Related 2D Materials Research Articles

Chemical Patterning of Graphene, Graphite and MoS2: From Microscale to Nanoscale

Chemical patterning of 2D materials is relevant in several different domains of science and technology with exciting possibilities in electronics, catalysis, sensing, and photonics. Despite intense efforts, spatially controlled, (multifunctional) covalent chemical patterning of graphene and related 2D materials is not straightforward. In my talk, I will present examples of covalent chemical patterning of graphene, graphite and MoS2 using diazonium chemistry. In the first case, spatially resolved multicomponent covalent chemical patterning of single layer graphene was achieved using a facile and efficient method. Three different functional groups could be covalently attached to the basal plane in dense, well-defined micrometer wide patterns using a combination of lithography and a self-limiting variant of diazonium chemistry requiring no need for graphene activation. The layer thickness of the covalent films could be controlled down to 1 nm. In the second case, i will present sub-10 nm chemical patterning of graphite achieved using the electrochemical diazonium chemistry. Here, an elegant combination of covalent and non-covalent chemistry was used to achieve 5-6 nm wide linear chemical patterns with excellent pattern transfer fidelity. In the last example, I will briefly share our initial results on the nanoscale chemical patterning of MoS2 using electrochemical diazonium chemistry. Throughout the discussion, i will highlight the critical role of scanning probe microscopy, namely STM, AFM and AFM-IR in providing critical spatial and spatio-chemical information at the nano and micrometer scale where conventional analytical techniques fail to provide accurate information.

Quantifying the thickness of WTe2 using atomic-resolution STEM simulations and supervised machine learning.

For two-dimensional (2D) materials, the exact thickness of the material often dictates its physical and chemical properties. The 2D quantum material WTe2 possesses properties that vary significantly from a single layer to multiple layers, yet it has a complicated crystal structure that makes it difficult to differentiate thicknesses in atomic-resolution images. Furthermore, its air sensitivity and susceptibility to electron beam-induced damage heighten the need for direct ways to determine the thickness and atomic structure without acquiring multiple measurements or transferring samples in ambient atmosphere. Here, we demonstrate a new method to identify the thickness up to ten van der Waals layers in Td-WTe2 using atomic-resolution high-angle annular dark-field scanning transmission electron microscopy image simulation. Our approach is based on analyzing the intensity line profiles of overlapping atomic columns and building a standard neural network model from the line profile features. We observe that it is possible to clearly distinguish between even and odd thicknesses (up to seven layers), without using machine learning, by comparing the deconvoluted peak intensity ratios or the area ratios. The standard neural network model trained on the line profile features allows thicknesses to be distinguished up to ten layers and exhibits an accuracy of up to 94% in the presence of Gaussian and Poisson noise. This method efficiently quantifies thicknesses in Td-WTe2, can be extended to related 2D materials, and provides a pathway to characterize precise atomic structures, including local thickness variations and atomic defects, for few-layer 2D materials with overlapping atomic column positions.

Ferroelectric phase transition driven by anharmonic lattice mode coupling in two-dimensional monolayer In2Se3

In this paper, to better understand the physical origin of spontaneous out-of-plane (OOP) polarization in the wildly investigated two-dimensional (2D) material monolayer (ML) ${\mathrm{In}}_{2}{\mathrm{Se}}_{3}$, we studied its lattice dynamics behaviors via performing a series of first-principles calculations. Cooperating with a fitted phenomenological model, it is clearly shown that, although the depolarization field is nonzero, spontaneous OOP polarization will survive through the anharmonic coupling with other lattices modes. Additionally, our results indicated that the conventional proper ferroelectric (FE) transition of in-plane (IP) polarization plays a role in the primary driving force for the transition in lattice. Tuning the stability of the IP polar modes can be considered an effective pathway of controlling the FE behavior in ML ${\mathrm{In}}_{2}{\mathrm{Se}}_{3}$ and related III-VI group 2D materials.

