Ordered dispersion of ZnO quantum dots in SiO2 matrix and its strong emission properties

Journal of Colloid and Interface Science 353 (2011) 30–38

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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis

Ordered dispersion of ZnO quantum dots in SiO2 matrix and its strong emission properties Shrabani Panigrahi, Ashok Bera, Durga Basak ⇑ Department of Solid State Physics, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India

a r t i c l e

i n f o

Article history: Received 18 May 2010 Accepted 18 September 2010 Available online 25 September 2010 Keywords: Quantum dots Nanocomposite Fluorescence Display Dispersion Interface

a b s t r a c t ZnO nanoparticles in the form of quantum dots (QDs) have been dispersed in SiO2 matrix using StÖber method to form ZnO QDs-SiO2 nanocomposites. Addition of tetraethyl orthosilicate (TEOS) to an ethanolic solution of ZnO nanoparticles produces random dispersion. On the other hand, addition of ZnO nanoparticles to an already hydrolyzed ethanolic TEOS solution results in a chain-like ordered dispersion. The photoluminescence spectra of the as-grown nanocomposites show strong emission in the ultraviolet region. When annealed at higher temperature, depending on the sample type, these show strong red or white emission. Interestingly, when the excitation is removed, the orderly dispersed ZnO QDs-SiO2 composite shows a very bright blue fluorescence visible by naked eyes for few seconds indicating their promise for display applications. The emission property has been explained in the light of structure– property relationship. Ó 2010 Elsevier Inc. All rights reserved.

1. Introduction Recently, the properties of the materials in the form of nanostructures have attracted much attention due to the opportunity to investigate the fundamental concepts of quantum mechanics and their various potential applications such as light emitting devices [1], nanoelectronics [2–4], including chemical sensors [5]. II–VI semiconductor nanostructures are the most popularly used ones in the optoelectronics [6,7]. Among these, being wide band gap and having large excitonic binding energy, ZnO have been established as a highly efficient emissive material used in the light emitting fillers, light emitting diodes (LED), etc. [8]. By adding metal nanoparticles, the emission property of ZnO can further be enhanced by harnessing the surface plasmon excitation [9]. However, one of the major problems one faces in this area is that the nanoparticles especially the quantum dots (QDs) usually grow continuously and agglomerate during storage [10,11]. Being susceptible to external environment, their fluorescence efficiency is quenched which is not desirable for the practical applications [12,13]. In order to overcome this barrier, the concept of nanocomposite by endowing the QDs in SiO2 matrix has been widely used [14]. SiO2 not only protects chemically and physically, but also provides water dispersibility, and biocompatibility. Therefore, ZnO QDsSiO2 is considered to be challenging systems for the quantum confinement of semiconductor nanocrystallites for a better control of

⇑ Corresponding author. Fax: +91 33 24732805. E-mail address: [email protected] (D. Basak). 0021-9797/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2010.09.055

shape, size and properties [15]. The preparation of such SiO2 coated QDs is usually done following two techniques: (1) StÖber method [16–20] and (2) reverse microemulsion method [21–25]. The former consists of two steps: (i) preparation of QDs and (ii) then dispersion of the QDs in the SiO2 matrix. A reverse microemulsion system, which consists of an oil phase, a surfactant phase, and an aqueous phase, is a thermodynamically stable isotropic dispersion of the aqueous phase in the continuous oil phase. This method is expensive, because the ratio of aqueous phase to oil phase and surfactant in these microemulsion systems is too small and therefore only a small amount of nanometer-sized particles can be obtained at the cost of a large amount of oil and surfactant. Various others techniques such as spray drying [26], RF sputtering [27,28] have also been demonstrated for ZnO QDs-SiO2 composite formation. Zhang and his co-workers have used another simple process by depositing ZnO thin films on SiOx/Si substrates and subsequent thermal annealing [29]. Kim et al. [30] have reported that the self-organized ZnO QDs could be grown on SiO2/Si substrates using sophisticated metal–organic chemical vapor deposition. StÖber method is preferred for preparing such oxide composites because it gives better the uniformity in the dispersion. In the previous reports, mainly the structural and optical properties of the composite systems have been studied extensively and in few, the fluorescent properties have been dealt. An interesting phenomenon of negative photoconductivity has been observed in case of ZnO–SiO2 composite by Panigrahi et al. [20]. ZnO QDs-SiO2/epoxy super nano composites have shown promising prospects as encapsulating material for the preparation of LED [31]. ZnO nanoparticles embedded in SiO2 matrix can emit various colors from blue to

