Search

Perfluorinated alkyl substances (PFAS), such as perfluorooctanesulfonic acid (PFOS) and perfluorooctanoic acid (PFOA), are pervasive organic contaminants that are widespread in aquatic environments, posing significant health risks to humans and wildlife. Due to their persistent nature, urgent removal is necessary. Conventional adsorbents are inefficient at removing PFOS and PFOA, highlighting the need for alternative materials. Herein, we present a synthetic method for quaternary ammonium cation-doped carbon nanoparticles (QACNs) using a solution plasma process for the efficient removal of PFOS and PFOA. QACN is formed simultaneously through a one-step discharge of nonequilibrium plasma at the interface of benzene and pyridinium chloride. The resulting material exhibited a high surface electrical charge and enhanced hydrophilicity as well as an amorphous structure of a nonporous nature, involving nanoparticles with an undefined shape. The obtained adsorbent demonstrated high adsorption efficiency and stability, adsorbing 998.45 and 889.37 mg g -1 of PFOS and PFOA, respectively, exceeding the efficiencies of conventional carbon-based adsorbents (80.89-313.15 mg g -1 ). The adsorption performance was dependent on the adsorbent dosage, pH of the solution, and the coexisting ionic species. Adsorption studies, including adsorption kinetics, isotherms, and thermodynamics, revealed that PFOS and PFOA were chemisorbed to the QACN surface, forming multilayers endothermically and spontaneously. Experimental and computational analyses revealed that adsorption primarily occurs via electronic interactions between the PFAS active sites and the quaternary ammonium group in the carbon framework. The slightly lower adsorption potential of the PFOS and PFOA fluorocarbon chains on the adsorbent was elucidated. Furthermore, the dispersibility of the adsorbent in solution significantly affected the adsorption performance. These findings highlight the potential of the novel synthetic method proposed in this study, offering a pathway for the development of highly effective carbon adsorbents for environmental remediation.

We demonstrate hydrothermally grown sodium hydrogen zirconium phosphate ((Na,H)-ZrP) crystals exhibiting high calcium ion selectivity. The standard Gibbs free energies for Ca 2+ exchange on (Na,H)-ZrP and γ-type ZrP were estimated to be -10.1 and -4.69 kJ mol -1 , respectively. The high Ca 2+ selectivity of (Na,H)-ZrP could be attributed to the size matching between the ion exchange site of (Na,H)-ZrP and Ca 2+ .

A sustainable society requires high-energy storage devices characterized by lightness, compactness, a long life and superior safety, surpassing current battery and supercapacitor technologies. Single-walled carbon nanotubes (SWCNTs), which typically exhibit great toughness, have emerged as promising candidates for innovative energy storage solutions. Here we produced SWCNT ropes wrapped in thermoplastic polyurethane elastomers, and demonstrated experimentally that a twisted rope composed of these SWCNTs possesses the remarkable ability to reversibly store nanomechanical energy. Notably, the gravimetric energy density of these twisted ropes reaches up to 2.1 MJ kg -1 , exceeding the energy storage capacity of mechanical steel springs by over four orders of magnitude and surpassing advanced lithium-ion batteries by a factor of three. In contrast to chemical and electrochemical energy carriers, the nanomechanical energy stored in a twisted SWCNT rope is safe even in hostile environments. This energy does not deplete over time and is accessible at temperatures ranging from -60 to +100 °C., (© 2024. The Author(s).)

It is critical to understand the laws of quantum mechanics in transformative technologies for computation and quantum information science applications to enable the ongoing second quantum revolution calls. Recently, spin qubits based on point defects have gained great attention, since these qubits can be initiated, selectively controlled, and read out with high precision at ambient temperature. The major challenge in these systems is controllably generating multiqubit systems while properly coupling the defects. To address this issue, we began by tackling the engineering challenges these systems present and understanding the fundamentals of defects. In this regard, we controllably generate defects in MoS 2 and WS 2 monolayers and tune their physicochemical properties via proton irradiation. We quantitatively discovered that the proton energy could modulate the defects' density and nature; higher defect densities were seen with lower proton irradiation energies. Three distinct defect types were observed: vacancies, antisites, and adatoms. In particular, the creation and manipulation of antisite defects provides an alternative way to create and pattern spin qubits based on point defects. Our results demonstrate that altering the particle irradiation energy can regulate the formation of defects, which can be utilized to modify the properties of 2D materials and create reliable electronic devices.

The ability to control the density and spatial distribution of substitutional dopants in semiconductors is crucial for achieving desired physicochemical properties. Substitutional doping with adjustable doping levels has been previously demonstrated in 2D transition metal dichalcogenides (TMDs); however, the spatial control of dopant distribution remains an open field. In this work, edge termination is demonstrated as an important characteristic of 2D TMD monocrystals that affects the distribution of substitutional dopants. Particularly, in chemical vapor deposition (CVD)-grown monolayer WS 2 , it is found that a higher density of transition metal dopants is always incorporated in sulfur-terminated domains when compared to tungsten-terminated domains. Two representative examples demonstrate this spatial distribution control, including hexagonal iron- and vanadium-doped WS 2 monolayers. Density functional theory (DFT) calculations are further performed, indicating that the edge-dependent dopant distribution is due to a strong binding of tungsten atoms at tungsten-zigzag edges, resulting in the formation of open sites at sulfur-zigzag edges that enable preferential dopant incorporation. Based on these results, it is envisioned that edge termination in crystalline TMD monolayers can be utilized as a novel and effective knob for engineering the spatial distribution of substitutional dopants, leading to in-plane hetero-/multi-junctions that display fascinating electronic, optoelectronic, and magnetic properties., (© 2022 Wiley-VCH GmbH.)

The present paper reviews recent activities on inverse analysis strategies in geotechnical engineering using Kalman filters, nonlinear Kalman filters, and Markov chain Monte Carlo (MCMC)/Hamiltonian Monte Carlo (HMC) methods. Nonlinear Kalman filters with finite element method (FEM) broaden the choices of unknowns to be determined for not only parameters but also initial and/or boundary conditions, and the use of the posterior probability of the state variables can be widely applied to, for example, the decision making for design changes. The relevance of the unknowns and the observed values and the selection of the best sensor locations are some of the considerations made while using the Kalman filter FEM. This paper demonstrates several real-world geotechnical applications of the nonlinear Kalman filter and the MCMC with FEM. Future studies should focus on the following areas: attaining excellent performance for long-term forecasts using short-term observation and developing a viable method for selecting equations that describe physical phenomena and constitutive models.

