Fluorine Phosphate Glasses Doped with Cadmium Sulfide and

Selenide Quantum Dots with High Quantum Efficiency at

Room-temperature

E. V. Kolobkova

1,2

, Zh. Lipatova

, M. S. Kuznetsova

and N. Nikonorov

Department of Optical Informatics Technologies and Materials, ITMO University, Saint-Petersburg, Russia

St. Petersburg State Institute of Technology, Technical University, St. Petersburg, Russia

Saint-Petersburg State University, St. Petersburg, Russia

Keywords: Quantum Dots, Fluorine-phosphate Glass, Luminescence, Absolute Quantum Yield.

Abstract: The results of the study of the luminescent properties of the CdS(Se) quantum dots (QDs) with the mean

size of 2-4 nm synthesized in the fluorine phosphate glass are discussed. The changes of the

photoluminescence absolute quantum yield (PL AQY) magnitude of the CdS(Se) QDs with various mean

sizes induced by the heat treatment are studied. It was found that the PL AQY of the CdSe QDs increases

monotonically to a maximum and then fells down. PL AQY magnitudes for glasses doped with CdS QDs

demonstrate weak dependence on the size. It was found that CdS(Se) QDs represents a series of excellent

emitters in the 600-750 nm spectral region. PL AQY in the glasses can reach 50-65%, which is equal to the

value in the colloidal nanocrystals and higher than it was reported earlier for the silicate glasses. The glass

matrix protects the QDs from external influence and their optical properties remain unchanged for a long

time.

1 INTRODUCTION

QDs are a type of nanomaterials with good

ﬂuorescent properties. The size-dependent emission

is probably the most attractive property of

semiconductor nanocrystals.

Among them, CdS and CdSe QDs are one of the

most promising materials because QDs have bright

luminescence in the visible range of the optical

spectrum. For example, CdSe QDs have shown

potential as superior biological labels (Han, M. et

al., 2001, Bruchez, M. et al., 1998 and Chan, W. C.

W., Nie, S. M., 1998), laser sources (Artemyev, M.

et al., 2001, Klimov, V. I.et al., 2000) and tunnel

diodes (Sundar, V. C. et al., 2000, Schlamp, M.C.,

Peng, X., Alivisatos,A.P., 1997).

Comparing with conventional ﬂ uorescent dyes

CdS(Se) QDs have a wide continuously distributed

excitation spectra, not only with symmetrical

distributed narrow emission spectra, but also many

other excellent properties such as adjustable color,

excellent photochemical stability and high threshold

of light bleaching (Qu, L., Peng, X. 2002).

However, colloidaly synthesized bare quantum

dots, including CdSe(S) QDs, usually have surface

defects, which diminish photoluminescence (PL)

absolute quantum yield. The best PL AQY reported

for the as-prepared nanocrystals at room temperature

is around 20% in the wavelength range between 520

and 600 nm and is about a few percent or lower in

the wavelength range above 600 nm and below 520

nm (Sundar, V. C. et al., 2000). In general, a low PL

AQY is considered as a result of the surface states

located in the bandgap of the nanocrystals, which act

as trapping states for the photogenerated charges.

These surface trapping states are originated from the

dangling bonds of some of the surface atoms (Fu,

H.; Zunger, A. 1997, Xu, K. M. et al., 2010 and Kim

J, M. et al., 2012). That`s why it is essential to

control the QDs surfaces to reduce the surface

defects by passivating the surface of QDs (Talapin,

D.V. et al., 2001). The core/shell structures solve

optical problems, such as low PL AQY, and improve

the stability of QDs. In (Talapin, D.V. et al., 2001)

was shown that the room-temperature quantum

efficiency of the band edge luminescence of CdSe

QDs can be improved to 40-60% by surface

328

Kolobkova E., Lipatova Z., Kuznetsova M. and Nikonorov N.

Fluorine Phosphate Glasses Doped with Cadmium Sulﬁde and Selenide Quantum Dots with High Quantum Efﬁciency at Room-temperature.

