2D Materials Research Articles

3D Printing Materials Mimicking Human Tissues after Uptake of Iodinated Contrast Agents for Anthropomorphic Radiology Phantoms.

(1) Background: 3D printable materials with accurately defined iodine content enable the development and production of radiological phantoms that simulate human tissues, including lesions after contrast administration in medical imaging with X-rays. These phantoms provide accurate, stable and reproducible models with defined iodine concentrations, and 3D printing allows maximum flexibility and minimal development and production time, allowing the simulation of anatomically correct anthropomorphic replication of lesions and the production of calibration and QA standards in a typical medical research facility. (2) Methods: Standard printing resins were doped with an iodine contrast agent and printed using a consumer 3D printer, both (resins and printer) available from major online marketplaces, to produce printed specimens with iodine contents ranging from 0 to 3.0% by weight, equivalent to 0 to 3.85% elemental iodine per volume, covering the typical levels found in patients. The printed samples were scanned in a micro-CT scanner to measure the properties of the materials in the range of the iodine concentrations used. (3) Results: Both mass density and attenuation show a linear dependence on iodine concentration (R2 = 1.00), allowing highly accurate, stable, and predictable results. (4) Conclusions: Standard 3D printing resins can be doped with liquids, avoiding the problem of sedimentation, resulting in perfectly homogeneous prints with accurate dopant content. Iodine contrast agents are perfectly suited to dope resins with appropriate iodine concentrations to radiologically mimic tissues after iodine uptake. In combination with computer-aided design, this can be used to produce printed objects with precisely defined iodine concentrations in the range of up to a few percent of elemental iodine, with high precision and anthropomorphic shapes. Applications include radiographic phantoms for detectability studies and calibration standards in projective X-ray imaging modalities, such as contrast-enhanced dual energy mammography (abbreviated CEDEM, CEDM, TICEM, or CESM depending on the equipment manufacturer), and 3-dimensional modalities like CT, including spectral and dual energy CT (DECT), and breast tomosynthesis.

Giant Valley Zeeman Splitting in Vanadium-Doped WSe2 Monolayers.

2D dilute magnetic semiconductors (DMS) based on transition metal dichalcogenides (TMD) offer an innovative pathway for advancing spintronic technologies, including the potential to exploit phenomena such as the valley Zeeman effect. However, the impact of magnetic ordering on the valley degeneracy breaking and on the enhancement of the optical transitions g-factors of these materials remains an open question. Here, a giant effective g-factors ranging between ≈-27 and -69 for the bound exciton at 4K in vanadium-doped WSe2 monolayers, obtained through magneto-photoluminescence (PL) experiments is reported. This giant g-factor disappears at room temperature, suggesting that this response is associated with a magnetic ordering of the vanadium impurity states at low temperatures. Ab initio calculations for the vanadium-doped WSe2 monolayer confirm the existence of magnetic ordering of the vanadium states, which leads to degeneracy breaking of the valence bands at K and K'. A phenomenological analysis is employed to correlate this splitting with the measured enhanced effective g-factor. The findings shed light on the potential of defect engineering of 2D materials for spintronicapplications.

AB–stacked bilayer β12–borophene as a promising anode material for alkali metal-ion batteries

As the lightest 2D material, monolayer borophene exhibits a specific charge capacity of 1860 mA h g−1 for Li-ion batteries, which is four times higher than that of graphite and is one of the highest specific charge capacities ever reported for 2D anode materials. Additionally, it showed high mechanical strength and a low diffusion barrier. However, monolayer borophene suffers from stability issues in its free-standing form, which restricts its real-life applications. Inspired by the recent experimental investigations, which proved the higher stability of bilayer borophene polymorphs (BBPs) over their monolayer counterparts, in this work, we investigated the dynamical and thermodynamical stabilities of both AA– and AB–stacked BBPs in their β12 phase using first-principles calculations. Between the two stacking patterns, we found that only the AB–stacked β12–BBP is both energetically and dynamically stable, and we further investigated its potential as a high-performance anode material for alkali metal-ion batteries. Our investigations show that AB–stacked β12–BBP exhibits good electrical conductivity before and after metal atom (Li/Na/K) adsorption onto it. Further, AB–stacked β12–BBP adsorbs the metal atoms strongly with adsorption energies ranging between −0.89 to −1.44 eV, indicating that there is a lesser possibility of forming dendrites on this anode. Similarly, it has a low diffusion energy barrier (~ 0.13–0.49 eV) for metal atoms, meeting the fast charge/discharge rate requirements. Moreover, it exhibits a reasonably low average metal-insertion voltage (0.43 to 0.65 V) and a specific charge capacity of 330–413 mA h g−1 that is comparable to graphite. All the above findings suggest that the AB–stacked bilayer β12– borophene can be a potentially favorable anode material.

