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Biexciton Binding Energy and Line width of Single Quantum Dots at Room Temperature
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Cite this: Nano Lett. 2021, 21, 13, 5760–5766
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https://doi.org/10.1021/acs.nanolett.1c01556
Published June 16, 2021

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Broadening of multiexciton emission from colloidal quantum dots (QDs) at room temperature is important for their use in high-power applications, but an in-depth characterization has not been possible until now. We present and apply a novel spectroscopic method to quantify the biexciton line width and biexciton binding energy of single CdSe/CdS/ZnS colloidal QDs at room temperature. In our method, which we term “cascade spectroscopy”, we select emission events from the biexciton cascade and reconstruct their spectrum. The biexciton has an average emission line width of 86 meV on the single-QD scale, similar to that of the exciton. Variations in the biexciton repulsion (Eb = 4.0 ± 3.1 meV; mean ± standard deviation of 15 QDs) are correlated with but are more narrowly distributed than variations in the exciton energy (10.0 meV standard deviation). Using a simple quantum-mechanical model, we conclude that inhomogeneous broadening in our sample is primarily due to variations in the CdS shell thickness.

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Since the first report of colloidal quantum-dot (QD) synthesis 30 years ago, (1) significant research effort has been devoted to reaching high emission efficiencies and pure emission colors tunable over the entire visible range. However, variations in particle size, which reduce the color purity because of inhomogeneous broadening, cannot be entirely avoided. Inhomogeneous and homogeneous broadening (2−6) of QD emission at room temperature, directly relevant for applications in displays and lamps, has been studied extensively on the ensemble and single-QD level. Previous studies have focused on QD emission in the regime of weak optical excitation, where QD fluorescence originates from the single-exciton excited state. Prospective new applications—such as lasing and high-power lighting—inherently demand stronger excitation, which creates multiexciton states with optical properties different from those of single excitons, (7,8) most notably a lower emission efficiency because of Auger recombination. (9,10)

The spectral characteristics of multiexcitons have remained largely unexplored. Studying multiexciton states is difficult because, under strong excitation, the simultaneous presence of trions, biexcitons, and higher multiexcitons obscures the investigation of any particular multiexciton state. Only at cryogenic temperatures can multi- and single-exciton emission be separated on the basis of polarization (11,12) or by spectral selection. (13) At room temperature, such selections are difficult because the energy splitting between fine-structure states and the spectral shifts between single excitons and multiexcitons are small (a few meV) in comparison to the emission line widths (tens of meV). Photon-correlation investigations of multiexciton (14−22) emission efficiencies at room temperature have revealed enormous heterogeneities with no apparent link to the single-exciton properties. (20,23−26) A detailed characterization of multiexciton emission energies and the variations thereof at the single-QD level is important for high-power applications (27−29) but has not yet been achieved.

In this Letter, we measure the biexciton spectrum of single QDs at room temperature. We employ a novel spectroscopic technique, which we call “cascade spectroscopy”. In this technique, we identify biexciton cascades from the emission of a single QD by cross-correlating the signal recorded by two time-resolved single-photon detectors. On one of the two detectors we achieve energy resolution, which allows us to reconstruct the biexciton emission spectra. We observe a repulsive biexciton of single CdSe/CdS/ZnS core–shell–shell QDs, commonly used for lasing, (27) consistent with power-dependent measurements on the ensemble scale and with previous studies of thick-shelled core–shell QDs. (30) The single-QD biexciton emission line width is 86 meV on average (full width at half-maximum; fwhm). This matches the line width of the single-QD exciton emission of our QDs and those of other groups. (3,28) We find single-QD biexciton shifts of Eb = 4.0 ± 3.1 meV, which are slightly negatively correlated with but more narrowly distributed than the single-QD exciton energies (variation of 10.0 meV). Comparing these correlations and variations to simple quantum-mechanical calculations of multicarrier states in QDs, we conclude that both are predominantly due to variations in the thickness of the CdS shell. We thus present the first method to characterize single-QD biexciton spectra at room temperature, which will guide the development of synthesis strategies aimed at minimizing inhomogeneous broadening of biexciton emission for high-power applications.

We first characterize the biexciton emission of our CdSe/CdS/ZnS core–shell–shell sample (section S1 in the Supporting Information) on the ensemble scale, as a micrometer-thick layer on a cover slip, using energy-resolved time-correlated single-photon counting. Figure 1a shows the setup, which we term the “galvo-APD” setup. A galvo mirror periodically varies the incidence angle of the QD emission onto a transmissive diffraction grating (300 lines/mm). Fourier imaging of the grating plane projects part of the QD spectrum onto an avalanche photodiode (APD) with a resolution Δλ ≈ aL/f, where a is the detector size (20 μm), L is the groove spacing (3.3 μm), and f is the focal distance (20 mm). These optical elements give a resolution of approximately Δλ = 3 nm (section S2.1 in the Supporting Information). Periodic scanning (Figure 1b, 0.5 Hz) measures the emission spectrum twice every 2 s. We perform a pulsed-excitation experiment (repetition rate 2.5 MHz), where we deduce the energy of each detected photon from the detection time T since the beginning of the experiment (see inset of Figure 1b). For each photon, we simultaneously measure the delay time t with respect to the previous laser pulse.

Figure 1

Figure 1. (a) Schematic of the galvo-APD setup. The QD emission is directed to a periodically rotating galvo mirror, which changes the angle of incidence on a transmission grating. Fourier imaging of the grating plane projects part of the QD emission spectrum onto an avalanche photodiode (APD). (b) Periodically rotating the galvo mirror with 0.5 Hz scans the emission spectrum of the QDs (pulsed excitation 2.5 MHz) over the APD twice every 2 s. We can convert the experimental time T to a photon energy (see inset). (c, d) Time-resolved emission spectra of a micrometer-thick layer of QDs on glass at increasing excitation-pulse energy. The white lines indicate the emission maximum as a function of delay time. At the highest power, the spectrum is slightly blue shifted at delay times of 0–2 ns because of the contribution of repulsive biexcitons. (e) Emission spectrum integrated between 0 and 0.6 ns delay times for n = 0.005 (red, panel c) and n = 0.23 (blue, panel d) excitons per pulse. By globally fitting a series of spectra for different fluences, we obtain an ensemble biexciton shift of E̅b = +14 meV.

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Figure 1c,d shows time-resolved emission maps, i.e. emission spectrum versus delay time, at different excitation powers, measured with our galvo-APD setup. At the lowest power (estimated number of excitons per QD per pulse n = 0.005, Figure 1c) the emission peaks at 2.002 eV on the interval t = 0–0.6 ns. It then slowly shifts to lower energies over the next 15 ns, likely due to inhomogeneous broadening of the emission (31) and exciton diffusion. (32) At higher powers (n = 0.23, Figure 1d) an additional slightly bluer contribution to the emission appears; the emission peaks at 2.007 eV on the interval t = 0–0.6 ns. Figure 1e shows the integrated emission spectra for t = 0–0.6 ns for the low fluence (red) and the high fluence (blue). This indicates a repulsive biexciton; i.e., the ensemble-scale biexciton shift is positive, E̅b = E̅BX – E̅X > 0, where E̅BX and E̅X are the ensemble-scale biexciton and exciton emission energies, respectively. We expect negligible contribution of triexcitons at the excitation fluences used. Nevertheless, extracting a quantitative value for E̅b remains difficult, because the majority of the signal originates from the exciton and the shift between exciton and biexciton emission is much smaller than the line width due to inhomogeneous and homogeneous broadening. A global-fit procedure (section S3 in the Supporting Information) yields a best estimate for the ensemble-scale biexciton shift of E̅b = 14 meV.

Experiments on single QDs allow us to unambiguously distinguish between biexciton and exciton photons and to directly extract the biexciton shift and the biexciton emission spectrum. We split the emission of a single QD using a Hanbury-Brown–Twiss (HBT) setup (Figure 2a), in which one of the detection channels uses a regular APD while the other uses the galvo-APD setup (70 lines/mm, Δλ = 13 nm), introduced in Figure 1a.

Figure 2

Figure 2. (a) Schematic of the cascade spectroscopy setup. The emission of a single QD is split equally over the energy-resolved “galvo APD” (Figure 1a) and a regular APD. (b) Photon correlation function g(2) of a single QD (repetition rate 2.5 MHz, estimated average number of excitons per pulse μ = 0.1) integrated over the entire experiment, showing a biexciton efficiency of ηBX = 10.5%. The highlights mark the delay time ranges where events of types 1–, 0–, 0+, and 1+ contribute. (c) Overview of possible ordered-emission events. The laser pulses are the vertical blue lines. Events wherein two photons (dots) are detected following two consecutive laser pulses are highlighted 1+ and 1–, depending on the order of the detector clicks. Two photons detected after a single laser pulse, indicating biexciton cascade emission, are highlighted 0+ and 0–. The first photon of a cascade is a biexciton photon (blue dots); all other photons most likely originate from the single-exciton state (red dots). (d, e) Energy-resolved g(2) (log y-scale) for low (d) and high (e) galvo-APD detection energies. Lower galvo-APD detection energies Edet = 1.95–1.99 eV yield a ratio N0+/N0– < 1, while for blue-shifted detection energies Edet = 2.08–2.16 eV, N0+/N0– > 1. This indicates a repulsive biexciton. The insets (linear y scale) are enlarged views to show N0+/N0– more clearly. We discarded afterglow events from panels b, d, and e (Figure S3 in the Supporting Information). (f) N0+/N1 as a function of galvo-APD detection energy Edet. The gray spectrum shows the exciton emission of this single QD. The error bars are calculated assuming shot noise. By fitting eq 2 to the data, we extract a single-QD biexciton shift of Eb = 6.8 ± 1.0 meV.

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From the energy-resolved measurement, we can reconstruct the same photon-correlation function g(2) as obtained with a conventional HBT setup, by simply integrating g(2) over the total experimental time of 30–60 min (section S4.2 in the Supporting Information). Figure 2b shows the result for a typical single QD from our batch. We performed a blind-time analysis to confirm that the zero-delay peak is due to biexciton-cascade events, while the side peaks are due to exciton–exciton pairs following consecutive laser pulses (Figure S4 in the Supporting Information). (33) The correlation function measured in the limit of weak excitation reveals the biexciton-to-exciton efficiency ratio ηBX/ηX from the number of photon-pair events in the zero-delay peak N0 and the number of photon-pair pairs in a side peak N1: (14)

(1)

We find a biexciton efficiency of ηBX = 10.5% (assuming ηX = 1 (14)), which is a typical value for thick-shell CdSe/CdS/ZnS core–shell–shell QDs. (14,26)

In our correlation analysis, the galvo APD acts as the “start” channel and the regular APD as the “stop” channel. In our definition of g(2), the biexciton cascade emission wherein the first photon arrived at the galvo APD thus appears in the zero-delay peak at positive delay times (highlighted with 0+ in Figure 2b,c); cascade emission wherein the first photon arrived at the regular APD appears in the zero-delay peak at negative delay times (highlighted with 0– in Figure 2b,c). Similarly, for the side peaks, the order of the detector clicks determines whether an exciton–exciton emission pair appears at positive or negative delay times (compare 1– and 1+ in Figure 2b,c).

The relative areas of the peaks in the correlation function g(2) for a specific galvo-APD detection energy Edet contain information about the biexciton emission spectrum. Indeed, the number of recorded photon pairs of types 1–, 0–, 0+, and 1+ depend on Edet and on the emission spectra of biexcitons and excitons. Successful detection of photon pairs of type 1–, 0–, and 1+ require the galvo APD to record an exciton photon (section S4.3 in the Supporting Information); thus, the number of detection events is proportional to the brightness of the exciton at Edet. In contrast, for events of type 0+ the galvo APD must record a biexciton photon; thus, the number of such events is proportional to the biexciton brightness at Edet. A comparison of g(2) for Edet = 1.95–1.99 eV (Figure 2d) and for Edet = 2.08–2.16 eV (Figure 2e) shows the consequence: the total number of events of types 0– and 0+, which we denote N0– and N0+, are unequal and dependent on Edet. More precisely, we observe that N0– > N0+ in Figure 2d and N0– < N0+ in Figure 2e (highlighted in the insets). Hence, a low Edet favors the recording of exciton photons on the galvo APD, while a high Edet favors the recording of biexciton photons. This proves that this single QD has a repulsive biexciton: i.e., the biexciton emission energy EBX is higher than the exciton emission energy EX. With a quantitative analysis, we can determine the biexciton shift Eb = EBX – EX on the single-QD level. We compare the number of events recorded of type 0+ (positive-time area of the zero-delay peak N0+) to the number of events recorded of types 1– and 1+ (side-peak areas N1– and N1+). To improve the signal-to-noise ratio, we take the average N1 = N1– + N1+. Realizing that N0+ scales with the biexciton emission spectrum IBX(E) peaking at EBX = EX + Eb and N1 with the exciton emission spectrum IX(E) peaking at EX, we can derive (section S4.1 in the Supporting Information)

(2)

We observe that N0+/N1 increases as the galvo-APD detection energy increases for Edet = 1.97–2.07 eV (Figure 2e). Outside this range of Edet, the detection efficiency of both biexciton and exciton emission is low and dark counts affect the measurement. We fit N0+/N1 as a function of detection energy Edet using eq 2 (dashed blue line), fixing the previously determined biexciton efficiency, assuming Gaussian exciton and biexciton spectra with equal line width (fwhm = 85 meV, gray exciton spectrum inset) and using the biexciton shift Eb as the only fit parameter. The solid blue line is calculated including the contribution of dark counts (section S4.3 in the Supporting Information), which yields a match with the data over the full range of Edet. We obtain a biexciton shift Eb = 6.8 ± 1.0 meV for this single QD.

Understanding the different cascade events that occur in our experiment, we can now reconstruct the single-QD biexciton spectrum directly from the positive-time area N0+ of the zero-delay peak. The absolute value of N0+ as a function of Edet is proportional to the biexciton emission spectrum IBX. Similarly, we obtain the exciton emission spectrum IX from N1 as a function of Edet. The contribution of photon pairs to N0+ and N1 originating from a transient low-efficiency gray state due to blinking is negligible (Figure S6 in the Supporting Information), since the count rate of photon pairs scales quadratically with emission efficiency. Figure 3a shows the biexciton and exciton emission spectra obtained with this analysis procedure of the same QD as described in Figure 2. The exciton spectrum (red dots) has a maximum at EX = 2.025 eV and a fwhm of 92 meV. The emission is broadened by spectral diffusion and slightly by the finite resolution of the setup (Figure S2 in the Supporting Information). Interestingly, we measure a biexciton spectrum (blue dots) with an equal fwhm of 89 meV, which confirms the assumption of equal exciton and biexciton line widths that we used in fitting eq 2 to the data in Figure 2e. From the difference in fitted exciton and biexciton emission maxima we obtain a biexciton shift of Eb = 7.2 ± 2.1 meV, very similar to the value found from the fit in Figure 2e.

Figure 3

Figure 3. (a) Biexciton (blue) and exciton (red) emission spectra of the single QD analyzed in Figure 2. The biexciton spectrum was extracted from the positive-time area N0+ of the zero-delay peak in the energy-resolved photon-correlation function. We measure a biexciton shift of Eb = 7.2 ± 2.1 meV. (b) A different QD with exciton with a larger biexciton shift of Eb = 12.7 ± 1.9 meV. (c) Biexciton and exciton line widths (fwhm) of 22 single QDs measured. Error bars are standard errors of the Gaussian fit.

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Figure 3b shows the emission spectra of a different QD from the synthesis batch. The exciton emission peaks at EX = 1.958 eV, and interestingly, the biexciton is significantly more repulsive: Eb = 12.7 ± 1.9 meV. Again, we observe equal line widths of the exciton and biexciton emission of 85 meV. Figure 3c shows the biexciton and exciton emission line widths obtained from 22 single-QD measurements (Figures S7–S9 in the Supporting Information). The single-QD line widths of biexciton and exciton emission are distributed between 70 and 100 meV fwhm, with a mean of 86 and 87 meV, respectively, and are strongly correlated; the biexciton and emission line widths of a single QD typically differ by no more than 10 meV fwhm. Equal line widths for the biexciton and exciton were reported before for epitaxially grown QDs over a wide temperature range from cryogenic temperatures up to room temperature. (34) Our experimental finding shows that the homogeneous emission broadening (determined by a combination of spectral diffusion, phonon coupling, and fine structure (3,34−36)) of different single QDs varies, but it affects the exciton and biexciton emission line widths by the same amount.

Figure 4a shows an overview of the biexciton shifts and exciton energies found. Of the 22 single-QD measurements, 15 were randomly selected QDs (blue dots in Figure 4a) and 7 QDs were specially selected for their low-energy emission (red dots in Figure 4a). We find that the variations in the exciton emission maximum (10.0 meV standard deviation) exceed the variations in the biexciton shift (3.1 meV) by almost a factor of 4. Additionally, we observe on average a slightly more repulsive biexciton Eb = +9.4 meV for the selected QDs in comparison to a weakly repulsive biexciton Eb = +4.0 meV for randomly selected QDs.

