CdS Quantum Dots via Room

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Production of Small, Stable PbS/CdS Quantum Dots via Room Temperature Cation Exchange Followed by a Low Temperature Annealing Processes Emek Goksu Durmusoglu, Melike Mercan Yildizhan, Mehmet Ali Gulgun, and Havva Yagci Acar J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b06153 • Publication Date (Web): 26 Oct 2017 Downloaded from http://pubs.acs.org on November 5, 2017

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The Journal of Physical Chemistry

Production of Small, Stable PbS/CdS Quantum Dots via Room Temperature Cation Exchange Followed by a Low Temperature Annealing Processes Emek G. Durmusoglu†,*, Melike M. Yildizhan§, Mehmet A. Gulgun§,ǁǁ, Havva Yagci Acar†,‡ † Koc University, Graduate School of Materials Science and Engineering, Rumelifeneri Yolu, Sariyer 34450, Istanbul, Turkey ‡ Koc University, Department of Chemistry, Rumelifeneri Yolu, Sariyer 34450, Istanbul, Turkey §Sabanci University, Faculty of Engineering and Natural Sciences, Istanbul 34956, Turkey ǁTurkey, Sabanci University Nanotechnology Application Center, Istanbul 34956, Turkey Corresponding Author: Emek G. Durmusoglu†,* E-mail: [email protected]; Abstract

Here, we discuss a simple low temperature process for the synthesis of small and stable PbS/CdS QDs with emission below 1100 nm. For this, small PbS QDs with emission below 1100 nm synthesized from PbCl2 in oleylamine with 1-dodecanethiol, as reported by our group recently, were used. A thin CdS shell was grown on PbS at room temperature (RT) via cation exchange (CE) which is a self-limiting process providing about 100 nm blue shift in the emission maxima, hence is quite practical for reaction control and production of predictable particles. RTCE

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process provides 6-9 times stronger emission than original PbS with better optical stability. Annealing of the PbS/CdS QDs in solid state at mild temperatures (50-100 °C) improves crystallinity of the particles.

Final ligand exchange on the annealed PbS/CdS with 1-

dodecanethiol (DT) enhances the long-term stability of particles further. The optimum overall process is determined as RTCE followed by annealing at 50 °C for 1 h and finished with ligand exchange with DT. Influence of these processes on QD structure and optical properties were studied as well as stability in chloroform and petroleum products (diesel and gasoline) for possible optical tagging applications of such liquids. Overall, a simple, controllable and scalable method is developed to produce highly stable, bright, size-tunable PbS/CdS QDs with emission detectable with low cost semiconductor detectors.


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Introduction The wet-chemical synthesis is a widely-used method to obtain highly monodisperse, stable and luminescent semiconductor quantum dots (QDs). Particle sizes smaller than Bohr radius provide discrete energy levels and widen the bandgap. This allows tuning of optical and electronic properties by simply changing the size of QDs. So, QDs have attracted significant attention with their unique size-dependent optical properties, which are dramatically different than bulk crystals. Within the IV−VI group semiconductors, lead chalcogenides (PbX, X = S, Se, or Te) are most studied near infrared (NIR) emitting QDs due to their bulk bandgap, which is 0.41 eV for PbS, 0.28 eV for PbSe, and 0.31 eV for PbTe at 300 K, and large exciton Bohr radius (18 nm for PbS).1 High absorption coefficient, emission in the near infrared region, multiple exciton generation2 and size tunability make lead sulfide colloidal QDs attractive for photovoltaic3, bioimaging4, LED5 and photodetector6 applications. Due to the large surface/volume ratio of QDs, surface chemistry plays an important role on the optical properties and stability of QDs.7 Most frequently used organic long-chain ligands provide colloidal stability and partial passivation to QDs. But, remaining dangling bonds at QD surface can act as charge trap sites and reduce the radiative emissions of QDs.8 Alternatively, passivation of core QD with another inorganic shell material emerged as a useful method to improve the luminescence intensity as well as resistance to photodegradation and improve colloidal stability. PbX/CdX (X = S, Se, or Te),9-11 CdS/ZnS,12 CuInS2/ZnS,13 and InP/GaP/ZnS14 are examples to such core-shell QDs. By passivating trap states and reducing dangling bonds at the core QD surface, such inorganic shells reduce non-radiative events and enhance lifetime and quantum yield of the QD.15-16 Furthermore, the shell acts as a physical barrier between the core QD and