International Interlaboratory Comparison of Thermogravimetric Analysis of Graphene-Related Two-Dimensional Materials

Research on graphene-related two-dimensional (2D) materials (GR2Ms) in recent years is strongly moving from academia to industrial sectors with many new developed products and devices on the market. Characterization and quality control of the GR2Ms and their properties are critical for growing industrial translation, which requires the development of appropriate and reliable analytical methods. These challenges are recognized by International Organization for Standardization (ISO 229) and International Electrotechnical Commission (IEC 113) committees to facilitate the development of these methods and standards which are currently in progress. Toward these efforts, the aim of this study was to perform an international interlaboratory comparison (ILC), conducted under Versailles Project on Advanced Materials and Standards (VAMAS) Technical Working Area (TWA) 41 "Graphene and Related 2D Materials" to evaluate the performance (reproducibility and confidence) of the thermogravimetric analysis (TGA) method as a potential new method for chemical characterization of GR2Ms. Three different types of representative and industrially manufactured GR2Ms samples, namely, pristine few-layer graphene (FLG), graphene oxide (GO), and reduced graphene oxide (rGO), were used and supplied to ILC participants to complete the study. The TGA method performance was evaluated by a series of measurements of selected parameters of the chemical and physical properties of these GR2Ms including the number of mass loss steps, thermal stability, temperature of maximum mass change rate (Tp) for each decomposition step, and the mass contents (%) of moisture, oxygen groups, carbon, and impurities (organic and non-combustible residue). TGA measurements determining these parameters were performed using the provided optimized TGA protocol on the same GR2Ms by 12 participants across academia, industry stakeholders, and national metrology institutes. This paper presents these results with corresponding statistical analysis showing low standard deviation and statistical conformity across all participants that confirm that the TGA method can be satisfactorily used for characterization of these parameters and the chemical characterization and quality control of GR2Ms. The common measurement uncertainty for each parameter, key contribution factors were identified with explanations and recommendations for their elimination and improvements toward their implementation for the development of the ISO/IEC standard for chemical characterization of GR2Ms.

Two‐Dimensional Materials and their Hetero‐Superlattices for Photocatalytic Hydrogen Evolution Reaction

AbstractSince the discovery of graphene, there has been intensive research on other two‐dimensional (2D) analogues. Some of them are elemental 2D materials such as phosphorene and bismuthene. Others are binary compounds fascinating in terms of energy applications are transition metal dichalcogenides (TMDCs: MoS2, MoSe2) and MXenes (Ti3C2, Nb2C3). 1T‐forms of MoS2 and MoSe2 are probably the best materials reported for dye‐sensitized photocatalytic hydrogen evolution reaction (HER). Here, we first highlight the phase engineering in 2D‐TMDCs to enhance hydrogen evolution activity. Generation of hetero‐superlattices of TMDCs with other HER active materials such as graphene, carbon nitride and borocarbonitrides, by utilizing coupling and electrostatic restacking strategies, appears to be advantageous for photocatalytic application. These hetero‐superlattices provide a counterpoint to the van der Waals heterostructures by means of covalent bonds. Ladder‐like networks of heterolayers generated due to cross‐linking enhance the interfacial area and charge‐transfer interactions, thereby improving HER activity. The use of 2D phosphorene as a photocatalytic material is limited by its ambient instability. Covalent functionalization of phosphorene surface with tris(pentafluorophenyl) borane and benzyl group increases ambient stability and dispersibility. The functionalized surface can be further utilized to cross‐link with amine or acid‐modified TMDCs. The HER activity obtained with phosphorene‐MoS2 superlattices is the highest reported among the phosphorene‐based systems, thus setting a new example for metal‐free catalysis. The ambient instability of MXenes and other related 2d materials under photocatalytic conditions can be resolved either by functionalization or by forming hetero‐superlattices.

Equilibrium phase diagrams of isostructural and heterostructural two-dimensional alloys from first principles

SummaryAlloying is a successful strategy for tuning the phases and properties of two-dimensional (2D) transition metal dichalcogenides (TMDCs). To accelerate the synthesis of TMDC alloys, we present a method for generating temperature-composition equilibrium phase diagrams by combining first-principles total-energy calculations with thermodynamic solution models. This method is applied to three representative 2D TMDC alloys: an isostructural alloy, MoS2(1-x)Te2x, and two heterostructural alloys, Mo1-xWxTe2 and WS2(1-x)Te2x. Using density-functional theory and special quasi-random structures, we show that the mixing enthalpy of these binary alloys can be reliably represented using a sub-regular solution model fitted to the total energies of relatively few compositions. The cubic sub-regular solution model captures 3-body effects that are important in TMDC alloys. By comparing phase diagrams generated with this method to those calculated with previous methods, we demonstrate that this method can be used to rapidly design phase diagrams of TMDC alloys and related 2D materials.

