2D Layered Materials

The 2D crystal structure of MoS2 gives rise to interesting properties for applications such as solid lubricants, optoelectronics, sensors and catalysis. Transition-metal doping has been shown to enhance performance in solid lubrication and catalysis. However, the mechanism for improvement, especially in coefficient of friction and reduction in wear for lubrication is unclear, which we address in this paper with theoretical calculations. Our previous work investigated Ni-doped MoS2 in the bulk 2H phase with structure, elasticity, thermodynamics, and Raman/IR spectroscopy. This work now explores other polytypes (1H and 1T monolayer, and 3R bulk) to determine favorable sites for Ni, the doping effect on crystal structure, electronic properties and the layer dissociation energy. The doping formation energy for Mo and S substitution or intercalation/adsorption shows the most favorable are tetrahedral intercalation, Mo-atop and hollow site in bulk phases, 1H and 1T respectively and suggests a possibility of phase change from 2H to 3R under substitution. In-plane change in lattice parameters were small but lattice parameters out of plane change notably. We find metallic behavior in 3R and 2H under Mo substitution, and in-gap states for Mo and S substitution in 1H, which could have interesting optoelectronic applications. Doping the 1T phase resulted in reconstructions leading to a metal-semiconductor transition. We find that Ni doping strengthens the layer binding which can explain the mechanism of low wear. Our systematic study shows the effect of doping concentration. This work gives insight into the previously unclear structure of Ni-doped MoS2, the relation of energy and structures of doped monolayers and bulk systems, the electronic properties under doping, and the effect of doping on interlayer interactions.

The surging demand for energy and staggering pollutants in the environment have geared the scientific community to explore sustainable pathways that are economically feasible and environmentally compelling. In this context, harnessing solar energy using semiconductor materials to generate charge pairs to drive photoredox reactions has been envisioned as a futuristic approach. Numerous inorganic crystals with promising nanoregime properties investigated in the past decade have yet to demonstrate practical application due to limited photon absorption and sluggish charge separation kinetics. Twodimensional semiconductors with tunable optical and electronic properties and quasi-resistance-free lateral charge transfer mechanisms have shown great promise in photocatalysis. Polymeric graphitic carbon nitride (g-C 3 N 4) is among the most promising candidates due to fine-tuned band edges and the feasibility of optimizing the optical properties via materials genomics. Constructing a two-dimensional (2D)/2D van der Waals (vdW) heterojunction by allies of 2D carbon nitride sheets and other 2D semiconductors has demonstrated enhanced charge separation with improved visible photon absorption, and the performance is not restricted by the lattice matching of constituting materials. With the advent of new 2D semiconductors over the recent past, the 2D/2D heterojunction assemblies are gaining momentum to design high performance photocatalysts for numerous applications. This review aims to highlight recent advancements and key understanding in carbon nitride based 2D/2D heterojunctions and their applications in photocatalysis, including small molecules activation, conversion, and degradations. We conclude with a forward-looking perspective discussing the key challenges and opportunity areas for future research.

tThe exfoliation of bulk-layered materials into 2D-nanosheets dramatically enhances their surface areaand opens up novel properties. The exploitation of these advantageous properties for use in energy storageis, however, conditioned by the effective utilization of the available surface area and the effective com-bination of 2D-nanosheets holding complementary properties. In this work, a 2D graphene-manganese oxide nanosheet hybrid, in which graphene was interleaved between manganese oxide nanosheets, was synthesized following a novel single-step liquid-phase co-exfoliation method. This active material was used to manufacture supercapacitor electrodes using a scalable spray deposition method. The interleaving of graphene between manganese oxide nanosheets enabled a better use of the pseudocapacitive properties of the manganese oxide nanosheets (the hybrid was tested in a 0.5 M K2SO4 aqueous electrolyte) achieving a geometric capacitance of 80 mF cm−2(single electrode), volumetric capacitances of 300 F cm−3(single electrode) and 40.9 F cm−3(symmetric device), and volumetric energy and power of 5.5 mWh cm−3 and 2,193.1 mW cm−3(symmetric device), respectively. This performance is promising in the context of micro-supercapacitor applications, where performance per unit area and per unit volume are relevant for miniaturization. This work demonstrates the remarkable versatility of liquid-phase exfoliation to synthesize hybrids with tailored properties providing the proof-of-concept work for further investigation and exploitation of 2D hybrids for supercapacitor applications.

