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The need for miniaturized and high-performance devices has attracted enormous attention to the development of quantum silicon nanowires. However, the preparation of abundant quantities of silicon nanowires with the effective quantum-confined dimension remains challenging. Here, we prepare highly dense and vertically aligned sub-5 nm silicon nanowires with length/diameter aspect ratios greater than 10,000 by developing a catalyst-free chemical vapor etching process. We observe an unusual lattice reduction of up to 20% within ultra-narrow silicon nanowires and good oxidation stability in air compared to conventional silicon. Moreover, the material exhibits a direct optical bandgap of 4.16 eV and quasi-particle bandgap of 4.75 eV with the large exciton binding energy of 0.59 eV, indicating the significant phonon and electronic confinement. The results may provide an opportunity to investigate the chemistry and physics of highly confined silicon quantum nanostructures and may explore their potential uses in nanoelectronics, optoelectronics, and energy systems.

Crystallographic dislocation has been well-known to be one of the major causes responsible for the unfavorable carrier dynamics in conventional semiconductor devices. Halide perovskite has exhibited promising applications in optoelectronic devices. However, how dislocation impacts its carrier dynamics in the ‘defects-tolerant’ halide perovskite is largely unknown. Here, via a remote epitaxy approach using polar substrates coated with graphene, we synthesize epitaxial halide perovskite with controlled dislocation density. First-principle calculations and molecular-dynamics simulations reveal weak film-substrate interaction and low density dislocation mechanism in remote epitaxy, respectively. High-resolution transmission electron microscopy, high-resolution atomic force microscopy and Cs-corrected scanning transmission electron microscopy unveil the lattice/atomic and dislocation structure of the remote epitaxial film. The controlling of dislocation density enables the unveiling of the dislocation-carrier dynamic relation in halide perovskite. The study provides an avenue to develop free-standing halide perovskite film with low dislocation density and improved carried dynamics., Crystallographic dislocation has proven harmful to the carrier dynamics in conventional semiconductors but it is unexplored in metal halide perovskites. Here Jiang et al. grow remote epitaxial perovskite films on graphene with density-controlled dislocations and confirm their negative impact.

Monolayer transition metal dichalcogenides (TMDs) possess superior optical properties, including the valley degree of freedom that can be accessed through the excitation light of certain helicity. While WS2 and WSe2 are known for their excellent valley polarization due to the strong spin-orbit coupling, the optical bandgap is limited by the ability to choose from only these two materials. This limitation can be overcome through the monolayer alloy semiconductor, WS2xSe2(1-x), which promises an atomically thin semiconductor with tunable bandgap. In this work, we show that the high-quality BN encapsulated monolayer WS0.6Se1.4 inherits the superior optical properties of tungsten-based TMDs, including a trion splitting of ~ 6 meV and valley polarization as high as ~60%. In particular, we demonstrate for the first time the emerging and gate-tunable interlayer electron-phonon coupling in the BN/WS0.6Se1.4/BN van der Waals heterostructure, which renders the otherwise optically silent Raman modes visible. In addition, the emerging Raman signals can be drastically enhanced by the resonant coupling to the 2s state of the monolayer WS0.6Se1.4 A exciton. The BN/WS2xSe2(1-x)/BN van der Waals heterostructure with a tunable bandgap thus provides an exciting platform for exploring the valley degree of freedom and emerging excitonic physics in two-dimension.

Nanostructures configured as triply periodic minimal surfaces, such as gyroidal systems, possess a broad range of potential applications as energy and storage materials and can be used as elementary building blocks for complex electronics. Here, we investigate the effect of structure size and material density on a number of sp 2 carbon gyroid structures. We describe a general Monte Carlo method for discovering atomic structures of carbon gyroids of arbitrary size that obey the prescribed space group symmetry for gyroid surfaces. A total of 19 structures are investigated, among which we find three types of ground state structures corresponding to a given number of atomic cycles present in the asymmetric unit cell. Each type is characterized by the distribution of non-hexagonal rings: type I structures present a minimum number of non-hexagonal rings (octagons), while type II (type III) structures include square and heptagon (pentagons and octagons) in addition to hexagons. We use density functional theory to establish how electronic, topological, geometric, and energetic properties of these gyroids vary with size and density. We determine that most studied systems feature occupied and unoccupied three-dimensional Dirac hyper-cones and metallic and semi-conducting behaviors.

