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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

Electron transport through iron porphyrin (FeP) molecules self-assembled on a gold (Au) substrate was investigated using conductive atomic force microscopy (AFM) to measure current–voltage (I–V) ch...

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.

Atmospheric pressure chemical vapor deposition (APCVD) is employed for the synthesis of layered vanadium disulfide. By tuning several critical growth parameters, we achieve VS2 flakes with lateral ...

Black phosphorus (b-P) is an allotrope of phosphorus whose properties have attracted great attention. In contrast to other 2D compounds, or pristine b-P, the properties of b-P alloys have yet to be explored. In this report, we present a detailed study on the Raman spectra and on the temperature dependence of the electrical transport properties of As-doped black phosphorus (b-AsP) for an As fraction x = 0.25. The observed complex Raman spectra were interpreted with the support of Density Functional Theory (DFT) calculations since each original mode splits in three due to P-P, P-As, and As-As bonds. Field-effect transistors (FET) fabricated from few-layered b-AsP exfoliated onto Si/SiO2 substrates exhibit hole-doped like conduction with a room temperature ON/OFF current ratio of ∼103 and an intrinsic field-effect mobility approaching ∼300 cm2 V-1 s-1 at 300 K which increases up to 600 cm2 V-1 s-1 at 100 K when measured via a 4-terminal method. Remarkably, these values are comparable to, or higher, than those initially reported for pristine b-P, indicating that this level of As doping is not detrimental to its transport properties. The ON to OFF current ratio is observed to increase up to 105 at 4 K. At high gate voltages b-AsP displays metallic behavior with the resistivity decreasing with decreasing temperature and saturating below T ∼100 K, indicating a gate-induced insulator to metal transition. Similarly to pristine b-P, its transport properties reveal a high anisotropy between armchair (AC) and zig-zag (ZZ) directions. Electronic band structure computed through periodic dispersion-corrected hybrid Density Functional Theory (DFT) indicate close proximity between the Fermi level and the top of the valence band(s) thus explaining its hole doped character. Our study shows that b-AsP has potential for optoelectronics applications that benefit from its anisotropic character and the ability to tune its band gap as a function of the number of layers and As content.

One primary concern in diluted magnetic semiconductors (DMSs) is how to establish a long-range magnetic order with a low magnetic doping concentration to maintain the gate tunability of the host semiconductor, as well as to increase Curie temperature. Two-dimensional van der Waals semiconductors have been recently investigated to demonstrate the magnetic order in DMSs; however, a comprehensive understanding of the mechanism responsible for the gate-tunable long-range magnetic order in DMSs has not been achieved yet. Here, we introduce a monolayer tungsten diselenide (WSe2) semiconductor with V dopants to demonstrate the long-range magnetic order through itinerant spin-polarized holes. The V atoms are sparsely located in the host lattice by substituting W atoms, which is confirmed by scanning tunneling microscopy and high-resolution transmission electron microscopy. The V impurity states and the valence band edge states are overlapped, which is congruent with density functional theory calculations. The field-effect transistor characteristics reveal the itinerant holes within the hybridized band; this clearly resembles the Zener model. Our study gives an insight into the mechanism of the long-range magnetic order in V-doped WSe2, which can also be used for other magnetically doped semiconducting transition metal dichalcogenides., 19 pages, 4 figures

We describe a flexible biosensor, based on Mn-doped MoS2, for low-level detection of dopamine in sweat and serum solution., 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 MoS2 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 (MntopMo) and Mn substituting a Mo atom (MnMo). At low dopamine concentrations, physisorption on MnMo dominates. At higher concentrations, dopamine chemisorbs on MntopMo, which is consistent with calculations of the dopamine binding energy (2.91 eV for MntopMo versus 0.65 eV for MnMo). Our results demonstrate that metal-doped layered materials, such as TMDs, constitute an emergent platform to construct ultrasensitive and tunable biosensors.

