Boron nitride nanosheet

Two-layer BN nanosheet.

Atomic-resolution images of a BN nanosheet prepared by CVD.^[1]

Boron nitride nanosheet is a two-dimensional crystalline form of the hexagonal boron nitride (h-BN), which has a thickness of one to few atomic layers. It is similar in geometry to its all-carbon analog graphene, but has very different chemical and electronic properties – contrary to the black and highly conducting graphene, BN nanosheets are electrical insulators with a band gap of ~5.9 eV, and therefore appear white in color.^[2]

Uniform monoatomic BN nanosheets can be deposited by catalytic decomposition of borazine at a temperature ~1100 °C in a chemical vapor deposition setup, over substrate areas up to about 10 cm². Owing to their hexagonal atomic structure, small lattice mismatch with graphene (~2%), and high uniformity they are used as substrates for graphene-based devices.^[2][3]

Structure

BN nanosheets consist of sp²-conjugated boron and nitrogen atoms that form a honeycomb structure.^[4][5] They contain two different edges: armchair and zig-zag. The armchair edge consists of either boron or nitrogen atoms, while the zig-zag edge consists of alternating boron and nitrogen atoms. These 2D structures can stack on top of each other and are held by Van der Waals forces to form few-layer boron nitride nanosheets. In these structures, the boron atoms of one sheet are positioned on top or below the nitrogen atoms due to electron-deficient nature of boron and electron-rich nature of nitrogen.^[5][6]

Synthesis

CVD

Chemical vapor deposition is the most common method to produce BN nanosheets because it is a well-established and highly controllable process that yields high-quality material over areas exceeding 10 cm².^[2][6] There is a wide range of boron and nitride precursors for CVD synthesis, such as borazine, and their selection depends on toxicity,^[6] stability,^[5][6] reactivity,^[6] and the nature of the CVD method.^[5][6][7]

Mechanical cleavage

A typical electron micrograph of BN nanosheets prepared by ball milling (scale bar 50 nm).^[8]

Mechanical cleaving methods of boron nitride use shear forces to break the weak van der Waals interactions between the BN layers.^[5] Cleaved nanosheets have low defect densities and retain the lateral size of the original substrate.^[5][6] Inspired by its use in the isolation of graphene, micromechanical cleavage, also known as the Scotch-tape method, has been used to consistently isolate few-layer and monolayer boron nitride nanosheets by subsequent exfoliation of the starting material with adhesive tape.^[5][6] The disadvantage of this technique is that it is not scalable for large-scale production.^[5][6][7]

Boron nitride sheets can be also exfoliated by ball milling, where shear forces are applied on the face of bulk boron nitride by rolling balls. This technique yields large quantities of low-quality material with poor control over its properties.^[5][6]

Unzipping of boron nitride nanotubes

BN nanosheets can be synthesized by the unzipping boron nitride nanotubes via potassium intercalation or etching by plasma or an inert gas. Here the intercalation method has a relatively low yield as boron nitride is resistive to the effects of intercalants.^[5][6]

Solvent exfoliation and sonication

Solvent exfoliation is often used in tandem with sonication to isolate large quantities of boron nitride nanosheets. Polar solvents such as isopropyl alcohol^[6] and DMF^[9] are more effective in exfoliating boron nitride layers than nonpolar solvents because these solvents possess a similar surface energy to the surface energy of boron nitride nanosheets. Combinations of different solvents also exfoliate boron nitride better than individual solvents.^[5] Many solvents suitable for BN exfoliation are rather toxic and expensive, but they can be replaced by water and isopropyl alcohol without significantly sacrificing the yield.^[5][6][9]

Chemical functionalization and sonication

Chemical functionalization of boron nitride involves attaching molecules onto the outer and inner layers of bulk boron nitride.^[6] There are three types of BN functionalization: covalent, ionic and or non-covalent.^[5] Layers are exfoliated by placing the functionalized BN into a solvent and allowing the solvation force between the attached groups and the solvent to break the van der Waal forces between BN layers.^[7] This method is slightly different from solvent exfoliation, which relies on the similarities between the surface energies of the solvent and boron nitride layers.

Solid state reactions

Heating a mixture of boron and nitrogen precursors, such as boric acid and urea, can produce boron nitride nanosheets.^[5][7] The number of layers in these nanosheets was controlled by temperature (ca. 900 ˚C) and the urea content.^[7]

Properties and applications

BN nanosheets are electrical insulators and exhibit a high thermal conductivity of 100–270 W/(m·K).^[4][5] They have a wide band gap of ~5.9 eV, which can be changed by the presence of Stone–Wales defects within the structure, by doping or functionalization, or by changing the number of layers.^[4][6] Owing to their hexagonal atomic structure, small lattice mismatch with graphene (~2%), and high uniformity, BN nanosheets are used as substrates for graphene-based devices.^[2][3] BN nanosheets are also excellent proton conductors. Their high proton transport rate, combined with the high electrical resistance, may lead to applications in fuel cells and water electrolysis.^[10]

References