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Effects of multiple organic ligands on size uniformity and optical properties of ZnSe quantum dots
Materials Research Bulletin 47 (2012) 1892–1897
Contents lists available at SciVerse ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Effects of multiple organic ligands on size uniformity and optical properties of ZnSe quantum dots J. Archana a,*, M. Navaneethan a, Y. Hayakawa a, S. Ponnusamy b, C. Muthamizhchelvan b a b
Research Institute of Electronics, Shizuoka University, 3-5-1 Johoku, Naka-ku, Hamamatsu, Shizuoka 432-8011, Japan Department of Physics, SRM University, Kattankulathur 603203, Tamil Nadu, India
A R T I C L E I N F O
A B S T R A C T
Article history: Received 5 October 2011 Received in revised form 21 February 2012 Accepted 17 April 2012 Available online 24 April 2012
The effects of multi-ligands on the formation and optical transitions of ZnSe quantum dots have been investigated. The dots are synthesized using 3-mercapto-1,2-propanediol and polyvinylpyrrolidone ligands, and have been characterized by X-ray diffraction, transmission electron microscopy (TEM), UV– visible absorption spectroscopy, photoluminescence spectroscopy, and Fourier transform infrared spectroscopy. TEM reveals high monodispersion with an average size of 4 nm. Polymer-stabilized, organic ligand-passivated ZnSe quantum dots exhibit strong UV emission at 326 nm and strong quantum confinement in the UV–visible absorption spectrum. Uniform size and suppressed surface trap emission are observed when the polymer ligand is used. The possible growth mechanism is discussed. ß 2012 Elsevier Ltd. All rights reserved.
Keywords: A. Nanostructures A. Semiconductors B. Chemical synthesis D. Luminescence D. Optical properties
1. Introduction The controllable synthesis of semiconductor nanocrystals and nanoparticles is very important for the development of optoelectronic devices [1–3]. They have been used for the fabrication of photovoltaic (e.g., solar) cells, lasers, and for the detection of biomolecules [6–11]. Semiconductor quantum dots are spherical nanoparticles that have diameters less than the bulk excitonic Bohr radius. Their energy bands have discrete levels and the electrons and holes are independently confined. Therefore, the size of a quantum dot plays a significant role in the optical properties [4]. For example, lead chalcogenides bulk crystals exhibit infrared emission, whereas 4–10 nm quantum dots exhibit strong optical transitions in the UV and visible regions [5]. Various methods have been developed to control the size and morphology of quantum dots, such as the electrochemical method [12], the microwave-assisted method [13], the wet chemical method [14,15], the sol–gel method [16], chemical deposition [17,18], and the hydrothermal method [19]. The wet chemical method involves easy processing, is inexpensive, and can be extended for large-scale synthesis. Moreover, the chemical kinetics of the reaction(s) can be controlled by organic additives, reaction
* Corresponding author. Tel.: +81 53 478 1338; fax: +81 53 478 1338. E-mail addresses: [email protected], [email protected] (J. Archana). 0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2012.04.027
temperature, and growth time. The wet chemical method has several drawbacks, however, such as particle aggregation due to Ostwald ripening and irregular shape formation. Ostwald ripening and irregular shape formation can be avoided by introducing organic ligands such as amines, thiols, and polymers [20–25] that have lone-pair electrons in valence orbitals. These organic ligands also induce high photoluminescence intensity from quantum dots. The influence of organic ligands in determining dot shapes and sizes (and hence their optical properties) has been explored by several groups. Nose et al. [26] have investigated the shift in the photoluminescence emission peak of CdSe dots having different sizes due to the presence of amine capping molecules with various chain lengths. Liu et al. [27] have demonstrated that the carrier mobility of PbSe nanocrystals can be tuned by thiol ligands. ZnSe is widely used in light-emitting diodes, photodetectors, color displays, etc. [28,29], because it has a direct band gap of 2.7 eV and an excitonic Bohr radius of 8.6 nm [30]. Zhijum et al. fabricated ZnSe quantum dot solar cells by a wet chemical method with an efficiency of 0.27%, and demonstrated the potential application for photovoltaic cells [31]. However, the control of particle size and uniformity is still challenging. In the present research, 3-mercapto-1,2-propanediol and polyvinylpyrrolidone have been used as capping ligands and stabilizing agents in the synthesis of ZnSe quantum dots by the chemical method. The effect of these multi-ligands on the morphology and optical properties of ZnSe quantum dots will be discussed below.
J. Archana et al. / Materials Research Bulletin 47 (2012) 1892–1897
2. Experimental details Chemical reagents were used without further purification. For the syntheses of 3-mercapto-1,2-propanediol capped ZnSe, 0.2 M of zinc acetate and 0.2 M of sodium selenite were dissolved in 50 ml of de-ionized water at 40 8C with vigorous stirring, followed by the addition of 0.5 ml of 3-mercapto-1,2-propanediol. The possible reaction is given as:
2ZnðCH3 COOÞ2 þ 2Na2 SeO3 ! 2ZnSe þ 4CH3 COONa þ 3O2 The reaction was allowed to proceed for 5 h and was then terminated by centrifugation in ethanol to separate unreacted residues from the product. The colloidal 3-mercapto-1,2-propanediol capped ZnSe quantum dots colloids were dried at 110 8C for 6 h. The same procedure was followed for the preparation of 3mercapto-1,2-propanediol capped ZnSe in polyvinylpyrrolidone, with the addition of 0.002 M of polyvinylpyrrolidone (approximate molecular weight of 40,000 g/M) to the reaction solution while stirring. The resultant 3-mercapto-1,2-propanediol capped ZnSe in polyvinylpyrrolidone product was centrifuged in ethanol and dried at 110 8C for 6 h. Structures were characterized by X-ray diffraction (XRD), using a X’per PRO (PANalytical) advanced X-ray diffractometer with Cu Ka radiation (l = 1.5406 A˚). Diffraction data in the range 20–808 were recorded. TEM images were acquired with a JEOL 2010 electron microscope operated at 200 kV. TEM samples were prepared by adding a droplet of ZnSe quantum dots (ultrasonically dispersed in ethanol) to a 150 mm carbon-coated copper mesh, followed by air drying. UV–visible absorption and emission spectra were measured with a Perkin Elmer Lambda 5 spectrophotometer and a Flurolog-3 spectrophotometer (Jobin Yvon) equipped with a Xenon lamp for 200–800 nm excitation. The photoluminescence (PL) spectra were obtained following 255 nm excitation from the Xenon lamp. The quality and composition of the ZnSe nanostructures were characterized by infrared (IR) spectroscopy (Perkin Elmer Spectrophotometer) in the range of 400–4000 cm1. Ethanol was used as solvent for the sample preparation. 3. Results and discussion 3.1. Structural studies The XRD patterns for (a) 3-mercapto-1,2-propanediol capped ZnSe dots and (b) 3-mercapto-1,2-propanediol capped ZnSe dots in polyvinylpyrrolidone are shown in Fig. 1. The comparative studies
Fig. 1. XRD patterns of (a) 3-mercapto-1,2-propanediol capped ZnSe quantum dots, (b) 3-mercapto-1,2-propanediol capped ZnSe quantum dots in polyvinylpyrrolidone.
