Exciton Fate in Semiconductor Nanocrystals at Elevated Temperatures

Article pubs.acs.org/JPCC

Exciton Fate in Semiconductor Nanocrystals at Elevated Temperatures: Hole Trapping Outcompetes Exciton Deactivation Clare E. Rowland† and Richard D. Schaller*,†,‡ †

Department of Chemistry, Northwestern University, 2145 Sheridan Rd., Evanston, Illinois 60208, United States Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois 60439, United States

‡

S Supporting Information *

ABSTRACT: The tens-of-percent photoluminescence (PL) quantum yields routinely obtained for colloidally prepared CdSe semiconductor nanocrystals (NCs) decrease substantially with temperature elevation. While such PL efficiency loss has direct consequences for applications ranging from lightemitting diodes and lasers to photovoltaics under solar concentration, the origin of this loss is currently not established, hindering synthetic efforts to design materials with robust performance. Here, for the first time, we utilize transient absorption and ultrafast PL in addition to static PL and time-correlated single photon counting, to characterize CdSe core-only and CdSe/ZnS core/shell NCs up to temperatures as high as 800 K. For multiple particle sizes, loss of PL efficiency as a function of temperature elevation is more severe and less reversible for core-only NCs than for core/shell NCs. Ultrafast measurements performed at elevated sample temperatures indicate that thermally activated trapping of individual carriers dominates the nonradiative loss of excitons. Through a combination of spectroscopic techniques, we identify the primary carrier loss process as hole trapping in particular. These findings support the notion that extrinsic trapping effects out-compete intrinsic exciton deactivation at high temperature and point to realizable improvements in thermally robust optoelectronic performance.

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INTRODUCTION Currently, much research aims to exploit the properties of colloidal quantum-confined semiconductor nanocrystals (NCs) in applications that range from light-emitting diodes (LEDs),1 solar cells,2 and lasers3 to biolabels,4 electronics,5 and various physical6 and chemical sensors.7 While size-tunable energy gaps, high photoluminescence (PL) quantum yields (QYs), and low-cost solution processing draw interest for many applications, the large surface-to-volume ratios of these materials impart inherently reduced thermodynamic and electronic stability in comparison to the bulk phase,8 properties that are particularly problematic for applications which experience temperature elevation such as high-current high-brightness LEDs,9 photovoltaics under solar concentration,10 and lasers.11 Furthermore, irreversible damage to NC PL upon exposure to elevated temperatures restricts device processing and material deposition to techniques that utilize only lower temperatures.12−14 Advances in comprehension concerning the material properties and electronic processes that dictate electron−hole pair (exciton) fate in NCs at elevated temperature are essential to the operation and longevity of such devices, the improved design of photoactive materials, and the determination of inherent performance limits. Much is known regarding the optoelectronic properties in CdSe NCs at both ambient and cryogenic temperatures,15−20 whereas examinations of NC photophysical properties at elevated temperature have been less common.21−27 Briefly, © 2013 American Chemical Society

excitons in CdSe NCs radiate from a thermally populated bright exciton state with a ∼20−25 ns lifetime at ambient temperatures, while near-microsecond lifetimes appear at temperatures below 10 K due to population of a lower-energy, weakly emitting, dipole forbidden dark-exciton state.17−20 PL energy as a function of temperature is well-described by the Varshni relation with a red-shift of emission for increasing temperature that occurs as phonons populate the lattice.18,22,25−27 A number of intrinsic and extrinsic effects can potentially lead to the loss of electron−hole pairs in NCs at elevated temperatures. For instance, possible intrinsic nonradiative processes (which take place within the quantum-confined electronic density of states) include interband relaxation via multiphonon generation, as well as thermally activated crossover between ground-state and excited-state potential energy surfaces. However, neither of these excited-state deactivation processes is expected to occur efficiently based upon low phonon energies (requiring the generation of large numbers of phonons) and small excited-state nuclear displacements (which suggest that curve crossings only occur at temperatures corresponding to >1000 K).28,29 Conversely, extrinsic quenching mechanisms such as thermally activated Received: June 6, 2013 Revised: July 22, 2013 Published: July 23, 2013 17337