The vertical integration of atomically thin-layered materials to create van der Waals heterostructures (vdWHs) has been proposed as a method to design nanostructures with emergent properties. In this work, epitaxial Bi

First-principles density functional theory with the time-dependent Bethe-Salpeter Equation (BSE) is used to calculate the second harmonic generation (SHG) spectra of five trigonal prismatic ($2H$-phase) transition metal dichalcogenide (TMD) monolayers, $\mathrm{Mo}{\mathrm{S}}_{2}, \mathrm{MoS}{\mathrm{e}}_{2}, \mathrm{W}{\mathrm{S}}_{2}, \mathrm{WS}{\mathrm{e}}_{2}$, and $\mathrm{MoT}{\mathrm{e}}_{2}$, as a function of biaxial and uniaxial strain. It is shown that it is important to take excitonic effects into account when studying the effects of strain on SHG. All the TMDs exhibit very strong SHG, with hexagonal monolayer $\mathrm{MoT}{\mathrm{e}}_{2}$ dominating over the other TMDs in the 0.5--1.4 eV region. These results are shown to agree with experimental data. While $\mathrm{W}{\mathrm{S}}_{2}$ appears to be a good candidate for strain-tolerant applications because the SHG is insensitive to strain, $\mathrm{WS}{\mathrm{e}}_{2}$ exhibits a strong, consistent strain response that points to potential strain-sensitive applications. Distinct subpeaks of the $C$ exciton are observed which, when compared with the electronic structure, are attributed to specific transitions in the Brillouin zone, with the most important contributions coming from the $K({v}_{1}{\ensuremath{\rightarrow}}_{}{c}_{2}),Q({v}_{1}{\ensuremath{\rightarrow}}_{}{c}_{1}),$ and $\mathrm{\ensuremath{\Gamma}}({v}_{1}\ensuremath{\rightarrow}{c}_{1})$ points. These findings open avenues of exploration for potential applications of these materials in strain-sensitive devices and improve our understanding of the $C$ exciton in TMDs.

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

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.

Herein an activated carbon (AC) with high surface area and microporosity was synthesized from the starch-rich mesocarp of the babassu coconut, an abundant biomass from north and northeastern Brazil. The synthesis of AC was realized by chemical activation using potassium hydroxide (KOH) coupled with pyrolysis at 750 °C and the produced material was further characterized by scanning (SEM) and transmission electron (TEM) microscopy, Fourier transform infrared (FTIR) and Raman spectroscopy, X-ray diffraction (XRD) and thermogravimetric analysis. SEM and TEM showed the formation of a thin-layer porous morphology of AC; whereas the nitrogen (N2) and carbon dioxide (CO2) adsorption experiments identified a high surface area and microporosity. Raman spectra obtained by various laser lines revealed that AC has graphite-like microstructures with characteristic bands with features dependent on laser excitation energy. The amorphous nature from AC was further proven by XRD whereas FTIR showed the presence of surface-active oxygen functional groups. This AC material, produced from Brazillian Biomass showed a great potential as a CO2 sensor using a optical technique, In-situ Raman spectroscopy and their D and G vibrational modes shifting in the presence of CO2.

We report a detailed investigation on Raman spectroscopy in vapor-phase chalcogenization grown, high-quality single-crystal atomically thin molybdenum diselenide samples. Measurements were performed in samples with four different incident laser excitation energies ranging from 1.95 eV ⩽ Eex ⩽ 2.71 eV, revealing rich spectral information in samples ranging from N = 1-4 layers and a thick, bulk sample. In addition to previously observed (and identified) peaks, we specifically investigate the origin of a peak near ω ≈ 250 cm-1. Our density functional theory and Bethe-Salpeter calculations suggest that this peak arises from a double-resonant Raman process involving the ZA acoustic phonon perpendicular to the layer. This mode appears prominently in freshly prepared samples and disappears in aged samples, thereby offering a method for ascertaining the high optoelectronic quality of freshly prepared 2D-MoSe2 crystals. We further present an in-depth investigation of the energy-dependent variation of the position of this and other peaks and provide evidence of C-exciton-phonon coupling in monolayer MoSe2. Finally, we show how the signature peak positions and intensities vary as a function of layer thickness in these samples.

We study the second-order nonlinear optical properties of several 2D materials through second harmonic generation (SHG) and sum frequency generation (SFG). SHG signals from 2D transition metal dichalcogenides (TMD) pumped at multiple fundamental wavelengths are measured and compared with theoretical analysis. We also use polarization-resolved second harmonic generation to characterize 2D materials and explore their biological applications. Using a narrow-band femtosecond laser beam and a supercontinuum, we measure the SFG of TMDs to characterize their second-order nonlinear susceptibility over a range of wavelengths.

