Graphene's exceptional gapless and linear electronic band structure close to the Dirac points gives rise to fascinating ultrafast phenomena, which are of great interest both for fundamental research as well as for technological applications. In particular, Auger scattering processes bridging the valence and the conduction band are very efficient in graphene. They change the charge carrier density and can give rise to a carrier multiplication (CM) that significantly increases the number of optically excited carriers. This is a promising many-particle phenomenon for highly efficient graphene-based photodetecting devices. In their Feature Article on pp. 2303–2310, Malic et al. present a review of the recent research on carrier multiplication in graphene and Landau-quantized graphene. The authors show theoretical predictions based on microscopic semiconductor Bloch equations and confront them with recent experimental pump-probe and angle-resolved photoemission measurements.

Rasool et al. (pp. 2351–2354) have developed a method to construct well-defined liquid containers with graphene windows. These containers are remarkably robust and can remain intact in the harsh imaging environments required for electron microscopy. The authors demonstrate that gold nanoparticles suspended in a phosphate buffer solution, a common biological buffer, can easily be imaged using a TEM. The confined nanoparticles can be imaged on multiple length scales, down to 3D atomic resolution motion. These liquid cell containers have broad applications and may be used to study complex nanoparticle assemblies with more complex motions in a fully hydrated state.

The remarkable electronic properties of graphene open up new carrier relaxation channels that can result in a significant multiplication of optically excited carriers. The research on this fundamentally interesting and technologically promising ultrafast many-particle phenomenon has been reviewed.

Piezo- and flexoelectricity are manifestations of electromechanical coupling in solids with potential applications in nanoscale materials. Michel et al. studied the in-plane strain and electric polarization response of static corrugations in 2D crystals by anharmonic lattice dynamics. In contradistinction to previous continuum mechanics-based work, the authors have found that contributions to the in-plane electric polarization due to anomalous flexoelectric effects are less important than those due to out-of-plane strains. The magnitude of the polarization is larger in stiffer materials such as 2D h-BN rather than in softer materials such as 2H-MoS₂.