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Two-dimensional (2D) semiconductors are promising candidates for optoelectronic application and quantum information processes due to their inherent out-of-plane 2D confinement. In addition, they offer the possibility of achieving low-dimensional in-plane exciton confinement, similar to zero-dimensional quantum dots, with intriguing optical and electronic properties via strain or composition engineering. However, realizing such laterally confined 2D monolayers and systematically controlling size-dependent optical properties remain significant challenges. Here, we report the observation of lateral confinement of excitons in epitaxially grown in-plane MoSe 2 quantum dots (~15-60 nm wide) inside a continuous matrix of WSe 2 monolayer film via a sequential epitaxial growth process. Various optical spectroscopy techniques reveal the size-dependent exciton confinement in the MoSe 2 monolayer quantum dots with exciton blue shift (12-40 meV) at a low temperature as compared to continuous monolayer MoSe 2 . Finally, single-photon emission (g 2 (0) ~ 0.4) was also observed from the smallest dots at 1.6 K. Our study opens the door to compositionally engineered, tunable, in-plane quantum light sources in 2D semiconductors., (© 2024. The Author(s).)

Low-dimensional materials, such as MoS 2 , hold promise for use in a host of emerging applications, including flexible, wearable sensors due to their unique electrical, thermal, optical, mechanical, and tribological properties. The implementation of such devices requires an understanding of adhesive phenomena at the interfaces between these materials. Here, we describe combined nanoscale in situ transmission electron microscopy (TEM)/atomic force microscopy (AFM) experiments and simulations measuring the work of adhesion ( W adh ) between self-mated contacts of ultrathin nominally amorphous and nanocrystalline MoS 2 films deposited on Si scanning probe tips. A customized TEM/AFM nanoindenter permitted high-resolution imaging and force measurements in situ . The W adh values for nanocrystalline and nominally amorphous MoS 2 were 604 ± 323 mJ/m 2 and 932 ± 647 mJ/m 2 , respectively, significantly higher than previously reported values for mechanically exfoliated MoS 2 single crystals. Closely matched molecular dynamics (MD) simulations show that these high values can be explained by bonding between the opposing surfaces at defects such as grain boundaries. Simulations show that as grain size decreases, the number of bonds formed, the W adh and its variability all increase, further supporting that interfacial covalent bond formation causes high adhesion. In some cases, sliding between delaminated MoS 2 flakes during separation is observed, which further increases the W adh and the range of adhesive interaction. These results indicate that for low adhesion, the MoS 2 grains should be large relative to the contact area to limit the opportunity for bonding, whereas small grains may be beneficial, where high adhesion is needed to prevent device delamination in flexible systems.

The widely studied class of two-dimensional (2D) materials known as transition metal dichalcogenides (TMDs) are now well-poised to be employed in real-world applications ranging from electronic logic and memory devices to gas and biological sensors. Several scalable thin film synthesis techniques have demonstrated nanoscale control of TMD material thickness, morphology, structure, and chemistry and correlated these properties with high-performing, application-specific device metrics. In this review, the particularly versatile two-step conversion (2SC) method of TMD film synthesis is highlighted. The 2SC technique relies on deposition of a solid metal or metal oxide precursor material, followed by a reaction with a chalcogen vapor at an elevated temperature, converting the precursor film to a crystalline TMD. Herein, the variables at each step of the 2SC process including the impact of the precursor film material and deposition technique, the influence of gas composition and temperature during conversion, as well as other factors controlling high-quality 2D TMD synthesis are considered. The specific advantages of the 2SC approach including deposition on diverse substrates, low-temperature processing, orientation control, and heterostructure synthesis, among others, are featured. Finally, emergent opportunities that take advantage of the 2SC approach are discussed to include next-generation electronics, sensing, and optoelectronic devices, as well as catalysis for energy-related applications., (© 2024 Wiley‐VCH GmbH.)

As thin films of semiconducting covalent organic frameworks (COFs) are demonstrating utility for ambipolar electronics, channel materials in organic electrochemical transistors (OECTs), and broadband photodetectors, control and modulation of their thin film properties is paramount. In this work, an interfacial growth technique is utilized to synthesize imine TAPB-PDA COF films at both the liquid-liquid interface as well as at the liquid-solid interface on a Si/SiO 2 substrate. The concentration of acetic acid catalyst in the aqueous phase is shown to significantly influence the thin film morphology of the liquid-solid growth, with concentrations below 1 M resulting in no film nucleation, concentrations of 1-4 M enabling smooth film formation, and concentrations greater than 4 M resulting in films with a higher density of particulates on the surface. Importantly, while the films grown at the liquid-liquid interface are mixed-orientation, those grown directly at the liquid-solid interface on the Si/SiO 2 surface have highly oriented COF layers aligned parallel to the substrate surface. Moreover, this liquid-solid growth process affords TAPB-PDA COF thin films with p-type charge transport having a transconductance of 10 μS at a gate voltage of -0.9 V in an OECT device structure.

