CHEMPHYSCHEM ARTICLES DOI: 10.1002/cphc.201402156
Exciton Coupling of Surface Complexes on a Nanocrystal Surface Xiangxing Xu,* Jianwei Ji, Guan Wang, and Xiaozeng You[a] Exciton coupling may arise when chromophores are brought into close spatial proximity. Herein the intra-nanocrystal exciton coupling of the surface complexes formed by coordination of 8-hydroxyquinoline to ZnS nanocrystals (NCs) is reported. It is studied by absorption, photoluminescence (PL), PL excitation
(PLE), and PL lifetime measurements. The exciton coupling of the surface complexes tunes the PL color and broadens the absorption and PLE windows of the NCs, and thus is a potential strategy for improving the light-harvesting efficiency of NC solar cells and photocatalysts.
1. Introduction
2. Results and Discussion
The synthesis of colloidal nanocrystals (NCs) has developed rapidly with their applications in electronics,[1, 2] energy,[3–5] and biology[6–8] in the past two decades. Generally, a colloidal NC consists of a crystalline nanocore and its surface-bonded ligands, which can be introduced by reactants, solvents, or ligand exchange.[2, 9–12] To better understand and utilize surface–ligand interactions of colloidal NCs and to distinguish them from normal metal–ligand complexes, an NC surface metal ion with its coordinated ligands can be regarded as a surface complex.[13–17] Therefore, the theories and methods of coordination chemistry, typically molecular orbital theory and ligand field theory, can be applied to predict, design, and discuss the properties of surface complexes. On the other hand, the NC surface may provide coordination fields that differ from those of normal complexes. It provides a unique orientation to investigate and check methods or hypotheses that are used to calculate and predict the properties of corresponding normal complexes. Exciton coupling is commonly observed in coordination compounds with two or more chromophores in close spatial proximity. To the best of our knowledge, there has been no such report on the exciton coupling of surface complexes or chromophores coordinated to NC surfaces. Herein, the intra-NC exciton coupling of surface complexes and its tuning of the fluorescence spectrum are demonstrated. The surface complex of 8-hydroxyquinoline (Q) and ZnS NCs (ZnSQ NCs) was selected as an illustrative example for its strong fluorescence, easy preparation, laboratory availability, and, most importantly, the fact that exciton coupling can be achieved with quinoline-based chromophores.
In solution an adsorption/desorption equilibrium of ligands with NCs exists.[18–23] It provides an opportunity to control the surface density of a certain ligand coordinated to the NC surface by ligand exchange. One method is to add a controlled amount of free ligand to the NC solution to give a final equilibrium with a desired degree of ligand exchange. We used this method to prepare ZnS-Q NCs with three different surface densities of the surface complex (see Experimental Section). Figure 1 A shows that, under UV excitation at 365 nm, the sample with low surface density of the surface complex (ZnS-Q-1) emits blue photoluminescence (PL), whereas the PL color of the samples with higher surface densities of the surface complex (ZnS-Q-2 and ZnS-Q-3) gradually changes to green. The pure Q ligand in methanol or ethanol solution is nonfluorescent. Transmission electron spectroscopy (TEM) measurements showed that the average size ( 3.7 nm) and crystalline structure of the zinc blende ZnS NCs are well preserved after ligand exchange (exemplified by ZnS-Q-2 in Figure 1 B). Owing to the size distribution of the NCs (Figure 1 B, inset) and because the number of ligands on the NC surfaces is hard to determine and may also vary significantly under different conditions, it remains a challenge to accurately measure the solution concentration of NCs.[24] Here, as an approximation, we assumed that the ZnS NCs are spheres with a diameter of 3.7 nm and that the oleic acid and oleylamine ligands amount to 35 wt % of the of the colloidal ZnS NCs. The Q/ZnS NC ratios used for ligand exchange can be estimated to be 1/10, 1/1, and 10/1, in accordance with the preparation of ZnS-Q-1, ZnS-Q-2, and ZnS-Q-3, respectively. The coordination of the Q ligands to the NC surface was characterized by Fourier transform infrared spectroscopy (FTIR) spectroscopy. Compared with pure Q, the CN stretching vibration of ZnS-Q shifts from 1380 to 1389 cm1, the OH bending band at 1223 cm1 disappears, and the relative intensity of the CO stretching band at 1108 cm1 increases, that is, the O and N atoms contribute to formation of the surface complexes. The residual CH2 bands at 2923 and 2852 cm1 and CH3 bands at 722 cm1 of the oleic acid and/or oleylamine ligands indicated partial exchange of the original ligands. On going from ZnS-Q-1 to ZnS-Q-3, the in-
[a] Dr. X. Xu, J. Ji, G. Wang, Prof. X. You State Key Laboratory of Coordination Chemistry School of Chemistry and Chemical Engineering Nanjing National Laboratory of Microstructures Nanjing University Nanjing 210093 (P. R. China) Fax: (+ 86) 25-83314502 E-mail: [email protected]
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To understand the prominent change in fluorescence color in Figure 1 A, PL, absorption, and PL excitation (PLE) spectra were measured (Figure 2). The PL spectra (Figure 2 A) showed a monotonic redshift of the PL peaks of ZnS-Q-1/2/3 NCs at 482/493/508 nm, respectively. The absorption band at 320– 460 nm is assigned to the surface complex.[30] The increasing absorption of the surface complex in the sequence of ZnS-Q1 < ZnS-Q-2 < ZnS-Q-3 again indicates its increasing surface density. The absorption band of the surface complex becomes broader and extends to longer wavelengths with increasing surface density (Figure 2 B). The main absorption peaks of ZnS-Q1/2/3 NCs in the range of 320– Figure 1. A) Images of ZnS-Q NCs dispersed in cyclohexane with increasing surface density of the surface complex 460 nm occur at 368/376/ from ZnS-Q-1 to ZnS-Q-3 (top) with their corresponding PL images (bottom, excitation: 360 nm). B) TEM image of 381 nm, respectively. This sugthe ZnS-Q-2 NCs. Inset: size distribution of the NCs. C) FTIR spectra of Q, ZnS, and ZnS-Q-1/2/3 NCs. gests that the PL redshift is probably due to exciton coupling, and not simply caused by Fçrster (or fluorescence) resocreased intensity of the bands of coordinated Q[25–29] at 1499, nance energy transfer (FRET) between the surface com1389, 1323, 1108, and 737 cm1 unambiguously confirmed inplexes.[31, 32] Further evidence comes from the PLE spectrum creasing surface density of the surface complex. The remaining oleic acid and/or oleylamine ligands help to maintain the dis(Figure 2 C). At low surface density of the surface complex, the persibility and stability of the NCs in nonpolar organic solvents excitation spectrum has two peaks at 310 and 368 nm. The (Figure 1 A) and thus facilitate further characterization and film 310 nm peak is attributed to the ZnS excitation band. This casting. means that energy is transferred from the ZnS NC core to the
Figure 2. A) PL spectra with excitation at 280 nm for ZnS NCs and 360 nm for ZnS-Q NCs. B) Absorption spectra and C) PLE spectra of ZnS and ZnS-Q-1/2/3 NCs, measured with the emission at the PL peaks of 390, 482, 493, and 508 nm, respectively. D) Schematic illustration of the proposed exciton-coupling model of the surface complex. E) Sketch of the PL mechanism of the ZnS-Q NCs with emission from the uncoupled and coupled surface complexes.