Figure 4

Figure 4. (a) Biexciton shift Eb versus exciton energy EX for randomly selected QDs (blue) and for QDs selected for their low-energy exciton emission (red). Error bars denote one standard error of the fit. We observe a slight negative correlation between biexciton shift and exciton energy. (b) Dependence of Eb and EX on CdSe core radius a and CdSe/CdS core–inner-shell radius b, calculated with an effective-mass model that includes Coulomb interactions between carriers as a perturbation to second order (section S5 in the Supporting Information). The QD geometry is given as grid lines for a (core radius, solid lines) and b (core–inner-shell radius, dashed lines). The gray-shaded area highlights the range of exciton energies and biexciton shifts found experimentally. The red point and arrows are the calculated exciton energy and biexciton shift for the independently determined values of a = 1.84 nm (from the absorption spectrum of the cores) and b = 4.1 ± 0.4 nm (from electron microscopy).

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To understand the variations in exciton energy and biexciton shift, we set up a simple quantum-mechanical effective-mass model of multicarrier states in a spherical core–shell–shell QD. Coulomb interactions between carriers are included as a perturbation to the particle-in-a-box confinement energies (section S5 in the Supporting Information). We compute the biexciton shift Eb and exciton energy EX of core–shell geometries with different CdSe core radii a and CdSe/CdS core–shell radii b. Figure 4b shows the results of the calculations in a way that facilitates comparison to the experimental data of Figure 4a: a grid of contour lines lays out which QD geometry (a, b) produces which values of Eb and EX.

We can determine the dominant geometrical contribution to inhomogeneous emission broadening in our QD sample by comparing the calculations (grid lines in Figure 4b) to the experimental distribution of QD properties (highlighted by the gray area). The experimental variation in EX exceeds the variation in Eb by almost a factor of 4, and they are slightly negatively correlated. This is consistent with energy variations due to a distribution of core–shell radii b, while a distribution of core radii a appears to be of lesser importance. Indeed, the solid contour lines in Figure 4b, running along directions of constant a, slope in approximately the same direction as the experimental distribution of single-QD emission energies. The calculations would predict that our experimental finding of the biexciton and exciton energies are due to a distribution of QD geometries with a = 1.82 ± 0.05 nm and b = 4.3 ± 0.3 nm. These calculated values match well with the values of a = 1.84 nm determined from the absorption spectrum of the cores and b = 4.1 ± 0.4 nm (mean ± standard deviation) from electron microscopy (red arrows in Figure 4b). A dominant contribution of variations in the CdSe/CdS core–shell size b to inhomogeneous broadening could be expected on the basis of our synthesis procedure and those of many others. (28,37,38) For the core synthesis, growth is fast and is stopped before size defocusing of a sets in. (39) Shells, on the other hand, are grown at a low growth rate to avoid secondary nucleation, which may lead to Ostwald ripening effects that widen the distribution of shell thicknesses. (40)

Using our effective-mass model, we can intuitively understand why variations in core–shell size b produce the observed distribution of biexciton and exciton energies. When Coulomb interactions are included as a first-order perturbation, the exciton energy is

(3)
where Econf, e and Econf, h are the particle-in-a-box confinement energies of the single-electron (e) and single-hole (h) 1S states, respectively, and Jij is the Coulomb interaction between carriers i and j. Similarly, the biexciton shift including Coulomb interactions to first order is
(4)

This description yields analytical expressions for the dependence of EX(1) and Eb(1) on a and b in the limit that b ≫ a (section S5.1 in the Supporting Information). As inhomogeneous broadening in our QD batch appears mostly due to variations in b, we can understand it from the derivatives of eqs 3 and 4 with respect to b

(5)
(6)
where ℏ is the reduced Planck’s constant, me the effective mass of the electron, e the elementary charge, ε0 the permittivity of the vacuum, and ε the relative permittivity of the QD material. Evaluating eqs 5 and 6 at a core–shell radius of b = 4.1 nm yields the prediction that a distribution of b produces variations in exciton energy of ∂EX(1)/∂b = −88 meV nm–1 (decrease with increasing shell thickness) and variations in biexciton binding of ∂Eb(1)/∂b = 26 meV nm–1 (increase with increasing shell thickness). The variations in the exciton energy ∂EX(1)/∂b are mainly caused by variations in the confinement energy of the electron. Hence, our simple model explains the wider variations in single-QD exciton energy than in biexciton binding as well as the slightly negative correlations between the two parameters. The negative correlation can also explain the less repulsive biexciton on the single-QD level (on average Eb = +4.0 meV) in comparison to the ensemble level (Figure 1c–e, E̅b = +14 meV). Since both the biexciton efficiency (26) and the biexciton shift (eq 6) increase with increasing shell thickness, we expect a biexciton-efficiency-weighted biexciton shift in ensemble measurements. This highlights the necessity of single-QD measurements to successfully characterize biexciton emission. Equations 3–6 can be used to predict and understand the inhomogeneous broadening of exciton and biexciton energies for other QD geometries.

To summarize, we have measured the biexciton shift and biexciton line width of single CdSe/CdS/ZnS QDs using cascade spectroscopy. We found that the line widths of biexciton and exciton emission are similar, although they vary between different single QDs. Variations in the biexciton shift are small in comparison to variations of the exciton energy, and the two parameters are slightly negatively correlated. We can rationalize these variations with simple quantum-mechanical calculations, which point to a distribution of CdS shell thicknesses as the dominant contributor to inhomogeneous broadening. Our method and findings are relevant for a fundamental understanding of multicarrier interactions in colloidal QDs. Moreover, they can serve as a guide for synthesis procedures of core–shell nanocrystals, which will particularly benefit their use in high-power applications.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.nanolett.1c01556.

  • Experimental methods, details of data processing and calibration procedures, details of quantum-mechanical calculations, a derivation of the energy-resolved g(2), and an overview of all single-QD experiments (PDF)

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Author Information

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  • Corresponding Author
  • Authors
    • Sander J. W. Vonk - Debye Institute, Utrecht University, Princetonplein 1, 3584 CC Utrecht, The Netherlands;  Orcidhttps://orcid.org/0000-0002-4650-9473
    • Bart A. J. Heemskerk - Debye Institute, Utrecht University, Princetonplein 1, 3584 CC Utrecht, The Netherlands
    • Robert C. Keitel - Optical Materials Engineering Laboratory, ETH Zurich, Leonhardstrasse 21, 8092 Zurich, Switzerland;  Orcidhttps://orcid.org/0000-0002-9412-8034
    • Stijn O. M. Hinterding - Debye Institute, Utrecht University, Princetonplein 1, 3584 CC Utrecht, The Netherlands
    • Jaco J. Geuchies - Optoelectronic Materials Section, Faculty of Applied Sciences, Delft University of Technology, van der Maasweg 9, 2629 HZ Delft, The Netherlands;  Orcidhttps://orcid.org/0000-0002-0758-9140
    • Arjan J. Houtepen - Optoelectronic Materials Section, Faculty of Applied Sciences, Delft University of Technology, van der Maasweg 9, 2629 HZ Delft, The Netherlands;  Orcidhttps://orcid.org/0000-0001-8328-443X
  • Author Contributions

    The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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This work was supported by the Dutch Research Council NWO (OCENW.KLEIN.008). F.T.R. acknowledges financial support from The Netherlands Organisation for Scientific Research NWO (VENI grant number 722.017.002) and The Netherlands Center for Multiscale Catalytic Energy Conversion (MCEC). J.J.G. and A.J.H. acknowledge support from European Research Council Horizon 2020 ERC Grant Agreement 678004 (Doping on Demand). R.C.K. acknowledges support from the European Research Council (ERC; FP/2007-2013), ERC Grant Agreement No. 339905 (QuaDoPS Advanced Grant). Peter Helfferich is thanked for technical support setting up the galvo-APD setup.