DOI: 10.5220/0006266203280333

In Proceedings of the 5th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2017), pages 328-333

ISBN: 978-989-758-223-3

passivation with inorganic (ZnS) or organic

(alkylamines) shells. Annealing of the samples in

different environments (oxygen, hydrogen, and air)

seems to reduce the PL emission, due to the

activation of non-radiative defect states.

Due to the failure of orange-red emitting

materials in general, efforts in colloidal CdS/CdSe

QDs formation were mainly concentrated on the

wavelength region between 600 nm (orange) and

650 nm (red). Nevertheless, the stability and the

reproducibility of the PL AQY are both not

predictable. With some inorganic and organic

surface passivation after the synthesis, the PL QY of

the colloidal CdSe nanocrystals is boosted more than

50% in the range 520-600 nm, but the efficiency for

the orange-red color window is still low. Especially

for red (around 650 nm), the PL QY of the

nanocrystals in solution was nearly zero (Qu, L.,

Peng, X. 2002).

Semiconductor nanoparticles CdS and CdSe

dispersed in a silicate glass matrix are attracting

much attention (Borreli, N. F. et al., 1987, Su, Z., et

al., 1996 and Xu, K. et al., 2010). The possibility of

QDs formation in the optical material creates

significant benefits for their application. Currently,

optical transparent glasses doped with nanocrystals

are of the great interest for the modern element base

of photonics. These materials combine the best

properties of nanocrystals and glasses (possibilities

of pressing and molding, spattering, pulling optical

fibers). In addition, the glass matrix protects the

QDs from external influence. In the silicate glasses,

the PL spectra consist of two bands: a less intense

high-energy band, and a lower energy broader band.

First band occurs at a wavelength 10-20 nm higher

than the absorption edge and is due to direct

electron-hole recombination. This peak shifts to the

higher wavelength with increasing particle size.

Second band is due to surface defects and occurs at

800-900 nm spectral region. PL AQY is less than

30% for CdS(Se) QDs in the silicate glasses and

decreases as a size of QD increases (Borreli, N.F. et

al., 1987).

In this study, we used a fluorine phosphate (FP)

glass featuring with a number of advantages

compared to conventional silicate glasses, including

low temperature synthesis, possibility for a wide

range adjustment of the formed quantum dots

concentration, low temperature and time of heat

treatment and higher spatial distribution

homogeneity. Two-stage heat treatment afforded

quantum dots with narrow size distribution. The CdS

and CdSe QDs dispersed in a fluorine phosphate

glasses are attracting much attention as nonlinear

materials (Vaynberg, B. et al., 1996 and Lipovskii,

A.A. et al., 1999), but information about

luminescent properties is not available.

In this study, we represent the luminescent

properties of a fluorine phosphate glass doped with

CdS and CdSe QDs.

2 EXPERIMENTAL

In order to investigate the effect of the QDs sizes on

the PL properties, the fluorine

-phosphate (FP)

glasses 0.25Na

O-0.5P

-0.05ZnF

-0.1Ga

0.05AlF

-0.05NaF (mol. %) doped with CdS or

CdSe were synthesized. The glass synthesis was

conducted in an electric furnace at 1050

C in the Ar-

atmosphere using the closed glassy carbon crucibles.

After quenching, the glasses were annealed at 320 C

for 1 hour to release thermal stress, cut into pieces of

10.0 x 10.0 mm, and then were optically polished.

Planar polished samples 0.4-1.0 mm thick were

prepared for further investigation. The glass

transition temperature measured with STA 449 F1

Jupiter (Netzsch) differential scanning calorimeter

was found to be 390±3 C. Samples were heat treated

using a muffle furnace (Nabertherm) with program

temperature control to induce formation of CdS(Se)

QDs at 410

C. The optical density spectra of the

studied FP glass samples were recorded using a

double-beam spectrophotometer Lambda 650

(Perkin Elmer) in the 1.5-5 eV spectral region with

0.1 nm resolution. For registration of the emission

spectra excited at λ =405 nm (3.06 eV) was used an

EPP2000-UVN-SR (Stellar Net) fiber spectrometer.