Two-dimensional (2D) inorganic/organic hybrids provide a versatile platform for diverse applications, including electronic, catalysis, and energy storage devices. The recent surge in 2D covalent organic frameworks (COFs) has introduced an organic counterpart for the development of advanced 2D organic/inorganic hybrids with improved electronic coupling, charge separation, and carrier mobility. However, existing synthesis methods have primarily focused on few-layered film structures, which limits scalability for practical applications. Herein, we present a general synthesis approach for a range of COF/inorganic 2D material hybrids, utilizing 2D inorganic materials as both catalysts and inorganic building blocks. By leveraging the intrinsic Lewis acid sites on the inorganic 2D materials such as hexagonal boron nitride (hBN) and transition metal dichalcogenides, COFs with diverse functional groups and topologies can grow on the surface of inorganic 2D materials. The controlled 2D morphology and excellent solution dispersibility of the resulting hybrids allow for easy processing into films through vacuum filtration. As proof of concept, hBN/COF films were employed as filters for Rhodamine 6G removal under flow-through conditions, achieving a removal rate exceeding 93%. The present work provides a simple and versatile synthesis method for the scalable fabrication of COF/inorganic 2D hybrids, offering exciting opportunities for practical applications such as water treatment and energy storage., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Co-published by University of Science and Technology of China and American Chemical Society.)

Twisted 2D layered materials have garnered much attention recently as a class of 2D materials whose interlayer interactions and electronic properties are dictated by the relative rotation/twist angle between the adjacent layers. In this work, we explore a prototype of such a twisted 2D system, artificially stacked twisted bilayer graphene (TBLG), where we probe, using Raman spectroscopy, the changes in the interlayer interactions and electron-phonon scattering pathways as the twist angle is varied from 0° to 30°. The long-range Moiré potential of the superlattice gives rise to additional intravalley and intervalley scattering of the electrons in TBLG, which has been investigated through their Raman signatures. Density functional theory (DFT) calculations of the electronic band structure of the TBLG superlattices were found to be in agreement with the resonant Raman excitations across the van Hove singularities in the valence and conduction bands predicted for TBLG due to hybridization of bands from the two layers. We also observe that the relative rotation between the graphene layers has a marked influence on the second order overtone and combination Raman modes signaling a commensurate-incommensurate transition in TBLG as the twist angle increases. This serves as a convenient and rapid characterization tool to determine the degree of commensurability in TBLG systems.

Current silicon technology is on the verge of reaching its performance limits. This aspect, coupled with the global chip shortage, makes a solid case for steering our attention toward the accelerated commercialization of other electronic materials. Among the available suite of emerging electronic materials, two-dimensional materials, including transition metal dichalcogenides (TMDs), exhibit improved short-channel effects, high electron mobility, and integration into CMOS-compatible processing. While these materials may not be able to replace silicon at the current stages of development, they can supplement Si in the form of Si-compatible CMOS processing and be manufactured for tailored applications. However, the major hurdle in the path of commercialization of such materials is the difficulty in producing their wafer-scale forms, which are not necessarily single crystalline but on a large scale. Recent but exploratory interest in 2D materials from industries, such as TSMC, necessitates an in-depth analysis of their commercialization potential based on trends and progress in entrenched electronic materials (Si) and ones with a short-term commercialization potential (GaN, GaAs). We also explore the possibility of unconventional fabrication techniques, such as printing, for 2D materials becoming more mainstream and being adopted by industries in the future. In this Perspective, we discuss aspects to optimize cost, time, thermal budget, and a general pathway for 2D materials to achieve similar milestones, with an emphasis on TMDs. Beyond synthesis, we propose a lab-to-fab workflow based on recent advances that can operate on a low budget with a mainstream full-scale Si fabrication unit.

