Elevated Temperature Photophysical Properties and Morphological

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Letter

Elevated Temperature Photophysical Properties and Morphological Stability of CdSe and CdSe/CdS Nanoplatelets Clare E. Rowland, Igor Fedin, Benjamin T. Diroll, Yuzi Liu, Dmitri V. Talapin, and Richard D. Schaller J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b02793 • Publication Date (Web): 28 Dec 2017 Downloaded from http://pubs.acs.org on December 31, 2017

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The Journal of Physical Chemistry Letters

Elevated Temperature Photophysical Properties and Morphological Stability of CdSe and CdSe/CdS Nanoplatelets

Clare E. Rowland,1,2 Igor Fedin,3 Benjamin T. Diroll,2 Yuzi Liu,2 Dmitri V. Talapin,2,3 Richard D. Schaller1,2*

1

2

3

Department of Chemistry, Northwestern University, Evanston, IL 60208

Center for Nanoscale Materials, Argonne National Laboratory, Argonne, IL 60439

Department of Chemistry and James Frank Institute, University of Chicago, Chicago, IL 60637

Abstract

Elevated temperature optoelectronic performance of semiconductor nanomaterials remains an important issue for applications. Here we examine two-dimensional CdSe nanoplatelets (NPs) and CdS/CdSe/CdS shell/core/shell sandwich NPs at temperatures ranging from 300-700 K using static and transient spectroscopies as well as in-situ transmission electron microscopy. NPs exhibit reversible changes in PL intensity, spectral position, and emission linewidth with temperature elevation up to ~500 K, losing a factor of ~8 to 10 in PL intensity at 400 K relative to ambient. Temperature elevation above ~500 K yields thickness dependent, irreversible degradation in optical properties. Electron microscopy relates stability of the core-only NP morphology up to 555 K and 600 K for 1 ACS Paragon Plus Environment

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the 4 and 5 monolayer NPs, respectively, followed by sintering and evaporation at still higher temperatures. Reversible PL loss, based on differences in decay dynamics between time-resolved photoluminescence and transient absorption, results primarily from hole trapping in both NPs and sandwich NPs.

Table of Contents Graphic

Quantum-confined, colloidally prepared semiconductor nanomaterials offer sizetunable bandgaps, high photoluminescence (PL) quantum yields, and scalable synthesis.12

Spherically shaped quantum dots (QDs), owing to discrete electronic density of states and

≤5% dispersion in ensemble particle size distributions for some compositions, have found commercial application in displays owing to narrow PL spectral width, robustness, and high brightness,3-4 and offer a broad range of prospective applications including solar cells, lasers, single-photon sources, and solid-state lighting.5-9 Exhibiting still narrower ensemble emission linewidths than presently attainable QD ensembles, semiconductor nanoplatelets (NPs) present negligible dispersion in the confinement direction owing to essentially perfect synthetic control over number of crystal monolayers (MLs).10-11 With demonstrated tunability from NP thickness, compositional alloying, and ad-growth of other 2 ACS Paragon Plus Environment

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semiconductors,10-19 these nanomaterials offer advances over colloidal QDs for a broad range of technologies. Due to large surface to volume ratios, nanomaterials suffer reduced thermodynamic stability relative to the bulk phase,20-22 and at the same time, proximal interfaces can degrade optical properties owing to carrier trapping.23-24 For applications that involve elevated operating temperature, including high brightness light-emitting diodes (LEDs),2531

concentrated solar cells,32-33 and lasers,3, 6, 19, 34-35 such thermal stability characteristics

present an inherent, ongoing challenge. PL quenching is known to occur at elevated temperature for various examined QD compositions,36-46 which relates deterioration of optoelectronic performance more broadly under such conditions. Our prior examinations of CdSe and InP QDs, respectively, point to electron and hole trapping as culprits for PL quenching with temperature elevation,40, 42 while replacement of native organic synthetic ligands with a wide bandgap shell (such as ZnS) or inorganic capping ligands (such as S2-) has led to improved PL retention, relating that surface details certainly impact performance in this regime. Here, we investigate the optoelectronic and morphological stability of NPs at elevated temperature with an eye toward whether the increased total surface area per particle relative to spherical QDs, faceting, thickness, and core/shell “sandwich” motif12, 47 of NPs impacts optical behavior with temperature via in-situ characterizations. While differences in morphology might not be expected to result in changes to temperature-dependent optical properties like electron or hole trapping per se, differences in thermodynamic stability can affect the temperature to which synthesized geometries retain their structural integrity and, by extension, the photoluminescent properties for which they are designed. Further, the larger surface area per particle presented by a NP