Nitrogen-doped multiwalled carbon nanotubes (CNx-MWNTs) have been decorated with γ-Al 2 O 3 nanoparticles by a novel method. This process involved a wet chemical approach in conjunction with thermal treatment. During the particle anchoring process, individual CNx-MWNT nanotubes agglomerated into bundles, resulting in arrays of aligned CNx-MWNT coated with γ-Al 2 O 3 . Extensive characterization of the resulting γ-Al 2 O 3 /CNx-MWNT bundles was performed using a range of electron microscopy imaging and microanalytical techniques. A possible mechanism explaining the nanobundle alignment is described, and possible applications of these materials for the fabrication of ceramic composites using CNx-MWNTs are briefly discussed.

This manuscript reports temperature- and power-dependence of in-plane E 1 2g and out-of-plane A 1g Raman modes in a continuous WS 2 monolayer film (CWMF) prepared using low pressure chemical vapor deposition (LPCVD), suspended on a perforated substrate. The frequencies of these two phonon modes vary linearly with temperature (77–523 K) and power (2–14 μW), and the modes soften as local temperature increases. The first-order temperature coefficients for E 1 2g and A 1g modes are 0.0124 and 0.0112 cm −1 /K, respectively, and power coefficients are 0.1228 and 0.1381 cm −1 /μW. Using the 1D Balandin’s approach, we have calculated the thermal conductivity of the suspended CWMF at room temperature to be ca. 20 and 16 W/mK when considering the E 1 2g and A 1g modes, respectively. Our results provide thermal properties values of CWMF, which are very important for developing monolayer WS 2 electronic devices.

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.

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. Although WS

Two-dimensional transition metal dichalcogenides (TMD), such as WS2 and MoS2, have been shown to exhibit large second order optical nonlinearity due to their non-centrosymmetric crystalline symmetry in few odd- and mono-layers, and resonance enhancement. Here we study the second-order nonlinear susceptibility of 2D TMDs through second harmonic generation (SHG) and sum frequency generation (SFG). Using a wavelength-tunable femtosecond laser, we can characterize SHG of TMDs to obtain the second-order nonlinear susceptibility at multiple wavelengths. Along with the experimental studies, theoretical investigation of the second-order nonlinear susceptibility is also performed. With this we explore the estimation of the second-order nonlinear susceptibility of 2D TMD layered materials based on their first-order susceptibility through the experimental and theoretical verification of Miller’s Rule for these materials. Additionally, we characterize the second-order nonlinear susceptibility of 2D TMD alloys through the SFG spectroscopy.

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

The isolation of graphene in 2004 from graphite was a defining moment for the "birth" of a field: two-dimensional (2D) materials. In recent years, there has been a rapidly increasing number of papers focusing on non-graphene layered materials, including transition-metal dichalcogenides (TMDs), because of the new properties and applications that emerge upon 2D confinement. Here, we review significant recent advances and important new developments in 2D materials "beyond graphene". We provide insight into the theoretical modeling and understanding of the van der Waals (vdW) forces that hold together the 2D layers in bulk solids, as well as their excitonic properties and growth morphologies. Additionally, we highlight recent breakthroughs in TMD synthesis and characterization and discuss the newest families of 2D materials, including monoelement 2D materials (i.e., silicene, phosphorene, etc.) and transition metal carbide- and carbon nitride-based MXenes. We then discuss the doping and functionalization of 2D materials beyond graphene that enable device applications, followed by advances in electronic, optoelectronic, and magnetic devices and theory. Finally, we provide perspectives on the future of 2D materials beyond graphene.

Summary Monolayers of transition metal dichalcogenides (TMDs), such as MoS2 and WS2, have recently triggered worldwide research interest due to their remarkable optical and electronic properties. More fascinatingly is the fact that these monolayers could also adopt various morphologies with exposed edges that include triangular, hexagonal or star-shaped clusters, in addition to nanoribbons. Exciting progress has been recently achieved in the synthesis, characterization, device fabrication and functionalization of these monolayer and few-layer TMDs. This article firstly reviews the properties of bulk and monolayer/few-layer TMDs. The “top-down” and “bottom-up” synthesis routes for different TMDs are then summarized. Raman spectroscopy is now becoming a key tool used to characterize atomically thin TMDs, and this review will show the latest advances using this spectroscopic technique. Here we also summarize the most relevant characterization techniques, optical/electronic device fabrication, functionalization and defect engineering of TMDs. There are numerous opportunities for applications and multiple challenges to overcome, and this review will be instructive and useful to researchers working in the area of 2-dimensional materials, as well as scientists and engineers interested in their applications in electronics, optics, catalysis, energy and many others.

