Hexagonal boron nitride (h-BN): structure, band gap and uses

A single layer of h-BN is a flat honeycomb, like graphene, but its two kinds of corner are occupied by different atoms: every boron is bonded to three nitrogen atoms and every nitrogen to three borons. The lattice constant is 2.504 Å, 1.8 % larger than graphene’s 2.460 Å (Yankowitz et al. 2012; Liu et al. 2022), so the two lattices almost match.

In the bulk crystal the layers are held together by van der Waals forces, 3.33 Å apart. They stack AA′: the hexagons sit directly above one another, but each boron lies over a nitrogen of the next layer, so the cell (hexagonal, P6₃/mmc, a = 2.504 Å, c = 6.661 Å) holds two layers (from its crystal file).

The chemistry changes everything electronic: graphene is a semimetal with no band gap, while h-BN is an insulator with a wide one, measured as an indirect gap of 5.955 eV (Cassabois et al. 2016).

1. Bulk single crystals. High-purity h-BN crystals are grown under high pressure and high temperature (Watanabe et al. 2004). Flakes from such single crystals were the substrates in the first high-quality graphene-on-h-BN devices (Dean et al. 2010).
2. Mechanical exfoliation. Adhesive tape peels thin flakes and single layers off a bulk crystal; boron nitride was among the first 2D crystals made this way (Novoselov et al. 2005). The flakes are then picked up and placed with polymer stamps (Wang et al. 2013; Pizzocchero et al. 2016).
3. Chemical vapour deposition. Large-area films two to five atomic layers thick can be grown by CVD (Song et al. 2010).
4. Liquid exfoliation. BN, like MoS₂ and other layered compounds, can be dispersed in common solvents into thin flakes and cast into films (Coleman et al. 2011).

• Wide band gap. An indirect gap of 5.955 eV (Cassabois et al. 2016); few-layer CVD films have an optical gap of 5.5 eV and are highly transparent over a broad wavelength range (Song et al. 2010).
• Deep-ultraviolet light. h-BN crystals luminesce strongly around 215 nm, with evidence of room-temperature lasing at that wavelength (Watanabe et al. 2004).
• A clean, flat surface. Atomically smooth and relatively free of dangling bonds and charge traps, with large optical phonon modes and a large electrical band gap (Dean et al. 2010).
• Strong and stiff. Mono- and few-layer h-BN are among the strongest electrically insulating materials, and unlike graphene their strength hardly changes with the number of layers (Falin et al. 2017).
• Oxidation resistance. Monolayer h-BN survives heating in air up to 850 °C (Li et al. 2014).
• Proton conductor. A single layer lets thermal protons through, with the highest room-temperature proton conductivity of the crystals tested (Hu et al. 2014).

Graphene on the usual SiO₂ substrate is highly disordered. Dean et al. (2010) placed graphene on single-crystal h-BN and found mobilities and carrier inhomogeneity almost an order of magnitude better, with less roughness, doping and chemical reactivity. This made h-BN a widely used substrate for 2D devices.

Wrapping graphene between two h-BN flakes and contacting it only along its edge gave devices with ballistic transport over more than 15 µm at low temperature (Wang et al. 2013), and the same pick-up methods build stacks of h-BN, graphene and other layers in any order: van der Waals heterostructures (Geim & Grigorieva 2013). Graphene aligned on h-BN also forms a moiré superlattice, because of the 1.8 % lattice mismatch (Yankowitz et al. 2012).

• Substrate and encapsulation. hBN/graphene/hBN stacks with edge contacts (Wang et al. 2013), made in batches by hot pick-up (Pizzocchero et al. 2016); h-BN-encapsulated MoS₂ transistors are stable even at elevated temperatures (Lee et al. 2015).
• Dielectric layers in heterostructures. A thin, flat insulator between the other layers of van der Waals heterostructures (Dean et al. 2010; Geim & Grigorieva 2013).
• Deep-ultraviolet emitters. Its 215 nm emission made h-BN a candidate for compact ultraviolet lasers (Watanabe et al. 2004).
• Proton-conducting membranes. Monolayer h-BN for hydrogen technologies (Hu et al. 2014).
• High-temperature coatings and reinforcement. Better oxidation resistance than graphene (Li et al. 2014) and good interlayer integrity for mechanical reinforcement (Falin et al. 2017).

For a paper, slide or poster, draw one layer as a honeycomb with two colours of atom (boron and nitrogen alternating), and a side view of two or more layers to show the AA′ stacking and the 3.33 Å spacing. The figures above show h-BN among the other 2D materials and in an encapsulated graphene stack, and open as editable LabCanvas figures. The structure drawing is a high-resolution SVG of a single h-BN layer that opens in the LabCanvas figure editor. The crystal structure drawing is a high-resolution SVG from the h-BN crystal file and opens in the figure editor, where you can change the style, the number of cells and the view. The 3D model opens in the LabCanvas 3D Studio, where you can stack layers, change the stacking and export a high-resolution image.