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ZnS type: preparation and optical studies
Synthetic Metals 139 (2003) 649–652
Low polydispersity core/shell nanocrystals of CdSe/ZnSe and CdSe/ZnSe/ZnS type: preparation and optical studies P. Reiss a,∗ , S. Carayon b , J. Bleuse b , A. Pron a a
CEA Grenoble, Département de Recherche Fondamentale sur la Matière Condensée, Service des Interfaces et des Matériaux Moléculaires et Macromoléculaires/Laboratoire Physique des Métaux Synthétiques, 17 rue des Martyrs, 38054 Grenoble cedex 9, France b CEA Grenoble, Département de Recherche Fondamentale sur la Matière Condensée, Service de Physique des Matériaux et des Microstructures/Laboratoire Nanophysique et Semiconducteurs, 17 rue des Martyrs, 38054 Grenoble cedex 9, France
Abstract A new method for the preparation of core/shell semiconductor nanocrystals of the A(II)B(VI) family is described, which involves the use of safe and cheap reagents at each synthesis stage, namely cadmium oxide as the Cd source and zinc stearate as the Zn source. It enables the fabrication of highly luminescent CdSe/ZnSe core/shell nanocrystals exhibiting a polydispersity <10%. In addition, a new double shell system is presented—CdSe/ZnSe/ZnS. An intermediate shell of ZnSe between the CdSe core and the outer shell of ZnS significantly lowers the lattice mismatch parameter assuring in this manner a “smooth” passage between the two crystallographically different components (CdSe and ZnS). At the same time the outer ZnS shell improves the charge carrier confinement in the core due to its larger band gap as compared to ZnSe. Photoluminescence spectroscopy of the CdSe/ZnSe/ZnS nanocrystals corroborates these arguments, as a significant increase in the fluorescence intensity is observed relatively to the CdSe/ZnSe and CdSe/ZnS systems. © 2003 Elsevier B.V. All rights reserved. Keywords: Core/shell semiconductor nanocrystals; In situ absorption spectroscopy; Photoluminescence; CdSe
1. Introduction Semiconductor nanocrystals of A(II)B(VI) (where A = Cd, Zn and B = S, Se, Te) have been the subject of an extensive research interest in the last two decades [1]. Their most interesting peculiarity, which stimulates their applications in very distant research areas, is the possibility of a precise tuning of their absorption and emission spectra by adjusting their size. After their grafting to molecules or individual biological objects, they are favourite candidates for biological labeling since they show much better photostability than routinely used organic dyes [2]. Recently, a new application of semiconductor nanocrystals has been proposed. It combines nanocrystal science and polymer electronics: A(II)B(VI) nanocrystals have been used in polymer solar cells as admixtures facilitating exciton dissociation and electron transport [3,4]. Some applications of nanocrystals require a high luminescence efficiency. This parameter can be improved by the deposition of a passivating “shell” consisting of
∗ Corresponding author. Tel.: +33-438-78-9719; fax: +33-438-78-5113. E-mail address: [email protected] (P. Reiss).
0379-6779/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0379-6779(03)00335-7
a larger gap semiconductor on the surface of a “bare” nanocrystal. The preparation of such “core/shell” systems requires careful selection of both core and shell materials with the goal to optimize the passivation and to minimize structural defects induced by the mismatch of their lattice parameters. Due to the strong size dependence of their physical properties, a narrow size distribution of the nanocrystals is of crucial importance, independently of their application. This point triggered an intensive research devoted to the development of several synthetic routes enabling the reproducible preparation of quasi-monodisperse colloidal nanocrystals [5–8]. Unfortunately, they are based on the high temperature decomposition of pyrophoric organometallic reagents. In the last 2 years, safer reagents have been proposed for the preparation of core materials [9,10], while the shell growth still requires organometallic precursors. In our previous communication we have described a new method for the preparation of highly luminescent CdSe/ZnSe nanocrystals, in which non-organometallic, safe reagents were used for both core and shell preparation [11]. In this paper, we demonstrate how core/shell engineering can be exploited for the design of nanocrystals with improved optical properties.