Quantum transport studies are performed on doped and functionalized 8- and 11-armchair graphene nanoribbons (aGNRs) by means of density functional theory. Substitutional doping is performed by introducing boron, nitrogen, oxygen, silicon, phosphorus, and sulfur atoms within the lattice of the aGNRs. Other functional groups such as borane, amine, hydroxyl, thiol, silane, silene, phosphine, and phosphorane groups are also introduced at the nanoribbon's edge. The dopant position and the nanoribbon's width strongly influence the current–voltage characteristics, and generally, the narrow 8-aGNRs and edge-doped 11-aGNRs show deteriorated transport properties, mainly due to the formation of irregular edges that create highly localized states disrupting several conducting bands. On the other hand, the inside-doped 11-aGNRs are barely affected, mainly because these systems preserve the edge's structure, thus edge conduction bands still contribute to the electron transport. Our results suggest that wider graphene nanoribbons could be functionalized at the inner sections without significantly compromising their transport characteristics while retaining the chemical reactivity that characterize doped nanocarbons. Such characteristics are highly desirable in fuel cells where doped graphene is used as a catalyst support or as a metal-free catalyst.

The mechanical properties of four different families of ordered porous graphene or giant Schwarzites, up to 12,288 atoms per cubic cell, were studied theoretically in order to shed light on the properties of newly synthesized graphene-like foams. It is shown that as the Schwarzite grows in size, the structure becomes flatter and not only more energetically stable, but also more elastically stable, thus opening the possibility of being synthesized in the near future. The mechanical properties such as bulk modulus, Young's modulus, and Poisson's ratio have been calculated with first principles for the smaller cells and with empirical methods for the larger cells. The bulk and Young moduli decrease as the structures grow. The “P” and the “I-WP” geometries favor smaller values of Poisson's ratio, likely to be synthesized experimentally. For the larger gyroid “G” and “D” cases, elastic instabilities appear, and these can be alleviated by breaking the symmetry of the associated space group. In addition, ripples in the graphene sheet stabilize the giant “D” family as the crystal cell dimensions increase. Finally, based on density functional theory calculations, the electronic properties of the high genus I-WP were examined for the first time finding semiconducting, semimetallic, and metallic behaviors.

In ReS$_2$ a layer-independent direct band-gap of 1.5 eV implies a potential for its use in optoelectronic applications. ReS$_2$ crystallizes in the 1T$^{\prime}$-structure which leads to anisotropic physical properties and whose concomitant electronic structure might host a non-trivial topology. Here, we report an overall evaluation of the anisotropic Raman response and the transport properties of few-layered ReS$_2$ field-effect transistors. We find that ReS$_2$ exfoliated on SiO$_2$ behaves as an $n$-type semiconductor with an intrinsic carrier mobility surpassing $��_i$ ~30 cm$^2$/Vs at $T = 300$ K which increases up to ~350 cm$^2$/Vs at 2 K. Semiconducting behavior is observed at low electron densities $n$, but at high values of n the resistivity decreases by a factor > 7 upon cooling to 2 K and displays a metallic $T^2$-dependence. This indicates that the band structure of 1T$^{\prime}$-ReS$_2$ is quite susceptible to an electric field applied perpendicularly to the layers. The electric-field induced metallic state observed in transition metal dichalcogenides was recently claimed to result from a percolation type of transition. Instead, through a scaling analysis of the conductivity as a function of $T$ and $n$, we find that the metallic state of ReS$_2$ results from a second-order metal to insulator transition driven by electronic correlations. This gate-induced metallic state offers an alternative to phase engineering for producing ohmic contacts and metallic interconnects in devices based on transition metal dichalcogenides., 25 pages, 5 figures

An efficient method for anchoring silver nanoparticles (Ag-NPs) on the surface of nitrogendoped multi-walled carbon nanotubes (CNx-MWCNTs) is reported. The process involves the attachment of biotin ...

