Related Two-dimensional Materials Research Articles

First-principles study on the microstructure, band structure, mechanical and optical properties of AB-stacked bilayer graphene under constant neutron irradiation with different electric field magnitudes and directions

Carbon-based devices have superior performance and lower power consumption compared to silicon-based devices. However, graphene and related two-dimensional materials are easily influenced by radiation due to their extremely high surface-to-volume ratio. Under constant neutron irradiation conditions, there have been no studies on the effects of the magnitude and direction of an external electric field on various properties of the AB system. This study systematically investigated the neutron irradiation effects on the AB system and the influence of the different electric fields on the properties of the AB system under constant irradiation conditions using the density functional theory and ab initio molecular dynamics method that assigns energy to the primary knock-on atom. The results indicate that under constant neutron irradiation with different electric fields, the changes in system configuration dominate the influence on band structure, while the impact of surface ripples is relatively limited. Reducing the interlayer spacing of the system can make the configuration more stable, which makes the band structure under the electric field more stable. The primary factors influencing the Young's modulus (Y) of the system are changes in the microstructure and the appearance of surface ripples, while the influence of system configuration is relatively limited. Under the irradiation conditions, the electric field can to some extent accurately modulate the band gap of the system. However, it will also correspondingly reduce the system's Y to a certain extent, causing negative impacts on the electrical and thermal conductivity.

Navigating the frontiers of graphene quality control to enable product optimisation and market confidence

With graphene and related two-dimensional (2D) materials now enhancing products used in everyday life, the scale of industrial production of many different types of 2D nanomaterials requires quality control (QC) processes that can be performed rapidly, non-destructively, in-line and in a cost-effective manner. These materials must be repeatably produced with targeted material properties, to reduce the costs associated with nonconformity of products, and so multiple QC methods that can monitor different material properties are required. Herein, we describe different measurands and associated techniques that either have the potential to be used for QC, or are already being used in this way, whether that off-line, at-line or in-line. The advantages and disadvantages of different techniques are detailed, as well as possible solutions that can ensure confidence in these methods and lead to measurement traceability in this growing industry.

Advancing carbon dioxide capture: Unravelling structure-property-performance dynamics in graphene related two-dimensional materials

Efficient design of adsorbents for carbon dioxide (CO2) capture is imperative in addressing the challenges caused by climate change. In this study, a comprehensive exploration of solid adsorbents derived from graphene-related two-dimensional materials (GR2Ms) with tunable surface chemistry and structures was undertaken to unravel the intricate structural-property-activity relationships essential for optimizing their CO2 adsorption. Nine key GR2Ms and carbon materials, including two types of (few-layer graphene (FLG), graphene oxide (GO) reduced graphene oxide (rGO) materials), expanded graphite (Exp Gft), rGO-Exp Gft and graphite (Gft), were methodically characterized alongside benchmarked materials such as activated carbon (AC) and molecular sieves (MS). Characterization parameters, i.e. morphology, particle size, interlayer distance, crystallite size, defect density, specific surface area (SSA), total pore volume, micropore volume and pore size, are correlated with CO2 adsorption capacity. Results revealed that the dominant properties influencing CO2 adsorption in GR2Ms are hierarchical porous morphology, defect density (up to 2.38 × 1011 cm−2), SSA (up to ∼271 m2/g), total pore volume (up to 0.86 cm3/g), and micropore volume (up to 0.03 cm3/g), all of which have established positive linear relationships. Conversely, crystallite size (0.38–48 nm) exhibited an inverse (non-linear) relationship with CO2 adsorption capacity under ambient conditions (25 °C and 1 atm). Particle size (8–52 μm), interlayer distance (0.33–0.89 nm) and pore size (4.2–13.3 nm) of GR2Ms showed negligible impact on CO2 adsorption capacity. Remarkably, porous rGO emerged as the top-performing CO2 adsorbent with an enhanced adsorption capacity (5.38 mmol/g) , in comparison with activated carbon (AC, 1.79 mmol/g) and molecular sieve (MS, 1.48 mmol/g). This study establishes a comprehensive structure-property-performance relationship, highlighting the significance of a three-dimensional (3D) hierarchical, open and interconnected pore structure within the graphene network, along with the largest pore volume and minimum crystallite size of rGO. These findings underscore the critical role of structural and porosity attributes of GR2Ms in CO2 adsorption, providing fundamental insights into the interplay between the structure and properties on their CO2 adsorption behavior. This study emphasizes the key pillars for effective adsorbent design and underlines the necessity of optimised control over the structural characteristics and porosity of GR2Ms to enhance their CO2 adsorption capabilities, contributing towards the ambitious net-zero target by 2050.