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the surrounding medium, protecting the core from environmental factors such as oxidation.17 This becomes more important as crystal size gets smaller. SILAR (Successive ionic layer adsorption and reaction) is the commonly used process to grow a shell upon a core in the photovoltaic applications.18-19 But, cation exchange (CE) method is emerging as a popular method, lately. This method requires only addition of the second cation, which will grow the shell in the expanse of the initial core, more or less keeping the anion component untouched. It is fast, easy, safe and produces more defect free interface, hence strong enhancement in the optical properties.20 So, cation exchange is becoming a useful method to obtain colloidal core/shell particles, which are often inaccessible by one-step synthetic routes.20 CdS is one of the most widely used shell material on PbS cores to enhance optical properties and stability. CdS has a larger bandgap (bulk bandgap 2.49 eV) than PbS, and hence acts as a potential wall confining both electrons and holes within the PbS core.16 Also, the small lattice mismatch between CdS (zinc blende, with lattice parameter 5.82 Å) and PbS (rock salt, with lattice parameter 5.93 Å) crystals, allows sharing of a face-centered cubic sublattice of S atoms and provide a good compatibility at the interface.9 Cation exchange is a diffusion controlled process and hence, typically performed at and above 100 °C to ensure desorption of surface cation and adsorption of the new ones, to avoid defective grain boundary formation and remnant impurities of the outgoing cation.20 But, such high temperatures generally leads to Ostwald ripening of QDs due to the thermal instability and hence cause broadening of photoluminescence emission peak of QDs during the course of heating necessary for the cation exchange process.2123

Many studies revealed that thicker shells formed at high temperatures cause a decrease in

quantum yield due to defects occurring at the interface and within the shell.15, 22-23 Hens et. al. reported that they obtained core-shell QDs with a limited shell growth in a room temperature


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cation exchange process and proposed that vacant positions at QD surface plays a key role in the exchange process.15 In addition, Alivisatos et. al. extensively investigated the cation exchange mechanism and reported thermodynamic limitations for RTCE of Pb+2 with Cd+2.24 We are interested in development of colloidal, small and stable NIR QDs with a strong emission below 1100 nm as optical probes for luminescent labeling, which can be easily detected with the most easily accessible and economic Si detector. There are few examples to such ultrasmall sized PbS.25-31 We have previously reported the synthesis of ultrasmall PbS QDs with strong emission below 1100 nm via green synthetic method based on the Cademartiri`s.32 This process produced PbS QDs with oleylamine (OLA) and 1-dodecanethiol (DT) mixed coating. Here, we targeted the development of PbS/CdS core/shell particles to improve the long-term stability of the ultrasmall QDs and enhance the emission tenability with the desired range (7001100 nm). So, we will discuss a three step process to achieve such goal in this study; Initial RTCE to form the PbS/CdS, followed by solid state annealing of the PbS/CdS QDs at low temperatures and ligand exchange with dodecanethiol (DT) in the last step to enhance optical properties and stability of small PbS/CdS QDs (emission below to 880 nm). Since, in the RTCE process, shell grows in the expanse of the core, the emission maxima of the core/shell particles would be kept below 1100 nm. Annealing is usually utilized to increase the crystallinity, remove impurities and reduce crystal and interface defects of the QD based films in photovoltaics.9, 18, 20 The influence of annealing temperature and time on the luminescence properties (peak position, peak width, emission intensity), absorbance, luminescence lifetime, crystallinity and stability of core-shell QD will be discussed. Lastly, impact of the ligand exchange process and coating PbS/CdS QDs with 1-dodecanethiol on the long-term stability in a typical solvent for such QDs, chloroform, and in harsh, chemically active long-chain hydrocarbon solutions (gasoline and


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diesel) of core/shell NIR QDs will be demonstrated. The later is important for tagging such valuable hydrocarbon products with luminescent QDs. Experimental Section Materials. Lead chloride (98%), sulfur (99.99%), oleylamine (OLA) (technical grade, 70%), 1-dodecanethiol (DT) (98%), cadmium oxide (CdO) (99.5%), oleic acid (OA) (99%), 1octadecene (ODE) (98%) were purchased from Sigma-Aldrich. Toluene, chloroform, ethanol was all ‘for synthesis’ quality and were purchased from Merck. Gasoline (Pb-free, 95 octane) and EuroDiesel were provided by OPET A.Ş. Colloidal PbS/CdS QD Synthesis. OLA/DT capped PbS QDs (PbS-OLA/DT) were synthesized as described in our previous study.32 PbS/CdS core/shell QDs were synthesized by cation exchange method adopted from Pietryga et al.11 As an example, 0.257 g (2 mmol) CdO, 3 mL (9.5 mmol) OA, and 12 mL ODE were heated to 250 °C under argon until whole CdO precursors had dissolved. After clear solution obtained, solution was cooled to 150 °C under Ar flow to remove moisture. The PbS core solution from above was degassed by Ar flow for ∼30 min. Immediately after the temperature of the PbS solution was set to desired temperature (room temperature or 100 °C in this work), the cadmium oleate solution was injected under heavy stirring. After the desired growth time, the reaction was quenched in cold water and cold ethanol was added. The solution was centrifuged at 6000 rpm and QDs were precipitated and then resuspended in chloroform. As can be seen in Figure S2, this cleaning step removes the excess OLA. Annealing of PbS/CdS QDs. Precipitated core-shell QDs annealed in vacuum oven at desired temperature and duration as a powder.


