Radiative Cooling Properties of Portlandite and Tobermorite: Two Cementitious Minerals of Great Relevance in Concrete Science and Technology.

Although concrete and cement-based materials are the most engineered materials employed by mankind, their potential for use in daytime radiative cooling applications has yet to be fully explored. Due to its complex structure, which is composed of multiple phases and textural details, fine-tuning of concrete is impossible without first analyzing its most important ingredients. Here, the radiative cooling properties of Portlandite (Ca(OH)2) and Tobermorite (Ca5Si6O16(OH)2·4H2O) are studied due to their crucial relevance in cement and concrete science and technology. Our findings demonstrate that, in contrast to concrete (which is a strong infrared emitter but a poor sun reflector), both Portlandite and Tobermorite exhibit good radiative cooling capabilities. These results provide solid evidence that, with the correct optimization of composition and porosity, concrete can be transformed into a material suitable for daytime radiative cooling.

Low density phases of TiO2 by cluster self-assembly.

The interest in titanium dioxide (TiO 2 ) phases is growing due to the number of applications in cosmetics, food industry and photocatalysis, an increase that is driven by its exceptional properties when engineered at the nanoscale like in the form of nanoparticles. Our goal is to discover unknown low-density phases of TiO 2 , with potential for applications in various fields. We then use well-known TiO 2 clusters as fundamental building blocks to be self-assembled into unique structures to study their distinct characteristics. Density functional calculations are employed to relax the structures and identify the most stable TiO 2 structures within an energy range of 0.1 eV per atom from the rutile and anatase phases, which are confirmed, validating our methodology. Going beyond conventional phases, we found two-dimensional TiO 2 structures, previously explored in separate studies, and showing typical structures of transition metal dichalcogenide layers, that forge a bridge between different TiO 2 structures. It is noteworthy that our investigation uncovered an entirely novel class of TiO 2 structures featuring hexagonal cages like beehive channels, opening novel phases with huge potential. These discovered low-density phases are interesting, particularly the hexagonal cage structures with remarkable large gaps, because they introduce other dimensions for uncharted applications in the ever-growing TiO 2 landscape.

Dimerization Effects and Negative Strain Energy in Silicon Monosulfide Nanotubes.

We report on the construction and characterization of silicon monosulfide nanotubes that were obtained by rolling up two-dimensional materials isoelectronic to phosphorene in the recently discovered layered Pmma and ß phases. We relaxed and studied the nanotube structures using computational methods within density functional theory (DFT). We found that the nanotubes with a thick Pmma layer remain stable at room temperature, and their electronic properties depend on their diameters. Small-diameter nanotubes display metallic character, while nanotubes with increasing diameter show semiconducting ground states due to the dimerization in the silicon-silicon distances that opens a gap, leading to interesting optical properties in the near-infrared region. Furthermore, we discovered ß SiS monolayer nanotubes having negative strain energies, similar to the well-known imogolite inorganic nanotubes. The combined thermal stability, compelling optical properties, and diverse applications of these silicon monosulfide nanotubes underscore the demand for novel synthesis methods to fully explore their potential in various fields.

Tuning Magnetoconductivity in LaMnO3 NPs through Cationic Vacancy Control.

The inclusion of La-Mn vacancies in LaMnO3 nanoparticles leads to a noticeable change in conductivity behavior. The sample retains its overall insulator characteristic, with a typical thermal activation mechanism at high temperatures, but it presents high magnetoconductivity below 200 K. The activation energy decreases linearly with the square of the reduced magnetization and vanishes when the sample is magnetized at saturation. Therefore, it turns out that electron hopping between Mn3+ and Mn4+ largely contributes to the conductivity below the Curie temperature. The influence of the applied magnetic field on conductivity also supports the hypothesis of hopping contribution, and the electric behavior can be explained as being due to an increase in the hopping probability via spin alignment.

Localization versus delocalization of d-states within the [Formula: see text]MnGa Heusler alloy.

We present calculations based on density-functional theory with improved exchange-correlation approaches to investigate the electronic structure of [Formula: see text]MnGa magnetic shape memory alloy prototype. We study the effects of hybrid functionals as well as a Hubbard-like correction parameter U on the structural, electronic and magnetic properties of the alloy. We show that the previously successful application of U on Mn should be extended by including U on Ni to describe the d localized electrons more accurately and in better agreement with experiments. The bonding interactions within this intermetallic alloy are analysed including the role of non-transition metal. We found that the strongest and most stabilizing bond is formed between the Ga-Ni pairs due to the delocalized s-s and p-s orbital hybridization. Our findings suggest that minimization of the over-delocalization error introduced by standard semi-local exchange-correlation functionals leads to a better description of the [Formula: see text]MnGa alloy. Furthermore we propose that the experimental total magnetic moment of Ni-Mn-Ga alloys could be increased after carefully selected heat treatment procedures.

