Effects of Moisture and Synthesis-Derived Contaminants on the Mechanical Properties of Graphene Oxide: A Molecular Dynamics Investigation

This paper reports on the effects of the chemical composition of graphene oxide (GO) sheets on the mechanical properties of bulk GO. Three key factors were analyzed: (i) the oxygenated functional groups’ concentration, (ii) the content of intersheet water (moisture), and (iii) the presence of residual contaminants observed from the synthesis of GO. Molecular dynamics simulations using the reactive force field ReaxFF were conducted to model tensile strength, indentation, and shear stress tests. The structural integrity of the carbon basal plane was the primary variable that determined mechanical behavior of GO slabs. Hydrogen-bond networks played an essential role in the tensile fracture mechanism, delaying the onset of fracture whenever strong hydrogen bonds existed in the intersheet space. The presence of interlayer sulfate ion contaminants negatively impacted the tensile strength, stiffness, and toughness of GO. Moreover, it was observed that intersheet sulfate ions improved the resistance to fracture of GO at low sulfur concentrations, while lower fracture strains were observed beyond a critical concentration. Alike the tensile stress findings, the indentation properties were determined by the integrity of the carbon basal plane. Our findings agree with experimental mechanical property measurements and reveal the importance of considering synthesis-derived contaminants in molecular models of GO

Formation and Interlayer Decoupling of Colloidal MoSe₂ Nanoflowers

We report the colloidal synthesis of substrate-free MoSe₂ nanostructures with a uniform flower-like morphology and tunable average diameters that range from approximately 50–250 nm. The MoSe₂ nanoflowers contain a large population of highly crystalline few-layer nanosheets that protrude from a central core. Aliquot studies and control experiments indicate that the nanoflowers are generated through a two-step process that involves the formation of a core in the early stages of the reaction followed by outward nanosheet growth that can be controlled based on the concentrations of reagents. The effects of laser-induced local heating, bulk-scale heating using a temperature stage, and nanostructuring on the ability to trigger and tune interlayer decoupling were also investigated. Notably, laser-induced local heating results in dynamic and reversible interlayer decoupling. Such capabilities provide a pathway for achieving quasi-two-dimensional behavior in three-dimensionally structured and colloidally synthesized transition metal dichalcogenide nanostructures

Discovery of Wall-Selective Carbon Nanotube Growth Conditions <i>via</i> Automated Experimentation

Applications of carbon nanotubes continue to advance, with substantial progress in nanotube electronics, conductive wires, and transparent conductors to name a few. However, wider application remains impeded by a lack of control over production of nanotubes with the desired purity, perfection, chirality, and number of walls. This is partly due to the fact that growth experiments are time-consuming, taking about 1 day per run, thus making it challenging to adequately explore the many parameters involved in growth. We endeavored to speed up the research process by automating CVD growth experimentation. The adaptive rapid experimentation and <i>in situ</i> spectroscopy CVD system described in this contribution conducts over 100 experiments in a single day, with automated control and <i>in situ</i> Raman characterization. Linear regression modeling was used to map regions of selectivity toward single-wall and multiwall carbon nanotube growth in the complex parameter space of the water-assisted CVD synthesis. This development of the automated rapid serial experimentation is a significant progress toward an autonomous closed-loop learning system: a Robot Scientist

Atomically Thin Layers of Graphene and Hexagonal Boron Nitride Made by Solvent Exfoliation of Their Phosphoric Acid Intercalation Compounds

The development of scalable and reliable techniques for the production of the atomically thin layers of graphene and hexagonal boron nitride (h-BN) in bulk quantities could make these materials a powerful platform for devices and composites that impact a wide variety of technologies (<i>Nature</i> <b>2012</b>, <i>490</i>, 192–200). To date a number of practical exfoliation methods have been reported that are based on sonicating or stirring powdered graphite or h-BN in common solvents. However, the products of these experiments consist mainly of few-layer sheets and contain only a small fraction of monolayers. A possible reason for this is that splitting the crystals into monolayers starts from solvent intercalation, which must overcome the substantial interlayer cohesive energy (120–720 mJ/m²) of the van der Waals solids. Here we show that the yield of the atomically thin layers can be increased to near unity when stage-1 intercalation compounds of phosphoric acid are used as starting materials. The exfoliation to predominantly monolayers was achieved by stirring them in medium polarity organic solvents that can form hydrogen bonds. The exfoliation process does not disrupt the sp² π-system of graphene and is gentle enough to allow the preparation of graphene and h-BN monolayers that are tens of microns in their lateral dimensions

