Improving the Quantum Yields of Semiconductor Quantum Dots

Apr 10, 2009 - Timothy V. Duncan, Miguel Angel Méndez Polanco, YooJin Kim and .... Yian Zhu , Zhen Li , Min Chen , Helen M. Cooper , Gao Qing (Max) L...
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J. Phys. Chem. C 2009, 113, 7561–7566

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Improving the Quantum Yields of Semiconductor Quantum Dots through Photoenhancement Assisted by Reducing Agents Timothy V. Duncan,† Miguel Angel Me´ndez Polanco,† YooJin Kim, and So-Jung Park* Department of Chemistry, UniVersity of PennsylVania, Philadelphia, PennsylVania 19104 ReceiVed: December 19, 2008; ReVised Manuscript ReceiVed: March 8, 2009

Surface modification of semiconductor quantum dots (QDs) often causes a drastic reduction of photoluminescence quantum yield (QY). Here, we report an efficient way to improve photoluminescence (PL) characteristics of silica-coated QDs using the combination of light-induced PL enhancement (photoenhancement) and the small molecule reducing agent, dithiothreitol (DTT). The photoenhancement process in the absence of a reducing agent is usually accompanied by a blue shift and broadening of the PL spectrum as well as a subsequent rapid PL quenching due to the competitive photo-oxidation process. The addition of DTT augments the degree of photoenhancement and inhibits the spectral shift and broadening. The photoenhancement assisted by DTT reported here should provide a simple and useful means of preparing stable, highly luminescent water-soluble silica-coated QDs that have PL QYs comparable to those exhibited by organic-soluble QDs. I. Introduction Semiconductor quantum dots (QDs) have received extensive attention owing to their unique and useful optical properties, including tunable emission wavelengths, large extinction coefficients, and high photostability.1 Recent improvements in synthetic methods2-4 for making high quantum yield (QY) CdSe nanocrystals have accelerated the exploitation of QDs in a variety of applications, ranging from biosensing5-10 to optoelectronics.11-15 In many of these applications, QD surfaces are often modified to control solubility or to provide additional functionalities. Among numerous functionalization methods reported to date, silica coating has long been used to make water-soluble nanoparticles.16-21 The silica-coating approach is particularly attractive for biological and medical applications of nanoparticles for several reasons. First, silica surfaces can be easily functionalized with biological molecules through well-established silanization chemistry.19,22-24 Second, various types of nanoparticles and small molecules can be simultaneously incorporated into the silica matrix to generate multifunctional nanoparticles.25,26 Third, silica-coated nanoparticles of varied shell thicknesses can be readily synthesized with narrow size distributions.19,27 However, the photoluminescence (PL) QYs of QDs often severely decrease with the growth of the silica shell due to the generation of surface defect sites during the coating process.18,25,27 The degradation of QD emission efficiency is frequently observed in other post-synthesis surface modifications as well.6,28,29 Indeed, PL quenching by surface defect sites has been a huge hurdle to overcome in light emission-based applications of QDs. Here, we report a reliable method to recover the high QY of CdSe QDs subsequent to coating with silica, using the combination of photoenhancement and the electron-donating ligand dithiothreitol (DTT). Photoenhancement of QDs involves the gradual increase of the PL QY in response to exposure to light and is frequently observed in QDs with low QY.30-40 While * To whom correspondence should be addressed. E-mail: sojungp@ sas.upenn.edu. Fax: +1-215-573-6743. † These authors contributed equally to this work.