References

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    Nature Chemistry (2013), 5 (7), 602-606CODEN: NCAHBB; ISSN:1755-4330. (Nature Publishing Group)
    The spectral linewidth of an ensemble of fluorescent emitters is dictated by the combination of single-emitter linewidths and sample inhomogeneity. For semiconductor nanocrystals, efforts to tune ensemble linewidths for optical applications have focused primarily on eliminating sample inhomogeneities, because conventional single-mol. methods cannot reliably build accurate ensemble-level statistics for single-particle linewidths. Photon-correlation Fourier spectroscopy in soln. (S-PCFS) offers a unique approach to investigating single-nanocrystal spectra with large sample statistics and high signal-to-noise ratios, without user selection bias and at fast timescales. With S-PCFS, we directly and quant. deconstruct the ensemble linewidth into contributions from the av. single-particle linewidth and from sample inhomogeneity. We demonstrate that single-particle linewidths vary significantly from batch to batch and can be synthetically controlled. These findings delineate the synthetic challenges facing underdeveloped nanomaterials such as InP and InAs core-shell particles and introduce new avenues for the synthetic optimization of fluorescent nanoparticles.
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  3. 3
    Cui, J.; Beyler, A. P.; Coropceanu, I.; Cleary, L.; Avila, T. R.; Chen, Y.; Cordero, J. M.; Heathcote, S. L.; Harris, D. K.; Chen, O. Evolution of the Single-Nanocrystal Photoluminescence Linewidth with Size and Shell: Implications for Exciton-Phonon Coupling and the Optimization of Spectral Linewidths. Nano Lett. 2016, 16 (1), 289– 296,  DOI: 10.1021/acs.nanolett.5b03790
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    3
    Evolution of the Single-Nanocrystal Photoluminescence Linewidth with Size and Shell: Implications for Exciton-Phonon Coupling and the Optimization of Spectral Linewidths
    Cui, Jian; Beyler, Andrew P.; Coropceanu, Igor; Cleary, Liam; Avila, Thomas R.; Chen, Yue; Cordero, Jose M.; Heathcote, S. Leigh; Harris, Daniel K.; Chen, Ou; Cao, Jianshu; Bawendi, Moungi G.
    Nano Letters (2016), 16 (1), 289-296CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    The optimization of luminescence spectral linewidths in semiconductor nanocrystal prepns. involves minimizing both the homogeneous and inhomogeneous contributions to the ensemble spectrum. Although the inhomogeneous contribution can be controlled by eliminating interparticle inhomogeneities, far less is known about how to synthetically control the homogeneous, or single-nanocrystal, spectral linewidth. Soln. photon-correlation Fourier spectroscopy (S-PCFS) was used to measure how the sample-averaged single-nanocrystal emission linewidth of CdSe core and core/shell nanocrystals change with systematic changes in the size of the cores and the thickness and compn. of the shells. The single-nanocrystal linewidth at room temp. is heavily influenced by the nature of the CdSe surface and the epitaxial shell, which have a profound impact on the internal elec. fields that affect exciton-phonon coupling. The results explain the wide variations, both exptl. and theor., in the magnitude and size dependence in previous reports on exciton-phonon coupling in CdSe nanocrystals. Also, the findings offer a general pathway for achieving the narrow spectral linewidths required for many applications of nanocrystals.
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  4. 4
    Utzat, H.; Shulenberger, K. E.; Achorn, O. B.; Nasilowski, M.; Sinclair, T. S.; Bawendi, M. G. Probing Linewidths and Biexciton Quantum Yields of Single Cesium Lead Halide Nanocrystals in Solution. Nano Lett. 2017, 17 (11), 6838– 6846,  DOI: 10.1021/acs.nanolett.7b03120
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    4
    Probing Linewidths and Biexciton Quantum Yields of Single Cesium Lead Halide Nanocrystals in Solution
    Utzat, Hendrik; Shulenberger, Katherine E.; Achorn, Odin B.; Nasilowski, Michel; Sinclair, Timothy S.; Bawendi, Moungi G.
    Nano Letters (2017), 17 (11), 6838-6846CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    CsPbX3 (X = Cl, Br, I) perovskite nanocrystals (PNCs) have recently become a promising material for optoelectronic applications due to their high emission quantum yields and facile band gap tunability via both halide compn. and size. The spectroscopy of single PNCs enhances the understanding of the effect of confinement on excitations in PNCs in the absence of obfuscating ensemble averaging and can also inform synthetic efforts. Single PNC studies were hampered by poor PNC photostability under confocal excitation, precluding interrogation of all but the most stable PNCs, leading to a lack of understanding of PNCs in the regime of high confinement. The 1st comprehensive spectroscopic study of single PNC properties using soln.-phase photon-correlation methods are reported, including both highly confined and blue-emitting PNCs, previously inaccessible to single NC techniques. With min. perturbative soln.-phase photon-correlation Fourier spectroscopy (s-PCFS), the ensemble emission linewidth of PNCs of all sizes and compns. is predominantly detd. by the intrinsic single NC linewidth (homogeneous broadening). The single PNC linewidth dramatically increases with increasing confinement, consistent with what was found for II-VI semiconductor nanocrystals. With soln.-phase photon antibunching measurements, the biexciton-to-exciton quantum yield ratio (BX/X QY) in the absence of user-selection bias or photodegrdn. were surveyed. The BX/X QY ratio depends both on the PNC size and halide compn., with values between ∼2% for highly confined bromide PNCs and ∼50% for intermediately confined iodide PNCs. The results suggest a wide range of underlying Auger rates likely due to transitory charge carrier sepn. in PNCs with relaxed confinement.
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  5. 5
    Gellen, T. A.; Lem, J.; Turner, D. B. Probing Homogeneous Line Broadening in CdSe Nanocrystals Using Multidimensional Electronic Spectroscopy. Nano Lett. 2017, 17 (5), 2809– 2815,  DOI: 10.1021/acs.nanolett.6b05068
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    5
    Probing Homogeneous Line Broadening in CdSe Nanocrystals Using Multidimensional Electronic Spectroscopy
    Gellen, Tobias A.; Lem, Jet; Turner, Daniel B.
    Nano Letters (2017), 17 (5), 2809-2815CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    The finite spectral line width of an ensemble of CdSe nanocrystals arises from size and shape inhomogeneity and the single-nanocrystal spectrum itself. This line width directly limits the performance of nanocrystal-based devices, yet most optical measurements cannot resolve the underlying contributions. We use two-dimensional electronic spectroscopy (2D ES) to measure the line width of the band-edge exciton of CdSe nanocrystals as a function of radii and surface chem. We find that the homogeneous width decreases for increasing nanocrystal radius and that surface chem. plays a crit. role in controlling this line width. To explore the hypothesis that unpassivated trap states serve to broaden the homogeneous line width and to explain its size-dependence, we use 3D ES to identify the spectral signatures of exciton-phonon coupling to optical and acoustic phonons. We find enhanced coupling to optical phonon modes for nanocrystals that lack electron-passivating ligands, suggesting that localized surface charges enhance exciton-phonon coupling via the Frohlich interaction. Lastly, the data reveal that spectral diffusion contributes negligibly to the homogeneous line width on subnanosecond time scales.
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  6. 6
    Van Der Bok, J. C.; Dekker, D. M.; Peerlings, M. L. J.; Salzmann, B. B. V.; Meijerink, A. Luminescence Line Broadening of CdSe Nanoplatelets and Quantum Dots for Application in W-LEDs. J. Phys. Chem. C 2020, 124 (22), 12153– 12160,  DOI: 10.1021/acs.jpcc.0c03048
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    6
    Luminescence Line Broadening of CdSe Nanoplatelets and Quantum Dots for Application in w-LEDs
    van der Bok, Johanna C.; Dekker, Daphne M.; Peerlings, Matt L. J.; Salzmann, Bastiaan B. V.; Meijerink, Andries
    Journal of Physical Chemistry C (2020), 124 (22), 12153-12160CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Nanoplatelets (NPLs) of CdSe are an emerging class of luminescent materials, combining tunable and narrow emission bands with high quantum yields. This is promising for application in white light LEDs (w-LEDs) and displays. The origin of the narrow spectral width of exciton emission in core NPL compared to core-shell NPL and quantum dot (QD) emission is not fully understood. Here we investigate and compare temp.-dependent emission spectra of core and core-shell CdSe NPLs and QDs. A wide temp. range, 4-423 K, is chosen to gain insight into contributions from homogeneous and inhomogeneous broadening and also to extend measurements into a temp. regime that is relevant for operating conditions in w-LEDs (T ≈ 423 K). The results show that temp.-induced homogeneous broadening does not strongly vary between the various CdSe nanostructures (ΔEhom ≈ 60-80 meV at 423 K) indicating that electron-phonon coupling strengths are similar. Only for the smallest QDs is stronger coupling obsd. The origin of the narrow bandwidth reported at 300 K for core CdSe NPLs is attributed to a very narrow inhomogeneous line width. At 423 K, the spectral width of NPL exciton emission is still narrower than that of QDs. A comparison with traditional w-LED phosphors is made to outline advantages (tunability, narrow bandwidth, high efficiency) and disadvantages (color shift, stability issues) of NPLs for application in w-LEDs.
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  7. 7
    Bae, W. K.; Park, Y. S.; Lim, J.; Lee, D.; Padilha, L. A.; McDaniel, H.; Robel, I.; Lee, C.; Pietryga, J. M.; Klimov, V. I. Controlling the Influence of Auger Recombination on the Performance of Quantum-Dot Light-Emitting Diodes. Nat. Commun. 2013, 4, 3661,  DOI: 10.1038/ncomms3661
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    7
    Controlling the influence of Auger recombination on the performance of quantum-dot light-emitting diodes
    Bae, Wan Ki; Park, Young-Shin; Lim, Jaehoon; Lee, Donggu; Padilha, Lazaro A.; McDaniel, Hunter; Robel, Istvan; Lee, Changhee; Pietryga, Jeffrey M.; Klimov, Victor I.
    Nature Communications (2013), 4 (), 3661/1-3661/8CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
    Development of light-emitting diodes (LEDs) based on colloidal quantum dots is driven by attractive properties of these fluorophores such as spectrally narrow, tunable emission and facile processibility via soln.-based methods. A current obstacle towards improved LED performance is an incomplete understanding of the roles of extrinsic factors, such as nonradiative recombination at surface defects, vs. intrinsic processes, such as multicarrier Auger recombination or electron-hole sepn. due to applied elec. field. Here, we address this problem with studies that correlate the excited state dynamics of structurally engineered quantum dots with their emissive performance within LEDs.We find that because of significant charging of quantum dots with extra electrons, Auger recombination greatly impacts both LED efficiency and the onset of efficiency roll-off at high currents. Further, we demonstrate two specific approaches for mitigating this problem using heterostructured quantum dots, either by suppressing Auger decay through the introduction of an intermediate alloyed layer, or by using an addnl. shell that impedes electron transfer into the quantum dot to help balance electron and hole injection.
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  8. 8
    Grim, J. Q.; Christodoulou, S.; Di Stasio, F.; Krahne, R.; Cingolani, R.; Manna, L.; Moreels, I. Continuous-Wave Biexciton Lasing at Room Temperature Using Solution-Processed Quantum Wells. Nat. Nanotechnol. 2014, 9 (11), 891– 895,  DOI: 10.1038/nnano.2014.213
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    8
    Continuous-wave biexciton lasing at room temperature using solution-processed quantum wells
    Grim, Joel Q.; Christodoulou, Sotirios; Di Stasio, Francesco; Krahne, Roman; Cingolani, Roberto; Manna, Liberato; Moreels, Iwan
    Nature Nanotechnology (2014), 9 (11), 891-895CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
    Soln.-processed inorg. and org. materials have been pursued for more than a decade as low-threshold, high-gain lasing media, motivated in large part by their tunable optoelectronic properties and ease of synthesis and processing. Although both have demonstrated stimulated emission and lasing, they have not yet approached the continuous-wave pumping regime. Two-dimensional CdSe colloidal nanosheets combine the advantage of soln. synthesis with the optoelectronic properties of epitaxial two-dimensional quantum wells. Here, we show that these colloidal quantum wells possess large exciton and biexciton binding energies of 132 meV and 30 meV, resp., giving rise to stimulated emission from biexcitons at room temp. Under femtosecond pulsed excitation, close-packed thin films yield an ultralow stimulated emission threshold of 6 μJ cm-2, sufficient to achieve continuous-wave pumped stimulated emission, and lasing when these layers are embedded in surface-emitting microcavities.
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  9. 9
    Hollingsworth, J. A. Heterostructuring Nanocrystal Quantum Dots toward Intentional Suppression of Blinking and Auger Recombination. Chem. Mater. 2013, 25 (8), 1318– 1331,  DOI: 10.1021/cm304161d
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    9
    Heterostructuring Nanocrystal Quantum Dots Toward Intentional Suppression of Blinking and Auger Recombination
    Hollingsworth, Jennifer A.
    Chemistry of Materials (2013), 25 (8), 1318-1331CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    A review. At the level of a single particle, nanocrystal quantum dots (NQDs) fluoresce intermittently or blink. They are also characterized by an efficient nonradiative recombination process known as Auger recombination (AR). Recently, new approaches to NQD heterostructuring were developed that directly impact both blinking and AR, resulting in dramatic suppression of these unwanted processes. The 3 successful hetero-NQD motifs are reviewed here: (1) interfacial alloying, (2) thick or giant shells, and (3) specific type-II electronic structures. These approaches, which rely on modifying or tuning internal NQD core/shell structures, are compared with alternative strategies for blinking suppression that rely, instead, on surface modifications or surface-mediated interactions. Finally, in each case, the unique synthetic approaches or challenges addressed that have driven the realization of novel and important functionality are discussed, along with the implications for development of a comprehensive materials design strategy for blinking and AR-suppressed heterostructured NQDs.
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  10. 10
    Pandey, A.; Guyot-Sionnest, P. Multicarrier Recombination in Colloidal Quantum Dots. J. Chem. Phys. 2007, 127 (11), 111104,  DOI: 10.1063/1.2786068
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    10
    Multicarrier recombination in colloidal quantum dots
    Pandey, Anshu; Guyot-Sionnest, Philippe
    Journal of Chemical Physics (2007), 127 (11), 111104/1-111104/4CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
    The rates of multicarrier recombination are measured in colloidal quantum dots with and without hole surface trapping and with hole extn. in type II core/shell systems. We report that hole trapping or the phys. sepn. of electronic and hole wavefunctions into different semiconductor domains of a type II system have an insignificant effect on the multicarrier recombination dynamics. These observations are inconsistent with the accepted Auger transition mechanism.
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  11. 11
    He, Y. M.; Iff, O.; Lundt, N.; Baumann, V.; Davanco, M.; Srinivasan, K.; Höfling, S.; Schneider, C. Cascaded Emission of Single Photons from the Biexciton in Monolayered WSe2. Nat. Commun. 2016, 7, 13409,  DOI: 10.1038/ncomms13409
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    11
    Cascaded emission of single photons from the biexciton in monolayered WSe2
    He, Yu-Ming; Iff, Oliver; Lundt, Nils; Baumann, Vasilij; Davanco, Marcelo; Srinivasan, Kartik; Hoefling, Sven; Schneider, Christian
    Nature Communications (2016), 7 (), 13409CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
    Monolayers of transition metal dichalcogenide materials emerged as a new material class to study excitonic effects in solid state, as they benefit from enormous Coulomb correlations between electrons and holes. Esp. in WSe2, sharp emission features have been obsd. at cryogenic temps., which act as single photon sources. Tight exciton localization has been assumed to induce an anharmonic excitation spectrum; however, the evidence of the hypothesis, namely the demonstration of a localized biexciton, is elusive. Here we unambiguously demonstrate the existence of a localized biexciton in a monolayer of WSe2, which triggers an emission cascade of single photons. The biexciton is identified by its time-resolved photoluminescence, superlinearity and distinct polarization in micro-photoluminescence expts. We evidence the cascaded nature of the emission process in a cross-correlation expt., which yields a strong bunching behavior. Our work paves the way to a new generation of quantum optics expts. with two-dimensional semiconductors.
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  12. 12
    Huang, X.; Chen, L.; Zhang, C.; Qin, Z.; Yu, B.; Wang, X.; Xiao, M. Inhomogeneous Biexciton Binding in Perovskite Semiconductor Nanocrystals Measured with Two-Dimensional Spectroscopy. J. Phys. Chem. Lett. 2020, 11 (23), 10173– 10181,  DOI: 10.1021/acs.jpclett.0c03153
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    12
    Inhomogeneous Biexciton Binding in Perovskite Semiconductor Nanocrystals Measured with Two-Dimensional Spectroscopy
    Huang, Xinyu; Chen, Lan; Zhang, Chunfeng; Qin, Zhengyuan; Yu, Buyang; Wang, Xiaoyong; Xiao, Min
    Journal of Physical Chemistry Letters (2020), 11 (23), 10173-10181CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    Perovskite semiconductor nanocrystals have emerged as an excellent family of materials for optoelectronic applications, where biexciton interaction is essential for optical gain generation and quantum light emission. However, the strength of biexciton interaction remains highly controversial due to the entangled spectral features of the exciton- and biexciton-related transitions in conventional measurement approaches. Here, we tackle the limitation by using polarization-dependent two-dimensional electronic spectroscopy and quantify the excitation energy-dependent biexciton binding energy at cryogenic temps. The biexciton binding energy increases with excitation energy, which can be modeled as a near inverse-square size dependence in the effective mass approxn. considering the quantum confinement effect. The spectral line width for the exciton-biexciton transition is much broader than that for the ground state to exciton transition, suggesting weakly correlated broadening between these transitions. These inhomogeneity effects should be carefully considered for the future demonstration of optoelectronic applications relying on coherent exciton-biexciton interactions.
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    Fernée, M. J.; Tamarat, P.; Lounis, B. Cryogenic Single-Nanocrystal Spectroscopy: Reading the Spectral Fingerprint of Individual CdSe Quantum Dots. J. Phys. Chem. Lett. 2013, 4 (4), 609– 618,  DOI: 10.1021/jz302142d
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    13
    Cryogenic Single-Nanocrystal Spectroscopy: Reading the Spectral Fingerprint of Individual CdSe Quantum Dots
    Fernee, Mark J.; Tamarat, Philippe; Lounis, Brahim
    Journal of Physical Chemistry Letters (2013), 4 (4), 609-618CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    A review. Spectroscopically resolved emission from single nanocrystals at cryogenic temps. provides unique insight into phys. processes that occur within these materials. At low temps., the emission spectra collapse to narrow lines, revealing a rich spectroscopic landscape and unexpected properties, completely hidden at the ensemble level. Since these techniques were 1st used, the technol. of nanocrystal synthesis has matured significantly, and new materials with outstanding photostability were reported. In this perspective, cryogenic spectroscopy of single nanocrystals probes the fundamental excitonic structure of the band edge, revealing spectral fingerprints that are highly sensitive to a range of photophys. properties as well as nanocrystal morphol. In particular, spectral and temporal signatures of biexciton and trion emission are revealed, and their relevance to emerging technologies is discussed. Overall cryogenic single nanocrystal spectroscopy can be used as a tool for understanding fundamental photophysics and guiding the synthesis of new nanocrystal materials.
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  14. 14
    Nair, G.; Zhao, J.; Bawendi, M. G. Biexciton Quantum Yield of Single Semiconductor Nanocrystals from Photon Statistics. Nano Lett. 2011, 11 (3), 1136– 1140,  DOI: 10.1021/nl104054t
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    14
    Biexciton Quantum Yield of Single Semiconductor Nanocrystals from Photon Statistics
    Nair, Gautham; Zhao, Jing; Bawendi, Moungi G.
    Nano Letters (2011), 11 (3), 1136-1140CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Biexciton properties strongly affect the usability of a light emitter in quantum photon sources and lasers but are difficult to measure for single fluorophores at room temp. due to luminescence intermittency and bleaching at the high excitation fluences usually required. Here, the authors observe the biexciton (BX) to exciton (X) to ground photoluminescence cascade of single colloidal semiconductor nanocrystals (NCs) under weak excitation in a g(2) photon correlation measurement and show that the normalized amplitude of the cascade feature is the ratio of the BX to X fluorescence quantum yields. This imposes a limit on the attainable depth of photon antibunching and provides a robust means to study single emitter biexciton physics. In NC samples, the BX quantum yield is considerably inhomogeneous, consistent with the defect sensitivity expected of the Auger nonradiative recombination mechanism. The method can be extended to study X,BX spectral and polarization correlations.
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  15. 15
    Beyler, A. P.; Bischof, T. S.; Cui, J.; Coropceanu, I.; Harris, D. K.; Bawendi, M. G. Sample-Averaged Biexciton Quantum Yield Measured by Solution-Phase Photon Correlation. Nano Lett. 2014, 14 (12), 6792– 6798,  DOI: 10.1021/nl5027953
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    15
    Sample-Averaged Biexciton Quantum Yield Measured by Solution-Phase Photon Correlation
    Beyler, Andrew P.; Bischof, Thomas S.; Cui, Jian; Coropceanu, Igor; Harris, Daniel K.; Bawendi, Moungi G.
    Nano Letters (2014), 14 (12), 6792-6798CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    The brightness of nanoscale optical materials such as semiconductor nanocrystals is currently limited in high excitation flux applications by inefficient multiexciton fluorescence. A soln.-phase photon correlation measurement was devised that can conveniently and reliably measure the av. biexciton-to-exciton quantum yield ratio of an entire sample without user selection bias. This technique can be used to study the multiexciton recombination dynamics of a broad scope of synthetically underdeveloped materials, including those with low exciton quantum yields and poor fluorescence stability. This method was applied to measure weak biexciton fluorescence in samples of visible-emitting InP/ZnS and InAs/ZnS core/shell nanocrystals, and to demonstrate that a rapid CdS shell growth procedure can markedly increase the biexciton fluorescence of CdSe nanocrystals.
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  16. 16
    Amgar, D.; Yang, G.; Tenne, R.; Oron, D. Higher-Order Photon Correlation as a Tool to Study Exciton Dynamics in Quasi-2D Nanoplatelets. Nano Lett. 2019, 19 (12), 8741– 8748,  DOI: 10.1021/acs.nanolett.9b03442
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    16
    Higher-Order Photon Correlation as a Tool To Study Exciton Dynamics in Quasi-2D Nanoplatelets
    Amgar, Daniel; Yang, Gaoling; Tenne, Ron; Oron, Dan
    Nano Letters (2019), 19 (12), 8741-8748CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Colloidal semiconductor nanoplatelets, in which carriers are strongly confined only along one dimension, present fundamentally different excitonic properties than quantum dots, which support strong confinement in all three dimensions. In particular, multiple excitons strongly confined in just one dimension are free to rearrange in the lateral plane, reducing the probability for multibody collisions. Thus, while simultaneous multiple photon emission is typically quenched in quantum dots, in nanoplatelets its probability can be tuned according to size and shape. Here, we focus on analyzing multiexciton dynamics in individual CdSe/CdS nanoplatelets of various sizes through the measurement of second-, third-, and fourth-order photon correlations. For the first time, we can directly probe the dynamics of the two, three, and four exciton states at the single nanocrystal level. Remarkably, although higher orders of correlation vary substantially among the synthesis' products, they strongly correlate with the value of second order antibunching. The scaling of the higher-order moments with the degree of antibunching presents a small yet clear deviation from the accepted model of Auger recombination through binary collisions. Such a deviation suggests that many-body contributions are present already at the level of triexcitons. These findings highlight the benefit of high-order photon correlation spectroscopy as a technique to study multiexciton dynamics in colloidal semiconductor nanocrystals.
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  17. 17
    Fisher, B.; Caruge, J. M.; Zehnder, D.; Bawendi, M. Room-Temperature Ordered Photon Emission from Multiexciton States in Single CdSe Core-Shell Nanocrystals. Phys. Rev. Lett. 2005, 94 (8), 087403  DOI: 10.1103/PhysRevLett.94.087403
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    17
    Room-Temperature Ordered Photon Emission from Multiexciton States in Single CdSe Core-Shell Nanocrystals
    Fisher, Brent; Caruge, Jean Michel; Zehnder, Don; Bawendi, Moungi
    Physical Review Letters (2005), 94 (8), 087403/1-087403/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
    The authors report room-temp. ordered multiphoton emission from multiexciton states of single CdSe(CdZnS) core(-shell) colloidal nanocrystals (NCs) that are synthesized by wet chem. methods. Spectrally and temporally resolved measurements of biexciton and triexciton emission from single NCs are also presented. A simple 4 level system models the results accurately and provides ests. for biexciton and triexciton radiative lifetimes and quantum yields.
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  18. 18
    Nasilowski, M.; Spinicelli, P.; Patriarche, G.; Dubertret, B. Gradient CdSe/CdS Quantum Dots with Room Temperature Biexciton Unity Quantum Yield. Nano Lett. 2015, 15 (6), 3953– 3958,  DOI: 10.1021/acs.nanolett.5b00838
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    18
    Gradient CdSe/CdS Quantum Dots with Room Temperature Biexciton Unity Quantum Yield
    Nasilowski, Michel; Spinicelli, Piernicola; Patriarche, Gilles; Dubertret, Benoit