Edge–Edge Interactions in Stacked Graphene Nanoplatelets

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 <i>via</i> 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

Reversible Intercalation of Hexagonal Boron Nitride with Brønsted Acids

Hexagonal boron nitride (h-BN) is an insulating compound that is structurally similar to graphite. Like graphene, single sheets of BN are atomically flat, and they are of current interest in few-layer hybrid devices, such as transistors and capacitors, that contain insulating components. While graphite and other layered compounds can be intercalated by redox reactions and then converted chemically to suspensions of single sheets, insulating BN is not susceptible to oxidative intercalation except by extremely strong oxidizing agents. We report that stage-1 intercalation compounds can be formed by simple thermal drying of h-BN in Brønsted acids H₂SO₄, H₃PO₄, and HClO₄. X-ray photoelectron and vibrational spectra, as well as electronic structure and molecular dynamics calculations, demonstrate that noncovalent interactions of these oxyacids with the basic N atoms of the sheets drive the intercalation process

Spectroscopic Signatures for Interlayer Coupling in MoS₂–WSe₂ van der Waals Stacking

Stacking of MoS₂ and WSe₂ monolayers is conducted by transferring triangular MoS₂ monolayers on top of WSe₂ monolayers, all grown by chemical vapor deposition (CVD). Raman spectroscopy and photoluminescence (PL) studies reveal that these mechanically stacked monolayers are not closely coupled, but after a thermal treatment at 300 °C, it is possible to produce van der Waals solids consisting of two interacting transition metal dichalcogenide (TMD) monolayers. The layer-number sensitive Raman out-of-plane mode A²_1g for WSe₂ (309 cm^–1) is found sensitive to the coupling between two TMD monolayers. The presence of interlayer excitonic emissions and the changes in other intrinsic Raman modes such as E″ for MoS₂ at 286 cm^–1 and A²_1g for MoS₂ at around 463 cm^–1 confirm the enhancement of the interlayer coupling

Field-Effect Transistors Based on Few-Layered α‑MoTe₂

Here we report the properties of field-effect transistors based on a few layers of chemical vapor transport grown α-MoTe₂ crystals mechanically exfoliated onto SiO₂. We performed field-effect and Hall mobility measurements, as well as Raman scattering and transmission electron microscopy. In contrast to both MoS₂ and MoSe₂, our MoTe₂ field-effect transistors are observed to be hole-doped, displaying on/off ratios surpassing 10⁶ and typical subthreshold swings of ∼140 mV per decade. Both field-effect and Hall mobilities indicate maximum values approaching or surpassing 10 cm²/(V s), which are comparable to figures previously reported for single or bilayered MoS₂ and/or for MoSe₂ exfoliated onto SiO₂ at room temperature and without the use of dielectric engineering. Raman scattering reveals sharp modes in agreement with previous reports, whose frequencies are found to display little or no dependence on the number of layers. Given that MoS₂ is electron-doped, the stacking of MoTe₂ onto MoS₂ could produce ambipolar field-effect transistors and a gap modulation. Although the overall electronic performance of MoTe₂ is comparable to those of MoS₂ and MoSe₂, the heavier element Te leads to a stronger spin–orbit coupling and possibly to concomitantly longer decoherence times for exciton valley and spin indexes

Excited Excitonic States in 1L, 2L, 3L, and Bulk WSe₂ Observed by Resonant Raman Spectroscopy

Resonant Raman spectroscopy (RRS) is a very useful tool to study physical properties of materials since it provides information about excitons and their coupling with phonons. We present in this work a RRS study of samples of WSe₂ with one, two, and three layers (1L, 2L, and 3L), as well as bulk 2H-WSe₂, using up to 20 different laser lines covering the visible range. The first- and second-order Raman features exhibit different resonant behavior, in agreement with the double (and triple) resonance mechanism(s). From the laser energy dependence of the Raman intensities (Raman excitation profile, or REP), we obtained the energies of the excited excitonic states and their dependence with the number of atomic layers. Our results show that Raman enhancement is much stronger for the excited A′ and B′ states, and this result is ascribed to the different exciton–phonon coupling with fundamental and excited excitonic states

Low-Temperature Solution Synthesis of Transition Metal Dichalcogenide Alloys with Tunable Optical Properties

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_<i>x</i>W_1–<i>x</i>Se₂ and WS_2<i>y</i>Se_{2(1–<i>y</i>)}, exhibiting fully tunable metal and chalcogen compositions that span the MoSe₂–WSe₂ and WS₂–WSe₂ 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