Alloying in 2D results in the development of new, diverse, and versatile systems with prospects in bandgap engineering, catalysis, and energy storage. Tailoring structural phase transitions using ...alloying is a novel idea with implications in designing all 2D device architecture as the structural phases in 2D materials such as transition metal dichalcogenides are correlated with electronic phases. Here, this study develops a new growth strategy employing chemical vapor deposition to grow monolayer 2D alloys of Re‐doped MoSe2 with show composition tunable structural phase variations. The compositions where the phase transition is observed agree well with the theoretical predictions for these 2D systems. It is also shown that in addition to the predicted new electronic phases, these systems also provide opportunities to study novel phenomena such as magnetism which broadens the range of their applications.
Re doping is demonstrated to be a controllable way to tailor crystal structure and magnetic properties in chemical vapor deposition (CVD)‐synthesized 2D transition metal dichalcogenides. Extra electrons from Re atoms induce a 2H–1T′ phase transformation in monolayer Mo1–xRexSe2 alloys. These results show that chemical doping is a promising pathway to tune structural and magnetic properties of other CVD‐grown MX2 (M = Mo, W; X = S, Se).
The challenge in the electrosynthesis of fuels from CO2 is to achieve durable and active performance with cost-effective catalysts. Here, we report that carbon nanotubes (CNTs), doped with nitrogen ...to form resident electron-rich defects, can act as highly efficient and, more importantly, stable catalysts for the conversion of CO2 to CO. The unprecedented overpotential (−0.18 V) and selectivity (80%) observed on nitrogen-doped CNTs (NCNTs) are attributed to their unique features to facilitate the reaction, including (i) high electrical conductivity, (ii) preferable catalytic sites (pyridinic N defects), and (iii) low free energy for CO2 activation and high barrier for hydrogen evolution. Indeed, DFT calculations show a low free energy barrier for the potential-limiting step to form key intermediate COOH as well as strong binding energy of adsorbed COOH and weak binding energy for the adsorbed CO. The highest selective site toward CO production is pyridinic N, and the NCNT-based electrodes exhibit no degradation over 10 h of continuous operation, suggesting the structural stability of the electrode.
Nitrogen‐doped carbon nanotubes (NCNTs) have been considered as a promising electrocatalyst for carbon‐dioxide‐reduction reactions, but two fundamental chemistry questions remain obscure: 1) What are ...the active centers with respect to various defect species and 2) what is the role of defect density on the selectivity of NCNTs? The aim of this work is to address these questions. The catalytic activity of NCNTs depends on the structural nature of nitrogen in CNTs and defect density. Comparing with pristine CNTs, the presence of graphitic and pyridinic nitrogen significantly decreases the overpotential (ca. −0.18 V) and increases the selectivity (ca. 80 %) towards the formation of CO. The experimental results are in congruent with DFT calculations, which show that pyridinic defects retain a lone pair of electrons that are capable of binding CO2. However, for graphitic‐like nitrogen, electrons are located in the π* antibonding orbital, making them less accessible for CO2 binding.
Useful defects: The electrochemical activity of nitrogen‐doped multiwalled carbon nanotubes (see picture) used for the reduction of CO2 was improved by tuning the nitrogen defect sites in the wall structure. Pyridinic nitrogen defects supported the selective formation of CO. DFT calculations confirmed the experimental results.
With the advent of graphene, the most studied of all two-dimensional materials, many inorganic analogues have been synthesized and are being exploited for novel applications. Several approaches have ...been used to obtain large-grain, high-quality materials. Naturally occurring ores, for example, are the best precursors for obtaining highly ordered and large-grain atomic layers by exfoliation. Here, we demonstrate a new two-dimensional material 'hematene' obtained from natural iron ore hematite (α-Fe
O
), which is isolated by means of liquid exfoliation. The two-dimensional morphology of hematene is confirmed by transmission electron microscopy. Magnetic measurements together with density functional theory calculations confirm the ferromagnetic order in hematene while its parent form exhibits antiferromagnetic order. When loaded on titania nanotube arrays, hematene exhibits enhanced visible light photocatalytic activity. Our study indicates that photogenerated electrons can be transferred from hematene to titania despite a band alignment unfavourable for charge transfer.
Dual photoluminescence (PL) emission characteristics of Mn2+ doped ZnS (ZnS:Mn) quantum dots (QDs) have drawn a lot of attention recently. However, here we report the effect of thermal annealing on ...the PL emission characteristics of uncapped ZnS:Mn QDs of average sizes ~2–3nm, synthesized by simple chemical precipitation method by using de-ionized (DI) water at room temperature. As-synthesized samples show dual PL emissions, having one UV PL band centred at ~400nm and the other in the visible region ~610nm. But when the samples are isochronally annealed for 2h at 100–600°C temperature range in air, ~90% quenching of Mn2+ related visible PL emission intensity takes place at the annealing temperature of 600°C. X-ray diffraction data show that the as-synthesized cubic ZnS has been converted to wurtzite ZnO at 600°C annealing temperature. The nanostructural properties of the samples are also determined by transmission electron micrograph, electron probe micro-analyser and UV–vis spectrophotometry. The photocatalytic property of the annealed ZnS:Mn sample has been demonstrated and photo-degradation efficiency of the as-synthesized and 600°C annealed ZnS:Mn sample has been found out to be ~35% and ~61%, respectively, for the degradation of methylene blue dye under visible light irradiation. The synthesized QDs may find significant applications in future optoelectronic devices.
•Effect of annealing on dual emission from ZnS:Mn quantum dots has been reported.•Quenching (~90%) of Mn2+ related PL intensity is observed at 600°C temperature.•Cubic ZnS has been converted to wurtzite ZnO at 600°C annealing temperature.•Visible light driven degradation of MB dye has been demonstrated.