The quantum age is just around the corner. As quantum systems become more stable, robust, and mainstream, tackling the challenge of high-throughput manufacturing will require further developments in materials synthesis, characterization, assembly, and diagnostics. As the building blocks of future technologies scale down to atomic and molecular scales, a paradigm shift in manufacturing will begin to take shape. Inspired by a quantum manufacturing world that elevates the Materials Genome Initiative to the next level, a "human-in-the-loop" framework for high-throughput manufacturing, which addresses key opportunities and challenges to be overcome, is outlined., (© 2022 Wiley-VCH GmbH.)

Layered Transition Metal Dichalcogenides (TMDs) are an important class of materials that exhibit a wide variety of optoelectronic properties. The ability to spatially tailor their expansive property-space (e.g., conduction behavior, optical emission, surface interactions) is of special interest for applications including, but not limited to, sensing, bioelectronics, and spintronics/valleytronics. Current methods of property modulation focus on the modification of the basal surfaces and edge sites of the TMDs by the introduction of defects, functionalization with organic or inorganic moieties, alloying, heterostructure formation, and phase engineering. A majority of these methods lack the resolution for the development of next-generation nanoscale devices or are limited in the types of functionalities useful for efficient TMD property modification. In this study, we utilize electron-beam patterning on monolayer TMDs (MoSe 2 , WSe 2 and MoS 2 ) in the presence of a pressure-controlled atmosphere of water vapor within an environmental scanning electron microscope (ESEM). A series of parametric studies show local optical and electronic property modification depending on acceleration voltage, beam current, pressure, and electron dose. The ultimate pattern resolution achieved is 67 ± 9 nm. Raman and photoluminescence spectroscopies coupled with Kelvin Probe Force Microscopy reveal electron dose-dependent p-doping in the patterned regions, which we attribute to functionalization from the products of water vapor radiolysis (oxygen and hydroxyl groups). The modulation of the work function through patterning matches well with Density Functional Theory modeling. Finally, post-functionalization of the patterned areas with an organic fluorophore demonstrates a robust method to achieve nanoscale functionalization with high fidelity.

Organic electrochemical transistors (OECTs) and OECT-based circuitry offer great potential in bioelectronics, wearable electronics and artificial neuromorphic electronics because of their exceptionally low driving voltages (<1 V), low power consumption (<1 µW), high transconductances (>10 mS) and biocompatibility 1-5 . However, the successful realization of critical complementary logic OECTs is currently limited by temporal and/or operational instability, slow redox processes and/or switching, incompatibility with high-density monolithic integration and inferior n-type OECT performance 6-8 . Here we demonstrate p- and n-type vertical OECTs with balanced and ultra-high performance by blending redox-active semiconducting polymers with a redox-inactive photocurable and/or photopatternable polymer to form an ion-permeable semiconducting channel, implemented in a simple, scalable vertical architecture that has a dense, impermeable top contact. Footprint current densities exceeding 1 kA cm -2 at less than ±0.7 V, transconductances of 0.2-0.4 S, short transient times of less than 1 ms and ultra-stable switching (>50,000 cycles) are achieved in, to our knowledge, the first vertically stacked complementary vertical OECT logic circuits. This architecture opens many possibilities for fundamental studies of organic semiconductor redox chemistry and physics in nanoscopically confined spaces, without macroscopic electrolyte contact, as well as wearable and implantable device applications., (© 2023. The Author(s).)

The development of high-precision large-area optical coatings and devices comprising low-dimensional materials hinges on scalable solution-based manufacturability with control over exfoliation procedure-dependent effects. As such, it is critical to understand the influence of technique-induced transition metal dichalcogenide (TMDC) optical properties that impact the design, performance, and integration of advanced optical coatings and devices. Here, we examine the optical properties of semiconducting MoS 2 films from the exfoliation formulations of four prominent approaches: solvent-mediated exfoliation, chemical exfoliation with phase reconversion, redox exfoliation, and native redox exfoliation. The resulting MoS 2 films exhibit distinct refractive indices ( n ), extinction coefficients ( k ), dielectric functions (ε 1 and ε 2 ), and absorption coefficients (α). For example, a large index contrast of Δ n  ≈ 2.3 is observed. These exfoliation procedures and related chemistries produce different exfoliated flake dimensions, chemical impurities, carrier doping, and lattice strain that influence the resulting optical properties. First-principles calculations further confirm the impact of lattice defects and doping characteristics on MoS 2 optical properties. Overall, incomplete phase reconfiguration (from 1T to mixed crystalline 2H and amorphous phases), lattice vacancies, intraflake strain, and Mo oxidation largely contribute to the observed differences in the reported MoS 2 optical properties. These findings highlight the need for controlled technique-induced effects as well as the opportunity for continued development of, and improvement to, liquid phase exfoliation methodologies. Such chemical and processing-induced effects present compelling routes to engineer exfoliated TMDC optical properties toward the development of next-generation high-performance mirrors, narrow bandpass filters, and wavelength-tailored absorbers., Competing Interests: Competing interestsThe authors declare no competing interests., (© This is a U.S. Government work and not under copyright protection in the US; foreign copyright protection may apply 2023.)