Valley-Polarized Topological Phases with In-Plane Magnetization.

The coexistence of valley polarization and topology has considerably facilitated the applications of 2D materials toward valleytronics device technology. However, isolated and distinct valleys are required to observe the valley-related quantum phenomenon. Herein, we report a new mechanism to generate in-plane magnetization direction-dependent isolated valley carriers by preserving or breaking the mirror symmetry in a 2D system. First-principle calculations are carried out on a prototype material, W2MnC2O2 MXene, to demonstrate the mechanism. A valley-coupled topological phase transition among Weyl semimetal, valley-polarized quantum anomalous Hall insulator, and topological semimetal is observed by manipulating the in-plane magnetization directions in W2MnC2O2. Monte Carlo simulations of W2MnC2O2 show that the estimated Curie temperature is around 170 K, indicating the possibility of observing valley-polarized topological states at higher temperatures. Our finding provides a generalized platform for investigating the valley and topological physics, which is extremely important for future quantum information processing applications.

Theoretical Design of Thorium Nitride MXenes.

Actinides with 5f6d7s valence orbitals feature special physicochemical properties different from those of the other elements. Actinide-based two-dimensional (2D) materials combine the distinctive physics of actinides with the quantum size effect of 2D materials, but relevant studies are scarce. Since Th has a valence electron configuration of 6d27s2 like Ti, and Ti-based MXenes show excellent stability and versatile applications, whether Th can form stable MXenes has become an intriguing question. Herein, we designed Th2N, Th3N2, and Th4N3 MXenes and investigated their physical properties, functionalization, and potential applications using density functional theory. Their stabilities are validated by global minimum search, phonon spectra, ab initio molecular dynamics, enthalpy of formation, and energy above the hull. All the Th-N MXenes exhibit metallic properties and are stabilized by the electrostatic interaction between Th and N ions, as well as the covalent bonding interaction between the Th 6d/5f and N 2p/2s orbitals. The H-, O-, and F-functionalization3N2 MXene improve its stability while preserving its metallicity, and the O-functionalized Th3N2 MXene shows promising catalytic activity for hydrogen evolution. The thorium nitride MXenes enrich the family of actinide-based 2D materials and extend our understanding of the structures and properties induced by actinide elements in low-dimensional materials.

Mechanical and Ionic Characterization for Organic Semiconductor-Incorporated Perovskites for Stable 2D/3D Heterostructure Perovskite Solar Cells.

Hybrid metal halide perovskite (MHP) materials, while being promising for photovoltaic technology, also encounter challenges related to material stability. Combining 2D MHPs with 3D MHPs offers a viable solution, yet there is a gap in the understanding of the stability among various 2D materials. The mechanical, ionic, and environmental stability of various 2D MHP ligands are reported, and an improvement with the use of a quater-thiophene-based organic cation (4TmI) that forms an organic-semiconductor incorporated MHP structure is demonstrated. It is shown that the best balance of mechanical robustness, environmental stability, ion activation energy, and reduced mobile ion concentration under accelerated aging is achieved with the usage of 4TmI. It is believed that by addressing mechanical and ion-based degradation modes using this built-in barrier concept with a material system that also shows improvements in charge extraction and device performance, MHP solar devices can be designed for both reliability and efficiency.

Growth Behavior of Ni on Hydrogen-Etched WS2 Surface.

Transition metal dichalcogenides (TMDs) are 2D materials in which the layers are stacked together by van der Waals forces. Although TMDs are expected to be promising for electronic applications, forming a uniform electrode on them is challenging because of the low adhesion forces between metals and TMDs. This study focuses on improving the quality of metal electrodes by introducing atomic H to create surface defects, using Ni on WS2 as an example. The detailed effects of H etching and subsequent Ni growth were investigated using scanning tunneling microscopy (STM) and synchrotron-based X-ray photoemission (XPS) techniques. Our studies reveal that introducing point defects of ∼3.05 × 1011 cm-2 on the WS2 surface, results in a significant shift in Ni growth from the Volmer-Weber to a near Frank-van der Merwe mode. The origin of the change is the bond formation between the Ni and W atoms, which is expected to realize ohmic contact. The optimization of metal-TMD interfaces offers valuable insights for advanced applications.