    Nano Letters (2015), 15 (6), 3953-3958CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Auger recombination is a major limitation for the fluorescent emission of quantum dots (QDs). It is the main source of QDs fluorescence blinking at the single-particle level. At high-power excitation, when several charge carriers are formed inside a QD, Auger becomes more efficient and severely decreases the quantum yield (QY) of multiexcitons. This limits the efficiency and the use of colloidal QDs in applications where intense light output is required. Here, the authors present a new generation of thick-shell CdSe/CdS QDs with dimensions >40 nm and a compn. gradient between the core and the shell that exhibits 100% QY for the emission of both the monoexciton and the biexciton in air and at room temp. for all the QDs the authors obsd. The fluorescence emission of these QDs is perfectly Poissonian at the single-particle level at different excitation levels and temps., from 30 to 300 K. In these QDs, the emission of high-order (>2) multiexcitons is quite efficient, and the authors observe white light emission at the single-QD level when high excitation power is used. These gradient thick shell QDs confirm the suppression of Auger recombination in gradient core/shell structures and help further establish the colloidal QDs with a gradient shell as a very stable source of light even under high excitation.
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  19. 19
    Bischof, T. S.; Correa, R. E.; Rosenberg, D.; Dauler, E. A.; Bawendi, M. G. Measurement of Emission Lifetime Dynamics and Biexciton Emission Quantum Yield of Individual InAs Colloidal Nanocrystals. Nano Lett. 2014, 14 (12), 6787– 6791,  DOI: 10.1021/nl502180w
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    19
    Measurement of Emission Lifetime Dynamics and Biexciton Emission Quantum Yield of Individual InAs Colloidal Nanocrystals
    Bischof, Thomas S.; Correa, Raoul E.; Rosenberg, Danna; Dauler, Eric A.; Bawendi, Moungi G.
    Nano Letters (2014), 14 (12), 6787-6791CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    The understanding of the photophysics of visible-emitting colloidal nanocrystals (NCs) has long been aided by single-mol. studies of their emission. Until recently, no suitable detection technologies have existed for corresponding studies of shortwave-IR (SWIR) emitters. Now, the use of superconducting nanowire single-photon detectors (SNSPDs) enables the detailed study of SWIR NC emission dynamics at the single-emitter level. Here, the authors report a detailed anal. of the emission dynamics of individual InAs/CdZnS NCs emitting in the SWIR region. The authors observe blinking akin to A and type B blinking previously obsd. in visible-emitting CdSe NCs. The authors det. the intrinsic radiative lifetime of several InAs/CdZnS NCs and find examples ranging from 50-200 ns, indicative of a quasi-type-II electronic structure. The authors also measure g(2)0 for several of these NCs and find that their biexciton emission quantum yields vary from <1% up to 43%.
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  20. 20
    Zhao, J.; Nair, G.; Fisher, B. R.; Bawendi, M. G. Challenge to the Charging Model of Semiconductor-Nanocrystal Fluorescence Intermittency from off-State Quantum Yields and Multiexciton Blinking. Phys. Rev. Lett. 2010, 104 (15), 157403,  DOI: 10.1103/PhysRevLett.104.157403
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    20
    Challenge to the Charging Model of Semiconductor-Nanocrystal Fluorescence Intermittency from Off-State Quantum Yields and Multiexciton Blinking
    Zhao, Jing; Nair, Gautham; Fisher, Brent R.; Bawendi, Moungi G.
    Physical Review Letters (2010), 104 (15), 157403/1-157403/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
    Semiconductor nanocrystals emit light intermittently; i.e., they blink, under steady illumination. The dark periods were widely assumed to be due to photoluminescence (PL) quenching by an Auger-like process involving a single addnl. charge present in the nanocrystal. Results challenge this long-standing assumption. Close examn. of exciton PL intensity time traces of single CdSe(CdZnS) core(shell) nanocrystals reveals that the dark state PL quantum yield can be 10 times less than the biexciton PL quantum yield. The authors observe spectrally resolved multiexciton emission and find that it also blinks with an on/off ratio >10:1. These results directly contradict the predictions of the charging model.
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  21. 21
    Shulenberger, K. E.; Bischof, T. S.; Caram, J. R.; Utzat, H.; Coropceanu, I.; Nienhaus, L.; Bawendi, M. G. Multiexciton Lifetimes Reveal Triexciton Emission Pathway in CdSe Nanocrystals. Nano Lett. 2018, 18 (8), 5153– 5158,  DOI: 10.1021/acs.nanolett.8b02080
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    21
    Multiexciton Lifetimes Reveal Triexciton Emission Pathway in CdSe Nanocrystals
    Shulenberger, Katherine E.; Bischof, Thomas S.; Caram, Justin R.; Utzat, Hendrik; Coropceanu, Igor; Nienhaus, Lea; Bawendi, Moungi G.
    Nano Letters (2018), 18 (8), 5153-5158CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Multiexcitons in emerging semiconducting nanomaterials play a crit. role in potential optoelectronic and quantum computational devices. We describe photon resolved single mol. methods to directly probe the dynamics of biexcitons and triexcitons in colloidal CdSe quantum dots. We confirm that biexcitons emit from a spin-correlated state, consistent with statistical scaling. Contrary to current understanding, we find that triexciton emission is dominated by band-edge 1Se1S3/2 recombination rather than the higher energy 1Pe1P3/2 recombination.
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  22. 22
    Park, Y. S.; Malko, A. V.; Vela, J.; Chen, Y.; Ghosh, Y.; García-Santamaría, F.; Hollingsworth, J. A.; Klimov, V. I.; Htoon, H. Near-Unity Quantum Yields of Biexciton Emission from CdSe/CdS Nanocrystals Measured Using Single-Particle Spectroscopy. Phys. Rev. Lett. 2011, 106 (18), 187401,  DOI: 10.1103/PhysRevLett.106.187401
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    22
    Near-unity quantum yields of biexciton emission from CdSe/CdS nanocrystals measured using single-particle spectroscopy
    Park, Y.-S.; Malko, A. V.; Vela, J.; Chen, Y.; Ghosh, Y.; Garcia-Santamaria, F.; Hollingsworth, J. A.; Klimov, V. I.; Htoon, H.
    Physical Review Letters (2011), 106 (18), 187401/1-187401/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
    Biexciton luminescence (PL) quantum yields (Q2X) of individual CdSe/CdS core-shell nanocrystal quantum dots with various shell thicknesses are derived from independent PL satn. and 2-photon correlation measurements. A near-unity Q2X was obsd. for some nanocrystals with an ultrathick 19-monolayer shell. High Q2X's are not universal and vary widely among nominally identical nanocrystals indicating a significant dependence of Q2X upon subtle structural differences. The measurements indicate that high Q2X's are not required to achieve complete suppression of PL intensity fluctuations in individual nanocrystals.
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    Park, Y. S.; Bae, W. K.; Pietryga, J. M.; Klimov, V. I. Auger Recombination of Biexcitons and Negative and Positive Trions in Individual Quantum Dots. ACS Nano 2014, 8 (7), 7288– 7296,  DOI: 10.1021/nn5023473
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    23
    Auger Recombination of Biexcitons and Negative and Positive Trions in Individual Quantum Dots
    Park, Young-Shin; Bae, Wan Ki; Pietryga, Jeffrey M.; Klimov, Victor I.
    ACS Nano (2014), 8 (7), 7288-7296CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Charged exciton states commonly occur both in spectroscopic studies of quantum dots (QDs) and during operation of QD-based devices. The extra charge added to the neutral exciton modifies its radiative decay rate and also opens an addnl. nonradiative pathway assocd. with an Auger process whereby the recombination energy of an exciton is transferred to the excess charge. Single-dot spectroscopic studies were conducted of Auger recombination in thick-shell (giant) CdSe/CdS QDs with and without an interfacial alloy layer using time-tagged, time-correlated single-photon counting. In luminescence (PL) intensity trajectories of some of the dots, the authors resolve 3 distinct states of different emissivities (bright, gray, and dark) attributed, resp., to the neutral exciton and neg. and pos. trions. Simultaneously acquired PL lifetime trajectories indicate that the pos. trion is much shorter lived than the neg. trion, which can be explained by a high d. of valence band states and a small hole localization radius (defined by the QD core size), factors that favor an Auger process involving intraband excitation of a hole. A comparison of trion and biexciton lifetimes suggests that the biexciton Auger decay can be treated in terms of a superposition of 2 independent channels assocd. with pos.- and neg.-trion pathways. The resulting interdependence between Auger time consts. might simplify the studies of multicarrier recombination by allowing 1, for example, to infer Auger lifetimes of trions of 1 sign based on the measurements of biexciton decay and dynamics of the trions of the opposite sign or, alternatively, est. the biexciton lifetime based on studies of trion dynamics.
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    Rabouw, F. T.; Lunnemann, P.; Van Dijk-Moes, R. J. A.; Frimmer, M.; Pietra, F.; Koenderink, A. F.; Vanmaekelbergh, D. Reduced Auger Recombination in Single CdSe/CdS Nanorods by One-Dimensional Electron Delocalization. Nano Lett. 2013, 13 (10), 4884– 4892,  DOI: 10.1021/nl4027567
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    24
    Reduced Auger Recombination in Single CdSe/CdS Nanorods by One-Dimensional Electron Delocalization
    Rabouw, Freddy T.; Lunnemann, Per; van Dijk-Moes, Relinde J. A.; Frimmer, Martin; Pietra, Francesca; Koenderink, A. Femius; Vanmaekelbergh, Daniel
    Nano Letters (2013), 13 (10), 4884-4892CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Progress to reduce nonradiative Auger decay in colloidal nanocrystals has recently been made by growing thick shells. However, the physics of Auger suppression is not yet fully understood. Here, we examine the dynamics and spectral characteristics of single CdSe-dot-in-CdS-rod nanocrystals. These exhibit blinking due to charging/discharging, as well as trap-related blinking. We show that one-dimensional electron delocalization into the rod-shaped shell can be as effective as a thick spherical shell at reducing Auger recombination of the neg. trion state.
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    Zhao, J.; Chen, O.; Strasfeld, D. B.; Bawendi, M. G. Biexciton Quantum Yield Heterogeneities in Single CdSe (CdS) Core (Shell) Nanocrystals and Its Correlation to Exciton Blinking. Nano Lett. 2012, 12 (9), 4477– 4483,  DOI: 10.1021/nl3013727
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    25
    Biexciton Quantum Yield Heterogeneities in Single CdSe (CdS) Core (Shell) Nanocrystals and Its Correlation to Exciton Blinking
    Zhao, Jing; Chen, Ou; Strasfeld, David B.; Bawendi, Moungi G.
    Nano Letters (2012), 12 (9), 4477-4483CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Biexciton (BX) nonradiative recombination processes in single semiconductor nanocrystals (NCs) were explored using confocal fluorescence microscopy and 2nd-order photon intensity correlation. More specifically, the authors measure the luminescence blinking and BX quantum yields (QYs) and study the correlation between these 2 measurements for single core (shell) CdSe (CdS) nanocrystals (NCs). NCs with a high on time fraction are significantly more likely to have a high BX QY than NCs with a low on fraction, even though the BX QYs of NCs with a high on fraction vary dramatically. The BX QYs of single NCs are also weakly dependent upon excitation wavelength. The weak correlation between exciton on fractions and BX QYs suggests that multiple recombination processes are involved in the BX recombination. To explain the results, the authors propose a model that combines both trapping and an Auger mechanism for BX recombination.
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    Rabouw, F. T.; Vaxenburg, R.; Bakulin, A. A.; van Dijk-Moes, R. J. A.; Bakker, H. J.; Rodina, A.; Lifshitz, E.; Efros, A. L.; Koenderink, A. F.; Vanmaekelbergh, D. Dynamics of Intraband and Interband Auger Processes in Colloidal Core-Shell Quantum Dots. ACS Nano 2015, 9 (10), 10366– 10376,  DOI: 10.1021/acsnano.5b04491
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    26
    Dynamics of Intraband and Interband Auger Processes in Colloidal Core-Shell Quantum Dots
    Rabouw, Freddy T.; Vaxenburg, Roman; Bakulin, Artem A.; van Dijk-Moes, Relinde J. A.; Bakker, Huib J.; Rodina, Anna; Lifshitz, Efrat; Efros, Alexander L.; Koenderink, A. Femius; Vanmaekelbergh, Daniel
    ACS Nano (2015), 9 (10), 10366-10376CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Conventional colloidal quantum dots (QDs) suffer from rapid energy losses by nonradiative (Auger) processes, leading to sub-ns lifetimes in all excited states but the lowest-energy single exciton. Suppression of interband Auger decay, such as biexciton Auger recombination, has been achieved with the design of heterostructured core-shell QDs. Auger-like processes are also believed to be responsible for rapid intraband hot-electron cooling in QDs. However, the simultaneous effect of shell growth on interband Auger recombination and intraband hot-electron cooling has not been addressed. Here we investigate how the growth of a CdS shell affects these two relaxation processes in CdSe/CdS core-shell QDs. Using a combination of ultrafast pump-push-probe spectroscopy on the QD ensemble and anal. of the photon statistics from single QDs, we find that Auger losses in the biexciton state are suppressed with increasing shell thickness, while hot-electron cooling remains unaffected. Calcns. conducted within an eight-band k·p model confirm the exptl. dependence of the biexciton Auger decay on the shell thickness, and provide insights into the factors detg. the cooling rate of hot carriers.
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    Klimov, V. I.; Ivanov, S. A.; Nanda, J.; Achermann, M.; Bezel, I.; McGuire, J. A.; Piryatinski, A. Single-Exciton Optical Gain in Semiconductor Nanocrystals. Nature 2007, 447 (7143), 441– 446,  DOI: 10.1038/nature05839
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    Single-exciton optical gain in semiconductor nanocrystals
    Klimov, Victor I.; Ivanov, Sergei A.; Nanda, Jagjit; Achermann, Marc; Bezel, Ilya; McGuire, John A.; Piryatinski, Andrei
    Nature (London, United Kingdom) (2007), 447 (7143), 441-446CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
    Nanocrystal quantum dots have favorable light-emitting properties. They show photoluminescence with high quantum yields, and their emission colors depend on the nanocrystal size-owing to the quantum-confinement effect-and are therefore tunable. However, nanocrystals are difficult to use in optical amplification and lasing. Because of an almost exact balance between absorption and stimulated emission in nanoparticles excited with single electron-hole pairs (excitons), optical gain can only occur in nanocrystals that contain at least two excitons. A complication assocd. with this multiexcitonic nature of light amplification is fast optical-gain decay induced by non-radiative Auger recombination, a process in which one exciton recombines by transferring its energy to another. Here we demonstrate a practical approach for obtaining optical gain in the single-exciton regime that eliminates the problem of Auger decay. Specifically, we develop core/shell hetero-nanocrystals engineered in such a way as to spatially sep. electrons and holes between the core and the shell (type-II heterostructures). The resulting imbalance between neg. and pos. charges produces a strong local elec. field, which induces a giant (~100 meV or greater) transient Stark shift of the absorption spectrum with respect to the luminescence line of singly excited nanocrystals. This effect breaks the exact balance between absorption and stimulated emission, and allows us to demonstrate optical amplification due to single excitons.
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    Chen, O.; Zhao, J.; Chauhan, V. P.; Cui, J.; Wong, C.; Harris, D. K.; Wei, H.; Han, H. S.; Fukumura, D.; Jain, R. K. Compact High-Quality CdSe-CdS Core-Shell Nanocrystals with Narrow Emission Linewidths and Suppressed Blinking. Nat. Mater. 2013, 12 (5), 445– 451,  DOI: 10.1038/nmat3539
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    28
    Compact high-quality CdSe-CdS core-shell nanocrystals with narrow emission linewidths and suppressed blinking
    Chen, Ou; Zhao, Jing; Chauhan, Vikash P.; Cui, Jian; Wong, Cliff; Harris, Daniel K.; Wei, He; Han, Hee-Sun; Fukumura, Dai; Jain, Rakesh K.; Bawendi, Moungi G.
    Nature Materials (2013), 12 (5), 445-451CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
    High particle uniformity, high luminescence quantum yields, narrow and sym. emission spectral lineshapes and min. single-dot emission intermittency (known as blinking) were recognized as universal requirements for the successful use of colloidal quantum dots in nearly all optical applications. Synthesizing samples that simultaneously meet all these 4 criteria proved challenging. The synthesis of such high-quality CdSe-CdS core-shell quantum dots in an optimized process that maintains a slow growth rate of the shell through the use of octanethiol and Cd oleate as precursors is reported. In contrast with previous observations, single-dot blinking is significantly suppressed with only a relatively thin shell. The elimination of the ensemble luminescence photodarkening that is an intrinsic consequence of quantum dot blinking statistical ageing is demonstrated. The small size and high luminescence quantum yields of these novel quantum dots render them superior in vivo imaging agents compared with conventional quantum dots. It is anticipated that these quantum dots will result in significant improvement in the performance of quantum dots in other applications such as solid-state lighting and illumination.
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    Xing, G.; Mathews, N.; Lim, S. S.; Yantara, N.; Liu, X.; Sabba, D.; Grätzel, M.; Mhaisalkar, S.; Sum, T. C. Low-Temperature Solution-Processed Wavelength-Tunable Perovskites for Lasing. Nat. Mater. 2014, 13 (5), 476– 480,  DOI: 10.1038/nmat3911
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    Low-temperature solution-processed wavelength-tunable perovskites for lasing
    Xing, Guichuan; Mathews, Nripan; Lim, Swee Sien; Yantara, Natalia; Liu, Xinfeng; Sabba, Dharani; Graetzel, Michael; Mhaisalkar, Subodh; Sum, Tze Chien
    Nature Materials (2014), 13 (5), 476-480CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
    Low-temp. soln.-processed materials that show optical gain and can be embedded into a wide range of cavity resonators are attractive for the realization of on-chip coherent light sources. Org. semiconductors and colloidal quantum dots are considered the main candidates for this application. However, stumbling blocks in org. lasing include intrinsic losses from bimol. annihilation and the conflicting requirements of high charge carrier mobility and large stimulated emission; whereas challenges pertaining to Auger losses and charge transport in quantum dots still remain. Herein, the authors reveal that soln.-processed org.-inorg. halide perovskites (CH3NH3PbX3 where X = Cl, Br, I), which demonstrated huge potential in photovoltaics, also have promising optical gain. Their ultra-stable amplified spontaneous emission at strikingly low thresholds stems from their large absorption coeffs., ultralow bulk defect densities and slow Auger recombination. Straightforward visible spectral tunability (390-790 nm) is demonstrated. Importantly, in view of their balanced ambipolar charge transport characteristics, these materials may show elec. driven lasing.
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    Bisschop, S.; Geiregat, P.; Aubert, T.; Hens, Z. The Impact of Core/Shell Sizes on the Optical Gain Characteristics of CdSe/CdS Quantum Dots. ACS Nano 2018, 12 (9), 9011– 9021,  DOI: 10.1021/acsnano.8b02493
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    30
    The Impact of Core/Shell Sizes on the Optical Gain Characteristics of CdSe/CdS Quantum Dots
    Bisschop, Suzanne; Geiregat, Pieter; Aubert, Tangi; Hens, Zeger
    ACS Nano (2018), 12 (9), 9011-9021CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Colloidal quantum dots (QDs) are highly attractive as the active material for optical amplifiers and lasers. Here, we address the relation between the structure of CdSe/CdS core/shell QDs, the material gain they can deliver, and the threshold needed to attain net stimulated emission by optical pumping. On the basis of an initial gain model, we predict that reducing the thickness of the CdS shell grown around a given CdSe core will increase the maximal material gain, while increasing the shell thickness will lower the gain threshold. We assess this trade-off by means of transient absorption spectroscopy. Our results confirm that thin-shell QDs exhibit the highest material gain. In quant. agreement with the model, core and shell sizes hugely impact on the material gain, which ranges from 2800 cm-1 for large core/thin shell QDs to less than 250 cm-1 for small core/thick shell QDs. On the other hand, the significant threshold redn. expected for thick-shell QDs is absent. We relate this discrepancy between model and expt. to a transition from attractive to repulsive exciton-exciton interactions with increasing shell thickness. The spectral blue-shift that comes with exciton-exciton repulsion leads to competition between stimulated emission and higher energy absorbing transitions, which raises the gain threshold. As a result, small-core/thick-shell QDs need up to 3.7 excitations per QD to reach transparency, whereas large-core/thin shell QDs only need 1.0, a no. often seen as a hard limit for biexciton-mediated optical gain. This makes large-core/thin-shell QDs that feature attractive exciton-exciton interactions the overall champion core/shell configuration in view of highest material gain, lowest threshold exciton occupation, and longest gain lifetime.
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    Rabouw, F. T.; Kamp, M.; Van Dijk-Moes, R. J. A.; Gamelin, D. R.; Koenderink, A. F.; Meijerink, A.; Vanmaekelbergh, D. Delayed Exciton Emission and Its Relation to Blinking in CdSe Quantum Dots. Nano Lett. 2015, 15 (11), 7718– 7725,  DOI: 10.1021/acs.nanolett.5b03818
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    Delayed Exciton Emission and Its Relation to Blinking in CdSe Quantum Dots
    Rabouw, Freddy T.; Kamp, Marko; van Dijk-Moes, Relinde J. A.; Gamelin, Daniel R.; Koenderink, A. Femius; Meijerink, Andries; Vanmaekelbergh, Daniel
    Nano Letters (2015), 15 (11), 7718-7725CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    The efficiency and stability of emission from semiconductor nanocrystal quantum dots (QDs) is neg. affected by blinking on the single-nanocrystal level, i.e., random alternation of bright and dark periods. The time scales of these fluctuations can be as long as many seconds, orders of magnitude longer than typical lifetimes of exciton states in QDs. The authors study luminescence from QDs delayed over microseconds to milliseconds. The results prove the existence of long-lived charge-sepd. states in QDs. The authors study the properties of delayed emission as a direct way to learn about charge carrier sepn. and recovery of the exciton state. A new microscopic model is developed to connect delayed emission to exciton recombination and blinking from which bright periods in blinking are in fact not characterized by uninterrupted optical cycling as often assumed.
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    Akselrod, G. M.; Prins, F.; Poulikakos, L. V.; Lee, E. M. Y.; Weidman, M. C.; Mork, A. J.; Willard, A. P.; Bulović, V.; Tisdale, W. A. Subdiffusive Exciton Transport in Quantum Dot Solids. Nano Lett. 2014, 14 (6), 3556– 3562,  DOI: 10.1021/nl501190s
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    Subdiffusive Exciton Transport in Quantum Dot Solids
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    Nano Letters (2014), 14 (6), 3556-3562CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
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    A facile and robust synthesis of CdxZn1-xS graded shells on CdSe nanoparticles that are prepd. by interface alloying between CdS and ZnS shells at elevated temps. is reported. Alloying provides systematic control over the electronic structure and enables switching between Type-I and quasi-Type-II configurations. Good control of particle shape, shell thickness, and compn. is achieved by slowly adding Zn oleate and octane thiol via syringe pump to readily prepd. CdSe/CdS particles. The resultant quantum dots exhibit PL quantum yields of up to 97% and superior robustness toward environmental influences and quenching agents. Alloying promotes a blue-shift of both the absorption and PL spectra compared to pure CdSe/CdS particles and an increased Stokes shift, opening a new synthetic pathway to stable, green-emitting core/shell/shell quantum dots. High PL quantum yields are correlated to a narrow distribution of single-particle lifetimes and suppressed fluorescence intermittency. A new method to characterize the PL intermittency of single quantum dots based on the autocorrelation function of their PL time trajectories is introduced.
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    Hot-injection synthesis is renowned for producing semiconductor nanocolloids with superb size dispersions. Burst nucleation and diffusion-controlled size focusing during growth have been invoked to rationalize this characteristic yet exptl. evidence supporting the pertinence of these concepts is scant. By monitoring a CdSe synthesis in-situ with X-ray scattering, we find that nucleation is an extended event that coincides with growth during 15-20% of the reaction time. Moreover, we show that size focusing outpaces predictions of diffusion-limited growth. This observation indicates that nanocrystal growth is dictated by the surface reactivity, which drops sharply for larger nanocrystals. Kinetic reaction simulations confirm that this so-called superfocusing can lengthen the nucleation period and promote size focusing. The finding that narrow size dispersions can emerge from the counteracting effects of extended nucleation and reaction-limited size focusing ushers in an evidence-based perspective that turns hot injection into a rational scheme to produce monodisperse semiconductor nanocolloids.
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    CdSe and CdSe/ZnSe core shell nanocrystals were prepd. via the inverse micelle technol. with TOP/TOPO/HDA surfactants, and their high crystallinity was confirmed by using X-ray diffraction (XRD) and high-resoln. transmission electron microscopy (HRTEM) analyses. Ostwald ripening behavior of the nanocrystals was monitored by using the red-shift in UV-visible absorbance peaks, and their size variation was estd. by employing a quantum confinement effect equation. Lifshitz-Slyozov-Wagner (LSW) kinetics analyses were performed by using the size variation according to ripening temp. and time period. Arrhenius-type plots were created by using the slopes of the LSW curves for the CdSe and CdSe/ZnSe nanocrystals, resp., and the activation energy values for the ripening were evaluated for the nanocrystals. At a low-temp. region, the CdSe and CdSe/ZnSe samples seem to show dissocn. of Cd-Se and Zn-Se surface at. bonds, resp., while at a high-temp. region above 266 °C, both samples seem to show active dissocn. of both Cd-Se and Zn-Se lattice at. bonds. The CdSe-ZnSe shows relatively low activation energy for the ripening, compared to the bare CdSe possibly due to weak Zn-Se at. bonds. One can complete the Ostwald ripening kinetics equation by using two kinetics variables, derived in this study, for the estn. of the size of CdSe/ZnSe core/shell nanocrystals. Also, this approach can be applied to ripening kinetics of other core/shell nanocrystal systems.
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  • Abstract