Since the isolation of graphene in 2004, two-dimensional (2D) materials research has rapidly evolved into an entire subdiscipline in the physical sciences with a wide range of emergent applications. The unique 2D structure offers an open canvas to tailor and functionalize 2D materials through layer number, defects, morphology, moiré pattern, strain, and other control knobs. Through this review, we aim to highlight the most recent discoveries in the following topics: theory-guided synthesis for enhanced control of 2D morphologies, quality, yield, as well as insights toward novel 2D materials; defect engineering to control and understand the role of various defects, including in situ and ex situ methods; and properties and applications that are related to moiré engineering, strain engineering, and artificial intelligence. Finally, we also provide our perspective on the challenges and opportunities in this fascinating field., Competing Interests: The authors declare no competing financial interest., (© 2022 The Authors. Published by American Chemical Society.)

Organic/inorganic heterostructures present a versatile platform for creating materials with new functionalities and hybrid properties. In particular, junctions between two dimensional materials have demonstrated utility in next generation electronic, optical, and optoelectronic devices. This work pioneers a microwave facilitated synthesis process to readily incorporate few-layer covalent organic framework (COF) films onto monolayer transition metal dichalcogenides (TMDC). Preferential microwave excitation of the monolayer TMDC flakes result in selective attachment of COFs onto the van der Waals surface with film thicknesses between 1 and 4 nm. The flexible process is extended to multiple TMDCs (MoS 2 , MoSe 2 , MoSSe) and several well-known COFs (TAPA-PDA COF, TPT-TFA-COF, and COF-5). Photoluminescence studies reveal a power-dependent defect formation in the TMDC layer, which facilitates electronic coupling between the materials at higher TMDC defect densities. This coupling results in a shift in the A-exciton peak location of MoSe 2 , with a red or blue shift of 50 or 19 meV, respectively, depending upon the electron donating character of the few-layer COF films. Moreover, optoelectronic devices fabricated from the COF-5/TMDC heterostructure present an opportunity to tune the PL intensity and control the interaction dynamics within inorganic/organic heterostructures.

Piezoelectricity in low-dimensional materials and metal-semiconductor junctions has attracted recent attention. Herein, a 2D in-plane metal-semiconductor junction made of multilayer 2H and 1T' phases of molybdenum(IV) telluride (MoTe 2 ) is investigated. Strong piezoelectric response is observed using piezoresponse force microscopy at the 2H-1T' junction, despite that the multilayers of each individual phase are weakly piezoelectric. The experimental results and density functional theory calculations suggest that the amplified piezoelectric response observed at the junction is due to the charge transfer across the semiconducting and metallic junctions resulting in the formation of dipoles and excess charge density, allowing the engineering of piezoelectric response in atomically thin materials., (© 2022 Wiley-VCH GmbH.)

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) exhibit compelling dimension-dependent exciton-dominated optical behavior influenced by thickness and lateral quantum confinement effects. Thickness quantum confinement effects have been observed; however, experimental optical property assessment of nanoscale lateral dimension monolayer TMDCs is lacking. Here, we employ ex situ spectroscopic ellipsometry to evaluate laterally coalescing monolayer metalorganic chemical vapor deposited MoS 2 . A multisample analysis is used to constrain Bruggeman and Maxwell-Garnett effective medium approximations and the effective dielectric functions are derived for laterally coalesced and uncoalesced MoS 2 films (∼10-94% surface coverage for ∼10-140 nm lateral grain sizes). This analysis demonstrates the ability to probe MoS 2 optical exciton behavior at growth-relevant grain sizes in relation to chemical vapor nucleation density, ripening, and lateral growth conditions. Our analysis is pertinent toward expected in situ epitaxial 2D TMDC film growth metrology to enable the facile development of monolayer films with targeted process-dependent optical properties.

