Functionalization Of 2D Materials Research Articles

Control of Charge Carriers and Band Structure in 2D Monolayer Molybdenum Disulfide via Covalent Functionalization.

The fine-tuning of electro-optic properties is critical for high-performing technologies. This is now obtainable with advanced nanostructures, particularly two-dimensional (2D) monolayer materials such as molybdenum disulfide (MoS2). Using spin-polarized periodic density functional theory (DFT), we find that the direct band gap (K → K') can be chemically tuned with covalently bound functional groups. With an electron-withdrawing group such as fluorine, we observe one occupied α and one unoccupied β band, which correspond to the addition of an α electron and a β hole, confirmed with the spin difference (Qα - Qβ) being 1. By increasing the electron-donating behavior with the replacement of F by H and then by Me, the occupied (valence) α band shifts upward in energy relative to the Fermi energy, and the unoccupied β shifts down until they are in contact with the Fermi energy. In addition, both α and β unoccupied (conduction) bands of the MoS2 shift down, relative to the Fermi energy, until they are in contact with the Fermi and the system can be described as metallic. The MoS2 + F system is thus a small gap semiconductor (0.96 eV), and the MoS2 + H and MoS2 + Me gaps are 0.21 and 0 eV (metallic), respectively. Spin density calculations illustrate the semilocalized nature of the α spin; however, this is not formed from the radical of the functionalizing group, but rather the resulting unpaired electron is on the sulfur atom after radical abstraction to form a covalent bond with the group. Five- and six-membered heterocycles were studied and further confirm these observations. Distinct from typical functional groups such as phenyl, we find evidence for the covalent bonding of pyrrole, cyclopentadiene, and pyridine to a sulfur atom of the MoS2 surface, from the new α and β bands in the band structure. The charge carrier nature of the 2D monolayers of functionalized MoS2 can be further tuned with charge doping (hole or electron), such that even the metallic systems can be returned to semiconducting states, but importantly as p-type conductors. Semilocalization of the spin states and control of the band gap can be generalized to other covalently functionalized 2D materials and appears suitable for electronic applications, such as photoluminescence devices, contact-free transistors, and quantum communication.

Performance Enhancement of Solution-Processed Organic Memories by Exploiting Synergistic Organic–Inorganic Hybrid Composites

Organic–inorganic hybrid composites (HCs) comprising functionalized 2D materials have been given exceptional importance and have been actively employed as memory materials toward development of nex...

Functionalization of 2D materials for enhancing OER/ORR catalytic activity in Li\u2013oxygen batteries

A major barrier toward the practical application of lithium-oxygen batteries is the high overpotential caused by the precipitation of oxygen-reduction products at the cathode, resulting in poor cyclability. By combining first-principle calculations and reactive molecular dynamics simulations, we show that surface functionalization of 2D MXene nanosheets offers a high degree of tunability of the catalytic activity for oxygen-reduction and oxygen-evolution reactions (ORR/OER). We show that the controlled creation of active vacancy sites on the MXene surface enhances ORR in excess of a factor of 60 compared to graphene-based cathode materials. Furthermore, we find that increasing the ratio of fluorine vs. oxygen termination of the functionalized Ti4N3-MXene catalyst reduces the charge overpotential by up to 70% and 80% compared with commercial platinum-on-carbon and graphene catalysts, respectively. These results provide direct guidance toward the rational design of functionalized 2D materials for modulating the catalytic activity for a wide range of electrocatalytic applications.

A biomimetic nanoleaf electrocatalyst for robust oxygen evolution reaction

Oxygen evolution reaction (OER) is a key process in various advanced technologies for renewable energy conversion, such as water splitting and metal-air batteries. However, as a four-electron coupled reaction, the OER is kinetically sluggish and limited by its high overpotential and low efficiency. The design of novel nanostructured electrocatalysts is highly desirable to promote OER kinetics. Herein, a bio-inspired nanoleaf electrocatalyst has been successfully achieved for the first time by in situ growing ultrathin NiCo layered double hydroxide (LDH) nanosheets on CuO nanowires. Attributed to the mechanical support of CuO nanowire veins, the NiCo LDH lamina presents a large lateral size (more than 10 μm) and unique hierarchical structure that consisted of ultrathin nanosheets with numerous exposed edges. The CuO veins distributed across the LDH lamina can serve as the fast path for charge transfer and significantly promote the LDH conductivity. Compared to the conventional NiCo LDH nanosheets, the novel nanoleaves with enlarged electrochemical surface area, edge-rich active sites, and improved conductivity exhibit greatly enhanced OER performances with an impressive 9.3 fold enhanced activity, much lower overpotential of 262 mV at 10 mA cm−2, as well as good stability and flexibility. The biomimetic nanoleaf structures and the corresponding design strategy can be broadly applied to other functional 2D materials for advanced applications.

