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Giant PbSe/CdSe/CdSe Quantum Dots: Crystal Structure-limited Ultrastable Near-infrared Photoluminescence from Single Nanocrystals Christina J. Hanson, Nicolai F. Hartmann, Ajay Singh, Xuedan Ma, William J. I. DeBenedetti, Joanna L. Casson, John K. Grey, Yves J. Chabal, Anton V. Malko, Milan Sykora, Andrei Piryatinski, Han Htoon, and Jennifer A. Hollingsworth J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b03705 • Publication Date (Web): 18 Jul 2017 Downloaded from http://pubs.acs.org on July 18, 2017
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Giant PbSe/CdSe/CdSe Quantum Dots: Crystal Structure-limited Ultrastable Near-infrared Photoluminescence from Single Nanocrystals Christina J. Hanson,† Nicolai F. Hartmann,† Ajay Singh,† Xuedan Ma,† William J. I. DeBenedetti,‡ Joanna L. Casson,ʆ John K. Grey,§ Yves J. Chabal,¥ Anton V. Malko,‡ Milan Sykora,†† Andrei Piryatinski,††† Han Htoon,† and Jennifer A. Hollingsworth†* † Materials Physics and Applications Division: Center for Integrated Nanotechnologies, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 ‡Department of Physics, The University of Texas at Dallas, Richardson, Texas, 75080 ʆ Chemistry Division, Los Alamos, NM, 87545 §Department of Chemistry, University of New Mexico, Albuquerque, New Mexico, 87544 ¥ Department of Materials Science and Engineering, The University of Texas at Dallas, Richardson, Texas, 75080 †† Chemistry Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 †††Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 ABSTRACT: Toward a truly photostable PbSe quan-
tum dot (QD) we apply the thick-shell or “giant” QD structural motif to this notoriously environmentally sensitive nanocrystal system. Namely, using a sequential application of two shell-growth techniques—partial-cation exchange and successive ionic layer adsorption and reaction (SILAR)—we are able to overcoat the PbSe QDs with sufficiently thick CdSe shells to impart new single-QD-level photostability as evidenced by suppression of both photobleaching and blinking behavior. We further reveal that the crystal structure of the CdSe shell (cubic zinc-blende or hexagonal wurtzite) plays the key role in determining the photoluminescence properties of these giant QDs, with only cubic nanocrystals sufficiently bright and stable to be observed as single emitters. Moreover, we demonstrate that crystal structure and particle shape (cubic, spherical or tetrapodal) and, thereby, emission properties, can be synthetically tuned by either withholding or including the coordinating ligand, trioctylphosphine, in the SILAR component of the shell-growth process.
INTRODUCTION Lead chalcogenide quantum dots (QDs) are infrared active semiconductor nanocrystals with wideranging potential applications, including field effect transistors,1 photodetectors,2 photovoltaics,3 light emitting diodes,4 and as gain media for realization of amplified spontaneous emission5,6 and lasing.7 In particular, PbSe QD photoluminescence (PL) wavelengths can be tuned from the near-infrared8 to the
mid-infrared (~4000 nm) by simple particle-size tuning.9 The extreme size-dependent spectral tunability (compared, for example, to CdSe QDs) is attributable to PbSe’s large exciton Bohr radius (46 nm).9 Emission in the near-infrared is compatible with common fiber-optics based telecommunication technologies (1300-1550 nm) and overlaps with a second optical transparency window for biological tissue (up to ~1350 nm),10 while mid-infrared sources are targeted for novel lasers needed in medical diagnostics, environmental monitoring and military applications.11 Despite their potential, practical application of PbSe QDs has been stymied by a lack of long-term stability in air. Specifically, PbSe QDs kept in air, at room temperature and, especially, in the presence of light are known to exhibit rapid photooxidative degradation, including both loss in PL intensity and spectral blue-shifting as the effective QD size shrinks with oxidation of surface lead and selenium.12-14 Significant effort has been dedicated to enhancing the stability of PbSe QDs. Approaches have included inorganic shelling, e.g., to form PbSe/CdSe core/shell QDs,15 and inorganic or organic surface treatments. The latter have entailed halide passivation using PbX2 (X = Cl, Br, I) precursors during PbSe synthesis as well as post-synthesis exposure to molecular chloride16 or halide salts,17 or improved organic passivation using phosphonic acid ligands.18 Enhanced stability has been confirmed by following PL and absorption trends as a function of time and storage conditions and by demonstrating improved device performance.18