Zigzag edges of monolayer MoS2 and other transition-metal (TM) dichalcogenides are experimentally shown to exhibit strong photoluminescence. Atomic models that have been proposed for these edges, however, are all metallic. Here, we address this puzzle by using first-principles calculations. We found that a more generic electron counting model (ECM) can be developed, which, when coupled with the ability of TM atoms at edges to change their valency from 4+ to 5+, can quantitatively account for the band gap opening at the zigzag edges. Due to the ECM, a 3× periodicity along the zigzag edge is necessary to open the band gap. Moreover, consistent with experiment, oxygen adsorption is shown to open even larger band gaps than intrinsic edges.

Quantum molecular dynamics (QMD) and density functional theory are employed in this work in order to study the structural and electronic properties of carbon, boron nitride or hybrid BNC nanoshells. The studied nanoshells can be formed by stacking two zigzag graphene nanoribbons, two zigzag boron nitride nanoribbons or one zigzag graphene nanoribbon on a boron nitride nanoribbon. In all cases only one of the edges of the ribbon is passivated, while the other one is left unpassivated. Our QMD results show that these nanoribbons collapse just a few femtoseconds after the beginning of the simulation, forming a coalesced structure in the shape of a shell. Our band structure calculations revealed that this structures may be metallic or semiconductor, depending on its stoichiometry. Furthermore, a spin splitting for energies near the Fermi level is predicted for both the pure carbon and the hybrid BNC-nanoshell systems. We further show that when a transverse electric field is applied to these systems, the nanoshell structure tends to open up. This effect can lead to the application of these nanoshells for molecular storage. As a proof of concept, We investigate this storage effect for the H2 molecule.

Crystallographic dislocation has been well-known to be one of the major causes responsible for the unfavorable carrier dynamics in conventional semiconductor devices. Halide perovskite has exhibited promising applications in optoelectronic devices. However, how dislocation impacts its carrier dynamics in the 'defects-tolerant' halide perovskite is largely unknown. Here, via a remote epitaxy approach using polar substrates coated with graphene, we synthesize epitaxial halide perovskite with controlled dislocation density. First-principle calculations and molecular-dynamics simulations reveal weak film-substrate interaction and low density dislocation mechanism in remote epitaxy, respectively. High-resolution transmission electron microscopy, high-resolution atomic force microscopy and Cs-corrected scanning transmission electron microscopy unveil the lattice/atomic and dislocation structure of the remote epitaxial film. The controlling of dislocation density enables the unveiling of the dislocation-carrier dynamic relation in halide perovskite. The study provides an avenue to develop free-standing halide perovskite film with low dislocation density and improved carried dynamics.

High-quality SiOx nanoflowers have been generated by heating mixed SiC/Co powder films in a CO atmosphere. The structural features associated with different stages of the growth were monitored by SEM, TEM, EDX, and XRD analysis. A plausible mechanism for the formation of exquisite flowerlike structures is proposed in which SiC reacts with CO, the Co particles acting as a catalytic template, to yield amorphous SiOx branches. Numerous loops and branches were also observed which may arise either as a result of bifurcation or coalescence. Use of Co2O3 instead of Co as catalyst leads to abnormal flowers exhibiting secondary growth.

Transport and field-emission properties of assynthesized CNx and BNCx (x < 0.1) multi-walled nanotubes were compared in detail. Individual ropes made of these nanotubes and macrofilms of those were tested. Before measurements, the nanotubes were thoroughly characterized using high-resolution and energy-filtered electron microscopy, electron diffraction and electron-energy-loss spectroscopy. Individual ropes composed of dozens of CNx nanotubes displayed well-defined metallic behavior and low resistivities of ∼ 10-100 kΩ or less at room temperature, whereas those made of BNCx nanotubes exhibited semiconducting properties and high resistivities of ∼ 50-300 MΩ. Both types of ropes revealed good field-emission properties with emitting currents per rope reaching ∼ 4 μA (CNx) and ∼ 2 μA (BNCx), albeit the latter ropes severely deteriorated during the field emission. Macrofilms made of randomly oriented CNx or BNCx nanotubes displayed low and similar turn-on fields of ∼ 2-3 V/μm. 3 mA/cm2 (BNCx) and 5.5 mA/cm2 (CNx) current densities were reached at 5.5 V/μm macroscopic fields. At a current density of 0.2-0.4 mA/cm2 both types of compound nanotubes exhibited equally good emission stability over tens of minutes; by contrast, on increasing the current density to 0.2-0.4 A/cm2, only CNx films continued to emit steadily, while the field emission from BNCx nanotube films was prone to fast degradation within several tens of seconds, likely due to arcing and/or resistive heating.

