1. Introduction 3. Experimental setup 4 ...

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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

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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

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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