Understanding the unorthodox stabilization of liquid phase exfoliated molybdenum disulfide (MoS2) in water medium.

Molybdenum disulfide is a highly esteemed 2D material with interesting applications in nanoelectronics, composites, biotechnology and beyond. Its production through liquid-phase exfoliation in H2O is low-cost and eco-friendly. Herein, we present a detailed experimental and theoretical investigation seeking to explain the peculiar stability of MoS2 in H2O medium. By combining different microscopic (SEM, AFM and OM), spectrometric (Raman, UV-vis and AFM-FTIR), scattering (DLS) and ab initio simulation techniques, an edge-functionalization hypothesis for the excellent solvent properties of water for producing few-layer MoS2 has been demonstrated.

Electron Doping of Ultrathin Black Phosphorus with Cu Adatoms.

Few-layer black phosphorus is a monatomic two-dimensional crystal with a direct band gap that has high carrier mobility for both holes and electrons. Similarly to other layered atomic crystals, like graphene or layered transition metal dichalcogenides, the transport behavior of few-layer black phosphorus is sensitive to surface impurities, adsorbates, and adatoms. Here we study the effect of Cu adatoms onto few-layer black phosphorus by characterizing few-layer black phosphorus field effect devices and by performing first-principles calculations. We find that the addition of Cu adatoms can be used to controllably n-dope few layer black phosphorus, thereby lowering the threshold voltage for n-type conduction without degrading the transport properties. We demonstrate a scalable 2D material-based complementary inverter which utilizes a boron nitride gate dielectric, a graphite gate, and a single bP crystal for both the p- and n-channels. The inverter operates at matched input and output voltages, exhibits a gain of 46, and does not require different contact metals or local electrostatic gating.

Raman spectroscopy of a few layers of bismuth telluride nanoplatelets.

We can shape the electronic and phonon properties of Bi2Te3 crystals via the variation of the number of layers. Here, we report a Raman study with the aid of first-principles calculations on few-layered Bi2Te3 systems ranging from 5 to 24 nm layer thickness using 1.92, 2.41 and 2.54 eV excitation energies. We examine how the frequency position, intensity and lineshape of the main Raman modes (A11g, E2g, and A21g) behave by the variation of the layer thickness and excitation energy. We observed a frequency dispersion on the number of layers of the main modes, indicating changes in the inter- and intra-layers interaction. A resonant Raman condition is reached for all modes for samples with 11 and 18 nm thickness because of van Hove singularities at the electronic density of states. Also, the Breit-Wigner-Fano line shape of the A21g mode shows an increase of electron-phonon coupling for thick layers. These results suggest a relevant influence of numbers of layers on the Raman scattering mechanics in Bi2Te3 systems.

Enhanced linear dichroism of flattened-edge black phosphorus nanoribbons.

Black phosphorus is a material with an intrinsic anisotropy in electronic and optical properties due to its puckered honeycomb lattice. Optical absorption is different for incident light with linear polarization in the armchair and zigzag directions (linear dichroism). These directions are also used in the cuts of materials to create black phosphorus nanoribbons. Edges of nanoribbons usually have small reconstruction effects, with minor electronic effects. Here, we show a reconstruction of the armchair edge that introduces a new valence band, which flattens the puckered lattice and increases the linear dichroism extrinsically in the visible spectrum. This enhancement in linear dichroism is explained by the polarization selection rule, which considers the parity of the wave function to a reflection plane. The flattened-edge reconstruction originates from the inversion of chirality of the P atoms at the edges and significantly alters the entire optical absorption of the material. The flattened edges have potential applications in pseudospintronics, photodetectors and might provide new functionalities in optoelectronic and photonic devices.

3d transition metal coordination on monolayer MoS2: a facile doping method to functionalize surfaces.

Two-dimensional materials (2DM) have attracted much interest due to their distinct optical, electronic, and catalytic properties. These properties can be tuned by a range of methods including substitutional doping and, as recently demonstrated, by surface functionalization with single atoms, thus increasing the 2DM portfolio. We theoretically and experimentally describe the coordination reaction between MoS2 monolayers and 3d transition metals (TMs), exploring their nature and MoS2-TM interactions. Density functional theory calculations, X-ray photoelectron spectroscopy (XPS), and photoluminescence (PL) spectroscopy point to the formation of MoS2-TM coordination complexes, where the adsorption energy for 3d TMs resembles the crystal-field (CF) stabilization energy for weak-field complexes. Pearson's theory for hard-soft acid-base and ligand-field theory were used to discuss the periodic trends of 3d TM coordination on MoS2 monolayer surfaces. We found that softer acids with higher ligand field stabilization energy, such as Ni2+, tend to form bonds with more covalent character with MoS2, which can be considered a soft base. On the other hand, harder acids, such as Cr3+, tend to form more ionic bonds. Additionally, we studied the trends in charge transfer and doping observed from XPS and PL results, where metals like Ni led to n-type doping. In contrast, Cu functionalization results in p-type doping. Therefore, the formation of coordination complexes on TMD's surface is a potentially effective way to control and understand the nature of single-atom functionalization of TMD monolayers without relying on or creating new defects.

Spontaneous chemical functionalization via coordination of Au single atoms on monolayer MoS2.

