods have to be used prior to measurement.
Sten Haastrup et al 2018 2D Mater. 5 042002
We introduce the Computational 2D Materials Database (C2DB), which organises a variety of structural, thermodynamic, elastic, electronic, magnetic, and optical properties of around 1500 two-dimensional materials distributed over more than 30 different crystal structures. Material properties are systematically calculated by state-of-the-art density functional theory and many-body perturbation theory ( and the Bethe–Salpeter equation for ∼250 materials) following a semi-automated workflow for maximal consistency and transparency. The C2DB is fully open and can be browsed online (http://c2db.fysik.dtu.dk) or downloaded in its entirety. In this paper, we describe the workflow behind the database, present an overview of the properties and materials currently available, and explore trends and correlations in the data. Moreover, we identify a large number of new potentially synthesisable 2D materials with interesting properties targeting applications within spintronics, (opto-)electronics, and plasmonics. The C2DB offers a comprehensive and easily accessible overview of the rapidly expanding family of 2D materials and forms an ideal platform for computational modeling and design of new 2D materials and van der Waals heterostructures.
Adam J Watson et al 2021 2D Mater. 8 032001
Two-dimensional (2D) materials offer opportunities to explore both fundamental science and applications in the limit of atomic thickness. Beyond the prototypical case of graphene, other 2D materials have recently come to the fore. Of particular technological interest are 2D semiconductors, of which the family of materials known as the group-VI transition metal dichalcogenides (TMDs) has attracted much attention. The presence of a bandgap allows for the fabrication of high on–off ratio transistors and optoelectronic devices, as well as valley/spin polarized transport. The technique of chemical vapor deposition (CVD) has produced high-quality and contiguous wafer-scale 2D films, however, they often need to be transferred to arbitrary substrates for further investigation. In this review, the various transfer techniques developed for transferring 2D films will be outlined and compared, with particular emphasis given to CVD-grown TMDs. Each technique suffers undesirable process-related drawbacks such as bubbles, residue or wrinkles, which can degrade device performance by for instance reducing electron mobility. This review aims to address these problems and provide a systematic overview of key methods to characterize and improve the quality of the transferred films and heterostructures. With the maturing technological status of CVD-grown 2D materials, a robust transfer toolbox is vital.
Thomas Schmaltz et al 2024 2D Mater. 11 022002
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.
About: Graphene Roadmap Briefs
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.
Zhong Lin et al 2016 2D Mater. 3 022002
Two-dimensional transition metal dichalcogenides (TMDs), an emerging family of layered materials, have provided researchers a fertile ground for harvesting fundamental science and emergent applications. TMDs can contain a number of different structural defects in their crystal lattices which significantly alter their physico-chemical properties. Having structural defects can be either detrimental or beneficial, depending on the targeted application. Therefore, a comprehensive understanding of structural defects is required. Here we review different defects in semiconducting TMDs by summarizing: (i) the dimensionalities and atomic structures of defects; (ii) the pathways to generating structural defects during and after synthesis and, (iii) the effects of having defects on the physico-chemical properties and applications of TMDs. Thus far, significant progress has been made, although we are probably still witnessing the tip of the iceberg. A better understanding and control of defects is important in order to move forward the field of Defect Engineering in TMDs. Finally, we also provide our perspective on the challenges and opportunities in this emerging field.
Maryam Khosravian et al 2024 2D Mater. 11 035012
Twisted van der Waals materials have risen as highly tunable platforms for realizing unconventional superconductivity. Here we demonstrate how a topological superconducting state can be driven in a twisted graphene multilayer at a twist angle of approximately 1.6 degrees proximitized to other 2D materials. We show that an encapsulated twisted bilayer subject to induced Rashba spin–orbit coupling, s-wave superconductivity, and exchange field generates a topological superconducting state enabled by the moiré pattern. We demonstrate the emergence of a variety of topological states with different Chern numbers, that are highly tunable through doping, strain, and bias voltage. Our proposal does not depend on fine-tuning the twist angle, but solely on the emergence of moiré minibands and is applicable for twist angles between 1.3 and 3 degrees. Our results establish the potential of twisted graphene bilayers to create topological superconductivity without requiring ultraflat dispersions.
