Near-Infrared Photoluminescence and Thermal Stability of PbS

Aug 28, 2016 - Here, we report a comprehensive study of the thermal stability of lead sulfide (PbS) QDs. Despite a bulk melting temperature of 1100 °...
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Near-Infrared Photoluminescence and Thermal Stability of PbS Nanocrystals at Elevated Temperatures Robert C. Keitel, Mark C. Weidman, and William A. Tisdale* Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States S Supporting Information *

ABSTRACT: Near-infrared quantum dots (QDs) are attractive materials for biological, defense, and telecommunications applications. Many of these applications depend on QD luminescence at elevated temperatures, which has not been thoroughly explored in infrared-emitting QD materials. Here, we report a comprehensive study of the thermal stability of lead sulfide (PbS) QDs. Despite a bulk melting temperature of 1100 °C, in situ transmission electron microscopy (TEM) revealed coalescence of neighboring PbS QDs at temperatures as low as 150 °C. While TEM, X-ray diffraction, and thermogravimetric analysis showed no changes to the QD structure below 150 °C, the photoluminescence of densely packed QD films irreversibly decreased after exposure to temperatures as low as 50 °C. In contrast, embedding the QDs in cross-linked poly(lauryl methacrylate) dramatically improved the stability of photoluminescence to thermal cycling, delaying the onset of irreversible luminescence quenching beyond 120 °C and maintaining measurable photoluminescence intensity and single-exponential decay dynamics above 200 °C. Time-resolved spectroscopy revealed increased rates of nonradiative recombination at elevated temperatures, indicating more efficient phonon-mediated decay processes. These results show that PbS QDs present similar challenges to visible-emitting QDs in applications requiring operation at elevated temperatures but encourage further efforts in thermal stabilization.

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INTRODUCTION Over the past decades, synthetic effort has led to high quality, solution processable, monodisperse quantum dots (QDs) emitting in the visible1,2 and in the infrared region.3−5 The interest in QDs arises due to their unique size-dependent optical and electrical properties opening up a variety of applications. The narrowband and tunable light emission make QDs highly interesting for light emitting devices, either directly as the active material6,7 or as fluorophores for generating white light via downconversion.8 PbS QDs,9,10 in particular, have attracted recent interest due to the near-infrared bandgap, leading to their use in solution processable photovoltaics,11−13 high performance IR photodetectors and night vision systems,14,15 light-emitting devices,7 biolabeling,16 and optical telecommunication.7,17,18 While many authors have focused on the temperature dependence of quantum dot photoluminescence at low temperatures,19−22 only a limited number of studies have addressed their high temperature properties, despite the practical importance of such information. Ihly et al. examined the long-term thermal stability of PbS QDs, showing oxidation and coalescence to be the major degradation mechanisms.23 The thermal stability of polymer embedded CdSe based quantum confined materials was studied by Zhao et al., finding both reversible and irreversible quenching processes occurring depending on the temperature range.24 © 2016 American Chemical Society

In this work, we study the thermal stability of PbS QDs at elevated temperatures, with a particular emphasis on lightemitting properties. Surface ligand desorption was investigated using thermogravimetric analysis (TGA). The high temperature behavior of the cores was studied using temperature dependent X-ray diffraction (XRD) and in situ transmission electron microscopy (TEM). The QD cores were found to be structurally stable up to temperatures of 150 °C. Despite the apparent structural stability below this temperature, we observed that the photoluminescence (PL) of densely packed QD thin films underwent irreversible degradation after exposing the film to very mild temperatures, as low as 50 °C. After embedding the QDs in a cross-linked poly(lauryl methacrylate) matrix the temperature stability was increased well beyond 120 °C. Though the PL recovered fully upon cooling to room temperature, emission at elevated temperature was partially quenched due to faster rates of nonradiative recombination.