Nanoscale pores in graphene oxide (GO) control various important functions. The nanoporosity of GO is sensitive to low-temperature heating. Therefore, it is important to carefully process GO and GO-based materials to achieve superior functions. Optimum pretreatment conditions, such as the pre-evacuation temperature and time, are important during gas adsorption in GO to obtain accurate pore structure information. This study demonstrated that the pre-evacuation temperature and time for gas adsorption in GO must be approximately 333-353 K and 4 h, respectively, to avoid the irreversible alteration of nanoporosity. In situ temperature-dependent Fourier-transform infrared spectra and thermogravimetric analysis-mass spectrometry suggested significant structural changes in GO above the pre-evacuation temperature (353 K) through the desorption of "physically adsorbed water" and decomposition of unstable surface functional groups. The nanoporosity of GO significantly changed above the aforementioned pre-evacuation temperature and time. Thus, standard pretreatment is indispensable for understanding the intrinsic interface properties of GO.

Two-dimensional materials (2DM) have attracted much interest due to their distinct optical, electronic, and catalytic properties. These properties can be tuned by a range of methods including substitutional doping and, as recently demonstrated, by surface functionalization with single atoms, thus increasing the 2DM portfolio. We theoretically and experimentally describe the coordination reaction between MoS 2 monolayers and 3d transition metals (TMs), exploring their nature and MoS 2 -TM interactions. Density functional theory calculations, X-ray photoelectron spectroscopy (XPS), and photoluminescence (PL) spectroscopy point to the formation of MoS 2 -TM coordination complexes, where the adsorption energy for 3d TMs resembles the crystal-field (CF) stabilization energy for weak-field complexes. Pearson's theory for hard-soft acid-base and ligand-field theory were used to discuss the periodic trends of 3d TM coordination on MoS 2 monolayer surfaces. We found that softer acids with higher ligand field stabilization energy, such as Ni 2+ , tend to form bonds with more covalent character with MoS 2 , which can be considered a soft base. On the other hand, harder acids, such as Cr 3+ , tend to form more ionic bonds. Additionally, we studied the trends in charge transfer and doping observed from XPS and PL results, where metals like Ni led to n-type doping. In contrast, Cu functionalization results in p-type doping. Therefore, the formation of coordination complexes on TMD's surface is a potentially effective way to control and understand the nature of single-atom functionalization of TMD monolayers without relying on or creating new defects.

We investigate a laser direct-write method to synthesize and deposit metastable, mixed transition metal oxides and evaluate their performance as oxygen evolution reaction catalysts. This laser processing method enabled the rapid synthesis of diverse heterogeneous alloy and oxide catalysts directly from cost-effective solution precursors, including catalysts with a high density of nanocrystalline metal alloy inclusions within an amorphous oxide matrix. The nanoscale heterogeneous structures of the synthesized catalysts were consistent with reactive force-field Monte Carlo calculations. By evaluating the impact of varying transition metal oxide composition ratios, we created a stable Fe 0.63 Co 0.19 Ni 0.18 O x /C catalyst with a Tafel slope of 38.23 mV dec -1 and overpotential of 247 mV, a performance similar to that of IrO 2 . Synthesized Fe 0.63 Co 0.19 Ni 0.18 O x /C and Fe 0.14 Co 0.46 Ni 0.40 O x /C catalysts were experimentally compared in terms of catalytic performance and structural characteristics to determine that higher iron content and a less crystalline structure in the secondary matrix decrease the charge transfer resistance and thus is beneficial for electrocatalytic activity. This conclusion is supported by density-functional theory calculations showing distorted active sites in ternary metal catalysts are key for lowering overpotentials for the oxygen evolution reaction.

Atomically thin 2D materials provide an opportunity to investigate the atomic-scale details of defects introduced by particle irradiation. Once the atomic configuration of defects and their spatial distribution are revealed, the details of the mesoscopic phenomena can be unveiled. In this work, we created atomically small defects by controlled irradiation of gallium ions with doses ranging from 4.94 × 10 12 to 4.00 × 10 14 ions/cm 2 on monolayer molybdenum disulfide (MoS 2 ) crystals. The optical signatures of defects, such as the evolution of defect-activated LA-bands and a broadening of the first-order ( E ' and A ' 1 ) modes, can be studied by Raman spectroscopy. High-resolution scanning transmission electron microscopy (HR-STEM) analysis revealed that most defects are vacancies of few-molybdenum atoms with surrounding sulfur atoms (V x Mo+ y S ) at a low ion dose. When increasing the ion dose, the atomic vacancies merge and form nanometer-sized holes. Utilizing HR-STEM and image analysis, we propose the estimation of the finite crystal length ( L fc ) via the careful quantification of 0D defects in 2D systems through the formula L fc = 4.41/ η ion , where η ion corresponds to the ion dose. Combining HR-STEM and Raman spectroscopy, the formula to calculate L fc from Raman features, I (LA)/ I ( A ' 1 ) = 5.09/ L fc 2 , is obtained. We have also demonstrated an effective route to healing the ion irradiation-induced atomic vacancies by annealing defective MoS 2 in a hydrogen disulfide (H 2 S) atmosphere. The H 2 S annealing improved the crystal quality of MoS 2 with L fc greater than the calculated size of the A exciton wave function, which leads to a partial recovery of the photoluminescence signal after its quenching by ion irradiation.

Single-layer heterostructures exhibit striking quasiparticle properties and many-body interaction effects that hold promise for a range of applications. However, their properties can be altered by intrinsic and extrinsic defects, thus diminishing their applicability. Therefore, it is of paramount importance to identify defects and understand 2D materials' degradation over time using advanced multimodal imaging techniques. Here we implemented a liquid-phase precursor approach to synthesize 2D in-plane MoS 2 -WS 2 heterostructures exhibiting nanoscale alloyed interfaces and map exotic interface effects during photodegradation using a combination of hyperspectral tip-enhanced photoluminescence and Raman and near-field nanoscopy. Surprisingly, 2D alloyed regions exhibit thermal and photodegradation stability providing protection against oxidation. Coupled with surface and interface strain, 2D alloy regions create stable localized potential wells that concentrate excitonic species via a charge carrier funneling effect. These results demonstrate that 2D alloys can withstand extreme degradation effects over time and could enable stable 2D device engineering.

In two-dimensional crystals, fractures propagate easily, thus restricting their mechanical reliability. This work demonstrates that controlled defect creation constitutes an effective approach to avoid catastrophic failure in MoS 2 monolayers. A systematic study of fracture mechanics in MoS 2 monolayers as a function of the density of atomic vacancies, created by ion irradiation, is reported. Pristine and irradiated materials were studied by atomic force microscopy, high-resolution scanning transmission electron microscopy, and Raman spectroscopy. By inducing ruptures through nanoindentations, we determine the strength and length of the propagated cracks within MoS 2 atom-thick membranes as a function of the density and type of the atomic vacancies. We find that a 0.15% atomic vacancy induces a decrease of 40% in strength with respect to that of pristine samples. In contrast, while tear holes in pristine 2D membranes span several microns, they are restricted to a few nanometers in the presence of atomic and nanometer-sized vacancies, thus increasing the material's fracture toughness.

