Lattice Dynamics and Contraction of Energy Bandgap in Photoexcited Semiconducting Boron Nitride Nanotubes

• Published in: ACS nano 2019-10, Vol.13 (10), p.11623-11631
• Main Authors: Li, Zhongwen, Xiao, Rui-Juan, Xu, Peng, Zhu, Chunhui, Sun, Shuaishuai, Zheng, Dingguo, Wang, Hong, Zhang, Ming, Tian, Huanfang, Yang, Huai-Xin, Li, Jian-Qi
• Format: Article
• Language: English
• ISSN: 1936-0851; 1936-086X
• DOI: 10.1021/acsnano.9b05466

Structural dynamics and changes in electronic structures driven by photoexcited carriers are critical issues in both semiconducting and optoelectronic nanodevices. Herein, a phase diagram for the transient states and relevant dynamic processes in multiwalled boron nitride nanotubes (BNNTs) has been extensively studied for a full reversible cycle after a fs-laser excitation in ultrafast TEMs, and the significant structural features and evolution of electronic natures have been investigated using pulsed electron diffraction and femtosecond-resolved electron energy-loss spectroscopy (EELS). It is revealed that nonthermal anisotropic alterations of the lattice apparently precede the phonon-driven thermal transients along the radial and axial directions. Ab initio calculations support these findings and show that electrons excited from the π to π* orbitals in the BN nanotubes weaken the intralayer bonds while strengthening the interlayer bonds along the radial direction. Importantly, time-resolved EELS measurements show contraction of the energy bandgap after fs-laser excitation associated with nonthermal structural transients. This fact verifies that laser-induced bandgap renormalization in semiconductors can essentially be correlated with both the rapid processes of excited carriers and nonthermal lattice evolution.