Aqueous Synthesis of Thiol-Capped CdTe Nanocrystals: State-of

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J. Phys. Chem. C 2007, 111, 14628-14637

FEATURE ARTICLE Aqueous Synthesis of Thiol-Capped CdTe Nanocrystals: State-of-the-Art Andrey L. Rogach,* Thomas Franzl, Thomas A. Klar, Jochen Feldmann, Nikolai Gaponik,* Vladimir Lesnyak, Alexey Shavel, Alexander Eychmu1 ller, Yuri P. Rakovich, and John F. Donegan Photonics and Optoelectronics Group, Physics Department and Center for NanoScience (CeNS), Ludwig-Maximilians-UniVersita¨t Mu¨nchen, Amalienstrasse 54, D-80799 Munich, Germany, Physical Chemistry, TU Dresden, Bergstrasse 66b, D-01062 Dresden, Germany, and Semiconductor Photonics Group, Department of Physics, and CRANN Research Institute, Trinity College Dublin, Dublin 2, Ireland ReceiVed: March 29, 2007; In Final Form: June 4, 2007

We report on the state-of-the art synthesis and improved luminescence properties of thiol-capped CdTe nanocrystals (NCs) synthesized in water. The optimized pH (12) and molar ratio of thiol to Cd ions (1.3:1) increases the room-temperature photoluminescence quantum efficiency of as-synthesized CdTe NCs capped by thioglycolic acid (TGA) to values of 40-60%. By employing mercaptopropionic acid (MPA) as a stabilizer, we have synthesized large (up to 6.0 nm in diameter) NCs so that the spectral range of the NCs’ emission currently available within this synthetic route extends from 500 to 800 nm. Sizing curve for thiol-capped CdTe NCs is provided. In contrast to CdTe NCs capped by TGA, MPA-capped CdTe NCs show up to 1 order of magnitude longer (up to 145 ns) emission decay times, which become monoexponential for larger particles. This phenomenon is explained by considering the energetics of the Te-related traps in respect to the valence-band position of CdTe NCs. The correlation between luminescence quantum efficiencies, luminescence lifetimes, and Stokes shifts of CdTe NC fractions is demonstrated, being in agreement with a model proposed previously that connects the emission properties of NCs with their surface quality determined by the Oswald ripening conditions during growth. imaging, and plasmonics.

1. Introduction Direct aqueous synthesis of II-VI semiconductor nanocrystals (NCs) like CdS,1 CdSe,2 CdTe,3 CdHgTe,4 HgTe,5 and ZnSe6 by employing different short-chain thiols as stabilizing agents provides a useful alternative to widely used synthetic routes in highly boiling organic solvents.7-10 The use of thiols allows us to control the kinetics of the NCs synthesis, passivate surface dangling bonds, and provides stability, solubility, and surface functionality of the nanoparticles. CdTe NCs capped by thioglycolic acid (TGA),11,12 1-thioglycerol,3,12 mercaptoethylamine,12 or L-cysteine12 have found several applications in material science and nanotechnology. Among others is the fabrication of polymer-NC13-18 and glass-NC19 light-emitting composites, which are robust and easily processable. Applications in optoelectronics cover the light-emitting diodes (LEDs),13,20,21 microarrays of multicolored light-emitting pixels,22 and photosensitive films.23,24 This is closely connected to photonic applications in which CdTe NCs play a role of tunable lightsources coupled to optical modes of photonic crystals25,26 and heterocrystals,27 spherical microresonators,28,29 photonic molecules,30 and waveguides.31 The ability of CdTe NCs to interact with neighboring nanoparticles or molecules gives rise to the fabrication of FRET-based32,33 and nanoplasmonic34 devices as well as sensors.35-37 The demands in light emitters that are * Corresponding authors. E-mail: [email protected] (ALR); [email protected] (NG).

compatible with water and the most-common biological buffers38 open for thiol-capped CdTe NCs such fields as biolabeling39-42 and bioimaging.43-45 They have been used as building blocks for self-organizing superstructures like luminescent nanowires,46-51 nanotubes,52 or nanosheets,53 for chemiluminescence generation,54 for fabrication of temperature-sensitive nanoassemblies,55 as luminescent components of multifunctional microbeads,56-61 and polymer microcapsules.61-65 Several improvements of the conventional aqueous synthetic method3,12 (referred to as “standard” further on) for thiol-capped CdTe NCs have been reported recently by different groups, including the hydrothermal synthesis,66,67 illumination,68 ultrasonic69 or microwave irradiation70,71 treatment, the use of an inert atmosphere,72 variation of reagent concentrations67,73-75 and pH,74,75 and the use of mercaptopropionic acid (MPA) as the capping agent.76 In this paper, we summarize the recent activities of our groups aiming on further improvements of the aqueous synthesis and the quality and luminescence properties of resulting thiol-capped CdTe NCs. In particular, our results demonstrate in correspondence to the recently published findings67,73 that the use of an optimized molar ratio of Cd ions to thiol stabilizer indeed allows us to increase the room-temperature photoluminescence quantum efficiency (PL QE) of as-synthesized TGA-capped CdTe NCs to values of 40-60%. We furthermore stress several advantages of MPA as the capping agent in the aqueous synthesis of CdTe NCs: An expansion of the range of available particle sizes, the possibility of shifting the emission wavelength

