J. Phys. Chem. B 2006, 110, 23175-23178
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Synthesis and Spectroscopic Investigations of Cu- and Pb-Doped Colloidal ZnS Nanocrystals Oliver Ehlert,† Andres Osvet,*,‡ Miroslaw Batentschuk,‡ Albrecht Winnacker,‡ and Thomas Nann*,†,§ Freiburg Materials Research Centre, Albert-Ludwig UniVersity Freiburg, Stefan-Meier-Straβe 21, 79104 Freiburg i. Br., Germany, Institute For Materials Science 6, UniVersity of Erlangen-Nuremberg, Martensstrasse 7, 91058 Erlangen, Germany, and School of Chemical Sciences and Pharmacy, UniVersity of East Anglia, Norwich NR4 7TJ, United Kingdom ReceiVed: July 19, 2006; In Final Form: September 19, 2006
A novel organometallic synthesis method for the preparation of colloidal ZnS nanoparticles is presented. This method enables the synthesis of undoped ZnS nanocrystals as well as doping with Cu, Pb, or both. The particles can be covered with an undoped layer of ZnS, forming core/shell-type particles with the ZnS:Pb, ZnS:Cu, or ZnS:Cu,Pb cores. The particles were characterized via TEM, XRD, dynamic light scattering, and optical spectroscopy. We investigated the extrinsic surface defects and their coverage with an additional ZnS layer in detail by temperature-dependent luminescence and luminescence lifetime spectroscopy.
I. Introduction Semiconductor nanocrystals such as CdSe/ZnS core/shell, CdSe/ZnS/SiO2 core/shell/shell,1-3 or InP4,5 nanocrystals have received major interest in the scientific community because of their unique and superior properties, such as weak photobleaching or size-dependent emission wavelengths. Potential fields of application are biological imaging,6-10 light converters for lightemitting diodes,11,12 and others. ZnS colloidal nanocrystals have also been reported, but usually the morphology-related issues of undoped particles are reported. The doped ZnS nanocrystals (NC) have been synthesized by the precipitation method using sulfide ions and different stabilizers in water or other suitable solvents (for a review of this big scientific field, see ref 13 and references therein). These nanoparticles are mostly yielded as nanocrystalline powders and are not redispersible in any solvents without agglomeration. Because of the possibility of doping the ZnS lattice with transition metal and rare-earth ions, a new field of nanoparticle research would be opened if there was a synthesis method, which allows the doping of colloidal ZnS during the nanoparticle synthesis without precipitation of the doping metals’ sulfide. In the following, we present a new and facile synthetic method for the synthesis of colloidal ZnS, ZnS: Pb core, ZnS:Pb/ZnS core/shell, ZnS:Cu,Pb core, and ZnS:Cu,Pb/ZnS core/shell nanocrystals. The structure of the particles was studied by optical absorption, transmission electron microscopy (TEM), and X-ray diffraction analysis (XRD). Dopingrelated properties were studied by temperature-dependent steadystate photoluminescence and PL lifetime spectroscopy. II. Experimental Section A. Synthesis of Doped and Undoped ZnS Nanocrystals. All chemicals were of the highest purity grade available and * Corresponding author. E-mail:
[email protected] (T.N.);
[email protected] (A.O.). † Albert-Ludwig University Freiburg. ‡ University of Erlangen-Nuremberg. § University of East Anglia.
