Photoluminescence Properties of Self-Assembled Monolayers of

Article pubs.acs.org/JPCC

Photoluminescence Properties of Self-Assembled Monolayers of CdSe and CdSe/ZnS Quantum Dots H. Yokota, K. Okazaki, K. Shimura, M. Nakayama, and D. Kim* Department of Applied Physics, Graduate School of Engineering, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan ABSTRACT: The photoluminescence (PL) properties of selfassembled monolayers (SAMs) of CdSe and CdSe/ZnS quantum dots (QDs) were investigated. From measurements of the temperature dependence of PL spectra, it is demonstrated that the thiol group, which is a constituent of the reagent for formation of the SAM, degrades the PL properties during the SAM formation process. With an additional dipping treatment in a Cd(ClO4)2 aqueous solution with pH = 10, the band-edge PL intensity is increased remarkably. The improvement of PL properties is attributed to QD surface modification with a Cd(OH)2 layer. On the other hand, the defect-related PL band is not observed at all, even at 10 K, in the SAMs of CdSe/ZnS core/shell QDs. This fact indicates that the ZnS shell layer prevents degradation of the PL properties during the SAM process. CdS,24,25 metal nanoparticles such as Au26 and Ag,27 and magnetic nanoparticles of Fe3O4.28 On the other hand, the formation of a self-assembled monolayer (SAM) has been used to attach oil-soluble QDs to substrates. The multilayers of QDs can be fabricated by sequential dipping of the substrate into dithiol solutions and QD dispersions. Although some successful preparations of SAMs of semiconductor QDs such as CdS29 and CdSe QDs have been reported,30−32 little attention has been paid to their PL properties. It is well-known that the thiol group influences PL properties of CdSe QDs.33,34 In ref 34, the strong quenching of PL of CdSe QDs by thiol ligands is reported. It is demonstrated that the photogenerated holes are trapped by the thiol ligands on the surface of the QDs, which quenches PL of the QDs and initiates photooxidation of the surface ligands. Thus, changes of PL properties during the SAM formation of CdSe QDs should be studied cautiously. This is connected with our motivation in the present work. In the present paper, we report on the PL properties of the SAM of CdSe QDs. In order to reveal the influence of the thiol group on PL properties, we measured the temperature dependence of PL spectra. The relative intensity of the defect-related PL band to that of the band-edge PL is increased at low temperatures, demonstrating that PL properties are degraded during the SAM formation process. By dipping the SAM of the CdSe QDs in an aqueous solution containing Cd2+ ions, the band-edge PL intensity is increased again. Furthermore, SAM formation from CdSe/ZnS core/shell QDs was also performed. It is demonstrated that the existence

1. INTRODUCTION Semiconductor quantum dots (QDs) have been intensively investigated from a scientific viewpoint to understand the intrinsic characteristics, especially those related to the quantum confinement effect on physical and/or chemical properties of QDs, as well as from interest in applications to new functional materials.1−4 The breakthrough in synthesizing colloidal QDs with a high photoluminescence (PL) yield has led to an explosive increase of QD studies and opened up possibilities for various applications such as biomolecular imaging,5 QD lasing,6,7 QD solar cells,8,9 and so on. CdSe QDs have been a model material for the QD studies10−16 because the synthesis of size-controlled QDs with high PL yields has been wellestablished.17−19 From the viewpoint of application for optoelectronic devices, it is essential to extract QDs from the solution and to disperse them homogeneously into matrices such as polymer films or glasses. Furthermore, in order to understand the PL mechanism, it is necessary to conduct systematic studies of the temperature dependence of PL spectra. Such a systematic investigation requires film samples because of a temperature control from low to room temperature. A typical method for preparing film samples is spin-coating and/or drop-casting of a mixed solution of QDs and polymer solutions. In comparison with these popular methods, a layer-by-layer (LBL) assembly process is a simple and convenient method for preparing highly homogeneous multilayers of QDs.20,21 For water-soluble QDs, the LBL assembly is based on the sequential adsorption of negatively charged colloidal QDs and positively charged polyelectrolytes. Many successful preparations of multilayers of QDs by LBL assembly have been reported so far for semiconductor QDs such as CdTe22,23 and © 2012 American Chemical Society

Received: December 4, 2011 Revised: February 6, 2012 Published: February 13, 2012 5456

dx.doi.org/10.1021/jp2116609 | J. Phys. Chem. C 2012, 116, 5456−5459

The Journal of Physical Chemistry C

Article

of the ZnS shell prevents degradation of PL properties due to the thiol group.