photoenhancement can be a simple way to improve the QY of QDs, this process is usually accompanied by spectral broadening and a blue shift due to concurrent photo-oxidation, which impedes its practical utilization. Here, we show that the addition of DTT can promote the photoenhancement process of silicacoated QDs and prevent the broadening and blue shift of emission peaks. While a great deal of work has been devoted to understanding the effect of surface ligands on the photophysical properties of QDs29,32,41-47 and numerous reports suggest that photoenhancement involves the rearrangement of surface-molecules and atoms,30-32,34,35,38-40 there have been few studies endeavoring to connect the two effects and elucidate how surface binding molecules affect the photoenhancement process. This work demonstrates the synergistic effect of light and small molecule reducing agents in improving the QY of silica-coated QDs and shows that photoenhancement aided by small reducing molecules can be used to address the practical problem of low PL QY in silica-coated QDs in a reliable, reproducible fashion. II. Experimental Section Materials. Trioctylphosphine oxide (TOPO, 99%), 1-hexadecylamine (HDA, 98%), cadmium oxide (CdO, 99.99+%), diethyl zinc (Zn(Et)2), trioctylphosphine (TOP, 90% technical grade), selenium (powder, 100 mesh, anhydrous, 99.99%), tetraethyl orthosilicate (TEOS, 99.999%), Igepal Co520, ammonium hydroxide (28% solution in water, 99.99%), and 3-aminopropyltrimethoxysilane (APS, 97%) were purchased from Aldrich. Bis(trimethylsilyl)sulfide ((TMS)2S) was purchased from Fluka. Tetradecylphosphonic acid (TDPA, 97%) was purchased from Alfa Aesar. Dithiothreitol (DTT, 99%) was purchased from Sigma. All solvents were purchased from Fisher; High-performance liquid chromatography grade solvents were used exclusively. Ultrapure water (18 MΩ, Barnstard) was used for all reactions and procedures. All materials were used as received without further purification unless otherwise noted. Instrumentation. Transmission electron microscopy (TEM) was performed on a JEOL TEM-2010 operating at 200 kV accelerating voltage to determine particle size. Fluorescence emission measurements were recorded with a Fluorolog-3

10.1021/jp811245r CCC: $40.75  2009 American Chemical Society Published on Web 04/10/2009

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spectrofluorometer (HORIBA Jobin Yvon, Inc.) utilizing an R928 PMT detector. Synthesis of CdSe/ZnS Core/Shell Nanocrystals. The CdSe and CdSe/ZnS (core-shell) nanocrystals were synthesized following a modified literature method.4,48,49 For the core CdSe nanocrystal preparation, TOPO (1.0 g, 2.59 mmol), HDA (0.50 g, 2.07 mmol), TDPA (0.12 g, 0.43 mmol), and cadmium oxide (26 mg, 0.02 mmol) were loaded in a three-neck flask equipped with a stir bar, and the flask was sealed and purged with nitrogen before the temperature was raised to 290 °C. When the solution became clear, a 1.0 M solution of selenium in trioctylphosphine (TOP) (79 mg Se in 1 mL TOP) was quickly injected into the reaction pot to commence nucleation and growth of the nanocrystals. The temperature was set to 250 °C and the reaction mixture was stirred for a period of time determined by the final desired emission color, typically on the order of a few minutes. The mean nanoparticle diameter and molar concentration were determined from the peak maximum and absorption intensity, respectively.50 For the core/shell structures, a volume of solution containing approximately 1.6 × 10-7 moles of CdSe nanocrystals was evaporated to dryness and a mixture of TOPO (2.0 g, 5.17 mmol), HDA (1.5 g, 6.21 mmol), and TOP (1.8 mL, 1.5 g, 4.05 mmol) was added into a three-neck flask. The flask was sealed and purged with inert gas and then the reaction temperature was raised to 150 °C. Meanwhile, a solution of Zn(Et)2 (48 µL, 58 mg, 0.47 mmol) and (TMS)2S (86 µL, 73 mg, 0.41 mmol) in TOP (7 mL, 5.8 g) was prepared. Once the CdSe reaction temperature was equilibrated, 4.0 mL of the Zn(Et)2/(TMS)2S solution was added to the reaction pot via syringe in dropwise fashion over about 20 min. The core/shell nanocrystals were then annealed at 90 °C for ∼2 h. The reaction mixture was cooled to room temperature, and the CdSe/ZnS nanocrystals were purified by precipitation with methanol (10 mL) three times and then once with acetone (10 mL). These purified CdSe/ZnS nanocrystals (QDs) were redispersed in cyclohexane and characterized by transmission electron microscopy, UV-vis, and photoluminescence (PL) spectroscopy. Synthesis of Silica-Coated QDs (CdSe/ZnS@SiO2). The silica-coated CdSe/ZnS (core/shell) nanocrystals were synthesized by following a modified literature method.25 Igepal CO520 (458 mL, 0.44 g) was dispersed in cyclohexane (9 mL) in a small reaction vessel by sonication and vortexed until a clear solution was formed. Then, 320 µL of a QD solution in cyclohexane (O.D.(λ ) 537 nm) ) 0.32) was added. The mixture was vigorously stirred for about 10 min with a magnetic stirrer. Then, ammonium hydroxide (28% solution in water, 80-90 µL) was added to the mixture with vigorous stirring. After about 5 min, TEOS (60 µL, 56 mg, 0.27 mmol) was added, and the reaction was continued for 9 h. The resulting CdSe/ ZnS@SiO2 nanoparticles were precipitated by the addition of methanol (ratio of QD solution to methanol ) 9:6) and centrifugation (9000 rpm, 30 min). The supernatant was decanted, the precipitates were redispersed in 5 mL ethanol, and APS (40 µL, 41 mg, 0.23 mmol) was added to the solution to render an amine-functionalized silica surface. The mixture was stirred overnight, and the product was precipitated by centrifugation at 11000 rpm for 30 min and the precipitates were dispersed in 6 mL of water. QY Determination. The fluorescence QY of a given batch of CdSe/ZnS particles was determined relative to rhodamine6G (QYs ) 0.95 in ethanol51) using the relation52