10.1021/jp072463y CCC: $37.00 © 2007 American Chemical Society Published on Web 09/18/2007

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Andrey L. Rogach received his Ph.D. degree in physical chemistry from the Belarusian State University in Minsk in 1995. From 1995 to 1996 he was a postdoctoral fellow at the Institute of Physical Chemistry, University of Hamburg with Prof. H. Weller, where he also worked in 1997-2002 as a visiting scientist, Alexander-von-Humboldt Fellow, and finally as a staff scientist. He was a short-term research fellow at British Telecom Laboratories (Ipswich, UK) in 1998-1999, and visiting faculty at the Oklahoma State University (with Prof. N. Kotov) in 1999-2000. Since 2002, he has been a lead scientist at the Photonics & Optoelectronics group at the Ludwig-MaximiliansUniversita¨t (LMU), Munich. He was holder of the Walton Award of the Science Foundation Ireland at Trinity College Dublin (Ireland) in 2005-2006. His research focuses on different aspects of the synthesis, assembly, spectroscopy, and applications of semiconductor and metal nanocrystals.

Thomas Franzl studied physics at the Ludwig-Maximilians-Universita¨t in Munich, and has been a Ph.D. student in the Photonics and Optoelectronics group at the same university since 2002, working on optical spectroscopy of semiconductor nanocrystals. Thomas A. Klar received a diploma in physics from the LudwigMaximilians-Universita¨t in Munich in 1997 and a Dr. rer. nat. from the Max-Planck-Institute for Biophysical Chemistry in Go¨ttingen in 2001. He currently holds an “Assistent” position at the Photonics and Optoelectronics Group at LMU Munich. Dr. Klar is a member of the German Physical Society (DPG), the Materials Research Society (MRS), and the European Optical Society (EOS). In 2001, he received the Helmholtz award together with S. Hell for their work on sub-Abbe resolution in far field microscopy. His current research interests include spectroscopy of hybrid systems containing solid-state nanoparticles. Jochen Feldmann received his Ph.D. degree from the University of Marburg in 1990. He was a postdoc at AT&T Bell Laboratories in 1990-1991 and returned to the University of Marburg where he completed his habilitation in 1994. Since 1995, he has led the Photonics and Optoelectronics group at the Ludwig-Maximilians-Universita¨t in Munich. His scientific contributions have been recognized by numerous distinctions: the Gerhard Hess Prize of the Deutsche Forschungsgemeinschaft (DFG) in 1994, the Walter Schottky Prize of the Deutsche Physikalische Gesellschaft (1995), and the Leibniz Prize of the DFG (2001). His current research interests are focused on optical spectroscopy of semiconductor nanocrystals, conjugated polymers and their hybrid systems, as well as on bio-sensing applications of metal nanoparticles.

further to the infrared, and an extended exciton decay, which becomes purely monoexponential for the largest NCs. The last phenomenon is explained by considering the energetics of the traps associated with surface Te atoms with respect to the valence-band position of the CdTe NCs. Both the emission

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Nikolai Gaponik received his Ph.D. in chemistry (2000) from the Belarusian State University studying electrochemical and photoelectrochemical properties of composite materials based on conducting polymers and semiconductor nanocrystals. He was a visiting scientist at the Ludwig-Maximilians-Universita¨t in Munich with Dr. M. Gao and Prof. J. Feldmann in 2000. In October 2000, he has joined the group of Prof. H. Weller at the University of Hamburg first as a DAADfellow and finally as a research scientist. Since 2005, he has been a senior scientist at the TU Dresden. His current research focuses on the synthesis, assembly, and applications of semiconductor nanocrystals. Vladimir Lesnyak received his Ph.D. in chemistry (2005) from the Belarussian State University. Since November 2006, he has joined the group of Prof. Alexander Eychmu¨ller at the TU Dresden as a postdoctoral fellow. Alexey Shavel studied chemistry at the Belarusian State University and worked in the National Academy of Sciences of Belarus as a research scientist from 1996 until 2002. In 2005, he received his Ph.D. under the guidance of Prof. Alexander Eychmu¨ller at the University of Hamburg and joined the group at the TU Dresden as a postdoctoral fellow. Since 2006, he has worked in the group of Prof. Luis LizMarzan at the University of Vigo. Alexander Eychmu1 ller studied physics at the University of Go¨ttingen. In 1987, he received his Ph.D. working on proton-transfer reactions under the supervision of Dr. K.-H. Grellmann and Prof. A. Weller at the Max-Planck-Institute for Biophysical Chemistry. After a postdoctoral year at UCLA working with Prof. M. A. El-Sayed on gas-phase metal clusters, he joined the group of Prof. A. Henglein at the HahnMeitner-Institute in Berlin where he became involved in research on semiconductor quantum dots. In 1994, he moved with Prof. H. Weller to the University of Hamburg focusing his scientific interests on the photophysical and structural properties of semiconductor nanocrystals. The habilitation in 1999 and the venia legendi in 2000 were followed by an offer of a chair in Physical Chemistry and Electrochemistry at TU Dresden in 2005. Yury Rakovich received his diploma in physics from the Belarusian State University and his Ph.D. in physics from the National Academy of Sciences of Belarus in 1995. He worked as a lecturer in Physics at the Brest State Technical University until 1997 and moved to the University of Minho (Portugal) in 1998. He joined the School of Physics in Trinity College Dublin in 2001, where he works now as a senior scientist in the Semiconductor Photonics Group with Prof. J.F. Donegan. His current research focuses on optics of semiconductor nanocrystals, three-dimensional microcavities, and photonic molecules, FLIM.