Light‐Responsive Hybrid Systems: Functionalization of 2D Materials with Photosensitive Molecules: From Light‐Responsive Hybrid Systems to Multifunctional Devices (Advanced Optical Materials 16/2019)

Light‐Responsive Hybrid Systems: Functionalization of 2D Materials with Photosensitive Molecules: From Light‐Responsive Hybrid Systems to Multifunctional Devices (Advanced Optical Materials 16/2019)

Анализ фазовых равновесий в тройной системе Ge–P–Sn

На основании анализа характера фазовых равновесий в двойных системах, ограняющих диаграмму состояний тройной системы Ge – P – Sn, предложены теоретически возможные схемы ее фазового субсолидусного разграничения. Исследование методом рентгенофазового анализа образцов, принадлежащих политермическим сечениям Sn4P3-Ge, Sn4P3-GeP, показало, что разделение трехкомпонентной диаграммы состояния ниже солидуса осуществляется с помощью сечений Sn4P3-Ge, Sn4P3 -GeP и SnP3-GeP. Построенная по данным дифференциального термического анализа фазовая диаграмма сечения Sn4P3-Ge представляет диаграмму эвтектического типа с координатами эвтектической точки 800 К, 15 mol % Ge.
 
 
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2DMatPedia, an open computational database of two-dimensional materials from top-down and bottom-up approaches

Two-dimensional (2D) materials have been a hot research topic in the last decade, due to novel fundamental physics in the reduced dimension and appealing applications. Systematic discovery of functional 2D materials has been the focus of many studies. Here, we present a large dataset of 2D materials, with more than 6,000 monolayer structures, obtained from both top-down and bottom-up discovery procedures. First, we screened all bulk materials in the database of Materials Project for layered structures by a topology-based algorithm and theoretically exfoliated them into monolayers. Then, we generated new 2D materials by chemical substitution of elements in known 2D materials by others from the same group in the periodic table. The structural, electronic and energetic properties of these 2D materials are consistently calculated, to provide a starting point for further material screening, data mining, data analysis and artificial intelligence applications. We present the details of computational methodology, data record and technical validation of our publicly available data (http://www.2dmatpedia.org/).

Functionalization of 2D Materials with Photosensitive Molecules: From Light‐Responsive Hybrid Systems to Multifunctional Devices

Abstract2D materials possess exceptional physical and chemical properties that render them appealing components for numerous potential applications in (opto)electronics, energy storage, sensing, and biomedicine. However, such unique properties are hardly tunable or modifiable. The functionalization of 2D crystals with molecules constitutes a powerful strategy to adjust and modulate their properties, by also imparting them new functions. In this framework, the combination of 2D materials with photosensitive molecules is a viable route for harnessing their light‐responsive nature. The latter takes full advantage of the extremely high sensitivity of 2D materials to subtle changes in the local environment and the capacity of photosensitive molecules to modify their intrinsic properties when exposed to electromagnetic fields. The hybrid molecule–2D materials can preserve the unique optical and electrical properties of 2D layers and can exhibit additional light‐tunable features. In this Progress Report, the protocols that can be pursued for the 2D material functionalization and switching mechanisms in photosensitive systems are reviewed, followed by an in‐depth discussion on their tunable optical properties and their exploitation when integrated in novel photoswitchable electronic devices. The opportunities and associated challenges to be tackled for the development of unprecedented and high‐performance light‐responsive devices are discussed.

Ionic Bombardment to Tune the Electrochemical Properties of a Semiconductor.