Duncan et al.

QYx )

Asn2x Fx Axns2Fs

QYs

where Fx and Fs are the total integrated fluorescence intensities of the unknown and emission standard, respectively, Ax and As are the corresponding wavelength-specific absorbances, and QYs is the fluorescence quantum yield value for the standard fluorophore. The quantity (nx/ns)2 represents the solvent refractive index correction. Fluorescence spectra were corrected to account for the wavelength-dependent efficiency of the detection system which was determined using the spectral output of a calibrated light source. The standard error in QYs determined by this method is typically taken as (10% of the reported value.52 For the photoenhanced samples, the QYs were determined by comparing the integrated area in the emission spectrum (λexc ) 400 nm) for each sample with that of the initially prepared QDs in cyclohexane; for this determination, the absorbances of photoenhanced samples were assumed to remain unchanged during the silica coating. Fluctuations in the instrument response (lamp intensity, etc.) on a day-to-day basis were accounted for by using water Raman or rhodamine-6G standard spectra prior to a given day’s measurements. III. Results and Discussion Effect of Silica Coating on PL Characteristics of CdSe/ ZnS QDs. CdSe nanocrystals were synthesized via literature methods,4,48,49 using TOPO and HDA as coordinating surfactants. Particle size was estimated by using an empirically determined calibration curve previously reported in the literature.50 The CdSe particles were coated with approximately 5 monolayers of ZnS shell using TOPO and HDA as surfactants. The QDs had an emission peak maximum of 550 nm and an emission QY (in cyclohexane) of 30%. The CdSe/ZnS QDs were subsequently coated with silica by hydrolyzing and condensing TEOS in the presence of the core-shell QDs. A representative TEM image of such particles is shown in the inset of Figure 1, which shows that each particle contains one QD with narrow shell thickness distributions. The light emission properties of QDs are sensitive to surface modifications, and the PL intensity usually degrades upon silanization, especially when a thick silica shell is deposited.18,25,27 The same general behavior was observed in our study. The effect of silanization on the QD PL properties was assessed by measuring PL spectra at different stages of the silanization reaction (Figure 1). PL spectra were acquired from aliquots that were removed from the reaction mixture immediately after the reaction was initiated (labeled as 0 h), at halfway through the coating process (4.5 h), just before finalizing the silica coating (9 h), and after purification (Purified, in water). Emission QYs were determined at each of these steps relative to the QY of the same QDs prior to the silanization reaction. As shown in Figure 1, the QY drops markedly during the silica coating reaction from 30% to approximately 0.5% for the final CdSe/ ZnS@SiO2 dispersed in water. The QY decay is not accompanied by a significant blue shift of the PL λmax, indicating that the QY decay is caused mainly by the exchange of coordinating surfactants rather than the etching or oxidation of QD surface. As mentioned above, reduced QYs in silica-coated QDs is a commonly occurring problem and has been reported by other researchers.18,25,27 We note that thin silica coating with mercapto silane or amino silane19 does not significantly reduce the QY. In fact, priming QDs with amino silane before thick silica shell