C3) are heated to 1000–1100 °C for 2–4 hr in an argon flow. Carbon nanotubes are formed when metal powders, e.g. Fe, Co, Y and Ni, are also present.

In this work, strong third order nonlinear optical properties exhibited by WS2 layers are presented. Optical Kerr effect was identified as the dominant physical mechanism responsible for these third order optical nonlinearities. An extraordinary nonlinear refractive index together with an important contribution of a saturated absorptive response was observed to depend on the atomic layer stacking. Comparative experiments performed in mono- and few-layer samples of WS2 revealed that this material is potentially capable of modulating nonlinear optical processes by selective near resonant induced birefringence. In conclusion, we envision applications for developing all-optical bidimensional nonlinear optical devices.

Resonant Raman spectroscopy is a powerful tool for providing information about excitons and exciton-phonon coupling in two-dimensional materials. We present here resonant Raman experiments of single-layered WS2 and WSe2 using more than 25 laser lines. The Raman excitation profiles of both materials show unexpected differences. All Raman features of WS2 monolayers are enhanced by the first-optical excitations (with an asymmetric response for the spin-orbit related XA and XB excitons), whereas Raman bands of WSe2 are not enhanced at XA/B energies. Such an intriguing phenomenon is addressed by DFT calculations and by solving the Bethe-Salpeter equation. These two materials are very similar. They prefer the same crystal arrangement, and their electronic structure is akin, with comparable spin-orbit coupling. However, we reveal that WS2 and WSe2 exhibit quite different exciton-phonon interactions. In this sense, we demonstrate that the interaction between XC and XA excitons with phonons explains the different Raman responses of WS2 and WSe2, and the absence of Raman enhancement for the WSe2 modes at XA/B energies. These results reveal unusual exciton-phonon interactions and open new avenues for understanding the two-dimensional materials physics, where weak interactions play a key role coupling different degrees of freedom (spin, optic, and electronic).

We report the use of transition metal nanoparticles (Ni or Co) to longitudinally cut open multiwalled carbon nanotubes in order to create graphitic nanoribbons. The process consists of catalytic hydrogenation of carbon, in which the metal particles cut sp(2) hybridized carbon atoms along nanotubes that results in the liberation of hydrocarbon species. Observations reveal the presence of unzipped nanotubes that were cut by the nanoparticles. We also report the presence of partially open carbon nanotubes, which have been predicted to have novel magnetoresistance properties.(1) The nanoribbons produced are typically 15-40 nm wide and 100-500 nm long. This method offers an alternative approach for making graphene nanoribbons, compared to the chemical methods reported recently in the literature.

A combination of theoretical techniques, high-resolution microscopy, and energy-dispersive X-ray spectroscopy shows the role sulfur plays in branching phenomena during carbon nanotube (CNT) network growth. A model is proposed in which small amounts of sulfur are enough to trigger the growth of a bud in a CNT, leading to kink formation and subsequent branch growth.

The radial distribution function (RDF) of different single-walled carbon nanotubes (SWCNTs) has been calculated showing characteristic patterns. Features related to diameter, chirality and doping in these nanostructures are analyzed. We also investigate the induced chirality of two samples, prepared with the chemical vapor deposition (CVD) method and the high-pressure Co conversion (HIPCo) procedure, respectively, by comparing the experimental RDF signal obtained from the neutron scattering technique with the RDFs of our simulations. We found armchair and zigzag nanotubes in the sample, but we cannot discard the presence of other chiralities. We believe that this is a useful method to study chiralities induced by different methods of production. Data up to Q = 30 A − 1 were used in the Fourier transform, giving a high real space resolution of Δ r ∼ 0.2 A.