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were done to monitor the effects of 3-mercapto-1,2-propanediol and polyvinylpyrrolidone on the morphology of the nanoparticles. All peaks in the diffraction patterns are indexed to the cubic structure of ZnSe where the lattice parameter (a = 5.67 A˚) is in good agreement with the standard JCPDS card no: 80-0021. The full-widths at half-maxima of the diffraction peaks of 3-mercapto1,2-propanediol capped ZnSe quantum dots in polyvinylpyrrolidone were broadened relative to those of 3-mercapto-1,2propanediol capped dots. Further XRD analysis revealed no additional products in the samples. Both samples exhibit the cubic phase with similar 2u values; thus the long chain polymer ligand polyvinylpyrrolidone does not influence the crystal structure of ZnSe quantum dots. Crystallite sizes of both 3-mercapto-1,2-propanediol capped ZnSe and 3-mercapto-1,2-propanediol capped ZnSe in polyvinylpyrrolidone have been calculated for the strong and prominent XRD plane (1 1 1) using the well-known Scherrer’s equation (1). t¼
kl ; b cos u
(1)
where t is the average crystallite size, k is the geometric factor (=0.90), l is the X-ray wavelength (1.5406 A˚), b is the half-width of the diffraction peak, and u is the angle of the diffraction peak. The calculated crystallite size for 3-mercapto-1,2-propanediol capped ZnSe quantum dots is 3.2 nm and that for 3-mercapto-1,2propanediol capped ZnSe quantum dots in polyvinylpyrrolidone is 2.7 nm. These rough estimates of particle sizes imply a possible effect of polymer stabilizer on quantum confinement. 3.2. Morphological studies More conclusive evidence of sizes, shapes and structures of the nanoparticles were documented with TEM images. Fig. 2(a) shows the TEM image of 3-mercapto-1,2-propanediol capped ZnSe spherical particles without aggregation over the area of 200 nm 200 nm. Particle sizes in the range 3–6 nm can be seen in the high resolution (HRTEM) images in Fig. 2(b) and (c), and are consistent with the size estimated from the XRD data and Eq. (1). The columns of the atoms were periodically arranged. Lattice fringes had the characteristic cubic pattern of the h1 1 1i projection of the zinc blende crystal structure and most of the quantum dots exhibited no defects such as stacking faults or coalescence. The lattice spacing of the 3-mercapto-1,2-propanediol capped ZnSe quantum dots was found to be 0.33 nm, consistent with the calculated lattice spacing from the XRD pattern (JCPDS no: 800021) [32]. A selected area electron diffraction (SAED) pattern from a 3mercapto-1,2-propanediol capped ZnSe quantum dots is shown in Fig. 2(d). Ring patterns indicate the cubic phase of ZnSe and are indexed to the (1 1 1), (2 2 0) and (3 1 1) planes. No patterns for defects or other lattices were observed. A histogram of particle size distribution is presented in the inset of Fig. 2(c), where it is evident that the sizes of the quantum dots were dispersed from 3.6 to 6.0 nm, with most near 5 nm. As discussed below, this variation in quantum dot size strongly affected the optical properties. Fig. 3(a)–(c) shows TEM and HRTEM images for 3-mercapto1,2-propanediol capped ZnSe in polyvinylpyrrolidone, respectively. These images clearly depict the formation of monodispersed ZnSe quantum dots. The size of the quantum dots is in the range 3.5–4.5 nm, which is less than that of bulk excitonic Bohr radius of 8.6 nm. Most of the quantum dots were 4 nm as shown in the histogram. The small deviation (approximately 1 nm) between the particle size observed by HRTEM analysis and that calculated from the XRD pattern and Eq. (1) is likely due to the strain-induced broadening effect of nanocrystals. The array of lattice fringes in the HRTEM images confirms the crystalline nature of the quantum
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Fig. 2. (a) TEM image, (b, c) HRTEM images and (d) SAED pattern of 3-mercapto-1,2-propanediol capped ZnSe quantum dots (inset of c is histogram of particle size distribution).