Andres Castellanos-Gomez et al 2014 2D Mater. 1 011002
The deterministic transfer of two-dimensional crystals constitutes a crucial step towards the fabrication of heterostructures based on the artificial stacking of two-dimensional materials. Moreover, controlling the positioning of two-dimensional crystals facilitates their integration in complex devices, which enables the exploration of novel applications and the discovery of new phenomena in these materials. To date, deterministic transfer methods rely on the use of sacrificial polymer layers and wet chemistry to some extent. Here, we develop an all-dry transfer method that relies on viscoelastic stamps and does not employ any wet chemistry step. This is found to be very advantageous to freely suspend these materials as there are no capillary forces involved in the process. Moreover, the whole fabrication process is quick, efficient, clean and it can be performed with high yield.
George Gorgolis and Costas Galiotis 2017 2D Mater. 4 032001
Graphene based aerogels (GAs) are 3D scaffold materials that can be lighter than air. Due to their fascinating properties, such as, high mechanical strength and electrical conductivity, thermal resistance and adsorption capacity, they have attracted a lot of interest currently. This review, covers the main routes for obtaining GAs namely, hydrothermal reduction/self-assembly, chemical reduction, template-directed reduction, cross-linking and sol–gel processes. Potential application fields for example in energy storage and environmental protection are also discussed. Finally, the future prospects of this exciting field based on the results published so far are examined.
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Shoaib Khalid et al 2024 2D Mater. 11 031003
Like in any other semiconductor, point defects in transition-metal dichalcogenides (TMDs) are expected to strongly impact their electronic and optical properties. However, identifying defects in these layered two-dimensional materials has been quite challenging with controversial conclusions despite the extensive literature in the past decade. Using first-principles calculations, we revisit the role of chalcogen vacancies and hydrogen impurity in bulk TMDs, reporting formation energies and thermodynamic and optical transition levels. We show that the S vacancy can explain recently observed cathodoluminescence spectra of MoS2 flakes and predict similar optical levels in the other TMDs. In the case of the H impurity, we find it more stable sitting on an interstitial site in the Mo plane, acting as a shallow donor, and possibly explaining the often observed n-type conductivity in some TMDs. We also predict the frequencies of the local vibration modes for the H impurity, aiding its identification through Raman or infrared spectroscopy.
Mathis Cameau et al 2024 2D Mater. 11 035023
Dirac nodal line (DNL) semimetals are a novel class of topological materials in which the valence and conduction bands touch along lines in the reciprocal space, with linear dispersion. These materials attract a growing attention, but the experimental realizations for two-dimensional systems are sparse. This article reports the first experimental realization of a two-dimensional hexagonal monolayer Cu2Ge, grown by evaporation of Ge on a Cu(111) substrate. Through a combination of low-energy electron diffraction XPS and ARPES measurements, it is shown that the surface presents all characteristics expected from calculations for a free-standing Cu2Ge monolayer. More specifically, the preservation of the two concentric nodal lines around the Γ point indicates weak interactions between the Cu2Ge surface and its Cu(111) substrate, making it an ideal system for the study of DNL materials.
Sanfeng Wu 2024 2D Mater. 11 033004
In strongly correlated quantum materials, electrons behave in ways that often extend beyond the confines of conventional Fermi-liquid theory. Interesting results include the observation of low-temperature metallic behavior in systems that are highly resistive. Here we provide an overview of experiments in which insulators exhibit characteristics of a metal such as the Shubnikov–de Haas-like quantum oscillations, focusing on recent findings in the correlated insulating states of two-dimensional WTe2. We discuss the status of current research, clarify the debates and challenges in interpreting the experiments, rule out extrinsic explanations and discuss promising future directions.