used as received without further purification. The preparations were made under standard Schlenk techniques. This synthesis procedure is based on a recently published procedure,14 apart from that we do not use an injection method but a homogeneously diluted solution that is heated to the desired reaction temperature. In a typical experiment, 300 µmol of Zn-acetate (99.99%, Aldrich) was dissolved with 600 µmol of oleic acid (Aldrich, 99%) in 20 mL of trioctylamine (TOA, Fluka 99%) at 80 °C and degassed. Afterward, 1.6 mmol of 1-hexadecanethiol (Merck) was added, and the flask was heated to 300 °C under vigorous stirring. After 150 min, the temperature was rapidly lowered to room temperature by means of a flow of compressed air (within 2-3 min). The nanoparticles were isolated by addition of excess acetone (99.9%, CHROMASOLV Plus for HPLC, Aldrich), centrifuged, and washed several times with acetone. The as-prepared nanocrystals can easily be redispersed in heptane or chloroform. For the doping procedures, 1.5 µmol of copper-acetate (Aldrich) and/or 1.5 µmol of leadstearate (Strem-Chemicals) were added and dissolved before the addition of the thiol. B. Synthesis of Shelled Nanocrystals. The shelling procedure was done with slight modifications of well-known literature methods. The nonshelled nanocrystals were isolated by evaporation of the solvent and redispersed and degassed in 4 g of trioctylphosphineoxide (TOPO, Aldrich 99%) and 2.5 mL of trioctylphosphine (TOP, Strem Chemicals 97%) at 80 °C. The temperature was then raised to 130 °C, and 300 µmol of diethylzinc (Aldrich) and 300 µmol of 1,1,1-3,3,3-hexamethyldisilathiane (Aldrich) in 2 mL of TOP were added dropwise within 10 min. The temperature was decreased to 90 °C, and the flask was stirred overnight. Afterward, 5 mL of 1-butanol was added, and the particles were allowed to stir for 4 more hours at 60 °C. They were isolated by addition of anhydrous methanol and centrifugation. C. Characterization. A 120 kV TEM (Zeiss LEO 912) was used for the average particle size determination of the NCs. A 300 kV high-resolution TEM setup (Philips CM 300 UT) with a resolution of 0.172 nm was used for imaging and determination
10.1021/jp064570i CCC: $33.50 © 2006 American Chemical Society Published on Web 10/26/2006
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Figure 1. Scheme of the synthesis method mentioned in this paper. The doping procedure takes place directly during the nucleation of the nanoparticles. The shelling procedure saturates the surface-bound doping-atoms with additional ZnS.
Figure 3. The X-ray diffraction pattern of the core-type ZnS:Cu,Pb nanoparticles. The red lines indicate the positions of the reflexes from the cubic phase of ZnS (JCPDS 5-566).
Figure 2. High-resolution TEM image of the core-type ZnS:Cu,Pb nanocrystals. The average particle diameter was estimated to be 5 nm by means of TEM and 4.7 nm from the Scherrers equation.
of the lattice layers of the nanoparticles. The content of doping ions was determined with an ion-coupled plasma device (Spectro Analytical Instruments). Absorbance measurements were performed on a TIDAS diode array spectrometer. Fluorescence spectra were excited and measured with an FL 3095 monochromator (both J&M, Aalen, Germany). The low-temperature spectra were analyzed with an HRS-2 monochromator with a dispersion of 1.2 nm/mm and recorded with a bialkali-cathode photomultiplier. The spectra are corrected for the system response. Decay time of the photoluminescence was measured under the excitation of a 308 nm XeCl excimer laser with pulse duration of 30 ns, and the decay curves were recorded with the photomultiplier and a digital real-time oscilloscope. For the dynamic light scattering measurements, a Malvern Zetasizer Nano-ZS was used working at 633 nm wavelength. Care was taken for choosing the right parameters, such as the index of refraction of ZnS at this wavelength. III. Results and Discussion Figure 1 shows the scheme of our synthetic procedure as mentioned in the Experimental Section. In Figure 2a, the HRTEM micrograph of the as-prepared ZnS:Cu,Pb codoped core nanocrystals is shown. A particle size of 5 nm with a narrow size distribution of about 6-7% was found. The morphology is dot-like as it was expected by the use of TOA as stabilizer.15 The well-resolved lattice layers have a separation of 0.31 nm, which corresponds to the separation of the 〈111〉 layers of the zinc blende ZnS structure. The cubic structure is also confirmed by the X-ray diffraction pattern (Figure 3) of a layer of dried nanoparticles, which show the (111), (220), and (311) indices, respectively. The Scherrers equation was used to estimate the particle size from the width of the X-ray peaks, and a diameter of 4.7 nm was obtained. Energy-dispersive X-ray analysis showed traces of Pb and Cu in ZnS, but overlap with the lines of Zn and S and the small relative amount of dopants did not enable determination of the contents. For quantitative analysis, the inductively coupled plasma (ICP) method was applied. The NCs
Figure 4. Absorbance spectra of the as-prepared ZnS nanocrystals as a function of the reaction time at 300 °C as mentioned in the Experimental Section.