Surface functionalization of metallic and semiconducting 2D transition metal dichalcogenides (TMDs) have mostly relied on physi- and chemi-sorption at defect sites, which can diminish the potential applications of the decorated 2D materials, as structural defects can have substantial drawbacks on the electronic and optoelectronic characteristics. Here, we demonstrate a spontaneous defect-free functionalization method consisting of attaching Au single atoms to monolayers of semiconducting MoS 2 (1H) via S-Au-Cl coordination complexes. This strategy offers an effective and controllable approach for tuning the Fermi level and excitation spectra of MoS 2 via p-type doping and enhancing the thermal boundary conductance of monolayer MoS 2 , thus promoting heat dissipation. The coordination-based method offers an effective and damage-free route of functionalizing TMDs and can be applied to other metals and used in single-atom catalysis, quantum information devices, optoelectronics, and enhanced sensing., (Copyright © 2020 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC).)

Dilute magnetic semiconductors (DMS), achieved through substitutional doping of spin-polarized transition metals into semiconducting systems, enable experimental modulation of spin dynamics in ways that hold great promise for novel magneto-electric or magneto-optical devices, especially for two-dimensional (2D) systems such as transition metal dichalcogenides that accentuate interactions and activate valley degrees of freedom. Practical applications of 2D magnetism will likely require room-temperature operation, air stability, and (for magnetic semiconductors) the ability to achieve optimal doping levels without dopant aggregation. Here, room-temperature ferromagnetic order obtained in semiconducting vanadium-doped tungsten disulfide monolayers produced by a reliable single-step film sulfidation method across an exceptionally wide range of vanadium concentrations, up to 12 at% with minimal dopant aggregation, is described. These monolayers develop p-type transport as a function of vanadium incorporation and rapidly reach ambipolarity. Ferromagnetism peaks at an intermediate vanadium concentration of ~2 at% and decreases for higher concentrations, which is consistent with quenching due to orbital hybridization at closer vanadium-vanadium spacings, as supported by transmission electron microscopy, magnetometry, and first-principles calculations. Room-temperature 2D-DMS provide a new component to expand the functional scope of van der Waals heterostructures and bring semiconducting magnetic 2D heterostructures into the realm of practical application., Competing Interests: The authors declare no conflict of interest., (© 2020 The Authors. Published by Wiley‐VCH GmbH.)

Reverse osmosis membranes of aromatic polyamide (PA) reinforced with a crystalline cellulose nanofiber (CNF) were synthesized and their desalination performance was studied. Comparison with plain PA membranes shows that the addition of CNF reduced the matrix mobility resulting in a molecularly stiffer membrane because of the attractive forces between the surface of the CNFs and the PA matrix. Fourier transform-infrared spectroscopy and X-ray photoelectron spectroscopy results showed complex formation between the carboxy groups of the CNF surface and the m- phenylenediamine monomer in the CNF-PA composite. Molecular dynamics simulations showed that the CNF-PA had higher hydrophilicity which was key for the higher water permeability of the synthesized nanocomposite membrane. The CNF-PA reverse osmosis nanocomposite membranes also showed enhanced antifouling performance and improved chlorine resistance. Therefore, CNF shows great potential as a nanoreinforcing material towards the preparation of nanocomposite aromatic PA membranes with longer operation lifetime due to its antifouling and chlorine resistance properties.

Stacking layers of atomically thin transition-metal carbides and two-dimensional (2D) semiconducting transition-metal dichalcogenides, could lead to nontrivial superconductivity and other unprecedented phenomena yet to be studied. In this work, superconducting α-phase thin molybdenum carbide flakes were first synthesized, and a subsequent sulfurization treatment induced the formation of vertical heterolayer systems consisting of different phases of molybdenum carbide-ranging from α to γ' and γ phases-in conjunction with molybdenum sulfide layers. These transition-metal carbide/disulfide heterostructures exhibited critical superconducting temperatures as high as 6 K, higher than that of the starting single-phased α-Mo 2 C (4 K). We analyzed possible interface configurations to explain the observed moiré patterns resulting from the vertical heterostacks. Our density-functional theory (DFT) calculations indicate that epitaxial strain and moiré patterns lead to a higher interfacial density of states, which favors superconductivity. Such engineered heterostructures might allow the coupling of superconductivity to the topologically nontrivial surface states featured by transition-metal carbide phases composing these heterostructures potentially leading to unconventional superconductivity. Moreover, we envisage that our approach could also be generalized to other metal carbide and nitride systems that could exhibit high-temperature superconductivity., Competing Interests: The authors declare no competing interest.

Two-dimensional transition metal dichalcogenides (TMDs) emerged as a promising platform to construct sensitive biosensors. We report an ultrasensitive electrochemical dopamine sensor based on manganese-doped MoS 2 synthesized via a scalable two-step approach (with Mn ~2.15 atomic %). Selective dopamine detection is achieved with a detection limit of 50 pM in buffer solution, 5 nM in 10% serum, and 50 nM in artificial sweat. Density functional theory calculations and scanning transmission electron microscopy show that two types of Mn defects are dominant: Mn on top of a Mo atom (Mn topMo ) and Mn substituting a Mo atom (Mn Mo ). At low dopamine concentrations, physisorption on Mn Mo dominates. At higher concentrations, dopamine chemisorbs on Mn topMo , which is consistent with calculations of the dopamine binding energy (2.91 eV for Mn topMo versus 0.65 eV for Mn Mo ). Our results demonstrate that metal-doped layered materials, such as TMDs, constitute an emergent platform to construct ultrasensitive and tunable biosensors., (Copyright © 2020 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC).)

The role of additives in facilitating the growth of conventional semiconducting thin films is well-established. Apparently, their presence is also decisive in the growth of two-dimensional transition metal dichalcogenides (TMDs), yet their role remains ambiguous. In this work, we show that the use of sodium bromide enables synthesis of TMD monolayers via a surfactant-mediated growth mechanism, without introducing liquefaction of metal oxide precursors. We discovered that sodium ions provided by sodium bromide chemically passivate edges of growing molybdenum disulfide crystals, relaxing in-plane strains to suppress 3D islanding and promote monolayer growth. To exploit this growth model, molybdenum disulfide monolayers were directly grown into desired patterns using predeposited sodium bromide as a removable template. The surfactant-mediated growth not only extends the families of metal oxide precursors but also offers a way for lithography-free patterning of TMD monolayers on various surfaces to facilitate fabrication of atomically thin electronic devices.

