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Concentration-Dependent Photoinduced Photoluminescence Enhancement in Colloidal PbS Quantum Dot Solution Teng Zhang,† Haiguang Zhao,*,† Daria Riabinina,†,‡ Mohamed Chaker,† and Dongling Ma*,† INRS-E´nergie, Mate´riaux et Te´le´communications, UniVersite´ du Que´bec, 1650 Lionel-Boulet, C.P. 1020, Varennes (Qc) J3X 1S2 Canada, and De´partement de physique, UniVersite´ de Montreal, C.P.6128, Succ. Centre-Ville, Montre´al (Qc) H3C 3J7 Canada ReceiVed: March 19, 2010; ReVised Manuscript ReceiVed: May 3, 2010
The concentration-dependent photoinduced photoluminescence (PL) enhancement of PbS quantum dots (QDs) is observed for the first time in the PbS colloidal solution. The enhancement in PL efficiency is attributed to the photooxidation of PbS QD surface based on the optical, transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) measurements. The decrease in PbS QD concentration leads to the increase in the amplitude of PL enhancement and to the decrease in the time required to reach the maximum PL enhancement. For the lowest investigated QD concentration of 58.7 nmol/L, PL efficiency can be increased by as much as 60 times. The critical concentration for realizing the considerable PL enhancement is found to be ∼200 nmol/L. Further investigation shows that the concentration-dependent PL enhancement is related to spontaneous ligand desorption and therefore more efficient photooxidation in low-concentration samples. Furthermore, it was found that the enhanced PL can remain during storage. It suggests that the postsynthesis treatment of UV illumination can serve as an alternative, while simple and highly efficient way in improving the PL efficiency of PbS QDs when the coverage of surface ligands is appropriately controlled. 1. Introduction Colloidal quantum dots (QDs) have attracted great attention because of their unique optical properties related to the quantum confinement effect.1 Their photostability, bright photoluminescence (PL), size-dependent absorption, narrow PL, and broad absorption band make QDs promising candidates for many potential applications, such as biosensing, bioimaging, photodetectors, and photovoltaics.2-7 Compared with their visible and ultraviolet (UV) light-emitting counterparts, PbS QDs with tunable band gaps in the near-infrared (NIR) region are of particular interest for in vivo deep-tissue imaging and NIR optoelectronics.8-11 High-temperature organometallic synthetic route for colloidal PbS QDs was first reported by Hines and Scholes using surface ligands of oleic acid (OA).12 Lobo et al. showed later on that OA ligands preferentially bind to lead atoms on the QD surface while leaving sulfur atoms unpassivated, which then serve as possible hole traps.13 The use of trioctylphosphine (TOP) as the second type of ligands in addition to OA allows us to produce the PbS QDs with quantum yields as high as 80% due to the surface passivation of both Pb and S sites by OA and TOP, respectively.14 Although the PL of QDs is often significantly enhanced through achieving better surface passivation either by using multiple types of surface ligands during synthesis as mentioned above or by forming an inorganic shell with a larger band gap, such as in the case of visible-emitting CdSe QD,15,16 it was recently demonstrated that for certain semiconductor materials (such as CdSe and CdSe@ZnS) the quantum efficiency of QDs can be enhanced via lamp or laser irradiation.17-21 By photo* Corresponding authors. Tel: (450) 929-8120. Fax: (450) 929-8102. E-mail:
[email protected] (H.Z.);
[email protected] (D.M.). † Universite´ du Que´bec. ‡ Universite´ de Montreal.
