Can graphene improve the thermal conductivity of copper nanofluids?

Copper (Cu) nanofluids (NFs) have attracted attention due to their high thermal conductivity, which has conferred a wide variety of applications. However, their high reactivity favors oxidation, corrosion and aggregation, leading them to lose their properties of interest. Copper capped by graphene (Cu@G) core@shell nanoparticles (NPs) have also attracted interest from the medical and industrial sectors because graphene can shield the Cu NPs from undesired phenomena. Additionally, they share some properties that expand the range of applications of Cu NFs. In this work, new Morse potentials are reported to reproduce the behavior of Cu@G NPs through molecular dynamics. Coordination-dependent Morse parameters were fitted for C, H, and Cu based on density functional theory calculations. Then, these parameters were implemented to evaluate the thermal conductivity of Cu@G NFs employing the Green-Kubo formalism, with NPs from 1.5 to 6.1 nm at 100 to 800 K, varying the size, the number of layers and the orientation of the graphene flakes. It was found that Cu@G NFs are stable and have an improved thermal conductivity compared to the Cu NFs, being 3.7 to 18.2 times higher at 300 K with only one graphene layer and above 26.2 times higher for the graphene-trilayered NPs. These values can be higher for temperatures below 300 K. Oppositely, the size, homogeneity and orientations of the graphene flakes did not affect the thermal conductivity of the Cu@G NFs.

COVID-19 mitigation by digital contact tracing and contact prevention (app-based social exposure warnings).

A plethora of measures are being combined in the attempt to reduce SARS-CoV-2 spread. Due to its sustainability, contact tracing is one of the most frequently applied interventions worldwide, albeit with mixed results. We evaluate the performance of digital contact tracing for different infection detection rates and response time delays. We also introduce and analyze a novel strategy we call contact prevention, which emits high exposure warnings to smartphone users according to Bluetooth-based contact counting. We model the effect of both strategies on transmission dynamics in SERIA, an agent-based simulation platform that implements population-dependent statistical distributions. Results show that contact prevention remains effective in scenarios with high diagnostic/response time delays and low infection detection rates, which greatly impair the effect of traditional contact tracing strategies. Contact prevention could play a significant role in pandemic mitigation, especially in developing countries where diagnostic and tracing capabilities are inadequate. Contact prevention could thus sustainably reduce the propagation of respiratory viruses while relying on available technology, respecting data privacy, and most importantly, promoting community-based awareness and social responsibility. Depending on infection detection and app adoption rates, applying a combination of digital contact tracing and contact prevention could reduce pandemic-related mortality by 20-56%.

The unexpected effect of vacancies and wrinkling on the electronic properties of MoS2 layers.

We report a combined experimental/theoretical approach to study the connection of S-vacancies and wrinkling on MoS2 layers, and how this feature produces significant changes in the electronic structure and reactivity of this 2D material. The MoS2 material, when used as a catalyst in operative conditions, was found to be mainly composed of thin and short 1-5 layer sheets instead of a poorly crystalline structure, as it was previously assumed. Notably wrinkled structures with S-vacancies were also found through transmission electron microscopy. Atomistic simulations revealed a natural connection between sulfur-vacancies, wrinkling and folding. Density functional calculations further revealed that such curved structures present a lower electronic band-gap and a higher reactivity towards thiophene compared to the planar MoS2 counterpart.

A density functional study on the reactivity enhancement induced by gold in IrAu nanoalloys.

IrAu nanoalloys have been proven to have remarkable reactivity for several reactions. In this work, mixed IrAu nanoalloys of 8, 27, 48 and 64 total atoms were studied in different atomic compositions (Ir m Au n ) using Density Functional Theory (DFT). A notable segregation tendency is observed, where Ir atoms are located in the inner part and Au atoms in the outermost region of the nanostructure. We found that IrAu nanoalloys present a distinctive synergistic effect with respect to reactivity. In addition, the projected density of electronic states (PDOS) energies were analyzed by examining the d-band shift to estimate the reactivity of various IrAu nanoalloys. Furthermore, the adsorption energies for the CO molecule in the domains of the Ir-Au interface were evaluated. In this sense, the addition of Au atoms to Ir clusters increases the reactivity of Ir by generating unoccupied orbitals near the Fermi level as indicated by the PDOS study.

Effects of oxidation on the plasmonic properties of aluminum nanoclusters.

The scouting of alternative plasmonic materials able to enhance and extend the optical properties of noble metal nanostructures is on the rise. Aluminum is endowed with a set of interesting properties which turn it into an attractive plasmonic material. Here we present the optical and electronic features of different aluminum nanostructures stemming from a multilevel computational study. Molecular Dynamics (MD) simulations using a reactive force field (ReaxFF), carefully validated with Density Functional Theory (DFT), were employed to mimic the oxidation of icosahedral aluminum nanoclusters. Resulting structures with different oxidation degrees were then studied through the Time-Dependent Density Functional Tight Binding (TD-DFTB) method. A similar approach was used in aluminum nanoclusters with a disordered structure to study how the loss of crystallinity affects the optical properties. To the best of our knowledge, this is the first report that addresses this issue from the fully atomistic time-dependent approach by means of two different and powerful simulation tools able to describe quantum and physicochemical properties associated with nanostructured particles.

Mechanochemical stability of sub-nm ZnO chains.

Formation of monoatomic chains by axial stretching of zinc oxide nanowires is investigated using molecular dynamics and supported by density functional calculations. Special focus is made on the mechanical properties of these structures. Using a state-of-the-art force field it was found that O2 species are commonly formed within the chain. This species drastically weakens the chain strength. Previous simulations, based on a pair potential, failed to predict O2 formation. Moreover, the superductility of zinc oxide nanowires observed in earlier studies, was found to be an artifact of the pair potential. Simulations revealed that the chain length before rupture (usually of 6 atoms) is independent of the nanowire diameter. The electronic structure and the charge distribution of the chains were also studied.

Spontaneous formation of metallic nanostructures on highly oriented pyrolytic graphite (HOPG): an ab initio and experimental study.

We have investigated the decoration of step-edges of HOPG by Ag, Au and Pt using experimental and theoretical approaches. Metallic nanowires can be formed on bare or functionalized step-edges. Energy dispersion analysis indicates the presence of oxygenated groups. The experiments showed that nanowires can be obtained with the three metals along the step-edges, but the shapes and morphologies are very different. We have found that the interaction between the metal wires and the carbon follows the sequence: Pt > Au > Ag. The electronic redistribution between the atoms participating in the bond between the metallic nanowire and the step-edges shows a complicated pattern. The density of electronic states projected on the different atoms indicates that there are different orbitals participating in the bonds.