J. Phys. Chem. C 2009, 113, 7015–7018
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Layer-by-Layer Assembly of Colloidal CdS and ZnS-CdS Quantum Dots and Improvement of Their Photoluminescence Properties D. Kim,* S. Okahara, K. Shimura, and M. Nakayama Department of Applied Physics, Graduate School of Engineering, Osaka City UniVersity 3-3-138, Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan ReceiVed: December 02, 2008; ReVised Manuscript ReceiVed: March 12, 2009
Thin films of colloidal CdS and ZnS-CdS alloy quantum dots (QDs) were fabricated by layer-by-layer (LBL) self-assembly to synthesize blue luminescent QD thin films. The assembly of negatively charged colloidal QDs and positively charged poly(diallyldimethylammonium chloride) results in QD/polymer multilayers. The relative intensity of the band-edge photoluminescence (PL) to that of the defect-related PL was decreased during the LBL assembly process. With an additional dipping treatment in a Cd(ClO4)2 aqueous solution with pH 10, the band-edge PL intensity was recovered. The PL decay profiles before and after the dipping treatment were considerably different. From this fact, we conclude that the enhancement of the band-edge PL intensity originates from the reduction of nonradiative recombination processes at the QD surface. 1. Introduction Semiconductor quantum dots (QDs) have been intensively investigated to understand their characteristic chemical/physical properties.1,2 They now have become a promising candidate for a novel high-efficiency luminescence material and for a new class of fluorescent labels because they have substantially high photoluminescence (PL) yield and wide-ranging color tunability. CdSe QDs have been a model material for QD studies3-9 since the synthesis of size-controlled QDs with high PL yield has been well-established.10-12 Those QDs are synthesized in hightemperature solvent and are soluble in oil. Another typical method for synthesizing QDs is a colloidal method that results in water-soluble QDs,13-15 which is suitable for biological systems. It is important to extract QDs from the solution and to disperse them into polymer films from the viewpoint of application for optoelectronic devices. Furthermore, to understand the PL mechanism, it is necessary to conduct systematic studies of the temperature dependence of PL spectra and PL decay profiles. Such a systematic investigation needs film samples because of temperature control from low temperatures 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. Compared to these popular methods, a layer-by-layer (LBL) assembly process is a simple and convenient method for preparing highly homogeneous QD/polymer multilayers as pointed out by Decher and co-worker16,17 and also by Kotov.18,19 Many successful preparations of QD/polymer multilayers by LBL assembly have been reported so far using semiconductor QDs such as CdTe,20,21 HgTe,22 CdS,23 CdSe,24,25 and TiO2,26,27 metal nanoparticles such as Au28 and Ag,29 and magnetic nanoparticles of Fe3O4.30 Especially, in ref 20 Mamedov et al. successfully fabricated “graded semiconductor films” using different-sized CdTe QDs by LBL assembly. Furthermore, in ref 21 a cascaded energytransfer structure made of CdTe QDs was proposed, and a * To whom correspondence should be addressed. E-mail:
[email protected].
