Phosphorescence by Trapping Defects in Boric Acid Induced by Thermal Processing

Abstract

The phosphorescence of boric acid (BA, H3BO3) at room temperature is a puzzling phenomenon subject to controversial interpretations although the role of structural defects has not yet been considered. Heat treatments of boric acid cause its transformation into the metaboric phase and amorphous boron oxide (B2O3). The structural changes after thermal processing can create defects that become centers of luminescence and recombination channels in the visible range. In the present work, commercial boric acid is thermally processed at different temperatures. Samples treated between 200 and 400 °C exhibit remarkable phosphorescence in the visible range. At ≈480 and 528 nm, two distinct phosphorescent emissions occur, associated with trapped charge carriers recombinations identified by thermoluminescence (TL) and electron paramagnetic resonance spectroscopy (EPR). The structural and optical studies suggest that the activation of boric acid phosphorescence after heat treatment is correlated with the presence of defects. The afterglow results from a trapping and detrapping process, which delays the recombination at the active optical centers. Time‐dependent density functional study (DFT) of defective BOH molecules and clusters shows the emergence of near UV and blue optical transitions in absorption. These defects trigger the photoluminescence in thermally processed boric acid samples.

Visible-Light-Excited Room-Temperature Phosphorescent Carbon Dots via Surface Amination in a Microreactor for Multimode Anticounterfeiting

“…Furthermore, when the concentration of CDs decreases to 1% or lower, the visible afterglow time reaches a maximum value of around 10 s. As the concentration of CDs decreases to a lower level, the luminescent intensity decreases, and the afterglow time remains the same. Although there are some reports indicating that BA may also exhibit phosphorescence, , and phosphorescence from BA itself is weak and hardly observed in this research (Figure S6: pictures of BA under and after UV), the extensive lifespan and color regulation space in this work should be attributed to CDs. This time difference provides new anticounterfeiting ideas.…”

From Defects to Photoluminescence in h-BN 2D and 0D Nanostructures

“…Several research groups have observed the rise of such fluorescence, which is explained by the possible presence of defects. Based on experiments performed on boron nitride nanostructures, it was hypothesized that this effect may also be directly related to the formation of structural defects in boron oxide . The investigation of boron oxide phase transitions during heat treatments at increasing temperatures revealed the occurrence of structural modifications when H 3 BO 3 melts and afterward recrystallizes (Figure ).…”

“…Gray and light blue areas highlight the temperature interval between the two H 3 BO 3 dehydration processes. Reproduced from ref . Copyright 2023 Wiley.…”

“…(a) EPR spectra of sample treated at 250 °C, (a) under UV irradiation, (b) after exposing the EPR cell to room temperature, and (c) after 1 day at room temperature. (b) Sketch of the processes related to the spectra evolution in panel (a) and involving a boron-containing structural defect (BO x ), an isolated proton, and a further not assigned defect X. Reproduced from ref . Copyright 2023 Wiley.…”

B–O–O–B Impurity Induces Ultralong Room-Temperature Phosphorescence of Boric Acid

“…Ultralong room-temperature phosphorescence (RTP) has important applications in the fields of optoelectronics, light-emitting diodes, biological imaging, anticounterfeiting technology, etc. − Conventional RTP phosphors usually rely on inorganic compounds containing rare earth/transition metals, which have strong spin–orbit coupling for the intersystem crossing between singlet and triplet excitons. ,− Pure organic phosphors with long-lived RTP have attracted increasing interest recently as a result of their advantages of low cost, easy fabrication, non-toxicity, and high biological compatibility. ,, Especially, some pure single-component molecular crystals were also reported to be able to emit RTP, e.g., the crystals of carbazole derivatives, triazine derivatives, and boric acid (BA). − The origin of RTP in these single-component molecular crystals has aroused heated debate in the community. RTP of them is originally attributed to H aggregation in the molecular packing for the carbazole derivatives and triazine derivatives and to weak conjugation between the n electrons of O atoms for BA by their finders. − Chen et al and Wu et al later argue that impurities in the commercial molecular crystal sources should be responsible for RTP in the carbazole derivatives and BA; i.e., the crystals used in previous experiments are not pure actually. , Ding et al further report that even trace impurity (<0.1%) can achieve bright RTP in molecular crystals .…”