The purpose of this work is to study in detail the effects of monoatomic and gas cluster ion beam (GCIB) on a semiconducting electrode (InP) thanks to the correlation between X-Ray Photoelectron Spectroscopy (XPS) and electrochemistry. Such approach allows estimating the overall change in the electrical, chemical and structural properties of the modified semiconductor (SC). Recent developments in ion sputtering source enable a better control of the perturbations induced at the surface of the material. Furthermore, anodic dissolution will be presented as a tool to recover the initial characteristics of the modified electrode. The interaction of ions with matter and especially surfaces has been an active topic for both fundamental and applied research over the past decades and is still relevant today. Thanks to ion bombardments, various types of surfaces can be chemically, mechanically, optically or electrically modified. However, the understanding of the inherent phenomena and physical/chemical mechanisms involved remains incomplete. When ions, whose energy range between 1 and 10 000 eV, interact with a surface, many processes can occur and the energy distribution (dissipation) could differ markedly from one material to another. Nevertheless, whatever the source of modifications considered, the common fundamental question is the nature of the effects induced by the perturbation and its evaluation (area, depth). The consequences of ion bombardment on the surface of a SC may have repercussions: formation of ordered nanostructures via low-energy ion-beam irradiation as a bottom-up nano-manufacturing technique; ion implantation for SC device technology; optical lithography using ion beam or even functionalization of 2D materials. This topic has gained specific interest in the field of surface analysis where depth-profiling is required (e.g. XPS, Auger Spectroscopy or Secondary Ion Mass Spectroscopy). In contrast with monoatomic ions (e.g. Ar+), cluster ions (e.g. Arn +) do not penetrate deeply into target materials. Therefore, the energy of their impact is dissipated within the first few nanometers of the surface, resulting in low sputtering yields and minimal surface damage. However, the effects of these new generations of GCIB are not yet fully understood. The strong originality of our approach is to investigate the surface modifications induced not only by UHV characterization technique (XPS) but also with the use of solid/liquid interface electrochemical response for which SC have specific behavior. These efficient and quantitative tools provide relevant information on the modification performed at the SC surface: metallization, amorphization, preferential sputtering. The high number of parameters that govern the interfacial electrochemistry of the SC/electrolyte system (band bending, SC doping level, double layer capacitance, space charge layer, SC electrical impedance, photopotential) make this approach pertinent and sensitive to evaluate the perturbation amplitude. An example is given on the figure where the metallization of an InP electrode can be observed in vacuum after successive Arn + etching sequences by the shift of the Fermi level (XPS valence band region) and in solution by the narrowing of the depletion region and the flattening of the Mott-Schottky plot. In addition, a very important characteristic of SC interfacial electrochemistry is its availability to generate in situ anodic dissolution at nanoscale. Thereby, it represents an accurate and quantitative tool to estimate the thickness of the modified region. Moreover, using well-controlled dissolution, the initial properties can be retrieved. These results are compared to those obtained after annealing under UHV. Thus, the electrochemical responses will be associated directly to surface chemical information such as: loss of stoichiometry, Ar implantation, shift of the Fermi Energy level position, photo-peak broadening. Figure 1

Strain engineering in functional 2-dimensional materials

Two-dimensional (2D) materials have attracted growing interest in the past decade because of their extraordinary properties and great potential in a wide range of applications. Strain is regarded as a simple yet powerful tool to modulate the properties of 2D materials, as it directly affects lattice structures and thus alters electronic structures. In this tutorial, we first discuss the fundamentals of strain engineering in 2D materials from macro and atomic perspective and then review some approaches to strain engineering as well as their merits and drawbacks. After that, we examine in detail how strain modulates physical and chemical properties in various types of 2D materials. In the last section of this tutorial, the applications of strain engineering in functional 2D materials are exhibited.

Local strain mapping of GO nanosheets under in situ TEM tensile testing

Local strain mapping of 2D materials at nanoscale under continuous deformation can enable a variety of studies on 2D materials including but not limited to more accurate characterization of physical properties, heterogeneity characterization of functionalized 2D materials, and strain engineering of 2D materials. Here, using graphene oxide (GO) as a representative material, we report a technique that achieves local strain mapping of 2D materials at nanoscale through gold nanoparticle decoration over GO under in situ TEM tensile testing and imaging. Our results reveal that GO at different uniaxial tensile strain levels exhibits a heterogeneous local strain distribution. Using this technique, the permanent deformation of GO was also quantified to be ∼1% on average. The technique can be readily applied to the strain mapping of other 2D materials.

"Snowing" Graphene using Microwave Ovens.

Developing a simple and industrially scalable method to produce graphene with high quality and low cost will determine graphene's future. The two conventional approaches, chemical vapor deposition and liquid-phase exfoliation, require either costly substrates with limited production rate or complicated post treatment with limited quality, astricting their development. Herein, an extremely simple process is presented for synthesizing high quality graphene at low-cost in the gas phase, similar to "snowing," which is catalyst-free, substrate-free, and scalable. This is achieved by utilizing corona discharge of SiO2 /Si in an ordinary household microwave oven at ambient pressure. High quality graphene flakes can "snow" on any substrate, with thin-flakes even down to the monolayer. In particular, a high yield of ≈6.28% or a rate of up to ≈0.11 g h-1 can be achieved in a conventional microwave oven. It is demonstrated that the snowing process produces foam-like, fluffy, 3D macroscopic architectures, which are further used in strain sensors for achieving high sensitivity (average gauge factor ≈ 171.06) and large workable strain range (0%-110%) simultaneously. It is foreseen that this facile and scalable strategy can be extended for "snowing" other functional 2D materials, benefiting their low-cost production and wide applications.