Modification of kitchen blenders into controllable laboratory mixers for mechanochemical synthesis of atomically thin materials

Graphene and related two-dimensional materials (2DMs) have shown promise across numerous technology areas including flexible electronics, energy storage and pollution remediation. Research into novel applications of these atomically thin materials relies on access to synthesis techniques for producing 2DMs with suitable quality and quantity. Liquid-phase exfoliation is a mechanochemical approach that can achieve this and produce defect-free nanomaterial dispersions which are compatible for downstream use (e.g. inkjet printing, coatings). Here, using kitchen blenders to deliver shear-driven exfoliation, we develop a range of inexpensive hardware solutions that can allow researchers to synthesise 2DMs using a controllable, sustainable and scalable process. Extensive modifications were necessary as the onboard electronics lack the experimental controls (temperature, speed, characterisation) for scientific research and precision synthesis. The technical aspects (including the many lessons learned) of the modifications are discussed and a simple selection process is proposed for creating bespoke mechanochemical processors for any application in the hope that this encourages experimentation. Specific builds with detailed notes, cost breakdown and associated files are provided in the Open Science Framework (OSF) repository, OpenLPE associated with this article.

Irradiation methods for engineering of graphene related two-dimensional materials

The research community has witnessed an exceptional increase in exploring graphene related two-dimensional materials (GR2Ms) in many innovative applications and emerging technologies. However, simple, low-cost, sustainable, and eco-friendly methods to manufacture large quantities and high-quality GR2Ms still remain an unsolved challenge. To address limitations of conventional wet chemical-based exfoliation methods using graphite resources, the top-down irradiation approach has proven to be an ultrafast, effective, and environmentally friendly technology for scalable exfoliation, production, and processing of GR2Ms providing new properties for emerging applications. Significant advancements have been made for preparation of broad range of GR2Ms from graphite, such as graphene, graphene oxide, and reduced graphene oxide, and their doped, functionalized and modified forms over the past two decades, thanks to the availability of photon and ion irradiation techniques, such as microwave, infrared, ultraviolet, solar, x-ray, gamma, laser, and plasma. This review presents recent advances on the application of these various irradiation techniques and highlights their mechanism, differences in properties of prepared GR2Ms, and their advantages and disadvantages in comparison with other conventional methods. The review provides an insight into the irradiation strategies and their prospective applications to produce, at a large scale, low-cost, high-quality GR2Ms for practical applications in transparent electrodes, optoelectronic devices, sensors, supercapacitors, protective coatings, conductive inks, and composites.

Magnetic and spin transport properties of a two-dimensional magnetic semiconductor kagome lattice Nb<sub>3</sub>Cl<sub>8</sub> monolayer