remarkable enhancement of PL intensity was realized. Because CdS has a higher band gap energy, 2.5 eV, than that of CdTe and CdSe, multilayers of CdS QDs will be a promising candidate for application to photonic devices in the blue and UV region. It is well-known that the defect-related PL band is usually dominant, and the intensity of the band-edge PL is very weak in colloidal CdS QDs in aqueous solutions.31,32 It was reported that a surface-modification process by adding Cd2+ ions to aqueous solutions of colloidal CdS QDs markedly improves the PL properties,33,34 which is due to the formation of a Cd(OH)2 layer on the surface of the CdS QD. Because the PL properties of colloidal CdS QDs are highly sensitive to surface structures, changes of PL properties during the LBL assembly process of colloidal CdS quantum dots should be studied carefully. This is connected with our motivation in the present work. In the present paper, we report on PL properties of CdS QD films fabricated by LBL assembly. The relative intensity of the defect-related PL band to that of the band-edge PL was increased during the LBL assembly process; that is, the PL properties were degraded. When CdS-LBL films are dipped in an aqueous solution containing Cd2+ ions, the band-edge PL intensity increased again. We discuss the PL recovery from the viewpoint of PL dynamics. Furthermore, the LBL assembly using ZnS-CdS alloy QDs was also performed. In alloy semiconductor systems, the band gap energies can be controlled by changing the alloy composition. Thus, by fabricating alloy QDs, we can control the optical properties by the QD size and by the alloy composition. Because the band gap energy of ZnS is higher than that of CdS, the PL wavelength becomes shorter by using ZnS-CdS alloy QDs. 2. Experimental Section The colloidal CdS QDs were prepared by injecting a H2S gas (0.2 mmol) into 100 mL of an aqueous solution containing 2 mL of 0.1 M Cd(ClO4)2 and 2 mL of 0.1 M sodium hexametaphosphate (HMP), which is a dispersing agent of colloids. In PL spectra of CdS QDs prepared by the colloidal method, a surface-defect-related PL band usually had been observed as a main PL band. Surface modification of QDs was
10.1021/jp810577p CCC: $40.75 2009 American Chemical Society Published on Web 04/07/2009
7016 J. Phys. Chem. C, Vol. 113, No. 17, 2009 performed to improve their PL properties. The QD surface was modified by the addition of 6 mL of 0.1 M Cd(ClO4)2 after the pH of the solutions was adjusted to fall within the alkaline region, which leads to the formation of a Cd(OH)2 layer on the surface of QDs.33 The PL quantum yield of the samples was estimated by comparison with solutions of perylene in ethanol, following previously reported methods.34,35 The solution sample of the CdS QDs has a quantum yield of ∼10%. A photoetching treatment was performed to prepare CdS QDs at a smaller size by irradiating a monochromatic light to the sample solution.36,37 For the photoetching treatment of the QDs, a 500 W Xe lamp was used as a light source. Monochromatic light (466 nm) was obtained by using interference filters: The full-width at half of the intensity maximum of the monochromatic light was ∼10 nm. After the photoetching treatment, the surface modification with a Cd(OH)2 layer was applied. ZnS-CdS alloy QDs were prepared by injecting H2S gas (0.2 mmol) into 100 mL of aqueous solutions containing 0.1 M Zn(ClO4)2 and 0.1 M Cd(ClO4)2 with a constant total volume of 2 mL. The volume ratio of Zn(ClO4)2 and 0.1 M Cd(ClO4)2 solutions was between 0 and 0.4. The size distribution of QDs was controlled by using a photoetching technique. After the photoetching, the QD surface was modified by the addition of 6 mL of 0.1 M Cd(ClO4)2 after adjusting the pH of the solutions to fall within the alkaline region: This process leads to the formation of a Cd(OH)2 layer on the surface of the ZnS-CdS QDs.38,39 The substrates of quartz used for the LBL deposition 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 after cleaning. The LBL assembly was performed by sequential dipping of the quartz substrates in aqueous solutions of positively charged poly(diallydimethylammonium) chloride (PDDA, Mw ) 100000-200000) and negatively charged colloidal QDs. The 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 excitationlight source, and the emitted PL was analyzed with a single monochromator with a spectral resolution of 0.5 nm. For measurements of PL decay profiles, third-harmonic-generation (THG) light (355 nm) of a laser diode-pumped yttrium aluminum garnet (YAG) laser with a pulse duration of 20 ns and a repetition of 10 kHz was used as the excitation light. The PL decay profiles were detected with a streak-camera system. 3. Results and Discussion Figure 1a shows the absorption and PL spectra of surfacemodified CdS QDs prepared by the colloidal method. The absorption structure is observed in a higher energy region than that of a CdS bulk crystal (the latter having a band gap energy of ∼2.5 eV), indicating the formation of CdS QDs. However, in the absorption spectrum, no distinguishing peak structure is observed, owing to a broad size distribution of QDs. The broken curve indicates the PL spectrum before the surface modification. The broad PL band that originates from defects is dominant. The PL spectrum after the surface modification is depicted by the solid curve. The band-edge PL is strongly activated by the surface modification and is observed as a main PL band. It was reported that the surface-modification process by adding Cd2+ ions to aqueous solutions of colloidal CdS QDs markedly
Kim et al.
