- Email: [email protected]
Luminescence studies and formation mechanism of symmetrically dispersed ZnO quantum dots embedded in SiO2 matrix
ARTICLE IN PRESS Journal of Luminescence 129 (2009) 605–610
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Luminescence studies and formation mechanism of symmetrically dispersed ZnO quantum dots embedded in SiO2 matrix Prashant K. Sharma a,, Ranu K. Dutta a, Manvendra Kumar a, Prashant K. Singh a,b, Avinash C. Pandey a,c a b c
Nanophosphor Application Centre, University of Allahabad, Allahabad 211002, India Department of Bio-Technology, University of Allahabad, Allahabad 211002, India Department of Physics, University of Allahabad, Allahabad 211002, India
a r t i c l e in fo
abstract
Article history: Received 9 June 2008 Received in revised form 10 December 2008 Accepted 5 January 2009 Available online 21 January 2009
Surface effects significantly influence the functionality of semiconductor nanocrystals. In the current work we present synthesis of ZnO quantum dots (QD) vis-a-vis symmetrically dispersed ZnO quantum dots embedded in SiO2 matrix and discussed their optical properties to understand the role of the surface effects. These nanomaterials were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), fourier transform infrared (FTIR), absorption (UV–visible) and photoluminescence (PL) spectroscopy. TEM studies confirm the formation of ZnO nanophosphors inside the SiO2 matrix in highly symmetrical manner. These symmetrically dispersed ZnO@SiO2 nanophosphors exhibited enhanced stable emission over uncoated sample and would permit the conjugation of the nanocrystals to biological entities after functionalization. Furthermore, the mechanism behind the formation of symmetrically dispersed ZnO quantum dots embedded in SiO2 matrix was discussed in detail. & 2009 Elsevier B.V. All rights reserved.
Keywords: Symmetrically mono dispersed Embedded quantum dots Bio-imaging
1. Introduction In recent years, the scientific community has paid much attention to the synthesis and characterization of II–VI semiconductor materials at nanometer scale, due to their great potential to test fundamental concepts of quantum mechanics [1,2] and because of their key role in various applications in piezoelectric transducers, gas sensors, light-emitting devices, photonic crystals [3], nanoelectronics [4] photodetectors, photodiodes, optical waveguides, transparent conductive films, varistors, solar cells, transparent UV protection films, chemical sensors and biological (drug delivery, bio-imaging, etc.) systems [5–7]. ZnO nanocrystals are now being used as building blocks for bio/ nanohybrids in biological systems as biomarkers and resonant energy-transfer sensors. Construction of these bio/nanohybrids is a challenging, ongoing problem. An understanding of how these nanomaterials function in these biological systems is needed so that tailored nanooptics with these systems can be exploited. Surface effects radically influence the functionality of semiconductor nanocrystals. Passivation with ligands or high band gap semiconductor shells is necessary to reduce surface trap densities, enhance quantum yield and increase photostability. Because of its large band gap and large exciton binding energy [8], ZnO can also
Corresponding author. Tel./fax: +91 532 2460675.
E-mail address: [email protected] (P.K. Sharma). 0022-2313/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2009.01.004
be used for new applications in bio-imaging by making some surface modifications. ZnO has high surface-to-volume ratio at nanometer scale and hence surface defects play an important role in its properties. An essential surface modification needs to be done for each of the desired applications. It is found that ZnO quantum dots (QD) embedded in SiO2 matrix give emission peak in the visible range and is highly intense. Emission in visible wavelength range is preferable for bio-imaging. Besides all these the most important feature is the biocompatibility of ZnO@SiO2 for possible in vivo bio-imaging unlike the other quantum dots based on Cd or heavy metals which are used conventionally. Zn is one of the most important trace elements found in various parts of the body like the muscle, bone, skin and plasma of blood [9]. A few milligrams of Zn is required everyday by the body and a considerable amount of it is present in various parts of the body. It is a very important co-factor for several enzymes like carbonic anhydrase, alcohol dehydrogenase, carboxypeptidases A and B, etc. In the present paper, we are demonstrating a simple chemical technique to synthesize ZnO quantum dots and symmetrically dispersed ZnO quantum dots embedded in SiO2 matrix with excellent structural and optical properties. We particularly emphasized on luminescence studies of these symmetrically dispersed ZnO quantum dots embedded in SiO2 matrix as the same can be further exploited for nano-biotechnology as bio markers/bio-imaging, etc. and chemical sensors. Further, we have tried to explore the mechanism behind the formation of
ARTICLE IN PRESS 606
P.K. Sharma et al. / Journal of Luminescence 129 (2009) 605–610
symmetrically dispersed ZnO quantum dots embedded in SiO2 matrix.