Munis Khan et al 2024 2D Mater. 11 035022
Monolayer graphene is a promising material for a wide range of applications, including sensors, optoelectronics, antennas, EMR shielding, flexible electronics, and conducting electrodes. Chemical vapor deposition (CVD) of carbon atoms on a metal catalyst is the most scalable and cost-efficient method for synthesizing high-quality, large-area monolayer graphene. The usual method of transferring the CVD graphene from the catalyst to a target substrate involves a polymer carrier which is dissolved after the transfer process is completed. Due to often unavoidable damage to graphene, as well as contamination and residues, carrier mobilities are typically 1000–3000 , unless complex and elaborate measures are taken. Here, we report on a simple scalable fabrication method for flexible graphene field-effect transistors that eliminates the polymer interim carrier, by laminating the graphene directly onto office lamination foils, removing the catalyst, and depositing Parylene N as a gate dielectric and encapsulation layer. The fabricated transistors show field- and Hall-effect mobilities of 7000–10 000 with a residual charge-carrier density of 1 cm−2 at room temperature. We further validate the material quality by terahertz time-domain spectroscopy and observation of the quantum Hall effect at low temperatures in a moderate magnetic field of T. The Parylene encapsulation provides long-term stability and protection against additional lithography steps, enabling vertical device integration in multilayer electronics on a flexible platform.
Jithin T Surendran et al 2024 2D Mater. 11 031002
A van der Waals heterostructure containing an atomically thin monolayer (ML) transition-metal dichalcogenide as a single-photon emitting layer is emerging as an intriguing solid-state quantum-photonic platform. Here, we report the utilization of spin-coating of silica nanoparticles for semi-deterministically creating the spectrally isolated, energetically stable, and narrow-linewidth single-photon emitters in ML-WS2. We also demonstrate that long-duration low-temperature annealing of the photonic heterostructure in the vacuum removes the energetically unstable emitters that are present due to fabrication-associated residue and lead to the emission of single-photons in a 25 nm narrowband visible spectral range centered at ~620 nm. This work may pave the way toward realizing a hybrid-quantum-photonic platform containing a van der Waals heterostructure/device and an atomic-vapor system emitting/absorbing in the same visible spectral range.
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Sarah C Gillespie et al 2024 2D Mater. 11 022005
Heterostructures (HSs) based on two-dimensional transition metal dichalcogenides (TMDCs) are highly intriguing materials because of the layers' pronounced excitonic properties and their nontrivial contributions to the HS. These HSs exhibit unique properties that are not observed in either of the constituent components in isolation. Interlayer excitons (IEs), which are electron–hole pairs separated across the HSs, play a central role in determining these HS properties and are of interest both fundamentally and for device applications. In recent years, a major focus has been on understanding and designing HSs composed of two or more TMDC materials. Less attention has been paid to HSs 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 IEs. Recent advances have started to uncover highly interesting phenomena, and with improved understanding these HSs offer great potential for device applications such as photodetectors and miniaturized optics.
I Cheliotis and I Zergioti 2024 2D Mater. 11 022004
Over the years, two-dimensional (2D) materials have attracted increasing technological interest due to their unique physical, electronic, and photonic properties, making them excellent candidates for applications in electronics, nanoelectronics, optoelectronics, sensors, and modern telecommunications. Unfortunately, their development often requires special conditions and strict protocols, making it challenging to integrate them directly into devices. Some of the requirements include high temperatures, precursors, and special catalytic substrates with specific lattice parameters. Consequently, methods have been developed to transfer these materials from the growth substrates onto target substrates. These transfer techniques aim to minimize intermediate steps and minimize defects introduced into the 2D material during the process. This review focuses on the transfer techniques directly from the development substrates of 2D materials, which play a crucial role in their utilization.