dots. In Fig. 3(a), the 3-mercapto-1,2-propanediol capped ZnSe nanocrystals are homogenously distributed on the chains of a network that was formed by the polymerization of polyvinylpyrrolidone. These particles were uniform and no aggregation was observed when polyvinylpyrrolidone was used to stabilize ZnSe nanocrystals. Since the long chain polarized molecules were coated on the surface of the nanocrystals, dispersive forces stabilized the nanocrystals in the colloid solution. This kind of polymer network was extended throughout the dispersion, thus the ZnSe nanocrystals exhibited excellent stability. Fig. 3(d) shows the SAED pattern of the 3-mercapto-1,2-propanediol capped ZnSe quantum dots in polyvinylpyrrolidone. It clearly depicts the signature of zinc blende with the (1 1 1), (2 2 0), (3 1 1) planes and no rings. Thus the polymer ligands determined size confinement without any change in crystalline phase. 3.3. Optical properties The UV–visible absorption spectrum of 3-mercapto-1,2-propanediol capped ZnSe quantum dots is shown in Fig. 4(a). It exhibits a UV absorption between 260 and 310 nm. Two excitonic absorption peaks were observed at 264 and 301 nm as shown in the inset of Fig. 4(a); they signify a shift toward higher energy relative to that of bulk ZnSe (460 nm). A possible reason for the two peaks is the existence of different sizes of quantum dots as seen in
the TEM and HRTEM images (Fig. 2). In principle, the optical absorption spectrum is a collection of cumulative transitions over a wide range of spectral responses. However, the histogram of the particle size distribution distinctly reveals the existence of two different sizes (3 and 6 nm). Multi-exciton generation has been extensively investigated in lead and cadmium chalcogenides quantum dots from the near infrared to the visible regions. Very recently, Zhang et al. [33] observed multi-exciton generation in ZnSe quantum dots with optical transitions at 269 nm and 307 nm. In the present work, the existence of multiple optical transitions may be due to the larger size distribution of ZnSe quantum dots. The emission spectrum for 3-mercapto-1,2-propanediol capped ZnSe quantum dots consists of multiple emission peaks in the near band-edge region as shown in Fig. 5(a). A broad emission peak is centered at 378 nm with two satellite peaks of less intensity. The existence of multiple emission peaks within the bandgap of 0.3 eV confirms the quantum size effect of the two differently sized quantum dots. The Stokes shift in the absorption edge and luminescence peak center of the 3-mercapto-1,2-propanediol capped ZnSe quantum dots was 0.49 eV. The deep level emission of 503 nm may be related to defects or dangling bonds on surface of the quantum dots. The optical absorbance spectrum of 3-mercapto-1,2-propanediol capped ZnSe quantum dots in polyvinylpyrrolidone is shown in Fig. 4(b). The excitonic absorption peak was observed at 288 nm and no other optical transitions or excitonic peaks were observed,
J. Archana et al. / Materials Research Bulletin 47 (2012) 1892–1897
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Fig. 3. (a) TEM image, (b, c) HRTEM images and (d) SAED pattern of 3-mercapto-1,2-propanediol capped ZnSe quantum dots in polyvinylpyrrolidone (inset of c is histogram of particle size distribution).
unlike the 3-mercapto-1,2-propanediol capped ZnSe quantum dots. The spectrum thus indicates a highly monodispersed distribution of quantum dots. The photoluminescence spectrum of 3-mercapto-1,2-propanediol capped ZnSe quantum dots in polyvinylpyrrolidone is
shown in Fig. 5(b). The single 326 nm emission peak is observed at near band edge luminescence in the deep ultra violet region. The Stokes shift of the optical transition is about 0.03 eV. No other deep-level emission has been observed. The photoluminescence intensity of the 3-mercapto-1,2-propanediol capped ZnSe quantum dots in polyvinylpyrrolidone increased twofold relative to
Fig. 4. UV–visible absorption spectra of (a) 3-mercapto-1,2-propanediol capped ZnSe quantum dots and (b) 3-mercapto-1,2-propanediol capped ZnSe quantum dots in polyvinylpyrrolidone.
Fig. 5. Photoluminescence spectra of (a) 3-mercapto-1,2-propanediol capped ZnSe quantum dots and (b) 3-mercapto-1,2-propanediol capped ZnSe quantum dots in polyvinylpyrrolidone.
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Fig. 6. Formation mechanism of organic ligands capped ZnSe quantum dots.
that of 3-mercapto-1,2-propanediol capped ZnSe quantum dots. This provided further evidence of stabilization by polymer ligands and the passivation of dangling bonds on the surface of the quantum dots. 3.4. Growth mechanism and functional group analysis Fig. 6 shows the schematic representation of the growth mechanism for the ZnSe quantum dots. Organic ligands and stabilizers were used to control the reaction kinetics and to prevent particle aggregation and Ostwald ripening. The ligands passivate the surface, stabilize the solution, and create a large potential barrier between neighboring dots that confines carriers to individual dots. Thus ligands play a pivotal role in the growth and nucleation of quantum dots. Moreover, the sulfhydryl group of the 3-mercapto-1,2-propanediol ligands has a strong affinity for
the Zn2+ ion in the ZnSe quantum dots. Therefore, the coordination between S and Zn dominates and the thiol group becomes more stable. By increasing the length of the polymer backbone and the density of hydrophobic side chains, a dramatic ‘‘steric hindrance’’ effect occurs and results in the homogenous formation of ZnSe quantum dots. The grafted polyvinylpyrrolidone molecular chains will thus stabilize the particles by steric repulsion and prevent particle aggregation. Moreover, on the particle surface dangling bonds and traps will gradually decrease during the reaction time, allowing the formation of stable surface states. The ZnSe quantum dots thus will not grow larger due to the important role of the polymer network chain length in controlling the critical radius. Functional groups in the ZnSe quantum dots were confirmed by FTIR spectra as displayed in Fig. 7(a) and (b). Characteristic vibrations of 3-mercapto-1,2-propanediol and polyvinylpyrrolidone (PVP) are observed in the spectra. Vibration peaks at 886 cm1 and 2600 cm1 correspond to the thiol S–H stretch. Peaks at 2886 cm1 and 3354 cm1 correspond to the CH2–S and O–H thiol stretches, respectively. These vibrations confirm the surface passivation of 3-mercapto-1,2-propanediol on the ZnSe nanoparticles. Whereas, the spectrum of thiol-capped ZnSe in PVP displays the weakened vibrations regions of 2300 and 1600 cm1 as shown in the inset. The data indicate the presence of PVP on the quantum dots, with PVP characteristic vibrations such as C5 5O and C–N at 1656 cm1 and 1333 cm1, respectively. Both thiol and PVP molecules effectively passivate the surface of ZnSe quantum dots. 4. Conclusions
Fig. 7. FTIR spectra of (a) 3-mercapto-1,2-propanediol capped ZnSe quantum dots and (b) 3-mercapto-1,2-propanediol capped ZnSe quantum dots in polyvinylpyrrolidone.