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2.1. Materials

(200) (112) (201)

(103)

(102)

(110)

(a)

(iii)

(ii)

Zinc acetate-2-hydrate [Zn(CH3COOH)22H2O; Sigma–Aldrich, 99.999%], hexamethylenetetramine (HMT) [(CH2)6N4, Merck, 99.5%], ammonium solution [NH3.H2O (wt. 25%), Merck, 99.5%], tetraethyl orthosilicate [TEOS; Sigma–Aldrich, 99.999%] and ethanol [C2H5OH; Merck, 99.5%] were used as received. All the aqueous solutions were prepared using de-ionized water.

(i)

30

2.2. Methods

40

50

60

70

2 θ (Degree)

(b) Sample A Annealed Sample A

Intensity (a.u.)

The composite was prepared in two steps – preparation of ZnO nanoparticles and then encapsulation of the particles with SiO2. For the 1st step, zinc acetate-2-hydrate [Zn(CH3COOH)22H2O] and hexamethylenetetramine (HMT) were mixed thoroughly in de-ionized water for 1 h under constant stirring and was heated at 90 °C. After 2 h, ZnO nanoparticles were grown as a white precipitate which was then isolated by centrifuging, followed by washing and drying under vacuum. As a 2nd step, these as-grown nanoparticles (0.2 g) were then added to a mixed solution of 20 ml ethanol, 9 ml distilled water and 0.5 ml NH3H2O under ultrasonic stirring condition. Then, 0.5 ml TEOS was added to the mixture drop-wise. After some time, the precipitate was isolated and washed by ethanol and water. Finally, it was dried under vacuum. This composite is hence forth termed as sample A. A slightly different procedure was applied for the preparation of another composite where at first, a solution of ethanol, distilled water and NH3H2O (the amounts were kept same as above) was prepared and then TEOS was added to the solution under constant stirring. ZnO nanoparticles were then added to the solution. After 3–4 h, the precipitate was isolated, washed and dried following the above procedure. This composite is named as sample B. The as-grown samples were annealed at 600 °C for 2 h in oxygen atmosphere for better crystallization. The structural studies of the as-synthesized product was done with the X-ray diffractometer using Cu Ka radiation (k = 1.5406 nm). The surface morphology was examined by a high resolution transmission electron microscopy [HRTEM, Model: JEOL

(101)

(100) (002)

(iv)

(002)

2. Materials and methods

JSM-2010]. Fourier transform infrared spectroscopy (FT-IR) of the samples have been measured using KBr pellet as the references by Shimadzu FT-IR 8400S spectrophotometer. A He-Cd laser (Kimmon Koha Co. Ltd.; Model: KR 1801C) with a wavelength of 325 nm was used for the optical excitation of the sample. A high-resolution spectrometer (Horiba Jobin Yvon, Model: iHR 320) together with a photomultiplier tube was used to detect the photoluminescence (PL) from the samples. The decay kinetics was recorded by using Horiba Jobin Yvon Fluromax-4 spectroflurometer. All measurements were performed at room temperature.