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P. Reiss et al. / Synthetic Metals 139 (2003) 649–652
2. Experimental Standard airless techniques have been applied for all syntheses as trioctylphosphine (TOP) and bis(trimethylsilyl) sulfide ((TMS)2 S) are air sensitive. 2.1. Reagents Anhydrous solvents and reagents (Aldrich) were of the highest purity available. Trioctylphosphine oxide (TOPO), trioctylphosphine and hexadecylamine (HDA) were additionally purified by distillation. Dodecylphosphonic acid (DDPA) was synthesized according to a literature method [12].
2.4. Photoluminescence spectroscopy and fluorescence quantum yield (QY) measurements Nanocrystals dispersed in toluene, put in 1 mm light path fluorescence cells, were excited with a continuous wave, Ar++ laser, emitting at 365 nm. The photoluminescence was detected with a liquid-nitrogen cooled, Si CCD, following a 46 cm focal length monochromator. The excitation density was of the order of 3 mW cm−2 . The QY was measured on a dedicated apparatus with Si photodetectors, under the same excitation conditions, except for a 10 mm light path, relatively to rhodamine 6G in ethanol (QY 94%, Exciton Inc.). The description of this apparatus and of the measurement principle will be published elsewhere.
2.2. CdSe core synthesis (yield ≥300 mg) Two hundred and fifty-seven milligrams (2 mmol) of CdO is placed into a three-neck flask containing 8 ml of TOPO and 12 ml of HDA. The mixture is heated to ca. 270 ◦ C under argon flow. Upon addition of 1.2 ml (4.2 mmol) of DDPA a colorless solution is formed. At 250 ◦ C, 15 ml of a 0.3 M solution (4.5 mmol) of Se powder in TOP is swiftly injected. To monitor the nanocrystal growth process by UV-Vis absorption spectroscopy, a fiber dip probe resides in the reaction flask, which is connected to an I.D.I.L. AVS-2000 spectrometer. For room temperature spectra (HP 8452A), small aliquots are removed from the reaction mixture periodically. At the desired nanocrystal size, the reaction is stopped by removal of the heating bath. Purification is carried out by two cycles of precipitation with MeOH and redispersion of the nanocrystals in toluene. 2.3. Synthesis of core/shell systems Two milliliters of a dispersion containing ca. 3 × 10−7 mol of CdSe core nanocrystals in toluene are dispersed in 8 ml of HDA. A 0.2 M zinc stock solution in toluene is prepared by dissolving 1.9 g of zinc stearate in 15 ml of toluene upon gentle heating (the mixture remains cloudy). At room temperature 4 ml of this solution are mixed with 4 ml of a 0.2 M solution of Se powder in TOP (synthesis of CdSe/ZnSe) or with a solution containing 168 mg of (TMS)2 S in 4 ml of toluene (synthesis of CdSe/ZnS) and placed in a syringe. The mixture is injected within 1 h into the reaction vessel containing the core nanocrystals at 190–200 ◦ C. For the synthesis of CdSe/ZnSe/ZnS nanocrystals identical precursor solutions and the same amount of core nanocrystals were used. An injection of 2 ml of the ZnSe precursor mixture (1:1) during 15 min is followed by an injection of 6 ml of the ZnS precursor mixture (1:1) within 45 min. The growth of the shell is monitored by photoluminescence spectroscopy and additionally evidenced by transmission electron microscopy (not shown here).
3. Results and discussion 3.1. CdSe core nanocrystals In recent attempts to replace dimethylcadmium in the synthesis of monodisperse CdSe nanocrystals [5] by cheaper and safer reagents, cadmium oxide, carbonate and carboxylates were proposed [9,10]. On the other hand, through addition of hexadecylamine to the routinely used coordinating solvent trioctylphosphine oxide a significant lowering of nanocrystals polydispersity was observed [13]. Here, we adopted an approach combining both modifications (see Section 2). Additionally, a new cadmium source, cadmium dodecylphosphonate, is used which is formed in situ in the reaction medium from CdO and dodecylphosphonic acid and facilitates the formation of CdSe nanocrystals in the strongly confined size regime (diameter 2–5 nm). The established preparation conditions assure a relatively slow nanocrystal growth rate, which in turn facilitates precise control of the nanocrystal size. This is demonstrated in Fig. 1 where UV-Vis absorption spectra of the growing nanocrystals, measured in situ in the reaction mixture, are presented. Slow evolution of the spectra occurs over a period of 10 min—a timescale sufficiently large to enable the termination of the growth at the desired nanocrystal size. The distinct excitonic peak, appearing from ca. 1 min after precursor injection on, indicates a narrow nanocrystal size distribution throughout the whole growth process.1 Low polydispersity is also corroborated by photoluminescence spectroscopy as the emission line width rests essentially constant, 27–31 nm full-width half-maximum (FWHM), during the crystal growth [11]. This is an important improvement since the polydispersity, and by consequence the emission line width, increase nearly linearly with the nanocrystal size if pure TOPO is used. Thus, the presented 1 Note that the absorption bands presented in Fig. 1, measured at 250 ◦ C, are broader than those measured at room temperature and exhibit a constant bathochromic shift of ca. 22 nm with respect to the latter.