We have synthesized ultra-fine monodispersed ZnSe quantum dots with an average size of 4 nm by an inexpensive, lowtemperature chemical route involving multiple ligands. Enhanced
J. Archana et al. / Materials Research Bulletin 47 (2012) 1892–1897
UV emission was observed in 3-mercapto-1,2-propanediol capped ZnSe quantum dots in polyvinylpyrrolidone. A size-dependent optical bandgap was observed and the Stokes shift was reduced by the steric hindrance effect of the polymer network. Possible reaction and growth mechanisms have been presented. The synthesized ZnSe quantum dots can be used in bio-labeling and quantum dot solar cells applications. The present approach is highly suitable for multi-gram synthesis of monodispersed semiconductor quantum dots. References [1] V.L. Colvin, M.C. Schlamp, A.P. Alivisatos, P.L. McEuen, Nature 370 (1994) 354–357. [2] W.C.W. Chan, S. Nie, Science 281 (1998) 2016–2018. [3] J. Weng, J. Ren, Curr. Med. Chem. 13 (2006) 897–909. [4] A. Mostafa, E. Sayed, Acc. Chem. Res. 37 (2004) 326–333. [5] Y. Wang, A. Suna, W. Mahler, R. Kasowski, J. Chem. Phys. 87 (1987) 7315–7322. [6] S. Coe, W.K. Woo, M.G. Bawendi, V. Bulvoic, Nature 420 (2002) 800–803. [7] J.M. Caruge, J.E. Halpert, V. Wood, V. Bulvoic, M.G. Bawendi, Nat. Photonics 2 (2008) 247–250. [8] B. Sun, E. Marx, N.C. Greenham, Nano Lett. 3 (7) (2003) 961–963. [9] W.U. Huynh, J.J. Dittmer, A.P. Alivisatos, Science 295 (2002) 2425–2427. [10] R.A. Sperling, P. Riveragil, F. Zhang, M. Zanella, W.J. Parak, Chem. Soc. Rev. 37 (2008) 1896–1908. [11] P. Alivisatos, Nat. Biotechnol. 22 (2004) 47–52. [12] D.S. Xu, X.S. Shi, L.G. Guo, L. Gui, Y.Q. Tang, J. Phys. Chem. B 104 (2000) 5061–5063. [13] J. Zhu, O. Palchik, S. Chen, A. Gedanken, J. Phys. Chem. B 104 (31) (2000) 7344–7347.
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[14] B.O. Dabbousi, M.G. Bawendi, O. Onituska, M.F. Rubner, Appl. Phys. Lett. 66 (1995) 1316–1318. [15] D.L. Klein, R. Roth, A.K.L. Lim, A.P. Alivisatos, P.L. McEuen, Nature 389 (1997) 699–701. [16] G. Li, M. Nogami, J. Appl. Phys. 75 (1994) 4276–4278. [17] C.D. Lokhande, P.S. Patil, H. Tributsch, A. Ennaoui, Sol. Energy Mater. Sol. Cell 55 (1998) 379–393. [18] S. Gorer, A. Albu-yaron, G. Hodes, Chem. Mater. 7 (6) (1995) 1243–1256. [19] X. Liu, J. Ma, P. Peng, W. Zheng, Langmuir 26 (12) (2010) 9968–9973. [20] J. Archana, M. Navaneethan, S. Ponnusamy, C. Muthamizhchelvan, Y. Hayakawa, Mater. Lett. 63 (2009) 1931–1934. [21] J. Archana, M. Navaneethan, S. Ponnusamy, C. Muthamizhchelvan, Y. Hayakawa, Mater. Lett. 64 (2010) 2094–2097. [22] C. Unni, D. Philip, K.G. Gopchandran, Opt. Mater. 32 (2009) 169–175. [23] A.L. Rogach, A. Kornowski, M. Gao, A. Eychmuller, H. Weller, J. Phys. Chem. B 103 (1999) 3065–3069. [24] L. Qi, H. Colfen, M. Antonietti, Nano Lett. 1 (2) (2000) 61–65. [25] I. Sondi, O. Siiman, S. Koster, E. Matijevic, Langmuir 16 (7) (2000) 3107–3118. [26] K. Nose, H. Fujita, T. Omata, S.O. Yao-Matsuo, H. Nakamura, H. Maeda, J. Lumin. 126 (2007) 21–26. [27] Y. Liu, M. Gibbs, J. Puthussery, S. Gaik, R. Ihly, H.W. Hillhouse, M. Law, Nano Lett. 10 (2010) 1960–1969. [28] M.B. Saab, E. Estephan, T. Coitre, C. Larroque, C. Gergely, J. Phys. Chem. C 114 (2010) 18509–18515. [29] D.J. Kim, K.K. Koo, Cryst. Growth Des. 9 (2009) 1153–1157. [30] H.P. Wanger, W. Langbein, J.M. Hvam, G. Bacher, T. Kummell, A. Forchel, Phys. Rev. B 57 (3) (1998) 1797–1800. [31] Z. Ning, H. Tian, C. Yuan, Y. Fu, H. Qin, L. Sun, H. Agren, Chem. Commun. 47 (2011) 1536–1538. [32] D. Zhu, X. Jiang, C. Zhao, X. Sun, J. Zhang, J.J. Zhu, Chem. Commun. 46 (2010) 5226– 5228. [33] L.J. Zhang, X.C. Shen, H. Liang, J.T. Yao, J. Phys. Chem. C 114 (50) (2010) 21921– 21927.
