Synthesis and characterization of brightly photoluminescent CdTe nanocrystals

Ruth Osovsky

Synthesis and characterization of brightly photoluminescent CdTe nanocrystals

Surface Science, 2007

Synthesis and characterization of brightly photoluminescent CdTe nanocrystals J. Kolny-Olesiak b , V. Kloper a , R. Osovsky a , A. Sashchiuk a , E. Lifshitz a, * a Department of Chemistry and Solid State Institute, Technion, Haifa, Israel b Energy and Semiconductor Research Laboratory, Department of Physics, University of Oldenburg, Germany Available online 4 January 2007 Abstract A new synthesis procedure for the preparation of spherical shaped CdTe nanocrystals (NCs) is presented, exhibiting bright lumines- cence with exceptionally high quantum eﬃciency (up to 85%). The growth of these NCs occurs in a non-coordinating solvent, octa- decene, with the addition of oleic acid/tri-octylphosphine stabilizers, CdO as a precursor for the Cd monomers and additional Cd metal particles as a supplementary Cd reservoir source. The dependence of the crystalline quality and the optical properties of the CdTe NCs, on the initial Cd:Te precursors’ molar ratio, and the reaction duration were investigated. It was demonstrated that the NCs’ prop- erties improved signiﬁcantly as the initial Cd:Te molar ratios are increased. The obtained NCs’ properties were correlated with measure- ments of the Cd 0 concentration in Cd metal particles, CdTe NCs and in Cd monomer solutions. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Colloidal; CdTe; Nanocrystals; PL; Quantum eﬃciency 1. Introduction In the last two decades a considerable progress has been made in the fundamental [1] and applied physical research of semiconductor nanocrystals (NCs) [2,3]. Colloidal CdTe NCs, in particular, show an increasing interest due to their large Bohr radius (7.3 nm) and the relatively small band gap (1.475 eV), leading to a pronounced quantum size ef- fect in a range of CdTe NCs with a diameter up to 10 nm. In addition, the absorption and emission spectra of these NCs can reach the near-IR spectral regime. The synthesis of high-quality NCs requires speciﬁc conditions of monomer reactivity and stabilizing ligands in order to control the monodispersity and optical properties of the growing NCs [4]. In this paper we present a new synthesis procedure, using two sequential stages for the preparation of high- quality CdTe NCs with photoluminescence (PL) quantum eﬃciency (QE) up to 85%. Using crystalline Cd particles, formed in situ in the ﬁrst stage as an additional source for Cd monomers, the growth of CdTe NCs occurred in a non-coordinating solvent, octadecene (ODE), during the second stage, with OA/TOP stabilizers. 2. Experimental A Te precursor solution was prepared by dissolving Te (0.0128 g) in tri octylphosphine (TOP, 0.2112 g), and octa- decene (ODE) to a total amount of 2 mL. The Cd precur- sor solution was prepared by mixing CdO (0.0256 g) and an oleic acid (OA, 200 lL)/ODE(10 mL) mixture. When heated under Ar ﬂow at 300 °C all CdO was dissolved in the OA/ODE mixture. Further heating to 310 °C, led to a formation of a grey precipitate, which was characterized as crystalline Cd particles, and considered as the ﬁrst stage of the reaction. The second stage of the reaction included an instantaneous injection of the Te precursor solution into the Cd solution at 310 °C. The morphology and size of the Cd precipitate and CdTe NCs were determined by high 0039-6028/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2006.12.013 * Corresponding author. Tel.: +972 48293987. E-mail address: ssefrat@tx.technion.ac.il (E. Lifshitz). www.elsevier.com/locate/susc Surface Science 601 (2007) 2667–2670