High pressure/temperature was applied on samples of pristine multi-walled carbon nanotubes (MWCNT), functionalized nanotubes (f-MWCNT), and nanotubes doped with nitrogen (CN x MWNT). Cylindrical compact pellets of f-MWCNT with diameters of about 6 mm were obtained under pressure of 4.0 GPa at room temperature and at 400 °C, using graphite as pressure transmitting medium. The best pellet samples were produced using nitric and sulfuric acids for the functionalization of MWCNT. The effect of high pressure/temperature on CNT was investigated by several spectroscopy and characterization techniques, such as Raman spectroscopy, X-ray powder diffraction, X-ray photoelectron spectroscopy, N2 adsorption/desorption isotherms, and transmission electron microscopy. It was found that MWCNT maintain their main features in the compacted pellets, such as integrity, original morphology, and structure, demonstrating that high-pressure/temperature compaction can indeed be used to fabricate novel CNT self-supported materials. Additionally, the specific surface area and porosity are unchanged, which is important when using bulk CNT in adsorption processes. Raman analysis of the G’-band showed a shift to lower wavenumbers when f-MWCNT were processed under high pressure, suggesting that CNT are under tensile stress.

It is a challenge to fabricate graphene bulk materials with properties arising from the nature of individual graphene sheets, and which assemble into monolithic three-dimensional structures. Here we report the scalable self-assembly of randomly oriented graphene sheets into additive-free, essentially homogenous graphene sponge materials that provide a combination of both cork-like and rubber-like properties. These graphene sponges, with densities similar to air, display Poisson's ratios in all directions that are near-zero and largely strain-independent during reversible compression to giant strains. And at the same time, they function as enthalpic rubbers, which can recover up to 98% compression in air and 90% in liquids, and operate between -196 and 900 °C. Furthermore, these sponges provide reversible liquid absorption for hundreds of cycles and then discharge it within seconds, while still providing an effective near-zero Poisson's ratio.

We show extensive theoretical studies related to the generation and characterization of 2D and 3D ordered networks using 1D units that are connected covalently. We experimentally created multi-terminal junctions containing 1D carbon blocks in order to study the most common morphologies and branched structures that could be used in the theoretical design of network models. We found that the mechanical and electronic characteristics of ordered networks based on carbon nanotubes (ON-CNTs) are dominated by their specific super-architecture (hexagonal, cubic, square, and diamond-type). We show that charges follow specific paths through the nodes of the multi-terminal systems, which could result in complex integrated nanoelectronic circuits. The 3D architectures reveal their ability to support extremely high unidirectional stress when their mechanical properties are studied. In addition, these networks are shown to perform better than standard carbon aerogels because of their low mass densities, continuous porosities, and high surface areas.

A compilation and an implementation of different methodologies to simulate NPT ensembles on finite systems is presented. In general, the methods discussed can be classified in two different groups depending on how the external pressure is applied to the system. The first approach is based on including the pressure with its conjugate thermodynamical variable, the volume, in the Lagrangian of the system. For this group four different volume definitions were considered and we assess their validity by studying the structural properties of small systems as function of pressure. In particular, we focus on the stability of the C 60 molecule as well as the amorphization process of a diamond-like cluster under pressure. In the second group, the finite system (C 60 ) is embedded in a classical fluid which serves as a pressure reservoir. We take the latter method as reference because it is closest to the experimental situation. The difficulties and the regimes where these methods can be used are also discussed.

We demonstrate that it is possible to encapsulate ferromagnetic nanowires of Fe, FeCo and FeNi inside carbon nanotubes via chemical vapor deposition methods. These wires exhibit extremely high coercive fields when compared with the bulk phases. We review the state-of-the art characterization carried out on these novel wires and discuss the importance of having aligned arrays of carbon nanotubes filled with ferromagnetic materials, towards the development of novel magnetic storage devices. In this context, we will show from the experimental and theoretical stand points, that the wire shape, aspect ratio and inter-wire distances play a crucial role in the fabrication of novel storage components. In addition, we theoretically show that pure carbon nanostructures such as carbon nanotori, perforated fullerenes and nanoporous graphitic structures, exhibiting negative Gaussian curvature introduced by the presence of non-hexagonal rings, behave as strong paramagnets experiencing large magnetic moments when an external magnetic field is applied. The latter results could explain some of the magnetic properties observed experimentally in carbon nanofoams and polymerized C 60 phases. We envisage that magnetism in different families of nanostructures will be playing a key role in the development of emerging technologies in the present century.