Contents lists available at SciVerse ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Effects of multiple organic ligands on size uniformity and optical properties of ZnSe quantum dots J. Archana a,*, M. Navaneethan a, Y. Hayakawa a, S. Ponnusamy b, C. Muthamizhchelvan b a b
Research Institute of Electronics, Shizuoka University, 3-5-1 Johoku, Naka-ku, Hamamatsu, Shizuoka 432-8011, Japan Department of Physics, SRM University, Kattankulathur 603203, Tamil Nadu, India
A R T I C L E I N F O
A B S T R A C T
Article history: Received 5 October 2011 Received in revised form 21 February 2012 Accepted 17 April 2012 Available online 24 April 2012
The effects of multi-ligands on the formation and optical transitions of ZnSe quantum dots have been investigated. The dots are synthesized using 3-mercapto-1,2-propanediol and polyvinylpyrrolidone ligands, and have been characterized by X-ray diffraction, transmission electron microscopy (TEM), UV– visible absorption spectroscopy, photoluminescence spectroscopy, and Fourier transform infrared spectroscopy. TEM reveals high monodispersion with an average size of 4 nm. Polymer-stabilized, organic ligand-passivated ZnSe quantum dots exhibit strong UV emission at 326 nm and strong quantum confinement in the UV–visible absorption spectrum. Uniform size and suppressed surface trap emission are observed when the polymer ligand is used. The possible growth mechanism is discussed. ß 2012 Elsevier Ltd. All rights reserved.
Keywords: A. Nanostructures A. Semiconductors B. Chemical synthesis D. Luminescence D. Optical properties
1. Introduction The controllable synthesis of semiconductor nanocrystals and nanoparticles is very important for the development of optoelectronic devices [1–3]. They have been used for the fabrication of photovoltaic (e.g., solar) cells, lasers, and for the detection of biomolecules [6–11]. Semiconductor quantum dots are spherical nanoparticles that have diameters less than the bulk excitonic Bohr radius. Their energy bands have discrete levels and the electrons and holes are independently confined. Therefore, the size of a quantum dot plays a significant role in the optical properties [4]. For example, lead chalcogenides bulk crystals exhibit infrared emission, whereas 4–10 nm quantum dots exhibit strong optical transitions in the UV and visible regions [5]. Various methods have been developed to control the size and morphology of quantum dots, such as the electrochemical method [12], the microwave-assisted method [13], the wet chemical method [14,15], the sol–gel method [16], chemical deposition [17,18], and the hydrothermal method [19]. The wet chemical method involves easy processing, is inexpensive, and can be extended for large-scale synthesis. Moreover, the chemical kinetics of the reaction(s) can be controlled by organic additives, reaction
* Corresponding author. Tel.: +81 53 478 1338; fax: +81 53 478 1338. E-mail addresses: [email protected], [email protected] (J. Archana). 0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2012.04.027
temperature, and growth time. The wet chemical method has several drawbacks, however, such as particle aggregation due to Ostwald ripening and irregular shape formation. Ostwald ripening and irregular shape formation can be avoided by introducing organic ligands such as amines, thiols, and polymers [20–25] that have lone-pair electrons in valence orbitals. These organic ligands also induce high photoluminescence intensity from quantum dots. The influence of organic ligands in determining dot shapes and sizes (and hence their optical properties) has been explored by several groups. Nose et al. [26] have investigated the shift in the photoluminescence emission peak of CdSe dots having different sizes due to the presence of amine capping molecules with various chain lengths. Liu et al. [27] have demonstrated that the carrier mobility of PbSe nanocrystals can be tuned by thiol ligands. ZnSe is widely used in light-emitting diodes, photodetectors, color displays, etc. [28,29], because it has a direct band gap of 2.7 eV and an excitonic Bohr radius of 8.6 nm [30]. Zhijum et al. fabricated ZnSe quantum dot solar cells by a wet chemical method with an efficiency of 0.27%, and demonstrated the potential application for photovoltaic cells [31]. However, the control of particle size and uniformity is still challenging. In the present research, 3-mercapto-1,2-propanediol and polyvinylpyrrolidone have been used as capping ligands and stabilizing agents in the synthesis of ZnSe quantum dots by the chemical method. The effect of these multi-ligands on the morphology and optical properties of ZnSe quantum dots will be discussed below.
J. Archana et al. / Materials Research Bulletin 47 (2012) 1892–1897
2. Experimental details Chemical reagents were used without further purification. For the syntheses of 3-mercapto-1,2-propanediol capped ZnSe, 0.2 M of zinc acetate and 0.2 M of sodium selenite were dissolved in 50 ml of de-ionized water at 40 8C with vigorous stirring, followed by the addition of 0.5 ml of 3-mercapto-1,2-propanediol. The possible reaction is given as:
2ZnðCH3 COOÞ2 þ 2Na2 SeO3 ! 2ZnSe þ 4CH3 COONa þ 3O2 The reaction was allowed to proceed for 5 h and was then terminated by centrifugation in ethanol to separate unreacted residues from the product. The colloidal 3-mercapto-1,2-propanediol capped ZnSe quantum dots colloids were dried at 110 8C for 6 h. The same procedure was followed for the preparation of 3mercapto-1,2-propanediol capped ZnSe in polyvinylpyrrolidone, with the addition of 0.002 M of polyvinylpyrrolidone (approximate molecular weight of 40,000 g/M) to the reaction solution while stirring. The resultant 3-mercapto-1,2-propanediol capped ZnSe in polyvinylpyrrolidone product was centrifuged in ethanol and dried at 110 8C for 6 h. Structures were characterized by X-ray diffraction (XRD), using a X’per PRO (PANalytical) advanced X-ray diffractometer with Cu Ka radiation (l = 1.5406 A˚). Diffraction data in the range 20–808 were recorded. TEM images were acquired with a JEOL 2010 electron microscope operated at 200 kV. TEM samples were prepared by adding a droplet of ZnSe quantum dots (ultrasonically dispersed in ethanol) to a 150 mm carbon-coated copper mesh, followed by air drying. UV–visible absorption and emission spectra were measured with a Perkin Elmer Lambda 5 spectrophotometer and a Flurolog-3 spectrophotometer (Jobin Yvon) equipped with a Xenon lamp for 200–800 nm excitation. The photoluminescence (PL) spectra were obtained following 255 nm excitation from the Xenon lamp. The quality and composition of the ZnSe nanostructures were characterized by infrared (IR) spectroscopy (Perkin Elmer Spectrophotometer) in the range of 400–4000 cm1. Ethanol was used as solvent for the sample preparation. 3. Results and discussion 3.1. Structural studies The XRD patterns for (a) 3-mercapto-1,2-propanediol capped ZnSe dots and (b) 3-mercapto-1,2-propanediol capped ZnSe dots in polyvinylpyrrolidone are shown in Fig. 1. The comparative studies
Fig. 1. XRD patterns of (a) 3-mercapto-1,2-propanediol capped ZnSe quantum dots, (b) 3-mercapto-1,2-propanediol capped ZnSe quantum dots in polyvinylpyrrolidone.
