True two-dimensional (2D) materials are only as thick as one atom. Ever since the first group of scientists isolated a single sheet of graphite, known as graphene, interest in 2D materials has significantly increased. This increased interest has been particularly focused on graphene, but there are various other materials that can either be directly synthesized or exfoliated into 2D sheets from bulk samples.
Hexagonal boron nitride is one of these materials. It is isoelectric with graphene and has a similar honey comb structure. However, the key difference between it and graphene is that hexagonal boron nitride (h-BN) has a band gap of over 5 eV.
This band gap means that h-BN is a useful material for several applications including electrical devices, optics and durable coatings. When layered with graphene, h-BN can improve the electrical performance of graphene in field transistors and other similar devices.
Before the h-BN can be widely used in these applications, it is important to understand the nanostructure, including the behavior and character of edge states and defects.
Scientists 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 for the first time.
The scientists reported on the dynamic defect structures characterized along grain boundaries. These included pentagon/heptagon defects and holes created by electron beam damage.
h-BN sheets were grown on the surface of copper films using a low-pressure chemical vapor deposition (LPCVD) technique. Grain boundaries and defects are induced in the h-BN layer by the polycrystalline structure of the copper growth substrate. It is therefore necessary to characterize these defects to understand the behavior of the material.
A layer of h-BN was isolated for TEM analysis. The h-BN layer was isolated by using iron chloride to dissolve the copper film. The layer was transferred to an E-chipTM. Figure 1 shows a TEM image of the h-BN transferred to the E-chipTM.
Figure 1. TEM image at 450 °C showing holes, 0 L, singe layer, 1 L, and a double layer, 2 L, of h- BN
As indicated in the image, single and two-layer sheets were observed. The sample was inserted into the TEM and the temperature was then increased to 800 oC in order to remove any residue and contamination present on the h-BN sheet. However, because of the thermal instabilities in the material at higher temperatures, a temperature of 450 oC was used for these experiments.
Imaging at 450 oC rather than at room temperature was observed to offer improved stability at high-resolution. This is because the elevated temperature prevented the build-up of hydrocarbon contamination on the h-BN film. The TEAM 0.5 TEM was used for all imaging at NCEM. The microscope is an FEI Titan Cubed with spherical aberration correctors for both the image and probe forming optics and a monochromator. The aberration correctors and monochromator produced the high-resolution needed to resolve the h-BN lattice structure of h-BN.
By operating the TEM in bright field mode at 80 kV, beam damage was minimized. Under these conditions, the resolution was approximately 1 Å. The key to preserving the resolution needed to examine the h-BN’s defect structure at high temperatures was the high thermal stability.
A polycrystalline copper film was used as the substrate during LPCVD growth of the h-BN. Small islands of h-BN nucleate on a variety of grains on the Cu substrate. They grow until they contact other islands of material and this creates defects and grain boundaries in the sheet.
Pentagon-heptagon (5/7) defects were observed along grain boundaries (Figure 2), compared to the 6 membered rings that produce pristine h-BN. The 5/7 defects were unexpected, because boron-boron (B-B) and nitrogen-nitrogen (N-N) bonds arise from these defects. These include local dipole moments.
The B-B and N-N bonds observed in 5/7 defects are not energetically favorable in pristine h-BN. However, they are favored along grain boundaries in certain instances as predicted by theoretical calculations. 4/8 defects were also observed, and these form along ripples in the sheet. The defect structure has been predicted in BN nanotubes, and, compared to the 5/7 defects, is more energetically likely to form in a curved section of the lattice.
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.
Due to knock-on damage, the h-BN lattice structure was unstable. However, beam damage was reduced by imaging with an acceleration voltage of 80 kV. When boron is preferentially ejected from the lattice by the beam, monovacancies occur.
These monovacancies can grow into larger, triangle-shaped holes. These may represent potential structures that can form in h-BN under normal conditions. However, under the beam, grain boundaries were more stable and more amenable to imaging for relatively longer periods.
In the last few years there has been a dramatic increase in the research on 2D materials. This research is posed for exponential growth in the near future. The properties of 2D materials, such as optical, chemical, electrical and mechanical, will help to develop novel devices ranging across a number of applications.
If precisely controlled in the material, engineering defects may allow tunable properties for certain applications. This is particularly true for h-BN. In order to analyze these materials in situ at the atomic scale in the TEM, a stable, low drift sample holder system is required.
Analysis and atomic resolution imaging of materials are facilitated by the Fusion heating and electrical biasing platform. This platform is also able to harness the resolution capabilities of advanced instruments like the TEAM 0.5.
This information has been sourced, reviewed and adapted from materials provided by Protochips.
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