2. EXPERIMENTAL SECTION CdSe QDs ∼3.3 nm in diameter were purchased from Sigma− Aldrich. CdSe/ZnS core/shell QDs ∼4 nm in diameter were purchased from Evident Technologies. The substrates of quartz were cleaned by immersion in fresh piranha solution [1/3 (v/v) mixture of 30% H2O2 and 98% H2SO4] for 20 min. (Caution: Piranha solution reacts violently with organic materials.) The substrates were rinsed with water and then used immediately for the sample preparation after cleaning. In the beginning of the sample preparation, SAMs that consists of (3mercaptopropyl)trimethoxysilane (MPTMS) molecules were deposited to anchor the QDs. SAM formation was performed by immersing the precleaned substrate in a MPTMS/toluene solution for 1 h. Next, the substrate was rinsed thoroughly with toluene. The SAM of the QDs linked by thiol molecules was prepared in a QD/toluene solution for 1 h. Finally, the samples were thoroughly rinsed with toluene and dried with a stream of nitrogen gas. The multilayer structure of the CdSe QDs was prepared by a layer-by-layer (LBL) assembly method. The LBL assembly was performed by sequential dipping of the substrates into the CdSe QD dispersions and dithiol solutions. For comparison with the SAM samples, we prepared polystyrene films containing CdSe QDs as follows. The QD solution was mixed with 7 wt % polystyrene/toluene solution. The mixed solution was spread on a glass, and then toluene was evaporated by heating at 60 °C for 1 h. It is noted that no thiol group was included in the film samples. Absorption spectra were measured with a double-beam spectrometer with a resolution of 0.2 nm. For PL measurements, the 325 nm line of a He−Cd laser was used as the excitation-light source, and the emitted PL was analyzed with a single monochromator with a spectral resolution of 0.5 nm. The sample temperature was controlled by use of a closed-cycle helium-gas cryostat.

Figure 1. (a) Absorption and PL spectra at room temperature of the solution sample of CdSe QDs. (b) Absorption spectra at room temperature of the SAM of CdSe QDs. (c) Optical density at 2.3 eV of the multilayer structures of CdSe QDs as a function of layer number.

3. RESULTS AND DISCUSSION Figure 1a shows the absorption and PL spectra at room temperature (RT) of the CdSe QDs dispersed in toluene solution. In the PL spectrum, the band-edge PL band is dominant, and a defect-related PL band is very weak. These results indicate the high crystallinity of the CdSe QDs. Figure 1b shows the absorption and PL spectra at room temperature of the SAM of the CdSe QDs. It is noted that the absorption and PL spectra of the SAM are comparable to those of the solution sample shown in Figure 1a. Figure 1c shows the optical density at 2.3 eV of the multilayer structures of CdSe QDs as a function of layer number. A linear dependence of optical density on layer number is clearly observed. These results indicate that the CdSe QDs were adsorbed homogeneously during the multilayer deposition. This is an advantage of the self-assembly process for preparing homogeneous QD thin films. It is well-known that the thiol group influences PL properties of CdSe QDs.33,34 Although the PL spectrum of the SAM of CdSe QDs is comparable to that of the solution sample as shown in Figure 1, it should be mentioned that measuring the temperature dependence of the PL spectra, especially at low temperatures, is important for revealing the influence of the thiol group on the PL properties, because PL measurements at

Figure 2. Temperature dependence of PL spectra of (a) SAM and (b) polystyrene film sample of CdSe QDs.

low temperatures are sensitive to defects or localized states. So far, most of the studies on PL properties of colloidal QDs have been limited to solution samples, and little attention has been paid to the temperature dependence of PL properties. Figure 2a shows the temperature dependence of PL spectra of the SAM of CdSe QDs. It is obvious that a defect-related PL band with a large Stokes shift of ∼0.5 eV is dominant at low temperatures, while the relative intensity of the band-edge PL increases with increasing temperature. For comparison, we measured the temperature dependence of PL properties of the CdSe QDs dispersed in polystyrene films. The film samples are free from the thiol group. As shown 5457



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Figure 3. Absorption and PL spectra of the SAM of CdSe QDs (a) before and (b) after the dipping treatment. Figure 4. Temperature dependence of PL spectra of the SAM of CdSe QDs after the dipping treatment.