Two-dimensional semiconductor materials with intrinsic magnetism have great application prospects in realizing spintronic devices with low power consumption, small size and high efficiency. Some two-dimensional materials with special lattice structures, such as kagome lattice crystals, are favored by researchers because of their novel properties in magnetism and electronic properties. Recently, a new two-dimensional magnetic semiconductor material Nb<sub>3</sub>Cl<sub>8</sub> monolayer with kagome lattice structure was successfully prepared, which provides a new platform for exploring two-dimensional magnetic semiconductor devices with kagome structure. In this work, we study the electronic structure and magnetic anisotropy of Nb<sub>3</sub>Cl<sub>8</sub> monolayer. We also further construct its <i>p-n</i> junction diode and study its spin transport properties by using density functional theory combined with non-equilibrium Green's function method. The results show that the phonon spectrum of the Nb<sub>3</sub>Cl<sub>8</sub> monolayer has no negative frequency, confirming its dynamic stability. The band gap of the spin-down state (1.157 eV) is significantly larger than that of the spin-up state (0.639 eV). The magnetic moment of the Nb<sub>3</sub>Cl<sub>8</sub> monolayer is 0.997 μ<sub>B</sub>, and its easy magnetization axis is in the plane and along the <i>x</i> axis direction based on its energy of magnetic anisotropy. Nb atoms make the main contribution to the magnetic anisotropy. When the strain is applied, the band gap of the spin-down states will decrease, while the band gap of the spin-up state is monotonously decreased from the negative (compress) to positive (tensile) strain. As the strain variable goes from -6% to 6%, the contribution of Nb atoms to the total magnetic moment gradually increases. Moreover, strain causes the easy magnetization axis of the Nb<sub>3</sub>Cl<sub>8</sub> monolayer to flip vertically from in-plane to out-plane. The designed <i>p-n</i> junction diode nanodevice based on Nb<sub>3</sub>Cl<sub>8</sub> monolayer exhibits an obvious rectification effect. In addition, the current in the spin-up state is larger than that in the spin-down state, exhibiting a spin-polarized transport behavior. Moreover, a negative differential resistance (NDR) phenomenon is also observed, which could be used in the NDR devices. These results demonstrate that the Nb<sub>3</sub>Cl<sub>8</sub> monolayer material has great potential application in the next generation of high-performance spintronic devices, and further experimental verification and exploration of this material and related two-dimensional materials are needed.

Magnetic and spin transport properties of a two-dimensional magnetic semiconductor kagome lattice Nb<sub>3</sub>Cl<sub>8</sub> monolayer

Two-dimensional semiconductor materials with intrinsic magnetism have great application prospects in realizing spintronic devices with low power consumption, small size and high efficiency. Some two-dimensional materials with special lattice structures, such as kagome lattice crystals, are favored by researchers because of their novel properties in magnetism and electronic properties. Recently, a new two-dimensional magnetic semiconductor material Nb<sub>3</sub>Cl<sub>8</sub> monolayer with kagome lattice structure was successfully prepared, which provides a new platform for exploring two-dimensional magnetic semiconductor devices with kagome structure. In this work, we study the electronic structure and magnetic anisotropy of Nb<sub>3</sub>Cl<sub>8</sub> monolayer. We also further construct its <em>p-n</em> junction diode and study its spin transport properties by using density functional theory combined with non-equilibrium Green's function method. The results show that the phonon spectrum of the Nb<sub>3</sub>Cl<sub>8</sub> monolayer has no negative frequency, confirming its dynamic stability. The band gap of the spin-down state (1.157 eV) is significantly larger than that of the spin-up state (0.639 eV). The magnetic moment of the Nb<sub>3</sub>Cl<sub>8</sub> monolayer is 0.997 μ<sub>B</sub>, and its easy magnetization axis is in the plane and along the <em>x</em> axis direction based on its energy of magnetic anisotropy. Nb atoms make the main contribution to the magnetic anisotropy. When the strain is applied, the band gap of the spin-down states will decrease, while the band gap of the spin-up state is monotonously decreased from the negative (compress) to positive (tensile) strain. As the strain variable goes from -6% to 6%, the contribution of Nb atoms to the total magnetic moment gradually increases. Moreover, strain causes the easy magnetization axis of the Nb<sub>3</sub>Cl<sub>8</sub> monolayer to flip vertically from in-plane to out-plane. The designed <em>p-n</em> junction diode nanodevice based on Nb<sub>3</sub>Cl<sub>8</sub> monolayer exhibits an obvious rectification effect. In addition, the current in the spin-up state is larger than that in the spin-down state, exhibiting a spin-polarized transport behavior. Moreover, a negative differential resistance (NDR) phenomenon is also observed, which could be used in the NDR devices. These results demonstrate that the Nb<sub>3</sub>Cl<sub>8</sub> monolayer material has great potential application in the next generation of high-performance spintronic devices, and further experimental verification and exploration of this material and related two-dimensional materials are needed.