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CHEMPHYSCHEM ARTICLES surface complex on excitation around 310 nm, because the good overlap of the PL spectrum of the ZnS NCs with the absorption of the surface complex satisfies the FRET requirement. The excitation peak at 368 nm is assigned to the excited state of the surface complex (Q*) without exciton coupling. With increasing surface density of the surface complex, two additional peaks at 330 and 410 nm arise, which are assigned to the higher- and lower-energy excited states (Q*’’ and Q*’, respectively) of coupled surface complexes (Figure 2 C–E). This is consistent with the fact that the probability that the distance between neighboring surface complexes on the same NC would lie in the exciton-coupling range increases with increasing surface density of the surface complex. The Q*’’ state also corresponds well to the absorption spectrum of ZnS-Q-3, in which a shoulder appears around 340 nm. The intensities of Q*’’ and Q*’ are approximately equal (see Figure 2 C). Unlike for a specific complex with a given structure, for which information on the molecular geometry can be derived from the intensity ratio of the higher- and lower-energy excited states of coupled chromophores,[31] no specific geometrical information can be extracted in this case, because the NCs offer surface coordination sites with diverse local fields. The apparent Q*’’ and Q*’ intensities should be the statistical result of coupled surface complexes with various structures. The relatively large energy gap between Q*’’ and Q*’ is comparable to those of some typical intramolecular exciton couplings in coordination compounds such as [M(dipyrrinato)3] (M = Co, Rh, Ga, or In) and [M(tp-azadp)2] (M = Co, Ni, Zn or Cu; tp-azadp = tetraphenylazadipyrromethene)[33–37] and indicates strong exciton coupling. From the crystal structure of ZnS, the shortest distance between two neighboring Zn atoms in the ZnS NC surface is 0.38 nm. Therefore, neighboring Q ligands could have very short spatial separations satisfying the condition for strong exciton coupling. This would possibly introduce electronic overlap between the two ligands, that is, the molecular orbital of the surface complex may be distributed over both ligands rather than confined to one or the other. Exciton coupling related to electronic overlap was found in CuII complexes.[37] The optical and electronic structure of the ZnQ2 complex as an electroluminescent material in organic light-emitting diodes, which has been calculated and measured,[38–41] which can be feasibly compared with the above assignment of the excitation states of the surface complex in ZnS-Q NCs. It is generally accepted that the strongest optical absorption of ZnQ2 at about 400 nm is the p!p* transition localized on the Q ligands and that the Zn atom contributes very little to the electron-density distribution of the frontier orbitals.[30, 42–44] Thus, Q*“ is assigned as the uncoupled excitation state of the surface complex in ZnS-Q NCs. The intramolecular and intermolecular interactions may also affect the photonic properties of ZnQ2 and thus induce a redshift of the p!p* transition.[30, 41] This is consistent with the exciton-coupling model. To interpret the 300–350 nm absorption/PLE band of ZnQ2, metal-to-ligand charge transfer (MLCT) was assumed.[30] For the 300–350 nm absorption/PLE band of AlQ3, the localized p!p* transition was suggested.[45] However, neither the MLCT nor the localized p!p* transition can explain the evolution of the Q*’’ peak in 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemphyschem.org the PLE spectra of ZnS-Q-1/2/3: the intensity of the Q*’’ band relative to that of Q* is rather low for identification at low surface concentration of the surface complex, but becomes stronger with increasing surface concentration of the surface complex. The exciton-coupling model proposed here perfectly explains the coupled emergence of the Q*’’ and Q*’ states in the PLE spectrum, with unambiguous assignment of the Q* band as uncoupled exciton. Moreover, it could be an alternative interpretation of the double peaks (ca. 340 and ca. 400 nm) observed in the absorption/PLE of the ZnQ2 complex.[30, 31, 46] Exciton coupling of uncoordinated 7-hydoxyquinoline or quinoline-derived chromophores in organic compounds[47, 48] and of coordinated quinoline chromophores in Cu/Zn complexes has been observed.[49–53] Albrecht et al. reported a lanthanum complex with three Q ligands showing exciton coupling of the coordinated Q at 280, 295, 350, and 480 nm in the absorption spectrum.[54] Telfer et al. and Kwong et al.[31, 55–57] also found exciton coupling in dinuclear and polynuclear complexes with structures that resemble those of neighboring surface complexes on an NC surface. All of these observations give support to the exciton-coupling model proposed for the ZnS-Q NCs. We note that, first, due to the size and shape distribution of the ZnS NCs, different surface Zn sites (i.e. the corners, edges, and sides of crystal planes) would have different coordination affinities to Q; second, interaction of the NC surface complexes with surface defect states[13] would effect changes in the absorption/PL/PLE spectra, but their characterization is beyond our technical capability. Theoretically, if the concentration of ZnS NCs and the molar [Q]/[NC] ratio are known, the number of Q ligands attached to one ZnS NC is usually described by a Poisson distribution [Eq. (1)]:[58–60] PðnÞ ¼ ex x n =n!
ð1Þ
where x is the average number of Q ligands per ZnS NC (estimated from a given molar ratio x = [Q]/[NC]) and n is the number of attached Q ligands on a given ZnS NC. Because of the distribution of attachment positions (i.e. the distribution of distances between Q ligands) of n Q ligands on a given ZnS NC, the number of two-, three-, or manifold-coupled surface complexes m2,3,… for this ZnS NC would have an distribution P’(n,m2,3…). Thus, the observed Q*’’ and Q*’ bands contain components not only from doubly coupled surface complexes, but also from triply and more highly coupled surface complexes. For this reason, the uncoupled and coupled surface complexes would generally coexist in all samples in different ratios that depend on the average [Q]/[NC] ratio. This intrinsic distribution matches well with the gradual change in the UV/Vis absorption and excitation spectra. Inter-NC exciton coupling of the surface complexes can be largely ruled out, because the NCs were well dispersed in nonpolar solvent (even centrifugation at 14 000 rpm for 10 min can not separate the NCs in the concentration range of 10 mg of NCs in 50 mL to 10 mL of cyclohexane), and thus the distance between NCs should be much larger than the critical coupling distance. NCs with the same surface density of the surface ChemPhysChem 0000, 00, 1 – 7
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www.chemphyschem.org Table 1. Fitting parameters of the PL decay curves from the double-exponential function. Sample
t1 [ns]
t2 [ns]
A1
A2
R2
tav [ns]
A1/A2
ZnS-Q-1 ZnS-Q-2 ZnS-Q-3
8.59 9.23 8.78
28.40 28.40[a] 28.40[a]
0.343 0.493 0.746
0.619 0.491 0.246
0.998 0.998 0.998
25.6 23.7 18.9
0.55 1.00 3.03
[a] Fixed.