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relative to QD, can potentially offer an increased probability for carrier trapping in the former. We relate below that NPs behave well over an intermediate temperature range of relevance to LEDs with regard to PL intensity and narrow spectral linewidth, but are more susceptible to the effects of still higher temperature in comparison to spherical CdSe QDs. Specifically, upon going from 300 to 400 K, PL intensity decreases by ~8 to 10x and exhibits no distinct dependence on the thickness of the NPs or presence of a CdS shell over this range, and emission linewidth remains narrow. Reversibility of PL quenching for this temperature range upon returning to ambient was complete. PL drops rapidly with further temperature elevation, to undetectable levels by ~450-500 K in the thinnest NPs, at temperatures lower than those reached by CdSe QDs, with a slowed loss for thicker NPs. Of the NPs examined, the CdS/CdSe/CdS shell/core/shell sandwiches exhibited detectable emission to the highest temperatures. Using a combination of transient absorption (TA) spectroscopy and time-resolved PL (trPL), we identify hole trapping as the primary origin of PL quenching, in similarity to CdSe QDs. Temperature dependent transmission electron microscopy (TEM) images directly show the core-only NP morphology remains intact to near 560 or 630K for 4 and 5 ML particles respectively, but deteriorates above this, which contributes heavily to irreversible loss of NP-like optical properties as confinement degrades via sintering. CdSe NPs were synthesized in octadecene from Cd(Ac)2 with TOPSe and Cd(myristate)2 with Se powder using previously described methods.48 CdSe NPs with a CdS layer sandwiching the CdSe core on either side were synthesized by beginning with a CdSe NP core, functionalizing the surface with S2-, and adding Cd(Ac)2 to complete the layer.48 Synthetic details are available in the Supporting Information. Time-integrated PL


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spectra were collected from drop-cast films mounted in an evacuated optical oven and excited by a 35 ps pulse-width, 405 nm diode laser operated between 100 kHz and 40 MHz. PL from the sample was focused into a 0.3 m grating spectrograph and thermoelectrically cooled CCD. Indicated PL intensities were ascertained either by integrating the spectral profiles, or by tracking the spectrally resolved PL maximum; both methods yielded similar findings. trPL data were produced by frequency doubling the output of a 35 fs, 2 kHz amplified titanium:sapphire laser to produce 400 nm photons that excited film samples under vacuum. trPL from the samples were directed to a 0.15 m grating spectrograph coupled to a single-photon sensitive streak camera, to generate both temporally- and spectrally-resolved data. Integrating the PL over the spectral region of interest yielded time-resolved PL decay traces, which were fit with exponential functions to determine decay time constants. To ensure that presented dynamics relate single exciton phenomena, and not multiexcitons processes, the data were collected in a regime where increasing or decreasing the laser power threefold did not influence the dynamics; the same principle was applied to collecting TA spectra and dynamics. TA spectroscopy was performed using 400 nm pump pulses at 1 kHz and a time-delayed white light probe at 2 kHz, produced by focusing 800 nm pulses into a sapphire plate. Reported decay times represent the time elapsed between excitation and when the bleach intensity reached 1/e of initial bleach amplitude. These values were also compared to exponential fits, and both analyses yielded similar results. Figure 1a shows PL spectra for a 4 ML thick CdSe NP drop cast film examined under vacuum as a function of increasing temperature. The PL spectrum steadily red-shifts with increasing temperature and decreases in intensity. Similar measurements, spectrally



