NANO LETTERS
High Quality ZnSe and ZnS Nanocrystals Formed by Activating Zinc Carboxylate Precursors
2004 Vol. 4, No. 11 2261-2264
Lin Song Li,† Narayan Pradhan,† Yunjun Wang,‡ and Xiaogang Peng*,† Department of Chemistry and Biochemistry, UniVersity of Arkansas, FayetteVille, Arkansas 72701, and Nanomaterials and Nanofabrication Laboratories (NN Labs), FayetteVille, Arkansas 72704 Received August 20, 2004; Revised Manuscript Received September 28, 2004
ABSTRACT Nearly monodisperse ZnSe and ZnS nanocrystals were formed in noncoordinating solvents using alternative precursors. The parameter window for the growth of high quality zinc chalcogenide nanocrystals was found to be much narrower than that of the corresponding cadmium chalcogenide nanocrystals. In contrast to syntheses performed in coordinating solvents, generic oxygen containing ligands, such as fatty acids, were found to be reasonable ligands for the formation of ZnSe and ZnS nanocrystals. Activation of the zinc precursor (zinc fatty acid salts) by alkylamines was found to be critical for the formation of high quality ZnSe nanocrystals. The photoluminescence (PL) quantum yield (QY) of ZnSe nanocrystals reached as high as 50% with a full width of half-maximum (fwhm) as narrow as 14 nm. The band gap PL of ZnS nanocrystals was always mixed with a deep-trap tail. The PL fwhm of the band gap emission of the ZnS nanocrystals was typically between 10 and 12 nm.
High quality colloidal semiconductor nanocrystals are nanometer sized fragments of the corresponding bulk crystals with nearly monodisperse size/shape distribution and strong band gap photoluminescence, which have been active targets for synthetic chemistry in the recent years due to their sizedependent properties and flexible processing chemistry.1-3 ZnSe (bulk band gap 2.7 eV) and ZnS (bulk band gap 3.6 eV) are wide band gap semiconductors. Due to quantum confinement, their nanocrystals are interesting emitting materials in the blue to the ultra violet range. Although high quality CdS nanocrystals with strong band gap emission in this window have become available recently,4,5 the intrinsic toxicity of cadmium places ZnSe and ZnS nanocrystals in an advantageous position. The wide band gaps of ZnSe and ZnS also make them an ideal choice as an inorganic passivation shell for a variety of semiconductor core/shell nanocrystals6-8 in order to improve the stability and emission properties of the semiconductor core nanocrystals with a relatively narrow band gap. These wide band gap semiconductor nanocrystals are also attractive hosts for the formation of doped nanocrystals.9-11 For these reasons, synthesis of high quality ZnSe and ZnS nanocrystals is still an attractive subject,9,12-18 although synthetic chemistry of cadmium chalcogenide nanocrystals has been well developed in the recent years.4,19-22 This work intended to develop synthetic * Corresponding author. E-mail:
[email protected]. † University of Arkansas. ‡ Nanomaterials and Nanofabrication Laboratories. 10.1021/nl048650e CCC: $27.50 Published on Web 10/14/2004
© 2004 American Chemical Society
approaches for ZnSe and ZnS nanocrystals using greener chemicals at elevated temperatures, which was achieved for the synthesis of ZnSe nanocrystals by the introduction of “activation of precursors” and “identical injection/growth temperatures”, especially the former. The synthesis of ZnSe and ZnS nanocrystals was conducted in noncoordinating solvents in this work. Octadecene (ODE) was used as the solvent for the injection solution by directly dissolving either elemental selenium or sulfur in it. For the selenium injection solution, about one equivalent tributylphosphine (TBP) or trioctylphosphine (TOP) was reacted with the selenium powder first to make a clear solution, and then diluted with a suitable amount of ODE. Tetracosane (TCA) and its mixture with ODE were used as the reaction solvents in most cases because of the high temperature requirements for the formation of ZnSe and ZnS nanocrystals. Fatty acids and aliphatic amines were found to be good ligands for the formation of ZnSe and ZnS nanocrystals. ZnO (dissolved by reacting with fatty acids) and zinc carboxylate salts were used as the precursors exclusively. Details of typical synthetic schemes for both ZnSe and ZnS nanocrystals are provided below. A typical procedure for the synthesis of ZnSe nanocrystals is as follows. Zinc stearate (0.0632 g, 0.1 mmol) was mixed with 0.2 mmol (0.054 g) octadecylamine (ODA) and 2 g tetracosane and 2 g ODE in a 25 mL three-neck flask. The mixture was heated to 330 °C under Ar flow. Selenium stock solution [0.048 g (0.6 mmol) of selenium powder dissolved
Figure 1. Growth kinetics of ZnSe nanocrystals with different ligands. The initial Zn/Se molar ratio was 1:6. The injection/growth temperatures are 330/310 °C, respectively. Decanoic acid: DA; stearic acid: SA; hexadecylamine: HDA; octadecylamine: ODA. [Zn]° represents the intial zinc precursor concentration of the reactions.
