Elena del Corro

Single Layer Molybdenum Disulfide under Direct Out-of-Plane Compression: Low-Stress Band-Gap Engineering

Tuning the electronic structure of 2D materials is a very powerful asset toward tailoring their properties to suit the demands of future applications in optoelectronics. Strain engineering is one of the most promising methods in this regard. We demonstrate that even very small out-of-plane axial compression readily modifies the electronic structure of monolayer MoS2. As we show through in situ resonant and nonresonant Raman spectroscopy and photoluminescence measurements combined with theoretical calculations, the transition from direct to indirect band gap semiconductor takes place at ∼0.5 GPa, and the transition to a semimetal occurs at stress smaller than 3 GPa.

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.

Strain Assessment in Graphene Through the Raman 2D′ Mode

Accurate and simple local strain assessment in graphene is one of the crucial tasks in device characterization. Raman spectroscopy is often used for that purpose through monitoring of the G and 2D modes. However, the shifts of those two bands might be biased, especially under uniaxial strain, by the effects of charge-transfer doping. Therefore, it is extremely desirable to use another Raman band, less affected by doping, but with a defined and measurable behavior under strain. The Raman 2D′ mode is in this sense the ideal feature for the evaluation of strain levels in stretched graphene monolayers, suitable for this task even under different experimental conditions. The sensitivity and accuracy of the approach through 2D′ mode is on the same level as through the G mode; however, the clear advantage of the 2D′ arises when doping effects are present in the sample.

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). The experiments were performed in a sapphire anvil cell and the morphological changes induced in the CNHs samples were monitored simultaneously by Raman spectroscopy and subsequently by transmission electron microscopy. CNHs samples subjected to a maximum stress of 8 GPa in a single direct compression cycle showed broadened Raman spectra, corresponding to carbonaceous regions with graphite-like structures, surrounded by debundled dahlia-like structures. However, samples subjected to a moderate stress single cycle (2 GPa) exhibited morphological changes from dahlia-like to bud-like structures. Finally, consecutive moderate stress cycles led to the aggregation of such bud spheres towards the formation of a laminar material with horn-like structures at the edges; a very promising configuration for targeted functionalization. This study demonstrates the advantages of using stress for pretreating CNHs samples for subsequent reactivity and functionalization studies.

Raman spectroscopy of aqueous methanol solutions under pressure

Recently, Dixit et al. (2002) used neutron diffraction to probe the molecular-scale structure of a concentrated methanol–water mixture (7:3 molar ratio) under ambient conditions. Their results suggest that the anomalous thermodynamics of water–alcohol systems arises from incomplete mixing at the molecular level, i.e. most of the water molecules exist as small hydrogen-bonded strings and clusters in a fluid of close-packed methyl groups. The application of high hydrostatic pressure together with Raman spectroscopy is a powerful technique to detect such structural changes in associated systems. In this work, we present measurements of Raman spectra of water–methanol mixtures under pressure at room temperature. We have used an anvil cell device with sapphire anvils to generate pressures up to 10 kbar. Our results allow us to suggest important changes on the cluster distribution as a function of pressure.

Graphene under direct compression: Stress effects and interlayer coupling

In this work we explore mechanical properties of graphene samples of variable thickness. For this purpose, we coupled a high pressure sapphire anvil cell to a micro-Raman spectrometer. From the evolution of the G band frequency with stress we document the importance the substrate has on the mechanical response of graphene. On the other hand, the appearance of disorder as a conse-quence of the stress treatment has a negligible effect on the high stress behaviour of graphene.

Graphite under non-hydrostatic conditions

The combination of Raman spectroscopy and high-pressure techniques provides a unique method for studying the mechanical and structural response of carbon-based materials. Existing Raman studies on graphite under pressure were restricted to the analysis of the so-called G band, because the Raman signal from the diamond anvils overlaps other characteristic Raman features, like the D band. Here, we present a Raman spectroscopy study of highly oriented pyrolytic graphite (HOPG) under uniaxial stress using moissanite anvils. The use of moissanite has allowed us to observe, for the first time, the evolution of the D band under extreme compression. We have employed several excitation wavelengths: 632.8, 532.0 and 488.0 nm, to further study dispersion effects. Our results have important implications on the interpretation of high-pressure Raman results on several families of carbon-based compounds.