P. Reiss et al. / Synthetic Metals 139 (2003) 649–652
Fig. 1. UV-vis absorption spectra of CdSe nanocrystals measured in situ in the reaction mixture using a fiber optics dip probe (T = 250 ◦ C). The 16 spectra represented are selected from a total set of 300 measurements carried out at time intervals of 2 s after precursor injection (increasing reaction time from the bottom (18 s) to the top (600 s) spectrum). The inset shows a room temperature spectrum of the same sample (reaction time 600 s).
method enables a reproducible, medium-scale (several hundreds milligrams per synthesis) preparation of monodisperse CdSe nanocrystals in the size range of 2–5 nm. 3.2. Core/shell CdSe/ZnS and CdSe/ZnSe nanocrystals In the selection of shell materials, for a given core, two parameters must be taken into consideration: the relative positions of the band gaps of the core and shell semiconductors and the lattice mismatch parameter. The former determines the charge carriers’ confinement, whereas the latter is crucial for an epitaxial type deposition of the shell layer on the core. In Table 1 band gap values and mismatch parameters for the three semiconductors investigated are collected. It is clear that the most widely used CdSe/ZnS system [6,7] assures better charge carrier confinement as compared to the CdSe/ZnSe one. This is especially true for the holes, as the valence band offset is only of the order of 0.1 eV for CdSe/ZnSe, whereas it attains 0.6 eV for CdSe/ZnS [14]. The rather large value of the lattice mismatch parameter of CdSe/ZnS may however give rise to structural defects at the core/shell interface, which cause photoluminescence quenching. On the contrary, the CdSe/ZnSe system offers better lattice matching. Moreover, the selected shell growth method may strongly influence the quality of the core/shell Table 1 Band gap values for selected A(II)B(VI) semiconductors and lattice mismatch parameters for their core/shell combinations [15] Shell material
Band gap (eV) (CdSe = 1.76)
Core material lattice mismatch (%)
ZnS
3.80
CdSe 10.6
ZnSe
2.72
ZnSe 4.6 CdSe 6.3
651
interface. For all above reasons we have decided to compare CdSe/ZnS and CdSe/ZnSe core/shell systems using the same shell growth method. CdSe/ZnSe particles were prepared according to the method we described in [11] where no organometallic precursors are used and zinc stearate is applied as Zn source. However, direct transfer of this method to the synthesis of a ZnS shell did not work. Elemental sulfur, dissolved in TOP, cannot be used as S source in analogy to the CdSe or ZnSe synthesis because it exhibits too low reactivity towards zinc stearate. In turn, we have achieved successful ZnS shell formation by applying thioacetamide (TAA) or bis(trimethylsilyl) sulfide, with preference for the latter because of its good solubility in toluene. As expected, difficulties were faced while attempting to grow an epitaxial type shell of ZnS onto CdSe nanocrystals. The photoluminescence spectrum of the crude solution shows, in addition to the distinct emission peak of the CdSe/ZnS nanocrystals at 580 nm, a second peak at ca. 400 nm. This peak can be attributed to the photoluminescence of separate “bare” ZnS nanocrystals, which are created in addition to the ZnS shell growth. Their formation is promoted by the large lattice mismatch parameter which also induces the presence of defects at the core/shell interface. These defects impair the passivation and decrease the fluorescence QY. 3.3. Core/double shell CdSe/ZnSe/ZnS nanocrystals The low lattice mismatch of CdSe/ZnSe and ZnSe/ZnS (see Table 