in 0.2 g of TBP and 0.3 g ODE] was injected. After the injection, nanocrystals grew at 310 °C to reach desired size. ZnSe nanorods were synthesized as follows. ZnO (0.3 mmol) was mixed with 1.2 mmol decanoic acid and 4 g tetracosane and loaded in a 25 mL three-neck flask. To this solution was injected 0.6 mmol Se in 0.5 g TOP when the temperature was heated to 350 °C under Ar flow; the growth temperature was kept the same to reach desired rods. To monitor the nanocrystal growth, a small amount of sample (∼0.2 mL) was taken via syringe from the flask during the reaction and diluted to an optical density of between 0.1 and 0.2 by addition of anhydrous chloroform. The resulting ZnSe nanocrystals can be dissolved in hexanes, chloroform, and toluene. Unreacted starting materials and side products were removed by the extraction and precipitation procedures reported previously.4 No size sorting was performed on any of the samples used for measurements reported here. A similar procedure was applied for the synthesis of ZnS nanocrystals. Zinc stearate (0.1264 g, 0.4 mmol) and a certain amount of tetracosane and ODE were loaded in a 25 mL three-neck flask. The mixture weight was 4 g in total and was heated to 340 °C under Ar flow. A solution of sulfur (0.0032 g, 0.1 mmol) in 1 g ODE was swiftly injected into this hot solution, and the reaction mixture was cooled to 300 °C for the growth of the ZnS nanocrystals. Synthesis of ZnSe nanocrystals in pure ODE (boiling point about 300-310 °C) as the noncoordinating solvent using fatty acids as the ligands yielded only nanocrystals with a decent size distribution, as we reported previously.4 Considering that zinc carboxylate salts are more stable than the corresponding cadmium carboxylate salts, TCA or its mixture with ODE was tested as the reaction solvent to increase the reaction temperature. A significant improvement was observed when the injection temperature and growth temperature were increased to 330 °C and 310 °C, respectively. At these temperatures, the ligand effects were studied. As shown in Figure 1, the growth of the nanocrystals was generally fast, which resulted in relatively large sized nanocrystals (judged by the first exciton absorption peak in the UV-vis 2262
Figure 2. Temporal evolution of the first exciton absorption peak vs reaction time. The reaction conditions were the same besides the presence of ODA in one reaction. Note: No ZnSe formation was observed for the reaction without amine in the first 10 min.
