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Upconversion of infrared to visible light in rare-earths doped phosphate

phosphors for photodynamic therapy application

By

Puseletso Pricilla Mokoena

(MSc)



A thesis submitted in fulfillment of the requirements for the degree

PHILOSOPHIAE DOCTOR

in the



Faculty of Natural and Agricultural Sciences

Department of Physics



at the



University of the Free State

South Africa



Promoter: Prof. O.M. Ntwaeaborwa

Co-Promoter: Prof. H.C. Swart

June 2017

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ii


Declaration by candidate


Mokoena Puseletso Pricilla, declare that the Doctoral Degree research thesis that I

herewith submit for the Doctoral Degree qualification at the University of the Free State is

my independent work, and that I have not previously submitted it for a qualification at

another instit

Mokoena Puseletso Pricilla, hereby declare that I am aware that the copyright is

vested in the

Mokoena Puseletso Pricilla, hereby declare that all royalties as regards intellectual

property that was developed during the course of and/or in connection with the study at the







Signature: Date..............................

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Figure 5.3 shows the EDS spectrum and mapping of Ba5(PO4)3OH powder.

5.3.3 UV-Vis diffuse reflectance spectra and Bandgap analysis

Diffuse reflection spectra of different phosphate phosphors ranging from 200 to 1000 nm are

shown in Figure 5.4. Figure 5.4 (a) shows the absorption peaks for (Ba5(PO4)3OH) host and

different rare earths ions (Eu
3+

and Yb
3+

). The absorption peaks in the range of 200-385 nm

are attributed to band-to-band transitions of the host lattice and crystal defects [12, 18]. The

other peak at 395 nm in the Ba5(PO4)3OH:Eu
3+

system is due to 4f-4f transitions of Eu
3+

ion

[26]. An additional peak observed at 977 nm for Ba5(PO4)3OH:Yb
3+

and

Ba5(PO4)3OH:Eu
3+

,Yb
3+

phosphors is assigned to Yb
3+

transitions from the ground state
2
F7/2

to excited state
2
F5/2 [27, 28]. The hump observed at 930 nm on all the spectrums is due to

the system fluctuation. Data plot of absorption versus energy in the absorption edge region is

shown in figure 5.4 (b), which is obtained from the corresponding diffuse reflectance

spectrum by using Kubelka-Munk function. By extrapolating the K-M function to K/S = 0,

the bandgap energies of Ba5(PO4)3OH, Ba5(PO4)3OH:Yb
3+

, Ba5(PO4)3OH:Eu
3+

, and

Ba5(PO4)3OH:Eu
3+

,Yb
3+

were estimated to be 3.7, 4.9, 3.9 and 4.7 eV respectively.

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Figure 5.4 (a) Reflectance spectra of Ba5(PO4)3OH, and Ba5(PO4)
3
OH with

different rare earths (Yb
3+

, Eu
3+

and Eu
3+

:Yb
3+

) and (b) bandgap energies of all

phosphor powders.

5.3.4 Photoluminescence properties of Eu
3+

/Yb
3+

co-doped Ba5(PO4)3OH

phosphor powder.

Figure 5.5 (a) show the PL excitation and emission of Ba5(PO4)3OH:Eu
3+

phosphor. The

excitation peaks are observed at different wavelengths ranging from 240-537 nm. The broad

intense excitation peak at 240 nm is attributed to O
2-

-Eu
3+

charge transfer band [29]. The

other excitation peaks located at ~319, 360, 382, 395 and 465-537 nm are assigned to
7
F0 →

(
5
H6,

5
D4,

5
L8,

5
L7, and

5
Dj (j =1,2)) transitions of Eu

3+
ion [30]. The emission peaks are

observed at ~589, 614, 651 and 699 nm attributed to
5
D0→

7
F1,

5
D0→

7
F2,

5
D0→

7
F3

and
5
D0→

7
F4 transitions of Eu

3+
ion respectively [31, 32]. The inset shows that the intensity

increases with the concentration of Eu
3+

from 0.1 mol.% to 3 mol.%, and at higher

concentrations of 5 and 7 mol.% the intensity decreases due to concentration quenching of

Eu
3+

.

Figure 5.5 (b) shows the emission spectrum of Ba5(PO4)3OH:Yb
3+

upon 980 nm laser

excitation. The intense blue emission peak at 475 nm and other minor peaks at 648 and 660

nm are observed and attributed to
2
F7/2→

2
F5/2 transitions of Yb

3+
ion [10].

Figure 5.5 (c) shows the emission spectrum of Ba5(PO4)3OH:Eu
3+

,Yb
3+

phosphor upon 980

nm laser excitation. Upconversion emission is not observed in the Ba5(PO4)3OH:Eu
3+



system, because singly Eu
3+

doped materials cannot absorb 980 nm photons, due to

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117


Combustion synthesis and characterization of Eu
3+

doped Ba5(PO4)3OH phosphors

P.P. Mokoena, H.C. Swart, O.M. Ntwaeaborwa

11.6 Publication

P.P. Mokoena, H.C. Swart, O.M. Ntwaeaborwa, Physica B: Condensed Matter,
In press, corrected proof, Available online 15 June 2017

11.7 Biography

Puseletso Pricilla Mokoena was born and raised in small village by the name of Namoha,

Qwaqwa in the Eastern Free State. She joined the University of the Free State (Qwaqwa

campus) in 2007 and obtained her bachelor B.Sc degree in 2011. In 2011, she enrolled

for B.Sc Honours (in Physics) and was also appointed as a student assistant. She joined

the University of the Free State (Bloemfontein campus) in 2012 to do Masters and her

project title was “narrowband UVB emission from gadolinium and praseodymium co-

activated calcium phosphate phosphors for phototherapy lamps” and she passed it with

Cum Laude. She was appointed as Research Assistant at the Centre for Microscopy in

2014. She started her Ph.D degree in 2014, working on “upconversion of infrared to

visible light in rare earth doped phosphate phosphors for photodynamic therapy

applications”. The aim of the project was to enhance the red luminescence of the

phosphors used to activate the photosensitizers that kills the cancerous cells. During the

period of her study, she was offered the opportunity to attend and give talks at national

and international conferences and laboratories. Her first international travel, she went to

Dubna, Russia in 2012 for participation in student practice in joint institute for nuclear

research (JINR) of research. She also went to Rhodes University for TL measurements at

physics department and PDT activity at chemistry department. She attended international

conference “3
rd

World Congress and Expo on Nanotechnology” in 2016 at Singapore. She

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also attended national conference
st
Annual Conference of the South African Institute

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