ZnS Quantum Dots with Suppressed “Blinking

Feb 7, 2017 - Spectrally resolved measurements reveal that the single-dot, room-temperature PL line width is much narrower (down to ∼60 meV) than th...
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Thick-Shell CuInS2/ZnS Quantum Dots with Suppressed “Blinking” and Narrow Single-Particle Emission Line Widths Huidong Zang,† Hongbo Li,† Nikolay S. Makarov,† Kirill A. Velizhanin,‡ Kaifeng Wu,† Young-Shin Park,†,§ and Victor I. Klimov*,† †

Chemistry Division and ‡Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States Center for High Technology Materials, University of New Mexico, Albuquerque, New Mexico 87131, United States

§

S Supporting Information *

ABSTRACT: Quantum dots (QDs) of ternary I−III−VI2 compounds such as CuInS2 and CuInSe2 have been actively investigated as heavy-metal-free alternatives to cadmium- and lead-containing semiconductor nanomaterials. One serious limitation of these nanostructures, however, is a large photoluminescence (PL) line width (typically >300 meV), the origin of which is still not fully understood. It remains even unclear whether the observed broadening results from considerable sample heterogeneities (due, e.g., to size polydispersity) or is an unavoidable intrinsic property of individual QDs. Here, we answer this question by conducting singleparticle measurements on a new type of CuInS2 (CIS) QDs with an especially thick ZnS shell. These QDs show a greatly enhanced photostability compared to core-only or thin-shell samples and, importantly, exhibit a strongly suppressed PL blinking at the single-dot level. Spectrally resolved measurements reveal that the single-dot, room-temperature PL line width is much narrower (down to ∼60 meV) than that of the ensemble samples. To explain this distinction, we invoke a model wherein PL from CIS QDs arises from radiative recombination of a delocalized bandedge electron and a localized hole residing on a Cu-related defect and also account for the effects of electron−hole Coulomb coupling. We show that random positioning of the emitting center in the QD can lead to more than 300 meV variation in the PL energy, which represents at least one of the reasons for large PL broadening of the ensemble samples. These results suggest that in addition to narrowing size dispersion, future efforts on tightening the emission spectra of these QDs might also attempt decreasing the “positional” heterogeneity of the emitting centers. KEYWORDS: Core/shell quantum dot, copper indium sulfide, photoluminescence line width, single-dot spectroscopy, suppressed blinking

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between I−III−VI2 and II−VI semiconductors. From the crystallography perspective, the tetrahedral structure of I−III− VI2 semiconductors can be considered as a superlattice of a zincblende (II−VI) structure composed of two interpenetrating face-centered cubic lattices distorted along the crystal c-axis. Furthermore, comparable bulk band-gaps and excitonic oscillator strengths between, for example, CdSe and CuInS2 (CIS) would suggest a similarity in their emission and absorption properties.15,24 However, contrary to these expectations, optical spectra and carrier dynamics in the QDs of these two materials are markedly different. Among the peculiarities of CIS QDs are very long radiative lifetimes (100−500 ns versus ∼20 ns in CdSe QDs), an extremely large Stokes shift between the PL band and the first apparent absorption feature (300− 500 meV versus 300 meV versus 60% on average) in the ON state with a single, well-defined intensity level. A two-state, “binary” blinking along with strong photon antibunching indicated by two-photon correlation studies reaffirm that the

measurements are conducted in the single-dot mode. Spectrally resolved studies reveal a surprisingly narrow PL line width down to 60 meV versus >300 meV in the ensemble samples. This observation suggests that despite a fairly tight size distribution ( 0 is the RC localization energy measured versus the bulksemiconductor valence band edge, rRC is the vector coordinate of the emitting recombination center, and ΔEeh(rRC) = Eeh,del − Eeh,loc(rRC) is the difference between the interaction energy of the band-edge (delocalized) electron and hole (Eeh,del) and that of the band-edge electron and the localized hole (Eeh,loc); see Section 1 of the Supporting Information. Here, we assume that the absolute energy of the hole localization center versus the vacuum level is QD-size-independent. Indeed using a semiclassical tunneling model, we estimate that for Eloc of ∼0.28 eV (determined based on the position of the PL band versus the bulk CIS valence band-edge), the hole localization radius is only ∼0.3 nm, which is much smaller than typical QD sizes. In Figure 3c, we show schematically the effect of the position of the hole localization site on the emission energy, and in Figure 3d, we display model calculations (illustrative in purpose) of hvPL as a function of rRC = |rRC| conducted for a spherical core/shell particle (radius a, shell thickness d = 0.2a) taking into consideration the interface polarization effects (see Section 1 of the Supporting Information). These calculations 1791