activating the ordered monolayer of 4.1 nm CdSe QDs in a variety of atmospheric gases including dry or wet argon, nitrogen, and oxygen, Cordero et al. have found that the enhancement of the PL intensity of CdSe QD monolayer is due to the presence of water molecules in ambient air. Water molecules adsorbed on the surface of QDs act to passivate surface traps, which results in increased luminescence.17 Nazzal et al. also confirmed that the adsorbed water molecules on the surfaces of CdSe QDs could dramatically enhance the PL efficiency.19 Furthermore, they proposed that laser illumination could optimize the interaction between ligands and surface atoms and thus improve PL properties.19 By studying the photoenhancement effect in colloidal CdSe and CdSe@ZnS core@shell QDs attached to a variety of surfactant molecules in aqueous and nonaqueous environments, Jones et al. supposed photoinduced rearrangement of ligand molecules on the QD surface or the addition of molecules such as methanol could stabilize surface trap states to cause an enhancement of PL efficiency.20 By investigating the photoinduced PL enhancement in monoand multilayer films of CdSe@ZnS QDs in a dry nitrogen atmosphere, Uematsu et al. suggested that the photoejection of electrons to the substrate can also be at the origin of photoactivation.21 From the PL variation of CdSe QDs under continuous illumination in the presence of different polymers and solvents, Biju et al. determined that static passivation of the surface defects by molecules in the environment plays an important role in the PL enhancement.22 All of these works demonstrate that photoactivation is an efficient way to improve PL efficiency of visible-emitting QDs, although the actual mechanism of this process is not necessarily the same under different conditions. Few reports have been published on the photoinduced PL enhancement in PbS QDs,23 and our understanding of the photoactivation mechanism in PbS QDs is still rather limited. In this work, we report, for the first time, the concentrationdependent PL enhancement behavior in PbS QDs induced by
10.1021/jp1025152 2010 American Chemical Society Published on Web 05/13/2010
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Figure 1. (a) PL spectra of PbS QDs before (original) and after 15, 60, and 120 min of UV irradiation. Inset: expanded PL spectra of original and 120 min irradiated PbS QDs. (b) Normalized PL and (c) absorption spectra of PbS QDs before (original) and after 15, 60, and 120 min of UV irradiation. The QD concentration is 58.7 nmol/L.
UV illumination. The origin of the observed PL enhancement upon UV illumination and subsequent PL darkening was discussed and found to primarily consist in the photooxidation of PbS QD surface. The “apparent” concentration-dependent behavior was found to be actually closely related to the ligand adsorption/desorption dynamics, which governs the photooxidation process of PbS QDs. 2. Experimental Section 2.1. Materials. Lead acetate trihydrate, octadecene (technical grade, 90%), bis(trimethylsilyl) sulfide (TMS)2S (synthesis grade), and TOP (technical grade, 90%) were all purchased from Sigma-Aldrich. OA (technical grade, 90%), hexane, and ethanol were purchased from Fisher Scientific. All chemicals were used as purchased. 2.2. Synthesis of PbS QDs. PbS QDs were synthesized according to the procedure reported by van Veggel et al. in the literature.14 In a typical synthesis, a mixture of lead acetate trihydrate (1 mmol), OA (1.2 mL), TOP (1 mL), and octadecene (15 mL) were heated to 150 °C for 1 h. Then, the system was cooled to ∼100 °C under vacuum for 15 min. Subsequently, the solution containing 0.5 mmol (TMS)2S, 0.2 mL of TOP and 4.8 mL of octadecene was quickly injected into the reaction flask at 130 °C; then, the reaction was quenched by cold water. PbS QDs were precipitated with ethanol, centrifuged to remove unreacted lead oleate and free OA molecules, and then redissolved in hexane. All sample preparation steps involving QDs were carried out under very low light to avoid uncontrolled photoactivation.
2.3. UV Illumination. PbS QDs in hexane were irradiated with UV light for various time by using a 4 W UV lamp (115 V, 60 HZ, model 22-UV, Optical Engineering). 2.4. Characterization. Absorption spectra were recorded with a Cary 5000 UV-visible-NIR spectrophotometer (Varian) with a scan speed of 600 nm/min. PL spectra were measured with a Fluorolog-3 system (Horiba Jobin Yvon) using an excitation wavelength of 670 nm. The relative quantum yield (RQY) was defined as the ratio of the integrated PL intensity to the absorbance at 670 nm, which is the excitation wavelength in fluorescence measurements. PbS QDs were also characterized by transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). For TEM measurements, a drop of PbS QDs was dried out on Cu/C grids and analyzed with a JEOL 2100F microscope. XPS spectra of PbS QDs were acquired using a VG Escalab 220i-XL equipped with an Al KR source. 3. Results and Discussion 3.1. Origin of PL Enhancement in PbS QDs under UV Illumination. Figure 1a illustrates the PL spectra of PbS QDs before (original) and after 15, 60, and 120 min of UV irradiation. Considering their low intensities, the expanded PL spectra for the initial and 120-min-irradiated samples are shown in the inset. Figure 1b shows the normalized PL spectra of PbS QDs before (original PbS QDs) and after 15, 60, and 120 min of UV irradiation. A gradual blue shift of the PL spectra can be clearly observed with illumination, just the same as that exhibited in their absorption spectra (Figure 1c). The PL peak maximum is located at ∼1080 nm for original nonirradiated PbS QDs,
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Figure 2. (a) PL peak positions of PbS QDs as a function of illumination time. (b) Relative quantum yield of PbS QDs as a function of illumination time. The solid and dashed lines were drawn to guide the eye. The QD concentration is 58.7 nmol/L.
