Colour emission tunability in Ho3+–Tm3+–Yb3+ co-doped Y2O3

upconverted phosphor

Anurag Pandey • Vineet Kumar Rai

Received: 10 July 2012 / Revised: 3 August 2012 / Published online: 25 September 2012

� Springer-Verlag 2012

Abstract The frequency upconversion (UC) emission

throughout the visible region from the Y2O3:Ho3?–Tm3?–

Yb3? co-doped phosphors synthesized by using low tem-

perature combustion process upon excitation with a diode

laser operating at 980 nm have been presented. The colour

emission tunability in co-doped phosphor has been

observed on increasing the pump power and seen by the

naked eyes. The tunability in colour emission has also been

visualized by CIE chromaticity diagram. The variation in

UC emission intensity of the 1G4 ? 3H6 (Tm3?) and5F3 ? 5I8 (Ho3?) transitions lying in the blue region has

been monitored with increase in the pump power and

marked that their ratio can be used to determine the tem-

perature. The developed phosphor has been used to record

fingerprints. The observed most intense visible colour

emission from the developed material may be used for

photodynamic therapy and as an alternative of traditional

fluorescent biolabels.

1 Introduction

The generation of visible light from rare-earth (RE) and

transition metal ions-doped materials are still very inter-

esting work among researchers from the last two decades.

This is due to their wide applications viz. display devices,

solar cells, fingerprint detections, medical diagnosis, tem-

perature sensors, etc. [1–7]. The best way of generating

visible emissions from the lanthanides-doped materials due

to their unique optical properties from the intra 4f-transi-

tions upon near infrared (NIR) excitations is frequency

upconversion (UC) [8–12]. The semiconductor quantum

dots due to their bright photoluminescence, size-dependent

tunable emission, broad ultraviolet excitation, narrow

emission and good photostability have shown a wide range

of applications, viz., gene delivery, cell labelling, energy

transfer, tissue imaging, etc. But the long-term uses of

quantum dots have been limited by its chemical instability

and inherent cytotoxicity. Therefore, it is very essential to

search the appropriate biolabels to overcome the limita-

tions of traditionally used biolabels. Recently, the fre-

quency upconversion through nonlinear optical processes

has shown much interest among the researchers in medical

field. Particularly, the lanthanides-doped UC nanoparticles

have been found as an alternate and excellent substitute for

traditional fluorescent biolabels. Photo dynamic therapy

(PDT) is a cancer treatment method in which a drug, called

photosensitizer is injected into blood stream which stays

into cells of whole body for several hours. The injected

sensitizer after certain period of time leaves the normal

cells but remains in cancer cells for a longer time. In order

to activate the sensitizer, excitation source of a particular

wavelength is incident on the affected part of body that

activates the photosensitizer remains in cancer cells and

release the active singlet oxygen that possibly kills the

cancer cells [13].

Several popular oxides have been used as host materials,

among them Y2O3 is most challenging because of its spe-

ciality as wide band gap, high melting point, low phonon

frequency, etc. [10]. The yttrium oxide has been considered

as an attractive host material because the Ho3?, Tm3? and

Yb3? ions have nearly the same ionic radii as that of the

Y3? ions. The triply ionized lanthanides doped into a

A. Pandey � V. K. Rai (&)

Laser and Spectroscopy Laboratory,

Department of Applied Physics,

Indian School of Mines, Dhanbad 826004,

Jharkhand, India

e-mail: vineetkrrai@yahoo.co.in; rai.vk.ap@ismdhanbad.ac.in

123

Appl. Phys. B (2012) 109:611–616

DOI 10.1007/s00340-012-5224-1

variety of hosts to obtain their emission characteristics and

as an ideal sensitizer to enhance their emission intensities

and energy transfer purposes have been used by different

researchers [14–16].

Colour emission tunability is one of the very interesting

properties of a material since it produces more than one

colour even from a single material upon suitable optical

excitation. A very few papers on colour emission tunability

for materials doped/co-doped with the Er3?, Yb3?, Ho3?

and Tm3? ions have been reported [17–25]. However, the

Yb3?/Ho3?/Tm3? ions co-doped in Y2O3 phosphor upon

980 nm excitation has not been synthesized and investi-

gated so for.

Here, we report the NIR to visible frequency UC in the

Ho3?–Tm3?–Yb3? co-doped Y2O3 phosphor prepared by

the combustion process. The colour emission tunability

with variation in the pump power has been monitored and

explained on the basis of CIE chromaticity diagram. The

temperature sensing measurement and finger print record-

ing of the material has been done and explained based on

the available data. The blue to red and green to red UC

emission intensity ratio which decides the purity of blue

and green emissions suitable for biological and medical

applications has been determined.