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Nano Letters have been done for several total QD radii and rRC ranging from 0−0.8a. In Figure 3d, the results of the modeling are shown as a function of the confinement energy found from the difference between the QD and the bulk-semiconductor (Eg,0) band gaps: Ec = Eg − Eg,0. According to these calculations, the emission energy for the centrally located defect (rRC = 0) is lower than that for the defect residing on the QD periphery, and the difference [defined by δEeh = |hvPL(0.8a) − hvPL(0)| quickly increases with increasing confinement energy. For the situation realized in our experiments (Ec of ca. 0.75 eV), δEeh is approximately 350 meV, which is similar to the ensemble PL line width. Thus, random positioning of the emissive center within the QD can indeed result in a large variation in the single-dot emission energy sufficient for explaining strong broadening of the ensemble PL. The random positioning of the emissive defect can also be at least one of the factors resulting in a considerable spread in single-dot PL lifetimes (τPL) (Figure 4a,b). According to the histogram in Figure 4b, the measured values of τPL vary from ∼75 to ∼325 ns, with the average of 216 ns, which is close to the 1/e PL lifetime of the QD ensemble (221 ns; Figure 2c). For the emission mechanism involving a localized recombination center, a variation in the single-dot PL lifetime is a natural consequence of the variation in the defect-position-dependent electron−hole overlap integral (illustrated in Figure 3c by red and blue profiles). This would result in a shorter lifetime for the centrally located defect versus the defect with a more peripheral location. For this mechanism, one can also expect to see correlations between the single-dot emission energy and the PL lifetime, i.e., lower PL energies should correspond to shorter values of τPL (Figure 3c). The plot of τPL versus hvPL, however, does not seem to show any apparent correlations (see Figure S8a), suggesting a possible contribution of other sources of lifetime heterogeneity. One such source is the influence of nonradiative decay channels that could distort the correlations expected for purely radiative recombination. An additional source is a dot-to-dot variation in the number of intragap recombination centers. In the case of the involvement of Cu2+ defects (pre-existing or produced via photoconversion of Cu1+ ions),28,29,31,32,51,52 the process of defect-assisted radiative recombination does not require a photogenerated hole, as Cu2+ can radiatively trap a conduction-band electron (i.e., Cu2+ can be considered as a state with a pre-existing hole).28,51 In this case, the recombination rate would increase with increasing number of emissive centers. The presence of multiple centers of radiative recombination in a QD is suggested, for example, by the observation of asymmetry in some of the single-dot emission spectra as illustrated by the example in Figure S4d. The involvement of multiple emissive defects can also increase the PL line width versus the situation of a single (preferred) radiative decay center, which might explain a large variation in the measured broadening of single-dot emission spectra (Figure 3b). To summarize, we have presented thick-shell CIS/ZnS QDs that show improved photostability and greatly reduced PL blinking at the single-dot level compared to more-traditional core-only or thin-shell samples. These properties allow us to conduct spectrally resolved single-particle measurements, which provide direct information on intrinsic PL broadening not obscured by heterogeneities of macroscopic samples. The narrow single-particle PL line widths, revealed by these

Figure 4. Single-dot PL dynamics of thick-shell CIS/ZnS QDs. (a) Individual QDs show single-exponential PL dynamics (“noisy” traces are the measurements; red smooth lines are single-exponential fits) with a large dot-to-dot variation in the characteristic time constant (varies from 80 to 328 ns for the examples shown in the plot). (b) The histogram of PL lifetimes based on the measurements of multiple thick-shell CIS/ZnS QDs (individual single-dot PL lifetimes are shown by blue open symbols at the top of the plot; the red solid circle is the average, and the red bars show the standard deviation). The average PL lifetime derived from single-dot measurements is 216 ns; this value is close to the 1/e ensemble lifetime (221 ns; Figure 2c).