Doping lies at the heart of modern semiconductor technologies. Therefore, for two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs), the significance of controlled doping is no exception. Recent studies have indicated that, by substitutionally doping 2D TMDs with a judicious selection of dopants, their electrical, optical, magnetic, and catalytic properties can be effectively tuned, endowing them with great potential for various practical applications. Herein, and inspired by the sol-gel process, we report a liquid-phase precursor-assisted approach for in situ substitutional doping of monolayered TMDs and their in-plane heterostructures with tunable doping concentration. This highly reproducible route is based on the high-temperature chalcogenation of spin-coated aqueous solutions containing host and dopant precursors. The precursors are mixed homogeneously at the atomic level in the liquid phase prior to the synthesis process, thus allowing for an improved doping uniformity and controllability. We further demonstrate the incorporation of various transition metal atoms, such as iron (Fe), rhenium (Re), and vanadium (V), into the lattice of TMD monolayers to form Fe-doped WS 2 , Re-doped MoS 2 , and more complex material systems such as V-doped in-plane W x Mo 1- x S 2 -Mo x W 1- x S 2 heterostructures, among others. We envisage that our developed approach is universal and could be extended to incorporate a variety of other elements into 2D TMDs and create in-plane heterointerfaces in a single step, which may enable applications such as electronics and spintronics at the 2D limit.

Functionalizing the surfaces of transition metal dichalcogenide (TMD) nanosheets with noble metals is important for electrically contacting them to devices, as well as improving their catalytic and sensing capabilities. Solution-phase deposition provides a scalable approach to the creation of metal-TMD hybrid systems, but controlling such processes remains challenging. Here we elucidate the different pathways by which gold and silver deposit at room temperature onto colloidal 1T-WS 2 , 2H-WS 2 , 2H-MoSe 2 , 2H-WSe 2 , 1T'-MoTe 2 and T d -WTe 2 few-layer nanostructures to produce several distinct classes of 0D-2D and 2D-2D metal-TMD hybrids. Uniform gold nanoparticles form on all of the TMDs. By contrast, silver deposits as nanoparticles with a bimodal size distribution on the disulfides and diselenides, and as atomically thin layers on the ditellurides. The various sizes and morphologies of these surface-bound metal species arise from the relative strengths of the interfacial metal-chalcogen bonds during the reduction of Au 3+ or Ag + by the TMDs.

Defect engineering is important for tailoring the electronic and optical properties of two-dimensional materials, and the capability of generating defects of certain types at specific locations is meaningful for potential applications such as optoelectronics and quantum photonics. In this work, atomic defects are created in single-layer WSe2 using focused ion beam (FIB) irradiation, with defect densities spanning many orders of magnitude. The influences of defects are systematically characterized. Raman spectroscopy can only discern defects in WSe2 for a FIB dose higher than 1 × 1013 cm-2, which causes blue shifts of both A'1 and E' modes. Photoluminescence (PL) of WSe2 is more sensitive to defects. At cryogenic temperature, the low-energy PL induced by defects can be revealed, which shows redshifts and broadenings with increased FIB doses. Similar Raman shifts and PL spectrum changes are observed for the WSe2 film grown by chemical vapor deposition (CVD). A four microsecond-long lifetime is observed in the PL dynamics and is three orders of magnitude longer than the often observed delocalized exciton lifetime and becomes more dominant for WSe2 with increasing FIB doses. The ultra-long lifetime of PL in single-layer WSe2 is consistent with first-principles calculation results considering the creation of both chalcogen and metal vacancies by FIB, and can be valuable for photo-catalytic reactions, valleytronics and quantum light emissions owing to the longer carrier separation/manipulation time.

One-dimensional defects in two-dimensional (2D) materials can be particularly damaging because they directly impede the transport of charge, spin, or heat and can introduce a metallic character into otherwise semiconducting systems. Current characterization techniques suffer from low throughput and a destructive nature or limitations in their unambiguous sensitivity at the nanoscale. Here we demonstrate that dark-field second harmonic generation (SHG) microscopy can rapidly, efficiently, and nondestructively probe grain boundaries and edges in monolayer dichalcogenides (i.e., MoSe 2 , MoS 2 , and WS 2 ). Dark-field SHG efficiently separates the spatial components of the emitted light and exploits interference effects from crystal domains of different orientations to localize grain boundaries and edges as very bright 1D patterns through a Čerenkov-type SHG emission. The frequency dependence of this emission in MoSe 2 monolayers is explained in terms of plasmon-enhanced SHG related to the defect's metallic character. This new technique for nanometer-scale imaging of the grain structure, domain orientation and localized 1D plasmons in 2D different semiconductors, thus enables more rapid progress toward both applications and fundamental materials discoveries.

Transition metal dichalcogenides (TMDs) are well known catalysts as both bulk and nanoscale materials. Two-dimensional (2-D) TMDs, which contain single- and few-layer nanosheets, are increasingly studied as catalytic materials because of their unique thickness-dependent properties and high surface areas. Here, colloidal 2H-WS 2 nanostructures are used as a model 2-D TMD system to understand how high catalytic activity and selectivity can be achieved for useful organic transformations. Free-standing, colloidal 2H-WS 2 nanostructures containing few-layer nanosheets are shown to catalyze the selective hydrogenation of a broad scope of substituted nitroarenes to their corresponding aniline derivatives in the presence of other reducible functional groups. Microscopic and computational studies reveal the important roles of sulfur vacancy-rich basal planes and tungsten-terminated edges, which are more abundant in nanostructured 2-D materials than in their bulk counterparts, in enabling the functional group selectivity. At tungsten-terminated edges and on regions of the basal planes having high concentrations of sulfur vacancies, vertical adsorption of the nitroarene is favored, thus facilitating hydrogen transfer exclusively to the nitro group due to geometric effects. At lower sulfur vacancy concentrations on the basal planes, parallel adsorption of the nitroarene is favored, and the nitro group is selectively hydrogenated due to a lower kinetic barrier. These mechanistic insights reveal how the various defect structures and configurations on 2-D TMD nanostructures facilitate functional group selectivity through distinct mechanisms that depend upon the adsorption geometry, which may have important implications for the design of new and enhanced 2-D catalytic materials across a potentially broad scope of reactions., (This journal is © The Royal Society of Chemistry 2019.)

The current state-of-the-art positive electrode material for chloroaluminate ion batteries (AIBs) or dual-ion batteries (DIBs) is highly crystalline graphite; however, the rate capability of this material at high discharge currents is significantly reduced by the modest conductivity of graphite. This limitation is addressed through the use of graphene-based positive electrodes, which can improve the rate capability of these batteries due to their higher conductivity. However, conventional methods of graphene production induce a significant number of defects, which impair the performance of AIBs and DIBs. Herein, we report the use of a defect-free graphene positive electrode, which was produced using the electrochemical exfoliation of graphite in an aqueous solution with the aid of Co 2+ as an antioxidant. The Co-treated graphene electrode achieved high capacities of 150 mAh g -1 in DIBs and 130 mAh g -1 in AIBs with high rate capability for both batteries. The charge-discharge mechanism of the batteries is examined using in situ Raman spectroscopy, and the results revealed that the intercalation density of [AlCl 4 ] - or [PF 6 ] - increased from a dilute staging index graphite intercalation compound (GIC) to a stage 1 GIC within the operating voltage window. The simple production method of high-quality graphene in conjunction with its high performance in DIBs should enable the use of graphene for DIB technologies.