Intensity (a.u.)

yellow–green depending on their size [32]. The fluorescence can be enhanced with an after-glow effect upon calcinations [33]. However, lack of detailed analyses of the photoluminescence properties and its correlation with the morphology leaves a scope of investigation which has motivated us to take up this study. Herein, first we report on the preparation of ZnO QDs-SiO2 composite by a StÖber method where the sequence of adding ZnO nanoparticles and tetraethyl orthosilicate (TEOS) to an ethanolic solution has been changed in order to investigate the change in the morphology. When TEOS is added to the ethanolic solution of ZnO nanoparticles, a random dispersion is observed. But when ZnO nanoparticles are added to the ethanolic solution of TEOS, a chain-like dispersion has been achieved which is not reported yet. In contrast to the earlier results [32,34,35], we have observed a strong excitonic emission in the as-grown nanocomposites without or with the presence of a low intense visible emission. When the as-grown nanocomposites are annealed at higher temperature, strong red and white emissions have been observed depending on the preparation technique. After removal of the optical excitation source, the composite with random dispersion does not exhibit any fluorescence while the composite with ordered dispersion shows a strong blue fluorescence which can be seen by naked eyes for few seconds. Thus, these composites are very much promising for display applications including LEDs.

FWHM 0.2466

FWHM 0.2877

33.5

34.0

34.5

35.0

35.5

2 θ (Degree) Fig. 1. (a) X-ray diffraction patterns of (i) sample A, (ii) annealed sample A, (iii) sample B and (iv) annealed sample B. (b) The figure shows only (002) peak of the wurtzite ZnO in case of as-grown and annealed sample A.

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3. Results and discussions 3.1. Structural and micro-structural analyses The XRD patterns of the as-grown and annealed samples A and B are shown in Fig. 1. All the peaks can be indexed to the hexagonal wurtzite phase of ZnO. No separate peak for SiO2 could be detected between 30° and 70° due to an amorphous nature of the matrix. Although there is no change in the peak intensities, the FWHM of the peaks for both the annealed samples decreases as compared to the as-grown samples. For example, FWHM of (0 0 2) peak of sample A is 0.2877 while the value for the annealed sample is 0.2466 (shown in Fig. 1b). This indicates better crystallinity of the annealed nanoparticles. The absence of any other peak in the XRD patterns of both the annealed samples indicates that no reaction has been taken place between ZnO and SiO2 unlike the formation of a ternary compound when ZnO–SiO2 was calcined at 700 °C by Li et al. [33]. However, they have observed no change in the crystalline structure when calcined their ZnO–SiO2 composite up to 500 °C. Since SiO2 is amorphous, no peak is appeared between 30° and 70° due to non-crystallinity of the matrix. The microstructure of the ZnO QDs-SiO2 composite sample A is revealed in the TEM images in Fig. 2. Fig. 2a shows an overall morphology which shows an irregular and lumped structure of the composite. The morphology observed with higher magnification

in Fig. 2b clearly shows a dispersion of the nanopartilces in the matrix of SiO2. The average diameter of the as-grown nanocrystals is around 5–7 nm. The dispersion though is homogeneous but has no ordered structure. When the sample is annealed at higher temperature, SiO2 matrix is formed with a regular shape either in the form of a sphere or a heart around the ZnO particles which is a more compact morphology (Fig. 2c). Because of the formation of a bigger SiO2 envelope around the ZnO particles, the average diameter of the nanoparticles has been decreased to around 3 nm. Since, the excitonic Bohr radius of ZnO is ca. 1.4 nm [36], these nanoparticles are further termed as QDs. The interface between ZnO and SiO2 is visible prominently. The HRTEM image in Fig. 2d shows the lattice fringes corresponding to the (1 0 0) plane of the wurtzite ZnO structure. The inset shows the corresponding fast Fourier transform (FFT) pattern of ZnO, which indicates hexagonal crystal structure. Similar micro-structural analyses of the sample B are represented in Fig. 3a–d. The overall morphology of this sample appears as the clusters of small spheres with uniform size as shown in Fig. 3a. Fig. 3b shows the higher resolution TEM image of the asgrown sample which shows the ZnO particles. In the annealed sample, the QDs of the size 5–6 nm form a radially ordered chain-like structure in the SiO2 matrix (arrows are marked along the chains in Fig. 3c). The HRTEM image in Fig. 3d shows the lattice fringes corresponding to the (1 0 0) plane of the wurtzite ZnO structure. The inset shows the corresponding fast Fourier

Fig. 2. (a) and (b) TEM micrographs of the sample A in two different magnifications. (c) TEM micrograph of the annealed sample A and (d) HRTEM image of the annealed sample A. The inset in (d) shows the corresponding FFT pattern of the hexagonal wurtzite structure of ZnO.