1) motivated us to prepare a three-component core/shell system in which ZnSe constitutes an inner shell between a CdSe core and a ZnS outer shell: Cd/ZnSe/ZnS nanocrystals. Thereby, the advantages of both pure CdSe/ZnSe and CdSe/ZnS systems can in principle be combined. Furthermore, the complete three-step synthesis can be accomplished without the use of any organometallic reagents (see Section 2). During ZnS shell growth on the surface of CdSe/ZnSe nanocrystals almost no formation of separate ZnS nanocrystals takes place, as the emission peak at ca. 400 nm is practically non-existent. In Fig. 2, the photoluminescence spectra of CdSe core nanocrystals and of the three different core/shell systems are compared. In each case, the same concentrations and volumes for the core nanocrystal dispersions and the shell precursor solutions were used. The amount of shell precursors necessary for the formation of 5 monolayer (ML) thick ZnSe and ZnS shells has been calculated on the basis of bulk lattice parameters. For the double shell system the ratio of ZnSe/ZnS precursors was 3/1, while keeping the same total amount of ZnB(VI) precursors as for the single shell systems. In Fig. 2, only the final stages of the shell growth, i.e. after complete injection of the precursor solutions, are presented. Deposition of a ZnSe or ZnS shell results in an increase of the QY, due to a better passivation of the core nanocrystal surface; yet the fluorescence of the CdSe/ZnS system amounts to only 84% of that of CdSe/ZnSe, because
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of organometallic reagents and allows the facile preparation of three types of highly luminescent nanocrystals of low polydispersity, namely CdSe/ZnSe, CdSe/ZnS and CdSe/ZnSe/ZnS. The latter is a new double shell system, which combines the advantages of the two other types: low lattice mismatch (CdSe/ZnSe) and good confinement of the charge carriers in the core (CdSe/ZnS). In consequence, this system exhibits the highest fluorescence efficiency of the three core/shell systems investigated.
References Fig. 2. Photoluminescence spectra of CdSe nanocrystals before and after passivation with different types of inorganic semiconductor shells. The relative fluorescence intensities can be compared as, for all shell preparations, the same CdSe core nanocrystals are used and the sample concentrations are identical.
of structural defects. On the contrary, deposition of a ZnS outer shell onto CdSe nanocrystals which are already covered with a relatively thin ZnSe layer (1–2 ML) significantly improves the fluorescence efficiency by a factor of ca. 1.7 as compared to CdSe/ZnS and by ca. 1.4 as compared to CdSe/ZnSe.2 While shell growth is in all cases accompanied by a bathochromic shift of the emission peak originating from partial exciton leakage into the shell, the emission line width, being 33–34 nm at FWHM, is not affected. 4. Conclusions To summarize, we have applied a new preparation method to passivate CdSe nanocrystals with different semiconductor shells. The procedure avoids at all stages the use
2 Absolute values for the QY have been measured as 17.6 ± 1.5% (CdSe/ZnSe), 14.8±1.5% (CdSe/ZnS) and 25.3±0.6% (CdSe/ZnSe/ZnS). We want to underline that these values do not represent upper limits for the three systems obtained with the described preparation method. On the contrary, no efforts to optimize the QY (adjusting the solvent system, annealing, etc.) have been undertaken in order to keep strict comparability of the different samples and to study exclusively effects arising from the shell material.