Figure 1. (a) Absorption and PL spectra of surface-modified CdS QDs prepared by the colloidal method. (b) Absorption spectra of the LBL films with a different number of bilayers of (PDDA/CdS)n. (c) Optical density at 2.68 eV of the LBL films as a function of the number of bilayers.
improves the PL properties,33,36 which is due to the formation of a Cd(OH)2 layer on the surface of the CdS QD. Figure 1b shows absorption spectra of CdS-LBL films with a different number of bilayers of (PDDA/CdS)n. For the LBL assembly process, solutions of the surface-modified CdS QDs were used. The broken curve denotes the absorption spectrum of the solution sample of the surface-modified CdS QDs for comparison. It is noted that the absorption spectra of the LBL films are comparable to that of the solution sample. Figure 1c shows optical density at 2.68 eV of the LBL films as a function of the number of bilayers of (PDDA/CdS)n. A linear dependence of optical density on the bilayer number (n) is clearly observed. These results indicate that the CdS QDs were adsorbed uniformly during the LBL deposition in proportion to the number of bilayers. This is a merit of the LBL assembly for preparing homogeneous QD thin films. The total layer thickness of (PDDA/CdS)n with n ) 10 was measured by spectroscopic
LBL Assembly of Colloidal CdS and ZnS-CdS QDs
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Figure 3. PL decay profiles of the band-edge PL of the LBL films before and after the dipping treatment. The broken curve denotes the PL decay profile of the solution sample of CdS QDs for the comparison. Figure 2. (a) Absorption and PL spectra of the as-grown LBL film. (b) Absorption and PL spectra of the LBL film after the dipping treatment in 0.6 mM Cd(ClO4)2 aqueous solution with pH 10.
ellipsometry: The thickness was ∼50 nm. Furthermore, the roughness of the film was measured by atomic force microscopy and had a root-mean-square roughness of 2.5 nm. Figure 2a shows the PL spectrum of as-grown CdS-LBL film with n ) 10. In comparison to the PL spectrum of the surface-modified CdS QDs shown in Figure 1a, it is obvious that the relative intensity of the defect-related PL to that of the band-edge PL increased during the LBL assembly process. Kim et al. reported that PL properties of colloidal CdS QDs are very sensitive to the pH value and that the best pH value is ∼10.40 In the LBL assembly process, the substrates were rinsed with water (pH 7) and were dipped in aqueous solutions of PDDA (pH 7) after the deposition of a layer of CdS QDs (pH 10). The dipping in aqueous solutions with pH 7 might cause the deviation from the optimum condition of the CdS/Cd(OH)2 structure and result in degradation of the PL properties. To improve the PL properties, an additional dipping treatment of the LBL-CdS film was performed in 0.6 mM Cd(ClO4)2 aqueous solution with pH 10. This process corresponds to the surface-modification treatment of CdS QDs in the LBL film with a Cd(OH)2 layer. As shown in Figure 2b, the relative intensity of the band-edge PL increased again. Thus, it is noted that the dipping treatment using Cd2+ ions is a key factor for the improvement of the PL properties of the CdS-LBL films. The band-edge PL was increased ∼10 times by the dipping treatment. Next, we discuss effects of the dipping treatment on the PL properties of the LBL films from the viewpoint of PL dynamics. Figure 3 shows the temporal profiles of the band-edge PL of the LBL film before and after the dipping treatment. For comparison, the temporal profile of the solution sample of the surface-modified CdS QDs was also depicted as a broken curve. The PL decay profile before the dipping treatment exhibits a faster decay as compared with that after the dipping treatment. In a previous paper,36 we reported the temperature dependence of photoluminescence dynamics in CdS QDs. The decay profile of CdS QDs can be explained quantitatively by a combination of one monoexponential and one stretched exponential functions:
A1 exp(-t ⁄ τ1) + A2 exp[-(t ⁄ τ2)β]
(1)
where τ1 is the decay time of the bright exciton state and τ2 corresponds to the decay time that is characterized by the dark exciton state and bound exciton state in quasi-thermal equilib-
Figure 4. (a) Absorption and PL spectra of the surface-modified CdS QD solutions after the photoetching treatment. (b) Absorption and PL spectra of the LBL film after the dipping treatment.
