Journal of Non-Crystalline Solids 357 (2011) 2428–2430

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Journal of Non-Crystalline Solids

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Influence of silver nanoclusters on formation of PbS quantum dots in glasses

Kai Xu a, Chao Liu a, Shixun Dai b, Xiang Shen b, Xunsi Wang b, Jong Heo a,⁎a Center for Information Materials, Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), San 31, Hyoja-dong, Nam-gu, Pohang,Gyeongbuk 790-784, Republic of Koreab College of Information Science and Engineering, Ningbo University, Ningbo 315-021, China

⁎ Corresponding author. Tel.: +82 54 279 2147; fax:E-mail address: jheo@postech.ac.kr (J. Heo).

0022-3093/$ – see front matter © 2010 Elsevier B.V. Adoi:10.1016/j.jnoncrysol.2010.11.091

a b s t r a c t

a r t i c l e i n f o

Article history:Received 9 July 2010Received in revised form 7 October 2010Accepted 2 November 2010Available online 20 December 2010

Keywords:PbS quantum dots;Silver clusters;Absorption;Photoluminescence;Thermal treatment

Heat-treatment was used to precipitate PbS quantum dots (QDs) in silicate glasses doped with differentamounts of Ag2O, and the influence of Ag2O on QDs was investigated. Under given heat-treatment conditions,the absorption coefficients and photoluminescence intensities of PbS QDs increased with the addition of Ag2O.Ag clusters formed by thermal treatment nucleated formation of PbS QDs in glasses.

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1. Introduction

Over the past two decades, glasses doped with semiconductorquantum dots (QDs) have attracted considerable interest foroptoelectronic applications due to the size-tunable optical propertiesof QDs. For example, glasses containing II–VI QDs have been used assharp-cut filters and non-linear optical devices [1–3]. Recently,narrow-band IV–VI QDs embedded in glasses have also beeninvestigated. These semiconductors have large exciton Bohr radii(PbS: 18 nm; PbSe: 46 nm) with the strong quantum confinement[4,5]. Therefore, glasses containing IV–VI QDs can have potentialapplications in optical switches and fiber-optic amplifiers fortelecommunication [6,7].

Thermal treatment has been used to form QDs in glasses, butprecise control of nucleation and growth of the QDs is difficult [8] andmay require complicated procedures. A two-step heat-treatmentmethod for the stepwise nucleation and growth of the nanocrystalscan provide improvement to a certain degree. In some cases, a longannealing at low temperature helped to improve the uniformity ofnucleation that led to the formation of small sized QDs with a narrowsize distribution [9]. Nevertheless, precise control of the nucleationstage has been difficult and QDs precipitated inside glass matricesgenerally have a much broader size distribution than those preparedby colloidal precipitation [6].

Ag nanoclusters are known to act as nuclei and promote theformation of oxide nanocrystals in glass matrices [10]. This result

suggests that Ag clusters in glasses may promote formation of QDs inglasses, and that adjusting Ag concentration may allow control of thenumber of nucleation sites. This paper reports the effect of Ag on theprecipitation of PbS QDs, and the effects of heat-treatment temperature(T) on the absorption coefficients and photoluminescence (PL)characteristic of the glasses containing them.

2. Experimental procedures

The nominal compositions of glasses were 50SiO2–35Na2O–5Al2O3–8ZnO–2ZnS–1PbO (in mol %) with an additional Ag2O of 10,20 and 30 ppm, respectively. After mixing, starting powders weremelted in a platinum crucible at 1350 °C for 45 min under an ambientatmosphere. Melts were quenched by pouring onto a brass mold andpressing with another plate. The as-made glass containing 30 ppm ofAg2O contained defects of dark-colored stripes. Other compositionsprovided transparent glasses and they were annealed at 350 °C for3 h, then heat-treated for 10 h at 440≤T≤480 °C to precipitate PbSQDs.