Figure 3. TEM images of PbS QDs (a) before and (b) after 120 min of UV irradiation. Insets: histograms of QD size. Gauss fitting curves are also displayed.
whereas its position is shifted to ∼1045 nm after 120 min of UV irradiation. The PL peak position as a function of irradiation time is shown in Figure 2a. The important variation of PL intensity with illumination was also observed. Figure 2b exhibits RQY of PbS QDs as a function of irradiation time. RQY significantly increases with irradiation time and reaches its maximum (60 times that of initial QDs) after 30 min of UV irradiation. Then, a drastic decrease is observed for irradiation times between 30 and 120 min. The blue shift of the PL peak has often been correlated with the photoaccelerated ligand etching or the photooxidation of QD surface.17,22,24 Although both lead to the reduction of the “effective” size of PbS, the former normally causes a more obvious decrease in the overall size of nanocrystals. To verify which mechanism dominates, TEM was performed on as prepared and irradiated PbS QDs. As shown in Figure 3, the size of PbS QDs is found to be equal to (3.7 ( 0.2 nm) and remains fairly same after UV illumination. This result clearly demonstrates that no considerable photoinduced ligand etching of the PbS QDs occurs during UV irradiation because otherwise, the overall QD size will decrease. In addition, if the ligand etching is the dominating mechanism, the reduction of the QD size should be ∼0.4 nm according to the absorption peak shift.25 Therefore, this mechanism cannot explain herein observed modifications of the optical properties of PbS QDs upon irradiation. Instead, the second mechanism of QD surface photooxidation mentioned above can be possibly responsible for the spectral blue shift.
In addition, the photoxidation is likely to contribute to the variation of PL intensity.15 For QDs under continuous irradiation, surface oxidation has been reported,17 leading to the formation of a partial or complete QD@oxide core@shell structure. This would lead to the restructuring of the QD surface and affect the PL efficiency either positively or negatively depending on the thickness of the shell, to some extent analogues to what were found for CdSe@ZnSe QDs.16 In that scenario, the PL efficiency initially increases with the formation of the ZnS shell related to better surface passivation and then decreases with increasing shell thickness due to newly created defects in the shell. To examine whether the photooxidation is the major factor influencing the variation of the optical properties of irradiated PbS QDs, we performed the XPS measurements of PbS QDs before and after 120 min of irradiation. Figure 4 shows the Pb 4f core-level spectra of PbS QDs before (a) and after 120 min (b) of UV irradiation. The spin-orbit splitting was all fixed at 4.86 eV, the same as that reported in literature,13 and the fitting appears quite good. Each peak is a superposition of two sets of peaks corresponding to unoxidized PbS and its oxidation products.26 The relative ratio of Pb atoms in PbS and oxidized components was evaluated by calculating the ratio between the areas of corresponding, decomposed peaks. It is found that even as-prepared PbS QDs are partially oxidized by ∼8%. After 120 min of irradiation, the relative degree of oxidation increases to ∼24%.
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Figure 5. PL enhancement factor of PbS QDs as a function of UV irradiation time at various QD concentrations. The dashed lines were drawn to guide the eye.
Figure 4. Pb 4f core-level spectra of PbS QDs (a) before and (b) after UV irradiation. The spin-orbit splitting was set at 4.86 eV. Each peak is a superposition of two sets of peaks corresponding to unoxidized PbS and its oxidation products.
It would also be reasonable to suppose that the important enhancement of the PL intensity observed in our experiments (Figure 2b) is due to the improved surface passivation by certain molecules in solution or ligand rearrangement on the surface initiated by illumination,20,22,23 but these mechanisms can absolutely not account for the consequent drop in PL intensity and the significant blue shift of PL peaks. As previously shown, TEM measurements reveal that the overall size of PbS QDs remains constant before and after UV irradiation and is equal to (3.7 ( 0.2) nm. By combining the TEM size and the oxidation degree (24% for 120 min irradiated QDs) from XPS and by assuming the uniform oxidation of the QD surface with either PbSO3 or PbSO4, it is elementary to find that the “effective” size of PbS decreases from 3.7 nm to ∼3.3 nm. The estimated size reduction by this simple calculation indeed corresponds well to the observed spectral shift, according to the data reported on the variation of the band gap of PbS QDs with sizes.25,27 The hypothesis of the PbS QD photooxidation can also explain well the PL intensity behavior as a function of UV irradiation time. During the first moments of irradiation, the surface oxidation leads to the decrease in surface defects and enables better surface passivation, as found in the process of coating CdSe with ZnS, which yields the increase in PL intensity.16,24 Further irradiation results in the formation of new surface defects in the oxide layer of QDs, eventually resulting in the significant drop of PL intensity.17,18,20,23 3.2. Concentration-Dependent PL Enhancement. Interestingly, it is found that the PL enhancement arising from UV illumination varies significantly with QD concentrations. Figure 5 plots the variation of PL enhancement factor of PbS colloidal solutions as a function of UV-irradiation time at five different concentrations. The PL enhancement factor is defined as (I1 - I0)/I0, where I0 and I1 are the initial and the enhanced PL intensities, respectively. The concentrations of PbS QDs have been calculated using the Beer-Lambert’s law: A ) εCL, where A is the absorbance at the peak position of the first exciton absorption for a given sample, C is the molar concentration of QDs, ε is the extinction coefficient per mole of QDs, and L is the light path length. For PbS QDs, ε is found to be proportional
Figure 6. Time required for obtaining the maximum PL enhancement factor as a function of the concentration of PbS QDs.
