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Stable, Ultralow Threshold Amplified Spontaneous Emission from CsPbBr3 Nanoparticles Exhibiting Trion Gain Yi Wang, Min Zhi, Yu Qiang Chang, Jian-Ping Zhang, and Yinthai Chan Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b01817 • Publication Date (Web): 16 Jul 2018 Downloaded from http://pubs.acs.org on July 16, 2018
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Nano Letters
Stable, Ultralow Threshold Amplified Spontaneous Emission from CsPbBr3 Nanoparticles Exhibiting Trion Gain Yi Wang,† Min Zhi,† Yu-Qiang Chang‡ Jian-Ping Zhang‡ and Yinthai Chan*,† †Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore ‡Department of Chemistry, Renmin University of China, 59 Zhongguancun Street, Beijing 100872, China
ABSTRACT: Wet-chemically synthesized cesium lead halide nanoparticles have many attractive properties that make them promising as optical gain media, but generally suffer from poor stability under ambient conditions and an optical gain threshold that is widely believed to be dictated by the need for biexcitons. These conditions make it impractical for such particles to be utilized as gain media given the need to undergo repeated stimulated emission processes at above-threshold pump intensities over long periods of time. We demonstrate that the surface treatment of CsPbBr3 nanoparticles with a mixture of PbBr2, oleic acid and oleylamine not only raises their fluorescence quantum yield to nearly unity and prolongs their stability in air from
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days to months, it also dramatically increases their trion photoluminescence lifetime from ~0.9 ns to ~1.6 ns. Via a combination of time-resolved photoluminescence and transient absorption spectroscopy, we provide evidence for trion gain at sufficiently low pump intensities in which the likelihood of predominantly biexciton-based gain is small. We then show that, in line with theoretical prediction, the amplified spontaneous emission (ASE) threshold of a thin film of surface treated CsPbBr3 nanoparticles reduces to a record low of ~ 1.2 µJ/cm2 with a corresponding average exciton occupancy per nanoparticle of 0.62. The ultralow pump threshold and increased stability allows for stable ASE over millions of laser shots, paving the way for the deployment of these nanoparticles as viable solution-processed optical gain media.
KEYWORDS: all-inorganic perovskites, colloidal nanocrystals, optical gain, trion state, amplified spontaneous emission
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Nano Letters
The prospect of wet-chemically synthesized semiconductor nanoparticles or colloidal quantum dots (CQDs) as the mainstay of gain materials in optical microresonators is compelling because of their size- or composition-tunable wavelength of emission over a large spectral range, relatively inexpensive cost of production and flexible surface chemistry that allows them to be deposited onto a myriad of microcavity architectures.1-3 Amongst the different classes of CQDs, all-inorganic cesium lead halide perovskites (CsPbX3, X = Cl, Br, I) are highly promising as materials for optical amplification and lasing due to their intrinsically high photoluminescence quantum yields (PLQY)4,5 that exceed those of their bulk counterparts,6 wavelength tunability via a facile anion-exchange process7-9 and large optical gain (> 450 cm−1).10,11 Subsequently, the amplified spontaneous emission (ASE) pump thresholds of thin films of CsPbX3 CQDs, which better reflects their intrinsic gain properties given the absence of optical feedback, is generally low (< 10 µJ/cm2).10,12,13 These attractive qualities make CsPbX3 CQD based gain media an attractive alternative to the more established II-VI semiconductor CQD systems. The degeneracy of the band edge states in CsPbX3 CQDs is two-fold,14 which likely arises from asymmetry in the cubic crystal structure and/or electron-hole exchange interactions.15 This means that for an ensemble of charge neutral CQDs in which the Poisson average exciton occupancy number per nanoparticle is 〈N〉, the threshold for optical gain is 〈N〉 ∼ 1.15, which implies that a substantial fraction of nanoparticles possess biexcitons (XX) and higher order excitons (multiexcitons). Where 〈N〉 < 1.15, the probability of photon re-absorption exceeds that of stimulated emission, which results in no net optical gain. The presence of XX and multiexcitons in nanoparticles inevitably introduces a fast, non-radiative Auger recombination pathway that stems from strong Coulombic interactions between the charges of different excitons. The XX Auger recombination process, in which the energy from the recombination of
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one exciton is non-radiatively transferred to one of the carriers of the other exciton, typically takes place on a timescale of tens to hundreds of picoseconds in CsPbX3 CQDs.16-18 Higher order multiexcitons have even shorter lifetimes due to faster Auger recombination rates. Given that the achievement of ASE depends on the gain build-up time τS being faster than the Auger dominated gain lifetime τG, the fast bi- and multiexcitonic Auger recombination rates set a relatively high limit on how low the pump fluence needs to be. Moreover, it is well known that as-synthesized CsPbX3 CQDs generally suffer from low stability in air,19 making it difficult for them to repeatedly undergo stimulated emission processes at above-threshold pump intensities over extended periods of time.