2. Experimental details Zinc acetate dihydrate (99.2%) [Zn(CH3COO)2 2H2O], lithium hydroxide (LiOH), tetra-ethyl-ortho-silicate (TEOS), ethanol and sulphuric acid (H2SO4), procured from E. Merck Limited, Mumbai 400018, India, were used to synthesize ZnO and ZnO quantum dots embedded in SiO2 matrix. All chemicals were of AR grade and were directly used without special treatment. In a typical co-precipitation method, 1 M Zn(CH3COO)2 2H2O was prepared in 50 ml of ethanol using vigorous stirring at 50 1C for 90 min, followed by cooling at room temperature. LiOH (0.59 g) was dissolved in 50 ml of ethanol and kept in ultrasonic bath, then cooled at room temperature. This solution was slowly added to the Zn2+ solution with vigorous stirring at room temperature. Yellow green emission was observed from the transparent
solution under an UV-lamp (lex ¼ 352 nm). The unwanted CH3COO and Li+ ions were removed by washing the solution repeatedly by n-heptane (volume ratio 1:2). The obtained precipitate was dried at room temperature resulting in white powders of ZnO nanoparticles. Some amount of this white ZnO powder was redispersed in ethanol. Now, the SiO2 matrix was prepared by mixing vigorously for 2 h the 10 ml of tetra-ethyl-ortho-silicate in 10 ml ethanol and 5 ml H2SO4 at 30 1C. The resulting transparent solution and the ethanolic suspension of ZnO were mixed slowly with constant stirring at 30 1C. The Si:Zn solution ratio was kept 30:1. After 2 h of constant stirring at 30 1C, the solution turned into transparent light yellow gel. This transparent light yellow gel was further subjected for ultrasonication for another 2 h. The resulting gel-like mixture was kept in air for few hours to give thick gel which was further dried and annealed at 500 1C in air, giving the final product. The prepared ZnO and ZnO quantum dots embedded in SiO2 matrix were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), fourier transform infrared (FTIR) spectroscopy, UV–visible spectroscopy and photoluminescence (PL) spectroscopy. XRD was performed on Rigaku D/max-2200 PC diffractometer operated at 40 kV/20 mA, using CuKa1 radiation with wavelength 1.54 A˚ in wide angle region from 301 to 701. The size and morphology of the prepared nanophosphors were recorded by transmission electron microscope model Technai 30 G2 S-Twin electron microscope operated at 200 kV accelerating voltage. Fourier transform infrared spectroscopic studies were performed with the FTIR spectroscope model Nicolet 20 SXB. Absorption spectra were recorded on Perkin Elmer Lambda 35 UV–visible spectrophotometer using halogen and deuterium lamp as sources for visible and UV radiations, respectively. PL studies were performed for lex ¼ 320 nm on a Perkin Elmer LS 55 luminescence spectrophotometer using a Xenon discharge lamp, equivalent to 20 kW for 8 ms duration as the excitation source at room temperature.
3. Results and discussion
Fig. 1. X-ray diffraction spectra of ZnO and symmetrically dispersed ZnO quantum dots embedded in SiO2 matrix. For symmetrically dispersed ZnO quantum dots embedded in SiO2 matrix, high background scattering suggests presence of SiO2 matrix.
Fig. 1 shows the XRD pattern of the ZnO and symmetrically dispersed ZnO quantum dots embedded in SiO2 matrix synthesized by co-precipitation method. For both the samples, XRD Spectra showed broad peaks at the positions 31.631, 34.501, 36.251, 47.501, 56.601, 62.801 and 67.921, which were in excellent
Fig. 2. TEM image of (A) bare ZnO quantum dots with spherical particles and (B) symmetrically dispersed ZnO quantum dots embedded in SiO2 matrix.
ARTICLE IN PRESS P.K. Sharma et al. / Journal of Luminescence 129 (2009) 605–610
agreement with the JCPDS file for ZnO (JCPDS 36-1451, a ¼ b ¼ 3.249 A˚, c ¼ 5.206 A˚) and indexed as the hexagonal wurtzite structure of ZnO having space group P63mc. All available reflections of the present phases fitted with the Gaussian distribution. The broadening of XRD peaks (i.e. Scherrer’s broadening) gave clear indication of formation of nanosized ZnO. Particle size ‘d’ of ZnO and symmetrically dispersed ZnO quantum dots embedded in SiO2 matrix were estimated by Debye–Scherrer’s equation d¼
0:9l B cos y
where d is the particle size, l is the wavelength of radiation used, y is the Bragg angle and B is the full-width at half-maxima (FWHM) on 2y scale. For ZnO samples, average particle size using
Fig. 3. FTIR spectra of ZnO quantum dots and ZnO quantum dots embedded in SiO2 matrix.
607
Debye–Scherrer’s equation was of the order of 25 nm. Furthermore, it can be also seen that for symmetrically dispersed ZnO quantum dots embedded in SiO2 matrix, XRD peaks were not as sharp as in the case of ZnO sample, i.e. slight decrease in crystallinity was observed, which indicates smaller particle size. Through Scherrer’s equation average particle size was of the order of 20 nm for symmetrically dispersed ZnO quantum dots embedded in SiO2 matrix. The high background scattering in XRD spectra suggested the presence of SiO2 matrix. These results were well supported by the TEM, FTIR and photoluminescence studies. TEM images were recorded by dissolving the assynthesized powder sample in ethanol and then placing a drop of this dilute ethanolic solution on the surface of copper grid. The morphology of the products is shown in Fig. 2. Fig. 2(A) is TEM image of bare ZnO quantum dots, while Fig. 2(B) is TEM image representing morphology of ZnO quantum dots embedded in SiO2 matrix. Fig. 2(A) clearly shows spherical particles with little agglomeration having sizes 25 nm. But for the case of ZnO@SiO2 quantum dots (Fig. 2(B)), symmetrically dispersed spherical particles encapsulated in the matrix of SiO2 were observed. These encapsulated structures were very symmetrically oriented throughout the whole TEM grid. The optical properties of the prepared materials were examined by fourier transform infrared, optical absorption and photoluminescence spectroscopy. The FTIR spectra were recorded by making pressed pellet of the prepared samples in potassium bromide (KBr). The corresponding FTIR spectrum is shown in Fig. 3. Clear Zn–O–Zn stretching modes were observed at 620 and 816 cm1 for both the samples and were well supported by available literatures [10,11]. These stretching modes were indicative of successful synthesis of ZnO in both the cases as already confirmed by XRD. Due to adsorption of CO few peaks at 1406 and 1572 cm1 were observed, while stretching modes at 1470 and 3310 cm1 were due to –OH groups present at the surface of symmetrically dispersed ZnO quantum dots embedded in SiO2 matrix. The well-known stretching modes of SiO2 were observed at 816 and 1270 cm1, indicating formation of ZnO quantum dots inside the matrix of SiO2. The broadening of these modes can be explained as overlapping of SiO2 peaks with ZnO peaks. These FTIR spectra were consistent with those from ZnO-loaded silica
Fig. 4. Optical absorption spectra of ZnO and ZnO quantum dots embedded in SiO2 matrix.
