Large-Scale Synthesis of InPZnS Alloy Quantum Dots with

Dec 26, 2011 - confirmed with TEM, elemental analysis, XRD, and XPS; particularly, P2p peaks for In−P and InP−S bonds in XPS analysis could give c...
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Letter pubs.acs.org/JPCL

Large-Scale Synthesis of InPZnS Alloy Quantum Dots with Dodecanethiol as a Composition Controller Taehoon Kim, Sung Woo Kim, Meejae Kang, and Sang-Wook Kim* Department of Molecular Science and Technology, Ajou University, Suwon 443-749, Korea S Supporting Information *

ABSTRACT: A method for the large-scale synthesis of InPZnS alloy QDs was developed, which tune the optical properties by controlling the dodecanethiol to palmitic acid ratio. The control of the surfactant ratio resulted in the change of alloy composition. The absorption and emission peaks showed a blue shift as the amount of DDT increased, which implied the increase of ZnS contents. The alloy structure was confirmed with TEM, elemental analysis, XRD, and XPS; particularly, P2p peaks for In−P and InP−S bonds in XPS analysis could give clear evidence. The alloy QDs were overcoated with a ZnS shell, and the quantum yields (QYs) were in the range of 20− 45% for all wavelengths. Finally, a cadmium-free QD yield in a single reaction was determined to be 2.74 g. SECTION: Nanoparticles and Nanostructures

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wavelengths).28,29 Recently, synthesis of InP/ZnS QDs using microfluidics showed the possibility for mass production.30 Herein, we report a method for the large-scale synthesis of InPZnS alloy QDs, whose emission wavelength and quantum yield are in the ranges of 490−630 nm and 15−40%, respectively. The emission wavelength could be tuned by the slow addition of tris(trimethylsilyl) phosphine (TMS3P) to the indium precursor solution containing mixed surfactants (dodecanethiol (DDT) and palmitic acid (PA)). The surfactant ratio was appropriately controlled because it played an important role in modifying the optical properties of QDs. On the laboratory scale, we could obtain 3 g of solid QDs after the workup process, implying that this method can be scaled up easily. The experimental setup for proposed QD synthesis is shown in Supporting Information Scheme S1. Indium acetate (1 mmol), zinc acetate (1 mmol), dodecanethiol, and palmitic acid were mixed in octadecene solvent, and the resulting solution was degassed and heated up to 210 °C. Subsequently, a solution of TMS3P, (1 mmol) in octadecene (5 mL) was added very slowly using a syringe pump, over a period of 5 h. Following the addition, the colorless solution progressively became light yellow (blue-emitting QDs), light orange (greenemitting QDs), and dark red (red-emitting QDs) in color. The optimum reaction temperature at which the highest quantum yield and full width at half-maximum (fwhm) of the emission spectrum could be obtained was 210 °C. When the reaction temperature exceeded 230 °C, a broad emission spectrum and precipitation of the product were observed. Three types of InPZnS alloy QDs were prepared using the same In/Zn

ntensive laboratory-scale research has been carried out on colloidal semiconductor nanoparticles (quantum dots, QDs), with emphasis on their synthesis, physical properties, and uses. Because QDs have potential applications in the production of light-emitting diodes,1−8 biomedical fluorophores,9−14 and solar cells,15−20 transfer of QD technology to the business sector is imminent, and this necessitates the development of a method for large-scale production. II−VI quantum dots and their core/shell structures, such as CdSe/ZnS, have been commerciallized using several methods, for example, the successive injection method,21,22 microfluidic continuous systems,23,24 and single-source precurors.25 However, to the best of our knowledge, mass production of III−V QDs, such as InP26,27 and InP/ZnS, the most attractive candidates for cadmium-free application, has not been realized. Well-size-controlled QDs can be synthesized by the “hotinjection method”, which involves nucleation by rapid injection of precursors at a high temperature followed by growth and aging of the QDs. Hot injection has been successfully used for the large-scale synthesis of CdSe QDs.21,22 Nevertheless, this method cannot be used to produce III−V QDs such as InP QDs because the time taken for injecting large quantities of precursors exceeds that for fast nuclei formation; this time lag results in extremely inhomogeneous QDs and precipitation. As alternatives, the “heating up method” and the “method based on in situ phosphine gas generation” were developed by the Reiss group, and they appeared to be promising for the commercialization of QDs.28,29 However, the QDs synthesized by the former method do not show long- wavelength emission (emission extending to the red region of the spectrum), while the latter method requires the use of highly toxic phosphine gas, and the QDs produced do not show short-wavelength emission (emission in the green-wavelength range or at shorter © 2011 American Chemical Society