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EXPERIMENTAL SECTION Nanocrystal Synthesis. Colloidal PbS QDs were synthesized following the method described in a previous paper.4 A Received: June 15, 2016 Revised: July 28, 2016 Published: August 28, 2016 20341

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average QD diameter was determined by measuring absorption of dilute dispersions of QDs in tetrachloroethylene and using a previously published sizing relation.4 The concentration was determined by diluting the stock dispersion with hexane and measuring the absorbance at 400 nm according to the method of Moreels et al.26 TGA. Thermogravimetric analysis (TGA) was measured with a TA Discovery Thermogravimetric Analyzer under N2 purging. Samples were prepared by drop-casting QD suspensions into platinum pans. Ligand exchange was performed overnight after deposition by placing the pan in a 0.1 M solution of the target thiol species in acetonitrile, followed by rinsing with acetonitrile. The samples were dried under flowing nitrogen at a temperature of 30 °C for 10 min before beginning each TGA scan. XRD. X-ray diffraction (XRD) data were obtained using a PANalytical X’Pert Pro XRPD with a Cu Kα source operated at 45 kV and 40 mA. A Bragg−Brentano parafocusing geometry with automatic divergence slit set to an irradiated length of 6 mm and a high-speed linear position sensitive detector were used. Samples were prepared by drop-casting QD suspensions on background-free silicon sample holders. Ex situ sample annealing was performed in a Fisher Scientific Isotemp Model 630F oven for 40 min at temperatures up to 190 °C. Higher temperatures were reached using a T-M vacuum products SS806NS-14 vacuum oven. TEM. Transmission electron microscopy (TEM) was conducted on a JEOL 2010 high resolution TEM under an acceleration voltage of 200 kV. Samples were drop cast onto TEM grids with an amorphous carbon support film. In situ heating experiments were performed using a temperaturecontrollable Gatan model 901 sample holder. The temperature was ramped up from room temperature and held for roughly 10 min every 5−10 °C in order to take images. A new area of the grid was imaged at each temperature to avoid beam damage. For TEM imaging of polymer-embedded samples, the QD/ monomer solution was cured in the shape of thin rods. The obtained polymer rod was then mounted in a Leica ultramicrotome and slices of ∼70 nm were cut using a diamond blade at an angle of 5°. The slices were transferred to TEM grids and imaged. Photoluminescence. Temperature dependent photoluminescence (PL) measurements were taken on QD thin films deposited via spin coating from a 20 mg/mL dispersion in toluene on cleaned glass substrates treated with (3mercaptopropyl)trimethoxysilane (Sigma-Aldrich, 95%). Spin coating was performed in an oxygen- and water-free glovebox, and the samples were mounted inside a steady-flow Janis ST100 optical cryostat and sealed inside the glovebox. The cryostat was then transferred to an optical table and evacuated. Polymer-embedded samples for PL measurements were mounted in the cryostat under ambient laboratory air. Two coaligned lasers were used for the optical characterization measurements. A pulsed 405 nm laser at a repetition rate of 62.5 kHz and a fluence of 2.3 nJ/cm2 was used for time correlated single photon counting. Photons emitted from the sample were focused onto a Micro Photon Devices InGaAs/InP single photon avalanche detector and synchronized with the laser using a Picoharp 300 time-correlated single photon counting module. After measuring the lifetime, a 785 CW laser with a power density of 2.4 W/cm2 was switched on to collect the CW photoluminescence spectrum. The PL signal was

large precursor ratio of Pb/S greater than 12:1 was used to synthesize highly monodisperse QDs. Lead(II) chloride (Alfa Aesar, 99.999%), oleylamine (Sigma-Aldrich, technical grade), sulfur (Sigma-Aldrich, ≥99.99%), and oleic acid (SigmaAldrich, 90%) were used as obtained. Briefly, in a four-neck flask, lead chloride was dissolved in oleylamine, degassed, and heated to 120 °C for 20 min under nitrogen. Meanwhile, sulfur was dissolved in oleylamine by heating to 120 °C while bubbling with nitrogen. The sulfur solution was cooled to room temperature while continuing the nitrogen bubbling. Once the lead chloride solution stabilized at 120 °C, the sulfur solution was rapidly injected. When the QDs reached their target size, the reaction was quenched by swiftly injecting cooled hexane and immersing the 4-neck flask in a water bath. The product was purified by repeated precipitation with oleic acid, followed by redispersal in hexane. During this process, the native ligand oleylamine is exchanged by oleic acid. In a final washing step, a mixture of butanol and methanol (2:1) was added to precipitate the nanoparticles and remove excess oleic acid. The QDs were again redispersed in hexane and stored for several days to precipitate remaining lead chloride before filtering through a 0.2 μm filter. Absorption and photoluminescence spectra for two QD batches are shown in Figure 1a,c, and corresponding TEM images are shown in Figure 1b,d.