Changing the structural and electronic properties of graphene and related two-dimensional materials using ion beam irradiation with nacl sacrificial layers

Changing the structural and electronic properties of graphene and related two-dimensional materials using ion beam irradiation with nacl sacrificial layers

Changing the structural and electronic properties of graphene and related two-dimensional materials using ion beam irradiation with NaCl sacrificial layers

Changing the structural and electronic properties of graphene and related two-dimensional materials using ion beam irradiation with NaCl sacrificial layers

Hydrogen storage on bidimensional GeC with transition metal adatoms

A density functional theory study is undertaken to explore H2 physisorption in bidimensional germanium carbide (GeC) decorated with transition metals (Cu, Ag and Au). Adsorption energy results show that the Au and Cu metal atoms are preferentially chemisorbed to a carbon atom, while Ag is adsorbed over a Ge–C bond. Hydrogen molecules are weakly adsorbed to the adatom, and the interaction mainly occurs between the charge density of the H–H bond and the slightly positively charged adatom. The Ag-decorated GeC monolayer possesses the maximum hydrogen storage capacity with seven molecules adsorbed on the adatom. We think this work could encourage theoretical and experimental studies of the GeC monolayer and related two-dimensional materials for green energy development applications.

(Invited) A Perovskite Solar Farm at Crete Enabled By the Utilization of 2D Layered Materials

The development of perovskite photovoltaics has matured to a point of industry-scale production; however, their operational long-term stability still remains the main obstacle impeding their commercialization. The utilization of solution processable graphene and related two-dimensional materials (GRMs), at different sections of perovskite solar cells represents an effective approach to enhance both the power conversion efficiency and the lifetime stability of such photovoltaic technology. On top of that, their compatibility with large-scale, solution-processable methods such as sheet-to-sheet and roll-to-roll lay the ground for their direct integration in pilot manufacturing lines, significantly improves the capital expenditure of this technology. In this context, a wide range of GRMs have been used as replacements for conventional charge transport layers and interlayers not only to improve the charge dynamics of the devices but also to protect the perovskite layer against diffusion of external agents, such as oxygen and moisture and the metal ion migration.In this talk, I will give an overview of our recent activities on the realization of highly efficient and stable perovskite solar cells and modules. I will give a holistic insight on how the hierarchical placement of GRMs in all the perovskite device can tune the transport layers work-function, passivate the interface/surface traps and most importantly protect the interfaces, resulting in a simultaneously improvement of the triangle of performance, stability and scalability of the perovskite PVs. Furthermore, I will discuss the implementation of a Solar Farm consisting of GRM enabled perovskite 0.5 sqm panels at the Hellenic Mediterranean University campus at Crete, in which outdoor field tests are currently performed. The latter is a pivotal information on the commercialization potential of this technology. I will also directly correlate the electrical measurements of the solar farm with environmental parameters, while at the same time, benchmark their outdoor performance against commercial PV panels installed at the same site.

Solution-processed two-dimensional materials for next-generation photovoltaics.

In the ever-increasing energy demand scenario, the development of novel photovoltaic (PV) technologies is considered to be one of the key solutions to fulfil the energy request. In this context, graphene and related two-dimensional (2D) materials (GRMs), including nonlayered 2D materials and 2D perovskites, as well as their hybrid systems, are emerging as promising candidates to drive innovation in PV technologies. The mechanical, thermal, and optoelectronic properties of GRMs can be exploited in different active components of solar cells to design next-generation devices. These components include front (transparent) and back conductive electrodes, charge transporting layers, and interconnecting/recombination layers, as well as photoactive layers. The production and processing of GRMs in the liquid phase, coupled with the ability to “on-demand” tune their optoelectronic properties exploiting wet-chemical functionalization, enable their effective integration in advanced PV devices through scalable, reliable, and inexpensive printing/coating processes. Herein, we review the progresses in the use of solution-processed 2D materials in organic solar cells, dye-sensitized solar cells, perovskite solar cells, quantum dot solar cells, and organic–inorganic hybrid solar cells, as well as in tandem systems. We first provide a brief introduction on the properties of 2D materials and their production methods by solution-processing routes. Then, we discuss the functionality of 2D materials for electrodes, photoactive layer components/additives, charge transporting layers, and interconnecting layers through figures of merit, which allow the performance of solar cells to be determined and compared with the state-of-the-art values. We finally outline the roadmap for the further exploitation of solution-processed 2D materials to boost the performance of PV devices.