Low-symmetry layered semiconductor In2Te5 for promising polarization-sensitive photodetector

Low-symmetry two-dimensional (2D) materials have attracted significant attention for polarization-sensitive photodetection due to the optoelectronic anisotropy. Here, we demonstrated the strong in-plane anisotropy of In2Te5 through electron density distribution calculations based on density functional theory and developed a polarization-sensitive photodetector. The photodetector shows a responsivity of 171.16 mA/W and a response time of 0.42 s under visible light illumination. Additionally, it presents a polarization-sensitive photoresponse with a dichroic ratio of 1.34. Our work reveals the anisotropic optoelectronic properties of In2Te5, potentially stimulating research interest in Group III-VI 2D materials (Pentatelluride M2Te5, M = Al, Ga, In, etc.).

Rapid yet Controlled Synthesis of 2D Covalent Organic Framework Nanocapsules as High-Performance Photocatalytic Carriers.

Synthesis and assembly of two-dimensional (2D) polymeric materials present a tricky trade-off between the high reaction rate and precise morphology control. Here we report a nanoconfined synthesis of imine-based 2D covalent organic frameworks (COFs) at the interface of oil-in-water (O/W) emulsion droplets stabilized by cationic surfactants. Highly uniform nanocapsules (NCs) could be prepared without adding extra catalysts at room temperature in just 4.5 h at a yield of 86%. The NCs have tunable average diameters and shell thicknesses, depending on the monomer and surfactant types/concentrations. Their BET-specific surface areas are up to 139.0 m2/g, mainly contributedby narrowly-distributed mesopores at ~5.0 nm and micropores at 1.4 nm.The surfactant plays the role of a catalyst during the reaction and interestingly, it also regulates the formation of mesopores and their sizes. Both theoretical and experimental studies confirm that the reaction has been accelerated by two orders of magnitude at the microdroplet interface, compared to that without emulsification. The resulting NCs could be well dispersed in water, and they have been demonstrated to be highly efficient nanocatalystsin application of water-based hydrogen evolution.Such microdroplet interface-confined synthesis may facilitate the future development of 2D polymeric materials for more advanced applications.

Observation of Néel-type magnetic skyrmion in a layered non-centrosymmetric itinerant ferromagnet CrTe1.38

Magnetic skyrmions are nanoscale vortex-like magnetization textures that hold great promise for next-generation memory and spintronic devices. While extensive research has focused on discovering such localized spin textures in bulk magnets and multilayers with heavy metals, there is a growing interest in finding them in two-dimensional (2D) magnetic materials. In this research work, we report two distinct phases of the 2D CrxTey family: non-centrosymmetric CrTe1.38 and centrosymmetric CrTe0.96, with a Curie temperature of around 200 and 300 K, respectively. Detailed magnetic study indicates a prominent out-of-plane magnetic anisotropy in CrTe1.38. In contrast, CrTe0.96 exhibits a weak uniaxial magnetic anisotropy. Lorentz transmission electron microscopy shows that the spontaneous ferromagnetic ground state of CrTe1.38 consists of Néel skyrmions, whereas CrTe0.96 exhibits Bloch domain walls, consistent with their crystalline symmetries. This research expands the quasi-2D CrxTey family and opens up new avenues for exploring non-trivial spin structures and their potential applications in spintronic devices.

Grain boundaries in single-layer magnetic material CrCl3

Grain boundaries in single-layer magnetic material CrCl3

Prediction of a novel two-dimensional superatomic Cd6S2 monolayer for photocatalytic water splitting.

Two-dimensional transition metal dichalcogenides possess a significant specific surface area, adjustable bandgaps, and excellent optical absorption properties, rendering them highly conducive to photocatalytic applications. Herein, a MoS2-like 2D superatomic Cd6S2 monolayer is predicted, wherein the octahedral Cd6 superatom unit connects with S atoms via six vertices. Chemical bonding analysis reveals that the remarkable dynamic, thermal, and mechanical stability of the Cd6S2 monolayer results from the covalent Cd-S bonds and the 6-center 8-electron (6c-8e) delocalized bond within the Cd6 core, which ensures the chemical octet rule for both the S atom and the Cd6 superatom. Demonstrating notable optical absorption coefficients and a strain-tuned energy band structure, the Cd6S2 monolayer emerges as a viable candidate for catalyzing the solar-powered splitting of water. This work offers an alternative avenue to modify or improve the properties of 2D materials for photocatalytic applications through superatomic assembly.