Figure 1. Absorption, PL, and TEM images of (a, b) 5.3 nm diameter and (c, d) 6.6 nm diameter PbS QDs.

Polymer Embedding. To embed PbS QDs in a crosslinked poly(lauryl methacrylate) matrix (PLMA), we followed a route similar to the one proposed by Bomm et al.25 Dried QDs were first brought into a nitrogen glovebox. A mixture of lauryl methacrylate (Sigma-Aldrich, 96%) and ethylene glycol dimethacrylate (Sigma-Aldrich, 98%; 4:1 by weight) was added to the QDs to make mixtures with 0.01, 0.1, or 1.0% QDs by weight. The mixture was stirred inside the glovebox for at least 30 min. Subsequently, 0.25 wt % of diphenyl(2,4,6trimethylbenzoyl)phosphine oxide (Sigma-Aldrich) was added as a UV-initiator, and the solution was stirred for another 30 min. The monomer solution was then dropcast onto a glass substrate and cured for 15 min under a UV lamp (365 nm). Nanocrystal Characterization. Optical absorption spectra were measured using a Cary 5000 UV−vis spectrometer. The 20342

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The Journal of Physical Chemistry C coupled via an optical fiber into a Bayspec SuperGammut NIR spectrometer. Photoluminescence lifetime and spectrum were first collected at 300 K before performing temperature-dependent studies. The temperature was then increased in steps of 20 K and held for 25 min before cooling back to room temperature. Lifetimes and spectra were collected at both the elevated temperature and again at room temperature after each temperature step to distinguish reversible from irreversible trends. See Figure S1 for the time−temperature history of each sample.

can estimate the ligand coverage. For oleic acid QDs, we estimate a surface coverage of 2.9 ± 0.2 ligands/nm2 for both QD sizes studied. This is in good agreement with values previously reported5,27 or assumed28 in literature, supporting the interpretation of full ligand loss at the step between 250 and 300 °C. When the native oleic acid ligand was exchanged for a series of alkanethiol ligands, only two mass loss features were observed (near 200 °C and near 450 °C), as shown in Figure 2b. The magnitude of the first mass loss event scales with the ligand molecular weight, suggesting that it represents the desorption of surface-bound alkanethiol ligands. For the thiol ligands, we obtained similar estimated surface coverages using the mass loss event occurring near 200 °C (see Figure S3), supporting our assignment of this feature in the TGA curve. The high-temperature TGA feature (near 450 °C) is independent of ligand length and very similar to the high temperature behavior seen in the OA-capped QDs, suggesting it is a mass loss related to an unknown chemical transformation of the inorganic core. In Figure 3, we show XRD patterns taken at room temperature of thin films of OA-capped QDs after annealing

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RESULTS AND DISCUSSION In Figure 2a we show a typical thermogravimetric analysis (TGA) curve of oleic acid-capped PbS QDs. Three distinct

Figure 3. X-ray diffraction patterns of 6.6 nm PbS QD films following annealing at the indicated temperature for 40 min and subsequent cooldown to room temperature. Vertical lines indicate expected peak positions for rock salt PbS.

Figure 2. Thermogravimetric analysis of (a) oleic acid-capped PbS QDs after purification (black) and after addition of excess oleic acid (red) and (b) a series of thiol ligand-exchanged PbS QDs.

at different temperatures for 40 min in air (the 210 °C sample was annealed in vacuum due to limitations of the air furnace). All major peaks were successfully matched with the peaks expected for cubic PbS. The QDs have an average diameter of 6.6 nm and peak width analysis using HighScore Plus yielded average crystal grain sizes around 5 nm at room temperature. Up to a temperature of 190 °C, no major changes to the XRD peak widths were observed, indicating the average crystallite size remained the same. At 190 °C, additional small and relatively narrow peaks were observed, which could be indexed to orthorhombic PbCl2. These peaks may originate from initially small, possibly amorphous residual lead chloride in the purified QD dispersion

mass loss events can be identified at temperatures near 150, 250, and 450 °C. A comparison with QDs from the same batch with a large excess of oleic acid added showed a change in the magnitude of the first mass loss event near 150 °C, while the magnitude of the other two mass loss events was unchanged (see Figure S2). This indicates that the first step in the TGA curve is likely related to loss of unbound surface ligands. We attribute the second mass loss event beginning near 250 °C to desorption of surface-bound ligands. Using a simple mathematical model where the inorganic cores are assumed to be perfect spheres with the mass density of bulk PbS uniformly covered by organic ligands with known molecular weight, we 20343