Dynamically-enhanced strain in atomically thin resonators

Graphene and related two-dimensional (2D) materials associate remarkable mechanical, electronic, optical and phononic properties. As such, 2D materials are promising for hybrid systems that couple their elementary excitations (excitons, phonons) to their macroscopic mechanical modes. These built-in systems may yield enhanced strain-mediated coupling compared to bulkier architectures, e.g., comprising a single quantum emitter coupled to a nano-mechanical resonator. Here, using micro-Raman spectroscopy on pristine monolayer graphene drums, we demonstrate that the macroscopic flexural vibrations of graphene induce dynamical optical phonon softening. This softening is an unambiguous fingerprint of dynamically-induced tensile strain that reaches values up to ≈4 × 10−4 under strong non-linear driving. Such non-linearly enhanced strain exceeds the values predicted for harmonic vibrations with the same root mean square (RMS) amplitude by more than one order of magnitude. Our work holds promise for dynamical strain engineering and dynamical strain-mediated control of light-matter interactions in 2D materials and related heterostructures.

Work function, deformation potential, and collapse of Landau levels in strained graphene and silicene

We perform a systematic {\it ab initio} study of the work function and its uniform strain dependence for graphene and silicene for both tensile and compressive strains. The Poisson ratios associated with armchair and zigzag strains are also computed. Based on these results, we obtain the deformation potential, crucial for straintronics, as a function of the applied strain. Further, we propose a particular experimental setup with a special strain configuration that generates only the electric field, while the pseudomagnetic field is absent. Then, applying a real magnetic field, one should be able to realize experimentally the spectacular phenomenon of the collapse of Landau levels in graphene or related two-dimensional materials.

Selective Cation Transport through Graphene in Nafion | Graphene | Nafion Membranes

Graphene and related two-dimensional (2D) materials are being intensively studied for use in membrane separators. The ultrathin nature of 2D materials combined with the opportunities they afford for intentional creation of nanometer-sized pores having controllable size and chemical character could allow for membrane separations with high flux and also high selectivity. Proton transport through graphene is especially interesting because it may occur through the intact pristine graphene rather than through defects / pores. Other cations, for example lithium or sodium, may pass through graphene but that transport is through to occur through defects / pores. This presentation will focus on selective cation transport through graphene layers that are embedded between Nafion membranes. Ion transmission through graphene is measured using a four-electrode cell in which two Luggin capillaries sense the potential difference that develops across the membrane in response to an ion current that is driven by a pair of platinum wire drive electrodes. Proton transmission in this cell configuration occurs more than 100 times faster than transport of any other ion. This difference is thought to reflect the different mechanisms by which protons and other cations pass though graphene. The lecture will discuss defect characterization in CVD graphene films, and will consider the role of defects, which are in fact nanopores, in allowing for selective transport of ions through graphene.

Uptake of formaldehyde onto doped phosphorene nanosheets: A cluster DFT study of single and co-adsorption states

Abstract To search for novel uptake platforms for the adsorption of toxic formaldehyde (H2CO), we investigate the adsorption properties of intrinsic, Al, and Cu adatom doped-phosphorene nanosheets using density functional theory calculations. Compared to intrinsic phosphorene and related two-dimensional materials, the doped-phosphorene remarkably increases the uptake of H2CO (in high and low concentrations of the compound, even in the presence of aerobic conditions). The stability of the intermolecular interactions is due to the strong chemical surface complexation, which is dominated by the interplay of polarization, charge-transfer, and/or permanent electrostatic effects. The uptake stability is decreased in the presence of low-magnitude electric fields, allowing recovery techniques of the adsorbent material. Then, Al and Cu doped-phosphorene nanosheets are expected to be excellent candidates for technologies related to the capture, filtration, removal, or remediation of H2CO in residential and industrial environments.

A Bright Future for Liquid Functional Materials?

Liquid metals were merely sci-fi fantasy a few decades ago, playing a prominent role in the Terminator film franchise. Current research is leading to an array of emerging functional liquids. Here, we discuss the exciting possibilities of such fluid materials.

Graphene and two-dimensional materials for silicon technology.