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Figure 1. PL QYs measured at different stages of the silica coating reaction and workup for the silica-coated QDs. The inset shows a representative TEM image of silica-coated QDs after ∼9 h of reaction time (scale bar ) 20 nm). A majority of particles possess a single, centrally located QD.

Figure 2. The PL QYs of silica-coated CdSe/ZnS QDs as a function of time exposed to room lights (solid lines) and kept in the dark (dashed lines) in an aqueous DTT solution (A) and neat water (B), respectively. The PL data demonstrate that DTT augments photoinduced enhancement and suppresses photoinduced quenching of the PL QYs of silicacoated CdSe/ZnS QDs. The excitation wavelength was 400 nm.

coating can reduce the degree of QY decay.21 However, the prefunctionalization usually leads to irregular silica coating and large size distributions. It is also worth mentioning that the QY decay during the silica coating depends highly on the thickness and grain boundaries of ZnS coating of CdSe/ZnS QDs. Photoenhancement of CdSe/ZnS@SiO2 with and without DTT. The response of the PL characteristics of CdSe/ZnS@SiO2 to ambient light exposure was monitored in the presence and absence of DTT. After purification, two portions of CdSe/ ZnS@SiO2 solutions were kept in water and another two were kept in aqueous DTT solutions (103 µM) at the same QD concentration (0.03 µM). One sample from each pair was left under room lights, and the other was stored in darkness, and PL spectra were measured for each sample at 1-day intervals (0-5 days) to follow the photoenhancement process. As depicted in Figure 2, silica-coated QDs undergo large PL intensity changes over time upon exposure to room light. Control samples left in the dark exhibited only a small degree of PL intensity changes during the same period of time (Figure 2, dashed lines), indicating that the main cause for the observed PL intensity enhancement is the exposure to light. Initially, both samples (with and without DTT) show a large PL intensity increase upon exposure to light (Figure 2, solid lines). However, the initial photoenhancement is followed by a subsequent PL drop to its pre-enhancement level in neat water without DTT (Figure 2B). Significantly, the photoenhancement is more pronounced in DTT solution and the enhanced PL intensity is maintained for a few weeks in DTT solution (Figure 2A). It is also important to note that DTT prevents the blue

Figure 3. Effect of light and DTT on the PL characteristics of silicacoated CdSe/ZnS QDs. Panel (A) plots representative PL spectra after 3 days of elapsed time for particles dispersed in water kept in the light (solid blue), in water kept in the dark (dashed blue), in aqueous DTT solution kept in the light (solid red), and in aqueous DTT solution kept in the dark (dashed red), as a function of elapsed time. Panels (B) and (C) plot the spectral bandwidths (fwhm) and peak maxima for samples under the above-described conditions as a function of elapsed time. All values were determined by fitting the spectral data to Gaussian functions. The excitation wavelength was 400 nm.

shift and peak broadening which typically accompany photoenhancement (Figure 3). In the absence of DTT, it is apparent that the modulation of PL characteristics is a result of two competing processes that lead to PL enhancement and quenching, respectively (Figure 2B). Initially, the enhancement process dominates; during the first 2 days, the QY was increased by a factor of 9.5, from 2.6 ( 0.4% to 24.6 ( 4.5%. After this period, however, the quenching process began to dominate and the PL QY dropped back down to 2.1 ( 0.9%, which is comparable to the preenhancement value. In addition, light-exposed QDs exhibit a substantial hypsochromic shift of their emission peak (∆λ ) 34 nm (1200 cm-1), Figure 3C) and a pronounced peak broadening, with the full width at half-maximum (fwhm) increasing from 45 nm (1470 cm-1) to 57 nm (2150 cm-1) over the course of the experiment (Figure 3B). There was also a nontrivial blue-shift and spectral broadening (∆λmax ) 12 nm (400 cm-1) and ∆fwhm ) 4 nm (230 cm-1)) observed in the control (dark) sample as well (Figure 3). The same general

