Researchers have been studying the growth and characterization of 2D materials. As the field expands beyond graphene, there is an increasing interest in the study of atomic planes of other Van der Waals solids and heterostructures, which are formed by arranging layers having complementary features to attain novel characteristics. Hexagonal boron nitride is one such material serving as a high quality substrate for graphene.
Chemical vapor deposition has proved useful in the preparation and production of graphene and also applied for synthesizing other two-dimensional materials such as hexagonal boron nitride and molybdenum sulfide. The researchers had shown previously that the Nanofab Agile tool from Oxford Instruments capable of PECVD and CVD processes can be utilized for monolayer graphene growth and other graphene- like allotropes. The researchers have recently designed a thermal CVD route for the synthesis of hexagonal Boron Nitride (hBN) using Nickel foils as catalyst in this tool.
The technique followed is given below:
- Standard semiconductor process gases including diborane (B2H6) and ammonia (NH3) were introduced in appropriate proportions over a nickel foil heated to -1000°C, which were pre-treated in a reducing atmosphere.
- Nickel catalyzes the reaction of these components resulting in the nucleation and growth of hBN islands.
- Quick sample transfer was enabled by the load-locked sample transfer exchange and also helped to accurately hinder the reaction to observe these growth fronts before forming a continuous film using an SEM and also on an Oxford Instruments Asylum Research MFP-3D Classic AFM
Figure 1. (a) SEM image showing a triangular hBN island growing on a Nickel crystal face. (b)AFM topography and (c) AFM lateral force image of a growing edge.
To confirm hBN deposition, Raman spectroscopy and X-Ray Photoelectron Spectroscopy (XPS) (Figure 2) was used. A sharp peak observed at approximately 1368cm-1 (excitation at 532nm) rises from the E2g phonon and symbolizes the h-BN phase. When the spectrum is examined more closely broad peaks that may occur due to un-wanted co-deposition of the cubic phase, amorphous BN soot or the carbon contaminated phase.
Figure 2. (a) Raman spectrum of hBN on Ni showing the characteristic peak ~1368 cm-1 (b)Zoomed in plot of Spectrum shown in (a) to elucidate the absence of non-hBN phase (c) XPS survey scan, (d) Nls(at 398.4 eV) and (e) B1s(at 191.03 eV)
X-ray photoelectron spectroscopy is used to characterize the chemical structures. XPS measurements were done using an Al-K X-ray source on the samples. Figure 2 shows the XPS spectrum for the transferred sample on Nickel, where two peaks at 191.03eV and 398.4eV are identified as the binding energies of the B 1s and N 1s electrons respectively.
Integrated intensity analysis and these energy values confirm the formation of hexagonal Boron Nitride presented in an equal stoichiometric ratio. Oxford Instruments features an extensive portfolio of deposition and characterization tools tailored towards research in the field of graphene and 2D materials.
This information has been sourced, reviewed and adapted from materials provided by Oxford Instruments Plasma Technology.
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