ARTICLE IN PRESS 608
P.K. Sharma et al. / Journal of Luminescence 129 (2009) 605–610
molecular sieves reported by Kwon et al. [10] and Gu et al. [12]. Optical absorption spectra were recorded by dispersing the samples in double distilled water and using double distilled water as reference. Fig. 4 shows the optical absorption spectra. The excitonic absorption peaks of both these samples were blue shifted as compared to the bulk band gap of 3.32 eV. The band gap of ZnO sample was found typically around 365 nm (Eg3.4 eV), while for ZnO quantum dots embedded in SiO2 matrix, large blue shift was observed in the excitonic absorption peak. The band gap corresponding to ZnO quantum dots embedded in SiO2 matrix was approximately around 337 nm (Eg3.6 eV). Figs. 5(a) and (b) shows the prepared ZnO quantum dots embedded in SiO2 matrix under ordinary and UV (352 nm excitation) lamps, respectively. While in both figures, letters A, B and C correspond to solid samples, samples dissolved in ethanol and samples dissolved in water, respectively. Samples represented in Fig. 5(a) showed pure
white color, while the samples represented by Fig. 5(b) showed greenish-yellow luminescence. Fig. 5 illustrates the photoluminescence spectra of synthesized ZnO and ZnO quantum dots embedded in SiO2 matrix. The well-known green emission [13,14] from ZnO nanoparticles had been seen shifted from 560 to 548 nm, which resulted in a red shift from the normal ZnO emission at 520 nm. This broad peak in the visible region was associated to the structural defects like interstitials [15,16], oxygen vacancies [17–19] and surface traps, although there was a total absence/quenching of excitonic and defect-associated emission peak intensities. From Fig. 6, it was clear that ZnO quantum dots embedded in SiO2 matrix had an intense peak at 369 nm, which showed electron–hole recombination after relaxation and a very broad emission around 542 nm due to recombination via surface-localized states, owing to the silica matrix, which stops the electron–hole recombination on the ZnO and reduces
Fig. 5. Shows prepared ZnO quantum dots embedded in SiO2 matrix under (a) ordinary and (b) UV (365 nm excitation) lamps. The letters A, B and C correspond to solid samples, samples dissolved in ethanol and samples dissolved in water. Photographs were taken with a digital camera just after completion of synthesis.
Fig. 6. Photoluminescence spectra of ZnO and ZnO quantum dots embedded in SiO2 matrix.
ARTICLE IN PRESS P.K. Sharma et al. / Journal of Luminescence 129 (2009) 605–610
609
the non-radiative transition, causing red shift from the normal ZnO emission at 520 nm. The band-edge emission of ZnO quantum dots embedded in SiO2 matrix was highly intense and had a better time stability, while any appreciable band-edge emission from non-coated ZnO was not found. In the ZnO quantum dots embedded in SiO2 matrix, the hole was confined within the ZnO, while the electron was delocalized over the entire embedded matrix. Unlike in metallic samples there were no shift in absorption or luminescence peak position, but the intensity changes were prominent. Studies have shown that the luminescence efficiency of nanoparticles strongly depends on the nature of the surface, because of large surface-to-volume ratio in smaller particles. In case of ZnO nanoparticles, surface states such as dangling bonds are usually involved in non-radiative processes, while O2 ions provide a critical pathway for the visible emission band [13,20–22]. In our case, the SiO2 matrix reduced the density of surface dangling bonds and O2 ions via Zn–O–Si. So the probability of non-radiative and visible emission further reduced, in contrast, the probability of UV emission will be increased. This was the main cause behind the enhanced UV emission in photoluminescence spectra. This could be further understood as the net charge of the Zn atom in the Zn–O–Si bond was positive. Positive centre in ZnO produced an attractive defect potential creating a donor state by attracting conduction band levels into the band gap [23]. Due to the weak attractive potential, the energy level of the donor state created by Zn–O–Si shifts to 3.4 eV (369 nm) and act as a luminescence centre for the 369 nm band; i.e., carriers excited in ZnO and SiO2 may be trapped at the interface states and consequently recombine to emit a band at 369 nm. In the ZnO@SiO2 matrix, some of the carriers excited in the SiO2 matrix may tunnel to the ZnO–SiO2 interface or to ZnO nanoparticles and recombine, enhancing the UV band. On the other hand, SiO2 provides a good coverage of the ZnO surface and acts as an energetic barrier preventing the escape of photo-generated carriers to outside the confined ZnO nanoparticles.