were dissolved to bare ions by means of nitric acid. The spectroscopic analysis of the plasma emission yielded the content of Cu and Pb of approximately 0.1 wt %. The size and growth of the nanocrystals over time was monitored by the means of absorbance spectroscopy (Figure 4). What can be observed is, that from a time of 150 min on, the absorption edge does not shift any more. As the excitonic radius of ZnS is exceeded, the possible size evolution cannot be monitored further via absorption spectroscopy. As an alternative method, dynamic light scattering (DLS) was used to check the particle size at longer synthesis times (Figure 5). As it can be seen in Figure 5, the hydrodynamic radius of the nanocrystals is not increasing further from a time of 150 min on. This result agrees with the absorption spectra. In the beginning of the reaction, the difference in hydrodynamic radius and radius detected by absorbance spectroscopy is about 0.7 nm, and in the end it is 1.4 nm, which is about the chain length of oleic acid. Yet because of the fact that the particle diameter is measured by DLS at the periphery of the capped NCs, we postulate that this 1.4 nm difference in diameter is originated by one molecule TOA on each side of the particle. The assumption that this discrepancy stems from oleic acid would give a doubled absolute deviation measured by DLS. The particle size distribution of 6-7% as mentioned above does not affect these measurements in a significant way. Figure 6 shows the photoluminescence spectra of undoped and Cu-doped nanoparticles. The blue luminescence of undoped particles (Figure 6, curve a) is interpreted as self-activated
Cu- and Pb-Doped Colloidal ZnS Nanocrystals
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Figure 7. Normalized room-temperature PL-spectra of the as-prepared ZnS:Pb core (a) and ZnS:Pb/ZnS (b) core/shell nanocrystals. Excitation wavelength 351 nm, T ) 300 K. Figure 5. The hydrodynamic radius detected by means of dynamic light scattering and the particle radius calculated from the absorbance spectra19 in Figure 4 of the as-prepared ZnS nanoparticles as a function of the reaction time. The dashed and solid lines represent simple sigmoid fits to the data points. It is obvious that, from a reaction time of 150 min on, no further detectable growth by dynamic light scattering can be observed. This cannot be monitored by absorbance spectra because of the small excitonic radius of ZnS of 2.3 nm (see also Figure 4).
Figure 8. PL-spectra of bare ZnS, ZnS:Cu,Pb core, and ZnS:Cu,Pb/ ZnS core/shell nanocrystals. As it can be seen, the pure particles only show the intense interstitial sulfur site-related blue emission around 420 nm.16 The doped particles have a broad green luminescence around 520 nm, and the shelled ones show a significant lowering of the red tail of the spectrum. This indicates the surface passivation of the doping atoms on the surface with ZnS. Excitation at 312 nm. Figure 6. Photoluminescence spectra of undoped (a) and Cu-doped (b) core-type ZnS nanoparticles. Excitation wavelength 351 nm, temperature 300 K.
