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Fluorescence Spectroelectrochemistry of Multilayer Film Assembled CdTe Quantum Dots Controlled by Applied Potential in Aqueous Solution Lihua Jin, Li Shang, Junfeng Zhai, Jing Li, and Shaojun Dong* State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Graduate School of the Chinese Academy of Sciences, Chinese Academy of Sciences, Changchun, 130022, China ReceiVed: September 04, 2009; ReVised Manuscript ReceiVed: December 09, 2009
The change of the photoluminescence (PL) property of CdTe quantum dot (QD) assembled multilayers along with applied potential has been investigated in aqueous solution. By using the in situ fluorescence spectroelectrochemistry technique, we examined the PL property of QDs upon applying potentials under different atmosphere media, in air and in nitrogen. It was found that when the sample was under nitrogen or exposed to air, the luminescence of QDs bear quenching by adding positive potentials. On the contrary, while applying negative potentials, the luminescence of QD was weakened only in the air condition. It seemed that the luminescence of CdTe QDs was hardly affected by the potentials under nitrogen. On the basis of the phenomena, electrochemistry and X-ray photoelectron spectroscopy (XPS) measurements were then performed to explain the possible origins of PL changes. The results clearly indicated that the variation of surface structure of QDs affected the PL property of CdTe QDs significantly. 1. Introduction Semiconductor nanocrystals (NCs), often referred to as quantum dots (QDs), have emerged as one of the most important classes of nanomaterials for over two decades. The variety of size- and shape-dependent optical and electrical properties, make QDs be potential to use for a wide range of applications ranging from biological imaging to novel electro-optical devices.1-6 In most applications such as light emitting diodes and photovoltaic panels, QDs are ultimately required to be stably immobilized on a substrate as films, which can effectively eliminate the issue of colloid diffusion.7,8 However, one major problem of QDs application as an electrochromic material is that the applied potentials can strongly affect the luminescence properties, and even irreversibly quench the luminescence. Consequently, a series of research has been directed toward better understanding this unusual phenomenon in order to engineer better devices.9-14 For example, Guyot-Sionnest and co-workers have studied the luminescence of charged CdSe QDs film and demonstrated that the electron could be injected into semiconductor NC thin films controllably and reversibly, accompanying the bleaching or recovery of the luminescence.10 Recently, Gooding et al. investigated a two-dimensional (2D) self-assembled monolayer of CdSe QDs by recording the changes of the photoluminescence (PL) spectra as a function of applied potential under atmospheres of either nitrogen or air. It was revealed that the PL response to the charge injection was strongly linked to the environmental effects such as gases and water vapor.14 Nevertheless, most of previous works have been conducted in anhydrous organic media, and scarce studies can be found to investigate the luminescent properties of QDs within a multilayer film in an aqueous environment.15 CdTe, as a useful group II-IV compound semiconductor, has been demonstrated to be one of the most robust and highly luminescence nanoparticle materials directly synthesized in an aqueous medium.16 Recently, the progress in device fabrication * To whom correspondence should be addressed. Fax: +86-043185689711. E-mail:
[email protected].
technology has motivated several attempts to use these CdTe QDs as elemental building blocks for the next generation of nanodevices.17,18 Thus, the necessity to deeply study their optical property change in response to charges becomes urgent. Hence, we investigated the influence on the luminescence of these QDs upon addition of applied potentials under two different atmosphere media: in air and in nitrogen. It was found that applied positive potential led to the irreversible quenching of the luminescence, while applying a negative potential could cause two different results. The PL was quenched in air, whereas it was hardly affected under nitrogen. These results indicated that the environment played a key role in the PL intensity of QDs. On the basis of these phenomena, we performed electrochemistry and X-ray photoelectron spectroscopy (XPS) studies to explain the observed spectroelectrochemistry results. Moreover, the present investigation, which is established in an aqueous solution, is of special interest for bioapplications of QDs. 2. Experimental Section 2.1. Materials. Mercaptosuccinic acid (MSA) was purchased from Aldrich. Sodium borohydride was purchased from Acros. Na2TeO3 was bought from Schering-Kahlbaum in Germany. Cadmium chloride, and trisodium citrate dehydrate were bought from Beijing Reagent Company. Polydiallyldimethylammonium chloride (PDDA, 50 w% in water, Mw ) 200 000) was obtained from Sigma. All other chemicals were of analytical grade, and the water used was doubly distilled water. Phosphate buffer solution (PBS) containing 10 mM Na2HPO4 and 10 mM NaH2PO4 (pH 7.2) was used as the electrolyte throughout the work. 2.2. Instrumentation. Spectroelectrochemical measurements (in situ fluorescence) were carried out in a modified fluorescence cell according to the previous report (1 cm-length quartz cell) at room temperature.19 The cell was capped with a Teflon plate, which also served as the electrode support. ITO electrode, platinum wire, and Ag/AgCl (saturated KCl) were used as working electrode, counter electrode, and reference electrode, respectively. The ITO electrodes with a geometric area of ∼1
10.1021/jp908574z 2010 American Chemical Society Published on Web 12/21/2009
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Figure 1. Absorption (A) and the normalized PL (B) spectra of the as-prepared CdTe QDs colloidal solution and the multilayer films (C). Excitation wavelength: 400 nm.