[1] J.R. Heath (Ed.), Acc. Chem. Res. 32 (1999), and references therein. [2] J.K. Jaiswal, H. Mattoussi, J.M. Mauro, S.M. Simon, Nat. Biotechnol. 21 (2003) 47. [3] N.C. Greenham, X. Peng, A.P. Alivisatos, Synth. Met. 84 (1997) 545. [4] W.U. Huynh, J.J. Dittmer, A.P. Alivisatos, Science 295 (2002) 2425. [5] C.B. Murray, D.J. Norris, M.G. Bawendi, J. Am. Chem. Soc. 115 (1993) 8706. [6] M.A. Hines, P. Guyot-Sionnest, J. Phys. Chem. 100 (1996) 468. [7] B.O. Dabboussi, J. Rodriguez-Viejo, F.V. Mikulec, J.R. Heine, H. Mattoussi, R. Ober, K.F. Jensen, M.G. Bawendi, J. Phys. Chem. B 101 (1997) 9463. [8] X. Peng, M.C. Schlamp, A.V. Kadavanich, A.P. Alivisatos, J. Am. Chem. Soc. 119 (1997) 7019. [9] Z.A. Peng, X. Peng, J. Am. Chem. Soc. 123 (2001) 183. [10] L. Qu, Z.A. Peng, X. Peng, Nano Lett. 1 (2001) 333. [11] P. Reiss, J. Bleuse, A. Pron, Nano Lett. 2 (2002) 781. [12] K. Sasse, in: E. Müller (Ed.), Methoden der organischen Chemie, Georg Thieme, Stuttgart, vol. 12/1, 1963, pp. 435 and 352–353. [13] D.V. Talapin, A.L. Rogach, A. Kornowski, M. Haase, H. Weller, Nano Lett. 1 (2001) 207. [14] S.-H. Wei, A. Zunger, Appl. Phys. Lett. 72 (1998) 2011. [15] O. Madelung (Ed.), Landolt-Börnstein series, Crystal and Solid State Physics, vol. 17b, Physics of II–VI and I–VII Compounds, Springer, Berlin, 1982.
Low polydispersity core/shell nanocrystals of CdSe/ZnSe and CdSe/ZnSe/ZnS type: preparation and optical studies P. Reiss a,∗ , S. Carayon b , J. Bleuse b , A. Pron a a
CEA Grenoble, Département de Recherche Fondamentale sur la Matière Condensée, Service des Interfaces et des Matériaux Moléculaires et Macromoléculaires/Laboratoire Physique des Métaux Synthétiques, 17 rue des Martyrs, 38054 Grenoble cedex 9, France b CEA Grenoble, Département de Recherche Fondamentale sur la Matière Condensée, Service de Physique des Matériaux et des Microstructures/Laboratoire Nanophysique et Semiconducteurs, 17 rue des Martyrs, 38054 Grenoble cedex 9, France
Abstract A new method for the preparation of core/shell semiconductor nanocrystals of the A(II)B(VI) family is described, which involves the use of safe and cheap reagents at each synthesis stage, namely cadmium oxide as the Cd source and zinc stearate as the Zn source. It enables the fabrication of highly luminescent CdSe/ZnSe core/shell nanocrystals exhibiting a polydispersity <10%. In addition, a new double shell system is presented—CdSe/ZnSe/ZnS. An intermediate shell of ZnSe between the CdSe core and the outer shell of ZnS significantly lowers the lattice mismatch parameter assuring in this manner a “smooth” passage between the two crystallographically different components (CdSe and ZnS). At the same time the outer ZnS shell improves the charge carrier confinement in the core due to its larger band gap as compared to ZnSe. Photoluminescence spectroscopy of the CdSe/ZnSe/ZnS nanocrystals corroborates these arguments, as a significant increase in the fluorescence intensity is observed relatively to the CdSe/ZnSe and CdSe/ZnS systems. © 2003 Elsevier B.V. All rights reserved. Keywords: Core/shell semiconductor nanocrystals; In situ absorption spectroscopy; Photoluminescence; CdSe
1. Introduction Semiconductor nanocrystals of A(II)B(VI) (where A = Cd, Zn and B = S, Se, Te) have been the subject of an extensive research interest in the last two decades [1]. Their most interesting peculiarity, which stimulates their applications in very distant research areas, is the possibility of a precise tuning of their absorption and emission spectra by adjusting their size. After their grafting to molecules or individual biological objects, they are favourite candidates for biological labeling since they show much better photostability than routinely used organic dyes [2]. Recently, a new application of semiconductor nanocrystals has been proposed. It combines nanocrystal science and polymer electronics: A(II)B(VI) nanocrystals have been used in polymer solar cells as admixtures facilitating exciton dissociation and electron transport [3,4]. Some applications of nanocrystals require a high luminescence efficiency. This parameter can be improved by the deposition of a passivating “shell” consisting of
∗ Corresponding author. Tel.: +33-438-78-9719; fax: +33-438-78-5113. E-mail address: [email protected] (P. Reiss).