Interactions between Reduced Graphene Oxide with Monomers of (Calcium) Silicate Hydrates: A First-Principles Study.

Graphene is a two-dimensional material, with exceptional mechanical, electrical, and thermal properties. Graphene-based materials are, therefore, excellent candidates for use in nanocomposites. We investigated reduced graphene oxide (rGO), which is produced easily by oxidizing and exfoliating graphite in calcium silicate hydrate (CSHs) composites, for use in cementitious materials. The density functional theory was used to study the binding of moieties, on the rGO surface (e.g., hydroxyl-OH/rGO and epoxide/rGO groups), to CSH units, such as silicate tetrahedra, calcium ions, and OH groups. The simulations indicate complex interactions between OH/rGO and silicate tetrahedra, involving condensation reactions and selective repairing of the rGO lattice to reform pristine graphene. The condensation reactions even occurred in the presence of calcium ions and hydroxyl groups. In contrast, rGO/CSH interactions remained close to the initial structural models of the epoxy rGO surface. The simulations indicate that specific CSHs, containing rGO with different interfacial topologies, can be manufactured using coatings of either epoxide or hydroxyl groups. The results fill a knowledge gap, by establishing a connection between the chemical compositions of CSH units and rGO, and confirm that a wet chemical method can be used to produce pristine graphene by removing hydroxyl defects from rGO.

From single-particle-like to interaction-mediated plasmonic resonances in graphene nanoantennas.

Plasmonic nanostructures attract tremendous attention as they confine electromagnetic fields well below the diffraction limit while simultaneously sustaining extreme local field enhancements. To fully exploit these properties, the identification and classification of resonances in such nanostructures is crucial. Recently, a novel figure of merit for resonance classification has been proposed1 and its applicability was demonstrated mostly to toy model systems. This novel measure, the energy-based plasmonicity index (EPI), characterizes the nature of resonances in molecular nanostructures. The EPI distinguishes between either a single-particle-like or a plasmonic nature of resonances based on the energy space coherence dynamics of the excitation. To advance the further development of this newly established measure, we present here its exemplary application to characterize the resonances of graphene nanoantennas. In particular, we focus on resonances in a doped nanoantenna. The structure is of interest, as a consideration of the electron dynamics in real space might suggest a plasmonic nature of selected resonances in the low doping limit but our analysis reveals the opposite. We find that in the undoped and moderately doped nanoantenna, the EPI classifies all emerging resonances as predominantly single-particle-like and only after doping the structure heavily, the EPI observes plasmonic response.

Out-of-plane magnetic anisotropy energy in the Ni3Bz3 molecule.

Organometallic complexes formed by transition metals clusters and benzene molecules have already been synthesized, and in selected cases display magnetic properties controlled by external magnetic fields. We have studied NinBzn complexes made of nickel atoms surrounded by benzene molecules and here we focus specifically on the magnetic molecule Ni3Bz3. By means of calculations including relativistic spin-orbit terms, we show that this molecule reveals a large magnetic anisotropy energy of approximately 8 meV, found with the easy axis perpendicular to the metal atom plane. Note that the matching bare Ni3 cluster has a similar magnetic anisotropy, however the easy axis is in-plane. Covering with benzene molecules thus switches the easy axis from in-plane for Ni3 to out-of-plane for Ni3Bz3. The large out-of-plane magnetic anisotropy of Ni3Bz3 suggests that this molecule could indeed be used as a part in the design of molecular magnetic memories.

A multi-scale approach for percolation transition and its application to cement setting.

Shortly after mixing cement grains with water, a cementitious fluid paste is formed that immediately transforms into a solid form by a phenomenon known as setting. Setting actually corresponds to the percolation of emergent network structures consisting of dissolving cement grains glued together by nanoscale hydration products, mainly calcium-silicate-hydrates. As happens in many percolation phenomena problems, the theoretical identification of the percolation threshold (i.e. the cement setting) is still challenging, since the length scale where percolation becomes apparent (typically the length of the cement grains, microns) is many times larger than the nanoscale hydrates forming the growing spanning network. Up to now, the long-lasting gap of knowledge on the establishment of a seamless handshake between both scales has been an unsurmountable obstacle for the development of a predictive theory of setting. Herein we present a true multi-scale model which concurrently provides information at the scale of cement grains (microns) and at the scale of the nano-hydrates that emerge during cement hydration. A key feature of the model is the recognition of cement setting as an off-lattice bond percolation process between cement grains. Inasmuch as this is so, the macroscopic probability of forming bonds between cement grains can be statistically analysed in smaller local observation windows containing fewer cement grains, where the nucleation and growth of the nano-hydrates can be explicitly described using a kinetic Monte Carlo Nucleation and Growth model. The most striking result of the model is the finding that only a few links (~12%) between cement grains are needed to reach setting. This directly unveils the importance of explicitly including nano-texture on the description of setting and explains why so low amount of nano-hydrates is needed for forming a spanning network. From the simulations, it becomes evident that this low amount is least affected by processing variables like the water-to-cement ratio and the presence of large quantities of nonreactive fillers. These counter-intuitive predictions were verified by ex-professo experiments that we have carried out to check the validity of our model.