resolution scanning electron microscopy (HR-SEM) and transmission electron microscopy (HR-TEM). The optical properties of CdTe NCs were studied using absorption and photoluminescence measurements. The amount of Cd atoms in each fraction was determined by an atomic absorption spectroscopy (AAS). The PL QE was measured by comparing the PL intensity with that of a commercial Rhodamine 6G with a known QE of 95%. All the measure- ments were carried out at room temperature. 3. Results and discussion Fig. 1A presents a HR-SEM image that reveals that the grey precipitate, formed during the ﬁrst stage of the reac- tion, consists of Cd particles with average size of 100 nm and with well pronounced facets. However, after a Te injection, the facets of the Cd particles disappear (Fig. 1B), suggesting the occurrence of a chemical reaction and a return of Cd monomers to the reaction solution. Fig. 1C shows a HR-TEM image of CdTe NCs with a diameter of 4.0 nm, indicating the formation of spherical NCs with a 5% size distribution with a cubic zinc blende structure (not shown). Fig. 1D shows the absorption spec- tra (dashed line) and the PL spectra (solid line) of various aliquots of the CdTe NCs. The lower exciton energy in the absorption spectra varies between 2.03 eV and 2.32 eV, corresponding to NCs’ sizes between 3.1 nm and 3.8 nm. These 1S-exciton bands have a full width at half maximum (FWHM) of 100 meV. The PL spectra of the 3.1 nm– 3.8 nm samples are Stokes shifted with respect to the corre- sponding 1S exciton absorption band by 75 meV and 36 meV respectively. In order to ﬁnd the optimal conditions for the fabrica- tion of high quality CdTe NCs, Cd atom concentrations and absorption data versus reaction time were investigated as a function of initial Cd:Te molar ratios. Fig. 2A shows the percentage of Cd atoms that were lost from the Cd pre- cursor solution to form the Cd particles. The half bold tri- angles (Fig. 2A) represent the percentage of Cd prior to an injection of the TOP:Te solution. The bold and open triangles represent the percentage of Cd atoms after an injection of TOP:Te, when the initial molar ratios are of Cd:Te = 1.25:1 and Cd:Te = 1:1.25, respectively. Fig. 2A shows that the percentage of Cd atoms in Cd precipitate increases gradually as long as the TOP:Te is avoided. However, after the Te injection, the growth of the Cd precipitate is stopped. The suppression of further Cd precipitation after the Te injection, suggests the occurrence of a Cd 0 M Cd 2+ equilibrium, which regu- lates the free Cd monomer concentration in the solution, meanwhile permitting a slower and controlled growth of spherical CdTe NCs, size focusing, excellent crystallinity and improved surface properties. The plots in Fig. 2A also show that the component of Cd monomers that form the precipitate of Cd particles does not inﬂuence the growth mechanism of CdTe NCs by diﬀerent initial Cd:Te molar 1 m B 8nm C 1.6 2.0 2.4 2.8 3.2 PL Intensity (a.u.) Energy (eV) 3.1 nm 3. 5 nm 3.8 nm Absorption (a.u.) D 200nm A Fig. 1. HR-SEM image of Cd 0 particles before (A) and after (B) the TOP:Te precursor injection. HR-TEM image of CdTe NCs (C). Absorption (dashed line) and PL (solid line) spectra of CdTe NCs with average diameters between 3.1 and 3.8 nm, all recorded at room temperature (D). 2668 J. Kolny-Olesiak et al. / Surface Science 601 (2007) 2667–2670