2 Experimental

2.1 Material preparation

The materials of high purity (99.90–99.99 %) have been

used without further purifications and synthesized by

combustion process. The Y2O3, Ho2O3, Tm2O3 and Yb2O3

were taken as raw materials while urea (NH2–CO–NH2)

was used as the organic fuel. In the synthesis process, the

concentration of dopants namely holmium and thulium

were fixed along with varying concentration of ytterbium.

The mixture ratios of reactants used were as follows:

1� x� y� zð Þ Y2O3 þ xHo2O3 þ yTm2O3 þ zYb2O3

where x and y were fixed at 0.001 and z varies from 0.01 to

0.07.

The desired amounts of Y2O3, Ho2O3, Tm2O3 and Yb2O3

materials were first dissolved in nitric acid properly to get

the transparent solution. The transparent solution was then

mixed with urea and stirred at 1,000 rpm with the help of

magnetic stirrer and heated at 80 �C. In 1–2 h, the solution

became a transparent gel and then rapidly heated into a

furnace preheated to 650 �C. Within 4 min, the solution

becomes foamed and a flame was produced which lasted for

few seconds. After that, the sample was immediately

removed from the furnace and resultant fluffy mass was

taken out. The fluffy mass of sample was grinded into fine

powder and finally the Y2O3:Ho3?–Tm3?–Yb3? phosphor

powders were used for further optical characterization.

2.2 Characterization

The as-synthesized fine Y2O3:Ho3?–Tm3?–Yb3? phosphor

powders were used for the characterization purposes. The

determination of average crystallite size have been performed

by X-ray powder diffraction pattern recorded by X-ray dif-

fractometer using Cu Ka1 radiation (k = 0.154 nm) within

the range 10�–80�.

The UC emission spectra have been recorded using a

Princeton triple grating monochromator (Acton SP2300)

attached with a photomultiplier tube (PMT) and personal

computer (PC). The samples were pumped by a diode

(CW) laser operating at 980 nm. All the measurements

were performed at room temperature.

3 Results and discussion

3.1 X-ray diffraction analysis

Figure 1 shows the X-ray diffraction (XRD) pattern of as-

synthesized Y2O3:Ho3?–Tm3?–Yb3? phosphor prepared by

combustion route in the range 10�–80�. The diffraction

peaks observed are well indexed to the cubic Y2O3 phase

(JCPDS file no. 25-1200). This shows that sample is well

crystallized, cubic-shaped with the lattice parameters

a = b = c = 10.604 A and a = b = c = 90�. The average

crystallite size of Y2O3:Ho3?–Tm3?–Yb3? phosphor has

been calculated using famous Debye–Scherrer equation:

d ¼ 0:89kbCosh

ð1Þ

where d the crystallite size, k the wavelength of X-ray

radiation, b the full-width half maxima (FWHM) and h is

diffraction angle. The average crystallite size was found in

Fig. 1 XRD pattern of Y2O3:Ho3?–Tm3?–Yb3? phosphor

612 A. Pandey, V. K. Rai

123

39–47 nm range that indicates that the developed phosphor

is a nano-material.

3.2 Upconversion emission study

The UC emission spectra of Y2O3:Ho3?–Tm3?–Yb3?

phosphor at varying concentrations of ytterbium from 1.0

to 7.0 mol % with fixed holmium and thulium concentra-

tions at 0.1 mol % have been recorded. Several UC emis-

sion bands for both the Ho3? and Tm3? ions have been

found. The maximum enhancement in UC emission inten-

sity for all bands in the co-doped phosphor of 0.1Tm3? ?

2.0Yb3? ? 0.1Ho3? RE ions concentrations has been

observed.

Figure 2 shows the UC emission spectrum of optimized

Y2O3:Ho3?–Tm3?–Yb3? phosphor in the 450–700 nm

wavelength range at room temperature using 980 nm

excitation. The UC emission bands have large peaks

corresponding to the 5F3 ? 5I8 (488 nm); 5F4 ? 5I8

(538 nm); 5S2 ? 5I8 (551 nm) and 5F5 ? 5I8 (668 nm)

transitions of Ho3? ions as well as 1G4 ? 3H6 (477 nm)

and 1G4 ? 3H4 (649 nm) transitions of Tm3? ions,

respectively, have been detected. As we know that UC

emission intensity (IUC) is directly proportional to pump

excitation power (P) by the relation [19],

IUC � Pk ð2Þ

where k represents the number of pump photons required to

excite the emitting states.