Stacking change in MoS2 bilayers induced by interstitial Mo impurities.

We use a theoretical approach to reveal the electronic and structural properties of molybdenum impurities between MoS2 bilayers. We find that interstitial Mo impurities are able to reverse the well-known stability order of the pristine bilayer, because the most stable form of stacking changes from AA' (undoped) into AB' (doped). The occurrence of Mo impurities in different positions shows their split electronic levels in the energy gap, following octahedral and tetrahedral crystal fields. The energy stability is related to the accommodation of Mo impurities compacted in hollow sites between layers. Other less stable configurations for Mo dopants have larger interlayer distances and band gaps than those for the most stable stacking. Our findings suggest possible applications such as exciton trapping in layers around impurities, and the control of bilayer stacking by Mo impurities in the growth process.

Quantum Nuclear Dynamics of Protons within Layered Hydroxides at High Pressure.

Studies of the structure of hydroxides under pressure using neutron diffraction reveal that the high concentration of hydrogen is distributed in a disordered network. The disorder in the hydrogen-bond network and possible phase transitions are reported to occur at pressures within the range accessible to experiments for layered calcium hydroxides, which are considered to be exemplary prototype materials. In this study, the static and dynamical properties of these layered hydroxides are investigated using a quantum approach describing nuclear motion, shown herein to be required particularly when studying diffusion processes involving light hydrogen atoms. The effect of high-pressure on the disordered hydrogen-bond network shows that the protons tunnel back and forth across the barriers between three potential minima around the oxygen atoms. At higher pressures the structure has quasi two-dimensional layers of hydrogen atoms, such that at low temperatures this causes the barrier crossing of the hydrogen to be significantly rarefied. Furthermore, for moderate values of both temperature and pressure this process occurs less often than the usual mechanism of proton transport via vacancies, limiting global proton diffusion within layers at high pressure.

Substitutional 4d and 5d impurities in graphene.

We describe the structural and electronic properties of graphene doped with substitutional impurities of 4d and 5d transition metals. The adsorption energies and distances for 4d and 5d metals in graphene show similar trends for the later groups in the periodic table, which are also well-known characteristics of 3d elements. However, along earlier groups the 4d impurities in graphene show very similar adsorption energies, distances and magnetic moments to the 5d ones, which can be related to the influence of the 4d and 5d lanthanide contraction. Surprisingly, within the manganese group, the total magnetic moment of 3 µB for manganese is reduced to 1 µB for technetium and rhenium. We find that compared with 3d elements, the larger size of the 4d and 5d elements causes a high degree of hybridization with the neighbouring carbon atoms, reducing spin splitting in the d levels. It seems that the magnetic adjustment of graphene could be significantly different if 4d or 5d impurities are used instead of 3d impurities.

Chemical Bonding of Transition-Metal Co13 Clusters with Graphene.

We carried out density functional calculations to study the adsorption of Co13 clusters on graphene. Several free isomers were deposited at different positions with respect to the hexagonal lattice nodes, allowing us to study even the hcp 2d isomer, which was recently obtained as the most stable one. Surprisingly, the Co13 clusters attached to graphene prefer icosahedron-like structures in which the low-lying isomer is much distorted; in such structures, they are linked with more bonds than those reported in previous works. For any isomer, the most stable position binds to graphene by the Co atoms that can lose electrons. We find that the charge transfer between graphene and the clusters is small enough to conclude that the Co-graphene binding is not ionic-like but chemical. Besides, the same order of stability among the different isomers on doped graphene is kept. These findings could also be of interest for magnetic clusters on graphenic nanostructures such as ribbons and nanotubes.

Co nanodot arrays grown on a GdAu2 template: substrate/nanodot antiferromagnetic coupling.

Controlling anisotropy and exchange coupling in patterned magnetic nanostructures is the key for developing advanced magnetic storage and spintronic devices. We report on the antiferromagnetic interaction between a Co nanodot array and its supporting GdAu2 nanotemplate that induces large anisotropy values in individual Co nanodots. In clear contrast with nonmagnetic Au substrates, GdAu2 triggers an earlier switch from out-of-plane anisotropy in monatomic high dots to in-plane when the dot height becomes biatomic.