Synthesis and characterization of high-quality single-crystal tungsten diselenide (WSe 2 ) films on a highly insulating substrate is presented. We demonstrate for the first time that the presence of gold (Au) nanoparticles in the basal plane of a WSe 2 film can enhance its Raman scattering intensity. The experimentally observed enhancement ratio in the Raman signal correlates well with the simulated electric field intensity using both three-dimensional electromagnetic software and theoretical calculation considering layered medium coupled-dipole approximation (LM-CDA). This work serves as a guideline for the use of Au nanoparticles on WSe 2 single-crystal thin films for surface enhanced Raman scattering (SERS) applications in the future.

The isolation of mechanically exfoliated graphene in 2004 1 sparked the research on two dimensional materials which keeps growing at a tremendous rate. It is often argued that the success of this research field is sustained by the potential of these materials to have a disruptive impact in different technological areas such as electronics and optoelectronics. However, at this stage most of the works on these systems are still focused on addressing fundamental questions and thus we might fall in the risk of selling empty promises and creating expectations in the society that might not be fulfilled in a near future. One of the real key factors behind the rise of this research field, on the other hand, is that mechanical exfoliation has democratized material science as high quality samples, showing an interesting plethora of physical phenomena, can be prepared in almost any laboratory even without specialized and expensive equipment. Just with a piece of a bulk layered material, a roll of tape and an optical microscope any trained researcher can isolate atomically thin layers of many different 2D materials ranging from wide band gap insulators to superconductors. Since 2010/2011 this relatively young field has experienced a new boost, originating from the works on the semiconducting 'cousins' of graphene, i.e. atomically thin, or, two dimensional, semiconductors). 2-4 Are we just entering new 'hype' phase, or are there intrinsic and profound reasons to justify this excitement of the scientific community.

Binary combinations of transition metal-layered double hydroxides (LDHs) have attracted considerable attention for supercapacitor applications owing to their extraordinary charge storage, low internal resistance , and superior electrochemical stability. In this study, nickel-cobalt LDH (Ni-Co LDHs) nanosheets were grown hierarchically on the surface of Ni foam (NF@Ni-Co LDHs) using a simple solvothermal approach without any binder elements. Interconnected nanosheets with a thickness of ~25 nm were grown uniformly on the nickel foam. The synergistic combination of Ni 2 + and Co 2 + /Co 3 + LDHs grown directly on nickel foam resulted in a remarkably high specific capacity of 1757 C g −1 (4392 F g −1) at a current density of 1 mA cm −2 (0.44 A g −1). Furthermore, a hybrid supercapacitor (HSC) device was fabricated using activated carbon as the anode and NF@Ni-Co LDHs as the cathode. The hybrid device exhibited high energy densities of 51.1 Wh kg −1 (at a power density of 777 W kg −1 and a current density of 1.0 A g −1) and 11.8 Wh kg −1 (at a power density of 12.1 kW kg −1 and current density of 20 A g −1). The electro-chemical stability of the HSC device was 72.2% after 10,0 0 0 charge/discharge cycles at a current density of 10 mA cm −2. The electrochemical performance of the Ni-Co LDHs nanosheets on Ni foam in a three-electrode system and as a hybrid device highlights its potential in energy storage applications.

The present low yielding growth techniques of semiconducting 2H phase molybdenum disulfide (MoS 2) hamper its widespread applications. In this article, we report a novel hydrothermal chemical approach to synthesize micron sized few layer 2H-MoS 2 on a large scale. Sodium molybdate and ammonium thiocyanate have been used as precursors to obtain template-free 2H-MoS 2 in solution. Detailed microscopic and spectroscopic characterizations reveal that the bottom-up synthesized few layer MoS 2 flakes are highly crystalline having the hexagonal 2H phase. Photodetector devices comprising a p-type silicon (p-Si)/n-MoS 2 heterostructure have been fabricated for the first time using solution processed 2H-MoS 2 synthesized by the bottom up approach. The heterojunction diode exhibits a high rectification ratio (>10 3) with broad band photoresponse over the visible range. Because of the visible light photoresponse, as-synthesized MoS 2 along with reduced graphene oxide (MoS 2 –RGO hybrids) have been utilized to study the potential of this two dimensional (2D) heterostructure for visible light driven photocatalytic Rhodamine B dye degradation. This study demonstrates the potential of solution processed MoS 2 for integration with silicon and growth of 2D heterostructures for visible light induced multifunctional applications.

