Growing coaxial nanotubes: Heterostructures of highly crystalline two-dimensional materials such as graphene, hexagonal boron nitride (hBN), and molybdenum disulfide (MoS ₂ ) are now routinely assembled from films or grown as layers. Xiang et al. report the growth of one-dimensional analogs of these heterostructures on single-walled carbon nanotubes (SWCNTs) through a chemical vapor deposition (see the Perspective by Gogotsi and Yakobson). Single-crystalline monolayers or multilayers of hBN or MoS ₂ were grown that maintained the electrical conductivity of the SWCNT. A monolayer of MoS ₂ was grown on a trilayer of hBN that encapsulated a SWCNT. Science , this issue p. 537 ; see also p. 506

In deformation processes, the presence of grain boundaries has a crucial influence on dislocation behavior; these boundaries drastically change the mechanical properties of polycrystalline materials. It has been considered that grain boundaries act as effective barriers for dislocation glide, but the origin of this barrier-like behavior has been a matter of conjecture for many years. We directly observe how the motion of individual dislocations is impeded at well-defined high-angle and low-angle grain boundaries in SrTiO₃, via in situ nanoindentation experiments inside a transmission electron microscope. Our in situ observations show that both the high-angle and low-angle grain boundaries impede dislocation glide across them and that the impediment of dislocation glide does not simply originate from the geometric effects; it arises as a result of the local structural stabilization effects at grain boundary cores as well, especially for low-angle grain boundaries. The present findings indicate that simultaneous consideration of both the geometric effects and the stabilization effects is necessary to quantitatively understand the dislocation impediment processes at grain boundaries.

Although the size effect in ferroelectric thin films has been known for long time, the underlying mechanism is not yet fully understood and whether or not there is a critical thickness below which the ferroelectricity vanishes is still under debate. Here, we directly measure the thickness-dependent polarization in ultrathin PbZr_0.2Ti_0.8O₃ films via quantitative annular bright field imaging. We find that the polarization is significantly suppressed for films <10-unit cells thick (∼4 nm). However, approximately the polarization never vanishes. The residual polarization is ∼16 μCcm⁻² (∼17%) at 1.5-unit cells (∼0.6 nm) thick film on bare SrTiO₃ and ∼22 μCcm⁻² at 2-unit cells thick film on SrTiO₃ with SrRuO₃ electrode. The residual polarization in these ultrathin films is mainly attributed to the robust covalent Pb–O bond. Our atomic study provides new insights into mechanistic understanding of nanoscale ferroelectricity and the size effects.