Luka Pirker et al 2024 2D Mater. 11 022003
This review delves into the intricacies of the interfaces formed between two-dimensional (2D) materials and metals, exploring a realm rich with fundamental insights and promising applications. Historically, our understanding of 2D materials emanated from studies employing dielectric substrates or suspended samples. However, integrating metals in the exfoliation and growth processes of 2D materials has opened up new avenues, unveiling various shades of interactions ranging from dispersive forces to covalent bonding. The resulting modifications in 2D materials, particularly transition metal dichalcogenides (TMDCs), offer more than a theoretical intrigue. They bear substantial implications for (opto)electronics, altering Schottky barrier heights and contact resistances in devices. We explore metal-mediated methods for TMDC exfoliation, elucidating the mechanisms and their impact on TMDC-metal interactions. Delving deeper, we scrutinize the fundamentals of these interactions, focusing primarily on MoS2 and Au. Despite the recent surge of interest and extensive studies, critical gaps remain in our understanding of these intricate interfaces. We discuss controversies, such as the changes in Raman or photoemission signatures of MoS2 on Au, and propose potential explanations. The interplay between charge redistribution, substrate-induced bond length variations, and interface charge transfer processes are examined. Finally, we address the intriguing prospect of TMDC phase transitions induced by strongly interacting substrates and their implications for contact design.
Thomas Schmaltz et al 2024 2D Mater. 11 022002
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.
About: Graphene Roadmap Briefs
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.
Mingfu Fu et al 2024 2D Mater. 11 022001
Among the allotropes of phosphorus, black phosphorus (BP) is one of the most thermodynamically stable structures. Due to its unique physical and chemical properties, BP has shown considerable potential in many applications, such as field-effect transistors, energy storage and conversion, and photocatalysis. However, low-dimensional BP is easily corroded by oxygen and water owing to the large specific surface area and unbonded lone pair electrons on the surface, which reduces its chemical stability in the environment. As a result, different passivation approaches, relying on noncovalent bonding, covalent functionalization, and surface coordination, are employed to enhance the stability and performance of BP. In this review, the degradation mechanisms of BP are first analyzed for the material in both its ground state and excited state. Subsequently, the promising strategies for improving stability are overviewed. A comprehensive and in-depth understanding of the oxidation mechanisms and protection strategies of BP will provide guidance for the large-scale applications of BP and its derivatives.
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Jürgens et al
Excitons in twisted bilayers of transition metal dichalcogenides have strongly modified dispersion relations due to the formation of periodic moiré potentials. The strong coupling to a light field in an optical cavity leads to the appearance of moiré polaritons. In this paper, we derive a theoretical model for the linear absorption spectrum of the coupled moiré polariton-phonon system based on the time-convolutionless (TCL) approach. Results obtained by numerically solving the TCL equation are compared to those obtained in the Markovian limit and from a perturbative treatment of non-Markovian corrections. A key quantity for the interpretation of the findings is the generalized phonon spectral density. We discuss the phonon impact on the spectrum for realistic moiré exciton dispersions by varying twist angle and temperature.
Key features introduced by the coupling to phonons are broadenings and energy shifts of the upper and lower polariton peak and the appearance of phonon sidebands between them. We analyze these features with respect to the role of Markovian and non-Markovian effects and find that they strongly depend on the twist angle. We can distinguish between the regimes of large, small, and intermediate twist angles. In the latter phonon effects are particularly pronounced due to dominating phonon transitions into regions which are characterized by van Hove singularities in the density of states.
Deng et al
The formation of morphotropic phase boundaries (MPBs) is a pivotal strategy in piezoelectric ceramics and crystals, primarily used to enhance the electromechanical coupling. However, the application of this strategy in van der Waals (vdW) piezoelectrics and ferroelectrics has been limited, largely due to challenges in achieving phase coexistence and enabling possible polarization rotation. In this study, we address this gap by synthesizing a Selenium doped vdW ferroelectric, CuInP2(S1-xSex)6, with a doping range of 0 ≤ x ≤ 0.15, to create an MPB. Our findings indicate the presence of an MPB near x=0.05, situated between monoclinic and trigonal phases. This boundary was confirmed using X-ray diffraction and transmission electron microscope techniques. Remarkably, the composition at x=0.05 shows a high dielectric constant (εr=13.8) and an impressive local effective piezoelectric coefficient (d33eff=51 pm/V) at 80 K. Additionally, an unusual softening of the Young's modulus was observed near MPB. These results are crucial for enhancing electromechanical coupling in vdW layered materials and herald new avenues for the design and optimization of piezoelectric and electromechanical properties in these materials.