Chemical doping constitutes an effective route to alter the electronic, chemical, and optical properties of two-dimensional transition metal dichalcogenides (2D-TMDs). We used a plasma-assisted method to introduce carbon-hydrogen (CH) units into WS 2 monolayers. We found CH-groups to be the most stable dopant to introduce carbon into WS 2 , which led to a reduction of the optical bandgap from 1.98 to 1.83 eV, as revealed by photoluminescence spectroscopy. Aberration corrected high-resolution scanning transmission electron microscopy (AC-HRSTEM) observations in conjunction with first-principle calculations confirm that CH-groups incorporate into S vacancies within WS 2 . According to our electronic transport measurements, undoped WS 2 exhibits a unipolar n-type conduction. Nevertheless, the CH-WS 2 monolayers show the emergence of a p-branch and gradually become entirely p-type, as the carbon doping level increases. Therefore, CH-groups embedded into the WS 2 lattice tailor its electronic and optical characteristics. This route could be used to dope other 2D-TMDs for more efficient electronic devices.

Graphene provides a unique platform for the detailed study of its dopants at the atomic level. Previously, doped materials including Si, and 0D-1D carbon nanomaterials presented difficulties in the characterization of their dopants due to gradients in their dopant concentration and agglomeration of the material itself. Graphene's two-dimensional nature allows for the detailed characterization of these dopants via spectroscopic and atomic resolution imaging techniques. Nitrogen doping of graphene has been well studied, providing insights into the dopant bonding structure, dopant-dopant interaction, and spatial segregation within a single crystal. Different configurations of nitrogen within the carbon lattice have different electronic and chemical properties, and by controlling these dopants it is possible to either n- or p-type dope graphene, grant half-metallicity, and alter nitrogen doped graphene's (NG) catalytic and sensing properties. Thus, an understanding and the ability to control different types of nitrogen doping configurations allows for the fine tuning of NG's properties. Here we review the synthesis, characterization, and properties of nitrogen dopants in NG beyond atomic dopant concentration.

We explored the effect of substitutional boron doping on the electrical conductivity of a metallicity-separated single walled carbon nanotube (SWCNT) assembly. Boron atoms were introduced into semiconducting (S)- and metallic (M)-SWCNT assemblies using high temperature thermal diffusion and the concentration of the doped boron atoms was controlled by the thermal treatment temperature. Depending on the conduction mechanism of the SWCNT assembly, both positive and negative effects upon boron incorporation are observed. For the S-SWCNT sheet, the electrical resistivity decreased by about 1 order on introduction of a small amount of boron atoms, due to the localized state for hopping conduction. In contrast, we observed an increase in the electrical resistivity on boron doping for M-SWCNTs. The pristine and boron doped metallic SWCNTs exhibited a tendency of decreasing electrical resistivity in the presence of an external magnetic field perpendicular to the film, which indicated two-dimensional weak localization behavior. A detailed analysis of the resistivity and the magnetoresistance implied that an increase in the inelastic scattering event at the doped boron site reduced the phase coherence length, leading to an increase in the electrical resistivity.

Atomic-defect engineering in thin membranes provides opportunities for ionic and molecular filtration and analysis. While molecular-dynamics (MD) calculations have been used to model conductance through atomic vacancies, corresponding experiments are lacking. We create sub-nanometer vacancies in suspended single-layer molybdenum disulfide (MoS 2 ) via Ga + ion irradiation, producing membranes containing ∼300 to 1200 pores with average and maximum diameters of ∼0.5 and ∼1 nm, respectively. Vacancies exhibit missing Mo and S atoms, as shown by aberration-corrected scanning transmission electron microscopy (AC-STEM). The longitudinal acoustic band and defect-related photoluminescence were observed in Raman and photoluminescence spectroscopy, respectively. As the irradiation dose is increased, the median vacancy area remains roughly constant, while the number of vacancies (pores) increases. Ionic current versus voltage is nonlinear and conductance is comparable to that of ∼1 nm diameter single MoS 2 pores, proving that the smaller pores in the distribution display negligible conductance. Consistently, MD simulations show that pores with diameters <0.6 nm are almost impermeable to ionic flow. Atomic pore structure and geometry, studied by AC-STEM, are critical in the sub-nanometer regime in which the pores are not circular and the diameter is not well-defined. This study lays the foundation for future experiments to probe transport in large distributions of angstrom-size pores.

2D materials cover a wide spectrum of electronic properties. Their applications are extended from electronic, optical, and chemical to biological. In terms of biomedical uses of 2D materials, the interactions between living cells and 2D materials are of paramount importance. However, biointerfacial studies are still in their infancy. This work studies how living organisms interact with transition metal dichalcogenide monolayers. For the first time, cellular digestion of tungsten disulfide (WS 2 ) monolayers is observed. After digestion, cells intake WS 2 and become fluorescent. In addition, these light-emitting cells are not only viable, but also able to pass fluorescent signals to their progeny cells after cell division. By combining synthesis of 2D materials and a cell culturing technique, a procedure for monitoring the interactions between WS 2 monolayers and cells is developed. These observations open up new avenues for developing novel cellular labeling and imaging approaches, thus triggering further studies on interactions between 2D materials and living organisms., (© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The strong in-plane anisotropy of rhenium disulfide (ReS 2 ) offers an additional physical parameter that can be tuned for advanced applications such as logic circuits, thin-film polarizers, and polarization-sensitive photodetectors. ReS 2 also presents advantages for optoelectronics, as it is both a direct-gap semiconductor for few-layer thicknesses (unlike MoS 2 or WS 2 ) and stable in air (unlike black phosphorus). Raman spectroscopy is one of the most powerful characterization techniques to nondestructively and sensitively probe the fundamental photophysics of a 2D material. Here, we perform a thorough study of the resonant Raman response of the 18 first-order phonons in ReS 2 at various layer thicknesses and crystal orientations. Remarkably, we discover that, as opposed to a general increase in intensity of all of the Raman modes at excitonic transitions, each of the 18 modes behave differently relative to each other as a function of laser excitation, layer thickness, and orientation in a manner that highlights the importance of electron-phonon coupling in ReS 2 . In addition, we correct an unrecognized error in the calculation of the optical interference enhancement of the Raman signal of transition metal dichalcogenides on SiO 2 /Si substrates that has propagated through various reports. For ReS 2 , this correction is critical to properly assessing the resonant Raman behavior. We also implemented a perturbation approach to calculate frequency-dependent Raman intensities based on first-principles and demonstrate that, despite the neglect of excitonic effects, useful trends in the Raman intensities of monolayer and bulk ReS 2 at different laser energies can be accurately captured. Finally, the phonon dispersion calculated from first-principles is used to address the possible origins of unexplained peaks observed in the Raman spectra, such as infrared-active modes, defects, and second-order processes.