intensity and emission lifetime of the NC fractions correlate well with a model proposed previously,12,77 which connects the emission properties of nanoparticles with their surface quality determined by the Oswald ripening conditions.78

14630 J. Phys. Chem. C, Vol. 111, No. 40, 2007 John F. Donegan received B.Sc. and Ph.D. degrees from the National University of Ireland, Galway. He had postdoctoral periods at Lehigh University and the Max Planck Institute for Solid State Research, Stuttgart. He was appointed to the academic staff in Trinity College Dublin in 1993. He leads the Semiconductor Photonics Group and is Director of Research in the School of Physics. He is also a principal investigator at the CRANN research institute in Trinity College Dublin. His research is in the area of Photonics, in particular the interaction of light with photonic structures: microspheres and photonic molecules coupled with nanocrystal emission, tunable laser structures based on slotted lasers, and two-photon absorption microcavity structures.

2. Experimental Section All chemicals used were of analytical grade or of the highest purity available. All solutions were prepared using Milli-Q water (Millipore) as a solvent. Al2Te3 lumps (Cerac Inc) or tellurium electrodes were used as a source for generation of H2Te gas. TGA or MPA (both from Fluka) were used as capping agents. UV-vis absorption spectra were recorded with a Cary 50 spectrophotometer (Varian). Integrated PL measurements were performed at room temperature using a FluoroLog-2 spectrofluorimeter (Instruments SA). The PL QE of CdTe NCs was estimated according to the procedure of ref 79 by comparison with Rhodamine 6G (laser grade, Lambda Physik) in ethanol (Uvasol, Merck) assuming its PL QE as 95%. Both integrated and time-resolved PL measurements were performed in ambient conditions at room temperature on diluted aqueous colloidal solutions of CdTe NCs having an optical density below 0.2 at the excitation wavelength of 400 nm. The frequency-doubled output (400 nm) of a Kerr-lens mode-locked titanium-sapphire (TiSa) laser (120 fs, 76 MHz) was used as the optical excitation source in time-resolved PL measurements. To derive decays over hundreds of nanoseconds, we used an APE pulse picker to reduce the repetition rate of the TiSa laser down to 3.8 MHz. The excitation power in both cases was far below the multiple excitation regime. The emission was dispersed both temporally and spectrally by using a streak camera with a maximal temporal resolution of 2 ps (C 5680, Hamamatsu) in combination with a Cromex spectrometer. Appropriate spectral filters were used to block any scattered light from the excitation beam. Microchannel plate measurements were performed using a Becker & Hickl Spc300 module with a time resolution of 100 ps. Alternatively, time-resolved PL spectra were measured using time-correlated single-photon counting (Time-Harp, PicoQuant). The samples were excited by 480-nm picosecond pulses generated by a PicoQuant, LDH-480 laser head controlled by a PDL-800B driver. The setup was operated with an overall time resolution of ∼150 ps. PL decays were measured to 5000-6000 counts in the peak and reconvoluted using nonlinear least-squares analysis (FluoFit, PicoQuant). High-resolution transmission electron microscopy (HRTEM) images were obtained on a Philips CM-300 microscope operating at 300 kV. Samples for TEM studies were prepared by dropping diluted solutions of CdTe NCs onto 400-mesh carboncoated copper grids with the solvent evaporated immediately in vacuum. Powder X-ray diffraction (XRD) spectra were recorded with a Philips X’Pert PRO MPD diffractometer (Cu KR radiation, variable entrance slit, Bragg-Brentano geometry, secondary monochromator). Samples for XRD were prepared by multiple washing of NCs out of excess organic moieties through repetitive precipitation in water/isopropanol solvent/ nonsolvent pair, followed by dropping of concentrated solutions of NCs on Si wafer supports and allowing them to dry completely.

Rogach et al. 3. Results and Discussion 3.1. Basics of the Aqueous Synthesis of CdTe Nanocrystals Employing Thiols as Capping Agents, and Its Recent Improvements. The basics of the aqueous synthesis of thiolcapped CdTe NCs remained essentially the same as reported previously.3,12 In a typical standard synthesis,12 0.985 g (2.35 mmol) of Cd(ClO4)2×6H2O (Alfa Aesar) was dissolved in 125 mL of water, and an appropriate amount of the thiol stabilizer was added under stirring, followed by adjusting the pH by dropwise addition of a 1 M solution of NaOH. The solution was placed in a three-necked flask fitted with a septum and valves and was deaerated by N2 bubbling for 30 min. Under stirring, H2Te gas was passed through the solution together with a slow nitrogen flow. CdTe NC precursors are formed at this stage (reaction 1); formation and growth of NCs (reaction 2) proceed upon refluxing at 100 °C under open-air conditions with a condenser attached. HS - R

Cd(ClO4)2 + H2Te 98 Cd - (SR)xTey + 2HClO4 (1) 100 °C

Cd - (SR)xTey 98 CdTe - nanocrystals

(2)

We refer interested readers to our previous publication for a schematic drawing of the experimental setup.12 The use of glass connections is strongly recommended because of the high reactivity of H2Te gas with rubber and common polymer tubes. We note that the synthetic procedure described above is easily up-scalable. Indeed, even in laboratory conditions reaction 2 routinely yields up to several grams of NC powder. H2Te gas can be generated for the synthesis of CdTe NCs as well as other tellurides, like HgTe5,80 or ZnTe,81 by at least two different methods: chemical decomposition of Al2Te3 powder or lumps according to reaction 3, and electrochemical reduction of Te electrode in acid media according to reaction 4.