Figure 3. PL spectra of ZnS-Q-3 NCs with concentrations of 1 and 10 mg mL1 in cyclohexane and in the solid state on a glass slide (excitation: 360 nm).
complex in 1 or 10 mg mL1 solutions and even dried ZnS-Q NCs showed no obvious changes in the PLE spectrum and only a small peak shift in the PL spectra ( 3 nm, Figure 3). This spectral pinning may due to the remaining oleic acid and/ or oleylamine on the NC surface providing efficient steric hindrance to prevent the approach of surface complexes from neighboring NCs. An inner filter effect inducing the redshift can also be excluded because of the large absorption/emission Stokes shift (Figure 2 A and B), the near independence of the PL peak from the ZnS-Q NC concentration, and concentrationindependent PL lifetime. In addition, considering the coordination-number requirement and steric effect introduced by the NC surface and the surface ligands, the coordination of a second ligand Q to a surface Zn atom should be disfavored. For instance, about half of the coordination space of a Zn atom in a crystal facet, as well as the orbitals, is occupied by the ZnS NC itself. For these reasons, the exciton coupling can only be induced by adjacent (two or more) surface complexes on the same ZnS-Q NC. Figure 4 shows the PL lifetimes of the ZnS-Q NCs in cyclohexane. The PL decay curves of the ZnS-Q NCs are well fitted to a double-exponential function [Eq. (2)]: IðtÞ ¼ A1 exp½ðt t0 Þ=t1 þ A2 exp½ðt t0 Þ=t2
ð2Þ
where t1 and t2 are the PL decay lifetimes and A1 and A2 the weighting parameters. The fitting parameters are given in Table 1. The average lifetime was determined by Equation (3):[61, 62]
Figure 4. PL lifetimes of ZnS-Q-1 at 482 nm, ZnS-Q-2 at 493 nm, and ZnS-Q3 at 508 nm with excitation at 379.2 nm.
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tan ¼ A1 t21 þ A2 t22 =ðA1 t1 þ A2 t2 Þ
ð3Þ
The average PL lifetimes of the ZnS-Q-1/2/3 NCs were calculated to be 25.6, 23.7, and 18.9 ns, respectively. They are longer than the PL lifetime of the ZnQ2 complex in the solid state (10.3 ns)[46] or in solution of dimethyl sulfoxide or ethanol (3.6 ns, 300 K).[63] The lower the surface density of the surface complex, the longer the average PL lifetime. If we assume that the slower component t2 is a constant (28.40 ns for ZnS-Q-1), the faster component t1 in ZnS-Q-2 is fitted to be 9.23 ns, and that in ZnS-Q-3 is 8.87 ns, which are approximately the same as the t1 value of ZnS-Q-1 (8.59 ns). We assign t1 to the PL decay lifetime of the exciton-coupled surface complexes and t2 to the uncoupled surface complexes. This explains the observation that the amplitude ratio A1/A2 increases remarkably with increasing surface density of the surface complex (see Table 1). The normalized PL decay curves were found to be the same when measured with NC concentrations of 1 and 10 mg mL1, which also gives rational support to the proposed mechanism of intra-NC exciton coupling of the surface complexes. The small deviations of the fitting curves from the measured curves of ZnS-Q-2 and ZnS-Q-3 were possibly due to fact that the lifetime varies with the coupling number of the surface complexes, in accordance with the P’(n,m2,3…) distribution discussed above. Therefore, three or more decay lifetimes could be used to achieve a more precise fitting. The decay curve of pure ZnS was not measured because of its very low absorption at the laser excitation wavelength of 379.2 nm, which overlaps with the PL peak of the pure ZnS NCs. The PL quantum yields of ZnS-Q-1, ZnS-Q-2 and ZnS-Q-3 NCs were measured to be approximately 33.0, 27.4, and 12.2 %, respectively, by using a methanolic solution of coumarin 6 as reference (quantum yield: 65 %). Generally, lifetime shortening can be induced by additional radiative and/or nonradiative transitions. Therefore, the decrease in quantum yield of ZnS-Q NCs accompanied by shortening of the PL lifetime tav indicates the existence of the additional nonradiative transition along with higher surface concentration of the surface complex. It is possible that surface-complex/ligand-related surface states or exciton coupling facilitate the nonradiative recombination. The accompanying PL band on the lower-energy side of the near-band-edge emission of an NC can be induced by the surface states (also known as deep traps or surface defect states), defect states inside the material, or excitation states of doping ions. Typically, the first is found in CdSe and CdS colloidal NCs,[64–66] the second is often observed in ZnO nanomateriChemPhysChem 0000, 00, 1 – 7
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CHEMPHYSCHEM ARTICLES als,[67, 68] and the last-named are well known as d-dots.[69, 70] Various models have been developed in studying these spectra. Differing from the traditional model of distributed deep trap states at the surface of the NC,[64–66] Kambhampati et al. recently reported a single-surface-state model to explain the defect emission of CdSe, CdS, and CdSe/ZnS NCs.[71–73] In principle, the deep-trap model and the single-surface-state model could also be used in explaining the observations reported in this work. Indeed, there is basically no difference between these models if the excitation state of the surface complex is defined as the trap state or as the single surface state. The feasibility of adopting the fluorescent-surface-complex model here is subject to following concerns: 1) the fluorescent species are well defined, which is less clear for the defect states corresponding to surface-defect structures; therefore 2) theory and methods of studying coordination complexes can readily be used for surface complexes; for example, exciton coupling in coordination complex is applied in studying interactions of the surface species; and 3) the fluorescence can be designed in parallel to that of the corresponding free-standing complexes (e.g. ZnS-Q and ZnQ2), so that the large database of functional complexes could be rationally used to guide NC modification and property control.
3. Conclusions ZnS NCs modified with Q were prepared. The surface complex exhibits prominent PL, especially in low surface concentration. Intra-NC exciton coupling of the surface complexes was proposed and characterized by absorption, PL, PLE, and PL lifetime measurements. By controlling the surface density of the surface complex, and thus the exciton coupling of the surface complexes, the fluorescence spectrum could be tuned from blue to green. Although a more extensive study of the ligandexchange dynamics, the exact [Q]/[NC] ratio- and quantum chemical calculations remain to be done, our preliminary results show that, in addition to the NC-to-NC and ligand-to-NC interactions, the ligand-to-ligand or, in this work, the surface complex-to-surface complex interaction should not be overlooked, because this could also dominate the properties of the colloidal NCs. Such colloidal NCs are potential active materials for light-emitting devices and fluorescent probes in biology. Additionally, the designability of the ligands, the energy transfer between the NC core and the surface complex, and the exciton-coupling effect reported here can be utilized to control the carrier-transport property and broaden the absorption and PLE windows of the NCs, and this is promising for improving the light-harvesting efficiency of NC solar cells and photocatalysts.