Thermoplastic polymers subjected to a continuous tensile stress experience a state of mechanical instabilities, resulting in neck formation and propagation. The necking process with strong localized strain enables the transformation of initially brittle polymeric materials into robust, flexible, and oriented forms. Here we harness the polymer-based mechanical instabilities to control the fragmentation of atomically thin transition metal dichalcogenides (TMDs). We develop a simple and versatile nanofabrication tool to precisely fragment atom-thin TMDs sandwiched between thermoplastic polymers into ordered and atomically perfect TMD nanoribbons in arbitrary directions regardless of the crystal structures, defect content, and original geometries. This method works for a very broad spectrum of semiconducting TMDs with thicknesses ranging from monolayers to bulk crystals. We also explore the electrical properties of the fabricated monolayer nanoribbon arrays, obtaining an on/off ratio of ∼10 6 for such MoS 2 arrays based field-effect transistors. Furthermore, we demonstrate an improved hydrogen evolution reaction with the resulting monolayer MoS 2 nanoribbons, thanks to the largely increased catalytic edge sites formed by this physical fragmentation method. This capability not only enriches the fundamental study of TMD extreme and fragmentation mechanics, but also impacts on future developments of TMD-based devices.

Nanostructures of layered transition metal dichalcogenide (TMD) alloys with tunable compositions are promising candidates for a broad scope of applications in electronics, optoelectronics, topological devices, and catalysis. Most TMD alloy nanostructures are synthesized as films on substrates using gas-phase methods at high temperatures. However, lower temperature solution routes present an attractive alternative with the potential for larger-scale, higher-yield syntheses of freestanding, higher surface area materials. Here, we report the direct solution synthesis of colloidal few-layer TMD alloys, Mo x W 1-x Se 2 and WS 2y Se 2(1-y) , exhibiting fully tunable metal and chalcogen compositions that span the MoSe 2 -WSe 2 and WS 2 -WSe 2 solid solutions, respectively. Chemical guidelines for achieving the targeted compounds are presented, along with comprehensive structural characterizations (X-ray diffraction, electron microscopy, Raman, and UV-visible spectroscopies). High-resolution microscopic imaging confirms the formation of TMD alloys and identifies a random distribution of the alloyed elements. Analysis of the tilt-angle dependency of the intensities associated with atomic-resolution annular dark field imaging line scans reveals the types of point vacancies present in the samples, thus providing atomic-level insights into the structures of colloidal TMD alloy nanostructures that were previously only accessible for substrate-confined films. The A excitonic transition of the TMD alloy nanostructures can be readily adjusted between 1.51 and 1.93 eV through metal and chalcogen alloying, correlating the compositional modulation to the realization of tunable optical properties.

Large-area (∼cm 2 ) films of vertical heterostructures formed by alternating graphene and transition-metal dichalcogenide (TMD) alloys are obtained by wet chemical routes followed by a thermal treatment at low temperature. In particular, we synthesized stacked graphene and W x Mo 1-x S 2 alloy phases that were used as hydrogen evolution catalysts. We observed a Tafel slope of 38.7 mV dec -1 and 96 mV onset potential (at current density of 10 mA cm -2 ) when the heterostructure alloy was annealed at 300 °C. These results indicate that heterostructures formed by graphene and W 0.4 Mo 0.6 S 2 alloys are far more efficient than WS 2 and MoS 2 by at least a factor of 2, and they are superior compared to other reported TMD systems. This strategy offers a cheap and low temperature synthesis alternative able to replace Pt in the hydrogen evolution reaction (HER). Furthermore, the catalytic activity of the alloy is stable over time, i.e., the catalytic activity does not experience a significant change even after 1000 cycles. Using density functional theory calculations, we found that this enhanced hydrogen evolution in the W x Mo 1-x S 2 alloys is mainly due to the lower energy barrier created by a favorable overlap of the d-orbitals from the transition metals and the s-orbitals of H 2 ; with the lowest energy barrier occurring for the W 0.4 Mo 0.6 S 2 alloy. Thus, it is now possible to further improve the performance of the "inert" TMD basal plane via metal alloying, in addition to the previously reported strategies such as creation of point defects, vacancies and edges. The synthesis of graphene/W 0.4 Mo 0.6 S 2 produced at relatively low temperatures is scalable and could be used as an effective low cost Pt-free catalyst.

We show that hexagonal domains of monolayer tungsten disulfide (WS 2 ) grown by chemical vapor deposition (CVD) with powder precursors can have discrete segmentation in their photoluminescence (PL) emission intensity, forming symmetric patterns with alternating bright and dark regions. Two-dimensional maps of the PL reveal significant reduction within the segments associated with the longest sides of the hexagonal domains. Analysis of the PL spectra shows differences in the exciton to trion ratio, indicating variations in the exciton recombination dynamics. Monolayers of WS 2 hexagonal islands transferred to new substrates still exhibit this PL segmentation, ruling out local strain in the regions as the dominant cause. High-power laser irradiation causes preferential degradation of the bright segments by sulfur removal, indicating the presence of a more defective region that is higher in oxidative reactivity. Atomic force microscopy (AFM) images of topography and amplitude modes show uniform thickness of the WS 2 domains and no signs of segmentation. However, AFM phase maps do show the same segmentation of the domain as the PL maps and indicate that it is caused by some kind of structural difference that we could not clearly identify. These results provide important insights into the spatially varying properties of these CVD-grown transition metal dichalcogenide materials, which may be important for their effective implementation in fast photo sensors and optical switches.

Defects play a significant role in tailoring the optical properties of two-dimensional materials. Optical signatures of defect-bound excitons are important tools to probe defective regions and thus interrogate the optical quality of as-grown semiconducting monolayer materials. We have performed a systematic study of defect-bound excitons using photoluminescence (PL) spectroscopy combined with atomically resolved scanning electron microscopy and first-principles calculations. Spatially resolved PL spectroscopy at low temperatures revealed bound excitons that were present only on the edges of monolayer tungsten disulfide and not in the interior. Optical pumping of the bound excitons was sublinear, confirming their bound nature. Atomic-resolution images reveal that the areal density of monosulfur vacancies is much larger near the edges (0.92 ± 0.45 nm -2 ) than in the interior (0.33 ± 0.11 nm -2 ). Temperature-dependent PL measurements found a thermal activation energy of ~36 meV; surprisingly, this is much smaller than the bound-exciton binding energy of ~300 meV. We show that this apparent inconsistency is related to a thermal dissociation of the bound exciton that liberates the neutral excitons from negatively charged point defects. First-principles calculations confirm that sulfur monovacancies introduce midgap states that host optical transitions with finite matrix elements, with emission energies ranging from 200 to 400 meV below the neutral-exciton emission line. These results demonstrate that bound-exciton emission induced by monosulfur vacancies is concentrated near the edges of as-grown monolayer tungsten disulfide.