Figure 1. Comparison of the mechanism for biexciton gain (left) in neutral CQDs and trion gain (right) in singly charged CQDs with doubly degenerate band edge states. Although the schematic illustrates the case of a positive trion, it is also possible that the trion is negative and comprises two electrons and a hole.
One promising alternative to the photo-generation of XX (or multiexcitons) to achieve ASE is the utilization of trions (X*), which are charged single excitons, for optical gain. While the occurrence of X* in II-VI CQDs generally require the setting up of a charge injection device (i.e. electrical doping)20 or use of a charge scavenger to extract carriers from photoexcited nanoparticles (i.e. photochemical doping),21 previous studies have shown that the formation of
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Nano Letters
X* in CsPbX3 CQDs takes place spontaneously under pulsed excitation at low pump fluence.18,22 Unlike neutral CQDs where optical gain is only achieved via biexciton (or higher order) excited states, the absorption of a single photon by a charged CQD immediately results in a condition of optical gain, as depicted in Figure 1. This reduces the theoretical gain threshold from 〈N〉 ∼ 1.15 (XX gain) to 〈N〉 ∼ 0.58 (X* gain) (see Figure S1 for derivation). Within the framework of statistical scaling, the Auger lifetime of X* is expected to be 4 times longer than that of XX,23-25 which should result in a longer gain lifetime. This implies that the ASE condition τS < τG should be more readily attained via X* than with XX. However, the scarcity of reports on ASE from mainly X* states in CsPbX3 CQDs suggests that it is not straightforward to harness X* for optical gain. Herein, we describe a facile post-synthesis surface treatment process that raises the PLQY of as-synthesized ∼11.9 ± 1.0 nm edge length cube-like CsPbBr3 CQDs from 60% to 95%, increases their stability in air from days to months and lengthens the average PL lifetime of X* in these nanoparticles from ∼0.9 ns to ∼1.6 ns. This results in a significant reduction of the ASE pump threshold from ∼3.8 µJ/cm2 to a record low value of ∼1.2 µJ/cm2. Importantly, the photostability of the ASE, defined as the number of laser excitation pulses needed for the integrated ASE intensity to drop to zero, increases nearly an order of magnitude from 1.3 × 106 to over 1.2 × 107 shots. These achievements pave the way for the deployment of all-inorganic perovskites as viable optical gain media.
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Figure 2. (a) Low resolution TEM image of 11.9 ± 1.0 nm edge length CsPbBr3 CQDs. Inset is a histogram of the particle edge length distribution of the sample. (b) The powder XRD spectrum of a thin film of CsPbBr3 CQDs (orange) and the reference standard for cubic CsPbBr3 (green). (c) Typical UV-vis absorption (dashed line) and PL (solid line) spectra of the CsPbBr3 CQDs dispersed in toluene. Inset shows the second-derivative of absorbance (α″) and PL intensity as a function of energy, which leads to a Stokes shift (∆S) of about 26 meV.
Cube-like CsPbBr3 CQDs with a mean edge length of ∼ 11.9 ± 1.0 nm, as illustrated by the transmission electron microscope image (TEM) and histogram analysis of their edge length distribution in Figure 2a, were synthesized according to a previously reported procedure26 with the exception that the concentration of surfactants employed was doubled to ensure better surface passivation. Given that the average particle edge length is approximately twice the Bohr exciton diameter (∼7 nm) for CsPbBr3, quantum confinement is expected to be weak in these CQDs. The XRD spectrum of the as-synthesized CQDs (Figure 2b) shows a well-defined set of peaks that match well with the reference standard for cubic CsPbBr3, which suggests that the synthesis was carried
out
successfully.
The
corresponding
steady-state
UV-vis
absorption
and
photoluminescence (PL) emission spectra for the CQDs dispersed in toluene are shown in Figure 2c. The band edge absorption profile is generally featureless, which is consistent with weak quantum confinement. The energy of the absorption band edge is inferred from the minimum of
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Nano Letters
the second-derivative of absorbance, as given in the inset of Figure 2c.27 The PL spectrum shows a single peak centred at 514 nm with a full width at half-maximum (FWHM) of ∼100 meV. The Stokes shift (∆S), defined as the energy difference between the absorption band edge (2.440 eV) and the PL peak (2.414 eV), is ∼26 meV which is similar to previous findings.28 The absolute PL quantum yield (PLQY) of as-synthesized CsPbBr3 CQDs, as determined by an integrating sphere, is on average 60 ± 6%. Although such a PLQY value cannot be considered low, it is indicative that the particle surface is either not completely passivated or contains atomic defects, which can result in shortened trion lifetimes.
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Figure 3. (a) The PL emission spectra of equal concentrations of untreated (blue shadow) and PbBr2 treated (red) CsPbBr3 CQDs. Inset is their corresponding UV-vis absorption spectra. (b) The TRPL traces of untreated (black) and PbBr2 treated (red) CsPbBr3 CQDs at an average exciton occupancy 〈N〉