measurements, indicate that large broadening of QD ensemble spectra is not intrinsic but is a result of dot-to-dot variations in the emission energy. The peculiarity of the emission mechanism in CIS QDs, which involves a localized center for radiative-recombination (presumably Cu-defect related), leads to an additional source of heterogeneity associated with random positioning of the emissive defect within the QD; our model calculations suggest that this effect can lead to a large (>300 meV) variation in the electron−hole interaction energy that translates into a varied PL energy. These findings suggest that, in principle, the line width of these toxic-elementfree QDs can be reduced to less than 100 meV, which would open a number of presently inaccessible applications including high-color-definition displays, “smart” lighting with a finely tunable spectrum and multichannel spectrally multiplexed sensing and labeling. A present challenge in achieving the narrow ensemble line widths is the development of chemical strategies that would allow for a reduction in a positional 1792

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37.4. Calculations of the atomic ratio in the studied CIS/ZnS QDs with the total apex-to-apex size L = 12.5 nm and the 6 ML (face-to-face; see inset of Figure 1c) ZnS shell yielded the Zn/ (Cu + In) ratio of 38, which was remarkably close to the value obtained from the ICPOES measurements. The PL quantum yield of the synthesized QDs was evaluated using two approaches: one involved absolute measurements with an integrating sphere, and the other utilized a comparison versus rhodamine 101 used as a reference. These methods produced mutually consistent results. In the case of the thickshell CIS/ZnS QD samples, the PL quantum yield was 50 ± 5%. Photostability Tests of Thin- and Thick-Shell CIS/ZnS QDs. Photostability tests were conducted on drop-cast films of thinand thick-shell CIS QD films. The excitation source was a continuous-wave 462 nm LED. The incident light intensity was 1.05 W/cm2. Considering the optical density of the QD film of 1.35 (at 462 nm), the absorbed power was 1.0 W/cm2. The PL intensity was recoded as a function of exposure time. For thinshell CIS/ZnS QDs, a significant PL quenching (by 70%) was observed in 2 h (see Figure S6). The thick-shell samples were considerably more robust and retained more than 50% of their original PL quantum yields after 8 h of exposure to LED radiation. Single-Dot Measurements. Single-dot measurements were performed using a home-built system described elsewhere.56 QDs mixed with poly(octadecyl methacrylate) in toluene were spun-cast onto cover glass slides to form upon drying a submonolayer film within the protective polymer matrix. A pulsed laser (PicoQuant; 405 nm wavelength and 40 ps pulse duration) was used to excite individual QDs. The laser repetition rate (500 kHz) was chosen to allow for “deexcitation” of QDs between adjacent pulses. To minimize contribution from multiexcitonic effects, the QDs were excited at low fluence that corresponded to the average per-pulse, perdot number of photogenerated excitons of no more than = 0.08, where was calculated as a product of the per-pulse photon fluences (j) and the absorption cross-section σ = 7.5 × 10−16 cm2 at 405 nm (determined based on pump-intensitydependent PL saturation measurements).15 An Olympus objective (100×, 0.9 NA) was used for both focusing pump light onto the sample and collecting PL from individual QDs. The second-order intensity correlation function (g(2)) was measured using a Hanbury-Brown and Twiss experiment, wherein the PL from the QD was split into two channels with a 50/50 nonpolarizing beam splitter and detected with two identical single-photon avalanche photodiodes (APDs) having a ∼350 ps time resolution. A multichannel, time-tagged, timeresolved (TTTR) mode provided by a module for time correlated single photon counting (PicoQuant HydraHarp 400) was utilized to record both a “macrotime” (time elapsed from the start of a measurement) and a “microtime” (time relative to a laser pulse) of photon arrivals for each detector independently. The TTTR data were analyzed using a LabVIEW software and allowed us to obtain PL intensity trajectories, PL dynamics, and the g(2) function simultaneously from the same measurement. In most of the single-dot PL-intensity-trajectory measurements, the bin size was 40 ms, which resulted in a sufficiently large signal-to-noise ratio (the ON/OFF PL intensity ratio was >3) and allowed us to clearly resolve switching between the ON and the OFF states. PL Lifetime Correction for Single CuInS2/ZnS. The background in single-dot PL intensity trajectories (shown by