All self-assembled nanostructures, like carbon nanotubes, exhibit structural imperfections that affect their electronic and mechanical properties and constitute a serious problem for the development of novel electronic nanodevices. Very common defects in nanotubes are pentagon-heptagon pairs, in which the replacement of four hexagons by two pentagons and two heptagons disrupts the perfect hexagonal lattice. In this work, we demonstrate that these defects can be eliminated efficiently with the help of femtosecond laser pulses. By performing nonadiabatic molecular dynamics simulations, we show that in the laser-induced electronic nonequilibrium the pentagon-heptagon pair is transformed back into four hexagons without producing any irreversible damage to the rest of the nanotube.

We demonstrate that carbon nanotori constructed by either coalescing C60 molecules along the 5-fold axis (incorporating pentagons and octagons) or by joining the ends of Haeckelite tubes (containing heptagons, hexagons, and pentagons) exhibit large magnetic moments when an external magnetic field is applied. In particular, we have used a π-orbital nearest-neighbor tight-binding Hamiltonian with the London approximation in order to study the influence of uniform external magnetic fields on various types of torous-like carbon nanostructures with negative and positive Gaussian curvature. We have calculated the ring currents and the induced magnetic moments on these structures. The results reveal the presence of an unexpected magnetic response, which is due to the presence of nonhexagonal carbon rings. Our results could also explain the existence of ferromagnetic nanocarbons. We envisage that coalesced peapods may exhibit unusual magnetic properties.

Here, we demonstrate that by coalescing double-walled carbon nanotubes (DWNTs), a novel and stable structure consisting of flattened tubules containing two single-walled tubes (SWNTs) is created. The process occurs due to the coalescence and reconstruction of the outer shells of DWNTs, leaving the inner cylinders almost intact, the latter being encapsulated inside the large diameter coalesced tube (bicable). We propose that the coalescence process is due to: (1) the thermal activity of the outer shells, which is driven by a surface energy minimization process, and (2) a zipping mechanism followed by atom reconstruction, in which two outer shells interact and anneal.

First-principles (FP) calculations of the electronic and vibrational properties of three different Haeckelite structures have been performed. The relatively low cohesive energies (when compared to C-60) of these phases suggest the possible synthesis of such novel carbon arrangements. In agreement with previous tight-binding calculations (Terrones, H.; Terrones, M.; Hernandez, E.; Grobert, N.; Charlier, J.-C.; Ajayan, P. M. Phys. Rev. Lett. 2000, 84, 1716), the Haeckelite structures exhibit a clear metallic behavior. In addition, within the ab initio framework, we predict the IR and Raman frequencies, which constitute the fingerprint of their structure and allow for their unambiguous identification. STM images and quantum conductances of various tubular Haeckelite structures are also calculated within a tight-binding framework. The three investigated Haeckelite structures are shown to be good candidates of conducting wires with great potential in nanoelectronics. The results presented here provide a catalog of properties that will aid in the identification of other Haeckelite structures as well as carbon systems containing pentagonal and heptagonal defects.

A fascinating structural transformation occurring inside single-walled carbon nanotubes (SWNTs) is the fullerene coalescence, which is responsible for forming stable zeppelinlike carbon molecules. We report in situ transmission electron microscope (TEM) observations revealing sequences of fullerene coalescence induced by electron irradiation on pristine nanotube peapods, together with extensive theoretical investigations of the microscopic mechanism underlying this process. TEM images indicate that the merging of fullerenes results in stable but corrugated tubules (5 to 7 Angstrom in diameter) confined within SWNTs. These observations have been confirmed using a combination of theoretical approaches based on molecular dynamics, empirical potentials, tight-binding methods, Monte Carlo techniques, and first principles calculations. We have fully elucidated the coalescence mechanism of fullerenes inside SWNTs under electron irradiation and thermal annealing. The process occurs via the polymerization Of C-60 molecules followed by surface reconstruction, which can be triggered either by the formation of vacancies (created under electron irradiation) or by surface-energy minimization activated by thermal annealing. These novel tubular forms of carbon contain hexagons, pentagons, heptagons, and octagons. The stability, electronic properties, and electron conductance of the novel tubules are strongly affected by the final geometry of the coalesced fullerene complex. The possibility of forming highly conducting and semiconducting tubular structures suggests new avenues in designing carbon nanowires with specific electronic characteristics.