Al2Te3 + 3H2SO4 f 3H2Tev + Al2(SO4)3

(3)

Te0 + 2H + + 2e- f H2Tev

(4)

For the experiments discussed in this paper, we have used reaction 3 because of its simplicity and availability of Al2Te3 in our groups. However, the limited amount of suppliers and the continuously increasing price for this reagent limit its availability for a lot of groups dealing with the synthesis of corresponding NCs, stimulating a search for alternative sources, for example, NaHTe solution obtained by reduction of Te powder with NaBH4.66,76 Although this method provides an alternative for Te source in the synthesis of CdTe NCs, the direct injection of H2Te gas is an easier, more controllable, cleaner, and more reproducible way to produce high-quality CdTe NCs. To avoid the use of Al2Te3, electrochemical methods can be used to produce H2Te gas, which was known from the beginning of last century82 and was generalized and reported in detail recently by Hodes et al.83,84 The use of the electrochemically generated H2Te gas for successful synthesis of both CdTe and HgTe NCs has been demonstrated recently.80,85 In our comparative study evaluating both chemical and electrochemical methods of generation of the H2Te gas, we have found that the final quality of synthesized CdTe NCs was independent of the method used. The electrochemical method adapted by ourselves (see details and schematics of equipment in the Supporting Information) allows for the control of the H2Te amount by measuring

Feature Article the charge passed through the cell, is applicable for the continuous generation of this gas, and can be scaled up easily. The improvement of the standard synthesis of aqueous CdTe NCs12 concentrated on the optimization of the thiol/Cd ratio and pH. Upon testing different TGA/Cd ratios between 2.45 (as was used in the standard synthesis of ref 12) and 1.1, it was found that decreasing the TGA/Cd ratio leads to a drastic increase of the PL QE of the CdTe NCs. In an attempt to understand this influence on the properties of the NCs, we compared the experimental data with the results of a numerical simulation of the solution composition.75 The results of this simulation show that the increase of the PL QE of CdTe NCs with the decrease in the TGA concentration can be attributed to the increase in the relative concentration of the Cd-SR complex (i.e., uncharged 1:1 complex of cadmium with TGA). This tendency has a natural limit at very-low values of the TGA/ Cd ratio (approaching 1) when the amount of stabilizer in the system becomes insufficient to stabilize NCs from aggregation. There is a competition of at least two different factors during the synthesis: (i) upon decrease in the concentration of TGA, the surface quality of the NCs improves as a result of the increase in the relative concentration of 1:1 Cd-SR complex in comparison with other possible complexes in solution and (ii) a sufficient amount of TGA as a stabilizer has to be present in solution to provide stability and surface passivation of the growing NCs. As a result, the optimum TGA/Cd ratio allowing us to produce CdTe NCs emitting with PL QE 40-60% at room temperature as prepared is slightly higher than 1 and the experimentally obtained optimal values are 1.30 (present work), 1.32,73 and 1.20.67 Synthesis of CdTe NCs employing thioglycerol as a stabilizer at a thioglycerol/Cd ratio of 1.3:1 provides as-prepared samples with PL QE in the range of 25%, which exceeds the values reported previously3 by an order of magnitude. We note that the solution of Cd precursors at low TGA/Cd ratios can be slightly turbid. This fact is additional indirect evidence of the domination of the uncharged, less-soluble CdSR complex. The turbidity of the solution does not disappear during refluxing, but this does not influence the ongoing reaction 2; filtration of the final solution of CdTe NCs removes the insoluble precipitate easily. The precipitate of Cd-SR may play an additional role as a source of cadmium. Gradual dissolution of the Cd-SR complex during NC growth could provide a constant rate of transport of Cd ions to the particles. A slow flux of the cadmium precursor provides the possibility of growing the NCs under diffusion control, which, as has been predicted theoretically,77,78 is preferable for narrowing the size distribution and may be a key factor for the dynamic improvement of the surface quality of the growing NCs. The pH value is another important factor that strongly influences the PL QE of thiol-capped NCs post-preparatively.11,74,76,86 Thus, it is reasonable to assume that the pH will influence the quality of the NCs during the synthesis as well. According to ref 12, the optimum pH value for the synthesis employing different capping ligands depends strongly on the nature of the stabilizer. The recommended value in the case of TGA was 11.2-11.8. For this stabilizer, it was found that an increase of pH of the initial solution is followed by a considerable acceleration of the NC growth. Moreover, as a result of this acceleration one can choose the synthetic conditions allowing the “focusing” of the NCs growth in term of narrowing their size distribution. For example, NCs synthesized at pH 12.0 and a TGA/Cd ratio of 1.3 possess a full width at half-maximum (fwhm) of the PL band of 39 nm, PL QE of 45%, and a Stokes