High‐throughput computational screening of layered and two‐dimensional materials

The successful exfoliation of graphene and other kinds of two‐dimensional (2D) materials from their corresponding three‐dimensional (3D) bulk counterparts has inspired researchers to screen layered bulk compounds as parent materials for potential 2D materials. With the rapid development of supercomputers and high‐performance computations, high‐throughput materials screening is a growing new power in materials science for the discovery of novel kinds of materials with desired functionality. Recently, many parent 3D bulks have been identified by high‐throughput screening from materials databases for potential 2D materials, and several 2D materials databases are established through numerous efforts. In addition, on the basis of high‐throughput computations for electronic properties and data‐mining algorithms, several functional layered and 2D materials, such as electrode materials, photohydrolytic catalysts, half metals, piezoelectric monolayers and heterostructures, have been achieved. In this review, we summarize the recent progress in the high‐throughput screening of parent candidates for 2D materials and their further applications, and the challenges and perspectives are also briefly discussed. We highly expect that this review could lead the way forward in the discovery of novel 2D materials and provide a guide for the further development of 2D materials.This article is categorized under: Structure and Mechanism > Computational Materials Science Electronic Structure Theory > Density Functional Theory

(Invited) Functionalization of 2D Materials: A Molecular Approach

In this presentation, I will focus on the functionalization and nanostructuring of graphite, graphene, and other 2D materials using two approaches. A first approach is based on molecular self-assembly at the interface between a liquid or air, and the 2D material. I will discuss concepts of nanostructuring emphasizing the effect of solvent, solute concentration and temperature, and self-assembly under nanoconfinement conditions. A second approach is based on grafting molecules on 2D materials via covalent chemistry. In addition to a discussion on the functionalization principles, it will be demonstrated how also nanolithography can be used to nanostructure such covalently modified 2D materials. Advanced interface specific methods such as scanning tunnelling microscopy (STM) and atomic force microscopy (AFM) provide structural and other types of information at the nanoscopic level. Figure 1

Moderate Bacterial Etching Allows Scalable and Clean Delamination of g-C3N4 with Enriched Unpaired Electrons for Highly Improved Photocatalytic Water Disinfection.

Delamination treatment is crucial in promoting the activity of bulk graphitic carbon nitride (g-C3N4). However, most of the currently used methods of exfoliating bulk g-C3N4 to achieve g-C3N4 thin layers suffer from low yield and environmental pollution. Herein, we developed a facile bacterial etching approach for the preparation of high-quality g-C3N4 nanosheets by exfoliating bulk g-C3N4 under room temperature. Morphology and physicochemical characterizations show that the bacteria-treated g-C3N4 (BT-CN) samples, especially BT-CN-2d, have a lamina-like two-dimensional (2D) in-plane porous structure, a significantly enlarged specific surface area (82.61 m2 g-1), and a remarkable narrow band gap (2.11 eV). X-ray photoelectron spectroscopy and electron paramagnetic resonance spectra confirm the dramatic enrichment of unpaired electron in the BT-CN-2d g-C3N4 nanosheets. EIS spectra and photocurrent tests indicate the fast electron transportation. As a result, the representative BT-CN-2d g-C3N4 photocatalyst shows an optimal visible light-driven photocatalytic performance in water disinfection (fourfold higher than bulk g-C3N4), as well as good cycle stability. This moderate and clean bacterial etching process can be realized in tens of gram scale in the laboratory and should be readily extended to kilogram scale. The present work provides fundamental knowledge about the scalable production of high-quality g-C3N4 by bioengineering method, offering extendable availability for designing and fabricating other functional 2D materials.

Highly Dispersed Mo2C Anchored on N,P‐Codoped Graphene as Efficient Electrocatalyst for Hydrogen Evolution Reaction

AbstractUniformly dispersed Mo2C were anchored on N, P‐codoped graphene (Mo2C‐N,P‐rGO) by a facile method, during which phytic acid was employed as cross‐linker to bridge the polyethyleneimine (PEI) modified graphene oxide (PEI‐GO) and Mo ions through electrostatic adsorption and coordination bonding. Thanks to this ingenious strategy, the aggregation of Mo2C that usually occurs at high calcination temperature was effectively suppressed. This well‐defined nanohybrid exhibits an excellent activity and high stability in hydrogen evolution reactions both in acidic and basic media due to the synergistic effects of the heteroatoms doped, accessibility of active sites, and conductivity of the two‐dimensional (2D) structure. The obtained Mo2C‐N,P‐rGO requires only small overpotentials of 71 and 95 mV to drive 10 mA cm−2 in alkaline and acidic solution, respectively. Benefiting from the protective effect of the coated carbon layers on the Mo2C surface, no obvious degradation was detected after 20 h of operation. This facile strategy may provide new opportunities for the precise design of functional 2D materials with unique properties.

