10146
2007, 111, 10146-10149 Published on Web 06/21/2007
An Approach for Optimizing the Shell Thickness of Core-Shell Quantum Dots Using Photoinduced Charge Transfer R. Vinayakan,† T. Shanmugapriya,‡ Pratheesh V. Nair,† P. Ramamurthy,‡ and K. George Thomas*,† Photosciences and Photonics, National Institute for Interdisciplinary Science and Technology§ (CSIR), TriVandrum 695 019, India, and National Centre for Ultrafast Processes, UniVersity of Madras, Taramani Campus, Chennai 600 113, India ReceiVed: April 11, 2007; In Final Form: June 8, 2007
Photoinduced charge-transfer dynamics between CdSe quantum dots (QDs) possessing varying monolayers of ZnS and hole scavengers such as phenothiazine (PT) and N-methylphenothiazine (NMPT) were investigated for optimizing the shell thickness of core-shell quantum dots. Spectroscopic investigations indicate that phenothiazine binds onto the surface of bare CdSe QDs, resulting in the photoluminescence quenching, and two monolayers of ZnS prevent the electron-transfer process. Methodologies presented here can provide quantitative information on the optimum shell thickness of core-shell QDs, which can suppress the undesired electron transfer and provide maximum luminescence yield.
Introduction The unique photophysical properties of semiconductor quantum dots (QDs) offer considerable promise as components in optoelectronic devices (for example, in photovoltaic devices)1 and as tags for bioimaging, labeling, and sensing.2,3 Compared to organic fluorophores, QDs possess (i) broad absorption bands, allowing the excitation over a wide range of wavelengths, (ii) large one-photon and two-photon absorption cross sections, (iii) narrow emission bands, with high tunability as a function of size, and (iv) long-lived luminescence.2e,4 These properties make them ideal candidates for biomedical applications. However, their interactions with biomolecules (for example, adenine and dopamine) result in luminescence quenching through an electrontransfer process, thereby limiting their use in biological systems.5 Bare QDs are highly susceptible to oxidation; for example, the exposed selenium sites on CdSe QDs react with molecular oxygen, resulting in the formation of SeO2 and thereby their degradation.5a,6a One of the most successful strategies adopted for imparting photostability and improving the photoluminescence (PL) efficiency is to overcoat CdSe QDs with a wide band gap inorganic material (for example, ZnS,6a-d ZnSe,6d,e and CdS6f,g). Photoinduced charge transfer in bare QDs7a-c can be prevented by overcoating with large band gap semiconductors. Overcoating can also prevent the leakage of core materials, avoiding the toxicity inside of the cell.7d However, the coreshell QDs with a large shell thickness are less effective for biological applications due to the decrease in luminescence quantum yield6 and the efficiency of light-induced energy transfer. In the initial stages of overcoating, there may be voids * To whom correspondence should be addressed. E-mail: georgetk@ md3.vsnl.net.in. † CSIR. ‡ University of Madras. § Formerly Regional Research Laboratory.
10.1021/jp072823h CCC: $37.00
on the surface, which can affect the photostability of QDs. This problem has been addressed by Bawendi and co-workers in the case of CdSe QDs overcoated with ZnS (CdSe/ZnS) by following the formation of a SeO2 peak in X-ray photoelectron spectroscopy as a function of shell thickness (analyzed by TEM and small-angle X-ray scattering).6a The peak corresponding to SeO2 is not observed for samples having >1.3 monolayers of ZnS upon exposure to air for 80 h, indicating a uniform coating.6a Recently, a method based on Raman spectroscopy has been proposed by Baranov et al. to control the quality of CdSe/ZnS QDs. Authors have suggested a morphological transition from semi-disordered coherent to incoherent epitaxial growth when the shell thickness is more than 2 monolayers.6b Since the synthesis of core-shell QDs is now routinely carried out in many laboratories, procedures which do not require any specialized instrumentation may be more desirable for the monitoring of the overcoating process. Photoinduced chargetransfer dynamics between QDs possessing varying monolayers and hole scavengers may be a convenient method for probing the overcoating process. Recently, Scaiano and co-workers have observed nonlinear effects in the quenching of the PL of CdSe QDs of varying size (2.4-6.7 nm) by free radicals, namely, TEMPO and 4-amino-TEMPO.8 On the basis of these studies, the authors have concluded that the quenching behavior involves an electron exchange mechanism and is dependent on the size of QDs. Herein, we report a methodology for probing the overcoating process by following the PL quenching of coreshell quantum dots, having varying shell thickness, in the presence of two hole acceptors (PT and NMPT). Experimental Section Details on the synthesis of QDs and the instrumental methods adopted for their characterization are presented in the Supporting Information. All of the spectroscopic studies were carried out © 2007 American Chemical Society
Letters
J. Phys. Chem. C, Vol. 111, No. 28, 2007 10147
Figure 1. HRTEM images of (A) CdSe QDs and (B) CdSe QDs overcoated with 3.9 monolayers of ZnS along with their photographs. (C) Absorption and PL spectra of bare CdSe QDs (solid line) and overcoated with 3.9 monolayers of ZnS (dashed line). (D) PL of bare CdSe QDs in toluene, upon successive additions of 1 mM PT; excited at 490 nm (OD ∼ 0.09).