Monolayers of transition metal dichalcogenides like MoS 2 have attracted a lot of attention for their potential applications in electronic and optoelectronic devices. However, reports on controlled growth of high quality MoS 2 layers are limited. Here, we demonstrate chemical vapor deposition (CVD) growth of uniform single to few-layers of MoS 2 . A back-gated field effect transistor based on MoS 2 shows p-type behavior with average mobility of 10 cm 2 V -1 s -1 .

Graphene and graphene-analogue nanosheets produced by efficient water-assisted liquid exfoliation of layered materials a b s t r a c t We report the efficient productions of single-and few-layered graphene and molybdenum disulphide (MoS 2) nanosheets using a surfactant-free, water-assisted liquid-phase exfoliation (LPE) of graphite and MoS 2 bulk powders in N-methylpyrrolidinone (NMP). Extensive absorption spectroscopic study and materials characterizations indicate that the addition of water to NMP was not only effective on the exfoliation yields but also the stability of the colloidal dispersions of exfoliated nanosheets in the water-NMP mixed solvent upto 18 months. Detailed micro Raman spectroscopic analysis suggests the defects of the exfoliated graphene nanosheets were reduced using the developed water-assisted LPE in NMP.

A porous membrane comprising stacked layers of nanosheets , each nanosheet comprising one to three atomic layers of a 2D material comprising or consisting of one or more transition metal dichalcogenides is provided. The nanosheets have pores and the membrane comprises a network of water permeation pathways including through pathways formed by the pores , horizontal pathways formed by gaps between the layers , and vertical pathways formed by gaps between adjacent nanosheets and stacking defects between the layers. Also provided is a method for making the membrane.

HDS are two dimensional layered anionic clays or hydrotalcite-like nanomaterials having diversified applications in catalysis, electrochemistry, separation technology and medicine  because of their layered structure and high anion exchange capacity. These nanomaterials are abbreviated by formulae: [M2+–M3+–X] where the M2+ and M3+ are cations and X- is an anion. The net positive charge, which is due to substitution of trivalent by divalent metal ions in the brucite-like layers M(OH)2, is stabilized by an equivalent negative charge of the interlayer solvated anions. These weakly bound anions on the layers are exchangeable by other anionic species resulting in vital variants in the interlayer space. The study aimed at synthesis of a layered nanomaterial and testing for controlled release. The ZnHNP was prepared by exchange of nitrate by orthophosphate ions in ZnHN synthesized by coprecipitation technique using zinc nitrate and zinc oxide. Both the precursors and the phosphate-exchanged phases were characterized by Fourier transformation infrared spectroscopy (IR) and slow release both nitrates and phosphates monitored by UV-vis spectroscopy. The anionic exchange showed that carbonate ions displaced both nitrates and phosphates in the ZnHNP. The nanosheets exhibited slow release properties hence a potential slow release fertilizer. This research applies nanotechnologies in use of slow/controlled release (SR/CR) fertilizers to the improvement of nutrient supply management while minimizing environmental, ecological and health hazards in agriculture as the whole nanosheets specie is fully degradable

Quasi van der Waals epitaxy (QvdWE) of III-V semiconductors on two-dimensional layered material, such as graphene, is discussed. Layered materials are used as a lattice mismatch/thermal expansion coefficient mismatch-relieving layer to integrate III-V semiconductors on any arbitrary substrates. In this chapter, the epitaxial growth of both III–V nanowires and thin films on two-dimensional layered materials is presented. Also, the growth challenges of thin film on two-dimensional materials using QvdWE are discussed through density functional theory calculations. Furthermore, optoelectronic devices of III-V semiconductors integrated on two-dimensional layered material based on QvdWE are overviewed to prove the future potential and importance of such type of epitaxy.