The luminescence was excited by semiconductor

lasers (=405 nm). All measurements were

performed at room temperature. Absolute quantum

yield measurements were carried out inside the

integrated sphere with Photonic Multichannel

Analyzer (PMA-12, Hamamatsu) at room

temperature. The measurement error for the absolute

quantum yield (AQY) was ±1%.

3 RESULTS AND DISCUSSIONS

3.1 Glasses Doped with Cadmium

Selenide Quantum Dots

The emission properties of semiconductor

nanocrystals can be characterized by three

fundamental parameters, which are the brightness,

the emission color, and the stabilityof the emission.

Fluorine Phosphate Glasses Doped with Cadmium Sulﬁde and Selenide Quantum Dots with High Quantum Efﬁciency at Room-temperature

329

A samples of the glass containing CdSe QDs

were prepared by heat treatment of a 0.25Na

O-

0.5P

-0.05ZnF

-0.1Ga

-0.02PbF

-0.08AlF

glass doped with 0.6 mol. % CdSe at T=410

during 20-60 min.

Due to quantum size effects, the band gap of

CdSe QDs increases from 2.2 eV to 3.0 eV, as the

size of the nanocrystals decreases from 4.0 nm to 2.0

nm. The emission color of the PL of the nanocrystals

shifts continuously from red (centered at 730 nm) to

orange (centered at 630 nm) as size of QDs decrease

(Fig1). QDs sizes were defined using data (Norris,

D. J. and Bawendi M. G. 1996).

In the PL spectra, the broad band with a large

Stokes shift is dominant, and the band-edge PL is

negligibly weak. The emission spectrum of samples

is dominated by "deep trap" emission, strongly red

shifted from the band edge (Fig2).

Figure 1: Absorption and luminescence spectra of the

glass doped with CdSe QDs with a sizes 2.0 nm (1), 3.0

nm (2), 4.0 nm (3). The excitation energy is 3.06 eV.

Figure 2: Dependence of the Stokes shift on the QDs size.

Figure 3: Dependence of the PL AQY for glasses doped

with CdSe QDs with sizes 2.0 nm (1), 3.0 nm(2), 4.0 nm

(3) on the excitation energy.

Figure 4: Dependence of the PL AQY on the CdSe QDs

sizes.

Fig. 3 demonstrates dependence of the AQY

magnitudes for glasses doped with CdSe QDs. The

QDs concentration in the glasses 2 and 3 is equal

(Fig.1), but AQY of the QD with size 3 nm is in two

times higher. The emission color of the PL of the

QDs with size 3 nm is red with λ

max

=700 nm

The PL AQY magnitudes for glasses doped with

CdSe QDs demonstrate nonlinear dependence on the

size (Fig. 4).The PL AQY of the QDs increases

monotonically to a maximum and then fells down to

30 % (Fig. 4). For convenience, the position with the

maximum PL AQY is called the bright point as in

(Qu, L., Peng, X. 2002). The bright point for CdSe

QDs in FP glass is observed for QDs with size 3.0

nm.

PHOTOPTICS 2017 - 5th International Conference on Photonics, Optics and Laser Technology

330

Figure 5: Dependence of the PL FWHM magnitudes for

glasses doped with CdSe QDs with different sizes.

The typical full width at half-maximum

(FWHM) of the PL peak of the CdSe QDs ensemble

at room temperature in FP glass, around 600-300

meV, is noticeably broader than that observed for

colloidal QDs (Qu, L., Peng, X. 2002).

FWHM

magnitudes decrease as QDs sizes increase (Fig 5).

The values of the PL AQY of the samples 1, 2

and 3 were measured several times during the year.

The results of the measurements coincided, which

confirmed the stability of luminescent

characteristics.

