Letter Cite This: J. Phys. Chem. Lett. 2018, 9, 6191−6197
pubs.acs.org/JPCL
What Is Beyond Charge Trapping in Semiconductor Nanoparticle Sensitized Dopant Photoluminescence? Prasenjit Manna,† Gouranga H. Debnath,† David H. Waldeck,*,‡ and Prasun Mukherjee*,† †
Centre for Research in Nanoscience and Nanotechnology, University of Calcutta, JD-2, Sector-III, Salt Lake, Kolkata 700106, West Bengal, India ‡ Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United States
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S Supporting Information *
ABSTRACT: A systematic comparison of the Ln3+ [Ln = Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb] photoluminescence in doped tin dioxide [Sn(Ln)O2] and doped titanium dioxide [Ti(Ln)O2] nanoparticles shows that the emission efficiency of trivalent lanthanide cations (Ln3+) in an oxide matrix can be improved by change of the cation site symmetry. An analysis of Ln3+ emission quantum yield and asymmetry ratio is used to identify the importance of symmetry breaking around the dopant site for enhancing the Ln3+ emission intensity. These findings identify an important criterion for engineering the luminescence intensity of dopant ions in semiconductor nanoparticle-based luminophores, which goes beyond the primary criterion of engineering the relative positions of the dopant energy levels with respect to the band edges of the host nanoparticle matrix.
D
important new applications in biological imaging, optoelectronics, sensing, laser technology, and telecommunications; among others.1,5−13 Toward the goal of understanding the underlying photophysical properties of Ln-doped, semiconductor NPs and developing design principles for producing materials with predictable properties, we have undertaken studies with hydrophobic Zn(Ln)S [Ln = Eu, Tb]14 and other II − VI metal chalcogenides,14 as a function of NP size and dopant density,15,16 hydrophilicity,17,18 and near bandgap matched Sn(Ln)O2 and Zn(Ln)S [Ln = Sm, Tb]19 NPs. These studies have revealed that the Zn(Tb)S, Ti(Nd)O2, and Ti(Sm)O2 NPs produce the strongest host sensitized Ln3+ photoluminescence; a number of other NPs, including [Zn(Eu)S, Ti(Eu)O2, Ti(Ho)O2, Ti(Er)O2, Ti(Tm)O2, Ti(Yb)O2], display a moderate sensitized emission intensity, and some, including Zn(Sm)S, Zn(Dy)S, Ti(Pr)O2, Ti(Gd)O2, Ti(Tb)O2, and Ti(Dy)O2, display no sensitized emission. Moreover, comparison of the Ln3+ emission lifetime with that for freely diffusing Ln3+ ions in bulk solvent reveals a significant lengthening of the emission lifetime in the NPs, suggesting a protection of the Ln3+ in the NPs from nonradiative decay processes that may originate from the nearby environment. In addition, these studies showed that the spectral overlap between the donor (semiconductor NPs) emission and acceptor (Ln3+) absorption is not a good predictor for the luminescence sensitization in these systems. The experimental findings described above can be rationalized by a photophysical model in which the Ln3+
espite their historic use as inorganic phosphors, trivalent lanthanide cations (Ln3+) are much less explored as a class of luminophores than are organic dye molecules. Because of the core like nature of their intraconfigurational electronic transitions, the lanthanides display sharp emission bands without significant spectral band overlap, possess long (microseconds to milliseconds) luminescence lifetimes, and are resistant to photobleaching. For example, the typical sharp emissions of Tb3+ around 490, 545, 585, and 620 nm do not overlap with each other; as well, the Tb3+ emission at 490 and 545 nm do not overlap significantly with other lanthanide ions, such as Eu3+ emission at 590 or 615 nm. These properties make the lanthanides attractive for multiplex assays, time-gated measurements that eliminate nanosecond-lived autofluorescence background signals, and long experimentation times. Using Ln3+ luminophores is often limited because of their low molar extinction (≤10 M−1cm−1 as compared to 104−105 M−1 cm−1 for typical organic fluorophores) and quenching of their emission by vibrational overtones of molecules in the nearby environment, including ligand and solvent molecules.1,2 An important new strategy for overcoming the limitations of lanthanide luminophores is to incorporate (dope) them in a compatible semiconductor nanoparticle (NP) matrix.3,4 Proper choice of the nanoparticle material allows for appreciable light absorption and subsequent transfer of the energy to the Ln3+ center to generate host sensitized Ln3+ emission, via an optical antenna effect. Moreover, multiple and distinct Ln3+ ions can be doped into a given host NP thus enabling multiplex assays; and the NP surface ligands can be modified to allow for targeted therapy applications, desirable solubility properties, etc. Such design flexibility is not available for molecular fluorophores and much less straightforward for molecular organic complexes. Lanthanide luminophores are finding © XXXX American Chemical Society
Received: September 12, 2018 Accepted: October 11, 2018 Published: October 11, 2018 6191
DOI: 10.1021/acs.jpclett.8b02807 J. Phys. Chem. Lett. 2018, 9, 6191−6197
Letter
The Journal of Physical Chemistry Letters Table 1. Electronic Nature of Ln3+ Emission Bands and Definition of Asymmetry Ratios GSa
Ln Pr Nd
4
Sm
6
4
G5/2
Eu
7
5
D0
Gd Tb
8
6
7
5
P7/2 D4
Dy
6
4
F9/2
Ho
5
5
S2 F5
H4 I9/2
H5/2
F0
S7/2 F6
H15/2
I8
3
P0 1 D2 4 F3/2
5
Er
4
I15/2
4
S3/2 F9/2 4 I13/2 1 D2 1 G4 3 H4 2 F5/2 4
Tm
3
Yb
2
H6
F7/2
FSa
ν (cm−1)
electronic natureb
H4 3 H4 4 I9/2 4 I11/2 4 I13/2 6 H5/2 6 H7/2 6 H9/2 7 F1 7 F2 7 F4 8 S7/2 7 F6 7 F5 7 F4 6 H15/2 6 H13/2 6 H9/2 5 I8 5 I8 5 I7 4 I15/2 4 I15/2 4 I15/2 3 F4 3 H6 3 H6 2 F7/2
20410 16670 11300 9350 7410 17860 16810 15625 16950 16260 14290 32260 20410 18350 17240 21050 17540 13070 18350 15380 10310 18180 15150 6490 21980 21280 12500 10240
ED ED ED ED ED MD MD + ED ED MD ED ED MD ED MD + ED MD ED ED MD ED ED ED ED ED MD ED ED ED MD
LSa
3
3
asymmetry ratio (AS) − −
∫ Iem[4G5/2→6H9/2] dν/∫ Iem[4G5/2→6H5/2] dν
∫ Iem[5D0→7F2] dν/∫ Iem[5D0→7F1] dν
− ∫ Iem[5D4→7F6] dν/∫ Iem[5D4→7F4] dν
∫ Iem[4F9/2→6H13/2] dν/∫ Iem[4F9/2→6H9/2] dν
−
∫ Iem [4S3/2→4I15/2] dν/∫ Iem [4I13/2→4I15/2] dν
−
−
GS, LS and FS stand for ground state, luminescent state and final state respectively. bThe electronic nature of the Ln3+ magnetic dipole (MD) transitions have been assigned following the reports by Carnall and Fields29 and Dodson and co-workers,28 with considerations from the selection rule for the magnetic dipole transitions.5 The transitions with radiative rates> 5 s−1 have been considered. The electric dipole (ED) transitions have been assigned based on the selection rule.5 The intensities of electric quadrupole (EQ) transitions being extremely low, with radiative rates