cm × 5 cm, which have a surface resistance of 30-60 Ω/cm2, were purchased from Nanbo Display Technology Co., Ltd. (Shenzhen, China). Electrochemical experiments were conducted with CHI832 electrochemical workstation (Shanghai Chenhua Instrument Corporation, China). PL measurements were carried out on a LS-55 luminescence spectrometer (Perkin-Elmer). Absorption measurements were performed with a Cary 500 UV-vis-NIR spectrometer (Varian). XPS measurements were carried out on an ESCALAB-MKII spectrometer (VG Co., U.K.) with Al KR X-ray radiation as the X-ray source for excitation. 2.3. Preparation of Water-Soluble CdTe QDs. Stable water-soluble CdTe QDs were prepared as described in previous papers.20 Briefly, cadmium chloride (CdCl2 · 2.5H2O, 0.04 M, 16 mL) was diluted to 200 mL in a one-necked flask, and trisodium citrate dihydrate (400 mg), Na2TeO3 (0.01 M, 4 mL), MSA (200 mg), and sodium borohydride (NaBH4, 400 mg) were added with stirring. When the color of the solution changed to green, the flask was attached to a condenser and refluxed under open-air conditions for 7 h. The resulting CdTe QDs were washed with ethanol and separated by centrifugation. Finally, the prepared CdTe QDs were dispersed in water. 2.4. Preparation of CdTe QDs Multilayers on Indium Tin Oxide (ITO). CdTe QDs multilayers were prepared using a spin-coater. Before modification, the ITO chips were ultrasonically washed with acetone, ethanol, and water. After being immersed in a solution of 1:1 (v/v) ethanol/NaOH (1 M) for 15 min, they were rinsed with water. The chips were dipped in a 5 mg/mL PDDA aqueous solution (pH 7.0) for 20 min to modify a monolayer of PDDA. Then the modified substrates were rinsed with water several times to remove the physically adsorbed PDDA, and dried under a stream of nitrogen. Then 50 µL aliquot of 0.5 mg/mL PDDA and 50 µL aliquot of CdTe QDs colloidal solution were alternately pipetted onto an ITO-coated chip and spun at 2700 rpm for 20 s, with intermediate water rinsing and N2 drying. Multilayer films could be formed by repeating these two steps in a cyclic fashion. (PDDA/CdTe)5 films on ITO substrates were selected for the spectroelectrochemistry investigation, and the thickness of the films determined by Dektak 6 M stylus profiler was estimated to be ca. 28-35 nm. Considering the possible error of the film thickness by this operation of the spin-coater, all the experiments were reproduced at least three times. 3. Results and Discussion 3.1. Characterization of CdTe QDs Colloidal Solution and Multilayer Films. Figure 1 presents typical absorption and PL
Figure 2. The emission spectra of CdTe QDs multilayers with different positive potentials being applied to the ITO working electrode in air (A) or nitrogen (C). The normalized PL peak intensity as a function of the applied potential in air (C) or nitrogen (D). The results were acquired 30 s after the potential step to the films. Excitation wavelength: 400 nm.