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were done to monitor the effects of 3-mercapto-1,2-propanediol and polyvinylpyrrolidone on the morphology of the nanoparticles. All peaks in the diffraction patterns are indexed to the cubic structure of ZnSe where the lattice parameter (a = 5.67 A˚) is in good agreement with the standard JCPDS card no: 80-0021. The full-widths at half-maxima of the diffraction peaks of 3-mercapto1,2-propanediol capped ZnSe quantum dots in polyvinylpyrrolidone were broadened relative to those of 3-mercapto-1,2propanediol capped dots. Further XRD analysis revealed no additional products in the samples. Both samples exhibit the cubic phase with similar 2u values; thus the long chain polymer ligand polyvinylpyrrolidone does not influence the crystal structure of ZnSe quantum dots. Crystallite sizes of both 3-mercapto-1,2-propanediol capped ZnSe and 3-mercapto-1,2-propanediol capped ZnSe in polyvinylpyrrolidone have been calculated for the strong and prominent XRD plane (1 1 1) using the well-known Scherrer’s equation (1). t¼
kl ; b cos u
(1)
where t is the average crystallite size, k is the geometric factor (=0.90), l is the X-ray wavelength (1.5406 A˚), b is the half-width of the diffraction peak, and u is the angle of the diffraction peak. The calculated crystallite size for 3-mercapto-1,2-propanediol capped ZnSe quantum dots is 3.2 nm and that for 3-mercapto-1,2propanediol capped ZnSe quantum dots in polyvinylpyrrolidone is 2.7 nm. These rough estimates of particle sizes imply a possible effect of polymer stabilizer on quantum confinement. 3.2. Morphological studies More conclusive evidence of sizes, shapes and structures of the nanoparticles were documented with TEM images. Fig. 2(a) shows the TEM image of 3-mercapto-1,2-propanediol capped ZnSe spherical particles without aggregation over the area of 200 nm 200 nm. Particle sizes in the range 3–6 nm can be seen in the high resolution (HRTEM) images in Fig. 2(b) and (c), and are consistent with the size estimated from the XRD data and Eq. (1). The columns of the atoms were periodically arranged. Lattice fringes had the characteristic cubic pattern of the h1 1 1i projection of the zinc blende crystal structure and most of the quantum dots exhibited no defects such as stacking faults or coalescence. The lattice spacing of the 3-mercapto-1,2-propanediol capped ZnSe quantum dots was found to be 0.33 nm, consistent with the calculated lattice spacing from the XRD pattern (JCPDS no: 800021) [32]. A selected area electron diffraction (SAED) pattern from a 3mercapto-1,2-propanediol capped ZnSe quantum dots is shown in Fig. 2(d). Ring patterns indicate the cubic phase of ZnSe and are indexed to the (1 1 1), (2 2 0) and (3 1 1) planes. No patterns for defects or other lattices were observed. A histogram of particle size distribution is presented in the inset of Fig. 2(c), where it is evident that the sizes of the quantum dots were dispersed from 3.6 to 6.0 nm, with most near 5 nm. As discussed below, this variation in quantum dot size strongly affected the optical properties. Fig. 3(a)–(c) shows TEM and HRTEM images for 3-mercapto1,2-propanediol capped ZnSe in polyvinylpyrrolidone, respectively. These images clearly depict the formation of monodispersed ZnSe quantum dots. The size of the quantum dots is in the range 3.5–4.5 nm, which is less than that of bulk excitonic Bohr radius of 8.6 nm. Most of the quantum dots were 4 nm as shown in the histogram. The small deviation (approximately 1 nm) between the particle size observed by HRTEM analysis and that calculated from the XRD pattern and Eq. (1) is likely due to the strain-induced broadening effect of nanocrystals. The array of lattice fringes in the HRTEM images confirms the crystalline nature of the quantum
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Fig. 2. (a) TEM image, (b, c) HRTEM images and (d) SAED pattern of 3-mercapto-1,2-propanediol capped ZnSe quantum dots (inset of c is histogram of particle size distribution).
