Atomically thin hexagonal boron nitride probed by ultrahigh-resolution transmission electron microscopy

We present a method to prepare monolayer and multilayer suspended sheets of hexagonal boron nitride (h-BN), using a combination of mechanical exfoliation and reactive ion etching. Ultrahigh-resolution transmission electron microscope imaging is employed to resolve the atoms, and intensity profiles for reconstructed phase images are used to identify the chemical nature (boron or nitrogen) of every atom throughout the sample. Reconstructed phase images are distinctly different for h-BN multilayers of even or odd number. Unusual triangular defects and zigzag and armchair edge reconstructions are uniquely identified and characterized.

Figure 1

(Color online) Boron nitride hexagonal lattice. Boron nitride maintains an $AAA$ stacking where boron and nitrogen atoms are alternately stacked on top of each other.

Figure 2

(Color online) Atomic structure of a one- to four-layer BN with its thickness map. (a) High resolution TEM image of BN. (b) The color gradient BN reconstructed phase image shown in (a). (c) A thickness map indicating the number of layers in boron nitride imaged here. The scale bar is 2 nm in this figure.

Figure 3

(Color online) The image of the sum of 20 unit cells in a one- to four-layer BN and its intensity line profile. (a) The resulting images from the summation of 20 unit cells in a one- to four-layer BN. The scale bar shows $1.5\text{Å}$. (b) The intensity line profile from the unit-cell images previously shown in (a). (c) A model for the edge-on structure of BN lattice with its mean atomic projected potential (shown on top of the lattice). (d) The simulated intensity modulations in an exit wave phase image for a single- to four-layer area in BN.

Figure 4

(Color online) Lattice structure in a monolayer of BN. (a) Reconstructed phase image of BN with a triangular monovacancy and a large vacancy previously pointed to by arrows in Fig. 2a. (b) A model indicating the position of boron and nitrogen atoms in the monolayer BN. This model shows the monovacancy to form as a result of a boron atom missing and the edges to be nitrogen terminating zigzag edges. (c) A model showing the boron and nitrogen atom positions in the large vacancy at the lower left side in (a). The scale bar is 1 nm.

Figure 5

(Color online) HRTEM image of BN sheet at the edge. BN sheets show both (a) zigzag and (b) armchair edges. The scale bar is 1 nm.