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Figure 1. Static and time-resolved PL of CdSe core-only NCs (a and b) and CdSe/ZnS core/shell NCs (c and d) display diminished static PL and excited-state lifetimes with increasing temperature.

absorption (TA), and ultrafast PL (uPL) to characterize these materials at temperatures from 300 to 800 K. Our measurements show that loss of PL efficiency for core-only NCs as a function of temperature elevation takes place more completely and irreversibly than for core/shell NCs. For the first time, we report elevated temperature TA and uPL measurements, which strongly suggest that thermally activated hole trapping constitutes the nonradiative loss of excitons in this material, dominating over rates of electron trapping or exciton deactivation. Overall, these findings indicate that extrinsic effects, rather than intrinsic effects, dominate exciton loss and point to realizable improvements in high temperature NC optoelectronic performance.

carrier ejection to trap states (ionization) or simultaneous electron−hole pair ejection (termed thermal escape) may dominate loss of exciton populations.18,30,31 Importantly, extrinsic quenching mechanisms likely depend upon material quality and interfacial properties that can be manipulated synthetically. In addition to these electronic processes, particle structure and composition details can change upon heating. For instance, ligand binding distributions, ligand decomposition, surface reconstructions, particle sintering, and Ostwald ripening can impact the reversibility or irreversibility of NC optoelectronic properties with temperature elevation.8 Several reports have examined the optical response of CdSe NCs as a function of temperature elevation. While most of these studies investigate temperatures below 400 K,21,22,27 even in this regime increases in temperature produce decreases in static PL intensity. Investigations of cyclical heating effects up to 353 K have also been reported, the results of which point to some recovery of lost PL upon cooling a sample back to ambient temperature,21−23 and a decline in quantum yield at ambient temperature that tracks with annealing temperature indicates permanent material damage with exposure to higher temperatures.24 Despite these efforts, to date, studies of NCs as a function of size and for the widely utilized core/shell arrangement, CdSe/ZnS, have not been reported, nor has the mechanism of PL quenching been identified. Here, we examine carrier dynamics as a function of temperature for multiple sizes of CdSe and CdSe/ZnS NCs. We utilize static PL, time-resolved PL (trPL), ultrafast transient

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EXPERIMENTAL SECTION Octadecylamine-capped CdSe and CdSe/ZnS NCs, labeled in figures according to the lowest-energy absorption peak observed at ambient temperature, were drop cast from toluene onto glass slides and mounted in an evacuated optical oven (see the Supporting Information for further NC characterization). For time-integrated PL and trPL measured using timecorrelated single photon counting (TCSPC), samples were photoexcited via a 35-ps pulsewidth, 405 nm diode laser operated between 2.5 and 40 MHz. PL was collected with a quartz lens and coupled into a fiber optic that routed photons to a 300 mm grating spectrograph. Static PL spectra were collected using a thermoelectrically cooled CCD. Integrated PL intensity was determined by fitting static PL spectra with a 17338

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Gaussian function and integrating the area. This routine was important for spectra where trap emission overlapped with the band-edge emission signal. TCSPC was recorded at the temperature-dependent PL spectrum maximum using an avalanche photodiode. In this work, we report excited-state lifetimes simply by the time required for the initial PL amplitude to reach an amplitude of 1/e, which emphasizes nonradiative processes. uPL and TA measurements were performed using a 35-fs amplified titanium:sapphire laser operating at 2 kHz. uPL was measured by photoexciting the sample with 35 fs, 3.1 eV photons. Emitted photons were collected with a lens and directed to a 150 mm spectrograph and single-photon sensitive streak camera. For TA measurements, a portion of the laser output at 1.55 eV was mechanically time-delayed and focused into a sapphire plate to produce a white light. Pump pulses at 3.1 eV were mechanically chopped to 1 kHz, overlapped with the probe pulse on the sample, and adjusted to an intensity corresponding to less than 1 exciton per NC on average. In one-way heating experiments, the oven temperature was increased incrementally from 300 to 800 K. At each measurement temperature, the sample was equilibrated for 10 min before optical data were collected. In cyclical heating experiments, the temperature was similarly ramped and equilibrated, however, here the temperature was returned to 300 K each time a new high temperature was reached.