rium. To estimate the decay time, the PL decay profiles were fitted using the convolution between the laser pulse profile with the Gaussian function and the decay function of eq 1. From the analysis, a notable change of τ1 was observed: 5 and 30 ns before and after the dipping treatment, respectively. The fast decay profile before the dipping treatment would reflect nonradiative recombination processes,40 whereas the relatively slow profile after the dipping treatment suggests that the nonradiative processes are reduced, which results in the enhancement of the band-edge PL intensity. As shown in Figure 2, the band-edge PL energy in the LBL film is ∼2.54 eV (488 nm), which is almost the same as that in previously reported LBL films of CdS QDs.23 A shorter PL wavelength is desirable for the application to blue luminescent materials. We have tried two approaches toward the synthesis of blue luminescent LBL films: (1) LBL assembly with use of smaller CdS QDs, where the quantum confinement effect leads to a shorter PL wavelength and (2) LBL assembly with use of ZnS-CdS alloy QDs, where the increase in ZnS composition results in an increase in band gap energy in ZnS-CdS alloy. The photoetching treatment makes it possible to make smaller CdS QDs.36,37 Figure 4a shows the absorption and PL spectra of the solution sample of the surface-modified CdS QDs after the photoetching treatment. Before the photoetching, no excitonic absorption peak is observed because of a wide size
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Kim et al. Acknowledgment. This work was supported in part by a Grant-in-Aid for Scientific Research from JSPS. References and Notes
Figure 5. PL spectra of the LBL films of CdS and ZnS-CdS QDs with different ZnS alloy compositions. The inset shows a photograph of the LBL film of CdS QDs illuminated by a 325 nm light.
distribution of the QDs as shown in Figure 1a. It is obvious that the absorption spectrum after the photoetching exhibits clear peak structures, indicating the reduction of the size-distribution width. The band-edge PL energy also shifts to the higher energy side, owing to the quantum confinement effect. Figure 4b shows the absorption and PL spectra of the LBL film of the surfacemodified CdS QDs after the photoetching treatment. The dipping treatment was also performed after the LBL assembly process. Absorption and PL spectra of the LBL film are almost consistent with those of the solution sample, indicating the success of the LBL self-assembly with use of the photoetched CdS QDs. The PL peak energy appears at ∼2.77 eV (448 nm). Thus, the blue shift of 230 meV is realized. Furthermore, the LBL assembly using ZnS-CdS alloy QDs was also performed to synthesize blue luminescent QD thin films. Figure 5 shows PL spectra for the LBL films of ZnS-CdS alloy QDs with different alloy compositions. It is obvious that the PL energy is blue-shifted with an increase in ZnS composition: the PL energy of the LBL film with the ZnS composition of 0.4 is 2.95 eV (420 nm). Thus, the present results indicate a potential of the LBL assembly of CdS and ZnS-CdS alloy QDs in the application to blue luminescent materials. 4. Conclusion We have investigated the PL properties of the thin films of colloidal CdS and ZnS-CdS alloy QDs. Thin films of the QDs were fabricated by the LBL assembly of negatively charged colloidal QDs and positively charged PDDA. The relative intensity of the band-edge PL to that of the defect-related PL was decreased during the LBL assembly process. Dipping treatment in an aqueous solution containing Cd2+ ions with pH 10 improves the PL properties of the LBL films. From the viewpoint of PL dynamics, it is demonstrated that the dipping treatment results in the reduction of the nonradiative recombination processes at the QD surface. The present results open up the possibility of the LBL thin films of the CdS and ZnS-CdS alloy QDs to application to blue luminescent materials.
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