dots. In Fig. 3(a), the 3-mercapto-1,2-propanediol capped ZnSe nanocrystals are homogenously distributed on the chains of a network that was formed by the polymerization of polyvinylpyrrolidone. These particles were uniform and no aggregation was observed when polyvinylpyrrolidone was used to stabilize ZnSe nanocrystals. Since the long chain polarized molecules were coated on the surface of the nanocrystals, dispersive forces stabilized the nanocrystals in the colloid solution. This kind of polymer network was extended throughout the dispersion, thus the ZnSe nanocrystals exhibited excellent stability. Fig. 3(d) shows the SAED pattern of the 3-mercapto-1,2-propanediol capped ZnSe quantum dots in polyvinylpyrrolidone. It clearly depicts the signature of zinc blende with the (1 1 1), (2 2 0), (3 1 1) planes and no rings. Thus the polymer ligands determined size confinement without any change in crystalline phase. 3.3. Optical properties The UV–visible absorption spectrum of 3-mercapto-1,2-propanediol capped ZnSe quantum dots is shown in Fig. 4(a). It exhibits a UV absorption between 260 and 310 nm. Two excitonic absorption peaks were observed at 264 and 301 nm as shown in the inset of Fig. 4(a); they signify a shift toward higher energy relative to that of bulk ZnSe (460 nm). A possible reason for the two peaks is the existence of different sizes of quantum dots as seen in
the TEM and HRTEM images (Fig. 2). In principle, the optical absorption spectrum is a collection of cumulative transitions over a wide range of spectral responses. However, the histogram of the particle size distribution distinctly reveals the existence of two different sizes (3 and 6 nm). Multi-exciton generation has been extensively investigated in lead and cadmium chalcogenides quantum dots from the near infrared to the visible regions. Very recently, Zhang et al. [33] observed multi-exciton generation in ZnSe quantum dots with optical transitions at 269 nm and 307 nm. In the present work, the existence of multiple optical transitions may be due to the larger size distribution of ZnSe quantum dots. The emission spectrum for 3-mercapto-1,2-propanediol capped ZnSe quantum dots consists of multiple emission peaks in the near band-edge region as shown in Fig. 5(a). A broad emission peak is centered at 378 nm with two satellite peaks of less intensity. The existence of multiple emission peaks within the bandgap of 0.3 eV confirms the quantum size effect of the two differently sized quantum dots. The Stokes shift in the absorption edge and luminescence peak center of the 3-mercapto-1,2-propanediol capped ZnSe quantum dots was 0.49 eV. The deep level emission of 503 nm may be related to defects or dangling bonds on surface of the quantum dots. The optical absorbance spectrum of 3-mercapto-1,2-propanediol capped ZnSe quantum dots in polyvinylpyrrolidone is shown in Fig. 4(b). The excitonic absorption peak was observed at 288 nm and no other optical transitions or excitonic peaks were observed,
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Fig. 3. (a) TEM image, (b, c) HRTEM images and (d) SAED pattern of 3-mercapto-1,2-propanediol capped ZnSe quantum dots in polyvinylpyrrolidone (inset of c is histogram of particle size distribution).
unlike the 3-mercapto-1,2-propanediol capped ZnSe quantum dots. The spectrum thus indicates a highly monodispersed distribution of quantum dots. The photoluminescence spectrum of 3-mercapto-1,2-propanediol capped ZnSe quantum dots in polyvinylpyrrolidone is
shown in Fig. 5(b). The single 326 nm emission peak is observed at near band edge luminescence in the deep ultra violet region. The Stokes shift of the optical transition is about 0.03 eV. No other deep-level emission has been observed. The photoluminescence intensity of the 3-mercapto-1,2-propanediol capped ZnSe quantum dots in polyvinylpyrrolidone increased twofold relative to
Fig. 4. UV–visible absorption spectra of (a) 3-mercapto-1,2-propanediol capped ZnSe quantum dots and (b) 3-mercapto-1,2-propanediol capped ZnSe quantum dots in polyvinylpyrrolidone.
Fig. 5. Photoluminescence spectra of (a) 3-mercapto-1,2-propanediol capped ZnSe quantum dots and (b) 3-mercapto-1,2-propanediol capped ZnSe quantum dots in polyvinylpyrrolidone.
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Fig. 6. Formation mechanism of organic ligands capped ZnSe quantum dots.
that of 3-mercapto-1,2-propanediol capped ZnSe quantum dots. This provided further evidence of stabilization by polymer ligands and the passivation of dangling bonds on the surface of the quantum dots. 3.4. Growth mechanism and functional group analysis Fig. 6 shows the schematic representation of the growth mechanism for the ZnSe quantum dots. Organic ligands and stabilizers were used to control the reaction kinetics and to prevent particle aggregation and Ostwald ripening. The ligands passivate the surface, stabilize the solution, and create a large potential barrier between neighboring dots that confines carriers to individual dots. Thus ligands play a pivotal role in the growth and nucleation of quantum dots. Moreover, the sulfhydryl group of the 3-mercapto-1,2-propanediol ligands has a strong affinity for
the Zn2+ ion in the ZnSe quantum dots. Therefore, the coordination between S and Zn dominates and the thiol group becomes more stable. By increasing the length of the polymer backbone and the density of hydrophobic side chains, a dramatic ‘‘steric hindrance’’ effect occurs and results in the homogenous formation of ZnSe quantum dots. The grafted polyvinylpyrrolidone molecular chains will thus stabilize the particles by steric repulsion and prevent particle aggregation. Moreover, on the particle surface dangling bonds and traps will gradually decrease during the reaction time, allowing the formation of stable surface states. The ZnSe quantum dots thus will not grow larger due to the important role of the polymer network chain length in controlling the critical radius. Functional groups in the ZnSe quantum dots were confirmed by FTIR spectra as displayed in Fig. 7(a) and (b). Characteristic vibrations of 3-mercapto-1,2-propanediol and polyvinylpyrrolidone (PVP) are observed in the spectra. Vibration peaks at 886 cm1 and 2600 cm1 correspond to the thiol S–H stretch. Peaks at 2886 cm1 and 3354 cm1 correspond to the CH2–S and O–H thiol stretches, respectively. These vibrations confirm the surface passivation of 3-mercapto-1,2-propanediol on the ZnSe nanoparticles. Whereas, the spectrum of thiol-capped ZnSe in PVP displays the weakened vibrations regions of 2300 and 1600 cm1 as shown in the inset. The data indicate the presence of PVP on the quantum dots, with PVP characteristic vibrations such as C5 5O and C–N at 1656 cm1 and 1333 cm1, respectively. Both thiol and PVP molecules effectively passivate the surface of ZnSe quantum dots. 4. Conclusions
Fig. 7. FTIR spectra of (a) 3-mercapto-1,2-propanediol capped ZnSe quantum dots and (b) 3-mercapto-1,2-propanediol capped ZnSe quantum dots in polyvinylpyrrolidone.