Reaction-driven restructuring of defective PtSe2 into ultrastable catalyst for the oxygen reduction reaction.

PtM (M = S, Se, Te) dichalcogenides are promising two-dimensional materials for electronics, optoelectronics and gas sensors due to their high air stability, tunable bandgap and high carrier mobility. However, their potential as electrocatalysts for the oxygen reduction reaction (ORR) is often underestimated due to their semiconducting properties and limited surface area from van der Waals stacking. Here we show an approach for synthesizing a highly efficient and stable ORR catalyst by restructuring defective platinum diselenide (DEF-PtSe2) through electrochemical cycling in an O2-saturated electrolyte. After 42,000 cycles, DEF-PtSe2 exhibited 1.3 times higher specific activity and 2.6 times higher mass activity compared with a commercial Pt/C electrocatalyst. Even after 126,000 cycles, it maintained superior ORR performance with minimal decay. Quantum mechanical calculations using hybrid density functional theory reveal that the improved performance is due to the synergistic contributions from Pt nanoparticles and the apical active sites on the DEF-PtSe2 surface. This work highlights the potential of DEF-PtSe2 as a durable electrocatalyst for ORR, offering insights into PtM dichalcogenide electrochemistry and the design of advanced catalysts.

Overview of the Design and Application of Photothermal Immunoassays.

Developing powerful immunoassays for sensitive and real-time detection of targets has always been a challenging task. Due to their advantages of direct readout, controllable sensing, and low background interference, photothermal immunoassays have become a type of new technology that can be used for various applications such as disease diagnosis, environmental monitoring, and food safety. By modification with antibodies, photothermal materials can induce temperature changes by converting light energy into heat, thereby reporting specific target recognition events. This article reviews the design and application of photothermal immunoassays based on different photothermal materials, including noble metal nanomaterials, carbon-based nanomaterials, two-dimensional nanomaterials, metal oxide and sulfide nanomaterials, Prussian blue nanoparticles, small organic molecules, polymers, etc. It pays special attention to the role of photothermal materials and the working principle of various immunoassays. Additionally, the challenges and prospects for future development of photothermal immunoassays are briefly discussed.

Easy Direct Functionalization of 2D MoS2 Applied in Covalent Hybrids with PANI as Dual Blend Supercapacitive Materials

AbstractThe pressing demand for more sustainable energy provision and the ongoing transition toward renewable resources underline the critical need for effective energy storage solutions. To address this challenge, scientists persistently explore new compounds and hybrids and, in such a dynamic research field, 2D materials, particularly transition metal di‐chalcogenides (TMDCs), show great potential for electrochemical energy storage uses. Simultaneously, also conductive polymers (CPs) are interesting and versatile supercapacitor materials, especially polyaniline (PANI), which is extensively studied for this purpose. In this work, a powerful method to combine TMDCs and PANI into covalently grafted hybrids starting from aniline functionalized few‐layers 1T‐MoS2, attained by a facile direct arylation with iodoaniline, is presented. The hybrids provide circa 70 F g−1 specific capacitance in a pseudo device setup, coupled with a robust capacitance retention of well over 80% for up to 5000 cycles. These findings demonstrate the potential of similar covalent composites to work as active components for novel, innovative energy storage technologies. At the same time, the successful synthesis marks the efficacy of direct covalent grafting of conductive polymer on the surface of 2D TMDCs for stable functional materials.

Large out-of-plane piezoelectric response and ultra-low polarization transition barriers in two-dimensional MoGe2N4/MoSi2N4 heterostructures

With the help of the first principle calculations, two-dimensional (2D) MoGe2N4/MoSi2N4 van der Waals (vdW) heterojunction was confirmed as a type II bandgap arrangement with distinct piezoelectric properties dependent on the stacking orders and interlayer interactions. This structural change is facilitated by slip interactions between the layers and is characterized by a process with a low energy barrier, allowing for efficient and responsive phase transition of structures. Notably, this heterojunction exhibits a particularly large out-of-plane piezoelectric coefficient, d33, within a finite thickness. Concurrently, the piezoelectric coefficient can be enhanced by adjusting the vertical strain to a specific value, and the out-of-plane piezoelectric coefficient d33 can be as high as 73.28 pm/V, which is larger than the values of the available 2D materials or their heterojunctions. In addition, we simulated a piezoelectric actuator on a two-dimensional scale, where the deformation of the upper surface of the piezoelectric brake was linearly modulated by adjusting the driving voltage. Thus, our research paves the way for the conceptualization and creation of cutting-edge nanoelectronic devices, electromechanical systems, and multifunctional atomic-scale appliances.