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behavior in the light and dark samples suggests that the responsible processes are thermodynamically favorable even in the dark and accelerated by the input of light energy. In contrast, in the presence of DTT, the quenching process was effectively suppressed, and the silica-coated QDs stored in aqueous DTT solution stayed bright well beyond the 5 day experimental time scale presented in Figure 2A. In addition, the QY (28.6% ( 1.3%) obtainable through photoenhancement was higher in the presence of DTT, showing an enhancement factor (enhanced QY divided by initial QY) of 13.6 (Figure 2A). Moreover, while the photoenhancement without DTT was accompanied by a substantial blue-shift and broadening of the PL spectra, the PL λmax and spectral bandwidth remain virtually unchanged in the aqueous DTT solution (Figure 3, red lines). To separate the effect of DTT from that of light in the PL intensity modulation, a control experiment was carried out in the dark for QDs in aqueous DTT solution (Figure 2A, dashed). Although QDs kept in the dark also showed a small amount of PL intensity increase over time in DTT, the enhancement factor is substantially smaller than the QDs under room light. This result indicates that DTT alone did not cause the drastic PL intensity change shown under light (Figure 2A). Mechanism Driving Light-Induced Dynamics of PL Characteristics of Silica-Coated CdSe/ZnS QDs in the Presence of DTT. To understand the photoenhancement of silica-coated QDs with and without DTT, two general questions are addressed: (1) What are the mechanisms that give rise to the increase in the PL quantum yield of QDs when they are left under room lights in water as well as the subsequent QY decline observed at longer exposure times? (2) What role does DTT play in both the photoenhancement and photoinduced quenching processes? Photoinduced PL Modulation in Neat Water. There have been a number of reports on light-induced PL enhancement observed in QDs.30-40 Although the exact mechanism is not known, the general consensus is that the photoenhancement involves the photoinduced optimization of the binding states of stabilizing surface molecules, which could be the original surfactants (i.e., TOPO and HDA) and/or other small molecules such as oxygen and water.30-32,34,35,38-40 The decay of QD PL that occurs after the initial photoenhancement is likely to be due to photo-oxidation of the QDs by 1 O2, which is in turn produced primarily via light-induced sensitization by the QDs.35,39,53 In general, the photo-oxidation of the QD surface leads to an increase in the density of surface defect states, which can act as PL quenchers, as well as a contraction in the effective QD size, which manifests itself as a blue-shift in the emission λmax. A spectral shift is also observed in the control experiment performed in the dark, but in smaller magnitude; this indicates that surface oxidation is promoted by light and 1O2 is likely to be the main species contributing to the oxidation and spectral shift. The observation of a blue-shift during the enhancement time frame (days 0-2) depicted in Figure 3 indicates that the two processes of enhancement and oxidation are in competition with each other. Photo-oxidation is apparently slower than enhancement, especially in core/shell QDs. However, the photo-oxidation ultimately becomes dominant at long observation times. This subsequent decay of the QY in combination with a spectral shift and peak broadening is often observed in various QD systems30,33-35,37,39,40,53 and has impeded the use of photoenhancement as a practical means to improve the QY of QDs. This result as well as other previous reports on photoenhancement demonstrate the complex nature of photoenhancement in QDs and show that, if photoenhance-

