Wang Jian, Wang Zhiqiang, Cho Hyunjin, Kim Myung Jong, Sham T K, Sun Xuhui
Canadian Light Source Inc., University of Saskatchewan, Saskatoon, SK S7N 2V3, Canada.
Nanoscale. 2015 Feb 7;7(5):1718-24. doi: 10.1039/c4nr04445b.
Chemical imaging, thickness mapping, layer speciation and polarization dependence have been performed on single and multilayered (up to three layers and trilayered nanosheets overlapping to form 6 and 9 layers) hexagonal boron nitride (hBN) nanosheets by scanning transmission X-ray microscopy. Spatially-resolved XANES directly from freestanding regions of different layers has been extracted and compared with sample normal and 30° tilted configurations. Notably a double feature σ* excitonic state and a stable high energy σ* state were observed at the boron site in addition to the intense π* excitonic state. The boron projected σ* DOS, especially the first σ* exciton, is sensitive to surface modification, particularly in the single layered hBN nanosheet which shows more significant detectable contaminants and defects such as tri-coordinated boron/nitrogen oxide. The nitrogen site has shown very weak or no excitonic character. The distinct excitonic effect on boron and nitrogen was interpreted to the partly ionic state of hBN. Bulk XANES of hBN nanosheets was also measured to confirm the spectro-microscopic STXM result. Finally, the unoccupied electronic structures of hBN and graphene were compared.
通过扫描透射X射线显微镜对单层和多层（多达三层以及三层纳米片重叠形成6层和9层）六方氮化硼（hBN）纳米片进行了化学成像、厚度映射、层形态分析和偏振依赖性研究。直接从不同层的独立区域提取了空间分辨的X射线吸收近边结构（XANES），并与样品法线和30°倾斜配置进行了比较。值得注意的是，除了强烈的π激子态外，在硼位点还观察到了双重特征的σ激子态和稳定的高能σ态。硼投影的σ态密度（DOS），特别是第一个σ*激子，对表面改性敏感，尤其是在单层hBN纳米片中，其显示出更显著的可检测污染物和缺陷，如三配位硼/氮氧化物。氮位点显示出非常弱或没有激子特性。对硼和氮的不同激子效应被解释为hBN的部分离子态。还测量了hBN纳米片的体相XANES，以证实光谱显微镜扫描透射X射线显微镜（STXM）的结果。最后，比较了hBN和石墨烯的未占据电子结构。