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RESULTS AND DISCUSSION Figure 1 shows static PL spectra and TCSPC-based trPL dynamics for both a core-only and a core/shell NC sample at several temperatures. Upon increasing the temperature from 300 K, static PL for both samples red-shifts (see also the Supporting Information, Figure S1) and emission intensity decreases. Here, measurements of the core-only sample do not appear above 500 K owing to loss of detectable PL photons, while measurable emission persists to temperatures as high as 800 K for the core−shell sample. trPL dynamics show that excited-state lifetimes decrease with increasing temperature for each sample. Taken together with the notable losses in PL intensity, the lifetime decreases with temperature signify increases in the rates of nonradiative processes. In Figure 2, we quantify integrated PL intensity and excitedstate lifetimes for three core sizes of core-only and core/shell NCs. For each core size, core/shell NCs retain higher integrated PL intensity for a given temperature than coreonly NCs. For instance, while the PL intensity of a core-only NC typically drops to ∼1% of the initial ambient-temperature magnitude by 450 K, core/shell NC samples studied here decrease to the same percentage only upon reaching ∼700 K. No distinct size dependence appears for the measured coreonly samples with temperature elevation, while the core/shell NCs exhibit a weak increase of stability with increasing PL wavelength. At ambient temperature, core-only samples (Figure 2b) exhibit shorter excited-state lifetimes than core/shell samples commensurate with lower emission quantum yield. Upon increasing temperature, both measured PL intensities and lifetimes decrease to a greater extent in the core-only samples. Decreases in PL intensity and excited-state lifetime with temperature can result from either irreversible or reversible effects. In one-way temperature ramping experiments, however, irreversible effects, which may include loss of surface ligands, particle sintering, and Ostwald ripening, remain indistinguishable from reversible effects, which can include thermally

Figure 2. (a) Integrated static PL for core-only (CdSe) and core/shell (CdSe/ZnS) NCs decreases with increasing temperature, but core/ shell NCs lose PL intensity less quickly and persist to higher temperatures than comparable core-only NCs. (b) With increased temperature, excited-state lifetimes decrease; this effect is more prominent in core-only NCs than in comparable core/shell NCs.

activated carrier or exciton ejection, intrinsic exciton deactivation, and potential population transfer into higherenergy exciton fine structure states of uncharacterized oscillator strength.15,16 Moreover, physical redistribution of NCs at high temperature and energy transfer within the films may play a role in PL quenching, particularly when coupled with irreversible effects that provide nonradiative decay paths. Thus, next we utilize a previously invoked cyclical approach to sample heating,21−23 where we return the sample to near ambient temperature, characterize the optical properties, and subsequently initiate another round of heating. Cyclical heating measurements reveal reversible processes, through recovery of static PL intensity upon cooling, and also expose irreversible changes, apparent because of incomplete recovery with regard to intensity, spectral position, and spectral shape. Such data for a core-only and core/shell sample with similar initial emission wavelength (near 640 nm), shown in Figure 3, reveal marked differences. The core-only NC sample begins to exhibit an irreversible loss of ambient-temperature PL at temperatures of just 375 K; by contrast, the core/shell sample shows near-complete PL intensity recovery for temperatures up to 600 K. Thus, not only does PL from core/shell NCs respond less to the effects of elevated temperature and persist to higher absolute temperature, but it also appears more reversible than similar core-only NCs. This reversibility is critical in high-temperature processing like that common in material deposition or other device fabrication 17339

