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).
Raman spectroscopy experiments on DWCNTs and HOPG subjected to non-hydrostatic conditions have been conducted in anvil cells to study the effect of the pressure/stress on the bands assigned to ...defects. These studies provide unique information to probe the mechanical behaviour of carbon materials.
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► High-pressure Raman spectra give key information on the mechanical behaviour of CMs. ► Pressure slopes help in resolving unambiguous spectral assignments. ► Alternative anvils (moissanite/sapphire) are more suitable than diamond to study CMs.
Raman spectroscopy experiments on double-wall carbon nanotube and highly oriented pyrolytic graphite (HOPG) samples subjected to non-hydrostatic conditions have been conducted in anvil cells to study the effect of the pressure/stress on the bands assigned to defects. Typical diamond anvils used in high pressure experiments have been substituted by moissanite (6H-SiC) and sapphire (Al
2O
3) anvils to allow the observation of the D band (around 1350
cm
−1) and the second-order Raman scattering without interference. We demonstrate that Raman experiments at high pressure provide unique information to probe the mechanical behaviour of carbon materials (CMs). We also show that this can be also a powerful technique to assign controversial spectral features such as those appearing in the second order region of the spectra of CMs. In HOPG samples we find that the D′/D band intensity ratio is independent of stress. The results indicate that an increase of non-hydrostatic stresses on HOPG generates graphitic domains with sizes around 20–30
nm when the sample is recovered to room conditions.
We report heat transport measurements on suspended single-layer graphene disks with radius of 150-1600 nm using a high-vacuum scanning thermal microscope. The results of this study revealed a ...radius-dependent thermal contact resistance between tip and graphene, with values between 1.15 and 1.52 × 108 KW−1. The observed scaling of thermal resistance with radius is interpreted in terms of ballistic phonon transport in suspended graphene discs with radius smaller than 775 nm. In larger suspended graphene discs (radius >775 nm), the thermal resistance increases with radius, which is attributed to in-plane heat transport being limited by phonon-phonon resistive scattering processes, which resulted in a transition from ballistic to diffusive thermal transport. In addition, by simultaneously mapping topography and steady-state heat flux signals between a self-heated scanning probe sensor and graphene with 17 nm thermal spatial resolution, we demonstrated that the surface quality of the suspended graphene and its connectivity with the Si/SiO2 substrate play a determining role in thermal transport. Our approach allows the investigation of heat transport in suspended graphene at sub-micrometre length scales and overcomes major limitations of conventional experimental methods usually caused by extrinsic thermal contact resistances, assumptions on the value of the graphene's optical absorbance and limited thermal spatial resolution.
Carbon particles have been obtained by the chlorination of chromocene (Cr(C5H5)2). Changes in their morphology and micro-nanostructure have been monitored at two different temperatures. At 400°C, ...filled materials (tubes and spheres) and agglomerated round particles are formed, whereas at 900°C closed-end tubes, hollow and solid spheres were produced. Transmission electron microscopy shows that these particles are formed of highly disordered graphene-like layers, which is confirmed by the absence of the 2D and 2G bands in the Raman spectrum. The calculated in-plane correlation length of these graphene-like layers is 1.2±0.1nm. In all the carbon particles, electron energy-loss spectroscopy shows a very similar sp2 carbon bonding content (89–98%) and mass density ranging from 1.6 to 1.8g/cm3, both below standard graphite. Textural studies performed on the sample prepared at 900°C show Type II adsorption isotherms with a surface area of 694m2/g.
A computational investigation of β-HP4N7 system is accurately performed using the density functional theory formalism coupled with quantum theory of atoms in molecules. The aim of this work is to ...understand the behavior of β-HP4N7 under compression. At ambient conditions, the N atoms occupy around 81% of the unit cell volume; for this reason the N atoms are the ones contributing significantly to the bulk properties. The particular tetrahedron (PN4) connection promotes high flexibility to this structure. The atoms composing the solid show significant differences in their compressibilities. The high compressibility (or low resistence to pressure) of the N atoms indicates that the reorganization of these atoms is the key in the densification process that takes place when pressure is increased. When compresing the β-HP4N7 from 0 to 130 GPa the formation of PN6 units is observed, leading to a phase transition. The analysis of the elastic constants reveals the stability of this new phase at 110 GPa. This study shows that β-HP4N7 could be a potential candidate for high pressure synthesis of new phases where P atoms would be 6-fold coordinated.
We performed simultaneous Raman spectroscopy and electrical conductivity measurements on self-standing aligned multi-walled carbon nanotubes sheets at varying inter-tube distances. A sapphire anvil ...cell is used here to modulate the inter-tube distance and promote the subsequent electronic tunneling phenomena. We observe a singular correlation between the intensity of the so called defect bands of carbon materials and their conductivity. This indicates that the conditions of the resonant processes that originate these bands are modified by the tunneling phenomena. Such an issue has never been reported before and has potential technological applications. Additionally, the provided AFM images evidence the debundling of the carbon nanotubes that had been described to occur after small compression.
The theory of atoms in molecules leads to a convenient partition of the crystalline space into atomic regions that are space filling and allow a decomposition of the bulk compressibility of a crystal ...as a volume-weighted sum of local compressibilities. Using available ab initio calculations for the complete alkali halides (AX) family in the rock salt phase and some selected spinels, we find that the pressure at the limit of stability of the crystal matches exactly those of the individual ions (or group of ions), pointing to the conclusion that the ionic volumes at the equilibrium define the relative compressibilities of the ions (or group of ions) in the crystal at all pressures. We also analyse the functional dependence between the ionic compressibilities and volumes for the AX family. We show that these ions exhibit universal behaviour when the local bulk moduli are correlated with the pressure referred to the spinodal pressure value, instead of volume. This fact allows us to define a generalised equation of state for the individual ions based on the spinodal instability hypothesis.
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