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1. Introduction
Long lasting phosphors (LLPs) are known for their persistent luminescence, which can emit
for hours after terminating irratation. LLPs exhibit potential in night-vision assistants,
security signs, and dial displays [1–5]. In recent years, transition metal-activated (such as
Cr
3+
or Mn
2+
) LLPs have attracted much interests because of their perfect near-infrared (NIR)
persistent luminescence, excellent chemical stability, and suitable emission range [6–10]. In
particular, NIR LLPs can be used as long-term optical probes for in vivo imaging systems
because these phosphors feature high signal-to-noise ratio and deep penetration and do not
require in situ excitation [11–13].
The majority of studies on NIR LLPs have focused on Cr
3+
. The 3d
3
electron
configuration of Cr
3+
can feature different types of emission depending on the crystal-field
environment of the host; Cr
3+
exhibits narrow-band emission at around 700 nm because of the
spin-forbidden
2
E→
4
A
2
transition or broadband emission ranging from 650 nm to 1600 nm
because of the spin-allowed
4
T
2
→
4
A
2
transition. Cr
3+
presents strong ability to substitute for
Ga
3+
(with a similar ionic radius) in octahedral coordination; as such, gallate materials used as
host have been intensively investigated [14–17]. Pan et al. developed a series of Cr
3+
-doped
zinc gallogermanate NIR persistent phosphors, which exhibit a super-long afterglow of more
than 360 h [16]. Allix et al. demonstrated a considerable improvement on NIR persistent
luminescence by partially substituting Ge and Sn to form the Zn
1+x
Ga
2−2x
(Ge/Sn)
x
O
4
solid
solution [17]. Recently, Li et al. reported an earth-abundant and inexpensive NIR
Received 15 Jan 2016; revised 8 Feb 2016; accepted 8 Feb 2016; published 25 Feb 2016
1 Mar 2016 | Vol. 6, No. 3 | DOI:10.1364/OME.6.000922 | OPTICAL MATERIALS EXPRESS 923