luminescence related to an interstitial sulfur ion16 because of the excess thiol used in our synthesis. The green emission with maximum at 500 nm, typical for Cu-doped ZnS, originates from the donor-acceptor (D-A) transitions from donor centers (e.g., interstitial S2-) to the ionized, randomly distributed, substitutional Cu centers (Cu2+). Because of the relatively large particle diameter of 5 nm, which is bigger than the exciton radius of ZnS of about 2.3 nm,17 no quantum confinement can be observed, and the spectrum is similar to the green luminescence of Cu in macroscopic ZnS powders (the G-Cu luminescence, data not shown). The width of the emission band, however, is larger by a factor of 1.4 in nanoparticles. In Pb-doped ZnS nanoparticles, the Pb-related luminescence dominates (Figure 7) over the self-activated luminescence. Two bands can be recognized, the green band with a maximum at 550 nm, and the red band peaking at 725 nm. The bands belong apparently to two different Pb2+-related luminescent centers. Bol and Meijerink18 interpret the red Pb-related emission in precipitation-synthesized ZnS as intraionic or charge-transfer luminescence of Pb2+ on regular Zn2+ sites and the green emission as belonging to Pb2+ on Zn2+ sites close to a defect. The effect of the undoped ZnS coating on Pb-doped ZnS nanoparticles can be seen in Figure 7 as well. The ratio of emission intensity related to regular and disturbed Pb2+ centers is in favor of the regular sites in particles covered with the
undoped shell, which avoids the location of Pb2+ ions near the surface of the particles. The different ratio of the red and green emission bands is evidence of the successful shelling procedure and can be observed with the naked eye: the emission of uncovered particles looks green, while the emission of particles covered with the shell looks white under UV excitation. In Figure 8, the spectra of pure ZnS nanoparticles (straight line), which show a characteristic blue luminescence,16 are shown together with the spectra of double-doped ZnS:Cu,Pb nanoparticles, covered (dashed line) or not covered with an undoped shell (dotted line). The emission of the doped particles is dominated by the green emission of Cu2+ luminescence centers with the maximum at 500 nm. The blue wing of the emission shows also the self-activated luminescence of ZnS; in the red part of the spectrum an additional band at about 700 nm is observed. The latter may be due to the Pb2+ centers, or strongly disturbed Cu2+ centers. The covering of the particles with a shell of undoped ZnS decreases the relative intensity of the red emission, thus speaking in favor of the red band being related to the disturbed Cu2+ centers. The temperature dependence of the integrated luminescence intensity of the green luminescence of Cu in ZnS:Cu,Pb particles is shown in Figure 9. The quenching starts at ca. 100 K, and the room-temperature intensity is ca. 20% of its low-temperature value. Estimating the activation energy for quenching by fitting the data with I(T) ) C/(1 + a*exp(-E/kT)) gives a value of E ) 0.05 eV. We interpret this temperature dependence as the result of thermal releasing of the trapped electrons from the
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Ehlert et al. with the bimolecular law, expected from the D-A nature of the emission. At low temperatures, however, the emission at longer times is stronger. This may be related to the release of electrons from very shallow traps, which are emptied at room temperature. The further study of the luminescence decay of the doped ZnS nanocrystals is in progress. IV. Conclusions
Figure 9. Temperature dependence of the time-integrated PL intensity at 500 nm of the ZnS:Cu,Pb core nanocrystals. Pulsed excitation at 308 nm.
A new and facile synthetic method for the preparation of Cu and/or Pb (co-)doped ZnS nanocrystals without injection of precursors was developed. The nanoparticles have the cubic structure and a particle size of about 5 nm with a narrow size distribution. The doping ions occupy regular lattice sites as confirmed by photoluminescence spectroscopy. The particles show blue (nondoped ZnS), green (ZnS:Pb core, ZnS:Cu,Pb core, and ZnS:Cu,Pb/ZnS core/shell), or white (ZnS:Pb/ZnS core/shell) luminescence. Covering of the particles with a layer of undoped ZnS reduces the effect of surface defects and, related to them, the quenching of luminescence. In Pb-doped ZnS particles, the fraction of Pb2+ on regular sites increases after the shelling procedure. This homogeneous synthetic route can be further exploited for the synthesis of other doped, sulfidebased semiconductor materials. Acknowledgment. We thank the Deutsche Forschungsgemeinschaft DFG (grant nos. NA 373/4-1 and WI 393) for financial support.