0379-6779/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0379-6779(03)00335-7
a larger gap semiconductor on the surface of a “bare” nanocrystal. The preparation of such “core/shell” systems requires careful selection of both core and shell materials with the goal to optimize the passivation and to minimize structural defects induced by the mismatch of their lattice parameters. Due to the strong size dependence of their physical properties, a narrow size distribution of the nanocrystals is of crucial importance, independently of their application. This point triggered an intensive research devoted to the development of several synthetic routes enabling the reproducible preparation of quasi-monodisperse colloidal nanocrystals [5–8]. Unfortunately, they are based on the high temperature decomposition of pyrophoric organometallic reagents. In the last 2 years, safer reagents have been proposed for the preparation of core materials [9,10], while the shell growth still requires organometallic precursors. In our previous communication we have described a new method for the preparation of highly luminescent CdSe/ZnSe nanocrystals, in which non-organometallic, safe reagents were used for both core and shell preparation [11]. In this paper, we demonstrate how core/shell engineering can be exploited for the design of nanocrystals with improved optical properties.
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P. Reiss et al. / Synthetic Metals 139 (2003) 649–652
2. Experimental Standard airless techniques have been applied for all syntheses as trioctylphosphine (TOP) and bis(trimethylsilyl) sulfide ((TMS)2 S) are air sensitive. 2.1. Reagents Anhydrous solvents and reagents (Aldrich) were of the highest purity available. Trioctylphosphine oxide (TOPO), trioctylphosphine and hexadecylamine (HDA) were additionally purified by distillation. Dodecylphosphonic acid (DDPA) was synthesized according to a literature method [12].
2.4. Photoluminescence spectroscopy and fluorescence quantum yield (QY) measurements Nanocrystals dispersed in toluene, put in 1 mm light path fluorescence cells, were excited with a continuous wave, Ar++ laser, emitting at 365 nm. The photoluminescence was detected with a liquid-nitrogen cooled, Si CCD, following a 46 cm focal length monochromator. The excitation density was of the order of 3 mW cm−2 . The QY was measured on a dedicated apparatus with Si photodetectors, under the same excitation conditions, except for a 10 mm light path, relatively to rhodamine 6G in ethanol (QY 94%, Exciton Inc.). The description of this apparatus and of the measurement principle will be published elsewhere.
2.2. CdSe core synthesis (yield ≥300 mg) Two hundred and fifty-seven milligrams (2 mmol) of CdO is placed into a three-neck flask containing 8 ml of TOPO and 12 ml of HDA. The mixture is heated to ca. 270 ◦ C under argon flow. Upon addition of 1.2 ml (4.2 mmol) of DDPA a colorless solution is formed. At 250 ◦ C, 15 ml of a 0.3 M solution (4.5 mmol) of Se powder in TOP is swiftly injected. To monitor the nanocrystal growth process by UV-Vis absorption spectroscopy, a fiber dip probe resides in the reaction flask, which is connected to an I.D.I.L. AVS-2000 spectrometer. For room temperature spectra (HP 8452A), small aliquots are removed from the reaction mixture periodically. At the desired nanocrystal size, the reaction is stopped by removal of the heating bath. Purification is carried out by two cycles of precipitation with MeOH and redispersion of the nanocrystals in toluene. 2.3. Synthesis of core/shell systems Two milliliters of a dispersion containing ca. 3 × 10−7 mol of CdSe core nanocrystals in toluene are dispersed in 8 ml of HDA. A 0.2 M zinc stock solution in toluene is prepared by dissolving 1.9 g of zinc stearate in 15 ml of toluene upon gentle heating (the mixture remains cloudy). At room temperature 4 ml of this solution are mixed with 4 ml of a 0.2 M solution of Se powder in TOP (synthesis of CdSe/ZnSe) or with a solution containing 168 mg of (TMS)2 S in 4 ml of toluene (synthesis of CdSe/ZnS) and placed in a syringe. The mixture is injected within 1 h into the reaction vessel containing the core nanocrystals at 190–200 ◦ C. For the synthesis of CdSe/ZnSe/ZnS nanocrystals identical precursor solutions and the same amount of core nanocrystals were used. An injection of 2 ml of the ZnSe precursor mixture (1:1) during 15 min is followed by an injection of 6 ml of the ZnS precursor mixture (1:1) within 45 min. The growth of the shell is monitored by photoluminescence spectroscopy and additionally evidenced by transmission electron microscopy (not shown here).