Two-dimensional chalcogenide semiconductors have recently emerged as a host material for quantum emitters of single photons. While several reports on defect- and strain-induced single-photon emission from 2D chalcogenides exist, a bottom-up, lithography-free approach to producing a high density of emitters remains elusive. Further, the physical properties of quantum emission in the case of strained 2D semiconductors are far from being understood. Here, we demonstrate a bottom-up, scalable, and lithography-free approach for creating large areas of localized emitters with high density (∼150 emitters/um 2 ) in a WSe 2 monolayer. We induce strain inside the WSe 2 monolayer with high spatial density by conformally placing the WSe 2 monolayer over a uniform array of Pt nanoparticles with a size of 10 nm. Cryogenic, time-resolved, and gate-tunable luminescence measurements combined with near-field luminescence spectroscopy suggest the formation of localized states in strained regions that emit single photons with a high spatial density. Our approach of using a metal nanoparticle array to generate a high density of strained quantum emitters will be applied to scalable, tunable, and versatile quantum light sources.

Multiplex electronic antigen sensors for detection of SARS-Cov-2 spike glycoproteins and hemagglutinin from influenza A are fabricated using scalable processes for straightforward transition to economical mass-production. The sensors utilize the sensitivity and surface chemistry of a 2D MoS 2 transducer for attachment of antibody fragments in a conformation favorable for antigen binding with no need for additional linker molecules. To make the devices, ultra-thin layers (3 nm) of amorphous MoS 2 are sputtered over pre-patterned metal electrical contacts on a glass chip at room temperature. The amorphous MoS 2 is then laser annealed to create an array of semiconducting 2H-MoS 2 transducer regions between metal contacts. The semiconducting crystalline MoS 2 region is functionalized with monoclonal antibody fragments complementary to either SARS-CoV-2 S1 spike protein or influenza A hemagglutinin. Quartz crystal microbalance experiments indicate strong binding and maintenance of antigen avidity for antibody fragments bound to MoS 2 . Electrical resistance measurements of sensors exposed to antigen concentrations ranging from 2-20 000 pg mL -1 reveal selective responses. Sensor architecture is adjusted to produce an array of sensors on a single chip suited for detection of analyte concentrations spanning six orders of magnitude from pg mL -1 to µg mL -1 ., Competing Interests: The authors declare no conflict of interest., (© 2022 Wiley‐VCH GmbH.)

Two-dimensional (2D) crystals have renewed opportunities in design and assembly of artificial lattices without the constraints of epitaxy. However, the lack of thickness control in exfoliated van der Waals (vdW) layers prevents realization of repeat units with high fidelity. Recent availability of uniform, wafer-scale samples permits engineering of both electronic and optical dispersions in stacks of disparate 2D layers with multiple repeating units. Here we present optical dispersion engineering in a superlattice structure comprising alternating layers of 2D excitonic chalcogenides and dielectric insulators. By carefully designing the unit cell parameters, we demonstrate greater than 90% narrow band absorption in less than 4 nm of active layer excitonic absorber medium at room temperature, concurrently with enhanced photoluminescence in square-centimetre samples. These superlattices show evidence of strong light-matter coupling and exciton-polariton formation with geometry-tuneable coupling constants. Our results demonstrate proof of concept structures with engineered optical properties and pave the way for a broad class of scalable, designer optical metamaterials from atomically thin layers., (© 2021. The Author(s), under exclusive licence to Springer Nature Limited.)

Heterogeneous integration strategies are increasingly being employed to achieve more compact and capable electronics systems for multiple applications including space, electric vehicles, and wearable and medical devices. To enable new integration strategies, the growth and transfer of thin electronic films and devices, including III-nitrides, metal oxides, and 2D materials, using 2D boron nitride (BN)-on-sapphire templates are demonstrated. The van der Waals (vdW) BN layer, in this case, acts as a preferred mechanical release layer for precise separation at the substrate-film interface and leaves a smooth surface suitable for vdW bonding. A tensilely stressed Ni layer sputtered on top of the film induces controlled spalling fracture that propagates at the BN/sapphire interface. By incorporating controlled spalling, the process yield and sensitivity are greatly improved, owed to the greater fracture energy provided by the stressed metal layer relative to a soft tape or rubber stamp. With stress playing a critical role in this process, the influence of residual stress on detrimental cracking and bowing is investigated. Additionally, a back-end selected area lift-off technique is developed which allows for isolation and transfer of individual devices or arbitrary shapes., (© 2021 Wiley-VCH GmbH.)