integrated and summarized in Figure 1b, were performed for 3 and 5 ML CdSe NPs as well as for 3/4/3 and 6/4/6 ML CdS/CdSe/CdS sandwich NPs (see Supporting Information, Figure S1). Decreasing PL intensity with increasing temperature is observed for each sample and is fairly consistent with prior reports for spherical colloidal semiconductor QDs.36-45 Although the change in PL intensity is largely independent of the both the number of MLs of CdSe and the presence or absence of CdS shell layers for lower temperatures, the temperature to which PL persists at detectable levels appears dependent on both. The 3 ML CdSe sample yields undetectable PL above 450 K, the 4 ML sample emits 1000-fold weaker by ~525 K, and the 5 ML sample reaches this level by 550 K, with the 3/4/3 and 6/4/6 ML samples following a similar trend to the 5 ML.


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Figure 1. (a) Static PL spectra of 4 ML NPs show a gradual decline in PL intensity with increasing temperature, using a logarithmic y-axis in the inset. (b) Integrated static PL spectra for 3, 4, and 5 ML NPs as well as 3/4/3 and 6/4/6 ML sandwich NPs show largely size-independent loss of PL up to 450 K. Beyond this temperature, PL becomes undetectable in the 3 ML NPs and, shortly thereafter, in the 4 ML NPs. The 5 ML NPs and sandwiches, however, persist to higher temperatures, with little difference in PL intensity among those three samples. 7 ACS Paragon Plus Environment


To discern whether physical (reversible) or chemical (irreversible) processes lead to degraded PL, we subject 4 ML, 3/4/3 ML, and 6/4/6 ML samples to cyclical heating measurements. Here, the same sample is measured at 300 K, then heated in steps and measured following 20 minutes of equilibration, before returning to room temperature at which point reversible vs irreversible PL degradation is noted. Data from these cyclical experiments are shown in Figure 2. In terms of PL, the three samples follow a similar trend until heated to 650 K, a temperature at which a discrepancy between the core and core/shell sandwich samples is observed. PL recovery once these samples have been returned to 300 K is markedly improved for the core/shell samples in comparison to the 4 ML NPs. Regarding structural integrity, we point to two optical metrics. Because NPs are quantum-confined in only one dimension, any change in that dimension, as would almost certainly occur in the event of structural reorganization (whether achieving a spheroid or sintered solid), should result in a change of emission wavelength at a given temperature owing to a change in confinement. We do observe a red-shift in PL with increasing temperature, consistent with lattice expansion that is reversible for lower temperatures.49 Upon return to 300 K, the detected emission wavelength is unchanged until the samples have been heated to above 650 K, at which point all three samples exhibit irreversible redshifts of about 20 nm and emit with greatly reduced efficiency even when returned to 300 K. Likewise, the PL full width at half maximum (FWHM), shown in Figures 2e and 2f (a plot with linear units of energy appears as Figure S2), can facilitate understanding of changes in the size distribution of the remaining fraction of emitting ensemble. While PL


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in NPs relates specifically to the number of monolayers, it is unlikely that emission could substantively broaden while retaining the same PL energy. In line with this expectation, we demonstrate that the FWHM at 300 K, following cyclical heating, remains constant. As with the emission wavelength, this holds true, at least in part, until the samples have been annealed at >650 K, at which point all three samples exhibit marked broadening. Both of these indices point to significant physical changes in the NPs, both cores and core/shell sandwiches, at temperatures in excess of 650 K.



Figure 2. Temperature cycling. (a) Comparing reversibility in PL loss between the 4 ML cores and 4 ML sandwiches and reveals greater integrity in the sandwiches at elevated temperature, particularly in recovery after heating to 650 K. When cycling temperature, a vertical black bar with label indicates the sample temperature attained prior to reverting temperature back to 300 K; the data point to the right of the vertical bar relates emission properties observed upon returning to 300 K. (b) The 6/4/6 ML sandwich sample exhibits the most consistent PL recovery upon return to 300 K . (c and d) Emission wavelength 10 ACS Paragon Plus Environment

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increases/red-shifts with increasing temperature, and returns to the initial emission wavelength upon cooling after annealing at temperatures as high as 650 K. (e and f) The full width at half maximum of emission spectra increases with increasing temperature but returns to the original values upon cooling, even after annealing at temperatures as high as 650 K.