High Crystal Quality 2D Manganese Phosphorus Trichalcogenide Nanosheets and their Photocatalytic Activity

AbstractTransition metal phosphorus trichalcogenides (MPX3, X = S, Se) are layered materials possessing high chemical diversity and wide range of applications in a broad wave length spectrum. Theoretical studies reveal that auspicious activity of photocatalytic water splitting can be realized from them. However, experimental efforts have so far been challenged with the synthesis bottleneck. Described herein is the general chemical vapor deposition (CVD) growth method and photocatalytic activity of these materials. A novel route to systematically grow MnPX3 nanosheets on flexible carbon fiber substrate is reported. The temperature profile of the CVD process is carefully optimized that confer a facile and successful conversion of oxide precursor to phospho‐trichalcogenide with high crystallinity. Moreover, the obtained manganese‐based phosphorus trichalcogenide nanosheets demonstrate promising activity in sacrificial agent‐free photocatalytic water splitting under simulated solar light (AM 1.5G). This study provides a significant stepping stone in exploring the fascinating world of functional 2D materials and pursuing performance enhancement.

Multimicrometer Noncovalent Monolayer Domains on Layered Materials through Thermally Controlled Langmuir-Schaefer Conversion for Noncovalent 2D Functionalization.

As functionalized 2D materials are incorporated into hybrid materials, ensuring large-area structural control in noncovalently adsorbed films becomes increasingly important. Noncovalent functionalization avoids disrupting electronic structure in 2D materials; however, relatively weak molecular interactions in such monolayers typically reduce stability toward solution processing and other common material handling conditions. Here, we find that controlling substrate temperature during Langmuir-Schaefer conversion of a standing phase monolayer of diynoic amphiphiles on water to a horizontally oriented monolayer on a 2D substrate routinely produces multimicrometer domains, at least an order of magnitude larger than those typically achieved through drop-casting. Following polymerization, these highly ordered monolayers retain their structures during vigorous washing with solvents including water, ethanol, tetrahydrofuran, and toluene. These findings point to a convenient and broadly applicable strategy for noncovalent functionalization of 2D materials in applications that require large-area structural control, for instance, to minimize desorption at defects during subsequent solution processing.

Metal Thio- and Selenophosphates as Multifunctional van der Waals Layered Materials.

Since the discovery of Dirac physics in graphene, research in 2D materials has exploded with the aim of finding new materials and harnessing their unique and tunable electronic and optical properties. The follow-on work on 2D dielectrics and semiconductors has led to the emergence and development of hexagonal boron nitride, black phosphorus, and transition metal disulfides. However, the spectrum of good insulating materials is still very narrow. Likewise, 2D materials exhibiting correlated phenomena such as superconductivity, magnetism, and ferroelectricity have yet to be developed or discovered. These properties will significantly enrich the spectrum of functional 2D materials, particularly in the case of high phase-transition temperatures. They will also advance a fascinating fundamental frontier of size and proximity effects on correlated ground states. Here, a broad family of layered metal thio(seleno)phosphate materials that are moderate- to wide-bandgap semiconductors with incipient ionic conductivity and a host of ferroic properties are reviewed. It is argued that this material class has the potential to merge the sought-after properties of complex oxides with electronic functions of 2D and quasi-2D electronic materials, as well as to create new avenues for both applied and fundamental materials research in structural and magnetic correlations.

Modulating Wettability of Layered Materials by Controlling Ligand Polar Headgroup Dynamics.

Integrating functionalized 2D materials into multilayer device architectures increasingly requires understanding the behavior of noncovalently adsorbed ligands during solution processing. Here, we demonstrate that the headgroup dynamics of polymerized monolayers of functional alkanes can be controlled to modify surface wetting and environmental interactions. We find that headgroup dynamics are sensitive to the position of the polymerizable diyne group; thus, the polymerization process, typically used to stabilize the noncovalent monolayer, can also be used to selectively destabilize chain-chain interactions near the headgroups, making the headgroups more solvent-accessible and increasing surface hydrophilicity. Conversely, interactions with divalent ions can be used to tether headgroups in-plane, decreasing surface hydrophilicity. Together, these results suggest a strategy for the rational design of 2D chemical interfaces in which the polymerization step reconfigures the monolayer to promote the desired environmental interactions.