Surface Science 601 (2007) 2667–2670 www.elsevier.com/locate/susc Synthesis and characterization of brightly photoluminescent CdTe nanocrystals J. Kolny-Olesiak b, V. Kloper a, R. Osovsky a, A. Sashchiuk a, E. Lifshitz b a,* a Department of Chemistry and Solid State Institute, Technion, Haifa, Israel Energy and Semiconductor Research Laboratory, Department of Physics, University of Oldenburg, Germany Available online 4 January 2007 Abstract A new synthesis procedure for the preparation of spherical shaped CdTe nanocrystals (NCs) is presented, exhibiting bright luminescence with exceptionally high quantum eﬃciency (up to 85%). The growth of these NCs occurs in a non-coordinating solvent, octadecene, with the addition of oleic acid/tri-octylphosphine stabilizers, CdO as a precursor for the Cd monomers and additional Cd metal particles as a supplementary Cd reservoir source. The dependence of the crystalline quality and the optical properties of the CdTe NCs, on the initial Cd:Te precursors’ molar ratio, and the reaction duration were investigated. It was demonstrated that the NCs’ properties improved signiﬁcantly as the initial Cd:Te molar ratios are increased. The obtained NCs’ properties were correlated with measurements of the Cd0 concentration in Cd metal particles, CdTe NCs and in Cd monomer solutions. 2006 Elsevier B.V. All rights reserved. Keywords: Colloidal; CdTe; Nanocrystals; PL; Quantum eﬃciency 1. Introduction In the last two decades a considerable progress has been made in the fundamental [1] and applied physical research of semiconductor nanocrystals (NCs) [2,3]. Colloidal CdTe NCs, in particular, show an increasing interest due to their large Bohr radius (7.3 nm) and the relatively small band gap (1.475 eV), leading to a pronounced quantum size effect in a range of CdTe NCs with a diameter up to 10 nm. In addition, the absorption and emission spectra of these NCs can reach the near-IR spectral regime. The synthesis of high-quality NCs requires speciﬁc conditions of monomer reactivity and stabilizing ligands in order to control the monodispersity and optical properties of the growing NCs [4]. In this paper we present a new synthesis procedure, using two sequential stages for the preparation of highquality CdTe NCs with photoluminescence (PL) quantum * Corresponding author. Tel.: +972 48293987. E-mail address: ssefrat@tx.technion.ac.il (E. Lifshitz). 0039-6028/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2006.12.013 eﬃciency (QE) up to 85%. Using crystalline Cd particles, formed in situ in the ﬁrst stage as an additional source for Cd monomers, the growth of CdTe NCs occurred in a non-coordinating solvent, octadecene (ODE), during the second stage, with OA/TOP stabilizers. 2. Experimental A Te precursor solution was prepared by dissolving Te (0.0128 g) in tri octylphosphine (TOP, 0.2112 g), and octadecene (ODE) to a total amount of 2 mL. The Cd precursor solution was prepared by mixing CdO (0.0256 g) and an oleic acid (OA, 200 lL)/ODE(10 mL) mixture. When heated under Ar ﬂow at 300 C all CdO was dissolved in the OA/ODE mixture. Further heating to 310 C, led to a formation of a grey precipitate, which was characterized as crystalline Cd particles, and considered as the ﬁrst stage of the reaction. The second stage of the reaction included an instantaneous injection of the Te precursor solution into the Cd solution at 310 C. The morphology and size of the Cd precipitate and CdTe NCs were determined by high 2668 J. Kolny-Olesiak et al. / Surface Science 601 (2007) 2667–2670 resolution scanning electron microscopy (HR-SEM) and transmission electron microscopy (HR-TEM). The optical properties of CdTe NCs were studied using absorption and photoluminescence measurements. The amount of Cd atoms in each fraction was determined by an atomic absorption spectroscopy (AAS). The PL QE was measured by comparing the PL intensity with that of a commercial Rhodamine 6G with a known QE of 95%. All the measurements were carried out at room temperature. 