Adsorption Behavior of Environmental Gas Molecules on Pristine and Defective MoSi2N4: Possible Application as Highly Sensitive and Reusable Gas Sensors.

Inspired by the recent practical application of two-dimensional (2D) nanomaterials as gas sensors, catalysts, and materials for waste gas disposal, herein, the adsorption behaviors of environmental gas molecules, including NO, CO, O2, CO2, NO2, H2O, H2S, and NH3, on the 2D pristine and defective MoSi2N4 (MSN) monolayers were systematically investigated using spin-polarized density functional theory (DFT) calculations. Our results reveal that all the gas molecules are physically adsorbed on the MSN surface with small charge transfer, but the electronic structures of NO, NO2, and O2 are obviously modified due to the in-gap states. The introduction of N vacancy on the MSN surface enhances the interaction between gas molecules and the substrate, especially for NO2 and O2. Interestingly, the adsorption type of NO and CO evolves from physisorption to chemisorption, which may be utilized in NO and CO catalytic reaction. Furthermore, the moderate adsorption strength and obvious changes in electronic properties of H2O and H2S on the defective MSN make them have promising prospects in highly sensitive and reusable gas sensors. This work offers several promising gas sensors based on the MSN monolayer and also provides a theoretical reference of other related 2D materials in the field of gas sensors, catalysts, and toxic gas disposal.

Interfacial charge and energy transfer in van der Waals heterojunctions

AbstractVan der Waals heterojunctions are fast‐emerging quantum structures fabricated by the controlled stacking of two‐dimensional (2D) materials. Owing to the atomically thin thickness, their carrier properties are not only determined by the host material itself, but also defined by the interlayer interactions, including dielectric environment, charge trapping centers, and stacking angles. The abundant constituents without the limitation of lattice constant matching enable fascinating electrical, optical, and magnetic properties in van der Waals heterojunctions toward next‐generation devices in photonics, optoelectronics, and information sciences. This review focuses on the charge and energy transfer processes and their dynamics in transition metal dichalcogenides (TMDCs), a family of quantum materials with strong excitonic effects and unique valley properties, and other related 2D materials such as graphene and hexagonal‐boron nitride. In the first part, we summarize the ultrafast charge transfer processes in van der Waals heterojunctions, including its experimental evidence and theoretical understanding, the interlayer excitons at the TMDC interfaces, and the hot carrier injection at the graphene/TMDCs interface. In the second part, the energy transfer, including both Förster and Dexter types, are reviewed from both experimental and theoretical perspectives. Finally, we highlight the typical charge and energy transfer applications in photodetectors and summarize the challenges and opportunities for future development in this field.

Review on Techniques for Thermal Characterization of Graphene and Related 2D Materials.

The discovery of graphene and its analog, such as MoS2, has boosted research. The thermal transport in 2D materials gains much of the interest, especially when graphene has high thermal conductivity. However, the thermal properties of 2D materials obtained from experiments have large discrepancies. For example, the thermal conductivity of single layer suspended graphene obtained by experiments spans over a large range: 1100–5000 W/m·K. Apart from the different graphene quality in experiments, the thermal characterization methods play an important role in the observed large deviation of experimental data. Here we provide a critical review of the widely used thermal characterization techniques: the optothermal Raman technique and the micro-bridge method. The critical issues in the two methods are carefully revised and discussed in great depth. Furthermore, improvements in Raman-based techniques to investigate the energy transport in 2D materials are discussed.

Elemental 2D Materials: Solution‐Processed Synthesis and Applications in Electrochemical Ammonia Production

AbstractGraphene and related elemental 2D materials have become core materials in nanotechnology and shown great promise for industrially important electrocatalysis reactions. Although excellent progress has been made over the past few years, research into the field of elemental 2D materials beyond graphene is still at an early stage. Importantly, recent research has revealed the promising efficacy of elemental 2D materials as effective nitrogen reduction reaction (NRR) electrocatalysts due to their many excellent properties including high surface activities, acting as active sites for effective functionalization and defect engineering. This review provides a comprehensive account of recent advances in elemental 2D materials with a major focus on the solution‐based synthesis routes and their applications in electrocatalytic NRR for ammonia (NH3) production. After a concise overview of elemental 2D materials, the advantages and challenges of currently available methods for the synthesis of these 2D materials are discussed. Then, the review focuses on the use of these emerging 2D materials in the electrocatalytic reduction of N2 for sustainable (NH3) synthesis. Finally, the challenges still to be addressed, and important perspectives in this attractive field are emphasized.

