Designing thermally stimulated 1.06 mu m Nd3+ emission for the second bio-imaging window demonstrated by energy transfer from Bi3+ in La-, Gd-, Y-, and LuPO4

Abstract

We report a general methodology to the rational design of thermally stimulated short-wave infrared (SWIR) luminescence between ∼900 and 1700 nm by a new combination of using efficient energy transfer from Bi
3+
to Nd
3+
and an adjustable hole trap depth via valence band engineering. Predictions from a vacuum referred binding energy (VRBE) diagram are combined with the data from optical spectroscopy and thermoluminescence to show the design concept by using bismuth and lanthanide doped rare earth ortho-phosphates as model examples. Nd
3+
with its characteristic
4
F
3/2
→
4
I
j
(j = 9/2, 11/2, 13/2) emission in the SWIR range is first selected as the emitting centre. The energy transfer (ET) processes from Bi
3+
or Tb
3+
recombination centres to Nd
3+
are then discussed. Photoluminescence results show that the energy transfer efficiency of Bi
3+
→ Nd
3+
appears to be much higher than of Tb
3+
→ Nd
3+
. To exploit this ET, thermally stimulated Bi
3+
A-band emission can then be designed by using Bi
3+
as a ∼2.7 eV deep electron trap in YPO
4
. By combining Bi
3+
with Tb
3+
, Pr
3+
, or Bi
3+
itself, the holes trapped at Tb
4+
, Pr
4+
, or Bi
4+
will release earlier than the electrons captured at Bi
2+
. On recombination with Bi
2+
, Bi
3+
in its excited state is formed generating Bi
3+
A-band emission. Due to the ET of Bi
3+
→ Nd
3+
1.06 μm Nd
3+
emission appears in YPO
4
. Herein, the thermally stimulated Nd
3+
SWIR emission is achieved by hole release rather than the more commonly reported electron release. The temperature when thermally stimulated Nd
3+
SWIR emission appears can further be engineered by changing the Tb
3+
or Pr
3+
hole trap depth in Y
1−x
Lu
x
PO
4
by adjusting x. Such valence band engineering approach can also be applied to other compounds like La
1−x
Gd
x
PO
4
and Gd
1−x
La
x
AlO
3
solid solutions. Our work opens the avenue to motivate scientists to explore novel SWIR afterglow phosphors in a design way instead of by trial and error approach.