3. Results and discussion 3.1. CdSe core nanocrystals In recent attempts to replace dimethylcadmium in the synthesis of monodisperse CdSe nanocrystals [5] by cheaper and safer reagents, cadmium oxide, carbonate and carboxylates were proposed [9,10]. On the other hand, through addition of hexadecylamine to the routinely used coordinating solvent trioctylphosphine oxide a significant lowering of nanocrystals polydispersity was observed [13]. Here, we adopted an approach combining both modifications (see Section 2). Additionally, a new cadmium source, cadmium dodecylphosphonate, is used which is formed in situ in the reaction medium from CdO and dodecylphosphonic acid and facilitates the formation of CdSe nanocrystals in the strongly confined size regime (diameter 2–5 nm). The established preparation conditions assure a relatively slow nanocrystal growth rate, which in turn facilitates precise control of the nanocrystal size. This is demonstrated in Fig. 1 where UV-Vis absorption spectra of the growing nanocrystals, measured in situ in the reaction mixture, are presented. Slow evolution of the spectra occurs over a period of 10 min—a timescale sufficiently large to enable the termination of the growth at the desired nanocrystal size. The distinct excitonic peak, appearing from ca. 1 min after precursor injection on, indicates a narrow nanocrystal size distribution throughout the whole growth process.1 Low polydispersity is also corroborated by photoluminescence spectroscopy as the emission line width rests essentially constant, 27–31 nm full-width half-maximum (FWHM), during the crystal growth [11]. This is an important improvement since the polydispersity, and by consequence the emission line width, increase nearly linearly with the nanocrystal size if pure TOPO is used. Thus, the presented 1 Note that the absorption bands presented in Fig. 1, measured at 250 ◦ C, are broader than those measured at room temperature and exhibit a constant bathochromic shift of ca. 22 nm with respect to the latter.
P. Reiss et al. / Synthetic Metals 139 (2003) 649–652
Fig. 1. UV-vis absorption spectra of CdSe nanocrystals measured in situ in the reaction mixture using a fiber optics dip probe (T = 250 ◦ C). The 16 spectra represented are selected from a total set of 300 measurements carried out at time intervals of 2 s after precursor injection (increasing reaction time from the bottom (18 s) to the top (600 s) spectrum). The inset shows a room temperature spectrum of the same sample (reaction time 600 s).
method enables a reproducible, medium-scale (several hundreds milligrams per synthesis) preparation of monodisperse CdSe nanocrystals in the size range of 2–5 nm. 3.2. Core/shell CdSe/ZnS and CdSe/ZnSe nanocrystals In the selection of shell materials, for a given core, two parameters must be taken into consideration: the relative positions of the band gaps of the core and shell semiconductors and the lattice mismatch parameter. The former determines the charge carriers’ confinement, whereas the latter is crucial for an epitaxial type deposition of the shell layer on the core. In Table 1 band gap values and mismatch parameters for the three semiconductors investigated are collected. It is clear that the most widely used CdSe/ZnS system [6,7] assures better charge carrier confinement as compared to the CdSe/ZnSe one. This is especially true for the holes, as the valence band offset is only of the order of 0.1 eV for CdSe/ZnSe, whereas it attains 0.6 eV for CdSe/ZnS [14]. The rather large value of the lattice mismatch parameter of CdSe/ZnS may however give rise to structural defects at the core/shell interface, which cause photoluminescence quenching. On the contrary, the CdSe/ZnSe system offers better lattice matching. Moreover, the selected shell growth method may strongly influence the quality of the core/shell Table 1 Band gap values for selected A(II)B(VI) semiconductors and lattice mismatch parameters for their core/shell combinations [15] Shell material
Band gap (eV) (CdSe = 1.76)
Core material lattice mismatch (%)
ZnS
3.80
CdSe 10.6
ZnSe
2.72
ZnSe 4.6 CdSe 6.3
651