Stanene is one of most important of 2D materials due to its potential to demonstrate room temperature topological effects due to opening of spin-orbit gap. In this pursuit we report synthesis and investigation of optical properties of stanene up to few layers, a two-dimensional hexagonal structural analogue of graphene. Atomic scale morphological and elemental characterization using HRTEM equipped with SAED and EDAX detectors confirm the presence of hexagonal lattice of Sn atoms. The position of Raman peak along with the inter-planar 'd' spacing obtained from SAED for prepared samples are in good agreement with that obtained from first principles calculations and confirm that the sheets are not (111) α-Sn sheets. Further, the optical signature calculated using density functional theory at ~191 nm and ~233 nm for low buckled stanene are in qualitative agreement with the measured UV-Vis absorption spectrum. AFM measurements suggest interlayer spacing of ~0.33 nm in good agreement with that reported for epitaxial stanene sheets. No traces of oxygen were observed in the EDAX spectrum suggesting the absence of any oxidized phases. This is also confirmed by Raman measurements by comparing with oxidized stanene sheets. Two dimensional (2D) layered materials have recently gained renewed interest due to their exotic electronic properties along with high specific surface area. The prospects of exploiting these properties in sensing, catalysis, energy storage, protective coatings and electrochromism have witnessed a paradigm shift towards the exploration of these sophisticated 2D materials. The exemplary performance of graphene 1 which is among the first of these elemental 2D materials have initiated a runaway effect in the pursuit of studying alternative 2D materials. Even though graphene has tunable exotic electronic properties 2 , the spin-orbit (SO) coupling is weak 3-5 limiting its applications as spin filters, topological insulators etc. Topological insulators by their very nature force the electrons to travel on the surface at very high speeds thereby finding useful applications in electronic and photonic devices. Exploration of group IV elements using first principles calculations have revealed that the SO coupling increases as the atomic weight of the basis atoms in the honeycomb lattice 6,7. Tin is one of the heaviest elements in this series having strong spin-orbit coupling making it a promising applicant for room temperature topological insulator 8. Thus there is an urgent need to discover novel 2D materials in the post graphene age to overcome its deficiencies. Here we report the synthesis of few-layer stanene (FLS) using ultra-fast laser-material interactions. FLS is analogous to few-layer graphene and can be visualized by replacing carbon atoms by tin on a graphene lattice. Structural characterization performed using high resolution transmission electron microscopy (HRTEM) equipped with energy dispersive X-ray analysis (EDAX) and selected area electron diffraction (SAED) detectors confirm the presence of hexagonal lattice of Sn atoms. EDAX and comparative Raman studies that oxide phases are absent and rules out the possibility of (111) α-Sn sheets. Height profile measured using atomic force micros-copy (AFM) suggests interlayer separation of ~3.3 Å and in good agreement with that of recently reported epitax-ial stanene. Further the UV-Vis spectrum and Raman spectrum are in good agreement with the optical spectra and phonon frequencies calculated using first principles techniques. The structural characterization along with optical signature suggests the synthesis of free standing stanene sheets. Results and Discussions Free standing stanene sheets were synthesized by impinging pulses from a tunable Ti:Saphire ultra-fast femto second laser (140 femto-second pulse width and 80 MHz repetition rate) on to a target in liquid medium. This interaction of femo-second laser pulse due to inverse Bremsstrahlung multiphoton absorption process 9 induce non-equilibrium conditions 10. This we hypothesize initiates the change of phase from tetragonal structure of

Photon harvesting by the active layers of bulk heterojunction (BHJ) polymer solar cells (PSCs) greatly influences the power conversion efficiency (PCE) of the devices. Inclusion of novel metal nanostructures in photoactive layers is one of the effective ways to enhance light trapping without increasing the device thickness. Here, we demonstrate a dramatic enhancement (up to 43%) of the power conversion efficiency (PCE) by exploiting gold nanostructures (AuNSs)-embedded graphene oxide (GO) as the hole transporting layer (HTL) in BHJ PSCs. The enhancement of the device performance could be attributed to the increase in both short circuit current density (J sc) and fill factor (FF). The increased FF is most likely the result of the enhanced charge collection through modified HTL and anode contact. Furthermore, the optical properties of the photovoltaic devices suggest that AuNSs cause light trapping via plasmonic effects resulting enhanced J sc .