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The Journal of Physical Chemistry C that are not detected in the samples annealed at lower temperatures, but which crystallize at elevated temperatures. However, it is also known from previous studies that the surface of PbS QDs synthesized from PbCl2 precursor is terminated with chloride ions.3−5 As the crystallization of PbCl2 is observed in a temperature range where also growth of QDs themselves takes place as indicated by peak narrowing, the surface of the QDs is also a possible source of PbCl2. Heating to 210 °C in vacuo resulted in larger PbCl2 peaks and a narrowing of the PbS peaks. Based on peak width analysis, we estimate that the average crystallite grain size has increased to over 8 nm following annealing at 210 °C. To better understand the apparent growth of thermally annealed PbS nanocrystals revealed by XRD, we performed TEM with in situ heating. Images highlighting the major temperature dependent changes are shown in Figure 4a−e. The first image in Figure 4a, taken at 30 °C, illustrates the quasispherical, monodisperse, and single-crystalline nature of the QDs. Up to temperatures around 150 °C no changes were observed. Upon heating to 170 °C, the center-to-center spacing between neighboring QDs was noticeably reduced and some merging of neighboring particles was observed (Figure 4b). However, as seen in the high-resolution inset, the crystalline lattices of neighboring nanocrystals remained distinct. By 175 °C, pairs of QDs with a continuous single crystal structure were observed (Figure 4c). This behavior is consistent with the previous work of van Huis et al., who observed formation of multi-QD single crystals by oriented attachment and subsequent rotation in hexylamine capped PbSe nanocrystal arrays.29 Interestingly, the processes observed here happened well below the desorption temperature of ligands (250 °C), as determined by TGA (Figure 2a), illustrating the lability of QD surface ligands as previously demonstrated both theoretically30 and experimentally.31,32 TEM images of the sample heated to 200 °C revealed large single crystals more than 12 nm in diameter (Figure 4d). Going even higher in temperature, larger single crystals with diameters >20 nm could be found (Figure 4e). Even at these high temperatures, the spacing of all visible lattice fringes still corresponded to the expected lattice spacing of bulk PbS. The TEM results are in good agreement with the XRD data. Besides a slight increase in the center-to-center distance between neighboring QDs (see Figure S4), no structural changes could be detected up to ∼150 °C. At temperatures around 180 °C a significant particle growth via oriented attachment was observed. TEM images of a sample heated ex situ under air at 190 °C confirmed that the in situ behavior observed in the vacuum environment of the TEM is reproduced under ambient conditions (see Figure S5). All of the structural changes reported here occur well below the melting point of bulk PbS (1114 °C). These observations are in line with classical theories of melting point suppression in nanocrystals, where the melting temperature decreases with decreasing particle size due to a reduction in surface and interfacial energy.29,33,34 The observation of coalescence before melting is consistent with a picture of surface phonon softening in nanocrystals,34,35 wherein weakly bound surface atoms can rearrange to form more thermodynamically stable structures. A study of thermal stability of PbS nanocrystalline films grown by chemical deposition performed by Sadovnikov et al. is in good agreement with our results for the structural stability.36 These authors observed nanocrystal growth in a similar temperature range (below 200 °C), but they also observed

Figure 4. In situ TEM. PbS QDs were imaged under active heating to (a) 30, (b) 170, (c) 175, (d) 200, and (e) 260 °C.

eventual formation of an oxide sulfate surface phase after annealing in air. Our observation of changes at lower temperatures is consistent with the use of smaller sized nanocrystals in our work. Photoluminescence (PL) is often the most sensitive reporter of thermal and structural changes in QDs.24 We measured the PL spectrum and transient PL decay of a thin film of 5.3 nm PbS QDs at elevated temperature (Figure 5a,b) and after cooling back to room temperature (Figure 5c,d). All samples 20344

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Figure 5. Photoluminescence properties in close-packed PbS QD thin films. (a, b) Photoluminescence spectra and transient decay traces collected at the indicated elevated temperature; (c, d) collected at room temperature after annealing at the indicated elevated temperatures for 25 min.