Two-dimensional materials are promising for a range of applications, as well as testbeds for probing the physics of low-dimensional systems. Tungsten disulfide (WS 2 ) monolayers exhibit a direct band gap and strong photoluminescence (PL) in the visible range, opening possibilities for advanced optoelectronic applications. Here, we report the realization of two-dimensional nanometer-size pores in suspended monolayer WS 2 membranes, allowing for electrical and optical response in ionic current measurements. A focused electron beam was used to fabricate nanopores in WS 2 membranes suspended on silicon-based chips and characterized using PL spectroscopy and aberration-corrected high-resolution scanning transmission electron microscopy. It was observed that the PL intensity of suspended WS 2 monolayers is ∼10-15 times stronger when compared to that of substrate-supported monolayers, and low-dose scanning transmission electron microscope viewing and drilling preserves the PL signal of WS 2 around the pore. We establish that such nanopores allow ionic conductance and DNA translocations. We also demonstrate that under low-power laser illumination in solution, WS 2 nanopores grow slowly in size at an effective rate of ∼0.2-0.4 nm/s, thus allowing for atomically controlled nanopore size using short light pulses.

Free-standing and flexible WS 2 /WO 2.9 /C hybrid membranes are synthesized and used as catalytic electrodes for electrochemical hydrogen evolution, exhibiting a high and stable catalytic activity. By virtue of the synergetic effect, a low onset overpotential of 20 mV and a Tafel slope of 36 mV dec -1 are achieved., (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

As a novel and efficient surface analysis technique, graphene-enhanced Raman scattering (GERS) has attracted increasing research attention in recent years. In particular, chemically doped graphene exhibits improved GERS effects when compared with pristine graphene for certain dyes, and it can be used to efficiently detect trace amounts of molecules. However, the GERS mechanism remains an open question. We present a comprehensive study on the GERS effect of pristine graphene and nitrogen-doped graphene. By controlling nitrogen doping, the Fermi level (E F) of graphene shifts, and if this shift aligns with the lowest unoccupied molecular orbital (LUMO) of a molecule, charge transfer is enhanced, thus significantly amplifying the molecule's vibrational Raman modes. We confirmed these findings using different organic fluorescent molecules: rhodamine B, crystal violet, and methylene blue. The Raman signals from these dye molecules can be detected even for concentrations as low as 10(-11) M, thus providing outstanding molecular sensing capabilities. To explain our results, these nitrogen-doped graphene-molecule systems were modeled using dispersion-corrected density functional theory. Furthermore, we demonstrated that it is possible to determine the gaps between the highest occupied and the lowest unoccupied molecular orbitals (HOMO-LUMO) of different molecules when different laser excitations are used. Our simulated Raman spectra of the molecules also suggest that the measured Raman shifts come from the dyes that have an extra electron. This work demonstrates that nitrogen-doped graphene has enormous potential as a substrate when detecting low concentrations of molecules and could also allow for an effective identification of their HOMO-LUMO gaps.

Molybdenum ditelluride, MoTe2 , is emerging as an important transition-metal dichalcogenide (TMD) material because of its favorable properties relative to other TMDs. The 1T ' polymorph of MoTe2 is particularly interesting because it is semimetallic with bands that overlap near the Fermi level, but semiconducting 2H-MoTe2 is more stable and therefore more accessible synthetically. Metastable 1T '-MoTe2 forms directly in solution at 300 °C as uniform colloidal nanostructures that consist of few-layer nanosheets, which appear to exhibit an approx. 1 % lateral lattice compression relative to the bulk analogue. Density functional theory calculations suggest that small grain sizes and polycrystallinity stabilize the 1T ' phase in the MoTe2 nanostructures and suppress its transformation back to the more stable 2H polymorph through grain boundary pinning. Raman spectra of the 1T '-MoTe2 nanostructures exhibit a laser energy dependence, which could be caused by electronic transitions., (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The pulsed laser deposition method was used to decorate appropriately single wall carbon nanotubes (SWCNTs) with PbS quantum dots (QDs), leading to the formation of a novel class of SWCNTs/PbS-QDs nanohybrids (NHs), without resorting to any ligand engineering and/or surface functionalization. The number of laser ablation pulses (NLp) was used to control the average size of the PbS-QDs and their coverage on the SWCNTs' surface. Photoconductive (PC) devices fabricated from these SWCNTs/PbS-QDs NHs have shown a significantly enhanced photoresponse, which is found to be PbS-QD size dependent. Wavelength-resolved photocurrent measurements revealed a strong photoconductivity of the NHs in the UV-visible region, which is shown to be due to multiple exciton generation (MEG) in the PbS-QDs. For the 6.5 nm-diameter PbS-QDs (with a bandgap (Eg) = 0.86 eV), the MEG contribution of the NHs based PC devices was shown to lead to a normalized internal quantum efficiency in excess of 300% for photon energies ≥4.5Eg. While the lowest MEG threshold in our NHs based PC devices is found to be of ~2.5Eg, the MEG efficiency reaches values as high as 0.9 ± 0.1.

Heteroatom doping is an efficient way to modify the chemical and electronic properties of graphene. In particular, boron doping is expected to induce a p-type (boron)-conducting behavior to pristine (nondoped) graphene, which could lead to diverse applications. However, the experimental progress on atomic scale visualization and sensing properties of large-area boron-doped graphene (BG) sheets is still very scarce. This work describes the controlled growth of centimeter size, high-crystallinity BG sheets. Scanning tunneling microscopy and spectroscopy are used to visualize the atomic structure and the local density of states around boron dopants. It is confirmed that BG behaves as a p-type conductor and a unique croissant-like feature is frequently observed within the BG lattice, which is caused by the presence of boron-carbon trimers embedded within the hexagonal lattice. More interestingly, it is demonstrated for the first time that BG exhibits unique sensing capabilities when detecting toxic gases, such as NO2 and NH3, being able to detect extremely low concentrations (e.g., parts per trillion, parts per billion). This work envisions that other attractive applications could now be explored based on as-synthesized BG.