at ambient conditions (unless specified), and the concentrations of QDs were in the micromolar range. Throughout the course of study, the QD solution remained homogeneous and stable, which was confirmed by following the absorption spectrum and PL yield. Results and Discussion CdSe QDs (diameter ∼ 4.2 nm) capped with trioctylphosphine oxide and a small percentage of trioctylphosphine were synthesized9 (referred as “bare QDs”) and overcoated with ZnS of varying thickness (0.65-3.9 monolayers) by following reported procedures6a (Supporting Information). The overcoated samples also possessed same capping agents, and the number of monolayers was estimated as reported by Bawendi and coworkers.6a Representative HRTEM images of bare CdSe QDs and overcoated with ZnS (3.9 monolayers) are presented in Figure 1A,B and other images of core-shell QDs as Supporting Information. Quantum dots obtained were nearly monodisperse, and an increase in overall size was observed upon overcoating. The absorption and emission spectra underwent a bathochromic shift with an increase in the number of ZnS monolayers, accompanied by a dramatic increase in the PL quantum yield (Figure 1C). The photographs illustrating the PL of bare CdSe and CdSe/ZnS QDs having 3.9 monolayers of ZnS are shown as insets of Figure 1A,B. The absorption and luminescence properties of bare CdSe and CdSe/ZnS QDs have been discussed by various groups.6a,b The bathochromic shift in the spectrum observed after overcoating is attributed to the partial leakage of the exciton into the ZnS shell.6a,b The luminescence yield of CdSe QDs increased with ZnS overcoating (quantum yield of ∼0.5 for 1.3-2.2 monolayers), which further decreased with shell thickness. The ZnS capping saturated the dangling bonds and suppressed the deep trap emission by passivating most of nonradiative sites on the surface of the bare QDs, resulting in an enhanced PL. The subsequent increase in the ZnS monolayer resulted in the reduction of PL, which is attributed to the
Figure 2. (A) Relative changes in the PL intensity of overcoated QDs, as a function of ZnS MLs, upon the addition of varying concentrations of PT, (9) 0 (b) 0.65, (2) 1.30, (1) 2.60 monolayers of ZnS. (B) Plot for log(I0/I) against varying concentrations of PT.
generation of misfit dislocations, resulting from the strain at the interface due to lattice mismatch between the CdSe and ZnS (12%).6a,b Ideally, the large band gap material should form a uniform monolayer over QDs; any voids on the surface can affect the photostability and also induce the leakage of the core material.7d We have analyzed the overcoated sample carefully by using
10148 J. Phys. Chem. C, Vol. 111, No. 28, 2007
Letters
Figure 4. Changes in the PL quantum yield (a) and static quenching constants (b) as a function of the number of ZnS MLs. Figure 3. Relative changes in the PL intensity of bare CdSe QDs upon the addition of various electron donors at various temperatures (a) PT at 298 K, (b) PT at 323 K, and (c) NMPT at 298 K, and the inset shows the plot for log(I0/I) against the concentration of PT at (a) 298 K and (b) 323 K.
SCHEME 1: Complexation of PT with (A) Bare and (B) ZnS-Overcoated CdSe QDs Having Voids
HRTEM, and these 2D images could not provide any information other than the increase in the overall size of the material. One of the objectives of the present investigation is to develop a simple methodology for estimating the optimum thickness of overcoating material, which can inhibit the undesired electron transfer without loosing the PL property. Comparison of redox potentials of CdSe QDs having a diameter of 4.2 nm (valence band edge redox potential, 1.61 V versus NHE)10 with those of hole scavengers (Eox(PT) ) 0.85 V and Eox(NMPT) ) 1.03V, both versus NHE)11 indicates that the process is thermodynamically feasible. Absence of any spectral overlap between the emission of QDs and the absorption of PT/NMPT rules out the possibility of an energy-transfer process. Upon addition of varying concentrations of PT (0-7 mM) to bare CdSe QDs, the absorption spectral profile remained more or less unaffected, ruling out the possibility of any chemical degradation of quantum dots. Interestingly, the PL of bare QDs underwent dramatic quenching upon addition of PT (Figure 1D). The Stern-Volmer plots for PL quenching, upon addition of PT, follows a nonlinear behavior (Figure 2A) and is attributed to the existence of more than one electronic state or a combination of static and dynamic quenching mechanisms.7b,12 Such nonlinear effects were observed in the quenching of CdSe QDs by various quenchers7b,12,13 and discussed based on several models,14 including the Perrin model.14a The relative PL intensities of unquenched to quenched QDs (I0/I) showed an exponential relationship with the concentration of PT, as shown in eq 18
I0/I ) eR[PT]
(1)
TABLE 1: Quenching Volume of Bare and Overcoated CdSe QDs number of ZnS MLs
static quenching constant (R, M-1)
quenching volume, cm3
0 0.65 1.30 2.60
277 184 42 12
46 × 10-20 31 × 10-20 7 × 10-20 2 × 10-20