The semiconductor-metal junction is one of the most critical factors for high-performance electronic devices. In two-dimensional (2D) semiconductor devices, minimizing the voltage drop at this junction is particularly challenging and important. Despite numerous studies concerning contact resistance in 2D semiconductors, the exact nature of the buried interface under a three-dimensional (3D) metal remains unclear. Herein, we report the direct measurement of electrical and optical responses of 2D semiconductor-metal buried interfaces using a recently developed metal-assisted transfer technique to expose the buried interface, which is then directly investigated using scanning probe techniques. We characterize the spatially varying electronic and optical properties of this buried interface with <20 nm resolution. To be specific, potential, conductance, and photoluminescence at the buried metal/MoS 2 interface are correlated as a function of a variety of metal deposition conditions as well as the type of metal contacts. We observe that direct evaporation of Au on MoS 2 induces a large strain of ∼5% in the MoS 2 which, coupled with charge transfer, leads to degenerate doping of the MoS 2 underneath the contact. These factors lead to improvement of contact resistance to record values of 138 kΩ μm, as measured using local conductance probes. This approach was adopted to characterize MoS 2 -In/Au alloy interfaces, demonstrating contact resistance as low as 63 kΩ μm. Our results highlight that the MoS 2 /metal interface is sensitive to device fabrication methods and provide a universal strategy to characterize buried contact interfaces involving 2D semiconductors.

A growing body of research focuses on engineering materials for electrochemical detection of dopamine (DA), a critical neurotransmitter involved in motor function, reward processes, and blood pressure regulation. Among various sensing materials, graphene is highly attractive due to its excellent electrical conductivity and, in particular, the π-π interaction between the aromatic rings of DA and graphene. However, the lowest detection limits reported solely using graphene are nominally 1 nM. To improve the sensor sensitivity, various strategies are being explored, including chemical functionalization, heterostructure/composite formation, elemental doping, and modification with biomolecules (aptamers, enzymes, etc. ). In this work, we demonstrate that commercially available graphene ink can exhibit selective and highly sensitive detection of DA by tuning the surface chemistry utilizing a simple, one-step annealing process. The annealing condition directly impacts the sensor response to DA, with the optimal conditions (30 min at 300 °C under 3% H 2 + Ar) yielding a distinguishable and selective response to DA down to 5 pM. X-ray photoelectron spectroscopy (XPS) confirms that the improved selectivity is due to the increased fraction of oxygen functionalities (in particular, C-OH), while Raman spectroscopy shows a higher degree of defectiveness for this condition compared to others. Evaluation of the interaction of three molecular components of DA ( i.e. , aromatic ring, hydroxyl groups, and amine group) with graphene confirms that the π-π interaction and -OH groups play a prominent role in the improved adsorption of DA on the graphene surface. Furthermore, we demonstrate a proof-of-concept, all-solution processable sensor on polyimide substrates using graphene ink. Tuning the sensor response by varying the annealing condition offers a simple avenue for developing sensitive, selective, and low-cost point-of-care biosensors, while low-temperature annealing ensures compatibility with flexible substrates, such as polyimide.

The era of two-dimensional (2D) materials, in its current form, truly began at the time that graphene was first isolated just over 15 years ago. Shortly thereafter, the use of 2D hexagonal boron nitride ( h- BN) had expanded in popularity, with use of the thin isolator permeating a significant number of fields in condensed matter and beyond. Due to the impractical nature of cataloguing every use or research pursuit, this review will cover ground in the following three subtopics relevant to this versatile material: growth, electrical measurements, and applications in optics and photonics. Through understanding how the material has been utilized, one may anticipate some of the exciting directions made possible by the research conducted up through the turn of this decade.

Anisotropy in a crystal structure can lead to large orientation-dependent variations of mechanical, optical, and electronic properties. Material orientation control can thus provide a handle to manipulate properties. Here, a novel sputtering approach for 2D materials enables growth of ultrathin (2.5-10 nm) tellurium films with rational control of the crystalline orientation templated by the substrate. The anisotropic Te 〈0001〉 helical chains align in the plane of the substrate on highly oriented pyrolytic graphite (HOPG) and orthogonally to MgO(100) substrates, as shown by polarized Raman spectroscopy and high-resolution electron microscopy. Furthermore, the films are shown to grow in a textured fashion on HOPG, in contrast with previous reports. These ultrathin Te films cover exceptionally large areas (>1 cm 2 ) and are grown at low temperature (25 °C) affording the ability to accommodate a variety of substrates including flexible electronics. They are robust toward oxidation over a period of days and exhibit the non-centrosymmetric P3 1 21 Te structure. Raman signals are acutely dependent on film thickness, suggesting that optical anisotropy persists and is even enhanced at the ultrathin limit. Hall effect measurements indicate orientation-dependent carrier mobility up to 19 cm 2 V -1 s -1 . These large-area, ultrathin Te films grown by a truly scalable, physical vapor deposition technique with rational control of orientation/thickness open avenues for controlled orientation-dependent properties in semiconducting thin films for applications in electronic and optoelectronic devices.