J. Phys. Chem. C, Vol. 111, No. 40, 2007 14631 shift as small as 100 meV when CdTe NCs are approximately 3 nm in diameter and possess a PL maximum at ca. 600 nm. Further growth proceeds more slowly and is accompanied by a slight broadening of the size distribution.75 A relatively fast NC growth leads to an insufficient quality reflected in low crystallinity of the resulting particles and a large amount of defects and surface states. Alternatively, a comparatively slow growth rate leads to a high content of sulfur (as a product of the TGA decomposition87) in the NCs and a higher probability of the NC’s oxidation. CdTe NCs synthesized in water can be transferred to nonpolar organic solvents like toluene through a partial ligand exchange with a long-chain thiol (1-dodecanethiol) in the presence of acetone.88 The transfer efficiency reaches 90% and depends on the component ratio of the 1-dodecanethiol/toluene/acetone mixture (typically 1:1:4 volume ratio), which in turn depends on the concentration of NCs and their average size. For any particular batch of NCs to be transferred, the correct ratio has to be found experimentally, with a typical variation of acetone content in the above-mentioned three-component system from 1:1:3 to 1:1:8. NCs can be transferred more efficiently when they are washed (e.g., by size-selective precipitation) from the reaction byproducts and the excess of short-chain thiol ligands. Thiol-capped CdTe NCs transferred into organics were used as photosensitizers of fullerenes,89,90 as building blocks for NCs/ polymer composites,88 and as the core material for the synthesis of stable and brightly emitting core-shell CdTe/ZnS nanoparticles.91 Their PL and absorption have been shown to be tuned by surface modification in the presence of dodecanethiol.92 Alternative methods of CdTe NCs transfer to organic media include the use of polymerizable surfactants,15,93,94 tetra-noctylammonium bromide,92 and ionic liquids.95 Furthermore, thiol-capped CdTe NCs are also available in polar organic solvents. Mercaptoethylamine-capped CdTe NCs synthesized in water are readily resoluble in dimethylformamide (DMF) after being precipitated by 2-propanol and dried. A direct synthesis in DMF is possible by taking cadmium lactate as a precursor and 1-thioglycerol as a ligand.96 The synthesis proceeds at higher temperature, the growth of the NCs is faster, and it takes only a few hours to obtain red-emitting samples. CdTe NCs precipitated from the crude solution immediately after synthesis by addition of excess of non-solvent (e.g., diethylether) are not only readily redissolvable in DMF but also in methanol. To the best of our knowledge, this is the only example of II-VI colloidal NCs being soluble in methanol as synthesized. Indeed, methanol, among other short-chain alcohols, is a commonly used non-solvent for size-selective precipitation of many types of organically and aqueously prepared NCs. 3.2. Structural Properties of Thiol-Capped CdTe Nanocrystals. Typical TEM and HRTEM images of thiol-capped CdTe NCs with sizes from 2 to 5 nm have been presented previously in refs 3, 12, and 96. Figure 1 shows TEM and HRTEM images of relatively large 5.5 ( 0.5 nm CdTe NCs capped with MPA. To avoid aggregation on the TEM grid, which is common for aqueous solutions of thiol-capped NCs, we transferred the size-selected sample to toluene using partial ligand exchange with 1-dodecanethiol by applying the procedure of ref 88. The images confirm the monodispersity of NCs; their nonspherical shape can be described within a truncated tetrahedral model proposed recently.53 The sizes of thiol-capped CdTe NCs synthesized in water we are referring to throughout this paper have been determined from the so-called “sizing curve”: the function of the size of thiol-capped CdTe NCs on the energy of the 1s-1s electronic

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Figure 3. XRD patterns of the CdTe NCs synthesized with a TGA/ Cd ratio of 1.3 (red) and 2.45 (black) after 20 h of synthesis (a). Panel b shows the influence of the TGA/Cd ratio on the relative position of the (111) XRD reflex of the CdTe NCs. Panel c presents the evolution of the relative shift of the (111) XRD reflex of the CdTe NCs during the synthesis.

Figure 1. TEM (top) and HRTEM (bottom) images of MPA-capped CdTe NCs, 5.5 nm average size, with a PL maximum at 780 nm.

Figure 2. Sizing curve for thiol-capped CdTe NCs synthesized in water. Filled circles represent sizes of NCs experimentally determined from powder XRD spectra; open circles represent sizes of NCs experimentally determined from TEM images. Solid line is a calculated dependence of the 1s-1s transition energy on CdTe NC size.

transition estimated from the position of the first absorption peak (Figure 2). The points (open circles) in Figure 2 were derived from statistical analyses of NC sizes obtained by TEM measurements. For the smallest NCs (filled circles in Figure 2) for which the precise TEM analyses are difficult, we used the sizes derived from the powder XRD spectra as described in detail in ref 3. Calculation of the 1s-1s transition energy of CdTe NCs (treated as spheres) as a function of their size has been done using an extended theoretical approach described in detail in ref 97 and is presented in Figure 2 as solid line. The extension over the common effective mass approximation includes the implementation of the Coulomb interaction and finite potential wells at the particle boundaries in water as the surrounding dielectric medium. The physical parameters of bulk CdTe put into the model can also be found in ref 3. The agreement between experiment and theory is quite well. However, it should be kept in mind that use of the physical parameters of bulk material for the calculations of quantum-sized NCs has limited validity and may explain the observable deviations from the experimental data. We note that the previously reported (ref 98) sizing curve for CdTe NCs has been derived for nanoparticles prepared by