in Figure 2b, the temperature dependence of PL spectra in the film sample is completely different from that in the SAM shown in Figure 2a. Although the defect-related PL is observed at low temperatures from 10 to 140 K, the band-edge PL is dominant. These results clearly demonstrate that the thiol group degrades the PL properties of the SAM of CdSe QDs. To improve the PL properties, an additional dipping treatment of the SAM of CdSe QDs was performed in a 0.6 mM Cd(ClO4)2 aqueous solution with pH 10 according to the method in ref 35. This process corresponds to surfacemodification treatment of the CdSe QDs with a Cd(OH)2 layer.35,36 Figure 3panels a and b show absorption and PL spectra of the SAM of CdSe QDs before and after the dipping treatment, respectively. By the dipping treatment, the absorption peak is shifted to the low-energy side. It is wellknown that the effective confinement size is increased by formation of a shell, which leads to the fact that the quantum confinement effect is weakened in core/shell QDs. Thus, the low-energy shift of the absorption energy after the dipping treatment is due to Cd(OH)2 shell formation on the CdSe QDs. The band-edge PL is increased ∼5 times by the dipping treatment. These results clearly indicate that the dipping treatment with Cd2+ ions is a key factor for improvement of the PL properties of the SAM of CdSe QDs. Furthermore, suppression of the defect-related PL at low temperatures is also expected after the dipping treatment. Figure 4 shows the temperature dependence of PL spectra of the SAM of CdSe QDs after the dipping treatment. It is obvious that PL spectra, especially at low temperatures, are drastically changed. The band-edge PL band is strongly activated by the dipping treatment and is observed as the main PL band, contrary to the fact that the defect-related PL band is dominant in the as-grown sample. It is considered that the thiol is still bound directly to the QD surface even after an additional dipping treatment in a Cd(ClO4)2 aqueous solution, though, as shown in Figure 4, the band-edge PL band is strongly activated by the dipping treatment. At present, an intrinsic nature of the effect of a Cd(OH)2 layer is not clear. However, there is a possibility that a “bound exciton state” caused by the Cd(OH)2 layer is formed on the low-energy side of the free exciton state, as discussed in ref 36. The bound exciton state would suppress a trapping process to defects, and the band-edge PL is observed as a main PL band. As shown in Figure 3, a remarkable difference is not observed in the line shape of two PL spectra at room temperature before and after the dipping treatment. We note that the influence of the thiol group on PL properties of the SAM structure is clarified by measuring the temperature dependence of PL spectra.

Figure 5. Temperature dependence of PL spectra of the SAM of CdSe/ZnS QDs. (Inset) Temperature dependence of the integrated PL intensity.

Formation of the Cd(OH)2 shell is a key factor for improvement of the PL properties of the SAM of CdSe QDs. It is well-known that CdSe/ZnS QDs are a model material for core/shell QDs since the synthesis of size-controlled QDs with high PL yields has been well-established. It is expected that the existence of the ZnS shell prevents degradation of the PL properties due to the thiol group. Thus, we prepared the SAM of CdSe/ZnS QDs. Figure 5 shows the temperature dependence of PL spectra of the SAM of CdSe/ZnS QDs. Only the band-edge PL is observed, and the defect-related PL is not observed at all, even at 10 K. The inset shows the temperature dependence of integrated PL intensity. We note that the PL intensity at room temperature was almost 60% of that at 10 K, which means that the thermal quenching effect is very small. As expected, these results indicate that the thiol group does not affect the PL properties of the CdSe/ZnS QDs, owing to the ZnS shell. Thus, the present results indicate a potential of the SAM of core/shell QDs for application to optoelectronic materials.

4. CONCLUSIONS We have investigated the PL properties of the SAM of CdSe QDs. The relative intensity of the defect-related PL band to that of the band-edge PL is increased at low temperatures, which indicates that the thiol group degrades PL properties during the SAM formation process. By an additional dipping treatment in a Cd(ClO4)2 aqueous solution with pH = 10, the band-edge PL intensity is increased remarkably. The improvement of PL properties is due to surface modification of the QDs with the Cd(OH)2 layer. In the SAM of CdSe/ZnS core/ shell QDs, the band-edge PL band is dominant, and the defectrelated PL band is not observed at all, even at 10 K. These 5458



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results indicate that the ZnS shell layer prevents degradation of PL properties during the SAM process.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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Photoluminescence Properties of Self-Assembled Monolayers of

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