Using Post‐Treatment Additives for Crystal Modulation and Interface Passivation Enables the Fabrication of Efficient and Stable Perovskite Solar Cells in Air

AbstractHigh‐performance perovskite solar cells (PSCs) fabricated in ambient air are considered inevitable for low‐cost commercial manufacturing. However, passivating perovskite film defects and controlling the crystallization process are critical for achieving high performance in PSCs. This study proposes using the novel 2D material MBene in green antisolvent to simultaneously modulate the crystallization and passivation defects of perovskites. MBene facilitates the passivation of uncoordinated Pb2+ ions, thereby enhancing the formation energy of vacancies within the perovskite film and adjusting the energy level structure. Moreover, MBene increases nucleation sites for perovskite, significantly extending crystal growth and improving film crystallinity, thereby reducing non‐radiative recombination. Consequently, champion devices treated with MBene achieve a power conversion efficiency (PCE) of 24.22% when fabricated in air, and exhibit superior humidity and long‐term stability. Furthermore, PSCs with added MBene exhibit significant long‐term stability under various environmental conditions, including humidity and heat. The study results lay a foundation for the development of MBene materials in photovoltaics, revealing their mechanism as a new type of 2D material in perovskites, and providing new insights for industrially producing efficient and stable solar cells.

Air stability of monolayer WSi2N4 in dark and bright conditions

Two-dimensional materials with chemical formula MA2Z4 are a promising class of materials for optoelectronic applications. To exploit their potential, their stability with respect to air pollution has to be analyzed under different conditions. In a first-principle study based on density functional theory, we investigate the adsorption of three common environmental gas molecules (O2, H2O, and CO2) on monolayer WSi2N4, an established representative of the MA2Z4 family. The computed adsorption energies, charge transfer, and projected density of states of the polluted monolayer indicate a relatively weak interaction between substrate and molecules resulting in an ultrashort recovery time of the order of nanoseconds. O2 and water introduce localized states in the upper valence region but do not alter the semiconducting nature of WSi2N4 nor its band-gap size apart from a minor variation of a few tens of meV. Exploring the same scenario in the presence of photogenerated electrons and holes, we do not notice any substantial difference except for O2 chemisorption when negative charge carriers are in the system. In this case, monolayer WSi2N4 exhibits signs of irreversible oxidation, testified by an adsorption energy of -5.5 eV leading to an infinitely long recovery time, a rearrangement of the outermost atomic layer bonding with the pollutant, and n-doping of the system. Our results indicate stability of WSi2N4 against H2O and CO2 in both dark and bright conditions, suggesting the potential of this material in nanodevice applications.

Advancing sensing frontiers: elevating performance metrics and extending applications through two-dimensional materials

Advancing sensing frontiers: elevating performance metrics and extending applications through two-dimensional materials

Boosting supercapacitive performance of SnS2 via trace Pb doping

High-performance electrode materials are crucial for enhancing the performance of supercapacitors. Among various candidates, pseudo-capacitive SnS2 is a promising one due to its high specific capacitance, earth-abundance, nontoxicity as well as low-cost. However, its actual electrochemical performance is restricted owing to the poor intrinsic conductivity and current fabrication processes on improving the conductivity are usually complicated. In this study, based on first-principles calculations, Pb doping is introduced to enhance the conductivity of SnS2. Pb-doped SnS2 nanosheets are synthesized via a simple one-step hydrothermal method. With trace Pb doping (Pbo.o1SnS2), an impressive 4-order-of-magnitude increase in conductivity was achieved compared to pristine SnS2. Furthermore, Pb-doped SnS2 nanosheets exhibit a superior mass-specific capacitance of 533.7 F g−1 at 50 mV s−1 and excellent long-term capacitance retention of 90.2 % over 100,000 cycles at 5 A g−1. This study presents a simple and effective approach to enhancing the supercapacitor performance of SnS2 and advances the practical applications of electrochemical energy storage devices based on 2D materials.