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Fig. 3. (a) and (b) TEM micrographs of the sample B in two different magnifications. (c) TEM image of the annealed sample B and (d) HRTEM image of the annealed sample B. The inset in (d) shows the FFT pattern of the hexagonal structure of ZnO.

transform (FFT) pattern which can be indexed to the hexagonal ZnO structure. No agglomeration of the QDs has been noticed even after 1 month which indicates excellent stability in the shape and size. To explain the difference in the morphology, we need to consider the sequence of adding the QDs and TEOS to the ethanolic solution. When ZnO QDs are added to the ethanolic solution and sonicated as a first step, the dots become separated to act as the seeds. When TEOS is added as a second step, hydrolysis and condensation of TEOS occurs around these seeds [37]. As a result, the SiO2 matrix is formed around the QDs as shown in Scheme 1a. For the other procedure, when TEOS is added to the ethanolic solution as a first step and ZnO QDs are added next followed by a stirring process of 2 h, the ZnO QDs do not get scope to disperse and are stabilized by an inter-chain bonding [38] (through to the inter molecular forces of attraction in the already hydrolyzed silica solution) giving rise to a chain-like structure as per the model shown in Scheme 1b. Different bonding scheme as proposed can further be supported by the FT-IR results as shown in Fig. 4. The absorption peaks at 3400–3500 cm1 can be attributed to the stretching vibrations of structural hydroxyl groups and one at about 1631 cm1 to the bending vibration of adsorbed water. The peak at about 780–800 cm1 and 1050–1150 cm1 are the symmetric stretching vibration and asymmetric stretching vibration of Si–O–Si respectively. The peaks at 440–480 cm1 correspond to the combination of bend vibration of Si–O–Si and stretching modes of Zn–O. The FT-IR spectra in Fig. 4 show that absorption due to bend vibration of Si–O–Si and stretching modes of Zn–O is

enhanced in case of annealed sample A compared to sample B due to the presence of more Si–O–Si bonding around ZnO particles. This indicates different bonding in sample A and sample B. 3.2. Optical analyses Fig. 5 shows the room temperature PL spectrum of the as-grown and annealed ZnO and ZnO QDs-SiO2 composites. Only ZnO nanoparticles also show a very strong UV peak at 384 nm and a moderately intense visible emission in 450–700 nm. The UV peak is due to the excitonic transition and the broad peak in the visible region is associated to the oxygen vacancies [39,40], surface traps [41] and interstitials structural defects [42]. When annealed, the intensity of the excitonic emission is reduced while that of the visible emission is increased. Though the result is in contradiction to the expectation that the optical properties should be improved with crystallinity after annealing, quenching of UV emission is often observed for ZnO nanostructures. Du et al. [43] also have observed the quenching of the excitonic emission and enhancement of the visible emission in ZnO film after annealing in oxygen environment. They predicted that oxygen vacancies, VO and interstitial zinc, Zni decrease while the zinc vacancies, VZn increase with increase in the pressure of oxygen. As the concentration of VZn increases, the optically produced carriers are mostly trapped by the vacancies causing the quenching of the UV emission. We have observed earlier that accumulation of defects on the surface and formation of different surface defect complexes (through interaction

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(a)

Si

(b) Si

ZnO

ZnO

ZnO

Si

O Si

O

ZnO ZnO

ZnO

ZnO

Si ZnO

Scheme 1. Models showing the mechanism of ZnO quantum dots dispersion in SiO2 matrix for (a) sample A and (b) sample B.