Theory of the effective Seebeck coefficient for photoexcited two-dimensional materials: Graphene

Thermoelectric phenomena in photoexcited graphene have been the topic of several theoretical and experimental studies because of their potential usefulness in optoelectronic applications. However, available theoretical descriptions of the thermoelectric effect in terms of the Seebeck coefficient do not take into account the role of the photoexcited electron density. In this work, we adopt the concept of effective Seebeck coefficient [G. D. Mahan, J. Appl. Phys. 87, 7326 (2000)] and extend it to the case of a photoexcited two-dimensional (2D) electron gas. We calculate the effective Seebeck coefficient for photoexcited graphene, we compare it to the commonly used ``phenomenological'' Seebeck coefficient, and we show how it depends on the photoexcited electron density and temperature. Our results are necessary inputs for any quantitative microscopic theory of thermoelectric effects in graphene and related 2D materials.

Infrared Polaritonic Biosensors Based on Two-Dimensional Materials.

In recent years, polaritons in two-dimensional (2D) materials have gained intensive research interests and significant progress due to their extraordinary properties of light-confinement, tunable carrier concentrations by gating and low loss absorption that leads to long polariton lifetimes. With additional advantages of biocompatibility, label-free, chemical identification of biomolecules through their vibrational fingerprints, graphene and related 2D materials can be adapted as excellent platforms for future polaritonic biosensor applications. Extreme spatial light confinement in 2D materials based polaritons supports atto-molar concentration or single molecule detection. In this article, we will review the state-of-the-art infrared polaritonic-based biosensors. We first discuss the concept of polaritons, then the biosensing properties of polaritons on various 2D materials, then lastly the impending applications and future opportunities of infrared polaritonic biosensors for medical and healthcare applications.

Enhancing the hydrogen evolution reaction by non-precious transition metal (Non-metal) atom doping in defective MoSi2N4 monolayer

Two-dimensional (2D) hydrogen evolution reaction (HER) electrocatalysts have attracted great attention due to their unique electronic properties and high activities. Recently, a new 2D monolayer material of MoSi2N4 has been successfully synthesized and its semiconducting property and excellent ambient stability have also been demonstrated (Science 2020, 369, 670). Here, a systematic screening of catalysts for HER among N- and Si-defective MoSi2N4-supported single non-precious transition metal (TM) and non-metal (NM) atom catalysts is performed by means of density functional theory (DFT) calculations. Interestingly, the single O/P/Fe/Nb atom doped N-(Si-) defective MoSi2N4 monolayer were found to possess excellent HER performance presenting a near-zero ΔGH, which is comparable to or even better than the state-of-the-art Pt-based materials. Moreover, the novel HER activities of some TM doped structures were explained by the “states filling” model. The energy level of the first available unoccupied states for accommodating hydrogen drops after the introduction of TM atom, which modulates the hydrogen binding strength. This work opens the door for the application of MoSi2N4 monolayer and other related 2D materials in the field of energy conversion.

Progress and Prospects on the Fabrication of Graphene-Based Nanostructures for Energy Storage, Energy Conversion and Biomedical Applications.

Graphene, a two-dimensional (2D) layered material has attracted much attention from the scientific community due to its exceptional electrical, thermal, mechanical, biological and optical properties. Hence, numerous applications utilizing graphene-based materials could be conceived in next-generation electronics, chemical and biological sensing, energy conversion and storage, and beyond. The interaction between graphene surfaces with other materials plays a vital role in influencing its properties than other bulk materials. In this review, we outline the recent progress in the production of graphene and related 2D materials, and their uses in energy conversion (solar cells, fuel cells), energy storage (batteries, supercapacitors) and biomedical applications.