spectra of CdTe QDs aqueous solution. The distinct absorption (A) and PL (B) peaks indicate a highly monodisperse sample. The absorption shoulder of CdTe QDs is located at 554 nm, while the emission spectrum displays an emission maximum around 583 nm upon excitation at 400 nm. The size of CdTe QDs in solution is estimated to be ca. 3.4 nm in virtue of the following empirical formula according to previous reports.21,22
D ) (9.8127 × 10-7)λ3 - (1.7147 × 10-3)λ2 + (1.0064)λ - 194.84 where D (nm) is the size of CdTe QDs, and λ (nm) is the wavelength of the first excitonic absorption peak of the corresponding sample. The emission spectrum of CdTe QDs multilayer films is characterized by curve C in Figure 1. A redshift of the PL peak has occurred as the number of bilayers increased, which is possibly due to interparticle interaction in the multilayer film. When CdTe QDs were deposited as a thin film, the bound polymer can hold QDs closer together in the film than in suspension, which will facilitate energy transfer from small to large QDs and result in an apparent red shift.23 3.2. Effect of Positive Potential in Air or N2. A series of spectra were recorded under the control of potential in the spectroelectrochemical cell. As shown in Figure 2A, with increasing the positive potential, the PL of QDs decreased gradually. Meanwhile, a blue shift in the maximum of the PL spectrum accompanied quenching was observed. The decrease was irreversible; returning the cell to open circuit did not lead to the recovery of the original PL intensity. Likewise, the PL of QDs was also found to be affected by applying positive potentials under nitrogen (Figure 2C). The PL decreased quickly as the applyied potentials were above 0.2 V, followed by a slow decrease at approximately 0.6 V. Analogous to the air condition, no recovery was observed upon reversal of the potential. According to the previous studies, the majority of II-VI binary semiconductors such as CdS and CdSe might undergo anodic decomposition, because valence band holes are thermodynamically unstable within the lattice.24 Therefore, it was postulated that the observed quenching phenomenon herein might also be due to the anodic unstableness of CdTe just as with other QDs. To verify these hypotheses, XPS was then
Effect of Applied Potential on PL of CdTe QD Multilayers
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Figure 3. XPS spectra of Te 3d for freshly prepared CdTe QDs multilayers (sample 1), and after applying positive potential (+1.2 V) to CdTe QD multilayers in air (sample 2), and under nitrogen (sample 3).
Figure 5. The emission spectra of CdTe QDs multilayers with different negative potentials being applied to the ITO working electrode in air (A). The normalized PL peak intensity as a function of the applied potential in air (B) or nitrogen (C).The results were acquired 30 s after the potential step to the film. Excitation wavelength: 400 nm.
Figure 4. XPS spectra of Te 3d5/2 of the CdTe QDs mulilayers before (A) and after (B) applying positive potential (+1.2 V) under nitrogen.
carried out to investigate the changes of surface structure of CdTe QD assembled multilayer film upon the applied potential. Herein, three samples were prepared for the analysis by XPS, i.e., sample 1 for freshly made CdTe QDs multilayers, and samples 2 and 3 for applying the positive potential CdTe QD multilayers in air and nitrogen, respectively. As shown in Figure 3, the Te 3d5/2 and Te 3d3/2 peak in all three samples, centered around 572 and 582.5 eV, which correspond to the values reported in CdTe bulk studies,25,26 showed no obvious difference. An inspection of the spectral region of sample 2 indicated that, after applying positive potential, a new valence state of Te appeared on the surface of QDs, with binding energies of about 575.5 and 585.9 eV. These new peaks were attributed to the oxidized Te surface atoms.27 Furthermore, applying positive potential also produced a new peak of Te0 in sample 3. According to the previous reports, the shift of 0.6 eV toward higher binding energy was related to the binding energy of Te0,28 and a rather good fit of Te 3d5/2 XPS spectra was observed (Figure 4). This result indicated that plenty of Te0 appeared at the surface of sample 3. Referring to early studies, Borchert and co-workers reported that tellurium surface atoms existing
in either unpassivated or oxidized tellurium forms could act as traps to decrease the PL efficiency.28 Therefore, the changes of surface structure of CdTe QDs explained the decrease of their luminescence effectively. Concurrently, the observed distinct blue-shift of PL peak position might also suggest the dissolution of QDs, with the size of QDs getting smaller.29 Furthermore, on the bais of the previous reports and XPS results herein, we tentatively postulated that unpassivated and oxidized tellurium might have different quenching rates on the PL of QDs. 3.3. Effect of negative potential in Air or N2. Figure 5A shows the PL result of CdTe QDs multilayers when the negative potential was added in open air. The applied potential was varied from 0 to -0.8 V, considering no interference with the electrolytic water in this scope. As seen, once the applied negative potential reached -0.4 V, the PL started to decrease distinctly, and a slight blue-shift was observed. Note that prior to charging, the PL was stable, and we did not observe visible change until the negative potential was applied. Evolution of the luminescence under an atmosphere of nitrogen is shown in Figure 5C, where a different result was observed: the PL intensity was hardly affected by the negative potential, which consequently suggested that the CdTe QDs were stable at the applyied negative potential in this scope. On the basis of the above observations, one presumption for explaining the quenching in air condition was the combination of the oxygen and the applied negative potential. Similarly, XPS spectra were then compared in the region of Te 3d5/2 in the absence and presence of the negative potential. As seen in Figure 6, the neutral valence state of Te increased distinctly on the surface upon applying negative potential. Excess Te0 was considered as the crucial component in decreasing the PL efficiency, which acted as traps to induce the nonradioactive recombination pathways to weaken the luminescence.28,30 Referred to previous reports, the presence of Te0 species was postulated to arise from surface degradation of QDs by the
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Figure 8. XPS spectra of Te 3d5/2 of the CdTe QDs mulilayers before (A) and after immerging into 0.1 mg/mL hydrogen peroxide solution (B).