where R represents the static quenching constant. A plot of log(I0/I) versus the concentration of PT shows a linear relationship (Figure 2B), and the R value is estimated to be 277 M-1. Experiments were further carried out with QDs possessing varying monolayers of ZnS, and it was found that the PL intensity remains unaffected with an increase in shell thickness (>2 monolayers). Relative changes in the PL intensity of CdSe QDs possessing 0.65, 1.3, and 2.6 monolayers of ZnS are presented in Figure 2A, and corresponding R values were deduced as 184, 42, and 12 M-1, respectively, from the linear fit plots (Figure 2B). The R values remained more or less the same for CdSe/ZnS possessing >2.6 monolayers (∼12 M-1). Interestingly, the PL of bare as well as overcoated CdSe QDs are practically unaffected upon addition of NMPT (trace c in Figure 3), even though the process is thermodynamically favorable, indicating that the hole scavenger fails to form a complex on the surface. The steric restrictions imposed by the bulky N-methyl group in NMPT may prevent complex formation, whereas the PT forms a ground-state complex, resulting in the PL quenching. A static quenching mechanism was further confirmed by investigating the temperature-dependent PL quenching studies (Figure 3) and the PL lifetimes in the absence and presence of PT. Upon increasing the temperature, a decrease in the static quenching constant was observed for both bare (R at 298 and 323 K is 290 and 151 M-1, respectively; inset of Figure 3) and overcoated samples (Supporting Information), which is attributed to the decrease in stability of the “QD-PT” complex at elevated temperatures. Time-correlated single-photon counting studies of bare CdSe were carried out in the absence and presence of PT (Supporting Information). The bare CdSe showed a triexponential decay before and after adding PT (up to 8 mM). The shorter components of 4 and 14 ns were attributed to the band edge luminescence, which contributed to a higher percentage. The longer lifetime (45 ns) component was attributed to the trap state luminescence of CdSe quantum dots. Lifetimes and amplitude components of CdSe QDs in the absence and presence of PT do not show any significant changes, indicating that the interaction is static in nature.12,15 These spectroscopic studies
Letters indicate that PT complexes with bare CdSe QD surface or through the voids in ZnS-overcoated CdSe QDs (Scheme 1). On the basis of these results, it is concluded that the large band gap ZnS overcoated on QDs may be inhibiting the electron tunneling into the CdSe core. Even though weak ligands such as pyridine4b are known to bind with ZnS, the present study does not provide any direct evidence for such interactions. In a recent study, Scaiano and co-workers have reported that the quenching radius is influenced by the size of quantum dots.8 The quenching radii can be better viewed as the action radii for the quencher as it approaches the QD surface and can be obtained by using Perrin analysis (R ) NAV, where NA is Avogadro’s number, and V is the quenching volume).8 In the case of a quencher molecule such as TEMPO, the quenching radius (0.9 nm) can reach across the nanoparticle when its size is smaller, where as for larger QDs, the interaction is restricted with the excitons near the surface. We have further estimated the quenching volume of the bare and overcoated QDs, and these results are presented as Table 1. The relative decrease in the quenching volume indicates that the overcoating prevents a charge-transfer interaction with PT. Optimizing the shell thickness, which can suppress the undesired electron-transfer process and provide maximum PL yield, is crucial for the design of core-shell QDs for biological applications. In order to obtain quantitative information on the optimum shell thickness, PL quantum yields (ΦL) and R values for CdSe/ZnS were plotted as a function of ZnS monolayers (Figure 4). The PL quantum yield dramatically increases with the number of ZnS monolayers (ΦL(bare) ) 0.08, ΦL(1.3) ) 0.44, and ΦL(2.6) ) 0.48) and further decreases slightly (ΦL(3.9) ) 0.44), whereas the static quenching constant dramatically decreases. On the basis of the ΦL and R values presented in Figure 4, it is concluded that two monolayers (corresponding to 0.65 nm) of ZnS is the optimum shell thickness for a 4.2 nm diameter CdSe quantum dot. Luminescent core-shell QDs, with optimum shell thickness, which do not involve any electron-transfer interactions, are ideal for bioimaging, and methodologies presented here are useful for probing the overcoating process in semiconductor QDs. Acknowledgment. The authors thank DRDO (K.G.T., P.R., P.V.N., S.P.), NIST (R.V., P.V.N., K.G.T.) and UGC (R.V.), Government of India, for financial support. We thank Professor T. Pradeep, IIT Madras, for HRTEM analysis. This is contribution No. NIST-PPD-242 from the NIST, Trivandrum, India. Supporting Information Available: Details on the synthesis and characterization of bare and ZnS-overcoated CdSe QDs, N-methylphenothiazine, and steady-state and time-resolved luminescence studies. This material is available free of charge via the Internet at http://pubs.acs.org.
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