interface. For all above reasons we have decided to compare CdSe/ZnS and CdSe/ZnSe core/shell systems using the same shell growth method. CdSe/ZnSe particles were prepared according to the method we described in [11] where no organometallic precursors are used and zinc stearate is applied as Zn source. However, direct transfer of this method to the synthesis of a ZnS shell did not work. Elemental sulfur, dissolved in TOP, cannot be used as S source in analogy to the CdSe or ZnSe synthesis because it exhibits too low reactivity towards zinc stearate. In turn, we have achieved successful ZnS shell formation by applying thioacetamide (TAA) or bis(trimethylsilyl) sulfide, with preference for the latter because of its good solubility in toluene. As expected, difficulties were faced while attempting to grow an epitaxial type shell of ZnS onto CdSe nanocrystals. The photoluminescence spectrum of the crude solution shows, in addition to the distinct emission peak of the CdSe/ZnS nanocrystals at 580 nm, a second peak at ca. 400 nm. This peak can be attributed to the photoluminescence of separate “bare” ZnS nanocrystals, which are created in addition to the ZnS shell growth. Their formation is promoted by the large lattice mismatch parameter which also induces the presence of defects at the core/shell interface. These defects impair the passivation and decrease the fluorescence QY. 3.3. Core/double shell CdSe/ZnSe/ZnS nanocrystals The low lattice mismatch of CdSe/ZnSe and ZnSe/ZnS (see Table 1) motivated us to prepare a three-component core/shell system in which ZnSe constitutes an inner shell between a CdSe core and a ZnS outer shell: Cd/ZnSe/ZnS nanocrystals. Thereby, the advantages of both pure CdSe/ZnSe and CdSe/ZnS systems can in principle be combined. Furthermore, the complete three-step synthesis can be accomplished without the use of any organometallic reagents (see Section 2). During ZnS shell growth on the surface of CdSe/ZnSe nanocrystals almost no formation of separate ZnS nanocrystals takes place, as the emission peak at ca. 400 nm is practically non-existent. In Fig. 2, the photoluminescence spectra of CdSe core nanocrystals and of the three different core/shell systems are compared. In each case, the same concentrations and volumes for the core nanocrystal dispersions and the shell precursor solutions were used. The amount of shell precursors necessary for the formation of 5 monolayer (ML) thick ZnSe and ZnS shells has been calculated on the basis of bulk lattice parameters. For the double shell system the ratio of ZnSe/ZnS precursors was 3/1, while keeping the same total amount of ZnB(VI) precursors as for the single shell systems. In Fig. 2, only the final stages of the shell growth, i.e. after complete injection of the precursor solutions, are presented. Deposition of a ZnSe or ZnS shell results in an increase of the QY, due to a better passivation of the core nanocrystal surface; yet the fluorescence of the CdSe/ZnS system amounts to only 84% of that of CdSe/ZnSe, because
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of organometallic reagents and allows the facile preparation of three types of highly luminescent nanocrystals of low polydispersity, namely CdSe/ZnSe, CdSe/ZnS and CdSe/ZnSe/ZnS. The latter is a new double shell system, which combines the advantages of the two other types: low lattice mismatch (CdSe/ZnSe) and good confinement of the charge carriers in the core (CdSe/ZnS). In consequence, this system exhibits the highest fluorescence efficiency of the three core/shell systems investigated.
References Fig. 2. Photoluminescence spectra of CdSe nanocrystals before and after passivation with different types of inorganic semiconductor shells. The relative fluorescence intensities can be compared as, for all shell preparations, the same CdSe core nanocrystals are used and the sample concentrations are identical.
of structural defects. On the contrary, deposition of a ZnS outer shell onto CdSe nanocrystals which are already covered with a relatively thin ZnSe layer (1–2 ML) significantly improves the fluorescence efficiency by a factor of ca. 1.7 as compared to CdSe/ZnS and by ca. 1.4 as compared to CdSe/ZnSe.2 While shell growth is in all cases accompanied by a bathochromic shift of the emission peak originating from partial exciton leakage into the shell, the emission line width, being 33–34 nm at FWHM, is not affected. 4. Conclusions To summarize, we have applied a new preparation method to passivate CdSe nanocrystals with different semiconductor shells. The procedure avoids at all stages the use
2 Absolute values for the QY have been measured as 17.6 ± 1.5% (CdSe/ZnSe), 14.8±1.5% (CdSe/ZnS) and 25.3±0.6% (CdSe/ZnSe/ZnS). We want to underline that these values do not represent upper limits for the three systems obtained with the described preparation method. On the contrary, no efforts to optimize the QY (adjusting the solvent system, annealing, etc.) have been undertaken in order to keep strict comparability of the different samples and to study exclusively effects arising from the shell material.
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