Lithium–oxygen batteries (LOBs) and direct methanol fuel cells (DMFCs) are both attractive technologies for the development of future energy storage and conversion devices, while the lack of highly ...active electrocatalysts has largely hampered their large-scale commercial applications. In the present work, we have demonstrated the controllable synthesis of nitrogen and sulfur codoped graphene (NS-G) nanosheets via a simple and cost-effective approach. Owing to their distinctive structural advantages, such as large surface areas, good flexibility, coexistence of N and S atoms with tunable doping contents and positions, and high electrical conductivity, the obtained NS-G sheets exhibit low discharge–recharge voltage gaps and high round-trip efficiencies as well as exceptional rate capability when used as cathode materials for LOBs. Furthermore, the NS-G layers are also identified to be an ideal substrate for the decoration of ultrafine Pt nanoparticles. Benefiting from the synergetic effects, exceptional electrocatalytic properties including high activity, strong poison tolerance, and outstanding long-term stability are observed for the resulting Pt/NS-G hybrid toward methanol oxidation reaction, which are significantly superior to those for Pt/undoped graphene and commercial Pt/C catalysts.
In the last several decades, significant efforts have been devoted to two-dimensional (2D) materials on account of their optical properties that have numerous applications in the optoelectronic world ...in the range of light-emitting diodes, optical sensors, solar energy conversion, photo-electrochemical cells, photovoltaic solar cells, and even the biomedical sector. First, we provide an outline of linear optical properties of 2D materials such as graphene, TMDs, h-BN, MXenes, perovskite oxide, and metal-organic framework. Then, we discuss the optoelectronic properties of the 2D materials. Along with these, we also highlight the important efforts in developing 2D optical materials with intensive emission properties at a broad wavelength from ultraviolet to near-infrared. The origin of this tunable emission has been discussed decoratively. Thickness and layer-dependent optical properties have been highlighted and are explained through surface defects, strain, vacancy, doping, and dangling bonds emerging due to structural change in the material. The linear and nonlinear optical properties in 2D MXene and perovskite oxides are also impressive due to their potential applications in next-generation devices with excellent optical sensitivity. Finally, technological innovations, challenges, and possible tuning of defects and imperfections in the 2D lattice are discussed.
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•2D Al70Co10Fe5Ni10Cu5 has been synthesized followed by liquid-phase exfoliation.•Optical properties of plasmonic 2D-QCs have been studied.•The plasmonic field enhancement property of ...2D-QCs has been demonstrated.•SPR-induced scattering properties of 2D-QCs have been exploited to obtain ic-RL.•OR-enhanced ic-RL has been demonstrated in the presence of resonator walls.
Photon trapping inside a gain medium using a dispersed two-dimensional (2D) passive scatterer is an impetus to obtain incoherent random lasing (ic-RL) emission due to non-resonant feedback. An optical resonator (OR) can be used to influence such lasing thresholds. Non-noble nanomaterials-based quasicrystals (QCs) are an intriguing research prospect due to their potential surface plasmon resonance (SPR) property and ability to be exfoliated into 2D. In this work, an aluminium-based multicomponent alloy (Al70Co10Fe5Ni10Cu5) has been synthesized via the arc melting method. Thereafter, ultrasonication-based liquid phase exfoliation was used to obtain 2D quasicrystals (2D-QCs). The SPR-induced light scattering properties of synthesized 2D-QCs were exploited to obtain ic-RL from DCM dye gain medium under 532 nm, 10 ns, 10 Hz pulsed laser pumping. The plasmonic field enhancement property of 2D-QCs which enables the gain medium to absorb photons outside its peak absorption band has been demonstrated. The transition from ic-RL to OR-enhanced ic-RL and vice versa in the presence of resonator walls has been achieved by tweaking the device architecture. In this way, the ability of 2D-QCs to be potential passive scatterers and the controllability of lasing thresholds in the presence of an OR has been demonstrated.
Despite its importance in the large-scale synthesis of transition metal dichalcogenides (TMDC) molecular layers, the generic quantum effects on electrical transport across individual grain boundaries ...(GBs) in TMDC monolayers remain unclear. Here we demonstrate that strong carrier localization due to the increased density of defects determines both temperature dependence of electrical transport and low-frequency noise at the GBs of chemical vapor deposition (CVD)-grown MoS2 layers. Using field effect devices designed to explore transport across individual GBs, we show that the localization length of electrons in the GB region is ∼30–70% lower than that within the grain, even though the room temperature conductance across the GB, oriented perpendicular to the overall flow of current, may be lower or higher than the intragrain region. Remarkably, we find that the stronger localization is accompanied by nearly 5 orders of magnitude enhancement in the low-frequency noise at the GB region, which increases exponentially when the temperature is reduced. The microscopic framework of electrical transport and noise developed in this paper may be readily extended to other strongly localized two-dimensional systems, including other members of the TMDC family.
The mechanical and optical properties generated due to the stacking of different atomically thin materials have made it possible to tune and engineer these materials for next-generation electronics. ...The understanding of the interlayer interactions in such stacked structures is of fundamental interest for structure and property correlation. Here, a combined approach of in situ Raman spectroscopy and mechanical straining along with molecular dynamics (MD) simulations has been used to probe one such interface, namely, the WS2/MoS2 heterostructure. Vertical heterostructures on poly(methyl methacrylate), when flexed, showed signs of decoupling at 1.2% strain. Theoretical calculations showed strain-induced stacking changes at 1.75% strain. The sliding characteristics of layers were also investigated using scanning probe microscopy based nanoscratch testing, and the results are further supported by MD simulations. The present study could be used to design future optoelectronic devices based on WS2/MoS2 heterostructures.
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