Experimental Section Synthesis of Zinc Blende ZnS NCs ZnCl2 (1 mmol) was mixed with oleic acid (4 mL), oleylamine (4 mL), and 1-octadecene (4 mL) in a 50 mL three-neck flask. On a Schlenk line the mixture was heated to 110 8C. Traces of H2O and 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemphyschem.org low-boiling impurities were removed by vacuum pumping for 30 min. The sulfur precursor was prepared by dissolving sulfur in oleylamine. Under the protection of an N2 flow, 2 mL of the sulfur solution (0.5 m) was swiftly injected. The mixture was heated to 240 8C for 30 min, the heater removed, and the reaction mixture cooled to room temperature. The ZnS NCs were collected by adding 60 mL of ethanol and centrifugation (4000 rpm, 10 min). The precipitated NCs were then redispersed in 4 mL of cyclohexane, washed again with ethanol, and centrifuged. The precipitated ZnS NCs were finally dried with an intense N2 blast for about 1 min and stored in cyclohexane or toluene with good dispersion to form transparent solutions. On the basis of the measured weight and theoretical weight of ZnS without ligands under the assumption of 100 % yield, the ligand content of the colloidal ZnS was about 18 wt %. Energy-dispersive X-ray spectroscopic measurements showed ligand fractions of up to 33–45 wt %.
Preparation of ZnS-Q NCs The ZnS-Q NCs with coordinated Q ligands were prepared by ligand exchange. The surface density of the surface complex (equal to that of the coordinated Q) of ZnS-Q NCs was controlled by means of the Q/ZnS ratio in the ligand-exchange procedure. Typically, 10 mg of ZnS NCs was dispersed in 100 mL of cyclohexane, and 20 mL of an ethanolic solution containing 1 108, 1 107, or 1 106 mol of Q was swiftly injected to form ZnS-Q-1, ZnS-Q-2, or ZnS-Q-3 NCs, respectively. After 1 min of shaking, 0.5 mL ethanol was added to precipitate the ZnS-Q-1/2/3 NCs. The NCs were collected by centrifugation (8000 rpm, 1 min), and any residual ligands and byproducts were removed by redispersing the NCs in 0.4 mL of cyclohexane, adding 1 mL of ethanol, and centrifugation. The final ZnS-Q NCs were readily redispersible and were stored in cyclohexane.
Instruments TEM images were measured on a JEM-2100 transmission electron microscope at an acceleration voltage of 200 kV. FTIR spectra were recorded on a Vector 22 spectrometer with a resolution of 2 cm1 by using KBr pellets. The absorption spectra were obtained by a Shimadzu UV-2700 UV/Vis spectrophotometer. The PL and PLE spectra were collected with a Hitachi F-4600 fluorescence spectrophotometer. PL lifetimes was measured with a Zolix Omini-l 300 fluorescence spectrophotometer and a picosecond pulsed diode laser (Edinburgh Instruments Ltd.) operating at 379.2 nm.
Acknowledgements This work was supported by the Major State Basic Research Development Program of China (Grant No. 2013CB922102, 2011CB808704), the National Natural Science Foundation of China (Grant No. 91022031, 21301089), and Jiangsu Province Science Foundation for Youths (BK20130562). The authors gratefully acknowledge Weikuan Duan and Prof. Wei Wei for the photoluminescence lifetime measurement. Keywords: exciton coupling · ligand exchange luminescence · nanoparticles · surface chemistry
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CHEMPHYSCHEM ARTICLES [3] O. E. Semonin, J. M. Luther, S. Choi, H.-Y. Chen, J. Gao, A. J. Nozik, M. C. Beard, Science 2011, 334, 1530 – 1533. [4] I. J. Kramer, E. H. Sargent, ACS Nano 2011, 5, 8506 – 8514. [5] A. H. Ip, S. M. Thon, S. Hoogland, O. Voznyy, D. Zhitomirsky, R. Debnath, L. Levina, L. R. Rollny, G. H. Carey, A. Fischer, K. W. Kemp, I. J. Kramer, Z. J. Ning, A. J. Labelle, K. W. Chou, A. Amassian, E. H. Sargent, Nat. Nanotechnol. 2012, 7, 577 – 582. [6] M. Bruchez Jr., M. Moronne, P. Gin, S. Weiss, A. P. Alivisatos, Science 1998, 281, 2013 – 2016. [7] W. C. W. Chan, S. M. Nie, Science 1998, 281, 2016 – 2018. [8] I. L. Medintz, H. T. Uyeda, E. R. Goldman, H. Mattoussi, Nat. Mater. 2005, 4, 435 – 446. [9] X. Wang, J. Zhuang, Q. Peng, Y. D. Li, Nature 2005, 437, 121 – 124. [10] J. S. Owen, J. Park, P.-E. Trudeau, A. P. Alivisatos, J. Am. Chem. Soc. 2008, 130, 12279 – 12281. [11] B. Fritzinger, I. Moreels, P. Lommens, R. Koole, Z. Hens, J. C. Martins, J. Am. Chem. Soc. 2009, 131, 3024 – 3032. [12] X. X. Xu, S. Stçttinger, G. Battagliarin, G. Hinze, E. Mugnaioli, C. Li, K. Mllen, T. Basch, J. Am. Chem. Soc. 2011, 133, 18062 – 18065. [13] A. A. Cordones, M. Scheele, A. P. Alivisatos, S. R. Leone, J. Am. Chem. Soc. 2012, 134, 18366 – 18373. [14] C. L. L, J. F. Gao, Y. Q. Fu, Y. Y. Du, Y. L. Shi, Z. M. Su, Adv. Funct. Mater. 2008, 18, 3070 – 3079. [15] T. Otsuka, Y. Chujo, J. Mater. Chem. 2010, 20, 10688 – 10695. [16] M. Tagaya, S. Motozuka, T. Kobayashi, T. Ikoma, J. Tanaka, Ind. Eng. Chem. Res. 2012, 51, 11294 – 11300. [17] L. J. Charbonnire, J. L. Rehspringer, R. Ziessel, Y. Zimmermann, New J. Chem. 2008, 32, 1055 – 1059. [18] A. M. Munro, I. Jen-La Plante, M. S. Ng, D. S. Ginger, J. Phys. Chem. C 2007, 111, 6220 – 6227. [19] X. H. Ji, D. Copenhaver, C. Sichmeller, X. G. Peng, J. Am. Chem. Soc. 2008, 130, 5726 – 5735. [20] C. A. Fields-Zinna, J. F. Parker, R. W. Murray, J. Am. Chem. Soc. 2010, 132, 17193 – 17198. [21] M. D. Donakowski, J. M. Godbe, R. Sknepnek, K. E. Knowles, M. Olvera de La Cruz, E. A. Weiss, J. Phys. Chem. C 2010, 114, 22526 – 22534. [22] A. J. Morris-Cohen, M. T. Frederick, L. C. Cass, E. A. Weiss, J. Am. Chem. Soc. 2011, 133, 10146 – 10154. [23] A. J. Morris-Cohen, V. Vasilenko, V. A. Amin, M. G. Reuter, E. A. Weiss, ACS Nano 2012, 6, 557 – 565. [24] W. W. Yu, L. H. Qu, W. Z. Guo, X. G. Peng, Chem. Mater. 