Olsen
We present a perspective on the status of antiferromagnetism in two-dimensional (2D) materials. Various types of spin-compensated orders are discussed and include non-collinear order, spin spirals and altermagnetism. Spin-orbit effects ultimately determine, whether compounds exhibit long range order, Kosterlitz-Thouless physics, or multiferroic properties and we discuss the basic magnetic prototypes that may arise in 2D materials depending on the magnetic anisotropy and ordering vector. A summary of 2D antiferromagnets that have been characterized experimentally is provided - with particular emphasis on magnetic anisotropies and Neel temperatures. We then outline the ingredients needed to describe the magnetic properties using density functional theory. In particular, the systematic determination of magnetic ground states from the generalized Bloch theorem and the magnetic force theorem, which may be used to calculate magnetic excitations from the Heisenberg model with parameters determined from first principles. The methods are exemplified by application to the monolayer helimagnet NiBr$_2$. Finally, we present a summary of predicted and prospective 2D antiferromagnets and discuss the challenges associated with the prediction of Néel temperatures from first principles.
S Kumar et al
2D transition metal dichalcogenides (TMDs) are leading materials for next-generation optoelectronics, but fundamental problems stand enroute to commercialization. These problems include firstly, the widely debated defect and strain-induced origins of intense low-energy broad luminescence peaks (L-peak) observed at low temperatures. Secondly, role of oxygen in tuning properties via chemisorption and physisorption is intriguing but challenging to understand. Thirdly, physical understanding of benefits of hBN encapsulation is inadequate. Using a series of samples, we decouple contributions of oxygen, defects, adsorbates, and strain on optical properties of monolayer MoS2. Defect-origin of L-peak is confirmed by temperature and power-dependent photoluminescence (PL) measurements, with a dramatic redshift ~ 130 meV for oxygen-assisted chemical vapour deposition (O-CVD) samples (c.f. exfoliated). Anomalously, O-CVD samples show high A-exciton PL at room temperature (c.f. exfoliated), but reduced PL at low temperatures, attributed to strain-induced direct-to-indirect bandgap-crossover in low-defect O-CVD MoS2. These observations are consistent with our density functional theory calculations, and supported by Raman spectroscopy. In exfoliated samples, charged O-adatoms are identified as thermodynamically favourable defects, and create in-gap states. Beneficial effect of encapsulation originates from reduction of charged O-adatoms and adsorbates. This experimental-theoretical study uncovers the type of defects in each sample, enables an understanding of the combined effect of defects, strain and oxygen on band structure, and enriches understanding of effects of encapsulation. This work proposes O-CVD for creating high-quality materials for optoelectronics.
Adanigbo et al
Research interests in two-dimensional (2D) materials have seen exponential growth owing to their unique and fascinating properties. The highly exposed lattice planes coupled with tunable electronic states of 2D materials have created manifold opportunities in the design of new platforms for energy conversion and sensing applications. Still, challenges in understanding the electrochemical characteristics of these materials arise from the complexity of both intrinsic and extrinsic heterogeneities that can obscure structure–activity correlations. Scanning electrochemical probe microscopic investigations offer unique benefits in disclosing local electrochemical reactivities at the nanoscale level that are otherwise inaccessible with macroscale methods. This review summarizes recent progress in applying techniques of scanning electrochemical microscopy (SECM) and scanning electrochemical cell microscopy (SECCM) to obtain distinctive insights into the fundamentals of 2D electrodes. We showcase the capabilities of electrochemical microscopies in addressing the roles of defects, thickness, environments, strain, phase, stacking, and many other aspects in the heterogeneous electron transfer, ion transport, electrocatalysis, and photoelectrochemistry of representative 2D materials and their derivatives. Perspectives for the advantages, challenges, and future opportunities of scanning electrochemical probe microscopy investigation of 2D structures are discussed.