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    Figure 1

    Figure 1. (a) Schematic of the galvo-APD setup. The QD emission is directed to a periodically rotating galvo mirror, which changes the angle of incidence on a transmission grating. Fourier imaging of the grating plane projects part of the QD emission spectrum onto an avalanche photodiode (APD). (b) Periodically rotating the galvo mirror with 0.5 Hz scans the emission spectrum of the QDs (pulsed excitation 2.5 MHz) over the APD twice every 2 s. We can convert the experimental time T to a photon energy (see inset). (c, d) Time-resolved emission spectra of a micrometer-thick layer of QDs on glass at increasing excitation-pulse energy. The white lines indicate the emission maximum as a function of delay time. At the highest power, the spectrum is slightly blue shifted at delay times of 0–2 ns because of the contribution of repulsive biexcitons. (e) Emission spectrum integrated between 0 and 0.6 ns delay times for n = 0.005 (red, panel c) and n = 0.23 (blue, panel d) excitons per pulse. By globally fitting a series of spectra for different fluences, we obtain an ensemble biexciton shift of E̅b = +14 meV.

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    Figure 2

    Figure 2. (a) Schematic of the cascade spectroscopy setup. The emission of a single QD is split equally over the energy-resolved “galvo APD” (Figure 1a) and a regular APD. (b) Photon correlation function g(2) of a single QD (repetition rate 2.5 MHz, estimated average number of excitons per pulse μ = 0.1) integrated over the entire experiment, showing a biexciton efficiency of ηBX = 10.5%. The highlights mark the delay time ranges where events of types 1–, 0–, 0+, and 1+ contribute. (c) Overview of possible ordered-emission events. The laser pulses are the vertical blue lines. Events wherein two photons (dots) are detected following two consecutive laser pulses are highlighted 1+ and 1–, depending on the order of the detector clicks. Two photons detected after a single laser pulse, indicating biexciton cascade emission, are highlighted 0+ and 0–. The first photon of a cascade is a biexciton photon (blue dots); all other photons most likely originate from the single-exciton state (red dots). (d, e) Energy-resolved g(2) (log y-scale) for low (d) and high (e) galvo-APD detection energies. Lower galvo-APD detection energies Edet = 1.95–1.99 eV yield a ratio N0+/N0– < 1, while for blue-shifted detection energies Edet = 2.08–2.16 eV, N0+/N0– > 1. This indicates a repulsive biexciton. The insets (linear y scale) are enlarged views to show N0+/N0– more clearly. We discarded afterglow events from panels b, d, and e (Figure S3 in the Supporting Information). (f) N0+/N1 as a function of galvo-APD detection energy Edet. The gray spectrum shows the exciton emission of this single QD. The error bars are calculated assuming shot noise. By fitting eq 2 to the data, we extract a single-QD biexciton shift of Eb = 6.8 ± 1.0 meV.

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    Figure 3

    Figure 3. (a) Biexciton (blue) and exciton (red) emission spectra of the single QD analyzed in Figure 2. The biexciton spectrum was extracted from the positive-time area N0+ of the zero-delay peak in the energy-resolved photon-correlation function. We measure a biexciton shift of Eb = 7.2 ± 2.1 meV. (b) A different QD with exciton with a larger biexciton shift of Eb = 12.7 ± 1.9 meV. (c) Biexciton and exciton line widths (fwhm) of 22 single QDs measured. Error bars are standard errors of the Gaussian fit.

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    Figure 4

    Figure 4. (a) Biexciton shift Eb versus exciton energy EX for randomly selected QDs (blue) and for QDs selected for their low-energy exciton emission (red). Error bars denote one standard error of the fit. We observe a slight negative correlation between biexciton shift and exciton energy. (b) Dependence of Eb and EX on CdSe core radius a and CdSe/CdS core–inner-shell radius b, calculated with an effective-mass model that includes Coulomb interactions between carriers as a perturbation to second order (section S5 in the Supporting Information). The QD geometry is given as grid lines for a (core radius, solid lines) and b (core–inner-shell radius, dashed lines). The gray-shaded area highlights the range of exciton energies and biexciton shifts found experimentally. The red point and arrows are the calculated exciton energy and biexciton shift for the independently determined values of a = 1.84 nm (from the absorption spectrum of the cores) and b = 4.1 ± 0.4 nm (from electron microscopy).