4. Formation mechanism of ZnO quantum dots embedded in SiO2 matrix We used tetra-ethyl-ortho-silicate for the formation of SiO2 matrix. The chemical structure of TEOS can be represented as in Fig. 7(a). TEOS is liquid at room temperature which slowly hydrolyzes into silicon dioxide and ethanol when in contact with ambient moisture. The key behind the understanding is to note that, in TEOS the silicon atom is already oxidized; the conversion of TEOS to SiO2 is essentially a rearrangement rather than an oxidation reaction, making the reaction mechanism simple. The overall reaction for the SiO2 matrix requires the removal of two oxygen atoms from TEOS as shown in Fig. 7(b). In the present case, formation of SiO2 matrix was probably the result of TEOS surface reaction. TEOS chemisorbs onto silanol groups (Si-OH) at the surface, as well as strained surface bonds. Mechanism is depicted in Fig. 7(c). TEOS will not adsorb onto the resulting alkyl-covered surface during intermediate reaction steps. So SiO2 matrix formation was probably limited by removal of these intermediate surface alkyl groups. As shown in Fig. 7(d), these groups can undergo elimination reactions with neighboring molecules to form Si–O–Si bridges. This process quickly occurs in the ambient reaction conditions: TEOS can be its own oxygen source, and SiO2 can be deposited in the form of a matrix from TEOS. However, additional oxygen atoms provided by ZnO increases the matrix formation rate, presumably this is the cause behind the formation of symmetrically dispersed ZnO quantum dots embedded in SiO2 matrix.
Fig. 7. (a) Chemical structure of tetra-ethyl-ortho-silicate (TEOS). (b) Removal of two oxygen atoms from TEOS. (c) Mechanism depicting TEOS chemisorbs onto silanol groups (Si-OH) surface. (d) Formation of Si–O–Si bridges by elimination reactions with neighboring molecules.
5. Conclusions In summary, we have successfully synthesized quantum dots of ZnO and symmetrically dispersed ZnO quantum dots embedded in SiO2 matrix at 373 K and studied their optical properties to understand the role of the surface effects. This embedded quantum dot exhibits enhanced photoluminescence. This can be
ARTICLE IN PRESS 610
P.K. Sharma et al. / Journal of Luminescence 129 (2009) 605–610
seen as one of the necessary and condemnatory constituent for bio-imaging, bio-sensors, etc. after the conjugation of the nanocrystals to biological entities, underlies the importance of the current work.
Acknowledgement Authors are thankful to DST and CSIR, India, for the financial assistance and to Dr. R.N. Bhargava, Nanocrystal Technology, USA, for his valuable discussions and support during the current work. References [1] [2] [3] [4] [5]
S.M. Prokes, K.L. Wang, Mater. Res. Sci. Bull. 24 (1999) 13. J. Hu, T.W. Odom, C.M. Lieber, Acc. Chem. Res. 32 (1999) 435. S. Nakamura, Science 281 (1998) 956. C.A. Mirkin, Science 286 (1999) 2095. A. Mendoza-Galvain, C. Trejo-Cruz, J. Lee, D. Bhattacharya, J. Metson, P.J. Evans, U. Pala, J. Appl. Phys. 99 (2006) 14306. [6] Honma, S. Hirakowa, K. Yamada, J.M. Bae, Solid State Ionics 118 (1999) 29. [7] T.S. Phely-Bobin, R.J. Muisener, J.T. Koberstein, F. Papadinmitrakopoulos, Synth. Met. 116 (2001) 439.
[8] V.A. Karpina, V.I. lazorenko, C.V. Lashkarev, V.D. Dobrowolski, L.I. Kopylova, V.A. Baturin, S.A. Lytuyn, V.P. Ovsyannikov, E.A. Mauvenko, Cryst. Res. Technol. 39 (2004) 980–992. [9] G. Thomas, Chemistry for Pharmacy and the Life Sciences: Including Pharmacology and Biomedical Science, Prentice-Hall, Padstow TJ Press, Englewoods Cliffs, NJ, 1996. [10] Y.J. Kwon, K.H. Kim, C.S. Lim, K.B. Shim, J. Ceram. Proc. Res. 3 (2002) 146. [11] Y. Xiong, L.Z. Zhang, G. Tang, G. Zhang, W.J. Chen, Luminescence 110 (2004) 17. [12] G. Gu, P.P. Ong, C. Chu, J. Phys. Chem. Solids 60 (1999) 943. [13] A. Van Dijken, E.A. Meulenkamp, D. Vanmaekelbergh, A. Meijerink, J. Lumin. 90 (2000) 123. [14] Z. Li, Y. Xiong, Y. Xie, J. Inorg. Chem. 42 (2003) 8105. [15] J. Zhang, L. Sun, J. Yin, H. Su, C. liao, C. Yon, Chem. Mater. 14 (2002) 4172. [16] M.H. Huang, Y.Y. Wu, H.N. Feich, N. Tran, E. Weber, P.D. Yang, Adv. Mater. 13 (2001) 113. [17] K. Vanheusden, W.L. Warren, C.H. Seager, D.R. Tallanl, J.A. Voigt, B.E. Gandu, J. Appl. Phys. 79 (1996) 7983. [18] M. Liu, A.H. Kitai, P. Mascher, J. Lumin. 54 (1992) 35. [19] X.L. Wu, G.G. Siu, C.L. Fu, H.C. Ong, Appl. Phys. Lett. 78 (2001) 2285. [20] J.F. Li, L.Z. Yao, C.H. Ye, C.M. Mo, W.L. Cai, Y. Zhang, L.D. Zhang, J. Cryst. Growth 223 (2001) 535. [21] A. Van Dijken, E.A. Meulenkamp, D. Vanmaekelbergh, A. Meijerink, J. Lumin. 87–89 (2000) 454. [22] A. Van Dijken, J. Makkinje, A. Meijerink, J. Lumin. 92 (2001) 323. [23] Y.M. Sun, P.S. Xu, C.S. Shi, F.Q. Xu, H.B. Pan, E.D. Lu, J. Electron. Spectrosc. 114–116 (2001) 1123.
