How long a material survives in ordinary air. Some 2D crystals keep for years on a shelf; others change within hours as oxygen, water and light attack them – and a measurement on a degraded sample says little about the real material.
Going deeper
Left: in air, oxygen and water attack a flake where its lattice is already interrupted – at edges, grain boundaries and vacancies – and light accelerates the reaction for many materials. Right: encapsulated between hBN sheets, which are themselves inert and nearly impermeable, the same flake is protected and keeps its properties far longer.
Why thin crystals are vulnerable
A bulk crystal exposes only a tiny fraction of its atoms to the air, and a thin oxide skin on the surface often protects the rest. A is all surface: every atom is exposed, and a reaction that would merely tarnish a bulk crystal can consume a 2D layer entirely. Degradation usually starts where bonds are already broken – edges, and – and spreads from there. For many materials oxygen and water act together, and light accelerates the reaction by creating charge carriers that take part in it.
How stability varies
The range is enormous. Graphene and hBN survive in air for years. monolayers such as MoS2 and WS2 are usable for weeks to months, but age visibly, oxidising along grain boundaries and edges and collecting organic contamination. Few-layer black phosphorus takes up water and degrades within hours; InSe and many tellurides degrade quickly too, and several are so sensitive that they are handled only in a glovebox. Thinner degrade faster than thicker ones of the same material, which makes stability a moving target as a sample is thinned.
Protecting a sample, and reporting how
The most effective protection is to exclude air from the start: exfoliate, stack and encapsulate in an inert glovebox, sealing the flake between hBN sheets or under a deposited oxide or polymer. devices of otherwise fragile materials can remain stable for months. Because degradation silently changes measurements – shifting , lowering , quenching emission – a result is only interpretable alongside how the sample was handled: the time it spent in air, under what light, and how it was protected. Stability measured on a thick flake does not carry over to a monolayer.
For specialists
Resistance to reaction with oxygen, water and light under ambient conditions, which often decides what can be measured at all. Few-layer black phosphorus takes up water and degrades visibly within hours; -grown TMDC monolayers age over months, oxidising along grain boundaries and collecting organic contamination. Report exposure time and handling with any result, and move to glovebox transfer and encapsulation when a material’s stability is short.