2003, 15, 2854 – 2860. [25] J. E. Tackett, D. T. Sawyer, Inorg. Chem. 1964, 3, 692 – 696. [26] C. Engelter, G. E. Jackson, C. L. Knight, D. A. Thornton, J. Mol. Struct. 1989, 213, 133 – 144. [27] T. Gavrilko, R. Fedorovich, G. Dovbeshko, A. Marchenko, A. Naumovets, V. Nechytaylo, G. Puchkovska, L. Viduta, J. Baran, H. Ratajczak, J. Mol. Struct. 2004, 704, 163 – 168. [28] H. C. Pan, F. P. Liang, C. J. Mao, J. J. Zhu, H. Y. Chen, J. Phys. Chem. B 2007, 111, 5767 – 5772. [29] X. H. Wang, M. W. Shao, L. Liu, Synth. Met. 2010, 160, 718 – 721. [30] L. M. A. Monzon, F. Burke, J. M. D. Coey, J. Phys. Chem. C 2011, 115, 9182 – 9192. [31] S. G. Telfer, T. M. McLean, M. R. Waterland, Dalton Trans. 2011, 40, 3097 – 3108. [32] K. E. Sapsford, L. Berti, I. L. Medintz, Angew. Chem. 2006, 118, 4676 – 4704; Angew. Chem. Int. Ed. 2006, 45, 4562 – 4589. [33] J. D. Hall, T. M. McLean, S. J. Smalley, M. R. Waterland, S. G. Telfer, Dalton Trans. 2010, 39, 437 – 445. [34] S. G. Telfer, J. D. Wuest, Chem. Commun. 2007, 3166 – 3168. [35] V. S. Thoi, J. R. Stork, D. Magde, S. M. Cohen, Inorg. Chem. 2006, 45, 10688 – 10697. [36] S. J. Smalley, M. R. Waterland, S. G. Telfer, Inorg. Chem. 2009, 48, 13 – 15. [37] A. Palma, J. F. Gallagher, H. Mueller-Bunz, J. Wolowska, E. J. L. McInnes, D. F. O’Shea, Dalton Trans. 2009, 273 – 279. [38] Y. Hamada, T. Sano, N. Fujita, T. Fujii, Y. Nishio, K. Shibata, Jpn. J. Appl. Phys. 1993, 32, L514. [39] N. Donz, P. Pechy, M. Grtzel, M. Schaer, L. Zuppiroli, Chem. Phys. Lett. 1999, 315, 405 – 410.
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ARTICLES Intra-nanocrystal exciton coupling of the surface complexes formed by coordination of 8-hydroxyquinoline (Q) to ZnS nanocrystals (NCs) tunes the photoluminescence (PL) color from blue to green (see picture), broadens the absorption and PL excitation windows of the NCs, and thus is a potential strategy for improving the light-harvesting efficiency of NC solar cells and photocatalysts.
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X. Xu,* J. Ji, G. Wang, X. You && – && Exciton Coupling of Surface Complexes on a Nanocrystal Surface
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Exciton Coupling of Surface Complexes on a Nanocrystal Surface Xiangxing Xu,* Jianwei Ji, Guan Wang, and Xiaozeng You[a] Exciton coupling may arise when chromophores are brought into close spatial proximity. Herein the intra-nanocrystal exciton coupling of the surface complexes formed by coordination of 8-hydroxyquinoline to ZnS nanocrystals (NCs) is reported. It is studied by absorption, photoluminescence (PL), PL excitation
(PLE), and PL lifetime measurements. The exciton coupling of the surface complexes tunes the PL color and broadens the absorption and PLE windows of the NCs, and thus is a potential strategy for improving the light-harvesting efficiency of NC solar cells and photocatalysts.
1. Introduction
2. Results and Discussion
The synthesis of colloidal nanocrystals (NCs) has developed rapidly with their applications in electronics,[1, 2] energy,[3–5] and biology[6–8] in the past two decades. Generally, a colloidal NC consists of a crystalline nanocore and its surface-bonded ligands, which can be introduced by reactants, solvents, or ligand exchange.[2, 9–12] To better understand and utilize surface–ligand interactions of colloidal NCs and to distinguish them from normal metal–ligand complexes, an NC surface metal ion with its coordinated ligands can be regarded as a surface complex.[13–17] Therefore, the theories and methods of coordination chemistry, typically molecular orbital theory and ligand field theory, can be applied to predict, design, and discuss the properties of surface complexes. On the other hand, the NC surface may provide coordination fields that differ from those of normal complexes. It provides a unique orientation to investigate and check methods or hypotheses that are used to calculate and predict the properties of corresponding normal complexes. Exciton coupling is commonly observed in coordination compounds with two or more chromophores in close spatial proximity. To the best of our knowledge, there has been no such report on the exciton coupling of surface complexes or chromophores coordinated to NC surfaces. Herein, the intra-NC exciton coupling of surface complexes and its tuning of the fluorescence spectrum are demonstrated. The surface complex of 8-hydroxyquinoline (Q) and ZnS NCs (ZnSQ NCs) was selected as an illustrative example for its strong fluorescence, easy preparation, laboratory availability, and, most importantly, the fact that exciton coupling can be achieved with quinoline-based chromophores.