We report the structural, vibrational, and electrical transport properties up to ∼16 GPa of 1T-TiTe 2 , a prominent layered 2D system. We clearly show signatures of two isostructural transitions at ∼2 GPa and ∼4 GPa obtained from the minima in c/a ratio concomitant with the phonon linewidth anomalies of E g and A 1g modes around the same pressures, providing a strong indication of unusual electron-phonon coupling associated with these transitions. Resistance measurements present nonlinear behavior over similar pressure ranges shedding light on the electronic origin of these pressure-driven isostructural transitions. These multiple indirect signatures of an electronic transition at ∼2 GPa and ∼4 GPa are discussed in connection with the recent theoretical proposal for 1T-TiTe 2 and also the possibility of an electronic topological transition from our electronic Fermi surface calculations. Between 4 GPa and ∼8 GPa, the c/a ratio shows a plateau suggesting a transformation from an anisotropic 2D layer to a quasi-3D crystal network. First-principles calculations suggest that the 2D to quasi-3D evolution without any structural phase transitions is mainly due to the increased interlayer Te-Te interactions (bridging) via the charge density overlap. In addition, we observed a first-order structural phase transition from the trigonal (P ¯ 3m1) to monoclinic (C2/m) phase at higher pressure regions. We estimate the start of this structural phase transition to be ∼8 GPa and also the coexistence of two phases [trigonal (P ¯ 3m1) and monoclinic (C2/m)] was observed from ∼8 GPa to ∼16 GPa.

The ability to detect light over a broad spectral range is central to practical optoelectronic applications and has been successfully demonstrated with photodetectors of two-dimensional layered crystals such as graphene and MoS2. However, polarization sensitivity within such a photodetector remains elusive. Here, we demonstrate a broadband photodetector using a layered black phosphorus transistor that is polarization-sensitive over a bandwidth from ∼400 nm to 3,750 nm. The polarization sensitivity is due to the strong intrinsic linear dichroism, which arises from the in-plane optical anisotropy of this material. In this transistor geometry, a perpendicular built-in electric field induced by gating can spatially separate the photogenerated electrons and holes in the channel, effectively reducing their recombination rate and thus enhancing the performance for linear dichroism photodetection. The use of anisotropic layered black phosphorus in polarization-sensitive photodetection might provide new functionalities in novel optical and optoelectronic device applications.

Two-dimensional membranes have gained enormous interest due to their potential to deliver precision filtration of species with performance that can challenge current desalination membrane platforms. Molybdenum disulfide (MoS2) laminar membranes have recently demonstrated superior stability in aqueous environment to their extensively-studied analogs graphene-based membranes; however, challenges such as low ion rejection for high salinity water, low water flux, and low stability over time delay their potential adoption as a viable technology. Here we report composite laminate multilayer MoS2 membranes with stacked heterodimensional one-to two-layer-thick porous nanosheets and nanodisks. These membranes have a multimodal porous network structure with tunable surface charge, pore size, and interlayer spacing. In forward osmosis our membranes reject more than 99% of salts at high salinities, and in reverse osmosis small-molecule organic dyes and salts are efficiently filtered. Finally, our membranes stably operate for over a month, implying their potential use in commercial water purification applications.

After the recent finding that CrI3, displays ferromagnetic order down to its monolayer, extensive studies have followed to pursue new two-dimensional (2D) magnetic materials. In this article, we report on the growth of single crystal CrCl3 in the layered monoclinic phase. The system after mechanical exfoliation exhibits stability in ambient air (the degradation occurs on a time scale at least four orders of magnitude longer than is observed for CrI3). By means of mechanical cleavage and atomic force microscopy (AFM) combined with optical identification, we demonstrate the systematic isolation of single and few layer flakes onto 270 nm and 285 nm SiO2/Si~(100) substrates with lateral size larger than graphene flakes isolated with the same method. The layer number identification has been carried with statistically significant data, quantifying the optical contrast as a function of the number of layers for up to six layers. Layer dependent optical contrast data have been fitted within the Fresnel equation formalism determining the real and imaginary part of the wavelength dependent refractive index of the material. A layer dependent (532 nm) micro-Raman study has been carried out down to two layers with no detectable spectral shifts as a function of the layer number and with respect to the bulk.

