Two-Dimensional Materials

Two-dimensional materials are crystalline solids only one or a few atoms thick, isolated from layered parent crystals, whose confinement to a single plane gives electronic, optical, and chemical properties unavailable in their bulk form.

Definition

A two-dimensional material is a sheet of a crystalline solid one or a few atomic layers thick, derived from a layered parent in which strong in-plane bonding coexists with weak interlayer forces, so that individual layers can be separated and behave as a distinct, confined material.

Scope

This topic covers the chemistry of atomically thin sheets: graphene as the archetype, the semiconducting transition-metal dichalcogenides such as molybdenum disulfide, hexagonal boron nitride, and emerging families like MXenes. It treats how such sheets are obtained — mechanical and liquid-phase exfoliation of layered crystals and bottom-up growth — their surface and edge chemistry, and the property changes, such as the emergence of a direct band gap on thinning, that confinement to two dimensions produces.

Core questions

• How are single atomic layers isolated from layered parent crystals?
• How do properties change when a layered material is thinned to one layer?
• What is the surface and edge chemistry of two-dimensional sheets?
• Beyond graphene, what families of two-dimensional materials exist?

Key concepts

• Graphene
• Transition-metal dichalcogenides
• van der Waals layering
• Mechanical and liquid-phase exfoliation
• Monolayer band-gap crossover
• Edge and surface functionalisation

Key theories

Exfoliation of layered crystals

Layered solids are held together in-plane by strong bonds but between layers by weak van der Waals forces, so individual sheets can be peeled off mechanically or separated in liquids by intercalation and sonication to give two-dimensional flakes.

Dimensional confinement and emergent properties

Reducing a layered crystal to a single sheet confines electrons to a plane, producing properties absent in the bulk — graphene's massless-carrier transport and the indirect-to-direct band-gap crossover seen when transition-metal dichalcogenides are thinned to a monolayer.

Mechanisms

In liquid exfoliation, solvent or intercalant molecules penetrate between layers and reduce the interlayer attraction so that agitation separates individual sheets; chemical functionalisation occurs preferentially at reactive edges and defect sites, where dangling bonds are most accessible.

Clinical relevance

Two-dimensional materials are studied for high-mobility and flexible electronics, transparent conductors, sensors with very high surface sensitivity, electrocatalysts for hydrogen evolution, and selective membranes, with the choice of material set by whether a conductor, semiconductor, or insulator is required.

History

The 2004 isolation of single-layer graphene by Novoselov and Geim, who used adhesive tape to cleave graphite, showed that a stable atomically thin crystal could exist and triggered the field. Subsequent work extended exfoliation to many other layered compounds and developed scalable liquid-phase routes, establishing two-dimensional materials as a broad family.

Key figures

• Andre Geim
• Konstantin Novoselov
• Jonathan Coleman

Seminal works

• novoselov2004
• geim2007
• nicolosi2013

Frequently asked questions

What makes graphene different from graphite?

Graphite is a stack of many graphene layers held together by weak van der Waals forces. A single isolated layer — graphene — confines its electrons to two dimensions, giving it distinctive properties such as extremely high carrier mobility that the three-dimensional stack does not show.

Why does molybdenum disulfide become a better light emitter as a single layer?

In the bulk, molybdenum disulfide has an indirect band gap, which makes light emission inefficient. When thinned to a single layer, confinement changes the band structure so that the gap becomes direct, allowing efficient absorption and emission of light.