Upconversion Nanoparticles. Remko R.M. Dijkstra1, Martijn Stopel1, Athira N. Raj2, Markus Haase2, Vinod Subramaniam1, Christian Blum1. 1. Introduction. 3.
Brightness Characterization of Single Core and Core/Shell 3+ 3+ Yb ,Er - Doped NaYF4 Upconversion Nanoparticles Remko R.M. Dijkstra , Martijn Stopel , Athira N. Raj , Markus Haase , Vinod Subramaniam , Christian Blum 1
1
2
2
1
1
University of Twente, Nanobiophysics Group, University of Twente, PO Box 217, 7500AE Enschede, The Netherlands 2 Universität Osnabrück, Anorganische Chemie I, Barbarastraße 7, D-49069, Osnabrück, Germany 1
1. Introduction
2. Research question
NaYF4: Er3+,Yb3+ nanoparticles: - Promising luminescent biomarkers - Extremely photostable [1] - Excited by near-infrared light ( ~ 980 nm) - Multiple emission bands in visible wavelength range
4
I9/2
4
I11/2
4
I13/2
5 0
4
S3/2
4
Energy Transfer Upconversion
S3/2 → 4I15/2
Low brightness
4
2
Er
Yb
3+
3+
H11/2→ 4I15/2
4
Particle brightness can be increased by applying a passive shell around the active core. However, the efficiency of bulk counterpart is never reached [2]. - Why is the bulk brightness never reached? - Why does brightness still increase even after applying very thick shells?
Our goal:
F5/2 2
I15/2
F9/2 → 4I15/2
2
980 nm
F9/2
F3/2
4
668 nm
10
4
Emission spectrum 4
417 nm 488 nm 525 nm 546 nm
15
Energy diagram
700 nm 847 nm
Energy (103 cm-1) 4 G 25 2H11/2 9/2 4 F5/2 4 F 20 2H 7/2 11/2
???
Core NaYF4 nanoparticle doped with Er3+,Yb3+ ions
F7/2 → 4I13/2
NaYF4: Er3+,Yb3+ nanoparticle with NaYF4 shell
Increased brightness
Give new insights for optimal core:shell synthesis by optically characterizing upconversion particles on a single particle level.
F7/2
4. Characterization method Typical scan images TEM image
3. Experimental setup
800
(a)
5
IR single mode fiber
Piezo scanning stage
700
0
600
976 clean-up filter
776 LP filter Objective 60x WI, NA 1.2
cps
cps
4000
(b)
3500
500 400
IR DC-mirror
3000
300 200
fiber coupled spectrometer
2500
100
Wedgebeamsplitter
0
(c)
Fianium supercontinuum white light laser
770 SP filter
50 nm
2000
(d)
1500
Power meter
1000
Flip mirror achromat lens Mirror
Computer with single-photon counting card
A single-photon counting scanning confocal microscope setup was used to optically characterize single particles with different core:shell configurations.
1:0
1:3
500
1:2
Single particle timetrace
1:7
4 μm Intensity (counts per 10 ms)
APD
0
50
50
40
40
30
30
20
20
10
10
0
0
2
4
t (s)
6
8
10
0
0
~ 300
50 100 150 200 250 Occurence
Brightness distribution
5. Brightness distribution 0.57 10 W/cm .
5
2
3.2 . 105 W/cm2
Four different core:shell configurations. Core:shell ratio is by volume.
particles
Intensity (counts/sec)
Occurrence fraction
Sample preparation: Particles were spin-coated in low concentration on glass coverslips such that single diffraction limited spots were spatially isolated in the obtained scan images.
6. Spectral measurements Single particle spectrum: - Background subtracted spectrum - Averaged over ~ 50 particles for each particle configuration
1698 counts/sec
1586 counts/sec
A shift towards higher intensities for increasing shell thickness is observed.
Key observation:
Broadening of the distibution for increasing shell thickness. We attribute this to the presence of Er3+ and Yb3+ dopants in the shell, which leads to additional energy transfer pathways to quenching sites on the particle surface. A suggested mechanism that leads to shell dopants is presented: Core particles
Add shell precursors
Ostwald ripening
Peak at ~ 700 nm: - Especially prominent in our core:shell spectra - Scarcely reported in literature - More research needed
389 counts/sec
7. Outlook
Dopants in shell
Quenching
986 counts/sec
For creating more efficient core:shell upconversion nanoparticles, it is important to focus on alternative shell synthesis methods. Additional characterization could be performed by obtaining the power dependence on a single particle scale, as the resulting brightness of particles strongly depends on the excitation power used, reflecting the multiphoton nature of the upconversion process.
[1] Shiwei Wu, Gang Han, Delia J. Milliron, Shaul Aloni, Virginia Altoe, Dmitri V. Talapin, Bruce E. Cohen, and P. James Schuck. Nonblinking and photostable upconverted luminescence from single lanthanidedoped nanocrystals. Proceedings of the National Academy of Sciences, 106(27):10917–10921, July 2009. [2] Markus Haase and Helmut Schäfer. Upconverting nanoparticles. Angewandte Chemie International Edition, 50(26):5808–5829, 2011.
NBP