high-temperature organic syntheses and does not include data for small NCs (first absorption maximum at wavelengths shorter than 570 nm), which are very easy to synthesize in water. The reduction of the amount of TGA at the synthesis stage leads to a reduction of the sulfur content in the NCs. XRD patterns of the CdTe NCs (Figure 3) show that at a TGA/Cd ratio of 1.3 and a pH 12, a smaller shift of the reflexes toward the position corresponding to cubic CdS is observed. It can be explained by the fact that decreasing the amount of the stabilizer in solution and the acceleration of the NCs growth leads to a decreasing probability of TGA hydrolysis87,96 and, as a result, to a lower sulfur content. As discussed in previous publications,12,68 the sulfur-enriched shell itself may be important for the improvement of the stability and luminescence properties of CdTe NCs. At the same time, a few monolayers of this shell is enough for the efficient protection of the NCs and further growth of CdS only reduce the NCs quality similar to the effect of the ZnS shell on the luminescence properties of CdSe NCs.99 3.3. Optical Properties of Thiol-Capped CdTe Nanocrystals. Typical absorption and PL spectra of size-selected12,100 fractions of TGA- and MPA-capped CdTe NCs are shown in Figure 4. The PL spectra of TGA-capped CdTe NCs are tunable in the range of 500-700 nm, whereas those of MPA-capped NCs are tunable between 530 and 800 nm. The MPA capping allows for controllable growth of CdTe NCs up to 6 nm in diameter. The energy gap of bulk CdTe estimated from the absorption measurements at 300 K is 1.43 eV or ca. 867 nm.101 The superior tunability of the absorption over the very-broad spectral range is important for the use of thiol-capped CdTe NCs as absorbers in solar cells,102 for choosing optimal donoracceptor pairs for FRET-based devices,32,103,104 and for tuning an optimal resonance condition in nanoplasmonics systems.34,105 Narrow PL spectra in combination with their tunability and high PL QE are of a special interest for biolabeling applications,62 imaging,43 and LEDs20 based on thiol-capped CdTe NCs. Figure 5 shows the PL decay curves for a size series of CdTe NCs capped with TGA and MPA. All decay curves for the TGA-capped CdTe NCs (Figure 5a) show multiexponential recombination kinetics, which has been observed frequently for different kinds of II-VI nanocrystals.106-110 A distribution of decay times causing nonexponential decays in CdTe NCs has been generally discussed in terms of a variation in the nonradiative decay rates caused by trap states.106,107,111,112 The average

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Figure 4. Set of typical PL (top) and absorption (bottom) spectra of TGA-capped and MPA-capped CdTe NCs demonstrates their tunability over a broad spectral range in the visible and near-infrared. Excitation wavelength is 450 nm. The inset shows a photograph of brightly emitting CdTe NCs of different sizes taken under UV-lamp excitation.

PL decay times increase steadily with increasing size of the TGA-capped NCs, a tendency that is generally observed for II-VI NCs.113 The same tendency is seen for the MPA-capped CdTe NCs, with the striking difference that, starting from the size of approximately 5 nm, the decay curves approach singleexponential behavior (Figure 5b) and become purely monoexponential for 6 nm, the biggest size we were able to grow (fitting curve in Figure 5c). The PL lifetime (τ1/e) reaches 145 ns for the largest MPA-capped CdTe NCs, which exceed typical average lifetimes of the TGA-capped NCs (10-20 ns) by 1 order of magnitude. The observed difference in PL decay curves for TGA-capped and MPA-capped CdTe NCs (Figure 5) could be caused by a different degree of surface passivation of the CdTe core by these two ligands. To clarify this, we have compared PL decays of several pairs of TGA-capped and MPA-capped CdTe NCs of similar sizes (e.g., emitting at similar wavelengths) and similar PL QE values. A typical case is presented in Figure 6. The decay curves are very similar for both stabilizers (which differ only in one additional -CH2- group in the case of MPA), indicating that it is a pure size effect that determines the monoexponential decays of the large CdTe NCs: The use of MPA as a stabilizer allows us to obtain CdTe NCs larger than 4 nm, which is hardly possible for TGA-capped NCs. We now turn to the question of what causes the transition from multiexponential to monoexponential decay curves with increasing size of CdTe NCs. The multiexponential PL decay results from a dispersion in trap energy levels, which leads to a spectrum of rate constants. For the CdTe NCs under discussion, the traps originate from Te surface atoms, as confirmed by XPS114 and electrochemical115 measurements. With increasing size of the CdTe NCs, the band gap decreases because of the weakening quantum confinement, and the valence-band position shifts up in energy. At the same time, the redox energy level of Te traps shifts to lower energies when the NC size increases.115 The latter effect is markedly dependent on the nature (here, on the chain length) of the thiol ligands.115 The position of the anodic peak (determined from the voltammograms of Au electrodes covered with CdTe NCs of different sizes) related to the surface trap energy of CdTe NCs is shifted

Figure 5. PL decays of TGA-capped (a) and MPA-capped (b) CdTe NCs of increasing sizes. Panel c shows a monoexponential fit for MPAcapped CdTe NCs of 6-nm diameter.