In solution an adsorption/desorption equilibrium of ligands with NCs exists.[18–23] It provides an opportunity to control the surface density of a certain ligand coordinated to the NC surface by ligand exchange. One method is to add a controlled amount of free ligand to the NC solution to give a final equilibrium with a desired degree of ligand exchange. We used this method to prepare ZnS-Q NCs with three different surface densities of the surface complex (see Experimental Section). Figure 1 A shows that, under UV excitation at 365 nm, the sample with low surface density of the surface complex (ZnS-Q-1) emits blue photoluminescence (PL), whereas the PL color of the samples with higher surface densities of the surface complex (ZnS-Q-2 and ZnS-Q-3) gradually changes to green. The pure Q ligand in methanol or ethanol solution is nonfluorescent. Transmission electron spectroscopy (TEM) measurements showed that the average size ( 3.7 nm) and crystalline structure of the zinc blende ZnS NCs are well preserved after ligand exchange (exemplified by ZnS-Q-2 in Figure 1 B). Owing to the size distribution of the NCs (Figure 1 B, inset) and because the number of ligands on the NC surfaces is hard to determine and may also vary significantly under different conditions, it remains a challenge to accurately measure the solution concentration of NCs.[24] Here, as an approximation, we assumed that the ZnS NCs are spheres with a diameter of 3.7 nm and that the oleic acid and oleylamine ligands amount to 35 wt % of the of the colloidal ZnS NCs. The Q/ZnS NC ratios used for ligand exchange can be estimated to be 1/10, 1/1, and 10/1, in accordance with the preparation of ZnS-Q-1, ZnS-Q-2, and ZnS-Q-3, respectively. The coordination of the Q ligands to the NC surface was characterized by Fourier transform infrared spectroscopy (FTIR) spectroscopy. Compared with pure Q, the CN stretching vibration of ZnS-Q shifts from 1380 to 1389 cm1, the OH bending band at 1223 cm1 disappears, and the relative intensity of the CO stretching band at 1108 cm1 increases, that is, the O and N atoms contribute to formation of the surface complexes. The residual CH2 bands at 2923 and 2852 cm1 and CH3 bands at 722 cm1 of the oleic acid and/or oleylamine ligands indicated partial exchange of the original ligands. On going from ZnS-Q-1 to ZnS-Q-3, the in-
[a] Dr. X. Xu, J. Ji, G. Wang, Prof. X. You State Key Laboratory of Coordination Chemistry School of Chemistry and Chemical Engineering Nanjing National Laboratory of Microstructures Nanjing University Nanjing 210093 (P. R. China) Fax: (+ 86) 25-83314502 E-mail: [email protected]
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To understand the prominent change in fluorescence color in Figure 1 A, PL, absorption, and PL excitation (PLE) spectra were measured (Figure 2). The PL spectra (Figure 2 A) showed a monotonic redshift of the PL peaks of ZnS-Q-1/2/3 NCs at 482/493/508 nm, respectively. The absorption band at 320– 460 nm is assigned to the surface complex.[30] The increasing absorption of the surface complex in the sequence of ZnS-Q1 < ZnS-Q-2 < ZnS-Q-3 again indicates its increasing surface density. The absorption band of the surface complex becomes broader and extends to longer wavelengths with increasing surface density (Figure 2 B). The main absorption peaks of ZnS-Q1/2/3 NCs in the range of 320– Figure 1. A) Images of ZnS-Q NCs dispersed in cyclohexane with increasing surface density of the surface complex 460 nm occur at 368/376/ from ZnS-Q-1 to ZnS-Q-3 (top) with their corresponding PL images (bottom, excitation: 360 nm). B) TEM image of 381 nm, respectively. This sugthe ZnS-Q-2 NCs. Inset: size distribution of the NCs. C) FTIR spectra of Q, ZnS, and ZnS-Q-1/2/3 NCs. gests that the PL redshift is probably due to exciton coupling, and not simply caused by Fçrster (or fluorescence) resocreased intensity of the bands of coordinated Q[25–29] at 1499, nance energy transfer (FRET) between the surface com1389, 1323, 1108, and 737 cm1 unambiguously confirmed inplexes.[31, 32] Further evidence comes from the PLE spectrum creasing surface density of the surface complex. The remaining oleic acid and/or oleylamine ligands help to maintain the dis(Figure 2 C). At low surface density of the surface complex, the persibility and stability of the NCs in nonpolar organic solvents excitation spectrum has two peaks at 310 and 368 nm. The (Figure 1 A) and thus facilitate further characterization and film 310 nm peak is attributed to the ZnS excitation band. This casting. means that energy is transferred from the ZnS NC core to the
Figure 2. A) PL spectra with excitation at 280 nm for ZnS NCs and 360 nm for ZnS-Q NCs. B) Absorption spectra and C) PLE spectra of ZnS and ZnS-Q-1/2/3 NCs, measured with the emission at the PL peaks of 390, 482, 493, and 508 nm, respectively. D) Schematic illustration of the proposed exciton-coupling model of the surface complex. E) Sketch of the PL mechanism of the ZnS-Q NCs with emission from the uncoupled and coupled surface complexes.
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CHEMPHYSCHEM ARTICLES surface complex on excitation around 310 nm, because the good overlap of the PL spectrum of the ZnS NCs with the absorption of the surface complex satisfies the FRET requirement. The excitation peak at 368 nm is assigned to the excited state of the surface complex (Q*) without exciton coupling. With increasing surface density of the surface complex, two additional peaks at 330 and 410 nm arise, which are assigned to the higher- and lower-energy excited states (Q*’’ and Q*’, respectively) of coupled surface complexes (Figure 2 C–E). This is consistent with the fact that the probability that the distance between neighboring surface complexes on the same NC would lie in the exciton-coupling range increases with increasing surface density of the surface complex. The Q*’’ state also corresponds well to the absorption spectrum of ZnS-Q-3, in which a shoulder appears around 340 nm. The intensities of Q*’’ and Q*’ are approximately equal (see Figure 2 C). Unlike for a specific complex with a given structure, for which information on the molecular geometry can be derived from the intensity ratio of the higher- and lower-energy excited states of coupled chromophores,[31] no specific geometrical information can be extracted in this case, because the NCs offer surface coordination sites with diverse local fields. The apparent Q*’’ and Q*’ intensities should be the statistical result of coupled surface complexes with various structures. The relatively large energy gap between Q*’’ and Q*’ is comparable to those of some typical intramolecular exciton couplings in coordination compounds such as [M(dipyrrinato)3] (M = Co, Rh, Ga, or In) and [M(tp-azadp)2] (M = Co, Ni, Zn or Cu; tp-azadp = tetraphenylazadipyrromethene)[33–37] and indicates strong exciton coupling. From the crystal structure of ZnS, the shortest distance between two neighboring Zn atoms in the ZnS NC surface is 0.38 nm. Therefore, neighboring Q ligands could have very short spatial separations satisfying the condition for strong exciton coupling. This would possibly introduce electronic overlap between the two ligands, that is, the molecular orbital of the surface complex may be distributed over both ligands rather than confined to one or the other. Exciton coupling related to electronic overlap was found in CuII complexes.[37] The optical and electronic structure of the ZnQ2 complex as an electroluminescent material in organic light-emitting diodes, which has been calculated and measured,[38–41] which can be feasibly compared with the above assignment of the excitation states of the surface complex in ZnS-Q NCs. It is generally accepted that the strongest optical absorption of ZnQ2 at about 400 nm is the p!p* transition localized on the Q ligands and that the Zn atom contributes very little to the electron-density distribution of the frontier orbitals.[30, 42–44] Thus, Q*“ is assigned as the uncoupled excitation state of the surface complex in ZnS-Q NCs. The intramolecular and intermolecular interactions may also affect the photonic properties of ZnQ2 and thus induce a redshift of the p!p* transition.[30, 41] This is consistent with the exciton-coupling model. To interpret the 300–350 nm absorption/PLE band of ZnQ2, metal-to-ligand charge transfer (MLCT) was assumed.[30] For the 300–350 nm absorption/PLE band of AlQ3, the localized p!p* transition was suggested.[45] However, neither the MLCT nor the localized p!p* transition can explain the evolution of the Q*’’ peak in 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemphyschem.org the PLE spectra of ZnS-Q-1/2/3: the intensity of the Q*’’ band relative to that of Q* is rather low for identification at low surface concentration of the surface complex, but becomes stronger with increasing surface concentration of the surface complex. The exciton-coupling model proposed here perfectly explains the coupled emergence of the Q*’’ and Q*’ states in the PLE spectrum, with unambiguous assignment of the Q* band as uncoupled exciton. Moreover, it could be an alternative interpretation of the double peaks (ca. 340 and ca. 400 nm) observed in the absorption/PLE of the ZnQ2 complex.[30, 31, 46] Exciton coupling of uncoordinated 7-hydoxyquinoline or quinoline-derived chromophores in organic compounds[47, 48] and of coordinated quinoline chromophores in Cu/Zn complexes has been observed.[49–53] Albrecht et al. reported a lanthanum complex with three Q ligands showing exciton coupling of the coordinated Q at 280, 295, 350, and 480 nm in the absorption spectrum.[54] Telfer et al. and Kwong et al.[31, 55–57] also found exciton coupling in dinuclear and polynuclear complexes with structures that resemble those of neighboring surface complexes on an NC surface. All of these observations give support to the exciton-coupling model proposed for the ZnS-Q NCs. We note that, first, due to the size and shape distribution of the ZnS NCs, different surface Zn sites (i.e. the corners, edges, and sides of crystal planes) would have different coordination affinities to Q; second, interaction of the NC surface complexes with surface defect states[13] would effect changes in the absorption/PL/PLE spectra, but their characterization is beyond our technical capability. Theoretically, if the concentration of ZnS NCs and the molar [Q]/[NC] ratio are known, the number of Q ligands attached to one ZnS NC is usually described by a Poisson distribution [Eq. (1)]:[58–60] PðnÞ ¼ ex x n =n!