Since graphene, a variety of 2D materials have been fabricated in a quest for a tantalizing combination of properties and desired physiochemical behavior. 2D materials that are piezoelectric, i.e., that allow for a facile conversion of electrical energy into mechanical and vice versa, offer applications for sensors, actuators, energy harvesting, stretchable and flexible electronics, and energy storage, among others. Unfortunately, materials must satisfy stringent symmetry requirements to be classified as piezoelectric. Here, 2D ultrathin single-crystal molybdenum oxide (MoO 2 ) flakes that exhibit unexpected piezoelectric-like response are fabricated, as MoO 2 is centrosymmetric and should not exhibit intrinsic piezoelectricity. However, it is demonstrated that the apparent piezoelectricity in 2D MoO 2 emerges from an electret-like behavior induced by the trapping and stabilization of charges around defects in the material. Arguably, the material represents the first 2D electret material and suggests a route to artificially engineer piezoelectricity in 2D crystals. Specifically, it is found that the maximum out-of-plane piezoresponse is 0.56 pm V -1 , which is as strong as that observed in conventional 2D piezoelectric materials. The charges are found to be highly stable at room temperature with a trapping energy barrier of ≈2 eV., (© 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Mechanical transfer of high-performing thin-film devices onto arbitrary substrates represents an exciting opportunity to improve device performance, explore nontraditional manufacturing approaches, and paves the way for soft, conformal, and flexible electronics. Using a two-dimensional boron nitride release layer, we demonstrate the transfer of AlGaN/GaN high-electron mobility transistors (HEMTs) to arbitrary substrates through both direct van der Waals bonding and with a polymer adhesive interlayer. No device degradation was observed because of the transfer process, and a significant reduction in device temperature (327-132 °C at 600 mW) was observed when directly bonded to a silicon carbide (SiC) wafer relative to the starting wafer. With the use of a benzocyclobutene (BCB) adhesion interlayer, devices were easily transferred and characterized on Kapton and ceramic films, representing an exciting opportunity for integration onto arbitrary substrates. Upon reduction of this polymer adhesive layer thickness, the AlGaN/GaN HEMTs transferred onto a BCB/SiC substrate resulted in comparable peak temperatures during operation at powers as high as 600 mW to the as-grown wafer, revealing that by optimizing interlayer characteristics such as thickness and thermal conductivity, transferrable devices on polymer layers can still improve performance outputs.

As elemental main group materials (i.e., silicon and germanium) have dominated the field of modern electronics, their monolayer 2D analogues have shown great promise for next-generation electronic materials as well as potential game-changing properties for optoelectronics, energy, and beyond. These atomically thin materials composed of single atomic variants of group III through group VI elements on the periodic table have already demonstrated exciting properties such as near-room-temperature topological insulation in bismuthene, extremely high electron mobilities in phosphorene and silicone, and substantial Li-ion storage capability in borophene. Isolation of these materials within the postgraphene era began with silicene in 2010 and quickly progressed to the experimental identification or theoretical prediction of 15 of the 18 main group elements existing as solids at standard pressure and temperatures. This review first focuses on the significance of defects/functionalization, discussion of different allotropes, and overarching structure-property relationships of 2D main group elemental materials. Then, a complete review of emerging applications in electronics, sensing, spintronics, plasmonics, photodetectors, ultrafast lasers, batteries, supercapacitors, and thermoelectrics is presented by application type, including detailed descriptions of how the material properties may be tailored toward each specific application., (© 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Since the isolation of graphene in 2004, there has been an exponentially growing number of reports on layered two-dimensional (2D) materials for applications ranging from protective coatings to biochemical sensing. Due to the exceptional, and often tunable, electrical, optical, electrochemical, and physical properties of these materials, they can serve as the active sensing element or a supporting substrate for diverse healthcare applications. In this review, we provide a survey of the recent reports on the applications of 2D materials in biosensing and other emerging healthcare areas, ranging from wearable technologies to optogenetics to neural interfacing. Specifically, this review provides (i) a holistic evaluation of relevant material properties across a wide range of 2D systems, (ii) a comparison of 2D material-based biosensors to the state-of-the-art, (iii) relevant material synthesis approaches specifically reported for healthcare applications, and (iv) the technological considerations to facilitate mass production and commercialization.

