2D Materials and Atomic Scale Imaging

True two-dimensional (2D) materials have a thickness of only one atom. Interest in 2D materials has increased significantly ever since the first group of researchers isolated a single sheet of graphite, known as graphene. Increased interest has been seen in the case of graphene in particular, but there are a number of other materials that can be synthesized directly or exfoliated into 2D sheets from bulk samples.

One such material is hexagonal boron nitride. This material is isoelectronic with graphene and has a similar honeycomb structure, however there is a key difference between hexagonal boron nitride (h-BN) and graphene in that the former has a band gap of over 5 eV.

This band gap makes h-BN a desirable material for several applications including durable coatings, electrical devices, and optics. When h-BN is layered with graphene, it can improve the electrical performance of graphene in field effect transistors and other similar devices.

However, before the widespread adoption of h-BN in these applications, it is important to understand the nanostructure, including the character and behavior of edge states and defects.

For the first time, researchers in Alex Zettl’s lab at UC Berkeley and at the National Center for Electron Microscopy at Lawrence Berkeley National Laboratory (NCEM, LBNL) used the Protochips system to characterize the atomic scale structure of CVD grown h-BN in situ in the TEM.

The team reported on the dynamic defect structures characterized along grain boundaries including heptagon/pentagon defects and holes induced by electron beam damage.

Experiment

Using a low-pressure chemical vapor deposition (LPCVD) technique, h-BN sheets were grown on the surface of copper films. Grain boundaries and defects are induced in the h-BN layer by the polycrystalline structure of the copper growth substrate, and it is necessary to characterize these defects to understand the material’s behavior.

A layer of h-BN was isolated for TEM analysis. This was done by dissolving the copper film using iron chloride, and transferring the layer to an E-chip™. A TEM image of h-BN transferred to the E-chip™ is shown in Figure 1.

Figure 1. TEM image at 450 °C showing holes, 0 L, singe layer, 1 L, and a double layer, 2 L, of h- BN

Single and two-layer sheets were observed as indicated in the image. The sample was inserted into the TEM and the temperature was increased to 800 °C to remove any residue and contamination present on the h-BN sheet. However, a temperature of 450 °C was used during the experiments because of the thermal instabilities in the material at higher temperatures.

It was observed that imaging at 450 °C, as opposed to room temperature, offered improved stability at high-resolution. This is because the buildup of hydrocarbon contamination on the h-BN film was prevented by the elevated temperature.

At NCEM, the TEAM 0.5 TEM was used for all imaging. The microscope is an FEI Titan Cubed with spherical aberration correctors for both the image and probe forming optics and a monochromator. The high-resolution needed to resolve the h-BN lattice structure of h-BN was provided by the aberration correctors and monochromator.

Beam damage was minimized by operating the TEM in bright field mode at 80 kV. The resolution was approximately 1 Å under these conditions. The high thermal stability of the system was key in preserving the resolution needed to examine the h-BN’s defect structure at high temperatures.

Discussion

During LPCVD growth of the h-BN, a polycrystalline copper film was used as the substrate. Small islands of h-BN nucleate on varied grains on the Cu substrate, and grow until they contact other islands of material. This creates defects and grain boundaries in the sheet.

As shown in Figure 2, pentagon-heptagon (5/7) defects were observed along grain boundaries, as opposed to the 6 membered rings that produce pristine h-BN. The 5/7 defects were unexpected, because nitrogen-nitrogen (N-N) and boron-boron (B-B) bonds result from these defects, which induce local dipole moments.

Although the N-N and B-B bonds observed in 5/7 defects are not energetically favorable in pristine h-BN, in certain instances they are favored along grain boundaries as predicted by theoretical calculations. 4/8 defects, which form along ripples in the sheet, were also observed. This defect structure has been predicted in BN nanotubes, and is more energetically likely to form in a curved section of the lattice than 5/7 defects.

Figure 2. TEM images taken at 450 °C showing a grain boundary in a layer of h-BN. Left panels shows two grains meet at an angle of 21°. Right panel shows a close up of the grain boundary and the defect structure, including 5/7 defects as indicated by the yellow pentagons and red heptagons.

The h-BN lattice structure was unstable as a result of knock-on damage, although imaging with an acceleration voltage of 80 kV helped to reduced beam damage. Monovacancies occur when boron is preferentially ejected from the lattice by the beam.

These monovacancies can grow into larger triangle-shaped holes, which may represent potential structures that can form in h-BN under normal conditions. However, grain boundaries were more stable under the beam, and amenable to imaging for comparatively longer periods.

Applications

The past few years have seen a dramatic increase in the research on 2D materials, and this research is poised for exponential growth in the near term. The properties of 2D materials, such as chemical, optical, mechanical, and electrical, will help to develop novel devices across a number of applications.

Particularly for h-BN, engineering defects, if precisely controlled in the material may allow tunable properties for certain applications. A stable, low drift sample holder system is needed to analyze these materials in situ at the atomic scale in the TEM.

Atomic resolution imaging and analysis of materials are facilitated by the Fusion heating and electrical biasing platform. This platform is also capable of harnessing the resolution capabilities of advanced instruments such as the TEAM 0.5.

This information has been sourced, reviewed and adapted from materials provided by Protochips.

For more information on this source, please visit Protochips.

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