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Luminescence studies and formation mechanism of symmetrically dispersed ZnO quantum dots embedded in SiO2 matrix Prashant K. Sharma a,, Ranu K. Dutta a, Manvendra Kumar a, Prashant K. Singh a,b, Avinash C. Pandey a,c a b c
Nanophosphor Application Centre, University of Allahabad, Allahabad 211002, India Department of Bio-Technology, University of Allahabad, Allahabad 211002, India Department of Physics, University of Allahabad, Allahabad 211002, India
a r t i c l e in fo
abstract
Article history: Received 9 June 2008 Received in revised form 10 December 2008 Accepted 5 January 2009 Available online 21 January 2009
Surface effects significantly influence the functionality of semiconductor nanocrystals. In the current work we present synthesis of ZnO quantum dots (QD) vis-a-vis symmetrically dispersed ZnO quantum dots embedded in SiO2 matrix and discussed their optical properties to understand the role of the surface effects. These nanomaterials were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), fourier transform infrared (FTIR), absorption (UV–visible) and photoluminescence (PL) spectroscopy. TEM studies confirm the formation of ZnO nanophosphors inside the SiO2 matrix in highly symmetrical manner. These symmetrically dispersed ZnO@SiO2 nanophosphors exhibited enhanced stable emission over uncoated sample and would permit the conjugation of the nanocrystals to biological entities after functionalization. Furthermore, the mechanism behind the formation of symmetrically dispersed ZnO quantum dots embedded in SiO2 matrix was discussed in detail. & 2009 Elsevier B.V. All rights reserved.
Keywords: Symmetrically mono dispersed Embedded quantum dots Bio-imaging
1. Introduction In recent years, the scientific community has paid much attention to the synthesis and characterization of II–VI semiconductor materials at nanometer scale, due to their great potential to test fundamental concepts of quantum mechanics [1,2] and because of their key role in various applications in piezoelectric transducers, gas sensors, light-emitting devices, photonic crystals [3], nanoelectronics [4] photodetectors, photodiodes, optical waveguides, transparent conductive films, varistors, solar cells, transparent UV protection films, chemical sensors and biological (drug delivery, bio-imaging, etc.) systems [5–7]. ZnO nanocrystals are now being used as building blocks for bio/ nanohybrids in biological systems as biomarkers and resonant energy-transfer sensors. Construction of these bio/nanohybrids is a challenging, ongoing problem. An understanding of how these nanomaterials function in these biological systems is needed so that tailored nanooptics with these systems can be exploited. Surface effects radically influence the functionality of semiconductor nanocrystals. Passivation with ligands or high band gap semiconductor shells is necessary to reduce surface trap densities, enhance quantum yield and increase photostability. Because of its large band gap and large exciton binding energy [8], ZnO can also
Corresponding author. Tel./fax: +91 532 2460675.
E-mail address: [email protected] (P.K. Sharma). 0022-2313/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2009.01.004
be used for new applications in bio-imaging by making some surface modifications. ZnO has high surface-to-volume ratio at nanometer scale and hence surface defects play an important role in its properties. An essential surface modification needs to be done for each of the desired applications. It is found that ZnO quantum dots (QD) embedded in SiO2 matrix give emission peak in the visible range and is highly intense. Emission in visible wavelength range is preferable for bio-imaging. Besides all these the most important feature is the biocompatibility of ZnO@SiO2 for possible in vivo bio-imaging unlike the other quantum dots based on Cd or heavy metals which are used conventionally. Zn is one of the most important trace elements found in various parts of the body like the muscle, bone, skin and plasma of blood [9]. A few milligrams of Zn is required everyday by the body and a considerable amount of it is present in various parts of the body. It is a very important co-factor for several enzymes like carbonic anhydrase, alcohol dehydrogenase, carboxypeptidases A and B, etc. In the present paper, we are demonstrating a simple chemical technique to synthesize ZnO quantum dots and symmetrically dispersed ZnO quantum dots embedded in SiO2 matrix with excellent structural and optical properties. We particularly emphasized on luminescence studies of these symmetrically dispersed ZnO quantum dots embedded in SiO2 matrix as the same can be further exploited for nano-biotechnology as bio markers/bio-imaging, etc. and chemical sensors. Further, we have tried to explore the mechanism behind the formation of
ARTICLE IN PRESS 606
P.K. Sharma et al. / Journal of Luminescence 129 (2009) 605–610
symmetrically dispersed ZnO quantum dots embedded in SiO2 matrix.