of the defects with the surface adsorbed species) makes as-grown ZnO nanowires devoid of defects which results in less intense visible emission. The heating of the nanowires causes surface modification. As a result of which defects again play key role in the PL emission properties [44]. In case of composites, both the samples A and B show very strong UV emission at 387 nm (shown in Fig. 5b and c). However, in contrast to the previous result [34], we have not observed any blue shift of the UV peak position in the composites. Rather, a slight red shift is observed which might be due to the strain in the QDs which has obscured the blue shift. It is well-known that the strain changes the band gaps and lifts the

degeneracy of the valence band, thus producing a red shift in the excitonic peak position [45,46]. The PL spectrum of sample B shows a broad emission in the visible region from 450 nm to 700 nm while the intensity of visible emission is very less in sample A (Fig. 5c). The nearly absence of the visible emission in case of sample A is due to larger interactions of the surface states with SiO2 matrix [47] as expected from the above explanation. In fact, as per the growth model, the strong interaction is expected in sample A which causes less defect band emission. Similar to ZnO, considerable change in the PL spectrum has been noticed when the composite samples are annealed at 600 °C. The PL spectrum of

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100

100

(a)

(b) 80

Transmittance (%)

Transmittance (%)

80

60

40

Annealed Sample A

20

0 4000

60

40

Annealed Sample B

20

3500

3000

2500

2000

1500

1000

0 4000

500

3500

3000

Wave number (Cm-1)

2500

2000

1500

1000

500

Wave number (Cm-1)

Fig. 4. FT-IR spectra of annealed (a) sample A and (b) sample B.

(a)

(b)

ZnO

Intensity (a.u.)

Intensity (a.u.)

AnnealedZnO

400

500

600

A Annealed A

400

700

Wavelength (nm)

(d)

Intensity (a.u.)

(c)

Intensity (a.u.)

500

600

700

Wavelength (nm)

B Annealed B

(477 nm)

( 396 nm ) (625 nm)

400

500

Wavelength (nm)

600

700

400

500

600

700

Wavelength (nm)

Fig. 5. (a) The photoluminescence spectra of as-grown and annealed ZnO nanoparticles. (b) and (c) The photoluminescence spectra of sample A – annealed sample A and sample B – annealed sample B respectively. The inset in (b) and (c) shows the corresponding chromaticity index (CIE) for the emissions. (d) The Gaussian curve fitting of the photoluminescence spectra of the annealed sample B.

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Table 1 CIE and CCT data for annealed sample A and annealed sample B. Name of the sample

x

y

CCT (K)

Annealed sample A Annealed sample B

0.41 0.31

0.38 0.32

3307 6736

8.9 eV

CB

CB

3.37 eV

Zni

V0..

Zni+ VZn

h

VB ZnO

VB SiO2 Fig. 6. Jablonski diagram of different transitions of probable recombinations.

the annealed sample A shows no excitonic emission but only a visible emission with a peak at around 620 nm (red emission). The annealed sample B shows a broad emission from 350 nm to 700 nm (white emission). The insets in Fig. 5b and c show the respective chromaticity diagrams deduced as per 1931 CIE x, y co-ordinates. The CIE co-ordinates of the corresponding emissions indicate red and white emissions respectively. The chromaticity co-ordinates (x, y) and correlated color temperatures (CCT) data are summarized in Table 1. It has been found that the defect structure and transition mechanism can also be modified by the amount and distribution of ZnO quantum dots in SiO2 matrix to yield distinct luminescence properties [26]. At the same time, it is also noted that when a host matrix is doped with a rare earth metal ions, the resultant emission shows some shift from that of the host emission [48]. Our results are in compliance with the former case. The broad visible emission of sample B can be fitted with a Gaussian curve fitting program as shown in Fig. 5d. It shows that the three emissions with peaks at 625 nm (red),1 477 nm (green) and 396 nm (blue) are superimposed to form the broad white emission. Therefore, our results indicate activation of different deep level de1

For interpretation of color in Figs. 1 and 5–8, the reader is referred to the web version of this article.

