Despite being only one-atom thick, defect-free graphene is considered to be completely impermeable to all gases and liquids^{1,2,3,4,5,6,7,8,9,10}. This conclusion is based on theory^3,4,5,6,7,8 and supported by experiments^1,9,10 that could not detect gas permeation through micrometre-size membranes within a detection limit of 10⁵ to 10⁶ atoms per second. Here, using small monocrystalline containers tightly sealed with graphene, we show that defect-free graphene is impermeable with an accuracy of eight to nine orders of magnitude higher than in the previous experiments. We are capable of discerning (but did not observe) permeation of just a few helium atoms per hour, and this detection limit is also valid for all other gases tested (neon, nitrogen, oxygen, argon, krypton and xenon), except for hydrogen. Hydrogen shows noticeable permeation, even though its molecule is larger than helium and should experience a higher energy barrier. This puzzling observation is attributed to a two-stage process that involves dissociation of molecular hydrogen at catalytically active graphene ripples, followed by adsorbed atoms flipping to the other side of the graphene sheet with a relatively low activation energy of about 1.0 electronvolt, a value close to that previously reported for proton transport^11,12. Our work provides a key reference for the impermeability of two-dimensional materials and is important from a fundamental perspective and for their potential applications.

Nanoscale materials offer much promise in the pursuit of high-efficient energy conversion technology owing to their exceptional sensitivity to external stimulus. In particular, experiments have demonstrated that flowing water over carbon nanotubes can generate electric voltages. However, the reported flow-induced voltages are in wide discrepancy and the proposed mechanisms remain conflictive. Here we find that moving a liquid–gas boundary along a piece of graphene can induce a waving potential of up to 0.1 V. The potential is proportional to the moving velocity and the graphene length inserted into ionic solutions, but sharply decreases with increasing graphene layers and vanishes in other materials. This waving potential arises from charge transfer in graphene driven by a moving boundary of an electric double layer between graphene and ionic solutions. The results reveal a unique electrokinetic phenomenon and open prospects for functional sensors, such as tsunami monitors.

Developments in semiconductor technology are propelling the dimensions of devices down to 10 nm, but facing great challenges in manufacture at the sub-10 nm scale. Nanotechnology can fabricate nanoribbons from two-dimensional atomic crystals, such as graphene, with widths below the 10 nm threshold, but their geometries and properties have been hard to control at this scale. Here we find that robust ultrafine molybdenum-sulfide ribbons with a uniform width of 0.35 nm can be widely formed between holes created in a MoS₂ sheet under electron irradiation. In situ high-resolution transmission electron microscope characterization, combined with first-principles calculations, identifies the sub-1 nm ribbon as a Mo₅S₄ crystal derived from MoS₂, through a spontaneous phase transition. Further first-principles investigations show that the Mo₅S₄ ribbon has a band gap of 0.77 eV, a Young’s modulus of 300GPa and can demonstrate 9% tensile strain before fracture. The results show a novel top–down route for controllable fabrication of functional building blocks for sub-nanometre electronics.