2. Experimental details Zinc acetate dihydrate (99.2%) [Zn(CH3COO)2 2H2O], lithium hydroxide (LiOH), tetra-ethyl-ortho-silicate (TEOS), ethanol and sulphuric acid (H2SO4), procured from E. Merck Limited, Mumbai 400018, India, were used to synthesize ZnO and ZnO quantum dots embedded in SiO2 matrix. All chemicals were of AR grade and were directly used without special treatment. In a typical co-precipitation method, 1 M Zn(CH3COO)2 2H2O was prepared in 50 ml of ethanol using vigorous stirring at 50 1C for 90 min, followed by cooling at room temperature. LiOH (0.59 g) was dissolved in 50 ml of ethanol and kept in ultrasonic bath, then cooled at room temperature. This solution was slowly added to the Zn2+ solution with vigorous stirring at room temperature. Yellow green emission was observed from the transparent
solution under an UV-lamp (lex ¼ 352 nm). The unwanted CH3COO and Li+ ions were removed by washing the solution repeatedly by n-heptane (volume ratio 1:2). The obtained precipitate was dried at room temperature resulting in white powders of ZnO nanoparticles. Some amount of this white ZnO powder was redispersed in ethanol. Now, the SiO2 matrix was prepared by mixing vigorously for 2 h the 10 ml of tetra-ethyl-ortho-silicate in 10 ml ethanol and 5 ml H2SO4 at 30 1C. The resulting transparent solution and the ethanolic suspension of ZnO were mixed slowly with constant stirring at 30 1C. The Si:Zn solution ratio was kept 30:1. After 2 h of constant stirring at 30 1C, the solution turned into transparent light yellow gel. This transparent light yellow gel was further subjected for ultrasonication for another 2 h. The resulting gel-like mixture was kept in air for few hours to give thick gel which was further dried and annealed at 500 1C in air, giving the final product. The prepared ZnO and ZnO quantum dots embedded in SiO2 matrix were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), fourier transform infrared (FTIR) spectroscopy, UV–visible spectroscopy and photoluminescence (PL) spectroscopy. XRD was performed on Rigaku D/max-2200 PC diffractometer operated at 40 kV/20 mA, using CuKa1 radiation with wavelength 1.54 A˚ in wide angle region from 301 to 701. The size and morphology of the prepared nanophosphors were recorded by transmission electron microscope model Technai 30 G2 S-Twin electron microscope operated at 200 kV accelerating voltage. Fourier transform infrared spectroscopic studies were performed with the FTIR spectroscope model Nicolet 20 SXB. Absorption spectra were recorded on Perkin Elmer Lambda 35 UV–visible spectrophotometer using halogen and deuterium lamp as sources for visible and UV radiations, respectively. PL studies were performed for lex ¼ 320 nm on a Perkin Elmer LS 55 luminescence spectrophotometer using a Xenon discharge lamp, equivalent to 20 kW for 8 ms duration as the excitation source at room temperature.
3. Results and discussion
Fig. 1. X-ray diffraction spectra of ZnO and symmetrically dispersed ZnO quantum dots embedded in SiO2 matrix. For symmetrically dispersed ZnO quantum dots embedded in SiO2 matrix, high background scattering suggests presence of SiO2 matrix.
Fig. 1 shows the XRD pattern of the ZnO and symmetrically dispersed ZnO quantum dots embedded in SiO2 matrix synthesized by co-precipitation method. For both the samples, XRD Spectra showed broad peaks at the positions 31.631, 34.501, 36.251, 47.501, 56.601, 62.801 and 67.921, which were in excellent
Fig. 2. TEM image of (A) bare ZnO quantum dots with spherical particles and (B) symmetrically dispersed ZnO quantum dots embedded in SiO2 matrix.
ARTICLE IN PRESS P.K. Sharma et al. / Journal of Luminescence 129 (2009) 605–610
agreement with the JCPDS file for ZnO (JCPDS 36-1451, a ¼ b ¼ 3.249 A˚, c ¼ 5.206 A˚) and indexed as the hexagonal wurtzite structure of ZnO having space group P63mc. All available reflections of the present phases fitted with the Gaussian distribution. The broadening of XRD peaks (i.e. Scherrer’s broadening) gave clear indication of formation of nanosized ZnO. Particle size ‘d’ of ZnO and symmetrically dispersed ZnO quantum dots embedded in SiO2 matrix were estimated by Debye–Scherrer’s equation d¼
0:9l B cos y
where d is the particle size, l is the wavelength of radiation used, y is the Bragg angle and B is the full-width at half-maxima (FWHM) on 2y scale. For ZnO samples, average particle size using
Fig. 3. FTIR spectra of ZnO quantum dots and ZnO quantum dots embedded in SiO2 matrix.
607
Debye–Scherrer’s equation was of the order of 25 nm. Furthermore, it can be also seen that for symmetrically dispersed ZnO quantum dots embedded in SiO2 matrix, XRD peaks were not as sharp as in the case of ZnO sample, i.e. slight decrease in crystallinity was observed, which indicates smaller particle size. Through Scherrer’s equation average particle size was of the order of 20 nm for symmetrically dispersed ZnO quantum dots embedded in SiO2 matrix. The high background scattering in XRD spectra suggested the presence of SiO2 matrix. These results were well supported by the TEM, FTIR and photoluminescence studies. TEM images were recorded by dissolving the assynthesized powder sample in ethanol and then placing a drop of this dilute ethanolic solution on the surface of copper grid. The morphology of the products is shown in Fig. 2. Fig. 2(A) is TEM image of bare ZnO quantum dots, while Fig. 2(B) is TEM image representing morphology of ZnO quantum dots embedded in SiO2 matrix. Fig. 2(A) clearly shows spherical particles with little agglomeration having sizes 25 nm. But for the case of ZnO@SiO2 quantum dots (Fig. 2(B)), symmetrically dispersed spherical particles encapsulated in the matrix of SiO2 were observed. These encapsulated structures were very symmetrically oriented throughout the whole TEM grid. The optical properties of the prepared materials were examined by fourier transform infrared, optical absorption and photoluminescence spectroscopy. The FTIR spectra were recorded by making pressed pellet of the prepared samples in potassium bromide (KBr). The corresponding FTIR spectrum is shown in Fig. 3. Clear Zn–O–Zn stretching modes were observed at 620 and 816 cm1 for both the samples and were well supported by available literatures [10,11]. These stretching modes were indicative of successful synthesis of ZnO in both the cases as already confirmed by XRD. Due to adsorption of CO few peaks at 1406 and 1572 cm1 were observed, while stretching modes at 1470 and 3310 cm1 were due to –OH groups present at the surface of symmetrically dispersed ZnO quantum dots embedded in SiO2 matrix. The well-known stretching modes of SiO2 were observed at 816 and 1270 cm1, indicating formation of ZnO quantum dots inside the matrix of SiO2. The broadening of these modes can be explained as overlapping of SiO2 peaks with ZnO peaks. These FTIR spectra were consistent with those from ZnO-loaded silica
Fig. 4. Optical absorption spectra of ZnO and ZnO quantum dots embedded in SiO2 matrix.