Duncan et al. ment is to be used as a method to achieve long-lasting high QYs, it is imperative that straightforward, efficient ways to inhibit the deleterious process of photo-oxidation be identified. Photoenhancement in the Presence of DTT. The addition of dilute aqueous DTT solutions to the silica-coated QD significantly affects the PL dynamics in two specific ways. First, DTT yields augmented degrees of light-induced enhancement of the QY over water alone. Second, DTT clearly inhibits oxidative photochemistry. DTT is a common reducing agent and can bind to the QD surface.43,54 Even in the absence of light, the PL intensity of silica-coated QDs increases over time in dilute (103 µM) aqueous DTT solution (Figure 2A). Apparently, DTT is small enough to diffuse to the QD/silica interface, where it can coordinate to the QD surface and passivate electron traps. Another possible role of DTT is to remove singlet oxygen. DTT is known to be a potent 1O2 scavenger,55,56 because of its ability to form a highly stable six-membered ring possessing an internal disulfide bond upon oxidation.57 DTT can suppress photooxidation by removing highly reactive 1O2 molecules that are generated at the photoexcited QD surface. This explanation is consistent with that of Yang et al., who, upon adding the potent 1 O2 scavengers histidine and sodium azide to aqueous solutions of CdTe QDs, observed that the QD PL maxima no longer shifted to higher energy during light irradiation.39 The prevention of photo-oxidation in silica-coated QDs is underscored by the lack of any hypsochromic shift of the PL peak maxima during irradiation in the presence of DTT (Figure 3). In summary, DTT contributes to time-dependent light-induced changes in the PL characteristics of silica-coated QDs (intensity, emission wavelength, etc.) via (i) an enhancement of the PL intensity through stabilization/passivation of the QD surfaces and (ii) inhibition of the subsequent PL quenching through efficient scavenging of photogenerated reactive 1O2. It is worth noting that PL improvement by DTT has been reported by other researchers. Ha et al.,43 have shown an immediate suppression of PL intermittency in single QDs exposed to dilute solutions of β-mercaptoethanol (BME) and DTT, both small molecule thiols. A subsequent study by Hollingsworth and co-workers44 demonstrated that the PL intensity enhancing capacity of small thiols is dependent on the solution pH; this effect was attributed to the fact that only the thiolate (S-) form of DTT is a potent reducing agent. While those reports did not consider the cooperative nature of light and DTT in improving the PL intensity, it is possible that the mechanism described here is applicable to those systems, as light was always applied to the samples in order to measure PL. Effect of Initial PL QY on the DTT-Assisted Photoenhancement of CdSe/ZnS@SiO2. Often, reports on QDs are seemingly contradictory because the behavior of QD PL properties can be entirely different depending on such intrinsic factors as QD size, the number and nature of surface defects, core or core-shell architectures, capping, and surfactant properties. To ascertain whether the mechanisms described above were general, another batch of CdSe/ZnS nanocrystals was subjected to similar photoenhancement experiments; these CdSe/ZnS nanocrystals possessed a QY of ∼65% in cyclohexane and an emission wavelength of 590 nm. From these nanocrystals, two batches of CdSe/ZnS@SiO2 were prepared. Similar drops in the emission QY were observed during these silanization reactions, and the final CdSe/ZnS@SiO2 nanoparticles exhibited QYs of 7% and 20% respectively. These two samples will be referred to as QD-7% and QD-20%, respectively. The QYs of

Improving the QYs of Semiconductor QDs

Figure 4. Photoenhancement of different batches of silica-coated CdSe/ ZnS QDs. The figure plots PL QYs of silica-coated CdSe/ZnS QDs in aqueous DTT solution (A) and in neat water (B) kept under room lights as a function of elapsed time. The black, red, and blue lines represent data acquired for samples QD-2.5%, QD-7%, and QD-20%, respectively.

Figure 5. Effect of DTT concentration on the photoenhancement of silica-coated CdSe/ZnS QDs (QD-20%). The figure plots PL QYs in 100 µM aqueous DTT solution (red), in 1 mM aqueous DTT solution (green), and in 10 mM aqueous DTT solution (blue), all kept in the light, as a function of elapsed time. Analogous data for the same particles acquired in neat water is plotted (black) for comparison. Associated changes in peak maxima and spectral bandwidths for all DTT samples were negligible.