ð1Þ
where x is the average number of Q ligands per ZnS NC (estimated from a given molar ratio x = [Q]/[NC]) and n is the number of attached Q ligands on a given ZnS NC. Because of the distribution of attachment positions (i.e. the distribution of distances between Q ligands) of n Q ligands on a given ZnS NC, the number of two-, three-, or manifold-coupled surface complexes m2,3,… for this ZnS NC would have an distribution P’(n,m2,3…). Thus, the observed Q*’’ and Q*’ bands contain components not only from doubly coupled surface complexes, but also from triply and more highly coupled surface complexes. For this reason, the uncoupled and coupled surface complexes would generally coexist in all samples in different ratios that depend on the average [Q]/[NC] ratio. This intrinsic distribution matches well with the gradual change in the UV/Vis absorption and excitation spectra. Inter-NC exciton coupling of the surface complexes can be largely ruled out, because the NCs were well dispersed in nonpolar solvent (even centrifugation at 14 000 rpm for 10 min can not separate the NCs in the concentration range of 10 mg of NCs in 50 mL to 10 mL of cyclohexane), and thus the distance between NCs should be much larger than the critical coupling distance. NCs with the same surface density of the surface ChemPhysChem 0000, 00, 1 – 7
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www.chemphyschem.org Table 1. Fitting parameters of the PL decay curves from the double-exponential function. Sample
t1 [ns]
t2 [ns]
A1
A2
R2
tav [ns]
A1/A2
ZnS-Q-1 ZnS-Q-2 ZnS-Q-3
8.59 9.23 8.78
28.40 28.40[a] 28.40[a]
0.343 0.493 0.746
0.619 0.491 0.246
0.998 0.998 0.998
25.6 23.7 18.9
0.55 1.00 3.03
[a] Fixed.
Figure 3. PL spectra of ZnS-Q-3 NCs with concentrations of 1 and 10 mg mL1 in cyclohexane and in the solid state on a glass slide (excitation: 360 nm).
complex in 1 or 10 mg mL1 solutions and even dried ZnS-Q NCs showed no obvious changes in the PLE spectrum and only a small peak shift in the PL spectra ( 3 nm, Figure 3). This spectral pinning may due to the remaining oleic acid and/ or oleylamine on the NC surface providing efficient steric hindrance to prevent the approach of surface complexes from neighboring NCs. An inner filter effect inducing the redshift can also be excluded because of the large absorption/emission Stokes shift (Figure 2 A and B), the near independence of the PL peak from the ZnS-Q NC concentration, and concentrationindependent PL lifetime. In addition, considering the coordination-number requirement and steric effect introduced by the NC surface and the surface ligands, the coordination of a second ligand Q to a surface Zn atom should be disfavored. For instance, about half of the coordination space of a Zn atom in a crystal facet, as well as the orbitals, is occupied by the ZnS NC itself. For these reasons, the exciton coupling can only be induced by adjacent (two or more) surface complexes on the same ZnS-Q NC. Figure 4 shows the PL lifetimes of the ZnS-Q NCs in cyclohexane. The PL decay curves of the ZnS-Q NCs are well fitted to a double-exponential function [Eq. (2)]: IðtÞ ¼ A1 exp½ðt t0 Þ=t1 þ A2 exp½ðt t0 Þ=t2
ð2Þ
where t1 and t2 are the PL decay lifetimes and A1 and A2 the weighting parameters. The fitting parameters are given in Table 1. The average lifetime was determined by Equation (3):[61, 62]
Figure 4. PL lifetimes of ZnS-Q-1 at 482 nm, ZnS-Q-2 at 493 nm, and ZnS-Q3 at 508 nm with excitation at 379.2 nm.