Synthesizing 3D carbon nanotube (CNT) networks with multifunctional characteristics has stimulated the interest from the scientific community since the 1990s. Here, the fabrication of a novel composite material consisting of 3D covalently interconnected multiwalled CNT with silicon carbide (SiC) nano and microparticles is reported. The material is synthesized by a two-step process involving the coating of CNT with silicon oxide (SiO x ) via chemical routes, followed by spark plasma sintering (SPS). SPS enables carbothermal reduction of SiOx and subsequent densification of the material into 3D composite blocks. Covalent interconnections of CNT are facilitated by a carbon diffusion process resulting in SiC formation as SiOx coated CNT are subjected to high temperatures. The presence of SiC in the sintered composite has been confirmed by Raman spectroscopy, as well as through energy filtered transmission electron microscopy maps. Interestingly, the 3D CNT composite exhibits high thermal conductivity (16.72 W m−1 K−1); and also a semiconducting behavior with an electron hopping mechanism associated to a 3D variable range hopping model. These findings demonstrate that it is indeed possible to fabricate SiC–CNT composites with enhanced physical properties that can be used as multifunctional materials.

Interest in 2D materials and van der Waals solids is growing exponentially across various scientific and engineering disciplines owing to their fascinating electrical, optical, chemical, and thermal properties. Whereas the micromechanical exfoliation technique has been adopted for rapid material characterization and demonstration of innovative device ideas based on these 2D systems, significant advances have recently been made in large-scale homogeneous and heterogeneous growth of these materials. This review reflects recent progress and outlines future prospects of these novel 2D materials. We provide a holistic overview of the different synthesis and characterization techniques, electronic and photonic device characteristics, and catalytic properties of transition metal dichalcogenides and their heterostructures. We also comment on the challenges that need to be overcome for full-scale commercial implementation of this novel class of layered materials.

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 ...

First-principles density functional theory calculations are performed on dopamine–graphene systems in the presence of an external electric field. The graphene lattice is also modified via substitutional boron- and nitrogen-doping, and via the introduction of defects (monovacancy and Thrower–Stone–Wales). The geometry optimization, electronic density of states, cohesive energy, electronic charge density, and wave functions are analyzed. Our results revealed that dopamine is anchored on the surface of graphene via a physisorption mechanism, and the cohesive strength varies as B-doped > N-doped > vacancy defect > Thrower–Stone–Wales defect. Boron-doped graphene exhibits valence states with dopamine molecules; furthermore, this system showed the strongest cohesive energy. When an electric field is applied, we observe shifts in the valence states near the Fermi level producing a decrease in the molecule–layer interaction. We envisage that the present results could help in developing novel biosensors based on d...

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.

Lattice {\em Kirigami}, ultra-light metamaterials, poly-disperse aggregates, ceramic nano-lattices, and two-dimensional (2-D) atomic materials share an inherent structural discreteness, and their material properties evolve with their shape. To exemplify the intimate relation among material properties and the local geometry, we explore the properties of phosphorene --a new 2-D atomic material-- in a conical structure, and document a decrease of the semiconducting gap that is directly linked to its non-planar shape. This geometrical effect occurs regardless of phosphorene allotrope considered, and it provides a unique optical vehicle to single out local structural defects on this 2-D material. We also classify other 2-D atomic materials in terms of their crystalline unit cells, and propose means to obtain the local geometry directly from their diverse two-dimensional structures while bypassing common descriptions of shape that are based from a parametric continuum., Comment: Published at PNAS (accepted on 3/31/2015, and online free of charge at www.pnas.org as of 4/27/2015.)

Inter-allotropic structural transformation of sp2 structured nanocarbon is a topic of fundamental and technological interest in scalable nanomanufacturing. Such modifications usually require extremely high temperature or high-energy irradiation, and are usually a destructive and time-consuming process. Here, we demonstrate a method for engineering a molecular structure of single-walled carbon nanotubes (SWNTs) and their network properties by femtosecond laser irradiation. This method allows effective coalescence between SWNTs, transforming them into other allotropic nanocarbon structures (double-walled, triple-walled and multi-walled nanotubes) with the formation of linear carbon chains. The nanocarbon network created by this laser-induced transformation process shows extraordinarily strong coalescence induced mode in Raman spectra and two-times enhanced electrical conductivity. This work suggests a powerful method for engineering sp2 carbon allotropes and their junctions, which provides possibilities for next generation materials with structural hybridization at the atomic scale.