3. Results and discussion Fig. 1A presents a HR-SEM image that reveals that the grey precipitate, formed during the ﬁrst stage of the reaction, consists of Cd particles with average size of 100 nm and with well pronounced facets. However, after a Te injection, the facets of the Cd particles disappear (Fig. 1B), suggesting the occurrence of a chemical reaction and a return of Cd monomers to the reaction solution. Fig. 1C shows a HR-TEM image of CdTe NCs with a diameter of 4.0 nm, indicating the formation of spherical NCs with a 5% size distribution with a cubic zinc blende structure (not shown). Fig. 1D shows the absorption spectra (dashed line) and the PL spectra (solid line) of various aliquots of the CdTe NCs. The lower exciton energy in the absorption spectra varies between 2.03 eV and 2.32 eV, corresponding to NCs’ sizes between 3.1 nm and 3.8 nm. These 1S-exciton bands have a full width at half maximum (FWHM) of 100 meV. The PL spectra of the 3.1 nm– 3.8 nm samples are Stokes shifted with respect to the corresponding 1S exciton absorption band by 75 meV and 36 meV respectively. In order to ﬁnd the optimal conditions for the fabrication of high quality CdTe NCs, Cd atom concentrations and absorption data versus reaction time were investigated as a function of initial Cd:Te molar ratios. Fig. 2A shows the percentage of Cd atoms that were lost from the Cd precursor solution to form the Cd particles. The half bold triangles (Fig. 2A) represent the percentage of Cd prior to an injection of the TOP:Te solution. The bold and open triangles represent the percentage of Cd atoms after an injection of TOP:Te, when the initial molar ratios are of Cd:Te = 1.25:1 and Cd:Te = 1:1.25, respectively. Fig. 2A shows that the percentage of Cd atoms in Cd precipitate increases gradually as long as the TOP:Te is avoided. However, after the Te injection, the growth of the Cd precipitate is stopped. The suppression of further Cd precipitation after the Te injection, suggests the occurrence of a Cd0 M Cd2+ equilibrium, which regulates the free Cd monomer concentration in the solution, meanwhile permitting a slower and controlled growth of spherical CdTe NCs, size focusing, excellent crystallinity and improved surface properties. The plots in Fig. 2A also show that the component of Cd monomers that form the precipitate of Cd particles does not inﬂuence the growth mechanism of CdTe NCs by diﬀerent initial Cd:Te molar B A 1 m D C PL Intensity (a.u.) 3.8 nm 8nm Absorption (a.u.) 2 0 0 nm 3. 5 nm 3.1 nm 1.6 2.0 2.4 2.8 3.2 Energy (eV) Fig. 1. HR-SEM image of Cd0 particles before (A) and after (B) the TOP:Te precursor injection. HR-TEM image of CdTe NCs (C). Absorption (dashed line) and PL (solid line) spectra of CdTe NCs with average diameters between 3.1 and 3.8 nm, all recorded at room temperature (D). 2669 J. Kolny-Olesiak et al. / Surface Science 601 (2007) 2667–2670 A 1:1.25 100 B 90 90 80 80 70 70 20 20 10 10 1.25:1 C 20 0 [Cd /CdO] % 30 100 10 0 0 0 50 100 150 200 Time (sec) 0 0 20 40 60 80 100 Time (sec) 0 50 100 150 200 Time (sec) Fig. 2. Plots of (Cd0/CdO) · 100 versus the reaction time: (A) in Cd0 precipitate before ( ) and after (m,n) the Te injection. The (m,n) designate two synthesis with diﬀerent Cd:Te ratios, as indicated in the text; (B) in CdTe NCs (s) and in monomers (h) for a synthesis with initial Cd:Te molar ratio of 1:1.25; and (C) in CdTe NCs (d) and in monomers (j) for a synthesis with initial Cd:Te molar ratio of 1.25:1; The arrows designate the time of Te precursor injection. ratios. Fig. 2B and C show Cd atom concentration of monomers (squares) and those that formed the CdTe NCs (circles) for two diﬀerent initial Cd:Te molar ratios. Fig. 2B and C show Cd atom concentration for samples with an initial Cd:Te molar ratio of 1:1.25 (Fig. 2B) and 1.25:1 (Fig. 2C). The plots in Fig. 2B and C show that in case of Cd excess only 75% of Cd monomers form the CdTe NCs NCs diameter (nm) A (Fig. 2C, bold circles), while in Te excess 90% of Cd monomers react to form the CdTe NCs (open circles in Fig. 2B). Fig. 3 represents plots of calculated [5] average diameter (3A), concentration (3B), absorption FWHM (3C) and PL QE (3D) of various CdTe NCs, versus the reaction time. The aliquots were taken from reactions with diﬀerent initial Cd:Te molar ratios as indexed in the inset of Fig. 3A. It should be noted that the preliminary CdO concentration was always constant. Fig. 3A shows that in case of excess Cd (bold symbols), the NCs grow to a certain size and then stop growing while their concentration drops (Fig. 3B). Fig. 3C shows also a constant size distribution broadening indicated by the FWHMs of the absorption peaks. This size distribution broadening can indicate that the CdTe NCs growth can be characterized by the Oswald ripening stage in which the larger particles continue to grow and the smaller ones get smaller and ﬁnally dissolve. There is a shortage of Te: due to the CdTe NCs growth, the Te is ﬁnally depleted. Therefore, as shown in Fig. 2C, only 75% of Cd monomers are used to form the CdTe NCs, and 20% of monomers stay unrelated in solution. In the case of excess Te (open symbols in Fig. 3A) fast NCs growth is observed, while their concentration is maintained constantly over time (Fig. 3B), and the NCs’ size distribution is focused (Fig. 3C). These NCs’ growth properties are correlated with results shown in Fig. 2B and suggest that about 90% of Cd monomers react to form the CdTe NCs while only 10% of monomers stay in solution. Even though the NCs B 3. 8 3. 6 Cd :Te 3. 4 2: 1 1. 5: 1 1. 25 :1 1: 1 1: 1.25 1: 1.5 1: 2 3. 2 3. 0 0 50 100 150 NCs concentration (a.u.) 40 0 200 50 Time (sec) 40 D 100 38 80 PL QE% Absorption FWHM (nm) C 36 34 32 28 0 100 150 Time (sec) 200 40 20 50 150 60 30 0 100 Time (sec) 200 0 50 1 00 150 200 Time (sec) Fig. 3. CdTe NCs diameter (A); the concentration of each aliquot (B); the FWHM of the NCs 1st absorption band (C); and PL QE (D) of CdTe NCs, prepared with various initial Cd:Te molar ratios as indicated in the inset of (A). 2670 J. Kolny-Olesiak et al. / Surface Science 601 (2007) 2667–2670 produced with an excess Te reaction have better size distribution and conservation of NCs concentration during the reaction time, the NCs produced in conditions with an excess of Cd grow more slowly, improving their surface passivation, which leads to improved optical properties (PL QE) as can be seen in Fig. 3D. The plots in Fig. 3D reveal that as the initial Cd:Te molar ratios are higher, the QE improved signiﬁcantly and reaches 85%. 4. Conclusions This paper presented the occurrence of in situ precipitation of Cd particles prior to the formation of CdTe NCs and the inﬂuence of this precipitation on the NCs growth. The experimental evidences shown above suggest that the existence of Cd0 precipitates induces a Cd0 M Cd2+ equilibrium with the reaction solution, leading to a regulation of the Cd supply during the NCs growth, revealing the formation of a uniform spherical shape CdTe NCs. The spectroscopic evidence; narrow absorption, emission lines, and the high emission QE revealed the formation of high quality samples resulting from the initial excess of Cd. Acknowledgments This project was supported by the German Israel Foundation (GIF) contact no. #156/03-12.6 and by the German-Israel Program (DIP) project no. #D 3.2 and Niederzaksen foundation project. Dr. A. Sashchiuk expresses her deep gratitude to the Ministry of Absorption of the State of Israel for the Giladi Fellowship. References [1] C. Burda, X. Chen, R. Narayanan, M.A. El-Sayed, Chem. Rev. 105 (2005) 1025. [2] A.J. Nozik, Physica E 14 (2002) 115. [3] A. Shavel, N. Gaponik, A. Eychmuller, Chem. Phys. Chem. 6 (2005) 449. [4] W.W. Yu, Y.A. Wang, X. Peng, Chem. Mater. 15 (2003) 4300. [5] W.W. Yu, L. Qu, W. Guo, X. Peng, Chem. Mater. 15 (2003) 2854.

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Synthesis and characterization of brightly photoluminescent CdTe nanocrystals