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  • References

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      CsPbX3 (X = Cl, Br, I) perovskite nanocrystals (PNCs) have recently become a promising material for optoelectronic applications due to their high emission quantum yields and facile band gap tunability via both halide compn. and size. The spectroscopy of single PNCs enhances the understanding of the effect of confinement on excitations in PNCs in the absence of obfuscating ensemble averaging and can also inform synthetic efforts. Single PNC studies were hampered by poor PNC photostability under confocal excitation, precluding interrogation of all but the most stable PNCs, leading to a lack of understanding of PNCs in the regime of high confinement. The 1st comprehensive spectroscopic study of single PNC properties using soln.-phase photon-correlation methods are reported, including both highly confined and blue-emitting PNCs, previously inaccessible to single NC techniques. With min. perturbative soln.-phase photon-correlation Fourier spectroscopy (s-PCFS), the ensemble emission linewidth of PNCs of all sizes and compns. is predominantly detd. by the intrinsic single NC linewidth (homogeneous broadening). The single PNC linewidth dramatically increases with increasing confinement, consistent with what was found for II-VI semiconductor nanocrystals. With soln.-phase photon antibunching measurements, the biexciton-to-exciton quantum yield ratio (BX/X QY) in the absence of user-selection bias or photodegrdn. were surveyed. The BX/X QY ratio depends both on the PNC size and halide compn., with values between ∼2% for highly confined bromide PNCs and ∼50% for intermediately confined iodide PNCs. The results suggest a wide range of underlying Auger rates likely due to transitory charge carrier sepn. in PNCs with relaxed confinement.
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      Nano Letters (2017), 17 (5), 2809-2815CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      The finite spectral line width of an ensemble of CdSe nanocrystals arises from size and shape inhomogeneity and the single-nanocrystal spectrum itself. This line width directly limits the performance of nanocrystal-based devices, yet most optical measurements cannot resolve the underlying contributions. We use two-dimensional electronic spectroscopy (2D ES) to measure the line width of the band-edge exciton of CdSe nanocrystals as a function of radii and surface chem. We find that the homogeneous width decreases for increasing nanocrystal radius and that surface chem. plays a crit. role in controlling this line width. To explore the hypothesis that unpassivated trap states serve to broaden the homogeneous line width and to explain its size-dependence, we use 3D ES to identify the spectral signatures of exciton-phonon coupling to optical and acoustic phonons. We find enhanced coupling to optical phonon modes for nanocrystals that lack electron-passivating ligands, suggesting that localized surface charges enhance exciton-phonon coupling via the Frohlich interaction. Lastly, the data reveal that spectral diffusion contributes negligibly to the homogeneous line width on subnanosecond time scales.
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      Van Der Bok, J. C.; Dekker, D. M.; Peerlings, M. L. J.; Salzmann, B. B. V.; Meijerink, A. Luminescence Line Broadening of CdSe Nanoplatelets and Quantum Dots for Application in W-LEDs. J. Phys. Chem. C 2020, 124 (22), 12153– 12160,  DOI: 10.1021/acs.jpcc.0c03048
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      Luminescence Line Broadening of CdSe Nanoplatelets and Quantum Dots for Application in w-LEDs
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      Journal of Physical Chemistry C (2020), 124 (22), 12153-12160CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Nanoplatelets (NPLs) of CdSe are an emerging class of luminescent materials, combining tunable and narrow emission bands with high quantum yields. This is promising for application in white light LEDs (w-LEDs) and displays. The origin of the narrow spectral width of exciton emission in core NPL compared to core-shell NPL and quantum dot (QD) emission is not fully understood. Here we investigate and compare temp.-dependent emission spectra of core and core-shell CdSe NPLs and QDs. A wide temp. range, 4-423 K, is chosen to gain insight into contributions from homogeneous and inhomogeneous broadening and also to extend measurements into a temp. regime that is relevant for operating conditions in w-LEDs (T ≈ 423 K). The results show that temp.-induced homogeneous broadening does not strongly vary between the various CdSe nanostructures (ΔEhom ≈ 60-80 meV at 423 K) indicating that electron-phonon coupling strengths are similar. Only for the smallest QDs is stronger coupling obsd. The origin of the narrow bandwidth reported at 300 K for core CdSe NPLs is attributed to a very narrow inhomogeneous line width. At 423 K, the spectral width of NPL exciton emission is still narrower than that of QDs. A comparison with traditional w-LED phosphors is made to outline advantages (tunability, narrow bandwidth, high efficiency) and disadvantages (color shift, stability issues) of NPLs for application in w-LEDs.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3cXoslOgsL4%253D&md5=39612897046b3a23087586d61cc602a8
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      Bae, W. K.; Park, Y. S.; Lim, J.; Lee, D.; Padilha, L. A.; McDaniel, H.; Robel, I.; Lee, C.; Pietryga, J. M.; Klimov, V. I. Controlling the Influence of Auger Recombination on the Performance of Quantum-Dot Light-Emitting Diodes. Nat. Commun. 2013, 4, 3661,  DOI: 10.1038/ncomms3661
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      Controlling the influence of Auger recombination on the performance of quantum-dot light-emitting diodes
      Bae, Wan Ki; Park, Young-Shin; Lim, Jaehoon; Lee, Donggu; Padilha, Lazaro A.; McDaniel, Hunter; Robel, Istvan; Lee, Changhee; Pietryga, Jeffrey M.; Klimov, Victor I.
      Nature Communications (2013), 4 (), 3661/1-3661/8CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
      Development of light-emitting diodes (LEDs) based on colloidal quantum dots is driven by attractive properties of these fluorophores such as spectrally narrow, tunable emission and facile processibility via soln.-based methods. A current obstacle towards improved LED performance is an incomplete understanding of the roles of extrinsic factors, such as nonradiative recombination at surface defects, vs. intrinsic processes, such as multicarrier Auger recombination or electron-hole sepn. due to applied elec. field. Here, we address this problem with studies that correlate the excited state dynamics of structurally engineered quantum dots with their emissive performance within LEDs.We find that because of significant charging of quantum dots with extra electrons, Auger recombination greatly impacts both LED efficiency and the onset of efficiency roll-off at high currents. Further, we demonstrate two specific approaches for mitigating this problem using heterostructured quantum dots, either by suppressing Auger decay through the introduction of an intermediate alloyed layer, or by using an addnl. shell that impedes electron transfer into the quantum dot to help balance electron and hole injection.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXhtVSrsrc%253D&md5=bf2a373aee50cce9ad983b4b3bc13b46
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      Continuous-wave biexciton lasing at room temperature using solution-processed quantum wells
      Grim, Joel Q.; Christodoulou, Sotirios; Di Stasio, Francesco; Krahne, Roman; Cingolani, Roberto; Manna, Liberato; Moreels, Iwan
      Nature Nanotechnology (2014), 9 (11), 891-895CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
      Soln.-processed inorg. and org. materials have been pursued for more than a decade as low-threshold, high-gain lasing media, motivated in large part by their tunable optoelectronic properties and ease of synthesis and processing. Although both have demonstrated stimulated emission and lasing, they have not yet approached the continuous-wave pumping regime. Two-dimensional CdSe colloidal nanosheets combine the advantage of soln. synthesis with the optoelectronic properties of epitaxial two-dimensional quantum wells. Here, we show that these colloidal quantum wells possess large exciton and biexciton binding energies of 132 meV and 30 meV, resp., giving rise to stimulated emission from biexcitons at room temp. Under femtosecond pulsed excitation, close-packed thin films yield an ultralow stimulated emission threshold of 6 μJ cm-2, sufficient to achieve continuous-wave pumped stimulated emission, and lasing when these layers are embedded in surface-emitting microcavities.
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      Hollingsworth, J. A. Heterostructuring Nanocrystal Quantum Dots toward Intentional Suppression of Blinking and Auger Recombination. Chem. Mater. 2013, 25 (8), 1318– 1331,  DOI: 10.1021/cm304161d
      9
      Heterostructuring Nanocrystal Quantum Dots Toward Intentional Suppression of Blinking and Auger Recombination
      Hollingsworth, Jennifer A.
      Chemistry of Materials (2013), 25 (8), 1318-1331CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      A review. At the level of a single particle, nanocrystal quantum dots (NQDs) fluoresce intermittently or blink. They are also characterized by an efficient nonradiative recombination process known as Auger recombination (AR). Recently, new approaches to NQD heterostructuring were developed that directly impact both blinking and AR, resulting in dramatic suppression of these unwanted processes. The 3 successful hetero-NQD motifs are reviewed here: (1) interfacial alloying, (2) thick or giant shells, and (3) specific type-II electronic structures. These approaches, which rely on modifying or tuning internal NQD core/shell structures, are compared with alternative strategies for blinking suppression that rely, instead, on surface modifications or surface-mediated interactions. Finally, in each case, the unique synthetic approaches or challenges addressed that have driven the realization of novel and important functionality are discussed, along with the implications for development of a comprehensive materials design strategy for blinking and AR-suppressed heterostructured NQDs.
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      Multicarrier recombination in colloidal quantum dots
      Pandey, Anshu; Guyot-Sionnest, Philippe
      Journal of Chemical Physics (2007), 127 (11), 111104/1-111104/4CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
      The rates of multicarrier recombination are measured in colloidal quantum dots with and without hole surface trapping and with hole extn. in type II core/shell systems. We report that hole trapping or the phys. sepn. of electronic and hole wavefunctions into different semiconductor domains of a type II system have an insignificant effect on the multicarrier recombination dynamics. These observations are inconsistent with the accepted Auger transition mechanism.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2sXhtFSrsLjM&md5=3c1db0edd33332c2254e3170e604b33b
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      He, Y. M.; Iff, O.; Lundt, N.; Baumann, V.; Davanco, M.; Srinivasan, K.; Höfling, S.; Schneider, C. Cascaded Emission of Single Photons from the Biexciton in Monolayered WSe2. Nat. Commun. 2016, 7, 13409,  DOI: 10.1038/ncomms13409
      11
      Cascaded emission of single photons from the biexciton in monolayered WSe2
      He, Yu-Ming; Iff, Oliver; Lundt, Nils; Baumann, Vasilij; Davanco, Marcelo; Srinivasan, Kartik; Hoefling, Sven; Schneider, Christian
      Nature Communications (2016), 7 (), 13409CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
      Monolayers of transition metal dichalcogenide materials emerged as a new material class to study excitonic effects in solid state, as they benefit from enormous Coulomb correlations between electrons and holes. Esp. in WSe2, sharp emission features have been obsd. at cryogenic temps., which act as single photon sources. Tight exciton localization has been assumed to induce an anharmonic excitation spectrum; however, the evidence of the hypothesis, namely the demonstration of a localized biexciton, is elusive. Here we unambiguously demonstrate the existence of a localized biexciton in a monolayer of WSe2, which triggers an emission cascade of single photons. The biexciton is identified by its time-resolved photoluminescence, superlinearity and distinct polarization in micro-photoluminescence expts. We evidence the cascaded nature of the emission process in a cross-correlation expt., which yields a strong bunching behavior. Our work paves the way to a new generation of quantum optics expts. with two-dimensional semiconductors.
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      Huang, X.; Chen, L.; Zhang, C.; Qin, Z.; Yu, B.; Wang, X.; Xiao, M. Inhomogeneous Biexciton Binding in Perovskite Semiconductor Nanocrystals Measured with Two-Dimensional Spectroscopy. J. Phys. Chem. Lett. 2020, 11 (23), 10173– 10181,  DOI: 10.1021/acs.jpclett.0c03153
      12
      Inhomogeneous Biexciton Binding in Perovskite Semiconductor Nanocrystals Measured with Two-Dimensional Spectroscopy
      Huang, Xinyu; Chen, Lan; Zhang, Chunfeng; Qin, Zhengyuan; Yu, Buyang; Wang, Xiaoyong; Xiao, Min
      Journal of Physical Chemistry Letters (2020), 11 (23), 10173-10181CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      Perovskite semiconductor nanocrystals have emerged as an excellent family of materials for optoelectronic applications, where biexciton interaction is essential for optical gain generation and quantum light emission. However, the strength of biexciton interaction remains highly controversial due to the entangled spectral features of the exciton- and biexciton-related transitions in conventional measurement approaches. Here, we tackle the limitation by using polarization-dependent two-dimensional electronic spectroscopy and quantify the excitation energy-dependent biexciton binding energy at cryogenic temps. The biexciton binding energy increases with excitation energy, which can be modeled as a near inverse-square size dependence in the effective mass approxn. considering the quantum confinement effect. The spectral line width for the exciton-biexciton transition is much broader than that for the ground state to exciton transition, suggesting weakly correlated broadening between these transitions. These inhomogeneity effects should be carefully considered for the future demonstration of optoelectronic applications relying on coherent exciton-biexciton interactions.
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      Fernée, M. J.; Tamarat, P.; Lounis, B. Cryogenic Single-Nanocrystal Spectroscopy: Reading the Spectral Fingerprint of Individual CdSe Quantum Dots. J. Phys. Chem. Lett. 2013, 4 (4), 609– 618,  DOI: 10.1021/jz302142d
      13
      Cryogenic Single-Nanocrystal Spectroscopy: Reading the Spectral Fingerprint of Individual CdSe Quantum Dots
      Fernee, Mark J.; Tamarat, Philippe; Lounis, Brahim
      Journal of Physical Chemistry Letters (2013), 4 (4), 609-618CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      A review. Spectroscopically resolved emission from single nanocrystals at cryogenic temps. provides unique insight into phys. processes that occur within these materials. At low temps., the emission spectra collapse to narrow lines, revealing a rich spectroscopic landscape and unexpected properties, completely hidden at the ensemble level. Since these techniques were 1st used, the technol. of nanocrystal synthesis has matured significantly, and new materials with outstanding photostability were reported. In this perspective, cryogenic spectroscopy of single nanocrystals probes the fundamental excitonic structure of the band edge, revealing spectral fingerprints that are highly sensitive to a range of photophys. properties as well as nanocrystal morphol. In particular, spectral and temporal signatures of biexciton and trion emission are revealed, and their relevance to emerging technologies is discussed. Overall cryogenic single nanocrystal spectroscopy can be used as a tool for understanding fundamental photophysics and guiding the synthesis of new nanocrystal materials.
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    14. 14
      Nair, G.; Zhao, J.; Bawendi, M. G. Biexciton Quantum Yield of Single Semiconductor Nanocrystals from Photon Statistics. Nano Lett. 2011, 11 (3), 1136– 1140,  DOI: 10.1021/nl104054t
      14
      Biexciton Quantum Yield of Single Semiconductor Nanocrystals from Photon Statistics
      Nair, Gautham; Zhao, Jing; Bawendi, Moungi G.
      Nano Letters (2011), 11 (3), 1136-1140CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Biexciton properties strongly affect the usability of a light emitter in quantum photon sources and lasers but are difficult to measure for single fluorophores at room temp. due to luminescence intermittency and bleaching at the high excitation fluences usually required. Here, the authors observe the biexciton (BX) to exciton (X) to ground photoluminescence cascade of single colloidal semiconductor nanocrystals (NCs) under weak excitation in a g(2) photon correlation measurement and show that the normalized amplitude of the cascade feature is the ratio of the BX to X fluorescence quantum yields. This imposes a limit on the attainable depth of photon antibunching and provides a robust means to study single emitter biexciton physics. In NC samples, the BX quantum yield is considerably inhomogeneous, consistent with the defect sensitivity expected of the Auger nonradiative recombination mechanism. The method can be extended to study X,BX spectral and polarization correlations.
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      Beyler, A. P.; Bischof, T. S.; Cui, J.; Coropceanu, I.; Harris, D. K.; Bawendi, M. G. Sample-Averaged Biexciton Quantum Yield Measured by Solution-Phase Photon Correlation. Nano Lett. 2014, 14 (12), 6792– 6798,  DOI: 10.1021/nl5027953
      15
      Sample-Averaged Biexciton Quantum Yield Measured by Solution-Phase Photon Correlation
      Beyler, Andrew P.; Bischof, Thomas S.; Cui, Jian; Coropceanu, Igor; Harris, Daniel K.; Bawendi, Moungi G.
      Nano Letters (2014), 14 (12), 6792-6798CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      The brightness of nanoscale optical materials such as semiconductor nanocrystals is currently limited in high excitation flux applications by inefficient multiexciton fluorescence. A soln.-phase photon correlation measurement was devised that can conveniently and reliably measure the av. biexciton-to-exciton quantum yield ratio of an entire sample without user selection bias. This technique can be used to study the multiexciton recombination dynamics of a broad scope of synthetically underdeveloped materials, including those with low exciton quantum yields and poor fluorescence stability. This method was applied to measure weak biexciton fluorescence in samples of visible-emitting InP/ZnS and InAs/ZnS core/shell nanocrystals, and to demonstrate that a rapid CdS shell growth procedure can markedly increase the biexciton fluorescence of CdSe nanocrystals.
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      Amgar, D.; Yang, G.; Tenne, R.; Oron, D. Higher-Order Photon Correlation as a Tool to Study Exciton Dynamics in Quasi-2D Nanoplatelets. Nano Lett. 2019, 19 (12), 8741– 8748,  DOI: 10.1021/acs.nanolett.9b03442
      16
      Higher-Order Photon Correlation as a Tool To Study Exciton Dynamics in Quasi-2D Nanoplatelets
      Amgar, Daniel; Yang, Gaoling; Tenne, Ron; Oron, Dan