Received: December 6, 2011 Accepted: December 26, 2011 Published: December 26, 2011 214

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Figure 1. Absorption and emission spectra of QDs by controlling DDT to PA ratios: (a) 1:1, (b) 1:3, (c) 1:20, and (d) only PA. Absorptions were measured after addition of 0.3 (black dotted line), 0.6 (black dashed line), and 1.0 mmol (gray solid line) of TMS3P solution and subsequent addition of zinc acetate (black solid line) and DDT (colorful solid line). (e) TEM images of blue-, (f) green-, and (g) red-emitting cores and their average size and size distribution. (h) The results of the emission wavelength, core size, surfactant ratio, and In to Zn ratio for each QD.

precursor ratio (1/1) and DDT to PA ratios controlled at 1:20, 1:3, and 1:1. Samples were taken after the addition of every 0.3 mmol portion of TMS 3 P solution and analyzed. For comparison, a sample without DDT was also prepared. The first excitionic peak in the absorption spectrum appeared at around 520, 480, and 450 nm in the case of the samples with DDT to PA ratios of 1:20, 1:3, and 1:1, respectively. These peaks showed a blue shift as the amount of DDT increased (Figure 1a−c). In every case, the absorption spectrum had a limited tunable window even though excess phosphine

precursor was added, indicating that the process is well-suited for large-scale production of InP−ZnS alloy QDs having a specific band gap. The absorption peak of the DDT-free comparison sample showed a gradual red shift from 480 to 600 nm when the amount of phosphine precursor added was increased. Photoluminescence data (PL) showed similar emission wavelengths as those determined from the absorption data (Supporting Information, Figure S1). The PL quantum yields (QYs) of the as-prepared QD samples were very low (below 1%) but increased slightly as the TMS3P solution was 215

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added. In addition, trap emission occurred between 600 and 800 nm. The QYs were measured by comparing the integrated area of emission with the emission of coumarin 153, rhodamine 110, and rhodamine 6G as standards. The QDs were coated with a ZnS shell by the sequential addition of zinc acetate and DDT, as mentioned in our previous report,31 to enhance the optical properties of core/shell QDs and thus improve the PL quantum yield. The detailed experimental procedure can be found in the Supporting Information. After ZnS coating, the QYs increased to 45%, while the position of the absorption peaks remained unchanged. Transmission electron microscopy (TEM) images of the three core samples were obtained without size selection after the synthesis; the images showed that the blue-emitting QDs have a larger particle size than QDs with longer wavelength. The mean particle diameter increased in the following order: blue-emitting QDs (4.5 nm) > green-emitting QDs (3.8 nm) > red-emitting QDs (2.8 mm). This indicated that the deviation of quantum confinement effects such as particle size dependence of the emission wavelength occurred if the composition of three QDs was the same. The sizes of QDs increased to 4.2 from 3.8 nm (green-emitting QDs) and to 3.4 from 2.8 nm (red-emitting QDs) after ZnS shell coating. (Figure S3, Supporting Information) Elemental analysis data by inductively coupled plasma−optical emission spectroscopy (ICP-OES) showed that In to Zn ratios are around 55/45 for blue-emitting dots, 60/40 for green-emitting dots, and 70/30 for red-emitting dots, which can explain the conflicting results of shorter-wavelength emission via the larger particle size by the increased amount of ZnS in the alloy QDs. Zinc ions in the redemitting dots may be attached and stabilized on the surface of the InP quantum dots, as mentioned in previous reports.31−33 A high-resolution image of the inset (Figure 1e) revealed a singlecrystalline state of alloy QDs. The core structures of the sample were elucidated by powder X-ray diffraction (XRD) analysis. The XRD patterns showed that the indexed peaks shifted toward those of bulk ZnS as the amount of DDT increased, indicating that the QDs prepared using DDT-containing surfactants were InP/ZnS core/shell or InP−ZnS alloy QDs, and not pure InP QDs. To confirm the exact structure, X-ray photoelectron spectroscopy (XPS) measurements were conducted on films prepared from QDs with emission wavelengths of 480 (DDT/ PA = 1/1), 530 (DDT/PA = 1/3), and 620 nm (pure PA). The spectra showed P2p peaks for In−P and InP−S at 128 and 132 eV, respectively (Figure 2a and b). The area ratio of two peaks decreased from 0.76 to 0.57 and then to 0.07 as the amount of DDT was decreased, indicating that a high concentration of DDT in the surfactant mixture resulted in a large number of InP−S bonds and the formation of large-sized, blue-emitting cores. The large number of InP−S bonds in the larger particles in turn implied that the QDs were not InP/ZnS core/shell but InP−ZnS alloys. The alloy structure can be permitted by the fact that both have the same zinc blende structure and the small difference of the lattice parameter (InP: 5.8687 Å; ZnS: 5.4053 Å). Meanwhile, the peak at 132 eV could be mistaken for the P2p peak of the InP−O bond due to the oxidized InP surface. To rule out the possibility of such an erroneous interpretation, XPS data before and after the formation of the ZnS shell were compared; if the aforementioned peak were indeed due to the InP−O bonds formed by oxidation, it would diminish in size or disappear after ZnS coating. However, the peak at 132 eV persisted even after the ZnS coating process (Supporting Information, Figure S2). Additionally, the spectrum of a general