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Notwithstanding improvements in the environmental stability of PbSe QDs, to date, PL properties have only been investigated for QD ensembles. Single-dot level emission properties have been experimentally inaccessible as a result of rapid photobleaching and the very long PL lifetimes (≥1 µs) that are characteristic of these colloidal nanocrystals.19,20 Even in the case of relatively faster emitting PbS QDs (~500-1000 ns solution-phase lifetimes21), radiative recombination of excitons is sufficiently slow to result in a weak PL signal, which when combined with low QD quantum yields (QYs), nanocrystal instability and high detector background noise, e.g., in the case of InGaAs array or avalanche photodiode (APD) detectors, prevents measurement of single QD emission.22 Observation of single-dot level PbS/CdS QD emission has required advanced detectors, such as the superconducting nanowire single photon detector (SNSPD).21 The ability to measure and, ultimately, make use of photons emitted by single PbSe QDs would be significantly enhanced if rapid photobleaching could be suppressed. For example, we have observed that when these QDs are spread onto glass substrates at low concentrations and interrogated at room temperature with laser excitation, they photobleach within seconds, rendering both single-QD PL studies and applications impractical. It is now well known that photobleaching and even fluorescence intermittency (blinking) is suppressed or eliminated in the case of some core/shell QDs for which the protective shell is very thick.23-27 Here, we show for the first time that the notoriously photo-unstable PbSe quantum emitter can be rendered ultra-photostable with the addition of a thick CdSe shell, exhibiting nonphotobleaching emission at the level of QD clusters and even for single nanocrystals spread bare on a glass slide and interrogated for over 1 hour using high laser-excitation power densities (50 to >300 W/mm2). Unexpectedly, we find that the shell crystal structure – zinc blende or wurtzite – determines the efficiency of PbSe/CdSe g-QD PL and, thereby, our ability to detect single-dot emission. We further show that cubic or hexagonal shell growth, as well as shell shape, can be synthetically directed. RESULTS AND DISCUSSION Thick-shell or “giant” PbSe/CdSe QDs (PbSe/CdSe g-QDs) were synthesized using sequentially applied partial-cation exchange9 and modified successive ionic layer adsorption and reaction (SILAR)26,28 shell-
Figure 1. (a)-(f) Low and high magnification annular dark-field scanning transmission electron microscopy (ADF-STEM; left) and TEM (right) images of rs-PbSe/ zb-CdSe/zb-CdSe g-QDs (a,b), rs-PbSe/zb-CdSe/wCdSe g-QDs (c,d) and rs-PbSe/zb-CdSe/w-CdSe tetrapods (e,f). Insets in (b) and (d) show the fast Fourier transforms (FFTs) for these g-QDs. (g) Powder x-ray diffraction (XRD) patterns of nanocrystals in a,b (top, blue trace), c,d (middle, orange trace) and e,f (bottom, red trace). The standard diffraction patterns of hexagonal and cubic bulk CdSe are included as references (PDFs: 01-071-4772 and 01-088-2346, respectively). The wurtzite:zinc-blende ratios (W:ZB) as determined by a Whole Pattern Fitting and Rietveld refinement method are shown for each pattern.
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growth procedures. Briefly, for example, PbSe QD cores (5.6 ± 0.5 nm diameter, 1561 nm 1S exciton absorption peak; Figure S1) suspended in toluene were mixed with excess cadmium oleate, heated to 90 °C and allowed to react for 2.5 h. This process resulted in a partial exchange of Pb for Cd, shrinking of the PbSe core and formation of a CdSe shell (Supporting Information, Synthetic Methods).9 In this way, total particle diameter remains approximately constant, while core size as determined by known diameter/PL-energy correlations becomes smaller. Here, the new reduced core sizes calculated based on the shifted PL peak positions was ~4.1 nm and (with corresponding post-exchange PL peak maximum: ~1410 nm), implying a CdSe shell thicknesses of 1 nm zb-CdSe shells (1.7 nm). Subsequent SILAR shelling using the Se-TOP precursor yielded PbSe/zb-CdSe/w-CdSe nanocrystals similar to that observed for the smaller-core preparations; however, the w-CdSe formed as tetrapoidal arms (Figure 1e,f ). We speculate that arm formation, rather than conformal shell growth, was
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promoted in this case due to the presence of well-established zinc-blende CdSe facets, only characteristic of sufficiently thick CdSe shells (i.e., ~5 MLs Vs.