Two-dimensional lateral heterojunctions based on monolayer transition-metal dichalcogenides (TMDs) have received increasing attention given that their direct band gap makes them very attractive for optoelectronic applications. Although bilayer TMDs present an indirect band gap, their electrical properties are expected to be less susceptible to ambient conditions, with higher mobilities and density of states when compared to monolayers. Bilayers and few-layers single domain devices have already demonstrated higher performance in radio frequency and photosensing applications. Despite these advantages, lateral heterostructures based on bilayer domains have been less explored. Here, we report the controlled synthesis of multi-junction bilayer lateral heterostructures based on MoS 2-WS 2 and MoSe 2-WSe 2 monodomains. The heterojunctions are created via sequential lateral edge-epitaxy that happens simultaneously in both the first and the second layers. A phenomenological mechanism is proposed to explain the growth mode with self-limited thickness that happens within a certain window of growth conditions. With respect to their as-grown monolayer counterparts, bilayer lateral heterostructures yield nearly 1 order of magnitude higher rectification currents. They also display a clear photovoltaic response, with short circuit currents ∼10 3 times larger than those extracted from the as-grown monolayers, in addition to room-temperature electroluminescence. The improved performance of bilayer heterostructures significantly expands the potential of two-dimensional materials for optoelectronics.

A novel top-down and universal optical technique for the effective decoration of two-dimensional (2D)
nanosheets (NS), graphene oxide (GO), boron nitride (BN) and tungsten disulfide (WS2), with noble
metallic nanoparticles (NPs) is reported. The NS–NP assemblies were formed under ambient conditions,
via a facile, rapid and solution compatible laser assisted process in the presence of a metallic precursor.
It is shown that a few seconds of irradiation is sufficient to decorate the NS lattice, while the NP density
can be readily controlled upon variation of the irradiation time. It is found that the laser induced
anchoring of Au NPs onto the NS basal plane and edges resulted in enhanced light harvesting that is
potentially useful for energy conversion and storage applications. To demonstrate the potential of the
approach for practical applications, the incorporation of WS2–Au NP assemblies into the photoactive
layer of ternary bulk heterojunction (BHJ) organic photovoltaic (OPV) cells is realized. The power
conversion efficiency (PCE) of the binary device consisting of a poly[N-90-heptadecanyl-2,7-carbazolealt-5,5-(40,70-di-2-thienyl-20,10,30-benzothiadiazole)] (PCDTBT) : [6,6]-phenyl-C71-butyric-acid-methylester (PC71BM) blend as the donor–acceptor pair was 5.6%, while after the employment of WS2–Au NPs
the efficiency enhancement of the ternary device was approximately 13% reaching a total PCE of 6.3%.
The facile, rapid and room temperature nature of the photo-induced method proposed here provides
unique opportunities for the cost-effective synthesis of bulk amounts of NS–NP assemblies in solution
for many optoelectronic applications.

Manganese oxide nanosheets were synthesized using liquid-phase exfoliation that achieved suspensions in isopropanol (IPA) with concentrations of up to 0.45 mg ml −1 . A study of solubility parameters showed that the exfoliation was optimum in N,N-dimethylformamide followed by IPA and diethylene glycol. IPA was the solvent of choice due to its environmentally friendly nature and ease of use for further processing. For the first time, a hybrid of graphene and manganese oxide nanosheets was synthesized using a single-step co-exfoliation process. The two-dimensional (2D) hybrid was synthesized in IPA suspensions with concentrations of up to 0.5 mg ml −1 and demonstrated stability against re-aggregation for up to six months. The co-exfoliation was found to be a energetically favorable process in which both solutes, graphene and manganese oxide nanosheets, exfoliate with an improved yield as compared to the single-solute exfoliation procedure. This work demonstrates the remarkable versatility of liquid-phase exfoliation with respect to the synthesis of hybrids with tailored properties, and it provides proof-of-concept ground work for further future investigation and exploitation of hybrids made of two or more 2D nanomaterials that have key complementary properties for various technological applications.