Glasses were cut into sections (~1.5 mm thick) and opticallypolished. Optical absorption spectra of the glasses at wavelengths (λ)300–2200 nm were recorded using a UV/Vis/NIR spectrophotometer(Perkin Elmer Lambda 750S). For the measurement of PL, an excitationsource of 750≤λ≤790 nm from a continuous-wave Ti:Sapphire laserwas used. A combination of a mechanical chopper of 50 Hz frequency, a1/4 m monochromator, an InGaAs detector and a lock-in amplifiersystem was used to record PL spectra. All measurements wereperformed at room temperature. A high-resolution transmissionelectron microscope (HR-TEM, JEOL JEM-2100F) was used under an

500 1000 1500 20000

1

2

3

Wavelength (nm)

As-made

440oC

450oC

460oC

470oC

480oC

As-made

440oC

450oC

460oC

470oC

a

Abs

orpt

ion

coef

ficie

nt (

cm-1

)

6

8b

ent (

cm-1

)

2429K. Xu et al. / Journal of Non-Crystalline Solids 357 (2011) 2428–2430

accelerating voltage of 200 kV to identify the crystal structure of theQDs.

3. Results

3.1. TEM analysis

A glass containing 20 ppm of Ag2O which had been heat-treated at480 °C was examined under HR-TEM (Fig. 1a). The QDs were nearlyspherical with an average radius of approximately 3.3 nm anddistribution of the size was ~10%. A TEM image (Fig. 1b) of a singlenanocrystal was obtained and the fast Fourier transform (FFT) patternwas obtained for a portion of this image (Fig. 1b, inset). The diffractionpattern was consistent with that of a bulk PbS crystal which has arock-salt crystal structure with a lattice constant of 5.9 Å. This resultconfirms that the QDs formed in the glasses are PbS.

3.2. Absorption and PL spectra

In glasses without Ag, heat-treatment at 440≤T≤480 °C caused theabsorption bands to appear at 695≤λ≤1580 nm (Fig. 2a). In glassescontaining 20 ppm Ag2O, heat-treatment at 440≤T≤480 °C caused

Fig. 1. (a) TEM image of PbS nanocrystals precipitated in glass containing 20 ppm ofAg2O and heat-treated at 480 °C for 10 h (scale bar: 20 nm). (b) TEM image of one PbSnanocrystal and fast Fourier transform pattern (inset) obtained from the area in thecircle.

480oC

500 1000 1500 20000

2

4

Wavelength (nm)

Abs

orpt

ion

coef

fici

Fig. 2. Absorption spectra of PbS QDs in glasses (a) without silver and (b) with 20 ppmof Ag2O after heat-treatment at various temperatures for 10 h.

these peaks to appear at 751≤λ≤1583 nm (Fig. 2b). At eachconcentration of Ag2O, the position of bands shifted to higher λ astemperature increased (Fig. 2; Table1). This phenomenondemonstratesthe strong quantum confinement effect in PbS QDs.

The radii of the PbS QDs (Table 1) calculated using the parabolicmodel [11] increased with T. At a given T, the presence of Ag had littleeffect on the QD radius, but absorption coefficients for glassescontaining 20 ppm of Ag2O were much higher than those for glasseswithout Ag. For instance, the absorption coefficient for the glass with20 ppm of Ag2O heat-treated at 480 °C was ~5.0 cm−1 while that forthe glass without silver was 1.0 cm−1 even subjected to the sameheat-treatment (Table 1).

In normalized PL spectra of PbS QDs in glasses containing 20 ppmof Ag2O the center wavelengths of PL from PbS QDs increased from970 nm at T=440 °C to 1615 nm at T=480 °C (Fig. 3); this trend was

Table 1Absorption peak position (λabs), absorption coefficient (α), and calculated average radius(R) of PbSQDsprecipitated inglasseswith0, 10 or20ppmAg2Obyheat-treatment for 10 hat different temperatures.