PLMA without forming agglomerates.25,44,45 We studied PLMA matrices embedded with 0.01, 0.1, and 1 wt % PbS QDs, as shown in Figure 6a. TEM images of thin sections of a sample

were prepared inside a nitrogen glovebox and heated inside a sealed, evacuated cryostat. PL measurements were made inside the cryostat to prevent exposure to oxygen or moisture. As the sample was heated, both PL intensity and lifetime monotonically decreased, and the PL peak showed a slight reversible blue-shift. The shortened lifetime combined with the weaker overall emission intensity indicates PL quenching via an enhancement of nonradiative decay pathways. Photoluminescence quenching with increasing temperature is commonly attributed to thermally activated nonradiative processes such as thermally activated trapping.20,22,37 In lead salt QDs, multiphonon processes are known to be highly efficient35,38 and likely drive these nonradiative decay channels at elevated temperatures. The blue shift observed originates from the temperature dependence of electron−phonon coupling in narrow band gap semiconductors.39,40 After cooling to room temperature, the PL peak position was recovered, but much of the overall PL intensity was permanently lost. Such behavior is usually attributed to an increase in surface trap state density, possibly resulting from permanent ligand removal.41 However, TGA measurements showed no significant ligand loss at these mild temperatures. We hypothesize that at mild temperatures only a small number of surface ligand-associated defects are created, which locally increase the rate of nonradiative recombination, but are not detectable by TGA. Through exciton diffusion,42 these localized “dark” QDs can quench excitons created on neighboring nanocrystals, dramatically reducing the overall photoluminescence from the QD film.43 The irreversible PL degradation observed already at very mild temperatures in vacuo is problematic for most PbS light emitting applications. To prevent this behavior, we investigated the use of a polymer matrix to suppress exciton diffusion and inhibit ligand loss. Previous work showed that CdSe/ZnS QDs capped with organic ligands can be embedded in cross-linked

Figure 6. PbS QDs embedded in cross-linked poly(lauryl methacrylate), PMMA. (a) Photographs of polymer-embedded samples with 0.01, 0.1, and 1.0 wt % QD loading. (b) TEM image of microtomed PbS QDs in PLMA (0.01 wt %). (c, d) Comparison of PL spectra and transient decay traces in solution, thin film, and PLMA.

with 0.01 wt % QDs, shown in Figure 6b, revealed that the QDs are individually dispersed within the matrix. In several images taken at random locations of the film, no agglomerates were detected. To determine the maximum QD weight loading in polymer matrix that still kept the QDs isolated, we compared PL spectra and lifetimes with those of QDs dispersed in tetrachloroethylene (TCE). The two lower weight loadings 20345

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Figure 7. Photoluminescence from PbS QDs embedded in cross-linked PLMA at 0.1 wt %. (a, b) Photoluminescence spectra and transient decay traces collected at the indicated elevated temperature; (c, d) collected at room temperature after annealing at the indicated elevated temperatures for 25 min.

Figure 8. Summary of the evolution of integrated PL intensity and 1/e lifetime with temperature for thin film and 0.1 wt % polymer embedded QDs.

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CONCLUSION The thermal stability of colloidal PbS QDs has been studied using both structural and photoluminescence characterization techniques. TGA revealed complete ligand loss of oleic acid at 300 °C and of alkanethiol ligands at 200 °C. XRD conducted after heat treatment showed the rapid increase in crystalline domain size, an indication of QD sintering, at temperatures above 200 °C. This finding was supported by TEM characterization using in situ heating, which showed the interdot spacing to decrease after 160 °C followed by alignment of the nanocrystals to form larger single crystals. Above 180 °C, multiple QDs coalesced into larger single-crystalline nanostructures. Despite the absence of obvious structural changes below 150 °C, photoluminescence from PbS QD thin films was irreversibly quenched after heating above room temperature. Embedding the QDs in a cross-linked PLMA polymer matrix improved the thermal stability of photoluminescence. Irreversible PL quenching was delayed to temperatures above 120 °C, and measurable PL intensity and single-exponential PL decay was maintained above 200 °C. This is in good agreement with the proposed mechanism for quenching in the thin film and enables access to a broader temperature range for PbS QD optical applications.