The development of silicon semiconductor technology has produced breakthroughs in electronics-from the microprocessor in the late 1960s to early 1970s, to automation, computers and smartphones-by downscaling the physical size of devices and wires to the nanometre regime. Now, graphene and related two-dimensional (2D) materials offer prospects of unprecedented advances in device performance at the atomic limit, and a synergistic combination of 2D materials with silicon chips promises a heterogeneous platform to deliver massively enhanced potential based on silicon technology. Integration is achieved via three-dimensional monolithic construction of multifunctional high-rise 2Dsilicon chips, enabling enhanced performance by exploiting the vertical direction and the functional diversification of the silicon platform for applications in opto-electronics and sensing. Here we review the opportunities, progress and challenges of integrating atomically thin materials with silicon-based nanosystems, and also consider the prospects for computational and non-computational applications.

MoS2 Quantum Dot/Graphene Hybrids for Advanced Interface Engineering of a CH3NH3PbI3 Perovskite Solar Cell with an Efficiency of over 20.

Interface engineering of organic-inorganic halide perovskite solar cells (PSCs) plays a pivotal role in achieving high power conversion efficiency (PCE). In fact, the perovskite photoactive layer needs to work synergistically with the other functional components of the cell, such as charge transporting/active buffer layers and electrodes. In this context, graphene and related two-dimensional materials (GRMs) are promising candidates to tune "on demand" the interface properties of PSCs. In this work, we fully exploit the potential of GRMs by controlling the optoelectronic properties of molybdenum disulfide (MoS2) and reduced graphene oxide (RGO) hybrids both as hole transport layer (HTL) and active buffer layer (ABL) in mesoscopic methylammonium lead iodide (CH3NH3PbI3) perovskite (MAPbI3)-based PSCs. We show that zero-dimensional MoS2 quantum dots (MoS2 QDs), derived by liquid phase exfoliated MoS2 flakes, provide both hole-extraction and electron-blocking properties. In fact, on one hand, intrinsic n-type doping-induced intraband gap states effectively extract the holes through an electron injection mechanism. On the other hand, quantum confinement effects increase the optical band gap of MoS2 (from 1.4 eV for the flakes to >3.2 eV for QDs), raising the minimum energy of its conduction band (from -4.3 eV for the flakes to -2.2 eV for QDs) above the one of the conduction band of MAPbI3 (between -3.7 and -4 eV) and hindering electron collection. The van der Waals hybridization of MoS2 QDs with functionalized reduced graphene oxide (f-RGO), obtained by chemical silanization-induced linkage between RGO and (3-mercaptopropyl)trimethoxysilane, is effective to homogenize the deposition of HTLs or ABLs onto the perovskite film, since the two-dimensional nature of RGO effectively plugs the pinholes of the MoS2 QD films. Our "graphene interface engineering" (GIE) strategy based on van der Waals MoS2 QD/graphene hybrids enables MAPbI3-based PSCs to achieve a PCE up to 20.12% (average PCE of 18.8%). The possibility to combine quantum and chemical effects into GIE, coupled with the recent success of graphene and GRMs as interfacial layer, represents a promising approach for the development of next-generation PSCs.

Surface-Mediated Aligned Growth of Monolayer MoS2 and In-Plane Heterostructures with Graphene on Sapphire.

Aligned growth of transition metal dichalcogenides and related two-dimensional (2D) materials is essential for the synthesis of high-quality 2D films due to effective stitching of merging grains. Here, we demonstrate the controlled growth of highly aligned molybdenum disulfide (MoS2) on c-plane sapphire with two distinct orientations, which are highly controlled by tuning sulfur concentration. We found that the size of the aligned MoS2 grains is smaller and their photoluminescence is weaker as compared with those of the randomly oriented grains, signifying enhanced MoS2-substrate interaction in the aligned grains. This interaction induces strain in the aligned MoS2, which can be recognized from their high susceptibility to air oxidation. The surface-mediated MoS2 growth on sapphire was further developed to the rational synthesis of an in-plane MoS2-graphene heterostructure connected with the predefined orientation. The in-plane epitaxy was observed by low-energy electron microscopy. Transmission electron microscopy and scanning transmission electron microscopy suggest the alignment of a zigzag edge of MoS2 parallel to a zigzag edge of the neighboring graphene. Moreover, better electrical contact to MoS2 was obtained by the monolayer graphene compared with a conventional metal electrode. Our findings deepen the understanding of the chemical vapor deposition growth of 2D materials and also contribute to the tailored synthesis as well as applications of advanced 2D heterostructures.