3.2 Glasses Doped with Cadmium

Sulfide Quantum Dots

As it was shown in (Lipatova, Zh.O., Kolobkova

E.V., Aseev,V.A. 2015) heat treatment has a

significant impact on properties of glasses doped

with CdS QDs. Absorption peaks due to conﬁned

excitons are clearly observed in the higher-energy

region compared with the band-gap energy of 2.5 eV

in a CdS bulk crystal. These results evidently

indicate the formation of CdS QDs. With increasing

of the heat treatment duration, the exciton-

absorption peak shifts to a lower-energy side. Based

on a theory of the quantum size effect in spherical

QDs (Martin J.L., Rivera R., Cruz S.A. 1998) the

mean radii of prepared CdS QDs are estimated to be

2.3 and 3.5 nm. The observation of the clear

absorption peaks indicates that the size-distribution

width of the CdS QDs is rather small (Fig 6).

Fig. 6 clearly shows effect of the heat treatment

on the absorption and emission spectra of FP glasses

doped with CdS QDs with sizes 2.3 and 3.5 nm. In

PL spectra, the broad PL band with a large Stokes

shift (1.2 eV) is dominant, and the band-edge PL is

negligibly weak. The emission spectrum of samples

is dominated by "deep trap" emission, strongly red

shifted from the band edge (Fig.6).

Fig. 7 demonstrates concentration and

size

dependence of the PL AQY magnitudes for glasses

doped with CdS QDs.

1,2 1,6 2,0 2,4 2,8 3,2 3,6 4,0

0,0

0,2

0,4

0,6

0,8

1,0

100

110

Figure 6: Absorption and luminescence spectra of the

glasses doped with CdS QDs with a sizes 2.3 nm (1), 3.5

nm (2). The excitation energy is 3.06 eV.

Concentration of the QDs in the glass 3 is in two

times higher than in glass 2. Comparison of the PL

AQY of the two glasses doped with different QDs

concentration (Fig. 7, curves 3 and 1) demonstrates

PL concentration quenching as number of QDs

increases above concentration threshold (Fig. 7).

The PL AQY magnitudes for glasses doped with

CdS QDs with sizes 2.3 -3.5 nm demonstrate weak

dependence on the size (Fig. 7). The PL AQY of the

CdS QDs increases to 65 % for QDs with size 2.3

nm and then slowly fells down to 60 % for QDs with

size 3.5 nm (Fig. 7). The values of the PL AQY of

the samples 1, 2 and 3 were measured several times

during the year. The results of the measurements

coincided.

Figure 7: Dependence of PL AQY magnitudes for glasses

doped with CdS QDs with sizes 2.3 nm (1), 3.5 nm (2) and

2.5 nm (3), respectively, on the excitation wavelength.

Fluorine Phosphate Glasses Doped with Cadmium Sulﬁde and Selenide Quantum Dots with High Quantum Efﬁciency at Room-temperature

331

Absolute quantum yield allows estimating

efficiency of converting UV light in the visible

range that is why it is an important parameter for

industrial applications of glasses doped with

CdS(Se) QDs as luminescence down shifting

material or phosphor.

4 CONCLUSIONS

The CdS(Se) nanocrystals synthesized in the

fluorine phosphate glass represents a series of

excellent emitters in the orange-red spectral region

(600-750 nm) in terms of their PL AQY and the

FWHM of the PL spectra, and they show the

stability of the emission for a long time.

The photoluminescence quantum yield of CdSe

QDs rises monotonically to a maximum value and

then decreases gradually with QDs size increase.

Such a maximum (a PL “bright point”) is in 650-750

nm spectral range.

The PL AQY magnitudes for glasses doped with

CdS QDs with sizes 2.3 -3.5 nm demonstrate weak

dependence on the size.

We suggest that origin of these dependences is

the difference in the interaction mechanisms

between CdSe, CdS quantum dots and glass-

network.

Experimental results suggest that the existence of

the PL bright point is general phenomenon of CdSe

QDs and likely is signature of an optimal surface

structure reconstruction of the nanocrystals grown in

a liquid (Qu, L., Peng, X. 2002) or in glass. Absolute

quantum yield magnitude of luminescence glasses

doped with CdS(Se) QDs can reach 50-65%, which

is in two times higher than it was reported earlier in

the silicate glasses. It opens up new prospects for

using such materials as phosphors for white LEDs

and down-convertors for solar cells.

ACKNOWLEDGEMENTS

Research was funded by Russian Science

Foundation (Agreement #14-23-00136).

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