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.

This review presents the state of the art in strain and ripple-induced effects on the electronic and optical properties of graphene. It starts by providing the crystallographic description of mechanical deformations, as well as the diffraction pattern for different kinds of representative deformation fields. Then, the focus turns to the unique elastic properties of graphene, and to how strain is produced. Thereafter, various theoretical approaches used to study the electronic properties of strained graphene are examined, discussing the advantages of each. These approaches provide a platform to describe exotic properties, such as a fractal spectrum related with quasicrystals, a mixed Dirac-Schr\"odinger behavior, emergent gravity, topological insulator states, in molecular graphene and other 2D discrete lattices. The physical consequences of strain on the optical properties are reviewed next, with a focus on the Raman spectrum. At the same time, recent advances to tune the optical conductivity of graphene by strain engineering are given, which open new paths in device applications. Finally, a brief review of strain effects in multilayered graphene and other promising 2D materials like silicene and materials based on other group-IV elements, phosphorene, dichalcogenide- and monochalcogenide-monolayers is presented, with a brief discussion of interplays among strain, thermal effects, and illumination in the latter material family., Comment: Review article, 66 pages, 65 figures. V2: accepted in Reports on Progress in Physics, with significant changes respect to the previous version

Hybrid systems consisting of ZnO nanoparticles (ZnO-NPs) anchored on the surface of nitrogen-doped multiwalled carbon nanotubes (CNX-MWNTs) have been synthesized. The anchoring process consists of ...

Two-dimensional (2D) nanocrystals of CdX (X = S, Se, Te) typically grown by colloidal synthesis are coated with organic ligands. Recent experimental work on ZnSe showed that the organic ligands can be removed at elevated temperature, giving a freestanding 2D sheet of ZnSe. In this theoretical work, freestanding single- to few-layer sheets of CdX, each possessing a pseudo honeycomb lattice, are considered by cutting along all possible lattice planes of the bulk zinc blende (ZB) and wurtzite (WZ) phases. Using density functional theory, we have systematically studied their geometric structures, energetics, and electronic properties. A strong surface distortion is found to occur for all of the layered sheets, and yet all of the pseudo honeycomb lattices are preserved, giving unique types of surface corrugations and different electronic properties. The energetics, in combination with phonon mode calculations and molecular dynamics simulations, indicate that the syntheses of these freestanding 2D sheets could ...

Nitrogen-doped multi-walled carbon nanotubes (CNxMWNTs) with multiple morphological defects were produced using a modified chemical vapor deposition (CVD) method. In a typical CNxMWNTs synthesis by CVD, an acetone trap is used to catch organic by-products from pyrolysis. In the present work, an aqueous solution of NaCl (26.82 wt%) was used in the trap, instead of acetone. Carbon nanotubes with sharp tips and lumps were found in the products. Scanning electron microscopy (SEM) and high resolution transmission electron microscopy showed the formation of nanoparticles of different shapes inside the nanotubes. The electronic and magnetic properties were studied using a physical properties measurement Evercool system (PPMS). With this simple change in the CVD-trap, it is possible to control the morphology of carbon nanotubes and metallic nanoparticles. Differences in gas flow are proposed as a possible mechanism to produce these changes in both nanoparticles and CNxMWNTs.

International audience; The dependence of the radial breathing modes (RBMs) and the tangential mode (G-band) of triple-wall carbon nanotubes (TWCNTs) under hydrostatic pressure is reported. Pressure screening effects are observed for the innermost tubes of TWCNTs similar to what has been already found for DWCNTs. However, using the RBM pressure coefficients in conjunction with the histogram of the diameter distribution, we were able to separate the RBM Raman contribution related to the intermediate tubes of TWCNTs from that related to the inner tubes of DWCNTs. By combining Raman spectroscopy and high-pressure measurements, it was possible to identify these two categories of inner tubes even if the two tubes exhibit the same diameters because their pressure response is different. Furthermore, it was possible to observe similar RBM profiles for the innermost tubes of TWCNTs using different resonance laser energies but also under different pressure conditions. This is attributed to changes in the electronic transition energies caused by small pressure-induced deformations. By using Raman spectroscopy, it was possible to estimate the displacement of the optical energy levels with pressure.

