Miriam Peña-Alvarez

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.

Decorating graphene oxide/nanogold with dextran-based polymer brushes for the construction of ultrasensitive electrochemical enzyme biosensors

A novel strategy was employed to prepare a water-soluble graphene derivative by using dextran-based polymer brushes as solubilizing agents. Graphene oxide was grafted with (3-mercaptopropyl) trimethoxysilane and further decorated with Au nanoparticles. This hybrid nanomaterial was then reduced and anchored with polysaccharide-based polymer brushes by chemisorption of an end-group thiolated dextran derivative on the Au nanoparticles. The resulting hybrid nonmaterial allowed highly stable aqueous dispersions to be obtained, which were used to coat glassy carbon electrodes for the preparation of a model tyrosinase electrochemical biosensor for catechol. The enzyme electrode showed excellent electroanalytical performance with fast response in about 5 s, a linear range of 100 pM-120 nM, a very high sensitivity of 45.9 A M-1 and a very low detection limit of 40 pM for catechol.

A Biradical Balancing Act: Redox Amphoterism in a Diindenoanthracene Derivative Results from Quinoidal Acceptor and Aromatic Donor Motifs

The reduced and oxidized states of an open-shell diindeno[b,i]anthracene (DIAn) derivative have been investigated by experimental and theoretical techniques. As a result of moderate biradical character and the ability of cyclopenta-fused scaffolds to stabilize both positive and negative charges, DIAn exhibits rich redox chemistry with four observable and isolable charged states. Structural and electronic properties of the DIAn system are brought to light by UV-vis-NIR and Raman spectroelectrochemical measurements. Aromatization of the diindeno-fused anthracene core upon successive single-electron injections is revealed through single-crystal X-ray diffraction of radical anion and dianion salts. We present a rare case where the pseudoaromatic/quinoidal ground state of a neutral biradical polycyclic hydrocarbon leads to a stable cascade of five redox states. Our detailed investigation of the transformation of molecular structure along all four redox events provides a clearer understanding of the nature of charge carriers in ambipolar organic field-effect transistors.

Pressure-Induced Conductivity in a Neutral Nonplanar Spin-Localized Radical

There is a growing interest in the development of single-component molecular conductors based on neutral organic radicals that are mainly formed by delocalized planar radicals, such as phenalenyl or thiazolyl radicals. However, there are no examples of systems based on nonplanar and spin-localized C-centered radicals exhibiting electrical conductivity due to their large Coulomb energy (U) repulsion and narrow electronic bandwidth (W) that give rise to a Mott insulator behavior. Here we present a new type of nonplanar neutral radical conductor attained by linking a tetrathiafulvalene (TTF) donor unit to a neutral polychlorotriphenylmethyl radical (PTM) with the important feature that the TTF unit enhances the overlap between the radical molecules as a consequence of short intermolecular S···S interactions. This system becomes semiconducting upon the application of high pressure thanks to increased electronic bandwidth and charge reorganization opening the way to develop a new family of neutral radical conductors.

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.

High pressure synthesis and stability of cobalt hydrides

In situ high-pressure high-temperature X-ray powder diffraction studies of the cobalt-hydrogen system reveal the direct synthesis of both the binary cobalt hydride (CoH) and a novel cobalt dihydride (CoH2). We observe the formation of fcc CoH at pressures of 4 GPa, which persists to pressures of 45 GPa. At this pressure, we see the emergence with time of a further expanded fcc lattice, which we identify as CoH2, where the hydrogen atoms occupy the tetrahedral vacancies. We have explored alternative synthesis routes of CoH2 and can lower the synthesis pressure to 35 GPa by the application of high temperature. CoH2 is stable to at least 55 GPa and decomposes into CoH below 10 GPa, releasing molecular hydrogen before further decomposing completely into its constituent elements below 3 GPa. As a first-row transition metal, cobalt has a relatively lower mass than other hydride-forming transition metals, and as a result, CoH2 has a high hydrogen content of 3.3 wt. % and a volumetric hydrogen density of 214 g/l.

High pressure chemistry and the mechanochemical polymerization of [5]-cyclo-p-phenylene

Evidence for the surprising formation of polymeric phases under high pressure for conjugated nanohoop molecules was found. This paper represents one of the unique cases, in which the molecular-level effects of pressure in crystalline organic solids is addressed, and provides a general approach based on vibrational Raman spectroscopy combining experiments and computations. In particular, we studied the structural and supramolecular chemistry of the cyclic conjugated nanohoop molecule [5]cyclo-para-phenylene ([5]CPP) under high pressures up to 10 GPa experimentally and up to 20 GPa computationally. The theoretical modeling for periodic crystals predicts good agreements with the experimentally obtained Raman spectra in the molecular phase. In addition, we have discovered two stable polymeric phases that arise in the simulation. The critical pressures in the simulation are too high, but the formation of polymeric phases at high pressures provides a natural explanation for the observed irreversibility of the Raman spectra upon pressure release between 6 and 7 GPa. The geometric parameters show a deformation toward quinonoid structures at high pressures accompanied by other deformations of the [5]CPP nanohoops. The quinonoidization of the benzene rings is linked to the systematic change of the bond length alternation as a function of the pressure, providing a qualitative interpretation of the observed spectral shifts of the molecular phase.

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.

Structures of lithium-zinc compounds at high pressures

Intermetallic lithium compounds have found a wide range of applications owing to their light mass and desirable electronic and mechanical properties. Here, by compressing pure lithium and zinc mixtures in a diamond-anvil cell, we observe a direct reaction forming the stoichiometric compound LiZn, at pressures below 1 GPa. On further compression above 10 GPa, we observe the formation of Li2Zn, which is the highest lithium content compound to be discovered in the Li-Zn system. Our results constrain the structures of these compounds and their evolution with pressure, furthering our understanding of potentially useful light volume-efficient energy storage materials.

Mechanochemistry in [6]Cycloparaphenylene: A Combined Raman Spectroscopy and Density Functional Theory Study

Raman spectroscopy under high pressures up to 10 GPa and density functional computations up to 30 GPa are combined to obtain insights into the behavior of a prototypical nanohoop conjugated molecule, [6]cycloparaphenylene ([6]CPP). Upon increasing pressure, the nanohoop undergoes deformations, first reversible ovalization and then at even higher pressures aggregates are formed. This irreversible aggregation is caused by the formation of new intermolecular σ-bonds. Frequencies and derivatives of the Raman frequency shifts as a function of pressure are well reproduced by the computations. The frequency behavior is tied to changes in aromatic/quinonoid character of the nanohoop. The modeling at moderate high pressures reveals the deformation of the [6]CPP molecules into oval-like and peanut-like shapes. Surprisingly the pressure derivatives of the observed Raman mode shifts undergo a sudden change around a pressure value that is common to all Raman modes, indicating an underlying geometrical change extended over the whole molecule that is interpreted by the computational modeling. Simulations predict that under even larger deformations caused by higher pressures, oligomerization reactions would be triggered. Our simulations demonstrate that these transformations would occur regardless of the solvent, however pressures at which they happen are influenced by solvent molecules encapsulated in the interior of the [6]CPP.