Figure 6. XPS spectra of Te 3d5/2 of the CdTe QDs mulilayers before (A) and after applying negative potential (-0.8 V) in air (B) and in nitrogen (C).
mL hydrogen peroxide solution for 10 min. Then, through investigating XPS spectra (Figure 8), it was found that excess Te0 was also present on the surface of this sample, as in Figure 6B, which supported the possible generation of hydrogen peroxide upon applying potential that caused the quenching of QDs. 4. Conclusions
Figure 7. Differential pulse voltammograms of PBS in air (A) and nitrogen (B) at a QD multilayer assembled ITO electrode with scan from 0 to -1.0 V.
existence of hydrogen peroxide.31 Hydrogen peroxide, as a strongly oxidizing agent and effective quenching agent, would affect the PL of QDs effectively.32,33 When the film was exposed to air, the existence of oxygen might be reduced by the applied potential and generated hydrogen peroxide as a product. Owing to the fact that the concentration of oxygen in the buffer solution was small, it was difficult to obtain a good electrochemistry for oxygen to prove its variation. Thus, differential pulse voltammetry (DPV) was used due to its ability to obtain a good signal over background. As shown in Figure 7, compared with that under nitrogen, a reduction peak of the oxygen at -0.4 V in the PBS was observed. This reduction potential was likely to be attributed to the generation of hydrogen peroxide (E0O2/ H2O2 ) -0.076 V).34,35 Moreover, in order to further validate the above presumption, another sample without applying potential was prepared and followed by immerging into 0.1 mg/
Herein, we investigated the PL properties of the CdTe QDs multilayers in aqueous solution as charge carriers were injected separately. It was found that, when a positive potential was applied to the film, the PL was irreversibly quenched. This was irrespective of whether the samples were under nitrogen or exposed to air. However, when applying negative potentials, the quenching was only observed in the air condition. Conversely, the PL intensity was hardly affected by the negative potential under nitrogen. These results clearly indicated that the environmental atmospheres had a considerable influence on the luminescence properties of the charged particles. Furthermore, we also performed XPS measurement to analyze the surface structure of QDs contrastively, and found a considerable amount of Te0 or oxidized Te present on the surface of the lowly luminescence CdTe QDs, which might explain the different observations upon applying different potentials. Moreover, these findings are thought to be of great importance for QDs in future bioapplications based on the PL of particles. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Nos. 20675076 and 20820102037). References and Notes (1) Colvin, V. L.; Schlamp, M. C.; Alivisatos, A. P. Nature 1994, 370, 354. (2) Chan, W. C.; Nie, S. M. Science 1998, 281, 2016. (3) Han, M. Y.; Gao, X.; Su, J. Z.; Nie, S. Nat. Biotechnol. 2001, 19, 631. (4) Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Science 2002, 295, 2425. (5) Achermann, M.; Petruska, M. A.; Kos, S.; Smith, D. L.; Koleske, D. D.; Klimov, V. I. Nature 2004, 429, 642. (6) Yang, J.; Zhou, Y. L.; Zheng, S. L.; Liu, X. F.; Qiu, X. H.; Tang, Z. Y.; Song, R.; He, Y. J.; Ahn, C. W.; Kim, J. W. Chem. Mater. 2009, 21, 3177–3182.
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