      Nano Letters (2019), 19 (12), 8741-8748CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Colloidal semiconductor nanoplatelets, in which carriers are strongly confined only along one dimension, present fundamentally different excitonic properties than quantum dots, which support strong confinement in all three dimensions. In particular, multiple excitons strongly confined in just one dimension are free to rearrange in the lateral plane, reducing the probability for multibody collisions. Thus, while simultaneous multiple photon emission is typically quenched in quantum dots, in nanoplatelets its probability can be tuned according to size and shape. Here, we focus on analyzing multiexciton dynamics in individual CdSe/CdS nanoplatelets of various sizes through the measurement of second-, third-, and fourth-order photon correlations. For the first time, we can directly probe the dynamics of the two, three, and four exciton states at the single nanocrystal level. Remarkably, although higher orders of correlation vary substantially among the synthesis' products, they strongly correlate with the value of second order antibunching. The scaling of the higher-order moments with the degree of antibunching presents a small yet clear deviation from the accepted model of Auger recombination through binary collisions. Such a deviation suggests that many-body contributions are present already at the level of triexcitons. These findings highlight the benefit of high-order photon correlation spectroscopy as a technique to study multiexciton dynamics in colloidal semiconductor nanocrystals.
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    17. 17
      Fisher, B.; Caruge, J. M.; Zehnder, D.; Bawendi, M. Room-Temperature Ordered Photon Emission from Multiexciton States in Single CdSe Core-Shell Nanocrystals. Phys. Rev. Lett. 2005, 94 (8), 087403  DOI: 10.1103/PhysRevLett.94.087403
      17
      Room-Temperature Ordered Photon Emission from Multiexciton States in Single CdSe Core-Shell Nanocrystals
      Fisher, Brent; Caruge, Jean Michel; Zehnder, Don; Bawendi, Moungi
      Physical Review Letters (2005), 94 (8), 087403/1-087403/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
      The authors report room-temp. ordered multiphoton emission from multiexciton states of single CdSe(CdZnS) core(-shell) colloidal nanocrystals (NCs) that are synthesized by wet chem. methods. Spectrally and temporally resolved measurements of biexciton and triexciton emission from single NCs are also presented. A simple 4 level system models the results accurately and provides ests. for biexciton and triexciton radiative lifetimes and quantum yields.
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    18. 18
      Nasilowski, M.; Spinicelli, P.; Patriarche, G.; Dubertret, B. Gradient CdSe/CdS Quantum Dots with Room Temperature Biexciton Unity Quantum Yield. Nano Lett. 2015, 15 (6), 3953– 3958,  DOI: 10.1021/acs.nanolett.5b00838
      18
      Gradient CdSe/CdS Quantum Dots with Room Temperature Biexciton Unity Quantum Yield
      Nasilowski, Michel; Spinicelli, Piernicola; Patriarche, Gilles; Dubertret, Benoit
      Nano Letters (2015), 15 (6), 3953-3958CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Auger recombination is a major limitation for the fluorescent emission of quantum dots (QDs). It is the main source of QDs fluorescence blinking at the single-particle level. At high-power excitation, when several charge carriers are formed inside a QD, Auger becomes more efficient and severely decreases the quantum yield (QY) of multiexcitons. This limits the efficiency and the use of colloidal QDs in applications where intense light output is required. Here, the authors present a new generation of thick-shell CdSe/CdS QDs with dimensions >40 nm and a compn. gradient between the core and the shell that exhibits 100% QY for the emission of both the monoexciton and the biexciton in air and at room temp. for all the QDs the authors obsd. The fluorescence emission of these QDs is perfectly Poissonian at the single-particle level at different excitation levels and temps., from 30 to 300 K. In these QDs, the emission of high-order (>2) multiexcitons is quite efficient, and the authors observe white light emission at the single-QD level when high excitation power is used. These gradient thick shell QDs confirm the suppression of Auger recombination in gradient core/shell structures and help further establish the colloidal QDs with a gradient shell as a very stable source of light even under high excitation.
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    19. 19
      Bischof, T. S.; Correa, R. E.; Rosenberg, D.; Dauler, E. A.; Bawendi, M. G. Measurement of Emission Lifetime Dynamics and Biexciton Emission Quantum Yield of Individual InAs Colloidal Nanocrystals. Nano Lett. 2014, 14 (12), 6787– 6791,  DOI: 10.1021/nl502180w
      19
      Measurement of Emission Lifetime Dynamics and Biexciton Emission Quantum Yield of Individual InAs Colloidal Nanocrystals
      Bischof, Thomas S.; Correa, Raoul E.; Rosenberg, Danna; Dauler, Eric A.; Bawendi, Moungi G.
      Nano Letters (2014), 14 (12), 6787-6791CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      The understanding of the photophysics of visible-emitting colloidal nanocrystals (NCs) has long been aided by single-mol. studies of their emission. Until recently, no suitable detection technologies have existed for corresponding studies of shortwave-IR (SWIR) emitters. Now, the use of superconducting nanowire single-photon detectors (SNSPDs) enables the detailed study of SWIR NC emission dynamics at the single-emitter level. Here, the authors report a detailed anal. of the emission dynamics of individual InAs/CdZnS NCs emitting in the SWIR region. The authors observe blinking akin to A and type B blinking previously obsd. in visible-emitting CdSe NCs. The authors det. the intrinsic radiative lifetime of several InAs/CdZnS NCs and find examples ranging from 50-200 ns, indicative of a quasi-type-II electronic structure. The authors also measure g(2)0 for several of these NCs and find that their biexciton emission quantum yields vary from <1% up to 43%.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXhvFKqurfP&md5=009a12653d0068000a6054dbb392ec39
    20. 20
      Zhao, J.; Nair, G.; Fisher, B. R.; Bawendi, M. G. Challenge to the Charging Model of Semiconductor-Nanocrystal Fluorescence Intermittency from off-State Quantum Yields and Multiexciton Blinking. Phys. Rev. Lett. 2010, 104 (15), 157403,  DOI: 10.1103/PhysRevLett.104.157403
      20
      Challenge to the Charging Model of Semiconductor-Nanocrystal Fluorescence Intermittency from Off-State Quantum Yields and Multiexciton Blinking
      Zhao, Jing; Nair, Gautham; Fisher, Brent R.; Bawendi, Moungi G.
      Physical Review Letters (2010), 104 (15), 157403/1-157403/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
      Semiconductor nanocrystals emit light intermittently; i.e., they blink, under steady illumination. The dark periods were widely assumed to be due to photoluminescence (PL) quenching by an Auger-like process involving a single addnl. charge present in the nanocrystal. Results challenge this long-standing assumption. Close examn. of exciton PL intensity time traces of single CdSe(CdZnS) core(shell) nanocrystals reveals that the dark state PL quantum yield can be 10 times less than the biexciton PL quantum yield. The authors observe spectrally resolved multiexciton emission and find that it also blinks with an on/off ratio >10:1. These results directly contradict the predictions of the charging model.
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    21. 21
      Shulenberger, K. E.; Bischof, T. S.; Caram, J. R.; Utzat, H.; Coropceanu, I.; Nienhaus, L.; Bawendi, M. G. Multiexciton Lifetimes Reveal Triexciton Emission Pathway in CdSe Nanocrystals. Nano Lett. 2018, 18 (8), 5153– 5158,  DOI: 10.1021/acs.nanolett.8b02080
      21
      Multiexciton Lifetimes Reveal Triexciton Emission Pathway in CdSe Nanocrystals
      Shulenberger, Katherine E.; Bischof, Thomas S.; Caram, Justin R.; Utzat, Hendrik; Coropceanu, Igor; Nienhaus, Lea; Bawendi, Moungi G.
      Nano Letters (2018), 18 (8), 5153-5158CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Multiexcitons in emerging semiconducting nanomaterials play a crit. role in potential optoelectronic and quantum computational devices. We describe photon resolved single mol. methods to directly probe the dynamics of biexcitons and triexcitons in colloidal CdSe quantum dots. We confirm that biexcitons emit from a spin-correlated state, consistent with statistical scaling. Contrary to current understanding, we find that triexciton emission is dominated by band-edge 1Se1S3/2 recombination rather than the higher energy 1Pe1P3/2 recombination.
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    22. 22
      Park, Y. S.; Malko, A. V.; Vela, J.; Chen, Y.; Ghosh, Y.; García-Santamaría, F.; Hollingsworth, J. A.; Klimov, V. I.; Htoon, H. Near-Unity Quantum Yields of Biexciton Emission from CdSe/CdS Nanocrystals Measured Using Single-Particle Spectroscopy. Phys. Rev. Lett. 2011, 106 (18), 187401,  DOI: 10.1103/PhysRevLett.106.187401
      22
      Near-unity quantum yields of biexciton emission from CdSe/CdS nanocrystals measured using single-particle spectroscopy
      Park, Y.-S.; Malko, A. V.; Vela, J.; Chen, Y.; Ghosh, Y.; Garcia-Santamaria, F.; Hollingsworth, J. A.; Klimov, V. I.; Htoon, H.
      Physical Review Letters (2011), 106 (18), 187401/1-187401/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
      Biexciton luminescence (PL) quantum yields (Q2X) of individual CdSe/CdS core-shell nanocrystal quantum dots with various shell thicknesses are derived from independent PL satn. and 2-photon correlation measurements. A near-unity Q2X was obsd. for some nanocrystals with an ultrathick 19-monolayer shell. High Q2X's are not universal and vary widely among nominally identical nanocrystals indicating a significant dependence of Q2X upon subtle structural differences. The measurements indicate that high Q2X's are not required to achieve complete suppression of PL intensity fluctuations in individual nanocrystals.
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    23. 23
      Park, Y. S.; Bae, W. K.; Pietryga, J. M.; Klimov, V. I. Auger Recombination of Biexcitons and Negative and Positive Trions in Individual Quantum Dots. ACS Nano 2014, 8 (7), 7288– 7296,  DOI: 10.1021/nn5023473
      23
      Auger Recombination of Biexcitons and Negative and Positive Trions in Individual Quantum Dots
      Park, Young-Shin; Bae, Wan Ki; Pietryga, Jeffrey M.; Klimov, Victor I.
      ACS Nano (2014), 8 (7), 7288-7296CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Charged exciton states commonly occur both in spectroscopic studies of quantum dots (QDs) and during operation of QD-based devices. The extra charge added to the neutral exciton modifies its radiative decay rate and also opens an addnl. nonradiative pathway assocd. with an Auger process whereby the recombination energy of an exciton is transferred to the excess charge. Single-dot spectroscopic studies were conducted of Auger recombination in thick-shell (giant) CdSe/CdS QDs with and without an interfacial alloy layer using time-tagged, time-correlated single-photon counting. In luminescence (PL) intensity trajectories of some of the dots, the authors resolve 3 distinct states of different emissivities (bright, gray, and dark) attributed, resp., to the neutral exciton and neg. and pos. trions. Simultaneously acquired PL lifetime trajectories indicate that the pos. trion is much shorter lived than the neg. trion, which can be explained by a high d. of valence band states and a small hole localization radius (defined by the QD core size), factors that favor an Auger process involving intraband excitation of a hole. A comparison of trion and biexciton lifetimes suggests that the biexciton Auger decay can be treated in terms of a superposition of 2 independent channels assocd. with pos.- and neg.-trion pathways. The resulting interdependence between Auger time consts. might simplify the studies of multicarrier recombination by allowing 1, for example, to infer Auger lifetimes of trions of 1 sign based on the measurements of biexciton decay and dynamics of the trions of the opposite sign or, alternatively, est. the biexciton lifetime based on studies of trion dynamics.
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    24. 24
      Rabouw, F. T.; Lunnemann, P.; Van Dijk-Moes, R. J. A.; Frimmer, M.; Pietra, F.; Koenderink, A. F.; Vanmaekelbergh, D. Reduced Auger Recombination in Single CdSe/CdS Nanorods by One-Dimensional Electron Delocalization. Nano Lett. 2013, 13 (10), 4884– 4892,  DOI: 10.1021/nl4027567
      24
      Reduced Auger Recombination in Single CdSe/CdS Nanorods by One-Dimensional Electron Delocalization
      Rabouw, Freddy T.; Lunnemann, Per; van Dijk-Moes, Relinde J. A.; Frimmer, Martin; Pietra, Francesca; Koenderink, A. Femius; Vanmaekelbergh, Daniel
      Nano Letters (2013), 13 (10), 4884-4892CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Progress to reduce nonradiative Auger decay in colloidal nanocrystals has recently been made by growing thick shells. However, the physics of Auger suppression is not yet fully understood. Here, we examine the dynamics and spectral characteristics of single CdSe-dot-in-CdS-rod nanocrystals. These exhibit blinking due to charging/discharging, as well as trap-related blinking. We show that one-dimensional electron delocalization into the rod-shaped shell can be as effective as a thick spherical shell at reducing Auger recombination of the neg. trion state.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXhsVSisbrM&md5=b9e3b4e95fd145dc6edf143c5ed55ee6
    25. 25
      Zhao, J.; Chen, O.; Strasfeld, D. B.; Bawendi, M. G. Biexciton Quantum Yield Heterogeneities in Single CdSe (CdS) Core (Shell) Nanocrystals and Its Correlation to Exciton Blinking. Nano Lett. 2012, 12 (9), 4477– 4483,  DOI: 10.1021/nl3013727
      25
      Biexciton Quantum Yield Heterogeneities in Single CdSe (CdS) Core (Shell) Nanocrystals and Its Correlation to Exciton Blinking
      Zhao, Jing; Chen, Ou; Strasfeld, David B.; Bawendi, Moungi G.
      Nano Letters (2012), 12 (9), 4477-4483CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Biexciton (BX) nonradiative recombination processes in single semiconductor nanocrystals (NCs) were explored using confocal fluorescence microscopy and 2nd-order photon intensity correlation. More specifically, the authors measure the luminescence blinking and BX quantum yields (QYs) and study the correlation between these 2 measurements for single core (shell) CdSe (CdS) nanocrystals (NCs). NCs with a high on time fraction are significantly more likely to have a high BX QY than NCs with a low on fraction, even though the BX QYs of NCs with a high on fraction vary dramatically. The BX QYs of single NCs are also weakly dependent upon excitation wavelength. The weak correlation between exciton on fractions and BX QYs suggests that multiple recombination processes are involved in the BX recombination. To explain the results, the authors propose a model that combines both trapping and an Auger mechanism for BX recombination.
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    26. 26
      Rabouw, F. T.; Vaxenburg, R.; Bakulin, A. A.; van Dijk-Moes, R. J. A.; Bakker, H. J.; Rodina, A.; Lifshitz, E.; Efros, A. L.; Koenderink, A. F.; Vanmaekelbergh, D. Dynamics of Intraband and Interband Auger Processes in Colloidal Core-Shell Quantum Dots. ACS Nano 2015, 9 (10), 10366– 10376,  DOI: 10.1021/acsnano.5b04491
      26
      Dynamics of Intraband and Interband Auger Processes in Colloidal Core-Shell Quantum Dots
      Rabouw, Freddy T.; Vaxenburg, Roman; Bakulin, Artem A.; van Dijk-Moes, Relinde J. A.; Bakker, Huib J.; Rodina, Anna; Lifshitz, Efrat; Efros, Alexander L.; Koenderink, A. Femius; Vanmaekelbergh, Daniel
      ACS Nano (2015), 9 (10), 10366-10376CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Conventional colloidal quantum dots (QDs) suffer from rapid energy losses by nonradiative (Auger) processes, leading to sub-ns lifetimes in all excited states but the lowest-energy single exciton. Suppression of interband Auger decay, such as biexciton Auger recombination, has been achieved with the design of heterostructured core-shell QDs. Auger-like processes are also believed to be responsible for rapid intraband hot-electron cooling in QDs. However, the simultaneous effect of shell growth on interband Auger recombination and intraband hot-electron cooling has not been addressed. Here we investigate how the growth of a CdS shell affects these two relaxation processes in CdSe/CdS core-shell QDs. Using a combination of ultrafast pump-push-probe spectroscopy on the QD ensemble and anal. of the photon statistics from single QDs, we find that Auger losses in the biexciton state are suppressed with increasing shell thickness, while hot-electron cooling remains unaffected. Calcns. conducted within an eight-band k·p model confirm the exptl. dependence of the biexciton Auger decay on the shell thickness, and provide insights into the factors detg. the cooling rate of hot carriers.
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    27. 27
      Klimov, V. I.; Ivanov, S. A.; Nanda, J.; Achermann, M.; Bezel, I.; McGuire, J. A.; Piryatinski, A. Single-Exciton Optical Gain in Semiconductor Nanocrystals. Nature 2007, 447 (7143), 441– 446,  DOI: 10.1038/nature05839
      27
      Single-exciton optical gain in semiconductor nanocrystals
      Klimov, Victor I.; Ivanov, Sergei A.; Nanda, Jagjit; Achermann, Marc; Bezel, Ilya; McGuire, John A.; Piryatinski, Andrei
      Nature (London, United Kingdom) (2007), 447 (7143), 441-446CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
      Nanocrystal quantum dots have favorable light-emitting properties. They show photoluminescence with high quantum yields, and their emission colors depend on the nanocrystal size-owing to the quantum-confinement effect-and are therefore tunable. However, nanocrystals are difficult to use in optical amplification and lasing. Because of an almost exact balance between absorption and stimulated emission in nanoparticles excited with single electron-hole pairs (excitons), optical gain can only occur in nanocrystals that contain at least two excitons. A complication assocd. with this multiexcitonic nature of light amplification is fast optical-gain decay induced by non-radiative Auger recombination, a process in which one exciton recombines by transferring its energy to another. Here we demonstrate a practical approach for obtaining optical gain in the single-exciton regime that eliminates the problem of Auger decay. Specifically, we develop core/shell hetero-nanocrystals engineered in such a way as to spatially sep. electrons and holes between the core and the shell (type-II heterostructures). The resulting imbalance between neg. and pos. charges produces a strong local elec. field, which induces a giant (~100 meV or greater) transient Stark shift of the absorption spectrum with respect to the luminescence line of singly excited nanocrystals. This effect breaks the exact balance between absorption and stimulated emission, and allows us to demonstrate optical amplification due to single excitons.
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    28. 28
      Chen, O.; Zhao, J.; Chauhan, V. P.; Cui, J.; Wong, C.; Harris, D. K.; Wei, H.; Han, H. S.; Fukumura, D.; Jain, R. K. Compact High-Quality CdSe-CdS Core-Shell Nanocrystals with Narrow Emission Linewidths and Suppressed Blinking. Nat. Mater. 2013, 12 (5), 445– 451,  DOI: 10.1038/nmat3539
      28
      Compact high-quality CdSe-CdS core-shell nanocrystals with narrow emission linewidths and suppressed blinking
      Chen, Ou; Zhao, Jing; Chauhan, Vikash P.; Cui, Jian; Wong, Cliff; Harris, Daniel K.; Wei, He; Han, Hee-Sun; Fukumura, Dai; Jain, Rakesh K.; Bawendi, Moungi G.
      Nature Materials (2013), 12 (5), 445-451CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
      High particle uniformity, high luminescence quantum yields, narrow and sym. emission spectral lineshapes and min. single-dot emission intermittency (known as blinking) were recognized as universal requirements for the successful use of colloidal quantum dots in nearly all optical applications. Synthesizing samples that simultaneously meet all these 4 criteria proved challenging. The synthesis of such high-quality CdSe-CdS core-shell quantum dots in an optimized process that maintains a slow growth rate of the shell through the use of octanethiol and Cd oleate as precursors is reported. In contrast with previous observations, single-dot blinking is significantly suppressed with only a relatively thin shell. The elimination of the ensemble luminescence photodarkening that is an intrinsic consequence of quantum dot blinking statistical ageing is demonstrated. The small size and high luminescence quantum yields of these novel quantum dots render them superior in vivo imaging agents compared with conventional quantum dots. It is anticipated that these quantum dots will result in significant improvement in the performance of quantum dots in other applications such as solid-state lighting and illumination.