Next we investigate stability of the 2D morphology at elevated temperature via insitu transmission electron microscopy (TEM). Whereas PL energy and PL linewidth necessarily indicate the presence of platelet structures up to at least 500 K, we aim to discern whether coalescence of NPs into lower energy structures such as spheres occurs, or whether sintering decreases surface energy principally with exposure to elevated temperature. Figure 3 shows a series of images at indicated temperatures for 4 ML NPs. Between 298 and 541 K, , individual NP structures, here principally as side-on views of NP stacks, appear morphologically unchanged. At 557 K, sintering (preferentially cofacial) appears to dominate an observable change in morphology, though some individual NPs persist; spheres are not observed. Still higher temperatures, such as 627 K, induce formation of CdSe droplets that also possibly volatilize, commensurate with loss of the NP structure. We note that presented images image proximal, but different areas of the same grid owing to noticeable electron-beam-induced damage at these elevated temperatures. The significant change in morphology with temperature elevation is consistent with irreversible losses of PL intensity, whereas persistence of PL linewidth relates emission from the greatly reduced number of 2D-confined NP structures for intermediate temperatures. Additional in-situ TEM studies of other samples are shown in Figures S3-



S6, which show that thicker NPs persist to higher temperature and the core/shell motif yields still higher stability with temperature elevation.

Figure 3. Temperature dependent transmission electron microscopy images, here for 4 ML CdSe NPs at subsequently increasing (indicated) temperatures, show intact side-on NP stacks at 298 and 541 K that begin to heavily sinter by 557 K, and form droplets that also volatilize by 627 K. Sintering occurs near 634 K for 5 ML NPs (see SI). Different but proximal regions of NPs are shown for each temperature owing to beam damage upon imaging which degraded examined regions.


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In order to compare PL losses in the core-only NPs and core/shell sandwiches to other compositions that can achieve substantial (tens of percent) photoluminescence quantum yields for ambient conditions,10, 12, 50-55 we refer to our earlier, related works on CdSe QD cores, CdSe/ZnS core/shells, InP cores, InP/ZnS core/shells, and Si cores.40-42 As related in Figure 4, the NPs, with or without CdS shell layers, exhibit similar or slightly greater robustness to temperature elevation than CdSe QD cores with organic ligands, but display substantially more degradation than core/shell CdSe/ZnS and InP/ZnS spheroidal QDs. Si QDs, with covalently bound ligands, exhibit robustness similar to core/shell materials and likewise outperform the NPs in retaining PL intensity. We suggest that this inferior resilience of NPs and sandwich NPs to temperature elevation could arise from electron-hole pair delocalization within the NP structure that offers exposure of carriers to a larger number of surface sites per particle in comparison to the 0D QD structure. As such, NPs as well as the sandwich structures, perform more similarly to CdSe QD cores in the low temperature (300-450 K) with regard to PL intensity, yet offer narrower PL linewidth.



Figure 4. Normalized integrated emission intensity is shown for several quantum-confined materials, including NPs discussed in this manuscript as well as CdSe, InP, and Si QD data drawn from other sources.41-43

Figure 5 shows in-situ, low-pump-fluence dynamics of carriers in NPs as a function of elevated temperature. For CdSe, TA primarily probes photo-excited electrons owing to disparate carrier masses, while time-resolved PL conveys dynamics of electron-hole pairs (see also Supporting Information, Figures S7 and S8). A TA map, shown in Figure 5a, measured for low-fluence excitation of a drop cast film of 3/4/3 NP sandwiches, shows a strong band-edge bleach near 640 nm. These data reveal electrons present in an excited state for fairly extended periods, particularly when compared with the trPL dynamics shown in Figure 5b. The TA bleach decay dynamics, tracking the bleach maximum, changes relatively little with increasing temperature (Figure 5c), presenting long, nanosecond lifetimes, whereas PL decays (Figure 5d) become progressively faster at higher temperature in the sub-nanosecond regime. (Supporting Information, Figure S9). Similar findings are shown in Figures 5e and 5f for 6/4/6 and 4 ML samples. The discrepancy between the timescale of radiative recombination and relaxation of the electron from the excited state clearly points to thermally activated hole trapping as the origin of reversible PL loss in this system, a portion of which we fail to resolve in Figure 5d. Temperature-induced perturbation of photo-excited electron dynamics do show some indication of systematic change across the measured samples. The 4 ML NP sample exhibits the greatest percentage decrease in TA decay time constant with increasing temperature, while the 6/4/6 sandwich sample shows the least change, with the 3/4/3