Figure 6. PL decays of TGA-capped and MPA-capped CdTe NCs of similar sizes (2.3 nm diameter) and similar PL QE values.

more strongly to lower energies with increasing particle size for MPA-capped NCs as compared to TGA-capped ones (Figure 7). Both effects, increase of the valence band energy of larger

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Rogach et al. TABLE 1: Comparison of Four Pairs of Samples Constituting LPL and HPL Fractions of CdTe NCs of Approximately the Same Size, Capped by TGA or MPA Stokes kr NC size λem shift PL QE τ1/e knr pair ligand (nm) (nm) (meV) (%) (ns) × 106 s-1 × 106 s-1 I

Figure 7. Dependence of the electrochemically measured115 redox potential of Te-related traps of TGA-capped (dots) and MPA-capped (squares) CdTe NCs on the optical band gap of NCs.

II

III IV

Figure 8. Scheme of energy levels of traps (dotted line) for CdTe NCs with increasing sizes. Upon the increase of NC size, the trap states shift down in energy according to Figure 7, whereas the valence band energy, EV, shifts up.

CdTe NCs and the decrease of the trap-state energies with increasing CdTe NC size, lead to a situation presented schematically in Figure 8: The energy levels of trap states appear below the top of the valence band of CdTe NCs (i.e., outside the band gap of CdTe NCs) above some critical size of the particles. This shift of the trap energy levels outside the band gap alters the recombination dynamics radically, resulting in the monoexponential PL decays observed for MPA-capped CdTe NCs larger than a critical size (Figure 5). Our estimation of the critical size from time-resolved PL measurements (PL maximum at app. 750 nm) and electrochemical studies from ref 115 (PL maximum at app. 790 nm) gives us the value in the region of 4.0-4.5 nm. This size is difficult to achieve for TGA-capped CdTe NCs, whereas the use of MPA as a stabilizing agent favors both the synthesis of larger NCs and allows for a more-dramatic shift of trap states outside the band gap of CdTe NCs (Figures 7 and 8), leading to the samples with monoexponential decays. We note that monoexponential decay curves were observed both by us for the large MPA-capped CdTe NCs and by Wuister et al. for CdTe NCs initially synthesized in organics in a mixture of trioctylphosphine and dodecylamine and then transferred to water by treatment with different thiols.111,116 This observation does not support a model of ref 110 recently derived for the deep-red emitting zinc-blende-type CdTe/CdS core-shell NCs with multiexponential decays, possibly because of the coating of the CdTe cores, differences in deriving the sizes of the cores, or both. 3.4. Correlation between PL Quantum Efficiencies, PL Lifetimes, and Stokes Shifts of CdTe Nanocrystal Fractions. It was shown recently that individual fractions of CdTe NCs separated from the same crude solution by size-selective precipitation could have different PL QE values, with a difference exceeding 1 order of magnitude.12,77,114 This phenomenon, which is typical not only for CdTe but also for other colloidally synthesized NCs like CdSe or InAs, was investigated both theoretically78 and experimentally,12,77,114 and it was attributed to a dynamical distribution of the Ostwald ripening growth conditions for the individual NCs in an ensemble. Most of the colloidal syntheses of semiconductor NCs are based on

TGA

synthesized at standard conditions of ref 12 2.85 596 158 3 3.7 3.0 597 114 20 17.4 11.7

45.8

synthesized at optimized conditions described in Section 3.1 TGA 3.0 590 93 19 14.5 3.05 597 79 49 43.9 11.2 11.6 synthesized at standard conditions of ref 12 MPA 2.4 547 213 7 7.8 2.45 542 158 12 10.7 MPA 3.2 645 190 4 10.3 3.5 646 87 20 34.4 5.9

23.2

the Oswald ripening phenomenon. The formation of NCs in solution is a dynamic process. After nucleation, further particle growth occurs via dissolution of smaller particles in favor of the growth of larger particles. At any stage of growth in an ensemble of NCs with a given size distribution, there is a specific ratio between growth and dissolution rates for a given particle size. Approximately in the middle of the size distribution, there always exists a fraction of particles with equal rates of growth and dissolution. The best conditions for the elimination of surface defects and to produce NCs with the highest surface quality are given at a net growth rate close to zero.78 The conditions of growth are thus important for the optical properties of NCs determined by their surface quality, in particular for the PL QE. The NCs (and the corresponding individual size-selected fractions) grown under conditions far away from the equilibrium of growth and dissolution have lower surface quality and thus a lower PL QE, and are denoted as “low photoluminescence” (LPL) fractions. Fractions grown under conditions close to the equilibrium have high surface quality and, as a consequence, a high PL QE value; these fractions are denoted as “high photoluminescence (HPL)”.77 It was of interest to investigate how the recombination dynamics of carriers in the individual selected fractions of CdTe NCs correlates with their PL QE value, with the latter being correlated to the surface quality of the NCs.77,78,114 Results of the comparison of four pairs of samples each constituting LPL and HPL fractions of TGA-capped and MPA-capped CdTe NCs of the same size (and same wavelength of emission) are summarized in Table 1. For the TGA-capped NCs, one pair of samples has been synthesized at the standard growth conditions of ref 12, whereas another pair at the optimized growth condition as discussed in subsection 3.1 and ref 75. For all pairs, we have observed a correlation between the value of Stokes shift and PL QE values of the NCs, with an increase in the QE value being accompanied by a reduction of

Figure 9. Stokes shift for different fractions of CdTe NCs vs PL QE value. Dashed line is a guide for the eye.