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    29. 29
      Xing, G.; Mathews, N.; Lim, S. S.; Yantara, N.; Liu, X.; Sabba, D.; Grätzel, M.; Mhaisalkar, S.; Sum, T. C. Low-Temperature Solution-Processed Wavelength-Tunable Perovskites for Lasing. Nat. Mater. 2014, 13 (5), 476– 480,  DOI: 10.1038/nmat3911
      29
      Low-temperature solution-processed wavelength-tunable perovskites for lasing
      Xing, Guichuan; Mathews, Nripan; Lim, Swee Sien; Yantara, Natalia; Liu, Xinfeng; Sabba, Dharani; Graetzel, Michael; Mhaisalkar, Subodh; Sum, Tze Chien
      Nature Materials (2014), 13 (5), 476-480CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
      Low-temp. soln.-processed materials that show optical gain and can be embedded into a wide range of cavity resonators are attractive for the realization of on-chip coherent light sources. Org. semiconductors and colloidal quantum dots are considered the main candidates for this application. However, stumbling blocks in org. lasing include intrinsic losses from bimol. annihilation and the conflicting requirements of high charge carrier mobility and large stimulated emission; whereas challenges pertaining to Auger losses and charge transport in quantum dots still remain. Herein, the authors reveal that soln.-processed org.-inorg. halide perovskites (CH3NH3PbX3 where X = Cl, Br, I), which demonstrated huge potential in photovoltaics, also have promising optical gain. Their ultra-stable amplified spontaneous emission at strikingly low thresholds stems from their large absorption coeffs., ultralow bulk defect densities and slow Auger recombination. Straightforward visible spectral tunability (390-790 nm) is demonstrated. Importantly, in view of their balanced ambipolar charge transport characteristics, these materials may show elec. driven lasing.
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    30. 30
      Bisschop, S.; Geiregat, P.; Aubert, T.; Hens, Z. The Impact of Core/Shell Sizes on the Optical Gain Characteristics of CdSe/CdS Quantum Dots. ACS Nano 2018, 12 (9), 9011– 9021,  DOI: 10.1021/acsnano.8b02493
      30
      The Impact of Core/Shell Sizes on the Optical Gain Characteristics of CdSe/CdS Quantum Dots
      Bisschop, Suzanne; Geiregat, Pieter; Aubert, Tangi; Hens, Zeger
      ACS Nano (2018), 12 (9), 9011-9021CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Colloidal quantum dots (QDs) are highly attractive as the active material for optical amplifiers and lasers. Here, we address the relation between the structure of CdSe/CdS core/shell QDs, the material gain they can deliver, and the threshold needed to attain net stimulated emission by optical pumping. On the basis of an initial gain model, we predict that reducing the thickness of the CdS shell grown around a given CdSe core will increase the maximal material gain, while increasing the shell thickness will lower the gain threshold. We assess this trade-off by means of transient absorption spectroscopy. Our results confirm that thin-shell QDs exhibit the highest material gain. In quant. agreement with the model, core and shell sizes hugely impact on the material gain, which ranges from 2800 cm-1 for large core/thin shell QDs to less than 250 cm-1 for small core/thick shell QDs. On the other hand, the significant threshold redn. expected for thick-shell QDs is absent. We relate this discrepancy between model and expt. to a transition from attractive to repulsive exciton-exciton interactions with increasing shell thickness. The spectral blue-shift that comes with exciton-exciton repulsion leads to competition between stimulated emission and higher energy absorbing transitions, which raises the gain threshold. As a result, small-core/thick-shell QDs need up to 3.7 excitations per QD to reach transparency, whereas large-core/thin shell QDs only need 1.0, a no. often seen as a hard limit for biexciton-mediated optical gain. This makes large-core/thin-shell QDs that feature attractive exciton-exciton interactions the overall champion core/shell configuration in view of highest material gain, lowest threshold exciton occupation, and longest gain lifetime.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXhs1ygtLfP&md5=edb2fc4e33dd57b4b7d80e1580af2143
    31. 31
      Rabouw, F. T.; Kamp, M.; Van Dijk-Moes, R. J. A.; Gamelin, D. R.; Koenderink, A. F.; Meijerink, A.; Vanmaekelbergh, D. Delayed Exciton Emission and Its Relation to Blinking in CdSe Quantum Dots. Nano Lett. 2015, 15 (11), 7718– 7725,  DOI: 10.1021/acs.nanolett.5b03818
      31
      Delayed Exciton Emission and Its Relation to Blinking in CdSe Quantum Dots
      Rabouw, Freddy T.; Kamp, Marko; van Dijk-Moes, Relinde J. A.; Gamelin, Daniel R.; Koenderink, A. Femius; Meijerink, Andries; Vanmaekelbergh, Daniel
      Nano Letters (2015), 15 (11), 7718-7725CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      The efficiency and stability of emission from semiconductor nanocrystal quantum dots (QDs) is neg. affected by blinking on the single-nanocrystal level, i.e., random alternation of bright and dark periods. The time scales of these fluctuations can be as long as many seconds, orders of magnitude longer than typical lifetimes of exciton states in QDs. The authors study luminescence from QDs delayed over microseconds to milliseconds. The results prove the existence of long-lived charge-sepd. states in QDs. The authors study the properties of delayed emission as a direct way to learn about charge carrier sepn. and recovery of the exciton state. A new microscopic model is developed to connect delayed emission to exciton recombination and blinking from which bright periods in blinking are in fact not characterized by uninterrupted optical cycling as often assumed.
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    32. 32
      Akselrod, G. M.; Prins, F.; Poulikakos, L. V.; Lee, E. M. Y.; Weidman, M. C.; Mork, A. J.; Willard, A. P.; Bulović, V.; Tisdale, W. A. Subdiffusive Exciton Transport in Quantum Dot Solids. Nano Lett. 2014, 14 (6), 3556– 3562,  DOI: 10.1021/nl501190s
      32
      Subdiffusive Exciton Transport in Quantum Dot Solids
      Akselrod, Gleb M.; Prins, Ferry; Poulikakos, Lisa V.; Lee, Elizabeth M. Y.; Weidman, Mark C.; Mork, A. Jolene; Willard, Adam P.; Bulovic, Vladimir; Tisdale, William A.
      Nano Letters (2014), 14 (6), 3556-3562CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Colloidal quantum dots (QDs) are promising materials for use in solar cells, light-emitting diodes, lasers, and photodetectors, but the mechanism and length of exciton transport in QD materials is not well understood. We use time-resolved optical microscopy to spatially visualize exciton transport in CdSe/ZnCdS core/shell QD assemblies. We find that the exciton diffusion length, which exceeds 30 nm in some cases, can be tuned by adjusting the inorg. shell thickness and org. ligand length, offering a powerful strategy for controlling exciton movement. Moreover, we show exptl. and through kinetic Monte Carlo simulations that exciton diffusion in QD solids does not occur by a random-walk process; instead, energetic disorder within the inhomogeneously broadened ensemble causes the exciton diffusivity to decrease over time. These findings reveal new insights into exciton dynamics in disordered systems and demonstrate the flexibility of QD materials for photonic and optoelectronic applications.
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    33. 33
      Deutsch, Z.; Schwartz, O.; Tenne, R.; Popovitz-Biro, R.; Oron, D. Two-Color Antibunching from Band-Gap Engineered Colloidal Semiconductor Nanocrystals. Nano Lett. 2012, 12 (6), 2948– 2952,  DOI: 10.1021/nl300638t
      33
      Two-Color Antibunching from Band-Gap Engineered Colloidal Semiconductor Nanocrystals
      Deutsch, Zvicka; Schwartz, Osip; Tenne, Ron; Popovitz-Biro, Ronit; Oron, Dan
      Nano Letters (2012), 12 (6), 2948-2952CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Photon antibunching is ubiquitously obsd. in light emitted from quantum systems but is usually assocd. only with the lowest excited state of the emitter. A fluorophore was devised that upon photoexcitation emits in either 1 of 2 distinct colors but exhibits strong antibunching between the 2. This work demonstrates the possibility of creating room-temp. quantum emitters with higher complexity than effective 2 level systems via colloidal synthesis.
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    34. 34
      Gindele, F.; Hild, K.; Langbein, W.; Woggon, U. Temperature-Dependent Line Widths of Single Excitons and Biexcitons. J. Lumin. 2000, 87, 381– 383,  DOI: 10.1016/S0022-2313(99)00409-3
      34
      Temperature-dependent line widths of single excitons and biexcitons
      Gindele, F.; Hild, K.; Langbein, W.; Woggon, U.
      Journal of Luminescence (2000), 87-89 (), 381-383CODEN: JLUMA8; ISSN:0022-2313. (Elsevier Science B.V.)
      The coupling of single zero-dimensional excitons and biexcitons to acoustic and optical phonons was studied in epitaxially grown Zn1-xCdxSe/ZnSe quantum structures. From the temp.-dependent line width of the exciton photoluminescence (PL) an enhanced coupling to acoustic phonons is found which the authors explain by the quantum confinement. The LO-phonon coupling is reduced compared to CdSe bulk material. The temp. dependence of the line width of the biexciton PL is similar, indicating a similar coupling strength of the exciton and the exciton-biexciton transition to the crystal lattice.
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    35. 35
      Gindele, F.; Hild, K.; Langbein, W.; Woggon, U. Phonon Interaction of Single Excitons and Biexcitons. Phys. Rev. B 1999, 60 (4), R2157– R2160,  DOI: 10.1103/PhysRevB.60.R2157
      35
      Phonon interaction of single excitons and biexcitons
      Gindele, F.; Hild, K.; Langbein, W.; Woggon, U.
      Physical Review B: Condensed Matter and Materials Physics (1999), 60 (4), R2157-R2160CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
      The exciton-phonon coupling of a single zero-dimensional exciton was studied in epitaxially grown CdSe/Zn1-xCdxSe quantum dot structures by analyzing its linewidth and phonon-assisted recombination. The single excitons do not couple to only a single phonon mode, but to a distribution of phonons, here consisting of the zone-center ZnSe longitudinal optic (LO) phonon and of mixed modes with an energy statistics centered between the ZnSe and CdSe LO phonon. Both exciton and corresponding biexciton reveal LO-phonon replica intensities of ∼3% of the resp. zero-phonon emission. These similar LO-phonon coupling strengths are evidence of a reduced polarity of the exciton states within the biexciton as the result of exciton-exciton correlation.
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    36. 36
      Korkusinski, M.; Voznyy, O.; Hawrylak, P. Fine Structure and Size Dependence of Exciton and Biexciton Optical Spectra in CdSe Nanocrystals. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 82 (24), 245304,  DOI: 10.1103/PhysRevB.82.245304
      36
      Fine structure and size dependence of exciton and biexciton optical spectra in CdSe nanocrystals
      Korkusinski, Marek; Voznyy, Oleksandr; Hawrylak, Pawel
      Physical Review B: Condensed Matter and Materials Physics (2010), 82 (24), 245304/1-245304/16CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      Theory of electronic and optical properties of exciton and biexciton complexes confined in CdSe spherical nanocrystals is presented. The electron and hole states are computed using atomistic sp3d5s* tight binding Hamiltonian including an effective crystal field splitting, spin-orbit interactions, and model surface passivation. The optically excited states are expanded in electron-hole configurations and the many-body spectrum is computed in the configuration-interaction approach. Results demonstrate that the low-energy electron spectrum is organized in shells (s,p,...), while the valence hole spectrum is composed of four low-lying, doubly degenerate states sepd. from the rest by a gap. As a result, the biexciton and exciton spectrum is composed of a manifold of closely lying states, resulting in a fine structure of exciton and biexciton spectra. The quasidegenerate nature of the hole spectrum results in a correlated biexciton state, which makes it slowly convergent with basis size. We carry out a systematic study of the exciton and biexciton emission spectra as a function of the nanocrystal diam. and find that the interplay of repulsion between constituent excitons and correlation effects results in a change of the sign of biexciton binding energy from neg. to pos. at a crit. nanocrystal size.
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    37. 37
      Hanifi, D. A.; Bronstein, N. D.; Koscher, B. A.; Nett, Z.; Swabeck, J. K.; Takano, K.; Schwartzberg, A. M.; Maserati, L. Redefining near-unity luminescence in quantum dots with photothermal quantum yield. Science 2019, 363 (6432), 1199– 1202,  DOI: 10.1126/science.aat3803
      37
      Redefining near-unity luminescence in quantum dots with photothermal threshold quantum yield
      Hanifi, David A.; Bronstein, Noah D.; Koscher, Brent A.; Nett, Zach; Swabeck, Joseph K.; Takano, Kaori; Schwartzberg, Adam M.; Maserati, Lorenzo; Vandewal, Koen; van de Burgt, Yoeri; Salleo, Alberto; Alivisatos, A. Paul
      Science (Washington, DC, United States) (2019), 363 (6432), 1199-1202CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      A variety of optical applications rely on the absorption and reemission of light. The quantum yield of this process often plays an essential role. When the quantum yield deviates from unity by significantly <1%, applications such as luminescent concentrators and optical refrigerators become possible. To evaluate such high performance, the authors develop a measurement technique for luminescence efficiency with sufficient accuracy <1 part per thousand. Photothermal threshold quantum yield is based on the quantization of light to minimize overall measurement uncertainty. This technique is used to guide a procedure capable of making ensembles of near-unity emitting CdSe/CdS core-shell quantum dots. The authors obtain a photothermal threshold quantum yield luminescence efficiency of 99.6 ± 0.2%, indicating nearly complete suppression of nonradiative decay channels.
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    38. 38
      Boldt, K.; Kirkwood, N.; Beane, G. A.; Mulvaney, P. Synthesis of Highly Luminescent and Photo-Stable, Graded Shell CdSe/CdxZn1–xS Nanoparticles by in Situ Alloying. Chem. Mater. 2013, 25 (23), 4731– 4738,  DOI: 10.1021/cm402645r
      38
      Synthesis of Highly Luminescent and Photo-Stable, Graded Shell CdSe/CdxZn1-xS Nanoparticles by In Situ Alloying
      Boldt, Klaus; Kirkwood, Nicholas; Beane, Gary A.; Mulvaney, Paul
      Chemistry of Materials (2013), 25 (23), 4731-4738CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      A facile and robust synthesis of CdxZn1-xS graded shells on CdSe nanoparticles that are prepd. by interface alloying between CdS and ZnS shells at elevated temps. is reported. Alloying provides systematic control over the electronic structure and enables switching between Type-I and quasi-Type-II configurations. Good control of particle shape, shell thickness, and compn. is achieved by slowly adding Zn oleate and octane thiol via syringe pump to readily prepd. CdSe/CdS particles. The resultant quantum dots exhibit PL quantum yields of up to 97% and superior robustness toward environmental influences and quenching agents. Alloying promotes a blue-shift of both the absorption and PL spectra compared to pure CdSe/CdS particles and an increased Stokes shift, opening a new synthetic pathway to stable, green-emitting core/shell/shell quantum dots. High PL quantum yields are correlated to a narrow distribution of single-particle lifetimes and suppressed fluorescence intermittency. A new method to characterize the PL intermittency of single quantum dots based on the autocorrelation function of their PL time trajectories is introduced.
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    39. 39
      Prins, P. T.; Montanarella, F.; Dümbgen, K.; Justo, Y.; Van der Bok, J. C.; Hinterding, S. O. M.; Geuchies, J. J.; Maes, J.; De Nolf, K.; Deelen, S. Extended Nucleation and Superfocusing in Colloidal Semiconductor Nanocrystal Synthesis. Nano Lett. 2021, 21 (6), 2487– 2496,  DOI: 10.1021/acs.nanolett.0c04813
      39
      Extended nucleation and superfocusing in colloidal semiconductor nanocrystal synthesis
      Prins, P. Tim; Montanarella, Federico; Duembgen, Kim; Justo, Yolanda; van der Bok, Johanna C.; Hinterding, Stijn O. M.; Geuchies, Jaco J.; Maes, Jorick; De Nolf, Kim; Deelen, Sander; Meijer, Hans; Zinn, Thomas; Petukhov, Andrei V.; Rabouw, Freddy T.; De Mello Donega, Celso; Vanmaekelbergh, Daniel; Hens, Zeger
      Nano Letters (2021), 21 (6), 2487-2496CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Hot-injection synthesis is renowned for producing semiconductor nanocolloids with superb size dispersions. Burst nucleation and diffusion-controlled size focusing during growth have been invoked to rationalize this characteristic yet exptl. evidence supporting the pertinence of these concepts is scant. By monitoring a CdSe synthesis in-situ with X-ray scattering, we find that nucleation is an extended event that coincides with growth during 15-20% of the reaction time. Moreover, we show that size focusing outpaces predictions of diffusion-limited growth. This observation indicates that nanocrystal growth is dictated by the surface reactivity, which drops sharply for larger nanocrystals. Kinetic reaction simulations confirm that this so-called superfocusing can lengthen the nucleation period and promote size focusing. The finding that narrow size dispersions can emerge from the counteracting effects of extended nucleation and reaction-limited size focusing ushers in an evidence-based perspective that turns hot injection into a rational scheme to produce monodisperse semiconductor nanocolloids.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3MXlsFGjur0%253D&md5=c54c3978c24659e9a4d4af677002e784
    40. 40
      Sung, Y.; Park, K.; Lee, Y.; Kim, T. Ripening Kinetics of CdSe/ZnSe Core/Shell Nanocrystals. J. Phys. Chem. C 2007, 111 (3), 1239– 1242,  DOI: 10.1021/jp066203c
      40
      Ripening Kinetics of CdSe/ZnSe Core/Shell Nanocrystals
      Sung, Yun-Mo; Park, Kyung-Soo; Lee, Yong-Ji; Kim, Tae-Geun
      Journal of Physical Chemistry C (2007), 111 (3), 1239-1242CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      CdSe and CdSe/ZnSe core shell nanocrystals were prepd. via the inverse micelle technol. with TOP/TOPO/HDA surfactants, and their high crystallinity was confirmed by using X-ray diffraction (XRD) and high-resoln. transmission electron microscopy (HRTEM) analyses. Ostwald ripening behavior of the nanocrystals was monitored by using the red-shift in UV-visible absorbance peaks, and their size variation was estd. by employing a quantum confinement effect equation. Lifshitz-Slyozov-Wagner (LSW) kinetics analyses were performed by using the size variation according to ripening temp. and time period. Arrhenius-type plots were created by using the slopes of the LSW curves for the CdSe and CdSe/ZnSe nanocrystals, resp., and the activation energy values for the ripening were evaluated for the nanocrystals. At a low-temp. region, the CdSe and CdSe/ZnSe samples seem to show dissocn. of Cd-Se and Zn-Se surface at. bonds, resp., while at a high-temp. region above 266 °C, both samples seem to show active dissocn. of both Cd-Se and Zn-Se lattice at. bonds. The CdSe-ZnSe shows relatively low activation energy for the ripening, compared to the bare CdSe possibly due to weak Zn-Se at. bonds. One can complete the Ostwald ripening kinetics equation by using two kinetics variables, derived in this study, for the estn. of the size of CdSe/ZnSe core/shell nanocrystals. Also, this approach can be applied to ripening kinetics of other core/shell nanocrystal systems.
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  • Supporting Information

    Supporting Information

    The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.nanolett.1c01556.

    • Experimental methods, details of data processing and calibration procedures, details of quantum-mechanical calculations, a derivation of the energy-resolved g(2), and an overview of all single-QD experiments (PDF)


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