Monolayer epitaxial graphene (EG), grown on the Si face of SiC, is an advantageous material for a variety of electronic and optical applications. EG forms as a single crystal over millimeter-scale areas and consequently, the large scale single crystal can be utilized as a template for growth of other materials. In this work, we present the use of EG as a template to form millimeter-scale amorphous and hexagonal boron nitride ( a -BN and h -BN) films. The a -BN is formed with pulsed laser deposition and the h -BN is grown with triethylboron (TEB) and NH 3 precursors, making it the first metal organic chemical vapor deposition (MOCVD) process of this growth type performed on epitaxial graphene. A variety of optical and non-optical characterization methods are used to determine the optical absorption and dielectric functions of the EG, a -BN, and h -BN within the energy range of 1 eV to 8.5 eV. Furthermore, we report the first ellipsometric observation of high-energy resonant excitons in EG from the 4H polytype of SiC and an analysis on the interactions within the EG and h -BN heterostructure., Competing Interests: Conflict of Interest: The authors declare no competing financial interest.

Flexible gallium nitride (GaN) thin films can enable future strainable and conformal devices for transmission of radio-frequency (RF) signals over large distances for more efficient wireless communication. For the first time, strainable high-frequency RF GaN devices are demonstrated, whose exceptional performance is enabled by epitaxial growth on 2D boron nitride for chemical-free transfer to a soft, flexible substrate. The AlGaN/GaN heterostructures transferred to flexible substrates are uniaxially strained up to 0.85% and reveal near state-of-the-art values for electrical performance, with electron mobility exceeding 2000 cm 2 V -1 s -1 and sheet carrier density above 1.07 × 10 13 cm -2 . The influence of strain on the RF performance of flexible GaN high-electron-mobility transistor (HEMT) devices is evaluated, demonstrating cutoff frequencies and maximum oscillation frequencies greater than 42 and 74 GHz, respectively, at up to 0.43% strain, representing a significant advancement toward conformal, highly integrated electronic materials for RF applications., (© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Regarding the improvement of current quantized Hall resistance (QHR) standards, one promising avenue is the growth of homogeneous monolayer epitaxial graphene (EG). A clean and simple process is used to produce large, precise areas of EG. Properties like the surface conductivity and dielectric loss tangent remain unstable when EG is exposed to air due to doping from molecular adsorption. Experimental results are reported on the extraction of the surface conductivity and dielectric loss tangent from data taken with a noncontact resonance microwave cavity, assembled with an air-filled, standard R100 rectangular waveguide configuration. By using amorphous boron nitride (a-BN) as an encapsulation layer, stability of EG's electrical properties under ambient laboratory conditions is greatly improved. Moreover, samples are exposed to a variety of environmental and chemical conditions. Both thicknesses of a-BN encapsulation are sufficient to preserve surface conductivity and dielectric loss tangent to within 10% of its previously measured value, a result which has essential importance in the mass production of millimeter-scale graphene devices demonstrating electrical stability., (© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Homogeneous monolayer epitaxial graphene (EG) is an ideal candidate for the development of millimeter-sized devices with single-crystal domains. A clean fabrication process was used to produce EG-based devices, with n-type doping level of the order of 10 12 cm -2 . Generally, electrical properties of EG, such as longitudinal resistivity, remain unstable when devices are exposed to air due to adsorption of molecular dopants, whose presence shifts the carrier density close to the Dirac point (<10 10 cm -2 ) or into the p-type regime. Here, we report experimental results on the use of amorphous boron nitride (a-BN) as an encapsulation layer, whereby EG can maintain its longitudinal resistivity and have its carrier density modulated. Furthermore, we exposed 12 devices to controlled temperatures of up to 85 °C and relative humidity of up to 85% and reported that an approximately 20 nm a-BN encapsulation thickness is sufficient to preserve their longitudinal resistivity to within 10% of the previously measured value. We monitored the electronic properties of our encapsulated and nonencapsulated EG samples by magnetotransport measurements, using a neodymium iron boron magnet. Our results have essential importance in the mass production of millimeter-scale graphene devices, with stable electrical properties.