Figure 10. Photoluminescence decay curves of Cu- and Pb-doped ZnS nanoparticles with (curves 2 and 4) and without (curves 1 and 3) an undoped ZnS shell. T ) 20 K (curves 3 and 4) and 300 K (curves 1 and 2). Excitation at 308 nm, detection at 500 nm.
donor levels. The energy E in the above equation is the energetic separation of the donor level and the conduction band. The photoluminescence decay of the green Cu emission after short-pulse (30 ns) excitation at 308 nm is strongly nonexponential and depends on the detection wavelength: the lower is the photon energy, the longer is the emission lifetime. Such a behavior is in accordance with the D-A nature of the Cu2+related luminescence. The decay curves for ZnS:Cu,Pb samples at room temperature (curves 1 and 2) and 20 K (curves 3 and 4) are shown in Figure 10. In addition to the thermal quenching, the effect of the undoped shell can also be recognized from Figure 10. At both temperatures, the decay of uncovered (coretype) samples is faster than the decay of particles covered with shell, which eliminates the effect of surface defects or states that provide nonradiative decay channels competing with the regular luminescence mechanism. The room-temperature time required for the emission to reach the value of 1/e is 0.35 µs for the particles with shell and 0.27 µs for the uncovered particles. At room temperature, the curves can be well fitted
References and Notes (1) Darbandi, M.; Thomann, R.; Nann, T. Chem. Mater. 2005, 17, 5720. (2) Gerion, D.; Pinaud, F.; Williams, S.; Parak, W.; Zanchet, D.; Weiss, S.; Alivisatos, A. J. Phys. Chem. B 2001, 105, 8861. (3) Nann, T.; Mulvaney, P. Angew. Chem., Int. Ed. 2004, 43, 5393. (4) Kumar, S.; Thomann, R.; Nann, T. J. Mater. Res. 2006, 21, 543. (5) Xu, S.; Kumar, S.; Nann, T. J. Am. Chem. Soc. 2006, 128, 1054. (6) Riegler, J.; Nick, P.; Kielmann, U.; Nann, T. J. Nanosci. Nanotechnol. 2003, 3, 380. (7) Riegler, J.; Nann, T. Anal. Bioanal. Chem. 2004, 379, 913. (8) Riegler, J.; Ehlert, O.; Nann, T. Anal. Bioanal. Chem. 2006, 384, 645. (9) Jaiswal, J. K.; Goldman, E. R.; Mattoussi, H.; Simon, S. M. Nat. Methods 2004, 1, 73. (10) Gao, X.; Cui, Y.; Levenson, R. M.; Chung, L. W. K.; Nie, S. Nat. Biotechnol. 2004, 24, 969. (11) Dabbousi, B. O.; Bawendi, M. G.; Onitsuka, O.; Rubner, M. F. Appl. Phys. Lett. 1995, 66, 1316. (12) Tessler, N.; Medvedev, V.; Kazes, M.; Kann, S.; Banin, U. Science 2002, 295, 1506. (13) Hu, H.; Zhang, W. Opt. Mater. 2005, 28, 536. (14) Jun, S.; Jang, E. Chem. Commun. 2005, 4616. (15) Li, Y.; Li, X.; Yang, C.; Li, Y. J. Phys. Chem. B 2004, 108, 16002. (16) Denzler, D.; Olschewski, M.; Sattler, K. J. Appl. Phys. 1998, 84, 2841. (17) Yoffe, A. D. AdV. Phys. 2002, 51, 799. (18) Bol, A. A.; Meijerink, A. Phys. Chem. Chem. Phys. 2001, 11, 2105. (19) Tiemann, M.; Weiss, O.; Hartikainen, J.; Marlow, F.; Linden, M. ChemPhysChem 2005, 6, 2113.