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DT Ligand Exchange. DT was added to chloroform suspension of PbS/CdS QD to replace the OLA at room temperature and stirred for 15 min (OLA:DT v:v-1:4). DT coated PbS/CdS QDs (PbS/CdS-DT) were precipitated in ethanol, removed from the supernatant by centrifugation and re-suspended in chloroform. These QDs are stored in ambient conditions. Cation Exchange at Room Temperature

PbS@CdS /DT

Figure 1. Schematic of PbS/CdS QD synthesis. Characterization. Absorbance spectra were taken in the range of 400–1200 nm by a Shimadzu 3600 PC UV-Vis-NIR spectrometer. Photoluminescence spectra were recorded by a homemade system, which uses a DPSS laser source working at 532 nm for excitation, a Newport Cornerstone 130 Monochromator and ‘Thorlabs PDF10A’ (1.4×10−15 W/Hz1/2) silicon detector (responsivity of the detector shown in Figure S3). Working range of this detector is 200-1100 nm. All PL data presented are absorption calibrated. Photoluminescence lifetime of PbS and PbS/CdS QDs in chloroform were measured by Horiba Fluorolog equipped with TCSPC Triple Illuminator. Spectral LED with 561 nm peak wavelength is used for excitation. High-resolution TEM (HRTEM) and scanning transmission electron microscopy (STEM) studies were carried out with a JEOL-ARM 200 CFEG operating at 200 kV. STEM micrographs were taken with High Angle Anular Dark Field HAADF) detector. X-ray photoelectron spectroscopy (XPS) measurements were performed using Thermo Scientific K-Alpha spectrometer using an Aluminum anode (Al Kα= 1468.3 eV) at electron takeoff angle of 90o (between the sample surface and the axis of the analyzer lens). Spectra were


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recorded using Avantage 5.9 data system. The binding energy scale was calibrated by assigning the C1 s signal at 284.5 eV. X-ray diffraction (XRD) pattern was obtained with a Bruker D8 Advance with DAVINCI design equipped with Diffractometer using a Cu Kα radiation source (λ= 0.15418 nm). For measurements 0.01236 increment, 16 second integration time, 40mA, 40kV and Si holder is used. FTIR spectra was taken with a Thermoscientific Nicolet iS10 instrument in the wavenumber range of 400-4000 cm-1. Results and Discussions Small PbS QDs coated with OLA and DT mixture with emission maxima between 1015-880 nm were synthesized via a modified Cademartiri method developed by our group.32 These core QDs were subjected to cation exchange process using Cd-oleate at room temperature. Figure 2ab shows the optical properties of a PbS core with emission maxima at 948 nm and the corresponding PbS/CdS QDs. RTCE resulted in a 40 nm blue shift in the emission maxima and ca 52 nm blue shift in the first excitonic peak seen in the UV spectrum. Such a blue shift in both absorbance and emission spectra is expected since desorption of Pb2+ from the surface decreases the effective crystal size of the PbS core. Crystal size of PbS cores were calculated from an empirical sizing curve developed by Cademartiri et al. and found as 2.70 and 2.55 nm for the PbS cores shown in Figure 2a-b, respectively.33 Hens et al. demonstrated that core sizes of the core/shell particles calculated based on the composition determined by ICP-OES correlates well with the size calculated from the absorbance data.22 This is also confirmed by many studies.34-35 So, with the RTCE, the PbS crystal shrank about 0.15 nm which would allow deposition of few monolayers of CdS. Presence of Cd was confirmed by XPS and EDX studies.


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Figure 2. Optical properties of (a) PbS, (b) PbS/CdS QDs (see supporting info for calculation of bandgap), different sized (c) PbS, (d) PbS/CdS QDs. In typical PbS synthesis using PbCl2 and elemental S, it is difficult to obtain a portfolio of small-sized PbS QDs with different emission maxima below 1100 nm, due to the fast reaction kinetics. Yet, we have managed to produce a relatively large number of PbS QDs with emission between 880-1015 nm with our modified method (Figure 2c). RTCE process applied to these particles produced a larger number of PbS/CdS QDs with emission within the sensitive detection range of cheap silicon detectors. In addition, particle with emission maxima at even 796 nm was achieved.