ARTICLE IN PRESS 608
P.K. Sharma et al. / Journal of Luminescence 129 (2009) 605–610
molecular sieves reported by Kwon et al. [10] and Gu et al. [12]. Optical absorption spectra were recorded by dispersing the samples in double distilled water and using double distilled water as reference. Fig. 4 shows the optical absorption spectra. The excitonic absorption peaks of both these samples were blue shifted as compared to the bulk band gap of 3.32 eV. The band gap of ZnO sample was found typically around 365 nm (Eg3.4 eV), while for ZnO quantum dots embedded in SiO2 matrix, large blue shift was observed in the excitonic absorption peak. The band gap corresponding to ZnO quantum dots embedded in SiO2 matrix was approximately around 337 nm (Eg3.6 eV). Figs. 5(a) and (b) shows the prepared ZnO quantum dots embedded in SiO2 matrix under ordinary and UV (352 nm excitation) lamps, respectively. While in both figures, letters A, B and C correspond to solid samples, samples dissolved in ethanol and samples dissolved in water, respectively. Samples represented in Fig. 5(a) showed pure
white color, while the samples represented by Fig. 5(b) showed greenish-yellow luminescence. Fig. 5 illustrates the photoluminescence spectra of synthesized ZnO and ZnO quantum dots embedded in SiO2 matrix. The well-known green emission [13,14] from ZnO nanoparticles had been seen shifted from 560 to 548 nm, which resulted in a red shift from the normal ZnO emission at 520 nm. This broad peak in the visible region was associated to the structural defects like interstitials [15,16], oxygen vacancies [17–19] and surface traps, although there was a total absence/quenching of excitonic and defect-associated emission peak intensities. From Fig. 6, it was clear that ZnO quantum dots embedded in SiO2 matrix had an intense peak at 369 nm, which showed electron–hole recombination after relaxation and a very broad emission around 542 nm due to recombination via surface-localized states, owing to the silica matrix, which stops the electron–hole recombination on the ZnO and reduces
Fig. 5. Shows prepared ZnO quantum dots embedded in SiO2 matrix under (a) ordinary and (b) UV (365 nm excitation) lamps. The letters A, B and C correspond to solid samples, samples dissolved in ethanol and samples dissolved in water. Photographs were taken with a digital camera just after completion of synthesis.
Fig. 6. Photoluminescence spectra of ZnO and ZnO quantum dots embedded in SiO2 matrix.
ARTICLE IN PRESS P.K. Sharma et al. / Journal of Luminescence 129 (2009) 605–610
609
the non-radiative transition, causing red shift from the normal ZnO emission at 520 nm. The band-edge emission of ZnO quantum dots embedded in SiO2 matrix was highly intense and had a better time stability, while any appreciable band-edge emission from non-coated ZnO was not found. In the ZnO quantum dots embedded in SiO2 matrix, the hole was confined within the ZnO, while the electron was delocalized over the entire embedded matrix. Unlike in metallic samples there were no shift in absorption or luminescence peak position, but the intensity changes were prominent. Studies have shown that the luminescence efficiency of nanoparticles strongly depends on the nature of the surface, because of large surface-to-volume ratio in smaller particles. In case of ZnO nanoparticles, surface states such as dangling bonds are usually involved in non-radiative processes, while O2 ions provide a critical pathway for the visible emission band [13,20–22]. In our case, the SiO2 matrix reduced the density of surface dangling bonds and O2 ions via Zn–O–Si. So the probability of non-radiative and visible emission further reduced, in contrast, the probability of UV emission will be increased. This was the main cause behind the enhanced UV emission in photoluminescence spectra. This could be further understood as the net charge of the Zn atom in the Zn–O–Si bond was positive. Positive centre in ZnO produced an attractive defect potential creating a donor state by attracting conduction band levels into the band gap [23]. Due to the weak attractive potential, the energy level of the donor state created by Zn–O–Si shifts to 3.4 eV (369 nm) and act as a luminescence centre for the 369 nm band; i.e., carriers excited in ZnO and SiO2 may be trapped at the interface states and consequently recombine to emit a band at 369 nm. In the ZnO@SiO2 matrix, some of the carriers excited in the SiO2 matrix may tunnel to the ZnO–SiO2 interface or to ZnO nanoparticles and recombine, enhancing the UV band. On the other hand, SiO2 provides a good coverage of the ZnO surface and acts as an energetic barrier preventing the escape of photo-generated carriers to outside the confined ZnO nanoparticles.