Geometric structures, energetics, and electronic properties of single-layer sheets, multilayer stacks, and single-walled nanotubes (SWNTs) of cadmium chalcogenides CdX (X = S, Se, Te) have been studied using ab initio density functional theory, along with spin–orbit coupling, van der Waals (vdW) interactions, and the GW approximation. Methodologies applied to the rationally designed materials have been validated through the experimental structural parameters and band gaps of 3D bulk zinc blende and wurtzite phases of CdX. The 2D single-layer sheet of CdS is found to be completely planar, while those of CdSe and CdTe are slightly corrugated, all showing a honeycomb lattice. The 2D sheets are destabilized with respect to the bulk zinc blende and wurtzite phases, but can be significantly stabilized by forming 3D multilayer stacks as a result of interlayer interactions. 1D (5,5) armchair and (9,0) zigzag SWNTs are also stabilized from their single-layer sheet counterparts. Both SWNTs consist of two concentric...

Besides graphene and hexagonal boron nitride, transition metal dichalcogenides (TMDs) also exhibit a layered structure in which the layers weakly interact via van der Waals forces. Semiconducting TMDs in bulk are indirect band gap materials. However, an isolated sheet exhibits a direct gap. This particular behavior makes them very attractive in terms of optical properties. Moreover, NbS2 and NbSe2 in bulk and their monolayers are metallic. Density functional theory calculations were carried out to study different TMD bilayer systems. First, different bilayer geometries with different stackings were considered. It was found that the indirect and direct band gaps compete; however, the indirect band gap always dominates. Surprisingly, bilayer heterostructures of different TMDs have been found to possess direct band gaps. Finally, heterobilayers composed of one metallic monolayer and a semiconducting layer are predicted as novel metallic van der Waals solids that might find applications in new two-dimensional nanodevices.

We investigate spin transport properties in a junction composed of a polyacetylene chain bridging two zigzag graphene nanoribbon (ZGNR) electrodes with antiferromagnetic (AF) and ferromagnetic (FM) ordering. The transport calculations are carried out using a nonequilibrium Green’s function (NEGF) technique combined with density functional theory (DFT). Previous studies have demonstrated that the ZGNRs exhibit a special AF ordering and half-metallicity at edge states, both of which can be destroyed by applying a strong external electric field. Moreover a stable FM state can be found in ZGNRs under an electric field. Here we demonstrate that the connection between the molecular bridge and nonequivalent carbon atoms (A/B) in the graphene sublattice of ZGNRs may occur in two bonding arrangements and can produce either metallic or semiconducting systems depending on the local coupling. By considering the carbon ring where the chain is attached, one connection resembles a para-linkage in benzene while the other...

Si/O/N-doped single-walled carbon nanotubes (SWNTs) are synthesized using aerosol-assisted chemical vapor deposition (AACVD). The samples are characterized by Raman spectroscopy, high-resolution transmission electron microscopy (HRTEM), electron energy loss spectroscopy (EELS), energy-dispersive X-ray spectroscopy (EDS), Auger electron spectroscopy (AES), and X-ray photoelectron spectroscopy (XPS). HRTEM and Raman spectroscopy studies indicate that doping plays a crucial role in the generation of stable small diameter SWNTs. In order to elucidate the role of the heterodoping (Si/N, O/N, and Si/O) on the electronic properties and stability of SWNTs, density functional theory (DFT) computations on semiconductor (10,0), semimetallic (9,0), and metallic (5,5) SWNTs are performed. It is found that in the heterodoped SWNTs substitutional nitrogen makes the inclusion of Si and O atoms more energetically favorable within the carbon lattice. Heterodoping with Si/O/N may have an important impact in the chemistry of...

High-resolution transmission electron microscopy studies show the dynamics of small graphene platelets on larger graphene layers. The platelets move nearly freely to eventually lock in at well-defined positions close to the edges of the larger underlying graphene sheet. While such movement is driven by a shallow potential energy surface described by an interplane interaction, the lock-in position occurs via edge-edge interactions of the platelet and the graphene surface located underneath. Here, we quantitatively study this behavior using van der Waals density functional calculations. Local interactions at the open edges are found to dictate stacking configurations that are different from Bernal (AB) stacking. These stacking configurations are known to be otherwise absent in edge-free two-dimensional graphene. The results explain the experimentally observed platelet dynamics and provide a detailed account of the new electronic properties of these combined systems.