Surface functionalization of metallic and semiconducting 2D transition metal dichalcogenides (TMDs) have mostly relied on physi- and chemi-sorption at defect sites, which can diminish the potential applications of the decorated 2D materials, as structural defects can have substantial drawbacks on the electronic and optoelectronic characteristics. Here, we demonstrate a spontaneous defect-free functionalization method consisting of attaching Au single atoms to monolayers of semiconducting MoS2(1H) via S-Au-Cl coordination complexes. This strategy offers an effective and controllable approach for tuning the Fermi level and excitation spectra of MoS2 via p-type doping and enhancing the thermal boundary conductance of monolayer MoS2, thus promoting heat dissipation. The coordination-based method offers an effective and damage-free route of functionalizing TMDs and can be applied to other metals and used in single-atom catalysis, quantum information devices, optoelectronics, and enhanced sensing.

Ultrasmall (<2 nm) Au@Pt Nanostructures: Tuning the Surface Electronic States for Electrocatalysis.

The ability to tune the electronic properties of nanomaterials has played a major role in the development of sustainable energy technologies. Metallic nanocatalysts are at the forefront of these advances. Their unique properties become even more interesting when we can control the distribution of the electronic states in the nanostructure. Here, we provide a comprehensive evaluation of the electronic surface states in ultrasmall metallic nanostructures by combining experimental and theoretical methods. The developed strategy allows the controlled synthesis of bimetallic nanostructures in the core-shell configuration, dispensing of the use of any surfactant or stabilizing agents, which usually inactivate important surface phenomena. The synthesized ultrasmall Au@Pt nanoarchitecture (∼1.8 nm) presents an enhanced performance catalyzing the hydrogen evolution reaction. First-principles calculations of projected and space-resolved local density of states of Au55@Pt92 (core-shell), Au55Pt92 (alloy), and Pt147 nanoparticles show a prominent increase in the surface electronic states for the core-shell bimetallic nanomaterial. It arises from a more-effective charge transfer from gold to the surface platinum atoms in the core-shell configuration. In pure Pt147 or Au55Pt92 alloy nanoparticles, a great part of the electronic states near the Fermi level is buried in the core atoms, disabling these states for catalytic applications. The proposed experimental-theoretical approach may be useful for the design of other systems composed of metallic nanoparticles supported on distinct substrates, such as two-dimensional materials and porous matrices. These nanomaterials find several applications not only in heterogeneous catalysis but also in sensing and optoelectronic devices.

Ultrasound exfoliation of graphite in biphasic liquid systems containing ionic liquids: A study on the conditions for obtaining large few-layers graphene.

Herein we describe a successful protocol for graphite exfoliation using a biphasic liquid system (water/dichloromethane, DCM) containing ionic liquids (ILs; 1,3-dibenzylimidazolium benzoate- and 1-naphthoate). The use of (surface active) IL and sonication led to stable DCM/water (O/W) emulsion, which enhanced graphene formation, suppressed its re-aggregation and decreased shear/cavitation damage. The O/W emulsion stabilization by the ILs was studied by dynamic light scattering (DLS), whereas their interaction with the graphene sheets were described by Density Functional Theory (DFT) calculations. Moreover, a comprehensive investigation on cavitation-based exfoliation in the O/W systems was performed to assess the importance of operational parameters, including, the type of ultrasound processor, ultrasound power and insonation, and the influence of the exfoliation medium.

Ionic Liquid-Silica Precursors via Solvent-Free Sol-Gel Process and Their Application in Epoxy-Amine Network: A Theoretical/Experimental Study.

This work describes the solvent-free sol-gel synthesis of epoxy-functionalized silica-based precursors in the presence of 1-butyl-3-methylimidazolium-based ionic liquids (ILs) containing different anions: chloride (Cl-) and methanesulfonate (MeSO3-). The IL-driven sol-gel mechanisms were investigated in detail using experimental characterizations (29Si NMR and ATR FTIR spectroscopy) and a theoretical computational method based on density functional theory (DFT). We observed complex IL influence on both hydrolysis and condensation steps, involving especially H-bonding and Coulomb coupling stabilization of the process intermediates. The obtained IL-silica precursors and their further xerogels were widely characterized (rheology measurements, MALDI TOF, 29Si NMR, ATR FTIR, and DFT simulation), which allowed observation of their precise silica structures and established their most energetically favorable conformations. The detected silica structures were dependent on the IL type and varied from highly condensed 3D cage-like to branched ladder-like and cyclic ones. The application of prepared IL-silica precursors as reinforcing additives into the epoxy-amine network led to an improvement in the organic/inorganic interphase interactions through chemical and physical bonding. Uniform and well-dispersed silica aggregates, in the size of ∼30 nm, were formed when ≤6.8 wt % of each IL-silica precursor was applied into the epoxy-amine network. The use of imidazolium-based ILs contributed to a significant improvement in thermomechanical properties of hybrids and reduced their UV absorption ability compared to that of the reference matrix. All hybrids exhibited an increase in energy to break (up to ∼53%), elongation at break (up to ∼43%), shear storage modulus in the rubbery region (up to 4 times), and thermo-oxidative stability.