We have synthesized ultra-fine monodispersed ZnSe quantum dots with an average size of 4 nm by an inexpensive, lowtemperature chemical route involving multiple ligands. Enhanced
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UV emission was observed in 3-mercapto-1,2-propanediol capped ZnSe quantum dots in polyvinylpyrrolidone. A size-dependent optical bandgap was observed and the Stokes shift was reduced by the steric hindrance effect of the polymer network. Possible reaction and growth mechanisms have been presented. The synthesized ZnSe quantum dots can be used in bio-labeling and quantum dot solar cells applications. The present approach is highly suitable for multi-gram synthesis of monodispersed semiconductor quantum dots. References [1] V.L. Colvin, M.C. Schlamp, A.P. Alivisatos, P.L. McEuen, Nature 370 (1994) 354–357. [2] W.C.W. Chan, S. Nie, Science 281 (1998) 2016–2018. [3] J. Weng, J. Ren, Curr. Med. Chem. 13 (2006) 897–909. [4] A. Mostafa, E. Sayed, Acc. Chem. Res. 37 (2004) 326–333. [5] Y. Wang, A. Suna, W. Mahler, R. Kasowski, J. Chem. Phys. 87 (1987) 7315–7322. [6] S. Coe, W.K. Woo, M.G. Bawendi, V. Bulvoic, Nature 420 (2002) 800–803. [7] J.M. Caruge, J.E. Halpert, V. Wood, V. Bulvoic, M.G. Bawendi, Nat. Photonics 2 (2008) 247–250. [8] B. Sun, E. Marx, N.C. Greenham, Nano Lett. 3 (7) (2003) 961–963. [9] W.U. Huynh, J.J. Dittmer, A.P. Alivisatos, Science 295 (2002) 2425–2427. [10] R.A. Sperling, P. Riveragil, F. Zhang, M. Zanella, W.J. Parak, Chem. Soc. Rev. 37 (2008) 1896–1908. [11] P. Alivisatos, Nat. Biotechnol. 22 (2004) 47–52. [12] D.S. Xu, X.S. Shi, L.G. Guo, L. Gui, Y.Q. Tang, J. Phys. Chem. B 104 (2000) 5061–5063. [13] J. Zhu, O. Palchik, S. Chen, A. Gedanken, J. Phys. Chem. B 104 (31) (2000) 7344–7347.
1897
[14] B.O. Dabbousi, M.G. Bawendi, O. Onituska, M.F. Rubner, Appl. Phys. Lett. 66 (1995) 1316–1318. [15] D.L. Klein, R. Roth, A.K.L. Lim, A.P. Alivisatos, P.L. McEuen, Nature 389 (1997) 699–701. [16] G. Li, M. Nogami, J. Appl. Phys. 75 (1994) 4276–4278. [17] C.D. Lokhande, P.S. Patil, H. Tributsch, A. Ennaoui, Sol. Energy Mater. Sol. Cell 55 (1998) 379–393. [18] S. Gorer, A. Albu-yaron, G. Hodes, Chem. Mater. 7 (6) (1995) 1243–1256. [19] X. Liu, J. Ma, P. Peng, W. Zheng, Langmuir 26 (12) (2010) 9968–9973. [20] J. Archana, M. Navaneethan, S. Ponnusamy, C. Muthamizhchelvan, Y. Hayakawa, Mater. Lett. 63 (2009) 1931–1934. [21] J. Archana, M. Navaneethan, S. Ponnusamy, C. Muthamizhchelvan, Y. Hayakawa, Mater. Lett. 64 (2010) 2094–2097. [22] C. Unni, D. Philip, K.G. Gopchandran, Opt. Mater. 32 (2009) 169–175. [23] A.L. Rogach, A. Kornowski, M. Gao, A. Eychmuller, H. Weller, J. Phys. Chem. B 103 (1999) 3065–3069. [24] L. Qi, H. Colfen, M. Antonietti, Nano Lett. 1 (2) (2000) 61–65. [25] I. Sondi, O. Siiman, S. Koster, E. Matijevic, Langmuir 16 (7) (2000) 3107–3118. [26] K. Nose, H. Fujita, T. Omata, S.O. Yao-Matsuo, H. Nakamura, H. Maeda, J. Lumin. 126 (2007) 21–26. [27] Y. Liu, M. Gibbs, J. Puthussery, S. Gaik, R. Ihly, H.W. Hillhouse, M. Law, Nano Lett. 10 (2010) 1960–1969. [28] M.B. Saab, E. Estephan, T. Coitre, C. Larroque, C. Gergely, J. Phys. Chem. C 114 (2010) 18509–18515. [29] D.J. Kim, K.K. Koo, Cryst. Growth Des. 9 (2009) 1153–1157. [30] H.P. Wanger, W. Langbein, J.M. Hvam, G. Bacher, T. Kummell, A. Forchel, Phys. Rev. B 57 (3) (1998) 1797–1800. [31] Z. Ning, H. Tian, C. Yuan, Y. Fu, H. Qin, L. Sun, H. Agren, Chem. Commun. 47 (2011) 1536–1538. [32] D. Zhu, X. Jiang, C. Zhao, X. Sun, J. Zhang, J.J. Zhu, Chem. Commun. 46 (2010) 5226– 5228. [33] L.J. Zhang, X.C. Shen, H. Liang, J.T. Yao, J. Phys. Chem. C 114 (50) (2010) 21921– 21927.