spectra) when fatty acids were used as the sole ligands. The sizes of the nanocrystals at the same reaction time decreased slightly by replacing dodecanoic acid (DA) (Figure 1A) with stearic acid (SA) (Figure 1B) as the ligands. This is consistent with the observations reported previously that the ligands with a relatively long hydrocarbon chain slow the reactions by increasing the steric effect of the cadmium monomers.4,23 However, the chain length effect observed here was much less significant than that previously observed from other IIVI and III-V systems (Figure 1).4,23 Concentration of the ligands was also varied under fixing reaction conditions. It was observed that a high fatty acid concentration typically resulted in no formation of any nanocrystals, indicating the activity coefficient of the monomers became too low to initialize the formation of a necessary amount of nuclei.22 All these results indicate that the zinc carboxylate precursors must be activated to achieve a desired balance between nucleation and growth. Aliphatic amines were chosen as the reagents to activate the zinc carboxylate precursors. It was noticed that white precipitation (likely ZnO) appeared at elevated temperature prior to the injection of selenium when the ratio between aliphatic amine and the precursor was higher than 6. (For the case of cadmium carboxylates, this ratio could be much higher because much lower temperatures were applied.21) Significant activation was observed as shown in Figure 1C,D, and E. In comparison to the pure fatty acid related reactions (Figure 1A and B), the starting size of the nanocrystals was much smaller, the concentration of the nanocrystals was higher, and the size distribution of the nanocrystals judged by the sharpness of the absorption peaks was significantly improved, at least for the relatively small sizes. These indicate that a balanced nucleation and growth is achieved. The activation of zinc precursor can be further demonstrated by observing reactions at relatively low temperatures. Figure 2 illustrates that, at about 200 °C, formation of ZnSe nanocrystals was not observed at all for about 10 min reaction if no amine was added into the system, as judged by UVvis absorption spectra of aliquots. On the contrast, the appearance of absorption features of ZnSe nanocrystals was observed almost immediately after the injection of Se solution. In addition, the growth of the formed ZnSe nanocrystals was also much slower if no amines presented, Nano Lett., Vol. 4, No. 11, 2004
Figure 3. Effects of injection/growth temperatures during the growth of ZnSe nanocrystals. Conditions: 0.025 mol/kg zinc stearate with 0.05 mol/kg ODA, Zn/Se-TBP molar ratio was 1:6.
as indicated by the rate of the red shift of the first exciton absorption peak (Figure 2). From the existing experimental observations, amine seems to attack the carbonyl group to release the zinc bonded to the carboxylate group. If no selenium was added, the reaction between amines and zinc fatty acid salts resulted in zinc oxide nanocrystals. This indicates that the formation of ZnSe nanocrystals may have gone through some oxide intermediates, such as zinc oxide monomers or very small clusters. The activation mechanism will be discussed in more detail in another publication. Because the mixture of amine and zinc carboxylate salts was not stable upon heating, an alternative method for introducing amines into the reaction system was developed, injecting the amines dissolved in the selenium injection solution. In this way, the amount of amine could be varied in a large concentration range. More importantly, the system was very stable, which made the reactions reproducible. In addition, the synthetic temperature could be lowered below 300 °C, which would eliminate TCA, which is solid and difficult to remove after synthesis at room temperature, from the solvent. The results revealed that this technique worked well using ODE (without using TCA) as the noncoordinating solvent, but the absorption and PL spectra of the nanocrystals obtained using this route were less sharp in comparison to those obtained by the reactions with amine inside the reaction flask. The range of nearly monodisperse ZnSe nanocrystal sizes, indicated by sharp absorption bands, grown with amine as the activation reagent was small (Figure 1C, D, and E). It is impractical to tune the achievable size range by using relatively high amine concentrations because high amine concentrations make the reaction system unstable, as mentioned above. It is a common consensus regarding the synthesis of semiconductor nanocrystals under elevated temperatures that the reaction should immediately cool for a certain period after the injection in order to stop the nucleation process promptly after the injection. In principle, a high growth temperature after the injection should promote a rapid growth of the nuclei formed immediately after the injection. For this reason, the effects of injection/growth temperatures were studied. As hypothesized, the size ranges of the resulting nanocrystals with sharp absorption bands using equal and high injection/growth temperatures (330 °C/ 330 °C, Figure 3 right) were indeed larger than those obtained using the more traditional temperature combination Nano Lett., Vol. 4, No. 11, 2004
Figure 4. (left) TEM images of the as-prepared ZnSe nanocrystals. (right top) Collections of UV absorption and PL spectra of as-prepared ZnSe nanocrystals. (right bottom) XRD pattern of the ZnSe nanocrystals.