(0.01, 0.1 wt %) overlap almost perfectly with the spectrum from the TCE solution, as shown in Figure 6c,d. Decay traces for those two concentrations show almost single exponential behavior and are again in reasonable agreement with data from solution. Compared to the dense film, the 1/e lifetime increased from 300 to 1500 ns. Additionally, the quantum yield (QY) of QDs embedded with a concentration of 0.1 and 0.01 wt % showed no significant difference, with values of 23 ± 4% and 24 ± 2%, respectively. Storing PLMA samples outside of the glovebox for 1 week had no detectable effect on the QY. QDs embedded with a concentration of ≥1.0 wt % partially precipitated during polymer curing. Optical properties of these films resembled something intermediate to the dispersed QD solution and the close-packed film (see Figures 6c,d and S6a− d). Temperature-dependent PL measurements of the polymerembedded samples, presented in Figure 7, showed significantly improved thermal stability compared to the close-packed thin films. As the sample was heated, the PL intensity decreased along with a spectral blue shift and shortened lifetime, consistent with the behavior in thin films. However, a significant difference to the thin film lies in the high temperature behavior. In the polymer-embedded samples, nearly single exponential decay traces and resolvable PL spectra are still obtained at temperatures above 200 °C. In contrast, the thin films samples showed no detectable photoluminescence above 200 °C. In addition, quenching of embedded QDs is completely reversible up to a temperature of at least 100 °C. In this range, no permanent quenching or spectral shift is detectable; both the spectrum and transient decay overlap almost completely. Beyond 120 °C, irreversible quenching was observed, accompanied by a red-shift of the PL spectrum (Figure S7). The red-shift was observed in a variety of different samples with QDs of different sizes, from different batches and at different concentrations. The red-shifted PL spectrum more closely resembles the PL spectrum of the thin films samples, possibly indicating aggregation of QDs within the polymer matrix.46 Close-packed films of QDs exhibit PL spectra red-shifted from QD dispersions due to solvatochromic effects and exciton diffusion to lower-energy sites within the film.42,47 Aggregation of QDs within the polymer matrix will lead to similar behavior, as shown in Figure 6c,d for the concentrated polymer samples and in the temperature-dependent characterization (Figure S6a−d). A direct comparison of the photoluminescence properties of densely packed QD thin films with QDs embedded in PLMA, shown in Figure 8, demonstrates the improved stability imparted by polymer embedding. While the room-temperature and elevated-temperature PL intensity from the thin film decreased monotonically after annealing steps, the roomtemperature PL intensity of the polymer-embedded sample was maintained at its initial value even when the sample was annealed to temperatures as high as 120 °C. After heating to temperatures as high as 160 °C, approximately 75% of the initial PL intensity was still retained. Similar trends were observed for the 1/e lifetime values, with the polymerembedded sample maintaining its initial long lifetime even after significant heating. We attribute the improved stability of the polymer-embedded samples to (1) suppressed exciton diffusion and (2) suppressed desorption of surface ligands. Unraveling the role of the polymer matrix in supporting favorable surface chemistry requires more work in the future.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b06053. Surface coverage for all ligands calculated from TGA data, more temperature-dependent in situ TEM images and analysis, TEM image of a sample annealed in air, sample time−temperature history for PL studies, and PL data for the 1.0 wt % PbS QDs in PLMA sample (PDF).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: 617-253-4975. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. Charles Settens for assistance with X-ray diffraction measurements. This work was supported by the MIT Energy Initiative Seed Fund Program. M.C.W. was supported by a National Science Foundation Graduate Research Fellowship under Grant No. 1122374. R.C.K. was supported by the Master Thesis Grant of the Zeno Karl Schindler Foundation. This work made use of the MRSEC Shared Experimental Facilities at MIT, supported by the National Science Foundation under Award Number DMR1419807. TGA measurements were performed in the ISN at MIT.

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