Raman Spectra of Double-Wall Carbon Nanotubes under Extreme Uniaxial Stress

We investigated the pressure dependence of the Raman frequencies and intensities of the D and G bands of double-wall carbon nanotubes under strong uniaxial conditions. Using moissanite anvils, we observed for the first time the evolution of the D band under extreme stress/pressure conditions. We find that the difference between D and G frequencies remains constant over the whole stress range. In addition, we observe that double-wall carbon nanotubes behave elastically up to the maximum uniaxial stress reached in our experiments, which is estimated to be about 12 GPa

Charge analysis in (RE)CrO4 scheelites by combined Raman spectroscopy and computer simulations

The quest for structure-property relationships in scheelite-type (RE)CrO4 compounds (where RE is a rare earth element) is a difficult task due to the number of exceptions found in RE empirical trends and the uncommon Cr(V) oxidation state. In this work, we experimentally and computationally analyse how the stretching vibrational frequencies ν1(Ag) and ν3(Eg) associated with the [CrO4] tetrahedral units evolve in the (RE)CrO4 crystal family (RE ¼ Nd, Gd, Dy, Ho, and Lu). Since previously reported Cr–O distances and volume changes along with the RE series are not sufficiently accurate to explain the monotonic decrease observed for the ν1(Ag) and ν3(Eg) frequencies, a deeper analysis was performed involving the well-known fact that the bond strength (force constant) decreases as the interatomic distance increases. Our results demonstrates that structural and spectroscopic parameters can be reconciled with classical solid state chemistry ideas when charge effects are considered. This analysis provides a new method for predicting chromium oxidation states from Raman spectroscopy that can be generalised to the study of other crystal families

Morphological changes in carbon nanohorns under stress: a combined Raman spectroscopy and TEM study

In this work, we present the first study of highly compressed carbon nanohorns (CNHs).</p

Highs and Lows of Bond Lengths: Is There Any Limit?

Two distinct points on the potential energy curve (PEC) of a pairwise interaction, the zero-energy crossing point and the point where the stretching force constant vanishes, allow us to anticipate the range of possible distances between two atoms in diatomic, molecular moieties and crystalline systems. We show that these bond-stability boundaries are unambiguously defined and correlate with topological descriptors of electron-density-based scalar fields, and can be calculated using generic PECs. Chemical databases and quantum-mechanical calculations are used to analyze a full set of diatomic bonds of atoms from the s-p main block. Emphasis is placed on the effect of substituents in C-C covalent bonds, concluding that distances shorter than 1.14 Å or longer than 2.0 Å are unlikely to be achieved, in agreement with ultra-high-pressure data and transition-state distances, respectively. Presumed exceptions are used to place our model in the correct framework and to formulate a conjecture for chained interactions, which offers an explanation for the multimodal histogram of O-H distances reported for hundreds of chemical systems

Pressure-Driven Metallization in Hafnium Diselenide

The quest for new transition metal dichalcogenides (TMDs) with outstanding electronic properties operating at ambient conditions draws us to investigate the 1T-HfSe2 polytype under hydrostatic pressure. Diamond anvil cell (DAC) devices coupled to in- situ synchrotron X-ray, Raman and optical (VIS-NIR) absorption experiments along with density functional theory (DFT) based calculations prove that: (i) bulk 1T-HfSe2 exhibits strong structural and vibrational anisotropies, being the interlayer direction especially sensitive to pressure changes, (ii) the indirect gap of 1T-HfSe2 trend to vanish by a -0.1 eV/GPa pressure rate, slightly faster than MoS2 or WS2, (iii) the onset of the metallic behavior appears at Pmet ~10 GPa, which is to date the lowest pressure among common TMDs, and finally (iv) the electronic transition is explained by the bulk modulus B0-Pmet correlation, along with the pressure coefficient of the band gap, in terms of the electronic overlap between chalcogenide p-type and metal d-type orbitals

Linear, Non-Conjugated Cyclic and Conjugated Cyclic Paraphenylene under Pressure

The n-paraphenylene family comprises chains of phenylene units linked together by C-C bonds that are between single- and double-bonded, and where n corresponds to the number of phenylene units. In this work, we compare the response of the optical properties of different phenylene arrangements. We study linear chains (LPP), cyclic systems (CPPs), and non-conjugated cyclic systems with two hydrogenated phenylenes (H4[n]CPP). Particularly, the systems of interest in this work are [6]LPP, [12]- and [6]CPP and H4[6]CPP. This work combines Raman and infrared spectroscopies with absorption and fluorescence (one- and two-photon excitations) measured as a function of pressure up to maximum of about 25 GPa. Unprecedented crystallographic pressure-dependent results are shown on H4[n]CPP, revealing intramolecular ¿-¿ interactions upon compression. These intramolecular interactions justify the H4[n]CPP singular optical properties with increasing fluorescence lifetime as a function of pressure