Oxygen vacancy levels mediated photophysical pathways of NIR-II responsive broadband upconversion

The photon upconversion (UC) is the process that has been known for a long time. Especially, lanthanide-based UC materials recently have attracted huge interest of scientific community due to their narrow visible emission bands upon near-infrared (NIR) excitation. However, the excitation in the high excitation power of NIR can cause highly bright and broad emission (B-UC). Such explosive B-UC was distinct at high excitation condition with UC wavelength encompassing the entire visible range. Previously, the proposed model of B-UC origination includes heat, multi-photon absorption, and accumulated thermal energy transfer. In this article, we will show various experimental and theoretical evidence supporting the idea that B-UC arises from photophysical pathways. Here, the key experimental evidence for photophysical pathways model of B-UC is to measure B-UC in the low-temperature environment (22 K). In addition, the density functional theory was used to regenerate energy levels which would be compared to the absorption spectral bands in Er2O3. As a result, we are confident of an electron tunneling model from 2P3/2 of Er3+ to oxygen vacancy level (VO) of Er2O3. Then, VO electrons are sequentially excited to the conduction band of Er2O3 by energy transfer from 4F9/2 of Er3+. Finally, the electrons return to the VO of Er2O3 with concomitant B-UC.