to the radius (r) of QDs (ε ) 19 600r2.32).25 All investigated samples were UV-irradiated for various time durations from 0 to 120 min. It is seen that the curve behavior of low-concentration samples is similar to that shown in Figure 2b, which demonstrates a significant initial increase in PL intensity with UV irradiation, followed by a dramatic drop at longer illumination times. The maximum of the PL enhancement factor for the lowest QD concentration (58.7 nmol/L) is reached after only 30 min of irradiation, whereas that for the highest concentration (616 nmol/ L) is reached after at least 120 min of irradiation. Figure 6 summarizes the time required to obtain the maximum PL enhancement factor by UV irradiation as a function of QD
Figure 7. Maximum PL enhancement factor for PbS QDs at various concentrations. The dotted lines were drawn to guide the eye.
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Figure 8. Schematic representation of the photoxidation process of QDs at different concentrations.
concentrations. The higher the QD concentration is, the more time the QDs need to achieve the maximum PL efficiency. Ligand dynamics in PbS QD solutions could be at the origin of this behavior.28,29 In the QD solution, the surface ligands can be either directly bonded to or physisorbed on the QD surface. During the preparation of low-concentration samples by dilution, a spontaneous ligand desorption may take place, which renders the QD surface more accessible to oxygen molecules and thus faster photooxidation. Also, the maximum PL enhancement factor varies as a function of QD concentration. For the lowest concentration (58.7 nmol/L), the PL enhancement is much more pronounced with the PL enhancement factor reaching as high as 60, whereas for the highest concentration (616 nmol/L), the maximum enhancement factor is only equal to 2. Figure 7 shows the maximum enhancement factor that can be obtained for PbS QDs at various concentrations. It displays a step-like function with high enhancement factors for the PbS concentration lower than 200 nmol/L, followed by a drastic decrease in the PL enhancement factor at 200 nmol/L and nearly identical low PL enhancement factors for the PbS QD concentrations above 350 nmol/L. The critical concentration for the efficient PL enhancement by UV irradiation of PbS QDs should therefore be ∼200 nmol/L. These data clearly reveal that at higher QD concentrations the PL enhancement mechanism is much less efficient than for low-concentration samples. The increase in QD concentration by a factor of 10 leads to the decrease in PL efficiency by a factor of 30. This phenomenon corroborates the hypothesis of the ligand desorption from the QD surface being a crucial parameter in dominating the variation of the photoxidation process in the samples of different concentrations. In the case of higher-concentration samples, the ligand coverage of QD surface is sufficient for protecting it from photooxidation. In the case of lower concentration samples, higher fractions of the QD surface can be oxidized, leading to higher fractions of surface sites being passivated by oxidation products and thus a larger increase in PL intensity. This proposed mechanism is schematically illustrated in Figure 8. It may be argued that the significant enhancement in lowconcentration samples is due to a low initial PL quantum efficiency related to the change of surface passivation during dilution. This argument is based on the fact that the PL enhancement is larger for QDs with lower initial quantum efficiency. To verify this hypothesis, we plotted the PL
Figure 9. PL intensity of PbS QDs as a function of QD concentrations.