The importance of international standards for the graphene community

If graphene and related 2D materials are to be used commercially, buyers need to have confidence in the measured properties of the material they obtain from suppliers. Scientists from international standards committees describe how the first joint ISO/IEC measurement standard, published this month, will help.

Linear regulation of electrical characteristics of InSe/Antimonene heterojunction via external electric field and strain

The study of the properties of two-dimensional (2D) materials and their heterostructures has become one of the mainstream directions of scientific attention. Herein, we systematically survey the structures, electronic properties adjustments of the InSe/antimonene heterostructure under external electric field and mechanical strain by using first principle calculations based on density functional theory. It is found that the AAII–stacking InSe/antimonene heterojunction possesses the maximum indirect-bandgap of 1.452 eV combined by van der Waals interaction among the four stacking orders, and antimonene/InSe heterojunction shows weak orbital hybridization. Moreover, the study of electrical properties shows that the band gap of the AAII–stacking heterojunction decreases linearly with the increase of electric field, and the maximum band gap is 1.461 eV at 0.05 eV/Å/e. The adjustment of electrical properties under mechanical strain is also explored: the band gap of the heterojunction decreases linearly as the absolute value of strain increases, and the maximum band gaps of the InSe/antimonene heterojunction are 1.658 eV and 1.686 eV at –3% uniaxial strain and –2% biaxial strain, respectively. The linear relationship between the band gap and the physical external fields shows great potential in tailoring band gap of the heterojunction, which may further broaden the applications of related 2D materials in electronic devices.

Graphene and Related 2D Materials for Desalination: A Review of Recent Patents

Abstract: The growing population and energy demand, coupled with the depleting fresh water resources resulted in great progress in sea water desalination (SWD) technologies. Nanopores of 2D materials, like graphene and its structural analogs, are the latest innovations in membrane technology for SWD. The performance of these novel atomically thin nanopores, as seen from various experimental and theoretical studies, is highly encouraging with reports of water permeability 2-3 orders of magnitude greater than the conventional reverse osmosis (RO). The potential for high efficiency and the low energy requirements of these nanopores for desalination led to tremendous efforts in fabrication and commercialization. We present here a review of the very recent patents associated with the preparation of these nanopores, the process and the efficiency of SWD. Keywords: 2D nanopores, Graphene, Membrane, Patents, Desalination.

Sensitivity to strains and defects for manipulating the conductivity of graphene

Implementing the quantum-mechanical Kubo-Greenwood formalism for the numerical calculation of dc conductivity, we demonstrate that the electron transport properties of a graphene layer can be tailored through the combined effect of defects (point and line scatterers) and strains (uniaxial tension and shear), which are commonly present in a graphene sample due to the features of its growth procedure and when the sample is used in devices. Motivated by two experimental works (He X. et al. Appl. Phys. Lett., 104 (2014) 243108; 105 (2014) 083108), where authors did not observe the transport gap even at large (22.5% of tensile and 16.7% of shear) deformations, we explain possible reasons, emphasizing on graphene's strain and defect sensing. The strain- and defect-induced electron-hole asymmetry and anisotropy of conductivity, and its nonmonotony as a function of deformation suggest perspectives for the strain-defect engineering of electrotransport properties of graphene and related 2D materials.

Editorial: Electronics and Optoelectronics of Graphene and Related 2D Materials

EDITORIAL article Front. Mater., 12 August 2020Sec. Thin Solid Films Volume 7 - 2020 | https://doi.org/10.3389/fmats.2020.00235

Excellent electronic transport in heterostructures of graphene and monoisotopic boron-nitride grown at atmospheric pressure

Hexagonal boron nitride (BN), one of the very few layered insulators, plays a crucial role in 2D materials research. In particular, BN grown with a high pressure technique has proven to be an excellent substrate material for graphene and related 2D materials, but at the same time very hard to replace. Here we report on a method of growth at atmospheric pressure as a true alternative for producing BN for high quality graphene/BN heterostructures. The process is not only more scalable, but also allows to grow isotopically purified BN crystals. We employ Raman spectroscopy, cathodoluminescence, and electronic transport measurements to show the high-quality of such monoisotopic BN and its potential for graphene-based heterostructures. The excellent electronic performance of our heterostructures is demonstrated by well developed fractional quantum Hall states, ballistic transport over distances around 10 µm at low temperatures and electron-phonon scattering limited transport at room temperature.