AVS 61st International Symposium & Exhibition
    Novel Trends in Synchrotron and FEL-Based Analysis Focus Topic Tuesday Sessions
       Session SA-TuA

Paper SA-TuA11
Layer Speciation and Electronic Structure Investigation of Hexagonal Boron Nitride Thin Film by Scanning Transmission X-ray Microscopy

Tuesday, November 11, 2014, 5:40 pm, Room 312

Session: Free Electron Laser and Synchrotron Studies at the Molecule-Surface Interfaces
Presenter: Jian Wang, Canadian Light Source Inc., Canada
Authors: J. Wang, Canadian Light Source Inc., Canada
Z. Wang, University of Western Ontario, Canada
H. Cho, Korea Institute of Science and Technology, Republic of Korea
M.J. Kim, Korea Institute of Science and Technology, Republic of Korea
T.-K. Sham, University of Western Ontario, Canada
X. Sun, Soochow University, China
Correspondent: Click to Email
Thin films of hexagonal boron nitride (hBN) exhibit a honey cone structure similar to that of graphene, with sp2 hybridized boron and nitrogen atoms alternately bonded in the basal plane. This unique structure leads to some excellent properties such as high chemical and thermal stabilities, enhanced thermal and electrical conductivity in the basal plane, and versatile doping capabilities. Thus hBN thin films have attracted increasing attentions in many fields. However, unlike graphene, the partly ionic B-N bond in hBN reduces electron-delocalization and creates a large band gap (5.2 eV) in the ultraviolet (UV), making hBN a wide band gap semiconductor and a promising deep UV light emitter. Therefore, an in depth understanding of the morphology and the electronic structure of individual hBN thin films will be of great importance in the development of sophisticated technologies. Synchrotron based scanning transmission X-ray microscopy (STXM) using a nanoscaled focused soft X-ray beam (~30 nm) provides an excellent combination of microscopic examination and chemical/electronic structure speciation via XANES spectroscopy for individual nanomaterials. In this work, chemical imaging, thickness mapping and layer speciation have been performed on a multilayered CVD hBN film with thickness from single layer up to 9 layers by STXM. Spatially-resolved XANES directly from discrete layers have been extracted and compared. Notably a double feature σ* exciton state and a stable high energy σ* state were observed at the boron site, and the boron projected σ* DOS, especially the first σ* exciton, is sensitive to surface modification, particularly in the single layer regions which show detectable contaminants and defects. Nitrogen site has shown no exciton character. The distinct exciton effect on boron and nitrogen was interpreted to the partly ionic state of hBN. Bulk XANES spectroscopy of hBN thin films was also measured to confirm the spectro-microscopic STXM result. Finally, we compare the XANES (i.e. unoccupied electronic structure) of hBN with that of graphene to elucidate the similarities and origins of the corresponding spectroscopic features.