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tan ¼ A1 t21 þ A2 t22 =ðA1 t1 þ A2 t2 Þ
ð3Þ
The average PL lifetimes of the ZnS-Q-1/2/3 NCs were calculated to be 25.6, 23.7, and 18.9 ns, respectively. They are longer than the PL lifetime of the ZnQ2 complex in the solid state (10.3 ns)[46] or in solution of dimethyl sulfoxide or ethanol (3.6 ns, 300 K).[63] The lower the surface density of the surface complex, the longer the average PL lifetime. If we assume that the slower component t2 is a constant (28.40 ns for ZnS-Q-1), the faster component t1 in ZnS-Q-2 is fitted to be 9.23 ns, and that in ZnS-Q-3 is 8.87 ns, which are approximately the same as the t1 value of ZnS-Q-1 (8.59 ns). We assign t1 to the PL decay lifetime of the exciton-coupled surface complexes and t2 to the uncoupled surface complexes. This explains the observation that the amplitude ratio A1/A2 increases remarkably with increasing surface density of the surface complex (see Table 1). The normalized PL decay curves were found to be the same when measured with NC concentrations of 1 and 10 mg mL1, which also gives rational support to the proposed mechanism of intra-NC exciton coupling of the surface complexes. The small deviations of the fitting curves from the measured curves of ZnS-Q-2 and ZnS-Q-3 were possibly due to fact that the lifetime varies with the coupling number of the surface complexes, in accordance with the P’(n,m2,3…) distribution discussed above. Therefore, three or more decay lifetimes could be used to achieve a more precise fitting. The decay curve of pure ZnS was not measured because of its very low absorption at the laser excitation wavelength of 379.2 nm, which overlaps with the PL peak of the pure ZnS NCs. The PL quantum yields of ZnS-Q-1, ZnS-Q-2 and ZnS-Q-3 NCs were measured to be approximately 33.0, 27.4, and 12.2 %, respectively, by using a methanolic solution of coumarin 6 as reference (quantum yield: 65 %). Generally, lifetime shortening can be induced by additional radiative and/or nonradiative transitions. Therefore, the decrease in quantum yield of ZnS-Q NCs accompanied by shortening of the PL lifetime tav indicates the existence of the additional nonradiative transition along with higher surface concentration of the surface complex. It is possible that surface-complex/ligand-related surface states or exciton coupling facilitate the nonradiative recombination. The accompanying PL band on the lower-energy side of the near-band-edge emission of an NC can be induced by the surface states (also known as deep traps or surface defect states), defect states inside the material, or excitation states of doping ions. Typically, the first is found in CdSe and CdS colloidal NCs,[64–66] the second is often observed in ZnO nanomateriChemPhysChem 0000, 00, 1 – 7
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CHEMPHYSCHEM ARTICLES als,[67, 68] and the last-named are well known as d-dots.[69, 70] Various models have been developed in studying these spectra. Differing from the traditional model of distributed deep trap states at the surface of the NC,[64–66] Kambhampati et al. recently reported a single-surface-state model to explain the defect emission of CdSe, CdS, and CdSe/ZnS NCs.[71–73] In principle, the deep-trap model and the single-surface-state model could also be used in explaining the observations reported in this work. Indeed, there is basically no difference between these models if the excitation state of the surface complex is defined as the trap state or as the single surface state. The feasibility of adopting the fluorescent-surface-complex model here is subject to following concerns: 1) the fluorescent species are well defined, which is less clear for the defect states corresponding to surface-defect structures; therefore 2) theory and methods of studying coordination complexes can readily be used for surface complexes; for example, exciton coupling in coordination complex is applied in studying interactions of the surface species; and 3) the fluorescence can be designed in parallel to that of the corresponding free-standing complexes (e.g. ZnS-Q and ZnQ2), so that the large database of functional complexes could be rationally used to guide NC modification and property control.
3. Conclusions ZnS NCs modified with Q were prepared. The surface complex exhibits prominent PL, especially in low surface concentration. Intra-NC exciton coupling of the surface complexes was proposed and characterized by absorption, PL, PLE, and PL lifetime measurements. By controlling the surface density of the surface complex, and thus the exciton coupling of the surface complexes, the fluorescence spectrum could be tuned from blue to green. Although a more extensive study of the ligandexchange dynamics, the exact [Q]/[NC] ratio- and quantum chemical calculations remain to be done, our preliminary results show that, in addition to the NC-to-NC and ligand-to-NC interactions, the ligand-to-ligand or, in this work, the surface complex-to-surface complex interaction should not be overlooked, because this could also dominate the properties of the colloidal NCs. Such colloidal NCs are potential active materials for light-emitting devices and fluorescent probes in biology. Additionally, the designability of the ligands, the energy transfer between the NC core and the surface complex, and the exciton-coupling effect reported here can be utilized to control the carrier-transport property and broaden the absorption and PLE windows of the NCs, and this is promising for improving the light-harvesting efficiency of NC solar cells and photocatalysts.
Experimental Section Synthesis of Zinc Blende ZnS NCs ZnCl2 (1 mmol) was mixed with oleic acid (4 mL), oleylamine (4 mL), and 1-octadecene (4 mL) in a 50 mL three-neck flask. On a Schlenk line the mixture was heated to 110 8C. Traces of H2O and 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemphyschem.org low-boiling impurities were removed by vacuum pumping for 30 min. The sulfur precursor was prepared by dissolving sulfur in oleylamine. Under the protection of an N2 flow, 2 mL of the sulfur solution (0.5 m) was swiftly injected. The mixture was heated to 240 8C for 30 min, the heater removed, and the reaction mixture cooled to room temperature. The ZnS NCs were collected by adding 60 mL of ethanol and centrifugation (4000 rpm, 10 min). The precipitated NCs were then redispersed in 4 mL of cyclohexane, washed again with ethanol, and centrifuged. The precipitated ZnS NCs were finally dried with an intense N2 blast for about 1 min and stored in cyclohexane or toluene with good dispersion to form transparent solutions. On the basis of the measured weight and theoretical weight of ZnS without ligands under the assumption of 100 % yield, the ligand content of the colloidal ZnS was about 18 wt %. Energy-dispersive X-ray spectroscopic measurements showed ligand fractions of up to 33–45 wt %.
Preparation of ZnS-Q NCs The ZnS-Q NCs with coordinated Q ligands were prepared by ligand exchange. The surface density of the surface complex (equal to that of the coordinated Q) of ZnS-Q NCs was controlled by means of the Q/ZnS ratio in the ligand-exchange procedure. Typically, 10 mg of ZnS NCs was dispersed in 100 mL of cyclohexane, and 20 mL of an ethanolic solution containing 1 108, 1 107, or 1 106 mol of Q was swiftly injected to form ZnS-Q-1, ZnS-Q-2, or ZnS-Q-3 NCs, respectively. After 1 min of shaking, 0.5 mL ethanol was added to precipitate the ZnS-Q-1/2/3 NCs. The NCs were collected by centrifugation (8000 rpm, 1 min), and any residual ligands and byproducts were removed by redispersing the NCs in 0.4 mL of cyclohexane, adding 1 mL of ethanol, and centrifugation. The final ZnS-Q NCs were readily redispersible and were stored in cyclohexane.
Instruments TEM images were measured on a JEM-2100 transmission electron microscope at an acceleration voltage of 200 kV. FTIR spectra were recorded on a Vector 22 spectrometer with a resolution of 2 cm1 by using KBr pellets. The absorption spectra were obtained by a Shimadzu UV-2700 UV/Vis spectrophotometer. The PL and PLE spectra were collected with a Hitachi F-4600 fluorescence spectrophotometer. PL lifetimes was measured with a Zolix Omini-l 300 fluorescence spectrophotometer and a picosecond pulsed diode laser (Edinburgh Instruments Ltd.) operating at 379.2 nm.
Acknowledgements This work was supported by the Major State Basic Research Development Program of China (Grant No. 2013CB922102, 2011CB808704), the National Natural Science Foundation of China (Grant No. 91022031, 21301089), and Jiangsu Province Science Foundation for Youths (BK20130562). The authors gratefully acknowledge Weikuan Duan and Prof. Wei Wei for the photoluminescence lifetime measurement. Keywords: exciton coupling · ligand exchange luminescence · nanoparticles · surface chemistry
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Received: March 21, 2014 Published online on && &&, 2014
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ARTICLES Intra-nanocrystal exciton coupling of the surface complexes formed by coordination of 8-hydroxyquinoline (Q) to ZnS nanocrystals (NCs) tunes the photoluminescence (PL) color from blue to green (see picture), broadens the absorption and PL excitation windows of the NCs, and thus is a potential strategy for improving the light-harvesting efficiency of NC solar cells and photocatalysts.
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X. Xu,* J. Ji, G. Wang, X. You && – && Exciton Coupling of Surface Complexes on a Nanocrystal Surface
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