1.INTRODUCTION
Afterglow phosphors are capable
of a long-term storage for varied excitation photons, including X-ray,
ultraviolet, and visible
light.[1-6] Their
potential application in
information encryption has been
widely demonstrated in a two-dimensional (2D) manner such as emergence
sign,[7] anti-counterfeiting
pattern,[8] and X-ray imaging
screen.[9]Up
to date, many afterglow materials have been developed such as
SrAl2O4: Eu2+,
Dy3+,[10] CaS:
Eu2+,
Sm3+,[11]SrSi2AlO2N3:Eu2+,
Ln3+.[12] These
traditional
phosphors usually have a high lattice energy, which requires a
high temperature calcination
procedure to form the target phase. Unfortunately, the calcination
process not only increases the production cost but also brings a severe
safety risk to the manufacturer. Moreover, it causes unwanted
agglomeration of powders that
leads to a severe scattering
issue.[13, 14] This poses
a great challenge to the
transparency of product and the ensuing
application of afterglow
phosphors in 3D information
storage or volumetric display.
To circumvent the scattering issue,
transparent
afterglow materials have emerged in the last decade. For instance,
Claire et al. synthesized a polycrystalline
ZnGa2O4 ceramic by combining high-energy
ball milling, solid-state reaction and spark plasma sintering methods.
The maximum of transmittance reached 78% in the near infrared
region.[15] Tang et al. successfully embedded
fluoride nanocrystals into an amorphous glass
matrix,
which exhibited a high transmittance
up to 80%.[16] One major caveat of these
nanocrystal-based strategies was the
low photoluminescence quantum
yield and ensuing poor afterglow
intensity as a consequence of the
abundant quenching sites on the surface. Recently, our group has
developed several transparent afterglow crystals based on chloride
double perovskite.[5, 17-20] Through proper doping
of activator ions, the single crystals exhibited a tunable afterglow
emission ranging from red to near-infrared light. Thanks to their low
phonon energy, the chlorides showed a high quantum efficiency up to 82%
and an afterglow duration of 2 hours. In addition, the doping of Tb ions
in Cs2NaScCl6 not only boosted the PL QY
up to 98.2%, but also activated an intense green afterglow up to 12
hours after ceasing X-ray excitation.[20] Despite
these progress, ultraviolet chargeable transparent crystals of long-term
afterglow remained elusive.
In this work, a transparent CsCdCl3 crystal was grown by
programmable cooling method in a hydrothermal reactor. The PL intensity
of pristine crystals was significantly by 4 folds by elevating the
temperature from 25 oC to 200 oC.
Such abnormal behavior was investigated and ascribed mainly to the
thermal enhanced absorption. After Mn2+ doping, the PL
QY exhibited a dramatical increase to nearly unity. Importantly, an
ultra-long afterglow over 12-hour was activated under UV excitation.