(330 °C/310 °C), Figure 3 middle) and those resulted from the reaction with equal and relatively low injection/growth temperatures (310 °C/310 °C, Figure 3 left). The photoluminescence (PL) of the ZnSe nanocrystals grown with fatty acids as the sole ligands was typically weak (PL quantum yield (QY) below 10%). However, the ZnSe nanocrystals grown with amines as activating reagents emitted very well, with PL QY as high as about 50% (measured using organic dyes LD423 as references). As shown in Figure 3 (top right), the PL was dominated by band gap emission and the full width at half-maximum (fwhm) of typical samples was about 12-15 nm. The narrow emission lines were consistent with the nearly monodisperse size distribution of the nanocrystals, as demonstrated by the transmission electron microscope (TEM) images in Figure 4. In addition to the control over the size and size distribution, shape control was also evidenced with some preliminary success by using high monomer concentration (Figure 4, bottom left). The crystallinity of the ZnSe nanocrystals was demonstrated by the X-ray powder diffraction (XRD) as shown in Figure 4. The XRD pattern of the nanocrystals resembles that of zinc blende crystals. The synthetic chemistry of ZnS nanocrystals was found to be somewhat different from that of ZnSe nanocrystals. Without the activation of aliphatic amines, nearly monodisperse ZnS nanocrystals (Figure 5) were formed using a traditional combination of injection/growth temperatures (340 °C/300 °C). The PL of the ZnS nanocrystals synthesized using fatty acids as the sole ligands was found to be about 10 times brighter than those formed with the presence of aliphatic amines in the reaction solution, although both types of ZnS nanocrystals showed significant contributions from the deep trap emission (Figure 5 inset). 2263
effects on the PL properties of zinc chalcogenide nanocrystals resemble those for cadmium chalcogenide nanocrystals. For both types of chalcogenide nanocrystals, amines were found to enhance the PL of the selenide nanocrystals and fatty acids provided better electronic passivation for the sulfide nanocrystals.24 Acknowledgment. This work was partially supported by the NSF and DoD. References
Figure 5. Temporal evolution of UV absorption spectrum of asprepared ZnS nanocrystals. The zinc stearate concentration was 0.025 mol/kg and the Zn/S molar ratio was 4:1. The injection/growth temperatures were 340/300 °C. (inset) PL spectrum of the aliquots taken at 50 min.
In summary, nearly monodisperse ZnSe and ZnS nanocrystals were formed in noncoordinating solvents using alternative precursors. Two new concepts, activation of precursors and same injection/growth temperatures, were introduced for the growth of ZnSe nanocrystals. Activation of precursors introduced here allows to apply generic compounds, such as zinc carboxylate salts for the growth of ZnSe nanocrystals. Identical growth and injection temperatures revealed that the balance between nucleation and growth is much more delicate than it has been believed for the growth of semiconductor nanocrystals. The window of the growth conditions of zinc chalcogenide nanocrystals, including monomer and ligand ratio, ligand concentration, and reaction temperatures, was found to be much narrower than that of the corresponding cadmium chalcogenide nanocrystals. This explains why nearly monodisperse zinc chalcogenide nanocrystals in the quantum confinement size regime have been challenging targets for either traditional organometallic routes, the alternative routes, or the singleprecursor routes. In contrast to syntheses performed in coordinating solvents,12 generic oxygen containing ligands, such as fatty acids, with a controlled concentration in noncoordinating solvents were found to be reasonable ligands for the formation of ZnSe and ZnS nanocrystals. The ligand
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NL048650E
Nano Lett., Vol. 4, No. 11, 2004