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Figure 3. Integrated static PL and PL center wavelength are shown here for CdSe core-only (a and b) and CdSe/ZnS core/shell (c and d) NCs subjected to cyclical heating. Data are plotted as a function of the measurement temperature, and symbols and colors correspond to the maximum temperature to which NCs were raised in the previous heating cycle. Core/shell NCs recover more resiliently after heating and persist to higher temperature than do core-only NCs. Both samples exhibit a linear red-shift in maximum emission wavelength with increased temperature that appears highly reversible.

extrinsic processes might contribute to loss of emitted photons, as hole trapping, electron trapping, and exciton deactivation will all contribute equally to changes in the PL dynamics. Conversely, TA bleaching dynamics in CdSe NCs predominantly exhibit sensitivity to the excited electron, since closely spaced hole states reduce contributions to absorption-based observation.33 Thus, TA provides a means of ascertaining (or precluding) the involvement of electron loss in PL quenching. When PL dynamics change in the absence of a similar change in TA dynamics, quenching can be deduced to arise from hole trapping because TA provides evidence of the continued presence of the electron (which would be absent in the event of either electron trapping or exciton deactivation). uPL and TA measurements for a core-only and core/shell NC sample measured at indicated temperatures are shown in Figure 4 and reveal that PL decay becomes faster with increasing temperature, consistent with the TCSPC data shown above. Moreover, the initial instantaneous PL intensity decreases with increasing temperature despite the high time resolution (here ∼10 ps). This change with temperature elevation suggests either that an unresolved, very fast (few picoseconds to subpicosecond) process yields the reduced intensity or that a portion of the NC ensemble ceases to emit. By contrast, TA data shows relatively small differences in the temperature regime explored. In particular, initial TA signals do not decrease by more than a factor of 2 for core-only or core/

steps. Figure 3 also shows that the peak emission wavelength for both samples appears highly reversible despite any intensity changes. We note that the irreversible loss of PL for the core-only NCs corresponds closely with thermo-gravimetric analysis (TGA) measurements performed on NCs capped with the same organic ligands, suggesting that decomposition and loss of ligands leads to reduced passivation of surface states in coreonly NCs.32 By contrast, the loss of reversibility in core/shell NCs at ∼700 K - well above the temperature at which TGA indicates total ligand loss - coincides with loss of core/shell particle integrity, according to previous small-angle X-ray scattering measurements.32 The highly reversible shifts in PL energy and line width (Supporting Information, Figure S2), despite intensity losses, suggest that any sintering, Ostwald ripening, or chemical compositional changes that might occur yields particles that fail to radiate efficiently. Additionally, powder X-ray diffraction patterns and Raman spectra of a coreonly sample taken before and after heating to 750 K show negligible physical and compositional changes (Supporting Information, Figure S3). In order to identify the carrier loss mechanism that leads to reduced PL intensity at high temperature, we performed a combination of uPL and TA measurements. Because PL only probes NCs that contain both an electron and a hole, PL measurements alone fail to discern which of the intrinsic and 17340

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Figure 4. uPL measurements for 600 nm CdSe core-only (a) and CdSe/ZnS core/shell (c) NCs exhibit faster decay and decreased initial PL intensity with increasing temperature. Early time normalized TA data (raw data shown in Supporting Information, Figure S4) measured at the position of the lowest-energy 1S absorption bleaching feature for core-only (b) and core/shell (d) NCs, in contrast to the uPL data, show little variation with temperature to 500 and 700 K for the core-only and core/shell NC samples, respectively.