Large-area Si-doped graphene (SiG) is controllably synthesized for the first time. A much-enhanced molecular-sensing performance is achieved when SiG is used as a probing surface. This will open up opportunities for developing high-performance sensors that are able to detect trace amounts of organic and fluorescent molecules. Furthermore, many fascinating properties predicted by theoretical calculations can be tested based on the as-synthesized SiG., (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

We have used in situ current-voltage measurements of cup-stacked carbon nanotubes (CSCNTs) to establish reversible strain induced (compressive bending) semiconducting to metallic behavior. The corresponding electrical resistance decreases by two orders of magnitude during the process, and reaches values comparable to those of highly crystalline multi-walled carbon nanotubes (MWCNTs) and graphite. Joule heating experiments on the same CSCNTs showed that the edges of individual cups merge to form "loops" induced by the heating process. The resistance of these looped CSCNTs was close to that of highly deformed CSCNTs (and crystalline MWCNTs), thus suggesting that a similar conduction mechanism took place in both cases. Using a combination of molecular dynamics and first-principles calculations based on density functional theory, we conclude that an edge-to-edge interlayer transport mechanism results in conduction channels at the compressed side of the CSCNTs due to electronic density overlap between individual cups, thus making CSCNTs more conducting. This strain-induced CSCNT semiconductor to metal transition could potentially be applied to enable functional composite materials (e.g. mechanical sensors) with enhanced and tunable conducting properties upon compression.

Electrospun biopolymer-derived nanofiber webs are promising scaffolds for growing tissue and cells. However, the webs are mechanically weak and electrically insulating. We have synthesized a polyethylene oxide (PEO) nanofiber web that is pliable, tough, and electrically conductive, by incorporating optically active, DNA-wrapped, double-walled carbon nanotubes. The nanotubes were individually trapped along the length of the PEO nanofiber and acted as mechanically reinforcing filler and an electrical conductor.

In this work, we carried out chemical oxidation studies of nitrogen-doped multiwalled carbon nanotubes (CNx-MWCNTs) using potassium permanganate in order to obtain nitrogen-doped graphene nanoribbons. Reaction parameters such as oxidation reaction, reaction time, the oxidizer to nanotube mass ratio, and the temperature were varied, and their effect was carefully analyzed. The presence of nitrogen atoms makes CNx-MWCNTs more reactive toward oxidation when compared to undoped multiwalled carbon nanotubes (MWCNTs). High-resolution transmission electron microscopy studies indicate that the oxidation of the graphitic layers within CNx-MWCNTs results in the unzipping of large diameter nanotubes and the formation of a disordered oxidized carbon coating on small diameter nanotubes. The nitrogen content within unzipped CNx-MWCNTs decreased as a function of the oxidation time, temperature, and oxidizer concentration. By controlling the degree of oxidation, the N atomic % could be reduced from 1.56% in pristine CNx-MWCNTs down to 0.31 atom % in nitrogen-doped oxidized graphene nanoribbons. A comparative thermogravimetric analysis reveals a lower thermal stability of the (unzipped) oxidized CNx-MWCNTs when compared to MWCNT samples. The oxidized graphene nanoribbons were chemically and thermally reduced and yielded nitrogen-doped graphene nanoribbons (N-GNRs). The thermal reduction at relatively low temperature (300 °C) results in graphene nanoribbons with 0.37 atom % of nitrogen. This method represents a novel route to preparation of bulk quantities of nitrogen-doped unzipped carbon nanotubes, which is able to control the doping level in the resulting reduced GNR samples. Finally, the electrochemical properties of these materials were evaluated.

We report a facile method to efficiently visualize the atomic carbon network of curved few-layered graphitic systems including folded bi-layer graphene, nanoribbon edges and multi-walled carbon nanotubes (straight and bent), via the processing of aberration-corrected high-resolution transmission electron microscopy (AC-HRTEM) images. This technique is also able to atomically resolve the structure of overlapping graphene layers with different orientations, thus enabling us to determine the stacking order of multiple graphene layers. To the best of our knowledge, we are the first to identify the stacking order of a misoriented 4-layer closed-edge graphene and a metal-semiconductor double-walled carbon nanotube junction.

The introduction of foreign atoms, such as nitrogen, into the hexagonal network of an sp(2)-hybridized carbon atom monolayer has been demonstrated and constitutes an effective tool for tailoring the intrinsic properties of graphene. Here, we report that boron atoms can be efficiently substituted for carbon in graphene. Single-layer graphene substitutionally doped with boron was prepared by the mechanical exfoliation of boron-doped graphite. X-ray photoelectron spectroscopy demonstrated that the amount of substitutional boron in graphite was ~0.22 atom %. Raman spectroscopy demonstrated that the boron atoms were spaced 4.76 nm apart in single-layer graphene. The 7-fold higher intensity of the D-band when compared to the G-band was explained by the elastically scattered photoexcited electrons by boron atoms before emitting a phonon. The frequency of the G-band in single-layer substitutionally boron-doped graphene was unchanged, which could be explained by the p-type boron doping (stiffening) counteracting the tensile strain effect of the larger carbon-boron bond length (softening). Boron-doped graphene appears to be a useful tool for engineering the physical and chemical properties of graphene.

We report a novel physicochemical route to produce highly crystalline nitrogen-doped graphene nanoribbons. The technique consists of an abrupt N(2) gas expansion within the hollow core of nitrogen-doped multiwalled carbon nanotubes (CN(x)-MWNTs) when exposed to a fast thermal shock. The multiwalled nanotube unzipping mechanism is rationalized using molecular dynamics and density functional theory simulations, which highlight the importance of open-ended nanotubes in promoting the efficient introduction of N(2) molecules by capillary action within tubes and surface defects, thus triggering an efficient and atomically smooth unzipping. The so-produced nanoribbons could be few-layered (from graphene bilayer onward) and could exhibit both crystalline zigzag and armchair edges. In contrast to methods developed previously, our technique presents various advantages: (1) the tubes are not heavily oxidized; (2) the method yields sharp atomic edges within the resulting nanoribbons; (3) the technique could be scaled up for the bulk production of crystalline nanoribbons from available MWNT sources; and (4) this route could eventually be used to unzip other types of carbon nanotubes or intercalated layered materials such as BN, MoS(2), WS(2), etc., (© 2012 American Chemical Society)

The coalescence process of fullerenes in the hollow core of single walled carbon nanotubes is systematically explored by the kinetic Monte Carlo method. Two elongation (or growth) modes via the coalescence (i) between an inner tube and fullerenes and (ii) between neighboring inner tubes are identified. It is found that the coalescence of two inner tubes mostly creates a very stable intra-tube junction which is composed of multiple pentagon-heptagon pairs. As a consequence, the study predicts that the inner tube of peapod derived double walled carbon nanotubes (DWNTs) must contain many intra-tube junctions. Careful high resolution transmission electron microscopy observation on peapod-grown DWNT sample provides experimental evidence of the presence of the junctions.