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Figure 3. (a) XRD of PbS and PbS/CdS QDs (syntheses at 100 °C) with vertical lines indicating the peak positions of PbS and CdS crystals, TEM images of (b) PbS QD, (c) PbS/CdS QD, (d) FFT pattern of PbS/CdS QD, (e) HR-TEM image of PbS/CdS QD, (f) Pb 4f, (g) Cd 3d, (h) S 2p XPS spectra of as made PbS/CdS QDs. XRD pattern of PbS/CdS QDs compared with that of PbS and CdS produced in house, indicates no CdS formation but a slight shift of PbS peaks to larger angles since CdS has smaller lattice constant than PbS crystal, suggesting the formation of a core/shell structure (Figure 3a).9 Besides, all XRD patterns fit well with the cubic structure. TEM images reveal monodisperse spherical PbS and PbS/CdS QDs (Figure 3b-c). A d-spacing of 0.290 nm was measured from the crystal in the circled area in Figure 3d, which agrees well with the literature value of PbS QDs.23 Since CdS shell is a few layers thick, we were unable to


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differentiate the CdS shell with the TEM. Yet, EDX shows the co-existence of Pb and Cd with a higher concentration of Pb (SFigure 1). Binding energies determined by XPS reveal different states or environments of Pb, S and Cd. In PbS there are two types of Pb and S. S 2p 3/2-2/1 at 161-162.8 eV belongs to Pb-S and 163164.6 eV corresponds to the S of DT (STable1). In case of Pb, two different Pb 4f 7/2-5/2 pairs exist at 136.88- 141.78 eV and 137.78-142.68 eV. We suggest that these correspond to surface versus inner core Pb, respectively. This is in agreement with the literature and actually Ma el. al., suggests that Pb with the lower binding energy represent uncoordinated surface Pb.36 Disappearance of this peak after CdS shell growth, actually confirms that the low binding energy Pb atoms were on the surface of the PbS. PbS/CdS QDs have only one type of Pb with binding energy at 138.28 – 143.18 eV and Cd 3d 5/2-3/2 pair at 404.78 – 411.58 eV (Figure 3f-h). These values correlate well with Pb-S and Cd-S bonds. PbS/CdS QDs have three types of S 2p 3/2-2/1 pair: The one at 162.9 – 64.4 eV may be Cd-S, another one at 166.1-167.3 eV may correspond to the S of DT and the last pair at 168.4 – 169.8 eV corresponds to oxidized sulfur such as –SO4. Yet, there is no corresponding Pb or Cd in their spectra for lead or cadmium sulfate.37 Actually, the core Pb and S peaks are slightly at higher binding energies, which may be due to the reduced crystal size of the PbS core during the cation exchange. Influence of Shell Growth Parameter on the Optical Properties of PbS/CdS QDs To investigate the influence of cation exchange temperature on the properties of core-shell QDs, procedure was performed both at traditional 100 °C and at room temperature using the same PbS cores keeping the other reaction parameters same. In first few minutes, a dramatic increase (ca 5.4-fold) in the emission intensity was detected in both cases with about a 105 nm blue shift which corresponds roughly to a 0.175 nm shell thickness (Figure 4a-b) (see Supporting


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Information for the size calculation). This may roughly corresponds to a two layers of CdS. Formation of such a thin shell seems to successfully passivate surface defects and reduce nonradiative events, causing such a dramatic enhancement of emission intensity. But, at 100 °C, cation exchange continued with time, as indicated by a further blue shift of the emission peak accompanied by a significant decrease in its intensity. The emission intensity fell below the intensity of the PbS core with about 200 nm blue shift, which corresponds roughly 0.35 nm shell thickness in 1h at 100 °C and afterwards stayed quite the same. This correlates with the previous reports indicating that a thick shell causes a drop in the emission intensity due to defects formed through the shell layer or growing lattice mismatch between PbS and CdS crystals (although there is a small lattice mismatch).34,

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Also, broadening in the emission peak with extended

reaction time indicates increasing polydispersity. However, at room temperature, shell growth process was found to be independent from the reaction time and showed a narrower emission peak, indicating slightly better size distribution (Figure 4c-d). After the initial blue shift and enhancement of the emission intensity observed within the first 5 minutes, no further shift in the peak position or change in the intensity was detected in 2h period. At room temperature, only weakly bound surface Pb2+ ions were displaced with Cd2+ ions, and further dissolution of Pb2+ from the core did not take place. Weller et. al. suggests that ca 30 % conversion takes place within the first minute of the CE due to surface reactions.39 Van Huis et al. also, indicates that the first two layers go under CE quite fast but the exchange rate of the fourth layer is nearly zero, based on classical molecular dynamics simulations performed on 4.7 nm PbSe core.40 Based on our observations, after the initial one or two layers, further migration of Cd+2 in and Pb+2 out is prevented at room temperature.