shell NC samples (see the Supporting Information, Figure S4). Furthermore, the decay dynamics appear to change very little as the temperature rises. At sufficiently high temperature (700 K), we do note that the core-only sample exhibits more rapid electron loss dynamics. In combination, the large differences between uPL and TA data suggest that hole trapping, an extrinsic process, dominates temperature-dependent exciton loss for both core-only and core/shell particles.33 These observations also indicate that intrinsic exciton loss processes as well as extrinsic thermal escape of electron−hole pairs do not occur with high efficiency for the temperature range examined, since these effects would also yield losses in TA bleach amplitude. Using the Arrhenius relation, knr = Ae−Ea/(kT), we further characterize the identified hole trapping process. Using the expression QY = kr/(kr + knr) we first calculate nonradiative rates, knr, obtained from integrated static PL intensities as functions of temperature (Figure 5). This relation tacitly assumes that kr does not change substantially for the temperature range studied,34,35 an assumption supported by nonradiative rates derived directly from uPL data which are, even at high temperature, in good agreement with the calculated nonradiative rates (see Figure 5). In fact, this correspondence between measured and calculated knr values indicates that any population of higher-lying electronic fine structure states does not substantially alter the radiative lifetime

Figure 5. Arrhenius plots of nonradiative rates calculated from quantum yield gives activation energies and attempt frequencies for CdSe and CdSe/ZnS NCs with indicated ambient-temperature emission wavelengths. Directly measured nonradiative rates determined from uPL measurements coarsely agree with the calculated nonradiative rates suggesting that the high temperature radiative lifetime deviates little from 20 ns.

from ∼20 ns (within a factor of ∼3). Moreover, a lack of trion decay signatures in TCSPC (700−1500 ns time scale) indicates a lack of static quenching.36 Values for activation energies, Ea, and prefactors, A, are summarized in Table 1. Considering 17341

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Foundation Graduate Research Fellowship under Grant No. DGE-0824162.

Table 1. Activation Energies and Attempt Frequencies from Arrhenius Fits of Data Collected between 300 and 375 K sample 550 coreonly 600 coreonly 640 coreonly 550 core/ shell 600 core/ shell 640 core/ shell

activation energy, Ea (meV)

ln (attempt frequency)

attempt frequency, A (s−1)

286(48)

29(2)

6.24 × 1012

224(38)

29(1)

5.05 × 1012

257(38)

29(1)

5.44 × 1012

194(21)

26(1)

1.50 × 1011

94(14)

21(1)

1.41 × 109

107(11)

21(1)

2.17 × 109

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nonradiative rates between 300 to 375 K, we obtain activation energies, Ea, that vary by a factor of 3 or less for core-only and core/shell NCs and are counterintuitively larger for core-only NCs than those obtained for core/shell NCs. Prefactors (attempt frequencies), A, exhibit quite large differences of 1012 and 109 s−1 for core-only and core/shell NCs, respectively. Such differences likely reflect the proximity of hole traps to holes in the NC core, whereas the ZnS shell layer in core/shell NCs increases the separation of holes from traps at the ZnSorganic interface.

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CONCLUSIONS In conclusion, we have characterized the PL intensity and carrier dynamics in core-only and core/shell CdSe as a function of elevated temperature. Core-only NCs lose PL more readily and more irreversibly with increasing temperature than similar core/shell NCs. This trend is also reflected in increasing nonradiative rates and attempt frequencies with temperature. Elevated temperature TA and uPL measurements suggest that the dominant nonradiative process contributing to exciton loss is extrinsic and involves hole trapping. That extrinsic carrier trapping out-competes intrinsic exciton deactivation opens the door for realizable improvements in high temperature NC optoelectronic performance and improved robustness to elevated temperature processing.

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ASSOCIATED CONTENT

S Supporting Information *

Emission wavelength and fwhm as a function of temperature; physical characterization including Raman, PXRD, and TEM. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], [email protected]. Notes

The authors declare no competing financial interests.

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ACKNOWLEDGMENTS Use of the Center for Nanoscale Materials was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC0206CH11357. We thank Dr. Yuzi Liu for assistance with TEM. C.E.R. acknowledges support by the National Science 17342

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