Systematic studies of structural changes in stacked cup carbon nanofibers by heat treatment from 1800 to 3000 °C are carried out. The most prominent feature upon heat treatment of these nanofibers is the formation of energetically stable loops between adjacent active end planes both on the inner and outer surfaces. The appearance of the jagged outer and inner surfaces at 3000 °C is due to a combinational effect, perhaps caused by improved stacking within domains connected by loops having limited crystallite size, accompanied by structural reorganization between domains. Consequently, a low degree of graphitizability is ascribed to this unusual stacked cup morphology of pristine carbon nanofibers.

Two-dimensional molybdenum disulfide (MoS2) is a promising material for optoelectronic devices due to its strong photoluminescence emission. In this work, the photoluminescence of twisted bilayer MoS2 is investigated, revealing a tunability of the interlayer coupling of bilayer MoS2. It is found that the photoluminescence intensity ratio of the trion and exciton reaches its maximum value for the twisted angle 0° or 60°, while for the twisted angle 30° or 90° the situation is the opposite. This is mainly attributed to the change of the trion binding energy. The first-principles density functional theory analysis further confirms the change of the interlayer coupling with the twisted angle, which interprets our experimental results.

Layer-by-layer stacking or lateral interfacing of atomic monolayers has opened up unprecedented opportunities to engineer two-dimensional heteromaterials. Fabrication of such artificial heterostructures with atomically clean and sharp interfaces, however, is challenging. Here, we report a one-step growth strategy for the creation of high-quality vertically stacked as well as in-plane interconnected heterostructures of WS2/MoS2 via control of the growth temperature. Vertically stacked bilayers with WS2 epitaxially grown on top of the MoS2 monolayer are formed with preferred stacking order at high temperature. A strong interlayer excitonic transition is observed due to the type II band alignment and to the clean interface of these bilayers. Vapour growth at low temperature, on the other hand, leads to lateral epitaxy of WS2 on MoS2 edges, creating seamless and atomically sharp in-plane heterostructures that generate strong localized photoluminescence enhancement and intrinsic p-n junctions. The fabrication of heterostructures from monolayers, using simple and scalable growth, paves the way for the creation of unprecedented two-dimensional materials with exciting properties.

Ternary two-dimensional dichalcogenide alloys exhibit compositionally modulated electronic structure, and hence, control of dopant concentration within each individual layer of these compounds provides a powerful tool to efficiently modify their physical and chemical properties. The main challenge arises when quantifying and locating the dopant atoms within each layer in order to better understand and fine-tune the desired properties. Here we report the synthesis of molybdenum disulfide substitutionally doped with a broad range of selenium concentrations, resulting in over 10% optical band gap modulations in atomic layers. Chemical analysis using Z-contrast imaging provides direct maps of the dopant atom distribution in individual MoS2 layers and hence a measure of the local optical band gaps. Furthermore, in a bilayer structure, the dopant distribution is imaged layer-by-layer. This. work demonstrates that each layer in the bilayer system contains similar local Se concentrations, randomly distributed, providing new insights into the growth mechanism and alloying behavior in two-dimensional dichalcogenide atomic layers. The results show that growth of uniform, ternary, two-dimensional dichalcogenide alloy films with tunable electronic properties is feasible.

Mixed-phase WxMoyCzS2 nanotubes are generated when N2/H2S is passed over a preoxidized mixture of WC and MO2C, maintained at an elevated temperature.

Using a simplified in-situ process, we have successfully generated relatively long IF WS2 nanotubes, in good yield and purity. The nanotubes are formed directly from WOx nanorods created from W and SiO2 during the first stage. Structural similarities and differences between IF nanotubes and carbon nanotubes have been analyzed. A noteworthy feature of the WS2 tubes is that some of their tips are fully open.