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The mechanism of CE process is actually under debate. Cation and vacancy diffusion as well as kick-off mechanism were suggested yet, in case of PbS/CdS the accepted mechanism is cation and vacancy exchange, which is supported by our observations. Discussion indicate unfavorable thermodynamics in exchange of Pb2+ with Cd2+, therefore the CE process is usually performed at elevated temperatures.24 But, size of the crystal and the type of the ligand is important. In our case, ultrasmall PbS core and weakly bound OLA15, 40 are probably positively affecting the CE process and providing successful CE at room temperature. Pb2+ is a borderline acid compared to soft Cd2+ and oleate is a harder base than OLA and sulfide. Hence, one may think that dissolved Pb2+ would preferentially be bound to oleate and Cd2+ would be bound sulfide and OLA, which would facilitate the reaction in the desired direction. All of these reports and considerations support our observations for the self-limiting RTCE process.


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Figure 4. PL spectra of shell growth process performed at (a) 100 °C, (b) RT, (c) PL spectra of PbS/CdS synthesized at RT and 100 °C, (d) PL spectra of the PbS core and PbS/CdS QDs prepared at different Pb:Cd:OA ratios. Figure 4c-d provides the comparison of the changes in emission spectra of the two different PbS cores with emission maxima at 1015 and 912 nm (3.16 and 2.65 nm) and the resulting PbS/CdS QDs prepared via RTCE. In both cases, ca 100nm blue shift in the peak maxima was observed with ca 5.8 and 9.0-fold increase in the emission intensity, respectively. More dramatic enhancement in the smaller particle emphasizes the importance of crystal size, especially in such a small size regime where more of the ions are on the surface.


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Room temperature process also, showed a narrower emission peak, indicating slightly better size distribution (Figure 4c). Hence, we conclude that tuning emission wavelength with the duration of the cation exchange at room temperature is not possible since the process is selflimiting, and RTCE process emerges as a simple, easily applicable, robust process for colloidal PbS QDs to provide strongly luminescent PbS/CdS QDs at ca 100 nm shorter wavelengths compared to the core PbS. Pb:Cd:OA ratio on the RTCE process and the final optical properties of core-shell QDs was also studied. Three different ratios were studied with two different Cd-Oleate formulations, basically targeting Pb:Cd ratio of 1:4 and 1:2 with different oleate amounts. PL spectra of the resulting PbS/CdS formed under identical conditions indicate that this variable is not that influential to tune the shell thickness and hence the emission wavelength (Figure 4d). This is in line with the self-limiting process at room temperature.15 Yet, about two fold stronger emission was observed at the Pb:Cd:OA ratio of 1:2:25.

Influence of Annealing on the Properties of Core-Shell Structures Although a thin layer of CdS shell obtained at RT was enough to enhance emission intensity with a controlled shift (ca 100 nm) to shorter wavelengths, resulting particles had poor crystallinity compared to PbS/CdS produced at 100 °C as indicated by XRD pattern of the particles(Figure 3a). High temperature process not only increases the diffusion of ions in and out but also within the crystal, providing better crystal structure. Poor crystallinity may reduce the stability of particles overtime and may influence optical properties negatively. Crystallinity may be improved with thermal annealing. Heat treatment process generally applied after SILAR process in PV applications to reduce interfacial defects between PbS and CdS layers.41-42 Yet,


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heat treatment of the colloidal QDs is not preferred due to possibility of Ostwald ripening, which generally reduces the luminescence.11,

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have better thermal stability.21 Due to higher thermal stability of core-shell QDs, mild heat treatment can be aid to remove impurities to core-shell QDs.20 Here, a relatively low temperature annealing of the PbS/CdS QDs were performed between 50-100 °C. PbS/CdS QDs were subjected to extensive washing before annealing, to remove all free Cd2+ from the material to prevent shell thickening (SFigure 2). If annealing is performed in solution, a large red shift was observed with almost completely diminished emission. This indicates Ostwald ripening during the annealing process in the colloidal form which is well-documented phenomenon for PbS QDs27, 43-44 and even seen during their storage.45-46 But, the ripening of core/shell particles are interesting and the mechanism is not clear yet. More interestingly, if PbS/CdS QDs are annealed in the powder form in a vacuum oven, again a red shift in absorbance and luminesce spectra were observed, but without a loss in the emission intensity. The difference is originating from the faster diffusion rate and homogeneity in the solution phase.