The pump power dependence for the possible appre-

ciable transitions to get the information about the number

of NIR photon involved has been monitored (Fig. 3). From

Fig. 3, the slope is found to be about *2; thereby allowing

the contribution of two NIR laser photons for the UC

emission in the visible region.

To understand the UC mechanism and processes

involved in detail, a schematic energy level diagram of

Ho3?–Tm3?–Yb3? ions with radiative electronic transi-

tions are shown in Fig. 4. Holmium ions in its ground state

absorb a 980 nm photon through ground state absorption

(GSA) and populate its 5I6 state through non-radiative

relaxation via the emission of four numbers of phonons.

After that, another NIR photon through excited state

absorption (ESA) populates the 5F4 and 5S2 levels from

where a radiative transition to ground state of Ho3? ions

gives green emissions at 538 and 551 nm, respectively.

A part of population stored in the 5S2 state decays non-

radiatively to the 5F5 level and then radiatively to ground

state giving red emission at 668 nm. The presence of Yb3?

ions enhances the efficiency of green (5F4/5S2 ? 5I8) and

red (5F5 ? 5I8) emissions through energy transfer ET-1

and ET-2 processes. A pair of Yb3? ions in its excited

(2F5/2) state transfer its excitation energy cooperatively to

the Ho3? and Tm3? ions and excited them to the 5F3

(Ho3?) and 1G4 (Tm3?) states, respectively. The excited

ions in these states relax radiatively to low lying states

and gives blue and red emissions corresponding to the5F3 ? 5I8 (Ho3?); 1G4 ? 3H6 (Tm3?) and 1G4 ? 3H4

(Tm3?) transitions, respectively.

The tuning in colour emission of developed phosphor

material with increasing pump power has been demon-

strated in CIE chromaticity diagram (Fig. 5). The colour of

emitted radiations from the co-doped phosphor has been

observed to change from green to blue with increasing the

Fig. 2 UC emission spectrum of Y2O3:Ho3?–Tm3?–Yb3? phosphor

on increasing power (a) 5 mW, (b) 100 mW and (c) 1,200 mW

Colour emission tunability in Ho3?–Tm3?–Yb3? co-doped Y2O3 613

123

pump power in the range 5 mW–1.8 W (Fig. 5). The

intensity ratio for the blue to red and green to red UC

emission bands were noted to be 1.7 and 3.9, respectively,

even at very low pump power (*5 mW) of the diode laser.

With increasing the pump power the blue to red and green

to red intensity ratio has been increased and the corre-

sponding ratio was observed to be *24 and *19,

respectively, at 1.8 W pump power of the diode laser. This

result indicates that high purity blue and green colour

emissions from the synthesized phosphor material can also

be used in minimally invasive photodynamic therapy and

fluorescent biolabelling [26, 27]. Because in PDT cancer

cells are irradiated with light flux of an appropriate

wavelength matched to the absorption spectrum of photo-

sensitizer and the absorption bands of photosensitizer lies

mainly throughout the visible region that suitably matched

with developed upconverted material. Irradiation with a

suitable wavelength may trigger selective photochemical

reaction that occurs at very small power densities

(0.01–50 W/cm2), resulting in desirable biological trans-

formation [28]. The power densities of light emitted

from synthesized Y2O3:Ho3?–Tm3?–Yb3? phosphor is off

course very small (*0.02–2.0 W/cm2 at pump excitation

of *0.33–117 W/cm2) but within the above range, i.e.

0.01–50 W/cm2 [28]. Therefore, the present developed

Y2O3:Ho3?–Tm3?–Yb3? phosphor may be of specific use

for PDT. The colour emission tunability with increase in

the pump power is shown in CIE colour coordinates

(Fig. 5) and the optical photographs of these colours are

shown in Fig. 2[a(i)], [b], [c(i)].

3.3 Fingerprint detection

The tunability in colour emission of as-synthesized

Y2O3:Ho3?–Tm3?–Yb3? phosphor with respect to excita-

tion power can be utilized in detection of finger prints. In

finger print detection technique, the phosphor material has

been spread on finger marks and NIR beam emitted from a

diode laser is allowed to fall on the marks. Figure 2[a(ii)]

and [c(ii)] shows the photographs of fingerprint developed

on a plane glass sheet using as-synthesized Y2O3:Ho3?–

Tm3?–Yb3? phosphor excited by 980 nm diode laser in

green and blue colours at 5 and 1,200 mW, respectively.