4. Formation mechanism of ZnO quantum dots embedded in SiO2 matrix We used tetra-ethyl-ortho-silicate for the formation of SiO2 matrix. The chemical structure of TEOS can be represented as in Fig. 7(a). TEOS is liquid at room temperature which slowly hydrolyzes into silicon dioxide and ethanol when in contact with ambient moisture. The key behind the understanding is to note that, in TEOS the silicon atom is already oxidized; the conversion of TEOS to SiO2 is essentially a rearrangement rather than an oxidation reaction, making the reaction mechanism simple. The overall reaction for the SiO2 matrix requires the removal of two oxygen atoms from TEOS as shown in Fig. 7(b). In the present case, formation of SiO2 matrix was probably the result of TEOS surface reaction. TEOS chemisorbs onto silanol groups (Si-OH) at the surface, as well as strained surface bonds. Mechanism is depicted in Fig. 7(c). TEOS will not adsorb onto the resulting alkyl-covered surface during intermediate reaction steps. So SiO2 matrix formation was probably limited by removal of these intermediate surface alkyl groups. As shown in Fig. 7(d), these groups can undergo elimination reactions with neighboring molecules to form Si–O–Si bridges. This process quickly occurs in the ambient reaction conditions: TEOS can be its own oxygen source, and SiO2 can be deposited in the form of a matrix from TEOS. However, additional oxygen atoms provided by ZnO increases the matrix formation rate, presumably this is the cause behind the formation of symmetrically dispersed ZnO quantum dots embedded in SiO2 matrix.
Fig. 7. (a) Chemical structure of tetra-ethyl-ortho-silicate (TEOS). (b) Removal of two oxygen atoms from TEOS. (c) Mechanism depicting TEOS chemisorbs onto silanol groups (Si-OH) surface. (d) Formation of Si–O–Si bridges by elimination reactions with neighboring molecules.
5. Conclusions In summary, we have successfully synthesized quantum dots of ZnO and symmetrically dispersed ZnO quantum dots embedded in SiO2 matrix at 373 K and studied their optical properties to understand the role of the surface effects. This embedded quantum dot exhibits enhanced photoluminescence. This can be
ARTICLE IN PRESS 610
P.K. Sharma et al. / Journal of Luminescence 129 (2009) 605–610
seen as one of the necessary and condemnatory constituent for bio-imaging, bio-sensors, etc. after the conjugation of the nanocrystals to biological entities, underlies the importance of the current work.
Acknowledgement Authors are thankful to DST and CSIR, India, for the financial assistance and to Dr. R.N. Bhargava, Nanocrystal Technology, USA, for his valuable discussions and support during the current work. References [1] [2] [3] [4] [5]
S.M. Prokes, K.L. Wang, Mater. Res. Sci. Bull. 24 (1999) 13. J. Hu, T.W. Odom, C.M. Lieber, Acc. Chem. Res. 32 (1999) 435. S. Nakamura, Science 281 (1998) 956. C.A. Mirkin, Science 286 (1999) 2095. A. Mendoza-Galvain, C. Trejo-Cruz, J. Lee, D. Bhattacharya, J. Metson, P.J. Evans, U. Pala, J. Appl. Phys. 99 (2006) 14306. [6] Honma, S. Hirakowa, K. Yamada, J.M. Bae, Solid State Ionics 118 (1999) 29. [7] T.S. Phely-Bobin, R.J. Muisener, J.T. Koberstein, F. Papadinmitrakopoulos, Synth. Met. 116 (2001) 439.
[8] V.A. Karpina, V.I. lazorenko, C.V. Lashkarev, V.D. Dobrowolski, L.I. Kopylova, V.A. Baturin, S.A. Lytuyn, V.P. Ovsyannikov, E.A. Mauvenko, Cryst. Res. Technol. 39 (2004) 980–992. [9] G. Thomas, Chemistry for Pharmacy and the Life Sciences: Including Pharmacology and Biomedical Science, Prentice-Hall, Padstow TJ Press, Englewoods Cliffs, NJ, 1996. [10] Y.J. Kwon, K.H. Kim, C.S. Lim, K.B. Shim, J. Ceram. Proc. Res. 3 (2002) 146. [11] Y. Xiong, L.Z. Zhang, G. Tang, G. Zhang, W.J. Chen, Luminescence 110 (2004) 17. [12] G. Gu, P.P. Ong, C. Chu, J. Phys. Chem. Solids 60 (1999) 943. [13] A. Van Dijken, E.A. Meulenkamp, D. Vanmaekelbergh, A. Meijerink, J. Lumin. 90 (2000) 123. [14] Z. Li, Y. Xiong, Y. Xie, J. Inorg. Chem. 42 (2003) 8105. [15] J. Zhang, L. Sun, J. Yin, H. Su, C. liao, C. Yon, Chem. Mater. 14 (2002) 4172. [16] M.H. Huang, Y.Y. Wu, H.N. Feich, N. Tran, E. Weber, P.D. Yang, Adv. Mater. 13 (2001) 113. [17] K. Vanheusden, W.L. Warren, C.H. Seager, D.R. Tallanl, J.A. Voigt, B.E. Gandu, J. Appl. Phys. 79 (1996) 7983. [18] M. Liu, A.H. Kitai, P. Mascher, J. Lumin. 54 (1992) 35. [19] X.L. Wu, G.G. Siu, C.L. Fu, H.C. Ong, Appl. Phys. Lett. 78 (2001) 2285. [20] J.F. Li, L.Z. Yao, C.H. Ye, C.M. Mo, W.L. Cai, Y. Zhang, L.D. Zhang, J. Cryst. Growth 223 (2001) 535. [21] A. Van Dijken, E.A. Meulenkamp, D. Vanmaekelbergh, A. Meijerink, J. Lumin. 87–89 (2000) 454. [22] A. Van Dijken, J. Makkinje, A. Meijerink, J. Lumin. 92 (2001) 323. [23] Y.M. Sun, P.S. Xu, C.S. Shi, F.Q. Xu, H.B. Pan, E.D. Lu, J. Electron. Spectrosc. 114–116 (2001) 1123.