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shifted to longer wavelengths with the increasing annealing temperature as follows; 65, 187 and 316 nm red shift upon annealing at 50, 75 and 100 °C, respectively, with no loss in the absorption value. Corresponding shifts in the emission maxima are 49 and 95 nm at 50 and 75 °C, respectively (Figure 5c). Emission maxima of the particles annealed at 100 °C fell outside of the detection limit of Si detector. According to sizing curve of Cademartiri el. al.33, annealing at 100 °C causes particles to grow from 2.82 nm to 3.99 nm. Because, this size difference is quite small, we could not observe such difference in the TEM images. Actually, decreasing band gap and coalescence of particles during annealing of QD-films have been reported before.47 However, what is critical here is that, at these annealing temperatures this process can be controlled, annealed particles can be suspended well in a carrier solvent again and the strong luminescence of the PbS/CdS QDs can be maintained. When annealing was performed at 50 °C (Figure 5b-c) no significant change in the emission intensity of the PbS/CdS was observed and this is independent from the duration of annealing. But, at 75 °C, there is about 52.9% loss in the emission intensity. Two suggestions to explain this observation are (i) the growing size of the QDs, which reduce the confinement22, 27 and (ii) surface perturbation due to detachment of organic surface ligands, which would cause surface defects. Both of these would reduce the QY.


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PbS Standart CdS Standart Annealing Temp. Unannealed 0 at 50 C 0 at 75 C 0 at 100 C

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Figure 6. (a) XRD patterns of PbS/CdS QD and annealed PbS/CdS QDs at different temperatures, (b) Enlarged area of XRD pattern showing the appearance of new peaks as indicated with arrows for clarification. Annealed PbS/CdS QDs showed improved crystallinity with no shift in the peak positions indicating no compositional change, regardless of the annealing temperature (Figure 6a). Major enhancement in the peak intensities was observed at 50 and 75 °C. No dramatic change was observed between 75 and 100 °C. RTCE was able to grow few layers of CdS on PbS, but the temperature was not enough to provide a good crystallization of the shell, which was achieved by post-synthetic annealing process. Annealing at 75 and 100 °C, created two new peaks at 39.2°


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and 46.9°, which is more significant at 100 °C. These peaks correlate well with the elemental S, which may form due to decomposition of surface ligand DT, coming from the modified Cademartiri method that we have utilized.32 Decomposition of ligands introduce surface trap sites, which may at least partially explain the loss in the emission intensity after annealing above 50 °C. Both luminescence properties and the XRD data indicate that annealing of the core/shell particles at 50 °C following the RTCE is a robust process, which improves the crystallinity and maintain the emission intensity. Long-Term Stability of PbS/CdS QD

PL Intensity (a.u.)

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Figure 7. Photoluminescence spectra of QDs shortly after the synthesis (solid lines) and after 1 year (dotted lines) in chloroform. Unannealed PbS/CdS (Curve A); annealed at 100 °C (Curve B); annealed at 100 °C and subjected to DT ligand exchange (Curve C). Long-term colloidal stability of these particles are essential for practical use and all particles had excellent colloidal stability in chloroform over a year, already. Another critical property is the optical stability. As made PbS/CdS QDs lose 93% of its emission intensity with 102 nm red


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shift in 1 year (Figure 7). Red shift of the emission peak in time coupled with low crystallinity of the as-made PbS/CdS suggests possibility of slow dissolution of the smaller particles and adding up to larger ones. On the other hand, PbS/CdS annealed at 100 °C showed a 66 nm blue shift with 68% decrease in the emission intensity in 1 year, indicating improved optical stability but not enough for practical purposes. Annealing enhanced the crystallinity, but also caused degradation of some surface bound DT at high annealing temperatures. This would cause some surface defects which correlates well with the loss in the emission intensity. In the long-term, partial loss of the coating may also cause surface oxidation and/or loss of surface ions as well, which would explain the blue shift in the emission peak along with the loss in the intensity. Yet, such loss is not as dramatic as in the case of un-annealed PbS/CdS, since the crystalline shell provides better stability.17 To improve the stability further, weakly bound OLA was displaced with DT after the annealing step. As usual, we observed PL intensity loss and redshift after the ligand exchange process. This is usually attributed to reduced inter-QD spacing with the shorter ligands as well as new intra-band defects formed during ligand exchange.42 Resulting QDs showed a dramatic improvement in the optical stability: only 30 nm red shift in the emission maxima with about 31% loss in its intensity after 1-year storage in chloroform. This suggests that Ostwald ripening and oxidation are mostly eliminated after the full process consisting of RTCE to form a thin CdS shell, annealing at low temperatures followed by DT ligand exchange.