Electronic properties of WS 2 nanotubes have been studied using the Density Functional Based Tight Binding method (DFTB). We show that WS 2 nanotubes are all semiconducting possessing a non zero moderate gap. Only for zig-zag ( n , 0) nanotubes the band gap tends to vanish for very small tube diameters ( n , n ) tubes exhibit an indirect gap of similar size as the direct gap in ( n , 0) nanotubes of comparable diameter.

Closed fullerenes and nanotubes exhibiting high topologies from genus 5 to genus 21 are proposed. These structures (holey-balls and holey-tubes) possess only hexagonal and heptagonal (negative Gaussian curvature) rings of carbon with no dangling bonds and no pentagonal rings (no positive Gaussian curvature). Holey-balls and holey-tubes can be seen as finite zeolites since they present interconnected channels suitable for locating guest molecules or atoms such as alkali metals. The geometry, stability and possible applications are studied.

We propose that the carbon particle structures which are transformed by ejection of pentagonal and neighbouring carborn rings, under high electron irradiation, are flexible and can be moulded via energy minimization into quasi-spherical arrangements. In our model, additional pentagonal and heptagonal rings are required to generate the stable quasi-spherical onion-like structures, which can be symmetric (crystalline) or non-symmetric (amorphous). The possibility to generate higher genus fullerene-like structures by introducing additional heptagonal rings on the outer shell of the quasi-spherical onion is also discussed.

Individual monolayers of metal dichalcogenides are atomically thin two-dimensional crystals with attractive physical properties different from their bulk layered counterpart. Here we describe the direct synthesis of WS2 monolayers with triangular morphologies and strong room-temperature photoluminescence (PL). Bulk WS2 does not present PL due to its indirect band gap nature. The edges of these monolayers exhibit PL signals with extraordinary intensity, around 25 times stronger than the platelets center. The structure and composition of the platelet edges appear to be critical for the PL enhancement effect. Electron diffraction revealed that platelets present zigzag edges, while first-principles calculations indicate that sulfur-rich zigzag WS2 edges possess metallic edge states, which might tailor the optical response reported here. These novel 2D nanoscale light sources could find diverse applications including the fabrication of flexible/transparent/low-energy optoelectronic devices.

Two-dimensional transition metal dichalcogenides (TMDs), an emerging family of layered materials, have provided researchers a fertile ground for harvesting fundamental science and emergent applications. TMDs can contain a number of different structural defects in their crystal lattices which significantly alter their physico-chemical properties. Having structural defects can be either detrimental or beneficial, depending on the targeted application. Therefore, a comprehensive understanding of structural defects is required. Here we review different defects in semiconducting TMDs by summarizing: (i) the dimensionalities and atomic structures of defects; (ii) the pathways to generating structural defects during and after synthesis and, (iii) the effects of having defects on the physico-chemical properties and applications of TMDs. Thus far, significant progress has been made, although we are probably still witnessing the tip of the iceberg. A better understanding and control of defects is important in order to move forward the field of Defect Engineering in TMDs. Finally, we also provide our perspective on the challenges and opportunities in this emerging field.

First-principles calculations are employed in the study of the electronic and quantum transport properties of hexagonally shaped boron nitride (h-BN) clusters embedded in either zigzag or armchair graphene nanoribbons. Chemical doping of the h-BN cluster was taken into consideration by using carbon atoms to replace either the boron (B27N24C3) or the nitrogen (B27N24C3) sites in the central ring. While the quantum conductance of the system with zigzag edges is found to be spin-dependent, it was observed that the system with an armchair edge requires an electron imbalance in order to show a spin-dependent conductance. Furthermore, the possibility of molecular adsorption onto these doped systems is studied. The effects of the attached molecules to the quantum conductance shows the potential of these hybrid systems for molecular sensing applications.

A method based on the decomposition of phenylacetylene and thiophene on a Ni-powder catalyst has been used to produce nanotubes, circular hollow structures, and other interesting graphitic morphologies. The structures obtained are analyzed using HREM. The stability of bent graphite sheets is studied.