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sandwich sample falling in-between. That the electron dynamics change with heating is indicative of at least some electron trapping taking place in addition to clear hole trapping as pointed to via discrepancy between TA and PL dynamics. Addition of CdS shells appears to mitigate the occurrence of that electron trapping, with a thicker shell seeming to provide more protection against electron trapping than the thinner shell.



Figure 5. (a) TA data from the a 3/4/3 ML sandwich shows strong band-edge bleach features near 640 and 590 nm. (b) trPL data for the 3/4/3 ML sandwich shows PL that decays much more rapidly than the 640-nm, band edge bleach in TA. (c) Decay traces at the band-edge bleach for the 3/4/3 sandwich sample as a function of temperature. (d) trPL dynamics show both rapid decay of PL and decreasing initial PL intensity with increasing temperature. (e) Fitted decay times (1/e times) of indicated samples quantify the decrease in the lifetime of the bleach feature with increasing sample temperature. (f) Decay constants from exponential fits quantify the decrease in radiative recombination with increasing sample temperature. Nanoplatelets maintain structural morphology for a large range of temperatures of relevance to applications, and the core/shell motif further bolsters such. Whereas the lower ratio of surface atoms to bulk in comparison to a spherical particle adds mechanical stability, the increased total surface area per particle leads to carrier trapping, seemingly whether that interface is inorganic-organic or inorganic-inorganic in the explored compositions. The core-ligand interface offers more efficient carrier trapping and irreversible degradation likely because the core-shell interface provides a barrier to the hole sampling the inorganic-organic interface. Degradation and decomposition of organic ligands, in particular, can yield efficient trap sites that the ligands, by design, normally passivate. We have shown that the NP morphology retains reasonable optoelectronic performance that is overall similar to thermally-driven PL quenching observed for other previously-examined semiconductor nanomaterials. As in the case of CdSe QDs, we identify the mechanism of quenching, which predominantly arises from hole trapping. NP


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thickness correlates with the temperature of total PL loss, where the 3 ML NPs, the thinnest studied, cease to emit approximately 100 K lower than the 5 ML NPs and the addition of a shell to form CdS/CdSe/CdS sandwiches improves PL reversibility with thermal cycling. Remarkably, the narrow photoluminescence linewidths of this material class are preserved through a large temperature range, only exceeding ther thermal limit by a factor of 1.5 to 3, which bolsters use in optical amplifiers, high color purity displays, and optically-based sensors. Further manipulations of NP interfacial regions can potentially improve PL efficiency with increasing temperature.

Associated Content Supporting Information Discussion of nanoparticle synthesis; emission maximum vs temperature, Figure S1; photoluminescence full width at half maximum vs temperature in comparison to thermal linewidth, Figure S2; temperature dependent transmission electron microscopy of other particle types, Figures S3, S4, S6; high resolution transmission electron microscopy of a CdS/CdSe/CdS nanoplatelet, Figure S5; transient absorption vs temperature of other particle types, Figure S7; time-resolve photoluminescence vs temperature of other particle types, Figure S8; position of transient absorption bleach maximum vs temperature of multiple particle types, Figure S9. (PDF)

Notes * [email protected], [email protected] The authors declare no competing financial interests.



Acknowledgement This work was performed, in part, at the Center for Nanoscale Materials, a U.S. Department of Energy Office of Science User Facility, and supported by the U.S. Department of Energy, Office of Science, under Contract No. DE-AC02-06CH11357. We acknowledge support from the NSF DMREF Program under awards DMR-1629383 and DMR-1629601.

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