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Figure 8. Optical stability of PbS-DT and PbS/CdS-DT QDs (a), (b) in gasoline and (c), (d) in diesel up to a month. QD concentration=100 ppm. Stability of such NIR QDs in petroleum products is a desirable feature also since there is an emerging interest for optical tagging of such valuable liquids.48-50 These liquids have autofluorescence below 700 nm. Therefore, labeling them with a NIR QD would be valuable in terms of material cost and privacy. Yet, they are much harsher environments for QDs due to chemically active additives compared to simple solvents such as chloroform. PbS and PbS/CdS QDs without DT ligand exchange lost emission in a week (data not shown) in gasoline and diesel but stability up to a month is desirable for all practical purposes. After DT ligand exchange PbS QDs (PbS-DT) showed only a 3 nm blue shift with 23% loss in the emission intensity in gasoline and PbS/CdS-DT showed 14 nm red shift with 13.3% loss in the emission intensity after 1 month


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(Figure 8a-b). Commercial diesel is harsher on QDs. While PbS-DT lost its emission almost completely, PbS/CdS-DT showed strong luminesce even after 1 month with ca 25% loss in the emission intensity with 17 nm blue shift, clearly indicating the benefit of having a CdS shell and DT ligand on the NIR emitting PbS. Lifetime spectroscopy is a useful technique to understand the mechanism of fluorescence by comparing exciton lifetimes. Generally, a biexponential decay is observed with QDs related to existence of radiative and non-radiative events. Fast component (τ1) arise from non-radiative events such as defect states, which are acting as hole or electron traps, and reducing the QY. Slow component (τ2) arises from hole and electron recombination and it means longer relaxation. Comparison of amplitudes of fast (B1) and slow (B2) components can provide information about the change in mechanism of fluorescence after processes such as shell growth and ligand exchange.


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(a) Normalized Intensity (counts)

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Figure 9. (a) Lifetime decays of PbS and PbS/CdS QDs, (b) Lifetime decays of annealed PbS/CdS QDs. Table 1. Influence of CdS Shell on Luminescence Lifetime

τ1 (µs)

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2.63 ± 0.018 65.55 ± 0.30 3.74 ± 0.026 34.45 ± 0.65

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Table 1 and Figure 9a show lifetimes of PbS and PbS/CdS QDs. The average lifetime (τav) increased from 3.01 µs for PbS QD to 3.73 µs for PbS/CdS. Also, the faster decay lifetime shortens with less contribution while the slower decay lifetime got longer with increased


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contribution. These are in agreement with the reports suggesting that shells increase the luminescence lifetime by reducing defect-related events.16, 21, 51 This is due to the confinement of exciton in the core with the larger bandgap shell in type-1 core-shell QDs.51-52 So, the faster decay shortens with less contribution while the slower decay lifetime got longer with increased contribution. Annealing had a strong impact in the luminescence mechanism (Figure 9b). Average lifetime of un-annealed PbS/CdS (3.73 µs) got longer with the annealing in a temperature dependent manner; 2.63, 2.52, 2.00 µs for PbS/CdS QD annealed at 50-75-100 °C, respectively (Table S2). Lifetime of both fast and slow decay events are reduced with increasing annealing temperature and the emission and absorption were shifted to longer wavelengths, indicating an increase in the size. As the size of the PbS QD increases, average lifetime usually decreases.22 Hens et. al., relates this to the weakening of the confinement regime.22 Exchange of OLA with DT after annealing at 50 °C reduced lifetime further from 2.63 µs to 2.32 µs (Table S3). After ligand exchange process, both events become faster with an increase in the amplitude of slower component. Influence of the ligand exchange on the average lifetime, emission peak position and intensity of PbS/CdS is much less than the effect observed for PbS.32 Many reports indicate shorter ligands cause shorter exciton lifetime due to smaller inter-QD spacing, which increases hole and electron mobility.42, 53-54 DT passivates QD surface better due to stronger binding to surface Cd atoms. Many reports relate ligand exchange with formation of new surface defects due to PL intensity loss after the exchange process.17,

42, 55

Besides, DT

ligands may act as a hole trap if the valance band of QD is below the redox potential of the thiol (about -5 eV), reducing the emissive coupling events.56 We observed PL intensity loss after


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ligand exchange, as well, but not sure about the reason, since actually emission should be originating from the confined core, but DT exchange was done on the surface of the shell.

Conclusions PbS/CdS core shell QDs were developed from ultrasmall PbS with a very thin CdS shell via room temperature cation exchange (CE) process using Cd-Oleate. Process is self-limiting and time independent at room temperature, which is very practical, compared to CE at 100 °C. Besides, RTCE provides narrower size distribution. Shell growth at RT provides effective passivation of the surface and confinement of the exciton within the core, causing 5.8-9 fold increase in the emission intensity and roughly a 100 nm blue shift in the emission maxima. So, it is possible to obtain a series of QDs with tuned emission wavelength by starting from a serious of PbS cores with well separated emission peaks. RT process provides a longer average decay of luminescence with stronger contribution of the core exciton coupling. Although control of the process was more difficult at 100 °C, CE at high temperature provides better crystallinity and stability. Hence, PbS/CdS QDs formed by RT CE process were subjected to mild heat treatment between 50-100 °C to improve crystallinity. A dramatic enhancement in crystallinity was accompanied with red shift in absorbance and luminescence peaks, which may indicate some coalescence of particles. Also, annealing at the higher end of this temperature range may cause some decomposition of surface DT coming from the synthesis of ultrasmall PbS. Annealing reduced the average lifetime further, yet also reversed the mechanism from more defect related to exciton coupling with increasing annealing time. Overall, we have concluded that RT CE process coupled with 1h annealing at 50 °C, provides a thin crystalline CdS shell around PbS causing luminescence intensity increase with overall ca 50 nm blue shift in the peak position.