heterogeneity in the distribution of emitting centers within a QD. Methods. Materials. Zinc acetate (Zn(Ac)2, 99.99%), zinc stearate (Zn(St)2, 90%), copper(I) acetate (CuAc, 99.99%), indium acetate (In(Ac)3, 99.99%), sulfur powder (S, 99.99%), dodecanethiol (DDT, 99.9%), oleylamine (OAm, 97%), octadecene (ODE, 90%), oleic acid (OA, 90%), and poly octadecyl methacrylate solution (POMA, 30−35% by weight in toluene) were all obtained from Sigma-Aldrich. Indium stearate (In(St)3) was prepared following the reported method from ref 53. All chemicals were used as received without further purification. Synthesis of CuInS2/ZnS (CIS/ZnS) Core−Shell Quantum Dots. We employed a hot-injection method for the synthesis of CIS cores, following the previous work 3 with some modifications. In a typical reaction, In(St)3 (386 mg, 0.4 mmol), CuAc (25 mg, 0.2 mmol), DDT (120 μL, 0.5 mmol), OA (126 μL, 0.4 mmol), and 10 mL of ODE were loaded into a three-neck flask. The mixture was heated to 80 °C and degassed for 30 min. Next, the temperature was raised to 180 °C under nitrogen flow. Then, 0.8 mL of S/ODE ([S] = 0.5 mol/L) was quickly injected into the mixture. The reaction was quenched in 5 min by cooling the solution to room temperature. This synthesis produced pyramidal CIS QDs with the mean apex-toapex size L = 4.6 nm, as determined based on the transmission electron microscopy (TEM) imaging (see Figure S1) and confirmed by the comparison to size-dependent PL measurements of ref 54. The growth of the ZnS shell was conducted using a two-step procedure.15,55 First, the ZnS stock solution, prepared by dissolving 2 mmol of Zn(Ac)2 in 16 mL of ODE and 4 mL of OAm, was added drop-wise to the CIS core solution heated to 210 °C at the rate of 4 mL/h for 5 h. This resulted in a thin, ∼1 ML ZnS shell (determined based on the magnitude of the blue shift of the PL band; see Figure 1a,b) produced by the replacement of the original, near-surface metal ions with Zn. During the second step of shell growth, the ZnS stock solution, prepared by dissolving 2 mmol of Zn(St)2 in 10 mL of ODE, 5 mL of OA, and 5 mL of DDT, was added drop-wise to the solution of thin-shell CIS/ZnS QDs heated to 230 °C at the rate of 4 mL/h for 5 h. Due to use of elevated temperature, the shell growth proceeds not via cation exchange but via the deposition of the additional thicker ZnS layer on top of the original thin shell. The final ZnS shell thickness was ∼6 ML (face-to-face distance; see inset of Figure 1c). As-synthesized thick-shell CIS/ZnS QDs were washed by repeated precipitation with ethanol and finally dispersed in hexane or chloroform for further characterization. QD Characterization. The samples for TEM studies were prepared by drop-casting a diluted dispersion of QDs onto carbon coated copper grids. TEM images were acquired with a JEOL JEM-2100 microscope operated at 200 kV and equipped with an image aberration corrector. The elemental analysis was carried out via inductively coupled plasma optical emission spectroscopy (ICPOES) using an ICAP 6000 spectrometer. Samples for the ICPOES measurements were prepared by dissolving a dry QD powder in fresh aqua regia. Chemical analysis of the CIS cores indicated a Cu/In atomic ratio of 1:1.49. A significant copper deficiency was intentional (helped increase the PL quantum efficiency) and resulted from the use of the 1:2 Cu/In precursor ratio. The analysis of the final thick-shell CIS/ZnS QDs indicated a Cu/In/Zn ratio of 1/ 1.45/91.7, which corresponded to the Zn/(Cu + In) ratio of 1793

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gray traces in Figure 2a,b) resulted primarily from polymer emission. In time-resolved PL measurements, it manifested as a fast initial component with the 4 ns time constant.57 The QDonly PL decays (shown in the main text) were obtained by subtracting the parasitic fast signal from the raw PL traces.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.6b05118. TEM images, measurements and analysis of single-dot PL blinking statistics; additional single-dot PL intensity trajectories, g(2) measurements, and single-dot PL spectra and dynamics; QD photostability studies; analysis of the distribution of single-dot PL energies; studies of correlations between single-dot PL lifetimes, PL peak energies, and PL line widths; and theoretical studies of the effect of electron−hole Coulomb interactions on PL energies of CIS-based QDs. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Young-Shin Park: 0000-0003-4204-1305 Victor I. Klimov: 0000-0003-1158-3179 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

The work on the synthesis of CuInS2/ZnS quantum dots was supported by the Center for Advanced Solar Photophysics (CASP), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences. Single-dot spectroscopic studies were supported by the Chemical Sciences, Biosciences and Geosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy.

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