Fig. 7. (a) Methanolic solution of only SiO2 under the laser light excitation. (b) and (c) The red and white emission from the methanolic solutions of annealed sample A and sample B respectively under the laser light excitation. (d) The blue fluorescence from the methanolic solution of annealed sample B when laser light is off. (e) and (f) The fluorescence from the annealed powder sample B designed in the forms of ‘IACS’ and ‘NANO’ when the laser is off.

fects after annealing. It would be worth to mention here that different defect centers are responsible for green, yellow and red emissions [49,50]. In case of bulk ZnO particles, the green–yellow

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emission is attributed to the radiative recombination of electrons from the conduction band (CB) to the deeply trapped holes at doubly charged oxygen vacancy, VÖ (green, 2.2 eV) is formed as a result of VÓ ? VÖ promoted by the hole trapping of the surface defects [51–53]. The red emission is attributed to the interstitial zinc [54] or sometimes to excess oxygen [55]. The blue emission is predicted to be responsible for a donor–acceptor pair (DAP) recombination between Zni (donor like nature) and VZn (acceptor like nature) [34]. With annealing, some of these defects become active as a result of which the PL spectrum of ZnO–SiO2 nanocomposites shows a white emission [26]. Different transitions showing probable recombinations have been shown in the Jablonski diagram in Fig. 6. Fig. 7 shows the colors of the samples in the presence of the laser excitation as recorded by a digital camera. No particular emission is observed from the methanolic solution of SiO2 (shown in Fig. 7a) as evidenced from the PL spectra of only methanol and methanol with SiO2 (not shown here). While an intense red emission in case of sample A and white emissions in case of sample B are distinctly visible as shown in the digital pictures in Fig. 7b and c. Interestingly, when the laser is removed, sample A does not show any fluorescence but the sample B shows a strong blue fluorescence (Fig. 7d) which is persistent for few seconds and visible by the naked eye. The after-glow in the form of certain letters ‘IACS’ and ‘NANO’ have been captured by a digital camera

37

and shown in Fig. 7e and f. An orange emission has been observed by Li et al. [32] for a ZnO/SiO2 core–shell structure. 3.3. Decay kinetics The strong blue fluorescence is visible since short-wavelength DAP luminescence generally have the longer decay life time [34]. The long-wavelength emissions involving the recombination between VO (donors) with VZn (acceptor) which have short decay life [34] as evidenced from the absence of any fluorescence in sample A after removal of the laser source. This is further confirmed from the time resolved luminescence decay experiments for the annealed samples at room temperature as shown in Fig. 8. The decay curve of the annealed sample can be well fitted with bi-exponential equation of the type y = a1exp(x/t1) + a2exp(x/t2), where the average life time was calculated by the equation P P Cav ¼ ð ai t2i = ai ti Þ. The average lifetime of sample A is found to be 0.00224 ms. While the life time of sample B is found to be 0.02 ms which is larger than that of the sample A. The surfaceto-volume ratio plays a crucial role in the PL emission [33]. Therefore, the difference between the QDs’ size and dispersion in the samples A and B might be the cause behind this difference in the emission properties observed here. 4. Conclusions

(a) Experimental data Exponential fitting λ ex= 370 nm

Intensity (a. u.)

1200

800

In summary, we have synthesized nanocomposite of ZnO QDsSiO2 by the StÖber method where QDs are uniformly dispersed in SiO2 matrix in an ordered structure using a previously hydrolyzed silica solution. The PL properties of the as-grown nanocomposites show a strong UV emission. Upon annealing, the sample A and sample B show strong red and white emissions respectively. The white emission is result of superimposition of red, green and blue emissions. After turning of the laser, the sample B shows a bright blue fluorescence visible by the naked eyes for few seconds. Our results show that these composites are very much promising for the display application.

400

Acknowledgments The authors are thankful to DAE-BRNS for the financial support for the work. S. Panigrahi and A. Bera are thankful to CSIR, New Delhi for awarding the junior research fellowship.

0 0.00

0.02

0.04

Time (ms)

References

(b)

Intensity (a.u.)

12000

8000

Experimental data Exponential fitting λ ex= 416 nm

4000

0.00

0.02

0.04

0.06

0.08

Time (ms) Fig. 8. (a) and (b) The experimental decay and the fitting curve of the emissions of the annealed samples A and B respectively.

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