Physical Review Applied

In electronics, cooling a device would seem to be as straightforward as putting a good thermal conductor on top. Graphene is one of the best thermal conductors and thus a natural choice, but realize that its thermal conductivity is a function of sample size $L$, and also strongly depends on temperature. Through extensive phonon-mode calculations, the authors show that graphene’s maximum thermal conductivity occurs around 100 K, and that $L$ of just 10 $\mu$m is sufficient for maximum effect. With these results, one can engineer a device with optimal $L$ for a required thermal conductivity at a given temperature.

Reverse-osmosis membranes made of composites of carbon nanotubes and aromatic polyamide have been exhaustively studied in recent years, due to their high salt rejection and permeability. However, there is still not complete agreement on the mechanism of diffusion of water through these interesting membranes. This study combines experiment and theory to propose a mechanism based on water hopping through the nanotubes dispersed within the membrane, and shows the great utility of multiscale simulations in this context. This understanding of water diffusion through nanocomposites is necessary for the optimization of next-generation desalination membranes.

Thermoelectric performance is expressed by the figure of merit $ZT$. However, as the electrical conductivity $\sigma$ in the numerator of the formula $ZT=S^2\sigma T/\kappa$ is related to the thermal conductivity $\kappa$ in the denominator, we still do not have a good guiding principle for obtaining a high $ZT$. The authors propose that $ZT$ is set by a universal function of a dimensionless parameter $\alpha$, which can be experimentally measured. The function reproduces most experimental results for most materials, whether they are one-, two-, or three-dimensional. From this we conclude that any semiconductor with $\alpha >4.5$ will show $ZT>2$, which could dramatically improve technology.

Given the importance of heat management in batteries and electronic circuitry, enhancing the efficiency of thermoelectric cooling materials could have a great technological impact. This research presents an approach for nanostructuring bismuth antimonide alloys, and yields an enhanced Seebeck coefficient for Bi${}_{1-x}$Sb${}_x$ nanocomposites. Thermopower clearly increases compared to that of bulk samples, even though the electron concentration is higher than in the bulk, which suggests that, consistent with theory, the increase results from size quantization. This result provides a promising strategy for improving the properties of this appealing thermoelectric alloy.

The states of π electrons localized in zigzag edges of graphene flakes exhibit a variety of electronic and magnetic properties. The authors review these edge-state spins, highlighting their use as probes in molecular sensors made of nanographene-based microporous carbon.

Can one size really fit all? The authors demonstrate the versatility of a graphene-based metasurface that is capable of actively controlling anomalous beam steering, focusing, cloaking, and illusion optics. These various functionalities are usually discussed in disparate fashion, but this work shows that they can all be described within a general framework for arbitrary surface morphology. This unified, simplified approach seems quite appealing for device design.

Progress in alkali-ion batteries based on graphite intercalation compounds (GICs) is hindered by a lack of microscopic understanding of the intercalation/de-intercalation process. To fill this gap, the authors use $ab$ $initio$ molecular dynamics calculations to investigate the dynamics of potassium intercalation in finite flakes of graphite. K atoms diffuse rapidly on a flake’s surface, then make an intriguing “U-turn” at the edge and enter the interlayer region, which widens substantially. This atomic-level insight is expected to benefit the development of potassium-ion batteries with high output voltage, energy density, and charge/discharge rate, plus low cost.

The continued miniaturization of electronics depends on our capability to engineer nanoscale heterostructures. This study focuses on the energetic response of anisotropic silicene upon injection or extraction of an electron, using density functional theory on varying armchair and zigzag edge lengths of rectangular silicene structures. The findings show that these finite structures are prone to behave as $n$-type semiconductors that could be used in low-dimensional heterojunctions. Moreover, a possible connection between the material’s energetic response and the distortion of its buckled lattice is discovered, which would be attractive in designing such devices.

In the quest for efficient light-emitting devices, improving the efficiency of photoluminescence in carbon nanotubes (CNTs) by utilizing their defects has become widespread. Here is a detailed investigation of the defects, electron and hole transport, and photoluminescence intensity of individual suspended CNTs. The results quantify the relationship between the $D/G$ -band Raman intensity ratio and photoluminescence intensity, and point to a potential limitation in using brightly luminescent CNTs for optoelectronic applications.

A great obstacle to technology using “forests” of highly ordered, vertically aligned carbon nanotubes (CNTs) is that collective properties of forests are inferior to those of individually grown freestanding CNTs. The pattern of changing growth rates seen in real-time measurements reveals that the carbon incorporation driving forest growth is governed by autocatalytic radical-chain polymerization of acetylene. Considering CNTs as a type of polymer renders the established methods of polymer science relevant for optimizing macroscopic platforms to retain the superior properties of individual CNTs in high-density energy storage, rapid heat dissipation, and advanced interconnects.