Although the main Raman features of semiconducting transition metal dichalcogenides are well known for the monolayer and bulk, there are important differences exhibited by few layered systems which ...have not been fully addressed. WSe2 samples were synthesized and ab-initio calculations carried out. We calculated phonon dispersions and Raman-active modes in layered systems: WSe2, MoSe2, WS2 and MoS2 ranging from monolayers to five-layers and the bulk. First, we confirmed that as the number of layers increase, the E', E″ and E2g modes shift to lower frequencies, and the A'1 and A1g modes shift to higher frequencies. Second, new high frequency first order A'1 and A1g modes appear, explaining recently reported experimental data for WSe2, MoSe2 and MoS2. Third, splitting of modes around A'1 and A1g is found which explains those observed in MoSe2. Finally, exterior and interior layers possess different vibrational frequencies. Therefore, it is now possible to precisely identify few-layered STMD.
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).
Substitutional phosphorus doping in single-wall carbon nanotubes (SWNTs) is investigated by density functional theory and resonance Raman spectroscopy. Electronic structure calculations predict ...charge localization on the phosphorus atom, generating nondispersive valence and conduction bands close to the Fermi level. Besides confirming sustitutional doping, accurate analysis of electron and phonon renormalization effects in the double-resonance Raman process elucidates the different nature of the phosphorus donor doping (localized) when compared to nitrogen substitutional doping (nonlocalized) in SWNTs.
Here, we present a temperature (T) dependent comparison between field-effect and Hall mobilities in field-effect transistors based on few-layered WSe2 exfoliated onto SiO2. Without dielectric ...engineering and beyond a T-dependent threshold gate-voltage, we observe maximum hole mobilities approaching 350 cm(2)/Vs at T = 300 K. The hole Hall mobility reaches a maximum value of 650 cm(2)/Vs as T is lowered below ~150 K, indicating that insofar WSe2-based field-effect transistors (FETs) display the largest Hall mobilities among the transition metal dichalcogenides. The gate capacitance, as extracted from the Hall-effect, reveals the presence of spurious charges in the channel, while the two-terminal sheet resistivity displays two-dimensional variable-range hopping behavior, indicating carrier localization induced by disorder at the interface between WSe2 and SiO2. We argue that improvements in the fabrication protocols as, for example, the use of a substrate free of dangling bonds are likely to produce WSe2-based FETs displaying higher room temperature mobilities, i.e. approaching those of p-doped Si, which would make it a suitable candidate for high performance opto-electronics.
Gold nanoparticles were selectively attached to chemically functionalized surface sites on nitrogen-doped carbon (CN x ) nanotubes. A cationic polyelectrolyte was adsorbed on the surface of the ...nanotubes by electrostatic interaction between carboxyl groups on the chemically oxidized nanotube surface and polyelectrolyte chains. Negatively charged 10 nm gold nanoparticles from a gold colloid suspension were subsequently anchored to the surface of the nanotubes through the electrostatic interaction between the polyelectrolyte and the nanoparticles. This approach provides an efficient method to attach other nanostructures to carbon nanotubes and can be used as an illustrative detection of the functional groups on carbon nanotube surfaces.
Coastal zones are dominated by economically important ecosystems, and excessive urban, industrial, agricultural, and tourism activities can lead to rapid degradation of those habitats and resources. ...Groundwater in the Eastern Yucatan Peninsula coastal aquifer discharges directly into the coastal ocean affecting the coral reefs, which are part of the Mesoamerican Coral Reef System. The composition and impacts of groundwater were studied at different coastal environments around Akumal (SE Yucatan Peninsula). Radium isotopes and salinity were used to quantify fresh groundwater and recirculated seawater contributions to the coastal zone. Excess Ra distribution suggests spatially variable discharge rates of submarine groundwater. High NO3− levels and high coliform bacteria densities indicate that groundwater is polluted at some sites. Dissolved phosphorous content is elevated in the winter and during the high tourism season, likely released from untreated sewage discharge and from aquifer sediments under reducing conditions.
•Groundwater and recirculated seawater contribution were quantified by Ra isotopes.•Groundwater springs are significant nutrient sources in Akumal coastal area.•The adjacent coral reef is impacted by anthropogenic sources of nutrients.
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 doped/defective graphene field-effect transistor devices.
Double-walled carbon nanotubes are needed in a pure, highly crystalline form before features such as their electronic properties, thermal transport and mechanical behaviour can be investigated. Here ...we fabricate a paper-like material that consists of hexagonally packed bundles of clean, coaxial carbon nanotubes whose double walls vary little in diameter; it is prepared in high yields using chemical-vapour deposition with a conditioning catalyst and two-step purification. Our results will enable investigation of the physical properties of double-walled carbon nanotubes, which are predicted to be superior to those of both their single- and multiwalled relatives.
Chlorine resistant reverse osmosis (RO) membranes were fabricated using a multi-walled carbon nanotube-polyamide (MWCNT-PA) nanocomposite. The separation performance of these membranes after chlorine ...exposure (4800 ppm·h) remained unchanged (99.9%) but was drastically reduced to 82% in the absence of MWCNT. It was observed that the surface roughness of the membranes changed significantly by adding MWCNT. Moreover, membranes containing MWCNT fractions above 12.5 wt.% clearly improved degradation resistance against chlorine exposure, with an increase in water flux while maintaining salt rejection performance. Molecular dynamics and quantum chemical calculations were performed in order to understand the high chemical stability of the MWCNT-PA nanocomposite membranes, and revealed that high activation energies are required for the chlorination of PA. The results presented here confirm the unique potential of carbon nanomaterials embedded in polymeric composite membranes for efficient RO water desalination technologies.
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