J. Phys. Chem. C, Vol. 113, No. 18, 2009 7565 4B) but showed similar enhancement behavior as the other particles in the presence of DTT (Figure 4A). The lack of PL intensity modulation without DTT is likely because wellpassivated QD-20% particles are more resistant to both the photoenhancement and photo-oxidation processes. The wellpassivated QDs still undergo further light-induced PL enhancement when DTT is present. From these studies, it is apparent that while the quantitative degree of photoenhancement in the presence of DTT is a complex function of the nature of silica layers and QDs, all of which can vary greatly from batch to batch, the same general trends are observed: DTT augments the photoenhancement effect and protects against photoinduced oxidative degradation. Concentration Dependence of DTT on the Photoenhancement of CdSe/ZnS@SiO2 Nanoparticles. Prior studies have shown that the effect of thiols on the PL characteristics of QDs are concentration dependent.43,44,46 To assess how DTT concentration affects the photoenhancement behavior, QD-20% nanoparticles were dispersed in aqueous DTT solutions with various DTT concentrations. The PL modulation of QDs in the DTT solutions was monitored, and the results are depicted in Figure 5 for DTT concentrations of 100 µM (red), 1 mM (green), and 10 mM (blue). This study showed that higher concentrations of DTT can have a negative effect on the PL characteristics of silica-coated QDs. Although all samples with DTT showed higher enhancement factors than the sample without DTT, 1 and 10 mM DTT concentrations result in generally lower QYs, particularly at longer light exposure times, compared to identical experiments performed using 100 µM DTT concentration. This observation is consistent with previous reports on the thiolate concentration dependence of QD PL characteristics.43,44 It has been suggested that while the initial effect of thiolate is to donate electrons to the QD surface, passivating surface electron trap sites, excess thiolate molecules can act as hole traps, which introduces new surface trap sites that can quench excitons and leads to thiolate desorption from the QD surface.44 Note that, unlike oxidation of the QD surface by O2, PL quenching via excess thiolates did not lead to any substantial shift of the PL λmax. Nevertheless, we note that even in DTT solutions of 10 mM, the degree of photoenhancement is still significantly greater than that of water alone (Figure 5). IV. Conclusions

Figure 6. The QYs before and after silica coating as well as at three time points during exposure to ambient room light, both in neat water and in 100 µM aqueous DTT solution. Photoenhancement of CdSe/ ZnS silica-coated QDs (QD-2.5%) in the presence of DTT results in near-complete recovery of precoating QYs of CdSe/ZnS QDs.

these silica-coated QDs are higher than those for the QDs discussed above (2.5%), in part due to the higher QY of their precursor CdSe/ZnS nanocrystals. The batch of CdSe/ZnS@SiO2 particles discussed in the preceding sections will be referred to as QD-2.5%. As presented in Figure 4, QD-7% showed similar photoinduced enhancement and quenching processes as QD-2.5% discussed above, demonstrating that the behavior is universal among low QY silica-coated QDs. The third batch of particles, QD-20%, which has a substantially higher QY, did not undergo significant changes in PL characteristics without DTT (Figure

We have found that the common reducing agent, DTT, can promote the light-induced enhancement of the PL QY of silicacoated CdSe/ZnS QDs. In general, photoenhancement, which is commonly observed for low QY QDs, only temporarily increases the QY and is accompanied by a spectral broadening and blue-shift due to competing light-induced surface oxidation processes. With the addition of DTT, not only was the enhancement factor increased (e.g., doubled for QD-20% on day 3, compared to the enhancement factor for analogous experiments without DTT) but also the spectral features remained virtually unchanged. This effect is general and was observed regardless of the QD core size, initial PL QY, and silica-coating characteristics. While DTT has been shown to increase the PL intensities of QDs, DTT alone did not drastically increase the QYs in samples which were kept in the dark, confirming the cooperative nature of light and DTT in enhancing PL. We postulate that DTT aids in photoenhancement by effectively passivating the electron traps on the QD surface and also by removing singlet oxygen, a powerful oxidant. The QY variation with and without DTT throughout the experiment is summarized in Figure 6. Silica coating is one of the widely

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