intensity of QDs at varying concentrations (Figure 9). It can be clearly seen that the PL intensity of QDs decreases linearly with decreasing QD concentrations. The result strongly indicates that the PL quantum efficiency is not affected by the dilution process, and all samples investigated in the current work have the same initial PL quantum efficiency. Obviously, although the dilution causes the desorption of some physisorbed ligands from the QD surface, the degree of surface passivation and, consequently, the density of surface defect states that strongly influences the PL quantum efficiency, does not change. To verify the role of the ligand dynamics in the concentrationdependent PL enhancement, excess amounts of OA (2.2 × 10-1 mol/L, 100 µL) were added to the lowest-concentration sample (58.7 nmol/L, 3 mL), and the PL enhancement factor was measured as a function of time. Figure 10 shows the variation of the PL enhancement factor of PbS QDs as a function of UV irradiation time before and after the addition of excess OA ligands. The phenomenon of substantially increased PL with UV illumination completely disappears when additional OA molecules are introduced to the solution. This result conversely proves that it is the ligand desorption behavior in the lowerconcentration PbS solutions that is responsible for the stronger PL enhancement under UV illumination. The addition of excess OA ligands to low-concentration samples can significantly increase the ligand surface coverage and prevent the oxidation process.
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Figure 10. PL enhancement factor of PbS QDs (58.7 nmol/L) as a function of UV irradiation time before and after the addition of excess OA ligands. The dashed lines were drawn to guide the eye.
In this work, we reported for the first time the concentration-dependent photoinduced PL enhancement in colloidal PbS solutions. To uncover the underlying mechanism, we first investigated the influence of UV irradiation on PL spectral characteristics, size, and chemical composition of PbS QDs. The possible mechanisms affecting the blue shift of PL peaks and variation in PL intensity of PbS QDs have been discussed and based on the integrated consideration of all the results; it is concluded that photoinduced PL enhancement of PbS QDs in solution is mainly due to the surface photooxidation of PbS QDs. The efficiency of PL enhancement in terms of time required to achieve the maximum PL enhancement and the maximum PL enhancement factor that can be obtained, however, differs significantly among QDs at different concentrations. It is found that this concentrationdependent PL enhancement behavior actually originates from the spontaneous variation of the coverage of ligands on the QD surface. Furthermore, the enhanced PL does not decay during storage. The work strongly suggests that the postsynthesis treatment of UV illumination can serve as an alternative, while simple and highly efficient way to increase the PL efficiency when the ligand coverage is set in a suitable range. Acknowledgment. We acknowledge the financial support from the Natural Sciences and Engineering Research Council of Canada, Fonds de la recherche sur la nature et les technologies, and the Canada Research Chair program. We also thank Horiba Jobin Yvon for its support on photoluminescence measurements.
Figure 11. PL intensity of PbS QDs synthesized freshly (original), UV-irradiated for 1 h, and stored for 12 h in air. The QD concentration is 58.7 nmol/L.
3.3. Stability of UV-Irradiated PbS QDs. It has been reported in the literature that an aging process may remove surface defects or lead to more organized ligand structures on the surface of QDs, which can increase their PL intensity.30 It has also been reported that the PL enhanced by illumination can rapidly decay back to near its original values after the light source is turned off.17,20 To investigate the stability of PL of UV-irradiated samples, we studied the evolution of PL spectra for PbS QDs stored in the dark in normal air environment during the period of 12 h. Figure 11 compares the PL intensity of freshly synthesized PbS QDs (58.7 nmol/ L), the same QDs but after 1 h of UV irradiation, and further after 12 h of room temperature aging in air. First, one can observe a large increase in PL intensity after UV irradiation, as shown and discussed in previous sections. Interestingly, a further increase in PL intensity was observed during 12 h of storage. It is worth recalling that for this specific sample, after being photooxidized with 1 h of UV illumination, further oxidation under UV light will lead to the decrease in PL intensity. The seemingly contradictory PL increase observed during storage, however, can be reasonably explained in the context of ligand readsorption and thus improved surface protection as well as annealing of the surface and thereby removal of defects to form less defective oxides on the QD surface in the absence of UV light.24
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Concentration-Dependent Photoinduced PL Enhancement (22) Biju, V.; Kanemoto, R.; Matsumoto, Y.; Ishii, S.; Nakanishi, S.; Itoh, T.; Baba, Y.; Ishikawa, M. J. Phys. Chem. C 2007, 111, 7924. (23) Peterson, J. J.; Krauss, T. D. Phys. Chem. Chem. Phys. 2006, 8, 3851. (24) Stouwdam, J. W.; Shan, J.; van Veggel, F. C. J. M. J. Phys. Chem. C 2007, 111, 1086. (25) Cademartiri, L.; Montanari, E.; Calestani, G.; Migliori, A.; Guagliardi, A.; Ozin, G. A. J. Am. Chem. Soc. 2006, 128, 10337. (26) Clifford, J. P.; Konstantatos, G.; Johnston, K. W.; Hoogland, S.; Levina, L.; Sargent, E. H. Nat. Nanotechnol. 2008, 9, 40.
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