Treatmenttemperature(°C)

λabs (nm) α (cm−1) R (nm)

0 10ppm

20ppm

0 10ppm

20ppm

0 10ppm

20ppm

440 695 702 751 0.05 0.14 0.48 1.2±0.1 1.2±0.1 1.4±0.1450 783 793 841 0.16 0.29 0.94 1.4±0.2 1.5±0.1 1.6±0.2460 976 978 977 0.28 0.71 1.91 1.9±0.3 1.9±0.2 1.9±0.2470 1232 1235 1256 0.75 1.42 3.65 2.4±0.3 2.4±0.3 2.5±0.4480 1580 1587 1583 1.00 2.10 5.05 3.4±0.3 3.4±0.3 3.4±0.3

800 1000 1200 1400 1600 18000.0

0.2

0.4

0.6

0.8

1.0N

orm

aliz

ed P

L

Wavelength (nm)

440oC

460oC

470oC

480oC

Fig. 3. Normalized PL spectra of PbS QDs in glasses containing 20 ppm of Ag2O afterheat-treatment at various temperatures for 10 h.

300 400 500 600 7000.0

0.5

1.0

1.5

2.0

2.5

3.0

Abs

orpt

ion

coef

ficie

nt (

cm-1

)

Wavelength (nm)

As-made

460oC

~365 nm

Fig. 4. Absorption spectra of glass without PbS QDs before and after heat-treatment at460 °C for 10 h.

2430 K. Xu et al. / Journal of Non-Crystalline Solids 357 (2011) 2428–2430

similar to the shift of the absorption bands to higher λ as T increased(Fig. 2).

4. Discussion

Silver clusters such as Ag7 or Ag9 formed in glasses cause anabsorption band at λ=~360 nm [12], but it is normally buried by the

400 600 800 1000 1200 14000.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0460oC

20 Ag2O

10 Ag2O

No Ag2O

Wavelength (nm)

Abs

orpt

ion

coef

ficie

nt (

cm-1

)

0

10

20

30

40

50

PL intensity (a. u.)

Fig. 5. Absorption and PL spectra of PbS QDs in glasses containing different contents ofAg2O after heat-treatment at 460 °C for 10 h.

absorption edge of glasses containing ZnS andPbO. Therefore, a glass thatdid not include ZnS and PbO was prepared to determine whether Agclusters formed. This glass had a nominal composition (in mol %) of50SiO2–35Na2O–5Al2O3–10ZnOandwasdopedwith 20 ppmofAg2O. Anabsorption band appeared at λ=365 nm when the glass was heat-treated at 460 °C for 10 h (Fig. 4). This absorption peak confirms theformation of Ag clusters such as Ag7 or Ag9 in this glass, and stronglysuggests that they also formed in the glasses that included ZnS and PbO.

The absorption coefficients and the PL intensities increasedsignificantly with the addition of Ag2O (Fig. 5). The absorptioncoefficients are directly related to the concentration of PbS QDs [9], sothe number of PbS QDs formed in glasses increased after Ag2O wasadded. We propose the following hypothesis to explain the results.First, increasing the concentration of silver increased the number ofAg clusters, which provide sites for nucleation of QDs. Second, thishigher number of nucleation sites led to the formation of largenumbers of PbS QDs in glasses. Third, the large number of QDsincreased the absorption coefficients and the intensities of the PLspectra. Thus, Ag clusters act as nucleation sites and promote thegrowth of PbS QDs in glasses.

5. Conclusions

PbS QDs were precipitated in glasses by heat-treatment and theeffect of Ag2O content on the precipitation of the QDs wasinvestigated. HR-TEM analysis confirmed the formation of the PbSQDs. Absorption coefficients and PL intensites of PbS QDs in glassesincreased significantly as the amount of Ag2O increased. Silverclusters were formed during the heat-treatment and providednucleation sites for PbS QDs.

Acknowledgements

This work was supported by the Priority Research Centers Programthrough the National Research Foundation of Korea (NRF) funded bythe Ministry of Education, Science and Technology (2009-0094037),and the Collaborative Research Project under the NRF—NationalNatural Science Foundation of China (NSFC) Cooperative Program(D00039).

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