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Although such core/shell QDs are much more stable than PbS core, DT ligand exchange provided further stability up to a year in chloroform. In case of diesel and gasoline, NIR emission of these QDs presents a great advantage since it is beyond the auto-fluorescence range of these liquids. For stability in the harsh conditions of these liquids, CdS shell and DT ligand exchange seems to be necessary. PbS/CdS-DT QDs provide strong luminescence within the typical shelf life (1 month) of these petroleum products. Overall, stable PbS/CdS QDs with emission between 800-1020 nm were synthesized from ultrasmall sized PbS QDs. Emission tunability, control of shell thickness and quality was demonstrated in a very simple and economic RT CE followed by low temperature annealing processes, which is high applicable to industrial scale.

Supporting Information Available The supporting information is available free of charge on the http://pubs.acs.org. Methods of size estimation, bandgap and quantum yield calculations, lifetime measurements; EDX mapping of PbS/CdS QD; FTIR spectra of PbS/CdS QD; responsivity of PDA10A silicon detector; XPS data; lifetime of annealed and ligand exchanged QDs.

Acknowledgements This project was funded by OPET A.Ş. We would like to thank Dr. Baris Yagci and Cansu Yıldırım at KUYTAM for their help in XPS and XRD analysis.


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50. Urey, H.; Yagci Acar, H.; Elbuken, C.; Can, B.; Akgun, O. V.; Uygurmen, F. K., A Method and an Apparatus for the Detection of a Tagging Material in Fluids 2016 51. Sanchez, R. S.; Binetti, E.; Torre, J. A.; Garcia-Belmonte, G.; Striccoli, M.; Mora-Sero, I., All Solution Processed Low Turn-on Voltage near Infrared Leds Based on Core-Shell PbSCdS Quantum Dots with Inverted Device Structure. Nanoscale 2014, 6, 8551-5. 52. Wheeler, D. A.; Fitzmorris, B. C.; Zhao, H. G.; Ma, D. L.; Zhang, J. Z., Ultrafast Exciton Relaxation Dynamics of PbS and Core/Shell PbS/CdS Quantum Dots. Sci. China Chem. 2011, 54, 2009-2015. 53. Gonfa, B. A.; Kim, M. R.; Delegan, N.; Tavares, A. C.; Izquierdo, R.; Wu, N.; El Khakani, M. A.; Ma, D., Towards High Efficiency Air-Processed near-Infrared Responsive Photovoltaics: Bulk Heterojunction Solar Cells Based on PbS/CdS Core-Shell Quantum Dots and TiO2 Nanorod Arrays. Nanoscale 2015, 7, 10039-49. 54. Speirs, M. J.; Balazs, D. M.; Fang, H. H.; Lai, L. H.; Protesescu, L.; Kovalenko, M. V.; Loi, M. A., Origin of the Increased Open Circuit Voltage in PbS-CdS Core-Shell Quantum Dot Solar Cells. J. Mater. Chem. A 2015, 3, 1450-1457. 55. Tsukasaki, Y.; Morimatsu, M.; Nishimura, G.; Sakata, T.; Yasuda, H.; Komatsuzaki, A.; Watanabe, T. M.; Jin, T., Synthesis and Optical Properties of Emission-Tunable PbS/CdS CoreShell Quantum Dots for in Vivo Fluorescence Imaging in the Second Near-Infrared Window. RSC Adv. 2014, 4, 41164-41171. 56. Wuister, S. F.; de Mello Donegá, C.; Meijerink, A., Influence of Thiol Capping on the Exciton Luminescence and Decay Kinetics of CdTe and CdDe Quantum Dots. J. Phys. Chem. B 2004, 108, 17393-17397.


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PbS-OLA/DT 1. RT Cation Exchange 2. Annealing 3. Ligand exchange

PbS

PL Intensity (a.u.)

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In Diesel PbS-DT 1 Day 1 Month PbS/CdS-DT 1 Day 1 Month

CdS 800

900 1000 Wavelength (nm)

1100

TOC Graphic


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CdS Quantum Dots via Room

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