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J. Phys. Chem. C, Vol. 111, No. 40, 2007 14635

Figure 10. Absorption and PL spectra (a), and corresponding PL decay curves (b) of LPL (black curve) and HPL (red curve) fractions of MPAcapped CdTe NCs synthesized at the standard conditions of ref 12 (pair IV from Table 1).

the Stokes shift (Table 1, Figure 9). Independent of the NCs’ size and growth conditions, the HPL fraction always showed a smaller value of Stokes shift as compared to that of the LPL fraction, which can be related to a wider distribution of trap states in the LPL NCs and to a stronger nonradiative energy dissipation in these samples with lower surface quality.77,78 Small Stokes shift values of HPL samples, which are only 3-4 times larger than room-temperature thermal energy (26 meV), are consistent with the mechanism of detrapping of carriers from extremely shallow trap levels.109 The finite size distribution of CdTe NCs in our samples imposes a lower limit on the value of the Stokes shift. The correlation between the values of the Stokes shift and the PL QE values of NCs can be used as a rapid technique to evaluate the quality of samples without involving the comparison with luminescence standards. We further analyzed PL decays for each pair of LPL and HPL CdTe NC fractions. Rather than fitting the experimental decay curves (Figure 10 presents typical LPL-HPL pair for MPA-capped NCs) to multiexponential functions, an estimate for the average lifetime was obtained from the time in which the emission intensity drops to 1/e of the initial value (τ1/e). The difference between the PL lifetimes of HPL and LPL fractions of NCs can reach over 1 order of magnitude (Table 1). HPL CdTe NC fractions show PL decays close to monoexponential, even in the case of TGA-capped NCs (Table 1), whereas the PL kinetics of the LPL fractions of NCs, also of MPA-capped ones, show multiexponential behavior. Deducing a value of τ1/e from the PL decay curves of HPL fractions, and taking into account the values of PL QE, we estimated relative contributions of radiative and nonradiative processes by calculating the radiative (kr) and nonradiative (knr) rate constants (Table 1). It is interesting that TGA-capped CdTe NCs show a value of kr 2 times that of the MPA-capped NCs with similar PL QE values. At the same time, nonradiative energy dissipation is typically stronger in TGA-capped NCs. As a result, the PL lifetimes observed in MPA-capped CdTe NCs are longer than those for TGA-capped ones. As was discussed above (Section 3.1), the concentrations of precursors play an important role in the enhancement of PL QEs of CdTe NCs and TGA-capped CdTe NCs with significantly improved PL QE values can be obtained under optimized synthetic conditions. Comparing the radiative and nonradiative rate constants of TGA-capped CdTe NCs emitting at similar wavelengths, but originating from syntheses at standard (PL QE 20%) and optimized (PL QE 49%) conditions (Table 1), we have found that the value of kr remains unaffected by the growth regime, whereas the values of knr differ substantially. Considering that nonradiative processes take place mainly out of deeper traps with energy considerably greater than the thermal energy, we can conclude that passivation of NC surface defect states

and the suppressing of the generation of structural defects during surface reconstruction77 are much-more efficient in NCs grown at low concentrations of Cd precursors (at the optimized synthetic conditions). 4. Summary and Outlook Optimized aqueous synthesis provides thiol-capped CdTe NCs with bright (40-60% quantum efficiency for as-prepared samples) emission tunable from 500 to 800 nm. By employing MPA as a stabilizer, large (up to 6 nm in diameter) NCs have been synthesized, showing up to 145-ns-long monoexponential PL decays. This phenomenon is explained by considering the energetics of the Te-related traps with respect to the valenceband position of CdTe NCs. The correlation between PL quantum efficiencies, PL lifetimes, and Stokes shifts of CdTe NCs agrees well with a model proposed previously that connects the emission properties of NCs with their surface quality determined by the Oswald ripening conditions. Brightly emitting water-soluble CdTe NCs with a flexible surface chemistry determined by easy choice of thiol capping ligands have already secured and will secure in the future a wide field of applications, ranging from life sciences to photonics and optoelectronics. In the field of biological imaging of cellular processes, the ability to fabricate NCs with well-defined surface passivation is important in studying transport processes within living cells. For transport into cells, particularly into the cell nucleus, the ability to alter the surface charge through changes in the NCs’ capping layer is extremely important. In photonics, one of the key requirements for the future is the possibility to produce single-photon emitters on demand. Here CdTe NCs with welldefined lifetimes through the control of the size and quantum efficiency can play a crucial role. Such emitters can be coupled to the modes of microcavity structures in which the quality factor can be tailored to change the fluorescence spectrum and dynamics of the emitters. These sources can play a vital role in the development of single-photon emitters for quantum computing applications. In addition, microscale lasers formed by combining NCs and spherical or cylindrical microcavity structures may become important sources particularly for sensing applications. Acknowledgment. We gratefully acknowledge the possibility of performing part of the experiments in the group and under the support of Prof. Horst Weller at the University of Hamburg. We thank Andreas Kornowski and Silvia Bartholdi-Nawrath (University of Hamburg) for valuable assistance with TEM measurements, Anna Helfrich (LMU) for excellent technical assistance, and Sergey Poznyak (Belarusian State University) for useful discussions. This work has been supported by the

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