The latent finger prints were visualized due to the UC

emission of powder sprinkled on finger print marks that can

be seen by naked eyes (shown in inset of Fig. 2). The

intense emissions from developed material in different

colour helped to detect the finger print marks easily.

Fig. 3 The logarithmic dependence of peak intensity of blue, green

and red UC emissions as a function of pump power

Fig. 4 Energy level diagram of Ho3?–Tm3?–Yb3? ions with possi-

ble transition scheme

Fig. 5 CIE chromaticity diagram of Y2O3:Ho3?–Tm3?–Yb3? phos-

phor at different pump powers

614 A. Pandey, V. K. Rai

123

3.4 Temperature sensor

Figure 6 shows the UC emission spectra of blue emission

band of the Y2O3: Ho3?–Tm3?–Yb3? co-doped phosphor

in the 450–510 nm region on 980 nm excitation at different

pump powers. It is observed that the UC emission intensity

ratio corresponding to the 1G4 ? 3H6 (Tm3?) and5F3 ? 5I8 (Ho3?) peaking at 477 and 488 nm, respectively,

shows a significant variation with increasing the pump

power. The energy difference between the two emission

transitions is DE & 473 cm-1. This shows that the 1G4 and5F3 levels are very close to each other. Therefore,

according to the fluorescence intensity ratio technique [29,

30], they can be used to study the temperature sensing

behaviour of the developed phosphor material. It has been

found that the intensity ratio of two closely spaced transi-

tions 1G4 ? 3H6 and 5F3 ? 5I8 (i.e. I477nm/I488nm) increa-

ses linearly with the input power densities (Fig. 7).

The intensity ratios of two transitions coming from the

closely spaced levels of Tm3? and Ho3? ions vary with

temperature according to:

I477nm

I488nm

¼ B expð�DE

kTÞ ð3Þ

where all the terms have their usual meanings [29, 30].

Now we consider that at very low pump power (i.e. power

density of 0.32 W/cm2 at 5 mW of beam spot size

*1.4 mm) the sample was at room temperature (27 �C).

The calculated value of constant B using the above equa-

tion is found to be 5.99. Then the temperature of sam-

ple corresponding to different values of intensity ratios

(I477nm/I488nm) at different power densities can easily be

calculated (Table 1).

Thus from the Table 1 we conclude that the developed

phosphor can also be used as high temperature sensor

probe.

4 Conclusion

Cubic shaped Ho3?–Tm3?–Yb3? co-doped Y2O3 phosphor

has been synthesized successfully by the combustion route.

NIR to visible UC emission has been observed. The colour

emission tunability from green to blue with increasing

pump power has been verified by the help of CIE chro-

maticity diagram. The UC-based finger print detecting

concept has been demonstrated and noted that the present

phosphor can be used in latent finger print detection pur-

pose. The Intensity ratio (I477nm/I488nm) of two closely

spaced blue UC emission bands of Tm3? and Ho3? ions

has been found to increase linearly with input pump power

Fig. 6 Variation of blue UC spectrum of Y2O3: Ho3?–Tm3?–Yb3?

phosphor on increasing pump power

Fig. 7 The plot of intensity ratio of blue UC emissions at different

powers densities

Table 1 Calculated temperature of sample for different intensity

ratios at different pump power densities

Power density

(W/cm2)

Intensity ratio

(I477nm/I488nm)

Calculated

temperature (K)

0.33 0.51 300.1

0.65 0.59 319.4

3.25 0.64 330.8

6.49 0.70 344.7

12.98 0.81 370.5

22.73 0.92 396.3

32.47 1.01 416.3

45.46 1.13 443.7

55.19 1.23 469.0

64.94 1.34 494.0

77.92 1.45 521.8

97.40 1.59 561.3

116.88 1.68 583.5

Colour emission tunability in Ho3?–Tm3?–Yb3? co-doped Y2O3 615

123

showing the potentiality of the present phosphor to be used

as high temperature sensor probe. Also, the highly pure

intense blue and green colour emissions from the devel-

oped phosphor upon excitation with 980 nm diode laser

provided the possibility that the present material can be

used for photodynamic therapy and also may be an alter-

native and substitute for traditional biolabels.

Acknowledgments Authors are grateful to the University Grants

Commission, New Delhi, India for providing the financial assistance.

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