Figure 2. (a) XRD pattern of QD cores. (b) XPS spectrum of blue-, (c) green-, and (d) red-emitting cores; P2p peaks for the In−P and InP−S bond were integrated.

InP/ZnS core/shell showed a small peak at 132 eV due to the InP−S bond of the core/shell interface (Supporting Information, Figure S2; red-emitting dots), confirming that the peak in question was attributable to the InP−S bonds. The emission of the alloy core/shell QDs (InPZnS alloy/ZnS shell) could be tuned precisely and reproducibly by controlling the DDT to PA ratio (Figure 3d). Figure 3a−c shows the absorption and emission spectra and photoimages of every sample. Under the no-DDT condition, red-emitting QDs with an emission wavelength of 630 nm (Figure 3a and b; dark blue color) were obtained, and the emission showed a gradual blue shift with an increase in the DDT content. The correlation between the emission wavelength and the DDT content is valid even in large quantities of gram scale. Green-emitting QDs showed the best yield of 45%, while blue-, yellow-, and redemitting QDs showed around 20% QY. Generally, alloy QDs are considered to have better stability than pure QDs.34 Stabilities of the InPZnS/ZnS and InP/ZnS QDs were compared, which were tested in the convection oven at 120 °C. Figure 3f shows that alloy InPZnS/ZnS QDs have better stability, as expected. The insets are the photographs of them after 72 h of aging. From the photoimage, the QD yield in a single reaction was determined to be 2.74 g (Figure 3e). In summary, a method for the large-scale synthesis of InPZnS alloy QDs was developed, which tune the optical properties by controlling the dodecanethiol to palmitic acid ratio. The control of surfactant ratio resulted in the change of alloy composition. The absorption and emission peaks showed a blue shift as the amount of DDT increased, which implied the increase of ZnS content. The alloy structure was confirmed with TEM, 216

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Figure 3. (a) Absorption and (b) emission spectra of alloyed InPZnS core−ZnS shell QDs. (c) Photograph image of each wavelength emitted QD under UV light. (d) The photograph image shows the amount of final product QDs in a single reaction. (e) QY and DDT/PA ratio graph against the emission wavelength. (f) Stability of the InPZnS/ZnS and InP/ZnS, which were tested at 120 °C. The insets are the photographs of them after 72 h of aging.

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elemental analysis, XRD, and XPS; particularly, P2p peaks for In−P and InP−S bonds in XPS analysis could give clear evidence. The alloy QDs were overcoated with ZnS shell, and the QY were in the range of 20−45% for all wavelengths. Finally, a cadmium-free QD yield in a single reaction was determined to be 2.74 g.

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ASSOCIATED CONTENT

* Supporting Information S

Detailed experimental procedure, the absorption and emission spectra of InPZnS alloy core QDs, and XPS data of alloy core and core/shell QDs. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +82-31-219-2522. Fax: +8231-219-1592.

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REFERENCES

ACKNOWLEDGMENTS

This work was supported by the National Research Foundation of Korea (NRF) Grant No. 2009-0067322, Priority Research Centers Program (2009-0093826), and the Industrial Core Technology Development Program funded by the Ministry of Knowledge Economy (No. 10035274), Republic of Korea. 217

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